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[Display omitted] • TiO 2 -anatase displays substantial activity and stability in gas-phase HBr oxidation. • Anatase and rutile exhibit similar kinetics, yet rutile is more active. • Self-doping with Br creates defect level enabling O 2 activation on both polymorphs. • O 2 activation is more facile on rutile due to shorter Ti cus distances. • Br 2 evolution is facilitated on rutile due to easier reduction of Ti4+ centers. This article investigates the activity of TiO 2 -rutile and TiO 2 -anatase polymorphs in the catalytic HBr oxidation, which is an enabling process to close the halogen loop in bromine-mediated transformation of natural gas to high-value chemicals and liquid fuels. The evaluation of rutile-, anatase-, and rutile-anatase TiO 2 catalysts, exhibiting the variable specific surface areas, revealed that anatase phase is also active in this reaction. Nonetheless, in contrast to photocatalytic processes in which anatase is typically more active than rutile, rutile exhibits ca. 2–5 times higher intrinsic rates of HBr oxidation than anatase. Thereby, the apparent activation energies and reaction orders with respect to HBr, O 2 , and H 2 O display similar values for the two polymorphs. The activity differences were rationalized by density functional theory analysis, which showed that HBr oxidation follows a similar defect-driven mechanism over the most stable rutile (110) and anatase (101) surfaces. Herein, HBr activates the catalyst through a self-doping mechanism that involves the substitution of surface oxygen by bromine with the concomitant reduction of Ti4+ to Ti3+ centers. This forms a defect level that is placed in the band gap and allows for the O 2 activation on the catalyst surface. While the HBr adsorption and H 2 O desorption display a similar energy profiles on both polymorphs, the O 2 activation and Br 2 evolution are more facile over rutile compared to anatase surface due to shorter distances between the coordinatively unsaturated Ti cus sites and easier reduction of Ti4+ centers upon product desorption, respectively. [ABSTRACT FROM AUTHOR]

Abstract: The mechanisms of direct N2O decomposition and N2O reduction by H2 or NH3 on RuO2 has been studied by means of steady-state catalytic tests at ambient pressure, transient experiments in a temporal analysis of products (TAP) reactor, and density functional theory (DFT) simulations. Bulk RuO2 is very active for N2O decomposition into N2 and O2, achieving full conversion at 723K. According to the DFT calculations, coordinatively unsaturated ruthenium (Rucus) sites are active for the decomposition and oxygen elimination from the surface is rate-limiting step. When pre-reducing RuO2 with H2, the de-N2O activity in the low-temperature region increases. This is attributed to the higher reactivity of surface oxygen vacancies created by H2 at bridge sites compared to Rucus sites. Co-feeding of hydrogen or ammonia with nitrous oxide strongly accelerates the rate of N2O elimination. TAP studies show that oxygen species formed upon N2O decomposition over RuO2 react with NH3 and H2 yielding N2, NO, N2O, and H2O. Both TAP experiments and DFT simulations show that the N-containing intermediates coming from ammonia adsorb much stronger on the catalyst surface than H-containing intermediates coming from hydrogen. This causes blockage of active sites for N2O decomposition by the former species, accounting for the higher efficiency of H2 as reductant for N2O. The presence of O2 in the feed cancels the reducing effect of H2 or NH3 due to the more favorable re-oxidation of reduced RuO2 by O2 compared to N2O. [Copyright &y& Elsevier]