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/H-2), and CO under atmospheric conditions and at elevated temperatures (370 degrees C). All catalyst precursors were prepared via oxalate coprecipitation in the absence of a classic support. Alcohol selectivities between 30 and similar to 40% (up to similar 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 C-6. Detailed catalysis and characterization studies were performed using a Co2Cu1 catalyst composition. The catalytic performances of the H-2- 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 H-2- 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 N-2 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) -> C-ad + 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.