Noble metal oxides

The thermally prepared oxides of the so-called rarer platinum metals are among the best electrocatalysts known for the oxygen gas evolution reaction from aqueous systems. Of these oxides, Ru02 exhibits the highest catalytic activity (at least in relatively short term tests) and has been investigated in most detail. Much of the published work on Ru02 has been stimulated by the success of Ru02-based anodes in chlor-alkali cells. 

Substantial amounts of chloride are present in the fixed ruthenium oxide-based films, e.g. about 4% for a preparation temperature of 400°C [194, 195]. The chlorine content has been observed to decrease slightly on going to the external surface as indicated by, for example, secondary ion mass spectrometry [196]. The exact location of chlorine in the bulk lattice is somewhat unclear at present. Oxygen content has been found to increase sharply over the last few monolayers at the external surface, as shown by SIMS [196] and XPS measurements for powders [197] and films [198], There is now evidence from several groups that suggests the existence of some Ru03 in the surface regions of ruthenium dioxide electrodes [196-200]. 

The basic rutile structure of Ru02 is exhibited by the thermally prepared films of Ru02. The films are microcrystalline in nature, and X-ray diffraction 

An attempt was made by Doblhofer et al. [210] to separate surface from bulk charging processes for thermally prepared Ru02 using the potential step technique. These authors [210] concluded that some bulk diffusion was involved, presumably involving protons, and estimated a diffusion coefficient of 10 19 cm2 s1. Weston and Steele [213] deduced a diffusion coefficient value for protons in porous powder electrodes of Ru02 which is approximately similar to the value of Doblhofer et al. [210]. Iwakura and co-workers [214], on the other hand, employed cyclic voltammetry in deduc- 

Oxygen evolution on Ru02 and Ru02/Ti02 mixed oxide electrodes is characterized by a relatively low Tafel slope, 40 mV decade 1, at low current densities. For plots of log j vs. E, an increase in slope is observed in these plots at high current densities. The deviation from Tafel linearity in this current density region has been related to, for example, uncorrected iR drops [227], i.e. within the film or between the oxide and Ti substrate. 

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A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Platinised tantalum Platinised niobium Platinised titanium Platinum Thermally deposited noble metal oxide on titanium High- silicon/ chromium iron... 

Usually noble metal NPs highly dispersed on metal oxide supports are prepared by impregnation method. Metal oxide supports are suspended in the aqueous solution of nitrates or chlorides of the corresponding noble metals. After immersion for several hours to one day, water solvent is evaporated and dried overnight to obtain precursor (nitrates or chlorides) crystals fixed on the metal oxide support surfaces. Subsequently, the dried precursors are calcined in air to transform into noble metal oxides on the support surfaces. Finally, noble metal oxides are reduced in a stream containing hydrogen. This method is simple and reproducible in preparing supported noble metal catalysts. 

If the initiation step, the activation of H2, is fast, as may be the case on noble metal oxides or highly defective oxide surfaces, the shrinking core or contracting sphere model applies (see Figure 2.3). The essence of this model is that nuclei of reduced metal atoms form rapidly over the entire surface of the particle and grow into a shell of reduced metal. Further reduction is limited by the transport of lattice oxygen out of the particle. The extent of reduction increases rapidly initially, but slows down as the metal shell grows. 

Instead of electrostatic (or physical) adsorption, metal uptake onto oxides might be considered chemical in nature. In chemical mechanisms, the metal precursor is envisioned to react with the oxide surface, involving as surface-ligand exchange [13,14] in which OH groups from the surface replace ligands in the adsorbing metal complex. In this section it will be shown that a relatively simple electrostatic interpretation of the adsorption of a number of catalyst precursors is the most reasonable one for a number of noble metal/oxide systems. 

The main classes of materials employed as catalysts are metals (generally transition and noble metals), oxides (including transition-metal oxides), transition-metal sulfides and zeolites. In the following sections, we discuss some of the more common structures and chemistry exhibited by catalytic systems. 

Sensor Cell Operating Mode. The simplest method of sensor operation is as a galvanic cell, whereby the sensor acts as a fuel cell and generates a current proportional to the gas concentration to be detected (1 ). However, when detecting certain species in air, it is difficult to obtain a counter-reference electrode in an acid system that will maintain the sensing electrode at a predetermined potential of approximately 1.0 V, to minimize interference. Counter-reference electrodes such as Pt/air (Op) or noble metal/ noble metal oxide structures have rest potentials in the 1.0 to... 

Hyponitrous acid (pKA1 = 7,05, pKA2 = 11.0) and its salts are obtained by (1) reduction of sodium nitrite widi (a) sodium amalgam, (b by electrolysis, (c) by stannous or ferrous salts (2) by reduction of alkyl nitrates (3) by reduction of hydroxylamine by noble metal oxides and (4) by reduction of sodium hydroxylamine monosnlfonale in alkaline solution. 

Permanent retention of the scavenger elements Cl and Br, as such, on noble metal oxidation catalysts is usually insignificant because of their volatility. In spite of this fact, it has been demonstrated [(66) and references therein] that scavengers by themselves can suppress the oxidation activity of Pt and Pd. 

In contrast to lead, the possible poisoning by metallic elements, derived from the vehicle system, is not easily documented. Many formulations of automotive catalysts contain both base and noble metals, but the detailed effect of such combinations on the particular reactions is rarely known with precision. One study was concerned with the effect of Cu on noble metal oxidation catalysts, since these, placed downstream from Monel NOx catalysts, could accumulate up to 0.15% Cu (100). The introduction of this amount of Cu into a practical catalyst containing 0.35% Pt and Pd in an equiatomic ratio has, after calcination in air, depressed the CO oxidation activity, but enhanced the ethylene oxidation. Formation of a mixed Pt-Cu-oxide phase is thought to underlie this behavior. This particular instance shows an example, when an element introduced into a given catalyst serves as a poison for one reaction, and as a promoter for... 

Shelef, M., Dalla Betta, R. A., Larson, J. A., Otto, K., and Yao, H. C., Poisoning of Monolithic Noble Metal Oxidation Catalysts in Automotive Exhaust Environment, Am. Inst. Chem. Eng., New Orleans Meet. (1973). 

The data analysis indicates that the catalyst reduced at 200 °C still contains Rh-0 contributions characteristic of Rh203, attributed to unreduced particles. The rate-determining step in the reduction of noble metal oxides is the nucleation... 

No systematic study of inert electrode materials has taken place to date and nothing is known about the anodic processes taking place in ionic liquids. It is probable that noble metal oxide coatings should be suitable but processes such as chlorine evolution will clearly have to be avoided for eutectic-based ionic liquids. The breakdown products of most cations are unknown but it is conceivable that some of them could be potentially hazardous. 

Table II. Structural Data for Some Strontium—Noble Metal Oxides...

Noble metal oxides, however, are expensive and usually have poor electrical conductivity. 

Silver Oxide, and Other Noble Metal Oxides.184... 

An increasing amount of attention is being given to oxides as possible anodes for oxygen evolution because of the importance of this reaction in water electrolysis. In this connection, numerous studies have been carried out on noble metal oxides, spinel and perovskite type oxides, and other oxides such as lead and manganese dioxide. Kinetic parameters for the oxygen evolution reaction at a variety of single oxides and mixed oxides are shown in Table 3. 

The kinetics of the OER on RUO2 were examined extensively (see Refs. 261-263 for reviews of the earlier literature) along with other noble metal oxides prepared by thermal decomposition of metal chlorides (see Section XVIII,B). OER studies on Ru and RUO2 in H2SO4 solutions using O-labeling and diflerential electrochemical mass spectrometry (264) indicate that the surface oxide layer participates in the OER on Ru and RUO2 electrodes, with formation of RuO " at Ru electrodes. 

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