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Poster Communications Year : 2016

determination of the most active sites for co hydrogenation over supported cobalt by selective poisoning with tin


1.Introduction Synthetic fuels and base chemicals can be obtained from both fossil and renewable sources through the conversion of synthesis gas (“syngas”, a mixture of carbon oxides and H2) [1]. The nature of the reaction mechanism of the Fischer-Tropsch synthesis over cobalt-based catalysts is still a debate, as well as the nature of the active sites [2,3]. IR-based techniques are useful in investigating syngas conversion because CO is both a reactant and a molecular probe often used to characterize metal surfaces. We recently reported that the most active cobalt sites were likely those associated with the formation of bridged CO(ads), which are thought to be located at the particle edges or steps [4]. This conclusion was reached by noting that the poisoning of cobalt by chloride affected more bridged CO(ads) than linear CO(ads) [4]. We report herein an operando diffuse reflectance FT-IR spectroscopy (DRIFTS) study in which the hydrogenation of CO is monitored on Co and Sn-Co catalysts. Sn-modified cobalt catalysts were synthesized to obtain similar poisoning effects, using an element less volatile than chlorine. Sn does not adsorb CO at the temperatures investigated, so the carbonyl band observed in the region 2100-1700cm-1 by FT-IR could be attributed to CO adsorbed only on cobalt atoms. 2.Experimental A 15 wt.% Co/Al2O3 catalyst used for the CO hydrogenation was prepared with the method reported by Shi et al.[5]. Commercial reagents of Co(NO3)2.6H2O, γ-alumina and citric acid were used. Two Sn-modified catalysts were prepared by adding tin (according to a yet undisclosed method) on the as-prepared 15 wt.% Co/Al2O3 catalyst, with Co/Sn molar ratios equal to 60 and 120. The hydrogenation of CO was performed using a modified high temperature DRIFT cell described in [4]. The reactor effluent was quantified using a 2m-pathlength FT-IR gas cell. The low mass of catalyst (30 mg) used in the reactor enabled to maintain low conversion and to operate essentially under differential conditions, making the DRIFTS cell a gradient-less reactor. 3.Results The rate of formation of methane, propene, and methanol were monitored for the three catalysts. The presence of tin lead to a significant activity loss of about 30% for the Co/Sn = 120:1 and 60% for the Co/Sn = 60:1. Such large activity losses for such low concentrations of poison suggest that most of the Sn remained at the particle surface, despite the fact that Co and Sn can form bulk alloyed phases. The operando DRIFTS spectra measured over a fresh alumina-supported cobalt sample exhibited both linear CO(ads), characterized by bands above 2000 cm-1, and bridged CO(ads), characterized by bands below 2000 cm-1. In contrast, Sn-modified sample exhibited a lower fraction of bands below 2000 cm-1, indicating that less bridged sites were available when Sn was present. A decomposition of the carbonyl signal was carried out to better characterize the changes in the DRIFTS band signal. The lowest number of Gaussian curves needed to obtain close fits of the original spectra was five (Figure 1.B). The band most affected by tin was that located at around 1860 cm-1, noted “Bridged CO(ads)”, the intensity of which decreased with increasing Sn content. It should be noted that this band may corresponds to various bridged and multi-bonded carbonyls. The plot of the relative loss of activity in product formation against the quantity of bridged carbonyls present on the surface of the catalyst appeared to yield an essentially linear relationship. 4.Discussion This work shows that it is possible to selectively poison cobalt using small concentrations of Sn. In addition, Sn appears to be preferentially titrating sites on which bridged CO(ads) are formed. This suggests that the sites associated with the bridged sites (possibly edges or steps) are the most active sites for Fischer-Tropsch synthesis. Step sites have already been proposed as being the most active sites for CO dissociation [6-9]. Methanol formation fitted less the linear relationship, possibly because it does not require CO dissociation and was shown to also involve formate species, located at the interface with the metal particles and an oxidic phase (CoOx or the support) [10]. 5.Conclusions The most active sites for CO hydrogenation over cobalt-based catalysts were shown to be related to sites on which CO adsorption led to bridged or multi-bonded species. Such sites could be selectively titrated by small concentrations of Sn. 6.References [1] A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (2007) 1692-1744. [2] N. Fischer, E. van Steen, M. Claeys, J. Catal. 299 (2013) 67-80. [3] J. Schweicher, A. Bundhoo, N. Kruse. J. Am. Chem. Soc. 134 (2012) 16135-16138. [4] A. Paredes-Nunez, D. Lorito, Y. Schuurman, N. Guilhaume, F.C. Meunier, J. Catal. 329 (2015) 229–236. [5] L. Shi, C. Zeng, Q. Lin, W. Niu, N. Tsubaki, Catal. Today 228 (2014) 206-211. [6] Q. Ge, M. Neurock, J. Phys. Chem. B. 110 (2006) 15368 [7] C.J. Weststrate, I.M. Ciobîca, A.M. Saib, D.J. Moodley, J.W. Niemantsverdriet, Catal. Today 228 (2014) 106. [8] P. van Helden, J.-A. van den Berg, I.M. Ciobîca, Catal. Sci. Tech. 2 (2012) 491. [9] R.A. van Santen, Acc. Chem. Res. 42 (2009) 57. [10] D. Lorito A. Paredes-Nunez, C. Mirodatos, Y. Schuurman, F. Meunier, Catal. Today, in press, doi:10.1016/j.cattod.2015.06.027.
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hal-01327959 , version 1 (07-06-2016)


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A. Paredes-Numez, N. Guilhaume, Y. Schuurman, Frédéric Meunier. determination of the most active sites for co hydrogenation over supported cobalt by selective poisoning with tin. 11th Natural Gas Conversion Symposium, NGCS 11, Jun 2016, Tromso, Norway. ⟨hal-01327959⟩
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