Alloys compensation effect

Fig. 26. Compensation effect plots for C2H, oxidation over Pd-Rh alloy films (O) and Pd-Ag alloy films (A) pure metals are indicated by solid symbols (73).

The points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

McKee (21, 195) and McKee and Norton (219, 249, 250) have reported compensation effects in the exchange reactions of methane on several pairs of binary noble metal alloy catalysts. For each combination of elements kinetic measurements were made at a number of different compositions. Although the compensation behavior was generally very similar, there were perceptible differences in the values of B and e calculated for the various alloy combinations. The parameters found, by use of the formulas given in Appendix II, are summarized in Table IV, A-E, and are subject to the following comments. In consideration of data for the Pd-Rh alloys, the point for... 

A compensation effect (Table IV, F) has also been found in reported data (251) for cyclopropane-deuterium exchange on Cu-Ni alloys. 

Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255) discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table III, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256). 

Experimental data on these effects are lacking, and important future work might be oriented in this direction. Some relatively fragmentary data on alloys do, however, show the potential importance of these considerations. Some of these are discussed in Ref. 91. There is some evidence that the Arrhenius preexponential term on copper is higher than that on nickel, whereas the reaction rate on nickel is the higher of the two, i.e., a compensation effect occurs. 

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

On the basis of extensive exploratory work,56,57 it proved possible before 1953 to make routine the x-ray emission spectrography of a high-temperature alloy for the determination of five major constituents. The work demonstrated conclusively that this x-ray method offered advantages in respect to convenience, speed, and precision over wet methods for these determinations. It proved that comparison of unknown and standard could satisfactorily compensate the absorption and enhancement effects encountered in these alloys. 

Similar effect of relatively stable cycling on the level of discharge capacity of 500 mA-h/g can be achieved, for example, using tin-based alloys with different metals (please, see the detailed investigation of such alloys in this book [5]). These metals perform probably the functions of elastic matrix in such alloys and gave possibility to compensate for the volumetric changes of Sn. 

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

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