Pt/CNF catalysts: Key parameters allowing high hydrogen selectivity and activity for stable supercritical water gasification of glycerol

Pt/carbon nanofibers (Pt/CNF), with various Pt loadings (0.15-9.65 wt.%) and Pt nanoparticle sizes in the 2.0-2.5 nm range, were investigated as a catalyst in the continuous supercritical water gasification (SCWG) of glycerol. The main gasification products were H 2 (55-65 vol.%) and CO 2 (30-40 vol.%), while the contents of CH 4 and CO in the gas were very low (below 5 and 1 vol.%, respectively). At low conversion, metal loading affected selectivity, lower-Pt loading (≤ 1 wt.%) showing higher selectivity for H 2 and CO 2 . At high conversion (89-96%) however, the impact of Pt loading on selectivity is negligible, H 2 selectivity being at its highest. Both the apparent Pt-normalized carbon conversion rate and apparent turnover frequency (TOF) increased markedly with decreasing Pt surface density. Deactivation of the catalysts was mainly visible at the beginning of the experiments (first 2 h) and was a consequence of the growth of Pt nanoparticles under SCWG conditions, which systematically stabilized to 3-4 nm diameter. After this “greening” phase, all reaction parameters were stable. The catalyst with the highest TOF, 0.15%Pt/CNF, exhibited a carbon conversion rate of 1310 ± 60 mol C ·mol Pt -1 ·min -1 , which represents almost a six-fold increase when compared to the most active methane-selective Ru/CNF catalyst previously reported, for which an optimal loading was reached at ca. 5 wt.% of Ru.