Study of Chemisorption of Carbon Monoxide on the Surface of Iron Catalysts
Abstract Generated abstract
This study examines the chemisorption of carbon monoxide on oxidized and hydrogen-reduced surfaces of an industrial iron-chromium catalyst using infrared spectroscopy, with phase characterization by X-ray structural analysis and gamma-resonance spectroscopy. On the oxidized hematite surface, CO adsorption produces carbonyl species, weakly bound CO, and, upon heating, more thermally stable carbonate and carboxylate structures. Reduction forms magnetite and shifts adsorption mainly toward multiple carbonyl species, while heating in CO promotes reactions with surface hydroxyl groups, formation of CO2, and possible formate, bicarbonate, and hydrocarbon-like surface structures. Catalytic testing under CO and water vapor indicates high selectivity for conversion and iron pentacarbonyl formation, but low selectivity for hydrocarbon synthesis.
Full Text
STUDY OF CARBON MONOXIDE CHEMISORPTION ON THE SURFACE OF IRON CATALYSTS
N. V. Usheva, N. A. Rubens, A. V. Kravtsov, S. I. Smolyaninov
Iron catalysts have found wide application in technological processes associated with the processing of carbon monoxide.
In this connection, the study of the forms of chemisorbed carbon monoxide on the surface of a catalyst is of undoubted interest. Investigation of the IR spectra of carbon monoxide adsorbed on the oxidized and reduced surface of an industrial iron–chromium conversion catalyst was carried out on a UR-10 spectrophotometer.
The phase composition of the catalyst was studied by X-ray structural analysis and \(\gamma\)-resonance spectroscopy.
It was shown that the initial catalyst sample is \(\alpha\)-\(Fe_2O_3\) with a rhombohedral type of crystalline structure.
Before carbon monoxide was admitted, the sample was subjected to conditioning in vacuum at \(450^\circ C\) for three hours. The initial spectrum of the conditioned sample contained several absorption bands in the region \(1200\)–\(1600\ \text{cm}^{-1}\). These bands may be associated with the presence in the sample of carbon or nitrogen impurities that were introduced during preparation of the catalyst.
When carbon monoxide was admitted onto the conditioned surface of the sample, the spectrum showed the appearance of several weak absorption bands characterizing carbonyl structures (\(2000\) and \(2095\ \text{cm}^{-1}\)). In addition to carbonyl structures, under these conditions weakly bound forms of carbon monoxide are also formed on the surface, which is confirmed by the presence in the spectrum of an absorption band in the region \(2175\ \text{cm}^{-1}\).
Heating in an atmosphere of CO to \(200^\circ C\) leads to a considerable increase in the intensity of the absorption bands characterizing carbonyl structures.
The large set of bands in the region of vibrations of the \(\nu C \equiv O\) bond of carbonyl structures is a consequence of the strong heterogeneity of the catalyst surface with respect to CO adsorption.
Heating the sample also leads to the appearance of a large number of intense absorption bands in the region 1800–1200 cm\(^{-1}\), i.e., to the formation of carbonate–carboxylate structures on the surface.
Groups of bands in the regions 1570–1520 cm\(^{-1}\) and 1410–1350 may characterize various carboxylate structures of the type:
\[ M^+\left[\mathrm{C}\begin{matrix} \cdots \mathrm{O}\\[-2pt] \cdots \mathrm{O} \end{matrix}\right]^- \]
Two other triplets in the regions 1780–1720 cm\(^{-1}\) and 1190–1125 cm\(^{-1}\) may be assigned to various structures of organic carbonate, in which the metal–oxygen bond approaches a covalent one. Carbonate and carboxylate structures are more thermostable than carbonyl structures.
The literature has repeatedly discussed the question of the possible participation of carbonate–carboxylate structures in the reaction of CO oxidation to CO\(_2\), with preliminary treatment of the surface with oxygen promoting their more intensive destruction.
Reduction of the catalyst with hydrogen leads to a significant decrease in its content of \(\alpha\)-Fe\(_2\)O\(_3\) and to the formation of stoichiometric Fe\(_3\)O\(_4\).
Interaction of carbon monoxide with the surface of the sample reduced for 3 hours at 400°C led predominantly to the formation of carbonyl structures on the surface (1920, 1965, 2015, 2050, 2095 cm\(^{-1}\)).
The high-frequency band at 2095 cm\(^{-1}\) may characterize subcarbonyl structures \(Me(CO)_x\). The intensity of this band increases sharply upon heating. Already at 100°C it exceeds the intensity of the absorption band at 2015 cm\(^{-1}\), which characterizes a linear form of CO adsorption.
In the initial spectrum of the reduced sample, several absorption bands were observed that characterize OH groups (3745, 3680, 3620, 3545 cm\(^{-1}\)).
Interaction of CO with surface hydroxyl groups begins only after 2 hours of heating in a CO atmosphere at 250°C. Under these conditions, a noticeable decrease is observed in the intensity of the absorption bands belonging to OH groups, and an absorption band appears at 2350 cm\(^{-1}\), corresponding to gaseous CO\(_2\). Since at the same time there is a decrease in the intensity of the absorption bands characterizing carbonyl struc-
then, probably, the latter take part in the reaction with OH groups, with formation of CO₂.
\[ \mathrm{CO}_{ads} + \mathrm{OH}^{-}_{surf.} \longrightarrow \mathrm{HCOO}^{-} \rightleftharpoons \mathrm{CO}_{2} + [\mathrm{H}] \]
In the interaction of CO with surface OH groups, in addition to formates, bicarbonate structures may also be formed, which can decompose with formation of CO₂.
\[ \mathrm{Me}^{+}\left[ \begin{matrix} \mathrm{O} \\ \mathrm{O} \end{matrix} \!\!\begin{matrix} \diagup\!\!\!\diagdown \\ \diagdown\!\!\!\diagup \end{matrix} \mathrm{C}-\mathrm{OH} \right]^{-} \]
In the case of the interaction of CO with the reduced surface of the iron–chromium catalyst, the spectra showed the appearance of absorption bands corresponding to vibrations of C–H bonds (3060, 2920, 2860, 2820 cm⁻¹) in saturated and unsaturated surface structures, which indicates the interaction of CO with hydrogen released during the decomposition of formate and bicarbonate structures.
The absorption bands at 2920 and 2855 cm⁻¹ are in good agreement with assignment to the antisymmetric and symmetric vibrations of the CH₂ and CH₃ groups in saturated hydrocarbon structures of the type
\[ \mathrm{M}-\mathrm{CH}_{2}-\mathrm{CH}_{3} \]
The absorption band observed in the spectrum at 3000 cm⁻¹ can be explained by the formation of a CH₂ group in structures of the type:
\[ \begin{matrix} \mathrm{CH}_{2}-\mathrm{CH}_{2} \\ | \qquad\quad | \\ \mathrm{M} \qquad \mathrm{M} \end{matrix} \qquad \mathrm{M}=\mathrm{CH}_{2} \]
Testing of the iron–chromium catalyst in the synthesis from carbon monoxide and water vapor (CO : H₂O = 3 : 1, T = 250°C, P = 50 atm) confirmed that it is highly selective in the conversion reaction and in the formation of iron pentacarbonyl, but has low selectivity in the reaction of synthesis of hydrocarbon structures.
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