The active oxygen for direct oxidation of methane to methanol in the presence of hydrogen

Methane was selectively converted to methanol by a mixture of oxygen and hydrogen at greater than or equal to 573 K and atmospheric pressure over Fe0.5Al0.5PO4 catalyst, while, in the absence of H-2, the conversion of methane was notably low and only a trace of methanol was formed. Characterization of the catalyst by XPS before and after the reaction suggested the redox of Fe(III)/Fe(II) on the catalyst surface during the oxidation of methane by a H-2-O-2 gas mixture. The adsorbed oxygen species generated on the catalyst in a H-2-O-2 gas mixture and its reactivity with methane were studied by in situ FT-IR spectroscopy. The absorption band at 895 cm(-1) was observed in a H-2-O-2 gas mixture when the temperature was raised above 573 K. The isotope substitution of O-16(2) with O-18(2) shifted the absorption band at 895 cm(-1) to 849 cm(-1). Three absorption bands at 895, 870 and 849 cm(-1) were observed when the mixture of O-16(2), (OO)-O-16-O-18 and O-18(2) was used with H-2 at greater than or equal to 573 K. These observations strongly suggest that the band at 895 cm(-1) is ascribed to a peroxide species adsorbed on the iron site of the catalyst. The intensity of the band due to this oxygen species decreased greatly in the presence of methane at greater than or equal to 473 K as a result of the reaction with methane. The new bands at 2936, 2870 and 1050 cm(-1), which were assigned to the stretching vibrations of CH3 groups and of C-O of methoxide species, appeared simultaneously. The absorption band at 3668 cm(-1), the stretching vibration of the adsorbed OH group, increased at the same time. The absorption bands at 3668, 2936 and 2870 cm(-1) were shifted to 2668, 2208 and 2124 cm(-1), respectively, when CD4 was used instead of CH4. These results suggest that the adsorbed peroxide reacts with methane at greater than or equal to 473 K, producing methoxide and OH groups as intermediates for the formation of methanol.