Catalysts for the thermo-catalytic cracking (TCC) process: Interactions between the yttria in yttria-doped alumina aerogel and the mono-oxide MoO3, CeO2, and bi-oxide MoO3–CeO2 species Original Research Article

The influence of yttria used as a doping agent for catalyst support, on the dispersion and the resistance to sintering of MoO3, CeO2 and bi-oxide MoO3–CeO2 species in the TCC catalysts has been investigated. Characterization techniques used included N2 adsorption, X-ray diffraction, Raman, thermogravimetric analysis, and iso-electric point (IEP).It was explicitly found that yttria-doped alumina aerogel is by far more capable of homogenously dispersing the active molybdenum species and significantly retarding their sintering at quite high-calcination temperature than conventional alumina and alumina aerogel. Characterization results of supported mono-oxide MoO3 catalysts have clearly indicated that the fraction of (surface) tetrahedral monomolybdate species increases in the detriment of crystalline MoO3 as we varied the support from conventional alumina to yttria-doped alumina aerogel with increasing amounts of the yttria loading. This can be attributed to the incorporation of yttria into alumina aerogel network, which led to a change in the support surface charge (IEP) and subsequently the stability of surface molybdate species. The dispersion degree of ceria (substituted ceria) on supported mono-oxide CeO2 was improved as well, upon using yttria-doped alumina aerogel. Such improvement was attributed to the formation of stronger Ce–Y–O bond, and/or the formation of CeO2–Y2O3 solid solution. In the bi-oxide MoO3–CeO2, characterization results have indicated the presence of surface interaction between Mo and Ce, probably through the formation of surface “Mo–O–Ce” type phase between the dispersed ceria and the molybdate monolayer. This interaction, which was highly favourable over yttria-doped alumina surface, contributed significantly to the overall surface stability of Mo–Ce catalysts.