Robbie Burch

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.

Metal-catalysed steam reforming of ethanol in the production of hydrogen for fuel cell applications

A range of oxide-supported metal catalysts have been investigated for the steam reforming of ethanol/water mixtures for the production of hydrogen. Alumina-supported catalysts are very active at lower temperatures for the dehydration of ethanol to ethene which, at higher temperatures, is converted into H2, CO, and CO2 as the major products and CH4 as a minor product. The order of activity of the metals is Rh>Pd>Ni=Pt. With ceria/zirconia-supported catalysts, the formation of ethene is not observed and the order of activity at higher temperatures is Pt≥Rh>Pd. By using combinations of a ceria/zirconia-supported metal catalyst with the alumina support it is shown that the formation of ethene does not inhibit the steam reforming reaction at higher temperatures. It is concluded that the support plays a significant role in the steam reforming of ethanol.

An investigation of alternative catalytic approaches for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

Abstract Catalytic systems for the direct production of hydrogen peroxide from hydrogen and oxygen are investigated, and the factors which make a successful process identified. The use of low metal loadings, an organic co-solvent (such as ethanol) and reduced palladium as the catalytic metal all lead to good activity and selectivity.

The effect of the addition of sodium compounds in the liquid-phase hydrodechlorination of chlorobenzene over palladium catalysts

Abstract The hydrodechlorination of chlorobenzene over supported palladium catalysts has been studied. The palladium catalysts deactivate as the reaction proceeds due to the HCl formed as by-product. The effect of the addition of sodium compounds has been analysed for the neutralisation of HCl. When NaOH was added to the reaction mixture, no beneficial effect was observed due to the detrimental effect of the alkaline medium on the textural and metallic properties of the catalysts. Doping the support with NaOH prior to impregnation with the metal precursor leads (after calcination and reduction) to catalysts with better activity and tolerance to deactivation, especially those obtained when using PdCl 2 as the metal precursor. Low metal dispersion and the capture of chloride by forming NaCl are the main factors contributing to the improved catalytic properties. Finally, doping the catalysts with NaOH or NaNO 3 , after reduction of the metal precursor leads to a moderate increase in initial activity and final conversion, although NaOH impregnation also gave rise to support corrosion and metal dispersion modification.

N2O and NO2 formation on Pt(111): A density functional theory study

Catalytic formation of N2O and NO2 were studied employing density functional theory with generalized gradient approximations, in order to investigate the microscopic reaction pathways of these catalytic processes on a Pt(111) surface. Transition states and reaction barriers for the addition of chemisorbed N or chemisorbed O to NO(ads) producing N2O and NO2, respectively, were calculated. The N2O transition state involves bond formation across the hcp hollow site with an associated reaction barrier of 1.78 eV. NO2 formation favors a fcc hollow site transition state with a barrier of 1.52 eV. The mechanisms for both reactions are compared to CO oxidation on the same surface. The activation of the chemisorbed NO and the chemisorbed N or O from the energetically stable initial state to the transition state are both significant contributors to the overall reaction barrier Ea, in contrast to CO oxidation in which the activation of the O(ads) is much greater than CO(ads) activation.

The mechanism of N2O formation via the (NO)(2) dimer: A density functional theory study

Catalytic formation of N(2)O via a (NO)(2) intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)(2) and cis-(NO)(2) configurations. A possible pathway involving (NO)(2) formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N(2)O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO + H(2) reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.

Bridging the Gap between CO Adsorption Studies on Gold Model Surfaces and Supported Nanoparticles

An in-depth understanding of CO adsorption on highly dispersed gold nanoparticles (AuNPs) is critically important to fully interpret the catalytic behavior of supported gold systems in processes such as CO oxidation, PROX (selective oxidation of CO in presence of a large excess of H2), [5–7] or LT-WGS (low-temperature water gas-shift) reactions. Despite its relevance, the quantitative data and fundamental information presently available on the CO–Au interaction mainly comes from studies carried out on model single-crystal and thin-film surfaces under experimental conditions far from those at which catalytic assays on supported gold systems are typically run. Probably because of the very weak and singular nature of the CO–Au interaction, which on the basis of both theoretical and experimental studies is generally acknowledged to take place on low-coordination surface sites, and the additional contribution of the support, a few studies have been carried out that were aimed at estimating the amount of CO adsorbed at low temperature on powdered or model supported AuNPs. To our knowledge, however, none of these have arrived at a detailed quantitative description of this process under conditions close to those occurring in real catalytic reactions. To bridge this gap, we have developed an approach in which AuNP size distributions, as determined from HAADFSTEM (high-angle annular dark-field scanning transmission electron microscopy) and quantitative CO adsorption data, as determined from volumetric adsorption at 308 K, under CO partial pressures ranging from 6.65 10 Pa to 3.99 10 Pa, are jointly analyzed with the help of a nanostructural model for the gold particles. This model could be deduced from the analysis of images recorded in a parallel HRTEM (highresolution transmission electron microscopy) study. As discussed herein, this approach gives a substantial experimental support to the extension to supported gold catalysts of the chemical principles governing the CO adsorption on model surfaces. We investigated two catalyst samples, 2.5 wt% Au/ Ce0.62Zr0.38O2 (Au/CZ) and 1.5 wt% Au/Ce0.50Tb0.12Zr0.38O2 x (Au/CTZ), which have significantly different gold particle size distributions. Two consecutive CO volumetric adsorption isotherms were recorded on the gold catalysts and the corresponding supports. Prior to running the second isotherms, samples were evacuated (residual pressure Pres< 1.33 10 4 Pa) for 30 min at 308 K. By processing the volumetric data in a similar way to earlier studies (for details, see the Supporting Information), the amounts of CO adsorbed on the AuNPs, on the surface cations of the supports (weak adsorption), and on the surface anions of the supports, which mainly consist of strongly chemisorbed carbonate species, could be determined from the difference of the two isotherms. The amount of CO adsorbed on the AuNPs at PCO= 1.33 10 4 Pa (100 Torr) was used as a measurement of the saturation coverage. The corresponding data are reported in Table 1 and the Supporting Information, Figure S1. Gold particle size distributions were determined for each of the investigated catalysts from the analysis of series of experimental ultra-high-resolution HAADF-STEM images (Supporting Information, Figure S2). In accordance with the physical principles lying behind the mechanism of image formation, this technique is particularly suitable for obtaining reliable metal particle size distributions in oxide-supported metal catalysts. Moreover, as recently shown, this technique can be fruitfully applied to a very fine characterization of AuNPs dispersed on mixed oxides of heavy elements, as is the case of those investigated herein. (Size distributions for Au/CZ and Au/CTZ catalysts are shown in the Supporting Information, Figure S2). [*] M. L pez-Haro, Dr. J. J. Delgado, J. M. Cies, E. del Rio, S. Bernal, Dr. M. A. Cauqui, Dr. S. Trasobares, Dr. J. A. P rez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016-288 E-mail: jose.calvino@uca.es

Preparation and characterisation of supported La0.8Sr0.2MnO3+x

Abstract Three supported La 0.8 Sr 0.2 MnO 3+ x catalysts were prepared, one supported on lanthanum-stabilised alumina and two supported on a NiAl 2 O 4 spinel. The catalysts were characterised using X-ray diffraction, transmission electron microscopy and surface area measurements following heat-treatments at temperatures up to 1200°C in air. In the alumina-supported catalyst, a reaction occurred between the active phase and the support at high temperatures, indicating that these materials would be unsuitable for high temperature catalytic combustion. Only in the NiAl 2 O 4 -supported catalysts were the supported perovskite phases found to be stable at high temperature. These catalysts showed good methane combustion activity.

Assessing the surface modifications following the mechanochemical preparation of a Ag/Al2O3 selective catalytic reduction catalyst

The surface modification of a mechanochemically prepared Ag/Al2O3 catalyst compared with catalysts prepared by standard wet impregnated methods has been probed using two-dimensional T1–T2 NMR correlations, H2O temperature programmed desorption (TPD) and DRIFTS. The catalysts were examined for the selective catalytic reduction of NOx using n-octane in the presence and absence of H2. Higher activities were observed for the ball milled catalysts irrespective of whether H2 was added. This higher activity is thought to be related to the increased affinity of the catalyst surface towards the hydrocarbon relative to water, following mechanochemical preparation, resulting in higher concentrations of the hydrocarbon and lower concentrations of water at the surface. DRIFTS experiments demonstrated that surface isocyanate was formed significantly quicker and had a higher surface concentration in the case of the ball milled catalyst which has been correlated with the stronger interaction of the n-octane with the surface. This increased interaction may also be the cause of the reduced activation barrier measured for this catalyst compared with the wet impregnated system. The decreased interaction of water with the surface on ball milling is thought to reduce the effect of site blocking whilst still providing a sufficiently high surface concentration of water to enable effective hydrolysis of the isocyanate to form ammonia and, thereafter, N2.

An investigation of the selective oxidation of NH3 to N2 in gasified biomass in the presence of excess CO and H2 using zeolite catalysts

The selective oxidation of NH3 to N2 in simulated biogas containing a large excess of CO and H2 has been examined using zeolite catalysts. Of the materials examined zeolite Beta gave the highest N2 yield (85% at 475–575°C), while ZSM5 produced 75% at 575°C, but HY was both less active and selective. In all cases N2 is formed via an internal selective catalytic reduction between NOx (derived from the oxidation of NH3) and NH3 adsorbed on Brønsted sites of the zeolite.