Nickel, compensation behavior

Combustion, 27 189, 190 reaction, sites for, 33 161-166 reaction scheme, 27 190, 196 Commercial isomerization, 6 197 CoMo catalysts, 40 181 See also Cobalt (nickel)-molybdenum-sulfide catalysts Compact-diffuse layer model, 30 224 Compensation behavior, 26 247-315 active surface, 26 253, 254 Arrhenius parameters, see Arrhenius parameters... 

There are other reports that mention compensation behavior on nickel catalysts (186-188), but since A values are not given in appropriate units, these data cannot be compared quantitatively with the above results. 

Arrhenius parameters for nickel carbide hydrogenation 162) is close to both lines on Fig. 3. Compensation behavior for reactions on the carbide phase must include an additional feature in the postulated equilibria, to explain the removal of excess deposited carbon, if the active surface is not to be poisoned completely. The relative reduction in the effective active area of the catalyst accounts for the lower rates of reaction on nickel carbide, and the difference in the compensation line from that of the metal (Fig. 3) is identified as a consequence of the poisoning-regeneration process. After any change in reaction conditions, a period of reestablishment of surface equilibria was required before a new constant reaction rate was attained (22). 

The general pattern of compensation behavior found for platinum-catalyzed reactions was appreciably different from that described above for nickel and palladium. Moreover, since some of the trends were less well defined, these may not represent meaningful obedience to Eq. (2). 

The possible role of nickel formate as an intermediate in the breakdown of formic acid on nickel has been extensively discussed (3, 232, 240b, 244) this is another catalytic reaction in which there is compensation behavior (Table III, R). While the observed obedience to Eq. (2) does not identify the reaction mechanism, it is probably significant that catalytic activity becomes apparent in the temperature range of onset of salt instability. Again it may be envisaged that the temperature dependence of effective concentration of nickel formate intermediate may vary with reaction conditions. 

Deren et al. (271) have reported compensation behavior in the oxidation of carbon monoxide on nickel oxide containing various amounts of chromium oxide the effect is attributed to the modification of the Fermi level at... 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

The following example illustrates one particular quantitative application of compensation behavior for the comparison of levels of activity between different systems. The Arrhenius parameters for the steam reformation reaction over nickel alumina catalysts (290) are log A = 17.25 and E = 29.0. The position of this point on compensation diagrams would appear to be more realistically represented by the compensation relation found for oxidation and exchange processes on nickel oxide (Table V, G) than that for cracking on the metal (Table I, A). One possible mechanistic explanation for this distinction is that the active catalyst is an oxide phase [possibly including NiAl204 (290)1... 

This pattern of behavior resembles those described for nickel and for palladium in recognizing two groups of exchange processes but differs in that the data for the lighter molecules do not coincide with the line for the (more broadly defined) cracking reactions. A compensation trend has also been described for exchange reactions of butanes (2/7) from these measurements we calculate e = 0.1126 and ac = 0.0116. 

The electrical properties of NiO, doped with monovalent ions (Li+) and trivalent ions (Ga +, Cr +) have been studied quite extensively. The study of Cr-doped NiO is of particular interest for understanding the oxidation behavior of Ni—Cr alloys. When trivalent Cr ions are introduced into a NiO crystal, the electrical compensation could occur by formation of doubly ionized nickel vacancies, according to... 

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