CO2; carbon capture and conversion; catalysis; electrocatalysis; photoelectrocatalysis; copolymerization
Correspondence
*Amin Taheri Najafabadi, Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall,Vancouver BC, V6T 1Z3, Canada.†E-mail: [email protected]
Received 28 November 2012; Revised 7 January 2013; Accepted 9 January 2013
1. INTRODUCTION
Energy and environment have been always classified as the topmankind challenges by the mid-century [1]. From this pros-pect, fossil fuels are double-edged swords, offering a numberof advantages as well as significantly negative environmentalimpacts associated with their undue consumption. From oneside, their versatile role in the current energy supply scenariosis unquestionable; where oil and natural gas supply close to90% of our current energy needs, bringing feasibility to somany industrial activities [2]. On the other side, notable carbondioxide (CO2) emissions as the result of increasing fossil fuelsconsumption is alarming, especially from the climate changeprospect. Burning 1 t of carbon in fossil fuels comes at the costof releasing more than 3.5 t of CO2; which its accumulation inthe atmosphere is now approaching 1 Tt [3].
Ample of reasons can be counted for the broad use offossil fuels despite the consequent destructive impacts on
the planet ecosystem; first, they are available in a widerange of physical formats (gas, liquid, and solid), spreadingalmost all over the world; second, a myriad of technologi-cal advances are made, implementing them for variousapplications in different scales; and, third, their superiorproperties as fuels, being portable with remarkably highenergy density comparing with other candidates [4].
A salient life quality factor recognized by United Nations’development program is human development index (HDI)[5]. The notable fact about this measure is its direct relationwith the energy consumption; the higher increase in fuel con-sumption, the more significant jump in HDI [6]! This impliesthe importance of striking a delicate balance in a reasonabletime scale between the energy usage and the environmentalimpacts of burning fossil fuels; which dictates meaningfulimprovements in energy utilization efficiency from one sideand serious environmental protection actions from the otherside. Concurrently, establishing a clear prospect for the
INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2013; 37:485–499
Published online 6 March 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3021
sustainable energies’ future and demand construction for therenewable energy supplies are considered as must-takensteps [7,8].
In the fossil-fuel-based economies, current remedies for theCO2 reduction from large-scale energy consumers (e.g. powerstations and cement works) mainly rely on carbon capture andstorage (CCS), having three proposed generic solutions: post-combustion capture, pre-combustion capture, and oxy fuelcombustion. [9]. All the aforementioned approaches are basedon various physical and chemical phenomena includingabsorption, adsorption, and cryogenic capture of CO2 [10].More recent and innovative practices such as employing or-ganometallic frameworks [11], ionic liquids [12], membranesystems [13], and microbiological-based capturing techniques[14] have been also examined for this purpose, but still distantto be feasible. Afterwards, the purified CO2 is sent for thephysical storage options such as deep ocean sequestration[15], geological storage [16], limited industrial uses (e.g.mineral processing and soda companies) [17], etc. However,researchers believe CCS strategy does not resolve the problempermanently, just shifting that from the atmosphere to some-where else [18]. In fact, long-term ecological impacts of usingthe earth as a gigantic reservoir for CO2 are off the beaten track[15,19]. Adding the potential hazards of CO2 leakage to theearth surface noticeably exacerbates the prospect of thisapproach [20]. Considering all the facts, the ultimate solutionof realistically facing with CO2 sequestration concerns is thechemical transformation of CO2 to useful products such asfuels, chemical commodities, polymers, etc. This ultimatelyleads to the utopia for the fossil-fuel-based economies withtaking full advantage of carbonaceous fuels while minimizingtheir negative environmental impacts. The intriguing idea ofuseful products synthesis from recovered CO2 substantiallyreduces carbon emissions, so-called carbon capture and con-version (CCC) [21].
This paper is a selective bundle of the latest encourag-ing combinations of science and engineering toward sus-tainable actions on alarming CO2 emissions, employingthis stable molecule as a feedstock to synthesize useful pro-ducts. From chemical engineering point of view with thematerial and process design approach, author has high-lighted the most salient technological advances towardchemical sequestration of CO2. It should be noted that po-tentially, ample of biological systems capable of bioCO2
capturing can be coupled with the major ideas of CO2
chemical transformation. Focusing mainly on catalytic,electrocatalytic, and photoelectrocatalytic techniques, au-thor has overlooked biological pathways for the sake ofconsistent narration. Readers are referred to the reviewsdone by Skjånes et al. [22] and Jones and Mayfield [14]for more detailed information.
2. CO2 CONVERSION TOCARBONACEOUS FUELS
Serving an important pattern, emission control enhancementsvia transportation chapter equate a substantial change in the
vehicle fuels. Generally, proposed alternatives in this sectionare electricity, hydrogen, and biofuels. Having electricity andhydrogen to replace fossil hydrocarbonaceous fuels necessi-tates a fundamental revolution in the energy infrastructure,while there are strong projections on biofuels’ utilizationconsidering their global supply limits, which lies betweenonly 20 and 30% of the transportation fuel needs [23]. In ad-dition, utilizing electricity and hydrogen as alternative fuelsrequires simultaneous decarbonization of their productionsources (i.e. upstream decarbonizing) [24]. Furthermore, bat-tery technologies mostly suffer from low gravimetric andvolumetric energy densities [25]. Figure 1 presents net sys-tems’ volumetric and gravimetric energy densities for vari-ous on-board energy carriers obtained via a recentcomprehensive ‘net systems analysis’ [26]. While hydro-gen’s gravimetric energy density exceeds that of batteriesand fossil fuels, its volumetric energy density is extremelylow comparing with carbonaceous liquid fuels such as dieseland gasoline. It’s noteworthy that enormous investments re-quired by the infrastructural changes (i.e. hydrogen produc-tion, distribution and refueling facilities) for switching tothe hydrogen-based economies further undermines the po-tential advantages.
The fourth choice, which is the author’s attitude in this pa-per, is initiated by the ideal state of CO2 chemical conversionto synthesize valuable products such as hydrocarbonaceousfuels, chemical commodities, polymers, etc. This option pro-vides the fascinating opportunity of utilizing primary energyfrom renewable green resources (e.g. electricity produced bysolar, wind, wave, geothermal, biofuel, or nuclear) to chem-ically store CO2. In association with hydrogen or methane,CO2 can be transformed into high-density fuels and othervaluable chemical commodities applicable to the currentinfrastructure. This decarbonizes fossil-fuel-based econo-mies without any revolutionary change in the infrastructurerequired by other alternatives’ replacement such as electricityand hydrogen.
Short-term solutions for the cumulative CO2 emissionsmostly put emphasis on the efficient energy utilization pro-grams. Nonetheless, in the long term, fossil fuels substitutionby the renewable energy resources is determining for theeffective global CO2 mitigation actions. Robust calculationsshow that some of the renewable resources such as solar
0 20 40 60 80 100 120 140 1600
5
10
15
20
25
30
35
40 Diesel
Ethanol
Gasoline
Hydrogen Gas
700 bar Hydrogen
Liquid Hydrogen
Liquid Natural Gas
Methanol
Natural Gas
250 bar Natural Gas
Batteries
MJ/Kg
MJ/
L
Figure 1. Net systems’ volumetric and gravimetric energydensities for various energy carriers.
CO2 chemical conversion to useful productsA. Taheri Najafabadi
energy by far exceed all the human energy needs [27].However, a great portion of these supplies are located inthe remote areas, far away from the consumers’ access.One option is the use of electricity as an energy carrier toaddress the transportation/distribution concern, which is notalways feasible due to the energy losses and other difficulties.Thus, employing other types of chemical energy carrierssuch as hydrogen and liquid carbonaceous fuels is moreattractive for renewable energy harnessing. Coupling theobtained green electricity or hydrogen with CO2 reductionpositively shakes the global carbon balance with the absenceof upstream CO2 emissions, where the fossil fuels arepredominantly burnt for their production [2,24].
Figure 2 illustrates a generic energy cycle of theaforementioned CO2 chemical conversion. It depicts theuse of sustainable electricity derived from renewableresources in order to electrolyze water, producing hydrogenfor fuel synthesis via Fischer–Tropsch process. This resultsin ideal ‘carbon-neutral’ state for the fossil-fuel-basedenergy cycles, offering a number of advantages; first, theproduced liquid fuels contain high energy densities well-suitedfor the industrial applications; second, such clean fuels do notcontain any sulfur as a hazardous environmental pollutant;and, third, methanol as the simplest carbonaceous fuel is acompetitive candidate for both fuel-cell- and combustion-based power systems [28–30].
2.1. Energy constraints
CO2 which is primarily the result of carbonaceous fuel ox-idation or combustion is highly stable, requiring noticeableenergy input and catalysis for its reduction. Hence, use ofCO2 as a feedstock calls for a closer look at the energybalance and exergy analysis. It should be noted that bothterms of enthalpy and entropy changes (ΔH0 and TΔS0,
respectively) of the Gibbs free energy change (ΔG0) arenot favorable for the CO2 chemical transformation [31].Hence, remarkable advances in the surface chemistry ofCO2 are only obtained through intelligent design ofinnovative catalytic techniques [29,32].
It is noteworthy that a wide range of industrial chemicalproduction lines are operating with high endothermicreactions; where steam methane reforming (SMR) to yieldsyngas (i.e. mixture of carbon monoxide and hydrogen) isa classic example (Equation 1).
CH4 þ H2O ! COþ 3H2 ΔH0 ¼ þ206:3 kJ mol�1
(1)
CO2 reforming of methane so-called ‘dry reforming’mimics the above-mentioned rational for the CO2 chemicalreduction with a hydrocarbon (Equation 2). This will befurther discussed in the upcoming sections especially withthe emphasis on the CO2 conversion from flue gas (i.e. thecombustion exhaust gas), producing chemical fuels as theoutput.
CH4 þ CO2 ! 2COþ 2H2 ΔH0 ¼ þ247:3 kJ mol�1
(2)
Comparing Equation 2 with Equation 1 indicates thatthe dry reforming is approximately 20% more energyconsumptive than SMR, which is not much more costlyto avoid running CO2 chemical reduction in large scales.Additionally, from thermodynamics point of view, co-reacting CO2 and other chemicals with higher Gibbs freeenergies (e.g. hydrogen or methane) facilities chemicalreduction of CO2. The chemical energy released by thesehydrogen-rich chemicals significantly enhances CO2
reducibility [33]. The advances of using hydrogen and
Figure 2. Ideal carbon-neutral energy cycle, coupling carbon capture technologies with renewable carbonaceous fuel production.
CO2 chemical conversion to useful products A. Taheri Najafabadi
methane for the catalytic CO2 conversion are reviewed inSections 2.2 and 2.3, respectively.
2.2. Catalytic reduction of CO2 withhydrogen
Methanol synthesis from CO2 is a major possible pathwayfor CO2 reduction using hydrogen as the co-reactant(Equation 3). This process can be paraphrased as thechemical liquefaction of hydrogen to increase its volumet-ric energy density! Carbon neutrality is achieved if theenergies for the water electrolysis (i.e. hydrogen produc-tion) and carbon capture/release are supplied from carbon-neutral resources (Figure 3).
CO2 þ 3H2 ! CH3OH þ H2O ΔH0 ¼ �131 kJ mol�1
(3)
In terms of material challenge in this field, CO2 and H2
only react with employ of multicomponent heterogeneouscatalysts at high temperatures [34,35]. In fact, the reversewater-gas shift reaction (RWGS) presented in Equation 4has a key role in the catalytic hydrogenation ofCO2. RWGS results in the preliminary conversion of theH2/CO2 stream to CO and H2O, where the produced wateracts as an inhibitor on the catalyst surface, jeopardizingthe complementary step of the methanol synthesis. Conse-quently, it is necessary to enhance the process efficiencyat lower temperatures with water tolerance. There havebeen some studies on the catalysts with improved yieldsof CO2 hydrogenation to methanol [36,37]. Cu-Zn oxidesare the most attractive candidates for this purpose combinedwith various additives such as ZrO2, Ga2O3, and SiO2 [37].These materials certainly increase the intrinsic reactivity,active surface area (with dispersion), thermal stability, andreusability; however, they have not exhibited enough watertolerance with low per pass conversions of about 10% [34].Thus, multicomponent heterogeneous catalysts (Cu/ZnO/ZrO2/Al2O3/SiO2) are employed, introducing better perfor-mance especially under real-time reaction standards.Nevertheless, in the case of using pure CO2 as a feedstock,the conversion rates are typically 3–10 times lower than
having CO/CO2 mixtures as the reagent [38]. Therefore,the feasibility studies on the methanol synthesis viaCO2 hydrogenation are mainly divided by two differentapproaches, having RWGS separated from methanol pro-duction step or integrating two stages in a single reactor.According, the first technique benefits from higher catalystperformance, lower gas recycle, and reactor size [39].
CO2 þ H2 ! COþ H2O ΔH0 ¼ þ41:2 kJ mol�1 (4)
In addition, a homogeneous catalytic method is recentlyintroduced for CO2-to-methanol conversion via hydrogena-tion. It basically employs nonmetal complexes for hydro-gen activation via so-called frustrated Lewis pairs (FLPs)in certain designed organometallic complexes [40,41].Figure 4 demonstrates the reversible CO2 uptake and re-lease by frustrated phosphine-borane Lewis pairs initiallysuggested by Mömming et al. Room temperature reductionof CO2 to CO and methanol by Al-based FLPs has beenalso suggested by Ménard and Stephan [42,43].
It is important to note that in terms of energy consump-tion, hydrogen production step by far outweighs otherpresented processes in Figure 3, twisting the CO2 hydroge-nation fortune (and any other renewable carbonaceousliquid fuels production) with green hydrogen production’s
Figure 3. A generic carbon-neutral cycle for the sustainable methanol synthesis coupled with green hydrogen production pathways.
Figure 4. Reversible CO2 uptake and release by frustratedphosphine-borane Lewis pairs for homogenous catalytic carbon
dioxide conversion [40].
CO2 chemical conversion to useful productsA. Taheri Najafabadi
advancements (i.e. cheap sustainable pathways to the pro-duction of hydrogen).
2.3. Catalytic reduction of flue gas withmethane
Use of flue gas as the direct feedstock instead of pre-separated and purified CO2, so-called tri-reforming, offers agreat advantage over dry reforming of captured CO2 usingmethane. In general, CO2 can be separated, recovered, andpurified from flue gases of fossil-fuel-based power plantsby two or more steps. This is mainly based on either absorp-tion or adsorption or membrane separation [10,44] whichadds significant cost to the CO2 conversion or sequestrationsystem. Tri-reforming is a synergetic combination of CO2
reforming, steam reforming, and partial oxidation of naturalgas. Table I presents the main reactions for syngas produc-tion by tri-reforming of natural gas [45]. The simultaneousoxy-CO2-steam reforming reactions in the tri-reforming pro-cess can produce synthesis gas (CO+H2) with controllableH2/CO ratios (1.5–2.0). Coke formation which is a seriousproblem in the CO2 reforming of methane can be alsoeliminated considering coke removal nature of the sidereactions [46]. In addition, tri-reforming benefits from moreauto-thermic reaction enthalpy than simple dry reformingof methane with CO2. Overall, the remarkable appeal hereis to use the CO2 present in the power plant exhausts directlyin the catalytic processes, generating syngas suitable forfuel synthesis. Figure 5 shows the tri-reformer schematicfor catalytic reduction of flue gas with methane.
In terms of materials, there are three major catalyst classessuggested for tri-reforming process [48]: (i) the non-noblegroups 8–10 transition metals (Fe, Co, and Ni), (ii) the noblegroups 8–10 transition metals (Ru, Rh, Ir, Pd, and Pt), and(iii) transition metal carbide catalysts (Mo2C and WC).Noble-metal-based nanoparticles have shown superioritieswith high resistance against coke formation [49]; however,their precious nature is a limiting factor. Ni-based catalystshave shown the closest activity to the noble metals, attractingintense attention due to their price advantage and wide avail-ability; nonetheless, nickel particles are prone to rapid cokeformation and catalyst deactivation due to the methane crack-ing reaction [50,51]. The adsorbed oxygen on the surfaceeliminates the deposited carbon, producing CO; nevertheless,in the case of non-noble metals such as Ni, the coke removalreaction does not take place efficiently due to the insufficientadsorbed O2 molecules, and/or high solubility of carbon inthe metal. Hence, extensive research is performed for oxygenactivation improvement on the surface of the catalyst. In this
regard, metal oxide composites such as CeO2/ZrO2 [52],Y2O3/ZrO2 [53], TiO2/ZrO2 [49], and Nb2O5/ZrO2 [54] asthe carriers enhance the catalyst performance relative to thepure oxides. In addition, the most recent studies by Asencioset al. have shown that synthesizing NiO-MgO-ZrO2 solidsolutions inhibits the Ni� deactivation more efficiently [48].Table II shows the reactions associated with the coke forma-tion and removal during tri-reforming process.
Both fixed-bed and fluidized-bed reactors have beenemployed for tri-reforming process, where the fluidized-bed configuration lowers carbon depositions in comparisonwith fixed-bed designs [55]. Furthermore, the positive im-pact of applying an external magnetic field on the catalyticactivity and process stability of tri-reforming has beenreported via experimental evaluations [56].
2.4. Electrocatalytic reduction of CO2
The electrochemical reduction of CO2 (ERC) was first ex-amined in the 19th century, producing formic acid usingzinc as the cathode material [57]. This topic received in-tense attention in the 1980s following the oil embargoesof the 1970s [58]. ERC represents a strategic pathway ofproducing useful chemicals from CO2 as a primary feed-stock [34].
Table I. Main reactions involved in catalytic reduction of flue gas with methane (i.e. tri-reforming process) (data from [47]).
Reaction Stoichiometry Enthalpy
CO2 Reforming of Methane (DRM) CH4 +CO2! 2CO+2H2 +247.3 (endothermic)Steam Reforming of Methane (SRM) CH4 +H2O!CO+3H2 +206.3 (endothermic)Partial Oxidation of Methane (POM) CH4 + 1/2O2!CO+2H2 �35.6 (exothermic)Catalytic Combustion of Methane (CCM) CH4 + 2O2!CO2 + 2H2O �880 (exothermic)
Figure 5. Tri-reforming process schematic for catalytic reduc-tion of flue gas with methane.
Table II. Reactions associated with the coke formation andremoval during tri-reforming process (data from [47]).
The pH above 9 causes CO2 sequestration as bicarbonate/carbonate, hindering CO2 participation in reduction mechan-isms. Bicarbonate/carbonate formations are presented by thereactions shown in Equation 7 and Equation 8.
CO2 aqð Þ þ OH�↔HCO3� K ¼ 3:3 � 107 298 Kð Þ (7)
HCO3� þ OH�↔CO3
2� þ H2O K ¼ 4:7 � 103 298 Kð Þ(8)
The reaction in Equation 6 is more thermodynamicallyfavored over that of Equation 5; however, formate can beefficiently produced on some specific cathode materialswhere the water reduction to hydrogen (Equation 6) iskinetically suppressed relative to the reaction in Equation 5.The preferred cathode materials are Hg, In, Pb, and Sn,which are also suitable candidates for the acidic reactionconditions [59]. Recently, a critical review has been pub-lished by Finn et al. [60], elaborating on active molecularcomplexes as more state-of-the-art alternatives for themetal-based CO2 electro-reduction. From electrochemistrypoint of view, the aforementioned metals possess highhydrogen overpotential, leading to low exchange currentdensities for the water reduction to hydrogen (Equation 6).This equates applying a cathode bias of at least �0.5 Vrelative to its Nernst equilibrium potential to run thewater reduction reaction at a reasonable rate [58]. Hence,an advantageous cathode material must have a remarkableexchange current density for the reaction in Equation 5, sig-nificantly higher than that of the water reduction to hydrogen(Equation 6) and other reactions (e.g. Table III). Thesecompetitive reactions can divert the electric current from thedesired pathway, behaving as so-called parasitic reactions.
The previous discussion arose from stoichiometry,thermodynamics and intrinsic kinetic activities of the ca-thodic CO2 electro-reduction. Nonetheless, any practicaluse of electrochemical reactors for this purpose mustaddress the mass transfer issue of CO2 to the cathodesurface. The major drawback is the low CO2 solubilityin water which is about 30 mM at 101 kPa(abs) and293 K. Aqueous electrolytes decrease this amountseriously where the ionic strength can be up to 10M. Themass transfer limiting current density for the CO2 electro-
reduction (Equation 5) can be estimated via Equation 9as iL=0.06 kAm�2. This assumes the typical value of1�10�5 m s�1 for the CO2 mass transfer coefficientto the cathode surface from a saturated electrolyte solution(electron stoichiometry coefficient ’n’=2, Faraday’snumber ’F’=96480 kC kmol�1, mass transfer coefficient’Km’=1�10�5 m s�1, and the bulk concentration ofreactant species ’Cb’=0.03 kmolm�3). This limiting currentis further declined by the use of dilute CO2 streams such asthose obtained by fossil fuels combustion in air near ambientpressure (e.g. 10–15 mol% CO2 at 100 kPa(abs)).
iL ¼ nFKmCb (9)
Generally speaking, geometric current densities (based onthe superficial electrode area) in the order of 0.1 kAm�2 aretypically insufficient to favor the industrial electrosynthesis.There are two main reasons for this rule of thumb related tothe stoichiometry and economics of electrochemical pro-cesses [61]. First, the reaction rates on the electrode areproportional to the current density via Faraday’s law(Equation 10), where the geometric surface area needed fora certain production rate is given by Equation 11 (r=specificreaction rate, i=current density, CE=current efficiency orelectrochemical selectivity for a specified reaction, A=totalelectrode area, and R0=total production rate). This simplymeans a significantly large surface is required to achievethe desired reaction rates when the geometric current densi-ties are not high enough.
r ¼ iCE=nF (10)
A ¼ nFR’=iCE (11)
Second, the fixed capital investment required for the elec-trochemical reactors manufacturing varies fromUS$10�103
to US$30�103 per square meter of geometric electrode(anode or cathode) area [59]. To afford this capital cost,industrial electrosynthesis processes are usually operated atgeometric current densities above 1 kAm�2 with at least50% current efficiency for the desired product [59]. For gas-eous reactants, gas space velocity as a typical rule of thumbshould be higher than 100 h�1. Hence, the aforementionedcriteria set tough framework for the design of a practical re-actor for electro-reduction of CO2 [58].
Table III. Half-cell reaction pathways for the carbon dioxideelectro-reduction (data from [47]).
The majority of the published work on the CO2 electro-reduction to formate/formic acid has centered mainly oncatalytic and mechanistic characteristics of the electrodereactions. The experiments are performed in small batchreactors (cells) under conditions distant from practical pur-poses [62–64]. Some of these studies acknowledge the im-portance of CO2 mass transfer [62], but few attempt toquantify its effect [34,59,65].
With respect to the feasible industrial application, perhapsthe most notable prior results are those of Udupa et al. [66],Mahmood et al. [67], and Todoroki et al. [64]. Theseresearchers reported, respectively: (i) the accumulation ofup to 3M sodium formate with about 80% current efficiencyat 0.2 kAm�2 superficial current density on a sparged,cation membrane batch reactor under ambient conditions; (ii)formate/formic acid current efficiencies near 100% at super-ficial current densities about 1.2 kAm�2 on a gas diffusioncathode under ambient conditions; and (iii) formate currentefficiencies near 100% at superficial current density around5.6 kAm�2 on a particulate electrode under CO2 pressureof 6000 kPa(abs) at 293K.
Following the successful employ of gas diffusion electro-des (GDEs) in the gas-fed fuel cells and other electrosynthesisprocesses [68,69], a few researches initiated the use of GDEsfor CO2 electro-reduction [59,70]. The results revealed thepromising role of GDEs in overcoming the counted issues as-sociated with CO2 mass transfer. However, they suffer fromtwo major problems which are temporal stability and liquid-phase products accumulation in the GDE pores [70]. Figure 6depicts a successful example of GDE application for thecontinuous CO2 electro-reduction reactor design under feasi-ble industrial conditions. Prior published work on continuousERC reactors is represented by that of Akahori et al. [71]and Li and Oloman [65]. Akahori et al. used a 1�10�4 m2
lead wire bundle cathode with a single-phase flow of thesaturated CO2 solution in a flow-by reactor. Employing a cat-ion membrane separator, they reported 100% formate currentefficiency at 0.02 kAm�2 (2 mA cm�2) under ambient con-ditions. Li and Oloman examined a trickle-bed electrochemi-cal reactor, utilizing pure CO2 streams to feed the electro-reduction process. They proved that the multi-cell trickle-bedelectrochemical reactors are potentially capable of CO2 reduc-tion to formate under industrial conditions. Each reactor canhandle gas feed rates up to about 3 tons of CO2 per day atthe superficial current densities of 2 to 4 kA m�2. Whipple
et al. [70] introduced another successful configuration basedon microfluidics approach with flexible operating conditionsincluding feed composition and electrolyte pH. They achieved89% of formate current efficiency and a current density of1 kA m�2. Furthermore, any water management issue atthe electrodes (flooding or dry-out) can be minimized usingthis fuel-cell-like arrangement (Figure 6). Their studies of-fered the microfluidic electrochemical cell as an effectiveapproach and a versatile analytical tool for the CO2
electro-reduction diagnosis [72].
2.5. Photoelectrocatalytic reduction of CO2
The photoelectrochemical conversion of CO2 and water isrecognized by the United States Department of Energy asone of the top three research priorities on advancedcatalysis for energy applications [73]. Hence, it is tried tomaneuver on this topic deeply, explaining the cutting-edgeadvances in nanoscale photocatalyst and photoreactordesign for CO2 photoreduction to fuels.
Like any other photocatalytic process, photoelectro-chemical reduction of CO2 by water is initiated by thesemiconductor’s light absorption with photons’ energygreater than its band gap (Eg). This absorption causes thegeneration of excited photoelectrons in the conductionband (CB) and holes in the valence band (VB) of the semi-conductor, depicted in Figure 7a. As indicated in Figure 7b,
Figure 6. Schematic of the microfluidic approach for CO2 electro-reduction [70].
Figure 7. Photoreduction of CO2 by water using Pt-TiO2 photo-catalyst; (a) photo-excitation in the electronic band structure ofthe photocatalyst, and (b) migration of generated electron-hole
pairs to the photocatalyst surface, driving redox reactions.
CO2 chemical conversion to useful products A. Taheri Najafabadi
once the photo-excited electron-hole pairs are created inthe bulk of the material, they must migrate separately tothe surface (paths i and ii in Figure 7b) and competestrongly with the electron-hole recombination (path iii inFigure 7b) that consumes the pairs, generating heat.Finally, at the surface of the semiconductor, absorbedCO2 and water molecules are reduced and oxidized byphoto-induced electrons and holes to produce methane(or methanol) and oxygen, respectively.
The significantly lower efficiency of the photocatalyticproduction of solar fuels comparing with photovoltaic so-lar systems is still the bottleneck of employing photocata-lytic energy conversion systems [74]. Unlike photovoltaicelectricity production, here a careful control over couplingthe photo-excitation process with the multi-electron redoxreactions is crucial. Hence, the photocatalyst candidatesfor this application should satisfy the redox potentials forthe rate-limiting steps of water oxidation (Equation 12)and CO2 reduction (Equation 13). Based on the thermody-namics, having a band gap of at least 2.88 eV is vital todrive the redox reactions with suitable band edges’ posi-tions. Consequently, wide-band-gap semiconductors arethe most suitable candidates for CO2 photoreduction.
2H2O lð Þ ! O2 gð Þ þ 4Hþ aqð Þ þ 4e� E0ox
¼ �1:23 V (12)
CO2 aqð Þ þ e� ! CO2� aqð Þ E0
red ¼ �1:65 V (13)
2.5.1. Photocatalyst materials: nano eraThe major drawback of utilizing wide-band-gap semi-
conductors is their poor activity under visible light irradia-tion. Strictly speaking, photons with the energies lowerthan the semiconductor band gap are incapable of generat-ing photo-excited electron-hole pairs, thus wasted as heat.Accordingly, only UV and near-UV portions of the solarspectrum can be employed by the wide-band-gap photoca-talysts such as TiO2 [75]. This results in the low absorptionof solar irradiation and consequently low conversion effi-ciency in the visible region. Although there are plenty ofsemiconductors available with lower band gaps (e.g. CdSand Fe2O3 with band-gap values of 2.4 and 2.3 eV, respec-tively), only a few of them meet all the criteria, especiallywith respect to band edges positions. This issue along withthe poor photo-corrosion resistivity of many semiconduc-tors in aqueous mediums set onerous conditions on the ma-terial selection for CO2 photocatalytic reduction [76,77].
Considering three important factors of cost, stability,and performance, there is still a noticeable affinity towardusage of TiO2 based on its superior photocatalyticproperties. Titania has been examined successfully for thephotocatalytic CO2 conversion to useful products in thegas- and aqueous-phase photoreactors. Various approachesare employed to synthesize more efficient TiO2-based mate-rials for this application [77,78]: (i) synthesis of the uniformsingle-phase titania crystalline structure, (ii) band-gap modi-fication of the UV-activated TiO2 particles, (iii) surfacecocatalyst deposition to catalyze redox reactions, (iv) use of
sensitization effect to harness visible light, and (v) enhancingnanofabrication techniques for better size and morphologycontrol.
It is vital to rule over the energy structure (i.e. band gap)of the semiconductor to develop active photocatalystsunder visible light. This is typically performed via fourstrategies: (i) ion doping [79], (ii) use of semiconductor com-posites [80], (iii) semiconductor alloy approach [81], and (iv)multi-photon transfer technique so-called z-scheme [82].Serving a salient example, the pairing of TiO2 with eitherCuO or, ideally, Cu2O has emerged as a promising candidatefor the direct photocatalytic CO2 conversion. Based on theenergy band diagram, Figure 8 indicates a possible CO2
reduction pathway for the electron-hole pairs in a TiO2-CuOsystem [80].
As mentioned earlier, the photocatalytic CO2 reductioninvolves surface gas evolution reactions. The photo-generatedelectrons and holes which successfully reach to the surface ofthe photocatalyst are able to reduce and oxidize surfaceabsorbed CO2 andwater molecules, respectively. Both surfaceabsorption and gas evolution reactions can be catalyzed withemploy of noble metals or transition metals and their oxides,so-called cocatalyst (e.g. Pt, Cu, andNiO) on the photocatalystsurface. These metallic compounds increase the gas evolutionefficiency based on the following roles: (i) capturing conduc-tion-band electrons or valence-band holes from the excitationsites (sink role) [83], and (ii) transferring charge carriers to thesurface absorbed molecules, reducing the activation energy ofthe redox reactions (injection role) [84].
The photo-generated charge transport is also a functionof crystal size and structure, nature and number of latticedefects, and the surface characteristics of photocatalyst.Having an efficient photo-induced charge carrier dynamicsrequires longer diffusion length comparing with theparticle size. Thus, the probability of reaching the chargecarriers to the surface increases with the particle sizedecrease [85]. Structural defects deteriorate the efficiencynotably, serving as trapping or recombination centersfor the electron-hole pairs, depending on the defects’ na-ture and location. Photocatalyst deposition over a high
Figure 8. Possible CO2 photoreduction pathways for the photo-generated electron-hole pairs in the TiO2-CuO photocatalyst.
CO2 chemical conversion to useful productsA. Taheri Najafabadi
surface area support is an advantageous approach for CO2
photocatalytic conversion. For instance, highly dispersedtetrahedrically coordinated TiO2 species incorporated intomesoporous MCM-type supports exhibit higher perfor-mances for CO2-to-fuel reduction relative to the unsup-ported TiO2 nanoparticles [77].
From photoreactor configuration prospect, an advanceddesign is having water dissociation to occur at a photoa-node, while CO2 reduction happens at a photocathode,which both are attached to the sides of a proton-conductingmembrane (Figure 9) [34]. Such a photoelectrocatalytic de-vice poses conceptual analogy to the proton-exchangemembrane fuel cells, benefiting from technologicaladvances of that mature technology. Next section aims toexpand the photoreactor design concept with the optoflui-dics approach for more efficient CO2 photoreduction.
2.5.2. Photoreactor design: optofluidics horizonOptofluidics is a discipline that combines the advan-
tages of microfluidics and optics.[86]. In fact, this is theapex of engineering for the solar-derived fuel synthesiswith the highest harnessing and conversion capabilities.The major strength of optofluidics is its simultaneous androbust control over fluids and light in small scales. Thisincludes implementing micro- and nano-fluidic channelsequipped with photonic elements such as waveguides,optical resonators, optical fibers, lasers, and metallicnanostructures [87].
The simultaneous control of fluids and light at smallscales offers several advantages: (i) the small enclosedpathways lower the mass transfer issues associated withthe long diffusion length of reactants before contactingphotocatalytic surface or photosynthetic micro-organism.This dramatically increases the power density, significantlylowering the operational costs (for instance, temperaturecontrol over the entire system is much more facile) [88];(ii) optofluidics aims to facilitate the simultaneous distribu-tion of light and fluid, offering adaptability and flexibilityfor solar collection and control; and (iii) fluid-based opticalmodifications using microfluidic handling techniques is
fairly straightforward without the complexities associatedwith moving solid components [89].
In the field of photocatalytic CO2 reduction, researchesprimarily focused on the material development rather thanthe science and engineering of photoreactor development.The principle as described earlier includes the photo-excitation of a semiconductor by photon absorption leadingin electron-hole generation. The holes oxidize water to ox-ygen, while the electrons are used for CO2 reduction tochemical commodities and fuels such as syngas, methane,and methanol. Perhaps, the most successful practice ofcombining the photocatalysis principle with the potentialsof the optofluidics for solar CO2 reduction is the photoreac-tor developed by Varghese et al. (Figure 10) [90]. Theauthors have improved the photocatalytic CO2 conversionrates by using the following strategies: (i) employing highsurface area titania nanotube arrays via anodizing titaniumfoil with a wall thickness low enough to accommodateefficient photo-generated charge transfer; (ii) modificationof the titania band gap via nitrogen doping to better absorbthe visible portion of the solar spectrum; and, (iii) impreg-nation of the cocatalyst nanoparticles (Pt-Cu alloys) onthe nanotube array surface to absorb the reactants, facilitat-ing redox process.
The interesting observation by Varghese et al. was thehigher CO concentration for Cu surface-loaded samplesand higher H2 concentration for Pt-loaded configurations.This clearly indicates that Cu is more active for CO2 re-duction, while Pt favors water oxidation. Hence, with adelicate dispersion of Cu-Pt cocatalysts on the nanotubearray surface, both reactions can be carried out atenhanced rates [90]. The hole diffusion length in titaniais �10 nm [91], and the electron diffusion length is�10 mm [92], while half of the TiO2 nanotube wallthickness in Varghese et al. work was approximately 10nm. Consequently, the high rate of CO2 conversion isattributed to the high surface area and nanoscale wall thick-ness of the nanotubes. This enables surface species toreadily receive both charge carriers generated near the sur-face via wave function overlap and those generated deep in-side the wall via diffusion. Stronger titania-hydrocarboninteraction is more likely when the hydrocarbon concentra-tion increases to higher values; hence, employing a flow-through photocatalytic membrane design is more preferredover a batch reactor as used for the Varghese et al. work [89].
Figure 9. Conceptual design of the proton-exchange membrane-based configuration for the photocatalytic carbon dioxide to
methanol conversion.
Figure 10. Solar-driven photocatalytic CO2 conversion to carbo-naceous fuels using nitrogen-doped titania nanotube arrays with
Cu and/or Pt cocatalyst nanoparticles [90].
CO2 chemical conversion to useful products A. Taheri Najafabadi
Based on the served example [90], it is concluded thatnanostructured photocatalyst arrays offer many advantagesincluding: (i) high surface area available for reaction, (ii)close proximity of point charges to the reactant fluid, (iii)rapid charge transfer, and (iv) low refractive index, whichminimizes incident light reflection. The optofluidics isalso implemented for the solar steam methanol reformingfor hydrogen production [93]. Lee et al. found that a1-mm-diameter capillary-based liquid reaction chamberwith nanoparticle cocatalysts performs more than 1000times better than the equivalent bulk reactor. The wettingnature of the capillaries was another notable characteristic,providing passive pumping of the reactants through thereaction islands.
Besides rendering photocatalytic reaction of interest,optofluidics can also provide an ideal environment for theprocess studies including sensing and analytical applica-tions. Furthermore, optofluidics not only brings the genericscaling benefits, but also promotes those of photocatalyticprocesses which are highly sensitive to the light, chemicaland thermal environment. This makes optofluidics moreintriguing in conjunction with photocatalytic processes,aiming to address the long-standing challenges in thisconcept [88].
3. CO2 CONVERSION TO POLYMERS
If CO2-to-fuel conversion is envisioned as an ambitious so-lution for CO2 chemical sequestration, the polymerizationapproach is more realistic. In fact, there is a myriad ofsuggested pathways from CO2 to useful materials whichare discussed in detail by Sakakura et al. [33] and Finnet al. [60]. However, the author has selected polymeriza-tion approach as the most industrially viable use of CO2
as a feedstock in large scales.Plastic production plants are among the major CO2
emitters. In fact, the carbon footprint of plastic (LDPE,PET, and polyethylene) is about 6 kg CO2 per kg ofplastic [94]. The idea here is to develop a green catalyticprocess that consumes CO2 as a feedstock for makingmore polymers, achieving net zero carbon emission inthe plastic industry. CO2 cannot be directly polymerized,but can yield polymer in the presence of certain epoxides(e.g. ethylene oxide, propylene oxide (PO), and cyclohexeneoxide), catalyzed by the well-designed metal complexes(Figure 11).
The very first thought of CO2 copolymerizationwas first triggered in 1969 by Inoue et al. at the TokyoInstitute of Technology with addressing poor catalystactivity (turnover number (TON): 6; turnover frequency(TOF): 0.12 h�1) [95]. Since this inspiring work,numerous studies performed on the various types ofcatalysts and processes to enhance their performancefor higher copolymerization yields. The major break-throughs were made in 2000s by the use of binarycatalytic systems of cobalt [96–98] and chromium
[99,100]. Recently, highly active catalytic systems(TON>20000; TOF>20000 h�1) are introduced forCO2/PO copolymerization, selectively producing highmolecular weight copolymers (Mn>300000) [97,101,102].The designed homogenous catalyst can be readily sepa-rated by filtration through a short pad of silica gel to yielda resin containing negligible amounts of metal residue(1–2 ppm) [33,101]. The separated catalyst can berecovered without a significant loss of catalytic performance.Hence, the aforementioned features are all positive for thecommercialization of the CO2/PO copolymer production.
SalenCo(III) complexes as catalysts have exhibitedseveral versatilities for CO2/epoxides copolymerization topolycarbonates: [97,98,101]: (i) excellent regioselectivityfor epoxide ring opening, (ii) high activity for the selectiveformation of copolymer with perfect alternating nature,(iii) broad substrate scope including epoxide bearingfunctional groups, (iv) immortal polymerization (a livingpolymerization), and (v) excellent activities at a very lowcatalyst loading and/or low CO2 pressures. These allowthe controlled synthesis of block and random terpolymers,as well as fine-tuning of regio- and stereo-chemistry.Figure 12 shows the chiral (R,R)-(salen)Co(III) complexas an active catalyst for CO2/PO copolymerization.
From industrial production prospect, operating a contin-uous CO2/PO copolymerization process is feasible basedon the aforementioned viable studies [97,101,102].Figure 13 depicts the suggested CO2 copolymerizationschematic with an effective catalyst recovery system. Thisresults in a biodegradable polycarbonate with excellentoptical properties (i.e. transparency), which up to 44% ofits weight is originated from CO2 conversion [102]. Theproduced polymer possesses impressive barrier character-istics towards O2 and H2O, comparable with Nylon andEVOH, being proper for the food packaging industry. Itcan be also used in the polymeric alloys with reasonableadhesive features. The CO2-derived polymer burnssmoothly in the air (as gently as wood or alcohol) without
Figure 11. CO2/propylene oxide copolymerization reaction, in-troducing polycarbonates as the main product and cyclic carbo-
nates as the by-product [97].
Figure 12. The chiral (R,R)-(salen)Co(III) complex as an activecatalyst for CO2/propylene oxide copolymerization [98].
CO2 chemical conversion to useful productsA. Taheri Najafabadi
toxic emission or ash residue, making its disposal by incin-eration practical and safe [60].
Major disadvantages are weak thermal properties andlow glass transition temperatures [97]. The synthesizedpolycarbonates typically start to decompose at 180�Cwhich is a limiting factor. However, the drawbacks canbe hindered via changing the epoxides and/or introducingterpolymerization process (i.e. use of three monomers inthe polymerization reaction), which improves the glasstransition temperatures, thermal properties, and otherpolymer characteristics [97]. The available literature onthe more comprehensive studies about CO2 copolymeriza-tion is limited, but growing in a fast-paced manner. This isone of the most scalable and economical candidates for theCO2 chemical transformation, offering a sustainable ap-proach to the plastic production industry, simultaneously.Importantly, green energy supply for the subsequent equip-ment design is a must to achieve carbon neutrality for theentire polymerization process.
4. CONCLUSION
In the past few decades, the main concern regardinghuman-made CO2 emissions, as the major cause of globalwarming, was the economical pathways toward efficientCO2 collection. Currently, mankind has moved one stepforward, demanding for the operative sequestration techni-ques of CO2, which is now purified and stored in anenormous order.
In this paper, the game-changing approach of CO2
chemical conversion to useful products from fuels tochemical commodities and polymers was elaborated. Anengineering insight to the latest advances in CO2 chemicaltransformation was presented with the emphasis on the en-ergy constraints, materials, and process design. This surelypromotes the environmentally benign use of carbonaceousfuels and derived hydrocarbon products, viewing purifiedCO2 as a strategic feedstock. However, to bring any ofthe aforementioned practices into the real life, achieving
notable progresses in the research and development is amust. Simultaneously, serious worldwide actions on theCO2 emission regulations, initiating more robust interna-tional collaborations among developed and developingcountries are crucial.
Last but not least, the fortune of CCC is entirely twistedwith the green energy incorporation from carbon neutralityprospect. Otherwise, it does not affect the global carbonbalance positively, where the unsustainable energyresources cause significant upstream CO2 emissions. Ulti-mately, the exergy analysis, and subsequently economics,determine the viability of this concept like any other feasi-ble industrial process.
NOMENCLATURE
Eg = band-gap energyCb = bulk concentrationCCC = carbon capture and conversionCCS = carbon capture and storageCCM = catalytic combustion of methaneDRM = CO2 reforming of methaneCB = conduction bandCE = current efficiencyERC = electrochemical reduction of carbon dioxiden = electron stoichiometry coefficientEVOH = ethylene vinyl alcoholF = Faraday’s numberFLP = frustrated Lewis pairHDI = human development indexLDPE = low density poly ethyleneKm = mass transfer coefficientiL = mass transfer limiting current densityMn = number-average molecular weightPOM = partial oxidation of methanePET = polyethylene terephthalatePO = propylene oxideRWGS = reverse water-gas shiftΔH0 = standard enthalpy changeΔS0 = standard entropy changeΔG0 = standard Gibbs free energy changeSMR = steam methane reformingA = total electrode areaR0 = total production rateTOF = turnover frequencyTON = turnover numberVB = valence band
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CO2 chemical conversion to useful products A. Taheri Najafabadi
