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化学选择性是评价催化剂性能最重要的参数之一,它直接决定了产物的经济价值及后续的分离成本.传统的负载型金属催化剂由于其金属粒径分布不均,且不同原子数组成的粒子通常具有特征产物选择性,从而限制化学选择性的提高;另一方面,对于金属多原子活性中心,反应物在催化剂表面可以存在多种吸附构型进而衍化为不同产物,产物可控性差.因此,获得金属尺寸均一,且具有原子分散的活性中心,即单原子催化剂,成为官能团多相催化转化高选择性的迫切需求.本课题组通过400 oC还原1%-Pd/ZnO得到PdZn金属间化合物,依据其规律排布的Pd-Zn-Pd单元获得Pd基单原子催化剂.该催化剂在乙烯化工中少量乙炔的加氢转化反应中获得令人欣喜的催化性能——兼具有乙炔的高转化率和乙烯的高选择性.结合微量吸附量热、理论计算等表征,Pd活性中心在PdZn金属间化合物中的特殊空间排布是其优异催化性能的根源,即乙炔以较强的σ键吸附在两个相邻的单Pd金属中心,易吸附活化加氢生成乙烯,而乙烯则吸附于单Pd金属中心,较弱的π键形式吸附有利于其脱附避免过渡加氢.基于前期研究,构筑具有均一单金属中心的负载型单原子催化剂是获得高选择性的另一有效方法,且较之于PdZn金属间化合物催化剂,该类单原子催化剂兼具有原子利用率最大化的优点.本文采用等体积浸渍法制备Pd/ZnO催化剂,通过降低Pd金属含量(1 wt%→0.1 wt%→0.01 wt%)并在较低的温度下(100 oC)还原(H2-TPR表明高温还原形成PdZn金属间化合物型合金)得到负载型单原子催化剂(Pd1/ZnO SAC).高分辨电镜结果表明,当Pd负载量由1%降至0.1%,金属纳米颗粒的粒径尺寸显著降低,而在0.01%-Pd/ZnO催化剂表面,Pd活性中心则以单原子状态分散于载体ZnO表面.X-射线吸收光谱及电子能谱表明,随着负载量的降低,Pd活性物种具有更高的正电性.该催化剂在乙炔选择性加氢反应中表现出更加优越的催化性能,具有与PdZn催化剂相当的高选择性,而更优的比活性.这归结于Pd1/ZnO单原子催化剂的Pdδ+单原子活性中心有助于其与乙炔的静电相互作用并吸附活化加氢生成乙烯,并促使乙烯以较弱的π键吸附,从而易于从催化剂表面脱附获得高选择性.
Chemical selectivity is one of the most important parameters to evaluate the catalyst performance, which directly determines the economic value of the product and the subsequent separation costs.Traditional supported metal catalysts due to uneven distribution of metal particle size, and particles of different atomic number Usually has the characteristic product selectivity, thus limiting the improvement of chemical selectivity; On the other hand, for the active sites of the metal polyatomic reactants there may exist a variety of adsorption configurations on the surface of the catalyst to derive different products with poor product controllability. , To obtain uniform size of the metal, and with the atomic dispersion of the active center, that is, monatomic catalyst, become an urgent need for high selectivity of heterogeneous catalytic conversion of functional groups.In this study, the PdZn intermetallic compound was obtained by reduction of 1% -Pd / ZnO at 400 oC , And Pd-Zn-Pd units were arranged according to their regular Pd-Zn-Pd units to obtain Pd-based monatomic catalysts.The catalyst obtained delightful catalytic performance in the small amount of acetylene hydroconversion in ethylene chemical industry - Rate and ethylene selectivity.Combined with trace amount calorimetry, theoretical calculations and other characteristics, Pd active center in PdZn intermetallic compounds special Among them, acetylene is adsorbed on two adjacent Pd metal centers by strong sigma bonds, and is easily adsorbed and activated to hydrogen to ethylene. Ethylene is adsorbed on the Pd metal center, Weak π bond form adsorption is conducive to its desorption to avoid transitional hydrogenation.Based on previous studies, to build a uniform single metal center supported monatomic catalyst is another effective method to obtain high selectivity, and compared to PdZn intermetallic Compound catalyst, which has the advantage of maximizing the atomic utilization rate.In this paper, the Pd / ZnO catalyst was prepared by the same volume impregnation method, by reducing the Pd metal content (1 wt% → 0.1 wt% → 0.01 wt%) and (Pd / ZnO SAC) was obtained at lower temperature (100 oC) (H2-TPR showed Pdn intermetallic compound was reduced by high temperature reduction) .High resolution electron microscopy results showed that when Pd loading was increased from 1% to 0.1%, the particle size of the metal nanoparticles decreased significantly, while on the surface of 0.01% -Pd / ZnO catalyst, Pd active sites were dispersed in the single-atom state on the surface of ZnO carrier.X-ray absorption spectra and electron energy The spectrum shows that as the loading decreases , The Pd active species has a higher positive charge.The catalyst showed more superior catalytic performance in the acetylene selective hydrogenation reaction with a rather high selectivity and better specific activity than the PdZn catalyst due to The Pdδ + monatomic active site of Pd1 / ZnO monatomic catalyst contributes to its electrostatic interaction with acetylene and its activation to hydrogenation to form ethylene, and promotes the adsorption of ethylene on the weak π bond, which makes it easy to obtain the desorption from the catalyst surface High selectivity.