Model catalysts carbon monoxide oxidation

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

M. Sheintuch, J. Schmidt, Y. Lecthman, and G. Yahav, Modelling catalyst-support interactions in carbon monoxide oxidation catalysed by Pd/Sn02, Appl. Catal. 49, 55-65 (1989). 

Two model reactions were applied to characterize the catalytic behavior of the zeolite-incorporated metal oxides carbon monoxide oxidation and methanol conversion. As is clear from what follows, the catalytic activity turned out to be the most sensitive properties toward the minor changes in a catalysts "biography". Therefore, some disagreements may rise on comparing the activity of supposedly similar samples with the same composition but of different preparation history. 

Akubuiro, E. C., Verykios, X. E. Lesnick, L. Dispersion and snpport effects in carbon monoxide oxidation over platinum. App/ied Catalysis 14, 215-227 (1985). Stara, L, Nehasil, V. MatoUn, V. The influence of particle size on CO oxidation on Pd/alumina model catalyst. Surface Science 331—333, 173—177 (1995). 

A mathematical model of a wall-catalyzed reactor is used to elucidate the effects of Inlet velocity and concentration, geometry of the duct, and axial conduction on the hysteresis, multiplicity and parametric sensitivity. The monolith reactor for carbon monoxide oxidation is the prototype considered, with wall catalyst being platinum on alumina on a ceramic substrate. 

Almost all catalytic converters have to contend with the decay or poisoning of the catalyst In some form and the catalytic monolith Is no exception. Indeed this Is notorious In the automotive application where the catalytic converter must survive 50,000 miles of operation and still perform adequately. Although we shall use the kinetics of carbon monoxide oxidation over a platinum catalyst as an obvious and Important example, our main objective Is to develop a model which can handle any catalyst decay question and to point out the differences In two types of poisoning. Thus our study comes within the third main division of the subject as laid out by Butt (1 ) In 1972 not the mechanism or rate determination but the effect of deactivation on the operation of the reactor. 

Oxidation Catalyst. An oxidation catalyst requires air to oxidize unbumed hydrocarbons and carbon monoxide. Air is provided with an engine driven air pump or with a pulse air device. Oxidation catalysts were used in 1975 through 1981 models but thereafter declined in popularity. Oxidation catalysts may be used in the future for lean bum engines and two-stroke engines. 

Kolbel et al. (K16) examined the conversion of carbon monoxide and hydrogen to methane catalyzed by a nickel-magnesium oxide catalyst suspended in a paraffinic hydrocarbon, as well as the oxidation of carbon monoxide catalyzed by a manganese-cupric oxide catalyst suspended in a silicone oil. The results are interpreted in terms of the theoretical model referred to in Section IV,B, in which gas-liquid mass transfer and chemical reaction are assumed to be rate-determining process steps. Conversion data for technical and pilot-scale reactors are also presented. 

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

Previous studies in conventional reactor setups at Philip Morris USA have demonstrated the significant effectiveness of nanoparticle iron oxide on the oxidation of carbon monoxide when compared to the conventional, micron-sized iron oxide, " as well as its effect on the combustion and pyrolysis of biomass and biomass model compounds.These effects are derived from a higher reactivity of nanoparticles that are attributed to a higher BET surface area as well as the coordination of unsaturated sites on the surfaces. The chemical and electronic properties of nanoparticle iron oxide could also contribute to its higher reactivity. In this work, we present the possibility of using nanoparticle iron oxide as a catalyst for the decomposition of phenolic compounds. 

Carbon Monoxide The presence of CO in a H2-rich fuel has a significant effect on anode performance because CO affects Pt electrodes catalysts. The poisoning is reported to arise from the dual site replacement of one H2 molecule by two CO molecules on the R surface (40, 41). According to this model, the anodic oxidation current at a fixed overpotential, with (ico) and without (in2) CO present, is given as a function of CO coverage (0co) by Equation (5-11) ... 

To demonstrate the potential available, simulations were carried out for the oxidation of carbon monoxide on a palladium shell catalyst with water desorption from 3A zeolite as a heat sink, based on experimentally validated model parameters for the individual steps (Figure 16). The calculations indicated that the reaction cycle time could be lengthened by a factor of 10, to a total 20 minutes, in comparison to a simple regenerative process with a similar amount of inert material instead of adsorbent in the fixed bed and for the same threshold for temperature deviation from the initial value. 

If a chemical reaction is operated in a flow reactor under fixed external conditions (temperature, partial pressures, flow rate etc.), usually also a steady-state (i.e., time-independent) rate of reaction will result. Quite frequently, however, a different response may result The rate varies more or less periodically with time. Oscillatory kinetics have been reported for quite different types of reactions, such as with the famous Belousov-Zha-botinsky reaction in homogeneous solutions (/) or with a series of electrochemical reactions (2). In heterogeneous catalysis, phenomena of this type were observed for the first time about 20 years ago by Wicke and coworkers (3, 4) with the oxidation of carbon monoxide at supported platinum catalysts, and have since then been investigated quite extensively with various reactions and catalysts (5-7). Parallel to these experimental studies, a number of mathematical models were also developed these were intended to describe the kinetics of the underlying elementary processes and their solutions revealed indeed quite often oscillatory behavior. In view of the fact that these models usually consist of a set of coupled nonlinear differential equations, this result is, however, by no means surprising, as will become evident later, and in particular it cannot be considered as a proof for the assumed underlying reaction mechanism. 

Rival Kinetic Models in the Oxidation of Carbon Monoxide over a Silver Catalyst by the Transient Response Method... 

The oxidation of carbon monoxide by nitrous oxide and oxygen over a silver catalyst at 20°C was analysed by both the Hougen -Watson procedure and the transient response method. The rival models derived from both procedures were clearly distinguished by the mode of the transient response curves of C02 or N caused by the concentration jump of CO, 02 or N20. 

Oxide catalysts are known to be effective for oxidation reactions. In this study, we wanted to produce carbon monoxide through partial oxidation of the biomass, as this could be expected to lead to a conversion of carbon monoxide into hydrogen via the water-gas shift reaction. An oxidization of the tarry product is also expected. By these two effects, improvement of the efficiency of the gasification is expected. Oxide catalyst is expected to enhance the oxidation reaction needed for this scenario. Since oxide catalyst is considerably cheaper than nickel catalyst, its use would make the whole gasification process more economical. Hence, we decided to examine the effect of oxide catalysts on gasification with partial oxidation using cellulose as a model compound. 

Carbon monoxide is a potential emission problem. Fortunately, it is a valuable industrial fuel and may be burned in a CO boiler to recover energy as steam and discharge carbon dioxide. However, it can also arise from the catalyst regenerators of some recent models of catalytic cracking, units which ran at temperatures too low to obtain complete oxidation of carbon to carbon dioxide. The most recent designs of regenerators operate at higher temperatures to achieve complete conversion of carbon to carbon dioxide. 

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