RAIRS and TPD study of CO and NO on beta-Mo2C

The chemisorption of CO and NO on bulk beta-Mo2C was studied using reflectance absorbance infrared spectroscopy (RAIRS) and thermal desorption measurements. The carbide foil substrate was cleaned in situ prior to each experiment by repeated annealing to 1400 K. Low coverage dissociation of CO yields a weak recombinative thermal desorption feature at approximately 1200 K and molecular desorption of CO occurs at 325 K for the lowest exposures studied. RAIRS results show that carbon monoxide on beta-Mo2C at 100 K is characterized by a single CO vibrational stretching frequency, typical of on-top adsorption, which increases from 2057 to 2072 cm(-1) as a function of increasing coverage. Preadsorption of oxygen leads to a broadening to higher frequencies but not to the appearance of any new absorbance bands. The vibrational spectrum for adsorbed NO at 100 K displays a band at 1780 cm(-1) and a shoulder at 1820 cm(-1). RAIRS spectra acquired as a function of temperature were used to monitor the decomposition of NO. Decomposition occurs over the 250-450 K range and results in high temperature CO and N-2 desorption peaks. The results for the chemisorption of NO and CO on molybdenum carbide are compared with literature data for the adsorption of these probe molecules on molybdenum and ruthenium surfaces. This comparison is used to comment on the physical basis for the correspondence in catalytic properties between early transition metal carbides and noble metal based catalysts. (C) 1997 Elsevier Science B.V.