Kinetic analysis employing a mechanism that captures the essential surface chemistry of the reaction allows quantitative interpretation of diverse experimental data. This approach is used with a Horiuti-Polanyi mechanism, modified by hydrogen activation steps, to describe the surface chemistry for ethylene hydrogenation over platinum catalysts. In this investigation, kinetic analysis provides a quantitative means of comparing, contrasting, and consolidating results from steady-state kinetic studies, deuterium tracing measurements, vibrational spectroscopy, and temperature programmed desorption. A noncompetitive pathway is dominant at low temperatures, involving sites for hydrogen adsorption that are not blocked by carbonaceous species. At higher temperatures and lower ethylene pressures, more surface sites become available for hydrogen adsorption, and the reaction shifts to a pathway involving competitive hydrogen and ethylene adsorption.
The adsorption of acetaldehyde and crotonaldehyde on the anatase and rutile polymorphs of TiO2 has been investigated with Fourier transform infrared spectroscopy (FTIR). Chemisorption of acetaldehyde on TiO2 involves a strong interaction between the surface and the carbonyl oxygen, causing a significant shift in the location of the ν(CO) vibrational mode to lower frequencies; no interaction with surface hydroxyl groups was observed. Capacities for acetaldehyde and crotonaldehyde adsorption under conditions relevant to aldolization reactions were determined in a novel reactor system providing simultaneous mass measurements and mass spectral analysis of gas-phase products. The coverage of acetaldehyde irreversibly adsorbed on TiO2 was similar to values previously reported for the adsorption of alcohols; coverages of crotonaldehyde were approximately 60% of those for acetaldehyde. Both gas-phase and surface analyses indicate that formation of crotonaldehyde by aldol condensation of acetaldehyde occurs on rutile TiO2 at temperatures as low as 313 K. This reaction was not observed on anatase at these conditions; higher temperatures were required. The production of crotonaldehyde on rutile at 313 K diminished with increasing exposure of acetaldehyde. Acetaldehyde and crotonaldehyde adsorbed in a similar fashion on both anatase and rutile, and either aldehyde could displace the other from the surface layer. Accordingly, the surface concentrations of adsorbed acetaldehyde and crotonaldehyde mirror those in the gas phase. Upon heating an adsorbed layer of acetaldehyde, small amounts of ethoxide and acetate species were formed, possibly from a Cannizzaro-type disproportionation reaction. The similarity of these results to those of studies on TiO2 single crystals illustrates the applicability of properly chosen metal oxide single-crystal surfaces as models for polycrystalline powders. Both demonstrate that the chemistry of aldehydes on TiO2 can be successfully explained in terms of the reactions of a few key surface species.
Metal oxide catalysts are active for aldol condensation reactions of aldehydes and ketones but typically exhibit rapid deactivation. In this study, aldol condensation with dehydration of acetaldehyde to produce crotonaldehyde was catalyzed by oxidized TiO2 anatase. Particular emphasis was placed on determination of product selectivities and reaction kinetics as a function of both conversion and the extent of catalyst deactivation. Condensation of acetaldehyde is rapid and selective on anatase TiO2, with turnover frequencies exceeding 0.03 s−1 and crotonaldehyde selectivities approaching 100%. The main side reactions generating volatile products are: (a) hydrogenation of the reactant and product aldehydes, (b) a secondary cross-esterification between crotonaldehyde and acetaldehyde to form ethyl crotonate, and (c) secondary condensations involving crotonaldehyde. Catalyst deactivation was observed to affect both the rate of reaction and the product selectivities, particularly at low values of time-on-stream. Secondary condensations that deposit nonvolatile organic species on the catalyst surface are responsible for the initial deactivation of the catalyst. The catalyst mass increase during the course of reaction was observed to be directly proportional to the rate of deactivation. Selectivity patterns were impacted by deactivation in a manner that could not be explained solely by the changing conversion levels associated with the deactivation process. We conclude that deactivation during aldol condensation on the anatase polymorph of titania alters both activity and selectivity of the active sites. As a result, care must be taken to account for catalyst deactivation when comparing both catalytic activities and selectivities.
As metal oxide reduction may be a limiting or otherwise important step in a reaction cycle, a complete description of the kinetics of the reduction can be critical to the successful choice of catalytic material. Unfortunately, such information is often lacking. Such is the case in our attempts to develop a catalytic cycle from the stoichiometric reductive carbonyl coupling reaction on reduced TiO2 surfaces. To provide the necessary reduction kinetics, reaction of the anatase and rutile forms of TiO2 with H2 has been studied from 573 to 773 K. A novel flow-through microreactor which provides time-resolved catalyst mass measurements to ±1 μg while maintaining a conventional, tubular reactor, gas−solid contacting pattern has been employed. A shift in the kinetic order with respect to H2 with increasing temperature occurs, from one-half order at 573 K to zero order at 673 K and above. A discontinuity was also observed within this same temperature range in Arrhenius plots of the reduction rates of both anatase and rutile TiO2; apparent activation energies determined were approximately 12 kcal mol-1 above and 29 kcal mol-1 below 623 K. Modification of the surface of anatase TiO2 with a sufficient loading of group VIII metals removes the Arrhenius plot discontinuity, increasing the rate of reduction and decreasing the apparent activation energy at low temperatures. A change in rate-determining step is indicated by these observations, and a mechanistic scheme which combines the current and previous observations within a single framework is proposed.
Microkinetic simulations have been carried out to describe the partial oxidation
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