Deactivating catalysts mechanism

The paper-impregnation drying oven exhausts contain high concentrations (10—20% LEL) of alcohols and some resin monomer. Vinyl resins and melamine resins, which sometimes also contain organic phosphate fire retardants, may be used for air filters. The organic phosphates could shorten catalyst life depending on the mechanism of reduction of catalyst activity. Mild acid leaching removes iron and phosphoms from partially deactivated catalyst and has restored activity in at least one known case. 

A remarkable feature of iridium enantioselective hydrogenation is the promotion of the reaction by large non-coordinating anions [73]. This has been the subject of considerable activity (anticipated in an earlier study by Osborn and coworkers) on the effects of the counterion in Rh enantioselective hydrogenation [74]. The iridium chemistry was motivated by initial synthetic limitations. With PFg as counterion to the ligated Ir cation, the reaction ceases after a limited number of turnovers because of catalyst deactivation. The mechanism of... 

In terms of the effect of water on the deactivation, several mechanisms have been identified, and they will influence the stability of the catalyst depending on the conditions and the support used. At high partial pressures of water oxidation is always a possibility, but the various reports are less clear to whether this is mainly surface oxidation of cobalt particles irrespective of particle size, or if small particles... 

Provide thermal sink In adiabatic reactors and in reactors where cooling is difficult and exothermic heat effects are large, it is often necessary to feed excess material to the reactor (an excess of one reactant or a product) so that the reactor temperature increase will not be too large. High temperature can potentially create several unpleasant events it can lead to thermal runaways, it can deactivate catalysts, it can cause undesirable side reactions, it can cause mechanical failure of equipment, etc. So the heat of reaction is absorbed by the sensible heat required to raise the temperature of the excess material in the stream flowing through the reactor. 

The present contribution is a description of the technique of INS spectroscopy of catalysts and a summary of some recent experimental results that illustrate the usefulness of neutron spectroscopy. These include the characterization of model systems, commercial catalysts, mechanisms of coke deposition and catalyst deactivation, and the identification of atomic hydrogen in the topmost atomic layers of... 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Thiophene metal poisoning as well as hydrogenation of ethylbenzene on metal catalysts require, as a first step, the chemisorption of both organic molecules on the metal active sites. Afterwards, catalyst deactivation can simply take place by the blocking of these sites or by further hydrogenolysis of thiophene and subsequent formation of an inactive surface metal sulfide. We believe that, in our conditions, this last mechanism is probably operating. This hypothesis is supported by the fact that butane was detected in our experiments and, furthermore, XPS analysis showed the formation of metal sulfides (S ) on the deactivated catalysts. 

Matsuoka et al. studied the deactivation of a V205/Ti02 catalyst due to formation of sulfates of ammonia on the catalyst, and subsequent regeneration, at 200-300 °C. The normal composition of their reaction gas was 200 ppm NO, 240 ppm NH3,50 ppm SO2, 3% O2,10% H2O, balance N2. The water soluble components of those catalysts which had been deactivated at 200 °C were found to contain NH4HSO4, NH4V(S04)2, and 2(804)3. No sulfates of Ti were detected. Both the surface area and pore volume of deactivated catalysts were found to be inversely proportional to the sulfur content of the catalyst. The percent reduction in the rate constant also correlated well with the extent of sulfur accumulation in the catalyst. The rate and extent of deactivation at 200 °C were observed to be independent of space velocity, but at 300 °C they were inversely proportional to the space velocity. To explain this, the authors proposed a different mechanism of deactivation at the higher temperature. 

S. Kasaoka, E. Sasaoka, and H. Nanba, "Deactivation Mechanism of Vanadium Pentoxide-Titanium Dioxide Catalyst by Deposited Alkali Salts and Regeneration Method of Deactivated Catalyst in Reduction of Nitric Oxide with Ammonia", Nippon Kagaku Kaishi. Japan, 1984,3, 486-494. 

In order to gain a deeper understanding of the mechanism of the deposit growth, and of its active intermediates in particular, the catalytic experiments were undertaken in which the deactivated catalyst was used. After the reaction at 350 C the catalyst was cooled down to room temperature and the deposit was carefully removed from its surface, then the catalyst was heated up to the same temperature and another catalytic test was initiated by switching on the flow of the reactants. 

Regeneration of a reaction-deactivated catalyst by H2 and by the SCO reaction probably occur by the same mechanism. Common to both regeneration methods is the exposure of the catalyst to H2. We propose that deactivation during CO oxidation is caused... 

In conclusion, we have found that AU/Y-AI2O3 catalysts can be deactivated both thermally and by CO oxidation. Successful regeneration of a thermally deactivated catalyst is accomplished by exposure to H2O, whereas a reaction-deactivated catalyst can be regenerated by exposure to H2 at room temperature. The latter can be regenerated also by the selective CO oxidation reaction, which is conducted in the presence of H2. The results can be explained with a reaction mechanism in which the CO oxidation reaction proceeds via the formation and decomposition of a surface formate and bicarbonate and an active site consisting of an ensemble of Au-hydroxyl group and metallic Au atoms. Deactivation is due to dehydroxylation of the Au-hydroxyl group or the formation of a rather inactive carbonate. 

The deactivation of bulk iron oxide during methane combustion has been studied. The observed deactivation behaviour has been explained as the result of two simultaneous deactivation mechanisms. In the initial phase of reaction both are in operation and the activity drops rapidly as a consequence of both catalyst sintering and of the depletion of lattice oxygen in the outer layers, due to a partial reduction of the catalytic surface. In later stages, catalyst deactivation is almost exclusively due to sintering imder reaction conditions. A kinetic model of deactivation is presented, together with the physicochemical characterization of fresh and partially deactivated catalysts. 

The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

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