金单原子催化剂上一氧化碳低温氧化

CO低温氧化对于基础研究和实际应用均具有重要意义.自上世纪八十年代日本的 Haruta教授发现氧化物负载金催化剂对 CO氧化的超高活性以来,负载金催化剂受到了广泛关注与深入研究,被认为是目前活性最高的 CO氧化催化剂.在诸多影响 CO氧化活性的因素中,纳米金的粒子尺寸是最重要因素之一.目前主流观点认为对于 CO氧化,纳米金有一个最优尺寸范围,在0.5–5 nm,而 Au原子/离子(Au3+, Au+)的活性则低一到两个数量级.因此,一般认为负载金单原子催
　　化剂对于 CO氧化反应的活性要比金纳米粒子和团簇低很多.然而,最近几年的理论与实验研究均表明,金单原子负载于合适的载体上可以显示出与金纳米粒子和团簇相当的活性.本文对这些新进展进行综述,阐述金单原子催化剂对 CO氧化的独特反应性能. Gates教授研究组进行了大量关于正价金对 CO氧化影响的研究,其中包括孤立的金原子(Au+).他们的研究发现, CO氧化活性随价态降低而降低,表明正价金对 CO氧化至关重要.此外,他们的研究也表明,孤立金原子对 CO氧化的活性(TOF)比金纳米粒子低一到两个数量级.然而,在他们的研究中,有几个因素可能导致催化剂的低活性.首先,他们一般采用非或弱还原性的载体.而载体的还原性对金催化剂上 CO氧化活性影响非常巨大.另外,他们所用的金原子前驱体为配合物,在催化剂制备与反应过程中配体并没有去除,可能也是导致催化剂活性低的原因之一.与此相反,张涛课题组近期采用氯金酸为前驱体,通过简单的吸附浸渍法制备了一系列负载金单原子催化剂.同时用相同的载体制备了负载金纳米粒子催化剂进行对比,可以排除载体等其它影响因素.对比结果显示,单原子催化剂均显示出与纳米粒子相当的 TOF(单位表面 Au原子)和更高的反应速率(单位重量金).首先制备了氧化铁负载金单原子催化剂,该催化剂在室温即展现出可观活性, TOF值与2–3 nm金粒子 TOF值相当(~0.5 s–1).更有趣也更重要的是,该催化剂在高温(200oC以上)展现出非常高的反应稳定性,在200oC反应100 h无失活.在300和400oC反应50 h也无失活发生,为开发高温稳定的金催化剂提供了新途径.其次制备了氧化钴负载金单原子催化剂,该催化剂以0.05%金负载量即可实现室温全转化,其 TOF值高达1.4 s–1.然而该催化剂在达到高活性之前必须首先在反应气氛中进行高温处理,这限制了其实用性.此外,催化剂需经反应气氛活化的原因尚待进一步研究.随之又制备了氧化铈负载金单原子催化剂,对富氢条件下 CO选择氧化不仅具有高活性,而且具有极高的 CO选择性.进一步研究结合理论计算表明,高选择性来自氧化铈负载的金单原子不能解离活化氢,对于氢气氧化活性极低,从而导致 CO氧化的高选择性.理论研究方面也有进展. Camellone等计算发现金原子可以取代 CeO2(111)面上的 Ce原子形成 Au+并促进 CO氧化.然而该金原子会扩散至氧空位形成带负电荷的 Auδ-,阻止 CO和 O2吸附,因而使催化剂失活.李隽课题组利用从头算分子动力学模拟首次发现氧化铈和氧化钛负载的 Au纳米粒子在 CO氧化过程中可以形成单原子的现象,并将之称为动态单原子催化剂. Yang等则计算了二维材料 BN负载 Au单原子催化 CO氧化并发现反应优先通过三原子 E-R机理进行.     

Effective platinum catalysts for low-temperature oxidation of CO

A number of platinum-based catalysts bonded on carbon fiber karbopon were prepared using alcohol solutions of H₂PtCl₆. The catalytic activity of the samples was determined in the low-temperature oxidation of carbon monoxide by oxygen. The effect of the solvent nature of the precursor on the activity of the catalysts was determined. The best results were obtained in the presence of 1.5% Pt/karbopon catalyst prepared using isobutyl alcohol; 100% CO conversion was achieved in respiratory treatment (12000 h^?1, room temperature).     

Support effect in high activity gold catalysts for CO oxidation

In this work, we present a detailed study concerning the evaluation of the metal-support interaction in high activity gold catalysts for CO oxidation. Using the colloidal deposition method, model catalysts were prepared, which allow the isolation of the effect of the support on the catalytic activity. Prefabricated gold particles were thus deposited on different support materials. Since the deposition process did not change the particle sizes of the gold particles, only the influence of the support could be studied. TiO2, Al2O3, ZrO2, and ZnO were used as support materials. Catalytic tests and high resolution transmission electron microscopy clearly show that the support contributes to the activity. However, our results are not in line with the distinction between active and passive supports based on the semiconducting properties of the oxidic material. The most active catalysts were obtained with TiO2 and Al2O3, while ZnO and ZrO2 gave substantially less active catalysts. Furthermore, the effect of other important parameters on the catalytic activity (i.e., particles size distribution, calcination temperature, and aging time for a Au/TiO2 catalyst) has also been studied. Using this preparation route, the catalysts show high-temperature stability, size dependent activity, and a very good long-term stability.     

Solid-solid transformation mechanism for nanocrystalline sodalite from pillared clay

We here report the synthesis of nanocrystalline sodalite by a solid-solid transformation from a solid gel mixture of Al2O3 pillared montmorillonite (Al2O3-PILM) and NaOH under an ambient atmosphere at 80 degrees C. HR-TEM clearly shows both the formation of sodalite nuclei by the solid-solid transformation of the montmorillonite matrix and the crystal growth of nanocrystalline sodalite through the rearrangement of delocalized nuclei.     

Single-atom catalysts for CO oxidation,CO2 reduction,and O2 electrochemistry

CO_x(x=1,2)and O₂ chemistry play key roles in tackling global severe environmental challenges and energy issues.To date,the efficient selective electrocatalytic transformations of COx-carbon chemicals,and O₂-hydrogenated products are still huge challenges.Single-atom catalysts(SACs)as atomic-scale novel catalysts in which only isolated metal atoms are dispersed on supports shed new insights in overcome these obstacles in CO_x and O₂ chemistry,including CO oxidation,CO₂ reduction reaction(CO₂RR),oxygen reduction reaction(ORR),and oxygen evolution reaction(OER).In this review,the unique features and advanced synthesis strategies of SACs from a viewpoint of fundamental synthesis design are first highlighted to guide future strategy design for controllable SAC synthesis.Then,the to-date reported CO₂RR,CO oxidation,OER,and ORR mechanism are included and summarized.More importantly,the design principles and design strategies of improving the intrinsic activity,selectivity,and stability are extensively discussed and the engineering strategy is classified as neighbor coordination engineering,metal-atom engineering,and substrate engineering.Via the comprehensive review and summary of state-of-the-art SACs,the synthesis–structure–property–mechanism–design principle relation can be revealed to shed lights into the structural construction of SACs.Finally,we present an outlook on current challenges and future directions for SACs in CO_x and O₂ chemistry.     

Size and support effects for CO oxidation on supported Pd catalysts

Science China Chemistry - Supporting Pd catalysts characterized significant different size distribution were obtained using PdCl2, [Pd(NH3)4](NO3)2 and Pd(acac)2 as precursors. High-resolution...     

Dielectric properties of polypyrrole/pillared clay nanocomposites

Dielectric constant ??? and loss factor ??? were measured in intercalated polypyrrole/aluminum pillared montmorillonite (PPy/Al-PMMT) clay nanocomposites in the frequency range 100 Hz to 1 MHz. The PPy/Al-PMMT nanocomposites were prepared by in situ polymerization of pyrrole in aqueous dispersion of varying amounts of (Al-PMMT) clay from 0.2 to 10%, using FeCl₃ · 6H₂O as an oxidant. Formation of the nanocomposite was studied by FTIR and intercalation of PPy in the clay galleries was confirmed by XRD. The nanocomposites exhibited very large values of ??? and ??? at low frequency which decreased with frequency and increased with the clay content in the samples. Electric modulus formalism exhibited a peak in the frequency dependence curves of imaginary part of the electric modulus due to conductivity relaxation process. The peak of conductivity relaxation shifted towards higher frequencies and the magnitude of relaxation decreased with the increase of MMT content in the composites.     

Low temperature CO oxidation over iron-containing MCM-41 catalysts

Different MCM-41 samples containing framework iron were prepared and tested in CO oxidation showing unprecedented high activities after reduction in hydrogen above 773 K.     

CO oxidation on Ta-Modified SnO2 solid solution catalysts

Co-precipitation method was adopted to prepare Sn–Ta mixed oxide catalysts with different Sn/Ta molar ratios and used for CO oxidation. The catalysts were investigated by N₂-Brunauer–Emmett–Teller (N₂-BET), X-ray diffraction patterns (XRD), H₂-temperature programmed reduction (H₂-TPR), Thermal Gravity Analysis – Differential Scanning Calorimetry (TGA–DSC) techniques. It is revealed that a small amount of Ta cations can be doped into SnO₂ lattice to form solid solution by co-precipitation method, which resulted in samples having higher surface areas, improved thermal stability and more deficient oxygen species on the surface of SnO₂. As a result, those Sn rich Sn–Ta solid solution catalysts with an Sn/Ta molar ratio higher than 4/2 showed significantly enhanced activity as well as good resistance to water deactivation. It is noted here that if tantala disperses onto SnO₂ surface instead of doping into its lattice, it will then have negative effect on its activity.     

Low temperature oxidation of CO over cluster-derived platinum-gold catalysts

The structural and catalytic properties of SiO2- and TiO2 -supported Pt-Au bimetallic catalysts prepared by coimpregnation were compared with those of samples of similar composition synthesized from a Pt2Au4(C{triple bond}CBut)8 cluster precursor. The smallest metal particles were formed when the bimetallic cluster was used as a precursor and TiO2 as the support. FTIR data indicate that highly dispersed Au crystallites in these samples, presumably located in close proximity to Pt, are capable of linearly coordinating CO molecules with a characteristic vibration observed at 2111 cm(-1). The cluster-derived Pt2Au4/TiO2 samples were the only ones exhibiting low-temperature CO oxidation activity, indicating that both the high dispersion of Au and the nature of the support are important factors affecting the catalytic activity for this system.     

Cu-Co-Fe oxide catalysts supported on carbon nanotubes for the oxidation of CO

The catalytic activity and adsorption characteristics of the surface of catalysts in the form of carbon nanotubes produced on nickel and cobalt oxides with the Cu-Co-Fe oxide system as supported active phase were studied. At carbon nanotubes produced on nickel oxide with (10 + 10) wt.% of the catalytically active phase total conversion of CO to CO₂ is realized at 47 °C. This sample has high specific surface area and a large volume of mesopores. It was shown that the increase in catalytic activity correlates with the increase in the amount of the α₂ form of CO₂. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 4, pp. 222–226, July–August, 2006.     

Supported metal catalysts based on fibrous carbon material for low-temperature CO oxidation

A number of supported-metal (Pt, Pd) catalysts based on Karbopon nonwoven carbon fibers were prepared by deposition from hydrosols and by ion exchange. The catalytic activity of the catalysts in low-temperature oxidation of carbon monoxide by atmospheric oxygen was determined. Catalyst samples were studied by transmission electron microscopy.     

Microwave synthesis of supported Au and Pd nanoparticle catalysts for CO oxidation

We report the microwave synthesis and characterization of Au and Pd nanoparticle catalysts supported on CeO2, CuO, and ZnO nanoparticles for CO oxidation. The results indicate that supported Au/CeO2 catalysts exhibit excellent activity for low-temperature CO oxidation. The Pd/CeO2 catalyst shows a uniform dispersion of Pd nanoparticles with a narrow size distribution within the ceria support. A remarkable enhancement of the catalytic activity is observed and directly correlated with the change in the morphology of the supported catalyst and the efficient dispersion of the active metal on the support achieved by using capping agents during the microwave synthesis. The significance of the current method lies mainly in its simplicity, flexibility, and the control of the different factors that determine the activity of the nanoparticle catalysts.     

酸洗Pt-Fe和Pt-Co催化剂在CO氧化反应中的对比研究

负载型纳米催化剂表面结构与其催化性能之间关系的研究一直受到广泛关注.由于其结构复杂使得人们在研究催化剂构效关系时遇到了很多困难.近年来,大量研究发现反转催化剂在众多反应中表现出优越的催化性能.反转催化剂是将过渡金属氧化物负载于其它金属表面.和传统金属/氧化物催化剂相比,反转催化剂更能突出氧化物在催化反应中的重要作用.众多研究表明,在氧化物-金属界面处存在特殊的作用,这种作用可以改变氧化物的电子特性和化学性质,进而产生较高的催化性能.傅强等人创建了金属氧化物负载于Pt表面的反转催化体系,其表现出了高的低温CO氧化反应性能.在氧化物和Pt之间的界面限域效应可以稳定氧化物中配位不饱和的金属阳离子.这种配位不饱和的氧化物提供了活化O2的活性位.目前,反转催化剂的研究主要集中在单晶模型体系中,在负载型催化剂中的研究还较少.我们以炭黑(CB)为载体,将还原后的Pt-Fe和Pt-Co催化剂经过酸洗制备了一种表面富Pt核为合金的结构.考察了酸洗后的Pt-Fe和Pt-Co催化剂经过不同温度氧化后的结构变化,并讨论了其结构与CO完全氧化反应(COOX)和CO选择氧化反应(CO-PROX)性能的关系.X射线粉末衍射(XRD),电感耦合等离子体发射光谱(ICP),透射电镜(TEM)和X射线光电子能谱(XPS)表征结果表明,还原后的Pt基催化剂经过酸洗可以选择性去除纳米粒子表面的3d过渡金属,形成表面富Pt体相为合金的结构.将酸洗后的Pt-Fe和Pt-Co催化剂在不同温度下空气中氧化,发现近表层的Fe(Co)会扩散到粒子表面上,形成过度氧化的Fe2O3(Co3O4)表面结构.氧化后的催化剂在COOX和CO-PROX反应中表现出截然不同的催化性能.酸洗后的Pt-Fe(Pt-Co)催化剂经过不同温度氧化后在COOX反应中活性都较差,室温下的CO转化率只有不到30%,CO完全转化的温度超过100oC,相当于纯Pt催化剂的活性.这说明Pt表面过度氧化的Fe2O3(Co3O4)对CO氧化反应的促进作用不明显.而氧化后的催化剂在CO-PROX反应中表现出较高的活性,250oC(或350oC)氧化后的酸洗Pt-Fe催化剂室温下的CO转化率接近100%,250oC(或350oC)氧化后的酸洗Pt-Co催化剂室温下的CO转化率也达到了70%.结合表征和反应结果,我们认为氧化处理形成的表面过度氧化的金属氧化物(Fe2O3,Co3O4)在COOX的催化性能较差.通入CO-PROX反应气后,气氛中大量H2的存在和Pt表面的氢溢流效应可以使得表面Fe2O3,Co3O4在室温下被还原成配位不饱和的FeO,CoO.这种配位不饱和的氧化物在表面Pt的限域作用和大量H2气氛下比较稳定,并且具有较强的活化解离O2的能力,进而提高了CO-PROX反应的活性.为了进一步证实催化剂表面氧化物与其催化性能的关系,我们在室温下进行了两种反应气的循环实验测试.测试结果表明,对于氧化后的酸洗Pt-Fe催化剂,COOX反应中的表面Fe2O3和CO-PROX反应中的表面FeO可以通过变换反应气氛实现两种氧化物的相互转变,并表现出完全不同的催化性能.对于氧化后的酸洗Pt-Co催化剂,CO-PROX反应中形成的CoO表面结构在COOX反应中也比较稳定,在两种反应气中表现出相似的催化性能.     

Design, synthesis and characterization of vanadia-doped iron-oxide pillared montmorillonite clay for the selective catalytic oxidation of H2S

A series of vanadia-doped iron-oxide-pillared clays (V/Fe-PILCs) with various amounts of vanadia were prepared and their performance for the selective catalytic oxidation of H(2)S was investigated. V/Fe-PILCs were characterized using X-ray diffraction (XRD), surface area- and pore volume measurements, chemical analysis, Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction by H(2) (H(2)-TPR). V/Fe-PILCs showed better catalytic performance than Fe-PILC without any significant SO(2) emissions. The H(2)S conversion over V/Fe-PILCs increased with increasing vanadia content up to 7 wt.%. However, it decreased at higher vanadia loading due to the decrease in surface area and the formation of the crystalline V(2)O(5) phase. The presence of water vapor in the reactant mixture resulted in a decrease of H(2)S conversion.     

Temperature hysteresis in CO oxidation on catalysts of various nature

The temperature hysteresis observed in the oxidation of CO on both supported and unsupported catalysts can be explained by the local overheating of active centers rather than by transitions from one steady state to another.     

The study of the active surface for CO oxidation over supported Pd catalysts

CO oxidation was investigated on various powder oxide supported Pd catalysts by temperature-programmed reaction.The pre-reduced catalysts show significantly higher activities than the pre-oxidized ones.Model studies were performed to better understand the oxidation state,reactivities and stabilities of partially oxidized Pd surfaces under CO oxidation reaction conditions using an in situ infrared reflection absorption spectrometer(IRAS).Three O/Pd(100)model surfaces,chemisorbed oxygen covered surface,surface oxide and bulk-like surface oxide,were prepared and characterized by low-energy electron diffraction(LEED)and Auger electron spectroscopy(AES).The present work demonstrates that the oxidized palladium surface is less active for CO oxidation than the metallic surface,and is unstable under the reaction conditions with sufficient CO.