Carroll O. Bennett

The Influence of Particle Size on the Catalytic Properties of Supported Metals

Publisher Summary Metals constitute a wide class of catalysts, and because catalysis occurs on the surface, there is an economic incentive, especially for precious metals, to obtain catalysts in the form of small metal particles. This, however, raises two main problems. One is fundamental in nature and addresses the question as to below which particle size the metallic properties are lost. The other is more practical and concerns the preparation and characterization of very small particles and their catalytic activity. The other problem concerns the practical ways of producing metals with particle sizes around the critical value calculated above. The easiest and most practical way to achieve this goal is to deposit the metal in low concentration on a high-surface-area support. The main purpose of using a carrier is, of course, to achieve a high dispersion of the metal and to stabilize it against sintering. In a number of reactions, however, the metal support is not inert and the overall process is actually a combination of two functions: that of the metal and that of the catalytically active support.

The Transient Method and Elementary Steps in Heterogeneous Catalysis

Abstract It is possible to consider the local rate of reaction in a fluid phase as a function of state variables, r(C, p, T). We assume that the concentrations of all the components C, which may, for example, be important in initiation and termination are included in the description. The rate must also be slow compared to other phenomena which might create a nonuniform mixture on the scale in which we are interested. If these restrictions are respected, we then can describe the rate in terms of a sequence of elementary steps, providing all the active intermediates are known. For gases the rate parameters (frequency factors, activation energy) for many elementary reactions have been measured; these matters are discussed, for example, in the book by Johnston [l]. The rate r (g-mole/cm3-sec) is in a certain sense a state function; it becomes related to a derivative or other quantities only as it appears in the conservation equations.

The transient method applied to the methanation and Fischer-Tropsch reactions over a fused iron catalyst

In the reduction of carbon monoxide with hydrogen to methane on a commercial iron catalyst, a transient method was used to study the reaction intermediates at 250/sup 0/C and atmospheric pressure. When a stream of the reactant mixture (10% carbon monoxide in hydrogen) flowing over the catalyst in a steady state was suddenly changed to pure hydrogen, a surface intermediate was converted to methane at an initial rate higher than that at the steady state. The intermediate species was deactivated by a similar sudden change to helium. The reactivity of the reaction intermediate was much higher than that of carbon added to the catalyst by carburization by CO in argon. The results indicate that the surface of the active catalyst is covered mostly by a carbon intermediate, whose hydrogenation represents the rate-determining step. The bulk of the catalyst is Hagg carbide, Fe/sub 2/C.

A transient infrared and isotopic study of methanation over Ni/Al2O3

Methanation of CO/H2 mixtures over a 10% Ni/Al2O3 catalyst has been studied by infrared spectroscopy and several isotopic techiques. Emission spectra revealed the presence of both linear and bridge-bonded CO during reaction, but no reactive HCO or CH species were observed. Isotopic substitution experiments were conducted in a gradientless microreactor. These show that a heterogeneous layer of CHx also accumulates. The probable value of x is zero. As much as 0.6 monolayer has been observed, but the amount depends on the reaction conditions. The global rate of methanation is controlled by a competition between the formation and the hydrogenation of this species, not by a single rate-determining step.

Carbon pathways in methanation and chain growth during the Fischer-Tropsch synthesis on Fe/Al2O3

The mechanism of methane and hydrocarbon formation from COH2 mixtures on an unpromoted 10 wt% Feγ-Al2O3 catalyst was studied by labeling the feed mixture with 13C or D. A reactive CH species was detected by COD2 and 13COH2 and found to deactivate slowly with time on stream. Only about 20% of the CH remained active after 1.5 h on stream at 285 °C, producing about 86% of the synthesis products. Also present on the Fe surface were several monolayers of hydrogen-free carbon, but this species was not a poison. Bulk carburization took place, but exchange with 13C on the surface was observed to be very slow, only about 3% of the overall synthesis rate, so that the bulk carbide did not participate appreciably in the synthesis. The transient incorporation of 13C into methane and higher hydrocarbons suggested that the same large portion of the hydrocarbons was produced from the small active portion of the CH species.

Nature of surface species on supported iron during CO/H2 reaction

Abstract The transient method has been used to study the H 2 CO reaction over 10% Fe Al 2 O 3 and 10% Fe SiO 2 at 1 atm and 285 °C in a differential reactor. Responses to perturbations in the feed composition to the reactor were measured by on-line mass spectrometry. Additional studies were made with the reactants CO He , C 2 H 4 H 2 , and C 2 H 4 He . The effect of water in the feed was also investigated. The nature of the species remaining on the surface or in the bulk of the iron was investigated by hydrogen and by oxygen flushing. The observations are explained by proposing a common surface carbon precursor to the formation of surface CH, surface carbidic carbon, surface graphitic carbon, and bulk iron carbides.

Experiments and processes in the transient regime for heterogeneous catalysis

Experiments in the transient regime are now widely used in mechanistic studies of heterogeneous catalysis. They permit the quantitative determination of the surface composition during catalysis and also give important information on the sequence of steps that underlie the observed global reaction. In particular, by computer-based modeling and simulation, the forward and backward rate constants of the steps can be estimated so that the behavior of the reacting system can be simulated by numerical methods over a wide range of variables. Results from transient experiments lead to reliable models for reactor design, and they are essential for reactors operating in the transient regime, a situation becoming increasingly common. In this review, I consider the general principles of common experiments in the transient regime: step response, pulse response, frequency response, and temperature-programmed response. The use of feeds of stable isotopes is widespread in transient experiments, greatly increasing the power of these methods. All these methods can be applied at atmospheric pressure and higher, and also under surface science conditions. Following a discussion of general principles, many examples have been chosen from the literature for more detailed case studies. Most of the reactions chosen have a practical application since the design of large-scale reactors requires quantitative information that is best obtained by transient experiments. Among the examples treated are those involving methane or carbon monoxide as reactants, and attention is also devoted to the reduction of the concentration of pollutants in exhaust gases. The response of the laboratory reactor is manifested by the concentration of the various components in the gas leaving the reactor as a function of time. Mass spectrometry is well suited for these measurements, but it is extremely useful to also measure the response of the surface phase composition of the catalyst. This measurement is usually made by infrared spectroscopy (including diffuse reflection from powders), but interesting results have also been obtained by Raman spectroscopy. X-ray spectroscopy, Mossbauer spectroscopy, and scanning tunneling microscopy. These are all applicable at atmospheric pressure; under vacuum conditions, there are many other techniques available. In the search for better catalysts or for a better understanding of structure-performance relations, the determination of the rate parameters of the elementary steps of a reaction is of great utility.

The CO/H2 reaction over nickel-alumina studied by the transient method

The methanation of carbon monoxide over NiAl2O3 has been studied at atmospheric pressure and in the temperature range of 180–240 °C using a once-through flow reactor at small conversions. When the steady-state reaction in COH2 was switched to H2 alone, a large peak of methane and a smaller peak of higher hydrocarbons were obtained. Considerable H2O was also obtained indicating the presence of substantial chemisorbed CO. The methane response is composed of two parts: a sharp reactive peak followed by a long tailing part. Separate transient experiments, involving COH2COHe, and C2H4He, indicate that the tailing part of the response is associated with chemisorbed CO and the sharp, reactive peak is associated with free carbon.

The COH2 reaction on RhAl2O3: II. Kinetic study by transient isotopic methods

Abstract Transient isotopic methods have been used to study the CO H 2 reaction over 5% Rh Al 2 O 3 in the temperature range 180–260 °C. The steady-state tracing method permits the determination of the surface coverage of CO and active carbon Cα during reaction, without the need for quenching in helium and subsequent titration by hydrogen. The results confirm that the sequence of steps for methane formation passes through a small reservoir of active carbon (θcα

Some geometric aspects of structure sensitivity

Abstract The structure of supported metal catalysts can often be related to the fraction of the total atoms which is exposed on the surface (FE). The catalytic property related to the structure is the turnover frequency (TOF), the rate per unit of surface atoms. The particular behavior of these atoms can then be expressed via their Taylor ratio, a function of the way the surface atoms form an active site. For particles of FE