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A set of physicochemical methods, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction, electron microscopy and X-ray absorption spectroscopy, was applied to study Pt/TiO2 catalysts prepared by impregnation using a commercial TiO2-P25 support and a support produced by pulsed laser ablation in liquid (PLA). The Pt/TiO2-PLA catalysts showed increased thermal stability due to the localization of the highly dispersed platinum species at the intercrystalline boundaries of the support particles. In contrast, the Pt/TiO2-P25 catalysts were characterized by uniform distribution of the Pt species over the support. Analysis of Pt4f XP spectra shows that oxidized Pt2+ and Pt4+ species are formed in the Pt/TiO2-P25 catalysts, while the platinum oxidation state in the Pt/TiO2-PLA catalysts is lower due to stronger interaction of the active component with the support due to strong interaction via Pt-O-Ti bonds. The Pt4f XP spectra of the samples after reaction show Pt2+ and metallic platinum, which is the catalytically active species. The study of the catalytic properties in ammonia oxidation showed that, unlike the catalysts prepared with a commercial support, the Pt/TiO2-PLA samples show higher stability during catalysis and significantly higher selectivity to N2 in a wide temperature range of 200&ndash, 400 &deg, C.

Ammonia oxidation experiments were conducted at high pressure (30 bar and 100 bar) under oxidizing and stoichiometric conditions, respectively, and temperatures ranging from 450 to 925 K. The oxidation of ammonia was slow under stoichiometric conditions in the temperature range investigated. Under oxidizing conditions the onset temperature for reaction was 850–875 K at 30 bar, while at 100 bar it was about 800 K, with complete consumption of NH3 at 875 K. The products of reaction were N2 and N2O, while NO and NO2 concentrations were below the detection limit even under oxidizing conditions. The data were interpreted in terms of a detailed chemical kinetic model. The rate constant for the reaction of the important intermediate H2NO with O2 was determined from ab initio calculations to be 2.3 × 102 T2.994 exp (−9510 K/T) cm3 mol−1 s−1. The agreement between experimental results and model work was satisfactory. The main oxidation path for NH3 at high pressure under oxidizing conditions is NH3⟶+OH NH2⟶+HO2,NO2 H2NO⟶+O2 HNO⟶+O2 NO ⟶+NH2 N2. The modeling predictions are most sensitive to the reactions NH2 + NO = NNH + OH and NH2 + HO2 = H2NO + OH, which promote the ammonia consumption by forming OH radicals, and to NH2 + NO = N2 + H2O and NH2 + NO2 = N2O + H2O, which are the main chain-terminating steps.

Ammonia oxidation to N2 over a Cu-SAPO-34 SCR catalyst was studied with the intent of determining if the mechanism is the direct oxidation of NH3 to N2 or proceeds through a two-step pathway where NH3 is oxidized to NO, with the NO then reacting with available NH3 through the SCR reaction to produce N2. NH3 oxidation data show that some NO is indeed formed and the NO selectivity is dependent on the amount of NH3 adsorbed on the surface, and thus on the adsorption time and temperature. This is due to the ratio of NO formed to adsorbed NH3 downstream along the monolith channel. NO selectivity increased as the amount of NH3 adsorbed on the catalyst surface decreased. Overall, the data demonstrate that NH3 to N2 can occur via the NO intermediate pathway, with subsequent reduction by NH3 to N2.

Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of N2O formation in the selective catalytic reduction of NO by NH3 (NH3-SCR). The promoted VWT and c-VWT catalysts all gave a significantly decreased N2O production at temperatures >400 °C compared to the unpromoted samples. However, such a promotion led to the loss in high temperature NO conversion, mainly due to the oxidation of NH3 to N-containing gases, particularly NO. Characterization of the unpromoted and promoted catalysts using X-ray diffraction (XRD), NH3 adsorption-desorption, and Raman spectroscopy techniques could explain the reason why the promotion showed much lower N2O formation levels at high temperatures. The addition of Fe2O3 to c-VWT resulted in redispersion of the V2O5 species, although this was not visible for 2.73% Fe2O3/VWT. The iron oxides exist as a highly-dispersed noncrystalline α-Fe2O3 in the promoted catalysts. These Raman spectra had a new Raman signal that could be tentatively assigned to Fe2O3-induced tetrahedrally coordinated polymeric vanadates and/or surface V-O-Fe species with significant electronic interactions between the both metal oxides. Calculations of the monolayer coverage of each metal oxide and the surface total coverage are reasonably consistent with Raman measurements. The proposed vanadia-based surface polymeric entities may play a key role for the substantial reduction of N2O formed at high temperatures by NH3 species adsorbed strongly on the promoted catalysts. This reaction is a main pathway to greatly suppress the extent of N2O formation in NH3-SCR reaction over the promoted catalysts.

International audience; Transition metal oxides (M=Mn, Co, Fe, Ni,…) are potential catalysts for application in combustion and depollution processes. Owing to huge improvements in their preparation, perovskites, spinels, hexaaaluminates, and some other oxide structures can replace noble metals in a number of processes. In this chapter, the most recent advances in the use of oxides for total oxidation (CO, methane, COV, wet air oxidation) and for the treatment of nitrogen compounds (NOx, NH3, urea) will be reviewed. In every case, the most probable mechanism (Langmuir–Hinshelwood, Eley–Rideal, Mars–van Krevelen…) and the nature of active sites (Mn+/Mn+1 ion pairs, acid–base sites…) as well as the role of reactive oxygen species will be examined in the light of recent results and up-to-date concepts. Finally the outstanding progresses in the oxide synthesis allow to apply these concepts to the development of extremely active and more stable catalysts.

In this work, the kinetics of the NH 3 /NO/O 2 reacting system over V 2 O 5 –WO 3 /TiO 2 catalysts was studied under conditions of NH 3 /NO ratio ≥ 1 and temperature up to 400 °C, where some ammonia oxidation is expected. NH 3 -SCR and NH 3 -oxidation tests were performed over powdered catalysts, at varying V-loads. Over a high V-load home-made catalyst, NO conversion passed through a maximum at increasing temperature, as commonly observed in SCR tests at the onset of ammonia oxidation; however, since the experiments were performed under excess ammonia, the high temperature decrease of NO conversion could not be associated with a lack of surface NH 3 . The experiments were interpreted as the evidence of an unselective NH 3 -oxidation route, giving rise to some NO production. NH 3 -oxidation tests, though, showed low selectivity to NO (which is in line with the current literature); however, a quantitative analysis of data showed that the observed product distribution is fully in line with an indirect-consecutive reaction scheme from NH 3 to N 2 , wherein NO is formed by NH 3 oxidation and is then rapidly converted to N 2 via NH 3 -SCR. The kinetic investigation was extended to commercial catalysts with low and medium V-loads. NH 3 -SCR tests showed increasing trends of NO conversion with temperature within the range 250–400 °C, thus they did not show any clear evidence for NH 3 oxidation. However, NH 3 -oxidation tests showed that the reaction was active above 350 °C; notably, over 95% selectivity to N 2 was observed. A quantitative analysis of the NH 3 -oxidation tests, based on the independent estimate of the intrinsic rate of NH 3 -SCR, showed that the observation of few ppm NO slip is fully in line with the assumption of a two-step kinetic scheme. A reaction path analysis shows that by neglecting the unselective nature of ammonia oxidation on V-catalysts the rate of this reaction is overestimated by a factor of 2. Preliminary evaluations suggest that the impact of NH 3 -oxidation on the integral performance of monolith reactors (operating at NH 3 /NO ratios close to 1) is non negligible, so that the correct identification of the reaction stoichiometry is crucial.