C. Gauthier

Effect of support material on the catalytic combustion of diesel soot particulates

Several potential support materials were mixed with soot and subsequently the combustion temperature of the soot in 15% O2 was determined. Al2O3 and SiO2 showed no catalytic effect, TiO2 and ZrO2 lowered the soot combustion temperature with 80–90 K, whereas CeO2, La2O2CO3 and V2O5 (reference catalyst) showed a substantial activity for soot combustion. After poisoning with sulfur dioxide, the catalytic effect of CeO2 was strongly inhibited, whereas La2O2CO3 and V2O5 retained their high activity.

Nitrogen dioxide effect in the reduction of nitric oxide by propane in oxidizing atmosphere

The role of nitrogen dioxide in the selective reduction of NO by propane over a Cu-MFI zeolite is investigated. NO2 and NO reductions were carried out under similar conditions of reaction. In the presence of oxygen, the reduction of NO by C3H8 does not differ significantly from that of NO2. In the absence of oxygen, the reduction of NO2 by propane occurs with a partial decomposition of the nitric dioxide molecule. Such a decomposition leads to the formation of oxygen, which is responsible for the increase in catalytic activity by comparison with the same reaction performed with NO. NO2 formed and released in the gas phase during the reduction of NO by propane in the presence of oxygen does not play a predominant role in the catalytic process.

Influence of the preparation method on the selective reduction of nitric oxide over Cu-ZSM-5. Nature of the active sites

Abstract Copper ions were introduced in a Na-ZSM-5 zeolite by using exchange, impregnation and precipitation procedures. The catalytic activity of the three solids was measured in the reduction of NO by propane in the presence of oxygen (0 to 10 vol.-%) in the 423–773 K temperature range. At a given temperature the activity for nitrogen formation remarkably increased with the oxygen content in the 0.5–2 vol.-% range, then slightly decreased for higher amounts of oxygen. For an oxygen content exceeding 0.5 vol.-% and regardless of the reaction temperature, the Cu-ZSM-5 sample prepared by exchange was the most active catalyst, the catalytic activity being expressed per gram of introduced copper. The adsorption of carbon monoxide followed by FT-IR spectroscopy suggested that copper was present as isolated Cu+ ions in the three catalysts. The dispersion of cuprous ions was the highest for the sample prepared by the exchange process. The catalytic activity appeared related to a high dispersion of isolated Cu+ ions in the MFI framework.

Low temperature diesel soots combustion using copper based catalysts modified by niobium and potassium promoters

Abstract Gravimetric temperature programmed oxidation was used to study the combustion of a diesel soot mixed with copper catalysts supported on La 2 O 3 or La 2 O 2 CO 3 . In a first step, different systems associating copper oxide with an other metal oxide were prepared and tested in presence of SO 2 . The association of copper and niobium was found the most active. The influence of alkali on the activity was also studied. It results that potassium is the most effective in lowering the combustion temperature domain in agreement with literature. Finally, CuNbK catalysts deposited on lanthanum oxide have an improved catalytic activity at low temperatures compared to CuVK or CuMoK/TiO 2 , reported in literature. For this catalyst, the maximum oxidation rate was observed at ca. 300°C with the combustion starting at about 250°C. A similar behaviour is obtained when replacing Nb by Ta or the support La 2 O 3 by either La 2 O 2 CO 3 or TiO 2 .

Preparation of alumina supported ceria. I: Selective measurement of the surface area of ceria and free alumina

In a general study of the role played by each component of three-way catalysts, a selective measurement of alumina and ceria surfaces area has been developed on model ceria-alumina supports. Three CeO2/Al2O3 solids (6.5, 13.4, 21 wt.% CeO2) were prepared by grafting Ce acetylacetonate on the surface of an alumina and calcination at 673 K. They were characterized by BET and X-ray diffraction measurements. The hydrogen temperature-programmed reduction (TPR) resulted in profiles similar to those of unsupported cerias, thus allowing an estimation of the equivalent surface area of ceria alone from the hydrogen uptake at low temperature. By adsorbing CO2 on the OH groups of the alumina surface, hydrogenocarbonates species were formed and the IR band at 1235 cm−1 was selected to determine quantitatively the unpertubed alumina surface. In that case, by comparison with the BET surface area, it was possible to deduce an other value of the ceria surface area. The two sets of values obtained by these independent methods were found in good agreement, thus giving a good support for the reliability of each method. They show that the unpertubed alumina surface decreases when increasing the cerium content. Only 20% of the original alumina surface is preserved in the support containing 21 wt.% CeO2.

Evolution of the ceria surface area of PtRh ceria-alumina catalysts submitted to hydrothermal aging at 1273 K

The accessible surface area of cerium oxide was measured on model ceria-alumina supported Pt and/or Rh catalysts submitted to an hydrothermal treatment at 1273 K (10% H2O in N2). Two distinct methods, developed for fresh catalysts, were utilized. They are based respectively on the hydrogen temperature programmed reduction (TPR) profiles and the infrared spectra of the specific adsorption of CO2 on the hydroxyl groups of alumina. After aging, the TPR curves have become flattened and it is difficult to have a good reliability in the deconvolution of the peak corresponding to the ceria surface reduction. On the support alone, the results of both methods are in agreement and indicate a decrease of the ceria surface area by a factor close to 4, whereas the total surface area is only divided by 2. It appears that cerium oxide sinters more rapidly than alumina. For the metallic catalysts, the calculated ceria surface areas values depend on the hypothesis done on the oxidation state of the precious metals before the TPR. If reliable results can be obtained in the case of rhodium with ORh=1.5, in presence of platinum, the mean oxidation state of platinum OPt=1 results in underestimated values. The ceria surface areas deduced from the infrared results are higher than those deduced from TPR. Although the former are uncertain, they are in good agreement with a lower coverage of the alumina surface by sintered cerium oxide particles.

Influence of the activation temperature on the metal accessibility in model three-way catalysts

Abstract The influence of the reduction temperature on the accessibility of the metallic surface was studied on model ceria-alumina supported platinum or rhodium catalysts. For a 0.5% Pt-Ce/Al solid, the H M values, deduced from hydrogen irreversible chemisorption, decrease deeply with Tr, the reduction temperature, from 60% at Tr = 300°C to 19% at Tr = 500°C. This can be attributed to strong interactions between ceria and platinum, since, the initial H2 chemisorption could be restored after reoxidation. The presence of BaSO4 in the support accelerates the loss of metallic area, because of sulfur poisoning of the platinum surface. For Tr = 300°C, the dispersion values were in agreement with those deduced from FTIR spectroscopy of adsorbed CO. In the case of rhodium, a 37% H/M dispersion was obtained, which did not change when Tr was increased from 300 to 500°C. For two industrial Pt-Rh three-way catalysts, the behaviour was found similar to that of platinum, the amount of chemisorbed hydrogen decreasing for Tr > 350°C. Thus, in the three-way catalysts characterization, the maximum metal accessibility is obtained after a reduction at 300°C.

Selective reduction of nitrogen oxide with hydrocarbons and hydrothermal aging of Cu-ZSM-5 catalysts

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H 2 O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N 2 increases with the number of isolated Cu 2+ /Cu + ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H 2 O) mixture, because of nitric acid formation linked to NO 2 which is either formed in the pipes of the apparatus or on the catalyst itself.

Influence of the copper dispersion on the selective reduction of nitric oxide over Cu/Al2O3: Nature of the active sites

Cu/Al2O3 solids with various Cu loadings, between 0.3 and 6.4 wt %, are used for the reduction of NO by propane in the absence and in the presence of oxygen (up to 10 vol. %) in the 423–773 K temperature range. At a given temperature and for high Cu loadings, the introduction of oxygen induces a decrease in the activity in nitrogen formation. For low Cu loadings the activity increases with the oxygen content in the 1–2 vol. % range, then slightly decreases for higher oxygen amounts. The nature of the Cu species accessible to CO and NO is determined by infrared spectroscopy. High Cu loadings favor the formation of bulk oxides at the surface of the support whereas low Cu loadings favor the formation of isolated Cu species. The selective reduction of NO is thus related to the presence of these isolated copper species easily reduced and reoxidized.