Synthesis and Treatment Parameters for Controlling Metal Particle Size and Composition in Cu/ZnO Materials—First Evidence of Cu3Zn Alloy Formation

Abstract

This paper deals with the study of the conditions of obtention of small metal copper particles on ZnO using easy synthesis methods and activation treatments, which were found “efficient” for the preparation of copper particles on silica, that is, deposition–precipitation with urea and incipient wetness impregnation. For the latter, four different copper precursors were used: copper(II) nitrate, acetylacetonate, bis-ethylenediamine, and tetraamine. Activation treatments were performed under different gas reducing conditions (H2 or CO in N2), temperatures between 280 and 350 °C, and isotherm between 1 min and 2 h, with or without a precalcination step. Transmission electron microscopy (TEM), UV–visible spectroscopy, and X-ray diffraction (XRD) techniques were used to characterize the samples at the different stages of the synthesis route. The TEM results showed that it is not as easy as on silica to make small copper particles on ZnO. However, some experimental conditions were found to lead to particles around 4 nm: impregnation with copper acetylacetonate or copper bis-ethylenediamine, followed by calcination before reduction in H2. The important result of this work is that depending on the final temperature of reduction under hydrogen, it was possible to form either metal Cu particles (280 °C) or Cu3Zn alloy particles (350 °C); this was directly proved by XRD and indirectly by UV–visible spectroscopy. However, the formation of Cu3Zn could be avoided if the reducing gas H2 was replaced by CO in N2. To our knowledge, this is the first time that the formation of Cu3Zn alloy particles is evidenced in Cu/ZnO samples.