AN IN-SITU FOURIER-TRANSFORM INFRARED STUDY OF FORMIC-ACID ADSORPTIONON A POLYCRYSTALLINE SILVER CATALYST

Infrared spectra are reported for the adsorption of formic acid on a p olycrystalline silver catalyst after various degrees of oxidation. Thr ee distinct chemisorbed species were identified, two of which correspo nded to adsorbed formate on Ag(110) and Ag(111) crystal planes and the other to adsorbed formate on a silver site modified by the presence o f subsurface oxygen. Moreover, it was discovered that subsurface oxyge n species were primarily located in the vicinity of grain boundary def ects. Desorption profiles suggested that coadsorbed oxygen moieties co uld destabilize chemisorbed formate. Because of the invariance in infr ared band positions associated with formate species, regardless of whe ther or not neighbouring atomic oxygen species were present, a kinetic stabiiization mechanism is proposed. The pivotal role of subsurface o xygen species found in the region of grain boundaries, in the mechanis m for the selective oxidation of methanol to formaldehyde, is emphasis ed. (C) 1994 Academic Press, Inc.