Characterization of oxygen-treated carbides of molybdenum and tungsten for n-hexane-dihydrogen reactions

Abstract

Oxygen treatment of tungsten carbide WC improved both activity and selectivity to branched hexanes in n-hexane-H₂ reactions and the effects became greater as the treatment temperature was raised. A marked increase in isomerization rates and a small change in hydrogenolysis rates for oxygen-treated catalysts were responsible for this result. The selectivity remained invariant with the change in n-hexane conversion. The oxygen treatment of molybdenum carbide Mo₂C, on the other hand, brought about isomerization activity by suppressing hydrogenolysis activity dominant over the untreated catalyst. The isomerization selectivity of Mo₂C catalysts was much less than that of WC and depended strongly on n-hexane conversion. The Mo₂C catalysts also showed rapid deactivation with time-on-stream. Several techniques (CO chemisorption, XRD, TPD/TPR and XPS) were employed to understand the effects of oxygen treatment on the catalytic behavior of WC and Mo₂C. The X-ray diffraction indicated that WC structure was maintained under oxygen below 673 K. The surface reactivity of oxygen-treated catalysts depended on parent metal (Mo or W), and treatment temperature. Treatment at higher temperatures not only deposited more oxygen onto the carbides, but also increased its stability towards reduction. Also, a greater number and a higher strength of acid sites were generated by oxygen treatment at higher temperature as evidenced by TPD of adsorbed NH3. The presence of this stable oxygen appeared to be a prerequisite to the isomerization selectivity and activity maintenance for oxygen-treated WC or Mo₂C.