Reversible reactions over non-metallic catalysts: A transient isotopic tracing of the isobutane-isobutene-hydrogen system over chromia

Whether a single rate-controlling step exists in complex reversible heterogeneous catalytic reactions is difficult to establish, especially with non-metallic catalysts, which often possess fewer active sites than supported metals. This problem is treated here using transient isotopic tracing with 13C marking for the system isobutane-isobutene-hydrogen over a chromia catalyst. A statistical approach is employed for modeling the simultaneous occurrence of traced isobutane and isobutene in the product stream using the Horiuti-Polanyi, mechanism. We find that a single rate-controlling step that consists of the conversion of the half-hydrogenated chemisorbed species i-C4H9l to chemisorbed isobutene i-C4H8l and the reverse exists. Such information should be useful for catalyst design and prediction of performance.