Platinum substrates, preparation

Voorhees and Adams141 obtained an active platinum black from the platinum oxide prepared by fusing a mixture of chloroplatinic acid and sodium nitrate at 500-550°C. The platinum oxide is readily reduced to an active black with hydrogen in a solvent in the presence or absence of substrate. The platinum oxide-platinum black thus prepared has been shown to be very active in the hydrogenation of various organic compounds and is now widely used as Adams platinum oxide catalyst. Frampton et al. obtained a platinum oxide catalyst of reproducible activity by adding a dry powder of a mixture of 1 g of chloroplatinic acid and 9 g of sodium nitrate in its entirety to 100... 

Synthesis of Polymers. Polyamic acid solutions were prepared by condensation of the aromatic anhydride and amine in N,N-dimethylacetamide (DMAc). Polyimide modified electrodes were made by casting or spin coating the precursor polyamic acid solution onto stainless steel or platinum substrates. Imidization was achieved by either heating the films to 400°C for 60 min or through a chemical dehydration process involving immersion in a 1 1 mixture of acetic anhydride and pyridine (6). BTDA-DAPI films were made by casting from a DMAc solution and heating to 100 C. 

In addition, the width and number of electrodes was optimized with regard to the preparation accuracy. The grain size of the platinum paste, the sinter process, and the substrate preparation has limited accuracy, resulting in variations in width and distance between the electrodes. These variations should have the smallest possible effect on the distribution of the IDC s capacity. Calculations showed that assuming a constant electrode distance of s = 150 jam, the average error will be sufficiently small, if the electrode width is >100 jam. Based on these calculations, b was taken as 125 jam. 

Recently it has been demonstrated that relatively high enantioselectivities can be obtained upon using platinum nanocolloids prepared in different ways [11-13]. These experiments clearly showed that high enantioselectivities can be obtained even on small platinum clusters (average particle size around 1.6 nm), where the accommodation of the 1 1 modifier-substrate surface complex (see Figure 1) is strongly hindered. Thus, these results created certain contraversion in the validity of the present models. 

All of the above modes of operation serve to increase sensitivity, and to some degree selectivity. They do, however, have some disadvantages when compared to conventional electrodes. With CMEs, the electrode preparation stage is obviously more detailed and time-consuming. Preparation of a conventional substrate is usually the first, and most important, step. For example, we have found that the analytical performance of polypyrrole coated on platinum substrate depends critically on the substrate preparation. Furthermore, the method of attaching the modifier, whether by chemisorption, covalent bonding or electropolymerisation, must also be carefully controlled if reproducible electrode surfaces are to be obtained. There is no doubt, however, that these time consuming procedures can be tolerated if the elect roan alyti cal chemist is provided with a surface which displays improved performance, reproducibility and stability over an extended time period. 

Generally, the preparation of these species is achieved by (i) reaction of a platinum alkynyl complex with a platinum substrate containing labile ligands and (ii) chloride-alkynide exchange reactions. 

To test this idea, two platinum substrate electrodes were prepared under identical conditions, and both were cathodically modified in the same DMF solution containing 0.1 mol.L- Nal. Then, one sample was immediately electrochemically oxidized without removing the sample from the solution that is, a positive potential was applied during 10 s (more positive than the peak potential corresponding to the anodic process), typically larger than -0.5 V/ SCE. The second sample was left only in contact with air for 1 h. Figure 2.14 compares the SEM images of the two produced platinum surfaces obtained... 

An interesting approach was demonstrated by Han et al. Microscopic spots of GOx were fabricated on platinum substrate electrode with a self-made concave platinum micrometer electrode. First, the concave platinum micrometer electrodes were prepared by means of phenol-allylphenol copolymer insulation and electrochemical etching. Using concave platinum micrometer electrode as reference and counter electrodes and platinum substrate as a working electrode, GOx on the concave platinum electrode was then electrochemically deposited and fixed on a platinum substrate to form microscopic spots with biological activity. The authors claim that the method is based on the local reduction of pH on the electrode surface. The diameter of the microscopic spots fabricated by this method was about 20 pm, and the microscopic spots retained the biological activity. 

Adams platinum, obtained by reduction of platinum dioxide [203, 204], has been used in a number of papers [203, 204, 210] on electrochemical oxidation. In [203] a platinum catalyst, prepared by dejmsition and subsequent reduction of oxidized forms of platinum on a carbon substrate, was used. Adams platinum is an active form of the catalyst, which is stable towards heat treatment and the action of contaminants [208]. 

At this point, the picture which evolves from all our preparative, kinetic, and mechanistic work with the carbenoid fragments [(dtbpm)Pt(O)] and [(dcpm)Pt(O)] and with different organosilanes suggests that the platinum center of these extremely reactive and electronically most unusual (vide supra) intermediates interacts simultaneously with all three atoms (i.e. with both bonds or bonding pairs) of H-C-Si substructures of organosilane substrates near or at the transition state (Scheme 4 [24]). 

The cA-PtCl2(diphosphine)/SnCl2 constitutes the system mostly used in catalyzed hydroformylation of alkenes and many diphosphines have been tested. In the 1980s, Stille and co-workers reported on the preparation of platinum complexes with chiral diphosphines related to BPPM (82) and (83) and their activity in asymmetric hydroformylation of a variety of prochiral alkenes.312-314 Although the branched/normal ratios were low (0.5), ees in the range 70-80% were achieved in the hydroformylation of styrene and related substrates. When the hydroformylation of styrene, 2-ethenyl-6-methoxynaphthalene, and vinyl acetate with [(-)-BPPM]PtCl2-SnCl2 were carried out in the presence of triethyl orthoformate, enantiomerically pure acetals were obtained. 

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