Selectivity structure sensitive reactions

As a result of disproportionation of CO on small particles, the selectivity of the CO-H2 reactions shifts from methanol on large particles to methane on small ones. The methanation activity increases as the metal particle size decreases, indicating that methanation is a structure-sensitive reaction on palladium. 

These results indicate that the same crystalline face does not necessarily exhibit the same catalytic properties with different molecules. Thus, the (010) face of a-Mo03 is selective for the formation of aldehydes from alcohols while it promotes essentially the deep oxidation of olefins. It is expected that the studies on structure-sensitive reactions will be made more quantitative using recent methods to determine the number of surface M=0 species (425 —7). It should be noted that the earlier observation on the specificity of Mo03 crystalline faces in propylene oxidation has been obtained on oriented Mo03-graphite catalysts (425k). Non-structure-sensitive reactions have also been reported (425k). 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

Nevertheless, two factors strongly influence the heat of sulfur chemisorption on metal surfaces relative coverage and crystallographic structure. Thus sulfur chemisorbs at high coordination sites and, as a result, a selective poisoning of structure-sensitive reactions, preferentially catalyzed by these sites, may occur. Such a simple geometrical model can be used to explain change in selectivities induced by sulfur adsorption. 

The particle-size effect is for both supports the largest for the selectivity towards the roll-over mechanism (via the di-G-T)1 intermediate, Figure ID), which is strongly increased with the larger particles. Hence, also the roll-over mechanism is a clearly structure-sensitive reaction. It is facilitated by large particles, and probably an ensemble of catalytically active, empty sites is needed for the formation of the di-G-r)1 intermediate. 

This paper describes some of our initial studies of the solution properties of sodium titanate powders in order to understand and control the metal-loading process in the preparation of a heterogeneous catalyst from the materials. The purposes of this study are 1) to define the pH and concentration regimes in which nickel is loaded onto sodium titanate as monomeric ions via ion exchange, as polymeric clusters via hydrolysis, or as discrete particles of colloidal nickel hydroxide, and 2) to characterize the catalysts with respect to the dispersion of the active metal under reaction conditions by measuring the activity and selectivity of the catalysts for a known "structure-sensitive" reaction, the hydrogenolysis of n-butane. 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

The formation of PO over Au-based catalysts is a structure-sensitive reaction. Only hemispherical Au particles with a suitable size (2-5 nm) will selectively produce PO [167,168,403] and 2.2 to 2.4 nm particle size seemed to be optimum in the early experiments [31]. The most effective type of Au nanoparticles is prepared by the DP technique, which brings them in strong contact with the support. Gold particles smaller than 2 nm show a shift in selectivity from PO to propane [7,169-171,403]. This switch of epoxidation to hydrogenation for particles under 2 nm size indicates that small Au clusters exhibit different behaviour in surface properties from that of metallic Au [171] (see Fig. 6.17). 

In catalysis one may expect the appearance of heterogeneous metallo-complex catalysts containing active centres distributed by the required way . This can result in preparation of highly selective catalysts for structurally sensitive reactions. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

The main effect of sintering is loss of active surface area and the resulting decrease in catalyst activity. However, a change in selectivity can also occur, especially in the case of structure-sensitive reactions. Extensive investigations of sintering have been carried out on highly dispersed metals such as Pt/Al203. 

The difference between the two definitions is that the rates vary with time or position, while the TOF only depends on the availability of the sites present. The activity of the catalysts and the selectivity of the reaction depend on the characteristics of the material. Some important features of these reactions should be remembered. The first one is related to the sensitivity of the chemical reaction to the catalyst structure. According to Boudart s theory, the supported catalysts consist of metal particles of different and variable sizes. With the increase of particle diameter, the concentrations of metal atoms and exposed sites vary significantly, indicating that no changes in surface structure have happened. The structure sensitive reactions (SSR) are those in which the intrinsic reaction rate relative to the number of surface active sites, i.e., the frequency of the reaction varies with the particle sizes, which does not happen in structure insensitive reactions (SIR). This means that in the structure sensitive reactions, the frequency of... 

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. ... 

The selective chemisorption of H2 and O2 for the determination of supported metal surface area, combined with the determination of specific activity, gives to the concept structural sensitive reaction and structural-non sensitive reaction in the field of heterogeneous catalysis. Within a certain range (l-5nm), changes in the size of metal particle may influence its surface structure. Experience has shown that the specific activity of some reactions changes with the surface structure of the metal. Such a reaction is called as structural sensitive reaction. Reaction in which specific activity does not change with the surface structure is called as structural non-sensitive reaction. Research results have shown that the non-structm-al sensitive reactions may become sensitive when the metal particles further become smaller in the highly dispersive state. 

The size of the metal particle plays an important role in the structure-sensitive reaction when site coordination (edge, kink, or terrace sites) or crystal orientation affect the activity and selectivity. In recent work, transient curves of gas-phase COg produced with CO-pulse injection on Pd/CeOg at 500-700 °C, established the structure sensitivity of the CO oxidation reaction on supported... 

This is a structure-sensitive reaction (RSS), and hence the intrinsic activity or turnover frequency (TOP) depends on the particle sizes. The hydrogenolysis reaction was also tested with the Pt/Al203 and promoted with Sn. The reaction was carried out at 1 atm and 573 K and a mixture of hydrogenolysis of methylcyclopentane (MCP)/H2 (1 10.5). The selectivity toward n-hexane, 2-methylpentane, and 3-methylpentane was determined for conversions less than 10 % and was presented in Table 3.2 [15]. 

The benzene hydrogenation was selected as a model reaction for determining the metallic surface sites, because it is a structure-sensitive reaction. The sole product was cyclohexane, as shown below. 

Abstract Nanostructured systems are of great interest from points of view of basic science and technological applications. Within the topic of catalysis, should be highlighted the properties associated with different morphologies, activities and selectivities, which are strongly affected by the shape and particle size in the case of crystalline metallic phases are oriented crystal faces. The reactions that are influenced by these factors (morphological) are known as structure sensitive reactions. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

The NO + CO reaction is only partially described by the reactions (2)-(7), as there should also be steps to account for the formation of N2O, particularly at lower reaction temperatures. Figure 10.9 shows the rates of CO2, N2O and N2 formation on the (111) surface of rhodium in the form of Arrhenius plots. Comparison with similar measurements on the more open Rh(llO) surface confirms again that the reaction is strongly structure sensitive. As N2O is undesirable, it is important to know under what conditions its formation is minimized. First, the selectivity to N2O, expressed as the ratio given in Eq. (7), decreases drastically at the higher temperatures where the catalyst operates. Secondly, real three-way catalysts contain rhodium particles in the presence of CeO promoters, and these appear to suppress N2O formation [S.H. Oh, J. Catal. 124 (1990) 477]. Finally, N2O undergoes further reaction with CO to give N2 and CO2, which is also catalyzed by rhodium. 

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

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