Catalysts models

Model catalysts such as An/Ti02(l 10) have been prepared by metal vapour deposition [42]. Figure A3.10.14... 

The reaction was studied in the absence, and presence, of (MeO)2AlMe as a model catalyst for the BINOL-AlMe system. The change in relative energy for the concerted hetero-Diels-Alder reaction, and formation of the hetero-Diels-Alder adduct 11 via a Mukaiyama aldol reaction, is shown in Fig. 8.13. The conclusion of the study was that in the absence of a catalyst the concerted reaction is the most... 

I.R. Harkness, C. Hardacre, R.M. Lambert, I.V. Yentekakis, and C.G. Vayenas, Ethylene oxidation over Platinum In situ electrochemical promotion using P"-A1203 and studies with a Pt(l 11)/Na model catalyst, J. Catal. 160, 19-26 (1996). 

The common underlying principle was shown in Figure 11.2. The electrochemical potential of electrons jl e(=Ep, the Fermi level) in the metal catalyst is fixed at that of the Fermi level of the support.37 This is valid both for electrochemically promoted model catalysts (left) and for seminconducting or ion-conducting-supported metal nanoparticles (right). 

Contributions of three types of Ga sites to propane conversion into aromatics were examined by using model catalysts, i.e., gallosilicate of MOR structure with deposited GaaOs particles. The rates of propane conversion and aromatics formation were correlated with the densities of three types of Ga sites determined by NH3-TPD, and it was shown that the propane conversion and the aromatics formation were limited by Ga sites on Ga20j surface. 

Figure 4.8. XPS wide-scan spectrum of a Rh/AIjO, model catalyst prepared by impregnating AI2O3 with a solution of RhClj in water. The photoelectron and Auger peaks (left) are given, along with a region of interest from the Rh 3d spectrum of the fresh and the...

Figure 4.15. Atomically resolved TEM images of a Cu/ZnO model catalyst in various gas environments together with the corresponding Wulff construction of the Cu particle (a,b) Cu nanocrystal faceted by (100), (110) and (111) surfaces the TEM image was recorded at 1.5 mbar of H2 at 220 °C with the electron beam parallel to the [Oil] zone-axis of copper. The insert shows EELS data at the Cu L2,3-edge...

Figure S.11. Comparison between the predictions of a micro-kinetic model and measurements on a Cu(lOO) model catalyst with a real methanol synthesis catalyst. The full line represents the ideal match between model and experiment. [Adapted from P.B. Rasmussen, P.M. Holmblad, T. Askgaard,...

Zhu L, Susac D, Teo M, Wong KC, Wong PC, Parsons RR, Bizzotto D, Mitchell KAR, Campbell SA (2008) Investigation of CoSa-based thin films as model catalysts for the oxygen reduction reaction. J Catal 258 235-242... 

The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

Although much progress has been made toward understanding the nature and probable catalytic behavior of active sites on CoMo/alumlna catalysts, much obviously remains to be accomplished. Detailed explanation of the acidic character of the reduced metal sites evidently most important In HDS, and presumably In related reactions, must await the Increased understanding which should come from studies of simplified model catalysts using advanced surface science techniques. Further progress of an Immediately useful nature seems possible from additional Infrared study of the variations produced In the exposed metal sites as a result of variations In preparation, pretreatment, and reaction conditions. 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

Despite the success in modeling catalysts with single crystals and well defined surfaces, there is a clear need to develop models with higher levels of complexity to address the catalytically important issues specifically related to mixed oxide surfaces. The characterization and design of oxide surfaces have not proven to be easy tasks, but recent progress in identification of the key issues in catalytic phenomena on oxide surfaces by in-situ characterization techniques on an atomic and molecular scale brings us to look forward to vintage years in the field. 

As said in the introduction, the CO-reduced system is active in ethylene polymerization and the resulting polymer is generally considered almost the same as that obtained with the industrial catalyst [4], Because of its simphcity, hereafter we will discuss only the polymerization on this model catalyst. 

Fig. 7 Schematic representation of the preparation of a Ziegler-Natta model catalyst...

Activation of the catalyst is usually performed by exposure to a co-catalyst, namely an aluminum alkyl. The model catalysts were successfully activated by trimethylalumimun (TMA) and triethylaluminum (TEA), commonly used for this purpose. The compounds were dosed from the gas phase either at room temperature for a prolonged time or for a much shorter time at a surface temperature of 40 K. Nominal 3400 L of TMA or TEA were exposed at room temperature. The chemical integrity of the co-catalyst was verified by IR spectroscopy of condensed films grown at low temperature on the substrates. The spectra were typical for condensed and matrix isolated species [119]. 

The model catalysts prepared according to the above-mentioned procedures were active for ethylene and propylene polymerization without additional cocatalyst present in the gas phase [21,23]. The TiClx/MgCl2-based catalysts have been proven catalytically active at 300 K and an ethylene pressure above... 

Fig. 12 Infrared spectrum of polyethylene film grown on the model catalyst...

Table 2 Comparison of IR frequencies between the model catalyst and data for literatme as well as the assignment of the frequencies taken from the literature ...

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