Michel Primet

Complete Oxidation of Methane at Low Temperature over Noble Metal Based Catalysts : A Review

Abstract This review examines recent developments in the complete oxidation of methane at low temperature over noble metal based catalysts in patents and open literature. The abatement of natural gas vehicle (NGV) methane emissions is taken as one example among possible applications. The review develops current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane under oxidising conditions, focusing on low-temperature reaction conditions. The influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of metal particle size and reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds are examined. The latest studies concerned with improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature in the presence of water and sulphur containing compounds are reported. Possible routes for preparing catalysts able to meet future regulations concerning methane emissions from lean-burn NGV vehicles are discussed.

Catalytic oxidation of methane over palladium supported on alumina : Effect of aging under reactants

Abstract A Pd/Al 2 O 3 catalyst was aged at 600°C under a methane, oxygen and nitrogen mixture with an oxygen to methane ratio of 4. The aged catalyst was more active than the fresh one, especially at low temperature (below 400°C). The temperature of half conversion was decreased by 70°C. Aging led to a decrease of the metal dispersion: the palladium particle size increased from 7-16 nm. The turnover number (activity per surface palladium atom) was strongly enhanced (factor 20 at 400°C) on the aged sample. In that sense, catalytic combustion of methane must be considered as a structure-sensitive reaction. The reactivity of adsorbed oxygen towards hydrogen increased with the metal particle size; this parallels the increase in the rate of methane oxidation with particle size. Temperature-programmed oxidation measurements suggested that palladium was in the form of bulk palladium oxide for reaction temperatures in excess of 400°C. The formation of bulk PdO appears to be also metal particle size dependent. An instability of the conversion as a function of time at a given temperature was observed. The instability was associated with the slow formation of bulk PdO.

Combustion of methane on CeO2–ZrO2 based catalysts

Abstract CeO 2 –ZrO 2 solid solutions have been prepared by precipitation of the corresponding hydroxides. They were calcined and aged at various temperatures and characterised by X-ray diffraction measurements, BET and TPR measurements as well by their activity in methane combustion. The Ce 0.67 Zr 0.33 O 2 solid exhibited the best thermal stability, the highest oxygen mobility and the best catalytic activity after ageing at 1000°C. It has been used to support active phases like platinum and manganese oxide. The fresh Pt/Ce 0.67 Zr 0.33 O 2 catalyst was much more active than the corresponding Pt/Al 2 O 3 solid although a deactivation on stream was observed in the 200–500°C temperature range. Nevertheless, the promoting effect of the Ce 0.67 Zr 0.33 support disappeared after ageing at 1000°C. In the case of MnO x supported onto the Ce 0.67 Zr 0.33 O 2 solid solution the activity of the fresh solid is similar to that of the MnO x /Al 2 O 3 catalyst. After ageing at 1000°C, the solid solution is decomposed, the BET area dramatically decreased and the catalytic properties almost disappeared. As far as temperature applications exceeding 1000°C are concerned, the CeO 2 –ZrO 2 solid solutions are not suitable supports for the catalytic combustion of methane.

Stabilization of alumina toward thermal sintering by silicon addition

Abstract The thermal stability of gamma alumina toward sintering at high temperatures (in the 1050–1220°C range) in the presence of steam has been studied from both structural and textural points of view. Improvement of the stabilization of alumina has been found be grafting silicon-containing compounds on the alumina surface. A reaction between the hydroxyl groups of alumina and the silicon-containing precursor was found to occur. The silicon-modified alumina is constituted by a layer of silica firmly bonded to the alumina support. The stabilization increases with increasing silicon loadings and reaches a plateau for ca. 3 wt% silicon.

Deactivation of nickel-based catalysts during CO methanation and disproportionation

The complex process of deactivation of Ni-based catalysts during CO methanation and disproportionation is studied by means of coupled kinetic and in situ physical methods (magnetism and infrared spectroscopy). Detailed analyses of the two main phenomena, namely metal sintering and carbon deposition, which contribute simultaneously to the overall deactivation are provided. The initial dispersion, the composition and pressure of the methanation feed, and the ability of the support to favor the water gas shift reaction are shown to play major roles in both the sintering mechanism and the surface poisoning by carbon.

Catalytic behaviour of Cl-free and Cl-containing Pd/Al2O3 catalysts in the total oxidation of methane at low temperature

Abstract Pd/Al2O3 catalysts prepared by the impregnation of alumina free of chlorine with H2PdCl4 and Pd(NO3)2 precursors were characterised and their catalytic activity in methane oxidation was measured under oxygen-rich conditions. The presence of residual chlorine ions either originating from the precursor or from impregnation with HCl was shown to strongly inhibit the conversion of methane. Residual chlorine ions could be slowly removed under reaction conditions in the form of HCl. This was observed by in situ measurement of HCl departure and catalytic activity. Experimental data were explained by a simple model in which residual chlorine would block PdO surface active sites during the reaction. The resulting catalyst, free of chlorine ions, was similar to the one prepared from the nitrate precursor in terms of dispersion and activity. Chlorine-free catalysts exhibited slow deactivation with time on stream, which was tentatively attributed to the slow conversion of the active PdO phase into a less active Pd(OH)2 phase. This interpretation was supported by the poisoning effect of water vapour observed upon addition of water to the feed while CO2 had no influence. Regeneration of the catalyst could be achieved by purging in dry carrier above 500°C, which could be ascribed to the decomposition of Pd(OH)2 into PdO.

Catalytic combustion of methane on substituted barium hexaaluminates

Abstract A sol–gel method using metallic barium, aluminum alkoxides and metal nitrates has been used to synthesize barium hexaaluminate partially substituted by either manganese, iron or both metal ions. The β-alumina structure was obtained by calcination under oxygen at 1200°C. X-ray analysis revealed that formation of a pure single phase BaMxAl12−xO19 occurred up to x=4 for Fe, x=3 for Mn and for Fe1Mn1 in the case of mixed substituted hexaaluminates. Incorporation of Mn in excess leads to another phase formation (manganese oxide or spinel). As far as the valence state of transition metal ions is concerned, the introduced Fe ions were always trivalent, whereas the Mn ones were either divalent or trivalent. In the latter case, the first Mn ions were introduced in the matrix essentially as Mn2+ and only for BaMn3Al9O19 does manganese exist exclusively as Mn3+, the higher the Mn concentration, the higher the proportion of Mn3+. All solids were aged at 1200°C under water and oxygen and showed a good thermal resistance. Activity for methane combustion has been measured for fresh and aged solids, light-off temperatures were observed in the 560–640°C range. However, the highest activity was obtained for catalysts containing either 3 Mn, 2 Fe or 1 Fe+1 Mn ions per unit cell. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.

About the nature of the Co-Cu interaction in Co-based catalysts for higher alcohols synthesis

As evidenced from magnetic measurements and FTIR spectroscopy of CO, the interaction of cobalt and copper in CoCu-based catalysts results in a surface CoCu alloy. It is proposed that this alloy is responsible for the orientation of the CO + H2 reaction towards higher alcohols, probably via a dual-site mechanism which involves both Co and Cu of the alloy.

Catalytic combustion of methane over copper- and manganese-substituted barium hexaaluminates

Copper- and manganese-substituted barium hexaaluminates were prepared by sol–gel method from metal alkoxides. The preparation conditions strongly influence the textural properties of the solids obtained. Manganese and copper occupy different crystallographic sites in the hexaaluminate structure: Mn3+ ions are located in octahedral sites, while Cu2+ enters only tetrahedral positions. The Cu sites are intrinsically more active than Mn sites for methane combustion, but the Cu-based catalysts are penalized by lower surface areas and by the lower limit of copper incorporation in the hexaaluminate matrix: manganese substitution for aluminium is possible up to ≈3 Mn per unit cell, while copper substitution is limited to about 1.3 Cu per unit cell. The catalytic activity of the Mn-substituted barium hexaaluminates increases with Mn substitution, the optimum composition being obtained when about 3 Mn ions per unit cell are incorporated, as regards the activity and the resistance to ageing at 1200°C in the presence of steam. This is related not only to the Mn content, but also to the higher Mn3+/Mn2+ ratio when the amount of manganese introduced increases, as shown by TPR and Auger parameter measurements. The catalytic activity can be correlated with the fraction of reducible manganese species.

Catalytic combustion of methane on aluminate-supported copper oxide

Abstract Copper oxide has been deposited onto high surface area magnesium aluminate spinel prepared from alumina and magnesium nitrate. The catalytic properties of such a solid have been investigated in methane combustion. At the laboratory scale a very good activity is observed (light-off of 530°C) and no CO is detected. Aging at 1000°C under water vapour has no influence on activity. The previous catalyst has been washcoated on monolith and tested on a rig either with methane or synthetic natural gas at very high GHSV under conditions close to those of a gas turbine. In that case also, a good activity was observed.