Influence of Phosphate on La-based Catalysts for Oxidative Coupling of Methane†

doi:10.7503/cjcu20170665

Abstract

Abstract:

A series of La-based catalysts promoted by different contents of P $O 4 3 -$ was synthesized by the citric acid method and their catalytic performance for oxidative coupling of methane(OCM) was investigated to clarify the effect of P $O 4 3 -$ on La-based catalyst. With characterizations such as X-ray powder diffraction(XRD), O₂-temperature programmed desorption(O₂-TPD) and X-ray photoelectron spectroscopy(XPS), we studied the physicochemical properties of the catalysts. The results showed that the phases of the catalysts were changed from La₂O₃, La₃PO₇ to LaPO₄ along with the decreased size when phosphate added. If the catalyst was composed of two phases(La₂O₃ and right amount of La₃PO₇), it showed better performances in OCM, which corresponded to the more quantities of surface defect sites and the larger ratio of electrophilic oxygen species/lattice oxygen species than other samples.

Key words: Phosphate, Oxidative coupling of methane, Lanthanum oxide, Lanthanum oxyphosphate

TrendMD:

BAI Yan, XIA Wensheng, WENG Weizheng, LIAN Mengshui, ZHAO Mingquan, WAN Huilin. Influence of Phosphate on La-based Catalysts for Oxidative Coupling of Methane^†[J]. Chem. J. Chinese Universities, 2018, 39(2): 247.

Figures/Tables 12

Fig.1 Catalytic performances of 0.07X-La catalysts in OCM^ a. La2O3; b. 0.07P-La; c. 0.07Si-La; d. 0.07S-La.

Table 1 Performances of La-based catalysts in OCM under selected temperatures*

Catalyst	T/℃	$X C H 4$ (%)	$X O 2$ (%)	$S C O x$ (%)	S_C2(%)	Y_C2(%)
Blank	700	0	0	0	0	0
	750	1.2	2.5	59.0	41.0	0.5
La₂O₃	550	29.6	94.3	55.9	44.1	13.1
	650	30.3	98.8	53.5	46.5	14.1
	700	29.8	100.0	57.0	43.0	12.8
	750	28.7	100.0	59.5	40.5	11.6
0.07P-La	550	29.5	94.1	53.2	46.8	13.8
	650	30.1	97.3	50.8	49.2	14.8
	700	31.7	98.8	50.8	49.2	15.6
	750	29.4	99.6	49.8	50.2	14.8
0.13P-La	550	31.0	97.6	48.0	52.0	16.1
	650	31.4	98.5	47.2	52.8	16.6
	700	31.4	98.9	48.2	51.8	16.3
	750	31.2	99.6	48.7	51.3	16.0
0.27P-La	550	29.3	96.6	51.7	48.3	14.2
	650	30.8	98.3	48.8	51.2	15.8
	700	31.0	98.7	48.5	51.5	16.0
	750	30.9	99.3	50.0	50.0	15.4
0.40P-La	550	29.0	99.1	59.9	40.1	11.6
	650	30.0	99.6	55.8	44.2	13.3
	700	30.8	99.7	55.4	44.6	13.7
	750	30.4	100.0	54.8	45.2	13.7
1.30P-La	650	1.2	2.2	100.0	0	0
	700	2.3	4.7	59.8	40.2	0.9
	750	4.0	8.0	52.7	47.3	2.0

Table 1 Performances of La-based catalysts in OCM under selected temperatures*

Catalyst	T/℃	$X C H 4$ (%)	$X O 2$ (%)	$S C O x$ (%)	S_C2(%)	Y_C2(%)
Blank	700	0	0	0	0	0
	750	1.2	2.5	59.0	41.0	0.5
La₂O₃	550	29.6	94.3	55.9	44.1	13.1
	650	30.3	98.8	53.5	46.5	14.1
	700	29.8	100.0	57.0	43.0	12.8
	750	28.7	100.0	59.5	40.5	11.6
0.07P-La	550	29.5	94.1	53.2	46.8	13.8
	650	30.1	97.3	50.8	49.2	14.8
	700	31.7	98.8	50.8	49.2	15.6
	750	29.4	99.6	49.8	50.2	14.8
0.13P-La	550	31.0	97.6	48.0	52.0	16.1
	650	31.4	98.5	47.2	52.8	16.6
	700	31.4	98.9	48.2	51.8	16.3
	750	31.2	99.6	48.7	51.3	16.0
0.27P-La	550	29.3	96.6	51.7	48.3	14.2
	650	30.8	98.3	48.8	51.2	15.8
	700	31.0	98.7	48.5	51.5	16.0
	750	30.9	99.3	50.0	50.0	15.4
0.40P-La	550	29.0	99.1	59.9	40.1	11.6
	650	30.0	99.6	55.8	44.2	13.3
	700	30.8	99.7	55.4	44.6	13.7
	750	30.4	100.0	54.8	45.2	13.7
1.30P-La	650	1.2	2.2	100.0	0	0
	700	2.3	4.7	59.8	40.2	0.9
	750	4.0	8.0	52.7	47.3	2.0

Fig.2 SEM images of samples^(A) La2O3; (B) 0.07P-La; (C) 0.13P-La; (D) 0.27P-La; (E) 0.40P-La; (F) 1.30P-La.

Table 2 Specific surface areas, pore volumes and pore sizes of samples

Catalyst	Surface area/(m²·g^-1)	Pore volume/(cm³·g^-1)	Pore size/nm	D_XRD/nm
Catalyst	Surface area/(m²·g^-1)	Pore volume/(cm³·g^-1)	Pore size/nm	La₂ $O 3 a$	La₃P $O 7 b$
La₂O₃	3.7	0.03	21.3	50.7
0.07P-La	3.9	0.02	14.8	30.1
0.13P-La	4.1	0.02	13.9	29.2	27.9
0.27P-La	9.6	0.08	25.2	26.3	25.8
0.40P-La	12.3	0.09	31.3		19.1
1.30P-La	19.2	0.16	29.5

Table 2 Specific surface areas, pore volumes and pore sizes of samples

Catalyst	Surface area/(m²·g^-1)	Pore volume/(cm³·g^-1)	Pore size/nm	D_XRD/nm
Catalyst	Surface area/(m²·g^-1)	Pore volume/(cm³·g^-1)	Pore size/nm	La₂ $O 3 a$	La₃P $O 7 b$
La₂O₃	3.7	0.03	21.3	50.7
0.07P-La	3.9	0.02	14.8	30.1
0.13P-La	4.1	0.02	13.9	29.2	27.9
0.27P-La	9.6	0.08	25.2	26.3	25.8
0.40P-La	12.3	0.09	31.3		19.1
1.30P-La	19.2	0.16	29.5

Fig.3 XRD patterns of the catalysts^a. La2O3; b. 0.07P-La; c. 0.13P-La; d. 0.27P-La; e. 0.40P-La; f. 1.30P-La.

Fig.4 FTIR spectra of the catalysts^a. La2O3; b. 0.07P-La; c. 0.13P-La; d. 0.27P-La; e. 0.40P-La; f. 1.30P-La.

Fig.5 Raman spectra of the samples^a. La2O3; b. 0.07P-La; c. 0.13P-La; d. 0.27P-La; e. 0.40P-La; f. 1.30P-La.

Fig.6 O2-TPD profiles of the catalysts^a. La2O3; b. 0.07P-La; c. 0.13P-La; d. 0.27P-La; e. 0.40P-La; f. 1.30P-La.

Fig.7 ESR spectra of the catalysts^a. La2O3; b. 0.07P-La; c. 0.13P-La; d. 0.27P-La; e. 0.40P-La; f. 1.30P-La.

Fig.8 XPS spectra for O1s of La2O3(A) and 0.13P-La(B)

Table 3 Curve-fitting results from XPS spectra for O1s of the catalysts*

Catalyst	E_b/eV, FWHM/eV, relative amount of oxygen species(%)				( $O 2 2 -$ +O^-)/O^2-
Catalyst	O^2-		O^-	$O 2 2 -$	C $O 3 2 -$
La₂O₃	528.7, 1.4, 12.4	530.2, 1.4, 12.8	531.1, 1.4, 48.8	532.1, 1.4, 26.0	5.0
0.13P-La	528.8, 1.4, 9.5	530.2, 1.4, 6.7	531.1, 1.4, 61.5	532.4, 1.4, 22.3	7.2

Table 3 Curve-fitting results from XPS spectra for O1s of the catalysts*

Catalyst	E_b/eV, FWHM/eV, relative amount of oxygen species(%)				( $O 2 2 -$ +O^-)/O^2-
Catalyst	O^2-		O^-	$O 2 2 -$	C $O 3 2 -$
La₂O₃	528.7, 1.4, 12.4	530.2, 1.4, 12.8	531.1, 1.4, 48.8	532.1, 1.4, 26.0	5.0
0.13P-La	528.8, 1.4, 9.5	530.2, 1.4, 6.7	531.1, 1.4, 61.5	532.4, 1.4, 22.3	7.2

Fig.9 Durability of 0.13P-La and La2O3 catalysts in OCM^Reaction conditions: n(CH4)/n(O2)=3, GHSV=30000 mL·g-1·h-1, mcat=100 mg, T=650 ℃. ◇◆ SC2; ☆★ XCH4; □? YC2.

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