Formic acid decomposition was used to study the surface chemical effects of oxygen, titania, and alumina adlayers on nickel through temperature-programmed reaction spectroscopy under ultra-high-vacuum conditions. Preexposing a clean, polycrystalline nickel surface to 1 langmuir of O2 at 300 K altered the decomposition rate of formate intermediates from 2 × 1013 exp(-96 kJ·mol-1/RT) s-1 on clean nickel to 3 × 1011 exp(-90 kJ·mol-1/RT) s-1. The CO2/CO selectivity ratio during formic acid decomposition increased to 11.8 from the value of 4.4 on clean nickel. Titania adspecies at a coverage of ca. 0.7 monolayers altered the decomposition rate to 4 × 1012 exp(-98 kJ·mol-1/RT) s-1 and decreased the CO2/CO selectivity to 1.8. For both oxygen and titania, activity and selectivity shifts exhibited smooth trends with increasing surface concentrations of the respective adspecies. Qualitatively similar results were found for alumina- and titania-containing Ni surfaces, although alumina appeared to be more poorly dispersed over the nickel surface than titania. These effects are rationalized in terms of a decomposition mechanism in which formic acid adsorbs dissociatively to form both formate and formyl intermediates. Surface oxygen or oxidic species are suggested to stabilize the formyl intermediates relative to the clean nickel surface. Isotopic labeling with 18O showed that whereas oxygen adatoms could participate directly in the decomposition process and appear in the product spectrum, titanium and aluminum bind oxygen strongly to the surface and prevent surface oxygen from appearing in the decomposition products.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry