Artificial Photosynthesis over Crystalline TiO2-Based
Arti?cial Photosynthesis over Crystalline TiO2-Based
Catalysts:Fact or Fiction?
Chieh-Chao Yang,?Yi-Hui Yu,?Bart van der Linden,?Jeffrey C.S.Wu,?and
Guido Mul*,?,§
Catalysis Engineering,Department of Chemical Engineering,Delft Uni V ersity of Technology, Julianalaan136,2628BL Delft,The Netherlands,Catalysis and Reaction Engineering Laboratory,Department of Chemical Engineering,National Taiwan Uni V ersity,No.1,Section4,
Roose V elt Road,Taipei,10617Taiwan,and PhotoCatalytic Synthesis Group,IMPACT Institute, Uni V ersity of Twente,P.O.Box217,7500AE Enschede,The Netherlands
Received February13,2010;E-mail:G.Mul@tnw.utwente.nl
Abstract:The mechanism of photocatalytic conversion of CO2and H2O over copper oxide promoted titania, Cu(I)/TiO2,was investigated by means of in situ DRIFT spectroscopy in combination with isotopically labeled 13CO2.In addition to small amounts of13CO,12CO was demonstrated to be the primary product of the reaction by the2115cm-1Cu(I)-CO signature,indicating that carbon residues on the catalyst surface are involved in reactions with predominantly photocatalytically activated surface adsorbed water.This was con?rmed by prolonged exposure of the catalyst to light and water vapor,which signi?cantly reduced the amount of CO formed in a subsequent experiment in the DRIFT cell.In addition,formation of carboxylates and(bi)carbonates was observed by exposure of the Cu(I)/TiO2surface to CO2in the dark.These carboxylates and(bi)carbonates decompose upon light irradiation,yielding predominantly CO2.At the same time a novel carbonate species is produced(having a main absorption at～1395cm-1)by adsorption of photocatalytically produced CO on the Cu(I)/TiO2surface,most likely through a reverse Boudouard reaction of photocatalytically activated CO2with carbon residues.The?nding that carbon residues are involved in photocatalytic water activation and CO2reduction might have important implications for the rates of arti?cial photosynthesis reported in many studies in the literature,in particular those using photoactive materials synthesized with carbon containing precursors.
Introduction
In photosynthesis,solar energy is converted to chemical energy by reaction of CO2and H2O to e.g.glucose and O2.It has been reported that titania-based catalysts induce arti?cial photosynthesis,yielding single-carbon molecules in photocata-lytic CO2reduction,such as CO,CH4,CH3OH,formaldehyde, and formic acid.Titania catalysts were?rst used in aqueous suspension for photoelectrocatalytic CO2reduction.1Hirano et https://www.wendangku.net/doc/ff3370200.html,ed copper-metal supported TiO2suspended in aqueous solution for photocatalytic CO2reduction.2,3CH3OH and HCHO were detected to be the main products.During illumination,trace amounts of formic acid were also detected in the liquid phase, while CO and CH4appeared in the gas phase.Tseng et al. con?rmed these data and reported that illumination of titania-supported copper catalysts(Cu/TiO2)in the presence of CO2in the liquid phase resulted in the formation of methanol.4,5For2wt%Cu/TiO2,the methanol yield reached12.5μmol/g-catalyst/h after20h of irradiation,which was～25times higher than that obtained for TiO2(sol-gel method)and3times higher than that for Degussa P25TiO2tested in the same system. Recently,Wu et al.also tested Cu(I)/TiO2materials in an optical-?ber reactor for gas phase photocatalytic CO2reduction.The maximum methanol yield for1.2wt%Cu(I)/TiO2was0.46μmol/g-catalyst/h under365nm UV irradiation.6Besides these studies on crystalline TiO2based catalysts,Ti-containing siliceous materials,such as TS-1,Ti-MCM-41,Ti-MCM-48,7-9 Ti-ZSM-5,10Ti-zeolite Y,11-13and Ti-SBA-15,14were found
?Delft University of Technology.
?National Taiwan University.
§University of Twente.
(1)Inoue,T.;Fujishima,A.;Konishi,S.;Honda,K.Nature1979,277
(5698),637.
(2)Hirano,K.;Inoue,K.;Yatsu,T.J.Photochem.Photobiol.A:Chem.
1992,64(2),255.
(3)Adachi,K.;Ohta,K.;Mizuno,T.Solar Energy1994,53(2),187.
(4)Tseng,I.H.;Chang,W.C.;Wu,J.C.S.Appl.Catal.,B2002,37(1),
37.
(5)Tseng,I.H.;Wu,J.C.S.;Chou,H.Y.J.Catal.2004,221(2),432.
(6)Wu,J.C.S.;Lin,H.M.;Lai,C.L.Appl.Catal.,A2005,296(2),
194.
(7)Zhang,S.G.;Fujii,Y.;Yamashita,K.;Koyano,K.;Tatsumi,T.;Anpo,
M.Chem.Lett.1997,(7),659.
(8)Anpo,M.;Yamashita,H.;Ikeue,K.;Fujii,Y.;Zhang,S.G.;Ichihashi,
Y.;Park,D.R.;Suzuki,Y.;Koyano,K.;Tatsumi,T.Catal.Today 1998,44(1-4),327.
(9)Ikeue,K.;Yamashita,H.;Anpo,M.;Takewaki,T.J.Phys.Chem.B
2001,105(35),8350.
(10)Yamashita,H.;Shiga,A.;Kawasaki,S.;Ichihashi,Y.;Ehara,S.;Anpo,
M.Energy Con V ers.Manage.1995,36(6-9),617.
(11)Anpo,M.;Yamashita,H.;Ichihashi,Y.;Fujii,Y.;Honda,M.J.Phys.
Chem.B1997,101(14),2632.
(12)Yamashita,H.;Fujii,Y.;Ichihashi,Y.;Zhang,S.G.;Ikeue,K.;Park,
D.R.;Koyano,K.;Tatsumi,T.;Anpo,M.Catal.Today1998,45
(1-4),221.
(13)Keita,I.;Nozaki,S.;Ogawa,M.;Anpo,M.Catal.Today2002,74
(3-4),241
.
Published on Web05/28/2010
10.1021/ja101318k 2010American Chemical Society 83989J.AM.CHEM.SOC.2010,132,8398–8406
to yield high methane production rates in gas phase photocata-lytic CO2reduction.The production yield of highly dispersed titanium oxide catalysts(inμmol/g-Ti/h)was increased10-300 times as compared to crystalline TiO2.Pt was found to further enhance the performance of Ti-MCM-48,resulting in an8-fold increase of CH4over CH3OH selectivity.Despite these numer-ous studies on photoreduction of CO2over TiO2based catalysts, relatively little is known about the surface chemistry and the mechanism of the reaction.Anpo et al.proposed a mechanism for isolated excited(Ti+III-O-I)*sites,based on EPR data,15 over which simultaneous reduction of CO2and decomposition of H2O are proposed to lead to CO and C radicals,and H and OH radicals,respectively.Subsequently,these photoinduced C, H,and OH radicals recombine to?nal products,such as CH4 and CH3OH.
IR studies focused on photoinduced CO2activation are rare. Rasko et al.16,17observed bent CO2-species on prereduced TiO2 upon illumination at190K,and proposed a mechanism for photon induced decomposition of these species into CO over a Rh/TiO2catalyst.The most comprehensive IR study to date focused on a Ti silicalite molecular sieve(TS-1).18CO was observed as the initial redox product of gaseous CO2photore-duction.Through labeled CO2and CH3OH experiments,the origin of CO was proposed to be the secondary photolysis of HCO2H,which was the2-electron reduction product of CO2 over photoexcited Ti centers generated by a LMCT transition (Ti+IV-O-II f Ti+III-O-I).
In the present study the surface chemistry of crystalline Cu(I)/ TiO2was further investigated employing a combination of DRIFT spectroscopy and isotopically labeled13CO2.The strong adsorption of CO on Cu(I)sites was used to identify the origin of this product,indicating that carbon residues are very important in determining the initial reactivity of photocatalysts active in CO2reduction.Moreover,a rich surface carbonate chemistry was observed for Cu(I)/TiO2,with an interconversion of CO2 induced carbonate formed in the dark to CO induced carbonate formed upon illumination.The implications of this study for studies in the literature using photoactive materials synthesized with carbon containing precursors will be discussed. Experimental Section
Material Preparation.Cu(I)/TiO2was synthesized by a modi?ed sol-gel method.The precursors titanium(IV)butoxide(TBOT,Ti-(OC4H9)4)and copper nitrate(Cu(NO3)2·2.5H2O)were used as received.17mL of TBOT,0.15g of(Cu(NO3)2·2.5H2O),2g of polyethylene glycol(PEG),and102mL of0.1M nitric acid(HNO3) were added to induce hydrolysis,and polycondensation was achieved by thermal treatment at353K for28h.The?nal sol was ?ltered,dried at423K for3h,and then calcined at773K for5h with a heating rate of1K/min.Based on elemental analysis,1% (weight basis)copper was deposited.The as-synthesized Cu(I)/TiO2 catalyst had a grass-green color.A reference Cu(I)/TiO2catalyst was prepared according to the same procedure,in the absence of polyethylene glycol(PEG).Finally,TiO2was also prepared following the same procedure,only without adding copper nitrate. In Situ Diffuse Re?ectance Infrared Fourier Transform Spectroscopy.Photocatalytic CO2reduction experiments were carried out using a Nicolet Magna860spectrometer,equipped with a liquid N2cooled MCT detector,and a three window DRIFTS (Diffuse Re?ectance Infrared Fourier Transform Spectroscopy)cell. Two ZnSe windows allowed IR transmission,and a third(Quartz) window allowed the introduction of UV/vis light into the cell.Prior to the illumination experiments,25mg of the as-synthesized catalyst were heated up to393K in He(30mL/min)for0.5h,to remove the majority of adsorbed water without changing the oxidation state of copper oxide.Before recording a background spectrum of the still grass-green catalyst,CO2(50vol%in He,20mL/min)was purged for20min.For experiments involving water vapor,CO2 was bubbled through a saturator at room temperature(300K),which added approximately4vol%water vapor to the CO2feed.During illumination,reactants were held stationary in the cell at room temperature(303K).In situ IR signals were thus recorded every 10min under UV/vis light irradiation(100W Hg lamp,λ:250-600 nm).
CO2(Linde Gas,99.995%),13CO2(ISOTEC,99.9%13C),CO (Linde Gas,5%in He),and13CO(ISOTEC,99%13C)were used as received.CO(or13CO)adsorption experiments were performed by introducing CO(2500ppm in He,20mL/min)over Cu(I)/TiO2 for20min.To estimate the CO adsorption capacity,He(30mL/ min)was used to?ush the catalyst and remove weakly adsorbed CO molecules.
To further illustrate the role of carbonates in photocatalytic conversion of CO2,an illumination experiment was conducted in the absence of CO2,after pre-exposure of the surface of the catalyst to CO2.Speci?cally a?ush-dose cycle of exposure of the catalyst to13CO2for20min,followed by?ush in He,was repeated four times,to increase the surface concentration of13C-labeled carbonates. Coking Experiments.The catalyst under investigation was also pretreated to achieve different degrees of coking.Coked catalysts were prepared by introducing a batch of fresh Cu(I)/TiO2catalyst (70mg)into an isobutane?ow at873K(30mL/min consisting of 1%C4H10and50%CO2).By varying the exposure time,variable amounts of coke were successfully deposited on Cu(I)/TiO2. Results
Illumination of Cu(I)/TiO2in Different Conditions.Figure1 shows DRIFT spectra of the Cu(I)/TiO2catalyst after80min of illumination in different atmospheres,against background spectra of the catalyst obtained after?ushing with the different respective gas compositions for20min.The spectra are dominated by an absorption band at2115cm-1,which can be assigned to the stretching mode of CO,consistent with literature.19-21CH4,often detected by gas chromatography in photocatalytic CO2reduction studies,is not observed,with quantities being too small to be detected in the gas phase in our DRIFT cell(with limited IR path length).Adsorption on the catalyst surface of methane is not expected.CH3OH,should be visible in an adsorbed state if formed in suf?cient quantity, but is not detected.In an inert(He)and oxidizing atmosphere (10%O2/He),a small quantity of CO evolved after80min of illumination.In the case of water vapor(spectrum1c),a signi?cantly higher intensity of the CO band at2115cm-1can be observed.By introducing CO2and water vapor(1d),the CO band broadens and blue-shifts to2117cm-1.The broadening of the CO band suggests that CO2coadsorption slightly alters the nature of the Cu(I)site.Without H2O cofeed(spectrum1e), CO2leads to a CO band of an even higher intensity,which might imply that in the presence of water subsequent hydrogenation of adsorbed CO takes place.13CO2was used to identify the
(14)Hwang,J.S.;Chang,J.S.;Park,S.E.;Ikeue,K.;Anpo,M.Top.
Catal.2005,35(3-4),311.
(15)Anpo,M.;Yamashita,H.;Ichihashi,Y.;Ehara,S.J.Electroanal.
Chem.1995,396(1-2),21.
(16)Rasko,J.;Solymosi,F.J.Phys.Chem.1994,98(29),7147.
(17)Rasko,J.Catal.Lett.1998,56(1),11.
(18)Ulagappan,N.;Frei,H.J.Phys.Chem.A2000,104(33),7834.(19)Hadjiivanov,K.;Klissurski,D.;Kantcheva,M.;Davydov,A.J.Chem.
Soc.,Faraday Trans.1991,87(6),907.
(20)Coloma,F.;Marquez,F.;Rochester,C.H.;Anderson,J.A.Phys.
Chem.Chem.Phys.2000,2(22),5320.
(21)Venkov,T.;Hadjiivanov,https://www.wendangku.net/doc/ff3370200.html,mun.2003,4(4),209.
J.AM.CHEM.SOC.9VOL.132,NO.24,20108399
Arti?cial Photosynthesis over TiO2-Based Catalysts A R T I C L E S
origin of the CO product.Spectrum1f shows two CO bands,at 2069and2117cm-1.The former one is assigned to adsorbed 13CO,in agreement with a calculation based on the harmonic equation22and spectra after dosing13CO over Cu(I)/TiO2,which will be described later.Unexpectedly,there is still a majority of12CO formed during illumination,despite the absence of 12CO2,demonstrating that carbon residues on the catalyst surface are involved in the reaction.Thermal gravimetric analysis(TGA) was performed for the as-synthesized catalyst,which showed no distinguishable weight loss,indicating that these residues are present in small quantities and cannot be easily removed by calcination.
In the absence of water vapor,the intensity of the band of adsorbed CO was enhanced(compare spectra1f and1g),in agreement with the experiment conducted with12CO2.As expected,in reference experiments over pure TiO2,CO absorp-tion bands in the2115cm-1region were absent,indicating that Cu(I)sites serve as a probe to visualize CO formation in IR spectroscopy.
Figure2shows the spectral development in the region of carbonate absorptions(1200-1800cm-1)during an experiment where Cu(I)/TiO2is illuminated in an atmosphere of13CO2 (compare Figure1g).In the presence of13CO2,irradiation enhances carbonate intensities.There are also decreasing bands observable(at approximately1650and1210cm-1),indicating that speci?c surface species are involved in the formation of CO.
To further evaluate the dynamics in the intensities of the carbonate vibrations,presaturation of the TiO2surface by treatment with13CO2was conducted.The13C-labeled carbonate containing surface was subsequently illuminated in a He
atmosphere.The spectral changes are displayed in Figure3.
Clearly12CO(2115cm-1)is formed upon illumination,together
with a minor amount of13CO,in agreement with the data shown
in Figure1.Rather than positive carbonate features,as observed
in the presence of gas phase13CO2,negative features are
observed in the spectral region of1800to1200cm-1,including
these at1650and1210cm-1,indicating that carbonates are
decomposing upon illumination.This mainly produces gas phase 13CO2,as is evident from IR absorption features at2280cm-1. In addition,the complex spectral signature in the carbonate
region contains positive contributions at～1560,1420,and ～1350cm-1,which can be assigned to the formation of carbonate species formed by(re)adsorption of CO,as will be
discussed in the following paragraph.Finally Figure3shows
signi?cant depletion in the hydroxyl region(3000to4000cm-1),
suggesting that hydroxyl groups and surface adsorbed water are
participating in the surface reactions.
Reference Spectra.To allow a better comprehension of the changes in the carbonate region(see Figures2and3),Figure4 shows the deconvolution of the region of the carbonate bands, formed by exposure of the Cu(I)/TiO2catalyst to CO2or13CO2, respectively.The corresponding band assignments are sum-marized in Table1.In agreement with literature data,23,24we assign the bands to bidentate carbonates(1363,1554cm-1for CO2and1319,1508cm-1for13CO2),monodentate carbonates (1409cm-1for CO2and1374cm-1for13CO2),bicarbonates (1481,1663cm-1for CO2and1461,1649cm-1for13CO2) and carboxylates(1663cm-1for CO2and1649cm-1for13CO2). The assignment at1663and1649cm-1is ambiguous,due to the dif?culty of distinguishing the contributions of bicarbonates or carboxylates.The preadsorbed carbonates were in a separate experiment exposed to D2O,to evaluate if speci?c carbonate bands would shift,providing further evidence for spectral assignment.Unfortunately,D2O has strong absorptions in the
(22)Harmonic equation was useful for calculating the theoretical peak shift
for isotopic molecules.The vibration frequency is inverse-proportional to the square root of reduced mass.
ν)
1
2π
, κμ,μ)m1×m2
m
1
+m
2
,ν∝ 1μ
ν
13CO )2117× (12×16)/(12+16)
(13×16)/(13+16)
)2069cm-1
(23)Davydov,A.A.Infrared spectroscopy of adsorbed species on the
surface of transition metal oxides;John Wiley&Sons:Chichester:
1990.
(24)Su,W.G.;Zhang,J.;Feng,Z.C.;Chen,T.;Ying,P.L.;Li,C.J.
Phys.Chem.C2008,112(20),7710
.
Figure1.FT-IR spectra of Cu(I)/TiO2obtained after80min of irradiation in the presence of(a)He,(b)10%O2/He,(c)water vapor,(d)12CO2and water vapor,(e)12CO2,(f)13CO2and water vapor,and(g)13CO2.
8400J.AM.CHEM.SOC.9VOL.132,NO.24,2010
A R T I C L E S Yang et al.
1200cm -1region,disguising any shifts in position of the 1210cm -1band.In addition,bands at 1650and 1210cm -1have been assigned to a bent CO 2-conformation,formed by CO 2adsorp-tion on Ti 3+-sites in the vicinity of Rh.16,17Following this assignment,the 1650and 1210cm -1bands observed in the present study might be associated with CO 2adsorption in the vicinity of the Cu(I)centers.Among the surface species,bidentate carbonates dominate the spectra and are thermally the most stable species.It must be noted that the control experiments (CO 2and 13CO 2)over pure TiO 2synthesized by the same sol -gel method showed similar surface species,which con?rms that most carbonates,bicarbonates,and carboxylates are ad-sorbed on titania,not on copper sites.
Figure 5contains reference spectra obtained by adsorption of CO and 13CO on the catalyst surface,in the absence or presence of CO 2.Cu(I)/TiO 2was ?rst exposed to CO,followed by a He ?ush.A strong adsorption of CO is observed,with a band composed of two contributions at 2107and 2115cm -1.By introducing 13CO 2the band at 2107cm -1rapidly disappears,and the band at 2115cm -1blue shifts to 2120cm -1,in agreement with observations reported in the literature.This blue shift was explained by a dynamic interaction between adsorbed CO and CO 2molecules from the gas phase.25,26After removing CO 2by purging with He,the band of adsorbed CO gradually red shifts back to 2115cm -1.The stability of adsorbed CO under illumination is shown in Figure 5b.Clearly desorption
is
Figure 2.Trend in carbonate formation over Cu(I)/TiO 2in the presence of 13CO 2during 80min of illumination.Time-pro?led DRIFT spectra between 0.4
and 80min of
illumination.
Figure 3.Time-pro?led FT-IR spectra of Cu(I)/TiO 2,preloaded with
13
CO 2.Spectra were recorded after illumination times of 10,30,60,and 80min,
respectively.
J.AM.CHEM.SOC.
9
VOL.132,NO.24,20108401
Arti?cial Photosynthesis over TiO 2-Based Catalysts A R T I C L E S
stimulated by illumination,in view of the signi?cant reduction in intensity of the band at 2070cm -1.A slight positive growth is observed at ～2115cm -1,again indicative of conversion of a carbon residue by surface adsorbed water.
Figure 6shows the (deconvoluted)carbonate intensities formed upon exposure of the Cu(I)/TiO 2catalyst to CO and 13
CO,respectively.While the features are similar to those obtained by adsorption of CO 2(compare Figure 4),intensity differences can be noted.In particular,bands at 1563,1419,and 1349cm -1are indicative of CO adsorbed on surface Ti(O)sites as bidentate and monodentate carbonates,at 1492cm -1as bicarbonates,and at 1665cm -1as the contribution of bicarbonates and carboxylates.A corresponding peak assignment
can be made for 13CO adsorbed on surface Ti(O)sites at 1569,1378,and 1315cm -1as bidentate and monodentate carbonates,at 1468cm -1as bicarbonates,and at 1645cm -1as the contribution of bicarbonates and carboxylates.
Illumination of Pretreated Cu(I)/TiO 2.To eliminate the contribution of surface carbon species,the catalyst was pre-treated for a prolonged period of time in moist air under UV illumination.The subsequent experiment with preloaded 13CO 2is shown in Figure 7.As compared to the fresh catalyst,much less CO is produced upon illumination.Figure 7also shows the amount of CO evolved for a catalyst that was prepared without PEG in the synthesis mixture.An even smaller CO formation rate is observed.
To further evaluate the in?uence of carbon residues on CO 2reduction rates over Cu(I)/TiO 2,coked catalysts were prepared with variable carbon content.By thermal gravimetric analysis (TGA)it was determined that coke amounts of 0.009,0.144,
(25)Tsyganenko,A.A.;Denisenko,L.A.;Zverev,S.M.;Filimonov,V.N.
J.Catal.1985,94(1),10.
(26)Hadjiivanov,K.;Reddy,B.M.;Knozinger,H.Appl.Catal.,A 1999,
188(1-2),355
.
Figure 4.Deconvolution of IR spectra obtained by adsorption of CO 2and
13
CO 2on the surface of Cu(I)/TiO 2.
Table 1.IR Spectral Assignment of Surface Species Formation after Inducing CO 2,
13
CO 2,CO,or
13
CO over Cu(I)/TiO 2
adsorbed molecule
IR peak position (cm -1)
species
vibration mode
CO 2
1363bidentate carbonate νas COO 1409monodentate carbonate νas COO 1481bicarbonate
νs COO 1554bidentate carbonate
νC d O 1663bicarbonate/CO 2-carboxylate νs COO 13
CO 2
1319bidentate carbonate νas COO 1374monodentate carbonate νas COO 1461bicarbonate
νs COO 1508bidentate carbonate
νC d O 1649bicarbonate/CO 2-carboxylate νs COO CO
1349bidentate carbonate νas COO 1419monodentate carbonate νas COO 1492bicarbonate
νs COO 1563bidentate carbonate
νC d O 1665bicarbonate/CO 2-carboxylate νs COO 13
CO
1315bidentate carbonate νas COO 1378monodentate carbonate νas COO 1468bicarbonate
νs COO 1569bidentate carbonate
νC d O 1645bicarbonate/CO 2-carboxylate νs COO
8402
J.AM.CHEM.SOC.
9
VOL.132,NO.24,2010
A R T I C L E S Yang et al.
and 0.297wt %were obtained after reaction times in the applied iso-butene/CO 2mixture of 10,30,and 60min,respectively.Figure 8a shows the DRIFT spectra of coked catalysts recorded after 80min of illumination in the presence of 13CO 2.Only the 12
CO band at 2117cm -1was observed for all the coked catalysts.Furthermore,compared to as-synthesized Cu(I)/TiO 2,coked catalysts show a smaller CO production after 80min of illumination.The more coke is present on the surface of the Cu(I)/TiO 2catalyst,the less CO is formed upon light irradiation.In the spectral region of carbonates (1200-1600cm -1),the same tendency is exhibited:less carbonates accumulate on the surface the higher the coke level of the applied catalyst.To further characterize the coked Cu(I)/TiO 2catalysts,DRIFT analyses of adsorbed CO were performed (Figure 8b),in the presence of gas phase CO (2500ppm),and after a subsequent He ?ush.A signi?cant amount of adsorbed CO can be observed in the presence of gas phase CO,which decreases as a function of increasing coke level.Furthermore,the stability of the adsorbed CO is smaller than observed for the as-synthesized Cu(I)/TiO 2catalyst (see Figure 5),in view of the signi?cant reduction in intensity after a He ?ush.It should be noted that the intensities of adsorbed CO are signi?cantly higher than those obtained after 80min of illumination in the presence of CO 2,which demonstrates that there are still enough Cu(I)sites
to
Figure 5.(a)CO (ads)-13CO 2(g)interaction.FT-IR spectra of Cu(I)/TiO 2after (i)2500ppm CO/He adsorption 20min,(ii)?ush with He 60min,(iii)2500
ppm 13CO 2/He 5min.(iv)2500ppm 13CO 2/He 60min,(v)?ush again with He 5min,and (vi)He 60min.(b)Time-pro?led IR spectra of Cu(I)/TiO 2preloaded with 13CO during 80-min light
irradiation.
Figure 6.Deconvolution of IR spectra obtained by adsorption of CO and
13
CO on the surface of Cu(I)/TiO 2.
J.AM.CHEM.SOC.
9
VOL.132,NO.24,20108403
Arti?cial Photosynthesis over TiO 2-Based Catalysts A R T I C L E S
allow CO adsorption.This shows that for coked catalysts the decreasing CO band (Figure 5a)is related to a smaller CO formation rate upon light irradiation.Finally,the intensity differences and changes in the carbonate region suggest that the Cu(I)/TiO 2surface is indeed largely covered by coke,leading to lower intensities and the lower stability of the CO induced carbonate species.
Discussion
Formation of CO by Carbon Assisted Photocatalytic CO 2Reduction.The experimental data presented herein demonstrate
that CO is formed in signi?cant quantities over Cu(I)/TiO 2catalyst during illumination in the presence of CO 2,in particular if PEG is applied in the synthesis https://www.wendangku.net/doc/ff3370200.html,ing isotopically labeled 13CO 2,it is shown that carbon residues and surface adsorbed water signi?cantly contribute to the formation of CO.The ratio of 12CO over 13CO is approximately 6.Two reactions
can be proposed to explain the formation of 12CO,i.e.the reverse Boudouard reaction,eq 1,and H 2O induced photocatalytic surface carbon gasi?cation,eq 2:
In view of the high 12CO over 13CO ratio,eq 2must have a predominant contribution to the products formed.The source of H 2O is probably surface adsorbed water,which is activated upon illumination.In addition it is well-known that surface OH groups are involved in oxidation reactions over TiO 2surfaces,which might also contribute to CO formation.
As stated in the introduction,usually additional products are obtained upon illumination of a Cu(I)/TiO 2catalyst in the presence of CO 2,including methanol or methane.In view
of
Figure 7.FT-IR spectra of Cu(I)/TiO 2preloaded with
13
CO 2after 80-min https://www.wendangku.net/doc/ff3370200.html,parison of (a)fresh Cu(I)/TiO 2(synthesized with PEG),(b)
Cu(I)/TiO 2cleaned by illumination in humid air for 14h,and (c)reference Cu(I)/TiO 2(synthesized without
PEG).
Figure 8.(a)FT-IR spectra after 80-min light irradiation in the presence of
13
CO 2for Cu(I)/TiO 2,and the coked analogue for 10,30,and 60min;(b)CO
adsorption capacity.Spectra of (i)coked 10min catalyst in the presence of CO,(ii)coked 60min catalyst in the presence of CO,(iii)coked 10min catalyst loaded with CO and then ?ushed by He for 30min,(iv)coked 60min catalyst loaded with CO and then ?ushed by He for 30min,(v)coked 10min catalyst after 80-min light irradiation loaded with 13CO 2,and (vi)coked 60min catalyst after 80-min light irradiation loaded with 13CO 2.
13
CO 2+’12C’f
13
CO +
12
CO (1)H 2O +’12C’f
12
CO +H 2
(2)
8404
J.AM.CHEM.SOC.
9
VOL.132,NO.24,2010
A R T I C L E S Yang et al.
the small quantities produced,we have not observed any spectral features that can be related to these species.However,it is to be expected that by consecutive reaction with H2,CO formed by eqs1and2can be converted to e.g.CH4.
Besides the isotopically labeled experiments,it was also demonstrated by prolonged exposure to UV-irradiation in the presence of water vapor that eq2plays a major role in the formation of CO over Cu(I)/TiO2.In a way it is surprising that adding additional coke did not lead to extensive formation of CO.Apparently the surface coverage achieved was that considerable that the surface had become highly hydrophobic. This limits the reactive adsorption of CO2and,more importantly, signi?cantly decreases the extent of surface hydration,as witnessed by the smaller intensity of the negative water bands in the1620-1680cm-1region(Figure8a).Clearly this is detrimental to the production of CO.Furthermore,the coke layer might simply prevent ef?cient light activation of the catalyst by absorption of light by the carbon layer,as was observed e.g. by Xia et al.27The surface chemistry of Cu(I)/TiO2in the photocatalytic CO2reduction is discussed in more detail in the following.
Surface Carbonate Chemistry.The Cu(I)/TiO2catalyst is very reactive toward CO2and CO at room temperature,leading to various carboxylate,bicarbonate,and carbonate species.Gener-ally IR intensities are higher upon exposure to CO2.When CO2 approaches Cu(I)/TiO2,carbonate species likely form on titania sites which are not fully coordinated,i.e.in the vicinity of the Cu(I)centers.This is in agreement with a study of Rasko et al.,who investigated CO2adsorption and photocatalytic de-composition over prereduced Rh/TiO2.16,17It was demonstrated in their study that,on prereduced Rh/TiO2,oxygen vacancies in the vicinity of Rh offered sites for carboxylate formation, speci?cally with a C atom of CO2linked to Rh and one O atom of CO2bonded to the oxygen vacancy of titania.It is generally accepted that,by illumination with light of suf?cient energy, electron-hole pairs are generated in titania.Rasko et al.propose that electrons are transferred to CO2,yielding CO,while holes are involved in neutralizing the Ti3+sites to Ti4+.It is thus proposed by Rasko et al.that bent adsorbed CO2species are the precursor of CO,with the oxygen atom being incorporated in the TiO2,changing the oxidation state from Ti3+to Ti4+.Also in our study we observe a depletion of the1650and1220cm-1 bands upon illumination,suggesting that these might be involved in the formation of CO by CO2dissociation.However,rather than being incorporated in the Cu(I)/TiO2lattice,we believe the remaining oxygen is predominantly transferred to residual carbon,yielding an additional CO molecule,according to eq1. The experiment conducted on a13CO2pretreated catalyst (Figure3)demonstrates that,in addition to carboxylates/ bicarbonates,CO2induced surface carbonates are also unstable upon illumination,yielding13CO2.As previously discussed,the large selectivity for12CO in the experiment is related to water induced reactions of carbon residues,according to eq2.This is followed by12CO adsorption on Cu(I)sites(2115cm-1)and surface TiO2sites,yielding12CO induced carbonates,explaining the growth of the demonstrated features(at1560,1420,and 1350cm-1)in Figure3.
It should be noted that the(bi)carbonate and carboxylate species formed are most likely a strong function of the extent of surface hydration of the applied TiO2catalysts.Morterra28 has evaluated the interaction of CO with TiO2surfaces.He demonstrated that removal of water molecules and decomposi-tion of surface hydroxyl groups(OH)lead to the formation of surface Lewis acid sites that reversibly chemisorb CO at ambient temperature.While Morterra removed water and hydroxyl groups by evacuation in a vacuum IR cell,it is obvious(Figure 3)that the degree of hydration is also largely affected by the illumination procedure.
Arti?cial Photosynthesis,Fact or Fiction?Many(recent) studies discuss the performance of modi?ed TiO2catalysts in the photocatalytic reduction of CO2.29Generally methane and/ or methanol are the products reported to be formed.In view of our study,the data reported in various related studies should be interpreted with caution.Frei and co-workers already observed for mesoporous materials that carbon residues can be involved in the production of primary products in the photo-catalytic reduction of CO2.30-32In view of our data,also for crystalline TiO2materials,water induced gasi?cation of these carbon residues might be affecting the product quantities and distribution(reaction2).This is particularly true if alkoxides (propoxide,butoxide)were used as the precursor33or if carbon supported TiO2was used.27Also in the synthesis procedure of N-doped TiO2,organic solvents were used,the residue of which might have contributed to the observed activity.34Clearly, contribution of these reactions leads to an overestimation of the rate of the real arti?cial photosynthesis reactions,such as 2CO2+4H2O f2CH3OH+3O2.It is highly recommended in future studies on photocatalytic CO2reactions to add a test of activity in the absence of CO2but in the presence of H2O to exclude the participation of catalyst associated carbon residues in the formation of products.
While there is no doubt that carbon residues contribute to catalytic performance,based on our study we cannot entirely exclude arti?cial photosynthesis.13CO was formed in various experiments,and it is necessary to isolate the reversed Boud-ouard reaction from true arti?cial photosynthesis.This would require careful evaluation of the production of O2.To close the oxygen balance is however extremely dif?cult,and usually irreproducible data are obtained due to?uctuations in back-ground oxygen pressures.The most conclusive evidence that CO is formed in the absence of carbon residues is again provided by Frei and co-workers18in a high vacuum IR cell,in the case of isolated Ti sites in mesoporous materials.Ozonation was a very ef?cient way of removing residual carbon in their study. The onward CO2reduction performed under13CO2and H2O showed merely13CO production,which is signi?cant evidence for true arti?cial photosynthesis.However,mechanistic studies on highly pure crystalline metal oxide systems are needed to de?nitively prove that arti?cial photosynthesis over these materials is fact as well as to further reveal the chemical pathway
(27)Xia,X.H.;Jia,Z.H.;Yu,Y.;Liang,Y.;Wang,Z.;Ma,L.L.Carbon
2007,45(4),717.(28)Morterra,C.J.Chem.Soc.,Faraday Trans.11988,84,1617.
(29)Koci,K.;Obalova,L.;Lacny,Z.Chem.Pap.2008,62(1),1.
(30)Lin,W.Y.;Frei,H.J.Am.Chem.Soc.2005,127(6),1610.
(31)Lin,W.Y.;Frei,H.J.Phys.Chem.B2005,109(11),4929.
(32)Lin,W.Y.;Frei,H.C.R.Chimie2006,9(2),207.
(33)Koci,K.;Obalova,L.;Matejova,L.;Placha,D.;Lacny,Z.;Jirkovsky,
J.;Solcova,O.Appl.Catal.,B2009,89(3-4),494.
(34)Varghese,O.K.;Paulose,M.;LaTempa,T.J.;Grimes,C.A.Nano
Lett.2009,9(2),731.
J.AM.CHEM.SOC.9VOL.132,NO.24,20108405
Arti?cial Photosynthesis over TiO2-Based Catalysts A R T I C L E S
of CO2reduction,including the complexity in the dynamics of surface carbonates.
Conclusions
Carbon residues largely participate in the formation of CO over Cu-promoted crystalline TiO2catalysts,as demonstrated by the combined use of DRIFT spectroscopy and13C labeled CO2.These residues are formed during the catalyst synthesis procedures,often involving the use of Ti-alkoxides and organic solvents,such as polyethylene glycol(PEG).Removal of these residues by thermal activation in air is incomplete,while prolonged exposure to water vapor and UV-irradiation is more ef?cient.Coking of Cu(I)/TiO2showed that extensive carbon coverage of Cu(I)/TiO2diminishes CO formation during il-lumination in the presence of CO2.
Acknowledgment.This work was supported by ACTS(NWO, The Netherlands),in the framework of an NSC-NWO project (Project Number NSC-97-2911-I-002-002).
Supporting Information Available:Additional data?gures. This material is available free of charge via the Internet at http:// https://www.wendangku.net/doc/ff3370200.html,.
JA101318K
8406J.AM.CHEM.SOC.9VOL.132,NO.24,2010
A R T I C L E S Yang et al.
操作系统第二章课后答案
第二章进程管理 2. 试画出下面4条语句的前趋图: S2: b:=z+1; S3: c:=a-b; S4: w:=c+1; 3. 程序在并发执行时，由于它们共享系统资源，以及为完成同一项任务而相互合作， 致使在这些并发执行的进程之间，形成了相互制约的关系，从而也就使得进程在执行期间出现间断性。 4. 程序并发执行时为什么会失去封闭性和可再现性？ 因为程序并发执行时，是多个程序共享系统中的各种资源，因而这些资源的状态是 由多个程序来改变，致使程序的运行失去了封闭性。而程序一旦失去了封闭性也会导致其再失去可再现性。 5. 在操作系统中为什么要引入进程概念？它会产生什么样的影响? 为了使程序在多道程序环境下能并发执行，并能对并发执行的程序加以控制和描述，从而在操作系统中引入了进程概念。 影响: 使程序的并发执行得以实行。 6. 试从动态性，并发性和独立性上比较进程和程序? a. 动态性是进程最基本的特性，可表现为由创建而产生，由调度而执行，因得不到资源 而暂停执行，以及由撤销而消亡，因而进程由一定的生命期；而程序只是一组有序指令的集合，是静态实体。 b. 并发性是进程的重要特征，同时也是OS的重要特征。引入进程的目的正是为了使其 程序能和其它建立了进程的程序并发执行，而程序本身是不能并发执行的。 c. 独立性是指进程实体是一个能独立运行的基本单位，同时也是系统中独立获得资源和 独立调度的基本单位。而对于未建立任何进程的程序，都不能作为一个独立的单位来运行。 7. 试说明PCB的作用?为什么说PCB是进程存在的唯一标志? a. PCB是进程实体的一部分，是操作系统中最重要的记录型数据结构。PCB中记录了操 作系统所需的用于描述进程情况及控制进程运行所需的全部信息。因而它的作用是使一个在多道程序环境下不能独立运行的程序(含数据)，成为一个能独立运行的基本单位，一个能和其它进程并发执行的进程。 b. 在进程的整个生命周期中，系统总是通过其PCB对进程进行控制，系统是根据进程 的PCB而不是任何别的什么而感知到该进程的存在的，所以说，PCB是进程存在的唯一标志。 8. 试说明进程在三个基本状态之间转换的典型原因. a. 处于就绪状态的进程，当进程调度程序为之分配了处理机后，该进程便由就绪状态变 为执行状态。 b. 当前进程因发生某事件而无法执行，如访问已被占用的临界资源，就会使进程由执行 状态转变为阻塞状态。 c. 当前进程因时间片用完而被暂停执行，该进程便由执行状态转变为就绪状态。 9. 为什么要引入挂起状态？该状态有哪些性质？ a. 引入挂起状态主要是出于4种需要（即引起挂起的原因）: 终端用户的请求，父进程 请求，负荷调节的需要，操作系统的需要。