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

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 (Delta H) 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 Delta H 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 Delta H, 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.