R. Doepper

In-situ surface and gas phase analysis for kinetic studies under transient conditions. The catalytic hydrogenation of CO2

Transient measurement techniques are applied for the kinetic investigation of the reaction mechanism of the carbon dioxide methanation, using diffuse reflectance infrared spectroscopy and mass spectrometry. The coupled information of the surface intermediates and the gas phase components time evolution leads to accurate identification of spectator species on the surface. Reaction intermediates, carbon monoxide and formates have been identified. The former is a key intermediate, and its hydrogenation leads to methane formation. The latter is fixed on the support, in equilibrium with a active formate species on the interface metal-support. A reaction mechanism is proposed including the formation step of the formates through a carbonate species.

In-situ characterisation of the surface intermediates for the ethanol dehydration reaction over γ-alumina under dynamic conditions

The dynamic behavior of the ethanol adsorption on g-alumina were investigated at 180 and 200 DegC by the transient-response method coupled with FT-IR data of the catalyst surface. The existence of three adsorbates was demonstrated: a reacting species which is the precursor for the formation of the gas-phase ethene; an inhibiting species responsible for the low steady-state reaction rate; and a spectator species accumulating on the catalyst surface. Their IR spectra indicate an ethoxide-like structure for the three adsorbates. Their C-H stretching bands can be depicted by four Lorentzians whose parameters indicate different surface environments. The surface concn. of the reacting species was detd. on the basis of the transient ethene response. A value of 0.77+-0.07 mol/kgcat was found at 180 DegC. The surface concn. of the spectator species was detd. by ex-situ thermogravimetric expts. A value of 0.13+-0.01 mol/kgcat was found at 180 DegC. The most likely structure of this species corresponded to an ethanol mol. coordinated in a bidentate manner on Al3+ cations, with stabilization of the alc.-hydroxyl group via lateral hydrogen bonding with an adjacent surface hydroxyl. [on SciFinder (R)]

Reduction of nitrogen oxides by carbon monoxide over an iron oxide catalyst under dynamic conditions

Redn. of NO and N2O by CO over a SiO2-supported iron oxide catalyst was studied by the transient response method, with different initial oxidn. states of the catalyst, i.e., completely reduced (Fe3O4), or oxidized (Fe2O3). The affect of CO pre-adsorption was also studied. From the material balance on the gas-phase species, it was shown that the compn. of the catalyst changes during relaxation to steady-state. The degree of redn. of the catalyst at steady-state could thus be estd. During the transient period, CO was shown to inhibit N2O as well as NO redns. by adsorption on reduced sites. Activity of the reduced catalyst was substantially higher as compared to the oxidized catalyst for both reactions. On this basis, it was attempted to keep the catalyst in a reduced state by periodically reducing it with CO. As a result, a significant increase in reactor performance with respect to steady-state operation could be achieved for N2O redn. by CO. Finally, the dynamic behavior of the N2O-CO and NO-CO reactions made it possible to evidence reaction steps, the occurrence of which could not be shown during previous investigations on the sep. interactions of the reactants with the catalyst. [on SciFinder (R)]

Reactor performance enhancement under periodic operation for the ethanol dehydration over γ-alumina, a reaction with a stop-effect

The possibility of enhancing reactor performance under forced cycling of the inlet feed was investigated for the ethanol to ethene dehydration reaction over c-alumina. This reaction presents a stop-e!ect, i.e. a drastic increase of the reaction rate for a stepwise decrease in the inlet concentration of ethanol (Koubek et al., 1980a, b; Moravek and Kraus, 1984). This phenomenon is due to an educt inhibition, leading to the existence of an optimal inlet concentration of ethanol under steady-state conditions. Periodic operation can be used advantageously to increase the performance of the ethene production up to values higher than the maximal steady-state value. The maximum enhancement is achieved for short non-symmetric periods, with the major part of the cycle under inert gas. It was shown by simulations that for an integral reactor the parameters of the cycle have to be adjusted to the residence time. Finally the two-sites model developed for the stop-e!ect (Golay et al., 1997, 1999) is able to predict qualitatively the in#uence of all the cycle parameters. ( 1999 Elsevier Science Ltd. All rights reserved.

Periodic operation applied to the kinetic study of CO2 methanation

Periodic operation was applied to the CO2 methanation reaction at 383 K on 2% Ru/TiO2. A continuous feed recycle reactor combined with a diffuse reflectance infrared cell and a mass spectrometer allowed to follow simultaneously the gas phase CO2 and CH4 as well as the adsorbed species (CO)a and (formate)a. Experiments consisting of periodic variations of CO2 in the hydrogen feed showed response curves with maxima/minima shifted in time in the sequence CO2, (formate)a --> (CO)a --> CH4. Similar delays measured for the (CO)a formation and hydrogenation indicate that both of these processes are limiting the overall reaction rate. A kinetic model was proposed and verified under periodic conditions. The main experimental trends, which are pronounced time lags between CO2, (CO)a and CH4, could be described satisfactorily.

Transient drift spectroscopy for the determination of the surface reaction kinetics of CO2 methanation

Transient expts. were applied to the study of the adsorbed CO intermediate, (CO)a, formed during CO2 methanation on 2 wt.% Ru/TiO2 catalyst at 383 K. Step-up expts. showed that the (CO)a formation steps are inhibited by H2O and enhanced by H2. Step-down expts. showed that the (CO)a hydrogenation is not influenced by the partial pressure of water. Based on the fact that water inhibits the overall CO2 methanation, it was deduced that the rate limiting process in the overall reaction is (CO)a formation. [on SciFinder (R)]

Dynamic kinetics of catalytic dehydrogenation of methanol to formaldehyde

Methanol dehydrogenation to formaldehyde was operated in a continuous fixed-bed reactor with external recycling in the presence of sodium carbonate catalyst supported on active carbon. Due to the importance of gas phase decompns. of methanol and mainly formaldehyde, selectivity of formaldehyde in such a reactor is small compared to that obtained in a fixed-bed reactor. Furthermore, the contribution of gas phase reactions to formaldehyde synthesis through global dehydrogenation of methanol complicates steady-state kinetic studies. However, transient and methanol isotope (CD3OD and CH3OD) transient expts. showed that a hydrogen species and also a methoxy and/or a hydroxymethyl intermediate(s) are strongly adsorbed on the catalyst surface. Based on these facts, a mechanism of methanol dehydrogenation on sodium carbonate catalyst supported on active carbon is proposed. [on SciFinder (R)]

Catalyst acid / base properties regulation to control the selectivity in gas-phase methylation of catechol

The reaction of catechol methylation by methanol in gas-phase over modified g-alumina was studied with the aim to correlate catalytic activity and selectivity to acid/base properties of the catalysts. Catalytic activity and selectivity towards guaiacol formation (O-alkylation) was found to increase with surface acidity. A 20-fold change in the O/C-methylation ratio was achieved by varying the catalyst acid/base properties, keeping const. the other reaction parameters. [on SciFinder (R)]

Bistability and oscillations of the NO/CO reaction on a Pt-Mo supported catalyst

Abstract The reaction of nitric oxide with carbon monoxide on a catalyst of 0.5 wt% Pt-3.4 wt% MoO3 supported on c~-A1203 has been investigated in different reactors at atmospheric pressure. A fixed-bed reactor with external recycle loop exhibited bistability and hysteresis as CO concentration or temperature were varied. In a tubular fixed-bed reactor non-isothermal periodic oscillations (ATm~x=40 K) were found in a specific pressure-concentration-temperature region (p-c-T region). The amplitude and period time of the oscillations changed smoothly inside the p-c-Tregion and died out suddenly at the borders of the region. The oscillations were of a relaxation type and lasted for more than 48 h. With fresh catalyst, a new quartz reactor and a specific pretreatment they were reproducible. It was shown that oscillations had kinetic origins and that the changes of the temperature synchronised the elementary oscillators. An autocatalytic model is proposed to explain the experimentally observed phenomena.

Selective methylation of catechol: catalyst development and characterization

A symposium report on the gas-phase methylation of catechol by methanol on g -Al2O3 modified by the basic elements: K, Li, Mg and Ca. Addn. of 7.5 at % Mg to g-Al2O3 was optimal and increased the 3-Me catechol selectivity from 0.26 to 0.65. X-ray diffraction expts. showed the diffusion of Li+ and Mg2+ cations into the g -Al2O3 bulk. This induces a change in the surface species and the surface acid-base properties. Ca2+ and K+ addn. to g-alumina was ineffective due to formation of basic oxide layers on the surface. [on SciFinder (R)]