Tailor-made preparation of Co–C, Co–B, and Co catalytic thin films using magnetron sputtering: insights into structure–composition and activation effects for catalyzed NaBH4 hydrolysis

Abstract

The magnetron sputtering (MS) methodology is a powerful tool for tailor-made fabrication of Co-based thin film catalysts with controlled microstructures and compositions for sodium borohydride (SBH) hydrolysis. In particular, Co–C catalysts were tested in this reaction and compared to Co–B and Co catalyst coatings. The microstructural and chemical analyses by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), Rutherford back scattering (RBS) and X-ray photoelectron spectroscopy (XPS) were used to characterize a complete library of thin film catalysts. Pure Co materials were characterized by their nanocrystalline microstructure, and grain refinement was achieved via an increase in the deposition pressure. The incorporation of boron or carbon via co-deposition results in amorphization and dispersion of the active metallic Co phase. The composition can be tuned while keeping a controlled microstructure, and a comparison of activity at 25 °C was performed on catalysts deposited on Ni foam substrates. A comparison of the initial activities showed that the Co–B samples were more active than the Co–C samples because of electronic effects. However, a strong activation was found for the Co–C catalysts after the first use. This effect was dependent upon the incorporation of cobalt boride (Co_xB) species on the catalysts' surface, as shown by XPS. After the first several uses, the activity of the Co–C samples (values up to 2495 mL min⁻¹ g_catalyst⁻¹) were as high as that of fresh Co–B, and the surface composition of both the catalysts was similar. This activation was not observed for the pure Co and was very weak for the Co–B catalysts. The use of polymeric (PTFE) substrates (flexible membranes) illustrated the versatility of the methodology to obtain catalytic membranes and allowed for a TEM microstructural analysis at the nanoscale. Catalytic activities at 60 °C were as high as 16.7 and 20 L min⁻¹ g_Co⁻¹ for the Co–C and Co–B membranes, respectively. We determined the optimized conditions to increase the catalytic activity of Co-based coatings prepared via magnetron sputtering.