The diffraction patterns mainly consisted of the peaks attributableto NiO and Al2O3phases for the as-calcined catalysts other than theCe-modified one. No characteristic diffraction lines of rare-earthelements were observed in their patterns, implying that rare-earthcompounds were in the amorphous state or too small in size to bedetected by XRD analysis. In the case of Ce-modified catalyst, thestrong peaks of CeO2phase were confirmed in addition to NiO andAl2O3phases. These results indicated that the sintering of CeO2par-ticles proceeded readily compared to other rare-earth compounds.In previous studies, the formation of NiAl2O4spinel via the solidstate reaction between NiO and Al2O3was reported for Ni/Al2O3catalysts [23,24]. Thus, NiAl2O4formation was expected in thisstudy though it was difficult to distinguish the diffraction peaksof NiAl2O4from those of NiO and Al2O3. After the reduction treat-ment at 600◦C, the Ni species was in the metallic state and the otherphases remained unchanged for all the catalysts (not shown).