Ni/SiO2 and Ni/ZrO2 catalysts for the steam reforming of ethanol and glycerol

Differently supported Ni catalysts were prepared. The supports were SiO2 (in the form of mesoporous SBA-15 and of amorphous dense nanoparticles) and ZrO2. Each sample was prepared either by liquid phase synthesis of the support followed by calcination at 800°C and impregnation with the active phase, or by direct synthesis through flame pyrolysis (FP). Many techniques have been used to assess the physical chemical properties of the catalysts, such as atomic absorption, N2 adsorption/desorption, TPR, XRD, XPS, SEM, TEM and FT-IR. The catalytic activity for the steam reforming (SR) of ethanol was determined by testing at 1 bar, 750, 500 and 625°C, with a 3:1 (mol/mol) H2O/ethanol mixture. SR of glycerol was carried out at 650°C with a 10 wt% aqueous solution. All the catalysts performed satisfactorily for ethanol SR when tested at 625 and 750°C. Full ethanol conversion was reached without any gaseous by-product and the C balance was almost always 100%. The best H2 productivity was obtained with 10%Ni/SiO2 prepared by flame pyrolysis at 625°C, i.e. 1.45 mol H2/h kgcat, corresponding to ca. 80 % of the theoretically achievable value. Higher activity, but by far higher coking was observed for the SBA-15-supported sample. Differences among these catalysts became more evident when tested at 500°C. Higher activity was observed for the silica-supported samples than for the ZrO2-carried ones. Indeed, the former were active also for methane reforming even at such low temperature. Silica also promoted more efficiently the WGS reaction at every temperature, so decreasing the CO/CO2 ratio. Coking became significant at 500°C: the FP-prepared samples were characterised by higher C balance and the latter was systematically higher for the ZrO2-supported samples than for the SiO2-based ones. By contrast, ethanol conversion was lower for the FP-prepared catalysts. Indeed, for the FP sample 10%Ni/SiO2 conversion was full at the beginning of the test, but progressively it diminished and stabilised to 80%. A corresponding increase of selectivity to acethaldehyde was observed. The latter by-product always appeared when catalyst deactivation was somehow noticed. The samples showed different textural, structural and morphological properties, as well as different reducibility and thermal resistance depending on the preparation method and support. The key parameter governing activity was metal-support interaction. In particular, the higher the latter, the higher was metal dispersion, leading to more stable small Ni clusters, characterised by higher catalytic activity. Surface acidity was also taken into account considering the effect of the different nature of acid sites (silanols or Lewis a.s.) on catalyst deactivation by coking. Silanols may be responsible of coke deposition, evidenced by a low C balance at reactor outlet. However, coke deposition occurred on the support only and it was not responsible of catalyst deactivation during time-on-stream. On the contrary, when Lewis acid sites were due to Ni particles, a progressive decrease of catalytic activity was observed until their complete coking. After this first time lapse, catalyst performance returned stable, but characterised by lower conversion. It should be noticed that such deactivation was reversible. Indeed, when increasing back the reaction temperature at 625°C, coke gasification occurred and optimal activity was again achieved. Medium Lewis acidity due to Zr(IV) surface sites seems less critical, carbon balance being always higher than for the silica supported samples. In spite of this, the FP-prepared 10%Ni/ZrO2 catalyst showed the lowest ethanol conversion, though not accompanied by significant coking, C balance being comparable to the that of the blank test. The reasons of its lower activity should be searched in the bigger Ni particle size and in turn to the lower metal/support interaction of the FP-prepared samples, which led to easier metal sintering. Similar conclusions were drawn for the SR of glycerol, even more strongly limited by catalyst deactivation. Independently of the nature of the support, for the tested catalysts it seems that a high treatment temperature during catalyst synthesis is required for good and stable performances. ZrO2 in general showed a good stability.