The effect of Ti4+ dopant in the 5 wt% Ni/Ce1-xTixO2-delta catalyst on the carbon pathways of dry reforming of methane studied by various transient and isotopic techniques

摘要

The carbon pathways in the dry reforming of methane with carbon dioxide (DRM) towards syngas at 750 degrees C have been investigated for the first time over 5 wt% Ni supported on Ti4+-doped ceria using various transient and isotopic experiments. The obtained results revealed important information about the effect of Ce1-xTixO2-delta support chemical composition (x = 0, 0.2 and 0.5) on the transient rates of "carbon" deposition via CH4/He (methane decomposition) and CO/He (Boudouard reaction), and on the rate of "carbon" removal via the participation of labile active oxygen of support under DRM reaction conditions. The alternative path of CO2 activation on the oxygen vacant sites of support was also probed by transient isothermal reduction (use of hydrogen) at 750 degrees C followed by transient CO2/He reaction. Quantification of the amount (mg C g(cat)(-1)) of inactive "carbon" accumulated as a function of time on stream in DRM and support chemical composition as well as of the origin of it (CH4 vs CO2 activation route), was made after conducting temperature-programmed oxidation (TPO) experiments coupled with the use of (CO2)-C-13. It was illustrated that 5 wt% Ni (d(Ni) similar to 20 nm) supported on Ce0.8Ti0.2O2-delta solid solution improves the H-2-yield and H-2/CO product gas ratio compared to 5 wt% Ni/CeO2 of similar Ni mean particle size but remarkably caused a drastic reduction in the rate of "carbon" deposition. After 12 h of DRM (20% CH4, CH4/CO2 = 1), Ni/Ce0.8Ti0.2O2-delta accumulates similar to 270 times less "carbon" the origin of which was the CH4 activation and not the CO disproportionation route. The 5 wt% Ni/Ce0.8Ti0.2O2 catalyst developed presented high CH4 and CO2 conversions (75% and 82%), a H-2-yiled of 64% and an H-2/CO product gas ratio of 1.3 after 100 h of DRM at 750 degrees C (40%CH4/40%CO2/He; GHSV = 30,000 h(-1)) with a relatively low amount (73 mg "C" g(cat)(-1) or 7.3 wt%) of inactive "carbon", which is considered to the best of our knowledge, as one of the lowest values reported in the literature for monometallic Ni supported catalysts under the examined DRM reaction conditions.