Ca, Sr co-Doped Ceria and Its Application in Oxidative Coupling of Methane†

doi:10.7503/cjcu20160467

Abstract

Abstract:

The Ca and Sr co-doped ceria catalysts were synthesized with a citric acid method and their performances in oxidative coupling of methane(OCM) were significantly enhanced when compared with the single or none doped ceria catalysts. By X-ray diffraction(XRD), CO₂ temperature programmed desorption(CO₂-TPD), O₂ temperature programmed desorption(O₂-TPD), X-ray photoelectron spectroscopy(XPS) characterizations, we found that the Ca and Sr co-doped catalysts were consisted of the phases CeO₂ and SrCO₃, along with Ca highly dispersed or doped in ceria. They exhibited the decreased size of ceria particles and the medium basicity. The number of electrophilic oxygen species and the ratio of electrophilic oxygen species to lattice oxygen species on the catalyst surfaces were changed in the order of CeSrCa>CeSr>CeO₂, corresponding to the C2 selectivity in OCM. In addition, the presence of SrCO₃was beneficial to improve on catalytic performances of OCM over Ce-based oxide catalysts.

Key words: Alkali earth, Dope, Ceria, Carbonate, Oxidative coupling of methane

TrendMD:

YANG Yanling, DONG Lingyu, XIA Wensheng, WAN Huilin. Ca, Sr co-Doped Ceria and Its Application in Oxidative Coupling of Methane^†[J]. Chem. J. Chinese Universities, 2016, 37(12): 2206.

Figures/Tables 12

Fig.1 Catalytic performances of Ce0.5Sr0.4M0.1Ox for OCM reation■ Ce0.5Sr0.5Ox; ▲ Ce0.5Sr0.4Mg0.1Ox; ● Ce0.5Sr0.4Ca0.1Ox; ▼ Ce0.5Sr0.4Ba0.1Ox.

Fig.2 Catalytic performances of Ce0.5Sr0.5-mCamOx(m=0—0.25) for OCM reation■ Ce0.5Sr0.5Ox; ▲ Ce0.5Sr0.45Ca0.05Ox; ● Ce0.5Sr0.4Ca0.1Ox; ★ Ce0.5Sr0.35Ca0.15Ox;▼ Ce0.5Sr0.3Ca0.2Ox; ◆ Ce0.5Sr0.25Ca0.25Ox.

Fig.3 Life test of the catalysts Ce0.5Sr0.5Ox(A) and Ce0.5Sr0.4Ca0.1Ox(B) in the oxidative coupling of methane a. Sel(C2); b. conv(CH4); c. Y(C2).

Table 1 Comparisons of OCM performances over some Ce-based and relevant catalysts at selected temperatures

Catalyst	T/℃	Conv(CH₄)(%)	Conv(O₂)(%)	Sel(CO_x)(%)	Sel(C2)(%)	Y(C2)(%)
Blank	750	1.2	2.3	59.0	41.0	0.5
	800	2.3	4.5	61.8	38.3	0.9
CeO₂	450	2.0	5.8	100.0	0	0
	500	17.9	100.0	98.0	2.0	0.4
	550	18.0	100.0	97.8	2.2	0.4
	700	19.1	100.0	95.1	4.9	0.9
Ce_0.5Sr_0.5O_x	500	1.1	2.9	100.0	0	0
	550	24.9	93.9	65.1	34.9	8.7
	650	28.6	100.0	52.8	47.2	13.5
	700	29.4	100.0	50.6	49.4	14.5
Ce_0.5Ca_0.5O_x	500	1.8	5.3	100.0	0	0
	550	24.3	98.0	63.7	36.3	8.8
	650	25.1	100.0	60.8	39.2	9.8
	700	24.4	100.0	64.0	35.9	8.8
Ce_0.5Sr_0.4Ca_0.1O_x	500	1.2	3.2	100.0	0	0
	550	28.1	97.0	54.8	45.2	12.7
	650	30.2	100.0	49.5	50.5	15.2
	700	30.4	100.0	48.5	51.5	15.7
Ce_0.5Sr_0.5O_x(MM)^*	550	0	0
(CeO₂+SrO)	700	1.6	4.1	85.4	14.6	0.2
	750	3.5	8.1	73.6	36.5	1.3
	800	9.3	19.9	52.0	48.0	4.5
Ce_0.5Sr_0.5O_x(MM)^*	550	19.1	98.5	92.8	7.2	1.4
(CeO₂+SrCO₃)	700	26.0	100.0	57.4	42.6	11.1
	750	27.5	100.0	52.4	47.6	13.1
	800	28.0	100.0	51.2	48.8	13.6
CaO	700	2.4	6.3	88.4	11.6	0.3
	750	6.2	14.9	65.7	34.3	2.1
	800	24.7	57.9	58.1	41.9	10.4
SrO	700	0.9	1.9	66.3	33.7	0.3
	750	2.3	4.0	40.4	59.7	1.4
	800	8.9	14.0	41.3	58.8	5.3
CaCO₃	700	18.1	53.4	66.4	33.6	6.1
	750	26.9	85.2	57.4	42.6	11.5
	800	28.7	98.8	58.0	42.0	12.1
SrCO₃	700	1.5	3.0	66.0	34.0	0.5
	750	3.1	6.3	67.8	32.2	1.0
	800	9.0	17.4	54.5	45.5	4.1

Table 2 Specific surface areas(SBET), porous volumes(Vp), porous sizes(Dp), CeO2 crystalline sizes(D) and lattice parameters(a) of the catalysts

Catalyst	S_BET/(m²·g^-1)	V_p/(cm³·g^-1)	D_p/nm	D/nm	a/nm
CeO₂	12.0	0.075	20.4	96.6	0.05394
Ce_0.5Sr_0.5O_x	3.0	0.018	21.5	48.3	0.05440
Ce_0.5Sr_0.4Ca_0.1O_x	5.5	0.028	18.8	18.7	0.05438

Fig.4 XRD patterns of the catalystsa. CeO2; b. Ce0.5Sr0.5Ox; c. Ce0.5Sr0.4Ca0.1Ox; d. CaCO3, PDF No. 99-0022; e. CeO2, PDF No. 01-081-0792;f. SrCO3, PDF No. 01-084-1778.

Fig.5 UV-Vis spectra of the catalystsa. CeO2; b. Ce0.5Sr0.5Ox; c. Ce0.5Sr0.4Ca0.1Ox.

Fig.6 O2-TPD profiles of the catalystsa. CeO2; b. Ce0.5Sr0.5Ox; c. Ce0.5Sr0.4Ca0.1Ox.

Fig.7 CO2-TPD profiles of the catalystsa. CeO2; b. Ce0.5Sr0.5Ox;c. Ce0.5Sr0.4Ca0.1Ox.

Fig.8 XPS Spectra for Ce3d of the catalystsa. CeO2; b. Ce0.5Sr0.5Ox;c. Ce0.5Sr0.4Ca0.1Ox.

Fig.9 XPS spectra for O1s of the catalysts(A) CeO2; (B) Ce0.5Sr0.5Ox; (C) Ce0.5Sr0.4Ca0.1Ox.

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

Catalyst	E_b/eV(FWHM^*)					Amount of oxygen species(%)				n( $O 2 2 -$ + $O 2 -$ )/ n(O^2-)
Catalyst	O^2-	$O 2 2 -$	$O 2 -$		C $O 3 2 -$		O^2-	$O 2 2 -$	$O 2 -$	C $O 3 2 -$
CeO₂	528.6(1.2)	529.7(1.2)	530.9(1.2)	532.1(1.2)		72.8	11.0	11.7	4.3	0.3
Ce_0.5Sr_0.5O_x	528.4(1.7)	530.3(1.7)		531.3(1.7)		23.6	65.5		10.9	2.8
Ce_0.5Sr_0.4Ca_0.1O_x	528.2(1.6)	530.5(1.6)		532.0(1.6)		22.7	66.1		11.2	3.0

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

Catalyst	E_b/eV(FWHM^*)					Amount of oxygen species(%)				n( $O 2 2 -$ + $O 2 -$ )/ n(O^2-)
Catalyst	O^2-	$O 2 2 -$	$O 2 -$		C $O 3 2 -$		O^2-	$O 2 2 -$	$O 2 -$	C $O 3 2 -$
CeO₂	528.6(1.2)	529.7(1.2)	530.9(1.2)	532.1(1.2)		72.8	11.0	11.7	4.3	0.3
Ce_0.5Sr_0.5O_x	528.4(1.7)	530.3(1.7)		531.3(1.7)		23.6	65.5		10.9	2.8
Ce_0.5Sr_0.4Ca_0.1O_x	528.2(1.6)	530.5(1.6)		532.0(1.6)		22.7	66.1		11.2	3.0

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