Catalyst deactivation sintering

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

In service, supported catalysts frequentiy undergo loss of activity over a period of time. In many cases, such catalyst deactivation is accompanied by the loss of accessible surface area of the active phase by sintering, by the accumulation of poisons, or by conversion of active sites to inactive species. 

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

As was noted for experiments HGR-12, HGR-13, and HGR-14, the rate of catalyst deactivation increased as the fresh gas feed rate increased. It is possible that higher rates of carbon deposition and metal sintering occur at the higher feed rates with resultant higher deactivation rates. 

For the catalyst activity factor (aj), several models have been proposed, depending on the origin of catalyst deactivation, that is, sintering, fouling, or poisoning. The following differential equation can semiempiricaUy represent different kinds of... 

The long-term stability of the Ru/Ti02 catalyst was studied under various reaction conditions and the spent catalysts were characterized for assessing the reasons of deactivation. It was observed that the rate exhibits a rapid reduction at the initial several hours of reaction, followed by a slow and continuous deactivation. Analysis of the spent catalyst, by H2 adsorption after removing surface carbon, showed that the initial rapid reduction of activity is mainly due to metal sintering, while the continuous and slow deactivation is related to the occurrence of the SMSl phenomenon at the later part of the catalyst bed, where reducing conditions prevail. In order to avoid these processes which lead to catalyst deactivation, Ti02... 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

Stefanov and coworkers—deactivation pathways for industrial Cu/Cr/Zn catalysts. Stefanov and coworkers250 published an XPS study indicating that the Cu-Cr-Zn catalyst deactivates via two pathways in an industrial reactor-sintering and poisoning by chlorine adsorption, which caused a deactivation of the catalyst from... 

An efficient, low temperature oxidation catalyst was developed based on highly disperse metal catalyst on nanostructured Ti02 support. Addition of dopants inhibits metal sintering and prevents catalyst deactivation. The nanostructured catalyst was formulated to tolerate common poisons found in environments such as halogen- and sulfur-containing compounds. The nanocatalyst is capable of oxidizing carbon monoxide and common VOCs to carbon dioxide and water at near ambient temperatures (25-50 °C). 

The catalyst remained active and did not start to deactivate for about 50 h on stream without any regeneration. After that, the catalyst activity started to drop slowly but steadily.The deactivation could be caused either by sintering of Pt nanoparticles or by coke deposited on the Pt atoms. If the catalyst deactivation was caused by coking, regeneration may reestablish catalyst activity. However, regeneration in air did not improve catalyst activity, which suggests that the cause of deactivation was not coke. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Based on this, it is apparent that the exothermic catalyst reactivation reactions need to be appropriately controlled to avoid Ni sintering/catalyst deactivation. 

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