Frederic Meunier

A review of the selective reduction of NOx, with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal catalysts

Abstract Research on the selective reduction of NOx with hydrocarbons under lean-burn conditions using non-zeolitic oxides and platinum group metal (PGM) catalysts has been critically reviewed. Alumina and silver-promoted alumina catalysts have been described in detail with particular emphasis on an analysis of the various reaction mechanisms that have been put forward in the literature. The influence of the nature of the reducing agent, and the preparation and structure of the catalysts have also been discussed and rationalised for several other oxide systems. It is concluded for non-zeolitic oxides that species that are strongly adsorbed on the surface, such as nitrates/nitrites and acetates, could be key intermediates in the formation of various reduced and oxidised species of nitrogen, the further reaction of which leads eventually to the formation of molecular nitrogen. For the platinum group metal catalysts, the different mechanisms that have been proposed in the literature have been critically assessed. It is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements. A simple mechanism which involves the dissociation of NO on a reduced metal surface to give N(ads) and O(ads), with subsequent desorption of N2 and N2O and removal of O(ads) by the reductant can explain many of the results with the platinum group metal catalysts, although an additional contribution from organo-nitro-type species may contribute to the overall NOx reduction activity with these catalysts. It is concluded, after the investigation of hundreds of catalyst formulations, that many of the fundamental questions relating to lean deNOx reactions have been addressed and the main boundary conditions have been established. It seems clear that catalysts with sufficient activity, selectivity or stability to satisfy the demanding conditions that appertain in automotive applications are still far away. The rapidly growing interest in NOx storage systems reflects this situation, and it now seems to be the case that acceptable direct NOx reduction catalysts may be very difficult to find for lean-burn applications.

Mechanistic differences in the selective reduction of NO by propene over cobalt- and silver-promoted alumina catalysts : kinetic and in situ DRIFTS study

The distribution of gaseous products and the nature of the surface species generated during the selective catalytic reduction of NO with C3H6 in the presence of excess O2 (i.e. C3H6-SCR) were studied over both a 0.4% Co/γ-Al2O3 catalyst and a sulphated 1.2% Ag/γ-Al2O3 catalyst. The results were compared with those previously reported for the C3H6-SCR over 1.2% Ag/γ-Al2O3 and γ-Al2O3. High concentrations of NO2 were observed in the product stream of the SCR reaction over the 0.4% Co/γ-Al2O3 and sulphated 1.2% Ag/γ-Al2O3 materials. The results show that (as in the case of the γ-Al2O3 and also probably that of the 1.2% Ag/γ-Al2O3) the NO2 was formed via an alternative route to the direct oxidation of NO with O2. The yields of NO2 were higher over the Co/γ-Al2O3 than over the other materials and in contrast to the other materials, no NH3 was produced over the Co/γ-Al2O3 catalyst. Based on these results and those of in situ DRIFTS experiments, a global reaction scheme incorporating organo-nitrogen species as key intermediates is proposed. In this scheme, NO, propene and oxygen react to form organo-nitro and/or organo-nitrito adsorbed species, the reaction products of which combine to yield N2. The results reported here suggest that Co preferentially promotes the formation of nitrito-compounds which can readily decompose to NO2, whereas Ag preferentially promotes the formation of nitro-compounds (from reaction of strongly bound ad-NOx species) which can decompose to isocyanates and ammonia. The sulphation of the 1.2% Ag/γ-Al2O3 reduced the surface concentration of strongly bound ad-NOx species which were thought to react with the reductant or derived species to yield the organo-nitrogen species.

Solar Powered Solid Adsorption Cold Store

Experimental results obtained on a 12 m/sup 3/ solar-powered solid-adsorption cold store are presented. These results are compared to the predictions of a simplified model. On the basis of this model, performance of a similar installation under various conditions (location, orientation) and other technological possibilities are given.

Oxidative dehydrogenation of propane over molybdenum-containing catalysts

Abstract Catalysts consisting of molybdena supported on different oxides (niobia, alumina, zirconia, silica, magnesia and titania) have been compared for the oxidative dehydrogenation of propane. Of these, titania-supported molybdena was found to be the most selective at isoconversion. A coverage of the titania support greater than a monolayer was necessary in order to achieve an improved selectivity towards propene, with an optimum at approx. 2.4 monolayers. In addition to giving propene and carbon oxides, the majority of the catalysts also had a low selectivity towards oxygenated compounds, mostly acrolein. Some loss of molybdenum was also found with most of the catalysts. Addition of the oxides of vanadium and niobium to the titania supported molybdena material gave an increase in the rate of the reaction without any loss in the selectivity to propene. This formulation gave a catalyst whose performance was close to a NiMoO 4 catalyst examined under similar experimental conditions.

Effective bulk and surface temperatures of the catalyst bed of FT-IR cells used for in situ and operando studies

The temperature prevailing in the catalyst bed of three different IR spectroscopic reaction cells was assessed by means of thermocouples, an optical pyrometer and reaction rate measurements. One of the cells was a custom-made transmission FT-IR cell for use with thin wafers and the two others were commercial Harrick and Spectra-Tech diffuse reflectance FT-IR (DRIFTS) cells used for the analysis of powdered samples. The rate of CO methanation measured over a 16 wt% Ni/alumina catalyst was used as a means to derive the effective temperature prevailing in the IR cells from that existing in a traditional (non-spectroscopic) reactor having a well-controlled temperature. The sample bed of these three IR cells exhibited a significantly lower temperature than that of the corresponding measure thermocouple, which was yet located in or close to the sample bed. The comparison of Arrhenius plots enabled us to determine a temperature correction valid over a large temperature range. The use of an optical pyrometer was assessed with a view to determining the temperature of the surface of the powdered beds and that at the centre of the wafer. The optical pyrometer proved useful in the case of the catalyst powder, which behaved as a black non-reflecting body. In contrast, the temperature reading was inaccurate in the case of the pressed wafer, probably due to the shiny surface and minute thickness of the wafer, which led to a significant portion of the IR radiation of the surroundings being reflected by and transmitted through the wafer. The optical pyrometer data showed that the temperature of the surface of the powdered beds was significantly lower than that of the bulk of the bed, and that the total flow rate and composition did not affect this value. This work emphasises that the effective bed temperature in spectroscopic cells can be significantly different from that given by measure thermocouples, even when located in the vicinity of the sample, but that the calibration curves derived from rate measurements can be used to overcome this problem.

Frequency response for nonisothermal adsorption in biporous pellets

The frequency response for nonisothermal adsorption in a biporous pellet is analyzed theoretically, using a mathematical model which includes heat and mass transfer resistances in both micropores and macropores. It is confirmed that, when the heat effect is involved, the out-of-phase component may exhibit a bimodal form. Moreover, it is shown that when both macropore diffusion and micropore diffusion resistances are comparable, macropore diffusion behaves like a surface barrier and leads to an intersection of the in-phase and out-of-phase response functions. When either micropore diffusion or macropore diffusion alone is dominant, the frequency response is essentially the same and, therefore, provides no information concerning the nature of the controlling diffusional resistance. Experimental data for light linear paraffins-5A, reported by Yasuda (1991, J. phys. Chem. 95, 2486–2492), are reanalyzed by the present nonisothermal model. It turns out that the reported experimental response can be equally well represented by a nonisothermal model using several different combinations of mass transfer resistances. It, therefore, appears that the bimodal behavior of the experimental out-of-phase data is caused by the heat effect, thus contradicting the conclusion of Yasuda that there are two adspecies with different mobilities. With the reported data, it is, however, not possible to extract reliable values for the intracrystalline diffusion coefficient, and the nature of the controlling mass transfer resistances cannot be established with certainty.

Mechanistic aspects of the steam reforming of methanol over a CuO/ZnO/ZrO2/Al2O3 catalyst

CO is not a primary but is a secondary product of the steam reforming of methanol over a CuO/ZnO/ZrO2/Al2O3 catalyst, CO is formed as a secondary product by the reverse WGS reaction only when the methanol has reacted.

Thermal Diffusivity and Adsorption Kinetics of Silica-Gel/Water

There have been performed experimental measurements of effective thermal conductivity of silica-gel for a stagnant cylindrical fixed bed under transient and steady state conditions in the presence of dry air at different pressures and for different amounts of adsorbed water. The Bauer-Schlünder and Kunii-Smith models have been used to identify the thermal solid conductivity of silica gel pellets from measurements of the conductivity in an adsorbent bed. Sorption rates of water vapor in silica gel were measured using a single-step thermal method by monitoring the sample surface temperature by infrared detection. In order to obtain the mass diffusivity it is necessary to match the numerical solution of the mathematical model to the experimental data.

Bridging the Gap between Surface Science and Industrial Catalysis

Understanding the structure-activity relationships of catalytic solids has always been hampered by the complexity and nonuniform structures of catalyst particles. Materials based on well-defined colloidal metal particles are ideal model solids to investigate such structure-activity relationships. A new paper by Tsang et al. in this issue indicates that highly selective alpha,beta-unsaturated aldehyde hydrogenation Pt-based catalysts can be obtained following the decoration of Pt nanocrystals with a second metal. The effects of the particle size, substrate shape, and electronic modifications were related to the sample activity. The possibility of depositing these tailor-made colloidal particles onto conventional supports opens exciting new routes toward rational catalyst design and ultraselective catalytic processes.

Influence of Traces of Water on Adsorption and Diffusion of Hydrocarbons in NaX Zeolite

Measurements have been performed on the influence of a small amount of water on adsorption properties of alkanes (C3 to C6) and olefins (C3 and C4) in NaX zeolite. Adsorption capacity and heat of adsorption have been measured by a Volume Step method. Kinetics has been measured by Thermal Frequency Response method and the results have been compared with the results obtained by PFG-NMR.It has been found that water reduces the adsorption capacity especially at low adsorbate concentration. The heat of adsorption is slightly reduced by water. The kinetics of alkanes is always reduced by the adsorbed water. On the contrary, the diffusion kinetics of olefins is improved or remains unchanged, depending of the sorbat concentration.