Quantitative Determination of the Catalytic Activity of Bulk Metal Oxides for Formic Acid Oxidation

The dissociative chemisorption of formic acid, HCOOH, to surface formate species, HCOO-M, on metal oxide catalysts was examined. The chemisorption studies reveal an average of ∼5–6 μmol/m2 of active surface sites for many of the metal oxide catalysts. From TPD experiments, the decomposition temperatures of the surface formate (Tp) were found to vary from 88 to 313°C. The turnover frequency (TOF) values for formic acid oxidation were found to vary over 11 orders of magnitude (10−3–108 s−1) at 250°C. No correlations were found between (i) H2-TPR onset temperatures and the enthalpies of formation of bulk metal oxides (−ΔHf), (ii) TOFs and H2-TPR onset temperatures, or (iii) TOFs and the isotopic dioxygen exchange rates with the metal oxide lattice oxygen. Weak inverse correlations were found for both TOFs and catalytic activities versus −ΔHf. A strong inverse relationship was found between the TOFs and the surface formate decomposition temperatures, supporting the assertion that the rate-determining step during formic acid oxidation is the decomposition of the surface formate intermediate.