The effect of doping transition metal oxides on copper manganese oxides for the catalytic oxidation of CO


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本文作者相关文章
	蔡丽娜

	胡臻皓

	Peter Brnton

	李文翠

Lina Cai^a, Zhenhao Hu^a, Peter Branton^b, Wencui Li^a

^a State Key Laboratory of Fine Chemicals, Department of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China;
^b Group Research and Development, British American Tobacco, Regents Park Road, Millbrook, Southampton SO15 8TL, UK

Article history: Received 2013-7-27;Accepted 2013-9-2;Published 2014-2-20
* Corresponding author. Tel/Fax: +86-411-84986355; E-mail: wencuili@dlut.edu.cn
Foundation Item: This work was supported by the Fudamental Resarch Funds for the Central Universities (DUT12ZD218) and the Specialized Research Fund for the Doctoral Program of Higher Education (20100041110017).

Abstract: A series of copper manganese oxides doped with transition metal oxides were prepared by co-precipitation using copper acetate and manganese acetate as precursors, ammonium bicarbonate as precipitant, and metal nitrates as dopants. The catalysts were characterized by N₂ adsorption-desorption, X-ray powder diffraction, temperature-programmed reduction, and in situ diffuse reflectance infrared Fourier transform spectroscopy. The results showed that doping transition metal oxides into copper manganese oxides can modify the CO adsorption ability of the catalyst and thus affect the catalytic oxidation of CO.

Key words: Copper manganese oxide Transition metal oxide Doping Carbon monoxide oxidation Diffuse reflectance infrared Fourier transform spectroscopy

1. Introduction

Catalytic oxidation of CO is one of the most effective methods of CO removal at low temperatures and has received considerable attention because of its many applications in industry and environmental fields. These include personal respiratory protective devices, CO₂ laser gas generation, proton exchange membrane fuel cells, and automobile emission controls ^{[1, 2, 3, 4, 5, 6]}. Compared with noble metal catalysts, non-noble metal oxide catalysts have the advantages of high availability and low cost. Among them, hopcalite based on manganese-copper mixed oxide, a well-known catalyst for CO oxidation, has attracted attention because of its low price and relatively high catalytic activity. However, it has poor low-temperature catalytic activity and moisture resistance ^{[7, 8, 9]}.

More recently, many attempts have been made to improve the catalytic activity of copper manganese oxides for CO oxidation, in particular by optimizing the preparation technologies and the improvement of preparation methods ^{[7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]}. Among these methods, doping a transition metal oxide into the copper manganese oxide catalysts could tune the oxygen mobility and the reduction ability of the catalysts, thus improving the catalytic activity. It was found that after doping a small amount of CeO₂ into copper manganese oxide, a highly dispersed CeO₂ phase could prevent sintering and aggregating of the catalyst. In addition, the reducibility was enhanced, the particle size was decreased, and the formation of the active sites for the oxidation of CO was improved significantly. Therefore, the activity of this rare earth-promoted catalyst was enhanced remarkably ^[7]. It was also reported that copper manganese oxide catalysts, prepared by co-precipitation, had improved O^2- availability in the lattice and an enhanced surface area. By adding low levels (~1.0 wt%) of Co, these materials can display much higher activity for CO oxidation compared with the current commercial copper manganese oxide catalysts at ambient conditions ^[8].

In our previous work ^[20], the combination effect of precipitant and precursor in the preparation has been studied. The precipitant shows the greatest influence on the crystalline phases of the catalyst while the precursor shows a greater effect on the number of catalytic active sites, both of which are directly related to the CO oxidation activity. In the present study, to further improve the catalytic activity of copper manganese oxide, we have prepared copper manganese oxide catalysts via co-precipitation by doping with transition metal oxides with (FeO_x or CeO₂) or without (ZnO) oxygen storage capacity. This study focuses on the influences of doping different transition metal oxide on CO adsorption and the resulting performance of the copper manganese oxide catalysts.

2. Experimental

2.1. Catalyst preparation

All chemicals used in this study were of analytical grade and used without further purification. The copper manganese oxide catalysts were prepared by co-precipitation using NH₄HCO₃ as the precipitant, the acetates of the copper and manganese as the precursors, and Fe(NO₃)₃, Ce(NO₃)₃, and Zn(NO₃)₂ as dopant. The typical procedure to synthesize the catalysts was as follows. A precipitant (30 mmol) was dissolved in deionized water (30 mL) with an initial pH value of ca. 8. The precursors (7.5 mmol) were mixed with deionized water (30 mL) with a 1/2 molar ratio of copper to manganese species. The mixed precursor solution was then added to the precipitant solution at 298 K with vigorous stirring. The resultant suspensions were aged for 30 min with continued vigorous stirring at 25 °C. Finally the precipitate was filtered, washed with deionized water and anhydrous alcohol, dried in air at 50 °C for 24 h, and then calcination at 300 °C for 2 h (denoted as CuMO_x-M, M = Fe, Zn, Ce) to obtain the final catalysts. The content of the doping transition metal oxide was fixed at 5 wt%. For comparison, the pure copper oxide and manganese oxide catalysts were prepared separately using the acetate as the precursor and NaOH as the precipitant, and keeping the other synthesis and after-treatment conditions the same as the copper manganese oxides. The corresponding samples were named CuO and MnO_x, respectively.

2.2. Catalyst characterization

X-ray diffraction patterns (XRD) were obtained with a Rigaku D/MAX-2400 diffractometer using Cu K_α radiation (40 kV, 100 mA, λ = 1.54056 Å). The textural characterizations of the samples were performed by nitrogen sorption at -196 °C using a Micromeritics Instrument Corporation Tristar 3000 device. Approximately 200 mg of the samples were heated to 200 °C under vacuum for 4 h to remove all adsorbed species. The surface area (S_BET) and pore size distribution were calculated using the BET method and BJH method, respectively. The total pore volume (V_total) was estimated from the amount adsorbed at a relative pressure of 0.99. The micropore volume was determined using the t-plot method. The morphologies of the catalysts were characterized with a FEI Quanta 450 instrument microscope equipped with a cooled energy-dispersive X-ray (EDX) spectrometer from Oxford Instruments for point-resolved elemental analysis. Hydrogen temperature programmed reduction (H₂-TPR) was performed by passing 8% H₂/Ar (50 mL/min) over a 20 mg sample (40-60 mesh size) at a heating rate of 10 °C /min to 900 °C. Before H₂-TPR, the samples were pretreated with Ar at 200 °C for 1 h. The system was then cooled to ambient temperature under Ar. The amount of hydrogen consumed (H₂ cons.) by each catalyst was calculated from the peak area of the H₂-TPR profile. In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) were recorded using a Nicolet 6700 FT-IR spectrometer at a resolution of 4 cm^-1 from 4000 to 640 cm^-1. Self-supporting disks were prepared from the sample powders and treated directly in the IR cell. The catalysts were connected to a vacuum-adsorption apparatus with a residual pressure below 10^-3Pa. Prior to CO adsorption (5 vol% CO and N₂ in balance), the catalysts were evacuated for 30 min at 200 °C. After flushing w ith pure He for 10 min, the CO spectrum was collected again.

2.3. Catalytic test

The activity of the copper manganese oxide catalysts for CO oxidation was measured in a quartz tubular fixed-bed flow reactor at atmospheric pressure using 200 mg of catalyst (40-60 mesh). The standard composition of the feed gas was 1% CO, 20% O₂, and 79% N₂ with a space velocity (SV) of 20000 mL/(h·g_cat). The temperature was ramped to the final temperature at a rate of 1 °C/min. The concentrations of CO were analyzed at the outlet of the reactor by a Techcomp GC 7890T gas chromatograph equipped with a thermal conductivity detector. Temperatures for 100% conversion of CO (T_100%) and 50% conversion of CO (T_50%) were used to evaluate the activity of the catalysts. Long-term stability test of the CuMnO_x-Fe sample was conducted under atmosphere 30 °C, and SV = 20000 mL/(h·g_cat).

3. Results and discussion

3.1. Structure analyses of copper manganese oxides

XRD analysis was used to determine the final phase of the copper manganese oxide catalysts doped with different metal oxides after heat treatment at 300 °C in static air for 2 h (Fig. 1). It can be seen that the main crystal phase composition of the catalysts are Mn₂O₃, CuO, and Cu_0.139Mn_0.861O₂ with lower crystallinity. The crystal phase composition of the catalysts did not change significantly by doping with transition metal oxides, indicating that this addition does not significantly alter the bulk composition of the catalyst. Moreover, the characteristic diffraction peaks of the doped transition metal or any derivative did not appear due to low doping contents (below 5 wt%) or highly dispersed or into the copper manganese oxide crystals lattice forming a solid solution ^[21].

Fig. 1. XRD patterns of the copper manganese oxide catalysts. (1) CuMnO_x; (2) CuMnO_x-Fe; (3) CuMnO_x-Zn; (4) CuMnO_x-Ce.

Figure 2 shows the N₂ adsorption-desorption isotherms and the corresponding pore size distributions of the copper manganese oxide catalysts doped with different transition metal oxides. All the samples showed the typical mesoporous structure, which illustrated IV type isotherms with a hysteresis loop at relative pressures (p/p₀) of 0.4-0.8. The pore sizes of CuMnO_x, CuMnO_x-Zn, and CuMnO_x-Ce catalysts were mainly concentrated at 4.5 nm, and CuMnO_x-Fe at 2.7 nm. The specific surface area and the total pore volume of CuMnO_x, CuMnO_x-Fe, and CuMnO_x-Zn were similar. But these of CuMnO_x-Ce were decreased slightly. These characterizations demonstrated that the specific surface area, pore structure, and phase structure of copper and manganese oxide catalysts were only slightly affected by doping with transition metal oxides.

Fig. 2. N₂ adsorption-desorption isotherms (a) and pore size distributions (b) of the copper manganese oxide catalysts. (1) CuMnO_x; (2) CuMnO_x-Fe; (3) CuMnO_x-Zn; (4) CuMnO_x-Ce. Curves (2), (3), and (4) in (a) offset vertically by 80, 160, and 240 cm³/g, STP, respectively.

Figure 3 shows the SEM images and corresponding EDX elemental mapping images of the CuMnO_x-Fe and CuMnO_x-Ce catalysts. The obtained catalysts showed a sphere morphology with diameters of 0.5-1.5 μm. The elemental mapping images showed that the doped catalysts contained the expected elements (Fe, Ce). The Fe and Ce species were evenly distributed in the obtained catalysts, revealing that the transition metal oxide could be doped into copper manganese oxide during the co-precipitation process.

Fig. 3. SEM images of the catalysts. (a) CuMnO_x-Fe; (b) CuMnO_x-Ce; (c) Mapped results of CuMnO_x-Fe; (d) Mapped results of CuMnO_x-Ce.

3.2. Catalytic performance of the copper manganese oxides for CO oxidation

Figure 4 displays the catalytic performance of the obtained catalysts for CO oxidation, and the corresponding activity is listed in Table 1. As shown in Fig. 4(a), the catalytic activity of all the catalysts increased as the reaction temperature and the activity towards CO oxidation strongly depends on the doped transition metal oxide, in the order CuMnO_x-Fe > CuMnO_x-Ce > CuMnO_x-Zn > CuMnO_x. The CuMnO_x catalyst without doping showed 44% of CO conversion at 30 °C and achieved complete conversion (T_100%) at 140 °C. Although there was no obvious improvement on the catalytic activity of CuMnO_x-Zn catalyst at 30 °C, the complete conversion temperature of CO shifted to 100 °C, exhibiting higher catalytic activity than that of CuMnO_x catalyst. The improvement in the catalytic activity of CuMnO_x-Ce sample was due to the ability of the included Ce to enhance the oxygen storage capacity and oxygen mobility in the catalysts ^[5]. The CO conversion over CuMnO_x-Fe catalyst at 30 °C was raised by 35% compared with the CuMnO_x catalyst, and this catalyst was able to achieve a complete conversion at 60 °C. This remarkable improvement in the catalytic activity could be attributed to the increase in defects, formed by doping with ferric oxide, which improves the adsorption of the reactants, CO and O₂ ^[8].

Fig. 4. (a) Catalytic activity of copper manganese oxides CuMnO_x (1), CuMnO_x-Fe (2), CuMnO_x-Zn (3), and CuMnO_x-Ce (4) for CO oxidation. (b) Long- term stability of CuMnO_x-Fe at 30 °C.

Table 1
Textural parameters and catalytic activity of the copper manganese oxide catalysts.

Furthermore, a long-term stability test (Fig. 4(b)) of a CuMnO_x-Fe sample was conducted under reaction atmosphere at 30 °C with a space rate of 20000 mL/(h·g_cat). The CO conversion gradually decreased with time and reached about 65% after 120 min.

3.3. Reduction properties of copper manganese oxides catalysts and doped catalysts

Figure 5 gives H₂-TPR profiles of fresh calcined samples. The obtained catalysts showed three asymmetric reduction peaks except for the CuMnO_x-Fe catalyst (four reduction peaks). The corresponding fitted H₂-TPR profiles were also shown in Fig. 5. The fitted data and the H₂ consumption data calculated from the integration of the corresponding peak areas are listed in Table 2. For CuMnO_x, the reduction peaks at 141.9 and 164.9 °C could be assigned to the step reductions of CuO → Cu₂O → Cu, which were significantly lower than the reported in the literature ^[22]. This illustrated that the reducibility of Cu species could be improved by the interaction between Cu and Mn species. Another two fitted peaks at higher temperature could be attributed to the step reduction of Mn₂O₃ → Mn₃O₄ → MnO ^[15].

Fig. 5. H₂-TPR profiles of the copper manganese oxide catalysts. (a) CuMnO_x; (b) CuMnO_x-Fe; (c) CuMnO_x-Zn; (d) CuMnO_x-Ce.

Table 2
H₂-TPR fitted results of the copper manganese oxide catalysts.

The TPR patterns of CuMnO_x-Zn and CuMnO_x-Ce were similar to CuMnO_x. However, there are four peaks over CuMnO_x-Fe. The fourth reduction peak could be assigned to the reduction of FeO_x species ^{[23, 24]}. Based on the integrated analysis of Figure 4 and Table 2, the reduction peak of the CuMnO_x-Zn catalyst shifted to lower temperature, demonstrating its enhanced reducibility. Moreover, the reduction peaks of CuMnO_x-Fe and CuMnO_x-Ce shifted to higher temperature and the hydrogen consumption amount decreased, confirming the decrease in the catalyst’s reducibility.

3.4. CO adsorption ability of the copper manganese oxide catalysts

To further demonstrate the impact of doping transition metal oxide on the copper manganese oxide catalysts, in situ DRIFTS of the obtained samples was performed to study the CO adsorption behavior on the catalysts’ surface. Pure CuO catalyst and MnO_x catalyst were also studied for comparison. As shown in Fig. 6, MnO_x showed three weak peaks, which can be assigned to CO weakly adsorbed on the Mn species, while CuO exhibited strong adsorption of CO centered at 2171, 2121, and 2055 cm^-1. The strong at 2121 cm^-1 band was attributed to the linear adsorption of CO on Cu⁺, which is the typical absorption mode on CuO catalysts ^{[25, 26]}. The weaker bands at 2055 and 2171 cm^-1 were assigned to the CO linearly adsorbed on Cu⁰ ^[27] and Cu²⁺ ^[28], respectively. The liner CO adsorption peak on copper manganese oxide catalysts is red-shifted to 2110 cm^-1. This can be attributed to the strong interaction between copper oxide and manganese oxide, which in turn weakens the C≡O bond. After doping with a transition metal oxide, the Cu⁺-CO adsorption peak of the catalysts does not change. This demonstrates that doping with transition metal oxide does not weaken the interaction between copper and manganese oxides. However, the intensity of the adsorption peak is enhanced, indicating that the number CO adsorption sites increases. The intensity of the vibrational absorption peak at 2110 cm^-1 was ordered CuMnO_x-Fe > CuMnO_x-Ce > CuMnO_x-Zn ≈ CuMnO_x. Combined with the H₂-TPR results, doping with Fe or Ce oxides could promote the interaction between the copper and manganese oxides, and this enhanced the Cu²⁺→Cu⁺ reduction. By increasing the Cu⁺ content, the CO adsorption capacity of the catalyst is increased significantly. The CO adsorption capacity of the zinc oxide catalyst was the same as the copper manganese catalyst, which is consistent with the catalytic activity data.

Fig. 6. DRIFTS spectra of CO adsorbed on the catalysts after 20 min at 25 °C. (1) MnO_x; (2) CuO; (3) CuMnO_x; (4) CuMnO_x-Zn; (5) CuMnO_x-Ce; (6) CuMnO_x-Fe.

Figure 7 shows the in situ DRIFTS spectra of catalysts adsorbed CO at 25 °C at different time intervals. The CuMnO_x-Fe catalysts show a strong adsorption of CO occurred at 2 min while the adsorption of CO was complete at 5 min; the other catalyst generally required about 8-10 min before reaching saturation. This implies that CuMnO_x-Fe can adsorb CO quickly during the reaction, thereby increasing the catalytic activity.

Fig. 7. DRIFTS spectra of the catalysts adsorbed CO at 25 °C at different time intervals. (a) CuMnO_x; (b) CuMnO_x-Fe; (c) CuMnO_x-Zn; (d) CuMnO_x-Ce.

4. Conclusions

Copper manganese oxide catalysts doped with Fe, Ce, and Zn oxides were prepared via co-precipitation. The addition of iron oxide and cerium oxide increase oxygen storage capacity of the copper manganese oxide catalysts, and can significantly improve CO adsorption capacity, and thus the performance of CO oxidation. Zinc oxide with no oxygen storage capacity, can increase the reduction performance of copper manganese oxide catalyst and improve the catalytic activity for CO oxidation. Catalytic tests showed that the CuMnO_x-Fe exhibited excellent catalytic performance with a 30%-40% enhancement in the CO conversion at 30 °C and total oxidation of CO can be achieved at 60 °C.

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过渡金属氧化物掺杂对铜锰氧化物催化CO氧化性能的影响

蔡丽娜^a, 胡臻皓^a, Peter Brnton^b, 李文翠^a

^a 大连理工大学化工学院精细化工国家重点实验室, 辽宁大连116024;
^b 英美联合烟草公司研发中心, 英国南安普顿SO15 8TL

收稿日期: 2013-7-27.接受日期: 2013-9-2.出版日期: 2014-2-20 .

基金来源：中央高校基本科研业务费专项资金（DUT12ZD218）；高等学校博士学科点专项科研基金（20100041110017）.

*通讯联系人. 电话/传真: (0411)84986355; 电子信箱: wencuili@dlut.edu.cn

摘要：以乙酸铜和乙酸锰为铜锰前驱体，以NH₄HCO₃为沉淀剂，相应金属硝酸盐为掺杂剂，采用共沉淀法制备了不同过渡金属氧化物掺杂的铜锰氧化物催化剂. 采用N₂物理吸附、X射线衍射，氢气-程序升温还原和原位红外漫反射光谱等方法对催化剂进行了表征，考察了系列催化剂上CO反应性能. 结果表明，掺杂过渡金属氧化物可以调变催化剂对CO的吸附能力，进而影响催化剂性能.

关键词：铜锰氧化物过渡金属氧化物掺杂一氧化碳氧化原位红外漫反射光谱

1. 前言

作为一种低温CO消除的主要方法之一, 催化氧化法广泛应用于呼吸防护装置的气体净化、潜艇以及航空航天器材等封闭体系中CO消除、CO₂激光器中气体的纯化、质子膜燃料电池气体纯化和汽车尾气控制等多个领域^{[1, 2, 3, 4, 5, 6]}. 与贵金属催化剂相比, 非贵金属催化剂具有储量丰富、廉价易得的优势. 其中, 铜锰氧化物(hopcalite), 作为一类价格低廉的过渡金属氧化物催化剂引起了广泛的关注, 如何提高铜锰氧化物的低温活性和抗水汽性能是拓宽其应用范围的关键^{[7, 8, 9]}.

目前, 提高铜锰氧化物催化活性的方法主要是合成工艺的优化和制备方法的改进^{[7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]}. 其中, 掺杂过渡金属氧化物可调变催化剂的供氧能力和氧化还原能力, 进而调变催化活性. 如将CeO₂高度分散于铜锰氧化物的表面, 可以减小催化剂的颗粒尺寸, 有效阻止铜锰氧化物的烧结, 提高催化剂的储放氧能力和晶格氧移动能力, 进而提高其催化活性^[7]. 在铜锰氧化物中掺杂约1.0 wt%的Co, 可以提高催化剂的比表面积和活性氧物种的数量, 进而提高它在常温常压条件催化CO氧化活性^[8].

我们前期研究了共沉淀过程中沉淀剂和前驱体的选择与匹配, 发现前驱体可以微弱改变催化剂的活性位数量, 而沉淀剂主要改变催化剂的晶相组成, 沉淀剂和前驱体匹配合适, 可获得高活性的铜锰氧化物催化剂^[20]. 在此基础上, 本文针对性选取具有储放氧能力的氧化铁、氧化铈和不具备储放氧能力的氧化锌对铜锰氧化物进行掺杂, 重点研究了不同过渡金属氧化物掺杂对铜锰氧化物催化剂CO吸附性能和催化性能的影响.

2. 实验部分

2.1. 催化剂的制备

本文中使用的药品均为分析纯, 使用前未经纯化. 以乙酸铜和乙酸锰为铜锰前驱体, 相应过渡金属(M)硝酸盐为掺杂剂, NH₄HCO₃为沉淀剂, 采用共沉淀法合成铜锰氧化物催化剂. 其中Cu/Mn比为1/2, 过渡金属氧化物的掺杂量为5 wt%. 按照相应的比例称取2.5 mmol乙酸铜、5 mmol乙酸锰和一定量金属硝酸盐(M = Fe, Ce, Zn), 加入到30 mL去离子水中, 磁力搅拌均匀. 将一定量的NH₄HCO₃ (M/NH₄HCO₃= 1/4)溶解到30 mL去离子水中, 初始pH约为8, 磁力搅拌均匀. 将上述盐溶液快速倒入NH₄HCO₃溶液中, 磁力搅拌反应0.5 h后, 沉淀产物经离心分离, 去离子水于50 °C洗涤后干燥24 h, 300 °C焙烧2 h后, 得到铜锰氧化物催化剂, 命名为CuMnO_x-M. 作为对比, 以醋酸盐为前驱体，氢氧化钠为沉淀剂, 采用沉淀法分别合成CuO催化剂和MnO_x催化剂, 并其它合成条件和后处理过程与铜锰氧化物催化剂保持一致.

2.2. 催化剂的表征

样品的比表面积、孔体积和孔径分布在麦克公司Tristar3000型物理吸附仪上测得, 测试前样品在200 °C真空脱气4 h. X射线衍射(XRD)在日本理学公司D/Max 2400型X射线衍射仪上测定, 扫描范围10°-80°, 扫描速率为10°/min, 步长为0.02°. 催化剂的形貌和元素分析由Hitachi S-4800型扫描电子显微镜(SEM)和其搭载的能量色散X射线光谱仪(EDX)完成. 程序升温还原(H₂-TPR)测试在麦克公司Auto Chem II 2920型化学吸附仪上进行, 采用热导检测器. 样品在测试之前, 先在200 °CAr气氛下吹扫2 h, 降至室温后在8% H₂/Ar气氛下程序升温到600 °C, 升温速率10 °C/min, 由峰面积计算得到耗氢量. 原位红外漫反射光谱测试(DRIFTS)在Nicolet 6700型红外光谱仪(MCT检测器)上进行, 采用KBr窗片, 样品测试前先在200 °C抽真空(< 10^-3 Pa)处理0.5 h, 降至25 °C后通入He吹扫2 min, 然后通入5% CO/N₂, 采集数据.

2.3. 催化剂的评价

催化剂的评价在常压固定床流动反应器中进行, 催化剂用量为200 mg (40-60目). 反应气氛为1% CO和20% O₂, 79% N₂为平衡气, 反应空速为20000 mL/(h·g_cat). 测试时保持程序升温速率为1 °C/min. 出口气体用天美气相色谱(GC7890T)分析, 检测器是热导池检测器(TCD). 采用完全转化温度T_100%和半转化温度T_50%来衡量催化剂的活性. CuMnO_x-Fe催化剂的稳定性测试温度为30 °C, 空速为20000 mL/(h·g_cat).

3. 结果与讨论

3.1. 铜锰氧化物的结构分析

图1为不同金属氧化物掺杂的铜锰氧化物催化剂的XRD谱, 由图可知, 共沉淀法制备的催化剂是由低结晶度的Mn₂O₃, CuO和Cu_0.139Mn_0.861O₂组成; 掺杂过渡金属氧化物后, 催化剂的晶相结构没有发生明显变化, 说明掺杂金属氧化物不会显著改变催化剂的体相组成, 且未出现掺杂过渡金属及其衍生物的特征衍射峰, 这可能是由于掺杂的过渡金属氧化物含量较低(约5 wt%), 且在铜锰氧化物中高度分散或进入到铜锰氧化物的晶格中形成了固溶体^[21].

图2为各掺杂铜锰氧化物催化剂的N₂物理吸附等温线和由吸附分支得到的相应孔径分布图. 由图可知, 在相对压力p/p₀为0.4-0.8处, 各样品吸附分支和脱附分支形成滞后环, 是IV型等温线的典型特征, 表明具有介孔特征. 另外, CuMnO_x的孔径分布主要集中在4.5 nm, 掺杂氧化铁后减小至2.7 nm, 而CuMnO_x-Zn和CuMnO_x-Ce孔径分布基本不变. 此外, 由表1可见, 与CuMnO_x相比, CuMnO_x-Fe和CuMnO_x-Zn的比表面积和总孔体积基本不变, 而CuMnO_x-Ce的略有降低. 结合XRD结果表明掺杂过渡金属氧化物对铜锰氧化物催化剂的比表面积、孔道结构和晶相结构影响较小.

图3是CuMnO_x-Fe和CuMnO_x-Ce催化剂的SEM照片和相应的面扫元素分布图. 由图可知, 所得催化剂为球形, 尺寸在0.5-1.5 μm. 由面扫元素分布图可以看出, 掺杂氧化铁和氧化铈的催化剂包含了预期的Fe和Ce, 且分布均匀, 说明共沉淀法可使Fe、Ce等元素均匀掺杂到铜锰氧化物催化剂中.

3.2. 铜锰氧化物催化剂上CO氧化反应性能

图4为铜锰氧化物催化剂上CO氧化反应活性图, 表1还列出了相应的活性数据. 由图可见, 催化剂活性均随着反应温度上升而提高, 各样品活性顺序为: CuMnO_x- Fe > CuMnO_x-Ce > CuMnO_x-Zn > CuMnO_x. CuMnO_xQ 97;催化剂的初始转化率只有44%, 完全转化温度(T_100%)为140 °C. 掺杂ZnO后, 催化剂的低温活性没有明显提高, 但T_100%降低至100 °C. 掺杂CeO₂后, 催化剂活性明显提高, 这是由于CeO₂可以增加催化剂的储放氧能力和氧移动能力^[5]. 其中, CuMnO_x-Fe催化剂表现出优异的催化CO氧化活性, 初始CO转化率比未掺杂的提高了35%, 60 °C即可实现CO完全转化, 可能与Fe进入铜锰氧化物晶体后形成更多缺陷有关, 因为缺陷有利于反应物种CO和O₂的吸附^[6]. 图4(b)为30 °C时CuMnO_x-Fe催化剂的稳定性测试. 可以看出,& #8197;CuMnO_x-Fe催化剂活性随时间延长而下降; 反应2 h后, CO转化率为65%.

3.3. 铜锰氧化物催化剂的还原性能

采用H₂-TPR表征了各催化剂的还原性能, 除CuMnO_x-Fe的TPR图出现4个峰外, 其余催化剂都出现3个不对称的还原峰, 对该谱图进行拟合结果见图5, 相应的拟合数据及其耗氢量列于表2. CuMnO_x催化剂的H₂-TPR谱可拟合为4个还原峰, 还原温度最低的两个峰可归属为CuO→Cu₂O, Cu₂O→Cu的还原, 峰值分别为141.9和164.9 °C, 比文献值大幅度降低, 说明铜锰复合后, 两者之间的相互作用增加了铜物种的还原能力^[22]. 还原温度较高的两个峰对应于Mn₂O₃→Mn₃O₄, Mn₃O₄→MnO的还原^[15].

CuMnO_x-Zn和CuMnO_x-Ce的TPR谱与CuMnO_x相似, 而CuMnO_x-Fe出现第四个还原峰可归属为FeO_x物种的还原^{[23, 24]}. 综合分析图4和表2可以看出, CuMnO_x-Zn催化剂的还原峰向低温方向移动, 表明掺杂ZnO后, 催化剂的还原能力提高. 而CuMnO_x-Fe和CuMnO_x-Ce中氧化铜和氧化锰物种的还原峰向高温方向移动, 同时各物种的耗氢量也有所降低, 表明掺杂Fe和Ce等金属氧化物后催化剂的还原性能下降.

3.4. 铜锰氧化物催化剂的CO吸附性能

为了进一步研究掺杂对铜锰氧化物催化剂的影响, 采用原位DRIFTS考察了CO在催化剂表面的吸附行为, 同时与CuO催化剂和MnO_x催化剂的CO吸附性能进行对比. 如图6所示, MnO_x只出现CO在Mn物种上的微弱吸附. CuO对CO的吸附作用较强, 分别在2171, 2121和2055 cm^-1出现吸附峰. 其中2121 cm^-1处较强的吸附峰可归属为Cu⁺-CO的线式吸附^{[25, 26]}, 这是CuO催化剂上CO的主要吸附形式; 2055和2171 cm^-1处强度较小的吸附峰可分别归属为Cu⁰-CO的线式吸附^[27]和Cu²⁺-CO的线式吸附^[28]. 与CuO催化剂相比, 未掺杂和掺杂后的CuMnO_x催化剂上Cu⁺-CO的线式吸附峰红移至2110 cm^-1, 这是由于氧化锰和氧化铜之间的强相互作用使Cu-C键键能增大, C≡O键能减小所致. 掺杂过渡金属氧化物后, Cu⁺-CO的线式吸附峰位置没有变化, 说明氧化铜和氧化锰间的相互作用并未减弱, 但吸收峰强度有所增加, 说明金属氧化物的掺杂使催化剂上CO吸附位数量增加. 掺杂后各催化剂在2110 cm^-1处吸收峰强弱顺序为CuMnO_x-Fe >CuMnO_x-Ce> CuMnO_x-Zn ≈ CuMnO_x. 结合H₂-TPR结果表明, 掺杂Fe和Ce两种金属氧化物有利于增强铜锰间相互作用, 有利于Cu²⁺→Cu⁺的还原, 增加了Cu⁺的比例, 使催化剂的CO吸附能力显著增加, 而掺杂ZnO的催化剂吸附CO能力变化不大, 这与催化活性数据基本一致.

图7给出了各催化剂在不同吸附时间时原位DRIFTS谱. 由图可见, 通入CO约0.5 min时, CuMnO_x-Fe催化剂上即可出现较强的CO吸附, 2 min时峰强度增加明显, 5 min后基本上实现CO的饱和吸附; 而其它催化剂一般需在8-10 min才可以达到饱和吸附. 可见, CuMnO_x-Fe催化剂达到吸附饱和所需时间最短, 在反应过程中可以实现CO的快速捕获, 进而提高反应活性.

4. 结论

采用共沉淀法合成了氧化铁、氧化铈和氧化锌掺杂的铜锰氧化物催化剂. 结果表明, 掺杂不具备储放氧能力的ZnO可以提高铜锰氧化物的还原性能, 进而提高催化剂的CO催化活性, 而掺杂具有储氧能力的氧化铁和氧化铈可以显著提高铜锰氧化物的CO吸附活性中心数量和CO吸附能力, 进而提高其催化CO氧化活性, 可使30 °C时CO转化率提高30%-40%, 完全转化温度降至60 °C.