Study Of Pd-decorated MWCNTs-promoted Pd-ZnO Catalysts For CO₂ Hydrogenation To Methanol

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.