Carbon monoxide metal-support interaction

The structure of supported rhodium catalysts has been the subject of intensive research during the last decade. Rhodium is the component of the automotive exhaust catalyst (the three-way catalyst) responsible for the reduction of NO by CO [1], In addition, it exhibits a number of fundamentally interesting phenomena, such as strong metal-support interaction after high temperature treatment in hydrogen [21, and particle disintegration under carbon monoxide [3]. In this section we illustrate how techniques such as XPS, STMS, EXAFS, TEM and infrared spectroscopy have led to a fairly detailed understanding of supported rhodium catalysts. 

The application of selective chemisorption to supported Pt catalysts is well established but there have been valuable additional studies of the use of hydrogen in the pulse-flow technique and of CO adsorption using TPD and carbon monoxide. Recently the usual assumption about the stoicheiometry for hydrogen adsorption, Ptg/H = 1 has been questioned. For the Council of Europe Pt-Si02 catalyst, where a weak metal-support interaction was postulated, 1.75 hydrogen atoms per surface metal atom were found at 300 K in two adsorbed forms (the formation of jSa was activated). Recent work on selective chemisorption applied to metals of catalytic interest other than platinum will now be examined. 

Titania-supported Metals. - After reduction at 473 K, platinum-group metals supported on Ti02 chemisorbed both hydrogen and carbon monoxide in quantities indicative of moderate-to-high dispersion, but following reduction at 773 K chemisorption was drastically lowered e.g., H/Mt <0.01 for Pt, Ir, and Rh, 0.05-0.06 for Pd and Ru, and 0.11 for Os). Agglomeration, encapsulation, and impurities were eliminated as possible causes and a strong metal-support interaction (SMSI) was proposed. Titania is not unique in its SMSI properties and 11 oxides used to support iridium were classified as follows ... 

It has been reported that the support has a major effect on the activity for the WGS reaction. Grenoble et al. (1981) suggested that the metal activates carbon monoxide whereas support sites are the principal sites for water activation. They suggest that the interaction of water molecules with the support can occur both through a dissociative... 

The choice depends on the type of molecules and strength or interaction with the surface sites. Carbon monoxide (CO) and hydrogen (H2) are the most often used but also NO, N2O, ethanol, and methanol. It gives us information about the dispersion of surface active sites, the nature and morphology of the metal sites, as well as metal-support interactions. The desorption profiles may be accompanied by several other profiles of products formed during the desorption. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

The phenomenon of metal transport via the creation of volatile metal carbonyls is familiar to workers using carbon monoxide as a reactant. It is often found that carbon monoxide is contaminated with iron pentacarbonyl, formed by interactions between carbon monoxide and the walls of a steel container. Thus, it is common practice to place a hot trap between the source of the CO and the reaction vessel. Iron carbonyl decomposes in the hot trap and never reaches the catalyst that it would otherwise contaminate or poison. Transport of a number of transition metals via volatile metal carbonyls is common. For example, Collman et al. (73) found that rhodium from rhodium particles supported on either a polymeric support or on alumina could be volatilized to form rhodium carbonyls in flowing CO. 

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

The activity of supported Pt catalysts for methanol synthesis from C0-H2 is considerably enhanced when the metal is supported on oxides which exhibit themselves appreciable activity for MeOH synthesis. Furthermore it is found that the rate of methanol formation on Pt-supported catalyst is increased when Th02, Ce02 were mechanically mixed with the Pt catalyst. Such behaviour is typical for bifunctional catalysts. It has already been reported that Th02, Ce02 adsorb carbon monoxide without dissociation. Such activated CO can be hydrogenated to form a formyl species, the formyl species interacting with lattice oxygen will produce a formate intermediate. 

---