Platinum catalysts dissolution

Corrosion (spontaneous dissolution) of the catalyticaUy active material, and hence a decrease in the quantity present. Experience shows that contrary to widespread belief, marked corrosion occurs even with the platinum metals. For smooth platinum in sulfuric acid solutions at potentials of 0.9 to 1.0 V (RHE), the steady rate of self-dissolution corresponds to a current density of about 10 A/cm. Also, because of enhanced dissolution of ruthenium from the surface layer of platinum-ruthenium catalysts, their exceptional properties are gradually lost, and they are converted to ordinary, less active platinum catalysts. 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

The hydrated ions of europium, ytterbium or samarium can be obtained by electrolytic reduction or by dissolution of MCI2 in water, and are reasonably stable. Colourless or pale yellow-green Eu (aq) is stable for more than ten days in the absence of oxidizing agents or of platinum catalysts. Pale ween Yb (aq) decays in water with /c = 2.4 x 10 s, and red Sm (aq) has = 0.6 X both reactions being first order. These rates of decom-... 

The volumetric method has very often been used with platinum catalysts for which quite satisfactory results are generally obtained it is usual to assume that the monolayer volume or amount, obtained as just described or by extrapolation corresponds to an H Ms (hydrogen atom to metal surface atom) ratio of 1 1. Some justification for this assumption is to be found, at least for particles of moderate size, in the adsorption stoichiometry shown by films and single crystals, but for very small particles and at high pressures the H/Mj ratio can exceed unity quite substantially this is especially so with rhodium" and iridium (see below). Care is however needed with palladium " " because of the risk of forming the hydride however, monolayer coverage is obtained at pressures below which dissolution starts. The base metals iron, cobalt and nickel have been... 

Moreover, in the specific case of ceria-supported platinum catalyst reduced at 1173 K or above, the HRTEM images have unequivocally shown the occurrence of an additional chemical/nano-structural effect consisting of the formation of an inter-metallic CePts phase. This implies the incorporation of heavily reduced cerium into the Pt lattice. No similar effect could be detected on Rh/Ce02 [353], By contrast, in Pd/Ce02 catalysts, though no inter-metallic phases have been observed, shifts in the XRD diffraction peaks assigned to Pd has been interpreted as due to some dissolution of Ce into the palladium lattice [401,415]. 

The mechanism of catalyst dissolution occurs to dilfering degrees, depending upon the degree to which platinum is alloyed and what other elements are included in the alloy. Work by Piela et al. [33] suggests that PtRu alloys, as employed in direct methanol fuel cell and reformate anodes to reduce sensitivity to CO, are extremely unstable and that operation leads to ruthenium... 

Platinum catalyst Platinum dissolution and deposition on cathode, Pt diffusion in MEA, hydrogen permeation Bi and Fuller, 2008... 

Platinum catalyst Potential static holding conditions and potential step conditions effect on platinum dissolution and carbon corrosion Shao et al., 2008... 

The platinum eleetroehemical dissolution rate in fuel cell operation is a critical durability issue due to loss of catalyst surface area vs. time. The dissolution behavior and solubility of platinum are governed by the chemical state of the platinum surfaee and the equilibrium platinum species in the solution. Temperature, pH, eleetrolyte composition, potential, and particle size are all major factors influencing the solubility and the dissolution rate. Potential cycling of Pt electrodes has been extensively investigated, and the platinum electrode dissolution rates from the literature are summarized in Table 23.5 [9, 33, 70-74], Both Pt(II) and Pt(IV) speeies were detected in sulfuric solution after potential cycling from... 

Several processes are available for the recovery of platinum and palladium from spent automotive or petroleum industry catalysts. These include the following. (/) Selective dissolution of the PGM from the ceramic support in aqua regia. Soluble chloro complexes of Pt, Pd, and Rh are formed, and reduction of these gives cmde PGM for further refining. (2) Dissolution of the catalyst support in sulfuric acid, in which platinum is insoluble. This... 

Yasuda K, Taniguchi A, Akita T, loroi T, Siroma Z. 2006a. Characteristics of a platinum black catalyst layer with regard to platinum dissolution phenomena in a membrane electrode assembly. J Electrochem Soc 153 A1599-A1603. 

The loss of the catalytically active surface of Raney nickel due to recrystallization is a continuously progressing process that can be retarded to some extent in Raney-nickel anodes by dispersing oxide ceramic materials like Zr02 and Ti03 in the nickel matrix. More serious is anodic oxidation for some metals additionally accompanied by dissolution of the catalyst to which even platinum is subject but which is an even more serious hazard for the less noble catalysts as silver and Raney nickel. 

Platinum Native, and as speriylite (PtAs2) Dissolution of native Pt in aqua regia, precipitation as (NH4)2PtClCatalyst adsorbent for gases manufacturing jewelry, electrical instruments, electrodes, crucibles, etc. 

Concern about air pollution has led to extensive investigations of catalysts for automobile exhausts. Elements such as lead (a potential poison), palladium and platinum can be determined after dissolution of the catalyst in acids such as aqua regia. Janouskova et al. [55] reported a detection limit of 0.8 ng for platinum in a catalyst using graphite furnace atomisation and 265.9nm. 

D-4642 Platinum in Reforming Catalysts Assay for platinum in a non-zeolitic reforming catalyst by dissolution of the catalyst and spectrophotometric absorbance for platinum... 

Historically, hexachloroplatinic acid in alcohol, so-called Speier s catalyst, was the first homogeneous catalyst and remained the most used for a long time. This platinum (IV) complex has been replaced by other (II) or (0) species, some of which are presented in Scheme 1. Their advantages are a reactivity that can be tuned by the choice of solvent or inhibitor and the absence of hydrochloride formation. Moreover, they are now commercially available as homogeneous solutions, which ensures a perfect dissolution of the complex, and some of them are colorless. 

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