engleski

Gold-catalysed Oxidation of Carbon Monoxide

Introduction Roles of different iron oxides as gold-supports in CO oxidation were doubtful so far. Namely, opposite opinions about influences of magnetite, maghemite and haematite on the mentioned reaction were published. During pre-treatment, there is always some possibility for transformations of those oxides to each other but reported results about their influences on the activity were contradicted. One of the most active catalysts for CO oxidation was prepared by co-precipitation when gold was supported on haematite1. Formation of magnetite2 and maghemite3 after pre-treatment in hydrogen decreased the activity of Au/Fe2O3 catalysts. But in some articles, appearance of maghemite with haematite or pure maghemite as gold supports was attributed for increased activity. So, potentials of those iron oxides for low-temperature oxidation of CO were investigated in this work. Experimental Au/FexOy catalyst was prepared by deposition-precipitation method from an aqueous solution of HAuCl4 3H2O on previously synthesized magnetite. Calcinations of the sample at 200 (M-200), 400 (M-400) and 600 K (M-600) caused transformations of magnetite to maghemite and to haematite. Catalysts were characterized by XRD, 57Fe and 197Au Mössbauer spectroscopy and TEM. Catalytic activity measurements were carried out in fixed-bed reactor. Adsorption of CO was analysed by FTIR spectroscopy and by DRIFTS in situ. Results and discussion The Mössbauer spectra showed that the samples calcined at 200 K (M-200) exhibits typical super paramagnetic behaviour. A small portion of maghemite in that sample can be proposed because of moderated magnetic field values. The overwhelming part of the M-400 spectra are characteristic for haematite but at 300 K there is still visible a small contribution of the super paramagnetic component. M-600 has typical haematite spectra but the sample is not completely homogeneous because slightly different MHF values can be distinguished. After introduction of CO, the FTIR spectra of M-200 shows increased bands at 3637 and ≈ 3400 cm− 1, which could be ascribed to H-bonded – OH, groups and adsorbed water4, respectively. Increase of peaks at 2958, 2870 and 1563 cm− 1, which originate from adsorbed residual citrates added during the synthesis, proves formation of adsorbed formates5, 6. Appearance of a small band at 2364 cm− 1, as a result of CO2 adsorption, indicates partial oxidation of very reactive formates what explains increased amount of water as well: CO + − OH → HCOO(ad) (1) 2 HCOO(ad) + O(support) → 2 CO2 + H2O (2) Water formed in this reaction can easily dissociate and increase amount of – OH groups on a surface of the oxide with uncoordinated metal cations and oxide anions. M-400, besides a band characteristic for H-bonded – OH groups (3629 cm− 1), has a band at 3670 cm− 1of free or almost free – OH groups7 because of less surface water in comparison with M-200. Increase of a band at 3480 cm− 1 is also a consequence of adsorbed water produced during the oxidation of formates (Eq. (2)). Differently from the sample M-200, appearance of the water on this sample did not cause increased amount of surface – OH groups because carbonate-type species (bands at 1617, 1420 and 1223 cm− 1) were formed after adsorption of CO2 (Eq. (2)) on – OH groups8. Namely, on this sample there are no adsorbed citrates, which were removed by calcinations at 673 K, and the development of the carbonates is possible. Also, that could be a reason for more intense oxidation of formates by lattice oxygen because it can occur more easily. In the same conditions, on M-600 there is only appearance of doublet at 2143 cm− 1, corresponds to gaseous CO which disappears after evacuation. It means there is no oxidation of CO and it could be explained by lack of adsorbed water and – OH groups. Fig.1. Conversion of CO as a function of catalyst temperature. At Fig. 1, which shows the catalytic activities of samples M-200, M-400 and M-600, it is obvious that sample M-600 is less active than the other two. It starts the reaction at 500 K, while samples M-200 and M-400 are active even at 340 K. Samples M-200 and M-400 show almost identical activity. (The reaction over M-200 was stopped at 473 K to prevent possible transformations of magnetite and maghemite, which usually occur at that temperature.) 100 % conversions over samples M-400 and M-600 are almost at the same temperature (583 K), but sample M-400 is more active at lower temperature: 50 % conversion over M-400 is at 515 K and over M-600 at 542 K. Those results are in accordance with preliminary tests1 with gold at the same supports. Conclusions A crucial role in the activity of iron oxides, which could be used as gold-supports (Au/FexOy) for CO oxidation at low temperature, plays the amount of surface − OH groups. It is independent of chemical composition but is dependent of calcinations temperature during the pre-treatment. Lower calcinations temperature means more − OH groups. In that case, CO reacts with − OH groups forming very reactive adsorbed formates, HCOO(ad). They can be oxidized to CO2 and water even in a vacuum by lattice oxygen and more easily in the reaction conditions.

CO oxidation; iron oxides; maghemite (γ -Fe2O3); haematite (α -Fe2O3)

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