Dry reforming of glycerol over Rh-based ceria and zirconia catalysts: New insights on catalyst activity and stability

Abstract

Effects of reaction temperature and feed composition on reactant conversion, product distribution and catalytic stability were investigated on syngas production by reforming of glycerol, a renewable waste, with CO2 on Rh/ZrO2 and Rh/CeO2 catalysts. For the first time in the literature, fresh and spent catalysts were characterized by in-situ FTIR, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray analysis techniques in order to unravel novel insights regarding the molecular-level origins of catalytic deactivation and aging under the conditions of glycerol dry reforming. Both catalysts revealed increased glycerol conversions with increasing temperature, where the magnitude of response became particularly notable above 650 and 700 °C on Rh/ZrO2 and Rh/CeO2, respectively. In accordance with thermodynamic predictions, CO2 transformation occurred only above 700 °C. Syngas was obtained at H2/CO ∼0.8, very close to the ideal composition for Fischer-Tropsch synthesis, and carbon formation was minimized with increasing temperature. Glycerol conversion decreased monotonically, whereas, after an initial increase, CO2 conversion remained constant upon increasing CO2/glycerol ratio (CO2/G) from 1 to 4. In alignment with the slightly higher specific surface area of and smaller average Rh-particle size on ZrO2, Rh/ZrO2 exhibited higher conversions and syngas yields than that of Rh/CeO2. Current characterization studies indicated that Rh/CeO2 revealed strong metal-support interaction, through which CeO2 seemed to encapsulate Rh nanoparticles and partially suppressed the catalytic activity of Rh sites. However, such interactions also seemed to improve the stability of Rh/CeO2, rendering its activity loss to stay below that of Rh/ZrO2 after 72 h time-on-stream testing at 750 °C and for CO2/G = 4. Enhanced stability in the presence of CeO2 was associated with the inhibition of coking of the catalyst surface by the mobile oxygen species and creation of oxygen vacancies on ceria domains. Deactivation of Rh/ZrO2 was attributed to the sintering of Rh nanoparticles and carbon formation.