Bimetallic Au-Ag/Al2O3 as efficient catalysts for the Hydrocarbon Selective Reduction of NOx from lean burn engine exhaust

International audienceCatalytic properties of Au-Ag/Al2O3 catalysts, prepared by successive impregnation, have been investigated for the hydrocarbon selective catalytic reduction (HC-SCR) of NOx. The performances of pre-reduced and aged catalysts were evaluated on synthetic reaction mixtures with gas compositions representative of the exhaust of Diesel powered engines. An optimal 2 wt.% Ag loading was chosen and the order of introduction during sequential impregnation of Ag or Au on respectively pre-impregnated Au/Al2O3 and Ag/Al2O3 substrates was investigated revealing drastic changes on the catalytic properties after aging. XPS analysis shows a significant silver enrichment irrespective of the preparation route and aging conditions whereas ex situ UV–vis DRS analysis reveals significant electronic disturbances which reflect different interactions between Au and Ag in fresh and aged Au-Ag/Al2O3 and Ag-Au/Al2O3 catalysts. As a consequence, significant rate enhancements in NOx conversion were observed with the superior performance of Au-Ag/Al2O3 but a lower resistance to deactivation at high temperature explained by different behaviour towards redispersion/agglomeration processes highlighted from TEM observations

Oxidation activity and <SUP>18</SUP>O-isotope exchange behavior of nickel oxide-stabilized cubic zirconia

A series of NiO-ZrO<SUB>2</SUB> samples with 2 to 40 mol% of NiO were prepared using a sol-gel synthesis technique and calcined at 873 K. XRD characterization of the samples revealed the stabilization of a cubic zirconia- (fluorite) phase containing nickel oxide up to 20 mol%. Bulk NiO characteristics were observed above 20 mol% loading of NiO. The linear decrease in lattice parameter up to 20 mol% of NiO indicates the probable incorporation of Ni<SUP>2+</SUP> into the lattice position of Zr<SUP>4+</SUP> ions. The NiO-ZrO<SUB>2</SUB> sample with 20% NiO retained its cubic phase even after prolonged heating at 1273 K, indicating 20 mol% as an optimum content of NiO for a thermally stable cubic zirconia phase. The BET surface areas of these samples were in the range of 40 to 70 m<SUP>2</SUP> g<SUP>−1</SUP>. XPS spectra along with XRD data indicated that at low (5 mol%) concentrations of NiO, Ni<SUP>2+</SUP> enters ZrO<SUB>2</SUB> lattice substitutionally creating oxygen vacancies. TPR of NiO-ZrO<SUB>2</SUB> showed the reduction of nickel oxide at 533 and 633 K, indicating nickel in two different environments, at a substitutional position and at a surface/interstitial position. <SUP>18</SUP>O-isotope exchange studies of these samples showed a partial heterogeneous exchange and the T<SUB>onset</SUB> was found to be lowest for the sample containing 20 mol% NiO. The activity for CH<SUB>4</SUB> and CO oxidation was investigated by <SUP>18</SUP>O-isotope exchange as well as catalytic studies in complete oxidation of CH<SUB>4</SUB> and CO. The activity for CH<SUB>4</SUB> oxidation was highest for the NiO-ZrO<SUB>2</SUB> sample with 20 mol% NiO. CH<SUB>4</SUB> and CO oxidation with <SUP>18</SUP>O isotope over NiO-ZrO<SUB>2</SUB> catalysts showed the formation of CO<SUB>2</SUB> with <SUP>16</SUP>O (amu 44), suggesting that the bulk oxygen is acting as an active species for methane oxidation. The structure of NiO-ZrO<SUB>2</SUB> samples with varying NiO content and its correlation with catalytic activity mechanism is discussed

Vapor phase oxidation of 4-fluorotoluene over vanadia-titania catalyst

The vapor phase oxidation of 4-fluorotoluene has been carried out over vanadia-titania catalysts with moderate conversion and selectivity for 4-fluorobenzaldehyde. Two series of V2O5/TiO2 catalysts with 1-10 mol% vanadia were prepared by sol-gel technique using vanadium and titanium peroxide as vanadia and titania precursors respectively and by impregnation technique using vanadium peroxide on anatase titania support. The samples were characterized by X-ray diffraction, NH3-TPD, FT-IR and BET surface area measurements. The XRD of the catalysts prepared by impregnation technique showed retention of the anatase titania whereas the catalysts prepared by sol-gel technique showed the formation of rutile titania with minor amount of anatase phase at lower vanadia content (1-3%), which completely transformed into anatase phase at higher vanadia loading. The samples prepared by sol-gel method showed higher acidity and surface area compared to the samples prepared by impregnation. Pyridine adsorption study by FT-IR revealed the presence of Lewis acidity at lower vanadia loading (1-3%) and presence of both Lewis as well as Bronsted acidity at higher vanadia loading. The catalytic activity for oxidation of 4-fluorotoluene increased with vanadia loading in the sol-gel catalysts. The catalysts prepared by impregnation technique were found to be less active. However the selectivity for 4-fluorobenzaldehyde decreased with increase in vanadia content. The influence of vanadia loading, reaction temperature and contact time on the catalytic activity for 4-fluorotoluene oxidation has been investigated. The structure of the catalyst and its catalytic activity has been correlated

Vanadium-based highly active and selective catalysts for oxidative dehydrogenation of ethyl lactate to ethyl pyruvate

International audiencePyruvates are important intermediates for various bioactive and pharmaceutical molecules. Synthesis of pyruvates is challenging due to low selectivity, as the pyruvates are prone to polymerisation. In the present work, oxidative dehydrogenation of ethyl lactate to ethyl pyruvate was carried out under very mild conditions using vanadium-based homogeneous and heterogeneous catalysts in the presence of aqueous t-butyl hydroperoxide as an oxidant. Homogenous vanadium-based catalyst, VO(acac)2 in acetonitrile solvent, gave excellent conversion (upto 83%) with 100% selectivity to ethyl pyruvate at room temperature. However, the heterogeneous catalyst, V2O5 exhibited very high activity for oxidative dehydrogenation of ethyl lactate only at higher temperature (80 °C). At higher temperature, significant TBHP decomposition was observed if all TBHP was added in one lot. In case of ethyl lactate dehydrogenation using V2O5 catalyst at 80 °C with two equivalents TBHP, 60% ethyl lactate conversion with 100% TBHP conversions were observed after 5 h when all TBHP was added initially in the reaction mixture. However, the ethyl lactate conversion at 80 °C, after 5 h increased to 72% when the same amount of TBHP was added batch wise over a period of 4 h, indicating improved conversion of TBHP to ethyl pyruvate. The heterogeneous catalyst, V2O5 exhibited up to 98% conversion with 100% ethyl pyruvate selectivity at 80 °C after 10 h with 3 equivalent TBHP added batch wise. The homogeneous catalyst could not be reused while V2O5 could be successfully recycled five times without catalytic performances loss. Oxidation proceeds by radical mechanism, as proved by experiment with radical scavenger

Highly loaded well dispersed stable Ni species in NiXMg2AlOY nanocomposites: Application to hydrogen production from bioethanol

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Rational preparation of Ag and Au bimetallic catalysts for the hydrocarbon-SCR of NOx: Sequential deposition vs. coprecipitation method

International audienceThis study emphasizes the importance of the preparation method for bimetallic Au–Ag catalysts supported on alumina in the selective reduction of NOx by hydrocarbons with gas feed compositions representative of diesel fuelled engine exhaust gas. An optimal balance between oxidative and reductive surface properties is obtained when Au and Ag are successively introduced. Significant re-dispersion processes take place when the catalyst runs at 500 °C leading to a gain in activity at low temperature and ascribed to a better interaction between Au and Ag species. Co-precipitation leads to a preferential formation of intermetallic Au–Ag particles which is detrimental to the catalytic performances. Aging at 500 °C leads to a significant particle sintering and a strengthening of the metallic character