Nickel carbide-hydrogen reaction

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). 

Fig. 3. Compensation plot for cracking reactions on nickel carbide (see text, Table I, B), line calculated by least squares method (see Appendix II). Points for reactant mixtures containing hydrogen, O line for cracking reactions on nickel metal (from Fig. 2) shown dashed.

Cracking reactions on nickel carbide in the absence of added hydrogen (22), hydrogenation of nickel carbide (162), and the reactions of water and of sulfur dioxide with this solid (163) exhibited a different compensation line (Fig. 3, full line, and Table I, B) from that for cracking reactions on the metal (Fig. 3, dashed line). When data for the reactions of propane on nickel carbide in the presence of some added hydrogen (O on Fig. 3) (22) are included in the calculation, the position of the line is almost unchanged, but the values of a are significantly increased (by the factor x 2, Table I, C). 

Following earlier work in which the intermediate in the formation of methane from carbon monoxide and hydrogen was found to be carbon, McCarty and Wise carried out a thorough study of the system. Four types of carbon were found to be formed from carbon monoxide on nickel at 550 50K. Chemisorbed carbon atoms reacted readily with hydrogen as did the initial layers of nickel carbide. Further deposits of the carbide, amorphous carbon, and crystalline elemental carbon were much less reactive and the kinetics of the reaction should be described by the established rate laws. Conversion of the more active to the less active forms of carbon occurred above approximately 600 K. 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

Which is the best catalyst for accelerating the reaction depends on the nature of the working materials. For the reaction of hydrogen or oxygen in potassium hydroxide solution, nickel or silver is suitable for carbonaceous fuels as well as for the reaction of oxygen in acid electrolytes platinum metals were up to the middle 60s, the only known catalysts. Precious metals are ruled out by price for wide application in fuel cells, and the search for cheaper catalysts has been actively pursued in many research laboratories. Many classes of inorganic substances (carbides, nitrides, oxides, sulfides, phosphides, etc.) have been investigated and, in particular, several chelates. 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

The carbonyl functionality of MIBK can be hydrogenated over nickel catalysts to yield methyl isobutyl carbinol (4-methyl-2-pentanol or methyl amyl alcohol) [108-11-2]. Industrial processes coproduce methyl isobutyl carbinol during the hydrogenation of mesityl oxide to MIBK. The product ratio of methyl isobutyl carbinol—MIBK during this reaction can be shifted toward methyl isobutyl carbinol by adopting a higher than normal pressure and H2 organic ratio (59). Methyl isobutyl carbinol is used as an ore flotation frother and to produce zinc dialkyl dithiophosphate lube oil additives. It is produced in the United States by Shell and Union Carbide ( 1.12/kg, October 1994). 

The hypothesis of formation of oxygenated compounds as intermediate products was rejected by Eidus on the basis of experiments on the conversion over cobalt of methyl and ethyl alcohols and formic acid which were found to form carbon monoxide and hydrogen in an intermediate step of the hydrocarbon synthesis (76). Methylene radicals are thought to be formed on nickel and cobalt catalysts (76) by hydrogenation of the unstable group CHOH formed by interaction of adsorbed carbon monoxide and hydrogen, while on iron catalysts methylene radicals are probably formed by hydrogenation of the carbide (78,81). Carbon dioxide was found to interact with the alkaline promoters on the surface of iron catalysts as little as 1 % potassium carbonate was found to occupy 30 to 40% of the active surface area. The alkali also promotes carbide formation and the synthesis reaction on iron (78). 

According to Eidus (90) carbides formed on cobalt or nickel catalysts are neither intermediate products nor catalysts promoting the formation of hydrocarbons from carbon monoxide and hydrogen. In the absence of hydrogen carbon monoxide poisoned the cobalt catalyst. Despite Eidus results, Braude and Bruns (42) supported Craxford s assumption that the carbide is formed by reaction of the metal (iron) with carbon monoxide and hydrogen. It was pointed out by Eidus (84a) that Braude and Bruns did not clearly distinguish between the carbide and free carbon... 

In addition to the sintering phenomena, the carbidization of the nickel particles and the formation of external carbon deposits, already revealed by temperature programmed hydrogenation carried out after the methanation reaction [33], may also contribute to the overall deactivation process. Further experiments aimed at quantifying this type of deactivation are in progress. 

Other reactions important to reforming are also considered in the reaction network in Figure 10, include the water-gas-shift reaction and its reverse, the reversible adsorption and decomposition of water, the desorption and adsorption of reforming products like CO, CO2, and H2, and the formation of hydrocarbons like CH. The formation of dissolved carbon, oxygen, and hydrogen in bulk nickel is also considered. Dissolved C, 0, or H may be important in the transport of those elements to or from interfaces with other solid phase (carbon, carbides, oxides, support). The possible formation of NiO from H2O is also shown. Finally, an important reaction to consider is the formation of a deactivating layer of carbons (6 or e carbon states). 

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