On the promotion effect of trivalent cations on zinc oxide and their influence on catalyst activity

Methanol synthesis is one of the important processes of the chemical industry. Because of the usage of the product molecule as fuel, platform chemical and hydrogen storage material. By using anthropogenic CO2 a sustainable carbon neutral chemical industry can be realized. The hydrogenation of CO and/or CO2 to methanol is performed on a Cu/ZnO (CZ) catalyst. As state of the art, the Cu/ZnO:Al (CZA) catalyst is highly active in this process. But up to nowadays, the role of aluminium as promotor is not completely understood. Therefore, the structural and electronic effect of Al and Ga as ternary metals in Cu/ZnO catalysts for the methanol synthesis was investigated. For this purpose, three zincian malachite-derived catalysts with the nominal Cu:Zn ratio of 70:30 were synthesized: an unpromoted, binary catalyst (CZ) and two ternary catalysts with either 3 mol% Al (CZA) or Ga (CZG). Both Al and Ga showed to have a strong impact on the catalyst’s properties. Alongside the catalyst treatment from the co-precipitated precursors phase to the activated and reduced catalyst, an improved microstructure and an increased BET surface area was found for the secondary promotor (Al or Ga) containing catalysts. Moreover, a sequence of chemisorption experiments allowed to quantify and differentiate between Cusurf and Znred surface species in the activated catalysts. Considering several analysis techniques like chemisorption method or diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), an additional electronic promotion of Al and Ga could be determined. This promotion effect is related to doped ZnO phases, as it was demonstrated by X-ray absorption near edge spectroscopy (XANES) and enhances the reducibility of the ZnO by forming more Znred sites. This effect is stronger for Al, leading to a more pronounced Zn overlayer on the Cu surface and strong metal support interactions (SMSI). In the methanol synthesis, this led to a performance in the order CZA>CZG>CZ. To further address the observed electronic effects of ZnO by doping with a trivalent cation like Al3+ or Ga3+, ZnO as a model support was synthesized according to the preparation method of the methanol synthesis catalyst. To determine the aluminium speciation and the solubility limit of the aluminium cation on zinc positions, a series of zinc oxides with varying aluminium contents was synthesized by a subsequent calcination of the co-precipitated precursors. The synthesis was inspired by the industrial synthesis of the methanol synthesis catalyst via crystalline precursors, here hydrozincite Zn5(OH)6(CO3)2 was employed. Short precipitate ageing time, low ageing temperature and low aluminium contents below 3 mol% metal were advantageous to suppress crystalline side-phases in the precursor, which caused an aluminium segregation and non- - 2 - uniform aluminium distribution in the solid. This was observed also after calcination at 320 °C by transmission electron microscopy (TEM), although zinc oxide was the only crystalline phase. At lower aluminium contents, however, the dopant was found preferably on the zinc sites of the zinc oxide lattice based on the AlZn • signal dominating the 27Al nuclear magnetic resonance (NMR) spectra. The solubility limit regarding this species was determined to be approximately x = 0.013 or 1.3 % of all metal cations. Annealing experiments showed that aluminium was kinetically trapped on the AlZn • site and segregated into zinc oxide and ZnAl2O4 spinel upon further heating. This shows that lower calcination temperatures such as applied in catalyst synthesis favour the aluminium doping on that specific site. Instead of aluminium gallium can also be used as a zinc oxide dopant. Because of the closer ionic radius of gallium and zinc, a better incorporation into the zincite lattice is assumed. Analogous to the aluminium series, a series with varying amounts of Ga was prepared and investigated regarding phase purity and solubility limit. Up to a doping level of 4 % Ga, the precursor phase was free of impurities indicating a homogeneous cation distribution. A further increase of the dopant result in a defective hydrozincite structure and an additional side phase, which was in analogy to the Al doping series determined to be a zaccagnaite-like phase. Decomposition at low temperature (around 320 °C) resulted in ZnO phase without any side phases. An increased Ga content >6 % increased the defects in the ZnO structure, as it was determined by powder X-ray diffraction (PXRD) and Raman spectroscopy. From 71Ga solid state NMR a solubility limit couldn’t be clearly derived due to strong line broadening. However, up to 6 % Ga content, the four-fold coordinated Ga environment was dominant. From band gap determination there was found a limit of 4 % Ga (xGa = 0.04 ) in ZnO to which the band gap was unaffected. Combining the results of PXRD analysis of the precursor phase together with the NMR, X-ray photoelectron spectroscopy (XPS) and UV–visible spectrophotometry (UVVis) results of the ZnO phase, a solubility limit around 4 % is expected. Beside the structural and compositional investigation of the copper based catalyst, the investigation of the active site of the methanol synthesis is still of interest but under controversial debate. Ammonia has been used as a probe molecule as it was found to inhibit the methanol synthesis in CO2 containing synthesis gas during reaction. The poisoning effect was investigated using an industrial type of copper/zinc oxide/alumina catalyst. During steady state methanol synthesis in a CO2/CO/H2 synthesis gas, isobaric tri-methylamine (TMA) and ammonia injections poisoned the methanol formation, with the poisoning of ammonia being significantly stronger than that of TMA. Together with density functional theory (DFT) calculations, a mechanism of ammonia poisoning could be derived: ammonia activation takes - 3 - place on adsorbed oxygen or hydroxyl groups followed by the formation of stable carbamate on the active site. Further hydrogenation of the carbamate to TMA was calculated to exhibit high barriers, thus being rather slow, explaining why ammonia poisoning has a longer term effect. Copper is known as a typical methanol formation catalyst. The combination of a FischerTropsch active metal like cobalt together with copper as a bi-metallic catalyst for alcohol formation was investigated as a suitable combination in the higher alcohol synthesis. Typically, higher alcohol synthesis (HAS) is performed from CO-containing synthesis gas with an approximately equimolar hydrogen-to-carbon monoxide (H2:CO) ratio. To investigate the effect of the Cu:Co composition in the presence of zinc for higher H2:CO ratios on HAS, a series of Cu-Co/ZnAl2O4 catalysts was synthesized from co-precipitated hydrotalcite-like precursors with different cobalt-rich Cu:Co ratios and compared to their monometallic pure cobalt and copper counterparts. The addition of copper strongly facilitated cobalt oxide reduction upon catalyst activation and resulted in much smaller domain sizes for the crystalline metallic phases. The catalysts were evaluated at H2:CO ratios of 4 at 20 bar or 60 bar in a temperature range between 200 °C and 380 °C at a relatively low space velocity. Copper addition resulted in an increased formation of higher alcohols and hydrocarbons. The monometallic catalysts produced mainly C1 products (CH4 on Co/ZnAl2O4 and CH3OH on Cu/ZnAl2O4), while the best catalyst with respect to ethanol yield reached a selectivity of 4.5 % and had a molar composition of Cu:Co ratio of 0.6 (x = 0.375). The microstructure of the bimetallic spent catalysts clearly confirmed a close interaction of both metal species. The pure cobalt catalyst showed strong coking, which was effectively suppressed on the coppercontaining samples. Despite these promotional effects of copper, the hydrocarbon selectivity dominated over the formation of (higher) alcohols on all cobalt-containing catalysts.


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