Microstructural and Chemical Evolution and Analysis of a Self-Activating CO2-Selective Cu- Zr Bimetallic Methanol Steam Reforming Catalyst

The microstructure of the CO2-selective self activating and self-stabilizing Cu-Zr bimetallic compound Cu51Zr14 has been studied by a combination of high-resolution electron microscopy and energy-dispersive X-ray spectroscopy both before and after entering the CO2 selective state in methanol steam reforming. Prior to catalysis, the phase composition of the catalyst is characterized by a microstructural mixture of Cu51Zr14 and metallic Cu. The structure appears in a distinct needle-like morphology with a characteristic micro-stucture of small Cu particles embedded in the intermetallic matrix. In contrast, entering the CO2-selective state goes along with oxidative decomposition investigated by differential thermal analysis (DTA), thermogravimetry (TG), and mass spectrometry (MS) and therefore massive structural and compositional changes of the Cu51Zr14 compound both in the near-surface and bulk regions. The final state is then composed of a structurally very heterogeneous sample with Zr-rich and Cu-rich regions within the material bulk with a characteristic lamellar structure. Most importantly, the catalytically relevant surface regions are drastically corroded and depleted in Zr and are characterized by a majority of Cu in intimate contact with oxidized ZrO2 exhibiting a well-ordered, predominantly tetragonal structure. This newly created Cu ZrO2 interface is believed to be the most significant descriptor steering the CO2 selectivity. In due course, this new method for self-adjustment of the microstructure starting from well-defined intermetallic compounds in the catalytic reaction mixture might pave the way for a more systematic approach of controlled oxidative decomposition of intermetallic compounds acting as promising catalyst precursors.