In-situ UV/vis and transient isotopic analysis of the role of oxidizing agent in the oxidative dehydrogenation of propane over silica-supported vanadia catalysts

In-situ UV/vis spectroscopy in combination with transient isotopic analysis was applied for elucidating the governing factors, which determine the superior performance of N2O as compared to O-2 in the oxidative dehydrogenation of propane (ODP) over VOx/MCM-41, VOx/MCM-48, and VOx/SiO2 catalytic materials. Characterization studies applying in-situ steady-state UV/vis spectroscopy evidenced higher reduction degree of surface VOx species at 773 K in a C3H8-N2O (C3H8/N2O = 1/1) feed than in a C3H8-O-2 (C3H8/O-2 = 2/1) flow at controlled conversions of propane (< 5%) and oxidizing agent (< 10%). The in-situ UV/vis method in a transient mode enabled to distinguish the ability of O-2 and N2O for reoxidation of reduced VOx species; N2O reoxidizes reduced VOx species slower than O-2. This property of N2O is very essential for suppressing nonselective consecutive propene oxidation to COx (mainly to CO). Additionally, the results of transient isotopic experiments in the temporal analysis of products reactor revealed that adsorbed (nonlattice) oxygen species are formed upon O-2 activation over reduced VOx species and participate in CO2 and CO formation. It is suggested that these nonselective species do not originate from N2O.