Effect of TiO2 crystalline phase composition on the physicochemical and catalytic properties of Pd/TiO(2) in selective acetylene hydrogenation

Pd/TiO(2) catalysts have been prepared using TiO(2) supports consisting of various rutile/anatase crystalline phase compositions. Increasing percentages of rutile phase in the TiO(2) resulted in a decrease in Brunauer-Emmett-Teller surface areas, fewer Ti(3+) sites, and lower Pd dispersion. While acetylene conversions were found to be merely dependent on Pd dispersion, ethylene selectivity appeared to be strongly affected by the presence of Ti(3+) in the TiO(2) samples. When TiO(2) samples with 0-44% rutile were used, high ethylene selectivities (58-93%) were obtained whereas ethylene losses occurred for those supported on TiO(2) with 85% or 100% rutile phase. X-ray photoelectron spectroscopy and electron spin resonance experiments revealed that a significant amount of Ti(3+) existed in the TiO(2) samples composed of 0-44% rutile. The presence of Ti(3+) in contact with Pd can probably lower the adsorption strength of ethylene resulting in an ethylene gain. Among the five catalysts used in this study, the results for Pd/TiO(2)-R44 suggest an optimum anatase/rutile composition of the TiO(2) used to obtain high selectivity of ethylene in selective acetylene hydrogenation.     

密度泛函理论计算研究表面应力对钯催化乙炔选择性加氢活性与选择性的影响（英文）

在石油催化裂解过程中,除了生成乙烯、丙烯及丁烯等烯烃,还会产生部分炔烃.目前工业上通常采用炔烃选择性加氢转化为对应的单烯烃,以除去其中炔烃.由于产品烯烃中的炔烃等杂质含量需极低,这就对用于加氢催化剂的活性和选择性提出了很高的要求,即催化剂需要选择性吸附炔烃并加氢,而不损失其中的烯烃.经过前期大量的基础研究工作,目前工业中炔烃选择性加氢应用最广泛的催化剂是负载型钯基催化剂.然而,单独的钯金属选择性并不理想,因而对其选择性以及活性进行调控成为了当前关注的研究课题.本文采用密度泛函理论计算结合微观反应动力学模拟手段,研究了钯金属表面应力存在条件下的活性与选择性,以及形成次表层物种的可能性和形成后的活性与选择性.研究发现,改变钯金属的晶格参数与表面应力,反应物、表面反应中间体和产物的吸附能都会产生相应的变化,且吸附能与晶格参数的变化存在线性关系,晶格参数越大,吸附越强.利用表面反应过渡态能量与初始态能量之间的线性关系,相应的乙炔加氢生成乙烯的反应速率可以通过微观反应动力学模拟得到.结果显示,不同晶格参数的钯催化剂催化乙炔加氢生成乙烯的反应活性位于相应火山型曲线的强吸附侧,即减弱乙炔和氢的吸附强度可提高乙烯的生成速率.在此基础上,本文研究了不同表面应力的钯催化剂在次表面吸附不同覆盖度碳原子和氢原子的情况,发现晶格参数越大越有利于碳原子和氢原子在次表面的吸附.同时,研究发现在次表面碳掺杂的条件下,不同表面应力条件下的钯催化剂的活性均有所增强.此外,由于乙烯在所有研究的钯催化剂表面脱附比进一步加氢容易,因而乙烯都可以选择性生成.     

负载型Pd/TiO2和Pd-Ag/TiO2催化剂的乙炔选择性加氢催化性能

利用程序升温还原(TPR)、X-射线衍射(XRD)、CO吸附-红外光谱(CO-IR)、电子顺磁共振(EPR)和微型催化反应评价等手段, 研究了负载Pd/γ-Al2O3, Pd/TiO2和Pd-Ag/TiO2催化剂的结构和乙炔选择性加氢催化性能. 结果表明, Pd/TiO2催化剂具有较Pd/γ-Al2O3催化剂更优良的乙炔选择性加氢催化性能, 这与Pd-TiO2之间的强相互作用密切相关. Pd-TiO2之间的强相互作用不仅使负载型钯金属催化剂具有较高的乙炔加氢催化选择性, 而且具有较高的乙炔加氢催化活性. Pd/TiO2催化剂中添加Ag 组分后, Pd金属可促进Ag+的还原并可能形成Pd-Ag合金, 催化剂的乙烯选择性虽有所增加, 但乙炔转化率和乙烯收率下降.     

Bimetallic Ru-Ni/TiO_2 catalysts for hydrogenation of N-ethylcarbazole:Role of TiO_2 crystal structure

Hydrogenation of N-ethylcarbazole(NEC), the hydrogen lean form of a liquid organic hydrogen carrier,on TiO_2 supported Ru-Ni bimetallic catalysts is investigated. Crystal structure of TiO_2 plays a critical role on the hydrogenation activity and selectivity towards fully hydrogenated product. Ru/anatase catalyst exhibits higher selectivity but lower reactivity compared to Ru/rutile catalyst. Ni addition significantly promotes the performance of Ru/anatase catalyst while causes severe performance deterioration of Ru/rutile catalyst. Commercial P25, a mixture of anatase and rutile phases in approximate ratio A/R~1/4,is found to be the best TiO_2 support for NEC hydrogenation. Ru/P25 catalyst outperforms both Ru/rutile and Ru/anatase and its activity can be further slightly improved by Ni addition. The unexpected synergism between the two different TiO_2 phases for Ru based NEC hydrogenation catalysts is related to metal-support interaction and Ru-Ni interaction.     

Molecule Saturation Boosts Acetylene Semihydrogenation Activity and Selectivity on a Core-Shell Ruthenium@Palladium Catalyst

Increasing selectivity without the expense of activity is desired but challenging in heterogeneous catalysis. By revealing the molecule saturation and adsorption sensitivity on overlayer thickness, strain, and coordination of Pd-based catalysts from first-principles calculations, we designed a stable Pd monolayer (ML) catalyst on a Ru terrace to boost both activity and selectivity of acetylene semihydrogenation. The least saturated molecule is most sensitive to the change in catalyst electronic and geometric properties. By simultaneously compressing the Pd ML and exposing the high coordination sites, the adsorption of more saturated ethylene is considerably weakened to facilitate the desorption for high selectivity. The even stronger weakening to the least saturated acetylene drives its hydrogenation such that it is more exothermic, thereby boosting the activity. Tailoring the molecule saturation and its sensitivity to structure and composition provides a tool for rational design of efficient catalysts.     

IB金属对Ni/SiO2催化剂乙炔选择性加氢反应性能的影响

乙烯是合成聚乙烯的原料,其主要来源是石油裂解气,其中少量的乙炔杂质会严重毒化生产聚乙烯的催化剂,因此需要将其去除.对于乙炔选择加氢反应,传统工业上使用的是Pd基催化剂,尽管其乙炔转化率很高,但对乙烯的选择性很低.我们前期的研究发现,IB族金属(Au,Ag和Cu)与Pd形成的合金单原子催化剂可以有效地提高乙烯的选择性.作为与Pd同组的非贵金属,Ni催化剂在多种催化加氢反应中显示出优异活性,而在乙炔选择加氢反应中,Ni是否能够替代贵金属Pd尚无定论.本文系统地研究了IB金属对Ni/SiO2催化剂乙炔选择性加氢性能的影响.与Pd/SiO2催化剂不同,单金属Ni/SiO2催化剂在低温下不具有活性.将IB金属添加到Ni/SiO2催化剂中,可以显著提高其催化活性以及对乙烯的选择性.其中,AuNix/SiO2和CuNix/SiO2催化剂的催化活性随还原温度升高而提高,而AgNix/SiO2催化剂对预处理温度不敏感.通过调变IB/Ni原子比和还原温度优化了催化剂的催化性能,发现优化后的三种催化剂(CuNi0.125/SiO2、AgNi0.5/SiO2和AuNi0.5/SiO2)的活性和选择性随反应温度升高表现出相似的变化趋势.催化稳定性考察结果显示,CuNi0.125/SiO2催化剂表现出最高选择性和稳定性;尽管AuNi0.5/SiO2的初始活性最高,但是稳定性最低.采用XRD、TPR和微量吸附量热等表征手段对不同IB金属对Ni基催化剂性质的影响进行了系统考察.以Cu-Nix/SiO2催化剂为例,H2-TPR测试结果表明,Cu-Ni双金属纳米颗粒的形成使得还原温度低于相应的单金属催化剂,表明铜和镍之间存在明显的相互作用.此外,通过TPR获得的CuNix/SiO2催化剂上的氢气消耗量与理论耗氢量相吻合,表明在还原处理的过程中双金属催化剂中的CuO和NiO可以被完全还原.乙炔的微量吸附量热结果表明,在CuNi0.125/SiO2,AgNi0.5/SiO2,AuNi0.5/SiO2和Ni0.5/SiO2催化剂上的初始吸附热分别为187,196,304和103 kJ/mol,即它们的初始乙炔吸附强度顺序为AuNi0.5/SiO2>AgNi0.5/SiO2>CuNi0.125/SiO2>Ni0.5/SiO2.该结果与三者的初始催化活性顺序一致,表明IB金属的加入可以增强乙炔在催化剂表面的吸附,从而提高催化活性.     

Catalysts Based on Fiberglass Supports: V. Absorption and Catalytic Properties of Palladium Catalysts Based on a Leached Silica-Fiberglass Support in the Selective Hydrogenation of an Ethylene–Acetylene Mixture

The absorption and catalytic properties of palladium catalysts (0.01% Pd) based on leached soda–silica fiberglass supports were studied in the selective hydrogenation of acetylene as the constituent of an ethylene–acetylene mixture. It was found that fiberglass catalysts exhibited much higher selectivity than traditional supported Pd catalysts. It was suggested that the high selectivity in the reaction of acetylene hydrogenation resulted from the selective absorption (diffusion) of acetylene in the bulk of fiberglass, where Pd microparticles are localized.     

Pd/ZnO在乙炔选择加氢反应中的不同催化机制研究(英文)

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.     

Atomically Dispersed Pd Atoms on a Simple MgO Support with an Ultralow Loading for Selective Hydrogenation of Acetylene to Ethylene

We report a simple and efficient Pd/MgO catalyst loaded with ppm level of Pd (7.8 ppm) for semi-hydrogenation of acetylene to ethylene. The catalyst showed excellent performance with high acetylene conversion (97%), high ethylene selectivity (89%) and good stability. Moreover, the atomically dispersed Pd atoms are inactive for ethylene hydrogenation. Isotopic and FTIR results suggest that H₂ dissociates at isolated Pd atoms in a heterolytic manner forming O−H bond, which may account for the high selectivity.     

Kinetics of toluene hydrogenation over platinum-titania catalysts in conditions of strong metal-support interaction

Toluene hydrogenation was investigated over different platinum-titania catalysts. It was found that the form of kinetic equation of the reaction does not depend on the modifications of titania (rutile, anatase) but the catalyst activity is different. The catalyst reduction at temperature higher than 300^oC leads to strong metal-support interaction (SMSI) which is manifested in a decrease of the reaction rate.     

Titania-supported iridium catalysts for dehydrogenative synthesis of benzimidazoles

In this review,development of supported catalysts for the dehydrogenative synthesis of benzimidazoles from primary alcohols and 1,2-phenyIenediamine derivatives is briefly summarized.Among them,titania-supported iridium catalysts showed excellent activities under mild reaction conditions.Remarkably,the low-temperature activity of iridium catalyst was significantly affected by titania supports,and the reaction of 1,2-phenylenediamine and benzyl alcohol in the presence of rutilesupported catalysts proceeded smoothly at 100℃to give 2-phenylbenzimidazole in high yields of up to 88%,On the other hand,catalysts supported on anatase generally showed poor activity at 100℃.A significant relationship between CO uptake and the activity of titania-supported catalysts has been reported,indicating that well-reduced iridium species on rutile would be responsible for the predominant catalytic activity.The present results suggest the importance of the selection of suitable titania supports for the iridium catalysts.     

Formation of C4 Oligomers in Hydrogenation of Acetylene over Pd/Al2O3 and Pd/TiO2 Catalysts

Selectivity of product formation has been tested in hydrogenation of acetylene over 0.3 wt.% Pd/-alumina and 0.5 wt.% Pd/TiO₂catalysts. Non-steady-state regime of catalyst operation was tested in pulse-flow experiments. Significant carbon poisoning appears to be a necessaryrequisite for selective formation of ethylene. The effect of hydrogen and acetylene partial pressure has been tested on the selectivity of C₄products. At 273–298 K the catalysts showed 26–35% selectivity for C₄ hydrocarbons and <2.5% for ethane production at conversionsof 30–40%. Deuterium distribution in ethylene and 1,3-butadiene and the deuterium content of the surface hydrogen pool have been compared and mechanismof diene formation has been discussed.     

Hydroxyl improving the activity,selectivity and stability of supported Ni single atoms for selective semi-hydrogenation

Atomically dispersed metal catalysts with high atomic utilization and selectivity have been widely studied for acetylene semi-hydrogenation in excess ethylene among others. Further improvements of activity and selectivity, in addition to stability and loading, remain elusive due to competitive adsorption and desorption between reactants and products, hydrogen activation, partial hydrogenation etc. on limited site available. Herein, comprehensive density functional theory calculations have been used to explore the new strategy by introducing an appropriate ligand to stabilize the active single atom, improving the activity and selectivity on oxide supports. We find that the hydroxyl group can stabilize Ni single atoms significantly by forming Ni₁(OH)₂ complexes on anatase TiO₂(101), whose unique electronic and geometric properties enable high performance in acetylene semi-hydrogenation. Specifically, Ni₁(OH)₂/TiO₂(101) shows favorable acetylene adsorption and promotes the heterolytic dissociation of H₂ achieving high catalytic activity, and it simultaneously weakens the ethylene bonding to facilitate subsequent desorption showing high ethylene selectivity. Hydroxyl stabilization of single metal atoms on oxide supports and promotion of the catalytic activity are sensitive to transition metal and the oxide supports. Compared to Co, Rh, Ir, Pd, Pt, Cu, Ag and Au, and anatase ZrO₂, IrO₂ and NbO₂ surfaces, the optimum interactions between Ni, O and Ti and resulted high activity, selectivity and stability make Ni₁(OH)₂/TiO₂(101) a promising catalyst in acetylene hydrogenation. Our work provides valuable guidelines for utilization of ligands in the rational design of stable and efficient atomically dispersed catalysts.

Hydroxyl group can stabilize significantly Ni single atom by forming Ni₁(OH)₂ complexes on anatase TiO₂(101), which displays high catalytic performance in acetylene semi-hydrogenation.     

Properties of Pd–Ag/C catalysts in the reaction of selective hydrogenation of acetylene

Samples of Pd/C and Pd–Ag/C, where C represents carbon nanofibers (CNFs), are synthesized by methane decomposition on a Ni–Cu–Fe/Al₂O₃ catalyst. The properties of Pd/CNF are studied in the reaction of selective hydrogenation of acetylene into ethylene. It is found that the activity of the catalyst in hydrogenation reaction increases, while selectivity decreases considerably when the palladium content rises. The obtained dependences are caused by the features of palladium’s interaction with the carbon support. At a low Pd content (up to 0.04 wt %) in the catalyst, the metal is inserted into the interlayer space of graphite and the catalytic activity is zero. It is established by EXAFS that the main share of palladium in catalysts of 0.05–0.1 wt % Pd/CNF constitutes the metal in the atomically dispersed state. The coordination environment of palladium atoms consists of carbon atoms. An increase in the palladium content in a Pd/CNF catalyst up to 0.3 wt % leads to the formation of highly dispersed (0.8–1 nm) Pd particles. The Pd/CNF samples where palladium is mainly in the atomically dispersed state exhibit the highest selectivity in the acetylene hydrogenation reaction. The addition of silver to a 0.1 wt % Pd/CNF catalyst initially probably leads to the formation of Pd–Ag clusters and then to alloyed Pd–Ag particles. An increase in the silver content in the catalyst above 0.3% causes the enlargement of the alloyed particles and the palladium atoms are blocked by a silver layer, which considerably decreases the catalytic activity in the selective hydrogenation of acetylene.     

Preparation and comparison of supported gold nanocatalysts on anatase, brookite, rutile, and P25 polymorphs of TiO2 for catalytic oxidation of CO

Nanosized anatase (< or = 10 nm), rutile (< or = 10 nm), and brookite (approximately 70 nm) titania particles have been successfully synthesized via sonication and hydrothermal methods. Gold was deposited with high dispersion onto the surfaces of anatase, rutile, brookite, and commercial titania (P25) supports through a deposition-precipitation (D-P) process. All catalysts were exposed to an identical sequence of treatment and measurements of catalytic CO oxidation activity. The as-synthesized catalysts have high activity with concomitant Au reduction upon exposure to the reactant stream. Mild reduction at 423 K produces comparably high activity catalysts for every support. Deactivation of the four catalysts was observed following a sequence of treatments at temperatures up to 573 K. The brookite-supported gold catalyst sustains the highest catalytic activity after all treatments. XRD and TEM results indicate that the gold particles supported on brookite are smaller than those on the other supports following the reaction and pretreatment sequences.     

Enhanced selectivity in acetylene hydrogenation by ligand modified Pd/TiO2 catalysts

Modification of Pd/TiO(2) catalyst by adsorption of triphenylphosphine and phenyl sulfide leads to markedly enhanced selectivity for acetylene hydrogenation in the presence of ethylene and excess hydrogen. Similar selectivities were maintained in cases where ligand decomposition took place and sulfur was retained on the catalyst surface.     

Study of the Influence Exerted by Zinc Additive on the Structure and Catalytic Properties of Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts for Liquid-Phase Hydrogenation of Acetylene

Effect of the phase composition of aluminum oxide [γ- and (δ + θ) phase] and introduction of zinc additives on the catalytic properties of 0.5% Pd/Al₂O₃ systems in the reaction of liquid-phase hydrogenation of acetylene into ethylene under an elevated pressure in a flow-through mode was studied. An increase in the activity of the Pd catalyst upon modification with zinc is only observed in the case of a system supported by the mixed phase of (δ + θ) aluminum oxide. XAFS spectroscopy was used to find that the increase in the activity and selectivity with respect to ethylene (in the presence of carbon monoxide) on the (0.5% Pd–0.62% Zn)/(δ + θ)-Al2O3 catalyst is correlated with the formation of the PdZn intermetallic compound.     

锐钛矿型TiO2担载的Pd催化剂用于乙炔选择加氢的催化性能及其表征

Anatase TiO₂ nanospindles containing 89% exposed {101} facets (TiO₂-101) and nanosheets with 77% exposed {001} facets (TiO₂-001) were hydrothermally synthesized and used as supports for Pd catalysts. The effects of the TiO₂ materials on the catalytic performance of Pd/TiO₂-101 and Pd/TiO₂-001 catalysts were investigated in the selective hydrogenation of acetylene to polymer-grade ethylene. The Pd/TiO₂-101 catalyst exhibited enhanced performance in terms of acetylene conversion and ethylene yield. To understand these effects, the catalysts were characterized by H₂ temperature-programmed desorption (H₂-TPD), H₂ temperatureprogrammed reduction (H₂-TPR), transmission electron microscopy (TEM), pulse CO chemisorption, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The TEM and CO chemisorption results confirmed that Pd nanoparticles (NPs) on the TiO₂-101 support had a smaller average particle size (1.53 nm) and a higher dispersion (15.95%) than those on the TiO₂-001 support (average particle size of 4.36 nm and dispersion of 9.06%). The smaller particle size and higher dispersion of Pd on the Pd/TiO₂-101 catalyst provided more reaction active sites, which contributed to the improved catalytic activity of this supported catalyst.     

Synthesis and characterization of Sibunit-supported Pd–Ga,Pd–Zn,and Pd–Ag catalysts for liquid-phase acetylene hydrogenation

Pd/Sibunit and Pd–M/Sibunit (M = Ga, Zn, or Ag) catalysts have been synthesized, and their catalytic properties in liquid-phase acetylene hydrogenation have been investigated. Doping of the palladium catalyst with a metal M leads to the formation of the Pd₂Ga, PdZn, or Pd_0.46Ag_0.54 bimetallic compound. The bimetallic particles are much smaller (1.6–2.0 nm) than the monometallic palladium particles (4.0 nm). Doping with zinc raises the ethylene selectivity by 25% without affecting the activity of the catalyst. Specific features of the effect of each of the dopants on palladium are reported.     

Catalytic Performance of Carbon Materials Supported Pd Nanoparticles in Selective Hydrogenation of Acetylene

Pd/C catalysts were prepared by deposited Pd nanoparticles (NPs) on different carbon supports including activated carbon (AC), graphite oxide (GO), and reduced graphite oxide (rGO) using sol-immobilization method. Through transmission electron microscopy, powder X-ray di raction, and X-ray photoelectron spectroscopy, the role of the carbon supports for the catalytic performances of Pd/C catalysts was examined in selective hydrogenation of acetylene. The results indicate that Pd/AC exhibited higher activity and selectivity than Pd/GO and Pd/rGO in the gas phase selective hydrogenation of acetylene. Thermal and chemical treatment of AC supports also have some effect on the catalytic performance of Pd/AC catalysts. The differences in the activity and selectivity of various Pd/C catalysts were partly attributed to the metal-support interaction.