Chemical catalyst preparation

Catalysis and Surface Science Developments in Chemicals from Methanol, Hydrotreating of Hydrocarbons, Catalyst Preparation, Monomers and Polymers, Photocatalysis and Photovoltaics, edited by Heinz Heinemann and Gabor A. Somorjai... 

Promoters are usually added to a catalyst during catalyst preparation (classical or chemical promotion). Thus if they get somehow lost (evaporation) or deactivated during prolonged catalyst operation, this leads to significant catalyst deterioration. Their concentration cannot be controlled in situ, i.e. during catalyst operation. As we will see in this book one of the most important advantages of electrochemical promotion is that it permits direct in situ control of the amount of the promoter on the catalyst surface. 

The preparation of catalysts is a mixture of art and science, but most of all much experience. Although the underlying chemistry is largely known, many catalyst preparation recipes are so complicated that it is not possible to write a complete scheme of chemical reactions in detail. 

In summary, large (>lpm) single crystal platelets of aurichalcite produced highly dispersed Cu and ZnO particles with dimensions on the order of 5 nm, as a result of standard catalyst preparation procedures used in the treatment of the precipitate precursors. The overall platelet dimensions were maintained throughout the preparation treatments, but the platelets became porous and polycrystalline to accommodate the changing chemical structure and density of the Cu and Zn components. The morphology of ZnO and Cu in the reduced catalysts appear to be completely determined by the crystallography of aurichalcite. 

For comparison, a 20wt% Pt/XC72 catalysts prepared commercially by E-TEK had an average diameter of 2.6 nm. The sputter deposited Pt had a standard deviation between 0.42 and 0.49 nm, whereas the commercial E-TEK catalyst has a standard deviation of 0.79 nm. Thus, the sputtering technique creates smaller and more uniformly dispersed Pt particles than those prepared chemically. It should be noted that the Pt/C samples having low loadings prepared via sputtering did not uniformly coat... 

Figure 9.7 Temperature-programmed reaction (TPR) spectra for CO oxidation at a series of model catalysts prepared by the soft landing of mass-selected Aun and AunSr cluster ions on MgO(lOO) thin films which are vacancy free (typically 1 % of a monolayer), (a) MgO (b) Au3Sr (c) Au4 (d) Au8. Also shown is the chemical reactivity R of pure Aun and AunSr clusters with 1 < n < 9. (Reproduced from Ref. 21).

Additional details on the catalyst preparations can also be found in the patents (2,3). Chemicals used in the catalyst preparations were obtained from the following sources ... 

Catalysts used in the trickle bed reactor were supplied by Engelhard Chemical Catalyst Group prepared industrially to match in-house prepared catalysts used in our initial screening. 

V-Mo-Zeolite catalysts prepared by solid-state ion exchange were studied in the selective catalytic reduction of NOx by ammonia. The catalysts were characterized by chemical analysis, X-ray powder diffraction, N2 adsorption (BET), DRIFT, UV-Vis and Raman, spectroscopy and H2 TPR. Catalytic results show that upon addition of Mo to V-ZSM-5, catalytic performance was enhanced compared to V-ZSM-5. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

The [S]cr value depends on the nature of the catalyst, its surface area per unit of weight, the ratio of the rates of hydroperoxide decomposition into free radicals and molecular products, hydrocarbon and dioxygen concentrations, the method of catalyst preparation, and the chemical treatment of the surface. 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Perhaps the one major drawback with DIPAMP is the long synthetic sequence required for its preparation, though shorter and cheaper methods are now available [12]. The ligand continues to be a player for the synthesis of amino acid derivatives at scale, including L-Dopa, as mentioned above [12, 25, 27-29]. Its continued use is a testament to the power of the initial discoveries, as well as showing that a chemical catalyst can achieve selectivities only previously seen with enzymes. 

Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is also used to determine the composition of catalysts based on weight changes and thermal behavior during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms of catalysts, and to investigate the acid-base properties of solid catalysts using probe molecules. However, these techniques lack chemical specificity, and require corroboration by other characterization methods. 

Similarly, heterogeneous catalyst prepared by immobilization of POMs on chemically modified hydrophobic Si02 has been applied to the selective epoxidation of various alkenes with 15% aqueous H202 without organic solvents [168],... 

Section I reviews the new concepts and applications of nanotechnology for catalysis. Chapter 1 provides an overview on how nanotechnology impacts catalyst preparation with more control of active sites, phases, and environment of actives sites. The values of catalysis in advancing development of nanotechnology where catalysts are used to facilitate the production of carbon nanotubes, and catalytic reactions to provide the driving force for motions in nano-machines are also reviewed. Chapter 2 investigates the role of oxide support materials in modifying the electronic stmcture at the surface of a metal, and discusses how metal surface structure and properties influence the reactivity at molecular level. Chapter 3 describes a nanomotor driven by catalysis of chemical reactions. 

The use of CLEA preparations of commercially available HNLs allowed for the enantiocomplementary production of cyanohydrins from a pyridinecarboxaldehyde at a much higher chiral purity than had previously been demonstrated with any chemical catalyst. The key to the success of this process was the use of the CLEA -immobilized biocatalysts that allowed reaction conditions to be chosen to minimize the negative effects of the nonspecific background reaction. 

From the data presented in Table 4 it may be concluded that the porous nature of the chemically modified silica remains more or less the same after immobilization of the cobalt(lll) complexes. In addition, there is a decrease in the surface area of the support following the incorporation of the metal complexes. The AAS data on Co(III)-CMS2 and Co(lll)-CMS3 appear to suggest that the extent of cobalt loading is dependent upon the initial amount of cobalt used. The cobalt loadings obtained for the catalysts prepared by H2O2 oxidation of CMS suspensions in 1 and 2 mmol cobalt(ll) solutions (in presence of 1 and 2... 

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