M. A. Ermakova

Effective catalysts for direct cracking of methane to produce hydrogen and filamentous carbon: Part I. Nickel catalysts

Abstract Data obtained by studying model catalytic systems were used to develop high-loaded nickel catalysts for direct cracking of methane into hydrogen and catalytic filamentous carbon (CFC). The approach to the synthesis of these catalysts can be the basis for development of catalytic nickel systems for commercial processing of natural gas. The catalysts were synthesized by fusing nickel nitrate with zirconium nitrate, or nickel nitrate with copper-doped aluminium nitrate followed by decomposition of the mixture at 300–450°C and its additional stabilization by silica. The silica textural promoter was formed by thermal decomposition of polyethoxysilane introduced into pores of the oxide matrix. The catalysts reduced by hydrogen comprised nickel in the amount of 85–90%. Unlike preparation of co-precipitated systems, synthesis of the catalysts mentioned does not include stages of filtration and wastewater treatment. Principal characteristic of the catalysts’ effectiveness for the reaction of decomposition of methane, i.e. carbon yield per gram of nickel, of the suggested systems was close or superior to that achieved before using the best nickel or nickel–copper catalysts prepared by co-precipitation.

XRD studies of evolution of catalytic nickel nanoparticles during synthesis of filamentous carbon from methane

The XDR technique was used for studying a series of high‐loaded (90%) nickel catalysts with silica as a textural promoter. These were catalysts for direct cracking of methane at 550°C. A relation between the initial average size of active catalyst particles, carbon yield and average methane conversion was demonstrated. Genesis of these catalysts was studied including oxide precursors, reduced catalysts prior to the reaction, as well as catalysts upon their contacting the reaction medium for various periods of time from 15 min to 2 h. The active catalyst particles were shown to merge or disperse at the outset of the reaction depending on their initial size. Anyway, close average sizes ranging from 30 to 40 nm were observed by the end of the first reaction hour for all the catalytic systems providing the carbon yield of 300–385 g/g Ni. The catalytic system was shown to self‐organize in the course of direct methane cracking, i.e., the catalyst particles transform in response to the reaction conditions. If the size of nickel particles cannot vary, these catalysts are inefficient for the given process.

Filamentous carbon templated SiO2–NiO aerogel: structure and catalytic properties for direct oxidation of hydrogen sulfide into sulfur

Silica aerogels comprising nickel oxide nanoparticles were synthesized with no use of supercritical drying. A high specific surface area (more than 1000 m2/g), mesoporous structure and considerable stability to sintering up to 900 °C are characteristic of these aerogels. The aerogels were synthesized using the sol–gel method. Filamentous carbon was templated by silica, tetraethoxysilane being used for supplying silica. Carbon was burnt later. Analysis of the aerogel structure revealed the presence of silica nanotubes and nanofibers. Aerogel testing for direct oxidation of H2S into S0 demonstrated as high as 60% conversion of hydrogen sulfide at almost 100% selectivity under stoichiometric conditions at the temperature range of 300–350 °C and 73% conversion at 100% selectivity at a considerable excess of oxygen at 160 °C.

Chemical properties of the surface of nanofibrous carbonaceous materials produced by catalytic methane decomposition

The chemical behavior of nanofibrous carbonaceous materials prepared by the catalytic decomposition of methane was studied by IR spectroscopy, X-ray photoelectron spectroscopy, and titration. Initial carbon was shown to be virtually devoid of functional groups on its surface. Treatment of carbonaceous samples with alkali, ammonia, or nitric acid modified the surface of carbon and increased the number of functional groups.

Synthesis of High Surface Area Silica Gels Using Porous Carbon Matrices

Porous silica was prepared using the sol-gel synthesis with porous carbon matrices as a pore-forming support. Tetraethoxysilane (TEOS) was hydrolyzed in an acid medium in the presence of a substoichiometric amount of water. Various carbon materials were used, among them Sibunit and catalytic filamentous carbon. Carbon matrices were impregnated with hydrolyzed TEOS and dried, then carbon was removed by burning out in air at 873 K. The obtained porous silica samples were studied by adsorption and electron microscopic methods. The specific surface area as high as 1267 m2g and pore volume as high as 5.7 cm3/g were determined for some silica samples. Thus deposited SiO2 was found to cover the carbon surface copying its surface. With CFC used as carbon matrix, silica nanotubes were obtained. Thermostability of such silica is significantly greater as compared to silica gels reported earlier.

Morphology and Texture of Silica Prepared by Sol–Gel Synthesis on the Surface of Fibrous Carbon Materials

Silica materials are synthesized by the sol–gel method including the deposition of tetraethoxysilane on various micro- and nanocarbon fibers. The use of nanofibrous carbon as a template makes it possible to prepare thermally stable mesoporous SiO2 samples with unusually high surface areas (up to 1255 m2/g) and high porosity (up to 5.6 cm3/g). These silica materials and aerogels prepared by supercritical drying have comparable pore volumes. It is found by high-resolution electron microscopy that a thin-wall matrix permeated by channels is a prevailing structure of silica materials. When some catalytic fibrous carbons are used as templates, silica nanotubes can be prepared.

Effect of Interactions between Components in Nickel–Silica Catalysts on the Yield of Carbon in Methane Decomposition

A number of 90% Ni–10% SiO2 catalysts for methane decomposition were studied at different stages of preparation and operation in the reaction using X-ray diffraction analysis, differential dissolution, temperature-programmed reduction, IR spectroscopy, and high-resolution electron microscopy. It was found that an increase in the interaction between components in the catalytic system decreased the ability of nickel to accumulate carbon in the decomposition of methane.