Oxidation amorphous alloys

Only about 10 elements, ie, Cr, Ni, Zn, Sn, In, Ag, Cd, Au, Pb, and Rh, are commercially deposited from aqueous solutions, though alloy deposition such as Cu—Zn (brass), Cu—Sn (bronze), Pb—Sn (solder), Au—Co, Sn—Ni, and Ni—Fe (permalloy) raise this number somewhat. In addition, 10—15 other elements are electrodeposited ia small-scale specialty appHcations. Typically, electrodeposited materials are crystalline, but amorphous metal alloys may also be deposited. One such amorphous alloy is Ni—Cr—P. In some cases, chemical compounds can be electrodeposited at the cathode. For example, black chrome and black molybdenum electrodeposits, both metal oxide particles ia a metallic matrix, are used for decorative purposes and as selective solar thermal absorbers (19). 

The passive films formed by the addition of sufficient amounts of valve metals to amorphous nickel-valve-metal alloys are exclusively composed of valve-metal oxyhydroxides or oxides such as TaOjCOH) , Nb02(OH) or TajO,. Consequently, amorphous alloys containing strongly passivating elements, such as chromium, niobium and tantalum, have a very high ability... 

The oxidation behaviour of amorphous alloys studied below their crystallisation temperature is not greatly different from that of crystalline metals, although the presence of large amounts of metalloids complicates the situation . ... 

The possible strategies are coprecipitation to prepare mixed hydroxides or carbonates [5], cosputtering of gold and the metal components of the supports by Ar containing O2 to prepare mixed oxides [23], and amorphous alloying to prepare metallic mixed precursors [24]. These... 

Diffusion coefficients in amorphous solids such as oxide glasses and glasslike amorphous metals can be measured using any of the methods applicable to crystals. In this way it is possible to obtain the diffusion coefficients of, say, alkah and alkaline earth metals in silicate glasses or the diffusion of metal impurities in amorphous alloys. Unlike diffusion in crystals, diffusion coefficients in amorphous solids tend to alter over time, due to relaxation of the amorphous state at the temperature of the diffusion experiment. 

Surface characterization includes also the study of the modification of a surface under cathodic load or after some pretreatments. The presence of residual surface oxides can explain some observations otherwise inexplicable. Activation in situ usually results in composite structures which are difficult to identify by X-ray, and may contain metallic and non-metallic components. Particularly crucial is the case of the surface structure of glassy metals or amorphous alloys. 

A common observation in most cases is that the surface of amorphous alloys, especially those containing Ti, Zr and Mo, is largely covered with inactive oxides which impart low electrocatalytic properties to the material as prepared [562, 569, 575], Activation is achieved by removing these oxides either by prepolarization or, more commonly and most efficiently, by leaching in HF [89, 152, 576]. Removal of the passive layer results in a striking enhancement of the electrocatalytic activity [89], but surface analysis has shown [89, 577] that this is due to the formation of a very porous layer of fine particles on the surface (Fig. 32). A Raney type electrode is thus obtained which explains the high electrocatalytic activity. Therefore, it has been suggested [562, 578] that some amorphous alloys are better as catalyst precursors than as catalysts themselves. However, it has been pointed out that the amorphous state appears to favor the formation of such a porous layer which is not effectively formed if the alloy is in the crystalline state [575]. 

A series of Pt-Si amorphous alloys has been compared with polycrystalline Pt. The activity has been found to be always less than the crystalline metal. Although no grains are present on the amorphous alloy surface, the presence of Si reduces the M-H adsorption strength which is the reason for the lower activity. Oxidation of the surface produces Si02. On oxidized surfaces spillover of hydrogen from Pt to Si02 has been observed [565]. 

Figure 5. Schematic arrangement of the surface of a partly crystallized E-L TM amorphous alloy such as Pd-Zr. A matrix of zirconia consisting of the two polymorphs holds particles of the L transition metal (Pd) which are structured in a skin of solid solution with oxygen (white) and a nucleus of pure metal (black). The arrows indicate transport pathways for activated oxygen either through bulk diffusion or via the top surface. An intimate contact with a large metal-to-oxide interface volume with ill-defined defective crystal structures (shaded area) is essential for the good catalytic performance. The figure is compiled from the experimental data in the literature [26, 27].

A material exhibiting high electrocatalytic activity in the HER was prepared by anodic oxidation of the amorphous alloy FegoCojoSiioBio in 30% aqueous KOH at 70°C (202). A dissolution-precipitation process involving the Fe is involved, giving an active surface oxide that is presumably reduced on subsequent evolution of Hj. A highly porous material results. 

Amorphous Ni-(40-x) at% Zr-x at% RE (x = 0, 1, 5 and 10 RE = Y, Ce and Sm) alloy ribbons of about 1 mm width and about 20 pm thickness were prepared by a single-roUer melt spinning method. The structure of the alloys prepared was confirmed by X-ray diffraction with Cu K radiation. The amorphous alloy ribbons were oxidized at 773 K in air for 5 hours and then reduced at 573 K imder flowing hydrogen for 5 hours. During this treatment the amorphous aUoys transformed to nickel catalysts supported on zirconia or zirconia-rare earth element oxides. 

Amorphous alloy ribbons of Ni-(40-x)Zr-xSm (x=0, 1, 3, 5, 7 and 10 at.% nominal composition) and Ni-(25, 35, 45, 55 and 65 at.%)Zr-5Sm were prepared by a melt-spinning method. The amorphous structure of these ribbons was confirmed by X-ray diffraction using Cu K radiation. Prior to catalytic reaction, the alloy specimens were oxidized in air at 773 K for at least 5 hours and subsequently reduced in flowing hydrogen at 573 K for at least 5 hours. 

CFCs were decomposed to HCl, HF, and CO2 at 150 °C to 350 °C by the reaction of H2O over amorphous alloy catalysts consisting of at least one element selected from the group of Ni and Co, at least one element selected from the group Nb, Ta, Ti, and Zr, and at least one element selected from the group Ru, Rh, Pd, Ir, and Pt. The alloys were activated by immersion in HF [105]. CFCs are decomposed by the reaction of water vapor at temperatures above 300 °C in the presence of iron oxide supported on activated carbon [106]. They are also decomposed by steam in... 

Fig. 4.6. Crystallization behavior of amorphous and partially oxidized Nie4Zr3e alloy investigated by DSC measurements. Curve A was measured for as-quenched amorphous alloy, a denotes the degree of oxidation of the sample. Heating rate 5 K/min...

High-temperature treatment of silicon- or titanium-containing alloys leads to the formation of crystalline titania or silica surface layers with thicknesses in the micrometer range, as demonstrated by Seo et al., who used these materials for the construction of heat exchangers with catalyti-cally active surfaces [166]. However, the thermal treatment of nickel alloys that also contain iron leads to a layer of amorphous iron oxide, which is not suitable for depositing catalytic materials [167]. 

It is usual to compare the infrared spectra of the electrochemically treated amorphous alloys to those of the chemically or thermally (at 380°C) prepared spinels to ascertain the nature and composition of the compound. With this methodology, it can be demonstrated that the hydrous oxide coating acts as a precursor of a spinel structure either from the electrolessly deposited amorphous alloys or after the square-wave potential treatment. 

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