Microstructure and Crystallographic Determination of Nanoporous Catalysts.

Abstract

Nanostructured materials, such as nanoporous catalysts with specific architectures, are currently being developed for energy applications. To improve the understanding of catalytic processes and optimize the catalytic reactions, information about the surface structure, composition and morphology of the active materials needs to be determined. Within the past few years, the dealloying method has been established as an efficient synthesis route towards high-quality monolithic nanoporous metals, which provides both compositional flexibility and a high level of morphological control. The mechanism of morphology evolution has been investigated in details for single-phase solid solution precursor alloys such as the AuAg system [1,2]. But the dealloying mechanism is less understood for more complex intermetallic starting alloys where both crystal structure and lattice constant change dramatically during dealloying.Nanoporous copper (npCu) prepared by dealloying of CuZn alloys has recently been used as catalyst for ethanol hydrogenation [3]. In the present work, we studied the grain morphology of different CuZn alloys via Electron BackScatter Diffraction (EBSD) at different steps of the dealloying process. Polished Cu 50 Zn 50 and Cu 20 Zn 80 alloy disks were studied before and after dealloying in HCl solution. Samples were then rinsed in deionized water and dried under vacuum. EBSD experiments were performed on a FEI Helios Nanolab 660 with an EDAX EBSD system and TEAM software. The electron beam was set to 30 kV and 13-26 nA, with a working distance of approximately 11.5 mm. Fiducials were produced to analyze the same ROI before and after dealloying.The selective etching of Zn during the dealloying process induces the presence of different macro-and microstructure which depend on alloy composition and on the presence of defects in the initial alloy. The dealloying kinetics was measured by weight loss during dealloying, SEM cross-sections and EDX maps before and after dealloying. During the dealloying, the formation of porosity induces roughness on the surface, as well as the presence of possible oxidation, two effects that hinder the acquisition of a decent EBSD pattern, as it is the case for Cu 5 Zn 8 or Cu 20 Zn 80 . In other cases, it is however possible to analyze the crystallography by EBSD. Especially, our results show that crystal grain structure and orientation of intermetallic with a majority component of bcc Cu 50 Zn 50 starting alloy is surprisingly preserved during dealloying in 5M HCl, despite the bcc Cu 50 Zn 50 to fcc npCu phase transformation (Figure 1). This is the first EBSD study for nanoporous metals from an intermetallic phase to clearly show this behavior.In addition to this, we studied nanoporous gold (npAu) catalysts for selective methanol oxidation. The ligament size of those samples is in the 50-70 m range, and the porosity of the materials corresponds to 70 %. The catalytic performance has been attributed to the presence of traces of silver (approximately 2108