J.R.H. Ross

Review of literature on catalysts for biomass gasification

Biomass gasification is a possible alternative to the direct use of fossil fuel energy. Biomass, a CO2 neutral source of renewable fuel, can contribute to the demand for heat, electricity and synthesis gas. However, there are inefficiencies in the technology, which at present render biomass gasification economically unviable. The presence of condensable organic compounds and methane in the product gas renders the gas unsuitable for specific applications. Elimination of the condensable organic compounds and methane by a suitably cheap technology will enhance the economic viability of biomass gasification. This paper contains an extensive literature review of the three main groups of catalysts, which have been evaluated for the elimination of these hydrocarbons. These three groups of catalysts are dolomite, alkali metals and nickel.

Methanol reforming for fuel-cell applications : development of zirconia-containing Cu-Zn-Al catalysts

Abstract The steam reforming of methanol to form mixtures of carbon dioxide and hydrogen, together with traces of carbon monoxide, is considered to be a potential source of hydrogen as the fuel for a fuel-cell to be used in mobile power sources. After outlining some of the constraints inherent in the use of the reaction and the types of catalysts which have been used by other investigators, this paper presents results on the preparation and testing of a series of copper-containing catalysts for this reaction. It is shown that the reaction sequence probably involves the formation of methyl formate which then decomposes to give CO 2 as the primary product; CO is formed by the reverse water–gas shift reaction and this only occurs to an appreciable extent when the methanol is almost completely converted. A number of different copper-containing catalysts are then described and it is shown that of these sequentially precipitated Cu/ZnO/ZrO 2 /Al 2 O 3 materials have the highest activities and stabilities for the steam reforming reaction.

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.

Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2 and of SO2 to SO3

Three types of platinum catalysts, Pt/SiO2, Pt/?-Al2O3 and Pt/ZrO2 were examined for the oxidation of NO to NO2 of SO2 to SO3. The activity order for both oxidation reactions was found to be: Pt/SiO2 > Pt/?-Al2O3 > Pt/ZrO2. The effect of Pt loading and Pt dispersion on the catalytic activity and selectivity was examined. Over Pt/SiO2, the specific activities were found to be strongly size-dependent, the larger Pt particles exhibiting higher specific activity than the smaller ones. Over the Pt/?-Al2O3 catalysts, however, the size-dependence appeared to be much less significant than that of the Pt/SiO2 catalysts. The results for Pt/ZrO2 catalysts, however, showed that the specific activities for both reactions were most probably size-independent. The catalytic selectivity for the oxidation of NO relative to that of SO2 appeared to be size-independent for all the catalysts. The interaction of NO, NO2 and SO2 with the catalysts and the effect of the support on the interaction were investigated using TPD technique.

Zirconia as a support for catalysts: influence of additives on the thermal stability of the porous texture of monoclinic zirconia

A single-phase monoclinic zirconia (the thermodynamically stable modification up to a temperature of 1170°C), having a specific surface area of 67 m2g?1 and a well-developed mesoporous texture, has been prepared by gel-precipitation followed by calcination at 450°C. A commercially available high-surface area monoclinic zirconia powder (SBET=71 m2g?1) has also been studied. It was found that the specific surface area and pore volume of monoclinic zirconia both decreased markedly on increasing the calcination temperature; despite the fact that the crystal structure was that of the stable modification, this did not seem to impart any substantial resistance to thermal sintering. The thermal stability of monoclinic zirconia could however be improved significantly by addition (by an impregnation technique) of various oxides: CaO, Y2O3, La2O3 all led to an improvement in the thermal stability up to 900°C while MgO exhibited stabilizing properties only up to 700°C; the best results were obtained with La2O3. All the additives investigated other than MgO were found to bring about a partial transition of the monoclinic to a fluorite-like phase of zirconia upon heat treatment; this phase has been shown in the case of the CaO-doped sample to be cubic zirconia and in the cases of the Y2O3- and La2O3-doped samples to be tetragonal zirconia. As little as 20?50% of a theoretical monolayer quantity of La2O3 was sufficient to give satisfactory thermal stability. The results can be explained by a model involving mass transport by a surface diffusion mechanism.

Stabilized tetragonal zirconium oxide as a support for catalysts: evolution of the texture and structure on calcination in static air

Single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y3+ or La3+ in ZrO2 to form solid solutions. The samples were prepared by controlled coprecipitation from aqueous solutions of the respective metal chlorides at room temperature and at a constant pH of 10, followed by calcination at 500°C (in the case of the Y3+ -doped sample) or 600°C (in the case of the La3+ -doped sample) to effectuate the crystallization into the tetragonal phase. The process of crystallization of the hydrous zirconia precursor was found to be retarded by the incorporation of Y3+ or La3+, the latter giving the greater effect. Upon crystallization, stabilized tetragonal samples were obtained with high specific surface areas (SBET ca. 88 m2 g?1 for both the samples) and well-developed mesoporous textures but without any microporosity. Both the Y3+ - and the La3+ -alloyed ZrO2 samples were found to fully retain the tetragonal phase upon calcination over the entire range of temperatures studied (up to 900°C). The thermal stability of the texture of zirconia was found to be considerably improved, in comparison with the undoped monoclinic material, by the stabilization of the crystal structure in the defect tetragonal form. In particular, incorporation of 5.4 mol-% of La3+ resulted in a support material which had a remarkable thermal stability. It is shown that the improvements in the thermal stability are derived from a strong inhibition of the processes of crystallite growth and the accompanying intercrystallite sintering and thus of the process of mass transport; the mass transport probably occurs by a mechanism of surface diffusion.

The effect of O2 addition on the carbon dioxide reforming of methane over Pt/ZrO2 catalysts

Abstract Pt/ZrO2, Pt/Al2O3 and Ni/Al2O3 catalysts were found to be active in the temperature range 550–800°C under the conditions for both CO2 reforming and partial oxidation of methane; of those, the Pt/ZrO2 material was found to be least prone to deactivation. Combined CO2 reforming and partial oxidation of methane was used to produce synthesis gas over the Pt/ZrO2 catalyst. Higher yields of synthesis gas were obtained at lower temperatures than would be obtained with CO2 reforming of methane alone. Combining the endothermic carbon dioxide reforming reaction with the exothermic partial oxidation reaction, hot spots in the catalyst bed were reduced significantly. Furthermore, the loss of activity of the catalyst with time on stream decreased with the amount of O2 added to the feed stream; a small amount of carbon deposition occurred in parts of the catalyst bed which were not exposed to oxygen. The reaction pathway for the partial oxidation and the combined partial oxidation and reforming reactions appeared to be methane combustion followed by reforming of the remainder of the methane by the resultant CO2 and steam.

Water-gas shift conversion using a feed with a low steam to carbon monoxide ratio and containing sulphur

Based on thermodynamic calculations a discussion was given on the need for operation of the water-gas shift (WGS) reaction with a low H2O/CO ratio when a H2-separation membrane is combined with the reaction and on the degree of possible formation of C and CH4 as the result of side reactions occurring in the mixture of CO, H2O, CO2 and H2 under WGS conditions. The behaviour of a number of materials including FeCr, CuZn, CoCr and Pt/ZrO2 were examined under the conditions where the ratio of H2O to CO in the feed was low, with and without the presence of H2S in the feed. The experimental results suggested that Pt/ZrO2 has potential for use in these conditions.

Evidence for the participation of surface nickel aluminate sites in the steam reforming of methane over nickel/alumina catalysts

The specific activities of various NiAl2O3 catalysts for the reaction of CH4 with H2O have been obtained and have been shown to vary markedly with catalyst preparation and to differ considerably from the specific activities of pure nickel. This has been explained by suggesting that the unreduced catalysts contain surface nickel aluminate phases which, on reduction, give monodispersed nickel atoms closely associated with alumina sites in addition to metallic crystallites arising from the reduction of nickel oxide. The results of exchange experiments using deuterium and H218O are presented in support of the suggestion that the monodispersed nickel atoms probably participate in the CH4 + H2O reaction.

The activity of supported vanadium oxide catalysts for the selective reduction of NO with ammonia

The activities of monolayer V2O5 catalysts for the selective reduction of NO with NH3 are compared with those of commercial available catalysts containing V and/or W. From steady state and pulse experiments it can be concluded that the reduction of surface sites proceeds either by NH3 + NO or by NH3 alone. The reoxidation of the reduced sites occurs by gaseous oxygen or NO. The experimental reaction stoichiometry can be explained in terms of suitable combinations of these four reactions.