materialsReviewAdvances in Photocatalytic CO2 Reduction withWater: A ReviewSamsun Nahar 1,3, *, M. F. M. Zain 1,2, *, Abdul Amir H. Kadhum 3 , Hassimi Abu Hasan 3 andMd. Riad Hasan 1,41234*Sustainable Construction Materials and Building Systems (SUCOMBS) Research Group, UniversitiKebangsaan Malaysia (UKM), UKM Bangi 43600, Malaysia; email@example.comDepartment of Architecture, Universiti Kebangsaan Malaysia (UKM), UKM Bangi 43600, MalaysiaDepartment of Chemical & Process Engineering, Universiti Kebangsaan Malaysia (UKM),UKM Bangi 43600, Malaysia; firstname.lastname@example.org (A.A.H.K.); email@example.com (H.A.H.)Department of Civil & Structural Engineering, Universiti Kebangsaan Malaysia (UKM),UKM Bangi 43600, MalaysiaCorrespondence: firstname.lastname@example.org (S.N.); email@example.com (M.F.M.Z.);Tel.: 60-112-817-5873 (S.N.); 60-173-333-870 (M.F.M.Z.)Academic Editor: Peter LoutzenhiserReceived: 15 February 2017; Accepted: 23 May 2017; Published: 8 June 2017Abstract: In recent years, the increasing level of CO2 in the atmosphere has not only contributed toglobal warming but has also triggered considerable interest in photocatalytic reduction of CO2 .The reduction of CO2 with H2 O using sunlight is an innovative way to solve the currentgrowing environmental challenges. This paper reviews the basic principles of photocatalysis andphotocatalytic CO2 reduction, discusses the measures of the photocatalytic efficiency and summarizescurrent advances in the exploration of this technology using different types of semiconductorphotocatalysts, such as TiO2 and modified TiO2 , layered-perovskite Ag/ALa4 Ti4 O15 (A Ca, Ba, Sr),ferroelectric LiNbO3 , and plasmonic photocatalysts. Visible light harvesting, novel plasmonicphotocatalysts offer potential solutions for some of the main drawbacks in this reduction process.Effective plasmonic photocatalysts that have shown reduction activities towards CO2 with H2 O arehighlighted here. Although this technology is still at an embryonic stage, further studies with standardtheoretical and comprehensive format are suggested to develop photocatalysts with high productionrates and selectivity. Based on the collected results, the immense prospects and opportunities thatexist in this technique are also reviewed here.Keywords: photocatalysis; CO2 reduction; visible light irradiation; plasmonic photocatalyst; surfaceplasmon resonance (SPR)1. IntroductionGlobal warming is viewed to be one of the vital environmental concerns that humankind isdealing with . CO2 contributes mostly to the worldwide climate change because it is more than 64%effective than other greenhouse gasses in the atmosphere . This chemically stable gas contributes tothe increase in global temperature through absorption and re-emission of infrared radiation. In the pastcentury, the temperature of the Earth’s surface increased by roughly 0.6 K; the warming trend revealsmore significant changes in last 20 years, according to the Intergovernmental Panel on Climate Change(IPCC) . The consequences of the greenhouse effect are global and severe, such as ice melting atthe Earth’s poles, the quick rising of sea level, and growing precipitation across the globe . To dealwith these issues, numerous studies have been conducted over the last few decades applying variousstrategies to control CO2 emission or convert it into other products.Materials 2017, 10, 629; ls
Materials 2017, 10, 6292 of 26There are at least three routes of lowering the amount of CO2 in the atmosphere: (i) direct reductionof CO2 emission; (ii) CO2 capture and storage (CCS); and (iii) CO2 utilization [5–7]. Lowering theCO2 emission may seem quite unrealistic because of the present human lifestyle and emergent use offossil fuel. The potential of CCS technology can be restrained because of the environmental risk ofleakage and the energy requirement for fuel compression and transportation. Among the renewableresources, solar energy is the most exploitable one by making available more energy to the Earth forevery hour than the total amount of energy humans consume in a year .Harvesting this abundant sunlight in solving environmental problems is a promising approachand one of the ultimate goals for sustainability of global development. In the long term, photocatalyticconversion of CO2 utilizing solar energy is the most appealing route for CO2 reduction [9,10]. In addition,CO2 reduction advances recycling of carbon source . The interest in this field of research has begunwith the work of Fujishima and Honda in 1972 . The advancements in nanotechnology, particularlythe synthesis of nanomaterials with different structures and morphologies [12,13], and the most recentapproach of using noble metals, such as Au or Ag, with surface plasmon resonance (SPR) to enhance thephotocatalytic efficiency of TiO2 or other semiconductors [14–16] have facilitated the progress.For real life application, a photocatalytic system must be capable of working under daylighteven when the sun is not directly overhead and show both long-time consistency and efficiency.The reduction process has to be promoted while suppressing any side reaction that can occur duringthe reaction, and H2 O should be used as an electron source . Unfortunately, a photocatalystthat satisfies all these requirements has not been reported yet. A considerable number of reviewpapers on this emerging topic have already been published. Some papers focus on the advances indeveloping novel photocatalysts with high photocatalytic activity [18–22], while others on studyingthe enhancement mechanisms and the influences of co-catalysts , the applications by highlightingon the reaction conditions, reactor design and analysis methods [18,24] and comprehensive discussionon general considerations that apply specifically to CO2 reduction [25,26]. Furthermore, extensivestudies on TiO2 -based photocatalysts [27–29] and noble metal nanoparticles dispersed plasmonicphotocatalysts have been published as well [30–32]. Nevertheless, the basic insight of photocatalyticCO2 reduction in presence of H2 O and comparison among the photocatalytic efficiency of differentphotocatalysts in this reaction has not been clearly documented to date. This review paper covers thebasic aspects of photocatalytic CO2 reduction process with H2 O, concentrating on recently reportedsemiconductor photocatalysts with high photoactivity, particularly on plasmonic photocatalysts.2. Photocatalysis and Photocatalytic Reduction of CO2 with H2 OThe word photocatalysis consists of two parts: photo and catalysis, “photo” means light and“catalysis” is the performance of a substance during the chemical transformation of the reactants tomodify the reaction rate without being changed ultimately . In practice, the word photocatalysisrefers to the acceleration of a photoreaction in the presence of a catalyst . In photocatalyticCO2 reduction system with water, both photo-reduction of CO2 and photo-oxidation of H2 O occursimultaneously under sunlight irradiation using a suitable photocatalyst. A variety of reactionconditions intensely affects the product distribution of this reaction, such as reactor geometry, catalysttype, sacrificial reagents, and even illumination type. Thus, predicting the product distribution ofa particular photocatalytic reaction is very challenging .The photocatalytic CO2 reduction is a very effective method considering that no additional energyis needed and no negative effect on the environment is produced. The use of cheap and abundantsunlight to transform this major greenhouse gas into other carbon containing products is also an idealapproach because of its low cost. Here, the high activation energy to break very stable CO2 moleculeis provided by solar energy . To date, many photocatalysts, including oxides and non-oxides,e.g., TiO2 , ZnO, Fe2 O3 , ZrO2 , SnO2 , BiWO3 , Ti-MCM-41, CdS, TNTs, ZnS, GaN, and SiC, have beenstudied for the photocatalytic reduction of CO2 with H2 O. A summary of different photocatalyticsystems employed in this technology since 2010 are given in Table 1.
Materials 2017, 10, 6293 of 26Table 1. Advances in photocatalytic systems for CO2 reduction with water since the year 2010.PhotocatalystRadiation Source0.5 wt %Cu/TiO2 -SiO2Xe lamp(2.4 mW cm 2 ,250–400 nm)ZnGa2 O4(RuO Pt)-Zn2 GeO4Ag/ALa4 Ti4 O15(A Ca, Ba and Sr)300 W Xe arc lamp300 W Xe arc lamp400 W Hg lampI-TiO2nanoparticles450 W Xe lampLiNbO3Natural sunlight orHg lamp(64.2 mW cm 2 )G-Ti0.91 O2 hollowspheresGraphene oxides(GOs)300 W Xe arc lamp300 W commercialhalogen lampMajor ProductsCommentsReferencesCO and CH4The synergistic combination of Cu depositionand high surface area of SiO2 supportenhanced CO2 photoreduction rates.CH4Strong gas adsorption and large specificsurface area of the mesoporous ZnGa2 O4photocatalyst contribute to its highphotocatalytic activity for converting CO2into CH4 .CH4In the presence of water, ultra-long andultrathin geometry of the Zn2 GeO4nano-ribbon promotes CO2 photo-reduction,which was significantly enhanced by loadingof Pt or RuO2 .CO, HCOOH,and H2On the optimized Ag/BaLa4 Ti4 O15photocatalyst, CO was the reported as the mainproduct. The molar ratio of O2 production(H2 CO:O2 2:1) demonstrated that waterwas consumed as a reducing reagent in thephotocatalytic process.COHigh photocatalytic activity was observedunder visible light and the efficiency of CO2photoreaction was much greater than undopedTiO2 due to the extension in the absorptionspectra of TiO2 to the visible light region andfacilitated charge separation.HCOOHThe MgO-doped LiNbO3 showed an energyconversion efficiency rate of 0.72% which waslower than that for the gas–solid catalyticreaction of LiNbO3 (2.2%).CH4 , COThe presence of G nanosheets compactlystacking with Ti0.91 O2 nanosheets allows therapid migration of photo-generated electronsfrom Ti0.91 O2 nanosheets into G and improvesthe efficiency of the photocatalytic process.CH3 OHAmong all GOs, GO-3 exhibited the highestefficiency as a photocatalyst for CO2 reductionunder visible light, and the conversion rate ofCO2 to CH3 OH on modified GO (GO-3) was0.172 mmol g 1 cat h 1 , which is six-foldhigher than that of pure TiO2 .W18 O49300 W Xe lampCH4The oxygen-vacancy-rich ultrathin W18 O49nanowires can be used to design materials withextraordinary photochemical activity becauseit displayed high CO2 reduction capability inpresence of water.Zn1.7 GeN1.8 O300 W Xe arc lampCH4Zn1.7 GeN1.8 O loaded with co-catalysts showedsignificantly higher conversion rate of CO2into CH4 .CH4The mesoporous TiO2 showed higher efficiencytowards CO2 reduction when loaded withnoble metal particles, and the order ofenhanced photocatalytic activity wasPt Au Ag. The optimum loading amountof Pt was 0.2 wt %.CH4The high photocatalytic activity of thisphotocatalyst was attributed to the improvedgas adsorption of the mesoporous structure,the chemisorption of CO2 on the photocatalystand the narrow bandgap of ZnAl2 O4 -modifiedZnGaNO to extend the light absorption.Pt-, Au-, orAg-loadedmesoporous TiO20.5 wt % Pt loadedZnAl2 O4 -modifiedmesoporousZnGaNO350 W Xe lamp300 W Xe lamp(λ 420 nm)
Materials 2017, 10, 6294 of 26Table 1. Cont.PhotocatalystRadiation SourceMajor ProductsCommentsReferencesGa2 O3 withmesopores andmacropores300 W Xe lamp(500 mW cm 2 )CH4Ga2 O3 with mesopores and macroporesshowed high photocatalytic activity due to itshigher CO2 adsorption capacity (300%) andincreased surface area (200%) compared to thebulk nanoparticles.Pt-TiO2 thinnanostructuredfilms400 W Xe lampCO and CH4The catalyst can be produced at an industrialscale for commercial application and showedhigh efficiency for selective CH4 formation.HNb3 O8350 W Xe lampCH4KNb3 O8 and HNb3 O8 were synthesized by theconventional solid-state reaction andperformed more effectively in photocatalyticCO2 reduction than commercial TiO2 .ZnO-basedmaterials8 W fluorescenttube (averageintensity7 mW cm 2 )CO, CH4 , CH3 OH,H2N-doping did not show any importantinfluence on the photocatalytic behavior ofZnO-based photocatalysts. The mesoporousstructure of ZnO favored CO and H2production, but catalysts with Cu showedan enhancement in the hydrocarbonproduction, mainly CH3 OH.CH4The use of a reactor with three opticalwindows, a combination of both bimetallicco-catalysts, and Ag@SiO2 nanoparticlesincreased the product formation significantlycompared to bare TiO2 .Ag, Pt, bimetallicAg–Pt andcore–shellAg@silica (SiO2 )nanoparticleswith TiO2100 W Hg lamp(330 nm)Carbon nanotubesNi/TiO2Nano-composites75 W visibledaylight lamp(λ 400 nm)CH4Compared to Ni/TiO2 and pure anatase TiO2,Ni/TiO2 incorporated with carbon nanotubesdemonstrated maximum CH4 product yield of0.145 mmol h 1 g 1 catalysts after 4.5 h ofirradiation under visible light.Pt/Cu/TiO2200 W Xe lampCH4 , CO, H2The addition of co-catalyst Pt decreases theselectivity for CO2 photo-reduction; however,loading Cu onto TiO2 increases the selectivityfrom 60 to 80%.CH4 , COPlasmonic photocatalyst Au/Pt/TiO2provided a more effective way to harvest solarenergy by consuming a high-energy photon inthe solar spectrum (UV region) and using it forcharge carrier generation. Moreover, it alsoutilized visible light to enhance thephotocatalytic activity.CH4Loading of montmorillonite on TiO2 enhancedthe surface area and reduced particle size, thusimproving charge separation, resulting inmaximum yield for CH4(441.5 mmol·g·cat 1 h 1 ).Au/Pt/TiO2500 W Xe lamp20 wt %montmorillonitemodified TiO2500 W Hg lamp(365 nm)0.5 wt %Pt/NaNbO3300 W Xe lamp(λ 300 nm)CH4 , CO, H2The cubic-orthorhombic surface-junctions ofmixed-phase NaNbO3 enhanced the chargeseparation, thereby improving itsphotoactivity.Ag supported onAgIO3 (Ag/AgIO3particles)500 W Xe arc lampCH4 and COIn the conversion of CO2 to CH4 and CO usingwater vapor, Ag/AgIO3 particles showed highand stable activity because of the surfaceplasmon resonance effect of Ag particles.CH4An intimate interface formation was suggestedbetween the C3 N4 and NaNbO3 nanowires ing-C3 N4 /NaNbO3 heterojunction photocatalyst,resulting in almost eight-fold higher CO2reduction than individual C3 N4 under visiblelight irradiation.g-C3 N4 /NaNbO3nanowires300 W Xe arc lamp
Materials 2017, 10, 6295 of 26Table 1. Cont.PhotocatalystIn2 O3 /g-C3 N4SnO2 x /g-C3 N4compositeRadiation SourceMajor ProductsCommentsReferences500 W Xe lampCH4The addition of In2 O3 nanocrystals ontog-C3 N4 surface improved the photocatalyticCO2 reduction process significantly due to theinterfacial transfer of photo-generatedelectrons and holes between g-C3 N4 and In2 O3.500 W Xe lampCO, CH3 OH, andCH4Enhancement in the surface area of g-C3 N4was observed by introducing SnO2 x . Improvephotocatalytic performance was attributed tothe increased light absorption and acceleratedthe separation of electron–hole pairs.AgX/g-C3 N4(X Cl and Br)nanocomposites15 Wenergy-savingdaylight bulb.CH4Under ambient condition and low-powerenergy-saving lamps, the optimal 30AgBr/pCN (protonated graphitic carbonnitride photocatalyst) sample showed highestphotocatalytic activity with significantenhancement in CH4 formation compared toindividual AgBr and pCN photocatalyst.Ag supported onAg2 SO3(Ag/Ag2 SO3 )500 W Xe lampCH4 and COPlasmonic photocatalyst Ag/Ag2 SO3 wasstable towards CO2 photoreduction after 10repetitive catalytic cycles with high efficiencyunder visible light irradiation.One of the major obstacles to this research progress is that most of the CO2 reducing photocatalystsare not visible light responsive . In this context, numerous types of photocatalysts have beendeveloped. A few of these catalysts performed under visible light irradiation with high conversionrate and selectivity, whereas other catalysts were weakly responsive under visible light and showeda low rate of reaction yield . The introduction of plasmonic metal onto semiconductor materials toenhance photocatalytic activity has been demonstrated to be very attractive in the visible region.In the following sections, the basic mechanisms and principles of measuring the efficiency ofa photocatalyst in photocatalytic CO2 reduction with H2 O are discussed.2.1. Theoretical ApproachPhotocatalysis means activating a semiconductor using sunlight or artificial light. Whena semiconductor material absorbs photons of sufficient energy, its electrons are excited from thevalence band (VB) to the conduction band (CB), creating electron–hole pairs. VB is the highest energyband occupied by electrons and CB is the lowest energy band in which there is no electron at theground state . These photo-generated electrons can move to the surface of a semiconductorand react with the adsorbed species on the surface. Meanwhile, electron–hole recombination is alsopossible . The efficiency of the photocatalytic reaction depends on the competition between thesetwo processes .The basic photocatalytic process can be summarized as follows:(i) Absorption of photons with suitable energy and generation of electron–hole pairs;(ii) Separation and transportation of electron–hole pairs (charge carriers); and(iii) The chemical reaction of surface species with charge carriers [68,69].This process is illustrated in Figure 1. As the charge recombination process ( 10 9 s) is usuallymuch faster than the reaction process ( 10 3 –10 8 s), acceleration of the electron–hole separation stepremarkably affects the reaction yield .
(i) Absorption of photons with suitable energy and generation of electron–hole pairs;(ii) Separation and transportation of electron–hole pairs (charge carriers); and(iii) The chemical reaction of surface species with charge carriers [68,69].This process is illustrated in Figure 1. As the charge recombination process ( 10 9 s) is usuallymuch fasterthanMaterials2017, 10,629 the reaction process ( 10 3–10 8 s), acceleration of the electron–hole separation6 stepof 26remarkably affects the reaction yield .Figure 1. Schematic diagram of photo-excitation and electron transfer process (adapted from ).Figure 1. Schematic diagram of photo-excitation and electron transfer process (adapted from ).Apart from the direct photon-excited charge carrier generation process in semiconductorsFigure 1, collisions, photon-electron interaction [70–72] or electron transfer from the SPR-excitedmetal nanoparticle [73,74] can also generate electron–hole pairs. However, all of the photo-excitedelectrons reaching the surface cannot reduce thermodynamically inert and very stable CO2 compound.This reduction reaction is endergonic and requires both hydrogen and energy . Thus, photocatalyticCO2 reduction using sunlight and water has the potential to be the most feasible means to removeatmospheric CO2 .The reduction potential for the various products of CO2 reduction at pH 7 is presented in Table 2.On the one hand, single-electron CO2 reduction reaction requires a highly negative potential of 1.9 eV,which makes the one-electron reduction process very unfavorable. On the other hand, the protonassisted multi-electron CO2 reduction reaction requires comparatively low redox potential (Table 2)and are more favorable. Photocatalysts can facilitate these reduction processes with lower potential.For this purpose, an ideal photocatalyst generally requires two characteristics: (i) the redox potential ofthe photo-excited VB hole must be sufficiently positive so that the hole can act as an electron acceptor;and (ii) the redox potentials of the photo-excited CB electron must be more negative than that of theCO2 /reduced-product redox couple.Upon absorbing radiation from the light source, photo-generated holes in the VB of thephotocatalyst oxidize H2 O. In addition, the photo-generated electrons in its CB form products such asHCOOH, HCHO, CH3 OH, and CH4 , by reducing CO2 . Here, the relation between the energy levels ofthe photocatalyst and the redox agent determines the type of reaction that takes place. Figure 2 showsthe CB, VB potentials, and bandgap energies of various semiconductor photocatalysts and relativeredox potentials of compounds involved in CO2 reduction. The final carbon containing products aredetermined by the specific mechanism to conduct the reaction. The number and rate of transferredelectrons from the photo-generated carriers to the reaction species in the reaction system also contributein this process .Table 2. Reduction potentials for the CO2 reduction process. E0 : Standard reduction potential.ReactionsE0 /eVCO2 e CO2CO2 2e 2H HCOOHCO2 2e 2H CO H2 OCO2 4e 4H HCHO H2 OCO2 6e 6H CH3 OH H2 OCO2 8e 8H CH4 2H2 O 1.9 0.61 0.53 0.48 0.38 0.24
Materials 2017, 10, 629CO2 e CO2CO2 2e 2H HCOOHCO2 2e 2H CO H2OCO2 4e 4H HCHO H2OCO2 6e 6H CH3OH H2OCO2 8e 8H CH4 2H2O 1.9 0.61 0.53 0.48 0.38 0.247 of 26Figure 2. Schematic representation of conduction band, valence band potentials, and band gapFigure 2. Schematic representation of conduction band, valence band potentials, and band gap energiesenergies of various semiconductor photocatalysts and relative redox potentials of the compoundsof various semiconductor photocatalysts and relative redox potentials of the compounds involved ininvolved in CO2 reduction at pH 7 (Adapted from ).CO2 reduction at pH 7 (Adapted from ).The most commonly used light source for photocatalysis is ultraviolet (UV) light. The high energycontent of UV light can effectively excite most photocatalysts. Thus, the majority of publications onphotocatalytic CO2 reduction processes are still based on using artificial UV light from high-powerlamp [75–77]. Only about 4% of solar energy is used by UV light where 43% of solar energy is occupiedby visible light; thus, a photocatalyst with a narrow bandgap that can use visible light is in highdemand [65,78]. At present, a significant number of studies focus on the direct use of visible lightboth from artificial and natural sources. Using visible light is more favorable than using UV lightbecause visible light is readily available from sunlight. However, the energy content of visible light isless competitive compared to UV light. Thus, in photocatalytic reduction, the visible light might notprovide for an adequate amount of energy for photo-excitation of the catalysts. As such, photocatalysisusing visible light and sunlight faces a great challenge .2.2. Measures of Photocatalytic EfficiencyThe photocatalytic CO2 reduction efficiency is generally measured by the yield of the product.Here, the general unit for R is mol·h 1 ·g 1 of catalyst and for the product either in molar units (µmol)or in concentration units (ppm).n(Product)R (1)Time m(Catalysts)In the catalyst-based measurements, the efficiency of the photocatalyst usually depends onthe amount of photocatalyst, the intensity of the light, lighting area, etc., so under the irradiationof light, the amount of product formed by per gram of photocatalyst within a certain timeperiod can be measured by its apparent quantum yield. It is calculated by using the amount ofproduct and the incident photon number as shown in the following equations [19,26]. When thephotocatalytic reduction reaction gives complex products, then the number of reacted electrons in theequation denotes the sum of the reacted electron to form each product [80,81]. Thus, in light-basedmeasurements, the quantum yield of CO2 photo-reduction into different products can be calculatedusing following equations:Overall quantum yield(%) Number of reacted electrons 100%Number of absorbed photons(2)
Materials 2017, 10, 629Apparent quantum yield(QY, %) 8 of 26Number of reacted electrons 100%Number of incident photons(Apparent) quantum yield of CO(%) 2 Number of CO molecules 100%Number of incident photons(3)(4)2 Number of HCOOH molecules 100%Number of incident photons(5)(Apparent) quantum yield of HCHO(%) 4 Number of HCHO molecules 100%Number of incident photons(6)(Apparent) quantum yield of CH3 OH(%) 6 Number of CH3 OH molecules 100%Number of incident photons(7)8 Number of CH4 molecules 100%Number of incident photons(8)(Apparent) quantum yield of HCOOH(%) (Apparent) quantum yield of CH4 (%) 3. Recent Photocatalysts for CO2 Reduction with H2 OThe first step towards enhancing the photocatalytic activity is the selection of a properphotocatalyst. It is a subject of considerable importance both for practical application of photocatalystsand understanding their mechanism. Photocatalysts could be categorized into two basic groups basedon their structures: homogeneous and heterogeneous photocatalysts.The seminal work by Lehn et al. demonstrated the selective CO2 reduction into CO by usingRe(I) diimine complexes ; since then, the use of metal complexes in photocatalysis has been greatlystudied for both CO2 reduction [83–86] and H2 O oxidation [87–89]. CO2 is efficiently reduced to formCO when homogeneous photocatalysts, such as Re complexes, are used in the presence of electrondonors, such as triethanolamine [80,90,91]. However, CO2 reduction and H2 O oxidation processesrequire distinct reaction conditions.As a result, carrying out both of the reaction simultaneously using a metal complex catalyst ina single system is a very difficult task. Reverse oxidation of organic products generated from thereduction of CO2 and the reverse reduction of O2 generated from the oxidation of H2 O terminatethe continuity of the reaction. Figure 3 summarizes these cases briefly . Figure 3a shows theadvantages of H2 O oxidation of a metal complex catalyst (H2 O oxidation site) with a sacrificial electronacceptor (SA). Figure 3b shows the advantages of CO2 reduction for a metal complex catalyst (CO2reduction site) with a sacrificial electron donor (SD). Figure 3c shows the problems encountered whencombining H2 O oxidation site and CO2 reduction site: (I) reverse oxidation of products such as organiccompounds; (II) electron transfer from H2 O oxidation site to CO2 reduction site; (III) need to beelectron storage; (IV) need to be active in H2 O; (V) easier reduction of O2 than CO2 ; and (VI) stabilityin H2 O . A number of challenges are encountered in constructing a homogeneous metal complexsystem for CO2 reduction along with H2 O oxidation. The inefficient electron transport betweenreduction and oxidation catalysts is one of the major difficulties in this process. Another drawback isthe short lifetimes of the one-electron-reduced species and the photo-excited state in the presence ofO2 generated by H2 O oxidation.Since the pioneering work of Fujishima, Honda, and their co-workers, where they reportedthe photocatalytic reduction of CO2 to organic compounds, such as HCOOH, CH3 OH, and HCHO,in the presence of various semiconductor photocatalysts, such as TiO2 , ZnO, CdS, SiC, and WO3 ,many heterogeneous semiconductor compounds, including metal oxides, oxynitrides, sulfides, andphosphides, had been investigated for this purpose [10,20]. TiO2 , BaLa4 Ti4 O15 , SrTiO3 , WO3 nanosheet,NaNbO4 , KNbO4 , Sr2 Nb2 O7 , Zn2 GeO4, and Zn2 SnO4 are the leading compounds in this list ofphotocatalysts and the list is increasing enormously in the last five years [1,9,10,18–20,28,64,65,93–98].Activation of an inert molecule such as CO2 requires contributions of both incident photons andeffectively excited electrons. Thus, the presence of reducing agents can assist the CO2 activationprocess. It takes advantage of H2 O oxidation and CO2 fixation when H2 O is used as the reducing
reduction of CO2 and the reverse reduction of O2 generated from the oxidation of H2O terminate thecontinuity of the reaction. Figure 3 summarizes these cases briefly . Figure 3a shows the advantagesof H2O oxidation of a metal complex catalyst (H2O oxidation site) with a sacrificial electron acceptor(SA). Figure 3b shows the advantages of CO2 reduction for a metal complex catalyst (CO2 reductionsite)withelectron donor (SD). Figure 3c shows the problems encountered whenMaterials2017,a10,sacrificial6299 of 26combining H2O oxidation site and CO2 reduction site: (I) reverse oxidation of products such asorganic compounds; (II) electron transfer from H2O oxidation site to CO2 reduction site; (III) need toagent.Appropriateandsuitablematerials have an important role inbeelectronstorage; incident(IV) needlightto beactivein Hsemiconductor2O; (V) easier reduction of O2 than CO2; and ssingandsensiblyengineeredstrong catalystmetalwithstability in H2O . A number of challenges are encountered in constructinga homogeneousgreat accessibilityareCOessentialto activate the very small molecules under ambient conditions .complexsystem for2 reduction along with H2O oxidation. The inefficient electron transportSomeofthedesirablepropertiesan efficientheterogeneousa highsurfacearea,between reduction and oxidationofcatalystsis oneof the majorphotocatalystdifficulties tion,thehighdrawback is the short lifetimes of the one-electron-reduced species and the photo-excited state in themobility ofcarriers,selectivity .presenceof chargeO2 generatedbyandH2Oproductoxidation.Figure 3. Advantages and disadvantages of metal complex catalysts for CO2 reduction with H2OFigure 3. Advantages and disadvantages of metal complex catalysts for CO2 reduction with H2 Ooxidation (adapted from ). (a) The advantages of H2O oxidation of a metal complex catalyst (H2Ooxidation (adapted from ). (a) The advantages of H2 O oxidation of a metal complex catalystoxidation site) with a sacrificial electron acceptor (SA); (b) the advantages of CO2 reduction for a metal(H
Materials 2017, 10, 629 2 of 26 There are at least three routes of lowering the amount of CO2 in the atmosphere: (i) direct reduction of CO2 emission; (ii) CO2 capture and storage (CCS); and (iii) CO2 utilization [5-7]. Lowering the CO2 emission may seem quite unrealistic because of the present human lifestyle and emergent use of fossil fuel. The potential of CCS technology can be restrained .
employed including chemical reduction using hydrazine or NaBH 4 , high-temperature annealing reduction , hydrothermal reduction using supercritical water , green chemistry method , and photocatalytic reduction using semiconductors [18-21]. Among them, the photocatalytic reduction is an environment-friendly and
photocatalyst disinfection [4 6], photocatalytic hydrogen producti on [7 11], photocatalytic reduction of CO 2 [12 15], photocatalyst wastewater treatment [16 20], and air purification [21 23]. However, photocatalytic technology is only at the laboratory stage, and there is still a long journey to apply this technology in practice .
Kemppi 2500 ac/dc 1 Tig, alu, CO2 250 Kemppi MLS 3003 ac/dc 1 Tig, alu, rustfri 250 Kemppi KMS 400 1 CO2 500 Kemppi Pro evo 4200 3 CO2 400 1 with welding tractor Kemppi Kempomig 4000W 1 CO2 400 Kemppi MLS 3000 3 TIG, alu, CO2 250 Fronius Transtig 3000 1 TIG 300 Kemppi Weld 400 2 CO2 400 Kempact MIG 2530 1 CO2 250 Kemppi Mat 320 1 CO2 320 Kemppi .
Kemppi 2500 ac/dc 1 Tig, alu, CO2 250 Kemppi MLS 3003 ac/dc 1 Tig, alu, rustfri 250 Kemppi KMS 400 1 CO2 500 Kemppi Pro evo 4200 3 CO2 400 1 med svejsetraktor Kemppi Kempomig 4000W 1 CO2 400 Kemppi MLS 3000 3 TIG, alu, CO2 250 Fronius Transtig 3000 1 TIG 300 Kemppi Weld 400 2 CO2 400 Kempact MIG 2530 1 CO2 250 Kemppi Mat 320 1 CO2 320 Kemppi 1 .
Photocatalytic reduction of CO 2 to methanol is not only to mitigate emissions but also provides alternative fuels under ambient conditions. In this work, hexagonal plate ZnO and copper-modified hexagonal plate ZnO nanostructures were synthesized and used as catalysts for photocatalytic reduction of CO 2 to methanol in water.
5 photocatalytic CO2 reduction under visible-light irradiation. Our results clearly establish that the coupling of 2D g-C3N4 nanosheets with basic 2D NiAl-LDH nanosheets plays a vital role in not only reducing charge recombination and improving the charge transfer but also improved CO2 adsorption and electron transfer to CO2.Moreover, NiAl-LDH coupled g-C3N4 nanosheets is a
The objective of this thesis was the photocatalytic reduction of CO 2 that is . The goals of the thesis are to design and characterize new catalysts that have high efficiency for the catalytic reduction of CO 2. After a brief introduction in Chapter 1 about the photocatalytic reduction of CO 2, a