活性炭表面化学性质对负载Cu颗粒性质及催化甲醇氧化羰基化反应的影响

doi:10.7503/cjcu20150995

摘要/Abstract

摘要：

分别以在氮气和氨气气氛下热处理得到的活性炭(AC)为载体, 采用浸渍焙烧法制备了Cu/AC催化剂, 考察了AC表面化学性质对催化剂组成、 Cu物种颗粒尺寸以及催化甲醇氧化羰基化反应性能的影响. 对AC载体和相应Cu/AC催化剂的表征结果表明, AC在氮气中于600 ℃热处理后表面大部分羧基被消除, 有利于催化剂中Cu物种的分散和减小颗粒尺寸; 进一步升高温度至800 ℃, AC表面部分内酯、酸酐、酚类和醚类官能团被消除, 导致催化剂中Cu物种发生团聚和烧结. 随着表面含氧官能团数量的减少, 前驱体与载体之间的相互作用力减弱, 促进Cu²⁺还原为Cu⁺或Cu⁰. 而氨气气氛下热处理会导致AC表面更多含氧官能团被消除, 但同时引入了吡啶氮、吡咯氮和4价氮等含氮官能团, 更利于催化剂中Cu物种的分散和减小颗粒尺寸. 随着表面含氮官能团数量的增加, 前驱体与载体之间的相互作用力增强, 抑制了Cu²⁺的还原. 实验结果表明, 催化剂的活性随着Cu物种的颗粒尺寸而改变, 当以氨气气氛下于800 ℃热处理的AC为载体时, Cu物种的颗粒尺寸最小(6.8 nm), 催化性能最佳, 催化反应的甲醇转化率、碳酸二甲酯(DMC)的时空收率和选择性分别为9.6%, 278.7 mg·g^-1·h^-1和68.3%.

关键词: 表面化学性质, Cu/AC催化剂, 催化剂组成, 颗粒尺寸, 氧化羰基化

Abstract:

Activated carbon(AC) was thermal treated under nitrogen and ammonia atmosphere, respectively, and used as a support to prepare Cu/AC catalyst by impregnation-calcination method. The influence of AC surface chemistry on the catalyst composition and Cu species particle size as well as the catalytic performace for oxidative carbonylation of methanol has been investigated. AC supports and their corresponding Cu/AC catalysts were characterized intensively by BET, XPS, XRD, H₂-TPR and TEM. The results indicate that the removal of most carboxylic groups by thermal treatment at 600 ℃ under nitrogen atmosphere can improve the Cu species dispersion and reduce the Cu species particle size. However, with further elevating the temperature to 800 ℃, part of lactones, carboxylic anhydrides, phenol and ether groups were removed that lead to the Cu species agglomeration and sintering. Moreover, the decreased AC surface oxygen containing groups has weakened the precursor-support interaction and promoted the reduction of Cu²⁺ to Cu⁺ or Cu⁰. Thermal treatment of AC under ammonia atmosphere results in that more surface oxygen containing groups were removed compared with nitrogen atmosphere. But nitrogen containing groups including pyridinic N(N-6), pyrrole nitrogen(pyrrolic-N) and quaternary nitrogen(N-Q) were introduced at the same time, which was conducive to the Cu species dispersion and reduction of particle size. Besides, the increased surface nitrogen containing groups has enhanced the precursor-support interaction and supressed the reduction of Cu²⁺ to Cu⁺ or Cu⁰. The evaluation result reveals that the catalytic activity of the catalyst varies with the change of the particle size of Cu species. As Cu supported on AC thermal treated at 800 ℃ under ammonia atmosphere, the catalyst shows the optimum catalytic performance, which is ascribed to the smallest Cu species particle size(6.3 nm). The conversion of methanol, space-time yield and selectivity of dimethyl carbonate(DMC) are 9.6%, 278.7 mg·g^-1·h^-1and 68.3%, respectively.

Key words: Surface chemistry, Cu/AC catalyst, Catalyst composition, Particle size, Oxidative carbonylation

中图分类号:

O643

TrendMD:

张国强, 郑华艳, 郝志强, 李忠. 活性炭表面化学性质对负载Cu颗粒性质及催化甲醇氧化羰基化反应的影响. 高等学校化学学报, 2016, 37(7): 1380.

ZHANG Guoqiang, ZHENG Huayan, HAO Zhiqiang, LI Zhong. Effect of Activated Carbon Surface Chemistry on the Properties of Cu Particles and the Catalytic Performance for Oxidative Carbonylation of Methanol^†. Chem. J. Chinese Universities, 2016, 37(7): 1380.

图/表 11

Table 1 Textual property of the AC supports and Cu/AC catalysts

Sample	S_BET/(m²·g^-1)	V_P/(cm³·g^-1)	D_P/nm	S_micro/(m²·g^-1)
AC	1597	0.80	2.00	1514
AC-N₂-600	1436	0.71	1.98	1364
AC-N₂-800	1480	0.73	1.98	1407
AC-NH₃-600	1442	0.72	2.01	1376
AC-NH₃-800	1465	0.73	2.00	1389
Cu/AC	1275	0.65	1.98	1190
Cu/AC-N₂-600	1227	0.61	1.98	1163
Cu/AC-N₂-800	1256	0.64	2.00	1184
Cu/AC-NH₃-600	1284	0.64	2.00	1195
Cu/AC-NH₃-800	1293	0.67	1.98	1201

Fig.1 XPS spectra of O1s(A—E) and N1s(A'—E') of the AC supports(A, A') AC-NH3-800; (B, B') AC-NH3-600; (C, C') AC; (D, D') AC-N2-600; (E, E') AC-N2-800.

Table 2 Carbon, oxygen and nitrogen contents of the AC samples obtained by XPS

Sample	Elemental content(mass fraction, %)
Sample	C	O	N
AC	91.1	7.9
AC-N₂-600	94.5	5.5
AC-N₂-800	95.6	4.6
AC-NH₃-600	96.5	2.7	0.8
AC-NH₃-800	97.0	1.6	1.4

Fig.2 Cu2p32 XPS(A—E) and Cu LMM Auger spectra(A'—E') of the Cu/AC catalysts(A, A') AC-NH3-800; (B, B') AC-NH3-600; (C, C') AC; (D, D') AC-N2-600; (E, E') AC-N2-800.

Table 3 Cu2p32 XPS and CuLMM AES curve-fitting analysis of catalysts

Catalyst	E_b of C $u 2 p 32$ /eV		Kinetic energy of CuLMM/eV		Area of C $u a 2 p 32$ (%)		Area of CuLMM^b(%)		C $u + c$ (%)	C $u 0 d$ (%)
Catalyst	Cu²⁺	Cu⁺+Cu⁰	Cu⁺	Cu⁰			Cu²⁺	Cu⁺+Cu⁰	Cu⁺	Cu⁰
Cu/AC-NH₃-800	934.2	932.4	915.5	917.4	63.7	36.3	43.8	56.2	16.0	20.3
Cu/AC-NH₃-600	934.2	932.4	915.5	917.4	57.2	42.8	44.0	56.0	18.8	24.0
Cu/AC	934.2	932.4	915.5	917.2	53.9	46.1	44.4	55.6	20.5	25.6
Cu/AC-N₂-600	934.2	932.4	916.0	918.0	33.3	66.7	65.3	34.7	43.5	23.2
Cu/AC-N₂-800	934.2	932.4	916.0	918.2	21.5	78.5	40.6	59.4	31.8	46.7

Table 3 Cu2p32 XPS and CuLMM AES curve-fitting analysis of catalysts

Catalyst	E_b of C $u 2 p 32$ /eV		Kinetic energy of CuLMM/eV		Area of C $u a 2 p 32$ (%)		Area of CuLMM^b(%)		C $u + c$ (%)	C $u 0 d$ (%)
Catalyst	Cu²⁺	Cu⁺+Cu⁰	Cu⁺	Cu⁰			Cu²⁺	Cu⁺+Cu⁰	Cu⁺	Cu⁰
Cu/AC-NH₃-800	934.2	932.4	915.5	917.4	63.7	36.3	43.8	56.2	16.0	20.3
Cu/AC-NH₃-600	934.2	932.4	915.5	917.4	57.2	42.8	44.0	56.0	18.8	24.0
Cu/AC	934.2	932.4	915.5	917.2	53.9	46.1	44.4	55.6	20.5	25.6
Cu/AC-N₂-600	934.2	932.4	916.0	918.0	33.3	66.7	65.3	34.7	43.5	23.2
Cu/AC-N₂-800	934.2	932.4	916.0	918.2	21.5	78.5	40.6	59.4	31.8	46.7

Fig.3 XRD patterns of the Cu/AC catalystsa. Cu/AC-NH3-800; b. Cu/AC-NH3-600; c. Cu/AC;d. Cu/AC-N2-600; e. Cu/AC-N2-800.

Fig.4 Representative TEM images of the Cu/AC catalysts(A) Cu/AC; (B) Cu/AC-N2-600; (C) Cu/AC-N2-800; (D) Cu/AC-NH3-600; (E) Cu/AC-NH3-800.

Fig.5 Size distribution of the Cu/AC catalysts(A) Cu/AC; (B) Cu/AC-N2-600; (C) Cu/AC-NH3-600; (D) Cu/AC-NH3-800.

Fig.6 H2-TPR profiles of the Cu/AC catalysts(A) Cu/AC-NH3-800; (B) Cu/AC-NH3-600; (C) Cu/AC; (D) Cu/AC-N2-600; (E) Cu/AC-N2-800.

Fig.7 Space time yield(A) and selectivity(B) of the DMC vs. time-on-stream ▼ Cu/AC; ● Cu/AC-NH3-600; ? Cu/AC-NH3-800; ▲ Cu/AC-N2-600; ★ Cu/AC-N2-800.

Fig.8 Dependence of the space time yield of DMC on the Cu particle size

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