Metal catalysts, monolayer

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... 

Here we shall consider a different concept, which has an interesting potential, particularly in liquid phase reactions used for the production of fine chemicals. The concept is schematically illustrated in Fig. 3. The modification of the metal catalysts is achieved by very small quantities (usually a sub-monolayer) of adsorbed auxiliaries (modifiers), which are either simply added to the reaction mixture (in-situ), or brought onto the catalyst surface in a... 

For the present purpose, we take the term ultrathin to refer to an evaporated metal film where the concentration of metal on the substrate is low enough for the film to consist of small isolated metal crystals. If the average concentration of metal atoms on the substrate is of the order of a monolayer or less, the metal crystals are small enough for ultrathin films to serve as models for highly dispersed metal catalysts, but where surface cleanliness and catalyst structure can be better controlled. 

Recently, ultrathin evaporated films have been used as models for dispersed supported metal catalysts, the main object being the preparation of a catalyst where surface cleanliness and crystallite size and structure could be better controlled than in conventional supported catalysts. In ultrathin films of this type, an average metal density on the substrate equivalent to >0.02 monolayers has been used. The apparatus for this technique is shown schematically in Fig. 8 (27). It was designed to permit use under UHV conditions, and to avoid depositing the working film on top of an outgassing film. ... 

The active components of many commercial supported heterogeneous catalysts are oxides or salts. Even for many metal catalysts, the precursors of metallic particles are also oxides or salts in some dispersed form. Hence the preparation of heterogeneous catalysts is deeply concerned in one way or another about the dispersion of oxides or salts on support surfaces. Furthermore, promoters or additives added to heterogeneous catalyst systems are also oxides or salts. Therefore, the spontaneous monolayer dispersion of oxides or salts on supports with highly specific surfaces as a widespread phenomenon will find extensive application in heterogeneous catalysis. Examples illustrative of this viewpoint are cited in the following sections. 

As was stated previously, metal cannot disperse as a monolayer onto catalyst supports. However, oxide precursors of metals can monolayer disperse on supports, and supported metal particles can be prepared from the monolayer-dispersed oxide by reduction. 

It is worth mentioning that spontaneous monolayer dispersion is also a very useful scientific basis underlying the process of regeneration of deactivated metal catalysts. Supported metal catalysts may sinter during use at elevated temperatures. Sintering will cause the metal catalyst to lose initial activity, and in order to recover it one has to find an effective way to redisperse the metal on the catalyst support. Applying what we have learned from our studies on spontaneous monolayer dispersion to... 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

Supported metal catalysts generally show an increase in catalytic activity compared to the pure oxide or metal. Yet these systems are not well characterised, owing to the fact that such catalysts typically consist of a range of different supported metal sites, from small clusters to monolayer islands, all with non-uniform distributions in size and shape. One way to begin to understand such complex systems is to attempt to capture some essential part of the full system by developing model catalysts experimentally or using computer modelling techniques. This chapter concentrates on the latter but in the context of the relevant experimental data. 

Bezerra et al extensively reviewed heat treatment and stability effects of various Pt/C, Pt-M/C, and C-supported Pt-free alloy catalysts, taking into account particle sizes and stiuctural parameters. Appropriate heat treatment of Pt/C catalysts improves ORR activity by stabilizing the carbon support against corrosion, which in turn increases the cathode life time. Depositing mixed-metal Pt monolayers on carbon-supported metal nanoparticles or Pt monolayers on noble/non-noble core-shell nanoparticles leads to enhanced electrode performance. RRDE experiments on the catalytic activity of Pt-M (M = Au, Pd, Rh, Ir, Re or Os) monolayers on carbon-supported Pd nanoparticles showed that an 80 20 PtiM ratio for the nuxed monolayers performs better than commonly nsed Pt/C catalysts. ... 

In the first case where metal surfaces provide active oxygen species to the support contact structure is not critical. The second case is often observed when supported metal catalysts are prepared by coprecipitation or sol-gel methods. Noble metals whose oxides are more stable than Pt oxides such as Pd and Ir are more readily buried in the bulk of metal oxide supports, and the metal oxide overlayers of a thickness of about a few monolayers are modified in their electronic and redox properties by underlying noble metal nanoparticles to become active at lower temperatures. 

In the early eighties, a new method for the deposition of transition metal ion monolayer on the surfaces of inorganic oxides was developed in our laboratory [3]. The essential step of this method is the selective reaction of the surface hydroxyl groups with transition metal alkoxide molecules [3,4]. The adopted procedure allows the design of catalysts containing a strictly determined number of transition metal monolayers or sandwich-type catalysts with different metals monolayers anchored to the surface of the carrier. 

The results of unsaturated carbonyl compounds hydrogenation in the presence of metal oxide - monolayer oxide - platinum (Pt loading 2 %) catalysts are collected in Table 3 (Pt from H2PtCl6) and Table 4 (Pt from Pt(acac)2). In all the cases, the selectivity of the platinum catalysts deposited on supports modified with transition metal oxide monolayers... 

Platinum catalysts deposited on the supports modified with transition metal oxide monolayers exhibited high activity and satisfactory selectivity in the hydrogenation of unsaturated aldehydes to the corresponding unsaturated alcohols. Platinum acetylacetonate was a more suitable catalyst precursor than hexachloroplatinic acid in the preparation of the transition metal-O-Pt catalytic system for the hydrogenation of cinnamaldehyde and... 

A chemisorbed monolayer of sulfur on a base metal catalyst (20 wt % base metal) with 50 m2 of active metal surface area per gram of active metal ( 150 A crystallite size) would be equivalent to <—0.1 and —0.5 wt % sulfur for monolithic (20 wt % washcoat) and particulate base metal catalysts respectively. For noble metal monolithic catalysts (0.3 wt % noble metal) with 150 m2 of active metal surface area per gram of active metal ( 20 A crystallite size), an adsorbed sulfur mono-layer would be equivalent to 0.03 wt %. These calculations are based on monolayer sulfur chemisorption, i.e. assuming the formation of a surface monosulfide phase. If sulfur were present as a surface oxysulfide or sulfate, the expected catalyst sulfur contents would be significantly lower than the values calculated for monosulfide formation. Thus, the catalyst sulfur contents expected from monolayer sulfur chemisorption (without bulk sulfiding) agree well with the sulfur contents observed in used catalysts. 

The ORR activity shows a volcano-type dependency on the t/-band center of different metal catalysts. The electrocatalyst surfaces should exhibit the optimum balance between the kinetics of 0-0 bonding breaking and the electro-reduction of the oxygenated intermediates or for 0-H formation. The highest activity is observed for Pt monolayer deposited on Pd substrate. 

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