ARTICLE pubs.acs.org/JPCC
Mechanistic Studies of WaterGas-Shift Reaction on Transition Metals Chia-Hao Lin, Chung-Liang Chen, and Jeng-Han Wang* Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan
bS Supporting Information ABSTRACT: A density functional theory (DFT) calculation has been carried out to investigate a watergas-shift reaction (WGSR) on a series of chemical related materials of Co, Ni, and Cu (from the 3d row); Rh, Pd, and Ag (from the 4d row); and Ir, Pt, and Au (from the 5d row). The result shows that WGSR mechanism involves the redox, carboxyl, and formate pathways, which correspond to CO* + O* f CO2(g), CO* + OH* f COOH* f CO2(g) + H*, and CO* + H* + O* f CHO* + O* f HCOO** f CO2(g) + H*, respectively. The reaction barriers in the three pathways are competitive and have a similar trend that groups 9 > 10 > 11 and 3d > 4d >5d. Thus, the bottom-right d-block metals (Cu, Pt, and Au) show better WGSR activity. The experimentally most observed formate can be attributed to its lower formation and higher dissociation barriers. Furthermore, the catalytic behavior on these active metal surfaces has been examined. The result shows that WGSR is mostly follows the redox pathway on Au(111) surface due to the negligible CO* oxidation barriers; on the other hand, all the three pathways contribute similarly in WGSR on Cu(111) and Pt(111) surfaces. Finally, the feasible steps of formyl in FischerTropsch synthesis (FTS), the combustion reaction, and formate pathway, CHO* f CH* + O*, CHO* f CO* + H*, and CHO* + O* f HCOO**, respectively, have also been studied. The result shows that activities of FTS and the WGSR have opposite trends on these metal surfaces.
1. INTRODUCTION Hydrogen, the most efficient and cleanest fuel, can be considered as the best answers for the limited fossil fuel and the problem of the climate change. Currently, hydrogen is mainly produced from the reforming of hydrocarbon feedstocks.1 The reformed fuel can be readily applied to fuel cells and other industrial applications unless the detrimental CO, a main side product from the reforming, can be efficiently removed. The watergas-shift reaction (WGSR), CO + H2O f CO2 + H2, ΔH298 = 41.2 kJ/mol, is considered to be one of the most powerful methods to remove CO along with the additional hydrogen production. WGSR has been widely applied in the chemical industry and attracted much scientific interests in the past few decades due to its importance. However, the detail mechanism of WGSR is not fully understood and some inconsistencies between experimental and computational results are still under debate. Numerous experimental and computational efforts have been conducted to find the best catalyst and elucidate the mechanism. As concluded from the recent reviews,24 WGSR shows the better performance on the metals of Cu, Pt, and Au supported on the oxides and their mechanisms mainly include the redox, carboxyl, and formate pathways. Experimentally, WGSR mechanisms on the promising catalysts for the supported Cu, Pt and Au have been extensively studied by various in situ techniques, STM, TPR, XRD, XANES, and DRIFTS, and common ex situ characterization methods, XPS, AES, UPS and TEM.526 These studies show that both oxides and metals have significant influences in WGSR activity. Oxides mainly help for water dissociation forming OH*, O*, and H*, and their oxygen vacancy plays an r 2011 American Chemical Society
important role in the reactions.57 The resultant fragments will oxidize CO*, which is preferentially adsorbed on metal surfaces, and eventually forming CO2(g) and H2(g) desorbing from surfaces. Computationally, WGSR mechanisms on the pure metal surfaces of Cu, Pt, and Au, and metal/oxide catalysts of Au/ CeO2, (Au, Cu)/TiO2, Pt/CeO2, and (TiOx, CeOx)/(Cu, Au) have been widely elucidated by first principles calculations to understand the optimal WGSR activities on Cu, Pt, and Au-based catalysts.5,6,8,11,14,2732 These works conclude that WGSR mostly follows the redox and carboxyl pathways, in which CO* interacts with O* forming CO2(g) and OH* forming COOH*, respectively, because of their energetic preference. The rate determining step of WGSR on pure metal surfaces is in the initial water dissociation. The high dissociation barriers of water can be reduced more than a half while the reaction is occurring on oxides or by a hydrogen abstracting process.27,28,33,34 Among these experimental and computational works, it is not yet clear whether the formate pathway plays an important role or is considered as a minor channel in WGSR. Experimentally, formate is the most observable intermediate and is bidentately adsorbed on the surface through its two O atoms. This intermediate shows a significant influence in WGSR activity on the catalysts, such as Pt/ZrO2 and Pt/CeO2.9,10,1517 On the other hand, some experiments2,1823 show that formate could not be the main reaction intermediate in the production of CO2 on Au/CeO2, Au/Ce-LaO2, Au/Ce-ZrO2, Pt/TiO2, and Pt/Al2O3 catalysts Received: April 13, 2011 Revised: August 24, 2011 Published: August 25, 2011 18582
dx.doi.org/10.1021/jp2034467 | J. Phys. Chem. C 2011, 115, 18582–18588
The Journal of Physical Chemistry C and might even degrade WGSR activity due to the blockage of surface active sites. Computationally, surface formate is considered only as a spectator in all of the computational works27,28,31,32,3537 due to its high formation and decomposition barriers. From these works, formate can be formed via three possible routes: (i) CO2* interacts with H2O* and its fragments,27,28 (ii) CO* reacts with OH* or H2O*,31,32 and (iii) CO* directly inserts into neighboring OH*,3537 showing that the formate pathway is energetically unfavorable compared to the redox and carboxyl ones. In addition, the resulted formate is hard to be desorbed from the surface or dissociated to CO2 because of its stronger adsorption energy and higher CH bond breaking barrier, respectively. Thus, formate is not considered as an active species in WGSR. Herein, we computationally investigate WGSR on the catalytically active metals of 3d (Co, Ni, and Cu), 4d (Rh, Pd, and Ag), and 5d (Ir, Pt, and Au) by density functional theory (DFT) calculations at the GGA-PW91 level to gain a fundamental understanding of WGSR mechanism. Although the (111) surfaces are not considered as the most active surfaces in WGSR experiments,24 these surfaces for Cu and Pt have been successfully applied in WGSR calculations27,28 and showed the promising results in the comparison with the experimental works on the interesting metal-based catalysts, such as Cu/ZnO and Pt/Al2O3. Also, the experimental works on the Cu(111) surface show similar rates and kinetic parameters to the results for the interesting Cu-based catalysts.25,26 In addition, the (111) surfaces has been commonly employed in the computational studies in heterogeneous catalytic reactions, such as ethanol decomposition,38 alcohol oxidations,39 ethanol synthesis,40 and oxygen oxidation reaction.41 Thus, the most stable (111) surfaces of these metals has been employed to computationally study WGSR. The key elementary steps in redox, carboxyl, and formate pathways have been systematically examined and compared on these metal surfaces. The resolved mechanism will be employed to clarify the catalytic trend on these metals and the preferential pathways in WGSR. In addition, the catalytic activities of WGSR, Fischer Tropsch synthesis (FTS) and the combustion reaction, the associated catalytic reactions, have also been preliminarily compared based on the resolved mechanism. These results can help us to understand the catalytic behavior in experiments, design better catalysts, and, therefore, move one step forward to enable hydrogen economy to the practical application.
2. METHODS The calculations are performed using Vienna ab initio simulation package (VASP),4244 implementing the DFT with a 3D periodic boundary condition. The exchange-correlation function is treated by the generalized gradient approximation45 with PerdewWang 1991 formulation (GGA-PW91).46 Combining the accuracy of augmented plane waves with the cost-effective pseudopotentials implemented in VASP, the projector-augmented wave method (PAW)47,48 is applied to the basis set. The kinetic cutoff energy of the plane-wave basis set is fixed at 600 eV. The Brillouin zone (BZ) integration is sampled at 0.05 2π Å1 interval in the reciprocal space by MonkhorstPack scheme.49 Higher cutoff energies (700 and 800 eV) and k-points values with smaller BZ sampling intervals (0.04 and 0.035 2π Å1) for the adsorption energy calculations of reactants (CO* and H2O*) on all the metal surfaces have been initially examined for the convergence test and show negligible difference ( 11, and delocalization, 3d < 4d
