Role of Co₂C in ZnO‐promoted Co Catalysts for Alcohol Synthesis from Syngas

Abstract

Controlling selectivity is a key goal in the design of a heterogeneous catalyst. Herein, we present detailed characterization and activity of silica‐supported cobalt catalysts modified by atomic layer deposition of ZnO. After reduction, the resulting catalysts exhibit substantial selectivity towards alcohol production during CO hydrogenation compared to catalysts containing only cobalt. The prepared catalysts have up to 46 % selectivity toward alcohols with 39 % of the alcohols corresponding to ethanol and other higher alcohols, albeit with reduced activity. In situ characterization of the catalyst by X‐ray diffraction and X‐ray absorption spectroscopy reveals details on the structural evolution in syngas, CO+H2, and shows that ZnO promotion of Co results in the formation of Co2C under catalytic conditions. A mechanism is proposed, supported by density functional theory calculations, which explains Co2C formation by the blocking of Co step sites by Zn species. The ZnO acts a dual promoter both by facilitating Co2C formation and by modifying the resulting Co2C. The Co2C formed from the ZnO‐promoted Co catalysts displays improved thermal stability and selectivity compared with similar Co2C catalysts without Zn.

Elucidating the K‐Edge X‐Ray Absorption Near‐Edge Structure of Cobalt Carbide

“…And, there is a white line peak at 7729 eV. The results are in good agreement with the spectrum proposed by Singh et al . (Figure S2.23).…”

“…In the case of the identification of cobalt carbide formation, some ex‐situ research has been reported . Particularly interesting is the study performed by Singh et al ., where a Co 2 C spectrum acquired at in‐situ conditions was reported. Among the different X‐ray spectroscopies, X‐ray Absorption Near‐Edge Structure (XANES) is particularly attractive because it allows to perform studies under in‐situ and ex‐situ conditions in a laboratory‐ and synchrotron‐based facility.…”

Understanding Support Effects of ZnO‐Promoted Co Catalysts for Syngas Conversion to Alcohols Using Atomic Layer Deposition

“…On the SiO 2 ‐supported catalyst, the addition of the ZnO promoter results in an 87 % loss of selectivity towards C 5+ hydrocarbons, a 29 % loss in selectivity towards C 2 −C 4 hydrocarbons, a 92 % increase in selectivity towards methane, and a more than 7‐fold improvement in selectivity towards methanol and higher oxygenates. The activity of the catalyst also decreases by over an order of magnitude, consistent with the decrease in CO uptake measured by chemisorption (Table 2) and in agreement with previous reports [11] . Similar trends are observed for the 4Zn/Co/C and 4Zn/Co/CeO 2 catalysts, with the primary exception of methane selectivity: there is a <1 % change in methane selectivity on the carbon support and a 42 % decrease on the CeO 2 .…”

“…The Zn XANES in both the 4Zn/Co/Al 2 O 3 (Figures 6a and S10a) and 4Zn/Co/15Al/SiO 2 (Figures 6b and S9b) as‐prepared catalysts show similar absorption edge positions and shape to the ZnO bulk reference. However, the features appear to be “smoothed out” on both catalyst samples, indicating the Zn is in a highly dispersed ZnO phase, consistent with the previously reported data for the SiO 2 catalyst [11] . During catalyst reduction, the ZnO transforms into a new phase, in which the features are different in the two catalysts.…”

Beyond Cars: Fischer‐Tropsch Synthesis for Non‐Automotive Applications

“…In addition to high‐quality hydrocarbons, the product slate of F‐T synthesis now can be extended to produce oxygenated chemicals, such as C 2+ alcohols and aldehydes, which are scarcely possible in traditional F‐T synthesis. These straight‐chain aldehydes and terminal alcohols are extensively employed as feedstocks for plasticizers, detergents, lubricants, food additives or perfumes . Rare reports concerning above product are observed in consideration of its great challenge.…”