Reaction mechanism in the photoreduction of with overCO2

CH4ZrO

2

Yoshiumi Kohno, Tsunehiro Tanaka, Takuzo Funabiki and Satohiro Yoshida

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto,606-8501, Japan

Received 3rd July 2000, Accepted 26th September 2000First published as an Advance Article on the web 26th October 2000

The surface species arising during the photoreduction of carbon dioxide with methane over zirconium oxide isobserved by infrared spectroscopy. Two deÐnite species have been expected to exist on the surface during thephotoreaction. One has been supposed to be a reaction intermediate and decomposed to CO at around 623 K,and the other has not been decomposed even at 673 K which should be a carbonaceous residue. From theresemblance of the IR spectral features, the latter species is assigned to the surface acetate ion. Severalproperties of the former species are found to be quite similar to those of the surface formate ion, which is areaction intermediate of the photoreduction of by over The former species is therefore assignedCO2 H2 ZrO2 .to the surface formate, which is also supposed to be the reaction intermediate of the photoreaction between

and The existence of another carbonaceous residue than the surface acetate is suggested. As no IRCO2 CH4 .bands assigned to the CÈH vibration are observed in the spectrum of the carbonaceous residues, the otherresidue is supposed to be a highly carbonaceous species. The EPR spectrum indicates the photoexcitation ofadsorbed to the anion radical, and the interaction of the radical with in the dark. OnCO2 CO2~ CO2~ CH4the basis of these results, a possible reaction mechanism in this reaction is proposed.

1. IntroductionCarbon dioxide is considered as one of the greenhouse e†ectgases, so that a decrease in the emission of the carbon dioxideis sought. One approach towards a decrease in its emission isto recycle carbon dioxide for use as a resource in the chemicalindustry before emission occurs. However, since carbondioxide has low reactivity because of its thermodynamic sta-bility, it is seldom used as it is. Catalytic conversion of carbondioxide to more reactive compounds is one of the promisingways forward.1 Normally the process requires somereductants.

Methane is the most abundant resource as a reductant onthe earth. Therefore, it is ideal that methane is used as areductant of carbon dioxide. However, methane is as poor inreactivity as carbon dioxide so that the reaction betweencarbon dioxide and methane is not easy.

There is an example of the direct reaction between carbondioxide and methane to produce CO and which is calledH2 ,

reforming of methane.2 Nickel3 and various nobleCO2metals4 are mainly used as catalysts. In that reaction one CO2molecule reacts with one methane to produce two CO andtwo hydrogen molecules. The reaction is highly endothermicso that the system has to be operated at high temperature. Inorder to achieve the reaction temperature, additional heatenergy should be supplied from an external system, or aninternal exothermic reaction such as methane combustion.Furthermore, it is generally known that coke forms at hightemperature and covers the surface of the metal catalyst,leading to deactivation during the reaction, although a systemsuppressing the formation of coke has been reported.5 Inorder to avoid the carbon deposition as well as the supply ofadditional energy to the reactor, the reaction carried out atlower temperature is to be desired.

Photocatalysis often helps the procedure of a thermody-namically disadvantageous reaction under mild reaction con-

ditions. We previously reported that is reduced to CO byCO2at room temperature under irradiation of In aH2 ZrO2 .6later study, we showed that is also e†ective as aCH4reductant for photoreduction of to CO.7 Although acti-CO2vation of by irradiation has been reported since,8,9 ours7CH4was the Ðrst study to show that reacts with underCH4 CO2irradiation at room temperature. The illuminating lightsupplies the large energy required to activate andCO2 CH4so that the highly endothermic reaction proceeds at roomtemperature. We have clariÐed that is activated to theCO2anion radical under irradiation of Therefore,CO2~ ZrO2 .10in the photoreaction of and is supposed toCO2 CH4 , CO2~be a key compound, although the reaction between the CO2~radical and is not elucidated.CH4It was also suggested that a reaction intermediate arises onthe surface during photoreaction between and onCO2 CH4From the fact that the surface species which may be aZrO2 .reaction intermediate did not include the carbon atom derivedfrom we guessed that is hardly oxidized to CO butCH4 , CH4remains on the surface to form carbonaceous residues.7 Inother words, two di†erent surface species (a reaction interme-diate and a carbonaceous residue) are supposed to exist on

during the photoreaction of and However,ZrO2 CO2 CH4 .these surface species have not been identiÐed yet.

In this study, we try to detect and identify these two surfacespecies mainly by IR spectroscopy. A 13C tracer experimentgreatly contributes to the clariÐcation of the formation of thesurface species. An EPR study together with the photoreac-tion at elevated temperature helps us to discuss the interactionbetween the radical and Based on this informa-CO2~ CH4 .tion, we propose the reaction mechanism of the photoreactionbetween andCO2 CH4 .

2. ExperimentalZirconium oxide used in this study was prepared using zir-

5302 Phys. Chem. Chem. Phys., 2000, 2, 5302È5307 DOI: 10.1039/b005315f

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conium oxychloride (Nacalai Tesque, GR) as a precursor. Anaqueous solution of zirconium oxychloride was precipitatedby 25 mass% The precipitate was washed with dis-NH3(aq).tilled water followed by Ðltration until the Ðltrate was nega-tive towards the test. The sample was dried at 373 K,AgNO3ground to a powder and calcined at 773 K for 5 h in a dry airstream. The XRD pattern indicated that the resulting powderwas zirconium oxide composed of a mixture of monoclinicand tetragonal phases.

Methane and carbon dioxide were puriÐed prior to use forthe photoreaction by vacuum distillation at the temperatureof liquid nitrogen. 13C-Labeled carbon dioxide (99 atom% of13C) was commercially supplied from ICON and used withoutfurther puriÐcation.

The photoreaction was carried out in a closed static systemconnected to a vacuum line. A sample of zirconium oxideweighing 0.3 g was spread on the Ñat bottom of a quartzreactor. For conditioning, the sample was heated at 673 K for30 min in air and evacuated for 30 min at the same tem-perature, followed by treatment with 8 kPa for 75 min andO2evacuation for 30 min at 673 K. A mixture of (150 lmol)CO2and (50 lmol) was admitted to the reactor, and the totalH2pressure in the reactor was ca. 25 kPa. A 500 W ultrahigh-pressure mercury lamp (Ushio Denki USH-500D) was used asa light source, and the reactor was illuminated from thebottom. The area subjected to illumination was 12 cm2. Forreactions at elevated temperatures, the reactor was heated bya temperature-controlled cylindrical electric furnace, allowingthe irradiation of the reactor from the bottom under heating.After each reaction for a given time, the gaseous productswere analyzed with an on-line TCD gas chromatograph(Shimadzu GC-8A) equipped with a column packed withMolecular Sieve 5A, and using argon as a carrier gas, whichcould detect and CO. In the tracer study using 13C-H2 , CH4labeled or the products were analyzed by quadru-CO2 CH4 ,pole mass spectrometer (ULVAC MSQ-101).

IR spectra were recorded with a Perkin-Elmer PARAGON1000 PC Fourier transformed IR spectrometer. The zirconiumoxide sample (ca. 80 mg) was pressed into a disk(diameter\ 10 mm) at a pressure of 4 MPa and suspended bya platinum wire in the cell as mentioned elsewhere.11 The cellallowed us to carry out heat treatment, treatment, theO2introduction of substrates, photoirradiation and measure-ments of spectra in situ. Before a measurement, the sampledisk was activated by evacuation at 673 K for 60 min, treat-ment with 8 kPa for 120 min and evacuation for 60 min atO2the same temperature, successively. Introduction of reactionsubstrates and was carried out as follows.(CO2 CH4) CO2(3.3 kPa) was introduced to the cell at room temperature fol-lowed by evacuation, and (4.7 kPa) was then introducedCH4to the cell. Acetic acid was adsorbed at room temperature fol-lowed by evacuation. A 250 W ultrahigh-pressure mercurylamp (Ushio Denki USH-250D) was used as a light source forphotoirradiation to the disk. For each spectrum, data of 200scans were accumulated at a resolution of 4 cm~1.

Electron paramagnetic resonance (EPR) spectra wererecorded using an in situ cell. Before measurements, the zir-conium oxide sample (0.3 g) was placed in the reaction part ofthe cell, was heated at 673 K for 30 min in air and was evac-uated for 30 min at the same temperature, followed by treat-ment with 8 kPa for 75 min and evacuation for 30 min atO2673 K. All the EPR spectra were recorded at room tem-perature with an X-band EPR spectrometer (JEOL JES-SRE2X) with 100 kHz Ðeld modulation. The g values and theamount of radical species were determined using an Mnmarker and TEMPOL (2,2,6,6-tetramethylpiperidine-1-oxyl)respectively. The e†ect of adsorption onto on theCO2 ZrO2EPR spectra was investigated by recording the spectra afterthe equilibrium adsorption of at room temperature fol-CO2lowed by evacuation. Some spectra were recorded under illu-

mination from the 500 W ultrahigh pressure mercury lampmentioned above. Particular attention was paid to theremoval of oxygen contamination to prevent the interferenceof the superoxide anion with the spectra. Prior to introductioninto the cell, methane was passed through a Pt catalyst bedmaintained at 473 K, and then through a liquid nitrogen trap.

was puriÐed by a freezeÈpumpÈthaw process with aCO2liquid nitrogen trap for several cycles.

3. Results

3.1. IR spectroscopy

We have already proposed the existence of the surface speciesarising during the photoreaction between carbon dioxide andmethane over zirconium oxide.7 The surface species consist oftwo distinct species : one is expected to be a reaction interme-diate which is decomposed to CO by heat at around 623 K,and the other has low reactivity and is not decomposed byheat even at 673 K, which we call carbonaceous residues. Inthis study, we tried to detect and identify the surface speciesby an IR spectroscopic method.

Fig. 1 shows the IR spectra of zirconium oxide (a) beforeand (b) after introduction of and By introducingCO2 CH4 .

onto several strong absorption bands appeared inCO2 ZrO2 ,the region between 1800 cm~1 and 1000 cm~1. They havealready been assigned to surface carbonate species.12,13 Evenwhen was introduced to adsorbed on noCH4 CO2 ZrO2 ,apparent changes were observed on the spectrum, indicatingthat no reaction proceeded in the dark between andCO2This agrees with the result that no product was detectedCH4 .under the coexistence of and on without pho-CO2 CH4 ZrO2toirradiation.7

Some spectral changes were detected after photoirradiationof with and The di†erence spectra ofZrO2 CO2 CH4 . ZrO2before and after photoirradiation for 36 h are illustrated inFig. 2. In the inset of Fig. 2, two peaks were observed at 2970cm~1 and 2878 cm~1. These absorption peaks were due to the

Fig. 1 IR spectra of (a) before and (b) after contact withZrO2 CO2and CH4 .

Fig. 2 Di†erence IR spectrum of before and after photoirra-ZrO2diation in the presence of and The inset illustrates theCO2 CH4 .same di†erence spectrum in the region from 2800 to 3000 cm~1.

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surface species arising during photoreaction. On the otherhand, in the range from 1800 cm~1 to 1000 cm~1, we couldobserve changes on the spectra but failed to assign thembecause of the interference of the strong absorption by thesurface carbonate species. However, it was already conÐrmedfrom TPD experiments that the surface species do not decom-pose to CO at 573 K whereas most of the surface carbonatedesorbs until the temperature reaches 573 K.14 Thus, if thesample is evacuated at 573 K, only carbonates will beremoved and the species which is formed during the photo-reaction will remain on the surface. The di†erence in thespectra between evacuated at 573 K and just afterZrO2 ZrO2pretreatment will therefore indicate the absorption bands ofthe surface species. The di†erence spectrum is shown in Fig. 3.It is a little complicated compared to spectra of pure adsorbedmolecules. Two di†erent surface species seem to give the spec-trum shown in Fig. 3.

Fig. 4 shows the spectrum of the surface species remainingon the sample after evacuation at 673 K in the region of 1800cm~1 to 1000 cm~1. The species which is not decomposed byheat at 673 K is considered as a carbonaceous residue. Twopeaks were observed at 1538 cm~1 and 1454 cm~1 in therange from 1800 cm~1 to 1000 cm~1, while no peak wasdetected between 3000 cm~1 and 2800 cm~1. The spectrum ofthe carbonaceous residue resembles the one obtained whenacetic acid was introduced onto the surface of at roomZrO2temperature. The spectrum of acetic acid adsorbed on isZrO2shown in Fig. 5. In the range between 1800 cm~1 and 1000cm~1, two absorption peaks at 1548 cm~1 and 1458 cm~1were observed. The acetic acid is expected to be dissociated onthe surface to form a surface acetate group.15 Therefore, weconcluded that the surface species remaining after evacuationat 673 K is the surface acetate. A slight di†erence of about 10cm~1 in the wavenumber between the surface species and theadsorbed acetic acid may be due to the slight di†erence in theadsorption mode of the acetate on caused by heat treat-ZrO2

Fig. 3 IR spectrum of the surface species on evacuated at 573ZrO2K after the photoreaction. The background of after treatmentZrO2was subtracted from the spectrum obtained after evacuation of thesample used for photoreduction at 573 K.ZrO2

Fig. 4 IR spectrum of the surface species remaining on the surfaceafter evacuation at 673 K. The background of after pretreat-ZrO2ment was subtracted from the spectrum obtained after evacuation ofthe sample used for photoreaction at 673 K.ZrO2

Fig. 5 IR spectrum of acetic acid adsorbed on the surface.ZrO2

ment at high temperature of 673 K. This is supported by thefact that the wavenumber of the C2O stretching band in thesurface acetate shifted to a lower value when the adsorbedacetic acid was evacuated at 673 K.

To ascertain whether the surface acetate was a reactionintermediate or not, we carried out a reaction using the aceticacid as a reactant. When 5 lmol of acetic acid was introducedto and irradiated for 6 h, only a trace amount ofZrO2 H2(0.08 lmol) was evolved and no CO was detected. When aceticacid was introduced to with 150 lmol of 0.2 lmolZrO2 CO2 ,of CO with 0.01 lmol of was detected after irradiation forH26 h. If the surface acetate was a reaction intermediate, muchmore evolution of CO should be expected. Therefore, we didnot consider the surface acetate as a reaction intermediate butas one of the carbonaceous residues.

The surface species arising after photoreaction shouldinclude the surface acetate. Since the appearance of the spec-trum in Fig. 3 is not identical to that in Fig. 4 or 5, the spec-trum found in Fig. 3 is supposed to be the superimposition ofthe spectra due to the surface acetate and another species.This is also expected from the fact that two absorption peakswere observed between 2800 cm~1 and 3000 cm~1 after pho-toreaction (shown in the inset of Fig. 2), whereas the surfaceacetate does not give any peaks in that region. The identiÐca-tion of the other surface species is discussed in a later section.

3.2. Origin of the carbon atom in the surface species : tracerstudy

The origin of the carbon atom contained in the products andsurface species was determined using carbon isotope (13C-labeled or The carbon atom of either orCO2 CH4). CO2 CH4was labeled by 13C, and the photoreaction between andCO2was carried out over The results are shown inCH4 ZrO2 .Table 1. The carbon monoxide evolved into the gas phaseduring photoirradiation was found to stem from the reductionof The CO produced by heat treatment after the photo-CO2 .reaction contained the carbon atom only from asCO2 ,reported previously.7 This shows that is reduced to COCO2by whereas is hardly oxidized to CO. On the otherCH4 , CH4

Table 1 The origin of the carbon atom contained in the productsa

Products Origin of carbon atom

CO produced by photoreaction From CO2A from CH4 bCO produced by heat treatment after Only from CO2photoreactionCO2 produced by O2 treatment after From CO2\ from CH4 c

evacuation at increased temperature

a The origin of the carbon atom was traced using 13C isotope experi-ments after photoreaction either between and or13CO2 12CH4between and for 6 h at room temperature. Substrates :12CO2 13CH4150 lmol, 50 lmol. b The signal assigned to CO originatingCO2 CH4from was very weak but the possibility of CO being formed fromCH4can not be excluded. c The signal for originating fromCH4 CO2 CH4was several times stronger than that from CO2 .

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hand, the product evolved by treatment after evacuation atO2773 K was the which originated mainly from TheCO2 CH4 .species not removed from the surface by evacuation at 773 Kare considered as the carbonaceous residue. Therefore, thisresult indicates that used for the photoreduction ofCH4 CO2is not released into the gas phase as a product, but remains onthe surface as a carbonaceous residue. However, the carbon-aceous residue is not only due to The results shown inCH4 .Table 1 indicate that part of the carbon involved in the car-bonaceous residue came from It suggests that some partCO2 .of the carbonaceous residue is made both of andCO2 CH4 .In other words, there are two di†erent species which couldcontribute to the carbonaceous residue.

3.3. Reactions at an increased temperature

The reaction was carried out at an increased temperature of673 K and compared to the results obtained at room tem-perature. The amount of product during the reaction at 673 Kis summarized in Table 2. Without photoirradiation, the reac-tion between and hardly proceeded over andCO2 CH4 ZrO2no products were detected. This agrees with the report that

itself has no activity for the (thermal) reforming ofZrO2 CO2methane, although it shows an excellent performance as thesupport of catalysts for the reaction.16 On the other hand, alarge amount of CO and was produced by the reaction atH2673 K under photoirradiation. As the photoreaction at roomtemperature for 5 h yielded 0.7 lmol of CO,7 the amount ofCO produced by the photoreaction at 673 K is ca. 40 timeslarger than that at room temperature. It is revealed that thephotoreaction of and on is strongly enhancedCO2 CH4 ZrO2by the increase in the reaction temperature.

3.4. EPR spectroscopy

We have previously reported that a anion radical arisesCO2by irradiation of with adsorbed on the surface.10ZrO2 CO2We have also conÐrmed in that study that the radicalCO2~disappears on contact with hydrogen without photoirradia-tion. This result indicates the interaction of hydrogen with thephotoexcited in the dark. Since surface formate has beenCO2found to be an intermediate of the photoreaction between

and over the reaction of hydrogen withCO2 H2 ZrO2 , CO2~to yield the surface formate is strongly suggested. In thisstudy, we recorded EPR spectra to try to observe the inter-action between the photoexcited species andCO2 CH4 .

Fig. 6(a) illustrates the EPR spectrum of withZrO2adsorbed under irradiation. According to the data in theCO2literature,17 we have assigned the sharp signal at g \ 2.002and g \ 1.996 to the radical species.10 The assignmentCO2~is also supported by the fact that the signal was split into twowhen was adsorbed instead of The amount of13CO2 12CO2 .the radical was determined using TEMPOL as a stan-CO2~dard, and calculated at 7.7 nmol. It has already been shownthat, once produced by photoirradiation, this radical species isstable and maintains its signal intensity to an almost constantlevel in the dark for at least 1 h.10

When the radical was exposed to in the dark,CO2~ CH4the signal assigned to the radical disappeared within 30CO2~min, as is shown in Fig. 6(b) and (c). This result suggests thatthe radical reacts with in the dark to yield theCO2~ CH4

Table 2 The amount of CO produced during the reaction carriedout at 673 Ka

Conditions Amount of CO produced/lmol

In the dark N.d.bUnder irradiation 26.6

a Substrates : 150 lmol 50 lmol ; reaction period 5 h. b NotCO2 CH4detected.

Fig. 6 EPR spectra of (a) under photoirradiation withZrO2adsorbed (b) 5 min after introduction of to (a) in the dark,CO2 , CH4and (c) 30 min after introduction of to (a). All spectra wereCH4recorded at room temperature.

surface species. This phenomenon is quite similar to thatobserved when is used for the reductant of hydrogenH2 CO2 :also reacts with on without photoirradiation.10CO2~ ZrO2There still remains a signal at around g \ 2 after 30 minfollowing the introduction of as is shown in Fig. 6(c).CH4 ,Because the appearance of such a signal is brought about byirradiation of freshly pretreated (i.e., without anyZrO2adsorbent on the surface),10 the signal remaining after CH4exposure at around g \ 2 is due to itself. According toZrO2the previous studies,10,18,19 the signal should be assigned to acolor center of As the paramagnetic center of wasZrO2 . ZrO2not inÑuenced by the photoexcitation of and the intro-CO2duction of to the surface radical, it is suggestedCH4 CO2~that there is no strong interaction between bulk andZrO2reactants. This aspect has also been observed in the photo-reaction between and overCO2~ H2 ZrO2 .

4. Discussion

4.1. IdentiÐcation of the reaction intermediate

From the resemblance of the shape of the IR spectra, we iden-tiÐed the surface species remaining on the surface after heattreatment at 673 K as the surface acetate. This species has lowreactivity and is considered as part of the carbonaceousresidue.

On the other hand, the existence of a reaction intermediateis expected. Since there is another surface species which isdecomposed to yield CO by heat at around 623 K, this speciescan be supposed to be the reaction intermediate. The di†er-ence IR spectrum between the sample evacuated at 573ZrO2K after the photoreaction and just after pretreatment (Fig. 3)should be the superimposition of the surface acetate and thereaction intermediate.

The known properties of the surface species considered asthe reaction intermediate are listed up as follows :

(1) It is decomposed to yield CO by heat at around 623 K.7(2) It shows absorption in the region between 2800 cm~1

and 3000 cm~1. The reason is that two absorption bands wereobserved at 2970 cm~1 and 2878 cm~1 in Fig. 2, whereas thesurface acetate showed no absorption in the region between2800 cm~1 and 3000 cm~1.

(3) It has an absorption band at around 1380 cm~1, for thesame reason mentioned above.

(4) It yields CO under the reaction condition, since it is thereaction intermediate.

On the other hand, we have already shown that the surfaceformate is the reaction intermediate in the photoreduction of

by over The surface formate is formed byCO2 H2 ZrO2 .14

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the reaction between the radical and and works asCO2~ H2 ,a reductant of other molecules to CO under irradia-CO2tion.10 The known properties of the surface formate are givenas follows : (1) It is decomposed to yield CO by heat at around623 K.6 (2, 3) It shows two absorption peaks in the regionbetween 2800 cm~1 and 3000 cm~1 as well as at 1390 cm~1and 1372 cm~1.14,20,21 (4) It works as a reductant of CO2under irradiation to yield CO.14 All of these properties agreewell with those of the surface species suggested to be a reac-tion intermediate in the photoreduction of by overCO2 CH4Accordingly, we can tentatively assign the reactionZrO2 .intermediate in the and reaction to the surfaceCO2 CH4formate. It is suggested that the surface formate is formed bythe reaction between and together with the surfaceCO2 CH4acetate, and that the surface formate reduces other mol-CO2ecules to CO under irradiation.

4.2. Carbonaceous residue

The surface acetate is considered as part of the carbonaceousresidue which does not react further but stays unchanged onthe surface. It is most natural to think that the surface acetatestems from the reaction of one and one molecule.CO2 CH4Therefore, if the carbonaceous residue were purely made ofthe surface acetate, it would contain both of the carbon atomsfrom and at the same ratio. However, from theCO2 CH4results obtained by the reaction using 13C-labeled orCO2it was found that the carbon atom contained in the car-CH4 ,bonaceous residue predominantly originates from ratherCH4than This indicates the existence of species other thanCO2 .the surface acetate on the surface as part of the carbonaceousresidue, which is mainly made of Such species seemedCH4 .not to be detected by an IR spectroscopic method in thisstudy. It may be a carbonaceous deposition species describedas The existence of the carbonaceous deposit is alsoÈ(CH

x)nÈ.

suggested by the result that the color of the sampleZrO2turned brown after the photoreaction between andCO2There are some articles reporting the IR bands of theCH4 .7surface carbonaceous deposit at 1550 cm~1 and 1450 cm~1,which are assigned to the CÈH deformation mode.22 Indeedwe also observed the absorption bands at 1550 cm~1 and1460 cm~1, however, in our case no bands are observed inCÈH stretching region. We thus judged that there are no car-bonaceous species giving detectable IR bands due to CÈHbonding. The carbonaceous deposit in this reaction may be avery highly carbonaceous one. The highly carbonaceousspecies has been reported to show an IR absorption at around1585 cm~1 which is assigned to the stretching vibration ofgraphitic carbon structures.23h25 In our case the absorption ofthe highly carbonaceous species may be hindered by thestrong absorption bands at around 1540 cm~1 due to thesurface acetate.

As the process for the formation of the carbonaceousdeposit, we can suggest that loses some hydrogen atomsCH4during the reduction of to the surface formate. ThisCO2process results in the formation of species which areÈ(CH

x)nÈ

derived from only. Since a little amount of hydrogen wasCH4detected as one of the products during the photoreaction, partof the hydrogen is not incorporated into HCOO~ but rel-eased to the gas phase as a molecule. In addition, there is aH2possibility that the reactive may react with toÈ(CH

x)nÈ CO2reduce it to CO directly. The process in which the È(CH

x)nÈ

loses hydrogen is highly speculative and not yet clariÐed. Inthe thermal reforming of the surface oxygenCO2 CH4 ,species is reported to play an important role in the oxidationof the species to CO.2 In the photoreaction betweenCH

xCO2and over the supply of the surface oxygen speciesCH4 ZrO2 ,

may malfunction at room temperature on the surface.ZrO2In summary, the carbonaceous residue is supposed to becomposed of two species ; the surface acetate and another

species suggested to be the carbonaceous deposit. The lattermay be made of the carbon from CH4 .

4.3. Proposed reaction mechanism

The EPR signal assigned to the radical disappeared onCO2~contact with methane. This result indicates the interaction ofmethane with the radical, and strongly suggests theCO2~reaction between the two. The reaction between andCO2does not proceed without irradiation even at 673 K. ItCH4also supports the suggestion that the radical reacts withCO2~because the radical is formed on onlyCH4 , CO2~ ZrO2under irradiation. It means that the radical is a keyCO2~species. On the other hand, it has rarely been reported thatmethane is dissociatively adsorbed on at room tem-ZrO2perature. Therefore, the radical is supposed to help theCO2~activation of Since the cleavage of the CÈH bond by aCH4 .radical species is not so surprising, the CÈH bond of mayCH4be cut by the radical to yield the surface formate or theCO2~surface acetate. The amount of the radical observed byCO2~EPR was very small ; however, considering that the radicalwill be produced continuously under reaction conditions, the

radical can reasonably be regarded as an active speciesCO2~in the reaction.As the formation mechanism of the surface acetate, we can

simply propose the reaction between and to yieldCO2~ CH4and OH~. The OH~ is produced by the reactionCH3COO~between hydrogen and surface oxygen species of con-ZrO2 ,suming a positive hole. In the same manner, the surfaceformate may also be produced by the reaction between CO2~and which results in the formation of HCOO~ andCH4 CH3species. The species does not react with the surfaceCH3oxygen to yield methoxy species, since no absorption bandsassigned to the surface methoxy species13,21 were observed inthe IR spectrum. It may react further and lose more hydrogenatoms to become a species, which is part of the carbon-CH

xaceous residue. During the further reaction, the hydrogen offorms the OH~ species with the surface oxygen ofCH3 ZrO2and the positive hole. In addition, part of the hydrogen ofis released to the gas phase as and the other part ofCH3 H2 ,

the hydrogen may be consumed to reduce to the surfaceCO2formate.As stated above, the surface acetate does not react by itself

or with but remains on the surface. Therefore, theCO2 ,surface acetate is not related to the reduction of to CO.CO2The surface formate is a reaction intermediate to yield gaseousCO. In the study investigating the photoreduction of byCO2over it has already been clariÐed that the surfaceH2 ZrO2 ,formate works as a reductant of to CO under irradiationCO2of The reaction mechanism following the formationZrO2 .14of the surface formate is common between the cases thateither or is used as a reductant ofH2 CH4 CO2 .

Based on the above discussion, we propose the reactionmechanism presented in Scheme 1 for the photoreduction of

with overCO2 CH4 ZrO2 .The carbon dioxide adsorbed on the surface of is pho-ZrO2toexcited under irradiation of to the anionZrO2 CO2~radical. The radical reacts with to yield both theCO2~ CH4surface acetate and the surface formate. The former does not

react further but stays on the surface. On the other hand, the

Scheme 1

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latter works as a reductant of another to CO under irra-CO2diation. During the reduction of by the surface formate,CO2the formate itself is perhaps oxidized to become the adsorbedspecies again. The production of CO proceeds via theCO2above two-step reaction (i.e., formation of surface formate and

reduction of to CO by the surface formate).CO2We observed the great enhancement of the activity in thephotoreaction of and at 673 K. Considering theCO2 CH4reaction mechanism as well as the fact that the surfaceformate is decomposed at around 623 K to yield CO,6,14 onecan conclude that the enhancement is caused by the decompo-sition of the surface formate by heat. At 673 K the surfaceintermediate need not work as a reductant of another CO2molecule. Instead, the surface formate itself is decomposed at673 K to yield CO directly. In other words, at 673 K thereaction mechanism is simpliÐed to a one step reaction.However, we cannot exclude the e†ect of the enhancement ofthe thermal step in the reaction by heat energy. The enhance-ment of the reaction at elevated temperature may be causedby the above two factors.

5. ConclusionIt was clariÐed that several surface species (formate, acetateand highly carbonaceous species) arise during the photoreac-tion between and on It is suggested that theCO2 CH4 ZrO2 .reaction mechanism is similar to that of photoreductionCO2by over except the point that a carbonaceousH2 ZrO2 ,residue is formed by the reaction of and This canCO2 CH4 .be rephrased as follows : the photoreduction of to COCO2proceeds regardless of the reductant, so long as the surfaceformate is formed.

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