The effect of La3+, Ti4+ and Zr4+ dopants on the mechanism of WGS on ceria-doped supported Pt catalysts

Abstract

Platinum nanoparticles supported on single CeO2 and TiO2 metal oxides and Ce0.8Ti0.2O2−δ solid solution were prepared to investigate the effect of Ti4+-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 (Zr4+, Ti4+ and La3+) and the concentration of active C-pool and H-pool of reaction intermediates as well as that of specific rate per gram basis (rCO, μmolg−1s−1) for the ceria-doped supported Pt is illustrated for the first time based on relevant results previously reported (Zr4+ and La3+-doped ceria). The 0.5wt% Pt supported on Ce0.8Ti0.2O2−δ (Ti4+-doped CeO2) exhibits significantly higher WGS activity in terms of CO conversion (%) and specific kinetic rate (μmolCOg−1s−1 or μmolCOcm−1s−1) compared to Pt/CeO2, Pt/TiO2, Pt/Ce0.8La0.2O2−δ and Pt/Ce0.5Zr0.5O2−δ 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 CO2 and H2 formation. A very good correlation between the concentration of active C-pool and the specific reaction rate, rCO (μmolg−1s−1) as a function of the dopant (Zr4+, La3+ and Ti4+) was found. The concentration of labile surface oxygen and its mobility in Ce0.8Ti0.2O2−δ compared to CeO2 (undoped), La3+ or Zr4+-doped ceria are also important factors. It is proposed that on Pt/CeO2-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 (Zr4+, La3+ and Ti4+).