Methanation reaction over nickel catalysts

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

J. Rostrup-Nielsen, Equilibria of decomposition reactions of carbon monooxide and methane over nickel catalyst, J. Catalysis, 27 343 (1972)... 

Steam reforming was the primary reaction over these nickel catalysts. The presence of hydrocarbons (G2 to G5) which would indicate cracking reactions occurred to the extent of less than 10% in the reaction products. The presence of methane, which would indicate partial reforming, did not exceed 5% in the reaction products. There does not appear to be any significant difference in product selectivity for the n-hexane steam reforming reaction over nickel on the 2 quite different supports—zeolite vs. alumina. Carbonaceous residues accumulated in the case of all the nickel catalysts where reforming activity was sustained and the carbon deposition on the zeolite catalysts compared favorably with G56. 

Matsumura, Y. and Nakamori, T. Steam reforming of methane over nickel catalysts at low reaction temperature. Applied Catalysis. A, General, 2004, 258 (1), 107. 

While the hydrogenation of the active surface carbon that forms from CO dissociation appears to be the predominant mechanism of CH4 formation, it is not the only mechanism that produces methane. Poutsma et al. [85] have detected the formation of CH4 over paliadium surfaces that do not readily dissociate carbon monoxide. They also observed methane formation over nickel surfaces at 300 K under conditions in which only molecular carbon m.onoxide appears to be present on the catalyst surfaces [81]. Vannice [86] also reported the formation of methane over platinurh, palladium, and iridium surfaces, and independent experiments indicate the absence of carbon monoxide dissociation over these transition-metal catalysts in most cases. It appears that the direct hydrogenation of molecular carbon monoxide can also occur but that this reaction has a much lower rate than methane formation via the hydrogenation of the active carbon that is produced from the dissociation of carbon monoxide in the appropriate temperature range. 

The reaction of direct cracking of methane over nickel catalysts has recently been receiving attention as an alternative route to the production of hydrogen from natural gas [5-10], This reaction is moderately endothermic, and is displaced towards the carbon formation at temperatures above 560 C ... 

Tn recent years ultrahigh vacuum methods have been applied to cata-lytic studies on initially clean metal surfaces having low surface area. In several instances (the hydrogenolysis of cyclopropane over platinum (I) and the catalytic methanation reaction over rhodium (2) and nickel (3)) a link between ultrahigh vacuum methods and conventional catalytic measurements was established. That is, specific reaction rates over low area cm ) catalyst samples agreed with specific reaction... 

Hoekman et al. [40] studied CO2 methanation reaction over Haldor Topspe commercially available methanation catalysts consisting of Ni and NiO on an alumina substrate with total nickel loading of 20-25% and an operating temperature range of 190-450 C in an extruded ring-shaped catalyst. Approximately 60% conversion of CO2 was observed at r= 300-350°C and stoichiometric CO2/H2 ratio. Aldana et al. [41] found that Ni over ceria-zirconia (prepared by sol-gel synthesis) shows an initial COj activity of almost 80%, with a CH4 selectivity of 97.3%, decreasing down to 84.7% after 90 hours of reaction. By IR operando analysis, they found that for Ni-ceria-zirconia catalysts the main mechanism for CO2 methanation does not require CO as reaction intermediate and the mechanism is based on CO2 adsorption on weak basic sites of the support. 

Methane or natural gas steam reforming performed on an industrial scale over nickel catalysts is described above. Nickel catalysts are also used in large scale productions for the partial oxidation and autothermal reforming of natural gas [216]. They contain between 7 and 80 wt.% nickel on various carriers such as a-alumina, magnesia, zirconia and spinels. Calcium aluminate, 10-13 wt.%, frequently serves as a binder and a combination of up to 7 wt.% potassium and up to 16 wt.% silica is added to suppress coke formation, which is a major issue for nickel catalysts under conditions of partial oxidation [216]. Novel formulations contain 10 wt.% nickel and 5 wt.% sulfur on an alumina carrier [217]. The reaction is usually performed at temperatures exceeding 700 °C. Perovskite catalysts based upon nickel and lanthanide allow high nickel dispersion, which reduces coke formation. In addition, the perovskite structure is temperature resistant. 

Kinetic of methane steam reforming reaction over nickel- and rhodium-based catalysts. Appl. Catal. A Gen., 387 (1-2), 147-154. 

Rostrup-Nielsen, J. R., Equilibria of Decomposition Reactions of Carbon Monoxide and Methane Over Nickel Catalysts, J. Catal., 1972, 27, 343-356. 

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

Kinetics. Extensive studies of the kinetics of methane synthesis were reported by White and co-workers (10,11, 12, 13, 14, 15). They studied the reaction between CO and hydrogen over a reduced nickel catalyst on kieselguhr at 1 atm and 300°-350°C (10). They correlated the rate of methane formation by the equation ... 

The synthesis of methane from C02 and hydrogen was studied by Binder and White (11) over a reduced nickel catalyst (Harshaw Ni-88). The surface reaction between the C02 and hydrogen appeared to be rate controlling. The rate of reaction can be correlated by either of the following rather awkward equations ... 

We expected to control the direction of OTM reaction over NiO by sur ce modification, namely making use of the interaction between NiO and other conq>onents to beget a synergistic effect. In this paper, two completely different behaviors of the oxidative transformation of methane were performed over the nickel-based catalysts because of the different modifications by alkali metal oxide and rare earth metal oxide and the different interactions between nickel and supports. Furthermore, the two completely different reactions were related with the acid-base properties of catalysts and the states of nickel present. 

The two predominant methods oPmaking synthesis gas are steam reforming and partial oxidation. Both are quite simple. The steam reforming method involves passing methane or naphtha plus steam over a nickel catalyst. The reaction, if methane is the feedstock, is ... 

Steam, at high temperatures (975-1375 K) is mixed with methane gas in a reactor with a Ni-based catalyst at pressures of 3-25 bar to yield carbon monoxide (CO) and hydrogen (H ). Steam reforming is the process by which methane and other hydrocarbons in natural gas are converted into hydrogen and carbon monoxide by reaction with steam over a nickel catalyst on a ceramic support. The hydrogen and carbon monoxide are used as initial material for other industrial processes. 

Because some refining processes require a minimum of carbon dioxide in the product gas, the oxides are reacted with hydrogen in a methanation step. This reaction takes place in the methanator over a nickel catalyst at elevated temperatures. 

SR of methane/natural gas is one of the largest catalytic processes in the world and is by far the most important method for producing industrial hydrogen today. The process is well described in literature and it is typically carried out at 800-950 °C over nickel-based catalysts." The main reactions are methane SR (11) and water-gas-shift (WGS) (12). 

Tile partially purilied synthesis gas leaves the C02 absorber containing approximately 0.1% CO2 and 0.5% CO. This gas is preheated at the methanator inlet by heat exchange with the synthesis-gas compressor interstage cooler and the primary-shift converter effluent and reacted over a nickel oxide catalyst bed in the methanator. The methanation reactions are highly exothermic and are equilibrium favored by low temperatures and high pressures. 

---