Platinum lead modified

Electrocatalytic oxidation of glyoxal on a platinum electrode modified by lead adatoms. 

P. C. C. Smits, B. F. M. Kuster, K. Van der Wiele, and H. S. Van der Baan, The selective oxidation of aldoses and aldonic acids to 2-ketoaldonic acids with lead-modified platinum-on-carbon catalysts, Carbohydr. Res., 153 (1986) 227-235. 

Lead-modified Platinum Catalysts The Lindlar Approach [19]... 

Selective electrocatalytic oxidation of sucrose on smooth and upd-lead modified platinum electrodes in alkaline medium. 

This work aims to investigate the electrocatalytic oxidation of sucrose on smooth and upd-lead modified platinum electrodes in order to find experimental conditions for selective electrosynthesis of high value added products. The preliminary results obtained show that the oxidation of sucrose on Pt-Pb electrodes leads mainly to C12 products, such as the I -monoacid and the 6-monoacid of sucrose. 

Figure 2. Voltammograms of an upd-lead modified platinum electrode, recorded at 50 mV.8, in 0.1 M NaOH, at room temperature, in presence of 0.01 M sucrose ...

However, such behaviour was not observed in a recent work by Laborde et al. [55], where hydrogen oxidation at a PAni film containing 0.1 mg cm of platinum takes place at low overpotentials as is usual for an electroactive material. In another paper [63] Laborde et al. observed the electro-oxidation of dissolved hydrogen at a co-polymer (polypyrrole-polydithiophene) electrode modified with 0.1 mg cm of platinum, leading to a very small overvoltage (Figure 10.2). 

Zhang M, Wilde CP (1995) The influence of organic adsorbates on the UPD process. Oxidation of formic acid at UPD lead-modified platinum electrodes. J Electroanal Chem 390 59-68... 

The direct electrooxidation of aqueous E>-g]uconic acid to l>arabinose on graphite has been performed in a very simple apparatus which may be suitable for practical application. The electrocatalytic oxidation of sucrose on smooth, lead-modified platinum electrodes has been examined with a view to finding experimental conditions for the selective electrosynthesis of value-added compounds. A paper in Bulgarian on the electrooxidation of diacetone-L-sorbose at low current densities in a nickel oxide electrolizer has been publi ed. The influence of the rize of palladium particles and their location on the support on their activity in the oxidation of glucose has been examined. An investigation of the effect of tonperature and pH on the platinum-catalysed oxidation of sucrose showed that changes in temperature affect mainly the reaction rate, where changes in pH alter the selectivity. ... 

This modified arrangement was used for measuring the e.m.f. s (between 0 and 6 mV) produced by two-phase Co + Cu alloys between 1025 and 1550 K, the electrode inside the tube now being, of course, redundant. The purpose of the CoO layers shown in Figure 5 was to prevent alloying between the platinum leads and the cobalt in the electrodes. 

Many authors have shown that the support could play a role, not only in changing particle size but also in modifying adsorption properties of the metals. Ceria could stabilize ionic species of platinum leading to a strong metal-support interaction. Bera et al. have compared the behavior of Pt/Ce02 and Pt/Al203 in TW catalysis." The enhanced activity observed in several reachons (CO-I-O2, CO - - NO and HC -f O2, Table 1.10) has been attributed to the formation of new sites (-0 Ce" +-0 Pt"+-0 with = 2 or 4). Ceria-supported catalysts are more active than alumina ones for all the reactions. NO as an oxidant is more sensitive in nature to support than O2. Moreover, ceria is a better promoter for oxidation of CO and propane than that of methane. Whatever the oxidant (NO or O2), methane oxidation remains difficult with a modest promotion by ceria. 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

The molecular modelling approach, taking into account the pyruvate—cinchona alkaloid interaction and the steric constraints imposed by the adsorption on the platinum surface, leads to a reasonable explanation for the enantio-differentiation of this system. Although the prediction of the complex formed between the methyl pyruvate and the cinchona modifiers have been made for an ideal case (solvent effects and a quantum description of the interaction with the platinum surface atoms were not considered), this approach proved to be very helpful in the search of new modifiers. The search strategy, which included a systematic reduction of the cinchona alkaloid structure to the essential functional parts and validation of the steric constraints imposed to the interaction complex between modifier and methyl pyruvate by means of molecular modelling, indicated that simple chiral aminoalcohols should be promising substitutes for cinchona alkaloid modifiers. Using the Sharpless symmetric dihydroxylation as a key step, a series of enantiomerically pure 2-hydroxy-2-aryl-ethylamines... 

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

While hydrosilylation of 1-alkenes and HSiCl3 with platinum catalysts provides linear products (1-trichlorosilylalkanes), palladium chloride modified with phosphines gives products carrying the trichlorosilyl group at the secondary carbon. This is highly remarkable because all other metal complexes studied so far lead to 1-substituted products. This regioselectivity leads to the possibility to carry out asymmetric hydrosilylation. 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

A catalytic agent can alter the speed of a chemical action, bat it cannot alter the condition of equilibrium.—Although the speed of a chemical reaction is modified by the presence of a catalytic agent, the final state of equilibrium is not affected. If otherwise, J. H. van t Hoff showed that we could allow these substances to react alternately with and without the catalytic agent this would involve a change in the quantity combined, and the energy thus obtained could be made to do work. This would lead to perpetual motion, which is assumed to be impossible. This deduction has been confirmed experimentally with hydrogen iodide with and without platinum black. Hence, adds W. Nernst, the catalyst must always affect... 

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