CuO/ZrO2 catalysts for the production of H2 through the water-gas shift reaction: Effect of calcination temperature of ZrO2
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摘要: 以经不同温度（120、250、350、450 ℃）焙烧处理的ZrO2为载体，采用沉积-沉淀法制备了系列CuO/ZrO2催化剂；考察了富氢气氛下催化剂的水煤气变换反应（WGS）催化性能。结果表明，CuO/ZrO2催化剂的催化活性随ZrO2载体焙烧温度的升高呈现先升高后降低的"火山型"变化趋势，在焙烧温度为250 ℃时取得最高值。采用X射线粉末衍射、N2物理吸附-脱附、N2O滴定、H2程序升温还原和CO程序升温还原及质谱跟踪等技术研究了系列ZrO2载体及CuO/ZrO2催化剂的结构和还原性能。结果表明，随着ZrO2焙烧温度的升高，一方面，CuO/ZrO2催化剂的Cu分散度逐渐降低，与ZrO2具有强相互作用的高分散活性Cu-[O]-Zr物种（"[]"表示ZrO2表面氧空位）逐渐减少；另一方面，Cu-[O]-Zr物种的还原能力逐渐增强，并诱导催化剂活性表面羟基的还原能力也相应增强（CO为还原剂），即降低了催化剂对WGS反应的起活温度。两方面的综合作用使得ZrO2载体焙烧温度为250 ℃（中等温度）时，CuO/ZrO2催化剂的WGS催化活性最高。
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关键词:
- CuO/ZrO2催化剂 /
- 水煤气变换反应 /
- ZrO2焙烧温度 /
- CO程序升温还原 /
- 表面羟基
Abstract: A series of CuO/ZrO2 catalysts were prepared by a deposition-precipitation method using ZrO2 calcined at various temperatures (120, 250, 350 and 450℃) as supports. The water-gas shift (WGS) reaction was carried out on these catalysts using H2 rich reactant gas (15% CO, 55% H2, 23% N2, 7% CO2). It was shown that the catalytic activity of the catalysts increased at first and then decreased with increasing calcination temperature of ZrO2. The catalyst supported on ZrO2 calcined at 250℃ showed the highest catalytic activity. The structure and reducibility of CuO/ZrO2 catalysts were studied by various techniques, such as XRD, N2-physisorption, N2O titration, H2-TPR and CO-TPR-MS. The results show that the Cu dispersion and the proportion of catalytically active Cu-[O]-Zr species ("[]" represents an oxygen vacancy on ZrO2 support) decrease with the increase of ZrO2 calcination temperature. The calcination of ZrO2 at higher temperature leads to an improvement of the reducibility of Cu-[O]-Zr species and hydroxyl groups on the CuO/ZrO2 catalysts, resulting in an easier onset of the surface WGS reaction between surface hydroxyl groups and CO reductant. The two factors reach a balance for the catalyst supported on ZrO2 calcined at 250℃ (moderate temperature), as is thought to be responsible for the highest WGS activity of this catalyst. -
图 1 经不同温度焙烧的ZrO2载体(a)及相应CuO/ZrO2催化剂(b)的XRD谱图
Figure 1 XRD patterns of ZrO2 calcined at different temperatures (a) and corresponding CuO/ZrO2 catalysts (b)
图 2 ZrO2焙烧温度与ZrO2载体晶粒粒径及催化剂Cu分散度间的关系
Figure 2 Dependence of the crystalline size of ZrO2 and Cu dispersion of the CuO/ZrO2 catalyst on the calcination temperature of the ZrO2 support
图 3 经不同温度焙烧的ZrO2及相应CuO/ZrO2催化剂的N2物理吸附-脱附曲线((a)、(c))和孔径分布((b)、(d))
Figure 3 N2 adsorption-desorption isotherms ((a), (c)) and pore size distribution ((b), (d)) of the ZrO2 supports calcined at different temperatures and corresponding CuO/ZrO2 catalysts
图 4 不同温度焙烧的ZrO2为载体的CuO/ZrO2催化剂的H2-TPR谱图
Figure 4 H2-TPR profiles of the CuO/ZrO2 catalysts prepared with the ZrO2 supports calcined at different temperatures
图 5 不同温度焙烧的ZrO2的H2O-TPD质谱谱图
Figure 5 H2O-TPD-MS profiles of the ZrO2 supports calcined at different temperatures
test conditions: carrier gas, He; m/z = 18; ZrO2 is not subjected to any pretreatments
图 6 不同温度焙烧的ZrO2为载体的CuO/ZrO2催化剂的CO-TPR-MS谱图
Figure 6 CO-TPR-MS profiles of the CuO/ZrO2
catalysts prepared with the ZrO2 supports calcined at different temperatures (CO2, m/z = 44; H2, m/z = 2)
图 7 不同温度焙烧的ZrO2为载体的CuO/ZrO2催化剂的WGS活性
Figure 7 WGS catalytic activity of the CuO/ZrO2 catalysts prepared with the ZrO2 supports calcined at different temperatures
reaction conditions: feed gas, 15%CO/55% H2/23% N2/7% CO2; space velocity, 4000 cm3/(g·h); molar ratio of steam to gas 0.4:1
表 1 经不同温度焙烧的ZrO2载体及相应CuO/ZrO2催化剂的结构性质
Table 1 Structural properties of the ZrO2 support calcined at different temperatures and corresponding CuO/ZrO2 catalysts
Sample Cu contenta
w/%dCub/% ACuc/% DZrO2d/nm ABET
/(m2·g-1)Pore volume
v/(cm3·g-1)Z-120 - - - 5.2 120 0.261 Z-250 - - - 5.7 124 0.282 Z-350 - - - 6.4 111 0.186 Z-450 - - - 9.2 59 0.147 C/Z-120 8.2 60.6 41.3 4.8 70 0.264 C/Z-250 8.2 57.8 39.4 5.6 80 0.270 C/Z-350 8.4 52.9 37.3 6.8 81 0.297 C/Z-450 8.3 30.2 20.8 9.7 53 0.254 a: measured by ICP-OES; b: Cu dispersion calculated from N2O titration results; c: Cu metal area calculated from N2O titration results; d: crystallite size of m-ZrO2 calculated by the Scherrer equation using the diffraction at 2θ=32.8° 
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表 2 不同温度焙烧的ZrO2为载体的CuO/ZrO2催化剂的还原性能
Table 2 Reducibility of the CuO/ZrO2 catalysts prepared withthe ZrO2 supports calcined at different temperatures
Sample H2-TPR peak temperature(℃) and
H2 consumption(μmol·g-1)aCO-TPR-MS peak
temperature t/℃Relative concentration
of active OHbpeak α peak β peak γ peak β peak δ C/Z-120 153(366) 170(595) 187(73) 208 287 0.84 C/Z-250 151(382) 164(479) 188(148) 163 254 1.00 C/Z-350 143(278) 153(290) 189(455) 159 233 0.96 C/Z-450 136(115) 144(146) 195(741) 157 230 0.58 a: values in parentheses are the H2 consumption calculated by fitting results; b: calculated from the H2 signal peak (m/z = 2) area in CO-TPR-MS profiles by assuming the relative concentration of active OH in C/Z-250 is 1.00
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