Stability and Promoter Effects of Carbon-Supported, Copper-Based Catalysts in Methanol Synthesis

Abstract

Nowadays, the chemical industry makes use of the Cu/ZnO/Al2O3 catalyst to convert CO2-enriched synthesis gas (CO/H2) to methanol. Within this Cu catalyst the ZnO promoter is needed to enhance the activity, but simultaneously influences the stability negatively. In this thesis the influence of Zn, Mn, and Cr promoters is investigated on the copper particle size and catalytic performance of carbon-supported, model catalysts. These model catalysts are synthesized via incipient wetness (co)-impregnation (IWI) of metal nitrates on pristine carbon nanotubes (CNTs), oxidized carbon nanotubes (oxCNTs), and high surface area graphite (HSAG). According to titrations the supports differ in Brønsted acidity by the following trend: oxCNT ≫ HSAG > CNTs. Upon oxidation of CNTs a smaller, mean Cu particle size is acquired (25 nm). Due to the irregular morphology of HSAG-supported catalysts this size increases to 12 nm. Addition of promoters only affect the initial mean Cu particle size of HSAG-supported catalysts: compared to the non-promoted catalyst MnO facilitates Cu particle growth, whereas Cr2O3 hampers this process. All promoters enhance the initial activity significantly during catalytic tests of 250 h: ZnO ≫ MnO > Cr2O3 ≫ non-promoted. However, addition of Cr2O3 results in the most stable catalyst. For most catalysts the methanol selectivity was ≥ 92% for the entire run. Clustering is observed for the MnO-Cu/oxCNT and Cr2O3-Cu/oxCNT catalysts with high-angle, annular, dark-field scanning-transmission electron microscopy by energy-dispersive X-ray detectors (HAADF-STEM-EDX). Fundamentally the most promising catalyst is the Cr2O3-promoted, HSAG-supported catalyst, which exhibit an initially small, mean Cu particle size, initially enhanced activity, and high catalyst stability.