Facile use of coal combustion fly ash (CCFA) as Ni-Re bimetallic catalyst support for high-performance CO₂...

The industrial waste coal combustion fly ash (CCFA) was used as a cheap catalyst support and by facile co-impregnation method, the active component Ni and promoter Re was loaded to form a bimetallic catalyst for high-performance CO₂ methanation. The physico-chemical properties of the prepared catalyst were further measured by a series of characterizations such as X-ray fluorescence (XRF), N₂ isothermal adsorption-desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) et al. The effect of reaction temperature and gas hourly space velocity (GHSV) on the catalytic performance was carefully investigated on a continuous fixed-bed reactor. The results showed that in comparison with the non-promoted monometallic Ni/CCFA catalyst, the bimetallic Ni-Re/CCFA catalyst displayed a superior activity, which could achieve 99.55% of CO₂ conversion and 70.27% of CH₄ selectivity under the condition of 400 °C, 2000 h^-1, 1 atm and H₂:CO₂:N₂ = 4:1:0.5, possibly owing to the higher Ni dispersion and more active sites in Ni-Re/CCFA. Besides, the addition of Re promoter was beneficial to enhance the catalyst anti-sintering and anti-coking abilities as reflected by the smaller Ni particle size growth and less carbon deposition amount in Ni-Re/CCFA. The in-situ diffuse reflection infrared Fourier transform spectrum (in-situ DRIFTS) was finally carried out to determine the CO₂ adsorption state and its methanation intermediates, from which a loop mechanism of CO₂ methanation process was proposed and depicted.