Annie Amariglio

Low temperature catalytic homologation of methane on platinum, ruthenium and cobalt

Abstract Adsorption of methane on metallic surfaces can proceed with hydrogen evolution if the temperature is sufficiently high (above approximately 100°C). Under flowing methane the desorbed hydrogen is continuously removed and the surface becomes increasingly covered with H- deficient CHx species. On Pt, Ru and Co, interruption of the methane flow followed by hydrogen flush causes the release of CH4 as well as higher hydrocarbons ranging from C2 to C7. On this basis a cyclic procedure is described for the selective homologation of methane at low temperature.

Catalytic activation of cobalt induced by oxidizing treatments in the methanation of carbon dioxide

A ribbon of Co catalyzing CO2 hydrogenation at atmospheric pressure and temperatures ranging from 200 to 500 °C has been taken as an example for studying the activating effect of pre-oxidation upon the activity of metals in hydrogenation reactions. The chemical state of the Co surface could be determined at any time by direct and quick transfer of the sample from the reactor into the ultrahigh vacuum chamber of an electron spectrometer. After cleaning and prolonged exposure to H2 (15 h, 1 bar, 500 °C) the sample displayed no visible activity up to 500 °C despite the absence of any visible contaminant on its surface. Activities ranging over several orders of magnitude could be induced in the metal by adequate pre-oxidations according to their severity. A large part of this activation was transient and its progressive decay required periods of time which were longer the more severe the pre-oxidation and the lower the temperature of reaction. The activity at any time after treatment was shown to depend not only on the oxidation but also on the subsequent reduction, since an increase of the rate of reduction resulted in a corresponding though transient increase of the activity. Reduction of most of the surface region was shown to occur very quickly, whereas deeper layers continued to reduce during the progressive decay of the activity. No visible contamination occurred during deactivation. The activation is interpreted in terms of the creation of surface defects by the alternate oxidations and reductions whereas the deactivation is considered to originate in the thermal restructuring of the surface.

Homologation of methane under non-oxidative conditions

This review presents results obtained in recent years concerning the catalytic conversion of methane into higher hydrocarbons using metal catalysts under non-oxidative conditions at moderate temperature. Although only a limited amount of work has been carried out in this area, the non-oxidative homologation of methane has already proved itself to be a novel and interesting way of addressing the problem of methane upgrading. Chemisorption of methane on transition metal surfaces has been studied for long on either ill-defined surfaces or single crystals. These studies, reviewed here, concerned mainly the kinetics and dynamics of chemisorption. In contrast, little has been known concerning the nature and reactivity of the adspecies. The reactivity of C1 adspecies originating from precursors other than methane (carbon monoxide, diazomethane, ketene, etc.) is better understood, which can be useful in investigating the reactivity of the hydrocarbonaceous adspecies resulting from methane. Most of the work concerning homologation of methane under non-oxidative conditions has been done by the groups of Amariglio in France and van Santen in the Netherlands. Both use two-step procedures in which metal catalysts are exposed first to methane and then to hydrogen. However, the procedures differ markedly in essentially two points: (i) the temperature of the first step and (ii) the pressure of methane. The Dutch group always uses a two-temperature cycle, decomposing dilute methane on Ru and Co at a rather elevated temperature and then carrying out hydrogenation at a much lower temperature and at atmospheric pressure. The French group, in contrast, has shown that homologation can be performed isothermally and at a moderate temperature on Pt, Ru and Co, using methane and hydrogen at atmospheric pressure. Consequently, in the two procedures the nature and reactivity of the surface species formed at the end of the exposure step are different. When the exposure to methane has been carried out at a moderate temperature and at atmospheric pressure, Cγ, is not formed (therefore no irreversible poisoning) and the products do not obey the Anderson-Schultz-Flory distribution. Also, thermodynamic limitations are circumvented by both groups but for reasons specific to each procedure. Finally, this review outlines prospects for future research and attempts briefly to estimate the potential commercial interest of the concept.

Direct conversion of methane to higher alkanes by platinum loaded zeolites

Abstract Direct methane conversion to higher hydrocarbons has been carried out on platinum loaded HX and HY zeolites by using a two-step reaction sequence. After exposure to flowing methane the samples were submitted to a flow of hydrogen, which resulted in the production of ethane, propane, n- and i-butane, n-, cyclo- and i-pentane, n-, cyclo- and other isomers of hexanes and traces of heptanes. On Pt/HX, the amount of homologated methane increased with temperature in spite of the fact that an increase of hydrogenolysia at the expense of higher alkanes took also place. The conversion, as referred to the adsorbed methane, did not vary significantly between 250 and 325 ° C (39% ). In the case of Pt/HY, the amount of homologated methane was maximum at about 275 °C and the yield decreased from 52 to 30% when the temperature increased from 250 to 325 °C. The distribution of the reaction products varied strongly with temperature and duration of the exposure to methane. Pt/HX was more active than Pt/ HY at temperatures above 250 °C, which is only partly caused by the higher methane adsorption capacity of Pt/HX. Some modifications of the Pt/zeolite samples are suggested in order to obtain higher activity.

Oscillating oxidation of propene on copper oxides

Bistability may be displayed by the system CuO used as a catalyst of propene oxidation under isothermal conditions and at low extents of reaction. This results from possible conversion of one Cu oxide into another under reacting conditions and from partial overlapping of the conditions of kinetic stability of both oxides. The phase changes of the solid are identified by X-ray analysis and are made visible by the strikingly different colors of the oxides. Under well-defined conditions, where the mass of the sample (Cu obtained from bulk CuO by reduction under a H2 flow at 400 °C for 2 h), grain size, residence time, and composition of the reactant mixture belong to an appropriate range, thermochemical oscillations are observed. CO2 formation, much more exothermic and oxygen-consuming than that of acrolein, is favored on CuO and causes overheating of the solid. Reducing conditions are then created, leading to Cu2O formation. On the latter oxide, acrolein is favored, so that less O2 is consumed and the temperature decreases, which allows the reoxidation of the solid. Particularities of the oscillating behavior can be interpreted in relation to the structural imperfections which are likely to result from repeated oxidations and reductions of the catalyst.

Induction of catalytic activity of rhodium in ethylene hydrogenation by the H2-O2 reaction

Abstract Rh ribbons have been used as catalysts for C 2 H 4 hydrogenation at atmospheric pressure and for temperatures ranging from about 100 up to 300 °C. The catalytic activity is shown to be heavily dependent on various oxidation-reduction treatments and on the presence of O 2 traces in the reactant mixture. Different experiments show that this behavior cannot be understood as the result either of contaminations and cleaning of the surface or of a modification of surface properties by chemisorbed O 2 . The best explanation is that of a structural sensitivity of the metal surface to the H 2 -O 2 reaction through the surface mobility it induces. The content of active sites is determined according to the rate of their chemical generation and that of their disappearance by thermal restructuring of the surface. Many kinetic features of the system (and especially transient behavior) are thereby accounted for and proof of significant rearrangements of the surface is obtained by optical and electron microscopy.

The steady state catalytic activity of nickel in ethylene hydrogenation as essentially due to the promoting effect of oxygen traces

Abstract The catalytic activity of nickel in ethylene hydrogenation is shown to result quasi-completely from active sites created by the partial or entire simultaneous conversion into water of the oxygen traces carried by the reacting mixture. The ethylene hydrogenation and the water synthesis appear then to be coupled upon the catalyst surface, the former proceeding on the active centers generated by the latter. As the rate of disappearance of active centers increases with temperature whereas the total oxygen conversion is limited, a maximum occurs in the steady-state activity versus temperature. This scheme is proved by the disappearance of this optimum, with a considerable lowering of activity, following a drastic purification of the reacting mixture. The maximum and activity both reappear when the purification becomes less rigorous.

Isothermal conversion of methane into higher hydrocarbons and hydrogen by a two-step reaction sequence involving a rhodium catalyst

Abstract A two-step reaction sequence for converting methane to higher hydrocarbons and hydrogen was investigated on a 5%Rh–5%CeO 2 /SiO 2 catalyst. Isothermal cycles were carried out from room temperature to 275°C and produced alkanes up to hexanes. In contrast with previous observations on platinum and ruthenium, the conversion was possible even at room temperature and two maxima versus temperature were evidenced. The first maximum occurred at about 100°C while the second one took place beyond 200°C. A noticeable selectivity to n -pentane was also observed. This work includes a study of the phenomena occurring during the exposure of the catalyst to flowing methane at atmospheric pressure (evolution of hydrogen at T ≥100°C and of ethane at T ≥150°C). A determination of the amounts of methane chemisorbed under various conditions of exposure as well as the evaluation of the H/C ratios of the corresponding surface species was achieved. These quantities were derived from temperature programmed desorption (up to 300°C) followed by reaction with hydrogen at room temperature and further temperature programmed surface reaction with hydrogen. Interpretations in line with those put forward in the case of platinum and ruthenium are proposed.

Oxidizing pretreatments as a means of activating cobalt in Co2 methanation; parallel kinetic probing and surface analysis

Abstract Pre-oxidations of Co conducted at T ⩾ 300°C results in strong rate enhancements of CO 2 hydrogenation. From direct transfer of the sample into an UHV vessel equipped with AES, XPS and UPS facilities it has been shown that the effect cannot be explained in terms of surface cleaning or of some influence of oxygen remaining in the sub-surface regionsurface distortions entailed by the reduction process and the opposite thermal restructuring seem to offer the best way of explanation.

Periodic operation of a catalyst as a means of overcoming a thermodynamic constraint. The case of methane homologation on metals

Abstract Periodic operation of a catalyst can be a way of overcoming a thermodynamic constraint. Homologation of methane is thermodynamically disfavored. However, a two-step procedure using metal catalysts under non-oxidative conditions allows the thermodynamic limitations to be circumvented. Metal catalysts, such as Pt, Co and Ru are exposed first to methane and then to hydrogen. In their dual-temperature procedure, van Santen et al. carry out the first step with dilute methane at a high temperature (usually 725 K), which allows the endothermic decomposition of methane to take place. Part of the C deposits may yield higher alkanes up to C4–C5 through the following hydrogenation at a much lower temperature (368 K). In contrast, we carry out these two steps at atmospheric pressure and at the same but moderate temperature (usually less than 570 K). In this case, chemisorption of methane is accompanied by release of hydrogen whereas coupling of H-deficient CHx adspecies may take place. Numerous higher alkanes up to C7-C8 are then removed by supplying hydrogen at ordinary pressure and at the same temperature as that of the first step. The driving force can be found in the energy which has to be supplied in order to compress part of the dilute hydrogen removed in the first step to make it usable in the second one. The influence of some key factors is studied.