Platinum solution stability

Figure 12.2 shows the case of the application of a square-wave symmetric routine within the platinum oxide stability region at medium frequencies in a strong acid solution. The development of the two types of a-oxides (ai and a2) for different program times can be seen clearly. No hydrous... 

The liquid phase hydrogenation of benzene on carrier-fixed ruthenium colloid catalysts suspended in an aqueous solution of sodium hydroxide proceeds with 59% cyclohexene selectivity at 50% benzene conversion. The catalysts are prepared by adsorbing a hydrophilic stabilized ruthenium metal colloid on lanthanum oxide. Protection of metal colloids with chiral molecules can lead to a new type of enan-tioselective catalyst combining good selectivity control with extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidinel °°l (Fig. 7). 

Boennemann and Braun found a very effective enantioselective colloidal catalyst. A platinum sol stabilized by DHCnd revealed very high enantioselectivity in the hydrogenation of ethyl pyruvate (EtPy) into ( )-(+)-ethyl lactate (i -EtLa) with an ee of 78% at 12°C and atmospheric pressure in a MflS i-homogeneous phase of a solution of AcOH + MeOH (Scheme 3.3.). 

The resistance of rhodium to chemical attack is remarkable, and surpasses that of platinum. Its domain of stability (as seen from Fig. 6.4) is extremely wide, and in the absence of complexing agents it is stable in aqueous solutions of all pH values. In the massive form it is unattacked by caustic alkalis, acids and oxidising agents, including aqua regia. When finely divided, however, it is attacked by concentrated sulphuric acid and aqua regia. 

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. 

Thiols are known to be excellent hgands for the stabilization of gold and platinum nanoparticles. In this respect, we did not observe any Iluxional behavior [31,52] in solution NMR experiments for thiols coordinated to the surface of noble metal particles (Fig. 8). However, in the case of rutheniiun, we foimd the slow catalytic formation of alkyl disulfides [31]. After exclud-... 

The size of the metal particles deposited from SMA is strongly dependent on the stabilization of the metal mi-croclusters present in the starting SMA solutions. Their stability can be conveniently modulated by addition of suitable ligands. Examples related to y-Al203-supported Platinum and Rhodium nanoparticles are reported here. 

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