Trends of Water Gas Shift Reaction on Close-Packed Transition Metal Surfaces

Abstract

The mechanism of the water gas shift reaction (WGSR) on the close-packed transition metal surfaces of Co, Ni, Cu (from the 3d row), Rh, Pd, Ag (from the 4d row), Ir, Pt, and Au (from the 5d row) has been systematically examined by periodic density functional theory (DFT) calculations. The comparison of potential energy surface (PES) concludes that WGSR activity is influenced by two kinds of elementary steps: O−H bond dissociation and C−O bond formation. Activation barriers (Ea) and reaction energies (ΔH) on a series of metal surfaces show good BEP relationship; however, their energetic trends are opposite in these two kinds of steps. In O−H bond dissociation steps, trends of Ea and ΔH are groups 9 < 10 < 11 and 3d < 4d < 5d. On the other hand, C−O bond formation steps on the surfaces of the lower-right metals in the d block (Cu, Ag, Pt, Au) have relatively lower Ea and ΔH, which is responsible for their high WGSR activity of metal/oxide catalysts. In addition, the fundamental of energetic trends has been examined from the analyses of adsorption energy, density of state (DOS), and charge density. The result shows that the surfaces of upper-left d-block metals (Co, Ni, Rh) with higher energy and smaller delocalization of their d orbitals yield a stronger adsorption energy with higher induced charges that will stabilize dissociating fragments to lower the barrier and retard desorptions to lift the barrier in O−H bond dissociation and C−O bond formation steps, respectively. The prediction of energetic trends in the present work is also appropriate for other catalytic reactions, such as ethanol decomposition and CO oxidation, and can help us scientifically design a better catalyst for the desired reaction.

CO-ADSORPTION OF CO, H2O AND MECHANISM OF WATER GAS SHIFT REACTION ON ZnO CATALYST SURFACE: A DENSITY FUNCTIONAL THEORY STUDY

“…One H-O2 bond length of H 2 O is slightly elongated from 1.00 Å to 1.08 Å after co-adsorption on the surface. Our calculations in co-adsorption energy (-1.41 eV) show a better result than from other study on metal surface [11,41]. This is able to say that the performances of WGSR mechanism on metal oxide surfaces (ZnO) in this works are more effectively than that on metal surfaces.…”

“…The first route, the previous studies [9,11] conclude that CO cannot directly react with stable H 2 O on the surface which we are thus not considering in this study. The second route, causing H 2 O is dissociated or 2OH recombined to O and 2H on the surface.…”

“…Thus, in addition to the toxic CO removal, water gas shift reaction is considered as a promising step for extra hydrogen production. It has been known that many experimental and theoretical calculations of this reaction were investigated in previous works [9][10][11][12][13]. However, the adsorption and mechanism calculations of WGSR on ZnO 0 1 10 catalyst surface fail to show what we will be mainly discussed in this study.…”

“…Hereafter, COOH cis formation has two scenarios of reaction on the surface including, (i) it decomposes to CO 2 and H, and (ii) the rotation of COOH cis to COOH trans configuration. However, it has been proved in previous investigations [10,11,42] to conclude that COOH cis rotatation to COOH cis configuration is more favorable than COOH cis decomposition. As such, we have consider step 3 as the rotation from COOH cis to COOH trans on ZnO (1010) catalyst surface.…”

Structuring Pt/CeO2/Al2O3 WGS catalyst: Introduction of buffer layer

“…However, whatever the mechanism most authors propose the water activation step as the rate limiting one for the WGS reaction on a wide selection of metallic active phases including Co, Cu, Ni, Ag, Rh, Ir, Au and Pt [18][19][20][21][22]. More precisely, Phatak et al [22] proposed the hydroxyl dissociation as rate-limiting step for Pt and other noble metals on comparing the binding energies of H2O, OH and H species on metals.…”