Carbon formation on nickel

Higher hydrocarbons show a higher tendency for carbon formation on nickel than does methane and, therefore, special catalysts either containing alkali or rare earths or based on an active magnesia support are required (refer to Section 5.3.2) [389] [425]. 

With low catalyst activity, the thermal cracking route (pyrolysis) may also take over in the reformer tube [389]. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts with low activity. The risk of carbon formation depends on the type of hydrocarbon with the contents of aromatics being critical. Ethylene formed by pyrolysis results in rapid carbon formation on nickel (refer to Section 5. 2). Ethylene may also be formed by oxidative coupling if air or oxygen is added to the feed - or by dehydration of ethanol. 

Czerwosz et al. s findings might be of particular interest to readers familiar with carbon formation on nickel and nickel-coated catalysts that had been exposed to hydrocarbons or carbon monoxide in hydrocarbon synthesis or in so-called re-forming reactions carried out in petroleum refineries. For example, the formation of filamentous carbon on such solids at temperatures in the same range as that used by Czerwosz et al. was reported by McCarthy in 1982 [115]. However, these authors did not analyze the carbon deposits by Raman spectroscopy, nor were they aware of the existence of fullerenes. Their concern was the removal of these carbons by steam or by combustion, because these carbons inactivated the catalyst. It was also unknown to them that these carbons had the lubricating properties that were demonstrated by Lauer and co-workers [60,62]. By using these catalysts under conditions of continuous wear, they could maintain the catalytic effect of the surface. 

Rostmp-Nielsen J, Trimm DL (1977) Mechanisms of carbon formation on nickel-containing catalysts. J Catal 48 155-165... 

The carbon formation on nickel catalysts is attributed to the formation of carbon filaments and coke, which explains the mechanism proposed by Rostrup-Nielsen [64]. 

Alstrup I, Tavares M T, Bernardo C A, Sprensen O, Rostrup-Nielsen J R, Carbon formation on nickel and nickel-copper alloy catalysts , Materiais and Corrosion,... 

In certain catalytic reactions, such as the reaction of carbon monoxide on nickel and of ethylene on nickel, carbonaceous deposits built up on the surface, and the rate of formation of these deposits varied greatly with the face exposed. In some cases, even when the deposit was very thick on certain faces, no carbon could be detected on others. 

The metal cap may influence the measurements. Various metal cap materials have been used, including the standard nickel, gold-plated nickel, or, more recently, silicon-coated nickel. Nickel is a good catalyst for carbon formation, and the measurements may therefore be falsified by carbon formation on the metal cap. Oxidation of reduced nickel results in similar problems. The experience with the recently developed silicon-coated caps is more promising. 

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. 

Carbon formation on steam reforming catalysts takes place in three different forms whisker-like carbon, encapsulated carbon, and pyrolytic carbon as described in Table 2.2 [1]. Whisker-like carbon grows as a fiber from the catalyst surface with a pear-shaped nickel crystal on the end. Strong fibers can even break down catalyst particles increasing the pressure drop across the reformer tubes [4], The carbon for whisker formation is formed by the reaction of hydrocarbons as well as CO over transition metal catalysts [1], The whisker growth is a result of diffusion through the catalyst and nucleation to form a long carbonaceous fiber. 

In view of the fact that we are here dealing with two sets of products formed simultaneously, a bimolecular process according to Equation (2) seems most likely for the rate-determining step. If such a reaction produced carbon monoxide and carbon dioxide, and fragments which, by further rapid steps involving two more molecules of formic acid, formed only carbon dioxide, the 3 1 ratio of these products would be obtained. The first reaction might involve the formation of a formate-like intermediate. Ruka (11) observed electron diffraction patterns of nickel formate on nickel exposed at 50° to formic acid near its saturation pressure, but no formate could be detected at higher temperatures and lower pressures. 

C. M. Finnerty, N. J. Coe, R. H. Cunningham, and R. M. Ormerod. Carbon formation on and deactivation of nickel-based/zirconia anodes in solid oxide fuel cells running on mehtane. Catalysis Today 46, (1998) 137-145. 

H. Amara, C. Bichara and F. DucasteUe. Formation of carbon nanostructures on nickel snr-faces A tight-binding grand canonical Monte Carlo stndy. Phys. Rev. B 73,2006,113404. 

Dry reduced nickel catalyst protected by fat is the most common catalyst for the hydrogenation of fatty acids. The composition of this type of catalyst is about 25% nickel, 25% inert carrier, and 50% soHd fat. Manufacturers of this catalyst include Calsicat (Mallinckrodt), Harshaw (Engelhard), United Catalysts (Sud Chemie), and Unichema. Other catalysts that stiH have some place in fatty acid hydrogenation are so-called wet reduced nickel catalysts (formate catalysts), Raney nickel catalysts, and precious metal catalysts, primarily palladium on carbon. The spent nickel catalysts are usually sent to a broker who seUs them for recovery of nickel value. Spent palladium catalysts are usually returned to the catalyst suppHer for credit of palladium value. 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

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