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    Topic review

    Carbon Dioxide Conversion

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    Submitted by: C. P. Huang


    Managing the concentration of atmospheric CO2 requires a multifaceted engineering strategy, which remains a highly challenging task. Reducing atmospheric CO2 (CO2R) by converting it to value-added chemicals in a carbon neutral footprint manner must be the ultimate goal. The latest progress in CO2R through either abiotic (artificial catalysts) or biotic (natural enzymes) processes is reviewed herein. Abiotic CO2R can be conducted in the aqueous phase that usually leads to the formation of a mixture of CO, formic acid, and hydrogen. By contrast, a wide spectrum of hydrocarbon species is often observed by abiotic CO2R in the gaseous phase. On the other hand, biotic CO2R is often conducted in the aqueous phase and a wide spectrum of value-added chemicals are obtained. Key to the success of the abiotic process is understanding the surface chemistry of catalysts, which significantly governs the reactivity and selectivity of CO2R. However, in biotic CO2R, operation conditions and reactor design are crucial to reaching a neutral carbon footprint. Future research needs to look toward neutral or even negative carbon footprint CO2R processes. Having a deep insight into the scientific and technological aspect of both abiotic and biotic CO2R would advance in designing efficient catalysts and microalgae farming systems. Integrating the abiotic and biotic CO2R such as microbial fuel cells further diversifies the spectrum of CO2R.

    1. Introduction

    Controlling atmospheric CO2 concentration is essential to the mitigation of global warming. Amongst all strategies, CO2 conversion to value-added chemicals should be a top choice. In principle, CO2 conversion can readily take place naturally (biotic) by cultivating plants or algae to absorb CO2 and artificially (abiotic) by using synthesized catalysts in a controlled system to accelerate electrochemical CO2 conversion. Photosynthesis is the most known naturally occurring CO2 conversion reaction [1]:
    The forward and backward reaction in Equation (1) indicates the photosynthesis and the respiration, respectively. The significance of Equation (1), in addition to the photosynthesis and natural respiration, is the involved chemical cycles of carbon, nitrogen, and phosphorus. This further implies that the reclamation of trace nutrients is an additional benefit to biotic CO2
    conversion. By contrast, artificial CO2 conversion requires energy input since CO2 has no heat value. Recent progress in catalysis greatly reduces the energy requirement in artificial CO2 reduction (CO2R).
    There are excellent reviews on the mechanistic aspects and effectiveness of CO2R catalysts [2,3,4,5,6]. Artificial CO2R can be achieved by processes, such as electrochemical [7,8,9], photochemical [10,11,12], and photoelectrochemical (PEC), by which electrons are transferred to CO2 molecules to increase their energy content [13,14]. Electrochemical CO2R is an electron transfer reaction between the cathode and the adsorbed CO2. Photochemical CO2R relies on the transfer of photogenerated electrons in the LUMO (the lowest unoccupied molecular orbital) of the catalyst to the HUMO (the highest occupied molecular orbital) of adsorbed CO2. In photoelectrochemical (PEC) CO2R, the photogenerated electrons are transported to the cathode in the presence of a bias electric field. The external electric field effectively suppresses the recombination loss of photogenerated charge carriers and enhances CO2R efficiency.
    In all modes of CO2R systems, catalysts (in the form of photosensitive or electric conductive) are needed to facilitate the CO2 reduction reactions. Therefore, the stability and surface chemistry of catalysts (electrodes) become important operation parameters. For instance, it is suggested that a thin γAl2O3
    overlayer could effectively stabilize the Faradaic efficiency and the partial current density of the SnO2 catalyst in an electrochemical CO2R reaction [8]. Chang et al. (2016) have reported that photogenerated holes, not electrons, are the primary cause of instability of Cu2O catalysts, which require Cu2O to be operated as dark cathodes [13]. High-CO-affinity electrocatalysts (i.e., Cr, Mn, and Fe-N-C) exhibited high carbon monoxide (CO) Faradaic efficiency. The pyridinic and hydrogenated (pyrrolic) nitrogen moieties of the carbonaceous support are active sites for CO2 adsorption [9]. Accordingly, a relatively basic surface (such as the presence of γAl2O3
    overlayer and pyridinic modification) would have a positive effect on CO2R efficiency enhancement, likely through increasing the affinity of the catalyst surface toward CO2 adsorption. Defects in a catalyst would introduce coordinately unsaturated sites (i.e., active sites for molecular chemisorption) and provide spatially supply channels for energy and electron transfers in photochemical CO2R [10]. Note that among all available CO2R processes, electrochemical CO2R is probably the simplest; therefore, it is the easiest and most sustainable (relative to chemical reduction) for operations and scaling-up.
    CO2R involves the consecutive transfer of one electron or hydrogen from the catalyst to CO2 and intermediates, which gradually reduces the carbon oxidation number stepwise. Only intermediates that possess even oxidation numbers are thermodynamically stable. This is why formic acid (FA, HCOOH), carbon monoxide (CO), formaldehyde (HCOH), methanol (CH3OH), and methane (CH4) are always found in CO2R reactions. This review focuses on recent advances of CO2R systems, both biotic and abiotic processes, and as far as the broad view of CO2 conversion is concerned, three significant implications are noted [15].
    Mitigating greenhouse gas effect: Electrocatalytic CO2R is usually conducted with high-purity CO2 based on thermodynamics considerations. This means that additional electricity is required for concentrating CO2 feedstock; however, the majority of electricity in modern society is produced through fossil fuels. Adopting electrocatalytic CO2R for the mitigation of the greenhouse effect would instead increase atmospheric CO2 concentration [2,16]. Therefore, increasing the deployment of renewable energy would be equally important in addition to the efficiency improvement of electrocatalytic CO2R from the prospect of greenhouse gas mitigation.
    Electrochemical conversion of CO2 to fuels: In order to achieve a carbon neutral footprint through electrocatalytic CO2R, produced hydrocarbons should not be fed to the combustion engines directly. Instead, they should be feed for fuel cells [15]. Again, increasing the efficiency of fuel cells for feeding CO2-derived methanol [17,18] or formic acid [19,20] is another issue of concern. Selectivity of CO2R catalysts is crucial too as it minimizes the carbon footprint required for subsequent separation and concentration of hydrocarbons produced from electrocatalytic CO2R [21].
    Electrochemical conversion of CO2 to a building block: Electrocatalytic CO2R to carbonaceous fuels usually leads to a positive carbon footprint as mentioned above. However, a carbon neutral footprint could be possibly reached by converting CO2 to a building block as an alternative. This is because CO2 is a C1 building block that requires no extra energy for its production [22]. For example, carbon monoxide produced by electrocatalytic CO2R could be directly used as the source for producing phosgene [23,24]. Similarly, as-obtained methanol and formic acid could be feedstocks for reversible chemical hydrogen storage and other applications [5,25].
    Apparently, converting CO2 to a building block for chemicals production could entice the pursuant of electrocatalytic CO2R. Indeed, all conceptual designs of an economically affordable CO2R business for achieving a negative carbon footprint have assumed that the CO2R catalysts could exhibit excellent reactivity and selectivity. For instance, Gai et al. (2016) performed a conceptual design of methanol production from CO2 at an industrial scale using ASPEN Plus® (Bedford, MA, USA) in which CO and H2 are produced from the electrolysis of CO2 and H2O [26]. They found that when CO2 conversion is less than 42%, the optimal methanol synthesis route is CO hydrogenation. Importantly, the achievement of a near zero carbon emission power plant is strongly built on the assumption that CO is the only intermediate and no additional energy is required for CO isolation [26]. Sun et al. (2019) have developed a 20 MWth solar–wind biodistributed energy system for simultaneously biomass cascade utilization, water resource conservation, waste heat recovery, and CO2 mitigation for hydrogen, formic acid, and grapheme production [27]. Again, in their framework, the energy efficiency is vulnerable to the compromised selectivity of electrocatalytic CO2R. Based on the above considerations, it is clear that the success of a CO2R industry strongly relies on multidiscipline cooperation and that technology is part of this. Additional bonuses such as creating jobs, building blocks for chemicals, and carbon right trading would make CO2R more sustainable. Integrating the knowledge of biotic and abiotic CO2R is another useful approach. A good example is the microbial electrosynthesis system (MES), in which the microbial is responsible for biotic CO2R, while engineering the electrode (abiotic CO2R) further improves the overall CO2R efficiency. Accordingly, recent advances in abiotic CO2R and biotic CO2R will be reviewed herein. Having a deep insight into the scientific and technological aspects of both abiotic and biotic CO2R would advance the design of efficient catalysts and the microalgae farming system. We first focus on the technology aspect of abiotic CO2R by discussing the reactivity and selectivity of CO2R catalysts and reaction mechanisms in both water and gas phases. The effect of the surface chemistry of synthesized catalysts on the reactivity and selectivity of CO2R will be addressed. This will be particularly beneficial to the rational design of high-efficient catalysts for CO2R conversion. CO2R through microalgae abstraction of CO2 is highly influenced by the bioactivity of selected microalgae. Separation and purification of various value-added chemicals obtained is another issue of concern, and further refinery of algal biomass is also included in this review. Additional considerations such as the involvement of other stakeholders that allow CO2R to be more sustainable are also discussed.

    2. The Chemistry of Abiotic CO2R

    2.1. Abiotic CO2R in Water Phase

    CO2R can occur in the water or gas phase. In the former system, carbonate species, namely, H2CO3*, HCO3, CO32−, are reduced. In the latter, gaseous CO2 is reacted with electron donors over catalysts and C1 or C2 compounds such as CO, formate, methanol, and oxalate are major products. Undoubtedly, the usage of rare and precious metals, such as Re and Pd, always leads to the highest CO2R efficiency [4]. The application of transition metals such as Fe, Mn, and Ni has received much attention recently because of material abundance and economical affordability [33,34].

    2.1.1. Effect of Cu Surface Chemistry on Abiotic CO2R

    Buckley et al. (2019) have studied the structure-reactivity relationships of electrocatalytic CO2R on modified Cu cathode surfaces [35]. The Cu cathode is first modified with long chain hydrocarbons so as to render the Cu surface hydrophobic. Modified electrodes are used to study the CO2R reaction in CO2-saturated KHCO3 (0.05 M) solution [36]. The Faradaic efficiency (ηF) of each species is calculated by Equation (2):
    where F is the faraday constant (96,485 C-mol−1), V is the volume of electrolyte (specifically the catholyte), C is the concentration of carbonate species (M), and Q is the total charge passing through the cathode during electrocatalysis CO2R. n is the number of electron transfer in the CO2R process. For example, n = 2 for CO2 reduction to CO or HCOOH (FA). The partial current density (ji) of certain species is the product of the Faradaic efficiency and total current density (jtotal):
    Figure 1 shows the replotted contour image of the current density of FA, CO, and H2 production in electrocatalytic CO2R over the modified Cu cathode [35]. Two interesting features are noted in Figure 1. First, FA is the major CO2R species on Cu-based cathodes as its value is about tenfold higher than that of CO. Second, the hot zone (the red area) of the hydrogen current occurs at the left-handed side of the figure that corresponds to the region with the lowest CO current. This clearly indicates that electrocatalytic hydrogen production profoundly competes with CO evolution. Importantly, the hot zone of hydrogen evolution appears in the region with a moderate FA current (0.5~2.5 mA/cm2). This means that FA formation is less affected by hydrogen evolution than that of the CO formation [13]. It must be mentioned that increasing operation potential inevitably leads to high H2 yield (Figure 2) [37]. Due to thermodynamics restrictions, FA and CO evolution in CO2R is suggested to be carried out at a relatively low potential condition.
    Figure 1. Current density of FA, CO, and H2 production in electrocatalytic CO2R over modified Cu electrode. Replotted data from Buckley et al. (2019) [35].
    Figure 2. Faradaic efficiency of CO, FA, and H2 production in electrocatalytic CO2R over boron-doped diamond (BDD) cathodes as a function of operation potential. Replotted data from Tomisaki et al. (2019) [37].
    Engineering electrochemical properties of cathode materials enhances the selectivity. Designing an efficient catalyst for CO2R is a highly technical challenge due to the strong completion from the hydrogen evolution reaction (HER) [3]. Factors such as catalytic reactivity, product selectivity, Faradaic efficiency, catalytic stability, and reduction mechanisms are crucial to controlling the efficiency of CO2R [38]. A systematical evaluation of the effect of catalyst structure on reaction selectivity is hence highly desirable [4]. Taking the electrocatalytic CO2R molecular system as an example, the energy required to dissociate an M–H bond to form a hydride is the key parameter in CO2R selectivity [2,4,39,40]. Figure 3 shows pathways regulating the transfer of either two protons (for CO evolution) or two electrons (for formic acid formation) and in both cases hydrogen evolution is always the major competitive side reaction.
    Figure 3. Illustration of CO2R pathways occurring at the protic and aprotic surface. Rearranged from Buckley et al. (2019) [35].
    In this framework, the preferential interaction between the catalytic metal center and CO2 over protons is responsible for the selectivity for CO evolution. While the moderate hydricity facilitates CO2 insertion into M–H bonds for FA production, strong hydride donors catalyze H2 formation [2,4,39,40]. It has been suggested that catalytic activity requires the presence of a weakly coordinating solvent molecule that can readily become dissociated during the catalytic cycle as to provide a vacant coordination site for water binding and assisting C−O bond cleavage [4]. Generally, H2 and FA formations are favorable reactions in aqueous solutions [39]. In fact, by plotting the hydricity as a function of individual free energy for the one-electron reduction of the parent species, a linear correlation appears indicating high FA selectivity over CO formation. Importantly, this correlation extends over a wide range of metals, ligand architectures, structural geometries, and overall charge of the metal hydride [40]. High overpotential is always found in CO2R [41]. Despite hydrogen evolution always being a competing reaction in CO2R, it is still worthy of scientific investigation on the hydrogenation of CO2 to FA and dehydrogenation of FA as a practical hydrogen storage pathway [5]. FA formation essentially increases the density of hydrogen gas [6]. This opens a practical alternative, such as the direct formic acid fuel cells (DFAFC) [42]. Major heterogeneous metal catalysts, such as In, Sn, Hg, and Pb exhibit high FA selectivity [5,6,41,42]. Electrochemical CO2R on polycrystalline Sn surfaces exhibits high FA selectivity too. Formation of *OCHO at Sn surfaces is the key intermediate for FA production due to optimal *OCHO binding energy. The results suggest that oxygen-bound intermediates are critical to understanding the mechanism of CO2 reduction to HCOO– on metal surfaces [43].
    Cu is known to have relatively low CO selectivity because the CO produced is further reduced to several multi-carbon oxygenates (i.e., ethanol, acetate, and n-propanol) [41]. Specifically, sulfur-modified copper catalysts (Cu–S) exhibit positive correlation between particle size and selectivity toward FA evolution [44]. Nanostructured porous dendritic Cu-based catalysts show stable and selective conversion of CO2 into FA at high current density with low overpotential [45]. The relatively low CO selectivity on Cu surfaces results from consecutive CO electroreduction activity [46]. That is, in CO2-saturated aqueous solutions, polycrystalline Cu catalysts produce a mixture of compounds. Indeed, H2 evolution is dominated at low overpotential, CO and FA formation mainly occurs at high overpotential, while hydrocarbons, ethanol, acetate, and n-propanol formation happen at the most extreme overpotentials [47,48]. In a CO2 free environment, CO is reduced to hydrocarbons and multi-carbon oxygenates over the Cu catalyst [49,50]. Interestingly, oxide-derived Cu (Cu catalysts prepared by reducing Cu2O) shows much higher H2 selectivity than polycrystalline Cu [51]. Similarly, aqueous electrochemical CO reduction to C2 products by face-to-face coordinated thiol-terminated metalloporphyrins on copper electrodes exhibits 83% Faradaic efficiency and 1.34 mA/cm2 current density at −0.40 V vs. RHE. This is a significant improvement in both selectivity and activity by one order of magnitude over parent copper surfaces or copper functionalized with porphyrins in an edge-on orientation [52]. In a similar system, oxide-derived copper (OD-Cu) electrodes exhibit a high CO reduction performance by producing ethanol and acetate with >50 % Faradaic efficiency at −0.3 V vs. RHE [53].
    In a short summary, the selectivity of abiotic CO2R in the aqueous phase is highly sensitive to the surface chemistry of Cu-family catalysts. CO2R occurring at the aprotic surface tends to yield CO as the major product. By contrast, CO2R happening at the protic surface is prone to produce hydrocarbons and multi-carbon oxygenates as the major products. In this case, the selectivity is strongly affected by the involved reaction pathway. In the former case, the reduction is achieved through the charge transfer, while in the latter case the hydride transfer is mainly responsible for the CO2R. The adsorption affinity between reduced intermediates and Cu-family catalysts is another critical factor regulating the CO2R selectivity. High adsorption affinity slows the desorption of the reduced intermediates, which enables their consecutive reduction. This explains that the hydrocarbons and multi-carbon oxygenates as the major products are found in this case. It is thus concluded that increasing CO2R selectivity could be achieved through the modification of surface hydrophobicity and adsorption affinity.

    2.1.2. Effect of Surface Chemistry of Non Cu-Family Catalysts on Abiotic CO2R

    In the iron-based CO2R, introducing the extra elemental Fe plate profoundly decreases the overpotential of the microbial electrosynthesis system (MES) [54]. In this MES system, all produced CO2 is reduced to formate at the cathode and vast hydrogen is produced during the digestion of waste activated sludge. This is attributed to the high selectivity toward formic acid evolution over CO and methane production to the reduction of H+ at the cathode due to the slow methanogensis in Fe-C MES [54]. In the case of photocatalytic CO2R, enhancing sunlight conversion efficiency is always accompanied with improving CO2R selectivity [55]. Similar to the strategy adopted in the dye-sensitized photoelectrochemical cells [56], increasing selectivity in FA formation is usually achieved through coordinating active metals with covalent organic frameworks. For example, the columnar orientation COFs (covalent organic frameworks) provides a high-efficient charge carrier transport through the ordered π-electronic pathway, which improves electron transfer from COF to metal moiety and thus increases the reactivity [55]. The results of density functional theory computations further reveal that COFs decorated with electron-donating substituents favor CO2 reduction by decreasing the hydricity of the Rh–H bond. This results in a lower hydride transfer barrier toward formic acid production [57] because the selectivity toward CO or HCOOH production is dependent on the coordination environment of the metal ion being capable of cleaving the C–O bond in the metal–CO2H intermediate [55]. Specifically, an electron-rich coordination environment breaks the C−O bond to form CO, whereas an electron-deficiency coordination environment tends to enhance the C−O bonding thereby enhancing FA formation [58]. That is, if the center metal in covalent-organic frameworks (COFs) is a strong π-donor, such as Co(II), it usually tends to promote CO evolution. By contrast, a weak π-donor, such as Zn(II), favors HCOOH production [55]. A catalyst exhibiting low adsorption energy for HCOO* (i.e., a large energy difference between the two adsorbed CO2 reduction intermediates, namely HCOO* and COOH* and large H* adsorption energy) would have high FA selectivity [59]. Ajmal et al. (2019) have studied the selectivity of the CO2 reduction reaction over bimetallic CuZn alloy catalysts and reported that the Faradaic efficiency and partial current density of FA on Cu0.5Zn0.5 (equal molar ratio of Cu and Zn) are enhanced by nearly 4 and 5 times, respectively, that of Cu foil [60]. The high selectivity of the CuZn bimetallic alloy catalyst is originated from the synergistic effect of Cu and Zn. In this case, the Zn (a weak π-donor) is likely to create a large energy difference for the adsorption of two CO2 reduction intermediates, namely, HCOO* and COOH*. An et al. (2019) have studied CO2 reduction over Sn/SnOx catalysts and reported a maximum FA Faradaic efficiency of 89% at −1.7 V (vs. Ag/AgCl) in a 0.1 M CO2-saturated KHCO3 solution [61]. The authors further concluded that Sn(IV) and Sn(II) species are mainly responsible for controlling the overpotential and suppressing H2 evolution toward improved FA selectivity. Chatterjee et al. (2019) have reported that nanoporous Pd-based alloys (np-PdX, X = Co, Ni, Cu, and Ag) exhibit FA selectivity following the order: np-PdAg > np-PdCu > Pd/C > np-PdNi > np-PdCo [62]. They have concluded that the composition-dependent behavior was governed by CO adsorption strength associated with the presence of transition metal alloying components near the Pd-skin surface and a composition-dependent change in the near surface H-sorption capacity. Interestingly, the free-standing np-PdCo and np-PdNi catalysts are able to sustain a high formate partial current density (>20 mA-cm−2) with high CO poisoning tolerance while exhibiting insignificant loss of the active area [62]. This further highlights the importance of durability and resistance of catalysts against CO poisoning during CO2R.
    The results of computational hydrogen electrode model simulation reveal a striking similarity in CO2R electrocatalytic activity for the Cu3 vs. Cu5 and Cu4 vs. Cu6 size-selected clusters [63]. The rate-limiting potential of Cu4 and Cu6 clusters in CO2R is the proton-electron (H+ + e) transfer to CO* (species adsorbed on clusters) to form CHO*, which is also the rate-limiting step on Cu surfaces. On the other hand, with respect to Cu3 and Cu5 clusters, removing OH* from the cluster surface (OH* S+OH)
    is the rate-limiting step in CO2R [63]. The above simulation unambiguously implies the role of surface defects, in addition to bulk electrocatalysts, in regulating CO2R reaction pathways. Indeed, the electrolysis of CO2 on 4-aminomethylbenzene-modified Pb electrodes exhibits a current density as high as 24.0 mA/cm2 (at −1.29 V vs. RHE) and a FA Faradaic efficiency greater than 80% [28]. Pt-based alloys having high-index facets generally show high specific catalytic activity over those having low-index facets [64]. Exposing the high-index facets of nanosized particles is promising to enhance Pt utilization and at the same time enriches crystalline defects in the CO2R catalyst [64]. Similarly, Pan et al. (2019) have reported that N,S-codoped carbon catalysts exhibit 92% CO Faradaic efficiency and CO current density of 2.63 mA/cm2 at a low overpotential of 0.49 V versus RHE [65]. Incorporating S in N-doped carbon introduces a high population of activate pyridinic N sites, which significantly decreases the free energy barrier for the formation of intermediate *COOH thereby enhancing CO adsorption toward high CO selectivity [65]. The high CO selectivity of the Pd85Cu15 catalyst is attributed to the presence of a larger number of low-coordination Cu sites than active monometallic Pd sites on the catalyst [66]. Accordingly, manipulating the size and chemical composition of bimetallic nanoparticles is critical to the selectivity of CO2R [66]. Results of density functional theory calculation indicate that high reactivity and selectivity are the outcome of defects that stabilize the *OCHO intermediate [67]. Surface modification of the Cu catalyst with protic, hydrophilic, and cationic hydrophobic species results in increasing the selectivity of H2, FA, and CO, respectively [35]. Table 1 summarizes the performance of various CO2R processes. Note that Faradaic efficiency alone is not sufficient to express the degree of selectivity because the current density of individual species is also an important characteristic of an efficient catalytic CO2R reaction. In addition, as mentioned above, a low faraday efficiency in CO/FA is usually accompanied by a high faraday efficiency in H2 evolution, which is another valuable product of CO2R in the aqueous solution.
    Table 1. Summary of CO2R performances included in this study.
    Base of Catalyst FE (%) in FA CO2R Condition Reference
    Cu modified with polymeric 38–45 −0.7 VRHE in 0.05 M K2CO3 and 4 mM KCl with 5 sccm CO2 [35]
    boron-doped diamond ~70 −2.1 V (vs. Ag/AgCl) in KCl aqueous solution [37]
    polycrystalline Sn ~70 −1.0 VRHE in 0.1 M KHCO3with 20 sccm CO2 [43]
    sulfur-modified copper ~80 −0.8 VRHE in 0.1 M KHCO3with 20 sccm CO2 [44]
    Cu ~85 −1.6 V vs. ferrocenium voltage in CO2-saturated [EMIM](BF4)/H2O (92/8 v/v) ionic liquid solution [45]
    Cu (1.5 cm ×
    3 cm) ~20 −0.8 VRHE in 0.1 M KHCO3 with 20 sccm CO2 [47]
    Cu ~20 −1.4 VRHE in 0.5 M KCl with 70 sccm CO2 [48]
    Cu2O@Cu ~40 −0.7 VRHE in 0.1 M KHCO3 with 5 sccm CO2 [51]
    Iron-graphite electrode pair ~18 −0.6 VAg/AgCl with CO2 saturated 0.5 MNaHCO3 in anaerobic sludge digestion process [54]
    Co incovalentorganic frameworks 1.02 mmol h1g1
    in CO MeCN with triethanolamineas sacrificial reducing agent and Ru(bpy)3Cl2·asphotosensitizerunder simulated sunlight [55]
    In0.6Bi0.2Sn0.2 alloy on a halide perovskite ~95 −1.3 VRHE in 0.5 M KHCO3with 20 sccmCO2under simulated sunlight [59]
    Cu0.5Zn0.5 ~60 −1.3 VRHE in CO2 saturated 0.1 M KHCO3 under simulated sunlight [60]
    SnOx/Sn ~80 −1.7 VAg/AgCl in 0.1 M CO2-saturated KHCO3 [61]
    Pd15Ni85 ~50 −0.5 VRHE in 1.0 M CO2-saturated KHCO3 [62]
    Pb modified with 4-aminomethylbenzene ~80 −1.3 VRHE in 1.0 M CO2-saturated KHCO3 [28]
    N,S-codoped carbon catalysts ~90 in CO −0.6 VRHE in 0.1 M KHCO3with 34 sccm CO2 [65]
    Pd85Cu15/C ~86 in CO −0.9 VRHE in 1.0 M CO2-saturated KHCO3 [66]
    β-Bi2O3 double-walled nanotubes
    ~90 −0.8 VRHE in 0.5M KHCO3with 20 sccm CO2 [67]
    3.7 nm Pd nanoparticles ~90 in CO −0.9 VRHE in 1.0 M CO2-saturated KHCO3 [68]
    boron-doped Pd catalyst ~70 −0.5 VRHE in 1.0 M CO2-saturated KHCO3 [69]
    Electrolytes also play a crucial role in determining CO2R selectivity. CO2R over the BDD electrode in the KClO4 electrolyte produces CO, whereas FA is the major product in the KCl electrolyte. This is because ClO4 promotes the adsorption of CO2
    intermediates [37]. Similarly, Eder et al. (2019) have reported that on Ru-based catalysts, hydrosilanes additives and KF stabilized formate intermediates (silylformate) forms form potassium formate with a turnover number of 110 mmol-formate/mmol-Ru [70]. Note again that the CO2R selectivity is a function of applied voltage, which gradually shifts from HCOO/HCOOH to CO/H2 with increasing overpotentials [68,69]. Based on the above consideration, a new approach for effective CO2R by an aluminum hydride-like reductant has been attempted [71]. The reductant is an organoaluminum complex containing a formal aluminum double bond (dialumene). Weetman et al. (2019) have demonstrated that dialumene improves the selective formation of formic acid equivalent via the dialuminum carbonate complex rather than the conventional aluminum–hydride-based cycle [71]. Likewise, Zhao et al. (2019) have reported that the KBH4 reduces CO2 to HCO2
    readily, accompanied by the release of activate intermediate species and H+. Further, CO2R is accelerated by a Cu/Ni bimetal catalyst that effectively regenerated the active boron species [72].
    Briefly, the surface chemistry of non-Cu-family catalysts also strongly influences the selectivity of abiotic CO2R in the aqueous phase. Unlike the Cu-family catalysts, the unique characteristics of non-Cu-family catalysts is that they can be tailored with a specific porous framework. In this configuration, the kinetics in the confined space govern the CO2R selectivity. The localized coordination environment is another factor regulating the overall CO2R selectivity, which can be modified via doping or introducing defects. Along with the modification is the adjustment in the energy levels that further affects the lifetime (stability) of the COR2 intermediate and consequently the CO2R selectivity.

    2.2. The Chemistry of Gaseous Phase Abiotic CO2R

    Several valuable chemicals such as CO, methane, methanol, low olefins, and long-chain carbohydrates could be produced from the gaseous phase CO2R reaction. To achieve selective CO2R, catalysts that are effective in activating both H2 and CO2 and stabilizing surface intermediates are needed. To this end, most catalysts are comprised of metallic sites, which are active in splitting adsorbed H2 (from H2 to H*) and exhibit a high affinity toward CO2 adsorption. Additional modification with alkali or noble metals can further change the surface acidity or aid in the formation of an extra alloy phase in the catalyst [73]. The metal/support interfacial sites are highly active in CO2 hydrogenation due to electron perturbation of the metal and partial reduction of metal oxide via the H-spillover mechanism [74,75].
    Wet impregnation and co-precipitation are the two most frequently used methods to synthesize heterogeneous catalysts for CO2 hydrogenation [76,77]. Nanosized catalysts can be synthesized by flame-spray pyrolysis [78]. The catalysts made by pyrolyzing metal organic precursors can achieve a complex nanostructure with high surface metal loadings [79,80] and the carbon sites can promote the adsorption and activation of CO2 [81]. Fixed bed reactor configuration is applied in the evaluation of catalytic performance, in which specific amounts of catalysts are distributed over inert particles, such as SiC or silica, to the minimize hot spot phenomenon. After pre-reduction of the catalysts in H2 flow, the stream that contains certain ratios of H2/CO2/inert gas with a specific gas hourly space velocity (GHSV) is injected into the fixed bed reactor. The CO2 conversion (XCO2) and selectivity (S) toward CO and low hydrocarbons are measured from on-line gas chromatography, while the long chain hydrocarbons are collected in a cold trap for further quantification.

    2.2.1. Hydrogenation of CO2 to CO and CH4

    Under ambient pressure, hydrogenation of CO2 over most metal catalysts produces either CO or CH4. The product CO can serve as a feedstock in the methanol synthesis process and the Fischer–Tropsch process for further carbohydrate synthesis [73]. Conversion of CO2 to methane could buffer the fluctuations in energy supply via the power-to-gas process that converts excess electricity to H2 as the reducing agent in CO2 methanation [78]. Equations (4) and (5) present the Sabatier reaction and reverse water–gas shift (RWGS), corresponding to the hydrogenation of CO2 to methane and CO, respectively. Thermodynamically, the former reaction dominates at T < 500 °C, while the latter at T > 500 °C. However, as presented in Table 2, the selectivity toward CO and methane is greatly altered on the heterogeneous catalysts with the combination of various metals and supports.
    Table 2. Performance of the selected catalysts for CO2 hydrogenation to CO and CH4.
    Catalyst T (°C) P (MPa) H2/CO2/Inert GHSV
    (mL g−1 h−1)
    XCO2 (%) SCO (%) SCH4 (%) Reference
    Ru/MnOx 300 0.1 22/5/73 150,000 25 10 90 [78]
    Ru/Al2O3 32 6 94
    Ru/CeO2 83 1 99
    Ru/ZnO 1 94 6
    PtCo/TiO2 300 0.1 67/33/0 36,000 8.2 99 1 [82]
    PtCo/CeO2 9.1 92 8
    PtCo/ZrO2 7.8 90 11
    Co/ZrO2 400 3.0 80/20/0 3600 92.5 < 1 > 99 [81]
    Co/SiO2 80.1 2 98
    Co/Al2O3 77.8 3 97
    Co/TiO2 30.9 96 4
    Ni/ZIF-8 a 420 0.1 80/20/0 15,000 43.8 97 3 [81]
    Fe/ZIF-8 a 43.8 97 3
    Ni/Fe/ZrO2 230 0.5 80/20/0 5000 82 14 86 [83]
    γ-Fe2O3 400 0.1 20/0.1/80 1,500,000 45 30 70 [84]
    Ni/MCM b 400 0.1 80/20/0 90,000 73.2 8 92 [85]
    In the presence of noble metals, CO2 hydrogenation to CO can be realized at a low temperature. This phenomenon is attributed to the lower activation energy of the hydrogenation process on the active sites of metals [86]. For instance, Dietz et al. (2015) have simulated RWGS at the (111) plane of several metals and found that Ni, Cu, and Rh favor the dissociation of CO2 → CO + O, while Ag, Pd, and Pt prefer the hydrogenation pathway: CO2 + H → COOH [86]. When Pt is loaded on silica and titania, the support itself enhances CO2 adsorption. The energy change of CO to HCO governs the selectivity toward CO, while the competition for *H2COH between hydrogenation and C–O bond cleavage affects the preferential production of CH4 or CH3OH [87]. The performance of Pt on RWGS is enhanced by the addition of a potassium promoter, which enables the formation of Pt-O(OH)-K interfacial intermediate that promotes the adsorption of the bicarbonate species, the precursor of CO via the formate pathway. CO2 conversion in the K-promoted Pt/zeolite system at 500 °C is 27.4%, which is 2-fold greater than the system without K-promoters [88]. Kattel et al. (2016) further reported that the interfacial sites between PtCo alloy and other reducible oxides (CeO2, TiO2 and ZrO2) are important to stabilizing surface intermediates [82]. Wang et al. (2015) have studied the mechanisms of CO2 hydrogenation over Pd/Al2O3 and concluded that RWGS and the Sabatier reaction do not take place at the same surface sites [89].
    Ru-based catalysts show great catalytic methanation at a low temperature. Dreyer et al. (2017) have investigated the hydrogenation of CO2 using Ru-based catalysts dispersed on different metal oxide supports, including Al2O3, ZnO, MnOx, and CeO2 [78]. They have found that methanation occurs by partial reduction at metal oxides supports, which increases the coverage of H* but strengthens the C–O bond of CO*. The highest CO2 conversion (83%) and methane selectivity (99%) at 300 °C is obtained by the Ru/CeO2 system [78]. Guo et al. (2018) have demonstrated the metal-support interactions and the effect of H-spillover on CO2 methanation [90]. By varying the degree of Ru dispersion from the size of a single atom to nanoparticle (4 nm) on Ru/CeO2, metal-support interaction is the strongest for a single-atom Ru/CeO2 that facilitates CO* activation, whereas H-spillover prevails in large Ru clusters and prevents the catalyst from poisoning by enhanced H2O removal [90]. Thus, controlling the size of Ru at around 1.2 nm achieves an appropriate balance between the two phenomena, leading to a turnover rate of 1.6-fold and 14-fold greater than single-atom and 4-nm Ru, respectively [90]. The interactions between Ru and TiO2 are fortified by syngas pretreatment at 600 °C, which leads to an increase in interfacial sites on Ru-TiO2 by encapsulation of Ru [91]. Hydrogenation of CO2 on nickel-based catalysts has been explored extensively. Dispersing Ni on SiO2 support is deterministic of selectivity, in which the 10 wt% Ni/SiO2 is effective in stabilizing the monodentate configured HCOO intermediate, which does not occur on catalysts of low Ni loadings (0.5 wt%) [92]. Bi et al. (2019) have demonstrated that impregnating Ni on an MCM zeolite with a sodium-free alkaline agent enhances the synergism between Ni and NiO during CO2 hydrogenation via H2 adsorption and CO2 activation, respectively [85]. The catalyst exhibits remarkable CO2 conversion (68.3%) and methane selectivity (91.4%) with high stability [85]. Doping Ni/ZrO2 with iron enhances the reducibility of Ni and ZrO2 owning to the electron-donating property of Fe(II), which in turn promotes the synergism effects of Ni-NiO and metal-support interactions [83,93].
    Based on the above discussions, it is noted that in addition to the reactivity of CO2R catalysts, the chemical environment of the support also plays an important role in CO2R selectivity in the gaseous phase. This is because the gaseous abiotic OC2R is conducted in a high temperature (in comparison with the condition of the aqueous CO2R). In this configuration, the dispersion of CO2R catalysts and consequently their contact with support is highly sensitive to the stability of support. A relative alkaline support such as alumina is beneficial for CO2R efficiency as CO2
    is regarded as a weak acid in this configuration. It is thus suggested that in addition to the characteristics of the CO2R catalyst, its dispersion over the support is another issue of concern for rationally engineering a CO2R catalyst with high selectivity and efficiency.

    2.2.2. Hydrogenation of CO2 to Methanol

    Methanol is a valuable CO2 reduction product as it can be served as biofuel, building blocks in organic synthesis, and fuels for a methanol-based fuel cell. Table 3 shows CO2 conversion to methanol at a pressure of 1–4 MPa over different metallic meal oxide catalysts. Catalyst Cu/Zn/Al2O3 plays an important role in the commercial methanol production from syngas. DFT studies reveal that the stabilization of the transition surface species is the key to achieving high selectivity in the syngas conversion. The formate pathway predominates the total process on the surface of Cu/ZnAl2O4 and Cu/Zn/Al2O3, in which the rate-limiting step is the formation of H2COO* and H2COOH* [94,95]. The CO production via RWGS is the major byproduct in methanol synthesis. Based on the H/D isotope substitution technique, it is known that methanol synthesis and RWGS occurrence take place at difference surface sites on Cu/ZnO/Al2O3, Cu/MgO, Cu/SiO2, and Pd/SiO2 [96]. Karelovic and Ruiz (2015) have suggested that ZnO loads with larger Cu particles tend to suppress the activity of RWGS as the specific methanol formation rate per surface Cu is independent of Cu particle size, while that of CO is enhanced by smaller Cu particles [97]. Ro et al. (2016) have studied the synergistic effect by dispersing Cu on ZrO2 and reported that the rate constant of CO2 conversion on Cu-ZrO2 interfacial is eight times greater than that on plain Cu [98]. Phongamwong et al. (2017) have added colloidal silica on Cu/Zn/ZrO2 as geometric spacers to enhance the stability and performance of the Cu-based catalysts in CO2 hydrogenation [99]. The authors have reported that 1% of SiO2 loading increases the methanol synthesis activity by 26% and retains 12% more activity after emerging from the steam for 96 h at 280 °C.
    Table 3. Performance of the selected catalysts for CO2 hydrogenation to methanol.
    Catalyst T (°C) P (MPa) CO2/H2/Inert GHSV
    (mL g−1 h−1)
    XCO2 (%) SCH3OH (%) Reference
    Cu/SiO2 250 4.1 72/24/4 3600 2.8 15 [100]
    Cu/ZnO 180 0.7 90/10/0 4000 0.9 94 [97]
    Cu/ZnO/ZrO2/SiO2 240 2.0 30/90/10 39,000 5.2 38 [99]
    Cu/ZnO/ZrO2/MgO/Al2O3 250 2.0 75/25/0 2000 12.1 36 [93]
    Pd/ZnO 250 2.0 25/75/0 3600 10.7 60 [101]
    Pd/SiO2 250 4.1 72/24/4 3600 3.0 23 [100]
    Pd-Cu/SiO2 6.6 34
    Pd-Cu/SiO2 250 5.0 75/25/0 30,000 1.6 27 [102]
    MnOx-Co3O4 250 1.0 60/20/20 120,000 45.1 22 [103]
    In2O3 270 4.0 60/20/20 15,000 1.1 55 [76]
    In2O3 330 4.0 60/20/20 15,000 7.1 40
    In2O3/ZrO2 300 5.0 80/20/0 16,000 5.2 >99 [104]
    Bimetallic Pd, such as PdZn and PdCu, exhibits a high methanol yield similar to Cu-based catalysts even at low temperatures [73]. The Pd dispersed in ZnO shows exceptional stability attributed to the particle size being maintained at 5 nm even after pre-reduction at 400 °C. While the colloidal dispersion technique is effective in stabilizing the interfacial sites of PdZn, its methanol formation yield is 40-fold greater than the same catalysts synthesized by the traditional wet impregnation method [101]. PdCu and PdCu3 bimetallic catalysts exhibit a methanol formation rate of 0.31 μmol gcat−1 s−1, which is 3.4-fold and 6.2-fold greater than monometallic Pd and Cu, respectively [100]. Results of DFT simulation further indicates that the (111) plane of PdCu is highly active in methanol evolution, particularly at the low-coordinated Pd on the stepped surface [102]. Furthermore, methanol evolution through the formate pathway again is the major catalytic reaction, which can be further promoted by adsorbing small amounts of water to lower the energy barrier through the H-shuttled mechanism [102]. A novel metal-free In2O3 catalyst is also known to exhibit high selectivity in reducing CO2 to methanol [76]. The oxygen vacancies on In2O3 are important to CO2 hydrogenation. Specifically, the In2O3 with optimal oxygen vacancy exhibits a methanol yield of 3.7 mol kg-cat−1 h−1, CO2 conversion of 7.1%, and methanol selectivity of 40%, respectively, at 330 °C and 4 MPa [75]. Martin et al. (2016) have reported that the selectivity toward methanol over In2O3 approaches 100% in the temperature range of 200–300 °C [104]. The methanol yield is further promoted by increasing the density of surface oxygen vacancy through Ar sputtering, syngas pretreatment, and ZrO2 support [104]. Likewise, incorporating a small fraction of Mn into the spinel Co3O4 structure greatly enhances the methanol selectivity in CO2 hydrogenation, likely attributed to the increase in surface basicity [103]. In brief, the significance of the thermal stability of the support in the CO2R to methanol is relatively less profound than that in the CO2R to CO and methane. This is because the former is usually carried out in a relatively lower temperature than that of the latter. In this case, the surface basicity becomes much more significant in affecting the reactivity of CO2R catalysts.

    2.2.3. Hydrogenation of CO2 to Low and Long Chain Chemicals

    CO2 conversion to lower olefins (C2–C4), building blocks, and other long chain hydrocarbons in the gasoline range (C5–C11) or diesel range (C12–C21) has been explored extensively. Again, catalysts that are reactive in both CO2 hydrogenation and the Fisher–Tropsch reaction will be capable of achieving the CO2 conversion objectives [105]. Table 4 lists the performance of CO2 hydrogenation to low and long chain hydrocarbons on Fe-based catalysts. Upon CO2 hydrogenation to different hydrocarbons, the Fe-based catalysts undergo consecutive phase transitions together with the creation of multivalent charges and this surface reconstruction process further diversifies distinct active sites on the catalyst [84]. The creation of multivalent charges results in the spinel iron (Fe3O4) being composed of Fe3O4, iron carbides, and α-iron, which enhance the activity of RWGS, carbon chain growth, and olefins’ secondary reactions [106]. It is therefore suggested that an appropriate fraction of Fe3O4 and Fe5C2 is necessary for the production of high olefin and paraffin [107,108]. Pretreatment of Co-Fe bimetallic under various reducing gases (i.e, H2, syngas, and CO) is another effective ex-situ modification strategy to precisely control the phase transition [77]. Under the selected reducing environment, CO activation leads to the formation of CoFe alloy and carburized phases (Co2C and FeCx) that shift the selectivity toward low hydrocarbons and oxygenate [77]. While most Fe-based catalysts produce lower hydrocarbons, delafossite (CuFeO2) exhibits high selectivity toward long chain hydrocarbons (C5+) and 85 wt% of the produced long chain hydrocarbons are in the gasoline and diesel range [109]. Doping alkali metals on Fe-based catalysts is also beneficial to chain growth propagation due to stronger COx adsorption [106]. Compared to Na-free Fe3O4, incorporating 1.18 wt% of Na greatly enhances the CO2 conversion and selectivity toward light olefin from 29.3 to 40.5% and 0.1 to 40.3%, respectively [110]. The presence of potassium promoter on Fe-Co/Al2O3 diminishes the density of hydrogen on metal surfaces, which in turn suppresses the hydrogenation of olefins [111].
    Table 4. Performance of the selected catalysts for CO2 hydrogenation to hydrocarbons.
    Catalyst T (°C) P (MPa) H2/CO2/Inert GHSV
    (mL g−1 h−1)
    XCO2 (%) SCO (%) Hydrocarbon Distribution (%) Reference
    CH4 C2=-C4= C2-C4 C5+
    Fe3O4 320 3 72/24/4 2000 29 17 60 <1 36 3 [110]
    Na-Fe3O4 320 3 72/24/4 2000 41 14 16 47 8 30
    Fe2O3 350 1.5 70/23/7 1150 23 21 18 82 [107]
    K-Fe3O4/Fe2O3 270 5 73/25/2 2700 37 14 24 42 9 29 [106]
    CuFeO2 300 1 75/25/0 1800 18 32 4 31 5 60 [109]
    CuFe2O4 300 1 75/25/0 1800 16 28 38 1 49 11
    K-Fe-Co/Al2O3 300 1.1 72/24/4 700 31 18 16 27 6 51 [111]
    Co/Fe oxide a 270 0.9 72/28/0 2000 27 14 82 15 <1 [77]
    Na-Co/Fe oxide a 270 0.9 72/28/0 2000 23 42 60 29 2
    Fe-MIL-88B b
    400 3 75/25/0 3600 46 18 32 23 18 27 [108]
    K-pyrolyzed Fe-MIL-88B b 400 3 75/25/0 3600 43 26 32 33 6 19
    In2O3/HZSM 340 3 73/24/3 9000 13 45 1 20 79 [112]
    Ga2O3/HZSM 340 3 73/24/3 9000 9 86 5 35 61
    Fe2O3/HZSM 340 3 73/24/3 9000 7 74 2 28 71
    Na-Fe3O4/HZSM 350 3 72/24/4 4000 33 26 8 18 74 [105]
    In2O3-ZrO2/SAPO 400 3 73/24/3 9000 36 85 4 76 17 3 [75]
    Bifunctional catalysts have emerged actively in synthesizing hydrocarbons from CO2 with great flexibility. Combined In2O3 and zeolite (HZSM-5) effectively synthesizes liquid fuels (C5+) from CO2 by suppressing undesired C1 products [112]. In the process, CO2 is first hydrogenated to methanol at the reduced site on In2O3. Methanol that diffuses to the acidic site of zeolite transforms to hydrocarbons by the hydrocarbon-pool mechanism, which results in 78.6% of C5+ hydrocarbons [112]. Replacing the zeolite HZSM-5 by SAPO-34 increases the selectivity toward lower olefins, probably as a result of changes in topology [74]. Notably, the bifunctional catalyst must be packed in granular instead of powder form as the surface acidity of the zeolite support is another important factor controlling the selectivity of CO2 hydrogenation toward hydrocarbons [112]. HZSM-5/Na-Fe3O4 composite also effectively converts CO2 to hydrocarbons in the gasoline range [110]. The above catalyst composites provide three distinct active sites: Fe3O4 sites for RWGS, Fe5C2 sites for FTS, and zeolites for oligomerization [105]. Na-Fe3O4/HZSM shows little deactivation; the CO2 conversion and C5+ selectivity remain at 27% and 54%, respectively, over 1000 h of operation [105]. Based on the above discussions, it is noted that the surface chemistry of the support is of equal importance to catalysts in CO2R. Interestingly, surface acidity instead of surface basicity is much more important in this case as the development of long chain chemicals strongly relies on the hydride transfer.
    In short, there are advances in the development of heterogeneous catalysts for CO2 hydrogenation in recent years due to a better understanding of the elementary reactions via DFT simulation, catalytic performance of metal-support, and metal-dopant interfacial sites. With appropriate modifications, CO2 can be hydrogenated to CO, CH4, CH3OH, and other low and long chain hydrocarbons. Further challenges are reducing cost and increasing durability of catalysts, reducing cost of renewable hydrogen sources, and carbon dioxide capture.

    The entry is from 10.3390/su13126962