DECOMPOSITION OF FORMIC-ACID ON NI (100)

The decomposition of HCOOD was studied on Ni(100). Low temperature adsorption of HCOOD resulted in the desorption of D2O, CO2, CO, and H2. The D2O was evolved below room temperature. CO2 and H2 were evolved in coincident peaks at a temperature above that at which h2 desorbed following H2 adsorption and well above that for CO2 desorption from CO2 adsorption; CO desorbed primarily in a desorption limited step. The decomposition of formic acid on the clean surface was found to yield equal amounts of H2, CO, and CO2 within experimental error. The kinetics and mechanism of the decomposition of formic acid on Ni (110) and Ni(100) single crystal surfaces were compared. The reaction proceeded by the dehydration of formic acid to formic anhydride on both surfaces. The anhydride intermediate condensed into islands due to attractive dipole-dipole interactions. Within the islands the rate of the decomposition reaction to form CO2 was given by: Rate = 6 × 1015 exp{-[25,500 + ω( c csat)]/RT} × c, where c is the local surface concentration, csat is the saturation coverage for the particular crystal plane, and ω is the interaction potential. The interaction potential was determined to be 2.7 kcal/mole on Ni(110) and 1.4 kcal/mole on Ni(100); the difference observed was due to structural differences of the surfaces relating to the alignment of the dipole moments within the islands. These attractive interactions resulted in an autocatalytic reaction on Ni(110), whereas the interaction was not strong enough on Ni(100) to sustain the autocatalytic behavior. Formic acid decomposition oxidized the Ni(100) surface resulting in the formation of a stable surface oxide. The buildup of the oxide resulted in a change in the selectivity reducing the amount of CO formed. This trend indicated that on the oxide surface the decomposition proceeded via a formate intermediate as on Ni(110) O. © 1979.