Mechanistic Understanding of Alloy Effect and Water Promotion for Pd

Subscriber access provided by UNIV OF DURHAM

Mechanistic Understanding of Alloy Effect and Water Promotion for Pd-Cu Bimetallic Catalysts in CO2 Hydrogenation to Methanol Xiaowa Nie, Xiao Jiang, Haozhi Wang, Wenjia Luo, Michael J. Janik, Yonggang Chen, Xinwen Guo, and Chunshan Song ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04150 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Mechanistic Understanding of Alloy Effect and Water Promotion for Pd-Cu Bimetallic Catalysts in CO2 Hydrogenation to Methanol Xiaowa Niea,#, Xiao Jiangb,#, Haozhi Wanga, Wenjia Luoe, Michael J. Janikc, Yonggang Chend, Xinwen Guoa,*, and Chunshan Songa,b,c,* a

School of Chemical Engineering, PSU-DUT Joint Center for Energy Research, State Key

Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024, P.R. China b

EMS Energy Institute, PSU-DUT Joint Center for Energy Research, Department of Energy and Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA c

PSU-DUT Joint Center for Energy Research and Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA

d

Network and Informationization Center, Dalian University of Technology, Dalian 116024, P.R. China e

School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P.R. China

#

X. Nie and X. Jiang are co-first authors who contributed equally to this work.

Corresponding Author Dr. Chunshan Song Email: [email protected] Dr. Xinwen Guo Email: [email protected]

-1-

ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Density functional theory (DFT) calculations on Pd-Cu bimetallic catalysts reveal that the stepped PdCu(111) surface with coordinatively unsaturated Pd atoms exposed on the top is superior for CO2 and H2 activation and for CO2 hydrogenation to methanol than the flat Cu-rich PdCu3(111) surface. The energetically preferred path for CO2 to CH3OH over PdCu(111) proceeds through CO2*HCOO*HCOOH*H2COOH*CH2O*CH3O*CH3OH*. CO formation from CO2 via reverse water-gas shift (RWGS) proceeds faster than CH3OH formation in terms of kinetic calculations, in line with experimental observation. A small amount of water, which is produced in-situ from both RWGS and CH3OH formation, can accelerate CO2 conversion to methanol by reducing the kinetic barriers for O-H bond formation steps and enhancing the TOF. Water participation in reaction alters the rate-limiting step according to the degree of rate control (DRC) analysis. Compared to CO2, CO hydrogenation to methanol on PdCu(111) encounters higher barriers, thus is slower in kinetics. Complimentary to the DFT results, CO2 hydrogenation experiments over SiO2-supported bimetallic catalysts show that the Pd-Cu(0.50) that is rich in PdCu alloy phase is more selective to methanol than the PdCu3-rich Pd-Cu(0.25). Moreover, advanced CH3OH selectivity is also evidenced on Pd-Cu(0.50) at specific water vapor concentration (0.03 mol%), whereas that of Pd-Cu(0.25) is not comparable. The present work clearly shows that the PdCu alloy surface structure has a major impact on the reaction pathway, and the presence of water can substantially influence the kinetics in CO2 hydrogenation to methanol.

Keywords: CO2 hydrogenation; Methanol; Pd-Cu bimetallic catalyst; Density functional density; Alloy effect; Water promotion

-2-


Page 2 of 65

Page 3 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1. Introduction With the increasing atmospheric CO2 concentration and the heightened concerns for climate change, global efforts are required to reduce emissions of greenhouse gas CO2.1,2 One of the approaches is to utilize CO2 as a carbon source in reactions with hydrogen produced from water using renewable energy to synthesize industrial chemical feedstocks and transportation fuels; doing so will reduce CO2 emissions and mitigate the dependence on fossil fuels.3-10 The synthesis of methanol from CO2 and H2 (CO2 + 3H2 CH3OH + H2O) has attracted increasing attention due to its potential application as a liquid energy/hydrogen carrier, as alternate fuel and commodity chemicals.4,6,11-24 The typical industrial CH3OH synthesis is conducted using syngas (CO-H2) conversion over a Cu-ZnO/Al2O3 catalyst between 5~10 MPa at 493-573 K.25 CH3OH synthesis from CO2 hydrogenation is exothermic; the conversion of CO2 to CH3OH is kinetically slow at low temperature and thermodynamically limited at high temperature, leading to a low CO2 conversion of 15~25% and low CH3OH yield of only 0.06 % at 0.1 MPa and 573 K.26 Current efforts focus on the development of highly active, selective and stable catalysts for methanol synthesis from CO2 hydrogenation.14-16 Cu-based materials have been studied for CH3OH synthesis from CO2,27-37 however, the catalytic activity is structure-dependent and sensitive to the composition of Cu-based catalysts. Previous studies have shown that adding promoters such as K, La, and Ba can improve the adsorption strength of CO2, stabilize surface intermediates, and enhance CH3OH selectivity.38-40 An appropriate support not only impacts the formation and stabilization of the active phase of the catalyst but also is capable of modulating the interactions among the adsorbates, catalyst surface and the promoter.41-49 A previous study by Yang and coworkers showed significant CH3OH promotion by the presence of small amount of water in low -3-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperature conversion of CO/H2 and CO2/H2 mixtures to methanol over Cu catalysts.50 Zhao, et al. performed DFT calculations to investigate the reaction network and effect of water on CO2 hydrogenation to methanol on Cu(111); they found that a significant decrease of the kinetic barrier for carboxyl (-COOH) formation from CO2 hydrogenation via a unique hydrogen transfer process with H2O involved, which leads to a favorable reaction pathway for CH3OH production via a -COOH species rather than going through a formate (-HCOO) intermediate.51 On the other hand, noble metal-based catalysts, such as Pd-based materials, have also been examined for the synthesis of methanol from CO2 and CO hydrogenation, and these catalysts show superior activity towards methanol production at lower temperatures.52-57 Our previous work showed enhanced activity for CO2 hydrogenation to CH3OH on Pd catalysts supported on mesoporous silica, wherein the nano-sized pore channels could confine the growth of Pd0 particles.58 However, current Pd catalysts yield smaller amount of CH3OH compared to Cu-based catalysts,59 albeit the Pd-based catalysts would be a potential candidate for low-temperature CH3OH synthesis from CO2 hydrogenation. Previous studies have shown that the combination of Pd with Cu exhibits superior CH3OH formation rate and selectivity than the monometallic Pd or Cu catalysts.60-63 The combination of Pd and Cu can lead to the formation of alloys which might produce particular active sites for CO2 activation and CH3OH formation.60,64 Therefore, it would be of great interest to investigate the effect of Pd-Cu alloy structures on CH3OH synthesis from CO2/H2. Moreover, understanding the mechanisms of CH3OH production from CO2 hydrogenation catalyzed by bimetallic Pd-Cu alloys at molecular level can provide useful information for future design of highly active, selective and stable catalysts. In our previous experimental work,63 we have enhanced CH3OH formation rate and selectivity with the bimetallic Pd-Cu catalysts, and strong bimetallic synergistic effect was observed when the

-4-


Page 4 of 65

Page 5 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Pd/(Pd+Cu) atomic ratios fall within the range of 0.25~0.34. Furthermore, the Pd-Cu composition that promotes CH3OH formation is different from that for enhancing CO formation, which suggests that certain alloy structures are more favored for CO2 hydrogenation to CH3OH. Detailed XRD analysis showed that alloys formation, particularly Pd-Cu and Pd-Cu3 type alloy structures, seems to be more favorable for the observed methanol promotion from CO2 hydrogenation. The experimental results provide a very interesting case for a computational study on the following questions. What types of Pd-Cu bimetallic alloy structures on the surface facilitate CO2 hydrogenation to CH3OH? What are the preferred pathways for CH3OH formation from CO2? What are the crucial factors for enhancing the CH3OH selectivity? In the literature, two possible paths have been proposed for CH3OH synthesis from CO2 hydrogenation over Cubased catalysts. One is through a formate (-HCOO) intermediate without formation of CO. The other possible path involves a carboxyl (-COOH) intermediate, through which CO2 is first converted to CO via the reverse water-gas shift (RWGS) reaction and then CO is hydrogenated to produce CH3OH eventually. The dominant pathway and key intermediates for CH3OH synthesis from CO2/H2 over Cu-based catalysts are still controversial to date.16,50,51,65,66 Adding Pd into Cu can modify the structure and surface electron properties of the Cu catalysts, which will impact the stability of intermediates and transition states and thus may alter the reaction path and modulate the selectivity. The objective of this paper is (i) to identify the superior Pd-Cu alloy for CO2 hydrogenation to CH3OH, (ii) to develop mechanistic understanding on how the particular Pd-Cu alloy structure facilitates CO2 activation and CH3OH formation, and (iii) to uncover the effect of water on CH3OH formation pathway and reaction kinetics. Experimental work was also conducted on characterizations and testing of Pd-Cu catalysts to help verify the DFT calculations.

-5-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2. Methods 2.1 Computational Details 2.1.1 Electronic Structure Methods DFT calculations were performed using Vienna ab initio simulation package (VASP) with the ion cores represented by the projector augmented wave (PAW) potentials provided by VASP.67-70 The exchange and correlation energies were treated with the spin-polarized generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerh of (PBE) functional.71 The cutoff energy for the plane-wave basis was set to 400 eV. Geometries were relaxed using a damped molecular dynamics method until the forces on all atoms were less than 0.03 eV/Å. Transition states were searched using the climbing image nudged elastic band (CINEB) method.72 The minimum energy path was examined using 5~8 images, including the initial and final state, during the transition state search. Each transition state was confirmed to have a single imaginary vibrational frequency along the reaction coordinate. Bader charge analysis was performed, using the code developed by Henkelman and co-workers.73 Conversional DFT method involves no nonlocal electron-electron correlations (dispersion forces) and therefore is unable to accurately describe the physisorption of molecules on the catalyst surfaces. The simplest and most popular is the DFT-D scheme proposed by Grimme, in which an empirical damped atom-atom R−6 potential term is added to the standard DFT energy at negligible additional computational expense.74,75 PBE-D3 is one of the widely used dispersion-corrected GGAs which consists of an atom pairwise sum over C6R−6 potentials, and is properly implemented in VASP.76-81 In this work, we used PBE-D3 to deal with the dispersion effect of the weakly adsorbed molecules on the catalyst surfaces.

-6-


Page 6 of 65

Page 7 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The binding energies of adsorbates on the Pd-Cu surfaces, BE(adsorbate), were calculated by BE(adsorbate) = Eadsorbate+surface – (Eadsorbate + Esurface), in which Eadsorbate+surface is the total energy of the adsorbate interacting with the Pd-Cu slab, Eadsorbate represents the energy of the adsorbate in gas phase, and Esurface is the energy of bare Pd-Cu surface. Zero point energy, heat capacity, and entropy were computed with standard methods, and then used to convert the electronic energies into free energies at temperature of 523 K to allow the comparison with experimental data. 2.1.2 Surface Models Our previous experimental study on bimetallic Pd-Cu catalysts for CO2 hydrogenation to methanol has identified the coexistence of alloy phase PdCu3(111) and PdCu(111) in the superior catalysts that show dramatically enhanced CH3OH formation rate and selectivity.63 The results of XRD showed the dependence of alloy type on catalyst composition, and the trend also appears to relate to the methanol formation activity. Therefore, it is suggested that that (111) facet should be the active sites and mainly contribute to the observed strong synergetic effect on methanol synthesis. Based on the experimental results, we chose PdCu(111) and PdCu3(111) as catalyst surface models in computation in this work, which provides a straightforward link between theory and experiment. The PdCu3 alloy has been found to be in the face-centered cubic (fcc) structure while the PdCu alloy is in the body-centered cubic (bcc) phase.63 Based on literature,82 the 1:1 PdCu composition at temperatures lower than 598 °C is stable in the CsCl structure which is a typical bcc structure; DFT calculations on formation energy demonstrated that the fcc phases are less stable than the bcc structure for 1:1 PdCu; the PdCu3 alloy was found to have ordered fcc structure. Therefore, in the present work, we used bcc PdCu and fcc PdCu3 bulk alloy structures (Supporting Information Fig. S1) to construct the (111) surfaces for the two types of Pd-Cu

-7-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

alloys, respectively. The lattice constant for bulk PdCu3 and

Page 8 of 65

PdCu were fixed at their

experimental values of 3.71 and 2.96 Å, respectively.83 For PdCu3(111), a p(3×3) surface slab including 4 atomic layers was used, with a vacuum thickness of 12 Å to avoid interactions between the repeating slabs. For PdCu(111), two types of (111) surfaces were considered, with Pd and Cu terminated, respectively. The Pd and Cu-terminated PdCu(111) are stepped surfaces, different from the PdCu3(111) which is a flat facet. A p(2×2) supercell including 8 atomic layers was used and the vacuum thickness was set to 12 Å. To sample the two-dimensional electronic Brillouin zone of the periodic supercell of PdCu3(111), we used a grid of 2×2×1 k-point within the Monkhorst-Pack scheme. A k-space mesh of 3×3×1 was employed for PdCu(111) surfaces. In our calculations, the bottom two atomic layers of PdCu3(111) and the bottom four atomic layers of PdCu(111) were fixed at their original atomic positions in the bulk structure while all remaining layers together with the adsorbates were fully relaxed during structural optimization. The optimized structures of PdCu3(111) and PdCu(111) surfaces are shown in Fig. S2. 2.1.3 Kinetic Parameters and Microkinetic Modeling Standard transition state theory (TST) was used to estimate the rate constant and equilibrium constant of surface reactions.84-86 Only the vibrational and electronic partition functions were taken into account for surface intermediates and transition states. The rate constants for forward and reverse reactions were calculated according to:

= ℎ

where kB, T, h, Ea, and ZPE are Boltzmann constant, reaction temperature, Planck’s constant, activation barrier calculated from DFT, and Zero Point Energy, respectively. The qTS and qIS or FS are the harmonic vibrational partition functions for the transition state and the initial (or final) state, respectively. The vibrational partition functions were obtained by: -8-


Page 9 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

q= "

1−

1

/

where vi is the vibrational frequency for each vibrational mode of the adsorbed intermediate and transition state calculated from DFT, i runs to N and N-1 for the initial (or final) state and transition state, respectively. The thermodynamic equilibrium constant (Keq) is obtained based on: #$% =

&'( $

The reaction kinetics are computed by the mean-field microkinetic modeling, based on the pseudo-steady state hypothesis. This approach involves solving a system of coupled ordinary differential equations describing the fractional surface coverages, and is widely used in heterogeneous catalytic reactions.22,87,88 2.2 Experimental Details Catalyst preparation. The Pd-Cu bimetallic catalysts were prepared by the coimpregnation method using an acetone solution of Pd(CH3COO)2 (Aldrich, > 99.9%) and Cu(NO3)2·2.5H2O (Alfa Aesar, ≥ 98 %) as described elsewhere.63 Amorphous silica was used as the support (Davisil Grade 62, particle size=75-250 × 10−6 m). To correlate our DFT results with experiments, the Pd/(Pd+Cu) ratio of 0.25 and 0.50 were chosen, wherein the metal components of the former were fully consumed in the formation of PdCu3 alloy as evidenced by characterization results,63 while the latter was selected based on the alloy phase diagram with PdCu as the major phase.63 Note that the total metal loading was fixed at 15.7 wt% (support weight basis) for comparison. The bimetallic catalysts were hereafter denoted as Pd-Cu(X), where the X represents the Pd/(Pd+Cu) atomic ratio.

-9-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Catalytic test. The detailed procedure for catalytic activity test is described elsewhere.63 Briefly, the gases, H2 (99.995 %) and CO2/H2/Ar (CO2/H2=1/3, 99.995 %) were used without further purification. For a typical activity test, 0.2 g of catalyst was charged and diluted by inert amorphous silica (ca. 0.42 g) with the same particle size to maintain a constant aspect ratio of 6.0. The catalyst was reduced at 573 K in H2 (ramp rate, 2.3 K min-1) for 2 h, followed by cooling down to 523 K. Then, the feed gas CO2/H2/Ar was employed to pressurize the system to 3 or 5 MPa, and the GHSV varied from 6000 to 30000 mL (STP) g-1 h-1. Gaseous products were periodically sampled with computer-controlled gas samplers and analyzed by online GC/TCD and GC/FID (SRI 8610C), wherein the former was used for analysis of Ar, CO and CO2, while the latter was used for analysis of CH3OH. Ar and CH4 were used as internal standards for GC/TCD and GC/FID analyses, respectively. Activity and selectivity reported in this paper were determined using the data obtained at 4-5 h on stream. In an attempt to verify the water effect on the catalytic property, a certain amount of water vapor was added to the feed gas. As reported by Campbell et al.,50 the presence of H2O evidently shortened the induction period of CH3OH synthesis with the H2O concentration ranging from 0.04 to 0.2 mol%, and its concentration also affected the CH3OH formation at steady state. Hence, the H2O concentration was varied from 0.03 to 0.1 mol% in this work. H2O vapor was fed into the system after the system has reached the desired reaction conditions, and the line was wrapped with heating tape and maintained at ca. 383 K. The detailed procedure for activity test and data analysis with water addition were similar as described above, and GHSV ranged from 6000 to 30000 mL (STP) g-1 h-1. Passivation. To preserve the bulk information, the freshly reduced catalysts were passivated using the gas mixture consisting of 0.970 vol% O2/99.030 vol% He (99.995%) at

-10-


Page 10 of 65

Page 11 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

ambient temperature for 4 h (ca. 20 mL (STP) min-1) and hereafter denoted as “reduced” catalysts. Characterization. XRD patterns of the reduced catalysts were collected using a PANalytical Empyrean X-Ray Diffractometer with Cu Kα (λ=0.154059 nm) radiation, and the detailed procedure was described elsewhere.63 The morphological properties of Pd-Cu bimetallic catalysts were studied by scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDS) using a FEI Talos F200X TEM at an accelerating voltage of 200 kV. The samples were dispersed in ethanol and sonicated for 10 min. A few droplets of the supernatant liquid were dropped on a carbon-coated molybdenum grid, followed by drying at ambient temperature. EDS maps were acquired in the Talos using Bruker Super-X quad EDS detectors at a beam current of 0.12 nA for approximately 5 min. A Standardless Cliff-Lorimer quantification was applied on the deconvoluted EDS line intensity data using the Bruker Esprit software. 3. Results and discussion To shed light on the effect of Pd-Cu alloy structures on CH3OH synthesis, we examined the structural properties and energetics for adsorption, activation, and dissociation of the reactant CO2 and H2, as well as the energy profiles associated with CO2 hydrogenation on PdCu3(111) and PdCu(111) surfaces.

-11-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b) Fig. 1. (a) The initial, transition, and final states associated with adsorption and dissociation of H2, and (b) the adsorption configurations of CO2 on PdCu3(111). (blue=palladium, orange=copper, carbon=grey, oxygen=red, white=hydrogen) 3.1 Adsorption, activation, and dissociation of H2 and CO2 On PdCu3(111), the energetically most stable adsorption of H2 occurs on the top Pd site, and the adsorption energy is -0.21 eV. Cu sites are unable to stabilize molecular H2. The dissociation of H2 from the adsorbed state to two H* atoms needs to overcome an energy barrier of 0.31 eV and the reaction is 0.45 eV exothermic. The dissociated H* atoms are favorably adsorbed at two hcp sites consisting of mixed Pd and Cu atoms. Optimized structures associated with H2 adsorption and dissociation processes on PdCu3(111) are shown in Fig. 1(a). CO2 molecule was found to weakly adsorb on the PdCu3(111) surface, on which CO2 linearly suspends above the Pd-Cu bridge site with the two O atoms facing towards the Pd and Cu atoms, respectively, as

-12-


Page 12 of 65

Page 13 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

show in Fig. 1(b). The adsorption energy for linear CO2 is only -0.02 eV from DFT (PBE). Dispersion correction increases the adsorption strength of linear CO2 to -0.26 eV. A bent configuration of CO2 adsorption was also found, in which the C atom interacts with the top Pd atom and one of the O atoms bound to the Cu atom within a Pd-Cu bridge bond (see from Fig. 1(b)). The adsorption for bent CO2 is endothermic, which is 0.37 eV. When dispersion correction was applied, the bent structure of adsorbed CO2 is unable to be stabilized and eventually optimized to the linear structure. Although dispersion correction increases the adsorption strength of CO2 which would be more appropriate to capture the weak van der Waals interactions between adsorbates and the metal surface, the observed adsorption features suggest that the attractive interactions between CO2 and the PdCu3(111) surface is still weak. The dissociation of CO2 to adsorbed CO* and O* on PdCu3 (111) unlikely occurs due to a large energy barrier of 1.94 eV which is endothermic by 0.35 eV.

(a)

(b)

-13-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c) (d) Fig. 2. (a) The initial, transition, and final states associated with adsorption and dissociation of H2 on the Pd-terminated PdCu(111), (b) adsorption configurations of CO2 on the Pd-terminated PdCu(111), (c) adsorption sites for H2 molecule on the Cu-terminated PdCu(111), and (d) CO2 adsorption on the Cu-terminated PdCu(111). (blue=first-layer palladium, green=second-layer palladium, orange=first-layer copper, pink=second-layer copper, carbon=grey, oxygen=red, white=hydrogen) On the Pd-terminated PdCu(111) surface, coordinatively unsaturated Pd atoms are exposed on top of the surface. Sub-layered Cu atoms connecting to top-layered Pd atoms produces stepped Pd-Cu bridge sites. This stepped surface with Pd atoms exposed on the top is likely a superior candidate for reactants adsorption and intermediates stabilization. The energetically most stable H2 adsorption is on the top-layered Pd site, and the resulting adsorption energy is 0.40 eV, 0.19 eV stronger than that calculated on the PdCu3(111) surface. H2 dissociation to two H* atoms has an energy barrier of 0.42 eV and is 0.45 eV exothermic. The dissociated H* atoms are stabilized at Pdtop-Pdsub-Cusub hollow sites. All states involved in H2 adsorption and dissociation on the Pd-terminated PdCu(111) surface are show in Fig. 2(a). For CO2 molecule, we found both linear and bent adsorption configurations, as illustrated in Fig. 2(b). The adsorption energies for these two structures are -0.09 and -0.08 eV calculated by PBE functional, and are -0.35 and -0.39 eV, respectively determined by PBE-D3. Dispersion correction does not change the adsorption configurations of CO2, only increasing the adsorption strength. The Pdterminated PdCu(111) surface is catalytically more active for both H2 and CO2 adsorption than

-14-


Page 14 of 65

Page 15 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

the PdCu3(111) surface. Direct dissociation of CO2 to adsorbed CO* and O* unlikely occurs on this surface due to a huge kinetic barrier of 2.82 eV which is endothermic by 0.88 eV. The Cu-terminated PdCu(111) surface exposes low-coordinated Cu atoms on top of the surface. We identified two unique adsorption sites for H2 molecule, which are on top Cu and sublayered Pd sites, as shown in Fig. 2(c). The adsorption energies of H2 on these sites are -0.19 and -0.11 eV, respectively, much lower than that (-0.40 eV) calculated on the Pd-terminated PdCu(111) surface. For CO2 adsorption on this surface (see Fig. 2(d)), we obtained an adsorption energy of -0.31 eV with PBE-D3, still weaker than that (-0.39 eV) calculated on the Pd-terminated surface. Therefore, the Pd-terminated surface is more active for reactant adsorption. The direct dissociation of CO2 on Cu-terminated PdCu(111) has an activation barrier of 2.48 eV and thus is kinetically inhibited. Our ongoing DRFITS study also provides some additional evidence on the effect of Pd-Cu composition on the surface species formed during CO2 hydrogenation. Three bands are discernible at 2054, 1922, and 1831 cm-1, corresponding to the linearly bonded CO species on either edges or corners (COL), di-coordinated bridging CO species (COB), and triply-bridging CO species on “hollow” sites in the lattice (COH), respectively.89-93 Among all, COL showed the strongest peak intensity. As reported, the proportion of COL varies with the particles size90. Considering the nano-sized particles identified from our previous work on the same catalyst, COL is the most abundant carbonyl species on the surface during reaction.63 In other words, Pd-terminated alloy particles are suggested to dominate the surface, which provides guidance for constructing the surface model in our computational work. Based on the computational results and experimental evidence, the Pd-terminated surface is expected to be the dominant active sites of PdCu(111) for CO2 conversion to methanol.

-15-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Although some experimental literature reported dissociative adsorption of CO2 on pure Cu,94-96 several theoretical studies on methanol synthesis from CO2 hydrogenation have demonstrated that CO2 direct dissociation is energetically unfavorable (highly endothermic) and has large kinetic barriers (1.3~1.8 eV) on flat Cu(111).16,97-99 On the Pd-Cu alloy surfaces studied in the present work, direct dissociation of CO2 to CO* and O* was found to be kinetically unfeasible due to huge energy barriers and the RWGS rate would be dominantly determined by CO2 hydrogenation to COOH* coupled with COOH* decomposition to CO*. 3.2 Effect of Pd-Cu alloy structure on the initial hydrogenation of CO2 We investigated the effect of Pd-Cu alloy structure on the initial hydrogenation of CO2 to the carboxyl (COOH*) and formate (HCOO*) intermediates on PdCu3(111) and the Pdterminated PdCu(111) surface. The energy profiles associated with these elementary steps are plotted in Fig. 3(a) and (b), with the structure of each state included.

-16-


Page 16 of 65

Page 17 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fig. 3. Energy profiles for initial hydrogenation of CO2 to form the trans-COOH* and HCOO* intermediates on (a) PdCu3(111) and (b) the Pd-terminated PdCu(111) surface. (The structure of each state is given. The activation barrier and reaction energy for each step are listed in the parenthesis.) The carboxyl (COOH*) has been found to be a key intermediate in water-gas shift (WGS) on Cu-based catalysts.100-102 On PdCu3(111), the COOH* adsorbs at the Pd-Cu bridge site in two configurations as the trans-COOH* and cis-COOH*, respectively, depending on the orientation of the hydroxyl group. These two configurations are almost equal in thermodynamic stability with only 0.03 eV difference in energy of adsorbed states and have an activation barrier of ~0.5 eV for mutual transformation. As the activated H* comes from the bimetallic surface, the structural preference tends to form the trans-COOH intermediate from CO2 hydrogenation, in which the surface H* directly attacks the O atom of CO2, as shown in Fig. 3(a). In parallel, CO2 hydrogenation also possibly generates a formate (HCOO*) intermediate via attacking the C atom

-17-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of CO2 by the surface H*. A stable bidentate HCOO* configuration was found, in which the two O atoms bond to surface Pd and Cu, respectively, as can be observed from Fig. 3(a). The adsorption energy for HCOO* is -2.59 eV, 0.72 eV stronger than that for trans-COOH*. The reaction energy for CO2 hydrogenation to trans-COOH is 0.38 eV endothermic and the energy barrier is 1.53 eV. CO2 hydrogenation to HCOO* requires to surmount a barrier of 1.29 eV and the reaction is 0.03 eV exothermic. Formation of HCOO* is energetically more favorable than COOH*. However, on PdCu3(111), CO2 adsorption is relatively weak and large hydrogenation barriers are observed for both COOH* and HCOO* formation. On the Pd-terminated PdCu(111) surface, we only considered a trans-COOH* configuration through formation of an O-H bond via CO2 hydrogenation. Trans-COOH* is stabilized in between two adjacent coordinatively unsaturated Pd atoms on the surface via bonding with the C and O atoms, as shown in Fig. 3(b). The other possible intermediate from hydrogenation of CO2 is the HCOO*. We found a stable bidentate configuration of HCOO* on PdCu(111), in which the two O atoms interact with two neighboring top-layered Pd atoms on the surface, as can be seen in Fig. 3(b). The adsorption energy of HCOO* is -2.75 eV, which is 0.60 eV stronger than that for trans-COOH*. The formation barriers for trans-COOH* and HCOO* are 1.32 and 0.62 eV, respectively, and the formed HCOO* is thermodynamically more stable than trans-COOH* whose formation also proceeds fast in kinetics. The energy profiles clearly demonstrate a drastic preference for HCOO* formation from CO2 hydrogenation on the PdCu(111) surface.

-18-


Page 18 of 65

Page 19 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fig. 4. Structural and charge properties of the initial and transition states associated with HCOO* formation on (a) the Pd-terminated PdCu(111) and (b) PdCu3(111)) surface. Due to the substantial kinetic preference for CO2 hydrogenation to HCOO* on PdCu(111), we further delved into the effect of alloy surface structure through geometric analysis and charge investigation. Fig. 4 shows the structural and charge properties of the initial and transition states associated with HCOO* formation on the two Pd-Cu alloy surfaces. In the initial state configuration on PdCu(111), the stepped surface with coordinatively unsaturated Pd atoms exposed on the top stabilizes the H* and CO2* in a favorable co-adsorption state for formation of the H-C bond (with distance of 3.05 Å), through which the transition state is quite facile to form due to small surface steric effect and closer H-C distance. The H-C distance in the TS is 1.61 Å. -19-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The low-coordinated Pd atoms stabilize the CO2 molecule in the bent structure and activate CO2 sufficiently upon adsorption onto the surface. However, in the initial state on PdCu3(111), the linear CO2* is more stable than the bent structure on the flat surface, and the co-adsorbed H* is 3.44 Å away from the C atom of CO2* which is stabilized at a hcp site consisting of mixed Cu and Pd atoms. In the transition state configuration, CO2 approaches to the surface H* by bending the C-O bond and the H* moves out of the hcp center to an adjacent top Cu site. Large structural variations were observed comparing the initial and transition states. In the formation process of the TS, the PdCu3(111) surface plays a major role in activating CO2 and drives CO2 approached to the surface H* which would require to overcome a high energy barrier. We observed that CO2 becomes bended during the TS formation and the closest distance between C of CO2 and the surface atoms is around 2.5 Å while CO2 is approximately 3.5 Å away from the surface in the initial state. These structure properties signify superior catalytic activity of the stepped Pd-rich PdCu(111) surface, on which CO2 adsorption and activation are more sufficient than the flat Curich PdCu3(111). Bader charge analysis for the initial and transition states involved in HCOO* formation on the two surfaces shows that the initial state of co-adsorbed H-CO2* is 0.53δnegatively charged on PdCu(111) while this state is only 0.29δ- negatively charged on PdCu3(111), showing more pronounced activation of the reactants on PdCu(111). In the formation process of transition state, there is less electrons transfer between the co-adsorbed state and the PdCu(111) surface as we observed that the charges on the TS is 0.51δ- which is 0.53δ- on the initial state. While on PdCu3(111), the bimetallic metal surface necessitates to transfer more electrons to the co-adsorbed state to activate CO2 which would require higher energy to form the TS (The charges on the TS is 0.52δ- which is 0.29δ- on the IS). These charge properties coincide

-20-


Page 20 of 65

Page 21 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

with the structural analysis which clearly reveal that the PdCu(111) is more favorable for the TS formation of HCOO*. Although the X-ray diffraction (XRD) and high-angle annular dark-field (HAADF) images indicate the uniform dispersion of Pd-Cu catalysts, the surface segregation is still possible under reaction conditions. Norskov and co-workers103 examined the surface segregation energies of transition-metal impurities for the close-packed surfaces of transition metal hosts and showed that introducing Pd into Cu host causes moderate surface segregation, indicating that the Pd atoms may enrich the surface region and form “core-shell” structures. A previous DFT study showed that Pd prefers to substitute top-layered Cu and the authors employed Pd/Cu(111) models with varied surface coverages of Pd to represent the Pd-Cu bimetallic catalysts.104 Some other computational studies with M/Cu (M=Pd and Au) catalysts used similar approach to construct the bimetallic models.105,106 To elucidate the effect of surface segregation of Pd-Cu catalysts on CO2 hydrogenation, we constructed a Pd-Cu bimetallic model based on Cu(111) and replaced the Cu atoms on the topmost layer by a monolayer (ML) Pd atoms, denoted as 1ML Pd/Cu(111), on which the adsorption of H2 and CO2 as well as initial hydrogenation of CO2 to HCOO* and COOH* were examined. Relevant results are provided in Figs. S3 and S4. We found that both H2 and CO2 adsorption are weaker than that on the Pd-terminated PdCu(111) alloy surface, implying that the Pd-Cu alloy is more active towards reactant adsorption and activation. With respect to the initial hydrogenation of CO2, the PdCu(111) alloy exhibits a dramatic preference for HCOO* formation with a low activation barrier of 0.62 eV whereas the activation barriers for COOH* and HCOO* formation are higher than 1 eV on the 1ML Pd/Cu(111) surface. These results indicate that the Pd-Cu alloy behaves differently from the “core-shell” structure, and the enhanced HCOO* formation on PdCu alloy would promote

-21-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CH3OH production. The Pd-Cu alloys are likely promising catalysts for CH3OH synthesis via CO2 hydrogenation, focusing on which we delved into the effect of alloy structure on CO2 hydrogenation to CH3OH in the present work. By comparison of the energy of pathways for COOH* and HCOO* formation on the two types of Pd-Cu alloy surfaces, the Pd-terminated PdCu(111) surface shows better capability for reactants adsorption, activation, and also exhibits superior performance for initial CO2 hydrogenation, particularly towards formation of HCOO* intermediate. The structures of Pd-Cu alloys significantly impact the surface chemistry in CO2 hydrogenation, and the stepped PdCu(111) with coordinatively unsaturated Pd atoms exposed on top of the surface is predicted to be a superior candidate for CO2 conversion to CH3OH (this prediction is supported by the experimental results discussed in Section 3.5.2). We therefore proceeded with the subsequent study of reaction network for methanol synthesis based on the Pd-terminated PdCu(111) surface. 3.3 Reaction network for CO2 hydrogenation to CH3OH over PdCu(111) CO2 and H2 in the feed can convert to CO and H2O via RWGS. Two possible pathways have been reported for CH3OH synthesis from CO2 and H2 over Cu-based catalyst.16,50,51,65,66 One is the “Formate” path, in which the reaction proceeds through CO2HCOOH2COO(or HCOOH)H2COOHCH2OCH3OCH3OH. The other path goes through RWGS and the subsequent conversion of CO to CH3OH via COCHOCH2OCH3OCH3OH which is referred as the “RWGS+CO-Hydro” path. Some previous studies suggested that CH3OH synthesis proceeds through the formate pathway on Cu surfaces;51,65,107,108 however, the catalytic activity is structure-dependent and sensitive to the composition of Cu-based catalysts. The present study focuses on the bimetallic effect of PdCu(111) alloy on the aforementioned two paths for CH3OH synthesis. -22-


Page 22 of 65

Page 23 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

3.3.1 The “Formate” path

Fig. 5. Surface reaction sequences for CO2 conversion to methanol in the “Formate” path and the “RWGS+CO-Hydro” path on PdCu(111). The results in section 3.2 show that HCOO* is the preferred intermediate from CO2 hydrogenation on PdCu(111), whose kinetic barrier is 0.62 eV; therefore, we first examined this “Formate” path. CO2 hydrogenation to HCOO* is quite facile, hence methanol production is rate limited by some other steps in the path. Fig. 5 (steps connected with red and black arrows) illustrates the surface elementary reactions of CO2 conversion to CH3OH in the “Formate” path where the optimized configurations of all the states involved in the path are shown in Fig. S5. Based on the elementary energetics, the preferred intermediate from the hydrogenation of HCOO* is HCOOH* rather than H2COO*. Subsequently, the conversion proceeds through formation of H2COOH*, CH2O*, CH3O* intermediates, and eventually produces CH3OH* on the catalyst surface. H2O can be produced from the decomposition of H2COOH* accompanied

-23-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by hydrogenation of the formed –OH* species. To examine the stability of H2O on the catalyst surface, we calculated the adsorption energy of H2O* on PdCu(111) which was found to be -0.39 eV. The dissociation of H2O* to surface –OH* and H* was also examined (optimized structures are provided in Fig. S6) and this process has a kinetic barrier of 1.05 eV and is 0.53 endothermic. Therefore, molecular H2O* is stable on the catalyst surface and its formation from hydrogenation of surface hydroxyl is energetically more favorable. Specific descriptions of other intermediates formation in this path are detained in the Supporting Information.

Fig. 6. Energy profiles of CO2 hydrogenation to methanol on PdCu(111) through the “Formate” and “RWGS+CO-Hydro” pathways. (Transition states of the highest barrier step in the two pathways are shown in the figure. For legibility consideration, H* was omitted from the labels after the adsorption of six H atoms in the first step.) The energy profile for CO2 hydrogenation to CH3OH on PdCu(111) through the “Formate” pathway is plotted in Fig. 6 (red lines). Optimized structures of the initial, transition, and final states are provided in Fig. S7, and key structural parameters are given in Table S1. Only -24-


Page 24 of 65

Page 25 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

HCOO* and CH3OH* formation are thermodynamically downhill, and other elementary steps are all endothermic. The effective barrier for the overall reaction from CO2 to methanol is determined to be 1.64 eV, and the elementary step of HCOO* hydrogenation to HCOOH* (an OH bond formation step) has the highest barrier of 1.28 eV, which may largely contribute to determine the rate for CH3OH formation. The formed HCOO* intermediate is quite stable on the PdCu(111) surface with an BE(HCOO) of -2.75 eV; hydrogenation on the O atom to form an O-H bond needs to break the strong O-Pd bond in adsorbed HCOO* which causes higher energy. 3.3.2 The “RWGS+CO-Hydro” path The other possible path for CH3OH synthesis goes through RWGS (CO2+H2CO+H2O) and subsequent CO hydrogenation to methanol via COCHOCH2OCH3OCH3OH. Fig. 5 (steps connected with blue and black arrows) illustrates the surface elementary steps of CO2 conversion to CH3OH in the “RWGS+CO-Hydro” path with the optimized geometries of all the states given in Fig. S5. Formation of trans-COOH* was found to have a kinetic barrier of 1.32 eV and is 0.20 eV endothermic on PdCu(111); its further conversion to CO* by breaking the strong C-O bond would be also slow in kinetics. COOH* decomposition along with hydrogenation of the formed –OH* species will produce H2O on the catalyst surface. Once CO* is formed, it will go through a series of hydrogenation steps to form the CHxO* species, and finally produce CH3OH*. Detained descriptions of these elementary steps are provided in the Supporting Information. The energy profile for CH3OH synthesis from CO2 on PdCu(111) through the “RWGS+COHydro” path is plotted in Fig. 6 (black lines). Optimized structures of the initial, transition, and final states are included in Fig. S7, and key structural parameters are given in Table S1. Decomposition of trans-COOH* to CO*+OH* encounters a large kinetic barrier of 1.32 eV, -25-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

comparable with that for trans-COOH formation from CO2 hydrogenation. The two elementary steps involved in RWGS both have substantial barriers and would be slow in kinetics for CO* formation. However, in CH3OH synthesis experiment,63 the formation rate of CO is faster than CH3OH, which seems to be in conflict with the current DFT results. We will reconcile this in the following section. As shown in Fig. 6, the effective barrier for the overall reaction from CO2 to CH3OH in the “RWGS+CO-Hydro” path is determined to be 1.86 eV, and the elementary steps of hydrogenation of CO2* to trans-COOH* and trans-COOH* to CO* both have large barriers around 1.3 eV, limiting the rate for CH3OH formation via CO intermediate. Provided that the rate-limiting step occurred before CO formation, the formation rate for CO vs. CH3OH would be identical. However, we observed faster formation rates of CO than CH3OH in experiment,63 indicating that either the “RWGS + CO-Hydro” path is not the dominant pathway for CH3OH production or there would be other factors impacting the kinetics for CO and CH3OH formation that we did not take into consideration in these DFT calculations. We will clarify these uncertainties in the following section on discussing the effect of water on methanol synthesis. 3.4 Effect of water on CH3OH formation path and rate over PdCu(111) Few experimental studies systematically investigated the effect of water on the rates of CO2 hydrogenation to methanol. Most DFT and microkinetic models do not consider water or waterderived -OH species in CO2 hydrogenation over Cu-based catalysts.16,65,66 A theoretical work by Zhao and co-workers51 examined the effect of water on the formation reactions of key HCOO* and COOH* intermediates from CO2 hydrogenation in CH3OH synthesis on Cu(111), and they found that the presence of water significantly promotes COOH* formation via a unique “Htransfer” process whereas the co-adsorbed water has a negligible effect on HCOO* formation. However, they only examined the water effect on two elementary steps for HCOO* and COOH* -26-


Page 26 of 65

Page 27 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

formation by including one explicit water molecule in their calculation model. Many previous DFT studies on electroreduction of CO2 on Cu catalysts have illustrated the important effect of solvent/H2O on the elementary energetics, and the reaction paths and rates could be drastically changed when H2O is involved.109-113 A recent experimental study by Yang et al.50 reported that the low-temperature conversions of CO/H2 and CO2/H2 mixtures to methanol over Cu catalyst are both assisted by the presence of small amount of water (mole fraction ~0.04-0.2%). They found that the water product from both methanol synthesis and RWGS serves to initiate both reactions in an autocatalytic manner in CO2/H2 mixtures. These studies suggest that the produced water in CO2/H2 reaction could impact the formation rate and selectively for methanol by modulating the stability of key intermediates/transition states, altering the elementary energetics, and thus influencing the dominant paths. To provide mechanistic insight, a thorough inspection of the impact of water on methanol synthesis over the bimetallic Pd-Cu catalysts is necessary. 3.4.1 H-transfer mechanisms with H2O inclusion In this work, we investigated the effect of water by including an explicit co-adsorbed H2O molecule in each elementary step for both the “Formate” and “RWGS+CO-Hydro” pathways, and examined the chemistry for C-O(H) bond scission, O-H bond formation, and C-H bond formation steps in methanol production. H2O participation can be grouped into two mechanisms, similar as those established in our previous work on CO2 electroreduction over Cu electrodes.109111

One is the “H-shuttled” mechanism in which the surface H* is transferred to a water molecule

which simultaneously shuttles another H to the adsorbate to accomplish the hydrogenation. The other is the “H2O-solvated” mechanism in which a water molecule is present to solvate the adsorbate nearby, and the hydrogenation occurs through direct transfer of the surface H* to the adsorbate. The “H-shuttled” mechanism reported in our previous work109-111 could also -27-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

rationalize the O-H bond formation and C-O(H) bond breaking steps, such as CO2*COOH* and COOH*CO*+OH*, in which the H2O molecule does not need to adsorb onto the surface and is present in gas phase to stabilize the adsorbate via H-bonding; an atomic H* is adsorbed onto the surface just underneath the H2O. The “H2O-solvated” mechanism is applicable for all CH bond formation reactions such as CO*CHO*, in which the H2O molecule is adsorbed onto the catalyst surface and present to solvate the adsorbate nearby via H-bonding. The transition state configurations of two representative steps (HCOO*HCOOH*; CO*CHO*) for CH3OH synthesis with H2O involved are shown in Fig. 7, in which the two “H-transfer” mechanisms are illustrated for the O-H and C-H bond formation reactions.

(a)

(b) Fig. 7. The transition state configurations of (a) HCOO*HCOOH* and (b) CO*CHO* steps involved in CH3OH formation with H2O included, from which the two “H-transfer” mechanisms are clearly shown for the O-H and C-H bond formation reactions on PdCu(111). 3.4.2 The effect of water on elementary energetics Fig. 8 illustrates the effect of H2O on elementary energetics for each step in the two paths. Optimized geometries for initial, transition, and final states with H2O involved are provided in -28-


Page 28 of 65

Page 29 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fig. S8, and key structural parameters are summarized in Table S2. The reaction energy and activation barrier for each elementary step in the two paths are listed in Table S3, without and with H2O included in the reaction for parallel comparison. In the “Formate” path, the reaction energy changes from that without H2O subtracting that with H2O lie in between 0~0.5 eV in absolute values for O-H bond formation and C-O(H) bond cleavage steps, and we observed 0.2~0.7 eV lowering of the kinetic barriers once H2O takes part in the reaction through the “Hshuttled” mechanism for these steps. For C-H bond forming steps, H2O inclusion in the reaction only slightly impacts the elementary kinetics and the barrier modifications were found to be less than 0.15 eV. In the “RWGS+CO-Hydro” path, we observed a similar trend for H2O effect on OH/C-O(H) and C-H bond chemistry. H2O inclusion in the reaction has negligible effect on C-H bond forming steps (|∆E| < 0.1 eV; |∆Eact| < 0.1 eV) but significantly decreases the kinetic barriers for O-H bond forming and C-O(H) bond breaking steps with |∆Eact| lie in between 0.4~0.7 eV, as shown in Fig. 8. These calculation results show that H2O greatly assists RWGS via hydrogenation of CO2* to CO* and H2O* through a COOH* intermediate. In the subsequent hydrogenation of CO*, only the final step of methanol formation from CH3O* hydrogenation is O-H bond formation which is substantially facilitated by H2O, as reflected by a 0.41 eV lowering of the kinetic barrier for this step. In the “Formate” path, if H2O is not included in the calculation, the HCOO* hydrogenation to HCOOH* has the highest barrier of 1.28 eV. However, this barrier reduces to only 0.58 eV when H2O participates in reaction, denoting a fast conversion of the HCOO* intermediate. Meanwhile, the highest barrier step changes to the hydrogenation of HCOOH* to H2COOH* which is a C-H bond formation step and is slightly impacted by H2O.

-29-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b) Fig. 8. The effect of H2O on reaction energetics for each elementary step in (a) the “Formate” and (b) “RWGS+CO-Hydro” paths. (∆E represents the reaction energy for the elementary step with water subtracted by that without water, and ∆Eact is defined in the same way for activation barrier.)

-30-


Page 30 of 65

Page 31 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

3.4.3 The effect of water on transition state formation We further analyzed the transition states for O-H bond formation and C-O(H) bond cleavage steps in the two pathways by examining the geometries and charge properties. In the formation processes for transition states associated with O-H bond formation, typical H3Oδ+ species is formed with 1 H2O inclusion, in line with our previous studies of CO2 electroreduction over Cu electrodes.88-90 Table 1 gives the charges on each part in the transition states for all O-H bond formation steps without and with H2O involved. Without H2O, the surface H* directly attacks the O atom of the reaming species (marked as ∆) and forms Hδ+ and ∆δ- assembly in the TS. Once H2O participates in the reaction, the TS contains a H3Oδ+ and a remaining species which is negatively charged (marked as ∆δ-). The electron interactions between the H3Oδ+ and the remaining species become stronger when H2O is involved, as evidenced by the increasing positive charges on the H3Oδ+ moiety and negative charges on the remaining parts for all O-H bond formation steps shown in Table 1, indicating a stabilization of the TS by H2O participation. With regard to C-O(H) bond cleavage steps, the H3O2* species is generated in the transition states under H2O existence which is negatively charged. H2O inclusion increases both the negative charges on the H3O2δ- moiety and also the total negative charges on the TS (see from Table 1), implying that the electron interactions between the adsorbates and the Pd-Cu surface become stronger with H2O participation. Such interactions enhance the stability of transition states and thus reduce the kinetic barriers for C-O(H) bond breaking steps. The charge densities for representative TS configurations of an O-H bond formation step (HCOO*+H*HCOOH*) and a C-O(H) bond cleavage step (COOH*CO*+OH*) with H2O involved are plotted in Fig. 9(a) and (b), respectively, from which we observed that the H3Oδ+ moiety in the TS (Fig. 9(a)) losses electron density and the excess charges are mainly localized in two areas: 1) between -31-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 65

H3O* and the remaining HCOO* moiety; 2) between H3O* and the Pd-Cu surface. In the TS for COOH* dissociation to CO*+OH* (Fig. 9(b)), we observed that the formed H3O2δ- species gains electron density and the Pd-Cu surface transfers electrons to the adsorbate H3O2* and CO*.

Table 1. Bader charges on each part in the TS configurations for all O-H bond formation and CO(H ) bond cleavage steps in the two reaction paths without and with H2O inclusion on PdCu(111). O-H bond formation

a)

HCOO* + H* HCOOH* + * CH3O* + H* CH3OH* + * CO2* + H* COOH* + * OH* + H* H2O* + * C-O(H) bond breaking H2COOH* + * CH2O* + OH* COOH* + * CO* + OH*

Without H2O H* ∆* a) 0.16 -0.57 0.13 -0.50 0.19 -0.50 0.02 -0.46 OH* ∆* a) -0.45 -0.08 -0.39 -0.13

With H2O H3O* ∆* a) 0.41 -0.69 0.55 -0.96 0.36 -0.58 0.24 -0.58 H3O2* ∆* a) -0.48 -0.08 -0.47 -0.11

“∆” represents the remaining species except the “H*”, “H3O*”, “OH*”, or “H3O2*” in the transition state configuration for each elementary step.

Fig. 9. Charge densities of the transition states for reactions of (a) HCOO* to HCOOH* and (b) COOH* to CO*+OH* with H2O involved on PdCu(111). (Yellow and cyan isosurfaces represent charge accumulation (i.e. gain of electron density) and depletion (i.e. loss of electron density) in the system, respectively) 3.4.4 The effect of water location and the number of water molecules

-32-


Page 33 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

To explore the effect of water location on the reaction, we in turn applied the “H2O-solvated” mechanism to examine CO2* hydrogenation to COOH* on PdCu(111) which is an O-H bond formation step (optimized structures are provided in Fig. S9(a)). The calculated activation barrier is 1.36 eV, 0.70 eV higher than that calculated based on the “H-shuttled” mechanism identified in this work. In addition, we also examined changing the H2O location for a typical C-H bond formation step of CO* hydrogenation to CHO*, in which the “H-shuttled” mechanism” was used instead of the pre-applied “H2O-solvated” mechanism (optimized structures are provided in Fig. S9(b)). We found that the activation barrier is 1.58 eV, still kinetically unfavorable than direct H-transfer in the “H2O-solvated” mechanism identified in this work, which has a barrier of 0.72 eV. These results further rationalize the computational models of H-transfer mechanisms employed in the present work for investigating H2O effect on surface reactions.

Fig. 10. The activation barriers for CO2* hydrogenation to COOH* and CO* hydrogenation to CHO* steps as s function of the number of H2O molecules on PdCu(111).

-33-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To investigate the effect of the number of H2O molecules on the reaction, we introduced more water (2, 3 and 6 molecules) into the system and examined the minima energy pathways for hydrogenation of CO2* to COOH* and of CO* to CHO* based on the preferred H-transfer mechanisms applied in this work. Optimized structures of all states involved in these calculations are provided in Fig. S10. The activation barriers for COOH* and CHO* formation as a function of the number of H2O molecules are presented in Fig. 10. For CO2* hydrogenation to COOH*, adding H2O into the system significantly promotes CO2 conversion by largely reducing the activation barrier as compared to the case without H2O (from 1.32 eV without H2O to 0.66 eV with 1 H2O). However, with the further increase of the number of H2O molecules from 1 to 6, we found that the kinetic barrier goes higher (from 0.66 eV with 1 H2O to over 0.9 eV with more H2O molecules); the case with 1 H2O shuttling the H significantly facilitates CO2 conversion to COOH*. With regard to CO* hydrogenation to CHO*, the presence of water and the number of H2O molecules have small effect on the kinetic barriers, as shown in Fig. 10. These results further rationalize the effect of H2O on different paths (i.e. O-H bond formation vs. C-H bond formation) in the present work. The key surface chemistry is well captured through the “Hshuttled” mechanism for O-H bond forming and C-O(H) bond breaking steps as the important role of water in accelerating the rate of H-shuttling in a proton transfer step is well-established in relevant studies.51,114-118 Meanwhile, the effect of H2O on C-H bond forming steps is small as it stabilizes the initial, transition, and final states through H-bonding in the “H2O-solvated” mechanism and does not take part in the reaction. Promoting effect of a small amount of H2O for CO2 conversion to CH3OH was verified from both available literature50,51 and our microkinetic modeling calculation, consistent with the results on the effect of the number of H2O molecules. Therefore, we expect the calculations with 1 H2O inclusion to be sufficient to uncover the effect

-34-


Page 34 of 65

Page 35 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

of the in-situ produced water on CO2 reactions based on which our computational and experimental results corroborate each other. 3.4.5 Energy profiles The energy profile for methanol synthesis via CO2/CO hydrogenation including water effect is illustrated in Fig. 11. In CO2/H2 mixture, the effective barrier for the overall reaction in CH3OH synthesis along the “Formate” path is calculated to be 1.07 eV, 0.40 eV lower than that along the “RWGS+CO-Hydro” path. Therefore, the energetically favorable path for CH3OH production

on

PdCu(111)

goes

CO2*HCOO*HCOOH*H2COOH*CH2O*CH3O*CH3OH*.

through With

water

participation, the highest barrier step changes to HCOOH* hydrogenation to H2COOH* (Eact = 0.82 eV) which is C-H bond formation step. Without H2O, the highest barrier step is the hydrogenation of HCOO* to HCOOH* (an O-H bond formation step) which encounters a kinetic barrier of 1.28 eV. Water greatly reduces the kinetic barriers by altering the “H-transfer” mechanism for O-H bond formation and C-O(H) bond cleavage steps. The Gibbs free energy profiles are plotted in Fig. 12, showing that CO production via RWGS has a highest barrier of 0.66 eV in the step of CO2* hydrogenation to COOH*, impacting the rate for CO formation. The hydrogenation of HCOOH* to H2COOH* has the highest barrier (0.95 eV) in the formate path for CH3OH formation. Comparing the kinetic barriers in slow steps for CO and CH3OH production (0.66 vs. 0.95 eV), CO formation should proceed faster than CH3OH formation. The effective free energy barrier for the overall reaction of CH3OH production from CO2 is 1.23 eV. If using CO/H2 as reaction mixture, methanol formation needs to overcome an effective free energy barrier of 1.62 eV (see from Fig. 12), thus is slower in kinetics than using CO2 as reagent. One crucial advantage with using CO2 as reagent is that the small amount of water in-situ -35-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

produced from both RWGS and CH3OH production could accelerate CO2 conversion and significantly promotes methanol production in an autocatalytic manner. To determine the activity and selectivity more accurately, the microkinetic modeling considering the reaction constant and coverage of surface species was carried out and discussed below.

Fig. 11. Energy profiles of methanol synthesis via CO2/CO hydrogenation on PdCu(111) including the effect of water. (For legibility consideration, H* was omitted from the labels after the adsorption of six H atoms in the first step.)

-36-


Page 36 of 65

Page 37 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fig. 12. Energy profiles based on Gibbs free energies of methanol synthesis via CO2/CO hydrogenation on PdCu(111) including the effect of water. 3.4.6 Microkinetic modeling on H2O effect To understand the effect of H2O on the kinetics of CO2 conversion to methanol, the microkinetic modeling was conducted based on DFT-determined energies and vibrational frequencies. Elementary steps included in the microkinetic model consist of the entire “Formate”, “RWGS”, and the “CO-Hydro” pathways. Relevant energetic and kinetic parameters under the condition without and with water are given in Tables S4 and S5, respectively. The microkinetic modeling was performed at 523 K under which the catalytic experiments were conducted. The turnover frequencies (TOFs) were estimated to show the effect of H2O on CO2 conversion to methanol and the details are described in the Supporting Information. The rate limiting step is identified by Campbell’s degree of rate control (DRC) method.119,120 Small variation on the activation barrier of each elementary step was set to analyze -37-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

its impact on the overall reaction rate. The DRC of each step in CO2 conversion to CH3OH was calculated and listed in Tables S4 and S5. Without H2O, the rate limiting step for CH3OH formation is the CH3O* hydrogenation to CH3OH* which has a DRC of 1.00. When H2O is involved in the reaction, both HCOOH* hydrogenation to H2COOH* and CH3O* hydrogenation to CH3OH* determine the rate, and the calculated DRC for these two steps are 0.73 and 0.65, respectively, showing that the contribution from HCOOH* hydrogenation to H2COOH* to the rate is larger. Comparing CO and CO2 hydrogenation with H2O inclusion, we observed that CO* hydrogenation to CHO* is energetically unfavorable due to a higher barrier and strong endothermic reaction energy. In addition, the subtractions between forward rate and reverse rate (rfwd - rrev) for CHO* and CH2O* formation are negative, indicating these two intermediates are unstable and the reverse reactions are favorable over the PdCu(111) surface. Therefore, the undesired product CO, is difficult to be converted to CH3OH via the “CO-Hydro” pathway, and CO2 would be the primary carbon source for CH3OH formation over Pd-Cu alloy catalysts. This result from modeling is in excellent agreement with our previous experimental finding.63 When water is involved in the reaction, the TOF for CO formation is 7.16×10-4 and that for CH3OH formation is 6.51×10-5, giving a selectivity factor of ~10 (CO vs. CH3OH) under reaction conditions which agrees well with the experimental results as CO is always selectively produced. Without water participation, the calculated TOF for CH3OH formation is pretty low, only 2.29×10-8, and water presence can enhance the TOF up to approximately 3000 times higher than that without water. The microkinetic modeling results clearly demonstrate a significant promotion of H2O on direct CO2 hydrogenation to CH3OH. The above computational study has shown that the stepped PdCu(111) surface with coordinatively unsaturated Pd atoms exposed on the top is catalytically more active than the Cu-

-38-


Page 38 of 65

Page 39 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

rich flat PdCu3(111) surface for CH3OH synthesis through CO2 hydrogenation. Moreover, we found that water involved in reaction accelerates CO2 conversion to methanol by reducing the kinetic barriers, altering the rate-limiting step, and enhancing the TOF. The energetically preferred

path

for

CH3OH

production

on

PdCu(111)

goes

through

CO2*HCOO*HCOOH*H2COOH*CH2O*CH3O*CH3OH*. CO* formation from CO2 hydrogenation through RWGS proceeds faster than formation of CH3OH* based on kinetic calculations. To verify our computational results, we also conducted an experimental study on CO2 hydrogenation over Pd-Cu bimetallic catalysts as discussed below. 3.5 Catalytic performance 3.5.1 Formation of Pd-Cu alloys SiO2-supported Pd-Cu(0.25) and Pd-Cu(0.50) were prepared, and these two Pd/(Pd+Cu) ratios were selectively chosen to mimic PdCu3-rich and PdCu-rich formulations, respectively. To verify the formation of desired alloy phase, the freshly reduced bimetallic catalysts were passivated and collected for further characterization. Fig. 13 shows the XRD patterns of reduced catalysts. As benchmark, monometallic catalysts were prepared and characterized as well. As depicted, monometallic Pd and Cu catalysts exhibit corresponding (111) lattice planes at 2θ=40.0 and 43.2 o, respectively. PdCu(0.25) and Pd-(0.50) show strong diffraction peaks in between, specifically at 2θ=42.7 and 42.0 o, respectively. The former diffraction of Pd-Cu(0.25) is sharp, corresponding to PdCu3(111) (PDF card No. 04-004-8211, ICDD), while that of Pd-Cu(0.50) is relatively broader. Deconvolution analysis demonstrates that Pd0, PdCu3(111), and PdCu(111) (PDF card No. 103082, ICSD) phases coexist, wherein PdCu is appreciably predominant. Using the full width at half

-39-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 65

maximum (FWHM), the crystallite size can be determined with Scherrer equation, and the results are presented in Table 2. Clearly, the alloy formation could considerably decrease the crystallite size of Cu phase; moreover, the average crystallite sizes of PdCu3 and PdCu approximates to each other. Even though other phases are identified, the PdCu still dominates in Pd-Cu(0.50). Therefore, the expected dominant alloy phase for respective bimetallic catalysts with comparable crystallite size is advantageous for comparing their catalytic property.

Fig. 13. XRD patterns of reduced monometallic and Pd-Cu bimetallic catalysts. (a) Cu; (b) PdCu(0.25); (c) Pd-Cu(0.50) with deconvoluted pattern in the inset (2θ: 35-50 o); (d) Pd. The Cu and Pd metal loadings for monometallic catalysts were 10 wt% and 5.7 wt%, respectively. Table 2. Crystallite phases and sizes for reduced mono- and bimetallic Pd-Cu(X) catalysts. Catalyst a)

2θ(111) / o

Metal Phase

Size b) / nm

Cu

43.2

Cu0

43

Pd-Cu(0.25)

42.7

PdCu3

4

Pd-Cu(0.50)

40.2

Pd0

55

41.8

PdCu

3

42.6

PdCu3

6

-40-


Page 41 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Pd

Pd0

40.0

3

a)

For monometallic catalysts, Pd=5.7 wt%, Cu=10 wt%. Crystallite sizes were determined using Scherrer equation based on (111) peak, wherein the dimensionless shape factor was close to unity.

b)

Fig. 14. HAADF images and overlapped EDS maps of reduced (a-b) Pd-Cu(0.25) and (c-d) PdCu(0.50). The alloy particles are also evidenced from the Z-contrast high-angle annular dark-field images (HAADF), as shown in Figs. 14 (a) and (c), and the particle sizes are consistent with the estimated results from XRD. The EDS maps of both catalysts reveal that Pd and Cu distribute uniformly in close proximity (Figs. 14 (b) and (d)), which indicates the strong interaction between metals due to the alloy formation. To examine the composition of alloy particles, -41-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 65

quantification analysis was carried out on well-shaped particles with strong counts, and detailed results are listed in Tables 3 and 4. As expected, Pd-Cu(0.25) and Pd-Cu(0.50) exhibit PdCu3rich and PdCu-rich alloy formulations, respectively. To confirm the alloy composition of the latter catalyst, both HAADF image and EDS map of another region were collected and analyzed (Supporting information, Fig. S11). A similar metallic distribution within the domain of the alloy particles were observed. Moreover, quantification analysis corroborates the dominance of PdCu-rich alloy particles in this composition (Table S9). Therefore, detailed characterization results show that the featured alloy environment with desired composition was successfully created for corresponding bimetallic catalysts. Table 3. Quantified phase compositions of alloyed Pd-Cu nanoparticles for reduced Pd-Cu(0.25) Particle #

Comp. / at. % a)

Cu/Pd at. ratio / at at-1

Cu-K

Pd-L

1

36.11

10.05

PdCu3.6

2

11.30

3.84

PdCu2.9

3

2.13

24.74

PdCu0.1

4

5.68

4.76

PdCu1.2

5

7.22

2.56

PdCu2.8

6

15.29

4.98

PdCu3.1

7

28.47

5.59

PdCu5.1

8

11.95

4.46

PdCu2.7

a)

Uncertainty of each element in quantitative analysis: O (3.96 at%); Si (2.23 at%); Cu (1.35 at%); Pd (0.95 at%). b) Values in the subscript mean the Cu/Pd ratios. Table 4. Quantified phase compositions of alloyed Pd-Cu nanoparticles for reduced Pd-Cu(0.50) Particle #

Comp. / at. % a)

Cu/Pd at. ratio / at at-1

Cu-K

Pd-L

1

5.29

6.40

PdCu0.8

2

7.58

4.40

PdCu1.7

3

4.60

5.93

PdCu0.8

-42-


Page 43 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

4

8.71

7.46

PdCu1.2

5

8.13

8.24

PdCu1.0

6

7.19

11.02

PdCu0.7

7

2.87

9.07

PdCu0.3

8

5.47

5.20

PdCu1.1

9

3.61

4.12

PdCu0.9

a)

Uncertainty of each element in quantitative analysis: O (4.77 at%); Si (2.94 at%); Cu (1.11 at%); Pd (1.35 at%). b) Values in the subscript mean the Cu/Pd ratios. 3.5.2 Effect of alloy phase on catalytic activity Table 5 shows the activity and selectivity of Pd-Cu(0.25) and Pd-Cu(0.50) tested in CO2 hydrogenation at 523 K and 5 MPa, and the GHSV was initially fixed at 30000 mL (STP) g-cat-1 h-1. Pd-Cu(0.25) presents a much higher CO space-time yield (STY) than CH3OH STY. Remarkably, CO formation rate shows a drastic reduction as the Pd amount increases in the bimetallic catalyst, namely Pd-Cu(0.50). As a result, Pd-Cu(0.50) shows an increased CH3OH selectivity in comparison to Pd-Cu(0.25), despite a slight drop in absolute formation rate and lower CO2 conversion. For comparison, the GHSV was decreased on Pd-Cu(0.50) to enhance the CO2 conversion to the same level as Pd-Cu(0.25). Clearly, Pd-Cu(0.50) still displays higher selectivity than Pd-Cu(0.25). Activity tests were also conducted at 3 MPa, and resultant activity performance is plotted in Fig. 15(b), as well as detailed activity and selectivity in Table S11. Similarly, Pd-Cu(0.50), with PdCu-rich alloy phase, still exhibits a better CH3OH selectivity in comparison to Pd-Cu(0.25) with the dominant alloy composition of PdCu3. Evidently, a higher CH3OH selectivity emerges on Pd-Cu(0.50) with PdCu-rich alloy composition, but with concomitant reduction of CO2 conversion and STY. As discussed in section 3.5.1, the PdCu(0.50), still consists of other phases other than the dominant PdCu alloy phase, such as Pd0 and PdCu3 alloy phase, which might result in the deviation of absolute activity away from the -43-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 65

expected. However, this set of comparative study is still promising, as evidenced by the drastic suppressed CO formation and increased CH3OH selectivity over the PdCu alloy. Of course, more effort is needed in the future, especially in the creation of pure PdCu phase and its correlation with catalytic property. The relative selectivity of these two catalysts agrees with DFT results in general, offering experimental evidence to support the strong promoting effect of PdCu alloy phase on CH3OH synthesis over that of PdCu3 alloy composition. Furthermore, the (111) facet of PdCu alloy identified from XRD characterization in Fig. 13 should be the crucial active sites and mainly contribute to the observed strong synergetic effect of Pd-Cu bimetallic catalyst on methanol synthesis. From DFT calculation, the stepped PdCu(111) surface exhibits superior property for both reactant adsorption and CO2 conversion to methanol, which is consistent with the experimental results. Table 5. Activity performance over Pd-Cu(X) catalysts a, GHSV / mL (STP) g-1 h-1

CO2 conv. /%

STY / mol kg-1 h-1

Selectivity / mol%

CH3OH

CO

CH3OH

CO

Pd-Cu(0.25)

30000

2.8

1.63

7.17

18.5

81.5

Pd-Cu(0.50)

30000

1.6

1.34

3.63

26.9

73.1

12000

2.4

0.73

2.30

24.0

76.0

Catalyst

a)

Reaction conditions: 523 K, 5 MPa, CO2/H2=1:3. Total metal loading was fixed at 15.7 wt%.

3.5.3 Effect of H2O on catalytic activity In our recent laboratory work, we found that Pd-rich Pd-Cu bimetallic catalysts favor the adsorption towards weakly-bonded H2 and CO2 at relatively lower temperature, which appears to correlate to CH3OH formation. Moreover, detailed XRD quantitative analysis elucidates that the Pd-rich Pd-Cu bimetallic catalysts primarily consist of more PdCu than PdCu3 phase. In the -44-


Page 45 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

present work, the effect of different alloy surface compositions, namely PdCu(111) and PdCu3(111) surfaces, on CO2 and H2 adsorption and activation was studied in detail through DFT calculations (section 3.1), and results suggest that Pd-terminated PdCu(111) surface shows better activity in terms of the adsorption and activation of reactants CO2 and H2. On the other hand, DFT calculation suggests that PdCu alloy composition, rather than PdCu3, enables a more favorable formation of CH3OH from CO2 hydrogenation by lowering the activation energy. To verify this prediction, the as-prepared Pd-Cu(0.25) and Pd-Cu(0.50), consisting of PdCu3 and PdCu alloy phase, respectively, were tested in CO2 hydrogenation. As expected, the latter (PdCu) displayed better CH3OH selectivity in comparison to the former (PdCu3). Therefore, considering the observed result with a superior selectivity to CH3OH over PdCu-rich Pd-Cu(0.50), the DFT results are supported by experimental observation. The DFT calculation also shows that the presence of a small amount of water can not only reduce the kinetic barriers through changing the “H-transfer” mechanism, but also alter the ratelimiting step and enhance the TOF for methanol formation. As aforementioned (section 2.2), previous work discovered that the addition of a certain amount of water was capable of shortening the induction period of CH3OH synthesis when the water vapor concentration ranged from 0.04 to 0.2 mol%.50 Hence, to verify the water impact, initial experiments were conducted without water and with water vapor concentration at 0.1 mol%, and the results are presented in Fig. 15(a) and tabulated in Table S10. Apparently, both Pd-Cu(0.25) and Pd-Cu(0.50) exhibit drops of CH3OH selectivity in the presence of 0.1 mol% water vapor, wherein the drop is more significant for the former with a reduction by almost 30 %, while little drop for the latter. Notwithstanding, Pd-Cu(0.50) with PdCu-rich alloy phase still shows higher CH3OH selectivity than Pd-Cu(0.25) with PdCu3-rich alloy phase. On the other hand, the promotional effect of H2O

-45-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

on the CH3OH synthesis on PdCu alloy-rich catalyst, namely Pd-Cu(0.50), is not observed as predicted from computational study. However, we do not think the current activity data contradicts our computational result. As reported by Campbell et al.,50 the CH3OH formation from CO2/H2 is an autocatalyzed process, whereas an excessive H2O addition results in the decrease of the CH3OH formation at steady state. Therefore, we further conducted experiments with lowered H2O concentration at 0.03 mol%, as illustrated in Fig. 15(b) and also shown in Table S11 for detailed data. Interestingly, both catalysts exhibit higher CH3OH selectivity upon the H2O addition at comparable level of CO2 conversion, as well as enhancement of CH3OH formation rate. The Pd-Cu(0.50), with PdCu-rich alloy phase, shows a more prominent improvement of CH3OH selectivity up to 52.4 %, while an increase of CH3OH formation rate from 0.12 to 0.34 mol kg-1 h-1 with H2O addition. These experimental results clearly demonstrate the important role of H2O in enhancing the selectivity toward CH3OH formation, and appear to be in line with our computational results. It should be pointed out that there may be other factors that can impact the effects of H2O, including (i) the produced H2O from CO2 conversion, (ii) the possible sites blocking, and (iii) the kinetic limitation of the RWGS reaction, which are of importance to further advance the understanding of PdCu alloy catalysis for CO2 conversion to methanol which should be considered in future work.

-46-


Page 46 of 65

Page 47 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fig. 15 Activity and selectivity of Pd-Cu bimetallic catalysts in the presence and absence of different water vapor concentrations (a) 0.1 mol% and (b) 0.03 mol%. Reaction conditions of (a): 523 K, 5 MPa, CO2/H2=1:3; GHSV= 30000 mL (STP) g-1 h-1. Reaction conditions of (b): 523 K, 3 MPa, CO2/H2=1:3, GHSV(Pd-Cu0.25)= 12000 mL (STP) g-1 h-1, GHSV(Pd-Cu0.50)= 6000 mL (STP) g-1 h-1. Total metal loading was fixed at 15.7 wt%. The DFT calculation showed that the formate route is a highly feasible pathway for CH3OH synthesis on PdCu(111) surface. The formate species plays a crucial role in the formation of CH3OH in this case. In our ongoing work on the effect of Pd-Cu composition on the surface species formed during the reaction by using DRIFTS, our results illustrate that the formate species (HCOO*) is readily formed during the reaction and is abundant on the surface. Moreover, the surface coverage of formate species varies with the Pd-Cu composition and appears to correlate well with the trend of CH3OH formation rate. Clearly, such consistence makes formate species a potential candidate as the key intermediate for CO2 hydrogenation on Pd-Cu bimetallic surface. In addition, methoxy species (CH3O*) was also identified from the DRIFT spectra through the H2 purge experiment after the system reached steady state. Recently, Copéret et al.121 carried out a study on identifying the key intermediates in CO2 hydrogenation over the zirconiasupported copper nanoparticles by kinetics, in situ IR, NMR, and isotopic labeling techniques,

-47-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 65

and they concluded that the formate (HCOO*) and methoxy (CH3O*) surface species are reaction intermediates for methanol formation. Such statement was also supported by their DFT calculations. Moreover, some recent studies20,37,122,123 revealed that the formate is a key intermediate in CH3OH synthesis via CO2 hydrogenation over Cu/ZnO and Cu/ZnO/Al2O3 catalysts

and

established

reaction

path

going

through

CO2HCOOHCOOHH2COOHCH2OCH3OCH3OH, similar as that proposed in the present work. Clearly, our own experimental observation and recent progress in this field offer useful information in support of the computational results in this work. The computational results show that CO formation from CO2 hydrogenation proceeds faster than formation of CH3OH with H2O involved in terms of kinetic calculation, and this conclusion is consistent with our experimental results as CO is always selectively produced. In our recent work, the CO2 was found to be a primary carbon source over Pd-Cu bimetallic catalysts when the CO2 conversion was extrapolated to zero.63 Thus, it is reasonable that the undesired product CO, enhanced by the existence of H2O, was difficult to be converted to CH3OH via the “RWGS + CO-Hydro” pathway due to the high barrier, as suggested by the calculation results, even though a large amount of CO was produced on each bimetallic catalyst with different alloy compositions. 4. Conclusions DFT calculation results clearly show that the structures of Pd-Cu alloys significantly impact the surface chemistry in CO2 reactions. The stepped PdCu(111) surface with low-coordinated Pd atoms exposed on top of the surface shows better capability for adsorption and activation of CO2 and H2, and also exhibits superior activity for initial CO2 hydrogenation than the Cu-rich flat PdCu3(111) surface. DFT results suggest that the Pd-Cu type alloys are promising catalysts for CH3OH synthesis through CO2 hydrogenation. -48-


Page 49 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Water was found to impact the surface chemistry and elementary energetics in CO2 hydrogenation to CH3OH over PdCu(111). The energetically preferred path for CH3OH production

proceeds

through

CO2*HCOO*HCOOH*H2COOH*CH2O*CH3O*CH3OH*. DFT results indicate that the presence of small amount of water can reduce the kinetic barriers through changing the “H-transfer” mechanism in hydrogenation process and also altering the rate-limiting step. CO2 conversion to CH3OH is significantly promoted in the presence of a small amount of H2O as the TOF is approximately 3000 times higher than the case without H2O. This is an additional but crucial advantage of using CO2 as reagent because water is always produced, via both RWGS and CH3OH formation, and water can enhance both CO2 conversion and CH3OH selectivity, similar to a co-catalytic function. CO formation from CO2 hydrogenation through RWGS proceeds faster than formation of CH3OH in terms of kinetic calculations, being consistent with experimental observations. Methanol production directly from CO hydrogenation on PdCu(111) encounters higher barriers, thus is slower in kinetics than using CO2 as reagent. Complimentary to computational results, CO2 hydrogenation over SiO2-supported bimetallic catalysts demonstrates that Pd-Cu(0.50) that is rich in PdCu alloy phase has a higher selectivity to methanol than PdCu3-rich Pd-Cu(0.25) catalyst. H2O impact was also examined by cofeeding water with different concentrations. A significant improvement on CH3OH selectivity with Pd-Cu(0.50) which has PdCu alloy phase was observed at the water vapor concentration of 0.03 mol%, whereas that on Pd-Cu(0.25) with PdCu3 alloy phase is not comparable. These experimental observations agree with the computational results.

-49-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

These results unambiguously show that Pd-Cu alloy structure has a major impact on the catalytic reaction pathway, and the presence of water can significantly influence the formation of CH3OH from CO2 hydrogenation. The present work opens up new avenue for future design of highly active and selective alloy catalysts for selective CO2 hydrogenation.

ASSOCIATED CONTENT Supporting Information. Descriptions of intermediate formation in the two pathways. Details in microkinetic modeling calculation. Reactants, intermediates, and products involved in CH3OH production. Initial, transition, and final states associated with each elementary step for CO2 conversion to CH3OH. Effect of H2O location and the number of H2O molecules on CO2 reactions. Key structural parameters, reaction energetics, and transition state imaginary vibrational frequencies. The energetic and kinetic parameters in microkinetic modeling. Additional experimental data. This material is available free of charge on the ACS Publication website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS

-50-


Page 50 of 65

Page 51 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

This work was financially supported by the National Key Research and Development Program of China (No. 2016YFB0600902), the National Natural Science Foundation of China (No. 21503027), the QianRen Program of the Chinese Government and the Pennsylvania State University. We acknowledge the Supercomputing Center of Dalian University of Technology for providing the computational resources for this work. The STEM/EDS was performed at the Materials Characterization Laboratory of the Penn State Materials Research Institute, for which the assistance of Jennifer Gray is gratefully acknowledged. We also thank Ms. Wenhui Li and Mr. Jianyang Wang for assistance in additional CO2 hydrogenation experiments on the water effect.

References (1) Knutson, T. R.; Tuleya, R. E. Impact of CO2-Induced Warming on Simulated Hurricane

Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization. J. Climate 2004, 17, 3477-3495. (2) Hansen, J.; Sato, M.; Ruedy, R.; Lo, K.; Lea, D. W.; Medina-Elizade, M. Global Temperature Change. P. Nati. Acad. Sci. 2006, 103, 14288-14293. (3) Ansari, M. B.; Park, S. E. Carbon Dioxide Utilization as a Soft Oxidant and Promoter in Catalysis. Energy Environ. Sci. 2012, 5, 9419-9437. (4) Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent Advances in Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40, 3703-3727. (5) Song, C. CO2 Conversion and Utilization: An Overview. In CO2 Conversion and Utilization, Song, C.; Gaffney, A. F.; Fujimoto, K., Eds. American Chemical Society: Washington D.C., 2002; Vol. 809, p 2-30. -51-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(6) Song, C. Global Challenges and Strategies for Control, Conversion and Utilization of CO2 for Sustainable Development Involving Energy, Catalysis, Adsorption and Chemical Processing. Catal. Today 2006, 115, 2-32. (7) Aresta, M. Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere. In Carbon Dioxide as Chemical Feedstock, Aresta, M. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2010; p 1-13. (8) Centi, G.; Perathoner, S. Opportunities and Prospects in the Chemical Recycling of Carbon Dioxide to Fuels. Catal. Today 2009, 148, 191-205. (9) Raudaskoski, R.; Turpeinen, E.; Lenkkeri, R.; Pongrácz, E.; Keiski, R. L. Catalytic Activation of CO2: Use of Secondary CO2 for the Production of Synthesis Gas and for Methanol Synthesis over Copper-Based Zirconia-Containing Catalysts. Catal. Today 2009, 144, 318-323. (10) Pontzen, F.; Liebner, W.; Gronemann, V.; Rothaemel, M.; Ahlers, B. CO2-Based Methanol and DME – Efficient Technologies for Industrial Scale Production. Catal. Today 2011, 171, 242250. (11) Xiaoding, X.; Moulijn, J. A. Mitigation of CO2 by Chemical Conversion: Plausible Chemical Reactions and Promising Products. Energy & Fuels 1996, 10, 305-325. (12) Porosoff, M. D.; Yan, B.; Chen, J. G. Catalytic Reduction of CO2 by H2 for Synthesis of CO, Methanol and Hydrocarbons: Challenges and Opportunities. Energy Environ. Sci. 2016, 9, 62-73. (13) Olah, G. A.; Mathew, T.; Prakash, G. K. S. Chemical Formation of Methanol and Hydrocarbon (“Organic”) Derivatives from CO2 and H2—Carbon Sources for Subsequent Biological Cell Evolution and Life’s Origin. J. Am. Chem. Soc. 2017, 139, 566-570. (14) Ye, J.; Liu, C.; Mei, D.; Ge, Q. Active Oxygen Vacancy Site for Methanol Synthesis from CO2 Hydrogenation on In2O3(110): A DFT Study. ACS Catal. 2013, 3, 1296-1306.

-52-


Page 52 of 65

Page 53 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(15) Ye, J.; Liu, C.; Ge, Q. DFT Study of CO2 Adsorption and Hydrogenation on the In2O3 Surface. J. Phys. Chem. C 2012, 116, 7817-7825. (16) Grabow, L. C.; Mavrikakis, M. Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation. ACS Catal. 2011, 1, 365-384. (17) Liu, C.; Liu, P. Mechanistic Study of Methanol Synthesis from CO2 and H2 on a Modified Model Mo6S8 Cluster. ACS Catal. 2015, 5, 1004-1012. (18) Zhang, L.; Zhao, Z. J.; Gong, J. Nanostructured Materials for Heterogeneous Electrocatalytic CO2 Reduction and their Related Reaction Mechanisms. Angew. Chem. Int. Ed. 2017, 56, 11326-11353. (19) Rodriguez, J. A.; Liu, P.; Stacchiola, D. J.; Senanayake, S. D.; White, M. G.; Chen, J. G. Hydrogenation of CO2 to Methanol: Importance of Metal–Oxide and Metal–Carbide Interfaces in the Activation of CO2. ACS Catal. 2015, 5, 6696-6706. (20) Kattel, S.; Yan, B.; Yang, Y.; Chen, J. G.; Liu, P. Optimizing Binding Energies of Key Intermediates for CO2 Hydrogenation to Methanol over Oxide-Supported Copper. J. Am. Chem. Soc. 2016, 138, 12440-12450. (21) Rubert-Nason, P.; Mavrikakis, M.; Maravelias, C. T.; Grabow, L. C.; Biegler, L. T. Advanced Solution Methods for Microkinetic Models of Catalytic Reactions: A Methanol Synthesis Case Study. AIChE J. 2014, 60, 1336-1346. (22) Choksi, T.; Greeley, J. Partial Oxidation of Methanol on MoO3(010): A DFT and Microkinetic Study. ACS Catal. 2016, 6, 7260-7277. (23) Zhang, W.; Chu, Y.; Wei, Y.; Yi, X.; Xu, S.; Huang, J.; Zhang, M.; Zheng, A.; Deng, F.; Liu, Z. Influences of the Confinement Effect and Acid Strength of Zeolite on the Mechanisms of

-53-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Methanol-to-Olefins Conversion over H-ZSM-5: A Theoretical Study of Alkenes-Based Cycle. Micropor. Mesopor. Mat. 2016, 231, 216-229. (24) Zhou, J.; Wang, Y.; Zou, W.; Wang, C.; Li, L.; Liu, Z.; Zheng, A.; Kong, D.; Yang, W.; Xie, Z. Mass Transfer Advantage of Hierarchical Zeolites Promotes Methanol Converting into paraMethyl Group in Toluene Methylation. Ind. Eng. Chem. Res. 2017, 56, 9310-9321. (25) Waugh, K. C. Methnaol Synthesis. Catal. Today 1992, 15, 51-75. (26) Liu, X.; Lu, G.; Yan, Z.; Beltramini, J. Recent Advances in Catalysts for Methanol Synthesis via Hydrogenation of CO and CO2. Ind. Eng. Chem. Res. 2003, 42, 6518-6530. (27) Nakamura, J.; Nakamura, I.; Uchijima, T.; Kanai, Y.; Watanabe, T.; Saito, M.; Fujitani, T. Methanol Synthesis over a Zn-deposited Copper Model Catalyst. Catal. Lett. 1995, 31, 325-331. (28) Arena, F.; Italiano, G.; Barbera, K.; Bordiga, S.; Bonura, G.; Spadaro, L.; Frusteri, F. SolidState Interactions, Adsorption Sites and Functionality of Cu-ZnO/ZrO2 Catalysts in the CO2 Hydrogenation to CH3OH. Appl. Catal., A: Gen 2008, 350, 16-23. (29) Saito, M.; Fujitani, T.; Takeuchi, M.; Watanabe, T. Development of Copper/Zinc OxideBased Multicomponent Catalysts for Methanol Synthesis from Carbon Dioxide and Hydrogen. Appl. Catal., A: Gen 1996, 138, 311-318. (30) Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Fernandez Sanz, J.; Rodriguez, J. A. Highly Active CopperCeria and Copper-Ceria-Titania Catalysts for Methanol Synthesis from CO2. Science 2014, 345, 546-550. (31) Tisseraud, C.; Comminges, C.; Pronier, S.; Pouilloux, Y.; Le Valant, A. The Cu–ZnO Synergy in Methanol Synthesis Part 3: Impact of the Composition of a Selective Cu@ZnOx

-54-


Page 54 of 65

Page 55 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Core–Shell Catalyst on Methanol Rate Explained by Experimental Studies and a Concentric Spheres Model. J. Catal. 2016, 343, 106-114. (32) Li, M. M.-J.; Zeng, Z.; Liao, F.; Hong, X.; Tsang, S. C. E. Enhanced CO2 Hydrogenation to Methanol over CuZn Nanoalloy in Ga Modified Cu/ZnO Catalysts. J. Catal. 2016, 343, 157-167. (33) Branco, J. B.; Ferreira, A. C.; Gonçalves, A. P.; Soares, C. O.; Almeida Gasche, T. Synthesis of Methanol Using Copper–f Block Element Bimetallic Oxides as Catalysts and Greenhouse Gases (CO2, CH4) as Feedstock. J. Catal. 2016, 341, 24-32. (34) Cai, W. J.; de la Piscina, P. R.; Toyir, J.; Homs, N. CO2 Hydrogenation to Methanol over CuZnGa Catalysts Prepared Using Microwave-Assisted Methods. Catal. Today 2015, 242, 193199. (35) Bali, F.; Jalowiecki-Duhamel, L. CO2 Hydrogenation to Methanol on Cu–ZrO2 Catalysts. In Global Warming: Engineering Solutions, Dincer, I.; Hepbasli, A.; Midilli, A.; Karakoc, T. H., Eds. Springer US: Boston, MA, 2010; p 315-327. (36) Ud Din, I.; Shaharun, M. S.; Subbarao, D.; Naeem, A. Synthesis, Characterization and Activity Pattern of Carbon Nanofibers Based Copper/Zirconia Catalysts for Carbon Dioxide Hydrogenation to Methanol: Influence of Calcination Temperature. J. Power Sources 2015, 274, 619-628. (37) Behrens, M.; Studt, F.; Kasatkin, I.; Kuhl, S.; Havecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B. L.; Tovar, M.; Fischer, R. W.; Norskov, J. K.; Schlogl, R. The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 336, 893-897.

-55-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38) Bansode, A.; Tidona, B.; von Rohr, P. R.; Urakawa, A. Impact of K and Ba Promoters on CO2 Hydrogenation over Cu/Al2O3 Catalysts at High Pressure. Catal. Sci. Technol. 2013, 3, 767778. (39) Song, Y.; Liu, X.; Xiao, L.; Wu, W.; Zhang, J.; Song, X. Pd-Promoter/MCM-41: A Highly Effective Bifunctional Catalyst for Conversion of Carbon Dioxide. Catal. Lett. 2015, 145, 12721280. (40) Guo, X.; Mao, D.; Lu, G.; Wang, S.; Wu, G. The Influence of La Doping on the Catalytic Behavior of Cu/ZrO2 for Methanol Synthesis from CO2 Hydrogenation. J. Mol. Catal. A: Chem. 2011, 345, 60-68. (41) Toyir, J.; de la Piscina, P. R.; Fierro, J. L. G.; Homs, N. Highly Effective Conversion of CO2 to Methanol over Supported and Promoted Copper-Based Catalysts: Influence of Support and Promoter. Appl. Catal. B: Environ. 2001, 29, 207-215. (42) Toyir, J.; Ramı́rez de la Piscina, P.; Fierro, J. L. G.; Homs, N. Catalytic Performance for CO2 Conversion to Methanol of Gallium-Promoted Copper-Based Catalysts: Influence of Metallic Precursors.Appl. Catal. B: Environ. 2001, 34, 255-266. (43) Liu, J.; Shi, J.; He, D.; Zhang, Q.; Wu, X.; Liang, Y.; Zhu, Q. Surface Active Structure of Ultra-Fine Cu/ZrO2 Catalysts Used for the CO2+H2 to Methanol Reaction. Appl. Catal. A: Gen. 2001, 218, 113-119. (44) Liu, X. M.; Lu, G. Q.; Yan, Z. F. Nanocrystalline Zirconia as Catalyst Support in Methanol Synthesis. Appl. Catal. A: Gen. 2005, 279, 241-245. (45) Słoczynski, J.; Grabowski, R.; Olszewski, P.; Kozłowska, A.; Stoch, J.; Lachowska, M.; Skrzypek, J. Effect of Metal Oxide Additives on the Activity and Stability of Cu/ZnO/ZrO2

-56-


Page 56 of 65

Page 57 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Catalysts in the Synthesis of Methanol from CO2 and H2. Appl. Catal. A: Gen. 2006, 310, 127137. (46) Raudaskoski, R.; Niemelä, M. V.; Keiski, R. L. The Effect of Ageing Time on CoPrecipitated Cu/ZnO/ZrO2 Catalysts Used in Methanol Synthesis from CO2 and H2. Top. Catal. 2007, 45, 57-60. (47) Słoczynski, J.; Grabowski, R.; Kozłowska, A.; Olszewski, P.; Stoch, J.; Skrzypek, J.; Lachowska, M. Catalytic Activity of the M/(3ZnO·ZrO2) System (M = Cu, Ag, Au) in the Hydrogenation of CO2 to Methanol. Appl. Catal. A: Gen. 2004, 278, 11-23. (48) Guo, X.; Mao, D.; Lu, G.; Wang, S.; Wu, G. Glycine–Nitrate Combustion Synthesis of CuO–ZnO–ZrO2 Catalysts for Methanol Synthesis from CO2 Hydrogenation. J. Catal. 2010, 271, 178-185. (49) An, X.; Li, J.; Zuo, Y.; Zhang, Q.; Wang, D.; Wang, J. A Cu/Zn/Al/Zr Fibrous Catalyst that is an Improved CO2 Hydrogenation to Methanol Catalyst. Catal. Lett. 2007, 118, 264-269. (50) Yang, Y.; Mims, C. A.; Mei, D. H.; Peden, C. H. F.; Campbell, C. T. Mechanistic Studies of Methanol Synthesis over Cu from CO/CO2/H2/H2O Mixtures: The Source of C in Methanol and the Role of Water. J. Catal. 2013, 298, 10-17. (51) Zhao, Y. F.; Yang, Y.; Mims, C.; Peden, C. H. F.; Li, J.; Mei, D. Insight into Methanol Synthesis from CO2 Hydrogenation on Cu(111): Complex Reaction Network and the Effects of H2O. J. Catal. 2011, 281, 199-211. (52) Poutsma, M. L.; Elek, L. F.; Ibarbia, P. A.; Risch, A. P.; Rabo, J. A. Selective Formation of Methanol from Synthesis Gas over Palladium Catalysts. J. Catal. 1978, 52, 157-168.

-57-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(53) Matsumura, Y.; Shen, W. J.; Ichihashi, Y.; Okumura, M. Low-Temperature Methanol Synthesis Catalyzed over Ultrafine Palladium Particles Supported on Cerium Oxide. J. Catal. 2001, 197, 267-272. (54) Kikuzono, Y.; Kagami, S.; Naito, S.; Onishi, T.; Tamaru, K. Selective Hydrogenation of Carbon Monoxide on Palladium Catalysts. Faraday Discuss. Chem. Soc. 1981, 72, 135-143. (55) Gotti, A.; Prins, R. Basic Metal Oxides as Co-catalysts in the Conversion of Synthesis Gas to Methanol on Supported Palladium Catalysts. J. Catal. 1998, 175, 302-311. (56) Liang, X. L.; Dong, X.; Lin, G. D.; Zhang, H. B. Carbon Nanotube-Supported Pd–ZnO Catalyst for Hydrogenation of CO2 to Methanol. Appl. Catal. B: Environ. 2009, 88, 315-322. (57) Collins, S. E.; Chiavassa, D. L.; Bonivardi, A. L.; Baltanás, M. A. Hydrogen Spillover in Ga2O3–Pd/SiO2 Catalysts for Methanol Synthesis from CO2/H2. Catal. Lett. 2005, 103, 83-88. (58) Koizumi, N.; Jiang, X.; Kugai, J.; Song, C. Effects of Mesoporous Silica Supports and Alkaline Promoters on Activity of Pd Catalysts in CO2 Hydrogenation for Methanol Synthesis. Catal. Today 2012, 194, 16-24. (59) Fujitani, T.; Nakamura, I. Methanol Synthesis from CO and CO2 Hydrogenations over Supported Palladium Catalysts. Bull. Chem. Soc. Jpn. 2002, 75, 1393-1398. (60) Subramanian, P. R.; Laughlin, D. E. Cu-Pd (Copper-Palladium). J. Phase Equilib. 1991, 12, 231-243. (61) Mierczynski, P.; Ciesielski, R.; Kedziora, A.; Zaborowski, M.; Maniukiewicz, W.; Nowosielska, M.; Szynkowska, M. I.; Maniecki, T. P. Novel Pd–Cu/ZnAl2O4–ZrO2 Catalysts for Methanol Synthesis. Catal. Lett. 2014, 144, 723-735. (62) Sahibzada, M. Pd-Promoted Cu/ZnO Catalyst Systems for Methanol Synthesis From CO2/H2. Chem. Eng. Res. Des. 2000, 78, 943-946.

-58-


Page 58 of 65

Page 59 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(63) Jiang, X.; Koizumi, N.; Guo, X.; Song, C. Bimetallic Pd–Cu Catalysts for Selective CO2 Hydrogenation to Methanol. Appl. Catal. B: Environ. 2015, 170-171, 173-185. (64) Zhang, C.; Oliaee, S. N.; Hwang, S. Y.; Kong, X.; Peng, Z. A Generic Wet Impregnation Method for Preparing Substrate-Supported Platinum Group Metal and Alloy Nanoparticles with Controlled Particle Morphology. Nano Lett. 2016, 16, 164-169. (65) Yang, Y.; Evans, J.; Rodriguez, J. A.; White, M. G.; Liu, P. Fundamental Studies of Methanol Synthesis from CO2 Hydrogenation on Cu(111), Cu Clusters, and Cu/ZnO(0001). Phys. Chem. Chem. Phys. 2010, 12, 9909-17. (66) Yang, Y.; White, M. G.; Liu, P. Theoretical Study of Methanol Synthesis from CO2 Hydrogenation on Metal-Doped Cu(111) Surfaces. J. Phys. Chem. C 2012, 116, 248-256. (67) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. (68) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558-561. (69) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using A Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (70) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (71) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396-1396. (72) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 99019004.

-59-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(73) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comp. Mater. Sci. 2006, 36, 354-360. (74) Grimme, S. Accurate Description of Van der Waals Complexes by Density Functional Theory Including Empirical Corrections. J. Comput. Chem. 2004, 25, 1463-1473. (75) Grimme, S.; Antony, J.; Schwabe, T.; Muck-Lichtenfeld, C. Density Functional Theory with Dispersion Corrections for Supramolecular Structures, Aggregates, and Complexes of (bio)Organic Molecules. Org. Biomol. Chem. 2007, 5, 741-758. (76) Xing, B.; Ma, J.; Li, R.; Jiao, H. Location, Distribution and Acidity of Al Substitution in ZSM-5 with Different Si/Al Ratios – A Periodic DFT Computation. Catal. Sci. Technol. 2017, 7, 5694-5708. (77) Sahu, P.; Ali, S. M. Dispersion Corrected Interaction of Polar and Nonpolar Fluids Confined within Carbon Nanotubes: Density Functional Theoretical Analysis Using Grimme's D3 Scheme. Inter. J. Quantum Chem. 2017, 25578. (78) Cukras, J.; Sadlej, J. The Influence of the Dispersion Corrections on the Performance of DFT Method in Modeling HNgY Noble Gas Molecules and Their Complexes. Chem. Phys. Lett. 2018, 691, 319-324. (79) Garg, P.; Choudhuri, I.; Pathak, B. Stanene Based Gas Sensors: Effect of Spin-Orbit Coupling. Phys. Chem. Chem. Phys. 2017, 19, 31325-31334. (80) Seitsonen, A. P.; Bryk, T. Melting Temperature of Water: DFT-Based Molecular Dynamics Simulations with D3 Dispersion Correction. Phys. Rev. B 2016, 94, 184111-184115. (81) Amaral, R. C.; Tereshchuk, P.; Seminovski, Y.; Da Silva, J. L. F. The Role of LowCoordinated Sites on the Adsorption of Glycerol on Defected Ptn/Pt(111) Substrates: A Density

-60-


Page 60 of 65

Page 61 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Functional Investigation within the D3 Van der Waals Correction. J. Phys. Chem. C 2017, 121, 3445-3454. (82) Sha, Y.; Yu, T. H.; Merinov, B. V.; Goddard, W. A. DFT Prediction of Oxygen Reduction Reaction on Palladium–Copper Alloy Surfaces. ACS Catal. 2014, 4, 1189-1197. (83) Alfonso, D. Density Functional Theory Studies of Sulfur Binding on Pd, Cu and Ag and Their Alloys. Surf. Sci. 2003, 546, 12-26. (84) Chorkendorff, I.; Niemantsverdriet, J. W. In Concepts of Modern Catalysis and Kinetics. WILEY-VCH: Weinheim, Germany 2003. (85) McQuarrie, D. A.; Simon, J. D., In Physical Chemistry: A Moleular Approach. University Science Books, Sausalito, California 1997. (86) Cao, X. M.; Burch, R.; Hardacre, C.; Hu, P. An Understanding of Chemoselective Hydrogenation on Crotonaldehyde over Pt(111) in the Free Energy Landscape: The Microkinetics Study Based on First-Principles Calculations. Catal. Today 2011, 165, 71-79. (87) Jørgensen, M.; Grönbeck, H. First-Principles Microkinetic Modeling of Methane Oxidation over Pd(100) and Pd(111). ACS Catal. 2016, 6, 6730-6738. (88) Gu, G. H.; Mullen, C. A.; Boateng, A. A.; Vlachos, D. G. Mechanism of Dehydration of Phenols on Noble Metals via First-Principles Microkinetic Modeling. ACS Catal. 2016, 6, 30473055. (89) Cabilla, G. Infrared Study of the Adsorption of Formic Acid on Clean and Ca-Promoted Pd/SiO2 Catalysts. Appl. Catal. A: Gen. 2003, 255, 181-195. (90) Cabilla, G. C.; Bonivardi, A. L.; Baltanas, M. A. Characterization by CO/FTIR Spectroscopy of Pd/Silica Catalysts and Its Correlation with Syn-Gas Conversion. Catal. Lett. 1998, 55, 147-156.

-61-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(91) Clarke, D. B.; Bell, A. T. An Infrared Study of Methanol Synthesis from CO2 on Clean and Potassium-Promoted Cu/SiO2. J. Catal. 1995, 154, 314-328. (92) Zhang, R.; Sun, Y.; Peng, S. In Situ FTIR Studies of Methanol Adsorption and Dehydrogenation over Cu/SiO2 Catalyst. Fuel 2002, 81, 1619-1624. (93) Cabilla, G. C.; Bonivardi, A. L.; Baltanás, M. A. FTIR Study of the Adsorption of Methanol on Clean and Ca-Promoted Pd/SiO2 Catalysts. J. Catal. 2001, 201, 213-220. (94) Eren, B.; Weatherup, R. S.; Liakakos, N.; Somorjai, G. A.; Salmeron, M. Dissociative Carbon Dioxide Adsorption and Morphological Changes on Cu(100) and Cu(111) at Ambient Pressures. J. Am. Chem. Soc. 2016, 138, 8207-8211. (95) Nakamura, J.; Rodriguez, J. A.; Campbell, C. T. Does CO2 Dissociatively Adsorb on Cu Surfaces? J. Phys.: Conden. Matter. 1989, 1, 149-160. (96) Kim, Y.; Trung, T. S. B.; Yang, S.; Kim, S.; Lee, H. Mechanism of the Surface Hydrogen Induced Conversion of CO2 to Methanol at Cu(111) Step Sites. ACS Catal. 2016, 6, 1037-1044. (97) Ou, L. Chemical and Electrochemical Hydrogenation of CO2 to Hydrocarbons on Cu Single Crystal Surfaces: Insights into the Mechanism and Selectivity from DFT Calculations. RSC Adv. 2015, 5, 57361-57371. (98) Zhang, X.; Liu, J. X.; Zijlstra, B.; Filot, I. A. W.; Zhou, Z.; Sun, S.; Hensen, E. J. M. Optimum Cu Nanoparticle Catalysts for CO2 Hydrogenation towards Methanol. Nano Energy 2018, 43, 200-209. (99) Muttaqien, F.; Hamamoto, Y.; Inagaki, K.; Morikawa, Y. Dissociative Adsorption of CO2 on Flat, Stepped, and Kinked Cu Surfaces. J. Chem. Phys. 2014, 141, 034702. (100) Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M. On the Mechanism of Low-Temperature Water Gas Shift Reaction on Copper. J. Am. Chem. Soc. 2008, 130, 1402-1414.

-62-


Page 62 of 65

Page 63 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(101) Peng, S. F.; Ho, J. J. The Mechanism of the Water-Gas Shift Reaction on Cu/TiO2(110) Elucidated from Application of Density Functional Theory. Phys. Chem. Chem. Phys. 2011, 13, 20393-20400. (102) Li, H. J.; Ho, J. J. Density Functional Calculations on the Hydrogenation of Carbon Dioxide on Fe(111) and W(111) Surfaces. J. Phys. Chem. C 2010, 114, 1194-1200. (103) Ruban, A. V.; Skriver, H. L.; Nørskov, J. K. Surface Segregation Energies in TransitionMetal Alloys. Phys. Rev. B 1999, 59, 15990-16000. (104) Liu, L.; Fan, F.; Jiang, Z.; Gao, X.; Wei, J.; Fang, T. Mechanistic Study of Pd–Cu Bimetallic Catalysts for Methanol Synthesis from CO2 Hydrogenation. J. Phys. Chem. C 2017, 121, 26287-26299. (105) Cho, J.; Lee, S.; Yoon, S. P.; Han, J.; Nam, S. W.; Lee, K. Y.; Ham, H. C. Role of Heteronuclear Interactions in Selective H2 Formation from HCOOH Decomposition on Bimetallic Pd/M (M = Late Transition FCC Metal) Catalysts. ACS Catal. 2017, 7, 2553-2562. (106) Jalili, S.; Zeini Isfahani, A.; Habibpour, R. Atomic Oxygen Adsorption on Au(100) and Bimetallic Au/M (M=Pt and Cu) Surfaces. Comput. Theor. Chem. 2012, 989, 18-26. (107) Rasmussen, P. B.; Holmblad, P. M.; Askgaard, T.; Ovesen, C. V.; Stoltze, P.; Nørskov, J. K.; Chorkendorff, I. Methanol Synthesis on Cu(100) from a Binary Gas Mixture of CO2 and H2. Catal. Lett. 1994, 26, 373-381. (108) Rasmussen, P. B.; Kazuta, M.; Chorkendorff, I. Synthesis of Methanol from a Mixture of H2 and CO2 on Cu(100). Surf. Sci. 1994, 318, 267-280. (109) Nie, X.; Esopi, M. R.; Janik, M. J.; Asthagiri, A. Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem. Int. Ed. 2013, 52, 2459-2462.

-63-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(110) Nie, X.; Luo, W.; Janik, M. J.; Asthagiri, A. Reaction Mechanisms of CO2 Electrochemical Reduction on Cu(111) Determined with Density Functional Theory. J. Catal. 2014, 312, 108-122. (111) Luo, W.; Nie, X.; Janik, M. J.; Asthagiri, A. Facet Dependence of CO2 Reduction Paths on Cu Electrodes. ACS Catal. 2016, 6, 219-229. (112) Montoya, J. H.; Shi, C.; Chan, K.; Norskov, J. K. Theoretical Insights into a CO Dimerization Mechanism in CO2 Electroreduction. J. Phys. Chem. Lett. 2015, 6, 2032-2037. (113) Cheng, T.; Xiao, H.; Goddard, W. A. Full Atomistic Reaction Mechanism with Kinetics for CO Reduction on Cu(100) from Ab Initio Molecular Dynamics Free-Energy Calculations at 298 K. P. Nati. Acad. Sci. 2017, 114, 1795-1800. (114) Jee, J. E.; Comas-Vives, A.; Dinoi, C.; Ujaque, G.; van Eldik, R.; Lledos, A.; Poli, R. Nature of Cp*MoO2+ in Water and Intramolecular Proton-Transfer Mechanism by Stopped-Flow Kinetics and Density Functional Theory Calculations. Inorg. Chem. 2007, 46, 4103-4113. (115) Szyja, B. M.; Smykowski, D.; Szczygiel, J.; Hensen, E. J.; Pidko, E. A. A DFT Study of CO2 Hydrogenation on Faujasite-Supported Ir4 Clusters: on the Role of Water for Selectivity Control. ChemCatChem 2016, 8, 2500-2507. (116) Luque-Urrutia, J. A.; Poater, A. The Fundamental Noninnocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-Based Catalysts. Inorg. Chem. 2017, 56, 14383-14387. (117) Polo, V.; Schrock, R. R.; Oro, L. A. A DFT Study of the Role of Water in the RhodiumCatalyzed Hydrogenation of Acetone. Chem. Commun. 2016, 52, 13881-13884. (118) Kovács, G. b.; Lledós, A.; Ujaque, G. Reaction Mechanism of the Gold(I)-Catalyzed Addition of Phenols to Olefins: A Concerted Process Accelerated by Phenol and Water. Organometallics 2010, 29, 3252-3260.

-64-


Page 64 of 65

Page 65 of 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(119) Campbell, C. T. Finding the Rate-Determining Step in a Mechanism. J. Catal. 2001, 204, 520-524. (120) Campbell, C. T. Future Directions and Industrial Perspectives Micro- and Macro-Kinetics: Their Relationship in Heterogeneous Catalysis. Top. Catal. 1994, 1, 353-366. (121) Larmier, K.; Liao, W. C.; Tada, S.; Lam, E.; Verel, R.; Bansode, A.; Urakawa, A.; Comas-

Vives, A.; Coperet, C. CO2 -to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface. Angew. Chem. Int. Ed. 2017, 56, 2318-2323. (122) Kattel, S.; Ramirez, P. J.; Chen, J. G.; Rodriguez, J. A.; Liu, P. Active Sites for CO2 Hydrogenation to Methanol on Cu/ZnO Catalysts. Science 2017, 355, 1296-1299. (123) Kattel, S.; Liu, P.; Chen, J. G. Tuning Selectivity of CO2 Hydrogenation Reactions at the Metal/Oxide Interface. J. Am. Chem. Soc. 2017, 139, 9739-9754.

Graphical Abstract

-65-


Mechanistic Understanding of Alloy Effect and Water Promotion for Pd

Recommend Documents