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. (C) 2015 Elsevier B.V. All rights reserved.