Knowing the heats of formation of adsorbed catalytic intermediates is an essential component in gaining the fundamental understanding of reaction mechanisms and the relationships between catalyst structure and activity necessary for rational design of heterogeneous catalysts. The current study utilizes the ultrahigh vacuum technique of single crystal adsorption calorimetry to determine the energetics of adsorbed formate species on Pt(111).

From previous literature, it is known that formic acid dosed on Pt(111) precovered with oxygen adatoms (O_ad) at 190K reacts completely to form adsorbed bidentate formate (HCOO_ad,bi) and gas phase water (H₂O_g) according to the reaction:

2 HCOOH_g + O_ad→ 2 HCOO_ad,bi + H₂O_g(1)

where HCOO_ad,bi bonds in a bridging configuration. The heat of Reaction (1) extrapolated to zero coverage was found to be -110 kJ/mol. A simple thermodynamic cycle that uses this and other known adsorption energies provides a heat of formation of HCOO_ad,bi of -418 kJ/mol.

Prior studies have shown that submonolayer coverages of formic acid on O-precovered Pt(111) at 130K first produce monodentate formate (HCOO_ad,mon) and adsorbed hydroxyl (OH_ad) via the reaction:

HCOOH_g + O_ad → HCOO_ad,mon + OH_ad (2)

where HCOO_ad,mon forms a single Pt—O bond to the surface. The heat of Reaction (2) extrapolated to zero coverage was found to be -93 kJ/mol, which provides a heat of formation of HCOO_ad,mon of -354 kJ/mol. At this same temperature, this monodentate formate converts to bidentate bonding, but at a rate that is slow relative to the 100 ms heat measurement time, so this slower reaction does not complicate thermodynamic measurement of the formation of this monodentate species. Together with our earlier measurements of the heats of formation of –OCH_3,ad and –OH_ad, these allow us to estimate reaction energies for conversion pathways between formate and methoxy intermediates in Pt catalysis.