Table 6 Comparison of NiO, Ni, and CeO₂ crystallite sizes between calcined, reduced, and spent gradient-coated spiral-structured Ni/CeO₂ catalysts after CO₂ methanation (CO₂/H₂ (1/4) feed flow rates ranging from 100 to 3000 ml/min at 350°C).

Sample	Crystallite	2θ (°)	Crystallite size (nm)^a
Calcined Ni/CeO₂	CeO₂	28.5	8.1
Calcined Ni/CeO₂	NiO	43.2	21.6
Reduced Ni/CeO₂	CeO₂	27.8	9.9
Reduced Ni/CeO₂	Ni	44.7	28.3
Spent Ni/CeO₂ from a single-passed reactor	CeO₂	28.7	9.4
	NiO	43.4	13.2
	Ni	44.7	29.0
Spent-R1 Ni/CeO₂ from a triple-passed reactor	CeO₂	28.5	9.6
	NiO	43.0	13.9
	Ni	44.4	32.0
Spent-R2 Ni/CeO₂ from a triple-passed reactor	CeO₂	28.5	9.2
	NiO	43.0	12.0
	Ni	44.4	21.4
Spent-R3 Ni/CeO₂ from a triple-passed reactor	CeO₂	28.5	9.1
	NiO	43.0	12.0
	Ni	44.5	33.4

^aCrystallite sizes were measured using the Scherrer equation from DIFFRAC.EVA software.

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