A FOURIER TRANSFORM-INFRARED REFLECTION ABSORPTION-SPECTROSCOPY STUDY OF THE FORMATION AND DECOMPOSITION OF CHEMISORBED FORMATE ON CLEAN AND POTASSIUM-MODIFIED RU(001)

The formation and decomposition of formate species on the clean and on potassium-modified Ru(001) surfaces have been investigated with time-resolved vibrational spectroscopy and thermal desorption mass spectrometry (TDMS). Utilizing Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) we have characterized chemisorbed formate produced by the decomposition of formic acid on clean Ru(001), Ru(001)-(square-root 3 X square-root 3)R30-degrees-K and on a K-multilayer adsorbed on Ru(001). The vibrational spectra show that formate is adsorbed on both clean Ru(001) and Ru(001)-(square-root 3 X square-root 3)R30-degrees-K with C2v symmetry indicative of a bridged or bidentate species. There are, however, characteristic differences in the vibrational spectra, which indicate that for the Ru(001)-(square-root 3 X square-root 3)R30-degrees-K surface the formate is directly bound to potassium. The vibrational spectrum of the latter species is found to be in good agreement with that of bulk potassium formate adsorbed on Ru(001). Based on the agreement with literature data for bulk formate, we propose a bonding model for the potassium formate monolayer, which also accounts for the observed contraction of the potassium monolayer resulting from the compound formation. The thermal decomposition of the various formate overlayers has been monitored by simultaneous thermal desorption mass spectrometry and time-resolved FT-IRAS. This combination allows us to correlate the desorbing gas-phase products with the appearance and disappearance of surface intermediates. In the case of formate adsorbed on the clean Ru(001), the C-H and C-O bond cleavage reactions occur simultaneously, leading to the production of equal amounts of CO and CO2. The simultaneous observation of desorbing CO2 (TDMS) and of adsorbed CO (IR) confirms earlier work, which postulated a mechanism involving a coupling of the C-H and C-O bond cleavage reaction channels of two neighboring formates. The presence of potassium changes dramatically the reaction pathway of the formate as it suppresses the C-H bond cleavage channel, leaving CO and OH as the main decomposition products. Compound formation with potassium also leads to thermal stabilization of the formate in comparison to formate adsorbed on the clean surface. However, formate adsorbed on the potassium-modified ruthenium substrate is found to be thermally less stable than formate adsorbed on clean Ru(001).