FeOxCoating on Pd/C Catalyst by Atom ic Layer Deposition Enhances the Catalytic Activity in Dehydrogenation of Form ic Acid

Jun-jie LiJun-ling Lu

Department of Chem ical Physics,Hefei National Laboratory for Physical Sciences at the M icroscale, iChEM(Collaborative Innovation Center of Chem istry for Energy Materials),CAS Key Laboratory of Materials for Energy Conversion,University of Science and Technology ofChina,Hefei230026,China

FeOxCoating on Pd/C Catalyst by Atom ic Layer Deposition Enhances the Catalytic Activity in Dehydrogenation of Form ic Acid

Jun-jie Li,Jun-ling Lu?

Department of Chem ical Physics,Hefei National Laboratory for Physical Sciences at the M icroscale, iChEM(Collaborative Innovation Center of Chem istry for Energy Materials),CAS Key Laboratory of Materials for Energy Conversion,University of Science and Technology ofChina,Hefei230026,China

Hydrogen generation from form ic acid(FA)has received significant attention.The challenge is to obtain a highly active catalyst under m ild conditions for practical app lications.Here atom ic layer deposition(ALD)of FeOxwas performed to deposit an ultrathin oxide coating layer to a Pd/C catalyst,therein the FeOxcoveragewas precisely controlled by ALD cycles. Transm ission electron m icroscopy and powder X-ray diff raction measurements suggest that the FeOxcoating layer im proved the thermal stability of Pd nanoparticles(NPs).X-ray photoelectron spectroscopy m easurem ent showed that deposition of FeOxon the Pd NPs caused a positive shift of Pd3d binding energy.In the FA dehydrogenation reaction,the ultrathin FeOxlayer on the Pd/C could considerably improve the catalytic activity,and Pd/C coated w ith 8 cycles of FeOxshowed an optim ized activity w ith turnover frequency being about 2 timeshigher than the uncoated one.The im proved activitieswere in a volcanoshape as a function of the number of FeOxALD cycles,indicating the coverage of FeOxis critical for the optim ized activity.In summary,simultaneous improvements of activity and thermal stability of Pd/C catalyst by ultra-thin FeOxoverlayer suggest to be an eff ective way to design active catalysts for the FA dehydrogenation reaction.

Form ic acid,Hydrogen generation,Atom ic layer deposition,FeOxcoating, Pd catalyst

I.INTRODUCTION

Hydrogen is one of themost prom ising energy carrier ow ing to its high abundance on earth and zero em ission [1].Considering the technological barriers in hydrogen storage and safety of releasing hydrogen,choosing appropriate hydrogen storage chem icals as hydrogen carriers is an attractive way for transportation[2,3].

Among the hydrogen storage chem icals,form ic acid (FA)has attracted considerable attention.FA is one of themajor products form ed during biom ass processing[4,5].It is liquid at room tem perature,and contains 4.4 w t%hydrogen.It is also non-toxic and can be stored safely at room temperature,which is suitable for hand ling and storage[4–8].There are two possible pathways for decomposition of form ic acid: the dehydrogenation pathway to form CO2and H2(HCOOH→CO2+H2),and the undesirable dehydration pathway to form H2O and CO(HCOOH→CO+H2O), in which the CO may poison the catalysts[3,5,9–16]. Therefore,it is highly demanded to design a catalyst w ith high catalytic activity and selectivity to catalyzeFA dehydrogenation to produce CO-free H2at near ambient tem perature[17–24].

Among the heterogeneous catalysts used in FA dehydrogenation,Pd-based catalysts showed superior catalytic activities[19,21,25–29].For instance,Zhu and co-workers synthesized theultra-fine Pd nanoparticles(～1.4 nm)on carbon nanospheres using anhydrous methanol,and reported a turnover frequency(TOF) as high as 7256 h?1at 60?C in the FA dehydrogenation[30].Jianget al.reported that the highly dispersed AgPd hollow spheres on the graphene(AgPd-Hs/G)showed a TOF value of 333 h?1at 25?C,which wasmuch higher than the Pd/G and AgPd/C catalysts. The synergistic effect between Pd and Ag and the hollow structure in which most of the atom s locate on the surface or the sub-surfaceof the sphere,both lead to the higher catalytic activity[31].Xuet al.reported that introducing Co into AgPd bimetallic nanoparticles to form(Co6)Ag0.1Pd0.9/RGO catalyst had a significant higher TOF value of 4711 h?1at 50?C.The prom otion was attributed to themuch smaller sized metal NPs by the sacrificial synthetic approach[32].

M odifying the Pd-based catalystsusingmetaloxide is another effective way to promote the catalytic activity in FA dehydrogenation[33–35].Zahmakiran and coworkers reported that the presence of M nOxNPs onPdAg alloy NPs could considerably accelerate hydrogenation of FA at room tem peraturew ith a remarkable activity of 330m olH2·m olcatalyst?1·h?1w ithout any additives.The activity promotion was attributed to that MnOxNPs provides sacrificial CO anchoring sites by form ing carbonates and increasing CO poisoning tolerance,suppressing the reconstruction and leaching of Pd NPs[35].Zhou and co-workers also found that the activitiesof Pd-Au/C and Pd-Ag/C catalystscould beenhanced enormously by co-deposition w ith CeO2,achieving the activitiesashigh as1640 and 548m L·m in?1·g?1at 91?C,nearly 10.3 and 3.4 times higher than that of Pd-Ag/C,respectively.The activity improvement was suggested due to the form ation of cationic Pd species induced by CeOx[34].

Atom ic layer deposition(ALD)is a thin fi lm grow th technique,which relies on sequential self-lim iting surface reactions between gaseous precursors and a substrate.ALD can produce fi lm s in a layer-by-layer fashion at the atom ic level[36,37],thereby allow ing atom iclevel precise control over the coverage of oxide fi lm on metal NPs[38–40].K imet al.reported that in the electrochem ical water oxidation,the catalytic activity increased after the TiO2ALD coating on Co/C,especially for the 60 cycle ALD(TiO2)-Co/C catalyst,which showed 2.5 timeshigher catalytic activity than the commercial Pt/C catalyst and was highly stable com pared to Co/C[41].In our previous work,we demonstrated that the catalytic activities of Au/A l2O3and Au/SiO2in the CO oxidation increased by the ALD TiO2coat. A volcano-like behavior of the catalytic activity as a function of the number of TiO2ALD cycles indicated that the catalytic activitiesof TiO2coated Au catalysts were highly dependt on the total length of perimeter sites[42].

Here,we precisely deposited FeOxonto a Pd/C catalyst using ALD for diff erent cycles to tune the coverage of FeOx.Transm ission electron m icroscopy(TEM)and powder X-ray diff raction(XRD)measurements showed that the ultrathin FeOxcoating layer can im prove the thermal stability of Pd NPs to some extent.X-ray photoelectron spectroscopy(XPS)m easurem ent revealed the positive shift of Pd3d binding energy ow ing to the strong Pd-FeOxinteraction.In FA dehydrogenation, we found the ultrathin FeOxlayer on the Pd/C catalyst could considerably im prove the catalytic activity, and the coverage of FeOxover layer on Pd NPs is very critical for the optim ized activity.

II.EXPERIM ENTS

A.Pd/C catalyst synthesis

The Pd/C catalyst was prepared using sodium borohydride(NaBH4,96%,Sinopharm Chem ical Reagent Co.Ltd.)as a reducing agent,and sodium citrate (Sinopharm Chem ical Reagent Co.Ltd.)as a stabilizing agent according to a procedure reported previously[43].Carbon black(Vulcan XC72R,Carbot Corp.)was used as the catalyst support as received. Typically,the Pd/C catalystwas synthesized as follows: 0.1mmol PdCl2(dissolved in 0.1mol/L HCl solution) and 0.8mmol sodium citratewas dissolved into 150m L water.400mg carbon black was then added.A fter stirring the m ixture for 20 m in,followed by 30 m in sonication,15m L of 0.1mol/L NaBH4solution was added drop w ise into the suspension under vigorous stirring; and the solution was further stirred at 25?C for another 8 h.Next,the precipitatewas fi ltered,and washed w ith deionized water for several times to remove the weakly bonded sodium citrate agent.The obtained materials were then dried overnight in a vacuum oven at 40?C to obtain the Pd/C catalyst.

B.FeOxALD coating

FeOxALD was carried out on a viscous flow reactor(GEMSTAR-6TMBenchtop ALD,Arradiance) at 150?C by alternatively exposing ferrocene(99.7%, Sigm a-A ldrich)and ultrahigh purity oxygen(99.999%) for different ALD cycles[44].The ferrocene precursor contained in a stainless steel reservoir washeated up to 90?C to get a reasonable vapor pressure and the inlet lineswere heated to 120?C to avoid any condensation. Ultrahigh purity N2(99.999%)was used as carrier gas at a flow rate of 200m L/m in.FeOxALD was executed on the Pd/C catalyst using the tim ing sequence of 90, 200,120,and 200 s for ferroceneexposure,N2purge,O2exposure and N2purge,respectively.The Pd/C samp lewith different ALD cycles of FeOxwas expressed asxcFePd/C,wherexis denoted as the number of FeOxALD cycles.

C.Characterization

The loadings of Pd and Fe were determ ined by an inductively coup led p lasma atom ic em ission spectrometer(ICP-AES).Themorphologiesof the Pd/C catalysts were characterized by TEM(JEOL-2010)at 200 kV. Powder XRD measurements were carried out on a Philips X’Pert Pro Super diff ractometer at the Structure Research Center at University of Scienceand Technology of China,w ith a Cu Kαradiation(λ=1.5418?A), operated at 40 kV and 50 m A.XPS measurements were taken on a Thermo-VG Scientific Escalab 250 spectrom eter,equipped w ith an alum inum anode(A l Kα=1486.6 eV,Hefei University of Technology).

D.Catalytic activity

A ll the catalysts were pretreated at 250?C in 10% O2in Ar for 1 h and 300?C in 10%H2in Ar for 1 h be-fore catalytic performance test.The catalytic activities of the Pd/C catalysts in FA decom position were evaluated in a gas generation setup,which can be found elsewhere[45,46].In brief,the catalyst and the FA (98%,Sinopharm Chem ical Reagent Co.Ltd.)/sodium formate(SF,99.5%,Sinopharm Chem ical Reagent Co. Ltd)aqueous solution were loaded into a two-necked round-bottom ed flask(50 m L),which was p laced in a water bath at 25?C under an ambient atmosphere.A gas burette fi lled w ith water was connected with the reaction flask to m easure the volum e of released gas. Typically,55mg catalystwas fi rst loaded into the reaction flask,and 5 m L FA substrate solution containing FA(0.6 m ol/L)and SF(0.6 mol/L)was then added under vigorously stirring.The volume of the generated gas was immediately monitored by recording the disp lacement of water in the gas burette against reaction time.Here general TOF values were calculated as the follow ing equation[17,30,32]:

Herengasis the m ole number of generated gas at a conversion below 20%,nPdis the mole number of Pd atoms,tis the reaction time.

E.On-line gas chrom atography m easurem ents

The gas burette was disconnected from the reaction flask and rep laced by an argon(A r)gas line.M eanwhile,a gas outlet from the flask was connected to an online gas chromatography instrument(GC,Shimadzu GC-2014)equipped w ith a flame ionization detector (FID)alongw ith am ethanator aswellasa therm alconductivity detector(TCD).A fter the reaction started,Ar was bubbled into the reaction solution at a flow rate of 15 m L/m in,and the generated gas was carried to the GC for analysis.A referencem ixture gas containing 1% CO and 1%CO2in Ar was also recorded for com parison.

III.RESULTS AND DISCUSSION

A.M orphology of catalysts

The loading of Pd and Fe in the catalysts are shown in Table I.TEM measurement was em p loyed to characterize the morphologies of these catalysts as shown in FIG.1.Besides the dom inant Pd NPs w ith a size of～3.1 nm,a considerable number of Pd NPs w ith a larger size of～6.3 nm were also frequently observed in the uncoated Pd/C catalyst(FIG.1(a)).These larger Pd NPswere likely formed through aggregation during the high tem perature pretreatm ent.In contrast,the Pd particle size was rather sim ilar,about 3.6 nm,highly dispersed in the 5cFePd/C,8cFePd/C and 12cFePd/C samp les(FIG.1(b)?(d)).Obviously,the FeOxcoating layer on the Pd NPs im proved the thermal stability of Pd NPs against sintering,although the FeOxwas extremely thin and could not be observed by TEM under current resolution[44].

FIG.1 TEM images of(a)Pd/C,(b)5cFePd/C, (c)8cFePd/C,and(d)12cFePd/C catalysts after calcination at 250?C for 1 h and then reduction at 300?C for another 1 h.W hite arrows highlight the Pd NPs w ith significant large sizes.

TABLE I The Pd and Fe loadings in these catalysts determ ined by ICP-AES.

XRD measurementswere further performed on these sam p les.As shown in FIG.2,a broad diff raction peak at～26?can be observed on all the sam p les,which can be attributed to the C(002)diff raction peak from the carbon black support[47].Sharp diff raction peaks at 40.1?and 46.8?,which can be assigned to the(111) and(200)of Pd,indicating the presence of large size of Pd NPs on the uncoated Pd/C samp le[43].However, no visible diff raction peaks of Pd can be found on the 2cFePd/C,5cFePd/C,8cFePd/C,and 12cFePd/C samp les,indicating ultrafine Pd NPswere highly dispersed on the C support[48].These results were consistent wellw ith the TEM images.

FIG.2 XRD patterns of Pd/C andxcFePd/C catalysts.

FIG.3 XPS spectra of the Pd/C,2cFePd/C,5cFePd/C, 8cFePd/C,and 12cFePd/C catalysts in the Pd3d region.

B.XPS studies

XPSwas carried out to study the electronic properties of the Pd NPs in these samples in the Pd3d region as shown in FIG.3.The binding energy of Pd3d5/2on the uncoated Pd/C sam p le was 335.8 eV,which is assigned to the zero-valence Pd[49,50].A fter depositing FeOxon Pd/Cby ALD,the Pd3d binding energy shifted to a higher value of～336.1 eV ow ing to the strong Pd-FeOxinteraction,confi rm ing the successful deposition of FeOxon Pd NPs.The positive shift of Pd3d binding energy by the FeOxcoating layer im plies that the Pd waselectron deficient on thexcFePd/C sam p les[51–54].

C.Catalytic activity

The activities of Pd/C andxcFePd/C catalystswere evaluated in FA dehydrogenation in a FA/SF aqueous solution at 25?C under ambient atm osphere.As shown in FIG.4(a),the 8cFe/C sam ple w ithout presence of Pd did not show any catalytic activity,indicating the FeOxitself had no contribution to the FA dehydrogenation reaction.On the other hand.The uncoated Pd/C generated about 70m L gas(CO2+H2)in 80m in.The initial activity of 2cFePd/C was higher than the uncoated Pd/C sam p le,but it quickly slowed down and generated 50 m L gas in 80 m in.The activity was further im proved as increasing the coverage of FeOx,and the m aximum activity was achieved on 8cFePd/C by generating 100 m L gas.Further increasing the number of FeOxALD cycles to 12(12cFePd/C)resulted a significant activity drop.

FIG.4(a)Plots of the volum e of the generated gases (CO2+H2)versus time over the Pd/C catalysts w ith and w ithout the FeOxcoating.(b)Calcu lated TOFs of catalystsbased on theamount of Pd.Reaction conditions:5m L aqueous solution of 0.6m ol/L FA+0.6m ol/L SF,55m g catalyst,reaction tem perature:25?C.The solid line in(b)is used to guide eyes.

We further calculated the initial TOFs based on the moles of Pd.The uncoated Pd/C showed an initial TOF of 126 h?1,which was in agreement w ith the previous work[55].A fter depositing diff erent cycles of FeOxon Pd/C by ALD,considerable im provem ent of catalytic activity was observed,and the TOF valueswere 178,220,262,and 142 h?1for the 2cFePd/C, 5cFePd/C,8cFePd/C,and 12cFePd/C,respectively.A clear volcano-shape of the catalytic activity as a function of the number of FeOxALD cycles was observed. The activity promotion by FeOxm ight be related w ith the electronic modu lation of Pd as observed by XPS shown in FIG.3,in line with the previous observation on CeOxpromoted Pd-Au/C and Pd-Ag/C catalysts, where the form ation of cationic Pd species was suggested to be responsib le for the activity prom otion[34]. On theother hand,the formed Pd-FeOxinterfacem ightalso p lay a certain role in the activity enhancement. The sharp catalytic activity decrease in the 12cFePd/C sam ple may be caused by the excessive FeOxcoating which blocks significantly Pd surface active sites and leads to the decrease of catalytic activity in FA dehydrogenation,indicating an appropriate FeOxcoverage is very critical for the activity promotion.

FIG.5 Online GC spectra of the generated gas from the FA/SF solution over the 8cFePd/C catalyst at 25?C using either(a)FID-m ethanator or(b)TCD.The reference m ixture gas of 1%CO and 1%CO2balanced in A r was also shown in(a)for com parison.

D.Com position of generated gas

In FA dehydrogenation,a low level of CO m ay be formed via the dehydration path way(HCOOH→CO+H2O).The undesired CO can severely poison the Pd catalysts and lead to catalyst deactivation[3,5,9–16].To determ ine whether the CO product was released in the gaseous products during the reaction,in situmonitoring of the composition of the released gasby an online GC was carried out on the 8cFePd/C catalyst. As shown in FIG.5(a),on ly the CO2peak can be observed by a FID-methanator detector,com pared to the referencem ixturegas(1%CO and 1%CO2).Clearly the formation of CO in FA dehyd rogenation was negligible, below the detection level even w ith a FID-methanator detector.On the other hand,using a TCD detector, we clearly observed H2and CO2in the released gas as shown in FIG.5(b).Therefore,the FA dehydrogenation path to CO2and H2formation was dom inant on 8cFePd/C during the reaction.

IV.CONCLUSION

In this work,FeOxALD was performed on a Pd/C catalyst to precisely deposit an oxide coating layer to the Pd NPs.According to the TEM and XRD results,we found that the FeOxcoating layer can im prove the thermal stability of Pd NPs to som e extent.The XPSmeasurement confi rmed the successful deposition of FeOxon the Pd NPs,as the binding energy shifted to a higher value at 336.1 eV in the Pd 3d5/2via the strong interaction between the FeOxlayer and Pd NPs. In the FA dehydrogenation reaction,the im provement of catalytic activity in a volcano-shape as a function of the number of FeOxALD cycleswas observed,and the 8cFePd/C samp le showed a highest activity of 262 h?1at room temperature,which was about 2 times higher than the uncoated Pd/C.The activity p romotion by FeOxis likely correlated w ith the electronicmodulation of Pd and the formed Pd-FeOxinterfaces.Taken together,we achieved considerable im provement of thermal stability and activity of Pd catalyst simu ltaneously by carefully controlling the coverage of FeOxcoating layer.

V.ACKNOW LEDGM ENTS

This work was supported by the National Natural Science Foundation of China(No.51402283 and No.21473169),One Thousand Young Talents Program under the Recruitment Program of G lobal Experts,the Fundam ental Research Funds for the Central Universities(No.WK 2060030017),and the Startup Funds from University of Science and Technology of China.

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ceived on March 12,2017;Accepted on March 27,2017)

?Author to whom correspondence shou ld be addressed.E-m ail: jun ling@ustc.edu.cn