Carboxylic acid deprotonation on the Ag(110) and Ag(111) surfaces

The surface chemistry of hydrocarbon carboxylic acids has been compared wit h that of perfluorinated carboxylic acids on the Ag(110) and Ag(111) surfac es. The hydrocarbon acids adsorb reversibly on the clean silver surfaces. O n Ag(110) their desorption energies increase linearly with increasing alkyl chain length (n) and are given by the expression DeltaE(des) = 10.6 + 0.9n kcal/mol. The surface chemistry of the perfluorinated acids is quite diffe rent from that of the hydrocarbon acids in the sense that they deprotonate to form perfluorocarboxylate species on both Ag(110) and Ag(111) surfaces. The perfluorocarboxylates are stable until the surface temperature reaches similar to 620 K, at which point they decompose yielding desorption Of CO2 and other decomposition fragments. On both Ag surfaces 2,2,2-trifluoroaceta te (CF3CO2) generated by deprotonation of 2,2,2-trifluoroacetic acid (CF3CO 2H) has been identified by its vibrational spectrum. The fact that fluorina ted acids deprotonate on the Ag surfaces while the hydrocarbon acids desorb during heating indicates that fluorination lowers the deprotonation barrie r, DeltaE(dep)(double dagger), to the point that it is lower than the desor ption energy, DeltaE(des).