Catalytic Hydrogenation of Carbon Dioxide to Methanol: Synergistic

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Catalytic Hydrogenation of Carbon Dioxide to Methanol: Synergistic Effect of Bifunctional Cu/perovskite Catalysts Matej Huš, Drejc Kopa#, and Blaž Likozar ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03810 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 26, 2018

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Catalytic Hydrogenation of Carbon Dioxide to Methanol: Synergistic Effect of Bifunctional Cu/perovskite Catalysts Matej Huˇs,∗,†,‡ Drejc Kopaˇc,† and Blaˇz Likozar† †Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia ‡Department of Physics, Chalmers University of Technology, Fysikgränd 3, SE-41296 Gothenburg, Sweden E-mail: [email protected]

Abstract As the increasing concentration of the atmospheric CO2 is being progressively recognised as a global environmental problem due to its greenhouse effect, the catalytic hydrogenation of carbon dioxide to methanol has been repeatedly put forward as a way of carbon fixation. Time and again have been copper-based heterogeneous catalysts shown to be best suited for this technological purpose, but their performance must be improved with secondary metal oxides, dopants and supports. Herein, first-principles surface simulations of a Cu phase with four prospective perovskite substrate materials were performed. Cu/CaTiO3 , Cu/SrTiO3 , Cu/BaTiO3 and Cu/PbTiO3 were systematically studied. After extensive density functional theory (DFT) calculations, aimed at elucidating their stable structure, mapping out a complex reaction network, and pinpointing the rate-determining mechanism steps, the results were fed into a kinetic

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Monte Carlo (kMC) set-up at industrially-relevant operating conditions (the temperature of 420–660 K, pressure 0.001–100 bar and different reactant ratios). It was found out that all studied systems outperformed the pure Cu. Among them, Cu/PbTiO3 was shown to offer very high selectivity and an overall good activity. With lead-containing metallic compounds being problematic due to their toxicity, Cu/SrTiO3 is a very good alternative, closely followed by Cu/BaTiO3 . In all instances, CH3 OH was observed to form via the formate route (from CO2 to HCOO, HCOOH, H2 COOH, H2 CO, H3 CO and CH3 OH), while CO is produced from CO2 through t-COOH and c-COOH. The direct dissociation pathway of CO2 or CO hydrogenation was not notable, as indicated by the linked multi-scale description.

Keywords CO2 , hydrogenation, methanol, copper, perovskite, multi-scale modelling, DFT, kMC

1

Introduction

Catalytic hydrogenation of carbon dioxide to methanol is an important method of CO2 fixation. Provided that hydrogen is obtained from green sources, e.g. hydrolysis of water with hydroelectricity, this is an environmentally friendly and carbon-neutral source of methanol. 1 Such methanol can be either used as a versatile chemical in chemical industry or as a convenient and easier-to-handle fuel than hydrogen. Transport, storage and usage of methanol are possible on the existing infrastructure with minimal changes, which is not true for the utilization of hydrogen. Hydrogenation of carbon dioxide to methanol is thermodynamically favourable at low temperatures and should in principle be possible with high conversion. 2 Unfortunately, as carbon dioxide is very inert, high temperatures, pressures and, most importantly, efficient catalysts are required. The quest for the best catalyst has been going on for decades and

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despite several “exotic” ones showing promising performance, the copper-based catalysts remain ubiquitous in the industry thanks to their low price, durability and sufficient activity. Interestingly, pure copper is not a very efficient catalyst for this reaction and neither are pure spinels, perovskites, ZnO, and other materials that are used as a secondary metal in common well-performing catalysts. 3–7 It has been shown that both metals are needed in an efficient catalyst. The origin of this synergistic effect remains elusive despite considerable research effort. It is known that a large accessible Cu surface is beneficial as long as it is in contact with zinc or other easily oxidisable metal. 8–10 There is a large amount of empirical knowledge on the correlation between the catalyst synthesis and its performance but few systematic studies on the theoretical understanding of these effects exist. Also, sites with low coordination number (adatoms, kinks, steps) and other irregularities (defects, vacancies, strain) seem to enhance the activity. 11–13 Mechanistically, this reaction has been often studied on copper surfaces. On Cu(100), Cu(111), Cu(211), Zn-doped Cu(100) and Cu(211), and more complex copper catalysts, the formate pathway (HCOO, H2 COO, H2 COOH, CH2 O, CH3 O) predominates. 2,14–22 Usually, the rate-determining step is the formation of H2 COO or HCOOH. The reverse water–gas shift (RWGS) reaction is the most important competing reaction, responsible for the production of CO. The catalytic performance is therefore most sensitive to the adsorption energies of CO, HCOO, 23 HCOOH and H2 COO. 24 In this paper, we explored the synergistic effect on the example of Cu/perovskite catalysts. Perovskites are promising materials for the production of methanol. 25,26 Based on available experimental data from the catalyst characterisation, we postulated an atomistic model of perovskite support in contact with the bulk copper. Catalysts with copper on SrTiO3 , BaTiO3 , CaTiO3 and PbTiO3 were studied. For every catalyst, density functional theory calculations were performed to obtain the atomistic insight into the electronic structure, adsorption modes and elementary reaction steps. Both, the principal formate reaction pathway and common side reactions (RWGS, CO2 dissociation, etc.) were taken into account.

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Kinetic Monte Carlo simulations were subsequently run to study the temporal evolution of the catalysed reaction and to obtain the kinetic data. It was found out that copper/perovskite bifunctional catalysts perform better than pure copper. Despite possessing a similar structure and exhibiting comparable thermodynamics, the four investigated catalysts show stark differences in performance. Lead- and strontiumbased perovskite materials were found to be the best secondary metals to supplement copper, while calcium-based perovskites performed the worse.

2 2.1

Computational details Electronic structure calculations

Plane-wave based density functional (DFT) computations were performed with Vienna Ab Initio Simulation package 5.4.1 27–30 (VASP 5.4.1). We used PAW 31,32 pseudopotentials containing gradient-corrected Perdew-Burke-Ernzerhof exchange correlation. 33,34 One-electron Kohn-Sham orbitals were expanded with a kinetic energy cut-off of 450 eV. Reciprocal space integration was approximated with Monkhorst-Pack k -point grid of 2×2×1 for catalytic surface and 16×16×16 for bulk calculations. Structures were relaxed (except for the bottom two perovskite layers, see Catalyst model) until forces dropped below 0.01 eV/˚ A for the stable structures and 0.03 eV/˚ A for the saddle points. A guess for transition states from nudge elastic band 35,36 method was refined with the dimer method 37–40 to the true structure. Vibrational analysis (for zero-point energy corrections, parition function calculations and saddle point verification), limited to the surface species and keeping the rest of the system fixed, was carried out by calculating Hessian matrix with finite difference approach with a step size of 0.02 ˚ A. Calculations were performed in a spin-polarized fashion. Dipole corrections in vacuum were used to counteract the errors introduced by periodic boundary conditions. 41,42 To compensate for a poor description of van der Waals interactions by vanilla GGA functionals, the D3 correction method by Grimme was employed. 43 Contrary to 4


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D2, which in many cases overbinds the adsorbates, D3 uses geometry-dependent dispersion coefficients.

2.2

Catalyst model

Bulk structures for fcc Cu and perovskites ATiO3 (A being SrTiO3 , BaTiO3 , CaTiO3 or PbTiO3 ) were optimised to obtain the cell parameters. As shown in Table 1, they are in good agreement with the experimental values. 44–47 The support was modelled as four layers of perovskite ATiO3 with an unreconstructed top A–O(001) surface, which is thermodynamically the preferred surface termination. 48,49 A supercell of 4×2 was chosen because its dimensions are commensurate with the copper lattice. From an experimental catalyst characterisation and previous theoretical work it is known that rather large copper islands must be present, which provide the sufficient interface area for the reaction. We envision an infinitely long copper deposition, cut along the (100) and (111) surfaces from the bulk copper structure, to be present on top of the perovskite structure (see Figure 1 for the structure). Several positions on the perovskite were considered to arrive at the global energy minimum. Ab initio molecular dynamics was performed on these structures to verify that they are kinetically stable. The reactions are then carried out at the interface between Cu and perovskite. The bottom two layers of the perovskite support were fixed in the calculations to mimic the bulk environment. The top two perovskite layers (those interacting with the copper phase and adsorbates), the copper phase and the adsorbates were free to relax to account for the surface relaxation effects and possible adsorbate-induced rearrangements.

2.3

Kinetics

Four types of events are possible: Langmuir-Hinselwood (LH) surface reactions, Eley-Rideal (ER) reactions, non-activated adsorption and activated adsorption. For each reaction, the reverse one is also included to ensure the macroscopic reversibility. The reaction rates are 5


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Table 1: Comparison of the calculated and experimental lattice constants for bulk Cu and perovskites. compound Cu SrTiO3 BaTiO3 CaTiO3 PbTiO3

calculated (˚ A) 3.630 3.932 4.039 3.899 3.968

experimental (˚ A) 3.61 3.90 44 4.00 45 3.82 46 3.97 47

Cu

Sr

O Ti

Figure 1: The atomistic model of the investigated catalysts with the copper–perovskite interface (three cells are shown). Left: top view, right: perspective view. Colour code: brown - Cu, green - Sr/Ba/Ca/Pb, grey - Ti, red - O. calculated as kf wd

EA (σ) Q]vib kB T exp − , = Qvib h kB T

(1)

for LH steps, where Qvib is the vibration partition function, kB is the Boltzmann constant, T is temperature, h is the Planck constant, and EA (σ) is the activation barrier. For the ER reactions, one must include the vibrational, rotational and translational partition functions for the gaseous reactant:

kf wd

Q]vib pA EA (σ) = lat gas gas gas √ exp − , kB T Qvib Qvib Qrot Qtrans 2πmkB T

(2)

where Qvib and Qtrans denote the vibration and translation partition function of the reactants and the transition state, p denotes the pressure, A is the effective area of the reaction site (in our case the surface of a 1×1 unit cell) and m is the mass of the gaseous species. An

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activated adsorption can be considered a special case of the ER mechanism:

kf wd

Q]vib pA EA (σ) √ = exp − , Qvib Qrot Qtrans 2πmkB T kB T

(3)

while a non-activated adsorption is a purely kinetic phenomenon

kf wd = √

pA . 2πmkB T

(4)

The theoretical background and more details have been published elsewhere, for instance in Ref. 50

2.4

Kinetic Monte Carlo simulations

Using the data from our ab initio calculations (see Results and Discussion), kinetic Monte Carlo (kMC) simulations were carried out to study the dynamics of surface reactions on a meso scale. These simulations provide theoretical estimates of the selectivity, production rate and turn-over frequency (TOF) for the catalyst under various operating conditions (such as the temperature, pressure, gas mixture composition). We follow a similar methodology and applications as in our previous works. 19,51 In this work, the kMC software package Zacros 2.0 was used. 50,52,53 Following the DFTobtained optimized perovskite structure, we define two distinct active sites: the interface between Cu and perovskite and the Cu site (except on Cu(111)). The lattice consisted of 50 × 50 cells, resulting in 5000 active sites. All energetics (species adsorption energies, reaction activation energies) was obtained from the DFT, while the pre-exponential factors were calculated in real time to account for the relevant operating conditions. To solve the problem of “fast reactions” (mostly adsorptions), we employed the Zacros stiffness scaling, allowing for a time-scale separation of fast quasi-equilibrated events, such as diffusion. The overall reaction model consists of 18 lattice intermediates and 6 gaseous species (see Table 2 for details), participating in 40 elementary reactions steps (counting also the 7


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reverse ones). Reactions occur on the interface between the Cu and perovskite, while H and CO adsorb, desorb and diffuse to and from the Cu sites. Species with two oxygen atoms were treated as bi-dentate, occupying two neighbouring interface sites. Lateral interactions between the neighbouring HCOO molecules were obtained from the DFT and taken explicitly into account. Due to their low coverage, lateral interactions of other intermediates were shown to be inconsequential in our model by preliminary testing.

3

Results and discussion

3.1

Adsorption

We list all calculated adsorption energies, including zero-point energy corrections, in Table 2. Negative values mean that the adsorption is an exothermic process. There are in principle three qualitatively different environments to which the adsorbates can bind. Most often, they orient to be in contact with the copper and perovskite surfaces simultaneously. This is to be expected since the interplay of both materials is necessary for the catalytic performance. H and CO, however, bind more strongly to copper. While H binds exclusively to copper and then migrates to the active site at the interface in a process of hydrogen spill-over, CO can also bind to the interface, as the difference between both adsorption modes is only around 0.1 eV. On the other hand, H2 O, H3 CO, and H2 CO prefer to bind to the perovskite surface. This can be explained by a strong electronegative character of their oxygen atom, which readily binds to undercoordinated surface Sr/Ba/Ca/Pb atoms. Geometries of the adsorbed species on Cu/SrTiO3 are shown in Figure 2. On other investigated catalysts, the structures are similar. H2 and CO2 interact with the catalyst in a non-specific way because their interaction is mostly van der Waals. H2 binds very weakly (0.07–0.13 eV). In an activated adsorption step, however, it dissociates and binds to copper as H atoms. Adsorption energies of H atoms relative to 12 H2 range from 0.35 eV on Cu/CaTiO3 to 0.60 eV on Cu/PbTiO3 . CO2 8


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Figure 2: Geometries of stable intermediates on Cu/SrTiO3 . From top left: CO2 , H/interface, H/Cu, CO/interface, CO/Cu, HCO, HCOO, t-COOH, c-COOH, H2 COO, H2 COOH, H2 CO, H3 CO, H2 COH, CH3 OH, O, OH, H2 O. Adsorbates on Cu/BaTiO3 , Cu/CaTiO3 , and Cu/PbTiO3 assume similar geometries.

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Table 2: Calculated zero-point corrected adsorption energy of reaction intermediates on the ZPE . Note that for H reported values are relative to 12 H2 . investigated catalysts, Eads Adsorbate H2 CO2 CO CO H2 CO HCOOH CH3 OH H2 O H H H HCOO H2 COO H2 COOH H3 CO t-COOH c-COOH HCO H2 COH O OH

Site non-specific non-specific interface copper perovskite interface interface perovskite interface copper perovskite interface interface interface perovskite interface interface interface interface interface interface

Cu/SrTiO3 −0.08 −0.42 −1.03 −1.14 −0.85 −0.96 −1.02 −1.03 −0.26 −0.50 −0.24 −3.23 −4.69 −3.07 −2.51 −2.49 −2.37 −2.03 −2.06 −5.30 −3.57

Cu/BaTiO3 −0.07 −0.37 −0.95 −0.98 −0.87 −0.88 −0.86 −0.88 −0.32 −0.60 −0.33 −3.01 −4.31 −2.58 −2.42 −2.54 −2.43 −2.09 −1.91 −5.12 −3.46

ZPE (in eV) Eads Cu/CaTiO3 −0.13 −0.45 −1.00 −1.03 −1.03 −1.05 −1.35 −1.30 −0.27 −0.35 −0.22 −3.56 −5.15 −3.28 −2.96 −2.55 −2.54 −2.09 −2.24 −5.17 −3.70

Cu/PbTiO3 −0.09 −0.26 −1.02 −1.17 −0.66 −0.66 −0.65 −0.59 −0.24 −0.50 −0.18 −2.80 −4.08 −2.33 −2.41 −2.36 −2.29 −1.96 −1.88 −5.11 −3.34

Cu −0.07 −0.26 — −0.72 −0.78 −1.31 −0.82 −0.41 — −0.07 — −3.00 −4.41 −3.11 −2.43 −1.60 — −1.73 −1.53 −4.72 −2.92

is a bigger molecule and thus its interaction is 0.26–0.45 eV. For its reaction with adsorbed hydrogen, both Langmuir-Hinshelwood and Eley-Rideal mechanisms are considered. CO binds to the copper and perovskite sites and shows slight preference for the former. It is bound strongly (0.95–1.17 eV). CH3 OH and HCOOH exhibit similar behaviour, both binding to the interface site with adsorption energies around 1 eV, except on Cu/PbTiO3 (0.66 eV). H2 CO and H2 O prefer the perovskite structure and again bind with approximately 1 eV, except on Cu/PbTiO3 . These interaction energies are sufficiently large that H2 CO and HCOOH react further. Looking at the less stable intermediates, we first notice that HCOO binds only in a bidentate configuration, whereas on pure Cu surface also a monodentate adsorption mode is possible. 24 It orients with both oxygen atoms towards the perovskite metal. Both, t-COOH and c-COOH bind to copper with the carbon atom and to perovskite with one oxygen atom. H2 COO and H2 COOH interact through the oxygen atoms; one binds to the perovskite surface and the other one to copper. H3 CO, O and OH bind through their oxygen atoms at the interface site, while H2 COH interacts with copper via its carbon atom and with the

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perovskite surface with the oxygen atom. For quantitative details on the strength of the interactions, see Table 2. The simulations showed (discussed in detail later on) that HCOO is the most abundant intermediate on the surface. To avoid poisoning the catalyst with that intermediate, repulsive interactions of two adjacent HCOO species were included in the model and calculated as

Einteraction = EHCOO∗ +HCOO∗ − 2EHCOO∗ + E ∗ ,

(5)

where EHCOO∗ +HCOO∗ is the total energy of the system with two co-adsorbed HCOO species, EHCOO∗ is the total energy of the system with one adsorbed HCOO and E ∗ is the energy of an empty catalyst. Interaction energies are 0.25, 0.26, 0.27, and 0.02 eV for Cu/SrTiO3 , Cu/BaTiO3 , Cu/CaTiO3 , Cu/PbTiO3 , respectively. These values are comparably low and according to preliminary calculations only affect the surface coverage to some extent but not the catalytic performance. Thus, it was not necessary to include other lateral interactions. A full model would encompass

1 2

17 · 18 of them, which would make it difficult to analyse.

The effect of lateral interactions on the transition states is included by the code through BEP relations. 52

3.2

Elementary reactions

The exact mechanism of CO2 hydrogenation to methanol is still a matter of speculation and probably depends upon the catalyst. Therefore, we worked with a comprehensive reaction network, accounting for all relevant reactions, as shown in Figure 3. We studied the formate pathway (often considered principal), RWGS pathway and possible side reactions (HCOOHvariant of the formate route, CH2 OH formation). Additionally, hydrogenation of CO is also included. All calculated elementary reaction steps are listed in Table 3. Besides LH reactions steps on the surface, the activated dissociative adsorption of hydrogen is included. Surface

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CO2 14

3

2

HCOO

t-COOH 15

5

4

c-COOH HCOOH

H2COO

16

CO

6 17

HCO

7

8 18

H2COOH

9

H2CO 10

11

CH2OH

CH3O 12

13

CH3OH

Figure 3: An overview of elementary reactions included in the model. The most probable pathway leading up to methanol is typeset in bold. For clarity, hydrogen, oxygen and hydroxyl are not explicitly written. Dashed arrows denote adsorption/desorption equilibria. diffusion was considered only for H and CO, because other species either bind too strongly or bind non-selectively (CO2 ). H and CO are quite mobile with diffusion barriers around 0.3 eV. Reaction rates were calculated as discussed in the Computational details section. The transition states for all elementary reaction steps are shown in Figure 4 and presented graphically in the potential energy surface (PES) diagrams in Figure 5.

3.2.1

Adsorption and diffusion

On all four catalysts, the molecular hydrogen weakly interacts with the copper surface. It easily dissociates and binds as H on copper in a fast reaction step (activation barrier 0.1 eV). To participate in the hydrogenation reaction, however, it must migrate to the interface site. The adsorption energy at the interface is 0.2–0.3 eV less favourable than on pure copper. This is the spill-over effect and presents one of the bottlenecks of this reaction. Diffusion of H and CO is explicitly taken into account. Both species are very mobile, having the diffusion

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Figure 4: Geometries of the transition states on Cu/SrTiO3 . From top left: H2 → H+H, CO2 → CO+O, CO2 +H→ HCOO, HCOO+H→ H2 COO, HCOO+H→ HCOOH, H2 COO+H→ H2 COOH, HCOOH+H→ H2 COOH, H2 COO→ H2 CO+O, H2 COOH→ H2 CO+OH, H2 CO+H→ H3 CO, H2 CO+H→ H2 COH, H3 CO+H→ H3 COH, H2 COH+H→ H3 COH, CO2 +H→ t-COOH, t-COOH→ c-COOH, c-COOH→ CO+OH, CO+H→ HCO, HCO+H→ H2 CO. Adsorbates on Cu/BaTiO3 , Cu/CaTiO3 , and Cu/PbTiO3 assume similar geometries. Colour code as in Figure 2.

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ACS Catalysis

Table 3: Calculated activation barriers and reaction energies for all considered elementary reactions on the investigated catalysts.

CH3OH

H2CO + 2 H

H2

H2

H3CO + H

+

H2COH + H

CO

H2

HCO + H H2CO

+

H2

H2CO + OH + H CO + 2 H H2CO + H2O

c -COOH + H H2COO

t -COOH + H

HCOOH H2CO + O + 2 H CO + OH + H H2COO + 2 H CO + H2O H2CO + O + 2 H H2COOH + H

-1

+

HCOOH

E (eV)

H2COH + H

H2

HCOO + H

H2CO + 2 H

1 HCOOH CO + OH + H H COO + 2 H H CO + O 2 2 +2H CO + H2O H2CO + O + 2 H H2COOH + H CO H2CO + OH + H H2

H2

CH3OH

H2COH + H

H2

H2CO + 2 H

H2

H3CO + H

+

+

+

H2

HCO + H H2CO

H2COO

t -COOH + H

+

HCOOH c -COOH + H

HCOO + H

H2

CO2

E (eV) -1 CH3OH

H2CO + 2 H

H2COH + H

H2

H2CO

+

+

CO2+ 2 H

HCO + H

0

Cu/PbTiO3

CO + 2 H H2CO + H2O

H2CO + OH + H CO + 2 H H2

CO + 2 H

H2CO + OH + H

H2COOH + H CO + H2O

H2

+

+

+

+

H2CO + H2O

H2

CO

H2CO + O + 2 H

H2CO + O

+

H2

H2COO + 2 H HCOOH + 2 H

+

CO + OH + H

c -COOH + H

t -COOH + H

+

H2COO HCOOH

CO2+ 2 H

HCOO + H

H2

CO2

+

-2

0

Cu/CaTiO3

0

Cu ∆E ZPE −0.14 +1.33 −0.05 +0.60 −0.24 −0.82 +0.02 +0.72 +1.03 −0.49 +0.05 −0.34 −0.89 +1.21 — −0.38 +0.29 −0.35 −0.50 −0.37 — —

-2

1

-1

H2

ZPE EA 0.26 1.46 1.19 0.97 0.54 0.49 0.72 1.21 1.41 0.54 0.75 0.70 0.29 1.91 — 0.78 0.39 0.41 0.53 0.89 0.17 0.22

Cu/BaTiO 3

CH3OH

+

H3CO + H

HCO + H

H2

H2CO

+

H2CO + H2O

H2CO + O

H2CO + O + 2 H CO CO + H2O

HCOOH H COO CO + OH + H 2 +2H +2H

t-COOH + H

H2

H2

H2COOH + H

-2

+

HCOOH

CO2 CO2 + 2 H

-1

H2

+

HCOO + H

E (eV)

H2

H2COO

+

+

Cu/PbTiO3 ZPE EA ∆E ZPE 0.09 −0.49 1.07 +0.55 0.21 −0.13 1.57 +1.05 0.71 +0.33 1.03 −0.25 0.82 +0.47 0.82 +0.19 0.36 −0.06 0.51 −0.44 1.00 −0.17 0.97 +0.00 0.83 −0.27 0.45 +0.14 0.51 +0.14 0.94 −0.22 1.08 +0.45 0.60 +0.11 0.96 −0.50 0.90 −0.06 0.33 — 0.21 —

1

Cu/SrTiO 3

0

Cu/CaTiO3 ZPE EA ∆E ZPE 0.10 −0.53 0.87 +0.70 0.25 −0.68 1.11 +0.75 0.79 +0.72 0.50 −0.09 0.74 −0.06 0.88 +0.84 0.73 +0.15 0.40 −0.60 0.54 −0.14 0.51 −0.12 0.41 −0.59 0.21 +0.16 0.49 +0.08 0.87 −0.32 0.50 +0.33 0.58 −0.11 0.61 −0.78 0.46 −0.39 0.22 — 0.24 —

CO2

1

Cu/BaTiO3 ZPE EA ∆E ZPE 0.07 −0.63 0.76 +0.72 0.27 −0.16 1.58 +1.09 0.62 +0.39 1.00 −0.18 0.96 +0.51 0.73 +0.21 0.12 −0.16 0.54 −0.17 1.06 +0.08 0.60 −0.12 0.92 −0.36 0.25 +0.15 0.47 +0.18 0.80 −0.15 0.69 +0.33 0.56 +0.09 0.75 −0.55 0.94 −0.15 0.35 — 0.27 —

CO2+ 2 H

Cu/SrTiO3 ZPE EA ∆E ZPE 0.11 −0.52 1.13 +0.51 0.29 −0.38 1.31 +0.88 0.73 +0.48 0.93 −0.35 0.80 +0.06 0.86 +0.42 0.42 +0.25 0.32 −0.34 0.92 −0.15 0.50 −0.24 0.53 −0.43 0.31 +0.18 0.51 +0.20 0.83 −0.40 0.46 +0.41 0.22 +0.01 0.86 −0.52 0.82 −0.25 0.24 — 0.24 —

Reaction H2 (g)→ 2 H CO2 → CO+O CO2 +H→ HCOO HCOO+H→ H2 COO HCOO+H→ HCOOH H2 COO+H→ H2 COOH HCOOH+H→ H2 COOH H2 COO→ H2 CO+O H2 COOH→ H2 CO+OH H2 CO+H→ H3 CO H2 CO+H→ H2 COH H3 CO+H→ H3 COH H2 COH+H→ H3 COH CO2 +H→ t-COOH t-COOH→ c-COOH c-COOH→ CO+OH CO+H→ HCO HCO+H→ H2 CO O+H→ OH OH+H→ H2 O H→ H (diffusion) CO→ CO (diffusion)

H3CO + H

# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

E (eV)

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

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-2

Figure 5: Potential energy surface for the formation of CH3 OH (solid lines) and CO (dashed lines) on the four investigated catalysts.

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ACS Catalysis

barrier 0.2–0.3 eV.

3.2.2

Formate pathway

The formate pathway is according to this and previous studies the predominant way of the production of methanol. In the first step, CO2 is hydrogenated to HCOO. This fast and moderately exothermic step is followed by hydrogenation to H2 COO or HCOOH. This second reaction is the rate-determining step for the production of methanol because it has both the highest activation barrier and is rather endothermic. Once this obstacle is overcome, H2 COO can either decompose into H2 CO or is further hydrogenated to H2 COOH, which also decomposes into H2 CO. Any HCOOH is also hydrogenated to H2 COOH. On all four catalysts, the activation barriers for the formation of HCOOH are actually lower but the reaction is endothermic. In all instances, H2 CO is hydrogenated to H3 CO and not H2 COH. Althouh both reactions are exothermic, the barrier for the formation of H3 CO is twice as low. Following the last hydrogenation, CH3 OH is formed, which then desorbs as the final production.

3.2.3

RWGS

RWGS is an undesired reaction pathway because it leads to the formation of CO. This pathway will be followed when CO2 is hydrogenated on the oxygen atom and t-COOH is formed. This reaction has a comparable activation energy to the competing HCOO formation, except on Cu/PbTiO3 where it is less favourable. The next step is its isomerization to c-COOH, which then decomposes into CO and OH. This reaction step occurs at the interface. Although barriers for the further hydrogenation of CO to HCO are not prohibitively high (0.4–0.7 eV), except on Cu/PbTiO3 , this step is very endothermic. Additionally, CO can diffuse away. CO has a low diffusion barrier and readily migrates to the copper surface, where it is inaccessible for the hydrogenation and desorbs. Thus, RWGS mostly creates CO. Were HCO formed, it would be hydrogenated to H2 CO and follow the formate pathway from

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there on.

3.2.4

Direct dissociation of CO2

CO could in principle also form directly from CO2 if it dissociated on the catalyst. This reaction is much less favourable than hydrogenation. Besides being strongly endothermic due to the formation of O, the activation barrier is prohibitively large. On Cu/BaTiO3 the barrier is the lowest at 0.76 eV and on Cu/SrTiO3 it is already 1.13 eV. As also shown by kMC, this reaction does not play an important role on the investigated surfaces.

3.2.5

Water formation

During the reaction, O and OH will form on the catalyst. Because hydrogen is also present, they will readily react to form water. The hydrogenation of O to OH and of OH to H2 O is exothermic and has the activation barriers between 0.46 and 0.94 eV. They are the largest on Cu/PbTiO3 and lowest on Cu/CaTiO3 . It is not likely that H2 O and OH would decompose by giving up hydrogen.

3.3

Kinetic modelling

kMC simulations were carried out to understand the temporal evolution of the system and the overall performance of the four catalysts. When dealing with a broad reaction network, such as in our case, kinetic modelling is paramount to make sense of the DFT data. We performed a series of simulations for all four investigated catalysts. To evaluate the effects of particular operating conditions and to gain an insight into the mechanism, reactions were studied at various pressures (from 1–60 bar), temperatures (420–600 K), and influx composition, all corresponding to realistic values for the industrial application. This allowed us to study the reaction in terms of selectivity, productivity, apparent activation barriers, reaction order, surface coverages and thus compare the four catalysts with pure copper.

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ACS Catalysis

3.3.1

Apparent activation energy

As shown in Figures 6 and 7, the rate of CO2 hydrogenation to either CH3 OH or CO follows the same regime in the whole range of the investigated temperatures. The rate of methanol production, measured as the TOF, is largest for Cu/PbTiO3 and Cu/BaTiO3 catalysts, followed by Cu/SrTiO3 , pure copper and Cu/CaTiO3 . The production of CO is the fastest on Cu/PbTiO3 , Cu/BaTiO3 , and Cu/SrTiO3 (in that order) and considerably slower on Cu/CaTiO3 . This predicts Cu/CaTiO3 to be a particularly bad catalyst for any hydrogenation of CO2 . Plotting the TOFs against the inverse temperature, we determined the apparent activation barrier (Eapp ) for the reactions using the Arrhenius law. For Cu/PbTiO3 and Cu/BaTiO3 , Eapp is smaller for methanol than for CO (88 vs. 102 and 89 vs. 82 kJ mol−1 , respectively). This has an influence on the selectivity, as will be shown later on. On Cu, the (Eapp ) is 104 kJ mol−1 , which is in agreement with previously reported values of 1.1 eV (106 kJ mol−1 ). 54 On the other two catalysts, Eapp for production of CO is lower. On Cu/CaTiO3 , the activation barriers for both reactions are high, making it unsuitable for catalysis. One should note that these are average Eapp for a complex reaction network with parallel paths and therefore cannot be approximated as the largest activation barriers in a particular reaction network.

17


102

-2

10

ln CH3OH TOF per site

10

Sr

Ba

-4

10

Ca

-6

10

10-8 10

400

450

Page 18 of 53

10

Pb

0

-10

500 550 T (K)

600

5 0 -5

-15 -20

650

Ba: 89 kJ/mol Sr: 143 kJ/mol Pb: 88 kJ/mol Ca: 186 kJ/mol Cu: 104 kJ/mol

-10

15

16

17

18

19

20

21

1/T (10-4 K-1)

Figure 6: The turnover frequency (TOF) for the production of CH3 OH on the investigated catalysts as a function of temperature at 2.5 bar CO2 and 7.5 bar H2 (left) allows for the determination of the apparent activation energy (right). 102

10

Ba Sr Pb

100 10

-2

10

-4

10

-6

ln CO TOF per site

CO TOF per site (s-1)

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 TOF per site (s-1)

ACS Catalysis

Ca

10-8 -10

10

400

450

500 550 T (K)

600

5 0 -5

-15 -20

650

Ba: 92 kJ/mol Sr: 123 kJ/mol Pb: 102 kJ/mol Ca: 151 kJ/mol Cu: 105 kJ/mol

-10

15

16

17

18

19

20

21

1/T (10-4 K-1)

Figure 7: The turnover frequency (TOF) for the production of CO on the investigated catalysts as a function of temperature at 2.5 bar CO2 and 7.5 bar H2 (left) allows for the determination of the apparent activation energy (right).

3.3.2

Selectivity

When hydrogenating CO2 to methanol, the RWGS is a competing reaction producing CO. On many catalysts with good conversion, this is the main product for kinetic reasons despite the thermodynamics favouring the methanol production. Thus, selectivity is an important measure of the usefulness of a particular catalyst. As seen in our reaction network and in the discussion on the event frequency later on, the first hydrogenation of CO2 can occur on the oxygen or carbon atom. 18


Page 19 of 53

When the carbon atom is first hydrogenated, HCOO is formed. It is ultimately hydrogenated to methanol, but HCOO itself is a very non-reactive intermediate with a high activation barrier for further reactions. It has even been described as a mechanistic deadend, 24 although our simulations show it does undergo hydrogenation. It can transform to either H2 COO or HCOOH, both eventually being transformed into methanol. When CO2 is first hydrogenated to t-COOH, a different mechanism is triggered. In that case, the C-O bond scission will follow and CO will be formed. The ensuing CO could be hydrogenated via HCO to methanol, but our simulations show that it rather desorbs. Thus, selectivity is in great part (but not exclusively) determined with the ratio between the activation barriers for the two first hydrogenations of CO2 . Comparing the DFT values, we see that they are comparable for all the catalysts except for Cu/PbTiO3 , where HCOO formation is much more likely. This is confirmed with kinetic studies. Figure 8 shows the selectivity towards methanol as a function of temperature and pressure, respectively. As expected, the selectivity is best for Cu-PbTiO3 catalyst, where not much CO is formed. Cu/SrTiO3 also exhibits good selectivity, reaching up to 80 % at lower temperatures and higher pressures. Cu/BaTiO3 is moderately selective, while Cu/CaTiO3 produces mainly (and slowly, as shown in the previous section) CO. 1.0

1.0

Pb

0.8

Pb

Sr

0.8 Sr

0.6

Ba

0.4 0.2 0.0 450

selectivity

selectivity

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

550 T (K)

600

Ba

0.4 0.2

Ca 500

0.6

0.0

650

Ca 20

40

60 p (bar)

80

100

Figure 8: The selectivity as a function of temperature at 2.5 bar CO2 and 7.5 bar H2 (left) and as a function of pressure at T = 580 K with H2 :CO2 = 1:1 (right).

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3.3.3

Reaction order

To obtain further insight into the reaction mechanism, we also study the reaction order. For a complex reaction network, it is difficult to obtain the reaction order without extensive kinetic modelling. Thus, we first study the effect of the CO2 partial pressure on the reaction rate. In Figure 9, we show a log–log plot of the TOF (for methanol and CO production) versus the CO2 pressure at a constant temperature and H2 pressure. For the most active Cu/PbTiO3 catalyst, the reaction order is approximately 0.8 for the production of methanol, while for Cu/PbTiO3 and Cu/SrTiO3 it is around 0.7. Cu/CaTiO3 is poorly active, the reaction rate not being strongly influenced by the CO2 concentration. For production of CO, the reaction order is 0.5-0.6 in all instances. At very high CO2 pressures, the reaction rate actually decreases. We studied the catalyst coverage and determined that this happens due to poisoning of the catalyst. At high pressures, the surface concentration of the adsorbed hydrogen drops precipitously, as HCOO starts to dominate the surface. This explains apparent negative reaction orders at high CO2 pressure. In Figure 10, the reaction order with respect to H2 is shown. We see that for Cu/PbTiO3 , Cu/SrTiO3 and Cu/BaTiO3 the reaction order is high (around 1.3–1.4), while Cu/PbTiO3 is not even active at very low pressures. At higher pressures, the reaction rate plateaus as the catalyst becomes saturated with hydrogen. At extremely high pressures, where the model accuracy becomes questionable, a negative reaction order is predicted due to catalyst saturation with hydrogen.

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0

-2

-2

-4

0.66

-6

0.69

-12

Sr

Ba

-8 -10

-0.95

0.05

Pb

0.81

-0.80

-14

Ca

ln CO TOF per site

0

-16

-6 -10 -12

-0.40

0.50

Sr

-8

-0.73 0.45

Ca

0.53

Pb

-14 -16

10-5 10-4 10-3 10-2 10-1 100 101 102 pCO (bar)

Ba 0.62

-4

10-5 10-4 10-3 10-2 10-1 100 101 102 pCO (bar) 2

2

Figure 9: The reaction order with respect to CO2 gas pressure for the production of CH3 OH and CO at T = 500 K and constant H2 pressure of 1 bar.

0

-0.53

-5 -10

1.29

Pb 1.40

1.45

Sr 0.21

Ca

Ba -15 -20

Ba

0

0.22

ln CO TOF per site


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

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0.42

-5 0.21

-10

Pb

Ca

0.40

-15 -20

10-5 10-4 10-3 10-2 10-1 100 101 102 pH (bar)

Sr

0.37

10-5 10-4 10-3 10-2 10-1 100 101 102 pH (bar) 2

2

Figure 10: The reaction order with respect to H2 gas pressure for the production of CH3 OH and CO at T = 500 K and constant CO2 pressure of 1 bar.

3.3.4

Surface coverage

One of the advantages of the kinetic Monte Carlo is that it shows the microscopic picture of the population of catalyst surface. In Figure 11, we plot the temporal evolution of the catalyst coverage for all four investigated catalysts at the same conditions. The rough picture is similar. In all instances, HCOO and H are the most abundant species. In case of the Ba-, Ca- and Sr-containing perovskites, a significant coverage of OH, H2 O and H3 CO is present, while the Pb-containing perovskite shows only the formation of H3 CO. As CO2 weakly binds to the surface and then quickly reacts (via both Langmuir-Hinshelwood and Eley-Rideal 21


ACS Catalysis

mechanisms), its surface concentration quickly becomes negligible. In all instances, hydrogen must first adsorb to the surface. After its surface concentration reaches a dynamic equilibrium (after approx. 10−8 s), the reaction proceeds. We see another change in regime after approximately 10−6 s, when the surface concentration of HCOO peaks and reaches a dynamic equilibrium. Afterwards, the catalyst operates in a steady state. After a second, the surface coverages stabilize. 0

coverage

10

coverage

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

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Ba

Ca

Pb

Sr

10-1

H* CO2** HCOO** HCOOH** H3CO* t-COOH** OH* H2O**

10-2

10-1

10-2

10-3 10-10

10-8

10-6

10-4

10-2

Figure 11: The surface coverage of the investigated catalysts at 10 bar and T = 480 K.

3.3.5

Charge transfer

From Table 2 we see that on average, all adsorbates interact most strongly with Cu/CaTiO3 and Cu/BaTiO3 and weakly with Cu/SrTiO3 and Cu/PbTiO3 . As we showed with the kMC modelling, this seems to affect the activity and selectivity of the catalyst. In Figure 12 we plot the average adsorption energy of all adsorbates on a particular catalyst in comparison to Cu/CaTiO3 n

∆Eads

1X Ca = Eads − Eads n i

22


(6)

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and the calculated charge transfer (Bader charge) from Cu to the perovskite. We also plot the selectivity at 520 K and 10 atm. We can see that in the case of Cu/CaTiO3 , the charge transfer is the strongest and electrons from copper move to the perovskite. This results in a stronger interaction of adsorbates and low selectivity towards methanol. The other extreme is Cu/PbTiO3 , where copper actually gains some electron density from the perovskite, which results in greater selectivity and weaker adsorbate interactions. 0.4

100 80

Pb Sr

60

0.2 40

Ba 0.1

20

Ca 0.0 -0.4

-0.2 0.0 0.2 0.4 charge transfer (e0)

Selectivity (%)

0.3 ∆Eads (eV)

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

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0.6

0

Figure 12: Correlation between the charge transfer from the copper island and the strength of the adsorption energies (more positive numbers indicate less favourable interaction; black squares) and selectivity (green circles) 10 bar and 525 K.

3.3.6

Event frequency

Lastly, we examine the relative frequency of each elementary step, as depicted in Figure 13, which gives a valuable insight into both the mechanism and ergodicity of the simulation. It is especially useful when there are parallel pathways, leading to the same product. We see that the simulation is well equilibrated. First, we see that products only desorb, which is the consequence of the kMC set-up (only H2 and CO2 in the gaseous phase). Where Eley-Rideal and Langmuir-Hinshelwood mechanisms are possible, the latter predominates (hydrogenation of the adsorbed CO2 as opposed to reacting directly from the gaseous phase). The direct dissociation of CO2 to CO and O is many orders of magnitude less frequent than hydrogenation, thus we can rule

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Page 24 of 53

out this mechanism of CO formation. The formation of t-COOH and HCOO is competitive and has direct implications for the selectivity, as already shown. HCOO, when formed, most readily converts into HCOOH and less so in H2 COO. Any H2 COO decomposes back into HCOO. HCOOH is hydrogenated into H2 COOH, which decomposes into H2 CO and is hydrogenated to methanol via H3 COH. Here we also see the reason why Cu/CaTiO3 performs poorly. It is the only catalyst, where H2 COO and H2 COH form in noticeable amounts. It is also the only catalyst where the formation of t-COOH is much more likely than the formation of HCOO, which has consequences for the selectivity. High reverse rates of all reactions results in lower TOFs for this catalyst.

Pb

Sr 10-4

10

-2

0

10

2

10

10-4

-2

10

Ba 0

10

2

10

Ca 10-4

-2

10

10

0

2

10

H+*↔*+H CO2 (g) ↔ CO2 CO (g) ↔ CO CH3OH (g) ↔ CH3OH H2O (g) ↔ H2O H2 (g) ↔ H2 H2 ↔ H + H H2 (g) ↔ H + H CO2 ↔ CO + O CO2 (g) ↔ CO + O CO2 + H ↔ HCOO CO2 (g) + H ↔ HCOO CO2 + H ↔ t-COOH CO2 (g) + H ↔ t-COOH HCOO + H ↔ H2COO HCOO + H ↔ HCOOH t-COOH ↔ c-COOH H2COO + H ↔ H2COOH HCOOH + H ↔ H2COOH c-COOH ↔ CO+OH CO + H ↔ HCO HCO + H ↔ H2CO H2COOH ↔ H2CO + OH H2COO ↔ H2CO + O H2CO + H ↔ H3CO H2CO + H ↔ H2COH H2COH + H ↔ CH3OH H3CO + H ↔ CH3OH O + H ↔ OH OH + H ↔ H2O forward reverse

Event frequency per site (s-1)

Figure 13: Event frequency for all elementary steps in the reaction network at T = 480 K and P = 10 bar for all four catalysts. Red steps indicate the forward reactions, while blue steps indicate the reverse reactions. 24


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3.4

Relevance for experimentalists

The aim of theoretical modelling is to explain observed phenomena and to give suggestions for experimentalists. Although this paper focuses on first-principles theoretical kinetic modelling, we can give some guidance for experimentalists. Our preliminary experimental work has shown that barium- and calcium-type perovskite catalysts can be effective for CO2 hydrogenation. As in more detail explained in the Supporting Information, the solution combustion method 55 using citric acid as a complexation agent and fuel produced viable Baand Ca-perovskite catalysts. Using the corresponding metal nitrates, citric acid, and titanium iso-propoxide yielded catalysts with BaTiO3 and CaTiO3 , and copper phases (Figures S1 and S2). Experimental testing showed that BaTiO3 exhibits greater activity than CaTiO3 (Figure S3 and S4), which is consistent with the theoretical modelling (quantitative agreement is not to be expected due to a complex nature of the catalysts). The testing shows that the temperature range 200-240 ◦ C has the greatest potential for good conversion into methanol. Based on the theoretical modelling and known stability of perovskites, we propose that Srand Pb-based perovskite catalysts be employed for CO2 hydrogenation. We propose to use the solution combustion method as described in the SI and to run the reaction at moderate reaction temperatures (below 250 ◦ C) and high hydrogen pressure (20 bar).

4

Conclusion

In this work, we studied four potential catalysts for the production of methanol with hydrogenation of carbon dioxide. For four different bimetallic catalysts with copper and perovskite components, namely Cu/SrTiO3 , Cu/CaTiO3 , Cu/BaTiO3 and Cu/PbTiO3 , first-principlesbased kMC simulations were performed. The results were compared with a pure Cu(111) facet. We used the DFT computations to find the most stable arrangement of each catalytic surface, emulating the interface between the Cu(111) and perovskite(100) surface. A

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complex network of elementary reaction steps was postulated. We have shown that among the investigated catalysts, Cu/PbTiO3 and Cu/SrTiO3 exhibit the highest selectivity. On Cu/PbTiO3 , CO2 is converted to methanol with almost no side products, while on Cu/SrTiO3 some CO is formed as a by-product. Both outperform pure copper. The other two catalysts are less suitable for the production of methanol, producing more CO. Cu/CaTiO3 has both very low selectivity and very low overall activity. The apparent activation energy was shown not to be simply the activation barrier of the slowest attainable step. A complex interplay of adsorption and parallel reactions manifests in the apparent activation energy. Lateral interactions are important only for the most persistent species, which in our case was shown to be HCOO. As the reaction progresses, HCOO would ultimately saturate the surface if its lateral interactions were not taken into account. One should also note that the calculated TOFs apply to the investigated interface. In real catalysts, most of the sites contribute little to the overall activity and a small fraction of the surface is responsible for most of the activity (mostly defects, steps, edges etc.). 4 Thus, the theoretically calculated TOFs cannot be simply transferred to the whole surface. To assess the synergies, we therefore compare the results with first-principles Cu(111) data and not experimental data. It was shown that the reaction order with respect to CO2 is 0.6 at low pressures and negative at high pressures, when the catalyst gets poisoned by HCOO. With respect to hydrogen, the catalysts exhibit higher reaction order (up to 1.40), while the rate also plateaus at high pressures. On the account of our work, we propose that Cu/PbTiO3 and Cu/SrTiO3 are the most promising copper/perovskite catalysts for the production of methanol from CO2 .

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Acknowledgement Funding from EU Framework Programme for Research and Innovation Horizon 2020 under grant agreement No. 727504 (FReSMe) and Slovenian Research Agency (ARRS) through Core Grant P2-0152 is greatly appreciated. M.H. acknowledges financial support from the Knut and Alice Wallenberg Foundation Project 2015.0057. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at C3SE (Gothenburg) and NSC (Linköping) and on resources provided by the National Institute of Chemistry (Ljubljana). We wish to thank Mr Anˇze Praˇsnikar for catalyst synthesis and testing.

Supporting Information Available Details on synthesis of the Ca- and Ba-containing perovskite catalysts and their productivity are given in the Supporting Information.

References (1) Song, C. Global Challenges and Strategies for Control, Conversion and Utilization of CO2 for Sustainable Development Involving Energy, Catalysis, Adsorption and Chemical Processing. Catalysis Today 2006, 115, 2 – 32, Proceedings of the 8th International Conference on Carbon Dioxide Utilization. ˇ (2) Huˇs, M.; Dasireddy, V. D.; Stefanˇ ciˇc, N. S.; Likozar, B. Mechanism, Kinetics and Thermodynamics of Carbon Dioxide Hydrogenation to Methanol on Cu/ZnAl2 O4 Spineltype Heterogeneous Catalysts. Applied Catalysis B: Environmental 2017, 207, 267 – 278.

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(3) Burch, R.; Golunski, S. E.; Spencer, M. S. The Role of Copper and Zinc Oxide in Methanol Synthesis Catalysts. J. Chem. Soc., Faraday Trans. 1990, 86, 2683–2691. (4) Behrens, M.; Studt, F.; Kasatkin, I.; K¨ uhl, S.; Hävecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B.-L.; Tovar, M.; Fischer, R. W.; Nørskov, J. K.; Schlögl, R. The Active Site of Methanol Synthesis over Cu/ZnO/Al2 O3 Industrial Catalysts. Science 2012, 336, 893–897. (5) Behrens, M.; Zander, S.; Kurr, P.; Jacobsen, N.; Senker, J.; Koch, G.; Ressler, T.; Fischer, R. W.; Schlögl, R. Performance Improvement of Nanocatalysts by PromoterInduced Defects in the Support Material: Methanol Synthesis over Cu/ZnO:Al. Journal of the American Chemical Society 2013, 135, 6061–6068. (6) van den Berg, R.; Prieto, G.; Korpershoek, G.; van der Wal, L. I.; van Bunningen, A. J.; Lægsgaard-Jørgensen, S.; de Jongh, P. E.; de Jong, K. P. Structure Sensitivity of Cu and CuZn Catalysts Relevant to Industrial Methanol Synthesis. Nature Communications 2016, 7, 13057. (7) Posada-Borbón, A.; Hagman, B.; Schaefer, A.; Zhang, C.; Shipilin, M.; Hellman, A.; Gustafson, J.; Grönbeck, H. Initial Oxidation of Cu(100) Studied by X-ray Photoelectron Spectroscopy and Density Functional Theory Calculations. Surface Science 2018, 675, 64 – 69. (8) Chinchen, G.; Waugh, K.; Whan, D. The Activity and State of the Copper Surface in Methanol Synthesis Catalysts. Applied Catalysis 1986, 25, 101 – 107. (9) Kurtz, M.; Bauer, N.; B¨ uscher, C.; Wilmer, H.; Hinrichsen, O.; Becker, R.; Rabe, S.; Merz, K.; Driess, M.; Fischer, R. A.; Muhler, M. New Synthetic Routes to More Active Cu/ZnO Catalysts Used for Methanol Synthesis. Catalysis Letters 2004, 92, 49–52. (10) D´ıez-Ram´ırez, J.; Dorado, F.; de la Osa, A. R.; Valverde, J. L.; Sánchez, P. Hydrogenation of CO2 to Methanol at Atmospheric Pressure over Cu/ZnO Catalysts: Influence 28


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of the Calcination, Reduction, and Metal Loading. Industrial & Engineering Chemistry Research 2017, 56, 1979–1987. (11) G¨ unter, M.; Ressler, T.; Bems, B.; B¨ uscher, C.; Genger, T.; Hinrichsen, O.; Muhler, M.; Schlögl, R. Implication of the Microstructure of Binary Cu/ZnO Catalysts for Their Catalytic Activity in Methanol Synthesis. Catalysis Letters 2001, 71, 37–44. (12) Kasatkin, I.; Kurr, P.; Kniep, B.; Trunschke, A.; Schlögl, R. Role of Lattice Strain and Defects in Copper Particles on the Activity of Cu/ZnO/Al2 O3 Catalysts for Methanol Synthesis. Angewandte Chemie 2007, 119, 7465–7468. (13) Hagman, B.; Posada-Borbón, A.; Schaefer, A.; Shipilin, M.; Zhang, C.; Merte, L. R.; Hellman, A.; Lundgren, E.; Grönbeck, H.; Gustafson, J. Steps Control the Dissociation of CO2 on Cu(100). Journal of the American Chemical Society 2018, 140, 12974–12979. (14) Nakatsuji, H.; Hu, Z. Mechanism of Methanol Synthesis on Cu(100) and Zn/Cu(100) Surfaces: Comparative Dipped Adcluster Model Study. International Journal of Quantum Chemistry 2000, 77, 341–349. (15) 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(000[1 with Combining Macron]). Phys. Chem. Chem. Phys. 2010, 12, 9909– 9917. (16) Grabow, L. C.; Mavrikakis, M. Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation. ACS Catalysis 2011, 1, 365–384. (17) Studt, F.; Abild-Pedersen, F.; Varley, J. B.; Nørskov, J. K. CO and CO2 Hydrogenation to Methanol Calculated Using the BEEF-vdW Functional. Catalysis Letters 2013, 143, 71–73.

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(18) Tang, Q.-L.; Zou, W.-T.; Huang, R.-K.; Wang, Q.; Duan, X.-X. Effect of the Components’ Interface on the Synthesis of Methanol Over Cu/ZnO from CO2 /H2: A Microkinetic Analysis Based on DFT + U Calculations. Phys. Chem. Chem. Phys. 2015, 17, 7317–7333. ˇ (19) Huˇs, M.; Kopaˇc, D.; Stefanˇ ciˇc, N. S.; Jurković, D. L.; Dasireddy, V. D. B. C.; Likozar, B. Unravelling the Mechanisms of CO2 Hydrogenation to Methanol on Cu-based Catalysts Using First-principles Multiscale Modelling and Experiments. Catal. Sci. Technol. 2017, 7, 5900–5913. (20) Kattel, S.; Ram´ırez, 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. (21) Nakamura, J.; Fujitani, T.; Kuld, S.; Helveg, S.; Chorkendorff, I.; Sehested, J. Comment on “active Sites for CO2 Hydrogenation to Methanol on Cu/ZnO Catalysts”. Science 2017, 357, 6354. (22) Palomino, R. M.; Ram´ırez, P. J.; Liu, Z.; Hamlyn, R.; Waluyo, I.; Mahapatra, M.; Orozco, I.; Hunt, A.; Simonovis, J. P.; Senanayake, S. D.; Rodriguez, J. A. Hydrogenation of CO2 on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal–Oxide Interface in Methanol Synthesis. The Journal of Physical Chemistry B 2018, 122, 794–800. (23) Wu, P.; Yang, B. Significance of Surface Formate Coverage on the Reaction Kinetics of Methanol Synthesis from CO2 Hydrogenation over Cu. ACS Catalysis 2017, 7, 7187– 7195. (24) Zhao, Y.-F.; Yang, Y.; Mims, C.; Peden, C. H.; Li, J.; Mei, D. Insight into Methanol Synthesis from CO2 Hydrogenation on Cu(111): Complex Reaction Network and the Effects of H2 O. Journal of Catalysis 2011, 281, 199 – 211.

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Page 34 of 53

Page 35 of 53

H2COH + H CH3OH

H2

H2CO + 2 H

H2CO + O

HCO + H

+

H2CO

H2

H3CO + H

CO + H2O

+


H2

H2

H2COOH + H

H2COO

H2


HCOO

+

+

HCOO + H

CO2

H2

HCOOH

t-COOH + H

+

+

H2CO + O + 2 H CO

Graphical TOC Entry

CO2+ 2 H

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

H2COO

CO2 t-COOH

c-COOH

HCOOH CO

H2COOH

HCO

35

H2CO

CH3O

HCOH

CH2OH


CH3OH

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 36 of 53

Cu

Sr

O Ti ACS Paragon Plus Environment

Page 37 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 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

Page 38 of 53

CO2 14

3

2

HCOO

t-COOH 15

5

4

c-COOH HCOOH

H2COO

16

CO

6 17

HCO

7

8 18

H2COOH

9

H2CO 10

11

CH2OH 13

CH3O 12


CH3OH

Page 39 of 53 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


E (eV)

E (eV)

-1 + +

H2

0

-1


-2

+ H2

H2COO +

+

+

+

H2

H2

+

CH3OH

1

CH3OH

H2COH + H

H2

H3CO + H

+

H2COH + H

H2

H3CO + H

H2CO + 2 H

CO

1

H2CO + 2 H

HCO + H H2CO


H2

HCO + H H2CO

+

HCOOH H2CO + O + 2 H CO + OH + H H2COO + 2 H CO + H2O H2CO + O + 2 H H2COOH + H +

CO + 2 H H2CO + H2O

c -COOH + H H2COO +

HCOOH CO + OH + H H COO + 2 H H CO + O 2 2 +2H CO + H2O H2CO + O + 2 H H2COOH + H CO H2CO + OH + H

HCOOH

t -COOH + H

H2

HCOO + H

0

HCOOH c -COOH + H

-1 CO2+ 2 H

CO2 +

t -COOH + H

Cu/CaTiO3

CO2

E (eV)

H2COH + H

ACS Catalysis

HCOO + H

H2

E (eV)

CH3OH

H3CO + H

HCO + H

Cu/SrTiO 3

CO2+ 2 H

+

CH3OH

1 H2

H2COH + H

H2CO + 2 H

H2CO

H2CO + O + 2 H CO

+

H2CO + 2 H

H2

HCO + H


CO + H2O

H2

H2CO

+

CO + 2 H

CO

H2COOH + H

H2CO + O H2

H3CO + H

H2

+

H2CO + H2O

+ H2

H2CO + OH + H

H2


H2

H2CO + O + 2 H

H2COO

+

H2COOH + H CO + H2O

H2CO + O

HCOOH

t-COOH + H

+

CO + OH + H

c -COOH + H

0 +

H2COO + 2 H HCOOH + 2 H

-2 H2

HCOO + H

-2

t -COOH + H

0

H2COO HCOOH

+ H2

HCOO + H

-1 CO2 + 2 H

+

CO2+ 2 H

CO2

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 CO2

1 Cu/BaTiO 3

Page 40 of 53

H2

-2

Cu/PbTiO3

H2

H2

H2

CH3OH TOF per site (s-1)

Page 41 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

102

Pb

100 10-2 10-4 10

Sr

Ba Ca

-6

10-8 10-10

400

ACS Paragon500 Plus Environment 450 550

T (K)

600

650


10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

Page 42 of 53

5 0 -5 Ba: 89 kJ/mol Sr: 143 kJ/mol Pb: 88 kJ/mol Ca: 186 kJ/mol Cu: 104 kJ/mol

-10 -15 -20

15

Environment 16 ACS Paragon 17 Plus18 19

1/T (10-4 K-1)

20

21

CO TOF per site (s-1)

2 Page 43 10 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

ACS Catalysis

10-2

Ba Sr Pb

10-4

Ca

100

10-6 10-8 10-10

400

ACS Paragon500 Plus Environment 450 550

T (K)

600

650

ln CO TOF per site

10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

Page 44 of 53

5 0 -5 Ba: 92 kJ/mol Sr: 123 kJ/mol Pb: 102 kJ/mol Ca: 151 kJ/mol Cu: 105 kJ/mol

-10 -15 -20

15

Environment 16 ACS Paragon 17 Plus18 19

1/T (10-4 K-1)

20

21

1.0 Page 45 of 53

selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

0.8

Pb Sr

0.6

Ba

0.4 0.2 0.0 450

Ca ACS Environment600 500Paragon Plus 550

T (K)

650

1.0

ACS Catalysis

Page 46 of 53

Pb

selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0.8 Sr 0.6 Ba

0.4 0.2 0.0

Ca 20 ACS Paragon 40 Plus Environment 60 p (bar)

80

100


Page 47 of053 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

-2 -4

0.66

-6

0.69

-12

0.05

Pb

0.81

-0.80

-14 -16

Sr

Ba

-8 -10

-0.95

Ca

-4 Paragon Plus-2Environment 10-5 10ACS 10-3 10 10-1 100 101 102 pCO (bar) 2

ln CO TOF per site

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

Ba

-2 0.62

-4 -6

-12

-0.40

0.50

Sr

-8 -10

Page 48 of 53

-0.73 0.45

Ca

0.53

Pb

-14 -16

-4 Paragon Plus-2 Environment 10-5 10ACS 10-3 10 10-1 100 101 102 pCO (bar) 2


Page 49 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACS Catalysis

0

0.22 -0.53

-5 -10

1.29

Pb 1.40

1.45

Sr 0.21

Ca

Ba -15 -20

-4 Paragon Plus-2 Environment 10-5 10ACS 10-3 10 10-1 100 101 102 pH (bar) 2

ACS Catalysis

ln CO TOF per site

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

0.42

-5

Sr

0.37 0.21

-10

BaPage 50 of 53

Pb

Ca

0.40

-15 -20

-4 Paragon Plus-2 Environment 10-5 10ACS 10-3 10 10-1 100 101 102 pH (bar) 2

Page 51 10of0 53

coverage

coverage

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

ACS Catalysis

Ba

Ca

Pb

Sr

10-1

10-2

10-1

10-2

-3

10

10-10

10-8

10-6

10-4

10-2


H* CO2** HCOO** HCOOH** H3CO* t-COOH** OH* H2O**

∆Eads (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0.3

100 Page 52 of 53

ACS Catalysis

80

Pb Sr

60

0.2 40

Ba 0.1

20

Ca 0.0 -0.4

ACS Paragon Environment -0.2 0.0 Plus0.2 0.4

charge transfer (e0)

0.6

0

Selectivity (%)

0.4

1 2 3 H+*↔*+H 4 CO2 (g) ↔ CO2 5 CO (g) ↔ CO 6 CH3OH (g) ↔ CH3OH 7 H2O (g) ↔ H2O 8 9 H2 (g) ↔ H2 10 H2 ↔ H + H 11 H2 (g) ↔ H + H 12 CO 13 2 ↔ CO + O 14 CO2 (g) ↔ CO + O 15 CO2 + H ↔ HCOO 16 CO2 (g) + H ↔ HCOO 17 CO2 + H ↔ t-COOH 18 19 CO2 (g) + H ↔ t-COOH 20 HCOO + H ↔ H COO 2 21 HCOO + H ↔ HCOOH 22 t-COOH ↔ c-COOH 23 H COO + H ↔ H2COOH 24 2 25 HCOOH + H ↔ H2COOH 26 c-COOH ↔ CO+OH 27 CO + H ↔ HCO 28 HCO + H ↔ H2CO 29 30H2COOH ↔ H2CO + OH 31 H2COO ↔ H2CO + O 32 H2CO + H ↔ H3CO 33 H2CO + H ↔ H2COH 34 35 H2COH + H ↔ CH3OH 36 H3CO + H ↔ CH3OH 37 O + H ↔ OH 38 OH + H ↔ H2O 39 40 41 42 43 44

Pb

Sr

Page 53 of 53

10-4

10

-2

Ba

ACS Catalysis 0

10

2

10

10-4

-2

10

0

10

2

10

ACS Paragon Plus Environment forward reverse

Event frequency per site (s-1)

Ca 10-4

-2

10

10

0

2

10

Catalytic Hydrogenation of Carbon Dioxide to Methanol: Synergistic

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