Water-gas shift

Water dissociation is a rate limiting step in many industrially important chemical reactions. In this investigation, climbing image nudged elastic band (CINEB) method, within the framework of density functional theory, is used to report the activation energies (E a) of water dissociation on Cu(111) surface with a vacancy. Introduction of vacancy results in a reduced coordination of the dissociated products, which facilitates their availability for reactions that involve water dissociation as an intermediate step. Activation energy for dissociation of water reduces by nearly 0.2 eV on Cu(111) surface with vacancy, in comparison with that of pristine Cu(111) surface. We also find that surface modification of the Cu upper surface is one of the possible pathways to dissociate water when the vacancy is introduced. Activation energy, and the minimum energy path (MEP) leading to the transition state remain same for various product configurations. CINEB corresponding to hydrogen gas evolution is also performed which shows that it is a two step process involving water dissociation. We conclude that the introduction of vacancy facilitates the water dissociation reaction, by reducing the activation energy by about 20%.

Platinum nanoparticles supported on single CeO 2 and TiO 2 metal oxides and Ce 0.8 Ti 0.2 O 2−ı solid solution were prepared to investigate the effect of Ti 4+-doping of ceria on important mechanistic and kinetic aspects of the Water–Gas Shift (WGS) reaction in the 200–300 • C range, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS, and (ii) the prevailing mechanistic path among " redox " and " associative " both proposed in the literature. The relationship between the chemical nature of dopant (Zr 4+ , Ti 4+ and La 3+) and the concentration of active C-pool and H-pool of reaction intermediates as well as that of specific rate per gram basis (r CO , mol g −1 s −1) for the ceria-doped supported Pt is illustrated for the first time based on relevant results previously reported (Zr 4+ and La 3+-doped ceria). The 0.5 wt% Pt supported on Ce 0.8 Ti 0.2 O 2−ı (Ti 4+-doped CeO 2) exhibits significantly higher WGS activity in terms of CO conversion (%) and specific kinetic rate (mol CO g −1 s −1 or mol CO cm −1 s −1) compared to Pt/CeO 2 , Pt/TiO 2 , Pt/Ce 0.8 La 0.2 O 2−ı and Pt/Ce 0.5 Zr 0.5 O 2−ı catalysts. This was explained mainly by: (i) the larger concentration of active C-pool of reaction intermediates formed around each Pt nanoparticle, and (ii) the higher reactivity of sites (k, s −1) along the Pt-support interface responsible for CO 2 and H 2 formation. A very good correlation between the concentration of active C-pool and the specific reaction rate, r CO (mol g −1 s −1) as a function of the dopant (Zr 4+ , La 3+ and Ti 4+) was found. The concentration of labile surface oxygen and its mobility in Ce 0.8 Ti 0.2 O 2−ı compared to CeO 2 (undoped), La 3+ or Zr 4+-doped ceria are also important factors. It is proposed that on Pt/CeO 2-doped catalysts the WGS reaction follows both the " redox " and " associative formate " mechanisms , where the extent of participation of each mechanism depends on the chemical nature of the dopant (Zr 4+ , La 3+ and Ti 4+).