Decomposition of formic acid on copper, nickel, and copper-nickel alloys: III. Catalytic decomposition on nickel and copper-nickel alloys

Abstract

Turnover rates for the catalytic dehydrogenation and dehydration of formic acid (HCOOH) on Ni SiO 2 and Ni and CuNi powders between 390 and 490K are reported. The decomposition rate constants calculated from these data are compared with values reported previously for the temperature-programmed decomposition (TPD) of HCOOH preadsorbed on Ni and CuNi. On these, catalytic dehydrogenation and dehydration seem to proceed through the same intermediate, believed to be a formate ion. The selectivity S, defined as the ratio of dehydrogenation to dehydration turnover rates, is independent of temperature, and between 4 and 6 on Ni and alloys with a surface Cu fraction less than 0.8. The observed selectivity is not affected by secondary reactions among the products. The intermediate appears to be bound to two adjacent Ni atoms. In contrast, TPD of HCOOH preadsorbed at low coverage on CuNi single crystals seems to involve a formic anhydride species that requires four adjacent Ni atoms, and decomposes with S equal to unity and a rate constant 5 × 10 3 times greater than that corresponding to the catalytic decomposition. Differences between catalytic and TPD data apparently arise from the effect of surface coverage on the relative surface density of formate and formic anhydride. Formate requires small nickel ensembles and predominates on fully covered surfaces typical of the catalytic reaction. This study demonstrates the differences in rate constant and S when reactive intermediates change with surface coverage because they require different ensembles for adsorption; in other words, they are structure sensitive.