Crystallite size Deactivation

The hydrogenation of para-substituted anilines over rhodium catalysts has been investigated. An antipathetic metal crystallite size effect was observed for the hydrogenation of /Moluidinc suggesting that terrace sites favour the reaction. Limited evidence was found for catalyst deactivation by the product amines. Catalysts with pore diameters less than 13.2 nm showed evidence of diffusion control on the rate of reaction but not the cis trans ratio of the product. 

An increase in the crystallites size of the HBEA zeolite brings a neat increase in the activity of the corresponding Pt-exchanged catalyst for n-C.% transformation (Table 1). However, this increase in activity is accompanied by a significant deactivation of the catalyst 90% with the 10-15 pm crystallite size, 60% with the 1-1.5 pm crystallite size, while no deactivation is observed with the 0.02 pm crystallite size. 

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

Owing to low copper content, copper-ceria catalysts are nonpyrophoric and stable, showing little or no deactivation during the experiments. The Cu0 2Ce0 x02 r catalyst prepared by coprecipitation method showed good catalytic activity for the WGS reaction. The Cu01Cc()9O2, catalyst prepared by sol-gel method was found to be less active, which could be due to lower number of active copper sites, or to different crystallite size and structure of copper-containing species. The copper-ceria catalysts were shown to be selective for the WGS reaction and no methanation reactions were observed over any catalyst under the experimental conditions used. 

The effects of space velocity, partial pressure, temperature, and catalyst crystallite size on the coke formation and the resulting deactivation in both MTO and the coke forming reactions were easily obtained in the TEOM experiments (39,40,86). Furthermore, the mass desorbed from the catalysts was determined by the transient mass response this information is important for the design of the stripping process in such applications as FCC and MTO. 

This work focuses on the investigation of the stability of catalytic activity in the liquid phase methanol synthesis process. Novel catalysts with a long-term stability have been developed by the addition of hydrophobic materials. The addition of hydrophobic materials were effective for slowing down the crystallite size growth and inhibition of deactivation of catalyst as compared with the original catalyst without modification. 

Samples of catalyst were removed from the pilot-plant reactor at various times during Run >3. Physical and chemical analyses were carried out on these samples and the results were compared with measurements on freshly-reduced catalyst, prior to exposure to synthesis gas. The analyses included BET surface area, S. Cl, Fe and Ni concentration on the catalyst. Cu and ZnO crystallite sizes, and Cu/Zn and Cu VCu ratios on the catalyst surface. The only strong correlation between the rate constant and any of these parameters is shown in Fig. 2, which reveals a striking dependence of the rate constant on the BET surface area. This relationship suggests that sintering of the overall catalyst surface is responsible for a large part of the observed deactivation. 

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

The most direct influence of the support is on dispersion and morphology. It is well known that nickel is better dispersed on S1O3 than AI>Oi, and the shape of crystallites may also be affeaed. Data in Table 2.4 do not address the question of crystallite size dependence, which has a much firmer foundation. Surface contamination is always present. Commercial reagents used in support preparation may contain impurities or provide ions which remain with the oxide. Subsequent deposition of active components may incorporate these ions as poisons. Acidic sites vary on different supports and during initial exposure to reagents produce carbonaceous deposits which in turn deactivate nickel " sites. A possibility that cannot be overlooked is spillover, a phenomena in which a reactive... 

Several conclusions may now be drawn from the present work. First of all, the crystallite size plays an important role in the diffusivity of reactant or product molecules, and subsequently in catalytic shape selectivity and deactivation rate. When the crystallite size increases, diffusion rate decreases, while shape selectivity and deactivation rate increase. These features obviously arise from an enhancement in the molecule pathway within the zeolite matrix, and from a decrease in the number of pore openings per unit weight when the crystallite size increases. 

The authors propose that the Pt surface is increasingly deactivated by platinum oxide as the crystallite size is reduced. At higher CO concentrations the low-rate region is encountered, and crystals of all sizes are supposed to be covered almost completely by CO. Recent work (142) has confirmed that the formation of platinum oxide at 220°C does occur on small particles of FE = 0.6. 

Deactivation, as noted above, is always accompanied by a significant decrease in pore volume and surface area and an increase in average crystallite size of the MgO and CaO components. These effects are still being investigated, and no quantitative data will be presented at this time. 

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