Theoretical Study Of Sulfur "Poisoning" Effect On Transition Metal Surface And Water-Gas Shift Reaction

Catalysts, which have played a vital role in human activities and chemical industries, maybe the unique way to deal with current energy and environmental crisis. Scientists have beenworking on the design of novel catalysts with high efficiency at low cost, high selectivity andhigh stability. However, the process is often hampered by the inherent complexity of thestructures on catalysts and the reaction mechanism.Residual sulfur (S) contained species have always act as a "poinsoner" on traditionaltransition metal catalysts (i.e., Pd, Pt), and would induce remarkably negative effects on theperformance of these catalysts. However, the negative effects "disappear" unexpectedly onthe Au(111) surface. This phenomenon is heavily curious to many scientists in heterogeneouscatalysis, and is expected to have potential applications in catalyst design. In addition, thewater-gas shift (WGS) reaction is an important chemical process for maximizing hydrogenproduction and removing residual CO in H₂fuel cell systems, and thus attracts increasinglyinterest in heterogeneous catalysis. In this dissertation, density functional theory calculationswere performed to understand the origin of several experimental observations aiming atproviding some theoretical guidance to the design of novel catalysts with high performance inexperiments. Several achievements have been made in the following:Firstly, through the ab initio atomic thermodynamics, the structural phase transition ofpotassium (K) adsorption on Au(111) with K coverage ranged from0.11ML to0.50ML iswell identified. Subsequently, we propose a possible surface configuration with astoichiometry of K₂Au for the (2×2) phase observed by LEED. Then, the promotional effectsof sodium (Na) on CO adsorption and dissociation are investigated to ascertain whether thereis a predominant long-range character in the AM-CO interactions. It is demonstrated that thepre-adsorbed Na atom does induce a long-range effect on CO adsorption, while the effect isinsufficient to destabilize the C-O bond to an extent for CO dissociation. The promoted COdissociation is mainly ascribed to the short-range O-Na attraction.Secondly, we focus on the sulfur (S) effects on transition metal surfaces. Through acomparative study of S and CO coadsorption Au(111) and Pd(111), we propose a chargeeffect model, which, for the first time, successfully explains the extraordinary S enhanced CO adsorption on Au surface. Via the model, we further predict that S can also stabilize NOadsorption on Au and Ag surfaces. Subsequently, we suggest that the PdAu bimetllic surfaceobtained from alloying Pd and Au may exhibit a better S poison-resistance with respect toPd(111) and a better activity in comparison with Au(111).Thirdly, through the comparison of H₂O and CO dissociation on a series of CuNi bimetallicsurfaces, the origin of Ni effects (both positive and negative ones) on water-gas-shift reactionis illustrated. Additionally, we suggest CuNi catalysts with highly dispersed Ni sites shouldexhibit high performance to WGS. We also propose a developed Cu-Ni system, namely,NiO_1-x/Cu, which may show both excellent activity and selectivity to WGS.