Corinne Petit

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Abstract Mixed LaNixFe(1−x)O3 perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi0.3Fe0.7O3.

Mechanism of catalytic destruction of 1,2-dichloroethane and trichloroethylene over γ-Al2O3 and γ-Al2O3 supported chromium and palladium catalysts

The destruction mechanisms of C2-chlorinated hydrocarbons have been investigated. The adsorption of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) on alumina and alumina supported catalysts was studied by FTIR-spectroscopy. Interpretation of the spectra suggests that the destruction of DCE occurs via HCl elimination to vinyl chloride, followed by an attack of a hydrogen and a surface oxygen on the double bond resulting in acetyl chloride. Subsequent elimination of a chloride ion gives acetaldehyde that can be further oxidized into acetate. With regard to the destruction of TCE, the spectra indicate that TCE is attacked by a basic oxygen and a hydrogen according to Markovnikov’s rule. This results in the formation of acyl chloride, which is further converted into acetate like species. In addition, the oxidation of TCE was examined over alumina supported catalysts. While alumina itself is not very active, palladium and chromium containing catalysts are. In the absence of water, the formation of tetrachloroethylene (PCE) was observed. Addition of water to the feed resulted in a decrease in the amount of PCE produced. Although water did not affect the TCE conversion over palladium it inhibited the oxidation reaction over chromium oxide. The activity of the chromia catalyst compared to alumina might be due to the supply of basic oxygen which can attack the double bond. The inhibitive effect of water on the conversion of TCE is probably due to blocking of active oxygen sites.

Absorption/desorption of NOx process on perovskites: performances to remove NOx from a lean exhaust gas

Abstract NOx adsorption/desorption capacities of perovskites (ABO3) were measured under representative exhaust gas mixture conditions at temperatures below 550°C, with A=Ca, Sr, Ba and B=Sn, Zr, Ti. The solids exhibited good NO2 sorption capacities with a reversible adsorption to desorption process according to the sequence Ba>Sr>Ca for A, while for element B the sequence Sn>Zr>Ti is observed. In the case of alkaline earth metals, the absorption behaviour proved to be directly related to their electropositivity (Ba>Sr>Ca). The key factors which control the absorption of NO2 are the bonding energy between the element B and the oxygen atom on one hand and the electropositivity of the element A on the other. The best result was obtained with the perovskite BaSnO3. The absorption of NO2 is favoured at low temperature and in the presence of water. The addition of platinum has no significant influence upon any NO2 absorption.

Removal of NOx: Part I. Sorption/desorption processes on barium aluminate

Abstract NO x adsorption/desorption capacities of barium aluminates were measured under representative exhaust gas mixture at temperatures below 550°C. The solid doped with Pt or not, exhibits good NO 2 sorption capacities with a reversible adsorption to desorption process. With bulk BaO, desorption was observed at high temperature. The different behaviour between the two catalysts is explained by the fact that strongly bonded carbonates are formed on bulk BaO while they do not exist on barium aluminate, therefore allowing the formation of nitrates which can be decomposed by a thermal process. SO 2 poisoning was also studied.

Application of heterogeneous gold catalysis with increased durability: Oxidation of CO and hydrocarbons at low temperature

Abstract2% Au/Al2O3 catalysts were prepared by a novel method involving Direct Anionic Exchange (DAE). The method produces strong bonding of the gold complex (HAuCl4) to the alumina support with no loss of gold during the subsequent steps of preparation. The complete removal of chloride from the catalyst was achieved by washing with concentrated ammonia. This procedure ensures a better activity and prevents sintering during calcination as shown by TEM. The catalysts were tested for the oxidation of CO and of saturated and unsaturated hydrocarbons (C1 to C3). The catalysts showed high activities over a range of concentrations and temperatures relevant to applications in automotive exhaust cleaning. Furthermore, a remarkable resistance to thermal ageing at 600°C in the absence or presence of water was observed, due to the presence of the strongly anchored nanosized gold particles obtained during the preparation step.

Support Effects in the Gold-Catalyzed Preferential Oxidation of CO

The study of support effects on the gold‐catalyzed preferential oxidation of carbon monoxide in the presence of hydrogen (PROX reaction) is possible only with careful control of the gold particle size, which is facilitated by the application of the direct anionic exchange method. Catalytic evaluation of thermally stable gold nanoparticles, with an average size of around 3 nm on a variety of supports (alumina, titania, zirconia, or ceria), clearly shows that the influence of the support on the CO oxidation rate is of primary importance under CO+O2 conditions and that this influence becomes secondary in the presence of hydrogen. The impact of the support surface structure on the oxidation rates, catalyst selectivity, and catalyst activation/deactivation is investigated in terms of oxygen vacancies, oxygen mobility, OH groups, and surface area on the oxidation rates, catalyst selectivity and catalyst activation/deactivation. It allows the identification of key morphological and structural features of the support to ensure high activity and selectivity in the gold‐catalyzed PROX reaction.

Absorption/desorption of NOx process on perovskites: Nature and stability of the species formed on BaSnO3

Abstract The formation of various species during the adsorption of NOx, issuing from a synthetic, Lean–Burn exhaust gas upon BaSnO3 was studied using FTIR and TGA. An exposure to CO2 does not lead to the formation of carbonates yet contact with NO2 does produce nitrates, which accounts for the high NOx capacity of such solids. N-bounded nitrate and bulk nitrate species were identified. These nitrates decompose upon heating with no loss of the perovskite structure. The process of absorption/desorption is reversible, repeatable and can be explained by the following equations: Absorption phase: BaSnO 3 +3 NO 2 ↔ Ba ( NO 3 ) 2 + SnO 2 + NO , Desorption phase: Ba ( NO 3 ) 2 + SnO 2 ↔ BaSnO 3 +2 NO 2 + 1 2 O 2 .

Removal of NOx: Part II. Species formed during the sorption/desorption processes on barium aluminates

Abstract The formation of various species formed during the adsorption of NO x issued from a synthetic lean-burn exhaust gas upon barium aluminates was studied using FTIR and TGA. The results have been systematically compared to those obtained with bulk BaO. Two factors are responsible for the difference observed during adsorption/desorption tests on the two solids. On one hand, stable carbonates are formed on BaO whereas no carbonate is formed on barium aluminate. On the other hand, the structure of the nitrates formed on these two compounds is very different, an N-bounded nitrate is formed on barium aluminate and not on bulk BaO.

The Mechanism of the Selective NOx Sorption on H3PW12O40·6H2O (HPW)

NOx absorption/desorption capacities of 12-tungstophosphoric acid hexa-hydrate were measured under representative exhaust gas mixture conditions. The amounts of NOx absorbed and then desorbed are high and equal to 46 of NO2 mgg−1 of HPW. The mechanism of absorption proceeds by substitution of lattice water molecules with formation of a [H+(NO2−,NO+)] complex. During the cooling phase and in the presence of water, around 100°C, reverse substitution occurs. Two possibilities to wash-coat HPW on a monolith are presented. The first one consists in a partial substitution of H+ by a monovalent cation while the second one consists of supporting HPW on a high surface oxide. The anchorage quality is related to the Brønsted acidity, the best candidates for the role of support are SnO2 and TiO2.

Steam reforming of methane on LaNixFe1–xO3 (0≤x≤1) perovskites. Reactivity and characterisation after test

Abstract LaNixFe1–xO3 perovskites (0≤x≤1) are efficient catalysts in steam reforming of methane (optimum ratio H2O/CH4=1) for syngas production. For low x values (x≤0.4), the three-metal structure is partly maintained with a strong interaction between free nickel and the perovskite, the carbon formation is limited and the regeneration of the three-metal perovskite by recalcination is possible. For higher x values (x>0.4), only a bimetallic LaFeO3 is maintained during the reaction and the catalysts perform as free nickel on LaFeO3 and La2O3. Coke formation becomes important and the regeneration gives two distinct perovskites, LaFeO3 and LaNiO3. The increase in H2O/CH4 from 1 to 3 enhances the oxidating power, leads to a decrease in the activity and favours CO2 formation.