Effects of Potassium and Manganese Promoters on Nitrogen-Doped Carbon Nanotube-Supported Iron Catalysts for CO₂ Hydrogenation

Nitrogen-doped carbon nanotubes (NCNTs) were used as a support for iron (Fe) nanoparticles applied in carbon dioxide (CO2) hydrogenation at 633 K and 25 bar (1 bar = 105 Pa). The Fe/NCNT catalyst promoted with both potassium (K) and manganese (Mn) showed high performance in CO2 hydrogenation, reaching 34.9% conversion with a gas hourly space velocity (GHSV) of 3.1 L·(g·h)−1. Product selectivities were high for olefin products and low for short-chain alkanes for the K-promoted catalysts. When Fe/NCNT catalyst was promoted with both K and Mn, the catalytic activity was stable for 60 h of reaction time. The structural effect of the Mn promoter was demonstrated by X-ray diffraction (XRD), temperature-programmed reduction (TPR) with molecular hydrogen (H2), and in situ X-ray absorption near-edge structure (XANES) analysis. The Mn promoter stabilized wüstite (FeO) as an intermediate and lowered the TPR onset temperature. Catalytic ammonia (NH3) decomposition was used as an additional probe reaction for characterizing the promoter effects. The Fe/NCNT catalyst promoted with both K and Mn had the highest catalytic activity, and the Mn-promoted Fe/NCNT catalysts had the highest thermal stability under reducing conditions

Determination of phase ratio in polymorphic materials by x-ray absorption spectroscopy: The case of anatase and rutile phase mixture in TiO2

We demonstrate that x-ray absorption spectroscopy (XAS) can be used as an unconventional characterization technique to determine the proportions of different crystal phases in polymorphic samples. As an example, we show that ratios of anatase and rutile phases contained in the TiO2 samples obtained by XAS are in agreement with conventional x-ray diffraction (XRD) measurements to within a few percent. We suggest that XAS measurement is a useful and reliable technique that can be applied to study the phase composition of highly disordered or nanoparticle polymorphic materials, where traditional XRD technique might be difficult