Giorgianni Gianfranco, Mebrahtu Chalachew, Schuster Manfred Erwin, Large Alexander Ian, Held Georg, Ferrer Pilar, Venturini Federica, Grinter David, Palkovits Regina, Perathoner Siglinda, Centi Gabriele, Abate Salvatore, Arrigo Rosa
University of Calabria, Via P. Bucci, I-87036 Rende (CS), Italy.
Phys Chem Chem Phys. 2020 Sep 14;22(34):18788-18797. doi: 10.1039/d0cp00622j. Epub 2020 Apr 24.
Hydrotalcite-derived Ni and Fe-promoted hydrotalcite-derived Ni catalysts were found to outperform industrial catalysts in the CO methanation reaction, however the origin of the improved activity and selectivity of these catalysts is not clear. Here, we report a study of these systems by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy elucidating the chemical nature of the catalysts surface under reaction conditions and revealing the mechanism by which Fe promotes activity and selectivity towards methane. We show that the increase of the conversion leads to hydroxylation of the Ni surface following the formation of water during the reaction. This excessive Ni surface hydroxylation has however a detrimental effect as shown by a controlled study. A dominant metallic Ni surface exists in conditions of higher selectivity towards methane whereas if an increase of the Ni surface hydroxylation occurs, a higher selectivity towards carbon monoxide is observed. The electronic structure analysis of the Fe species under reaction conditions reveals the existence of predominantly Fe(iii) species at the surface, whereas a mixture of Fe(ii)/Fe(iii) species is present underneath the surface when selectivity to methane is high. Our results highlight that Fe(ii) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.
水滑石衍生的镍催化剂以及铁促进的水滑石衍生镍催化剂在CO甲烷化反应中表现优于工业催化剂，然而这些催化剂活性和选择性提高的原因尚不清楚。在此，我们通过原位X射线光电子能谱和近边X射线吸收光谱对这些体系进行了研究，阐明了反应条件下催化剂表面的化学性质，并揭示了铁促进甲烷生成活性和选择性的机制。我们表明，转化率的提高会导致反应过程中形成水后镍表面发生羟基化。然而，如一项对照研究所示，这种过度的镍表面羟基化具有不利影响。在对甲烷选择性较高的条件下存在占主导的金属镍表面，而如果镍表面羟基化增加，则会观察到对一氧化碳的选择性更高。对反应条件下铁物种的电子结构分析表明，表面主要存在Fe(iii)物种，而当对甲烷的选择性较高时，表面以下存在Fe(ii)/Fe(iii)物种的混合物。我们的结果突出表明，Fe(ii)对维持镍处于金属态具有有益作用，而铁氧化态的扩展伴随着镍表面羟基化程度的增加，对甲烷选择性产生负面影响。
