Structure sensitivity catalyst deactivation

The Rh-BINAP catalysts are very sensitive to impurities such as oxygen, moisture, and carbon dioxide. If an excess of water ([H20]/[Rh] = 15) is present in the reaction mixture, the isomerization is stopped aftera few turnovers with precipitation of air-stable red-brown crystals, which were found to be [ Rh(BINAP) 3()I3-0H)2 C104. X-Ray analysis of this complex (H20 was replaced by DzO) revealed a unique structure of a triangular Rh3 core capped with two triply bridging OH groups (Fig. 3.1). This Rh(I)-trinuclear complex was totally inactive as an isomerization catalyst, which suggests a catalyst deactivation mechanism [12], The effect of several additives on the isomerization of 2 with [Rh(BINAP)(COD)]+ has been examined, and the results are summarized in Table 3.3. 

We have summarized these developments in two recent papers the first addresses the topic of structure sensitivity in combination with BEP relationships (14) and the second addresses the Sabatier principle (27). Sabatier-type volcano relationships can be deduced for activity as a function of adsorption energy, and they can also be used to predict trends regarding deactivation of Fischer-Tropsch catalysts by C—C recombination reactions (28). We refer to these texts as background information to the material presented here. 

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... 

When a catalyst is exposed to a stoichiometric gas mixture, whether oscillating or stationary, the resulting deactivation is very moderate when the temperature of the pretreatment is raised from 500°C to 900°C. This deactivation can only be proven by the values of the T50 recorded for the reaction between NO and HC and not for that between CO and O2. This confirms that the CO/O2 reaction is structure insensitive. Indeed, a moderated sintering of the metallic phase, as seen by microscopy, has no influence upon the reaction when compared to a catalyst aged from 500°C to 900°C. The implication is that the best way to observe an effect related to sintering is to consider the reactions involving HC or NO since they are usually structure sensitive [15,16]. 

Specific to the active metal, the exposed surface area and metal particle size most often decrease with deactivation. Specific adsorbents such as H2, CO, or H2-O2 are most often employed to estimate the metal surface area. It is not clear if VOC is a structure sensitive or structure insensitive reaction, i.e., whether the activity is proportional to exposed metal or not. In reducing environments the activity will be strictly proportional to exposed metal area for structure-insensitive reactions. In oxidizing environments, as in these reaction conditions, platinum may exist in a partially oxidized state. The techniques currently employed in catalyst characterization do not currently attempt to differentiate between exposed metal and exposed oxidized metal. 

The effect of changing active sites in deactivating catalysts is described by the structure-sensitive or structure-insensitive nature... 

Boudart et al. for supported metal catalysts to indicate whether the rate of reaction is nonlinearly or linearly proportional to the total surface area of the metal, but the concept can be extended to other types of catalysts as well. Thus, for acid catalysts, the rate of formation of a particular species may depend upon the total number of acid sites, or upon the number of a particular type, e.g., a particular range of acid strengths. In general, the deactivation process may diminish the number of a particular type of active sites, while the rate of formation of each product may depend upon other types of active sites (which may include part or all, or none, of the first type). The interrelation between these types denotes whether the various reactions are structure-sensitive or structure-insensitive, and determines how the selectivities change with... 

The structure-sensitive character of methane decomposition reaction has been confirmed, as has the importance of the metal dispersion on the catalyst performance over different supported Ni catalysts. TEM analyses of the spent catalysts reveal that both filamentous and encapsulating carbon species were formed under isothermal conditions at 823 K, the latter being responsible for catalyst deactivation. 

Bonura et alf studied several supported Ni catalysts for methane decomposition and found that both filamentous and encapsulating carbon species were formed under isothermal conditions at 823 K, the latter being responsible for catalyst deactivation. They also confirmed the structure-sensitive character of methane decomposition. The efficient use of these catalysts implies a high dispersion of metal phases which can be achieved by controlled segregation of the active phase. Different Ni mixed oxides such as Ni-Al hydrotalcite, Ni-La perovskites and Ni-Al spinels as catalysts precursors allow a high degree of Ni dispersion, of which that derived from hydrotalcite mixed oxide showed the highest activity for Hg production by methane decomposition. ... 

Bimetallic catalysts made up of group 8, 9, or 10 elements and tin have been reported in dehydrogenation and hydrogenation reactions of hydrocarbons. Seminal work by Boudart et al on structure-sensitive reactions fostered numerous studies over a wide range of catalyst compositions. It has been shown that tin acts as a promoter, thereby increasing dramatically selectivity and activity, while preventing deactivation by coke accumulation. A combination of both geometric and electronic effects on the active sites has been proposed to rationalise the role of tin. ... 

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