Effect of synthesis solution pH of Co/γ-Al₂O₃ catalyst on its catalytic properties for methane conversion to syngas

Amir Mosayebi^{1
, ,},
Reza Abedini²

1.
Department of Chemical Engineering, Tafresh University, Tafresh 39518 79611, Iran
2.
Faculty of Chemical Engineering, Babol Noshirvani University of Technology, Babol 47148 71167, Iran

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Corresponding author: Amir Mosayebi, E-mail: mosayebi@tafreshu.ac.ir

摘要

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Abstract: The cobalt nanoparticles over γ-Al₂O₃ support were prepared via chemical reduction of CoCl₂·6H₂O using NaBH₄ with various values of pH in the range of 11.92-13.80. Synthesized catalysts were studied through X-ray diffraction (XRD), N₂ adsorption/desorption (BET), H₂-temperature programmed reduction (H₂-TPR), H₂-chemisorption, O₂ pulse titration and temperature programmed oxidation (TPO) methods. Obtained results exhibited the synthesis solution pH showed a significant influence on the activity and selectivity in partial oxidation of methane reaction. The methane conversion, CO selectivity and H₂ yield were enhanced by increasing of the synthesis solution pH. Compared to other catalysts, the catalyst that synthesized at pH of 13.80, showed a superior ability in syngas production with a H₂/CO ratio of near 2 and also a proper stability against deactivation during the partial oxidation of methane.
- partial oxidation /
- Co/γ-Al₂O₃ /
- nanoparticles /
- selectivity /
- pH value

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Figure 1 XRD patterns of the synthesized catalysts before reduction behavior

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Figure 2 H₂-TPR profiles of synthesized catalysts

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Figure 3 Time-dependent of CH₄ conversion over the reduced Co/γ-Al₂O₃ catalysts

p= 1.5 MPa, GHSV=9000 h^-1, T=1023 K, CH₄/air=4

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Table 1 Composition of the prepared catalysts

Sample	NaOH concentration w/(mol·L^-1)	pH of synthesis solution
CA₁	0.01	11.92
CA₂	0.02	12.47
CA₃	0.1	13.04
CA₄	0.6	13.80

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Table 2 Textural parameters of the prepared catalysts

Sample	Surface area A/(m²·g^-1)	Total pore volume v/(cm³·g^-1)	Average pore size d/nm	Average CoO crystal size (XRD) d/nm	Average CoO crystal size (TEM) d/nm
γ-Al₂O₃	230	0.498	9.2	-	-
CA₁	185.2	0.441	8.2	7.6	7.9
CA₂	183.1	0.437	7.7	8.9	9.3
CA₃	179.6	0.439	7.8	10.3	10.2
CA₄	178.3	0.438	7.5	11.9	12.3

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Table 3 Acid sites of prepared samples determined by NH₃-TPD analysis

Sample	NH₃ uptake /(μmol·g^-1)
Sample	W	M	S
CA₁	72	139	127
CA₂	83	155	99
CA₃	89	161	87
CA₄	95	180	71
weak (W), medium-strength (M) and strong (S) acid sites

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Table 4 H₂-chemisorption and O₂ pulse titration results for the synthesized catalysts

Sample	Uncorrected dispersion /%^a	Uncorrected cobalt size d/nm^b	Reduction degree /%^c	Corrected dispersion /%^d	Corrected cobalt size d/nm^e
CA₁	7.81	12.29	71.6	10.91	8.82
CA₂	7.57	12.67	79.7	9.49	10.11
CA₃	7.32	13.12	83.2	8.79	10.92
CA₄	6.57	14.59	91.8	7.15	13.42
^a: uncorrected dispersion= 100×number of metallic cobalt atoms on the surface/total number of metallic cobalt atoms; ^b: uncorrected cobalt size were measured by H₂-chemisorption using 96/uncorrected dispersion; ^c: measured by O₂ pulse titration; ^d: corrected dispersion=100× (uncorrected dispersion/reduction degree); ^e: corrected cobalt size= (uncorrected cobalt size×reduction degree)/100

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Table 5 Results of catalytic activity in the partial oxidation of methane

Catalyst	Reaction temperature T/K	x_CH₄ /%	Selectivity s/%		H₂ yield w/%/CO ratio	H₂/CO ratio
Catalyst	Reaction temperature T/K	x_CH₄ /%	CO	CO₂	H₂ yield w/%/CO ratio	H₂/CO ratio
CA₁	923	40.33	31.08	19.73	33.98	1.76
	1023	51.23	41.57	13.58	52.71	1.8
CA₂	923	46.24	36.79	17.42	39.67	1.82
	1023	53.08	46.51	13.32	55.23	1.83
CA₃	923	58.66	40.24	13.24	49.14	1.91
	1023	65.09	48.19	10.23	60.59	1.88
CA₄	923	62.67	55.24	8.28	61.17	1.98
	1023	71.02	61.49	6.71	70.88	2.05

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Table 6 Coke deposition and deactivation as a function of synthesis solution pH over the Co/Al₂O₃ catalysts

Sample	Amount of coke formation /(mol×10^-4)	Deactivation /%
CA₁	2.423	66.1
CA₂	1.987	51.23
CA₃	1.744	38.46
CA₄	1.214	27.15

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