Oxidative coupling of methane to higher hydrocarbons and carbon-monoxide oxidation over titania-based catalysts.

摘要

Oxidative coupling of methane was studied over several titania-based catalysts. In addition, varying the operating parameters showed that under certain conditions significant gas phase reactions occur in the absence of catalysts. The trend dictated by the gas phase kinetics shows that the hydrocarbon selectivity falls as methane conversion increases. Specifically, at methane conversions of 2%, the hydrocarbon selectivity was around 65%, but when methane conversions were increased to 32%, the hydrocarbon selectivity decreased to 29%. A reaction pathway for gas phase oxidative coupling has been considered and compared to proposed catalytic pathways.; Catalytic results indicate lithium-doped titania catalysts are effective for oxidative coupling. The degree of promotion was studied by varying the lithium loading on the rutile crystal structure of titania. Increasing the lithium loading reduces the combustion capacity of the catalyst, lowers methane conversion, and increases hydrocarbon selectivity. A 16.2% lithium-titania catalyst had methane conversions around 15% with hydrocarbon selectivities about 75%. A series of titanate catalysts was also studied for their oxidative coupling activity. The lanthanum-titanate catalyst had the best hydrocarbon yields (ca. 10-12%) of the titanate catalysts and was more active at lower temperatures than any other catalysts studied. Routes for surface-catalyzed reactions are considered and related to catalyst characterization.; In addition, the catalytic oxidation of CO was investigated on Pt/TiO{dollar}sb2{dollar} catalysts to study the influence of SMSI effects and of using different titania crystal structures. Catalysts were prepared using rutile and anatase titania and were characterized by chemisorption, x-ray diffraction, and x-ray photoelectron spectroscopy. Reduction at 500{dollar}spcirc{dollar}C suppresses CO and H{dollar}sb2{dollar} chemisorption and leads to changes in Pt 4f{dollar}sb{lcub}7/2{rcub}{dollar} electron binding energies. Effects of reduction temperature and of support material on CO oxidation activity were compared through temperature programmed reaction experiments. Catalysts reduced at 200{dollar}spcirc{dollar}C show slightly higher activity and lower ignition temperatures than those reduced at 500{dollar}spcirc{dollar}C. Rutile-supported catalysts show much higher CO oxidation activity with lower ignition temperatures; the increased activity is speculated to result from a lower activation energy for oxygen desorption. A morphological model of metal-support interactions involving oxygen transfer from the support is proposed to coexist with the Langmuir-Hinshelwood mechanism.