Synthesis of Cu-Al spinels and its non-isothermal formation kinetics analysis
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摘要: 以氢氧化铜和拟薄水铝石为原料，通过固相法合成了Cu-Al尖晶石，研究了合成温度、合成时间、Cu/Al物质的量比对尖晶石的生成、晶相组成和还原性能的影响。结果表明，配位缺陷的Cu-Al表面尖晶石在400℃已经生成，难还原尖晶石Cu2+和易还原尖晶石Cu2+分别在700和800℃已生成，尖晶石含量随合成温度升高而不断增多，生成Cu/Al物质的量比不等的富Al尖晶石固溶体，至1200℃生成计量比尖晶石CuAl2O4，因此，尖晶石还原性能随合成温度显著变化。Al适度过量时（Cu/Al（molar ratio）=1：3），在950℃生成难还原Cu2+物种含量较高（约为25.9%）的尖晶石固溶体；Cu过量时在1200℃生成CuAlO2，两者都比计量尖晶石CuAl2O4难还原。另外，延长合成时间也能促进尖晶石生成。非等温动力学分析表明，Cu-Al尖晶石的生成随温度表现出三个动力学区域，即700-850、850-950和950-1200℃，表观活化能分别为85.2、304.4和38.1 kJ/mol。当温度低于950℃时生成的产物层较薄，反应物通过产物层的扩散可认为是一维扩散；超过950℃后产物层变厚，反应物接近于三维发射扩散。Abstract: Cu-Al spinels were synthesized by a solid phase method using Cu(OH)2 and pseudo-boehmite as the raw materials. The effects of synthesis temperature, synthesis time and Cu/Al molar ratio on the formation and properties of Cu-Al spinels were fully investigated by the thermogravimetry(TG/DTG), X-ray diffraction(XRD), H2 temperature programmed reduction(H2-TPR). The non-isothermal kinetics of Cu-Al spinel formation process were analyzed using Coats-Redfern method and two diffusion-controlled kinetic models. Characterization results showed that the Cu-Al surface spinels with unsaturated coordination formed easily at the temperature as low as 400℃, and the content of these surface spinel decreased sharply with the synthesis temperature rising. The hardly-reducible spinel Cu2+ species and easily-reducible spinel Cu2+ species were identified at the synthesis temperature of 700 and 800℃, respectively. The spinel content increased gradually with the synthesis temperature increasing, leading to the formation of Al-rich spinel solid solutions with different Cu/Al molar ratios. At a higher temperature of 1200℃, however, the formation of stoichiometric CuAl2O4 spinel was observed. Hence, the spinel reducibility varied dramatically with the synthesis temperature as illustrated by the drastic change of the molar ratio of hardly-reducible spinel Cu2+ species and easily-reducible spinel Cu2+ species. An appropriate excess of Al3+(Cu/Al=1:3) could result in the formation of spinel solid solution with more hardly-reducible spinel Cu2+ species, while an excess of Cu2+ would lead to the formation of delafossite-type CuAlO2. Both samples owned low reducibility as compared to the stochiometric CuAl2O4 spinel. Besides, a longer synthesis time would favor the spinel formation as well but to a limited extent. Non-isothermal kinetics analysis showed that the formation process of Cu-Al spinel owned three kinetic regions in terms of synthesis temperature, namely 700-850, 850-950 and 950-1200℃, and the apparent activation energies were determined to be 85.2, 304.4 and 38.1 kJ/mol, respectively. The diffusion of reactants via product layer could be considered as an one-dimensional diffusion below 950℃, whereas it was more likely to be a three-dimensional diffusion above 950℃, indicating that the product layer became much thicker.
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Key words:
- Cu-Al spinel /
- solid phase method /
- synthetic conditions /
- non-isothermal kinetics
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图 1 不同样品的TG-DTG曲线
(a): precursor CA2; (b): copper hydroxide; (c): pseudo-boehmite
Figure 1 TG-DTG curves of the samples
图 2 前驱体CA2及其焙烧样品CA2-t的XRD谱图
Figure 2 XRD patterns of the precursor CA2 and the calcined samples CA2-t
图 3 CA2-t的(400)晶面放大图
(a): t=400-1200 ℃; (b): t=400-700 ℃
Figure 3 Enlarged diffraction lines of (400) plane of CA2-t
图 4 CA2-t的H2-TPR谱图
(a): t=400-1200 ℃; (b): t=400-850 ℃
Figure 4 H2-TPR profiles of the synthesized samples CA2-t
图 5 Cu/Al物质的量比不等的尖晶石XRD谱图和H2-TPR谱图
(a): XRD patterns of CAx-950; (b): H2-TPR profiles of CAx-950;(c): XRD patterns of CAx-1200; (d): H2-TPR profiles of CAx-1200
Figure 5 XRD patterns and H2-TPR profiles of Cu-Al spinels with different Cu/Al molar ratios
图 6 CA3-t(t′)的物化性质随合成时间的变化
(a): cell parameters of Cu-Al spinel; (b): spinel content
Figure 6 Profiles of physicochemical properties of CA3-t(t′) versus synthetic time
图 7 合成尖晶石CA3-t(t′)的XRD谱图
(a): CA3-900(t′); (b): CA3-950(t′); (c): CA3-1000(t′)
Figure 7 XRD patterns of the synthesized spinels CA3-t(t′)
图 9 Cu-Al尖晶石非等温生成动力学模拟
Figure 9 Simulation of non-isothermal kinetics of Cu-Al spinel formation process
表 1 CA2-t Cu-Al尖晶石催化剂的物化性质
Table 1 Physicochemical properties of the synthesized Cu-Al spinel catalysts CA2-t
CA2-700 CA2-800 CA2-850 CA2-900 CA2-950 CA2-1000 CA2-1100 CA2-1200 dCuO /nm[a] 20.7 23.5 26.1 26.3 30.6 33.8 35.4 - dspinel /nm[a] -[e] - 6.9 9.8 16.9 22.8 31.8 34.9 a /nm[b] - - 0.8007 0.8044 0.8062 0.8065 0.8071 0.8078 I(440)/I(311) - - 0.33 0.26 0.20 0.21 0.19 0.20 x[c] 0.2866 0.2563 0.2285 0.1351 0.0613 0.0428 0.0196 0.0010 Xspinel /%[d] 10.9 18.4 25.6 52.4 76.8 83.6 92.4 99.6 [a]: the crystallite sizes of CuO and Cu-Al spinel; [b]: the cell parameters of Cu-Al spinel; [c]: x value in Cu1-3xV xAl2+2xO4(V: vacancy); [d]: the molar ratio of spinel Cu to total Cu; [e]: the data cannot be obtained by XRD characterization results 
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表 2 CAx-tCu-Al尖晶石催化剂的物化性质
Table 2 Physicochemical properties of the synthesized Cu-Al spinel catalysts of CAx-t
CA-950 CA2-950 CA3-950 CA9-950 CA-1200 CA2-1200 CA3-1200 CA9-1200 a/nm[a] 0.8061 0.8063 0.8053 0.7980 - 0.8077 0.8076 0.8073 dspinel /nm[b] 17.5 16.9 12.7 6.0 - 34.9 34.5 30.9 XCuO /%[c] 59.4 13.4 5.2 0 5.89 0.0 0.0 0.0 XCu-O-Al /%[c] 4.4 9.8 13.4 44.7 - 0.4 0.2 1.5 Xspinel /%[c] 36.2 76.8 81.3 55.3 - 99.6 99.8 98.5 (Cu/Al)spinel[d] 0.362 0.384 0.271 0.061 - 0.498 0.499[e] 0.493[e] [a]: the cell parameters of Cu-Al spinel; [b]: the crystallite sizes of CuO and Cu-Al spinel; [c]: the molar ratio of different Cu2+ to total Cu; [d]: the Cu/Al molar ratio of Cu-Al spinel phase; [e]: the value was obtained by the assumption that all excessive Al3+ ions were participated out from spinel solid solution in the form of α-Al2O3, as illustrated in CA3-1200[11]
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表 3 Cu-Al尖晶石的非等温合成动力学拟合
Table 3 Simulation results of Cu-Al spinel non-isothermal formation kinetics
t/℃ Ⅰ(700-850 ℃) Ⅱ(850-950 ℃) Ⅲ(950-1200 ℃) Model Jander G-B Jander G-B Jander G-B R2 0.986 0.986 0.991 0.986 0.969 0.992 E/(kJ·mol-1) 85.2 88.3 304.4 277.9 68.4 38.1 A/min-1 1.71 2.49 1.19×1011 5.98×109 1.98 4.34×10-2
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