Higher Alcohols through CO Hydrogenation over CoCu Catalysts: Influence of Precursor Activation

Bimetallic CoCu model catalysts were investigated for the synthesis of higher alcohols using catalytic CO hydrogenation according to the Fischer–Tropsch technology. Emphasis was placed on revealing the influence of the activation conditions. Accordingly, catalyst precursors were activated in argon, hydrogen, syngas (CO/H2), and CO under atmospheric conditions and at elevated temperatures (370 °C). All catalyst precursors were prepared via oxalate coprecipitation in the absence of a classic support. Alcohol selectivities between 30 and ∼40% (up to ∼50% for the sum of alcohols and alkenes) were obtained with an Anderson–Schulz–Flory (ASF) chain lengthening probability maximizing the slate up to C6. Detailed catalysis and characterization studies were performed using a Co2Cu1 catalyst composition. The catalytic performances of the H2- and syngas-activated Co2Cu1 catalyst were similar. While the CO-activated catalyst shows significantly higher catalytic activity and ASF chain lengthening probability, the alcohol selectivities are lower than those of H2- or syngas-activated ones. All catalysts required time on stream for several hours to achieve steady-state catalytic performance. Co2Cu1 catalysts were characterized by temperature-programmed decomposition (TPDec), in situ N2 physisorption (Brunauer–Emmett–Teller), transmission electron microscopy (TEM), and in situ X-ray photoelectron spectroscopy (XPS). The data indicate major restructuring occurs during activation. An “onion-like” graphitic carbon shell was observed via TEM for the CO-activated Co2Cu1 catalyst, which probably originated mainly from the Boudouard reaction (2CO + [ ]ad → Cad + CO2). This interpretation is in accordance with the TPDec profiles and XPS results. The latter also indicates that syngas and CO activation lead to higher than nominal Co/Cu surface ratios. The surface segregation of Co in the presence of CO atmospheres is interpreted on the basis of Co@Cu core–shell structured particles.