Effect of Different Chelating Agents on the Physicochemical Properties of Cu/ZnO Catalysts for Low-temperature Methanol Synthesis from Syngas Containing CO₂

Abstract

Nanostructured Cu/ZnO catalysts were prepared by a simple solid-state method, using cheap metal nitrates as raw materials. Special attention was paid to the influence of different chelating agents such as citric acid, formic acid and oxalic acid on the physicochemical properties of Cu/ZnO catalysts, which were characterized by detailed investigations. CuO crystallite size, oxygen vacancies and surface compositions were clearly affected by chelating agent types, leading to different CuO–ZnO interactions in the calcined catalysts as well as distinct reducibility. Different chelating agents also had significant effects on specific surface area, metallic Cu⁰ surface area, and Cu crystallite size, as well as ZnO (002) plane to ZnO (100) plane ratio (I₍₀₀₂₎/I₍₁₀₀₎) calculated based on the peak intensity of XRD patterns of the reduced samples, so influencing the catalytic activity for low-temperature methanol synthesis from syngas containing CO₂. The structure-activity correlations, particularly the relationships between space time yield (STY) of methanol with Cu⁰ surface area and I₍₀₀₂₎/I₍₁₀₀₎ ratio, were evaluated. Cu/ZnO catalyst prepared with oxalic acid showed the highest STY value, which was 2.3 and 5.3 times higher that of the catalysts obtained with citric acid and formic acid, respectively. The excellent catalytic performance was due to its greater Cu⁰ surface area, higher specific surface area and smaller Cu crystallite size. Stronger interactions between Cu and ZnO in the reduced samples were also favorable for enhancing catalytic activity for low-temperature methanol synthesis.