Reactor design for CO2 photo-hydrogenation toward solar fuels under ambient temperature and pressure
Abstract
Photo-hydrogenation of carbon dioxide (CO2) is a green and promising technology and has received much attention recently. This technique could convert solar energy under ambient temperature and pressure into desirable and sustainable solar fuels, such as methanol (CH3OH), methane (CH4), and formic acid (HCOOH). It is worthwhile to mention that this direction can not only potentially depress atmospheric CO2, but also weaken dependence on fossil fuel. Herein, 1 wt % Pt/CuAlGaO4 photocatalyst was successfully synthesized and fully characterized by ultraviolet-visible light (UV-vis) spectroscopy, X-ray diffraction (XRD), Field emission scanning electron microscopy using energy dispersive spectroscopy analysis (FE-SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET), respectively. Three kinds of experimental photo-hydrogenation of CO2 in the gas phase, liquid phase, and gas-liquid phase, correspondingly, were conducted under different H-2 partial pressures. The remarkable result has been observed in the gas-liquid phase. Additionally, increasing the partial pressure of H-2 would enhance the yield of product. However, when an extra amount of H-2 is supplied, it might compete with CO2 for occupying the active sites, resulting in a negative effect on CO2 photo-hydrogenation. For liquid and gas-liquid phases, CH3OH is the major product. Maximum total hydrocarbons 8.302 mu molg(-1) is achieved in the gas-liquid phase.
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catalysts Article Reactor Design for CO2Photo-Hydrogenation toward Solar Fuels under Ambient Temperature and Pressure Chun-Ying Chen 1, Joseph Che-Chin Yu 1, Van-Huy Nguyen 2, Jeffrey Chi-Sheng Wu 1,*, Wei-Hon Wang 3and Kamila Koˇcí 4 1Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan; [email protected] (C.-Y.C.); [email protected] (J.C.-C.Y.) 2Faculty of Chemical and Environmental Engineering, Lac Hong University, 812431, No. 10 Huynh Van Nghe, Buu Long, Bien Hoa, Dong Nai, Viet Nam; [email protected] 3Chung-Shan Institute of Science and Technology, Tao Yuan 32599, Taiwan; [email protected] 4Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava-Poruba, Czech Republic; [email protected] *Correspondence: [email protected]; Tel.: +886-2-23631994; Fax: +886-2-23623040 Academic Editor: Rajendra S. Ghadwal Received: 27 December 2016; Accepted: 8 February 2017; Published: 16 February 2017 Abstract: Photo-hydrogenation of carbon dioxide (CO 2 ) is a green and promising technology and has received much attention recently. This technique could convert solar energy under ambient temperature and pressure into desirable and sustainable solar fuels, such as methanol (CH 3 OH), methane (CH 4 ), and formic acid (HCOOH). It is worthwhile to mention that this direction can not only potentially depress atmospheric CO 2 , but also weaken dependence on fossil fuel. Herein, 1 wt % Pt/CuAlGaO 4 photocatalyst was successfully synthesized and fully characterized by ultraviolet-visible light (UV-vis) spectroscopy, X-ray diffraction (XRD), Field emission scanning electron microscopy using energy dispersive spectroscopy analysis (FE-SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET), respectively. Three kinds of experimental photo-hydrogenation of CO 2 in the gas phase, liquid phase, and gas-liquid phase, correspondingly, were conducted under different H 2 partial pressures. The remarkable result has been observed in the gas-liquid phase. Additionally, increasing the partial pressure of H 2 would enhance the yield of product. However, when an extra amount of H 2 is supplied, it might compete with CO 2 for occupying the active sites, resulting in a negative effect on CO 2 photo-hydrogenation. For liquid and gas-liquid phases, CH 3 OH is the major product. Maximum total hydrocarbons 8.302 µmol·g−1is achieved in the gas-liquid phase. Keywords: CO2reduction; Pt/CuAlGaO4; photo-hydrogenation; photocatalysis; solar fuels 1. Introduction Nowadays, it is important to develop an efficient and effective method for utilizing carbon dioxide (CO 2 ) greenhouse gas. The CO 2 captured from the atmosphere will be stored in the ground or the ocean [ 1 ]. Another option, which is a more attractive method for CO 2 consumption, is to convert it into useful hydrocarbon fuels directly [ 2 ]. In industry, CO 2 is widely used for Sabatier reaction [ 3 , 4 ] and reverse water-gas shift (RWGS) reaction [ 5 , 6 ]. Regarding the Sabatier reaction, CO 2 reacts with H 2 to form CH 4 and H 2 O at 300–400 ◦ C. As for RWGS reaction, CO 2 firstly reacts with H 2 to form CO and H 2 O; then CO can be easily converted into hydrocarbons by the Fischer-Tropsch reaction. However, both reactions require high temperature and high pressure to reduce CO 2 into hydrocarbons. Thus, it is inevitable to develop another method to conduct CO2reduction. Catalysts 2017,7, 63; doi:10.3390/catal7020063 www.mdpi.com/journal/catalysts
Catalysts 2017,7, 63 2 of 12 Thepossible CO 2 conversion processes, including biological [ 7 ], catalytic [ 8 – 11 ], photocatalytic [ 12 – 14 ], and electrocatalytic [ 15 , 16 ] conversion are summarized in Table 1. Among these processes, photocatalytic CO 2 reduction is one of the most promising technologies and has received much attention recently [ 14 , 17 – 21 ]. CO 2 as a C1-feedstock could be reduced to desirable and sustainable solar fuels at ambient conditions under light irradiation. Table 1. Summary of different processes of CO2reduction. Conversion Process Main Products Advantages Disadvantages Ref. Biological HCOOH: conversion efficiency of (22 ±9) ×10−2%Biological capability to synthesize liquid fuels Complicated and cumbersome biological processes [7] Catalytic (Heterogeneous catalysis) CH 4 : 3.8 × 10 4µ mol · h −1· g −1 catalyst High efficiency High temperature requirement [8] CH 3 OH: 8.8% of CO 2 conversion with the corresponded selectivity of 63% [9] CH3OH: 193.9 g·kgcat−1·h−1[10] Catalytic (Homogeneous catalysis) CH3OH: 79% yield [11] Photocatalytic (Heterogeneous catalysis) CH4: 0.56 µmol h−1·g−1catalyst Storage of solar energy Low efficiency [12] CH3OH: 4.6 µmol h−1·g−1catalyst [13] Photocatalytic (Homogeneous catalysis) CO: 12.66 h−1of TOF [14] Electrocatalytic Liquid fuels Converting CO2directly to liquid fuels (long-chain molecule) High energy barrier needs overcoming [15,16] Ideally, photocatalytic CO 2 reduction could provide an alternative and sustainable pathway to producing desired hydrocarbon products from renewable energy and CO 2 . However, it is noted that their efficiency is still relatively low. To enhance the photocatalytic performances, several factors—such as light harvesting, loss of photons, product separation, and charge carrier recombination—are considered in our group to design and developed in a new photo-reactor. In 2007, a circulated photocatalytic reactor was developed, which could provide a large specific surface area and uniformity of gas concentrations in the reactor [ 22 ]. In 2008, the optical-fiber photo-reactor, which could deliver light efficiently and uniformly to the surface of a photocatalyst, was also successfully designed [ 23 ]. It is important to note that there are two main sources to conduct CO 2 reduction, H 2 O (g) and H 2(g) . In a previous study, Abbott et al. calculated a series of changes of enthalpy ( ∆ H 0 ) and changes of Gibbs free energy ( ∆ G 0 ), respectively, for CO 2 reduction to form hydrocarbons [ 24 ]. The result clearly shows that adding H 2 largely decreases these values. That is, the CO 2 reducing reaction in which the involved H 2 requires lower energy and becomes more spontaneous. Therefore, in 2013, a novel twin reactor was successfully developed to hydrogenate CO 2 into CH 3 OH [ 20 ]. However, it is noted that its efficiency is still relatively low. In this study, 1 wt % Pt/CuAlGaO 4 was prepared using a well-known solid-state fusion and photo-deposition method, respectively. Detailed characterization of photocatalyst was conducted to reveal its structure. Three kinds of reactors—including gas phase reactor, liquid phase reactor, and gas-liquid reactor—were designed and employed to study the influence of hydrogen (H 2 ) and carbon dioxide (CO 2 ) on the photo-hydrogenation of CO 2 . Additionally, a possible reaction pathway of CO 2 photo-hydrogenation is also proposed based on the knowledge of products presented during the photocatalytic reaction. 2. Results and Discussion 2.1. Characterization of Photocatalysts All photocatalysts were fully characterized by several techniques. Brunauer-Emmett-Teller (BET), ultraviolet-visible light (UV-vis) spectroscopy; X-ray diffraction (XRD), transmission electron microscopy (TEM), and Field emission scanning electron microscopy (FE-SEM) using energy dispersive spectroscopy (EDS) analysis were used to reveal their structure and surface morphology
Catalysts 2017,7, 63 3 of 12 of photocatalysts. X-ray photoelectron spectroscopy (XPS) was used to reveal the chemical state of the species. Figure 1displays the UV-vis light absorption spectra of CuAlGaO 4 and 1 wt % Pt/CuAlGaO 4 photocatalysts, respectively. Clearly, all the photocatalysts have the absorption band in the range of 250–670 nm, which is consistent with that reported previously [ 25 ]. It notes that loading of the Pt neither significantly improves the light absorption nor changes the band gap. A possible reason comes from the fact that Pt loading is very small and highly dispersed on the supporting surface. Catalysts2017,7,633of12 Figure1displaystheUV‐vislightabsorptionspectraofCuAlGaO4and1wt%Pt/CuAlGaO4 photocatalysts,respectively.Clearly,allthephotocatalystshavetheabsorptionbandintherangeof 250–670nm,whichisconsistentwiththatreportedpreviously[25].ItnotesthatloadingofthePt neithersignificantlyimprovesthelightabsorptionnorchangesthebandgap.Apossiblereason comesfromthefactthatPtloadingisverysmallandhighlydispersedonthesupportingsurface. Figure1.Ultraviolet‐visiblelight(UV‐vis)spectraforCuAlGaO4and1wt%Pt/CuAlGaO4 photocatalysts. Figure2showstheXRDpatternsofCuAlGaO4,1wt%Pt/CuAlGaO4photocatalystsandtheir startingmaterialsofCuO,Al2O3,andGa2O3.Clearly,nopeakscanbeassignedtoeitherCuO,Al2O3, orGa2O3,indicatingthatcompletereactionoftheprecursorswasachievedbysolid‐statefusiona mixtureofAl2O3,Ga2O3,andZnOat1125Kfor12h.TheloadingofPtobviouslyretainsthestructural featuresofCuAlGaO4photocatalyst,butitscrystallitesappeartobeslightlydecreased.Both photocatalystshavepatternssimilartothatofthesingle‐phasespineltypestructureoftheCuAlGaO4 (JCPDSfile,cardNo.26‐0514).However,thereisnonoticeablecrystallinephaseobservedintheXRD patternofPt/CuAlGaO4photocatalystthatcanbeattributedtoPtelement.Asdiscussedabove,the Ptloadingmightbeverysmallandhighlydispersedonthesupportingsurface,whichisconsistent withtheUV‐visspectrum. Figure2.X‐raydiffraction(XRD)patternsofphotocatalysts:CuAlGaO4and1wt%Pt/CuAlGaO4,in comparedwithstartingmaterialsofCuO,Al2O3,andGa2O3. Figure 1. Ultraviolet-visible light (UV-vis) spectra for CuAlGaO4and 1 wt % Pt/CuAlGaO4photocatalysts. Figure 2shows the XRD patterns of CuAlGaO 4 , 1 wt % Pt/CuAlGaO 4 photocatalysts and their starting materials of CuO, Al 2 O 3 , and Ga 2 O 3 . Clearly, no peaks can be assigned to either CuO, Al 2 O 3 , or Ga 2 O 3 , indicating that complete reaction of the precursors was achieved by solid-state fusion a mixture of Al 2 O 3 , Ga 2 O 3 , and ZnO at 1125 K for 12 h. The loading of Pt obviously retains the structural features of CuAlGaO 4 photocatalyst, but its crystallites appear to be slightly decreased. Both photocatalysts have patterns similar to that of the single-phase spinel type structure of the CuAlGaO 4 (JCPDS file, card No. 26-0514). However, there is no noticeable crystalline phase observed in the XRD pattern of Pt/CuAlGaO 4 photocatalyst that can be attributed to Pt element. As discussed above, the Pt loading might be very small and highly dispersed on the supporting surface, which is consistent with the UV-vis spectrum. Catalysts2017,7,633of12 Figure1displaystheUV‐vislightabsorptionspectraofCuAlGaO4and1wt%Pt/CuAlGaO4 photocatalysts,respectively.Clearly,allthephotocatalystshavetheabsorptionbandintherangeof 250–670nm,whichisconsistentwiththatreportedpreviously[25].ItnotesthatloadingofthePt neithersignificantlyimprovesthelightabsorptionnorchangesthebandgap.Apossiblereason comesfromthefactthatPtloadingisverysmallandhighlydispersedonthesupportingsurface. Figure1.Ultraviolet‐visiblelight(UV‐vis)spectraforCuAlGaO4and1wt%Pt/CuAlGaO4 photocatalysts. Figure2showstheXRDpatternsofCuAlGaO4,1wt%Pt/CuAlGaO4photocatalystsandtheir startingmaterialsofCuO,Al2O3,andGa2O3.Clearly,nopeakscanbeassignedtoeitherCuO,Al2O3, orGa2O3,indicatingthatcompletereactionoftheprecursorswasachievedbysolid‐statefusiona mixtureofAl2O3,Ga2O3,andZnOat1125Kfor12h.TheloadingofPtobviouslyretainsthestructural featuresofCuAlGaO4photocatalyst,butitscrystallitesappeartobeslightlydecreased.Both photocatalystshavepatternssimilartothatofthesingle‐phasespineltypestructureoftheCuAlGaO4 (JCPDSfile,cardNo.26‐0514).However,thereisnonoticeablecrystallinephaseobservedintheXRD patternofPt/CuAlGaO4photocatalystthatcanbeattributedtoPtelement.Asdiscussedabove,the Ptloadingmightbeverysmallandhighlydispersedonthesupportingsurface,whichisconsistent withtheUV‐visspectrum. Figure2.X‐raydiffraction(XRD)patternsofphotocatalysts:CuAlGaO4and1wt%Pt/CuAlGaO4,in comparedwithstartingmaterialsofCuO,Al2O3,andGa2O3. Figure 2. X-ray diffraction (XRD) patterns of photocatalysts: CuAlGaO 4 and 1 wt % Pt/CuAlGaO 4 , in compared with starting materials of CuO, Al2O3, and Ga2O3.
Catalysts 2017,7, 63 4 of 12 Figure 3shows the SEM images with corresponded elemental spectra of CuAlGaO 4 and 1 wt % Pt/CuAlGaO 4 photocatalysts, respectively. For CuAlGaO 4 photocatalyst, it has cube-like particles and smooth surfaces. Most importantly, loading of Pt on CuAlGaO 4 could not change the shape and morphology of the photocatalyst. Additionally, a uniform distribution of the Pt particles was found with a narrow size range of 4–20 nm. Catalysts2017,7,634of12 Figure3showstheSEMimageswithcorrespondedelementalspectraofCuAlGaO4and1wt% Pt/CuAlGaO4photocatalysts,respectively.ForCuAlGaO4photocatalyst,ithascube‐likeparticlesand smoothsurfaces.Mostimportantly,loadingofPtonCuAlGaO4couldnotchangetheshapeand morphologyofthephotocatalyst.Additionally,auniformdistributionofthePtparticleswasfound withanarrowsizerangeof4–20nm. Figure3.Scanningelectronmicroscopy(SEM)imageswithcorrespondedelementalspectraof(a) CuAlGaO4and(b)1wt%Pt/CuAlGaO4photocatalysts. Table2showselementalanalysisofCuAlGaO4and1wt%Pt/CuAlGaO4,respectively.Itclearly pointsoutthatonly1wt%Pt/CuAlGaO4showedthePtsignal(Figure3).Ontheotherhand,the otherelementalsignals(O,Al,Cu,Ga)wereverysimilarbetweenCuAlGaO4and1wt% Pt/CuAlGaO4photocatalysts. Table2.Energydispersivespectroscopy(EDS)analysisforelementcompositionsofCuAlGaO4and 1wt%Pt/CuAlGaO4photocatalysts. ElementAtomicRatio(%) CuAlGaO4Pt/CuAlGaO4 O61.961.6 Al23.022.0 Cu12.813.0 Ga2.33.2 PtN/D10.2 1N/D—Notdetection. Figure4revealstheTEMimages,whichwereconductedtofurtherunderstandthepresenting ofPtloadingon1wt%Pt/CuAlGaO4photocatalyst.Ithasseveralwell‐dispersedPtnanoparticles withcorrespondedsizeofnear4–20nmarelocatedonthesurfaceofPt/CuAlGaO4photocatalyst. Figure 3. Scanning electron microscopy (SEM) images with corresponded elemental spectra of (a) CuAlGaO4and (b) 1 wt % Pt/CuAlGaO4photocatalysts. Table 2shows elemental analysis of CuAlGaO 4 and 1 wt % Pt/CuAlGaO 4 , respectively. It clearly points out that only 1 wt % Pt/CuAlGaO 4 showed the Pt signal (Figure 3). On the other hand, the other elemental signals (O, Al, Cu, Ga) were very similar between CuAlGaO 4 and 1 wt % Pt/CuAlGaO4photocatalysts. Table 2. Energy dispersive spectroscopy (EDS) analysis for element compositions of CuAlGaO 4 and 1 wt % Pt/CuAlGaO4photocatalysts. Element Atomic Ratio (%) CuAlGaO4Pt/CuAlGaO4 O 61.9 61.6 Al 23.0 22.0 Cu 12.8 13.0 Ga 2.3 3.2 Pt N/D 10.2 1N/D—Not detection. Figure 4reveals the TEM images, which were conducted to further understand the presenting of Pt loading on 1 wt % Pt/CuAlGaO 4 photocatalyst. It has several well-dispersed Pt nanoparticles with corresponded size of near 4–20 nm are located on the surface of Pt/CuAlGaO4photocatalyst.
Catalysts 2017,7, 63 5 of 12 Catalysts2017,7,635of12 Figure4.Transmissionelectronmicroscopy(TEM)imagesofphotocatalysts:(a)CuAlGaO4and(b) 1wt%Pt/CuAlGaO4photocatalysts. XPSPt4fspectraofCuAlGaO4and1wt%Pt/CuAlGaO4photocatalystsareshowninFigure5. Mostimportantly,therearetwopossiblepeaksat73.6–74.1and77.0–77.4eVhavebeenobservedfor 1wt%Pt/CuAlGaO4photocatalyst,whichmaybeattributedto4f7/2and4f5/2ofPtoxide, respectively[26]. Figure5.ThePt4fX‐rayphotoelectronspectroscopy(XPS)spectrafor(a)CuAlGaO4and(b)1wt% Pt/CuAlGaO4photocatalysts. Lastly,theBETsurfaceareaofCuAlGaO4and1wt%Pt/CuAlGaO4wasapproximately0.65and 0.86m2∙g−1,respectively.Bothvalueswerenotveryhighduetothesolid‐statefusionmethodwhich operatesatahightemperature,causingthephotocatalystaggregation. 2.2.PhotocatalyticCO2ReductionwithGas,Liquid,Gas‐LiquidPhaseReactors Inthisstudy,severalblanktestsinalltypeofreactors(gasphase,liquidphase,andgas‐liquid phasereactors)wereconductedtoanswerthequestionwhethertheformationofthereaction productscomefromphoto‐hydrogenationandnotfromCO2contaminationsorthephotocatalyst itself.TheresultoftestswasshowninTable3.TheresultclearlyshowsthatifCO2wasnotintroduced tothereactingsystem,noproductwasgeneratedevenwiththepresenceofthehydrogen, photocatalyst,andlightirradiation.Itisimportanttorememberthatthephotocatalystsynthesis processdidnotuseanycarbon‐containingprecursor.Thisblanktestresultisfurtherconfirmation thatthephotocatalystitselfcontainsnearlynocarbonresidue.Table3alsoclearlyshowsthat,without anyofthreeessentialelementsinperformingthephoto‐hydrogenationreaction,including(a)CO2, (b)photocatalystand(c)lightsource,noproductcouldbedetected. Figure 4. Transmission electron microscopy (TEM) images of photocatalysts: ( a ) CuAlGaO 4 and (b) 1 wt % Pt/CuAlGaO4photocatalysts. XPS Pt4f spectra of CuAlGaO 4 and 1 wt %Pt/CuAlGaO 4 photocatalysts are shown in Figure 5. Most importantly, there are two possible peaks at 73.6–74.1 and 77.0–77.4 eV have been observed for 1 wt % Pt/CuAlGaO 4 photocatalyst, which may be attributed to 4f 7/2 and 4f 5/2 of Pt oxide, respectively [26]. Catalysts2017,7,635of12 Figure4.Transmissionelectronmicroscopy(TEM)imagesofphotocatalysts:(a)CuAlGaO4and(b) 1wt%Pt/CuAlGaO4photocatalysts. XPSPt4fspectraofCuAlGaO4and1wt%Pt/CuAlGaO4photocatalystsareshowninFigure5. Mostimportantly,therearetwopossiblepeaksat73.6–74.1and77.0–77.4eVhavebeenobservedfor 1wt%Pt/CuAlGaO4photocatalyst,whichmaybeattributedto4f7/2and4f5/2ofPtoxide, respectively[26]. Figure5.ThePt4fX‐rayphotoelectronspectroscopy(XPS)spectrafor(a)CuAlGaO4and(b)1wt% Pt/CuAlGaO4photocatalysts. Lastly,theBETsurfaceareaofCuAlGaO4and1wt%Pt/CuAlGaO4wasapproximately0.65and 0.86m2∙g−1,respectively.Bothvalueswerenotveryhighduetothesolid‐statefusionmethodwhich operatesatahightemperature,causingthephotocatalystaggregation. 2.2.PhotocatalyticCO2ReductionwithGas,Liquid,Gas‐LiquidPhaseReactors Inthisstudy,severalblanktestsinalltypeofreactors(gasphase,liquidphase,andgas‐liquid phasereactors)wereconductedtoanswerthequestionwhethertheformationofthereaction productscomefromphoto‐hydrogenationandnotfromCO2contaminationsorthephotocatalyst itself.TheresultoftestswasshowninTable3.TheresultclearlyshowsthatifCO2wasnotintroduced tothereactingsystem,noproductwasgeneratedevenwiththepresenceofthehydrogen, photocatalyst,andlightirradiation.Itisimportanttorememberthatthephotocatalystsynthesis processdidnotuseanycarbon‐containingprecursor.Thisblanktestresultisfurtherconfirmation thatthephotocatalystitselfcontainsnearlynocarbonresidue.Table3alsoclearlyshowsthat,without anyofthreeessentialelementsinperformingthephoto‐hydrogenationreaction,including(a)CO2, (b)photocatalystand(c)lightsource,noproductcouldbedetected. Figure 5. The Pt 4f X-ray photoelectron spectroscopy (XPS) spectra for ( a ) CuAlGaO 4 and ( b ) 1 wt % Pt/CuAlGaO4photocatalysts. Lastly, the BET surface area of CuAlGaO 4 and 1 wt % Pt/CuAlGaO 4 was approximately 0.65 and 0.86 m 2· g −1 , respectively. Both values were not very high due to the solid-state fusion method which operates at a high temperature, causing the photocatalyst aggregation. 2.2. Photocatalytic CO2Reduction with Gas, Liquid, Gas-Liquid Phase Reactors In this study, several blank tests in all type of reactors (gas phase, liquid phase, and gas-liquid phase reactors) were conducted to answer the question whether the formation of the reaction products come from photo-hydrogenation and not from CO 2 contaminations or the photocatalyst itself. The result of tests was shown in Table 3. The result clearly shows that if CO 2 was not introduced to the reacting system, no product was generated even with the presence of the hydrogen, photocatalyst, and light irradiation. It is important to remember that the photocatalyst synthesis process did not use any carbon-containing precursor. This blank test result is further confirmation that the photocatalyst itself contains nearly no carbon residue. Table 3also clearly shows that, without any of three essential elements in performing the photo-hydrogenation reaction, including (a) CO 2 , (b) photocatalyst and (c) light source, no product could be detected.
Catalysts 2017,7, 63 6 of 12 Table 3. Summary of blank tests for photo-hydrogenation of CO2. No. Type of Reactors Experimental Conditions Yield of Products (µmol·g−1) CO2H2 (0.01 atm) Photocatalyst (1 wt % Pt/CuAlGaO4) Light Source 1Gas phase reactor X1O2O O BDL 3 2 O O X O BDL 3 O O O X BDL 4 Liquid phase reactor X O O O BDL 5 O O X O BDL 6 O O O X BDL 7Gas-liquid phase reactor X O O O BDL 8 O O X O BDL 9 O O O X BDL 1 X—absent in the photoreactor; 2 O—present in the photoreactor; 3 BDL—below the detection limit of gas chromatography. For CO 2 reduction, there are several possible C 1 products, such as formic acid, formaldehyde, methanol, and methane. Because of the high detection limit of formaldehyde by HPLC (3.3 ppm), we used the Nash reagent [ 27 ] to react with formaldehyde first, then analyzed the colored product diacetyldihydrolutidine (DDL) by UV-vis at 414 nm to quantify the formaldehyde. The detail was described in the previous literature [ 27 , 28 ]. Even the detection limit of this method was 0.17 µ mol · L −1 , which was about three orders lower than HPLC, formaldehyde was not observed under all investigated conditions. However, it is noted that although we did not detect formaldehyde, some of the formaldehyde might also be formed on the surface of the catalyst [ 29 , 30 ]. For the gas phase reactor, there is also no formic acid and methanol could be detected, and only methane evolution would be discussed in this case. The CH4yield under gas phase conditions was shown in Figure 6. Catalysts2017,7,636of12 Table3.Summaryofblanktestsforphoto‐hydrogenationofCO2. No.TypeofReactors ExperimentalConditions YieldofProducts (μmol∙g−1) CO2H2 (0.01atm) Photocatalyst (1wt%Pt/CuAlGaO4)LightSource 1 Gasphasereactor X1O2OOBDL3 2OOXOBDL 3OOOXBDL 4 Liquidphasereactor XOOOBDL 5OOXOBDL 6OOOXBDL 7 Gas‐liquidphasereactor XOOOBDL 8OOXOBDL 9OOOXBDL 1X—absentinthephotoreactor;2O—presentinthephotoreactor;3BDL—belowthedetectionlimitofgaschromatography. ForCO2reduction,thereareseveralpossibleC1products,suchasformicacid,formaldehyde, methanol,andmethane.BecauseofthehighdetectionlimitofformaldehydebyHPLC(3.3ppm),we usedtheNashreagent[27]toreactwithformaldehydefirst,thenanalyzedthecoloredproduct diacetyldihydrolutidine(DDL)byUV‐visat414nmtoquantifytheformaldehyde.Thedetailwas describedinthepreviousliterature[27,28].Eventhedetectionlimitofthismethodwas0.17μmol∙L−1, whichwasaboutthreeorderslowerthanHPLC,formaldehydewasnotobservedunderall investigatedconditions.However,itisnotedthatalthoughwedidnotdetectformaldehyde,someof theformaldehydemightalsobeformedonthesurfaceofthecatalyst[29,30].Forthegasphasereactor, thereisalsonoformicacidandmethanolcouldbedetected,andonlymethaneevolutionwouldbe discussedinthiscase.TheCH4yieldundergasphaseconditionswasshowninFigure6. Figure6.Thetime‐dependentyieldofCH4evolutionover1wt%Pt/CuAlGaO4photocatalystunder differentH2(g)partialpressuresinagasphasereactor.Linesarenotbasedonakineticmodelandare presentedforeye‐guidingonly. Mostinterestingly,H2playsanactiveroleonCH4yield. CO2(g)+4H2(g)→CH4(g)+2H2O(l) ∆H0=−259.9kJ/mol;∆G0=−132.4kJ/mol(1) ItisevidentthattheconversionofCO2intoCH4hasnegative∆H0and∆Gvalues,meaningthat theprocessisspontaneous,equilibriumfavorable,andexothermic.Inthisstudy,theenthalpy(∆H0) andGibbsfreeenergy(∆G0)valuesat298KforseveralinterestingreactionswerecalculatedbyAspen [31].Atfirst2h,theinitialratesofCH4yieldwere0.01,0.57,0.73,and0.40μmol∙g−1∙h−1that correspondtoH2partialpressure=0,0.01,0.1,0.2atm,respectively.Itisworthnotingthatincreasing thepartialpressureofH2wouldenhancetheyieldoftheproduct.AmongtheconditionsofH2partial pressure,0.1atmshowsthebestperformance.However,anextrasupplyofH2mightcompetewith CO2foroccupyingtheactivesites,bringinganegativeeffectonCO2photo‐hydrogenation.Asshown Figure 6. The time-dependent yield of CH 4 evolution over 1 wt % Pt/CuAlGaO 4 photocatalyst under different H2(g) partial pressures in a gas phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only. Most interestingly, H2plays an active role on CH4yield. CO2(g) + 4H2(g) →CH4 (g) + 2H2O(l) ∆H0=−259.9 kJ/mol; ∆G0=−132.4 kJ/mol (1) It is evident that the conversion of CO 2 into CH 4 has negative ∆ H 0 and ∆ Gvalues, meaning that the process is spontaneous, equilibrium favorable, and exothermic. In this study, the enthalpy ( ∆ H 0 ) and Gibbs free energy ( ∆ G 0 ) values at 298 K for several interesting reactions were calculated by Aspen [ 31 ]. At first 2 h, the initial rates of CH 4 yield were 0.01, 0.57, 0.73, and 0.40 µ mol · g −1· h −1 that
Catalysts 2017,7, 63 7 of 12 correspond to H 2 partial pressure = 0, 0.01, 0.1, 0.2 atm, respectively. It is worth noting that increasing the partial pressure of H 2 would enhance the yield of the product. Among the conditions of H 2 partial pressure, 0.1 atm shows the best performance. However, an extra supply of H 2 might compete with CO 2 for occupying the active sites, bringing a negative effect on CO 2 photo-hydrogenation. As shown in Figure 6, CH 4 evolution became stable after two hours in reaction, implying that the reactions in the system reached a dynamic equilibrium. A comparative experiment was also conducted in the liquid phase. The correlation between the amount of CH 4 , CH 3 OH, and HCOOH formed and the reaction time for the liquid phase are shown in Figure 7. Catalysts2017,7,637of12 inFigure6,CH4evolutionbecamestableaftertwohoursinreaction,implyingthatthereactionsin thesystemreachedadynamicequilibrium. Acomparativeexperimentwasalsoconductedintheliquidphase.Thecorrelationbetweenthe amountofCH4,CH3OH,andHCOOHformedandthereactiontimefortheliquidphaseareshown inFigure7. Figure7.Thetime‐dependentyieldof(a)CH4evolution,(b)CH3OHformation,and(c)HCOOH formationover1wt%Pt/CuAlGaO4photocatalystunderdifferentH2(g)partialpressuresinaliquid phasereactor.Linesarenotbasedonakineticmodelandarepresentedforeye‐guidingonly. ThemainproductsoftheliquidphasereactorwereCH4,CH3OH,andHCOOH,respectively. CO2(g)+3H2(g)→CH3OH(l)+H2O(l) ∆H0=−137.8kJ/mol;∆G0=−10.7kJ/mol(2) CO2(g)+H2(g)→HCOOH(l) ∆H0=−31.0kJ/mol;∆G0=+34.3kJ/mol(3) Itisclearthatallexothermicreactionsexhibitnegative∆H0values.Moreover,manyreactions arealsoassociatedwithnegative∆G0valuesand,asaconsequence,thereactionwillbe thermodynamicallyfavorable.Mostimportantly,CH3OHwasfoundtobethemajorproductunder currentcondition.ThisfactisthatCH3OHrequiresonlysixphotoelectronsforthephoto‐ hydrogenationofCO2whileCH4needstohaveeightphotoelectronsforreaction.Lastbutnotleast, anextraamountofH2providedmayalsosuppresstheyieldofproducts,whichisconsistentwiththe observationinthegasphase. ThecorrelationsbetweentheamountofCH4,CH3OH,andHCOOHformedandthereaction timeforthecombinedgas‐liquidphaseareshowninFigure8. Figure8.Thetime‐dependentyieldof(a)CH4evolution,(b)CH3OHformation,and(c)HCOOH formationover1wt%Pt/CuAlGaO4photocatalystunderdifferentH2(g)partialpressuresingas‐liquid phasereactor.Linesarenotbasedonakineticmodelandarepresentedforeye‐guidingonly. Figure 7. The time-dependent yield of ( a ) CH 4 evolution, ( b ) CH 3 OH formation, and ( c ) HCOOH formation over 1 wt % Pt/CuAlGaO4photocatalyst under different H2(g) partial pressures in a liquid phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only. The main products of the liquid phase reactor were CH4, CH3OH, and HCOOH, respectively. CO2(g) + 3H2(g) →CH3OH(l) + H2O(l) ∆H0=−137.8 kJ/mol; ∆G0=−10.7 kJ/mol (2) CO2(g) + H2(g) →HCOOH(l) ∆H0=−31.0 kJ/mol; ∆G0= +34.3 kJ/mol (3) It is clear that all exothermic reactions exhibit negative ∆ H 0 values. Moreover, many reactions are also associated with negative ∆ G 0 values and, as a consequence, the reaction will be thermodynamically favorable. Most importantly, CH 3 OH was found to be the major product under current condition. This fact is that CH 3 OH requires only six photoelectrons for the photo-hydrogenation of CO 2 while CH 4 needs to have eight photoelectrons for reaction. Last but not least, an extra amount of H 2 provided may also suppress the yield of products, which is consistent with the observation in the gas phase. The correlations between the amount of CH 4 , CH 3 OH, and HCOOH formed and the reaction time for the combined gas-liquid phase are shown in Figure 8. Similar to the result of the liquid phase, CH 3 OH still is the main product for gas-liquid phase. On the other hand, the CH 4 yield has a tendency to be suppressed after conducting the experiment for two hours. It is possible that an intermediate product of CO 2 photo-reduction, CO, could be generated as the reaction proceeds (Equation (4)). Following is the competing reaction pathways of photo-reduction of CO to CH 3 OH and HCOOH via H 2 and H 2 O, respectively (Equations (5) and (6)). This result implies that the presence of photocatalyst in the gas phase will promote the converting of CH4into another compound, such as CH3OH or HCOOH, resulting in a decrease of CH4yield. CO2(g) + CH4(g) →2CO(g) + 2H2(g) ∆H0=−247.5 kJ/mol; ∆G0= +170.8 kJ/mol (4)
Catalysts 2017,7, 63 8 of 12 CO(g) + 2H2(g) →CH3OH(l) ∆H0=−131.6 kJ/mol; ∆G0=−29.9 kJ/mol (5) CO(g) + H2O(g) →HCOOH(l) ∆H0=−24.8 kJ/mol; ∆G0= +15.1 kJ/mol (6) In brief summary, the possible reaction pathways of CO 2 photo-hydrogenation is illustrated in Scheme 1. This mechanism is proposed based on the knowledge of products presented during the photocatalytic reaction. Our observation indicates that CH 4 is the only product under gas phase condition. On the other hand, CH 4 , CH 3 OH, and HCOOH are found in the liquid and gas-liquid phase reactions. Although we could not measure an intermediate CO product quantitatively, we expect that CO might be generated during the reaction. In the gas-liquid phase reactor, the photocatalyst is well packed and dispersed on the quartz plate in the gas phase. Hence, it might promote the transformation of CO into CH 3 OH and HCOOH products. We do believe that conducting the experiment in different phases (such as gas, liquid, and gas-liquid phases) could not change the mechanism of the photo-hydrogenation of CO 2 . However, the presence of photocatalyst in different phases might accelerate different pathways of the reaction. Catalysts2017,7,637of12 inFigure6,CH4evolutionbecamestableaftertwohoursinreaction,implyingthatthereactionsin thesystemreachedadynamicequilibrium. Acomparativeexperimentwasalsoconductedintheliquidphase.Thecorrelationbetweenthe amountofCH4,CH3OH,andHCOOHformedandthereactiontimefortheliquidphaseareshown inFigure7. Figure7.Thetime‐dependentyieldof(a)CH4evolution,(b)CH3OHformation,and(c)HCOOH formationover1wt%Pt/CuAlGaO4photocatalystunderdifferentH2(g)partialpressuresinaliquid phasereactor.Linesarenotbasedonakineticmodelandarepresentedforeye‐guidingonly. ThemainproductsoftheliquidphasereactorwereCH4,CH3OH,andHCOOH,respectively. CO2(g)+3H2(g)→CH3OH(l)+H2O(l) ∆H0=−137.8kJ/mol;∆G0=−10.7kJ/mol(2) CO2(g)+H2(g)→HCOOH(l) ∆H0=−31.0kJ/mol;∆G0=+34.3kJ/mol(3) Itisclearthatallexothermicreactionsexhibitnegative∆H0values.Moreover,manyreactions arealsoassociatedwithnegative∆G0valuesand,asaconsequence,thereactionwillbe thermodynamicallyfavorable.Mostimportantly,CH3OHwasfoundtobethemajorproductunder currentcondition.ThisfactisthatCH3OHrequiresonlysixphotoelectronsforthephoto‐ hydrogenationofCO2whileCH4needstohaveeightphotoelectronsforreaction.Lastbutnotleast, anextraamountofH2providedmayalsosuppresstheyieldofproducts,whichisconsistentwiththe observationinthegasphase. ThecorrelationsbetweentheamountofCH4,CH3OH,andHCOOHformedandthereaction timeforthecombinedgas‐liquidphaseareshowninFigure8. Figure8.Thetime‐dependentyieldof(a)CH4evolution,(b)CH3OHformation,and(c)HCOOH formationover1wt%Pt/CuAlGaO4photocatalystunderdifferentH2(g)partialpressuresingas‐liquid phasereactor.Linesarenotbasedonakineticmodelandarepresentedforeye‐guidingonly. Figure 8. The time-dependent yield of ( a ) CH 4 evolution, ( b ) CH 3 OH formation, and ( c ) HCOOH formation over 1 wt % Pt/CuAlGaO 4 photocatalyst under different H 2(g) partial pressures in gas-liquid phase reactor. Lines are not based on a kinetic model and are presented for eye-guiding only. Catalysts2017,7,638of12 Similartotheresultoftheliquidphase,CH3OHstillisthemainproductforgas‐liquidphase. Ontheotherhand,theCH4yieldhasatendencytobesuppressedafterconductingtheexperiment fortwohours.ItispossiblethatanintermediateproductofCO2photo‐reduction,CO,couldbe generatedasthereactionproceeds(Equation(4)).Followingisthecompetingreactionpathwaysof photo‐reductionofCOtoCH3OHandHCOOHviaH2andH2O,respectively(Equations(5)and(6)). Thisresultimpliesthatthepresenceofphotocatalystinthegasphasewillpromotetheconvertingof CH4intoanothercompound,suchasCH3OHorHCOOH,resultinginadecreaseofCH4yield. CO2(g)+CH4(g)→2CO(g)+2H2(g) ∆H0=−247.5kJ/mol;∆G0=+170.8kJ/mol(4) CO(g)+2H2(g)→CH3OH(l) ∆H0=−131.6kJ/mol;∆G0=−29.9kJ/mol(5) CO(g)+H2O(g)→HCOOH(l) ∆H0=−24.8kJ/mol;∆G0=+15.1kJ/mol(6) Inbriefsummary,thepossiblereactionpathwaysofCO2photo‐hydrogenationisillustratedin Scheme1.Thismechanismisproposedbasedontheknowledgeofproductspresentedduringthe photocatalyticreaction.OurobservationindicatesthatCH4istheonlyproductundergasphase condition.Ontheotherhand,CH4,CH3OH,andHCOOHarefoundintheliquidandgas‐liquid phasereactions.AlthoughwecouldnotmeasureanintermediateCOproductquantitatively,we expectthatCOmightbegeneratedduringthereaction.Inthegas‐liquidphasereactor,the photocatalystiswellpackedanddispersedonthequartzplateinthegasphase.Hence,itmight promotethetransformationofCOintoCH3OHandHCOOHproducts.Wedobelievethat conductingtheexperimentindifferentphases(suchasgas,liquid,andgas‐liquidphases)couldnot changethemechanismofthephoto‐hydrogenationofCO2.However,thepresenceofphotocatalyst indifferentphasesmightacceleratedifferentpathwaysofthereaction. Scheme1.Thereactionpathwaysduringthephoto‐hydrogenationofCO2. Table4summarizestheproductyieldsattwohoursinthreekindsofreactorsunderdifferent H2partialpressures.WeseethatH2playsanactiveroleinthephotocatalyticreductionofCO2. Additionally,aremarkablesynergeticactivitywasclearlyobservedwhentheexperimentwas conductedingas‐liquidphaseunder0.01atmofH2.Inmoredetail,thetotalproductsinthegasand liquidphaseare0.400and0.243μmol∙g−1,respectively.Interestingly,about8.302μmol∙g−1isachieved inthegas‐liquidphase. Scheme 1. The reaction pathways during the photo-hydrogenation of CO2. Table 4summarizes the product yields at two hours in three kinds of reactors under different H 2 partial pressures. We see that H 2 plays an active role in the photocatalytic reduction of CO 2 . Additionally, a remarkable synergetic activity was clearly observed when the experiment was conducted in gas-liquid phase under 0.01 atm of H 2 . In more detail, the total products in the gas
Catalysts 2017,7, 63 9 of 12 and liquid phase are 0.400 and 0.243 µ mol · g −1 , respectively. Interestingly, about 8.302 µ mol · g −1 is achieved in the gas-liquid phase. Table 4. Summary of the product yields at 2 h in three kinds of reactors over 1 wt % Pt/CuAlGaO 4 under different H2partial pressures. Entry Phase H2Partial Pressure (atm) Product Yields (µmol·g−1) CH4CH3OH HCOOH Total HCs 1 1 Gas 0.00 0.012 ±0.010 BDL 2BDL 0.012 2 0.01 0.400 ±0.100 BDL BDL 0.400 3 0.20 0.780 ±0.300 BDL BDL 0.780 4Liquid 0.00 0.010 ±0.010 0.235 ±0.100 BDL 0.245 5 0.01 0.149 ±0.050 BDL 0.094 ±0.045 0.243 6 0.20 0.112 ±0.030 0.340 ±0.200 BDL 0.452 7 Gas-Liquid 0.00 0.014 ±0.010 0.285 ±0.100 BDL 0.299 8 0.01 0.480 ±0.200 7.352 ±2.100 0.470 ±0.100 8.302 9 0.20 0.666 ±0.120 0.445 ±0.110 0.145 ±0.010 1.255 1 Total hydrocarbons yield = CH 4 yield + CH 3 OH yield + HCOOH yield); 2 BDL—below the detection limit of gas chromatography. Lastly, the quantum efficiencies in three kinds of reactors under 0.01 atm of H 2 are also calculated. The highest quantum efficiency was in the gas-liquid phase reactor for 0.0011%, while the gas and liquid phases were about 0.0001% and 0.0005%, respectively. The possible reason for higher quantum efficiency in the gas-liquid phase reactor is that both H 2 in the gas phase and the proton in liquid phase could be utilized simultaneously, affording more chances to conduct CO2photo-reduction. 3. Materials and Methods 3.1. Preparation of Photocatalysts CuAlGaO 4 photocatalyst was firstly prepared by solid-state fusion method, which is mentioned in previous studies [ 19 , 25 ]. Firstly, copper oxide (CuO, Showa, Tokyo, Japan), aluminum oxide (Al 2 O 3 , Type A-5, Sigma-Aldrich, St. Louis, MO, USA), and β -gallium trioxide ( β -Ga 2 O 3 , ≥ 99.9%, Sigma-Aldrich) powders were mixed in the molar ratio of Cu/Al/Ga = 1:1:1 and pulverized in a mortar. Subsequently, the resulting mixture was calcined at 1150 ◦ C for 12 h, and then cooled to room temperature and further pulverized to obtain a CuAlGaO4powder. 1 wt % Pt/CuAlGaO 4 photocatalyst was prepared by the photo-deposition method as described in the same reference. The required amount of chloroplatinic acid hydrate (H 2 PtCl 6· xH 2 O, ≥ 99.9%, Sigma-Aldrich) solution was mixed with the as-prepared CuAlGaO 4 powder. Herein, the loading of Pt on CuAlGaO 4 is 1 wt %. The mixed solution was irradiated by a UV source (320–500 nm, EXFO S1500, EXFO Inc., Quebec City, QC, Canada) for 90 min to perform the photo-deposition process. After that, the solid product was centrifuged and washed with deionized water several times. Finally, the washed material was dried at 80 ◦C for 8 h to obtain 1 wt % Pt/CuAlGaO4powder. 3.2. Characterization of Photocatalysts X-ray diffractometer (XRD, Ultima IV, Rigaku, Tokyo, Japan) equipped with Cu K α (1.5418 Å) was used to verify the crystalline structure of photocatalysts. The UV-vis diffuse reflectance spectrum of the photocatalyst was fully recorded over the range 300–800 nm by a Cary 100 UV-visible spectrometer (UV-vis, Varian Cary 100, Agilent Technologies, Santa Clara, CA, USA). BaSO 4 was used as the reflectance standard. A field emission scanning electron microscope (FE-SEM, Nano SEM 230, FEI, Hillsboro, OR, USA) equipped with energy dispersive spectroscopy (EDS) was used to directly reveal the presence of the atomic elements in the photocatalysts. Transmission electron microscopy