F. Garin

Photocatalytic oxidation of butyl acetate in vapor phase on TiO2, Pt/TiO2 and WO3/TiO2 catalysts

The photocatalytic degradation of butyl acetate in the gas phase was investigated in a fixed-bed continuous annular reactor using a titanium dioxide semiconductor, Pt/TiO2 and WO3/TiO2 catalysts. Platinum was deposited on the titanium by adsorption of PtCl62− anions, and WO3/TiO2 was prepared by a conventional impregnation method using an aqueous solution of ammonium paratungstate. Different samples, with different nominal weight loadings in Pt and WO3, were tested. For each catalyst sample studied, air containing butyl acetate and water vapors in various molar ratios was fed at 200 cm3/min to the photoreactor. The roles of the reaction temperature and of H2O in the photocatalytic oxidation of butyl acetate and the influence of the content of TiO2, Pt, and WO3 were investigated. Although an initial decay in photocatalytic activity was observed, partly issued from an initial deactivation, total mineralization was achieved under all experimental conditions. For relative water humidity between 50 and 75% and for adequate TiO2 covering ratios, the photocatalytic activity of TiO2 toward the photocatalytic degradation of butyl acetate can be greatly improved by the addition of WO3, mainly due to a prevention of the initial deactivation and of photogenerated charge recombination. This optimized photocatalyst is very stable on stream and no regeneration treatments are needed. The influence of Pt and WO3 on photocatalytic activity are discussed with regard to chemical and electronic modifications of TiO2.

Mechanism of NOx decomposition

Abstract The mechanism of NOx decomposition is reviewed. This reaction is first studied on metal or oxide single crystals, in order to get solid basis from catalysis on well-defined surface structures. If we understand few things about this reaction, at least the catalytic surface is known. This statement is not 100% correct, as the surface is reconstructed during the catalytic reaction; and this point is considered in the manuscript. Then we analyse this reaction on metallic catalysts supported on metal oxides. The crystallites are more or less dispersed, and the support can play the role of a modifier or of a promoter. A general consensus emerges on (i) the chemisorption of the reactant(s) which causes adsorbate-induced restructuring of the surfaces, (ii) the substrate, which is influenced by the redox properties of the support and by the particle sizes of the metallic aggregates, (iii) the fact that low-coordinated surface atoms are the more active in NO bond breaking, and (iv) the general pathways of the reaction, where redox mechanisms involving nitrosyl species, as intermediates, moving to nitrite-nitrosyl or dinitrosyl species are proposed. All these points are discussed.

Conversion of butane over the solid superacid ZrO2/SO42− in the presence of hydrogen

The conversion of n-butane to isobutane on the solid superacid ZrO2/SO42− has been studied in a microflow reactor in a helium/hydrogen stream at constant hydrocarbon pressure under kinetic-controlled conditions. The effect of the partial pressure of hydrogen indicates a dual role for this component, namely that it is an inhibitor at high partial pressure, but is essential for maintaining the catalyst activity. The presence of platinum to maintain the catalyst activity is not necessary if the hydrogen pressure is high enough.

Conversion of butane over the solid superacid ZrO sub 2 /SO sub 4 sub 2 minus in the presence of hydrogen

The conversion of n-butane to isobutane on the solid superacid ZrO2/SO42− has been studied in a microflow reactor in a helium/hydrogen stream at constant hydrocarbon pressure under kinetic-controlled conditions. The effect of the partial pressure of hydrogen indicates a dual role for this component, namely that it is an inhibitor at high partial pressure, but is essential for maintaining the catalyst activity. The presence of platinum to maintain the catalyst activity is not necessary if the hydrogen pressure is high enough.

Isomerization and hydrogenolysis of C6 alkanes on PtAl2O3 catalysts and Pt single-crystal faces

Abstract The contact reactions of C 6 hydrocarbons (isomerization, dehydrocyclization, hydrocracking) were studied under the same experimental conditions, on platinum-alumina catalysts of low or high dispersion, on Pt(557), Pt(119), Pt(111) surfaces, and on a polycrystalline foil of platinum. The experiments on these well-characterized surfaces were performed up to atmospheric pressure in an isolation cell housed within the main UHV chamber equipped with LEED, AES, and ELS facilities. The reaction mixture was analyzed by GLC and part of it was used for mass spectrometric location of the 13 C in each molecule. The results have shown that isomerization by bond-shift or cyclic mechanisms and hydrogenolysis take place on single-crystal faces of platinum, simulating very well the data from PtAl 2 O 3 catalysts of large size aggregates of Pt. Isomerization by bond shift is much more important on stepped surfaces. The contributions of the relative percentages of bond-shift and cyclic mechanisms are discussed as a function of different models of adsorbed species evolving from one site to multisites of chemisorption, differences in local density of states of the edges, and differences in the electronic charges of the carbon atoms in the hydrocarbons. These models are correlated to the classical ones of Anderson, Rooney, and Gault. The peculiar properties of the highly dispersed 0.2% PtAl 2 O 3 catalyst were never simulated by single crystals of platinum.

Correlations between the surface structure of platinum single crystals and hydrocarbon skeletal rearrangement mechanisms: approach to the nature of the active sites

Abstract The isomerization and hydrocracking reactions of 2-methylpentane and n -hexane and the hydrogenolysis of methylcyclopentane were investigated on a clean Pt (311) single crystal at 350 °C under atmospheric pressure in an isolation cell housed within an ultrahigh vacuum chamber. The surface composition was monitored before and after the catalytic reactions by Auger electron spectroscopy. The results were compared with those obtained in the same experimental conditions on Pt (111), Pt (557), and Pt (119) surfaces, a polycrystalline foil, and two Pt Al 2 O 3 catalysts of both low and high dispersion. Experiments with labeled hexanes were also undertaken to determine the nature of the mechanisms involved in the isomerization reactions. The catalytic results show that each crystallographic face has its own peculiarities especially for (i) catalytic activity, (ii) selectivity as regards isomers produced, and (iii) selectivity as regards mechanisms. Conversely, the distribution of cracked products presents no significant structure sensitivity on “massive” metal surfaces although the highly dispersed alumina-supported platinum catalysts behave differently. Results with 13 C-labeled hydrocarbons suggest that the atoms in the so-called B 5 site configuration are responsible for the bond-shift plus cracking reactions and that the atoms located at the corners and edges are involved in the cyclic mechanism.

Durable changes of the catalytic properties of alumina-supported platinum induced by microwave irradiation

The isomerization of 2-methylpentane over a 0.2 wt% platinum dispersed on γ-alumina catalyst is studied when the catalyst is heated either by microwave irradiation or by a classical oven. The selectivity for isomerization is considerably increased, from 45 to 80%, when the catalyst has been previously treated by microwaves or when the catalytic reaction is performed under the electromagnetic field. The isomerization power of such an irradiated catalyst remains high even when it is subsequently tested according to classical conditions. It is consequently suggested that the microwaves induced durable and profitable modifications of the catalytic behaviour of alumina-supported platinum.

A study of the regeneration of fresh and aged SOx adsorbers under reducing conditions

Sorption of SO2 over a commercial SOx adsorber was studied at a laboratory scale using a fixed bed reactor. The adsorbent material contains mainly alumina, barium oxide and precious metals. Sulphates formed by SO2 sorption are reduced by carbon monoxide or hydrogen in the presence of water. The main species emitted during the reduction was H2S. The species COS was emitted at a very low level when carbon monoxide was used as a reducing agent. The total removal of sulphur was not possible in the absence of water, the sulphur remaining bonded to platinum (or palladium) sites. The role of water is to hydrolyse these sulphides, leading to H2S or COS. The species SO2 was also emitted when an aged catalyst was used, without a loss of the catalytic reduction efficiency. After a first sequence adsorption–regeneration, the adsorption capacity was not affected and the second regeneration was more efficient. Such a SOx adsorber can be placed upstream a NOx adsorber to avoid poisoning of this latter by SO2.

From Bifunctional Site to Metal–Proton Adduct Site in Alkane Reforming Reactions on Sulphated-Zirconia-Supported Pt or Pd or Ir Catalysts

Isomerization reactions of n-heptane, n-octane and n-nonane are studied on sulphated-zirconia-supported 0.2 wt% Pt, Pd or Ir catalysts. Evolutions of isomer selectivity versus total conversion and reaction temperatures are analysed. When total conversion (αT) is increased, isomer selectivity (%Sisom) is decreased and the slope of the curve %Sisom=f(αT) is more pronounced when the carbon number in the alkane is more important. At isoconversion, around 20%, below 473 K, cracking is favoured over isomerization reaction, and above 473 K it is the reverse. Moreover, with n-heptane, when the catalytic reaction occurred at 423 K and at low conversion, αT≤20%, we observed a large decrease in the isomer selectivity percentages on Pd/SZ and Ir/SZ compared to Pt/SZ. What is remarkable is that, at this low temperature, both metals are inactive in the carbon–carbon bond rupture. To explain these results the following points are raised: (i) an associative mechanism is proposed for the adsorption step of the alkane involving an agostic intermediate species where the carbon–hydrogen bonds act as ligands to the transition metal centres forming covalent C–H⋅⋅⋅M systems, and (ii) a metal–proton adduct site, which gathers metallic and acidic sites is suggested. This approach seems to better explain our results than the “traditional” bifunctional mechanism.

NO reaction over nanometer scale platinum clusters deposited on γ-alumina : an XAS study

Abstract An in situ EXAFS study has been done on a pre-reduced and pre-oxidised 1.16% Pt/γ-Al2O3 catalyst in order to determine the effect of 1% NO (balance N2) versus temperature at atmospheric pressure on the platinum particles. Changes were observed in the crystallite morphologies from 200°C due to sintering processes on the pre-reduced catalyst. On the pre-oxidised one, the phenomenon is much less pronounced. This study specifies the reactivity of platinum clusters deposited on γ-alumina versus NO.