Edouard Garbowski

Catalytic combustion of methane over palladium supported on alumina catalysts : evidence for reconstruction of particles

Abstract The physicochemical properties of palladium supported on alumina solids were studied before and after the catalytic combustion of methane by combining nanodiffraction, electron microscopy and Fourier transform IR (FT-IR) analysis. The catalytic activity was strongly increased after reaction at 600°C under the O 2 -CH 4 mixture under reducing as well as under oxidizing conditions, whereas the dispersion of the metallic phase decreased. Fresh Pd/Al 2 O 3 samples exhibited mainly Pd (111) surface crystal planes and an epitaxy with the Al 2 O 3 (222) planes may be considered. After reaction with the two oxygen-methane mixtures, the Pd (200) surface crystal planes developed at the expense of the Pd (111) ones, the formation of the Pd (200) crystal planes was evidenced by nanodiffraction as well as from the FT-IR spectra of adsorbed carbon monoxide. The increase in catalytic activity was connected with the presence of the Pd (200) surface planes. It was proposed that the presence of these latter planes allows the reversible transition from surface Pd 0 to surface PdO without drastic changes of lattice parameters.

Physicochemical properties of copper oxide loaded alumina in methane combustion

Copper oxide was deposited on gamma alumina with loadings between 2.1 and 9.2 wt % of CuO. The activity of these solids was measured in the catalytic combustion of methane before and after an aging test performed at 1340 K. The state and the environment of the copper species was followed by UV–VIS reflectance spectroscopy. Carbon monoxide adsorption on fresh and aged samples was performed and studied by infrared spectroscopy. For fresh samples, the number of active centres decreases when the copper loading increases. At low amounts of CuO, copper oxide does not have the spectral features of bulk oxide whereas the 9.2 wt % CuO sample behaves like bulk CuO. At moderate conversion levels the catalytic activity is directly proportional to the absorbance of the vCO band of carbon monoxide bonded to surface CuII ions of the copper oxide phase, showing that the sites responsible for the catalytic activity are revealed by carbon monoxide adsorption. After the ageing test, the catalytic activity strongly decreases because of the formation of copper aluminate, copper being in the oxidation state + 1 or +2 according to the stoichiometry of the reactants mixture. In that case, the remaining activity is also proportional to the absorbance of the vCO band of carbon monoxide bonded to CuII ions of the copper oxide phase.

Catalytic oxidation of methane over palladium supported on alumina: Influence of the oxygen-to-methane ratio

Abstract The oxidation of methane over a Pd/Al 2 O 3 catalyst was studied for oxygen-to-methane ratios (“ r ”) varying from 4 to 0.66. After aging under the reactants mixture at 600 ° C an increase in the catalytic activity was observed whatever the “ r ” value. In addition, the selectivity of the reaction depends on the oxygen-to-methane ratio. In oxygen deficient mixtures, the formation of carbon monoxide was observed at conversion levels corresponding to the total oxygen consumption. Palladium supported on alumina was found to be active in the methane steam reforming reaction at moderate temperature. This reaction was proposed to be responsible for carbon monoxide formation in the case of methane oxidation in lean mixtures.

Catalytic combustion of methane over bulk and supported LaCrO3 perovskites

LaCrO3 perovskite samples have been prepared in the bulk state as well as in the supported state. Whatever the nature of the precursors, bulk perovskites exhibit low specific areas, leading to a low activity in the catalytic combustion of methane; changes in catalytic activity have been connected with modifications of the B.E.T. areas of the bulk material.The activity of LaCrO3 is strongly enhanced by supporting the perovskite on carriers of high specific area like alumina or magnesium aluminate. In the fresh state, the increase in catalytic activity is due to the increase of the dispersion state of the perovskite phase. In the case of the Al2O3 support, spectroscopic measurements show that ca. 70% of the alumina surface is covered by LaCrO3 and catalytic activity is proportional to the fraction of alumina covered by LaCrO3.Supported perovskites have been aged at 1340 K under a stoichiometric (air–methane) mixture. For the LaCrO3/Al2O3 sample, the loss of activity has been correlated with a sintering of the perovskite phase onto the surface of alumina and does not follow the global area of this material. For the LaCrO3/MgAl2O4 sample, the loss of specific area agrees with the decrease in catalytic activity, suggesting that the coverage of MgAl2O4 by LaCrO3 is preserved, although sintering of the solid occurs.For supported perovskites in both fresh and aged states, no incorporation of Cr3+ ions into the support lattice was observed, probably because of the procedure of preparation.

Catalytic properties of copper oxide supported on zinc aluminate in methane combustion

Copper oxide was deposited on a zinc aluminate prepared from high specific area alumina and zinc nitrate. The activity of the solid obtained has been measured in the catalytic combustion of methane before and after an aging test performed at 1340 K. The environment of the copper species was followed by UV–VIS reflectance spectroscopy whereas the state of copper was deduced from temperature-programmed reduction experiments. By comparison with a series of CuO/Al2O3 catalysts, the CuO/ZnAl2O4 sample is slightly less active in the complete oxidation of methane. This decrease in catalytic activity is connected with the presence of bulk CuO particles on the zinc aluminate support instead of isolated copper ions in the case of the alumina carrier.Slight deactivation was observed after the aging test at 1340 K whereas the CuO/Al2O3 system was strongly deactivated under the same conditions. Spectroscopic and TPR experiments have shown that the copper oxide phase was preserved in the case of the zinc aluminate supported copper oxide. The formation of the copper aluminate phase at the origin of the dramatic loss in activity of the aged CuO/Al2O3 catalyst barely occurs when copper oxide is supported on zinc aluminate.

Chlorinated alumina. Acidic properties and catalytic activity towards n-butane isomerization

γ-Al2O3 has been chlorinated by CCl4 at 573 K in order to increase its acidity. Different physical methods, including infrared spectroscopy, thermogravimetry and thermodesorption of pyridine, have been used to calculate the actual number of acidic sites, according to their strength. Comparison with samples treated with HCl or AlCl3 leads to the conclusion that stronger Lewis sites are in the form of grafted AlCl3. n-Butane isomerization has been performed on such solids. Reaction occurs without the presence of metal and the higher the acidity the higher the activity and selectivity.

Spectroscopic studies of benzene hydrogenation on platinum-loaded zeolites. Part 1.—Benzene adsorption on supports

U.v. diffuse reflectance and i.r. spectroscopies have been used to study the benzene adsorption on silica, alumina, silica—alumina and Y zeolites containing cations or protons. C6H6 is strongly adsorbed on the zeolite support especially in the case of cation-exchanged zeolites.U.v. measurements show that benzene molecules are perturbed by the zeolite surface and that the aromatic ring is no longer planar. I.r. spectra confirm this observation and also show the presence of two types of adsorbed benzene. The first type, more or less strongly held, is directly bonded to the cation by formation of a π-complex and the second type corresponds to benzene molecules in interaction with the cation–C6H6 complex. This latter type might explain the new extra u.v. band observed at 215 nm which is assigned to a charge-transfer complex. In both cases, adsorption leads to a decrease in the aromatic character.

Spectroscopic studies of Ni(I) complexes with CO, O2 and no ligands in a NiCa-Y zeolite

Abstract Ni(I) ions formed in situ by reducing NiCa-Y zeolite by hydrogen at 200°C were allowed to react at room temperature with CO, O 2 or NO adducts and were studied by means of ESR spectroscopy with complementary aid of UV and IR spectroscopies. In the case of CO adduct, a weak interaction occurred between a Ni(I) ion and two coordinated CO molecules with a small unpaired spin delocalization towards CO ligands, the starting ions being restored by outgassing at 100°C. The ESR parameters of the Ni(CO) + 2 complex are as follows: g 1 = 2.225, g 2 = 2.075 anf g 3 = 2.062, a 1 ≈ 34 and a 2 ≈ a 3 = 31 Oc for 13 C hyperfine splitting. In the case of oxygen, dispersed Ni(O) atoms were shown to be slowly oxidized into Ni(I) ions coordinated to O 2 molecules and further to Ni(II) and O − 2 . In the case of NO, oxidation of Ni(O) and Ni(I) occurred very rapidly into Ni(II) coordinated with NO forming a Ni + NO + complex.

Spectroscopic studies of rhodium complexes with CO and CH3I ligands in a NaRhY zeolite

Abstract Carbon monoxide is able to reduce the Rh(III) ions introduced in a NaY zeolite to give rise to gem dicarbonyl species or Rh(I) ions Subsequent oxidative addition of CH 3 I leads to an acetyl complex of Rh (III) In the presence of CO, reductive elimination of CH 3 COI occurs: this acetyl halide can migrate towards the zeolitic support, and the initial Rh(I) (CO) 2 species is re-obtained.

Catalytic properties of chromium–palladium loaded alumina in the combustion of methane in the presence of hydrogen sulfide

Impregnation of γ-Al 2 O 3 by chromium(III) ions leads to surface chromate species after calcination under oxygen at 773 K. This solid is very active in methane combustion, however selectivity towards CO 2 is not complete because some CO is also produced even in the presence of a large excess of oxygen. To prevent the formation of CO, subsequent ultra-low palladium deposition (0.05–0.2 wt% Pd) has been performed on the chromate supported alumina. In this case the activity is unchanged but full selectivity towards CO 2 is now observed. For this bimetallic catalyst, palladium is present in the form of large PdO particles, which do not participate in the conversion of methane to CO 2 , mainly due to their size, however, they promote the oxidation of CO to CO 2 . The presence of H 2 S in the feed leads to a severe deactivation of the Pd/Al 2 O 3 catalyst owing to the formation of sulfate groups at the surface of PdO particles. On the contrary, the activity of the Cr/Al 2 O 3 catalyst was not dramatically affected, but selectivity towards CO was strongly improved. In the presence of H 2 S the chromate groups of the Cr/Al 2 O 3 are reduced to Cr 3+ ions probably in the form of Cr 2 O 3 . For the mixed catalyst containing both palladium and chromium, H 2 S addition has a limited effect on the global catalytic activity, whereas the formation of CO is fully suppressed. An electronic interaction between chromium oxide and palladium oxide is postulated leading to a weakening of the Pd–S bond of the adsorbed sulfate.