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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