Study of the reducibility of copper in CuO-ZnO catalysts by temperature-programmed reduction

Temperature-programmed reduction (TPR) has been used in this work to study the reduction of copper in CuO-ZnO catalysts with different Cu/Zn atomic ratios using H-2 as reducing agent. In all catalysts, CuO was completely reduced to metal. The influence exerted by ZnO on the reduction of copper was evaluated for a wide range of composition and a scale of reducibility was established. ZnO affects the hydrogen reduction of copper, CuO-ZnO samples showing a different behaviour with respect to the pure copper oxide. The reduction is always promoted and, in particular, catalysts with lower copper loading (Cu:Zn < 30:70 as atomic ratio) showed the highest reactivity and are characterised by the presence of two reducible copper species. Results of a kinetic analysis based on the TPR profiles confirmed the role played by ZnO as promoter of the copper oxide reduction. The effect of the preparation method on the catalyst reducibility was also verified and discussed in a specific case, H-2-CO2-H-2 redox cycles were carried out on some representative samples which, after the first reduction in H-2, were reoxidised with CO2 and then reduced again by H-2. These experiments revealed that a small percent of metal copper formed in the CuO-ZnO catalysts is oxidised by CO2 regardless of the catalyst composition, whereas metal copper formed by reduction of pure CuO is not reoxidised at a detectable level. Furthermore, it was evidenced that the small fraction of copper reoxidised by CO2 was extremely reactive, being reduced at temperatures much lower than those found for the reduction of the as-prepared catalysts, Both the TPR investigation and the H-2-CO2-H-2 redox cycles clearly assessed the presence of a synergistic effect arising by the contact of CuO with the ZnO particles.