Dynamics of redox behavior of nano-sized VO x species over Ti–Si-MCM-41 from time-resolved in situ UV/Vis analysis

Time-resolved in situ UV/Vis spectroscopy related the activity and selectivity of highly dispersed VO x species over Ti–Si-MCM-41 materials in propane oxidation to propene with the constants of reduction (K(C 3H 8)) and reoxidation (K(O 2), K(N 2O)) of these species. Time-resolved in situ UV/Vis spectroscopy has been successfully applied for analyzing the reduction degree of highly dispersed VO x (0.4–1.1 V/nm 2) species over Ti–Si-MCM-41 (Ti/Si = 0–1.5) materials in the oxidative dehydrogenation of propane (ODP) as well as for deriving the kinetic parameters of reduction (with C 3H 8) and reoxidation (with O 2 and N 2O) of these species. This quantitative kinetic study enabled to recognize at least two types of differently reducible VO x species in the samples with titanium. Three effects of titanium on redox properties of VO x species were identified: (i) the constant of reduction of oxidized VO x species by C 3H 8 increases, (ii) the constant of oxidation of reduced VO x species by O 2 decreases; and (iii) the constant of oxidation of reduced VO x species by N 2O increases. These effects are highly relevant for the ODP performance. The intrinsic ODP activity increases linearly with an increase in the constant of reduction of oxidized VO x species. Propene selectivity is determined by the ratio of the constants of reduction and reoxidation of VO x species; the higher the ratio, the higher the selectivity. This ratio can be tuned by the oxidizing agent (O 2 vs. N 2O) and by the Ti/Si ratio.