Oxidation of Pt

Oxidation of [Pt(C6Cl5)4]2- yields the unusual paramagnetic organo-metallic [Pt(C6Cl5)]4 with square planar coordination of platinum... 

The abilities of this approach will then be illustrated with two examples (i) the potential-induced lifting of the Au(lOO) surface reconstruction and (ii) the electrochemical oxidation of Pt(l 11). 

You H, Zurawski DJ, Nagy Z, Yonco RM. 1994. In-situ X-ray reflectiyity study of incipient oxidation of Pt(lll) surface in electrol)Te-solutions. J Chem Phys 100 4699-4702. 

The oxidation of Pt(II) complexes is thought to proceed in most cases via addition of an electrophile to the Pt(II) center (141,178-182). This process does not involve outer-sphere electron transfer. The initial product of the electrophile addition is possibly a five-coordinate Pt(IV) species, however, the observable Pt(IV) product is six-coordinate. Coordination of a sixth ligand, e.g. solvent, occurs (183). This may proceed concertedly with the addition of the electrophile, in particular if the sixth ligand is solvent, or alternatively after the addition of the electrophile, as shown in Scheme 35. 

Some evidence to suggest that peroxo complexes can be intermediates in the oxidation of Pt(II) by 02 has been presented. As shown in Scheme 41, a Pt(IV) peroxo complex was obtained by reacting cis-PtCl2(DMSO)2 and 1,4,7-triazacyclononane (tacn) in ethanol in the presence of air (200). An alkylperoxoplatinum(IV) complex is obtained in the reaction of (phen)PtMe2 (phen = 1,10-phenanthroline) with dioxygen and isopropyl-iodide. Under conditions that favor radical formation (light or radical initiators), an isopropylperoxoplatinum(IV) compound was obtained (201,202), depicted in Scheme 42. 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

A pronounced structural sensitivity of the oxidation of Pt surfaces is also seen in Fig. 1. The reaction takes place at the most positive potential on Pt(lll). This is probably due to effective blocking of the surface by oxy-anions with the trigonal symmetry, compatible with the (111) orientation. A detailed analysis of this reaction on Au(lll) has been recently performed by Angerstein-Kozlowska et al. (14). No such blocking is possible for the Pt(100) and Pt(110) surfaces with four-fold and two-fold symmetries. Consequently, the oxidation commences at more negative potentials, probably predominantly determined by the surface energy as found with Au (16). 

The complex anion [Pt(S-Me2SO)Cl3] undergoes an internal redox reaction in acidic media, and evidence for the formation of Pt(IV) species and Me2S has been presented (466). This may be an explanation for the deoxygenation of (CH2)4SO previously mentioned (164). The oxidation of Pt(II) to Pt(IV) with concomitant reduction of Me2SO to Me2S has been accomplished using hydrochloric acid (357), as shown in Eq. (28). 

One of the most important redox reactions of this class of compounds is the oxidation of Pt(2.0+)2 by 02, for this is the main cause of the appearance of the blue, purple, or dark red colors of the mixed-valence species. Although no detailed examination has been performed, the kinetics of the 02 oxidation of I Pt,1 (NHo)4(a-pyrrolido-nato)2]2+ into I Pt1 VNH3)4(a-pyrrolidonato)2(H20)2 I41 (Eq. (7)) was spec-trophotometrically examined (117). The study showed that the reaction proceeds over several days at room temperature. The first-order rate constants in acidic media were in the range of kobs = 4.2 x KT6 s 1 (pH = 0.23) - 1.13 x lO6 s"1 (pH = 2.1) (at 25°C, in air, I = 1.5 M) (117). 

The 02-oxidation of hydroquinone into quinone, which is very slow in the absence of a catalyst, was found to be accelerated by the addition of the ce-pyrrolinonate-bridged Pt(2.5 + )4 (19) (117). The detailed kinetic investigation revealed that the Pt(2.0+)2 species formed according to Eq. (1) plays a major role as the catalyst. The reaction rate of the quinone formation is higher than that of 02 oxidation of Pt(2.0+)2 into Pt(3.0 + )2 and was found to be rather linear to the hydroquinone concentration. Therefore, it was suggested that the quinone formation proceeds via a certain intermediate formed between the Pt(2.0+)2 species and molecular oxygen (e.g., peroxo species). The possible schematic mechanism is illustrated in Eq. (12). 

Nevertheless, for a condition in which methanol oxidation to COad is faster, such as at a higher temperature or in a higher concentration of methanol, higher methanol oxidation activity would be expected because of the higher catalytic activity on COad oxidation of Pt-Ru-Sn. 

Reduction of Pt(IV) by e and oxidation of Pt(II) by OH produces reactive Pt(III) species which can be detected by conductivity, polarographic and uv/visible spectral means. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Above this value a further oxidation of the surface by OH adsorption occurs, which becomes considerable above -0.2 V. A simplified reaction route of the step-by-step surface oxidation of Pt by OR is (77) ... 

When the air flow was temporarily substituted by a nitrogen flow for 15-20 minutes in the reaction represented by Figure 5a, the rate of alcohol oxidation did not increase. These experiments also prove that the reason of catalyst deactivation is not the over-oxidation of Pt° active sites, but a partial coverage of active sites by impurities (chemical deactivation). 

The oxygen cathode—for which platinum catalyst due to its outstanding structural and catalytic capability is the rule—is not used as an oxygen evolution anode in the electrolyzer operation mode because oxidation of Pt and fast catalyst deterioration would be the consequence. Therefore an oxygen cathode based on a platinum catalyst must operate as a -evolving cathode in the regenerative mode. 

The reaction in Eq. 13.5 can be thought of as an electrophilic attack by HgtUiotvlhe platinum-carbon bond. The oxidative addition reaction shows oxidation of Pt(II) to Pt(lV) with simultaneous expansion of the coordination number of Pt from A to 6. 

The analogous reactions of Pd(OEP) or Pd(TTP) did not result in formation of Pd(IV) porphyrins, but in excessive chlorination of the meso [295] or peripheral (p-pyrrole) positions [296] of the porphyrin rings. The oxidation of Pt(P) in the absence of hydrogen chloride did not lead to isolatable products. Of course, an oxoplatinum(IV) porphyrin would have been a very interesting compound. 

Figure 1. Cottrell plots obtained from oxidation of (+ ) Pt/Nafion-GOD-DMAFc, ( O ) Pt/Nafion-GOD-ClFc+ and ( ) Pt/Nafion-GOD-C6Fc+ in 0.1M phosphate buffer. DMAFc, CiFc+ and C6Fc+ were incorporated into the Nafion-GOD films from a 0.5 mM solution.

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