An IR study on the surface passivation of Mo2C/Al2O3catalyst with O2, H2O and CO2

The surface active sites of a fresh Mo2C/Al2O3 catalyst and their evolution under passivation conditions were characterized by IR spectroscopy using CO as the probe molecule. It was found that adsorption properties of CO on the fresh sample were quite different from those of the reduced passivated one. Moδ+ (0 < δ < 2) sites are mainly present on fresh Mo2C/Al2O3 catalyst as probed by a characteristic IR band at 2054 cm−1 of adsorbed CO. When the fresh Mo2C/Al2O3 catalyst in the IR cell is exposed to trace amounts of O2 or H2O in situ at RT, the intensity of the 2075 cm−1 band declines. It shows that the fresh Mo2C/Al2O3 catalyst can be oxidized by trace amounts of O2 or H2O easily and the oxidation capacity of H2O is weaker than that of O2. IR spectra of adsorbed CO on a reduced Mo2C/Al2O3 catalyst passivated by a 1% O2/N2 mixture show that two weak bands at 2180 and 2095 cm−1 appear, which suggests that the passivation layer cannot be completely reduced, even by H2-reduction at high temperatures. For reduced Mo2C/Al2O3 passivated by H2O or CO2, IR spectra of adsorbed CO give characteristic IR bands at 2081 and 2030 cm−1, which indicates that the surface Mo atoms are in a state of Moϕ+ (0 < ϕ < 3). Thus, we found that the Mo2C/Al2O3 passivated by H2O or CO2 can be regenerated by H2 treatment at 673 K and most of the active sites can be recovered. It is too active to control the passivation extent using O2 as an oxidant, while Mo2C/Al2O3 passivated by H2O or CO2 can be regenerated by simple reduction with H2.