IR spectroscopy in catalysis

DFT calculations combined with a computational screening method have previously shown that bimetallic Ni–Fe alloys should be more active than the traditional Ni-based catalyst for CO methanation. That was confirmed experimentally for a number of bimetallic Ni–Fe catalysts supported on MgAl 2 O 4 . Here, we report a more detailed catalytic study aimed at optimizing the catalyst performance. For this purpose, two series of mono and bimetallic Ni–Fe catalysts supported on MgAl 2 O 4 and Al 2 O 3 , respectively, were prepared. All catalysts were tested in the CO methanation reaction in the temperature interval 200–300 °C, and characterized using elemental analysis, N 2 physisorption measurements, XRD and TEM. Optimization of the catalyst performance was made by varying the Ni:Fe ratio, the total metal loading and the support material. For both support materials, the bimetallic catalysts with compositions 25Fe75Ni and 50Fe50Ni showed significantly better activity and in some cases also a higher selectivity to methane compared with the traditional monometallic Ni and Fe catalysts. A catalyst with composition 25Fe75Ni was found to be the most active in CO hydrogenation for the MgAl 2 O 4 support at low metal loadings. At high metal concentrations, the maximum for the methanation activity was found for catalysts with composition 50Ni50Fe both on the MgAl 2 O 4 and Al 2 O 3 supports. This difference can be attributed to a higher reducibility of the constituting metals with increasing metal concentration. The maximum of the catalytic activity and the highest selectivity to methane were observed for the sample with 20 wt% total metal loading. It appears that it is possible to increase substantially the efficiency of Ni-based methanation catalyst by alloying with Fe.

CO methanation over supported bimetallic Ni–Fe catalysts: From computational studies towards catalyst optimization

Structure and catalytic performance of Pt-promoted alumina-supported cobalt catalysts under realistic conditions of Fischer–Tropsch synthesis

e-Iron carbide is a promising catalyst for low-temperature Fischer–Tropsch synthesis but is difficult to synthesize. Here, the authors report a rapid-quenching process for the synthesis of nanocrystalline e-iron carbide, and evaluate the catalytic activity and selectivity of the material.

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ε -Iron carbide as a low-temperature Fischer–Tropsch synthesis catalyst

The synthesis of ultrasmall supported bimetallic nanoparticles (between 1 and 3 nanometers in diameter) with well-defined stoichiometry and intimacy between constituent metals remains a substantial challenge. We synthesized 10 different supported bimetallic nanoparticles via surface inorganometallic chemistry by decomposing and reducing surface-adsorbed heterometallic double complex salts, which are readily obtained upon sequential adsorption of target cations and anions on a silica substrate. For example, adsorption of tetraamminepalladium(II) [Pd(NH 3 ) 4 2+ ] followed by adsorption of tetrachloroplatinate [PtCl 4 2− ] was used to form palladium-platinum (Pd-Pt) nanoparticles. These supported bimetallic nanoparticles show enhanced catalytic performance in acetylene selective hydrogenation, which clearly demonstrates a synergistic effect between constituent metals.

A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.

The interaction of carbon monoxide and hydrogen with an alumina-supported iron catalyst has been studied at temperatures ranging from 300 to 523 K and at a pressure of 103 kPa. The species produced and adsorbed on the catalyst surface have been examined by infrared spectroscopy, while the chemical state of the iron within the catalyst was investigated by magnetization measurements and Mossbauer spectroscopy. At 300 K the main type of adsorbed species on the iron particle surface appeared to be molecular carbon monoxide and formate groups. The amount of the latter species increases considerably at elevated reaction temperatures up to 473 K. Changes in the infrared absorption bands of CO, and in the magnetization and Mossbauer data indicate that under the applied conditions the surface properties of the iron particles are affected by the synthesis gas, but that the bulk remains metallic. The increased Fischer-Tropsch activity at more elevated temperatures, viz. 473 and 523 K, is accompanied by significant changes in the in situ recorded infrared spectra. The formation of hydrocarbons is evident from the well developed absorption bands in the 3000-2800 cm−1 range, due to (a)symmetric stretching vibrations of CH2 and CH3 groups. Assignment of the various absorption bands in the 1700-1200 cm−1 wavenumber range is speculative due to (partial) coincidence of the bands and the gradual change in chemical composition of the catalyst particles. At higher temperatures the bulk of the iron particles reacts rapidly to a mixture of carbides. Initially the unstable e-Fe2C is formed which reacts to e′-Fe2.2C upon prolonged periods of time of reaction and/or increased reaction temperatures. These carbides appeared to be thermally unstable: upon heating in helium they are converted into a mixture of χ-Fe5C2 and metallic Fe. Interaction of the reduced iron catalysts with ethylene or methane at 523 K gives rise to absorption bands at 1555, 1345 and 1050 cm−1, which are assigned to CHx adsorbed species. Upon reaction with ethylene also a band at 2160 cm−1 was observed, which was assigned to an ethylidyne species.

Behaviour of a cyanide-derived Fe/Al2O3 catalyst during Fischer-Tropsch synthesis

A supported iron catalyst is prepared by precipitation of an iron hexacyanoferrate complex onto γ-alumina. Removal of the cyanide ligands by oxidation at 563 K results in the formation of highly dispersed iron(III) oxide particles. The reduction of the oxidic precursor proceeds via a support stabilised Fe 1 − x O phase as revealed by in situ magnetization measurements and Mossbauer spectroscopy. Upon exposure of the reduced and subsequently evacuated catalyst to carbon monoxide at room temperature, both linearly and bridged-bonded carbon monoxide species are observed by infrared spectroscopy at wavenumbers of 1985 cm −1 and 1980-1700 cm −1 , respectively. A fraction of the carbon monoxide reacts with irreversibly chemisorbed hydrogen to formate and small hydrocarbons, which remain bonded on the surface of the iron particles.

Preparation, reduction and CO chemisorption properties of cyanide-derived NixFe/Al2O3 catalysts

Fischer–Tropsch synthesis has been performed with a series of alumina-supported copper–iron catalysts, which are prepared via deposition–precipitation of stoichiometric cyanide complexes, viz. Cu 2 Fe(CN) 6 , Cu 3 [Fe(CN) 6 ] 2 and CuFe(CN) 5 NO onto γ-Al 2 O 3 . The catalysts are characterized in situ by magnetic measurements and Mossbauer spectroscopy. Upon exposure of the reduced catalysts to synthesis gas at 548 K, the metallic iron rapidly reacts to a mixture of ϵ′-Fe 2.2 C and χ-Fe 5 C 2 carbides. Prolonged reaction causes the contribution of χ-Fe 5 C 2 to increase at the expense of the ϵ′-Fe 2.2 C. Infrared spectroscopy performed with a Cu 2 Fe catalyst after intermittent Fischer–Tropsch reactions at increasingly higher temperatures indicates at 300 K the presence of both Fe- and Cu-like sites at the surface of the bimetallic particles. The number of Cu-like sites, however, decreases at 373 K and becomes negligible at 423 K. At 473 and 548 K beside absorption bands due to CO bridged-bonded on Fe-like sites, bands assigned to hydrocarbons, formate species and to carbonates are observed. The activity of the thus prepared copper–iron catalysts is substantially higher than that of a monometallic iron catalyst prepared from a complex cyanide. However, the activity decreases from an initial level of 135–95 to about 20–15 mmol C/(kg Fe s) within 252 ks, while the monometallic iron catalyst exhibits an activity of about 1.5 mmol C/(kg Fe s) after 252 ks. The Schulz–Flory coefficient rises from 0.36 to 0.45 with the copper content of the catalysts as well as the production of carbon dioxide and the selectivity for olefins.

Behaviour of cyanide-derived CuxFe/Al2O3 catalysts during Fischer–Tropsch synthesis

To assess the suitability of metal cyanide complexes as active precursors for supported catalysts, a series of homo- and heteronuclear cyanide complexes has been precipitated in the presence of alumina, titania, and silica supports. To establish the distribution of, the insoluble cyanide complexes on the support, the catalyst precursors were investigated by transmission electron microscopy. Conversion of the cyanide precursors into oxidic or metallic catalysts can be performed by thermal treatments in oxygen, argon, and hydrogen, respectively. Detailed results of the thermal treatment of a copper-iron cyanide precursor on alumina are presented. Oxidation of the cyanide precursors to highly dispersed oxides calls for treatment at relatively low temperatures, viz., about 573 K. The resulting oxide can subsequently be reduced smoothly to the corresponding (bi) metallic supported catalyst. Besides by electron microscopy, the catalyst (precursor) systems were characterized by Mossbauer spectroscopy, XRD, and thermal analysis. It was demonstrated that the proportion of the metals within the stoichiometric cyanide precursors was retained in the reduced bimetallic catalysts. Heteronuclear cyanide complexes are therefore very well appropriate to produce bimetallic catalysts of a uniform chemical composition of the individual supported alloy particles.

Preparation of supported mono- and bimetallic catalysts by deposition-precipitation of metal cyanide complexes

A copper-iron catalyst for the hydrogenation of carbon monoxide has been prepared using a supported stoichiometric cyanide complex. Conversion of the cyanide precursor to a metallic catalyst appeared to be a precious process. Copper and iron in the bimetallic particles easily separate by thermal treatment and upon exposure to carbon monoxide, as revealed from Mossbauer and infrared spectroscopy. During Fischer-Tropsch reaction the catalyst exhibits a rapid decline of activity. Magnetisation measurements on spent catalysts indicate that the deactivation is caused by a fast conversion of metallic iron to initially unstable carbides which transform ultimately to more stable carbides.

E. Boellaard

Papers

Behaviour of a cyanide-derived Fe/Al2O3 catalyst during Fischer-Tropsch synthesis

Preparation, reduction and CO chemisorption properties of cyanide-derived NixFe/Al2O3 catalysts

Behaviour of cyanide-derived CuxFe/Al2O3 catalysts during Fischer–Tropsch synthesis

Preparation of supported mono- and bimetallic catalysts by deposition-precipitation of metal cyanide complexes

Development of CuxFe/Al2O3 catalysts for the hydrogenation of carbon monoxide guided by magnetic methods, Mössbauer and infrared spectroscopy