Low-loaded 0.3 wt.% Pt/SiO2 & horbar;Al2O3 catalysts, either unmodified or modified with WO3 or Fe2O3, were investigated to elucidate the reaction pathways and mechanistic features of the NOX selective catalytic reduction by hydrogen (H-2-SCR) over a wide temperature range. At low temperatures (50-100 degrees C), catalytic activity is governed by H-2 activation, which is significantly enhanced by WO3 addition, whereas Fe2O3 inhibits NO reduction and consequently suppresses low-temperature N2O formation. In the intermediate temperature range (80-200 degrees C), distinct H-2-SCR behaviors are linked to the reactivity of in situ-formed NH3 toward residual NOX. This NH3-SCR type pathway is strongly promoted by WO3, while Fe2O3 hampers NH3 activation at low temperature, leading to increased NH3 slip. However, this reaction pathway remains limited to a narrow temperature window, as it requires the presence of unconverted NOX. At higher temperatures (> 150 degrees C), the NH3 + O-2 reaction, previously identified as the dominant deNO(X) route, is largely unaffected by the addition of WO3 or Fe2O3. Moreover, the undesired N2O formation in this high temperature range originates not only from the ammonia oxidation route, but a possible contribution from the decomposition of ammonium nitrate-like surface species was also evidenced.

This preliminary work opens a possible innovative route to investigating a paradigm shift in ammonia synthesis. It involves a hydrogen donor molecule (HDM) to favor ammonia synthesis at low pressure and temperature (1 bar, T < 200 °C), subsequently reducing the environmental footprint of NH3 synthesis.

The pathways and mechanistic aspects of H2-SCR over precious metal-based catalysts is still under debate. This study focusses on low loaded platinum-based catalysts (0.07–0.3%) in a large temperature range (50–500 °C), with special focus on (i) the role of NH3 as a possible intermediate species, (ii) the origin of the undesired N2O emission and (iii) the platinum sites involved in the H2-SCR deNOX reactions. Up to 60 °C, the N2O selectivity was close to 100%, with no influence of the presence of oxygen in the 50–100 °C temperature range. Ammonia formation was observed at relatively low temperatures (from 60 °C), but its reactivity was then limited. All these low temperature reactions were associated with the same platinum sites, probably a mix of edge and face sites. The maximum outlet NH3 was observed around 100 °C and the role of the NH3-SCR in the whole H2-SCR process appeared very limited. On the contrary, the ammonia oxidation by O2, which started near 120 °C, significantly contributed to the H2-SCR process and appeared responsible for the second N2O emission peak (150–500 °C). This reaction did not imply the same platinum sites and appears mainly dependant on the platinum particle size.

Because of its current and future intensive use, the synthesis of sustainable ammonia is of major concern. This study focuses on highlighting the mechanistic behaviours of NH3 production from molecular nitrogen and hydrogen on ruthenium materials supported on cerium-based ternary intermetallics. An interesting activity is reported for Ru/CeTX materials (T=Sc, Ti; X=Si, Ge) with an ammonia production rate around 0.60 mmol(NH3) h(-1) g(-1) at only 300 degrees C and 1 bar. While the activity at 400 degrees C appeared attributable to the N equivalent to N cleavage activation, this low-temperature activity is no longer correlated with this parameter, but rather to the behaviour regarding hydrogen and especially the ability of cerium-based intermetallic materials to desorb hydrogen from the structure, supporting an associative-type mechanism.

This study investigated (Ru-)Mo2C (MXene) materials for ammonia thermo-catalytic synthesis under atmospheric or moderate pressure. Under a H2-N2 (3:1) flow, the Mo2C MXene phase showed limited activity in ammonia synthesis at atmospheric pressure at 400 °C (0.01mmolh⁻¹ g⁻¹), which increased significantly with temperature and pressure, reaching 2.07mmolh⁻¹ g⁻¹ at 500°C and under 5bar. Interestingly, Mo-based MXene was able to generate an appreciable quantity of ammonia without promoters such as ruthenium, the most active metal for the ammonia synthesis reaction. Unexpectedly, the addition of Ru to Mo2C did not enhance its activity. The nitriding of the MXene under NH3 or N2 was then performed and characterized. A thermal treatment under NH3 (600 °C, 5bar) was efficient and, interestingly, nitriding also occurred in a lower extent under N2 (600 °C, 5bar) for the sample containing ruthenium. The N-containing MXene produced ammonia under pure H2 flow from temperatures lower than 250 °C. Consecutive nitriding treatments and ammonia production under pure H2 were successfully achieved and demonstrated for 5 cycles. This result is promising for chemical looping ammonia production process. This work highlights essential aspects that should be explored for future advances to consider using Mo2CTx MXene for the efficient thermal production of ammonia.

This study investigates the influence of the rare‐earth (R) element in Ru/RScSi electride‐like intermetallic catalysts for ammonia synthesis under mild conditions (300–450 °C; 1–5 bar). All materials present poor specific surface area and the grain size impacts the ammonia yield. The catalytic performances follow the La to Gd lanthanides series (Ru1.7/LaScSi=Ru1.7/CeScSi=Ru1.7/PrScSi>Ru1.7/NdScSi>Ru1.5/SmScSi>Ru1.3/GdScSi) and appear correlated with the formation of the hydride phase. Ru1.7/CeScSi shows remarkable catalytic activity under moderate condition (0.58 mmolNH3/h/g at 300 °C, 1 bar) associated with its reversible hydrogen storage–release properties. It is evidenced that RScSi materials assist the N2 dissociation in agreement with their electride character and that the hydrogen for NH3 formation mainly comes from gaseous hydrogen and not primarily from the hydride phase. It is suggested that NHX formation could be the rate determining step rather than the N2 cleavage over these catalysts.

The catalytic treatment of residual traces of refractory methane in the exhaust gas from NGV engines involves the use of expensive, highly loaded precious metal based catalysts. Recently, it was demonstrated that the association of a perovskite with a Pd/YSZ (Yttria-stabilized zirconia) catalyst improved both the oxygen exchange behavior on the YSZ support together with the methane oxidation on palladium. However, these interesting results were obtained in condition quite far from an exhaust pipe and this work aims to investigate the concept of this dual catalytic bed system in more realistic conditions. In this study, a mixed electronic/ionic conducting perovskite (CaTi0.7Fe0.3O3-δ) was associated to a pure oxygen ion conductor (YSZ) impregnated with 1% palladium. This dual-bed configuration promoted CH4 abatement in lean mixture, probably associated to the Pd/PdO active phase formation, but inhibiting effect of both CO2 and H2O was also observed.

The design of catalysts active for ammonia synthesis under mild conditions of temperature and pressure is a new grail for its production process based on green hydrogen and intermittent renewable energy sources. Ru/LaScSi and Ru/CeTiGe are investigated with an approach combining DFT calculations and 15N/14N homomolecular exchange experiments. Ru/LaScSi is the most active of both catalysts. It has electride-like properties which are enhanced when partially hydrogenated leading to higher activity in nitrogen equilibration reaction. Ru/CeTiGe, relatively active despite the absence of electride-like character, is proposed to produce NH3 following the associative mechanism with the assistance of absorbed H.

Catalytic methane combustion was carried out over YSZ, Pd/YSZ and Rh/YSZ between 200 and 700 degrees C. Despite similar light-off curves for Pd/YSZ and Rh/YSZ between 200 and 450 degrees C, isotopic exchange experiments using CD4 and 18O2 revealed different behaviour of both supported catalysts regarding the activation of reactant molecules. The use of C18O2 isotopic gas was shown to be more appropriate than 18O2 to evaluate the bulk oxygen mobility in YSZ at low temperatures. The exchange of gaseous CO2 with lattice YSZ oxygen atoms, which occurs via surface hydrogen carbonate intermediate species, allowed to demonstrate the contribution of bulk oxygen atoms of YSZ in Rh/YSZ at low temperatures.

Four catalysts—1%Pd-2%Mn/γ-Al2O3, 1%Pd/γ-Al2O3, 2%Mn/γ-Al2O3 and γ-Al2O3—were synthesized via a sol–gel method and characterized using various techniques to evaluate their physicochemical, textural, surface and acidic properties. They were used in the catalytic transformation of ozone and nitrogen oxides using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. Different consecutive gas sequences were followed to unravel the poisoning role of nitrogen oxides and the possible reactivation by ozone. It has been proven that on palladium and manganese-based catalysts, the inhibition effect of nitrogen oxides was due to the formation of monodentate nitrites, monodentate, bidentate and bridged nitrates, which are difficult to desorb and decompose into gaseous NOx, either by oxidation or by thermal treatment. Interestingly, monodentate nitrites could be eliminated if the catalyst went through a co-adsorption of NOx and ozone prior to exposure in clean ozone flow. This transformation could be the reason why the catalytic conversion of ozone could return to its original value before the poison effect of nitrogen oxides.

In the present work, a series of CaTi1-xFexO3-δ (0 < x < 0.5) materials are prepared using a modified Pechini method based on citric acid and a polyol as chelating agents. The synthesis conditions are optimized with respect to the specific surface area and phase purity by varying polyols (ethylene glycol, glycerol, and 1.6-hexanediol) and the ratio between citric acid, polyols, and cations. The impact of the polyols and the iron content (up to 40 mol % on the B site) is studied with respect to the oxygen exchange rate, reducibility using H2-TPR, and catalytic performance for methane total oxidation. A correlation between the oxygen exchange rate studied using 18O exchange in powdered samples of CaTi1-xFexO3-δ (0 < x < 0.5) and ferric sites determined using Mössbauer spectroscopy and H2-TPR is established. The oxygen activation and diffusion in CaTi1-xFexO3-δ (0 < x < 0.5) continuously increase in the studied range of Ti substitution. The methane oxidation performance does not increase above x = 0.3, showing that methane oxidation is not limited by surface oxygen activation and CH4 is activated by specific iron sites in Fe-doped perovskites.

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).

The partial substitution of zirconium for praseodymium in a ceria-zirconia support was studied for WO3 or Nb2O5 supported catalysts dedicated to the NOx SCR by NH3. This partial substitution favored the support reducibility, but both niobium and tungsten impregnation strongly inhibited the redox behaviors of the support. Concomitantly, Nb2O5 and WO3 also provided acidic sites but praseodymium noticeably inhibited the ammonia storage, especially for WO3-containing sample. Finally, praseodymium drastically decreased the deNO(x) performances. Additional NO and NH3 oxidation experiments were performed and the various redox and acidic behaviors of all the studied materials were suitable to provide an overview of the reactional pathways which begins by the oxidative dehydrogenation of NH3 on acid sites. The generated species react either with oxygen species or with gaseous NO, reflecting a strong competition between the NOx SCR and NH3 oxidation together with the formation of a key intermediate in the nitrogen oxides reduction.

The Selective Catalytic Reduction (SCR) is one of the most efficient process for NOx removal from Diesel exhaust gas. However, the urea/NH3-SCR process implemented in recent vehicles still suffers from a poor activity in the low temperature range (T < 250 degrees C). One main reason is its dependency against the NO2/NOx ratio, limiting the expected fast-SCR reaction in this temperature range. Recently, we shown that the addition of ethanol to ammonia led to a significant increase of the activity of a Ag/Al2O3 catalyst in this low temperature range. Moreover, in a dual-bed configuration (Ag/Al2O3 + WO3/Ce-x-ZryO2), a remarkable improvement was achieved at low temperature using only NO as NOx. The present work aims to highlight the DeNO(x) chemistry encountered over the WO3/Ce-x-ZryO2 catalyst in a bifunctional (EtOH + NH3) mixture. In addition to the fact that this process takes advantage of the low temperature NO2 formation over the upstream Ag/Al2O3 catalyst, this work also puts in evidenced unexpected interactions between NO2 and CH3CHO (resulting from ethanol oxidation over Ag/Al2O3) thus leading to NO emission.

To highlight the deactivation mechanisms encountered by minerals impurities from biodiesel, the effects caused by Na, P or (Na + P) additions were studied over a model Cu-FER catalyst. Na, P or (Na + P) were added by wet impregnation in water in a wide concentration range up to 2 wt-%. The catalytic behaviors were evaluated by NH3/NO oxidation and standard/fast NH3-SCR reactions. In addition, a combination of several characterization techniques (ICP AES, N-2 adsorption/desorption, XRD, NH3-TPD, NO adsorption monitored by FTIR and H-2-TPR) was applied to provide useful information regarding the deactivation mechanism caused by the minerals addition. Sodium and phosphorus interacted differently with the Cu-FER catalyst. Na addition induced a loss of Bronsted acid sites and a back-exchange of Cu2+ with Na,+ with formation of external CuO species, thus favoring the oxidation of NO and NH3. After phosphorus addition, the exchanged Cu2+ species remained moderately affected, but direct interactions with copper were evidenced which were primarily responsible for catalyst deactivation toward the oxidation reactions. After equimolar addition of phosphorus and sodium, both Na and P effects were observed. For the NH3-SCR process, the ammonia adsorption ability, which depends on both acidity and copper units, appeared the main key parameter driving the catalytic activity at low temperature (T <= 250 degrees C). Phosphorus appeared to be the major responsible for catalyst deactivation after (Na + P) co-poisoning.

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.