Steam Reforming of Ethanol Using Ni-Co Catalysts Supported on MgAl2O4: Structural Study and Catalytic Properties at Different Temperatures

A spectroscopic and microscopic investigation was made of the dynamics of bimetallic nanoparticles (NPs) in Co-Ni/MgAl2O4 catalysts used for the steam reforming of ethanol (SRE) reaction, considering the implications for catalytic performance, shedding light on the elusive effect of Co-Ni alloy in reforming reactions. X-ray absorption spectroscopy (XAS) analyses showed that contact with the reagent mixture led to major changes in the superficial structure of the Co-Ni nanoparticles, which were strongly dependent on temperature and the metal particle size. At room temperature, contact between the Co-Ni NPs and the reactants (ethanol and H2O) led to the formation of a Co-Ni oxide film over the Co-Ni metal core, with CoO/NiO = 1. For smaller Co-Ni NPs (about 5 nm), the oxide film showed a dynamic composition according to temperature, with Co migrating to the surface while being oxidized up to around 350 degrees C. A structure with a core of (Co,Ni) and a shell rich in CoO was formed, which could be reduced above 350 degrees C. The surface CoO was mainly reduced upon heating in the reaction stream, with migration into the NP cores. For larger Co-Ni NPs (about 10 nm), the oxide film remained stable up to 200 degrees C, being reduced by the ethanol stream at higher temperatures. At operating temperatures in the range of 350-400 degrees C, the surface structure of the Ni-Co alloy showed (Ni,Co)-O species, while these metal oxide species decreased with increases of the thermal treatment temperature and the nanoparticle size. At low temperatures, the Co-Ni nanoparticles were covered by CoO/NiO and were active for the oxidative dehydrogenation of ethanol. With the increase of temperature, surface CoO was reduced and incorporated in the Co-Ni nanoparticle core. These metal sites became active toward the reactant and catalyzed the dehydrogenation of ethanol, followed by C-C bond cleavage, resulting in the formation of CH4, CO, and H-2. The presence of CoO at the surface of the smaller Co-Ni nanoparticles suppressed carbon accumulation, compared to larger Co-Ni or Ni nanoparticles that had highly reduced surfaces.