Catalyst-surface transient response

Equations (11) and (12) enable the generation of the total isotopic transient responses of a product species given (a) the transient response that characterises hypothesized catalyst-surface behaviour and (b) an inert-tracer transient response that characterises the gas-phase behaviour of the reactor system. Use of the linear-convolution relationships has been suggested as an iterative means to verify a model of the catalyst surface reaction pathway and kinetics. I This is attractive since the direct determination of the catalyst-surface transient response is especially problematic for non-ideal PFRs, since a method of complete gas-phase behaviour correction to obtain the catalyst-surface transient response is presently unavailable for such reactor systems.1 1 Unfortunately, there are also no corresponding analytical relationships to Eqs. (11) and (12) which permit explicit determination of the catalyst-surface transient response from the measured isotopic and inert-tracer transient responses, and hence, a model has to be assumed and tested. The better the model of the surface reaction pathway, the better the fit of the generated transient to the measured transient. 

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

Figure 5.10. Transient response of catalyst work function O and potential Uwr upon imposition of constant currents I between the Pt catalyst (labeled26 C2) and the Pt counter electrode p"-A1203 solid electrolyte T = 240°C, p02 = 21 kPa Na ions are pumped to (I<0) or from (I>0) the catalyst surface at a rate I/F.26 Reprinted with permission from Elsevier Science.

The first set of results from the TAP reactor, as shown in Figure 3, shows the 1,2 C vinyl acetate (MW=88) response curve for the four catalyst samples. The transient response suggests that KOAc dramatically accelerated the desorption of the vinyl acetate off the surface of the catalyst (peak maximum at 7.5 seconds without KOAc, 5.5 seconds with KOAc on average). In addition, Au enhanced the desorption rate of the vinyl acetate, but to a much lesser extent. It is also seen that Au improved the production rate of the catalyst (peak areas Pd-Au > Pd and Pd-Au w/KOAc > Pd w/KOAc). 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

Apart from the above mentioned redox type reactions, we like to consider (in connection with work to be published by us elsewhere) another type of relaxations, due to the possible reorganisations of sorption intermediates on the catalyst surface, as suggested by some investigations in our laboratory. This structuring on the catalyst surface is equivalent to a change in the entropy of the system catalyst surface / adsorbed intermediates and seems to be responsible e.g. for the selectivity change under transient conditions in the oxidation of hydrocarbons. Actually this structural organization of the surface intermediates is also a rate process which can be observed under transient conditions. 

The following Eqns. are important for the mathematical analysis of the elements of the transient responses around the wavefront (see (5, 6 ). In these equations y. stays for the state variables, such as concentration in the ambient fluid, or on the catalyst surface, temperature etc., f is the relaxation function (e.g. 

The adsorption of NO, under lean conditions was studied by imposing a step change of NO and NO2 feed concentrations in the presence and absence of excess oxygen over the reference catalysts in a fixed-bed flow microreactor operated at 350 ° C and analyzing the transient response in the outlet concentrations of reactants and products [transient response method (TRM)[. The adsorption/desorption sequence was repeated several times in order to condition the catalytic systems fully due to the regeneration procedure adopted (either reduction with 2000 ppm H2 + He or TPD in flowing He), BaO was the most Ba-abundant species present on the catalyst surface. FT-IR spectroscopy was used as a complementary technique to investigate the nature of the stored NO species. 

Hegedus et al. (1980) carried out experiments over a Pt-alumina catalyst exposed to mixtures of NO, CO, and 02 by periodically switching the stoichiometry of the feedstream between reducing and oxidizing conditions. The concentrations of species on the catalyst surface were measured and found to be oscillating. The results were viewed as proof that the catalyst surface events determine the transient response characteristics of the system. 

We thus conclude that the mode of CO adsorption and the stoichiometry of adsorbed gases are quite different when CO + H2 are adsorbed over Ru/Ti02 catalyst and the bulk Ru metal at different temperatures. The presence of H2 has a large promotional effect on the CO uptake by Ru/Ti02 catalyst wMch resulted in the formation of RuH(CO)u type surface complexes. These surface transient species are envisaged to be responsible for the low temperature CO methanation activity of Ru/Ti02. In case of Ru metal, only linearly held CO adsorption states are formed which transform to CH4 via CO disproportionation reaction [6,7]. 

In Figure 4, the normalized transient responses for the masses 15, 17 and 40 ( CH, CH and Ar, respectively) are shown for an experiment using the Co/Re/AljOj catalyst. The relatively smooth curves make the calculation of the areas under the curves, and thus the surface residence times, straightforward. The two CO transients are omitted from the figure for simplification. 

Two pools of Cj intermediates are assumed to exist on the catalyst surface one pool is the direct precursor of methane and the initiator of the chain growth while the second pool derived directly from CO j is responsible for monomer addition to the growing chains. As mentioned in the previous section, the presence of two intermediates or heterogeneity in the Cj intermediate has been established for the production of methane. However, these intermediates should be connected in parallel according to the methane transient experiments, whereas in Figure 48 they are connected in series. 

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