Nanostructure and kinetic isotope effect of alkali-doped Pt/silica catalysts for water-gas shift and steam-assisted formic acid decomposition

•2%Pt/SiO2 doped with light alkali (Li, Na, K) displayed much higher WGS activity.•The Pt crystallites were highly dispersed (1–4 nm) and in contact with alkali.•A normal kinetic isotope effect was observed for WGS and formic acid decomposition.•The same decrease in NKIE was observed with Na-doping for WGS and FAD.•Mechanistic results are consistent with a weakening of the formate C–H bond by Na.

For alkali-doped silica supported Pt catalysts, STEM-EDX measurements indicate the alkali metal promoter is well dispersed over the support, existing both away from and in close proximity to 1–4 nm Pt crystallites. A normal kinetic isotope effect was identified in switching from CO + D2O to CO + H2O, and the magnitude of the KIE was identical to that of steam-assisted formic acid decomposition, where the label was placed at the C–H bond: HCOOH + H2O to DCOOH + H2O. Moreover, an identical decrease in the KIE of both reactions was identified following Na-doping, consistent with the proposal that Na electronically weakens the formate C–H bond. This finding suggests that an associative mechanism may operate, in line with previous findings over alkali doped Pt/zirconia. The results are consistent with a mechanism involving formate C–H bond breaking as the rate-limiting step of that pathway. However, one cannot rule out the presence of alternative (e.g., a related intermediate) or parallel pathways.

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