CeO2 Catalyzed Water-Gas Shift Reaction

Subscriber access provided by Kaohsiung Medical University

C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Theoretical Study on PdCu/CeO Catalyzed Water-Gas Shift Reaction: Crucial Role of the Metal/Ceria Interface and O-Enhancement Effects 2

Wenjia Luo, Yu Chen, Zheng Du, and Congmei Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10447 • Publication Date (Web): 23 Nov 2018 Downloaded from http://pubs.acs.org on November 26, 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 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

The Journal of Physical Chemistry

Theoretical Study on PdCu/CeO2 Catalyzed Water-Gas Shift Reaction: Crucial Role of the Metal/Ceria Interface and O2-enhancement Effects Wenjia Luoa*, Yu Chenb, Zheng Duc, and Congmei Chenc a

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

b

School of Mathematics and Information, China West Normal University, Nanchong, 637002, P.R. China c

National Supercomputing Center in Shenzhen, Shenzhen, 518055, P.R. China

Corresponding Author Dr. Wenjia Luo Email: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 Low-temperature water-gas shift (LT-WGS) is an important process for H2 production and purification. Recently, as promising LT-WGS catalysts, PdCu and PtCu bimetallic catalysts supported on ceria have been found. Existing studies showed that the addition of a small quantity of O2 not only promoted CO conversion but also enhanced the production of H2. In this study, by using density functional theory and microkinetic models, we investigated the reaction mechanisms of WGS reaction catalyzed by a PdCu/CeO2 catalyst and deciphered the underlying reasons for the O2-enhancement phenomenon. We compared the catalytic performance of the unsupported PdCu metal with that of the PdCu/CeO2 interface structures and found that only the PdCu/CeO2 interface structure was active towards WGS. The unsupported metal was incapable of activating water, while the metal and ceria sites on the interface structure work in tandem to activate water and promote CO conversion. With the addition of O2, WGS proceeded through an OH migration pathway, because O2 modulated the oxidation state of the ceria support and increased the mobility of OH. The catalytic performance was quantitatively estimated using microkinetic models, and the results were consistent with previous experimental measurements. This study provides insights into the separate catalytic roles of different components in the PdCu/CeO2 system and corroborates the important roles of the metal/oxide interfaces in WGS reactions.

Keywords: water-gas shift (WGS), metal/oxide interface, density functional theory (DFT), microkinetic model, PdCu bimetallic catalyst, ceria support


Page 2 of 53

Page 3 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


1. Introduction The water-gas shift (WGS) reaction (Eq. 1) is a hydrogen production process that is widely used in industry.1 H2O(g) + CO(g)→CO2(g) + H2(g)

(1)

Depending on the operating temperature, WGS catalysts can be divided into hightemperature WGS (HT-WGS) catalysts and low-temperature WGS (LT-WGS) catalysts. Since WGS is slightly exothermic, LT-WGS can achieve higher CO conversion and is especially useful in applications where CO must be removed from the H2 stream.2-3 Understanding the WGS reaction mechanism is crucial to develop highly efficient WGS catalysts and thus this mechanism has been investigated in numerous studies, especially by using theoretical methods.2, 4-8 Despite the fact that metal supported on oxides are often used as catalysts in experiments, most earlier theoretical studies focused on pure metals without considering the oxide support due to high computational cost. However, the oxide support may play an important role in the catalytic reaction. Some experimental results indicate that the oxide support could participate in the catalytic reaction, for example, by producing hydroxyl groups.9-13 In many recent theoretical studies, the effects of the oxide support in WGS reactions have been explicitly examined. In these studies, different structures including single metal atoms14-15, metal nanoclusters16-24, and one-dimensional metal nanowires25-26 supported on oxides have been modeled. The metal/oxide interfaces were found to produce unique catalytic activities that are different from either the pure metal or the oxide. PdCu or PtCu bimetallic alloys supported on ceria were found to be promising LTWGS catalysts in a series of studies.27-30 These catalysts have the following three 3 ACS Paragon Plus Environment


interesting features: 1) They achieve high CO conversion and are especially suitable for situations where low concentrations of CO must be removed from the H2 stream. 2) The ceria support can significantly promote the catalytic activity. 3) A small amount of O2 was surprisingly reported not to consume H2 but instead increase H2 yield after O2 was added to the feed gas for the intention of promoting CO conversion. From a mechanistic perspective, the PdCu/CeO2 or PtCu/CeO2 catalysts provide an opportunity to investigate the different roles of the metals and oxides in WGS catalytic reactions. There have been no first-principles studies on PdCu/CeO2 or PtCu/CeO2 that have explored their WGS catalytic reaction mechanisms and explained why the addition of O2 could enhance hydrogen production. In this study, first-principles methods, including density functional theory (DFT) and microkinetic models, were used to quantitatively investigate the reaction mechanisms and catalytic activities of PdCu/CeO2 or PtCu/CeO2 catalysts. Since PdCu/CeO2 and PtCu/CeO2 have rather similar catalytic performance, with PdCu/CeO2 being slightly better, we focused on PdCu/CeO2. The results are believed to be similar to those of PtCu/CeO2. To consider the effect of the ceria support, we built two different models including 1) an unsupported metal slab and 2) a metal/ceria interface model in which CeO2 directly participated in the reactions. Computational results presented in Section 3.2 and 3.3 suggest that only the second model is active in WGS reactions. By making the structure sufficiently large, it can be proved that the activity is not caused by size or edge effects. In Section 3.3.3, the predicted catalytic activity calculated from microkinetic models is compared with experimental measurements. Finally, the O2-enhanced WGS reaction mechanisms and the separate roles of different components in the PdCu/CeO2 4 ACS Paragon Plus Environment

Page 4 of 53

Page 5 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


catalyst material based on the theoretical results are discussed in Section 3.4.

2. Methods Plane wave DFT calculations were performed by using the Vienna Ab initio Simulation Package (VASP).31-32 Pseudopotentials implemented in the projector augmented wave (PAW) method were used to simulate the core electrons.33 The revised Perdew-Burke-Ernzerhof (RPBE) form of the generalized gradient approximation (GGA) was chosen as the exchange-correlation functional.34 In this work, we preferred RPBE over other functionals like Perdew-Burke-Ernzerhof (PBE)35 or Perdew-Wang 91 (PW91)36, because RPBE can predict the adsorption site and binding energy of CO on Pd with higher accuracy;34, 37 this accurate prediction is important for quantitative kinetic calculations. We also did not consider the dispersion effect, for example, by using Grimme’s DFT-D3 method38-39, since we found that DFT-D3, when coupled with RPBE, would overestimate the CO binding energy on Pd. For other surface reactions where adsorption or desorption is not involved, consideration of the dispersion effect is also unnecessary since DFT-D3 could produce similar results with DFT. Detailed comparisons between calculations with or without D3 are provided in the Supporting Information Section S2.1. In this work, two different models were built to represent the ceria-supported PdCu bimetallic catalyst; their structures are shown in Fig. 1.



(a)

(b) Fig. 1. Top and side views of two models of the PdCu/CeO2 catalyst. (a) An unsupported PdCu3(111) slab. (b) A PdCu/CeO2 interface. Pd, Cu, Ce, O and H are shown in blue, orange, green, red, and white, respectively. Boundaries of periodic unit cells in each structure are outlined in black lines. A vacuum space of 15 Å was placed above each structure. For clarity, atoms that belong to the next periodic unit cell but fall approximately on the boundaries are also shown. For clarity, in the top view of panel (a), atoms on the second layer are shown smaller, and the third-layer atoms are smallest. 6 ACS Paragon Plus Environment

Page 6 of 53

Page 7 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


The first model, as shown in Fig. 1(a), is a four-layer PdCu metal slab without ceria support. We used a 4×4 unit cell that represents the (111) facet of the PdCu3 stoichiometric alloy. The ratio of Pd to Cu (1:3) in this structure was in the range (from 1:5 to 2:5) that was shown to be optimal for LT-WGS in previous experiments.28 The lattice constant of PdCu3 was calculated to be 3.770 Å, which is close to the experimental value (3.705 Å).40 During structural relaxations, the bottom two layers were fixed in lattice positions, and the top two layers were allowed to relax. A 2×2×1 Monkhorst−Pack k-point mesh was used for this structure.41 Fig. 1(b) shows the second model. PdCu layers are supported on a CeO2 slab, which represents a CeO2(111) surface. We chose the CeO2(111) surface as the support because (111) is the most stable facet of CeO2.42-43 The CeO2(111) slab has a size of 5×2 and it consists of six layers of atoms (two layers of O-Ce-O units). Only the left half of the CeO2(111) slab was covered by three PdCu layers. The PdCu layers are periodic in the ydirection (direction of the shortest cell vector) and are four (bottom layer) and three atoms (top two layers) wide in the x-direction. The lattice constant of CeO2 was optimized to be 5.537 Å, which is slightly larger than the experimental value (5.411 Å).44 2×2 CeO2(111) has a unit cell length of 7.831 Å, which is very close to the size of a 3×3 PdCu3(111) unit cell (7.997 Å). Therefore, in our structure, the supported metal has a slight (2.1%) compressive strain in the y-direction since the unit cell lengths were fixed at the CeO2 values. There are eight Pd and 22 Cu atoms in each unit cell, resulting in a Pd:Cu ratio of 1:2.75. The distances between Pd atoms are maximized so that the Pd atoms are uniformly distributed among Cu instead of forming a cluster. As illustrated in Fig. 1(b), atoms with 7 ACS Paragon Plus Environment


a

symbol were fixed in the position determined separately from a periodic structure of

3-layer 3×3 PdCu3 layers supported on a 2×2 CeO2(111) slab (as shown in the upper-left corner of Fig. 2(c)). All other atoms were allowed to relax. We designated the right-hand side of the PdCu layers as indicated in Fig. 1(b), where PdCu and ceria formed an interface area, to be the active sites. The right-hand side slope of the PdCu layers, where all catalytic reactions take place, is also a (111) facet. The purpose of this model was to simulate the cases where the supported metal clusters are large (> 2 nm) in size. To ensure that this purpose can be fulfilled, we performed detailed analysis (Section S2.2) and found that further increase in the width and height of the PdCu layers, as well as additional CeO2 layers below, did not significantly change the calculated reaction energies; this result confirms that our structure is sufficiently large. Another important feature of this structure is that the PdCu layers are supported on an O-terminated instead of Ce-terminated CeO2(111) because the former one was found to be more stable through a thermodynamic analysis given in Section 3.1.1. A 1×2×1 k-point mesh was used for this structure. In the unsupported PdCu3 slab model, we used a plane wave cutoff energy of 450 eV without spin polarization. In the second model, since the Ce element is involved, we considered the onsite Coulomb correlation by adopting the DFT+U approach of Dudarev et al.45-46 An effective Hubbard-type U parameter of 4.5 eV was used for the 4f electrons of Ce. This is consistent with previous studies.47-48 In the second model, spin polarization calculations with a plane wave cutoff energy of 500 eV were performed. In each model, at least 15 Å of vacuum space was placed above the metal or metal/ceria structures. Dipole corrections were also included to minimize interactions between unit cells in the z-direction. All structures were relaxed using a limited memory 8 ACS Paragon Plus Environment

Page 8 of 53

Page 9 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


Broyden-Fletcher-Goldfarb-Shanno (LBFGS)49 algorithm until the forces on all atoms were less than 0.03 eV/Å. Kinetic barriers were located by the dimer method50 for reactions on the unsupported PdCu3(111) surface, while barriers for the reactions on the PdCu/CeO2 interface were calculated by using the climbing image nudged elastic band (Cl-NEB) method51-52. The dimer method is computationally more efficient, but it requires a rather accurate evaluation of the forces on the atoms; this, unfortunately, is unsuitable for the ceria systems, for which only the numerically more robust Cl-NEB methods can be used. Nonetheless, the transition state structures and energies found by the dimer method are almost identical to those located by Cl-NEB (a comparison is given in Section S2.3). The transition states were further confirmed by the existence of single imaginary vibrational modes along the reaction coordinates, with few exceptions, which are discussed in detail in Section S3. We used standard statistical thermodynamics and transition state theory to estimate the rate constants of each step.53-54 The mathematical equations that were used to calculate rate constants are provided in detail in Section S1. We then incorporated production rates of reactants into an ordinary differential equation (ODE) to build microkinetic models.55 By solving this ODE, steady-state behaviors of the reaction system were derived after the temperature and partial pressures of all reactants were specified. The reaction conditions in the microkinetic models were set to match those in experimental kinetics measurements29, which were 533 K (260 oC), PCO = 0.098 bar, and PH2𝑂= 0.23 bar. The effect of co-feeding O2 was also tested with the O2 pressure set to 0.014 bar in cases where the O2-enhancement effects were considered. The experimental kinetics measurement was performed under a differential condition such that all O2 was 9 ACS Paragon Plus Environment


fully consumed, but CO conversion was low.29 Therefore, we set the pressures of H2 and CO2, which are needed by the microkinetic model but do not appreciably affect the results as long as the conditions are far from equilibrium, arbitrarily to a low value of 0.01 bar corresponding to a 10% CO conversion rate. Further details of the microkinetic models are provided in Section 3.2.3 and 3.3.3. Our microkinetic models were derived completely from the results of first-principles calculations, with no adjustable parameters that were fit from the experimental kinetic measurements.

3. Results and Discussion 3.1 Structure of the Catalyst Surface under WGS Reaction Conditions 3.1.1 Oxidation State of the Catalyst Surface The nature of the catalyst surface, including the termination of CeO2 and the most abundant species on the PdCu metallic surface, is of paramount importance in the prediction of its catalytic activity. If we restrict our discussion on typical WGS reaction conditions without the explicit presence of gaseous O2, from the thermodynamic perspective, the oxidation state of catalyst surface will be dictated by the temperature and the partial pressures of two reactants, namely H2O and H2. We performed ab initio thermodynamics by using the method described previously56-57 to investigate the most stable surface configurations under different conditions. The surface phase diagrams for metallic PdCu alloy, ceria, and ceria supported PdCu are presented in Fig. 2.


Page 10 of 53

Page 11 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


(a)

(b)



Page 12 of 53

(c) Fig 2. Surface phase diagrams for (a) unsupported PdCu3 alloy, (b) ceria, and (c) PdCu/CeO2 interface under 533 K. ∆𝜇𝐻2𝑂 is defined by ∆𝜇𝐻2𝑂 = 𝜇𝐻2𝑂 ― 𝐸𝐻2𝑂, where 𝜇𝐻2𝑂 and 𝐸𝐻2𝑂 are the chemical potential of H2O and the DFT calculated energy of an 1

1

isolated H2O molecule at 0 K, respectively, and ∆𝜇𝐻 = 2∆𝜇𝐻2 = 2(𝜇𝐻2 ― 𝐸𝐻2); they are defined in the same way as in previous studies.56-57 Black dashed lines in each figure signify the H2O and H2 pressures (PH2𝑂= 0.23 bar and PH2=0.01 bar) relevant to the WGS reaction condition examined in this study.

Fig. 2(a) suggests that the clean PdCu3(111) is the most stable configuration of the PdCu3 bimetallic alloy surface under WGS reaction conditions (without considering other reactants, for instance, CO or CO2). It also means that the metallic surface is kept in a reduced state without appreciable coverage of OH* or atomic O* if only H2O and H2 are present in the gas phase. Fig. 2(b) compares the stability of various configurations of the CeO2 surface. The result is similar to that in a previous work by Fronzi et al.57, except that we also considered structures which can possibly occur in a hydrogen-rich environment. In Fig. 12 ACS Paragon Plus Environment

Page 13 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


2(b), CeO2, CeO2 + VO, CeO2:Ce refer to O-terminated stoichiometric CeO2, partially reduced CeO2 with half of the surface O atoms removed, and highly reduced CeO2 with all surface O atoms removed, respectively. Under H2 pressures higher than 10-12 bar, presence of H on the surface can be observed in the form of hydroxyl groups or atomic H. We used the notation CeO2:X to mean that the CeO2 surface is terminated by X, thus CeO2:OH, CeO2:OH+H, CeO2:H, and CeO2:Ce+H correspond to ceria surfaces terminated by 1 ML of OH groups, by 0.5 ML OH and 0.5 ML H, by 1 ML H, and by 0.5 ML H and 0.5 ML bare Ce atoms, respectively. This surface phase diagram reveals that under WGS conditions the ceria surface should be terminated by OH groups, which justifies our choice of using OH-terminated CeO2 to represent the ceria moiety of the PdCu/CeO2 catalyst surface as shown in Fig. 1(b). Fig. 3(a) and (b) focus on PdCu metal and CeO2 separately, whereas Fig. 3(c) investigates the metal/ceria interface. Under oxidative environment, PdCu metal is supported on fully oxidized, O-terminated CeO2, thus forming a PdCu-O-Ce sandwichlike structure as shown in the upper-left corner of Fig. 3(c). In contrast, under highly reductive environment all O atoms on the top layer of CeO2 are removed, consequently a PdCu-Ce interface as shown in the lower-right corner of Fig. 3(c) can be observed. Between these two extreme cases, it is also necessary to consider the possibility that O atoms between PdCu metal and CeO2 are partially removed. Therefore, the phase diagram in Fig. 3(c) also compares the stability of structures that contain 0.25 ML, 0.5 ML, and 0.75 ML of oxygen between PdCu metal and Ce. The diagram suggests that the 0.75 ML O structure is the most stable one under the WGS reaction condition. In other words, oxygen atoms are present between the PdCu phase and the ceria phase in most 13 ACS Paragon Plus Environment


areas, which is why we used the PdCu-O-Ce model as shown in Fig. 1(b) to represent the PdCu/CeO2 interface in this study. Furthermore, the diagram also proves that the presence of a small quantity of O vacancies between PdCu and ceria are thermodynamically favorable. Those O vacancies, especially if located on exposed areas which are accessible to gas phase reactants, are beneficial to the WGS catalytic activity because they can provide activation sites for H2O adsorption and dissociation as discussed in Section 3.3. In conclusion, the ab initio thermodynamics study found that under WGS reaction conditions, the most stable surface structures for PdCu metal and ceria should be the clean metallic surface and the OH-terminated CeO2, respectively, while the PdCu/CeO2 interface should form a PdCu-O-Ce structure with a small portion of O removed. However, there is a limitation in the ab initio thermodynamics study, as it considers only H2O and H2 as gas phase reactants. In fact, other reactants including O2 (in the case of O2-enhanced WGS) and CO are also present in the reactor. The influence of O2 can be more appropriately addressed by microkinetic models as demonstrated later in Section 3.2.3 and 3.3.3 because H2O, H2, and O2 are not in thermodynamic equilibrium with each other under reaction conditions. Meanwhile, the influence of CO adsorption on the PdCu metallic surface configuration is discussed in the following section.

3.1.2 CO Adsorption on the PdCu Metallic Surface On the PdCu3(111) surface, there are seven unique CO adsorption sites, which are shown in Fig. 3 along with their respective adsorption energies ∆Eads, which are calculated by Eq. 2; a more negative ∆Eads is indicative of stronger binding. 14 ACS Paragon Plus Environment

Page 14 of 53

Page 15 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


(2)

ΔEads = E[CO ∗ ] ―E[ ∗ ] ―E[CO(g)]

(a) ∆Eads = -0.98 eV

(b) ∆Eads = -0.88 eV

(c) ∆Eads = -0.80 eV

(e) ∆Eads = -0.32 eV

(f) ∆Eads = -0.45 eV

(g) ∆Eads = -0.65 eV

(d) ∆Eads = -0.82 eV

Fig 3. CO adsorption structures and energies at seven unique sites on PdCu3(111). (a) Pd top site, (b) Pd-Cu-Cu hcp site, (c) Pd-Cu-Cu fcc site, (d) Pd-Cu bridge site, (e) Cu hcp site, (f) Cu top site, and (g) Cu fcc site. For clarity, metal atoms on the second layer of PdCu3(111) are shown smaller, and third-layer atoms are smallest. In Fig. 3(a) through (d), the CO* molecule is adsorbed on or beside a Pd atom, and these four adsorption sites are categorized as Pd sites. On the other hand, in Fig. 3(e) through (g), the CO* molecule bonds only to Cu atoms, and these three adsorption sites are classified as Cu sites. The ∆Eads values in Fig. 3 clearly show that CO* adsorption at Pd sites are much stronger than at Cu sites. In terms of Gibbs free energy, CO* adsorption at a Cu site (as in Fig. 3(g)) is 0.35 eV less stable compared with the adsorption at a Pd site (as in Fig. 3(a)), which suggests that approximately only 0.05% of all CO* molecules on the metal surface will occupy Cu sites estimated by the rule of Boltzmann distribution when the coverage is low. For this reason, we would expect the CO* molecules to prefer Pd sites over Cu sites, and Cu sites can only bind to CO* when 15 ACS Paragon Plus Environment


Page 16 of 53

all Pd adsorption sites are filled. To estimate the surface CO coverage under WGS reaction conditions, we explicitly calculated the average and differential adsorption energies of CO on PdCu3(111) from low coverage (1/16 ML) to intermediate coverage (3/8 ML). The structures of several typical coverages are presented in Fig. 4. The adsorption energies are summarized in Table 1.

(b) (c) (a) Fig. 4 Adsorption geometries of CO on PdCu3(111) under (a) 1/16 ML, (b) 1/4 ML, and (c) 5/16 coverages. Table 1. Average and differential adsorption energies of CO on PdCu3(111) under different coverages and PCO that is in equilibrium with the coverages Coverage (ML)

Average Δ Eads (eV)a

Differential ΔEads (eV)b

ΔG𝑜ads (eV)c

PCO (bar) in Equilibriumd

1/16 2/16 3/16 4/16 5/16 6/16

-0.98 -0.98 -0.99 -1.00 -0.92 -0.69

-0.98 -0.99 -1.01 -1.02 -0.58 0.41

-0.11 -0.13 -0.14 -0.16 0.28 1.27

0.086 0.063 0.044 0.033 4.43×102 9.98×1011

a. Average ΔEads = (𝐸[𝑛𝐶𝑂 ∗ ] ―𝐸[ ∗ ] ―𝐸[𝐶𝑂(𝑔)])/𝑛, n is the number of CO molecules adsorbed on the surface within each unit cell under this coverage. b. Differential ΔEads = 𝐸[𝑛𝐶𝑂 ∗ ] ―𝐸[(𝑛 ― 1)𝐶𝑂 ∗ ] ―𝐸[𝐶𝑂(𝑔)]. n has the same meaning as above. c. ΔG𝑜ads is based on the differential ΔEads, considering the change in entropy, zero-point energy, and heat capacity upon CO adsorption under 1 bar and the WGS reaction temperature of 533 K.2729 Standard statistical mechanics methods were used to calculate these quantities based on DFT calculated vibrational modes, as explained in Section S1. d. Equilibrium PCO is the partial pressure of CO(g) that is in equilibrium with each coverage by assuming that the Gibbs free energy of CO(g) satisfies 𝐺 = 𝐺𝑜 + RTlnPCO


Page 17 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


A straightforward conclusion can be drawn from Fig. 3, Fig.4, and Table 1 that CO would and would only be adsorbed at Pd sites. Due to the possible attractive interactions between adsorbed CO molecules,58 the differential adsorption energies in Table 1 do not fit a strict monotonically increasing pattern. Instead, ΔEads reaches a minimum at 1/4 ML, which corresponds to the situation where all Pd atoms are filled by CO* molecules. Since the CO partial pressures under WGS reaction conditions are typically within the range of 0.1 to 1.0 bar, we expect that the surface coverage would be rather close to 1/4 ML, which means that nearly all Pd sites would be occupied by CO* and that Cu sites do not bind to CO*. The same conclusion also applies to the PdCu/CeO2 interface model, because on the PdCu/CeO2 interface, the ΔEads of CO at the last unfilled Pd site is around -1.00 eV but jumps to -0.44 eV after all Pd sites are occupied. Therefore, in this study, we calculated the reaction thermodynamics and kinetics with the explicit presence of CO* at all Pd sites.

3.2 WGS Reaction on the Unsupported PdCu Bimetallic Slab 3.2.1 Water Adsorption and Dissociation We have examined H2O dissociation pathways on the PdCu3(111) surface. The reaction energy profiles are shown in Fig. 5. The structures of all initial states (IS), transition states (TS) and final states (FS) are shown in Fig. 6. We found that water adsorption on PdCu3(111) is very weak (ΔEads = -0.05 eV). The dissociation of water into OH* and H* (Ea = 1.42 eV, ΔE = 0.64 eV, Fig. 6(a)), as well as the further dissociation of OH* into O* and H* (Ea = 1.61 eV, ΔE = 0.75 eV, Fig. 6(d)), are quite difficult. We also considered the reaction between water and an adsorbed O* to produce two OH*, 17 ACS Paragon Plus Environment


which is slightly exothermic in electronic energy (ΔE = -0.12 eV, Ea = 0.58 eV, Fig. 6(f)), but still endothermic if zero-point energy corrections are considered (see Section 3.2.3 and Table 2, entry 6). Finally, the H2(g) production from two H* atoms, as shown in Fig. 6(g), is 0.12 eV endothermic with a barrier of 0.82 eV.

Fig. 5. Energy profiles of H2O adsorption and dissociation on PdCu3(111) surface. Red numbers are activation barriers (without zero-point energy corrections).

(a)

(b)


Page 18 of 53

Page 19 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


(c)

(d)

(e)

(f)

(g) Fig. 6. Top and side views of IS, TS, and FS of (a) H2O dissociation catalyzed by a Cu site on a surface with all Pd sites occupied by CO*, (b) H2O dissociation catalyzed by a Cu site adjacent to a free Pd site, (c) H2O dissociation catalyzed by a free Pd site, (d) OH dissociation on a surface with all Pd sites occupied by CO*, (e) OH dissociation adjacent to a free Pd site, (f) H2O(g) + O* → 2 OH*, and (g) 2 H*→ H2(g) . The lengths of the bonds which are being broken or formed are shown along with each TS structure. In (a), (b), (c) and (f), the distances between the metal surfaces and the H2O molecules are also shown.



Page 20 of 53

Energy profiles in Fig. 5 and their corresponding reaction steps in Fig. 6(a), (d), (f), and (g) were based on the assumption that all Pd sites on the PdCu3(111) surface were occupied by CO* under WGS reaction conditions. However, under real reaction conditions, there may be a small number of CO-free Pd sites; their activity towards H2O splitting should also be examined. Therefore, we tested water dissociation on or beside a CO-free Pd atom. If water dissociation is catalyzed by a Cu atom adjacent to a CO-free Pd atom as shown in Fig. 6(b), the kinetic barrier is lowered slightly by 0.11 eV (Ea = 1.31 eV, ΔE = 0.49), possibly because the steric hindrance posed by a nearby CO* has been removed. On the other hand, if water dissociation is directly catalyzed by a CO-free Pd atom as shown in Fig. 6(c), the reaction is even more difficult (Ea = 1.48 eV, ΔE = 0.11 eV) than at Cu sites with co-adsorbed CO*. Similarly, OH* dissociation at a COfree site (Ea = 1.44 eV, ΔE = 0.72 eV, Fig. 6(e)) is 0.17 eV lower in kinetic barrier than its counterpart with co-adsorbed CO*. Nevertheless, the barriers for the reactions catalyzed by CO-free Pd sites are still quite high. These results suggest that CO-free sites do not show significantly improvement in water activation activity compared with sites with CO*. Because of their scarcity and inadequate activity, the CO-free sites were not considered in the microkinetic models discussed in Section 3.2.3.

3.2.2 CO Conversion We have considered two pathways of CO conversion listed below based on previous mechanistic studies.5-6, 10, 59 In these equations, the * symbol means that the species is adsorbed on the catalyst surface. Redox Mechanism: CO* + O* → CO2* + * 20 ACS Paragon Plus Environment

(3)

Page 21 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


Carboxyl Mechanism: CO* + OH* → COOH* + *

(4)

We did not investigate the formate pathway (Eq. 5) since formate is considered to be a spectator species formed between CO2* and H*, and it is not along the WGS reaction route.5, 12 HCOO* ⟷ CO2* + H*

(5)

The energy profiles along the CO conversion pathways are shown in Fig. 7. The structures of IS, TS, and FS for all steps are shown in Fig. 8.

Fig. 7. Energy profiles of CO conversion on PdCu3(111) surface.

(a)



(b)

(c)

(d) Fig. 8. Top and side views of IS, TS, and FS of (a) O* + CO* → CO2(g), (b) OH* + CO* → cis-COOH*, (c) cis-COOH* → trans-COOH*, and (d) trans-COOH* → H* + CO2. The IS of (c) and (d) are the same as the FS of (b) and (c), respectively, and are therefore not shown.

3.2.3 Microkinetic Models and the Effect of Co-feeding O2 Based on the DFT results presented in Section 3.2.1 and 3.2.2, we built microkinetic models to quantitatively predict the water-gas shift catalytic activity of the unsupported PdCu3(111) surface both without (termed as WGS) and with the addition of O2(g) (termed as OWGS). The list of elementary steps, reaction and activation energies, rate constants, and fluxes are presented in Table 2. Coverages of surface species in steady states are listed in Table 3.


Page 22 of 53

Page 23 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


Table 2. Energetic and kinetic parameters for elementary steps considered in the microkinetic models for WGS and OWGS on the unsupported PdCu3(111) surfacea. No

Elementary Step

∆EZPC EZPC a

kfwdc

(Reactions 1-15 were considered for both WGS and OWGS) 1 H2O(g) + * → (OH+H)* 0.54 1.22 1.21×10-06 2 CO(g) + * → CO* -0.86d 0.00 1.33×10+8 3 (OH+H)* + * →OH* + H* -0.05 0.00 1.11×10+13 4 OH* → (O+H)* 0.67 1.44 1.70×10-01 5 (O+H)* + * →O* + H* -0.29 0.00 1.11×10+13 6 H2O(g) + O*→ (OH+OH)* 0.01 0.66 4.06×10-01 7 (OH+OH)* + * →2 OH* 0.11 0.11 1.10×10+12 8 2 H*→ (H+H)* + * 0.00 0.00 1.16×10+13 9 (H+H)* → H2(g) + * -0.01 0.74 2.15×10+6 10 CO*+O*→ (CO+O)* + * 0.25 0.25 5.08×10+10 11 (CO+O)* → CO2(g) + * -0.95 0.81 2.69×10+05 12 CO* + OH*→ (CO+OH)* + * 0.00 0.00 1.11×10+13 13 (CO+OH)* → cis-COOH* 0.26 0.64 7.90×10+06 14 cis-COOH* → trans-COOH* 0.07 0.43 1.23×10+09 15 trans-COOH* → CO2(g) + H* -0.66 0.83 2.03×10+07 (Reactions 16 and 17 were considered for OWGS only) 16 O2(g) + * → O2* -0.64 0.00 1.24×10+08 17 O2* + * → 2 O* -0.50 0.08 4.52×10+11

krev

flux WGS

OWGS

1.14×10+07 3.64×10+07 3.78×10+12 1.00×10+06 1.87×10+10 1.31×10+07 1.11×10+13 1.11×10+13 2.01×10+00 1.11×10+13 3.89×10-12 1.11×10+13 7.03×10+10 3.05×10+09 2.90×10-08

8.16×10-08 8.16×10-08 8.16×10-08 5.71×10-11 5.71×10-11 -2.01×10-11 -2.01×10-11 8.16×10-08 8.16×10-08 7.72×10-11 7.72×10-11 8.15×10-08 8.15×10-08 8.15×10-08 8.15×10-08

-1.03×10-03 3.18×10+02 -1.03×10-03 -1.54×10-02 -1.54×10-02 -5.92×10-03 -5.92×10-03 -6.95×10-03 -6.95×10-03 3.18×10+02 3.18×10+02 2.55×10-03 2.55×10-03 2.55×10-03 2.55×10-03

7.69×10+09 1.02×10+08

-

1.59×10+02 1.59×10+02

a. Results obtained under 533 K, PCO = 0.098 bar, 𝑃𝐻2𝑂= 0.23 bar, 𝑃𝐻2 = 𝑃𝐶𝑂2 = 0.01 bar, and 𝑃𝑂2 = 0.014 bar (in OWGS only), as stated in Section 2. b. ∆EZPC and EZPC are reaction energies and activation energies with zero-point energy corrections; a their units are eV. c. kfwd and krev are the forward and reverse rate constants. Flux is the net reaction rate (forward rate minus reverse rate) when the system is in steady states. Their units are s-1site-1. d. The ∆EZPC and krev values of reaction 2 under the OWGS condition are slightly different and are -0.89 eV and 1.86×10+07 s-1site-1, respectively; this difference is explained in the text below.



Table 3. Steady-state coverages of surface species under WGS and OWGS conditions on the unsupported PdCu3(111) surface. No

Surface Species

1 2 3 4 5 6 7 8

Empty site (*) CO* OH* H* O* cis-COOH* trans-COOH* O2*

Coverage (ML) WGS OWGS 0.7365 0.3702 0.2634 0.2590 3.358×10-10 1.067×10-05 6.966×10-05 9.155×10-06 1.751×10-13 0.3695 1.352×10-14 8.403×10-10 5.442×10-15 3.391×10-10 8.366×10-05

Reactions 1 through 15 represent the dissociation of water and the conversion of CO, and they were included in the models for both WGS and OWGS. The reaction energies, activation barriers and rate constants of these steps are the same for both WGS and OWGS (reaction 2 is an exception and is explained below), whereas the fluxes of these steps under WGS and OWGS conditions are different and are therefore listed separately. For OWGS, two additional steps (reactions 16 and 17) were included to take the adsorption and dissociation of O2(g) into consideration. From Table 3 it can be seen that both CO* and O* can reach a moderate coverage (> 0.25 ML). In order to get accurate results, the adsorption energies of CO* and O* were considered to be dependent on their coverages in our models. Table 1 has already shown this dependency for CO* adsorption, and the ∆Eads values of O* (relative to O2(g)) were calculated to be -1.89, -1.84, -1.76, -1.67, -1.24, and -0.53 eV at 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 7/16 ML, respectively. Linear interpolations were used to estimate the adsorption energies of CO* and O* under other coverages. Consequently, the kinetic parameters of reactions 2 and 17 were expressed as functions of CO* and O* coverages, respectively, 24 ACS Paragon Plus Environment

Page 24 of 53

Page 25 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


while the parameters of other steps were independent of surface coverages. This is why the kinetic parameters of step 2 for WGS and OWGS slightly differ (see note d under Table 2) as the CO* coverages are not the same under these two conditions. This approach is necessary because without considering the coverage effects of CO* and O*, the microkinetic model would predict an unreasonably high CO* coverage (0.733 ML) under the WGS condition and O* dominance (0.991 ML) of the surface under the OWGS condition, which led to wrong estimations of the catalytic activity. Apart from the coverage effects of CO* and O*, another detail in our model is that the adsorption and dissociation of water were combined in a single step (step 1 in Table 3), and the forward rate of water dissociation was calculated by 𝑟1 = 𝑘1𝑝𝐻2𝑂𝜃 ∗ instead of 𝑟𝑑𝑖𝑠𝑠𝑜 = 𝑘𝑑𝑖𝑠𝑠𝑜𝜃𝐻2𝑂 ∗ 𝜃 ∗ . The reason for adopting this approach is that water adsorption on PdCu3(111) is so weak (ΔEads = -0.05 eV as reported previously in Section 3.2.1) that only physisorption is possible, as demonstrated in the IS structure of Fig. 6(a). Although the physisorption state can be used to represent H2O*, the water molecule is not fixed on the surface and still has translational and rotational degrees of freedom. Based on this structure, the kinetic constant 𝑘𝑑𝑖𝑠𝑠𝑜 calculated from Eq. S14 will be erroneous since only vibrational partition functions are considered in that equation. By contrast, in our models, it is assumed that an H2O(g) molecule dissociates immediately after it collides with the surface if it has sufficient kinetic energy. The advantage of this strategy is that it avoids considering the problematic H2O* state and the rate constant k1 can be correctly calculated based on the partition functions of the transition state and the gaseous H2O(g). By solving the microkinetic models, the catalytic activity of the unsupported PdCu3(111) was estimated; the most important results are summarized in Table 4. 25 ACS Paragon Plus Environment


Page 26 of 53

Table 4. Key results of the microkinetic models for WGS and OWGS on PdCu3(111)a Key Results

WGS

OWGS

H2 production TOF (s-1)

8.157×10-8

-6.947×10-3

CO2 production TOF (s-1)

8.157×10-8

3.184×102

Reaction Order of COb

0.006

0.041

Reaction Order of H2O

1.000

0.000

Apparent Barrier (kJ/mol)

142.4

98.4

a. Results obtained under 533 K, PCO = 0.098 bar, 𝑃𝐻2𝑂= 0.23 bar, 𝑃𝐻2 = 𝑃𝐶𝑂2 = 0.01 bar, and 𝑃𝑂2 = 0.014 bar (in OWGS only), as stated in Section 2. b. Reaction orders and apparent barriers were based on the CO2 production TOF.

Based on the microkinetic models, the unsupported PdCu3(111) surface should be inactive towards WGS. This is mainly caused by the high barriers and strong endothermicity in both water dissociation and OH dissociation that lead to a very low surface coverage of OH* or O*. (see Table 3). The major conversion route of CO without the presence of O2 is through the carboxyl pathway. Campbell’s degree of rate control (XRC) was used to determine the rate-controlling steps.60-61 For WGS on PdCu3(111), the steps with non-zero XRC values are step 1, water dissociation (XRC = 0.424) and step 15, COOH dehydrogenation (XRC = 0.625). It confirms that water dissociation is rate-limiting. The high barrier of the final dehydrogenation step also slows down the WGS reaction. For OWGS, the microkinetic model revealed that when O2 is added to the feed gas, a high CO oxidation activity can be observed on the surface. The rate-controlling step for CO oxidation is step 11 with XRC = 1.00, which means that the CO oxidation rate is completely determined by the kinetic barrier (0.81 eV with ZPC) in CO2 formation. Meanwhile, adding O2 has little effect on H2 production. Although a very slow H2 26 ACS Paragon Plus Environment

Page 27 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


consumption (TOF = -6.947×10-3, the negative sign means H2 is consumed instead of produced) can be observed, the rate is negligible. The fast CO conversion matches the experimental observation that O2 can be quickly consumed once added into the reactor.29 A limitation of our model is that the calculated CO conversion rate could be inaccurate. This inaccuracy is duo to the fact that after exposure to O2, the PdCu3(111) surface has been oxidized, while our results are derived from a metallic surface. However, other than obtaining a quantitatively correct CO oxidation TOF, the aim of this study is to estimate the WGS catalytic activity. For this purpose, the microkinetic model clearly reveals that the unsupported PdCu3(111) is inactive towards hydrogen production regardless of the presence of O2.



Page 28 of 53

3.3 WGS Reaction on the PdCu/CeO2 Interface 3.3.1 A Mars-van Krevelen Mechanism for WGS Catalyzed by the PdCu/CeO2 Interface One of the major obstacles for WGS on unsupported PdCu(111) is the high barrier of water dissociation. In contrast, many studies have found that water dissociation on both stoichiometric and reduced CeO2 are facile processes.57,

62

Therefore, we tested WGS

reaction catalyzed by a metal/ceria interface (as shown in Fig. 1(b)) to investigate whether ceria can provide the water activation activity. Fig. 9 shows the energy profile of a complete WGS reaction cycle at an interface site. Fig. 10 demonstrates the structures of IS, TS, and FS of all steps along the reaction pathway presented in Fig. 9.

Fig. 9. Energy profiles of WGS catalyzed by the PdCu/CeO2 interface. In each state, the notation A*/B* means A is adsorbed on the PdCu metal side of the interface, while B is adsorbed on the CeO2 side. A and B are adjacent to each other.


Page 29 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


IS

IS

TS

TS

FS

FS

TS

TS

FS

FS

(b) TS

TS

FS

FS

TS

FS

FS

1.44 Å

(a) IS

IS 1.85 Å

IS

IS

2.13 Å

(c) IS

IS

TS 1.01 Å

(d) Fig. 10. Side and top views of IS, TS, and FS of (a) H2O dissociation, (b) OH dissociation, (c) CO oxidation, and (d) H2 formation catalyzed by a PdCu/CeO2 interface site. For clarity, atoms below the topmost Ce layer are hidden in the top views.

The reaction proceeds with a Mars-van Krevelen-type mechanism.63 Assuming that there is an O vacancy on the CeO2 surface adjacent to the metallic phase, H2O can be adsorbed at this vacancy site with a ∆Eads of -0.22 eV (Fig. 10(a)). The following dissociation of H2O is rather facile with a barrier of only 0.24 eV and is exothermic by 0.51 eV. Further dissociation of OH* is endothermic by 0.39 eV with a barrier of 1.31 eV (1.18 eV with zero-point energy correction). We also considered the possibility that instead of dissociation, the OH* group might diffuse to the metal surface. However, results show that OH* migration to the metal surface is thermodynamically forbidden (∆E = 1.48 eV). 29 ACS Paragon Plus Environment


Consequently, dissociation is the only possible reaction route. The O* atom produced by OH* dissociation then reacts with a CO* molecule adsorbed on the PdCu metal surface to produce CO2* (Ea = 0.80 eV, ∆E = -0.15 eV). Interestingly, CO2 can be stably adsorbed at the interface site with a bent structure (Fig. 10(c)), and the desorption of CO2* is endothermic by 0.75 eV in electronic energy. Only when entropic contributions are considered, desorption of CO2* become favorable with ∆𝐺𝑜𝑑𝑒𝑠= -0.09 eV. CO2* desorption would also leave an O vacancy behind. Finally, two H* atoms that have been detached from H2O* and OH* bond with each other to form a H2(g) molecule (Ea = 0.78 eV, ∆E = 0.79 eV). The adsorption of an additional CO(g) molecule (∆Eads = -1.07 eV) replenishes the surface CO* species; subsequently, the surface of the catalyst is restored to its original state and completes a full catalytic cycle. Within this process, a lattice oxygen atom from the CeO2(111) surface is at first consumed by CO oxidation and then regenerated from H2O* and OH* dissociation. This is a characteristic feature of the Mars-van Krevelen mechanism. Compared with the unsupported PdCu3(111) surface, the PdCu/CeO2 interface has different catalytic activity. H2O* dissociation became facile, and OH* dissociation became the most difficult step. The quantitative evaluation of its catalytic performance is presented in Section 3.3.3. This WGS path coincides with the one proposed by Bunluesin et al.64


Page 30 of 53

Page 31 of 53

3.3.2 The OH Spillover Mechanism for the O2-enhanced WGS The energy diagram in Fig. 9 indicates that OH* dissociation is the slowest step along the WGS pathway. However, with the addition of O2, the slow OH* dissociation step can be avoided, and a new pathway is generated for CO conversion. The energy profiles of the O2-enhanced WGS (OWGS) reaction pathway is shown in Fig. 11. The IS, TS, and FS structures of all important steps along this pathway are given in Fig. 12.

*/ *

+H2O(g)

C O

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


CO

*/

H

O

A

0.24 +O2(g)

*

2

O (C

+H

O (C

/ )*

+H

O

)*

H

/ (O

O (C

A

*

H

B

)* 2 +O

0.48

B

+H

+O

/ (O )*

H

+O

-CO2(g)

)*

1.12 (H...OH)

+CO(g) 0.31

H

*/

(O

(H

H

+O

O +C

)*

/ (O )*

H

+O

)*

(H

O +C

+O

)*

H

/ )*

O

(H 0.91 (CO…OH) *

O (C

O 2 +H

)

O */

(H

+

*

+

tr

H O O C sci 0.43

O s-C n a

O

H

/O

*

1.03 -CO2(g)

)*

/O

*

(2

H

O +C

/ )* 2

O

* 2H

*/

O

*

Fig. 11. Energy profiles of OWGS catalyzed by the PdCu/CeO2 interface. In each state, the notation A*/B* for each state has the same meaning as in Fig. 10. The inset pictures show the geometries of molecular (marked as state A) and atomic oxygen (state B) adsorbed on the PdCu/CeO2 interface.



IS

IS

TS

TS

Page 32 of 53

FS

FS

FS

FS

1.93 Å

(a)

IS

IS

TS

TS

(b)

(c) TS

TS

TS

TS

TS

TS

1.42 Å

1.94 Å

FS

FS

FS

FS

FS

FS

(d) (f) (e) Fig. 12. Side and top views of IS, TS, and FS of (a) CO oxidation, (b) OH diffusion, (c) H + OH → H2O, (d) cis-COOH formation, (e) cis-COOH → trans-COOH, and (f) COOH dehydrogenation catalyzed by a PdCu/CeO2 interface site. The IS of (d), (e), and (f) are the same as the FS of (b), (d), and (e), respectively, and are therefore not shown.

The initial steps of OWGS, which are H2O adsorption and dissociation, are identical to WGS until the formation of OH* on the interface (Fig. 11, the state marked as ‘(CO+H)*/OH*’). An O2 molecule, if present in the gas phase, can be quickly adsorbed on the surface of the catalyst and dissociate. If one O2 molecule is adsorbed on the interface (∆Eads = -0.72 eV, Fig. 11 inset picture state A), the dissociation of molecular O2* into 32 ACS Paragon Plus Environment

Page 33 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


atomic O* is autonomous without a kinetic barrier, which is highly exothermic (∆E = -2.27 eV, Fig. 11 inset pictures state B). As a result, both the ceria and the PdCu metal near the interface are oxidized. The O* atom produced on the PdCu metal surface can be removed by reacting with a surface CO*; this step is rather facile (Ea = 0.48 eV, ∆E = -1.35 eV). As discussed in Section 3.3.1, the migration of OH* from the interface to the PdCu metal, without the addition of O2, is unfavorable (∆E = 1.48 eV), possibly because the migration of OH* produces an O vacancy on the ceria surface and reduces the ceria, which is endothermic. However, with the addition of O2, the situation is different since the ceria is maintained in a high oxidation state during the OH* migration process. In fact, the migration of OH* is favorable (Ea = 0.31 eV, ∆E = -0.26 eV, see Fig. 12(b)) when an additional O* atom is present on the interface. Therefore, instead of dissociation, the OH* group migrates to the metal surface and opens up the carboxyl pathway. After migrating to the metal surface, OH* reacts with CO* to produce cis-COOH* (Ea = 0.91 eV, ∆E = 0.16 eV, see Fig. 12(d)), which can readily transform into a more stable isomer, trans-COOH* (Ea = 0.43 eV, ∆E = -0.10 eV, see Fig. 12(e)). Alternatively, the migrated OH* may bond with an H* on the metal surface to regenerate H2O* (Ea = 1.12 eV, ∆E = 0.57 eV, see Fig. 12(d)). However, it is kinetically more difficult than cis-COOH* production and we found that the reaction rate to H2O* is three orders of magnitude smaller than COOH formation (see Table 5, entry 17). Dehydrogenation of trans-COOH* (Ea = 1.03 eV, ∆E = -0.06 eV, see Fig. 12(f)) produces an H* and a CO2* molecule that also has a bent structure and is adsorbed on the PdCu/CeO2 interface. Desorption of the CO2* molecule (∆Edes = -0.36 eV), which produces a free Pd site, can be followed by the replenishment of CO* via CO adsorption. 33 ACS Paragon Plus Environment


Page 34 of 53

The rest of the reaction pathway (after the last step shown in Fig. 11 and a CO adsorption step) is identical to the second half of the WGS pathway (see Fig. 9, starting from the state (CO+2H)*/O*). The overall reaction along the OWGS pathway is shown in Eq. 6. H2O + 3 CO + O2 → 3 CO2 + H2

(6)

This process can be considered as a WGS reaction coupled with a CO oxidation reaction. Please note that in this pathway the step with the highest barrier (trans-COOH* dehydrogenation) is still much faster than OH dissociation. This pathway has been previously proposed as the OH spillover mechanism.65 However, OH spillover may only be possible when the ceria support is maintained in a high oxidation state with the addition of O2. Therefore, this addition of O2 may enhance both CO conversion and H2 production. The quantitative evaluation of its activity is given in Section 3.3.3. In addition, we considered another plausible OWGS pathway in which H2O directly reacted with a surface adsorbed O* atom on the metal surface to produce two OH* groups. However, we found this pathway unfavorable if the competition between CO oxidation and H2O conversion was considered. A detailed discussion on this pathway is given in Section S5.


Page 35 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


3.3.3 Microkinetic Models for WGS and OWGS on the PdCu/CeO2 Interface We used microkinetic models to estimate the catalytic activity of the PdCu/CeO2 interface with and without the addition of O2. Detailed information of the microkinetic models, including the list of elementary steps, reaction and activation energies, kinetic constants, and fluxes of each step in steady states are presented in Table 5. Coverages of surface species in steady states are listed in Table 6. It is worth mentioning again that we used the notation A*/B* to refer to a state in which A* and B* are on the metal and ceria side of the interface, respectively, as explained earlier in Fig. 9. For example, the structure of CO*/H2O* is shown in the IS of Fig. 10(a), and the FS of Fig. 10(a) corresponds to (CO+H)*/OH*. One exception to this rule is that H* refers to a hydrogen atom adsorbed on the metal side of the interface, while the adsorbate on the ceria side is unspecified. Compared with the microkinetic models for the unsupported PdCu3(111) surface, the models for the PdCu/CeO2 interface are slightly different. This is because the PdCu3(111) surface was considered as a two-dimensional lattice of active sites, while the PdCu/CeO2 interface, which refers to the boundary line between the metallic and the ceria phases, was viewed as a one-dimensional array of active sites. The different arrangements of active sites lead to distinctions in the definitions of coverage. In the former case, coverage refers to the concentration of a species on the surface measured in the unit of monolayer (ML). By contrast, the coverage of a species on the PdCu/CeO2 interface means the percentage of active sites in this one-dimensional array occupied by this species and does not have a unit. Apart from this difference in the definition of coverage, the microkinetic models for PdCu3(111) and PdCu/CeO2(111) are similar to each other. 35 ACS Paragon Plus Environment


Page 36 of 53

For example, the forward rate of a surface reaction A*+B* → C* equals 𝑘𝑓𝑤𝑑𝜃𝐴 ∗ 𝜃𝐵 ∗ on both the metallic surface and the interface, because 𝜃𝐴 ∗ 𝜃𝐵 ∗ represents the probability that A* and B* encounter each other in both one-dimensional and two-dimensional cases, and the coverages of all species sum up to 1. A similar approach to represent the metal/oxide interface sites in microkinetic models was also used by Zhao et al.25 Table 5. Energetic and kinetic parameters for elementary steps considered in the microkinetic models for WGS and OWGS on the PdCu/CeO2 interfacea. No

∆EZPC EZPC a

Elementary Step

kfwd

krev

flux WGS

OWGS

(Reactions 1-9 were considered for both WGS and OWGS) 1

*/* + CO(g) → CO*/*

-1.00 0.00 1.33×10+08 2.00×10+06 2.14×10+01 2.003×10+02

2

CO*/* + H2O(g) → CO*/H2O*

-0.30 0.00 1.66×10+08 5.53×10+12 2.14×10+01 2.003×10+02

3

CO*/H2O* → (CO+H)*/OH*

-0.48 0.21 4.44×10+10 4.55×10+06 2.14×10+01 2.003×10+02

4

(CO+H)*/OH*→CO*/OH*+ H*

0.06

0.06 5.16×10+12 1.11×10+13 2.14×10+01 2.003×10+02

5

CO*/OH*→(CO+H)*/O*

0.25

1.18 2.56×10+01 1.11×10+04 2.14×10+01 -3.504×10-01

6

(CO+H)*/O*→CO*/O*+H*

0.01

0.00 1.11×10+13 2.71×10+13 2.14×10+01 -3.504×10-01

7

CO*/O* → */CO2*

8

*/CO2* → */* + CO2(g)

9

2H* → H2(g)

-0.12 0.77 2.21×10+05 2.57×10+04 2.14×10+01 2.003×10+02 0.78

0.78 6.85×10+08 1.06×10+08 2.14×10+01 2.003×10+02

-0.01 0.76 9.07×10+05 7.60×10-01 2.14×10+01 1.999×10+02

(Reactions 10-21 were considered for OWGS only) 10

-2.96 0.00 1.11×10+13 6.76×10-07

-

2.006×10+02

11 (CO+O)*/(OH+O)* →*/(OH+O)*+CO2(g) -1.29 0.48 1.92×10+08 1.36×10-12

-

2.006×10+02

12

*/(OH+O)*+CO(g) →CO*/(OH+O)*

-0.45 0.00 1.33×10+08 5.26×10+11

-

2.006×10+02

13

CO*/(OH+O)* →*/OH*+CO2(g)

-0.99 1.21 6.88×10+00 9.08×10-17

-

1.650×10-04

14

*/OH*+CO(g) →CO*/OH*

-0.87 0.00 1.33×10+08 1.65×10+07

-

1.650×10-04

15

CO*/(OH+O)* → (CO+OH)*/O*

-0.25 0.30 8.77×10+09 2.14×10+07

-

2.006×10+02

16

(CO+OH)* →cis-COOH*/O*

0.22

0.89 8.60×10+04 1.99×10+06

-

2.002×10+02

(CO+OH)*/O*+H* → CO*/O* + H2O(g) -0.57 1.04 2.69×10+03 1.10×10-10

-

3.895×10-01

17

CO*/OH*+O2(g) → (CO+O)*/(OH+O)*

18

cis-COOH*/O* →trans-COOH*/O*

-0.09 0.40 5.01×10+08 1.42×10+08

-

2.002×10+02

19

trans-COOH*/O* → (H+CO2)*/O*

-0.07 0.81 1.77×10+05 4.33×10+04

-

2.002×10+02

20

(H+CO2)*/O* →H*+CO2(g)+*/O*

-0.47 0.00 3.06×10+15 3.74×10+03

-

2.002×10+02

21

*/O*+CO(g) →CO*/O*

-0.94 0.00 1.33×10+08 1.22×10+07

-

2.002×10+02

a. Reaction conditions and the meanings and units of all columns are identical to Table 2.


Page 37 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


Table 6. Steady-state coverages of surface species under WGS and OWGS conditions on the PdCu/CeO2 interface. No

Surface Species

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

*/* CO*/* CO*/H2O* (CO+H)*/OH* CO*/OH* (CO+H)*/O* CO*/O* */CO2* H* (CO+O)*/(OH+O)* */(OH+O)* CO*/(OH+O)* */OH* (CO+OH)*/O* cis-COOH*/O* trans-COOH*/O* (H+CO2)*/O* */O*

Coverage WGS OWGS -02 2.005×10 1.889×10-04 0.1304 1.128×10-03 8.965×10-07 4.946×10-09 -03 8.746×10 4.276×10-06 0.8358 6.893×10-05 -06 2.100×10 3.166×10-05 1.006×10-04 9.072×10-04 3.100×10-05 5.840×10-07 -03 4.861×10 1.485×10-02 1.045×10-06 0.9706 2.399×10-05 8.776×10-05 9.769×10-03 3.217×10-04 1.134×10-03 3.500×10-09 8.692×10-04

As can be seen in Table 5, a total of nine elementary steps were considered for both WGS and OWGS, and 12 additional steps were included in the model to simulate the O2(g) adsorption/dissociation pathway and the OH* migration pathway under the OWGS condition. Some side reactions that lead away from COOH formation, including steps 13, 14, and 17, were also considered, although their fluxes are negligible. Based on the models, the catalytic activity of the PdCu/CeO2 was estimated, and the most important results are summarized in Table 7. It is also evident that under the OWGS condition, H2(g) production is primarily through the OH migration pathway because the direct dissociation of OH* (step 5) has a flux close to zero.



Page 38 of 53

Table 7. Key results of our microkinetic models for WGS and OWGS on the PdCu/CeO2 interface and comparisons with experimental measurements. Key Results

WGS

OWGS

MKMa

Expb

MKM

Expc

H2 production TOF (s-1)d

2.141

1.66

19.99

2.08

CO2 production TOF (s-1)

2.141

Reaction Order of CO(g)e

0.018

0.4

0.978

1.6

Reaction Order of H2O(g)

0.139

0.3

0.001

0.2

Apparent Barrier (kJ/mol)

95.6

25.7

29.1

60.11

a. Microkinetic model results obtained under the same conditions as in Table 2. b. Experimental results from Ref.30 and the observation that the H2 production rate in WGS was approximated 80% of that in OWGS (Figs. 11 and 12 in Ref.30) c. Experimental results from Ref.29 d. Calculated TOFs were multiplied by a factor of 0.1 in order to compare with experimental TOFs; the reason is given in the discussion below. e. Reaction orders and apparent barriers were based on H2 production rates.

In order to interpret the microkinetic results and to compare them with experimental measurements, the following four points are discussed. 1. The calculated TOFs in our microkinetic models were based on the PdCu/CeO2 interface sites and were 21.41 s-1 and 199.9 s-1 for WGS and OWGS, respectively. In contrast, the experimental TOFs were measured based on the whole metal surface area.29 Results in Section 3.2.3 have shown that the metal sites far from the support are inactive towards WGS; thus, we need to estimate the percentage of sites on the metal surface that can be considered as interface sites. From the field emission SEM image, it was found that the PdCu bimetallic clusters were less than 10 nm, or approximately 38 atoms wide.29 Assuming that the metal clusters have a circular shape, the number of interface sites would be proportional to its circumference, 2𝜋𝑟, with r = 19, and the number of total 38 ACS Paragon Plus Environment

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


metal sites should be proportional to its area 𝜋𝑟2. Therefore, the percentage of interface sites is approximately equal to 2/r, or 10%, and our TOF should be multiplied by a factor of 0.1 before comparing with the experimental values. In this sense, our models estimated TOFs of 2.141 and 20.0 for WGS and OWGS, respectively. The TOF for WGS is reasonably close to the experimentally observed TOF of 1.66, considering the accuracy of the DFT methods.66 For OWGS, the estimated TOF corresponds to a situation where O2(g) comes with unlimited supply and should not be directly compared with the experimental TOF; this topic will be further discussed below. 2. One of the limitations of microkinetic models is that a solution to the differential equations represents a steady state with the pressures of all gas phase reactants fixed at certain values. Unfortunately, the consumption of O2 is much faster than CO conversion because O2 can oxidize the surface, and the partial pressures of the gas phase O2 would drop to 0 shortly after O2 enters into the reactor. Our two microkinetic models for OWGS and WGS approximate the entry point and exit point of the reactor, where O2 is abundant and depleted, respectively. The overall performance of the catalyst under the OWGS condition, including TOF, reaction orders, and apparent barriers, is expected to be within the range defined by these two extreme conditions (between 2.14 s-1 and 20.0 s-1). A precise quantitative estimation of the overall TOF under the OWGS condition is difficult because it is necessary to consider a dynamic process of O2 accumulation, diffusion, and consumption on the catalyst surface instead of a steady state. Despite these limitations, our microkinetic models revealed that a more facile carboxyl pathway is opened with the addition of O2, which can qualitatively explain the enhanced H2 production activity.



3. Without O2 addition, the reaction orders of H2O and CO equaled 0.14 and 0.02, respectively. When O2 was abundant, the order of CO increased to nearly 1.0 while the order of H2O dropped to 0. This is consistent with experimental findings that with O2 addition, the order of CO increased significantly while the order of H2O decreased slightly. This is because with the addition of O2, the activation of water becomes easier (the difficulty in OH* dissociation is avoided), while a larger quantity of CO is needed to remove O* from the surface. Quantitatively, our predicted orders, especially the order of CO, are smaller than the experimental values. One of the possible reasons for this may be that after exposure to a pulse of O2 gas, the PdCu surface may have a rather high surface or subsurface O coverage. On the oxidized PdCu surface, the WGS activity may be suppressed until enough CO molecules are provided to reduce the PdCu cluster back to the metallic state. Since we did not explicitly consider a fully or partially oxidized PdCu cluster, the calculated reaction orders may be inaccurate. Another possible reason may be the inaccuracy of the DFT methods.67 An earlier study found that very accurate estimations of the adsorption and reaction energies, for example by using the HSE06 hybrid functional, were required to quantitatively match the experimental reaction orders.68 Nonetheless, our models correctly captured the trends of change in reaction orders after the addition of O2 and provided insights for the reaction mechanisms of WGS and OWGS. 4. Consistent with our expectation, the rate-controlling step of WGS is the slow OH* dissociation (Step 5, XRC = 0.926). In addition, hydrogen formation (Step 9, XRC = 0.079) also limits the overall rate slightly. With O2 addition, the rate-controlling steps for


Page 40 of 53

Page 41 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


hydrogen production shift to COOH formation (Step 16, XRC = 0.253) and dehydrogenation (Step 19, XRC = 0.765). In summary, our first-principles-based microkinetic models generally match well with the experimental findings. It can be confirmed that the addition of O2 promotes both CO conversion and H2 production. The latter effect may be achieved through an OH spillover mechanism.

3.4 Roles of Different Components in the PdCu/CeO2 Material 3.4.1 The Ceria Support This study found that the unsupported PdCu metal is inactive towards WGS mainly because it is unable to catalyze H2O dissociation. In contrast, the PdCu/CeO2 interface has a superior H2O splitting activity. Specifically, the PdCu metal surface has weaker binding with the OH* group, while the interface has stronger binding, which suggests more exothermic water splitting and a lower barrier. It is interesting to know what leads to this difference between metal surface sites and interface sites. This difference may be understood from their electronic structures. We used the crystal orbital Hamilton population (COHP) method to analyze the bonding between OH* and metal atoms.69-71 This technique projects the delocalized electron density generated from the plane wave DFT into local bonding and antibonding orbitals. Fig. 13 shows the pCOHP plots for OH* adsorbed on the metal and on the interface.



(a) (b) Fig. 13. pCOHP bonding analysis between the O atom and metal (Cu, Ce) atoms for an OH group adsorbed on a (a) PdCu3(111) surface and (b) PdCu/CeO2 interface. (a) is based on the IS structure of Fig. 6(d) and (b) is based on the IS structure of Fig. 10(b).

In the pCOHP plots, we followed the convention of plotting negative pCOHP along the positive x-direction. Therefore, areas enclosed on the right-hand side of the y-axis means bonding interactions, and antibonding interactions are represented by the areas to the left of the y-axis. Clearly, the O-Cu bond in an OH*/PdCu3(111) structure is unfavorable because of the large antibonding interaction ranging from -4 eV all the way to the Fermi level. In contrast, the O-Ce bond in the OH*/PdCu/CeO2 structure is strong with negligible antibonding interactions. Interestingly, the Cu atom of the interface site also contributes significant bonding while its antibonding contribution vanishes. Therefore, the ceria support alters the electronic structures of the PdCu metal, and thus the interface site is suitable for catalytic water dissociation.


Page 42 of 53

Page 43 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


3.4.2 The Pd and Cu Metals The role of the PdCu metal should be associated with the CO conversion and H2 production activities, as ceria alone is inactive towards WGS28 and previous studies also found that ceria alone failed to produce H2 from water.62 We also tested the adsorption of CO molecules on the ceria surface but found that the adsorption was rather weak (ΔEads = -0.09 eV), indicating that ceria failed to capture and convert CO. One question remains, what are the separate roles of Pd and Cu? One clear difference between Pd and Cu is that CO adsorption on Pd is stronger than on Cu. Under the experimental WGS conditions, CO adsorption at the Pd site is exothermic in Gibbs free energy, while CO adsorption on Cu (after all Pd sites are filled) is endothermic (Table 1). Pd, which acts as an anchor point, helps to capture CO molecules from the gas phase. It is interesting to ask what will happen if the Pd concentration in the PdCu alloy becomes high, or the PdCu alloy is replaced by pure Pd. To investigate the effect of Pd concentrations, we considered the rate-controlling step of WGS on the PdCu/CeO2 interface, i.e., the OH* dissociation. As shown in Fig 10(b), O-H bond breaking proceeds across a Pd-Cu bridge. If the Pd concentration value is rather low or high, OH* dissociation will be more likely to proceed across a Cu-Cu bridge or a Pd-Pd bridge, respectively. We have recalculated the reaction energies of OH* dissociation under these situations. The detailed results, given in Section S4, show that the OH* dissociation activity of a Cu-Cu bridge is identical to a Pd-Cu bridge, and a Pd-Pd bridge is even slightly more active given that these two Pd atoms are not occupied by CO* at the same time. However, if both Pd atoms are filled by CO*, OH* dissociation becomes significantly more difficult. This suggests that an extremely high Pd 43 ACS Paragon Plus Environment


concentration may lead to a high CO* surface coverage, which could suppress other steps including OH* dissociation and H2 formation. In summary, Pd atoms can enhance CO adsorption and make CO conversion faster. However, care should be taken to maintain the Pd concentration in a reasonable range to prevent CO poisoning of the catalyst surface.

4. Conclusions In this study, we performed DFT calculations and analyzed microkinetic models to investigate the WGS reaction mechanisms catalyzed by PdCu supported on ceria. Quantitative evaluations of the catalytic activities revealed that the unsupported PdCu3(111) surface is inactive towards WGS because the bimetallic surface cannot activate and dissociate water. The PdCu/CeO2 interface catalyzes the WGS reaction through a Mars-van Krevelen mechanism, through which an O vacancy on the CeO2 surface is at first filled by an OH* group dissociate from water and then regenerated by CO oxidation. The rate-limiting step along this pathway is the dissociation of OH*. The addition of O2, which has been shown experimentally to enhance H2 production, showed little effect on the unsupported PdCu3(111) surface. In contrast, for the PdCu/CeO2 interface, we proposed an OH* spillover mechanism through which H2 production may be promoted. In this pathway, OH* migration from the ceria surface to the PdCu metal surface becomes possible after the ceria has been re-oxidized by O2. OH* can then react with CO* through a carboxyl pathway to avoid the slow OH* dissociation step. Quantitative evaluations of the catalytic performance confirmed the enhancement


Page 44 of 53

Page 45 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


effect of O2. The estimated catalytic activity matched well with that in previous experimental studies. The results of this study suggest that the metal/ceria interface plays a crucial role in the WGS reactions for the PdCu/CeO2 material. The ceria support directly participates in the reactions. It is, however, unclear whether the metal/oxide interface also has a crucial role in other WGS reactions such as the Cu/ZnO catalyzed WGS reaction. Therefore, future studies should investigate the roles of the metal/oxide interfaces in the reaction mechanisms of WGS.

ASSOCIATED CONTENT Supporting Information. Statistical approaches used in microkinetic models. Discussions on the influences of various parameters. Alternative reaction pathways.

Acknowledgments Computational resources were provided by the National Supercomputing Center in Shenzhen. This work was financially supported by the Sichuan Provincial Grant for Scholars with Overseas Degrees and the National Science Foundation of China (Grant No. 21703176).



References 1. LeValley, T. L.; Richard, A. R.; Fan, M., The Progress In Water Gas Shift and Steam Reforming Hydrogen Production Technologies – A Review. Int. J.

Hydrogen Energy 2014, 39, 16983-17000. 2. Schumacher, N.; Boisen, A.; Dahl, S.; Gokhale, A. A.; Kandoi, S.; Grabow, L. C.; Dumesic, J. A.; Mavrikakis, M.; Chorkendorff, I., Trends in Low-Temperature Water–Gas Shift Reactivity on Transition Metals. J. Catal. 2005, 229, 265-275. 3. Park, E. D.; Lee, D.; Lee, H. C., Recent Progress in Selective CO Removal in a H2-rich Stream. Catal. Today 2009, 139, 280-290. 4. Boisen, A.; Janssens, T. V. W.; Schumacher, N.; Chorkendorff, I.; Dahl, S., Support Effects and Catalytic Trends for Water Gas Shift Activity of Transition Metals. J. Mol. Catal. A: Chem. 2010, 315, 163-170. 5. Grabow, L. C.; Gokhale, A. A.; Evans, S. T.; Dumesic, J. A.; Mavrikakis, M., Mechanism of the Water Gas Shift Reaction on Pt: First Principles, Experiments, and Microkinetic Modeling. J. Phys. Chem. C 2008, 112, 4608-4617. 6. Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M., On the Mechanism of LowTemperature Water Gas Shift Reaction on Copper. J. Am. Chem. Soc. 2008,

130, 1402-1414. 7. Mhadeshwar, A. B.; Vlachos, D. G., Microkinetic Modeling for WaterPromoted CO Oxidation, Water−Gas Shift, and Preferential Oxidation of CO on Pt. J. Phys. Chem. B 2004, 108, 15246-15258. 8. Madon, R. J.; Braden, D.; Kandoi, S.; Nagel, P.; Mavrikakis, M.; Dumesic, J. A., Microkinetic Analysis and Mechanism of the Water Gas Shift Reaction over Copper Catalysts. J. Catal. 2011, 281, 1-11. 9. Cui, Y., et al., Participation of Interfacial Hydroxyl Groups in the Water-Gas Shift Reaction over Au/MgO Catalysts. Catal. Sci. Technol. 2017, 7, 5257-5266. 10. Gorte, R. J.; Zhao, S., Studies of the Water-Gas-Shift Reaction with CeriaSupported Precious Metals. Catal. Today 2005, 104, 18-24. 11. Hilaire, S.; Wang, X.; Luo, T.; Gorte, R. J.; Wagner, J., A Comparative Study 46 ACS Paragon Plus Environment

Page 46 of 53

Page 47 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


of Water-Gas-Shift Reaction over Ceria Supported Metallic Catalysts. Applied

Catalysis A: General 2001, 215, 271-278. 12. Tibiletti, D.; Meunier, F. C.; Goguet, A.; Reid, D.; Burch, R.; Boaro, M.; Vicario, M.; Trovarelli, A., An Investigation of Possible Mechanisms for the Water–Gas Shift Reaction over A ZrO2-Supported Pt Catalyst. J. Catal. 2006,

244, 183-191. 13. Ang, M. L.; Miller, J. T.; Cui, Y.; Mo, L.; Kawi, S., Bimetallic Ni-Cu Alloy Nanoparticles Supported on Silica for the Water-Gas Shift Reaction: Activating Surface Hydroxyls via Enhanced CO Adsorption. Catal. Sci. Technol. 2016, 6, 3394-3409. 14. Ammal, S. C.; Heyden, A., Titania-Supported Single-Atom Platinum Catalyst for Water-Gas Shift Reaction. Chem. Ing. Tech. 2017, 89, 1343-1349. 15. Gu, X.-K.; Huang, C.-Q.; Li, W.-X., First-principles Study of Single Transition Metal Atoms on ZnO for the Water Gas Shift Reaction. Catal. Sci. Technol. 2017,

7, 4294-4301. 16. Aranifard, S.; Ammal, S. C.; Heyden, A., On the Importance of Metal-Oxide Interface Sites for the Water-Gas Shift Reaction over Pt/CeO2 Catalysts. J. Catal. 2014, 309, 314-324. 17. Ammal, S. C.; Heyden, A., Water-Gas Shift Activity of Atomically Dispersed Cationic Platinum versus Metallic Platinum Clusters on Titania Supports. ACS

Catal. 2017, 7, 301-309. 18. Chiang, H.-N.; Jiang, J.-C., Density Functional Theory Study of Water-GasShift Reaction on 3Cu/α-Al2O3(0001) Surface. J. Phys. Chem. C 2013, 117, 12045-12053. 19. Song, W.; Hensen, E. J. M., Mechanistic Aspects of the Water-Gas Shift Reaction on Isolated and Clustered Au Atoms on CeO2(110): A Density Functional Theory Study. ACS Catal. 2014, 4, 1885-1892. 20. Foppa, L.; Margossian, T.; Kim, S. M.; Muller, C.; Coperet, C.; Larmier, K.; Comas-Vives, A., Contrasting the Role of Ni/Al2O3 Interfaces in Water-Gas Shift and Dry Reforming of Methane. J. Am. Chem. Soc. 2017, 139, 17128-17139. 47 ACS Paragon Plus Environment


21. Vecchietti, J.; Bonivardi, A.; Xu, W.; Stacchiola, D.; Delgado, J. J.; Calatayud, M.; Collins, S. E., Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts. ACS Catal. 2014, 4, 2088-2096. 22. Yang, L.; Karim, A.; Muckerman, J. T., Density Functional Kinetic Monte Carlo Simulation of Water-Gas Shift Reaction on Cu/ZnO. J. Phys. Chem. C 2013, 117, 3414-3425. 23. Ren, Z.; Peng, F.; Chen, B.; Mei, D.; Li, J., A Combined Experimental and Computational Study of Water-Gas Shift Reaction over Rod-Shaped Ce0.75M0.25O2 (M = Ti, Zr, and Mn) Supported Cu Catalysts. Int. J. Hydrogen

Energy 2017, 42, 30086-30097. 24. Chen, Y.; Wang, H.; Burch, R.; Hardacre, C.; Hu, P., New Insight into Mechanisms in Water-Gas-Shift Reaction on Au/CeO2(111): A Density Functional Theory and Kinetic Study. Faraday Discuss. 2011, 152, 121-133. 25. Zhao, Z.-J.; Li, Z.; Cui, Y.; Zhu, H.; Schneider, W. F.; Delgass, W. N.; Ribeiro, F.; Greeley, J., Importance of metal-oxide interfaces in heterogeneous catalysis: A combined DFT, microkinetic, and experimental study of water-gas shift on Au/MgO. J. Catal. 2017, 345, 157-169. 26. Sun, K.; Kohyama, M.; Tanaka, S.; Takeda, S., Reaction Mechanism of the Low-Temperature Water-Gas Shift Reaction on Au/TiO2 Catalysts. J. Phys.

Chem. C 2017, 121, 12178-12187. 27. Kugai, J.; Miller, J. T.; Guo, N.; Song, C., Oxygen-Enhanced Water Gas Shift on Ceria-Supported Pd–Cu and Pt–Cu Bimetallic Catalysts. J. Catal. 2011, 277, 46-53. 28. Kugai, J.; Miller, J. T.; Guo, N.; Song, C., Role of Metal Components in Pd– Cu Bimetallic Catalysts Supported on CeO2 for the Oxygen-Enhanced Water Gas Shift. Applied Catalysis B: Environmental 2011, 105, 306-316. 29. Kugai, J.; Fox, E. B.; Song, C., Role of CeO2 Support for Pd-Cu Bimetallic Catalysts for Oxygen-Enhanced Water Gas Shift. Applied Catalysis A: General 2013, 456, 204-214. 48 ACS Paragon Plus Environment

Page 48 of 53

Page 49 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


30. Kugai, J.; Fox, E. B.; Song, C., Kinetic Characteristics of Oxygen-Enhanced Water Gas Shift on CeO2-Supported Pt–Cu and Pd–Cu Bimetallic Catalysts.

Applied Catalysis A: General 2015, 497, 31-41. 31. Kresse, G.; Furthmüller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15-50. 32. Kresse, G.; Hafner, J., Ab initio molecular dynamics for liquid metals. Phys.

Rev. B: Condens. Matter 1993, 47, 558. 33. Blöchl, P. E., Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953. 34. Hammer, B.; Hansen, L. B.; Nørskov, J. K., Improved Adsorption Energetics within Density-Functional Theory Using Revised Perdew-Burke-Ernzerhof Functionals. Phys. Rev. B 1999, 59, 7413. 35. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 36. Perdew, J. P.; Chevary, J.; Vosko, S.; Jackson, K. A.; Pederson, M. R.; Singh, D.; Fiolhais, C., Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev.

B 1992, 46, 6671. 37. Lazić, P.; Alaei, M.; Atodiresei, N.; Caciuc, V.; Brako, R.; Blügel, S., Density Functional Theory with Nonlocal Correlation: A Key to the Solution ofthe CO Adsorption Puzzle. Phys. Rev. B 2010, 81, 045401. 38. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A Consistent and Accurate Ab

Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. The Journal of Chemical Physics 2010, 132, 154104. 39. Grimme, S.; Ehrlich, S.; Goerigk, L., Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456-1465. 40. Alfonso, D. R.; Cugini, A. V.; Sholl, D. S., Density Functional Theory Studies of Sulfur Binding on Pd, Cu and Ag and Their Alloys. Surf. Sci. 2003, 546, 12-26. 49 ACS Paragon Plus Environment


41. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations.

Phys. Rev. B 1976, 13, 5188-5192. 42. Fronzi, M.; Soon, A.; Delley, B.; Traversa, E.; Stampfl, C., Stability and Morphology of Cerium Oxide Surfaces in an Oxidizing Environment: A FirstPrinciples Investigation. The Journal of Chemical Physics 2009, 131, -. 43. Wang, Z. L.; Feng, X., Polyhedral Shapes of CeO2 Nanoparticles. J. Phys.

Chem. B 2003, 107, 13563-13566. 44. Gerward, L.; Staun Olsen, J.; Petit, L.; Vaitheeswaran, G.; Kanchana, V.; Svane, A., Bulk Modulus of CeO2 And PrO2—an Experimental and Theoretical Study. J. Alloys Compd. 2005, 400, 56-61. 45. Anisimov, V. I.; Zaanen, J.; Andersen, O. K., Band theory and Mott insulators: Hubbard U instead of Stoner I. Phys. Rev. B 1991, 44, 943-954. 46. Dudarev, S.; Botton, G.; Savrasov, S.; Humphreys, C.; Sutton, A., Electronenergy-loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+ U Study. Phys. Rev. B 1998, 57, 1505-1509. 47. Kropp, T.; Paier, J.; Sauer, J., Interactions of Water with the (111) and (100) Surfaces of Ceria. J. Phys. Chem. C 2017, 121, 21571-21578. 48. Graciani, J., et al., Highly Active Copper-Ceria and Copper-Ceria-Titania Catalysts For Methanol Synthesis from CO2. Science 2014, 345, 546. 49. Sheppard, D.; Terrell, R.; Henkelman, G., Optimization methods for finding minimum energy paths. J. Chem. Phys. 2008, 128, 134106. 50. Henkelman, G.; Jónsson, H., A Dimer Method for Finding Saddle Points on High Dimensional Potential Surfaces Using Only First Derivatives. J. Chem.

Phys. 1999, 111, 7010. 51. Henkelman, G.; Jónsson, H., Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J.

Chem. Phys. 2000, 113, 9978. 52. 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, 9901. 50 ACS Paragon Plus Environment

Page 50 of 53

Page 51 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


53. McQuarrie, D. A.; Simon, J. D., Molecular Thermodynamics; Univ Science Books, 1999. 54. Chorkendorff, I.; Niemantsverdriet, J. W., Concepts of Modern Catalysis And

Kinetics; John Wiley & Sons, 2006. 55. Dumesic, J. A.; Rudd, D. F.; Aparicio, L. L.; Rekoske, J. E.; Trevino, A. A.,

The Microkinetics of Heterogeneous Catalysis; American Chemical Society: Washington, D.C., 1993. 56. Luo, W.; Asthagiri, A., Density Functional Theory Study of Methanol Steam Reforming on Co(0001) and Co(111) Surfaces. J. Phys. Chem. C 2014, 118, 15274-15285. 57. Fronzi, M.; Piccinin, S.; Delley, B.; Traversa, E.; Stampfl, C., Water Adsorption on the Stoichiometric and Reduced CeO2(111) Surface: A FirstPrinciples Investigation. Phys. Chem. Chem. Phys. 2009, 11, 9188-9199. 58. Gunasooriya, G. T. K. K.; van Bavel, A. P.; Kuipers, H. P. C. E.; Saeys, M., CO Adsorption on Cobalt: Prediction of Stable Surface Phases. Surf. Sci. 2015,

642, L6-L10. 59. Clay, J. P.; Greeley, J. P.; Ribeiro, F. H.; Nicholas Delgass, W.; Schneider, W. F., DFT Comparison of Intrinsic WGS Kinetics over Pd and Pt. J. Catal. 2014,

320, 106-117. 60. Campbell, C. T., Micro- and macro-kinetics: Their relationship in heterogeneous catalysis. Top. Catal. 1994, 1, 353-366. 61. Campbell, C. T., The Degree of Rate Control: A Powerful Tool for Catalysis Research. ACS Catal. 2017, 7, 2770-2779. 62. Hansen, H. A.; Wolverton, C., Kinetics and Thermodynamics of H2O Dissociation on Reduced CeO2(111). J. Phys. Chem. C 2014, 118, 27402-27414. 63. Mars, P.; van Krevelen, D. W., Oxidations Carried Out by Means of Vanadium Oxide Catalysts. Chem. Eng. Sci. 1954, 3, 41-59. 64. Bunluesin, T.; Gorte, R. J.; Graham, G. W., Studies of the Water-Gas-Shift Reaction on Ceria-Supported Pt, Pd, And Rh: Implications for Oxygen-Storage Properties. Applied Catalysis B: Environmental 1998, 15, 107-114. 51 ACS Paragon Plus Environment


65. Burch, R.; Chappell, R. J.; Golunski, S. E., Synergy Between Copper and Zinc Oxide During Methanol Synthesis. Transfer of Activating Species. J. Chem.

Soc. Faraday Trans. I 1989, 85, 3569-3578. 66. Walker, E.; Ammal, S. C.; Terejanu, G. A.; Heyden, A., Uncertainty Quantification Framework Applied to the Water–Gas Shift Reaction over PtBased Catalysts. J. Phys. Chem. C 2016, 120, 10328-10339. 67. Gautier, S.; Steinmann, S. N.; Michel, C.; Fleurat-Lessard, P.; Sautet, P., Molecular Adsorption at Pt(111). How Accurate are DFT Functionals? Phys.

Chem. Chem. Phys. 2015, 17, 28921-28930. 68. Zhao, Z.-J.; Li, Z.; Cui, Y.; Zhu, H.; Schneider, W. F.; Delgass, W. N.; Ribeiro, F.; Greeley, J., Importance of Metal-Oxide Interfaces in Heterogeneous Catalysis: A Combined DFT, Microkinetic, and Experimental Study of Water-Gas Shift on Au/MgO. J. Catal. 2017, 345, 157-169. 69. Deringer, V. L.; Tchougréeff, A. L.; Dronskowski, R., Crystal Orbital Hamilton Population (COHP) Analysis As Projected from Plane-Wave Basis Sets. J. Phys.

Chem. A 2011, 115, 5461-5466. 70. Maintz, S.; Deringer Volker, L.; Tchougréeff Andrei, L.; Dronskowski, R., Analytic projection from plane‐wave and PAW wavefunctions and application to chemical‐bonding analysis in solids. J. Comput. Chem. 2013, 34, 2557-2567. 71. Maintz, S.; Deringer Volker, L.; Tchougréeff Andrei, L.; Dronskowski, R., LOBSTER: A Tool to Extract Chemical Bonding from Plane‐Wave Based DFT. J.

Comput. Chem. 2016, 37, 1030-1035.


Page 52 of 53

Page 53 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


Table of Contents Graphic


CeO2 Catalyzed Water-Gas Shift Reaction

Recommend Documents