Surface modifiers catalysts

Considerable progress has been made in the development of supported Pt-based catalysts for formic acid oxidation, with a variety of Pt alloy, intermetallic, and surface-modified catalysts showing impressive increases in performance relative to Pt/C. The use of Au, Bi, Pb, or Sb as the second metal has been shown to be particularly beneficial, although it is not clear yet whether any of these metals combined with Pt will provide sufficient long-term durability, nor which type of modification of the Pt stmcture (alloy, intermetallic, or surface modified) is most suitable. 

As expected, the carbon heterogenous surface modification is indeed beneficial to the improvement in HDS performance of the Ti02-supported catalyst. With the surface-modified catalyst, MoNi/C/Ti02 exhibits an outstanding HDS performance, and the DBT conversion can reach up to 98%. By contrast, the conversion of MoNi/Ti02 is only about 65%. Moreover, the DBT conversion of MoNi/C/Ti02 remains constant throughout... 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

Figure 27. Probable pathways for propylene epoxidation over surface modified Ti-Si02 catalysts [88].

Figure 55.4 compares the Raman spectra of the two samples spectra were recorded at 380°C in a 15% O2/N2 stream, on equilibrated catalysts downloaded after reaction. Catalyst VN 1.06 was not oxidized in the air stream, whereas in the case of catalyst PA 1.00 bands typical of a phosphate, ai-VOP04, appeared in the spectrum. These bands were not present in the spectmm of the equilibrated catalyst recorded at room temperature. Indeed, the spectra of the two equilibrated catalysts were quite similar when recorded at room temperature. This result confirms that the surface of catalyst VN 1.06 is less oxidizable than that of catalyst PA 1.00. Therefore, the latter is likely more oxidized than the former one under reaction conditions. A treatment in a more oxidant atmosphere than the reactive n-butane/air feed modifies the surface of catalyst VN 1.06, and leads to the unsteady behavior shown in Figure 55.1. The same treatment did not alter the surface of the equihbrated catalyst P/V 1.00 that was already in an oxidized state under reaction conditions.

Baruwati, B., Gum, D. and Manorama, S.V. (2007) Pd on surface-modified NiFe204 nanopartides a magnetically recoverable catalyst for Suzuki and Heck reactions. Organic Letters, 9 (26), 5377—5380. 

In general, heterogeneous catalysts do not show the selectivity shown by chiral catalysts, although current research on surface modifiers has shown that even enantioselective reactions, albeit for a restricted range of substrates is becoming possible [3, 4]... 

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

The most conventional investigations on the adsorption of both modifier and substrate looked for the effect of pH on the amount of adsorbed tartrate and MAA [200], The combined use of different techniques such as IR, UV, x-ray photoelectron spectroscopy (XPS), electron microscopy (EM), and electron diffraction allowed an in-depth study of adsorbed tartrate in the case of Ni catalysts [101], Using these techniques, the general consensus was that under optimized conditions a corrosive modification of the nickel surface occurs and that the tartrate molecule is chemically bonded to Ni via the two carbonyl groups. There were two suggestions as to the exact nature of the modified catalyst Sachtler [195] proposed adsorbed nickel tartrate as chiral active site, whereas Japanese [101] and Russian [201] groups preferred a direct adsorption of the tartrate on modified sites of the Ni surface. 

One of the oldest mechanisms of interaction between adsorbed reactant and adsorbed TA has been proposed by Klabunovskii and Petrov [212], They suggested that the reactant adsorbs stere-oselectively onto the modified catalyst surface. The subsequent surface reaction is itself nonstere-ospecific. Therefore, the optically active product is a result of the initial stereoselective adsorption of the reactant, which in turn, is a consequence of the interactions between reactant, modifier, and catalyst. The entities form an intermediate chelate complex where reactant and modifier are bound to the same surface atom (Scheme 14.4). The orientation of the reactant in such a complex is determined by the most stable configuration of the overall complex intermediate. The mechanism predicts that OY only depends on the relative concentrations of keto and enol forms of the reactant,... 

The selectivity was enhanced by adding small amounts of anthraquinone-2-sulfonate (A2S), which decreased the formation of deoxy by-products. Thus, by adding 260 ppm of A2S with respect to arabinonic acid the selectivity to deoxy-products decreased from 4.2 to 1.6%. A2S acted as a permanent surface modifier since the catalyst was recycled with the same selectivity without further addition of A2S. The highest selectivity to arabitol was 98.9% at 98% conversion, with a reaction rate of 73 mmol h 1 gRU 1 at 80 °C. 

Other successful but limited surface modifying trials have been done such as Pt dispersed onto Ti02, Pt-Co and Mn perovskite , Pt-transition metal oxides , Pt-Au-Fe203 or Ru02- These materials are known as gas-solid interface carbon monoxide catalysts and have been... 

Zheng, S., Jentys, A., and Lercher, J.A. (2006) Xylene isomerization with surface-modified HZSM-5 zeolite catalysts an in situ IR study. ]. Catal, 241 (2), 304-311. 

These differences in the performance of Pt/Si02 and tin-modified catalysts agree with previously published information about how the modification of a platinum surface causes a decrease in the hydrogenation rate of acetophenone [114]. 

The surface nethoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabilities were studied by Temperature-Programmed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed ammonia was... 

Two series of catalysts were synthesized for subsequent evaluation as methane dimerization catalysts. The first series was alkali modified zinc oxide (6) and magnesium oxide catalysts (7), which were reported to be active for methane activation, while the second series was ion modified perovskites described by Machida and Enyo (8). The objective of the present study was to determine whether the aerosol technique could provide a wide range of ion substitutions as homogeneous solid solutions, and to determine whether moderately high surface area catalysts could... 

The fluorous silica concept involves the selective partitioning of a fluorous-modified catalyst between an organic liquid phase and the fluorinated surface phase. In the absence of CO2, the fluorinated catalyst prefers the fluorous surface phase and remains partitioned onto the silica. When CO2 pressure is added, the catalyst will partition off of the silica and into the GXL phase (containing reactants), where homogeneous reaction can take place. After the reaction is completed, the CO2 is removed and the catalyst will partition back onto the fluorous silica surface, which can be easily recovered by filtration. A cartoon schematic is shown as Figure 2. 

When compared with other heterogeneous catalysts, studies of surface conditions of these modified catalysts are quite difficult because the amounts of modifying reagent adsorbed on the catalyst are very small and the catalyst consists mostly of metal. Especially, the physical study of the adsorption mode of the modifying reagent is difficult because it is adsorbed as a mono-layer or close to it. In the next section, the surface conditions of MRNi will be discussed in connection with the adsorbed modifying reagent. 

In the modification with TA and NaBr, NaBr adsorbs on the surface areas where TA does not adsorb and unstably adsorb as already mentioned in Section IV,A, 1, e. The correlation diagram between the fractional saturating ratio of NaBr (/) on the surface of the catalyst and the EDA of the modified catalyst as shown in Fig. 32 can be deduced from the diagram of correlation between the EDA of the modified catalyst, the amounts of NaBr and TA adsorbed, and the concentration of NaBr in the modifying solution as shown in Fig. 11. 

It seems likely that the chromate species can exist on the silica surface and acts as parent for an active site. Thus, pairing of chromium atoms is not a requirement for polymerization. Chromium trioxide (Cr03) probably binds to the silica as chromate initially, at least at the ordinary 1 % loading. But some rearrangement to dischromate at high temperatures may occur. If so, it could account for the change in color from yellow to orange, and even to red in some modified catalysts. 

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