Formic acid-assisted synthesis of highly efficient Cu/ZnO catalysts: effect of HCOOH/Cu molar ratios

Abstract

A series of metallic Cu/ZnO catalysts were prepared by a novel formic acid-assisted solid-state combustion method with and without further reduction using metal nitrates and varied amounts of formic acid as raw materials. The states of the copper species of the as-burnt catalysts were mainly affected by two important catalyst preparation routes. One route was dissolution and reduction during the solid-state process; another was reduction during the calcination process. By gradually increasing the formic acid content, the copper species in the as-burnt catalysts changed from CuO to Cu₂O, and from Cu₂O to metallic Cu⁰. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Energy Dispersive Spectroscopy (EDS), Temperature-Programmed Reduction (TPR), and High-Resolution Transmission Electron Microscopy (HRTEM) analysis revealed that the precursors with a HCOOH/Cu²⁺ ratio greater than 12/1, after being burnt in argon atmosphere and without further reduction, were absolutely converted into metallic Cu⁰ and ZnO species. With gradually increasing formic acid content, the catalysts exhibited a larger BET surface area, larger metallic Cu⁰ area, and smaller Cu crystallite size. The activity of the as-prepared catalysts was tested for low-temperature methanol synthesis. The total carbon conversion increased with increasing formic acid content. The variation trend was in accordance with that of the metallic Cu⁰ surface area. The methanol selectivity was closely related to metallic Cu⁰ crystallite size. The Cu crystallites with a smaller size exhibited higher hydrogenation capability to yield methanol.