无表面活性剂条件下一锅法制备金属/氧化锌复合材料用于催化二氧化碳加氢制甲醇反应（英文）
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摘要
由于温室效应的危害和人类对可再生能源的需求,二氧化碳加氢还原制甲醇成为非常重要的一个化学反应。最近几年的研究中,过渡金属/氧化锌纳米复合材料作为催化剂被广泛应用于该反应,这是因为过渡金属/氧化锌纳米复合材料具有优秀的协同功能以及独特的光电子和催化性能。因此,发展该复合材料的尺寸可控制备方法显得很有价值。虽然使用物理方法可以大批量制备催化材料,但却难以实现金属和载体间的强相互作用力。因此,研究者们更多地倾向于使用化学方法来制备多组分催化剂材料。然而,为了获取活性金属相,该催化剂通常需要氢气还原步骤;同时,还需要表面活性剂来控制纳米粒子的尺寸,这就使得大多数复合纳米材料的合成需要很多步骤,从而导致金属/氧化锌纳米复合材料催化性能的不稳定性。因此,我们发明了一种在回流乙二醇中一锅法合成金属(钯,金,银,铜)/氧化锌纳米复合材料的制备方法,该制备方法不需要任何表面活性剂。在该方法的制备过程中,钯和氧化锌可以通过减少各自的表面能从而在之后的聚集中互相稳定彼此来实现粒子的尺寸控制。此外,碳酸氢钠可以通过调整碱性度来控制钯纳米粒子的尺寸。而乙二醇作为一种温和的还原剂可以将钯离子还原成钯纳米粒子,同时还可以作为该制备过程的溶剂。在制备过程中,钯粒子通过热还原而成核和聚集,氧化锌纳米粒子则通过醋酸锌的热分解而形成。本文中,我们通过X射线衍射来分析制备的复合纳米材料的相态,结果显示,没有杂相。我们使用透射电镜来研究材料的形貌和结构特征。此外,X射线光电子能谱分析被用来确认金属/氧化锌复合材料的成分组成,结果显示钯和氧化锌之间有金属和载体间的强相互作用力。为了确定复合材料的真实元素组成,我们对材料进行了电感耦合等离子体质谱分析,并且发现理论值和实验值相吻合。为了研究钯锌投料比和碳酸氢钠对钯粒子尺寸的影响,我们通过X射线衍射结果计算出不同钯锌投料比和碳酸氢钠反应量下钯粒子的尺寸,并进行比较分析,之后利用透射电镜图进行进一步直观验证。众所周知,Cu/ZnO/Al_2O_3纳米复合材料是二氧化碳加氢制甲醇的优良催化剂,本文中研究的其他金属/氧化锌复合材料也可以很好地催化该反应。所以,为了进一步研究所制备的不同金属/氧化锌复合材料,我们将其作为催化剂,研究了它们对二氧化碳加氢制甲醇的催化作用;结果显示,当钯锌投料比为1:9,反应条件为240oC,5 MPa时,其催化效果最好,二氧化碳转化率为30%,甲醇选择性为69%。其出色的催化表现可能是以下两个因素,其一是因为钯是氢气解离为活泼氢原子的良好催化剂;其二是因为钯和氧化锌之间的强的金属和载体间相互作用力可以使得氧化锌表面形成表面氧空穴。此外,我们发现大部分金属/氧化锌复合物都表现出很高的甲醇选择性,尤其是金/氧化锌催化剂,它的甲醇选择性达到了82%,只是二氧化碳转化率较低。最后,希望本文可以提供一种制备金属/氧化锌的简便易行的方法,且该方法可以为金属/氧化锌用于催化时提供干净的催化表面。
Catalytic hydrogenation of CO_2 to methanol is an important chemical process owing to its contribution in alleviating the impacts of the greenhouse effect and in realizing the requirement for renewable energy sources. Owing to their excellent synergic functionalities and unique optoelectronic as well as catalytic properties, transition metal/ZnO(M/ZnO) nanocomposites have been widely used as catalysts for this reaction in recent years. Development ofsize-controlled synthesis of metal/oxide complexes is highly desirable. Further, because it is extremely difficult to achieve the strong-metal-support-interaction(SMSI) effect when the M/ZnO nanocomposites are prepared via physical methods, the use of chemical methods is more favorable for the fabrication of multi-component catalysts. However, because of the requirement for an extra H2 reduction step to obtain the active metallic phase(M) and surfactants to control the size of nanoparticles, most M/ZnO nanocomposites undergo two-or multi-step synthesis, which is disadvantageous for the stable catalytic performance of the M/ZnO nanocomposites. In this work, we demonstrate facile one-pot synthesis of M/ZnO(M = Pd, Au, Ag, and Cu) nanocomposites in refluxed ethylene glycol as a solvent, without using any surfactants. During the synthesis process, Pd and ZnO species can stabilize each other from further aggregation by reducing their individual surface energies, thereby achieving size control of particles. Besides, NaHCO_3 serves as a size-control tool for Pd nanoparticles by adjusting the alkaline conditions. Ethylene glycol serves as a mild reducing agent and solvent owing to its capacity to reduce Pd ions to generate Pd crystals. The nucleation and growth of Pd particles are achieved by thermal reduction, while the ZnO nanocrystals are formed by thermal decomposition of Zn(OAc)_2. X-ray diffraction patterns of the M/ZnO and ZnO were analyzed to study the phase of the nanocomposites, and the results show that no impurity phase was detected. Transmission electron microscopy(TEM) was used to study the morphology and structural properties. In addition, X-ray photoelectron spectroscopy analysis was performed to further confirm the formation of M/ZnO hybrid materials, and the results confirm SMSI between Pd and ZnO. Inductively coupled plasma mass spectrometry was used to check the actual elemental compositions, and the results show that the detected atomic ratios of Pd/Zn were consistent with the values in the theoretical recipe. To investigate the effects of the Pd/Zn molar ratios and the added amount of Na HCO3 on Pd size, the average sizes of Pd particles were calculated, and the results were confirmed by TEM observation. The Cu/ZnO/Al_2O_3 composite is a widely known catalyst for hydrogenation of CO_2 to methanol, and other M/ZnO composites are also catalytic for this reaction. Therefore, different M/ZnO hybrids were further studied as catalysts for hydrogenation of CO_2 to methanol, among which Pd/ZnO(1 : 9) demonstrated the best performance(30% CO_2 conversion, 69% methanol selectivity, and 421.9 gmethanol·(kg catalyst·h)-1 at 240 °C and 5 MPa. The outstanding catalytic performance may be explained by the following two factors: first, Pd is a good catalyst for the dissociation of H_2 to give active H atoms, and second, SMSI between Pd and ZnO favors the formation of surface oxygen vacancies on ZnO. Moreover, most M/ZnO composites exhibit excellent performance in methanol selectivity, especially the Au/ZnO catalyst, which has the highest methanol selectivity(82%) despite having the lowest CO_2 conversion. Hopefully, this work would provide a simple route for synthesis of M/ZnO nanocomposites with clean surfaces for catalysis.
Catalytic hydrogenation of CO_2 to methanol is an important chemical process owing to its contribution in alleviating the impacts of the greenhouse effect and in realizing the requirement for renewable energy sources. Owing to their excellent synergic functionalities and unique optoelectronic as well as catalytic properties, transition metal/ZnO(M/ZnO) nanocomposites have been widely used as catalysts for this reaction in recent years. Development ofsize-controlled synthesis of metal/oxide complexes is highly desirable. Further, because it is extremely difficult to achieve the strong-metal-support-interaction(SMSI) effect when the M/ZnO nanocomposites are prepared via physical methods, the use of chemical methods is more favorable for the fabrication of multi-component catalysts. However, because of the requirement for an extra H2 reduction step to obtain the active metallic phase(M) and surfactants to control the size of nanoparticles, most M/ZnO nanocomposites undergo two-or multi-step synthesis, which is disadvantageous for the stable catalytic performance of the M/ZnO nanocomposites. In this work, we demonstrate facile one-pot synthesis of M/ZnO(M = Pd, Au, Ag, and Cu) nanocomposites in refluxed ethylene glycol as a solvent, without using any surfactants. During the synthesis process, Pd and ZnO species can stabilize each other from further aggregation by reducing their individual surface energies, thereby achieving size control of particles. Besides, NaHCO_3 serves as a size-control tool for Pd nanoparticles by adjusting the alkaline conditions. Ethylene glycol serves as a mild reducing agent and solvent owing to its capacity to reduce Pd ions to generate Pd crystals. The nucleation and growth of Pd particles are achieved by thermal reduction, while the ZnO nanocrystals are formed by thermal decomposition of Zn(OAc)_2. X-ray diffraction patterns of the M/ZnO and ZnO were analyzed to study the phase of the nanocomposites, and the results show that no impurity phase was detected. Transmission electron microscopy(TEM) was used to study the morphology and structural properties. In addition, X-ray photoelectron spectroscopy analysis was performed to further confirm the formation of M/ZnO hybrid materials, and the results confirm SMSI between Pd and ZnO. Inductively coupled plasma mass spectrometry was used to check the actual elemental compositions, and the results show that the detected atomic ratios of Pd/Zn were consistent with the values in the theoretical recipe. To investigate the effects of the Pd/Zn molar ratios and the added amount of Na HCO3 on Pd size, the average sizes of Pd particles were calculated, and the results were confirmed by TEM observation. The Cu/ZnO/Al_2O_3 composite is a widely known catalyst for hydrogenation of CO_2 to methanol, and other M/ZnO composites are also catalytic for this reaction. Therefore, different M/ZnO hybrids were further studied as catalysts for hydrogenation of CO_2 to methanol, among which Pd/ZnO(1 : 9) demonstrated the best performance(30% CO_2 conversion, 69% methanol selectivity, and 421.9 gmethanol·(kg catalyst·h)-1 at 240 °C and 5 MPa. The outstanding catalytic performance may be explained by the following two factors: first, Pd is a good catalyst for the dissociation of H_2 to give active H atoms, and second, SMSI between Pd and ZnO favors the formation of surface oxygen vacancies on ZnO. Moreover, most M/ZnO composites exhibit excellent performance in methanol selectivity, especially the Au/ZnO catalyst, which has the highest methanol selectivity(82%) despite having the lowest CO_2 conversion. Hopefully, this work would provide a simple route for synthesis of M/ZnO nanocomposites with clean surfaces for catalysis.
引文
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