Chemisorption of CO and NO on molybdenum carbide foils

Abstract

Reflectance absorbance infrared spectroscopy (RAIRS) and thermal desorption data are presented for the interaction of CO and NO with Mo₂C. The metal carbide samples were prepared from pure molybdenum foil by temperature programed reaction in a CH₄/H₂ gas flow. Annealing in UHV to 1400 K activates the carbide towards chemisorption by removing graphitic carbon from the surface. CO chemisorbs on the activated carbide yielding a CO stretching frequency of 2069 cm⁻¹ characteristic of on-top bonding. Partial dissociation of CO occurs on the Mo₂C surface and molecular desorption peaks are observed at 155 K and 300–350 K. In contrast, chemisorbed NO dissociates on heating the sample to 400 K. The resulting atomic oxygen combines with surface carbon to yield CO desorption peaks above 800 K. Some dissociation of NO also occurs at low temperatures. The RAIRS spectrum recorded following NO exposure at 104 K displays a broad peak at 1761 cm⁻¹ and a shoulder at 1806 cm⁻¹ indicative of on-top bonding. The results for CO and NO chemisorption are compared with literature data for ruthenium and molybdenum single crystal surfaces. This comparison suggests that carbide formation reduces the propensity to form the tilted CO surface species which are thought to be precursors to dissociation on early transition metals.