【作者】 梁雪莲；

【导师】 张鸿斌；

【作者基本信息】 厦门大学 ， 物理化学， 2009， 博士

【摘要】 温室效应对我们这个星球的环境和人类社会的生存发展已构成越来越严重的威胁，减少主要温室气体之一的CO₂的排放已成为一个紧迫的问题．通过加氢转化，将CO₂加工为有价值的燃料或化学品被认为是现时固定大量排放的CO₂的较好方法．在诸多CO₂加氢的可能产物中，甲醇因其既是非石油基洁净合成燃料，又是重要化工原料，还可作为燃料电池氢燃料的载体，因而成为首选的目标产物．催化剂的研制开发是实现该过程实用化的关键．较早报道可用于催化CO₂加氢制甲醇的催化剂是Cu基（尤其是CuO-ZnO-基和CuO-ZrO₂-基）催化剂；负载型Pd催化剂对CO₂加氢制甲醇也有相当高的催化活性和选择性，并以ZnO负载的Pd基体系为佳．一个具有实用意义的CO₂加氢制甲醇的过程要求所使用的催化剂具有高的性能；然而就我们所知，现有CO₂加氢制甲醇用催化剂的活性和选择性均比较低，开发兼具高活性高选择性的催化剂是实现该过程实用化的技术瓶颈．在另一前沿，多壁碳纳米管（MWCNTs）作为一类新型纳米催化材料，近年来引起国际催化学界日益增加的兴趣．这类纳米碳材料在结构上与中空的石墨纤维相近；它具有纳米级的管腔、由sp²-C构成的表面，并展现出良好的导电导热性及对H₂吸附活化的优异性能；迄今报道的催化应用领域涵盖选择加氢、氢甲酰化、脱氢、氧化、氨合成、F-T合成、甲醇／低碳醇合成、电催化和燃料电池等，研究结果已展现其作为某些类型催化剂的优良载体或促进剂的应用前景．本文开展CO₂加氢制甲醇用的MWCNTs促进的高效新型Pd-ZnO基催化剂的开发研究，从负载型Pd-ZnO催化剂的应用基础研究入手，在初步弄清MWCNTs的促进作用性质的基础上，进而研发出一类金属Pd修饰MWCNTs促进的高效新型Pd-ZnO催化剂，取得如下兼具理论意义和实用价值的重要进展．1．CO₂加氢制甲醇用的MWCNTs负载Pd-Zn催化剂的研究1．1 MWCNTs负载Pd-Zn催化剂的性能用一类自行制备的“鱼骨型（Herringbone-type）”MWCNTs（记为MWCNTs（h-type）或MWCNTs）作为载体，由分步等容浸渍法制备MWCNTs-负载的Pd-ZnO催化剂（记为x％Pd_iZn_j／MWCNTs，x％为质量百分数），在加压固定床连续流动反应系统，评价其对CO₂加氢制甲醇的催化性能，并与常规载体γ-Al₂O₃，活性炭（AC），以及“平行型（Parallel-type）”MWCNTs（记为MWCNTs（p-type））分别负载的体系作比较．结果表明，在组成经优化的16％Pd_0．100Zn₁／MWCNTs（h-type）催化剂上，在3．0 MPa，523 K，V（H₂）：V（CO₂）：V（N₂）=69：23：8和GHSV=1800 ml（STP）／（g·h）的反应条件下，所观测CO₂加氢的转化率达6．30％，相应的转化频率（TOF，即在单位时间（s）、单个表面Pd⁰-活性位上CO₂加氢转化的分子数）为1．15×10^-2s^-1．这个TOF值是相同反应条件下3种各具最佳Pd_0．100Zn₁负载量的参比催化剂，22％Pd_0．100Zn₁/MWCNT（p-type）、35％Pd_0．100Zn₁／AC和20％Pd_0．100Zn₁／γ-Al₂O₃上的相应TOF观测值(1．08×10^-2，0．98×10^-2和0．97×10^-2(s^-1))分别的1．06，1．17和1．1 8倍．1．2 MWCNTs负载Pd-Zn催化剂的表征表观活化能（E_a）的测量及催化剂的XRD、XPS和H₂-TPD比较表征研究揭示，用MWCNTs代替AC或γ-Al₂O₃作为催化剂载体并不引起CO₂加氢反应的E_a发生明显变化，但导致工作态催化剂表面以PdZn合金形态存在的Pd⁰-物种（一类与甲醇的生成密切相关的表面催化活性物种）的摩尔分率明显上升．在另一方面，MWCNTs-负载的Pd-ZnO催化剂在从室温至623 K温度范围能可逆地吸附大量的H₂，这一特点有助于在工作态催化剂表面营造较高稳态浓度活泼氢吸附物种的表面反应氛围，于是提高了表面加氢反应的速率．与常规载体AC或γ-Al₂O₃不同，MWCNTs起着作为催化剂的载体和促进剂的双重作用．与MWCNTs（p-type）管壁表面C原子属于类石墨平面面内原子的情形不同，MWCNTs（h-type）的管壁表面C原子属于类石墨平面边沿原子，有较多的表面悬键，化学活性较高，因而对H₂有较强的吸附活化能力，其促进作用较MWCNTs（p-type）显著．2．CO₂加氢制甲醇用Pd-修饰MWCNTs促进的高效新型Pd-Zn催化剂的研发2．1 Pd修饰MWCNTs-基纳米材料的研制以自行制备的MWCNTs作为基质，用微波助多元醇化学还原沉积法制备一类金属Pd-修饰的MWCNTs，记为y％Pd／MWCNTs（y％为质量百分数，2％（?）y％（?）8％）．所制得的复合材料的TEM／SEM观测结果显示，金属Pd颗粒相当均匀地负载／分散在MWCNTs表面，从TEM／SEM和XRD的图像可估计其粒径在10 nm以下．组成为5．0％Pd／MWCNTs的试样的EDX分析证实，C和Pd是该试样表面仅有的两种元素，其原子分率分别为99．4％和0．6％．H₂-TPD测试揭示，适量金属Pd对MWCNTs表面的修饰导致其对H₂的吸附容量显著增加，以组成为5．0％Pd／MWCNTs的试样对H₂吸附容量的增幅最大，达～85％(所作对比观测两试样的H₂-TPD曲线在273～723 K温度范围的相对面积强度比为：A_{5．0％Pd/MWCNTs}／A_MWCNTs=100／54)．2．2 Pd修饰MWCNTs促进的共沉淀型Pd-Zn催化剂的性能用上述制备的金属Pd-修饰的MWCNTs（y％Pd／MWCNTs）作为促进剂，制备y％Pd／MWCNTs-促进的共沉淀型Pd-ZnO催化剂，记为Pd_iZn_j-x％（y％Pd／MWCNTs）（x％和y％均为质量百分数）；在加压固定床连续流动反应系统，评价其对CO₂加氢制甲醇的催化性能，并与不含促进剂的原基质Pd_iZn_j和添加等量纯MWCNTs的参比体系作比较．结果表明，在组成经优化的Pd_0．100Zn₁-9．6％（5．0％Pd／MWCNTs）催化剂上，在5．0 MPa，543 K，1 5000 ml_STP／（h·g）和V（H₂）／V（CO₂）／V（V₂）=69／23／8的反应条件下，CO₂加氢的转化率达6．98％，相应的TOF为6．68×10^-2s^-1，这个TOF值是相同反应条件下两种参比催化剂，Pd_0．100Zn₁和Pd_0．100Zn₁-9．6％MWCNTs，的相应观测值(4．22×10^-2和5．40×10^-2(s^-1))分别的和1．58和1．24倍；前者的甲醇时空产率达343 mg／（h·g），是后两者的相应值(202和307 mg／（h·g）分别的1．70和1．12倍，也大幅度超过迄今文献报道的同类或可比的Pd-基催化剂（Pd-ZnO、Pd-Ga₂O₃等）或Cu-基催化剂（CuO／ZnO、CuO／ZrO₂等）的水平．这些结果表明，MWCNTs，尤其是Pd修饰的MWCNTs，确实可作为CO₂加氢制甲醇用的Pd-ZnO催化剂的高效促进剂．2．3 Pd修饰MWCNTs促进的共沉淀型Pd-Zn催化剂的表征TEM／SEM、N₂-BET和CO化学吸附的联合表征结果显示，含促进剂的催化剂(Pd_0．100Zn₁-9．6％（5．0％Pd／MWCNTs）和Pd_0．100Zr₁-9．6％MWCNTs)的粒度较小、分散度较高，而不含促进剂的原基质催化剂(Pd_0．100Zn₁)的粒度较大、比面较小．3种催化剂的氧化态N₂-BET比表面（SSA）依次为15．64，19．05，4．05（m²／g）；相应工作态的金属Pd表面(SA_pd)分别为2．120、2．346、1．975（m²/g）．实验发现，催化剂的最高还原温度对其催化性能有显著影响，以经受538 K、H₂-还原的催化剂的活性为佳；XRD观测证实，经适当高温度还原的催化剂金属Pd组分绝大部分以Pd-Zn合金相的形态存在；这些实验事实表明Pd-Zn合金微晶相是与CO₂加氢制甲醇更加密切相关的催化活性相．3种催化剂(Pd_0．100Zn₁-9．6％（5．0％Pd／MWCNTs），Pd_0．100Zn₁-9．6％MWCNTs和Pd_0．100Zn₁)的反应后试样的XPS分析显示，其表面pd⁰物种在总表面Pd量中的摩尔分率分别为57．3，52．6，48．6（mol％），前者是后两者分别的1．09和1．18倍；这个顺序与3种催化剂上CO₂加氢制甲醇的反应活性高低顺序相一致．H₂-TPD测试结果表明，MWCNTs，尤其是5．0％Pd／MWCNTs，促进的Pd_0．100Zn₁-基体系对H₂有更强的吸附／活化能力；所作对比观测3种试样的H₂-TPD曲线的相对面积强度比为：A_{Pd0．100Zn1-9．6％（5．0％Pd／MWCNTs）}／A_{Pd0．100Zn1-9．6％MWCNTs}/A_Pd0．100Zn1=100／86／81（在293～773 K温度范围）或100／87／75（在473～773 K温度范围），这个顺序与这3种催化剂上CO₂加氢制甲醇的反应活性高低顺序相一致．2．4 Pd-修饰MWCNTs-基纳米材料的促进作用本质上述催化剂评价结果显示，适当添加少量5．0％Pd／MWCNTs到Pd_0．100Zn₁基质催化剂中导致CO₂加氢转化率和甲醇产率大幅度增加；然而表观活化能（E_a）的测量结果显示，少量MWCNTs-基纳米材料促进剂的加入并不引起CO₂加氢转化的E_a发生明显变化，这可能暗示CO₂加氢反应速率决定步骤的反应途径并不因少量MWCNTs-基促进剂的加入而有所改变．在另一方面，所观测催化活性表面Pd-物种的摩尔分率的增加无疑有助于催化剂比活性（即单位质量催化剂的活性）的提高，然而所观测CO₂加氢转化率高达70％的增幅(6．98％vs．4．11％对于Pd_0．100Zn₁-9．6％（5．0％Pd／MWCNTs）和Pd_0．100Zn₁)很难单纯归因于活性金属Pd表面的增加（2．120 vs．1．975（m²／g），增幅仅为7．3％）所贡献；此外，从活性金属Pd表面积增加的角度出发也无法解释单个表面Pd⁰-活性位上CO₂加氢的转化频率（即TOF）的显著差别(6．68×10^-2s^-1vs．4．22×10^-2s^-1，增幅达58％)．因此在我们看来，在Pd修饰MWCNTs促进的Pd-ZnO催化剂上CO₂加氢高的活性与作为促进剂的MWCNTs-基纳米材料对H₂优良的吸附性能也密切相关．根据上述H₂-TPD结果，能够推断：在CO₂加氢的反应条件下，在工作态Pd_0．100Zn₁-9．6％（5．0％Pd／MWCNTs）催化剂表面存在着大量的氢吸附物种，营造具有较高稳态浓度活泼氢吸附物种的表面反应氛围，于是提高了表面加氢反应的速率．3．结论Pd修饰MWCNTs能够作为CO₂加氢制甲醇用的Pd-Zn催化剂的高效促进剂；作为助剂的MWCNTs-基纳米材料的促进效应主要表现在：1)通过影响催化剂的化学态使表面催化活性钯（pd⁰）的表面积有所增加；2)提高了催化剂对H₂（反应物之一）的吸附活化能力．4．本文的创新点研发出一种金属Pd修饰MWCNTs促进的高效新型Pd-ZnO催化剂，其对CO₂加氢制甲醇的催化活性和选择性高而稳定，CO₂加氢转化率及甲醇时空产率明显高于现有文献报道同类催化剂的水平，具有明显创新性．初步弄清作为助剂的MWCNTs-基纳米材料的促进作用机理，对于增进对MWCNTs-基纳米材料催化特性的认识具有重要理论意义．更多还原

【Abstract】 The greenhouse effect of carbon dioxide has been recognized to be one of themost serious problems in the world and a number of countermeasures have beenproposed so far.Catalytic hydrogenation of carbon dioxide to produce various kindsof chemicals and fuels has received much attention.Among the options considered,methanol synthesis by CO₂ hydrogenation has been also considered to play a role inthe transportation of hydrogen energy produced from natural such as solar energy,hydropower and so on.A number of Cu-based catalysts,especially CuO-ZnO and CuO-ZrO₂ basedcatalysts,have been found to exhibit interesting properties for synthesis of methanolfrom CO₂/H₂ feedstock.Supported Pd catalysts have also been found to displaycertain activity for hydrogenation of CO₂ to methanol.Nevertheless,a practicalmethanol synthesis process from CO₂ hydrogenation requires a high performancecatalyst,and to our best knowledge,the activity and selectivity of the existingcatalysts for CO₂ hydrogenation to methanol were still relatively low.Development ofcatalysts with high efficiency and selectivity has been one of the key objectives forR&D efforts.To the other front,multi-walled carbon-nanotubes （symbolized as MWCNTs inlater text）,as a novel nano-carbon material,have been drawing increasing attentionrecently.This new form of carbon is structurally close to hollow graphite fiber,exceptthat it has a much higher degree of structural perfection.MWCNTs possess severalunique features,such as graphitized tube-wall,nanometer-sized channel andsp²-C-constructed surface.They display high thermal/electrical conductivity,mediumto high specific surface areas,and excellent performance for adsorption of hydrogen,all of which render this kind of nanostructured carbon materials full of promise as anovel catalyst support and/or promoter.In the present work,a series of supported and promoted Pd-ZnO catalysts byMWCNTs or Pd-decorated MWCNTs,noted as x%Pd_iZn_j/MWCNTs and Pd_iZn_j-x%（MWCNTs or y%Pd/MWCNTs）,respectively,（where x% and y% both representedmass percentage） were prepared by methods of conventional impregnation orco-precipitation.Their catalytic performance for CO₂ hydrogenation to methanol wasevaluated,and compared with the related reference systems.The results should shed light on the understanding of the nature of promoter action by the MWCNTs-basednano-material and on the design of practical catalyst for CO₂ hydrogenation tomethanol.The progresses obtained in the present work were briefly described asfellows.1.Study of MWCNTs-supported Pd-Zn catalyst for hydrogenation of CO₂ tomethanol1.1 Performance of MWCNTs-supported Pd-Zn catalystUsing a kind of home-made“Herringbone-type”MWCNTs （noted asMWCNTs（h-type） in later text） as support,a type of MWCNTs（h-type）-supportedPd-ZnO catalysts were prepared by a stepwise incipient wetness method,and theircatalytic activity for CO₂ hydrogenation to methanol was evaluated,and compared tothat of the reference systems supported on activated carbon （AC） orγ-Al₂O₃.Theresults showed that over the composition-optimized catalyst,16%Pd_0.100Zn₁/MWCNTs（h-type）,under the reaction condition of 3.0 MPa,523 K,V（H₂）:V（CO₂）:V（N₂）=69:23:8 and GHSV=1800 ml_STP/（g·h）,the observedconversion of CO₂-hydrogenation reached 6.30%,with the corresponding turnoverfrequency (TOF,i.e.,the number of CO₂-molecule hydrogenated on unit site ofexposed Pd⁰ per second (s^-1)) being 1.15×10^-2s^-1.This value of TOF was 1.06 or 1.17or 1.18 times that (1.08×10^-2 or 0.98×10^-2 or 0.97×10^-2(s^-1) under the same reactioncondition) of the catalyst of 22%Pd_0.100Zn₁/MWCNTs（p-type） or 35%Pd_0.100Zn₁/ACor 20%Pd_0.100Zn₁/γ-Al₂O₃,all prepared by the same method with the respectiveoptimal Pd_0.100Zn₁-loading.1.2 Characterizations of MWCNTs-supported Pd-Zn catalystThe measurement of the apparent activation energy （E_a） and the comparativecharacterization-study of the catalysts by XRD,XPS and H₂-TPD et al.demonstratedthat using the MWCNTs in place of AC orγ-Al₂O₃ as the catalyst support displayedlittle change in the E_a for CO₂ hydrogenation,but led to an increase of surfaceconcentration of the Pd⁰-species in the form of PdZn alloys,a kind of catalytically active Pd⁰-species closely associated with the methanol generation.On the other hand,the MWCNTs-supported Pd-ZnO catalyst could reversibly adsorb a greater amount ofhydrogen at temperatures ranging from room temperature to 623 K.This uniquefeature would help to generate a micro-environment with higher concentration ofactive H-adspecies at the surface of the functioning catalyst,thus increasing the rateof surface hydrogenation reactions.It is evident that the MWCNTs played dual rolesas support and promoter.The“Herringbone type”MWCNTs possess more activesurface （with more dangling bonds）,and thus,higher capacity for adsorbing H₂,which make their promoting action more remarkable,compared to the“Parallel-type”MWCNTs.2.Development of Pd-decorated MWCNTs-promoted co-precipitated Pd-Zncatalyst for hydrogenation of CO₂ to methanol2.1 Preparation and characterization of metallic Pd-decorated MWCNTsWith the home-made MWCNTs（h-type） as substrate material,a type of metallicPd-decorated CNTs,noted as y%Pd/MWCNTs （2 mass%（?）y%（?）8 mass%）,wasprepared using an intermittent microwave irradiation-assisted polyol-reduction/deposition method.The TEM and SEM observations of the synthesized materialsshowed that metallic Pd-particles were quite uniform in shape and size and welldispersed on the MWCNTs surface,with the Pd-particle diameters being below 10 nm,as estimated from the corresponding XRD patterns.The EDX analysis demonstratedthat carbon and palladium were the only two elements at the surface of 5.0%Pd/MWCNTs,with atomic percentage of 99.4% and 0.6%,respectively.The H₂-TPDinvestigation revealed that the modification of an appropriate amount of metallicpalladium to the MWCNTs led to a significant increase in their hydrogen-adsorbingcapacity,with the material of 5.0%Pd/MWCNTs composition reaching a maximumincrement of 85% (i.e.,the ratio of relative area-intensity of the observed H₂-TPDprofiles A_{5.0%Pa/MWCNTs}/A_MWCNTs=100/54) in the temperature range of 273～723 K.2.2 Performance of Pd-decorated MWCNTs-promoted co-precipitated Pd-Zn catalystsUsing the CNTs or 5.0%Pd/MWCNTs as promoter,a type of co-precipitated Pd-ZnO catalysts,noted as Pd_iZn_j-x%（MWCNTs or y%Co/MWCNTs）,forhydrogenation of CO₂ to methanol was developed.It was experimentally shown thatthe CNTs,especially the Pd-decorated MWCNTs,promoted catalyst displayed highactivity for CO₂ hydrogenation and excellent selectivity for methanol formation.Overthe composition-optimized Pd_0.100Zn₁-9.6%（5.0%Pd/MWCNTs） catalyst underreaction condition of 5.0 MPa,543 K,V（H₂）/V（CO₂）/V（N₂）=69/23/8 and GHSV=15000 ml_STP/（h·g）,the observed conversion of CO₂ hydrogenation reached 6.98%,with the corresponding TOF being 6.68×10^-2 s^-1.This TOF value was 1.58 and 1.24times that (4.22×10^-2 and 5.40×10^-2 (s^-1)) of the two reference systems,Pd_0.100Zn₁ andPd_0．100Zn₁-9.6%MWCNTs,respectively,under the same reaction condition.Theformer’s methanol-STY reached 343 mg/（h·g）,which was 1.70 and 1.12 times that（202 mg/（h·g） and 307 mg/（h·g）） of the latter two systems,successively,showing theremarkable promoter effect by the metallic Pd-decorated MWCNTs-material.2.3 Characterizations of Pd-decorated MWCNTs-promoted co-precipitated Pd-ZncatalystsIt was experimentally found that the temperature for the reduction of catalystprecursor has a marked effect on the performance of the catalyst,with theperformance of catalyst undergoing H₂-reduction at 538 K being optimal.On theother hand,the post XRD analysis of the tested three catalysts revealed that most ofthe Pd-component existed in the form of PdZn-alloy in the catalysts undergoing theH₂-reduction at 538 K.This strongly implied that it was the PdZn-alloy crystallitephase that was closely associated with the selective formation of methanol.Nevertheless,it was also experimentally shown that the H₂-reduction at thetemperatures higher than 538 K lead to increasing particle-diameter of PdZn-alloycrystallites and decreasing the metallic Pd exposed area.Post XPS-analysis of the tested catalysts revealed that appropriate incorporationof a minor amount of the x%Pd/MWCNTs into the Pd_0.100Zn₁ host catalyst led to amarked increase of surface concentration of metallic palladium species （Pd⁰）,a kindof catalytically active Pd-species closely associated with the methanol generation.Theobserved relative content （mol%） of the surface Pd⁰-species in the total Pd-amount atthe surface of the three catalysts,Pd_0.100Zn₁-9.6%（5.0%Pd/MWCNTs）,Pd_0.100Zn₁-9.6%MWCNTs and Pd_0.100Zn₁,was 57.3,52.6 and 48.6 （mol%）,successively.Thissequence was in line with the sequence of the specific activity of the three catalystsfor CO₂ hydrogenation.H₂-TPD measurements showed that appropriate addition of a minor amount ofthe x%Pd/MWCNTs into the Pd₍0.100_Zn₁ host catalyst could improve the capacity ofthe catalyst for adsorbing hydrogen to a greater extent.The relative area-intensityratio of the H₂-TPD profiles taken on the three catalysts pre-reduced by hydrogen wasestimated to be A_{Pd0.100Zn1-9.6%（5%Pd/MWCNTs）}/A_{Pd0.100Zn1-9.6%MWCNTs}/A_Pd0.100Zn1=100/87/75 in the temperature region of 473～773 K.This was expected to also be thesequence of increase in concentration of hydrogen ad-species at the surface offunctioning catalysts,in line with the activity sequence observed on the correspondingthree catalysts for hydrogenation of CO₂ to methanol.2.4 Nature ofthe promoter action by MWCNTs-based nano-materialsThe aforementioned results of the catalyst evaluation showed that appropriateincorporation of a minor amount of the 5.0%Pd/MWCNTs into the Pd_0.100Zn₁ hostcatalyst led to a significant increase in conversion of CO₂ hydrogenation and yield ofmethanol.Yet the result of measurement of apparent activation energy （E_a） indicatedthat the addition of a minor amount of the MWCNTs-based promoter into the Pd_0.100Zn₁host catalyst did not cause a marked change in the Ea for CO₂ hydrogenation-conversion,most likely implying that the addition of a minor amount of the MWCNTs-basedpromoter to the Pd_0.100Zn₁ did not alter the reaction pathway of rate-determining step ofthe CO₂ hydrogenation reaction.On the other hand,the increment of the catalytically active surface Pd-species,observed by the XPS,was undoubtedly in favour of enhancing the specific activity ofthe catalysts （i.e.,activity of unit mass of catalyst）.Nevertheless,it would be difficultto believe that the observed large-sized increase of CO₂ hydrogenation-conversionover the catalyst promoted by the 5.0%Pd/MWCNTs was solely attributed to thedifference in their specific active Pd surface-area.Besides,the difference in the activePd surface-area could hardly justify the increase of the observed TOF.It appears that the high reactivity of CO₂ hydrogenation over the 5.0%Pd/MWCNTs promoted Pd_0.100Zn₁ catalyst was closely related to the peculiar propertiesof this kind of MWCNTs,especially their excellent performance for adsorption /activation of H₂.Based upon the above H₂-TPD results,it could be suggested that,under the reaction conditions of the CO₂ hydrogenation,there existed a considerablylarger amount of reversibly adsorbed H-species on the functioning Pd_0.100Zn₁-9.6%（5.0%Pd/MWCNTs） catalyst,which would generate a surface micro-environmentwith high stationary-state concentration of H-adspecies on the catalyst,thus increasethe rate of a series of surface hydrogenation reactions in the CO₂ hydrogenationconversion.3.Concluding RemarksThe present work showed that the MWCNTs,especially the metallicPd-decorated MWCNTs,could serve as an excellent promoter of the Pd-ZnO catalystfor CO₂ hydrogenation to methanol.The developed Pd_0.100Zn₁-9.6%（5.0%Pd/MWCNTs） catalyst achieved high single-pass-yield of methanol from CO₂hydrogenation,and demonstrated great potential in commercial use for convertingCO₂ into valuable chemicals.The results also shed some light on the understanding ofthe promoter action by the transition metal-decorated MWCNTs-additive and on thedesign of practical catalyst for CO₂ hydrogenation to methanol.For betterunderstanding of mechanism of the promoter action by the MWCNTs-based additives,further studies,especially in-situ characterization of reaction intermediates under theactual reaction condition,would be desirable.更多还原