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ARTICLEAn Apparatus for Investigating the Kinetics of Plasmonic CatalysisWenZhanga,b,YongZhoua*,WeiChenb,c,TianjunWangb,ZhaoxianQinb,GaoLib,ZefengRenb,XuemingYangb,ChuanyaoZhoub,c*a.Anhui Key Laboratory of Optoelectric Materials Science and Technology,Department of Physics,An-hui Normal University,Wuhu 241000,Chinab.State Key Laboratory of Molecular Reaction Dynamics,State Key Laboratory of Catalysis,Dalian Insti-tute of Chemical Physics,Chinese Academy of Sciences,Dalian 116023,Chinac.University of Chinese Academy of Sciences,Beijing 100049,China(Dated:Received on November 2,2022;Accepted on December 9,2022)Plasmonic catalysis,which is driven bythe localized surfaceplasmon resonanceof metal nanoparti-cles,has become anemerging field inheterogeneous catal-ysis.The microscop-ic mechanism of thiskind of reaction,however,remains controversial partly because of the inaccuracy of tempera-ture measurement and the ambiguity of reagent adsorption state.In order to investigate thekinetics of plasmonic catalysis,an online mass spectrometer-based apparatus has been built inour laboratory,with emphases on dealing with temperature measurement and adsorptionstate identification issues.Given the temperature inhomogeneity in the catalyst bed,threethermocouples are installed compared with the conventional design with only one.Such amultiple-point temperature measuring technique enables the quantitative calculation ofequivalent temperature and thermal reaction contribution of the catalysts.Temperature-pro-grammed desorption is incorporated into the apparatus,which helps to identify the adsorp-tion state of reagents.The capabilities of the improved apparatus have been demonstrated bystudying the kinetics of a model plasmon-induced catalytic reaction,i.e.,H2+D2HD overAu/TiO2.Dissociative adsorption of molecular hydrogen at Au/TiO2 interface and non-ther-mal contribution to HD production have been confirmed.Key words:Plasmonic catalysis,Reaction apparatus,Reaction mechanism,Driving force,Multiple-point temperature measureing,Adsorption state identificationI.INTRODUCTIONMetal nanoparticle catalysts are widely used in thefield of thermal catalysis,including scientific research13 and industrial applications 4,5.In the past tenyears,localized surface plasmon resonance(LSPR)ofmetal nanoparticles(NPs)has been introduced intoheterogeneous catalysis,and a new field,called plas-monic catalysis,has emerged 6,7.LSPR is the collec-tive oscillation of electrons in metal or dielectric materi-als interacting with external electromagnetic waves 8.The relaxation of LSPR generates hot electrons in tens Part of the special topic of the Chinese Chemical Societys 17thNational Chemical Dynamics Symposium”*Authors to whom correspondence should be addressed.E-mail:,CHINESE JOURNAL OF CHEMICAL PHYSICSVOLUME 36,NUMBER 3JUNE 27,2023DOI:10.1063/1674-0068/cjcp2211160249 2023 Chinese Physical Societyof femtoseconds and excited phonons in a few picosec-onds timescale 9.From the energy point of view,elec-trons and phonons are possible driving force in plas-monic catalysis,which is the reason why controversyexists in this field.Generally,hot electrons are proposed to inducechemical reactions on metal nanoparticle surfaces by in-jecting into the lowest unoccupied molecular orbitals(LUMO)of adsorbates in plasmonic catalysis 10,11.Such a mechanism makes plasmonic catalysis a promis-ing method to selectively control the reaction pathwaysby designing nanostructures to meet the specific chargetransfer requirements 10.This is advantageous overthermal catalysis where energies are transferred to allpossible reaction coordinates,triggering desired and un-desired reactions at the same time.The hot electron transfer mechanism is proposedmainly according to the following phenomena 1215:enhancement of the product yield or even opening ofnew reaction channels by light illumination;correlationof the product yield with LSPR absorption.Ascribingthe typical several times enhancement of the productyield to hot electron transfer,however,has been com-mented from the temperature point of view 1618.Forthe thermal reaction,the reaction rate k follows the Ar-rhenius equation:k=Aexp(EaRT)(1)where A,Ea,R and T stand for prefactor,activation en-ergy,ideal gas constant,and temperature,respectively.Due to the exponential dependence of k on the recipro-cal of T,it is critical to guarantee the accuracy of tem-perature measurement in order to compare the reactionrate quantitatively.Assuming an activation energy of 1eV,then a 50 K temperature deviation(this value istypical as measured in the results section)around 300 Kwill cause a 7-fold difference in the reaction rate.Suchdifference is comparable with those reported previouslywhich are relied on to draw the conclusion of hot elec-tron-induced reaction 13.Therefore,related work hasbeen questioned and the product yield differences aretotally attributed to the inaccuracy of temperaturemeasurement 17.This attribution,however,is alsodoubtful because the quantitative reaction rate differ-ence caused by temperature deviation is unknown.The temperature in the whole catalyst bed is usuallyinhomogeneous.In previous apparatuses,temperatureis measured at one point,which will inevitably causediscrepancy.Using a multiple-point temperature mea-suring(MPTP)method,Lius group have successfullycalculated the equivalent temperature in the catalystbed and quantitatively differentiated the thermal con-tribution in a plasmon-induced catalytic reaction 19,20.The adsorption state is the prerequisite to analyzethe reaction mechanism 21.Despite the importance,itis not well identified in plasmonic catalysis 7,13,14,22,23.Temperature-programmed desorption(TPD)isa powerful method to quantitatively measure the des-orption species and their desorption temperature.Therefore,it is widely used to analyze the adsorptionstate of reagents in heterogeneous catalysis 24 and sur-face science field 25.In order to precisely measure the catalyst tempera-ture and adsorption state of reagents in plasmoniccatalysis and then to quantitatively analyze the reac-tion kinetics,an online mass spectrometer-based appa-ratus with both MPTP and TPD has recently beenbuilt in our laboratory.It consists of reagent input,re-action,sampling,and detection systems.The capabili-ties of this apparatus have been demonstrated by mea-suring the characteristics of a model plasmon-inducedcatalytic reaction,i.e.,H2+D2HD over AuNPs/TiO2.II.EXPERIMENTAL APPARATUSFIG.1 displays the three-dimensional design of theplasmonic catalysis apparatus,which can be divided in-to four subsystems:a reagent intake system,a reactionsystem,a product sampling system,and a detection sys-tem.The connections between the last three systemsare schematically shown in FIG.2.A.The reagent intake systemThe reagent intake system which is mounted on thesupporting frame under the optical table,is labeled bythe red dotted line in FIG.1.The reagent intake sys-tem consists of source gas bottles,pressure reductionvalves,1/4-inch stainless steel tubes,connectors,con-trolling bellows-sealed valves,and mass flow con-trollers.The tubes can be baked by heating tape toabove 373 K and pumped by a 42 L/s turbo molecularpump(TS75W1001,Edwards).There are three gaschannels.Typically,two for reagents and one for carri-250Chin.J.Chem.Phys.,Vol.36,No.3Wen Zhang et al.DOI:10.1063/1674-0068/cjcp2211160 2023 Chinese Physical Societyer gas.All the channels can be pumped and filled inde-pendently without interruption.The gas flow speed ineach channel can be regulated by a mass flow controller.The three gas channels converge to a single tubewhich is connected to the reaction system by valve 1(FIG.3(a).B.The reaction systemThe reaction system,which is at the heart of plas-monic catalysis apparatuses,includes a reactor and alight source(FIG.3).The reactor comprises a reactionchamber,a reaction cup,gas inlet and outlet ports,aheating wire,a cooling system,a temperature con-troller,three thermocouples,and a light incoming win-dow.The reaction chamber is equipped with three gasports.The first one is the reacting and carrier gas en-trance,the second one is the sampling exit,and the lastone is connected to the pump and ventilating cabinet.Depending on the specific situation,the pressure atwhich reactions take place can vary from vacuum(1 104 mbar)to atmosphere.Schematic cut view of the commercially available re-actor(HVC-MRA-5,Harrick Scientific Products)isshown in FIG.3(b).The reaction cup(6.3 mm 4 mm)where the catalysts are placed is located at thecenter of the reaction chamber.Both the heater(ATK-024-3,Harrick Scientific Products)and cooler are be-low the reaction cup.By resistive heating and 295 Kwater cooling,the temperature of the reaction bed canbe tuned between 298 K and 1183 K.Compared withthe commonly used gas flush method,residual gas(forexample,O2 and H2O due to air exposure and reagentsin previous experiments)removal in the reagent inlet Sampling system Reagent intake systemDetection systemReaction system CapillaryPinholeFIG.1 Three-dimensional design of the plasmonic catalysis apparatus.CapillaryAngle Valve2mmPump Stations III&Turbopump I Pump Stations III&Turbopump IIPump Stations I Pump Stations I&IIChamber IDetection chamberReaction chamberChamber II3 mm(a)Reaction system(b)Sampling system(c)Detection system4ValvePinhole1101 mbar1104 mbar1106 mbar1103 mbarFIG.2 Connection and differential pump between(a)reaction,(b)sampling,and(c)detection systems.Chin.J.Chem.Phys.,Vol.36,No.3An Apparatus for Investigating the Kinetics of Plasmonic Catalysis251DOI:10.1063/1674-0068/cjcp2211160 2023 Chinese Physical Societyand reaction systems by baking and pumping is muchmore efficient.Originally,there is only one thermocouple Tc mount-ed between the reaction cup and the heater.Given theposition of the thermocouple and the inhomogeneity ofthe temperature across the catalysts,the temperaturemeasured by Tc will deviate significantly from the actu-al ones.The accuracy of temperature in plasmoniccatalysis experiments,however,is extremely crucial toanalyzing the reaction mechanism.In order to improvethe accuracy of temperature measuring,another twothermocouples,Tb and Tu,are added into the reactioncup,located at the bottom and the upper surface of thecatalysts,respectively.The equivalent temperature(Te)is then calculated according to the heat conduc-tion model.This method will be referred to as the multi-ple-point temperature measuring technique(MPTM)thereafter.Given the structure of the reactor shown inFIG.3(b),for the catalysts with a thickness of h along zdirection,the temperature can be assumed to changelinearly along z and remain homogeneous in the hori-zontal plane according to the heat conductance of aplate 26.Then the temperature of catalysts at a pointz(z=0 at the bottom)is:T(z)=Tb+Tu Tbhz(2)According to the Arrhenius equation,the thermal reac-tion rate of this part of catalysts(of thickness dz)is:dr=AexpEaRT(z)dz(3)where r is the reaction rate,A is the prefactor,Ea is theapparent activation energy and R is the ideal gas con-stant.Integration of Eq.(3)from 0 to h yields the totalthermal reaction rate r:r=h0Aexp(EaRT(z)dz(4)The total thermal reaction rate can also be written as:r=Aexp(EaRTe)(5)where Te is the equivalent temperature of the catalysts.Substituting Eq.(2)into Eq.(4),we haveexp(EaRTe)=1Tu TbTuTbexp(EaRT)dT(6)The equivalent temperature Te and the apparent acti-vation energy Ea in Eq.(6)can be obtained by iterativecalculation.Using the logarithmic representation of theArrhenius equation,lnr=lnA EaR1Te(7)The activation energy and the prefactor can also be de-termined by a linear fitting between lnr and 1/Te.In a plasmonic reaction,the total,thermal,and non-thermal reaction rates are defined as Ra,Rt and Rn,re-spectively.Then we have FIG.3 (a)Three-dimensional design of the reaction system.(b)Schematic cut view of the reactor.252Chin.J.Chem.Phys.,Vol.36,No.3Wen Zhang et al.DOI:10.1063/1674-0068/cjcp2211160 2023 Chinese Physical SocietyRa=Rt+Rn(8)The first term in Eq.(8)can be experimentally mea-sured.By determining A,Ea and Te via the above de-scribed method,the second term can be calculated ac-cording to the Arrhenius equation.Thus,we can get thethird term,i.e.,the non-thermal reaction rate,if there isany.The light source in the current work is a continuouswave(CW)diode laser(HXB520XXL110,MZlaser),forwhich the output spectrum is measured(Avaspec-Min4096CL-UVI10,AVANTES)and shown inFIG.4(b).The center wavelength of the laser is518.2 nm,which is located right in the LSPR region ofthe Au NPs investigated in the current work(FIG.5(c).The 800 mW output can be continuouslyattenuated by a neutral density filter(NDC-100C-2M;Thorlabs).The diode laser is installed 750 mm rightabove the reactor.In view of covering the top surface ofthe catalysts as much as possible,the original beamspot(13.1 mm2)is expanded by the cylindrical lensto 3.13.1 mm2(FIG.4(a).The intensity distribu-tion of the beam is measured by a beam viewer(Laser-Cam HR,Coherent)(FIG.4(c)and(d).The relative-ly homogeneous light intensity distribution along twoaxes is the prerequisite to assume homogeneous temper-ature across the horizontal surface.C.The product sampling and detection systemsWhile plasmonic catalytic reactions usually takeplace at atmospheric pressure,mass spectrometer,apowerful instrument to quantitatively analyze chemi-cals,has to work in vacuum condition(typically 1106 mbar to protect the filament and electron mul-tiplication detector).Therefore,gaseous products in thereactor should be introduced at a progressively reducedpressure into the detection chamber.Then a three-stagedifferential pump-based sampling and detection systemis built(FIG.2).Gases in the reactor pass through a capillary(innerdiameter of 250 m and typical length of 900 mm)intochamber I where the pressure maintains below 0.1mbar.After another two-stage differential pump,thevacuum in the 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