Tandem reactors and reactions for CO₂ conversion

Abstract

Carbon dioxide (CO₂) valorization is a promising pathway for mitigating greenhouse gas emissions from the chemical sector and reducing the reliance of chemical manufacturing on fossil fuel feedstocks. This Perspective discusses tandem catalytic paradigms for sustainable CO₂ conversion that have potential advantages over processes using single-functional catalysts. Recent progress is discussed for tandem catalysis using multifunctional catalysts in a single reactor, as well as tandem reactors involving multiple catalysts. Opportunities for further developing these tandem strategies for thermochemical and electrochemical processes in various configurations are presented to encourage research in this burgeoning field.

Main

Sustainable CO₂ conversion is a promising strategy for both carbon reduction and carbon utilization to mitigate CO₂ emissions and limit global warming to 1.5 °C. The reliance on fossil fuel-sourced feedstocks and the large amounts of energy required to generate the high temperatures and pressures at which most industrial-scale chemical reactions occur leave the modern chemical industry responsible for approximately 7% of global greenhouse gas emissions^1,2,3. By replacing fossil feedstock with CO₂ for the chemical supply chain, commodity chemicals, plastics, fertilizers, polymers and many other important products can be produced with greatly reduced environmental impacts, assuming that there is sufficient renewable energy available to power the reactors. Furthermore, developing catalytic processes that can use clean electricity, either by using electrochemical reactors or electrically powering thermochemical reactors, will allow the chemical industry to embrace renewable electricity to reduce CO₂ emissions.

CO₂ can be converted into a wide variety of products by reactions with reductants (such as H₂, protons and alkanes) in conjunction with external energy inputs, including thermo-, electro-, photo- or plasma-assisted processes, both of which are necessary to overcome the thermodynamic stability of CO₂. Simple products (for example, CO and CH₄) are facile to obtain using a catalyst containing a single catalytic function; however, longer-chain hydrocarbons, complex molecules and oxygenate molecules (such as olefins, aromatics, alcohols and carboxylic acids) are difficult to produce by direct CO₂ conversion owing to the complex reaction pathways involving many different bond scission/formation and electron-transfer steps. Because of this complexity, tandem reaction processes, wherein two or more distinct catalytic cycles are coupled in such a way that the products of one reaction can immediately be used in a subsequent reaction, have recently gained attention as a paradigm for CO₂ valorization. Traditionally, tandem catalysis has typically referred to the use of multifunctional catalysts with several different types of active site by virtue of their molecular structures (for example, metal-modified zeolites and core@shell nanoparticles), reactor beds composed of mixtures different catalysts or spatially separated catalyst beds within a single reactor (Fig. 1). There is also a burgeoning field of research focused on coupling two independent reactors sequentially such that the output of one reactor is fed directly into a second reactor^4,5,6,7. In this Perspective, ‘tandem catalysis’ refers to the conventional ‘one-pot’ approach of using multifunctional single catalysts or mixtures of multiple catalysts in a single reactor such that the intermediates produced by using the first catalyst are transported to the second catalyst where they further react to form the final products. The term ‘tandem reactors’ refers to a pair of sequentially coupled reactors wherein the products from the first reactor enter a second reactor operating with a different catalyst under different reaction conditions.

Fig. 1: An illustration of the different catalyst mixing scales relevant to tandem catalysis and tandem reactors.

The relationship between mixing scale, catalyst compatibility, thermodynamic compatibility of the coupled reactions and control over individual reaction conditions for tandem catalysts and tandem reactors are indicated by the arrows at the bottom of the figure. Multi-site catalysts can encompass a wide range of catalysts, and two common examples are core@shell nanoparticles (top) and metal-modified zeolites that contain distinct Lewis acid sites (LAS) and Brønsted acid sites (BAS) (bottom). In typical thermocatalytic reactors, catalyst mixtures can be employed by physically mixing two catalysts and loading them into a packed bed reactor, whereas in electrocatalytic reactors, these catalysts mixtures are typically employed by depositing the metallic catalysts onto a conductive substrate. A tandem EC–TC reactor pair is used to illustrate tandem reactors; however, we emphasize that this is only one example, and the tandem reactors category encompasses many other orderings and types of reaction. Throughout this paper, orange is used to indicate the first catalytic step and blue is used to represent the second catalytic step.

Tandem processes have several potential benefits: (1) separation and purification of intermediate products can be simplified in many cases, (2) safety risks, financial costs and environmental concerns associated with transporting and storing hazardous feedstocks are minimized, (3) availability of short-lived surface and/or gaseous intermediates that would not exist in disjoint reactors, and (4) a wider range of opportunities for thermodynamically and kinetically coupling unique reaction chemistries to produce complex and value-added products.

When engineering tandem reaction processes, the overall catalytic performance is optimized by adjusting the catalyst proximity to balance catalyst compatibility, proximity between active sites and control over the reaction sequence, as illustrated in Fig. 1. Optimizing the ratio between the individual catalytic functions or catalysts is also critical to prevent a rate imbalance that causes intermediates to accumulate and evolve toward undesired products. Catalysts containing multiple active sites, such as core–shell and zeolite-supported catalysts or nanoscale mixtures of catalysts, are often advantageous because the proximity of the distinct active sites leads to facile transport of intermediates and active species between the multiple types of active site^8,9,10. However, two catalysts in such close contact may sometimes lead to interference between the adjacent catalysts or active sites that alters the electronic properties of the catalysts with a negative impact on catalytic performance, leading to compromised catalytic activity, selectivity and stability. Because of this, macroscale granule mixtures or microscale powder mixtures often strike a good balance between proximity and compatibility, but, in cases where the deleterious catalyst interactions are too severe, two spatially separated catalyst beds must be used. This can be achieved either by tandem catalyst beds in a single reactor if the requisite reaction conditions for each catalyst bed are similar or by two separate tandem reactors if the individual reaction conditions are markedly different. Figure 1 illustrates this trade-off for tandem thermocatalytic processes and tandem electrocatalytic processes.

In this Perspective, we discuss the current progress of and opportunities for developing tandem processes to sustainably convert CO₂ into value-added, high-demand products. We use examples of tandem catalysis for thermochemical (TC) and electrochemical (EC) reactions, as well as tandem reactors for TC–TC, EC–EC, EC–TC and TC–EC processes, to illustrate the advantages of these tandem processes for CO₂ conversion. We also provide guidance on criteria for choosing between the single reactor configuration with a multifunctional catalyst and tandem reactors with multiple catalysts, as well as a brief overview of current progress in and opportunities for tandem catalytic processes for CO₂ conversion involving biocatalysis, photocatalysis and plasma-assisted catalysis.

Thermocatalytic tandem processes

Single-reactor tandem catalysis can be implemented to couple an endothermic reaction with an exothermic reaction that consumes the products or intermediates from the former, thus enhancing the equilibrium conversion or reducing the reaction temperature according to Le Chatelier’s principle. The utilization of a multifunctional catalyst or a mixture of multiple catalysts within a single reactor also favors tandem reactions involving short-lived intermediates. However, the distinct thermodynamic characteristics of the individual reactions result in different preferred reaction temperature windows (RTWs) for each reaction, so tandem reactors must be used in cases where the difference in RTWs would result in a positive Gibbs free energy change for the overall process. Tandem reactors also offer opportunities to separately optimize catalyst compositions and reaction conditions within each favorable RTW. As shown in the two examples in Fig. 2 involving the reactions of CO₂ and ethane (C₂H₆), the tandem strategy enables the simultaneous upgrading of two abundant feedstocks, CO₂ and C₂H₆ from large-reserved shale gas, to value-added products. The selection of either the single reactor or the tandem reactors configuration is based primarily on the difference in RTWs for the individual reactions.

Fig. 2: Tandem thermocatalytic strategies for CO₂ conversion.

In the single reactor strategy (top) Ga/ZSM-5/P first catalyzes the oxidative dehydrogenation of C₂H₆ with CO₂ to produce C₂H₄, and the C₂H₄ undergoes subsequent aromatization to form BTEX. In the tandem reactor strategy (bottom) PtSn₃/γ-Al₂O₃ catalyzes the oxidative dehydrogenation in the first reactor at 600 °C, and the resulting mixture of CO, C₂H₄ and H₂ undergoes hydroformylation at 200 °C over an RhCo₃/MCM-41 catalyst in the second reactor to produce propanal and propanol. Precise reaction mechanisms, particularly for the oxidative dehydrogenation and aromatization of CO₂, are complex, so the given stoichiometries represent the reaction pathways that are most likely to be dominant. T, temperature; P, pressure.

Single reactor with multifunctional catalysts

As shown in the top panel of Fig. 2, BTEX aromatics (benzene, toluene, ethylbenzene and xylene isomers) are produced from tandem reactions of CO₂-assisted oxidative dehydrogenation of C₂H₆ to produce C₂H₄ and its subsequent aromatization over a multifunctional catalyst of Ga- and P-modified Zeolite Socony Mobil 5 (ZSM-5)¹¹, where the Ga sites were mainly responsible for C–H bond activation of C₂H₆, ZSM-5 for aromatization of C₂H₄, and P modification for improving the stability of acid sites. The single reactor configuration was selected because both dehydrogenation and aromatization reactions could occur at a common temperature of 600 °C. In this case, tandem catalytic reactions of CO₂ and C₂H₆ enhanced the equilibrium yield of aromatics by consuming H₂ via the reverse water–gas shift reaction (CO₂ + H₂ → CO + H₂O) and improved catalyst stability by inhibiting carbon deposition via the reverse Boudouard reaction (CO₂ + C → 2CO).

The single reactor tandem catalysis strategy has also been applied to CO₂ hydrogenation to produce olefins, alcohols, carboxylic acids and aromatics. The balance between the thermodynamic compatibility of tandem reactions, catalyst proximity and compatibility of multiple catalysts plays a pivotal role in the catalytic performance. Xu et al. combined CuZnAl and K-CuMgZnFe oxides for CO₂ hydrogenation to improve the production of ethanol¹²; CuZnAl was identified to be active for the reverse water–gas shift reaction to supply CO* for the subsequent CO* insertion reaction toward CH_x* to form ethanol over K-CuMgZnFe. Li et al. developed a tandem catalyst composed of the ZnZrO solid solution and ZSM-5 that achieved a high single-pass CO₂ conversion and aromatic selectivity¹³; the tandem reaction proceeded with CO₂ → CH_xO* over ZnZrO, followed by CH_xO* → olefins → aromatics over ZSM-5 that required an intimate contact of active sites for the transfer of CH_xO* from ZnZrO to the micropores of ZSM-5. Gao et al. identified that the proximity of the reducible In₂O₃ and zeolites was crucial in achieving high selectivity for the production of gasoline-range hydrocarbons from CO₂ hydrogenation¹⁴, which involved the activation of CO₂ and H₂ by In₂O₃ to form methanol, followed by C–C coupling inside zeolite pores to produce hydrocarbons.

Tandem reactors

The bottom panel in Fig. 2 shows the application of tandem reactors to convert CO₂ and C₂H₆ to C₃ oxygenates (propanal and propanol). Thermodynamically, the direct conversion process is not feasible due to a highly positive Gibbs free energy change for the reaction (ΔG°) over the entire temperature range^15,16. The overall process can alternatively be split into two tandem steps: (1) concurrent CO₂-assisted oxidative dehydrogenation and dry reforming of C₂H₆ to produce C₂H₄, CO and H₂ at high temperatures (that is, 600–800 °C) and (2) subsequent hydroformylation reaction to produce C₃ oxygenates at low temperatures (that is, 200 °C). Therefore, a tandem reactor configuration was introduced to circumvent the thermodynamic gap between the above two steps by running the reactions at their respective preferred reaction temperatures. Xie et al. utilized the tandem reactor strategy by employing ceria-supported FeNi (FeNi/CeO₂) as a reforming and dehydrogenation catalyst in the first reactor and Mobil Composition of Matter No. 41 (MCM-41)-supported RhCo as a hydroformylation catalyst in the second reactor to produce C₃ oxygenates¹⁶, highlighting the promise of the tandem reactor strategy when the target sequential reactions are thermodynamically mismatched. Regarding the selection between tandem reactions or a tandem reactor, one should consider the thermodynamics of the overall reaction; that is, ΔG_overall° = ΔG_Rxn-A° + ΔG_Rxn-B° = (ΔH° − TΔS°)_Rxn-A + (ΔH° − TΔS°)_Rxn-B, where ΔH⁰, T and ΔS⁰ represent the enthalpy change for the reaction, temperature and the entropy change for the reaction, respectively. If ΔG_Rxn-B° cannot compensate for ΔG_Rxn-A° to achieve a negative ΔG_overall⁰ within a reasonable temperature range, conducting the two reactions separately in two reactors at their optimal temperature windows should be preferred.

The tandem reactor paradigm in Fig. 2 can be expanded to encompass a variety of other value-added, multi-carbon liquid products. The effluent from the first reactor comprises a mixture of C₂H₄, CO, H₂ and H₂O, in addition to unconverted C₂H₆ and CO₂, offering opportunities for integrating additional reaction chemistries in downstream reactors. For instance, carboxylic acids can be obtained through the exothermic hydrocarboxylation process (that is, C₂H₄ + CO + H₂O → CH₃CH₂COOH). Despite sharing thermodynamic similarities with hydroformylation, heterogeneous hydrocarboxylation has rarely been demonstrated successfully. Future endeavors should prioritize mechanistic studies of the kinetically relevant step(s) and subsequently develop proof-of-concept catalysts. Another area for using the tandem reactors is in upcycling plastic wastes with CO₂ into value-added chemicals. For example, tandem reactors can be employed for the dry (CO₂) reforming of waste plastics using a Ni–Co–Al catalyst in the first reactor for plastic pyrolysis, followed by catalytic dry reforming of the pyrolysis gases with CO₂ in the second reactor¹⁷. More applications of tandem reactors should be explored for the CO₂-assisted upgrading of plastics in situations where the individual reactions are thermodynamically unfavorable.

Electrocatalytic tandem processes

Both tandem catalysis and tandem reactors have been explored for the electrocatalytic valorization of CO₂. The tandem strategy promotes the production of multi-carbon products, primarily by controlling the production of CO and its subsequent C–C bond formation. As shown in the two examples in Fig. 3, the tandem strategy promotes electrocatalytic activity and selectivity for converting CO₂ to the target products. One of the most important criteria for selecting either the single reactor or tandem reactor configuration is based on the difference in the desired pH range for the individual reactions. Another consideration is that the tandem reactor scheme allows the optimization of catalysts and reaction conditions in each electrochemical reactor.

Fig. 3: Tandem electrocatalytic strategies for CO₂ conversion.

In the single reactor strategy (top) single-atom Ni anchored on nitrogen assembly carbon (Ni-NAC) mixed with Cu nanowires catalyze CO₂RR to produce CO, which immediately undergoes further reduction to produce C₂H₄ at Cu sites. Ni foam is used to catalyze the anodic oxygen reduction reaction (ORR) and 0.5 M KHCO₃ is used as the electrolyte. In the tandem reactor strategy (bottom) Ag catalyzes CO₂RR to produce CO in the first reactor, with IrO₂ as the ORR catalyst and a 1 M KHCO₃ electrolyte. CO is subsequently reduced using a Cu catalyst in the second reactor, with NiFeO_x as the ORR catalyst and a 1 M KOH electrolyte.

Single reactor with multifunctional catalysts

As with tandem thermocatalysis, there has been considerable progress in developing single reactor tandem electrocatalysis strategies for upgrading CO₂ to valuable C₂₊ products with high selectivity^18,19. Copper is the only element identified so far that can efficiently catalyze the CO₂ reduction reaction (CO₂RR) to C₂₊ products, and it is widely accepted that this reaction proceeds via the formation of a *CO surface intermediate followed by *CO dimerization to *OC–CO and subsequent reduction to form C₂₊ products²⁰. Because CO is a key intermediate in the CO₂ → C₂₊ reaction, CO reduction (COR) has been widely studied as a proxy for CO₂RR. Several metals have been identified as highly effective electrocatalysts for CO₂ → CO conversion. Consequently combining Cu with one of these catalysts can enhance C₂₊ production by the CO spillover phenomenon, whereby a CO-rich environment inhibits the competing H₂ evolution reaction (HER) that would otherwise reduce the selectivity for C₂₊ products on pure Cu²¹. Ag and Au are especially attractive options because of their high CO selectivity and immiscibility with Cu, which precludes changes to catalytic properties of either metal due to the formation of bimetallic alloys and maintains local phase separation of the two catalytic functions to enable tandem catalysis²².

Catalyst mixtures have been demonstrated successfully for CO₂ conversion to multi-carbon products, and these mixtures almost always consist of Cu mixed with a second CO-producing catalyst. In the simplest demonstrations, these catalysts are mixed together and deposited on a conductive substrate to create an electrode that can produce C₂₊ products with higher selectivity than pure Cu-based electrodes^23,24. Greater spatial separation of catalysts can be achieved by using one metal as a catalytically active and conductive substrate onto which the second metal is deposited^25,26,27. Segmented electrodes have also been studied recently as a strategy for achieving greater control over the separation between distinct electrocatalysts. In the simplest example, two catalysts can be deposited adjacent to each other on a conductive substrate to produce a high concentration of CO close to the inlet, which then flows over a C₂₊-producing catalyst²⁸. Segmenting the electrode in this way increases the residence time of CO within the electrochemical reactor, consequently leading to higher Faradaic efficiencies of C₂₊ products^28,29. Different segmentation patterns have been explored as a way to tune the C₂₊ product distribution, as well as methods for independently controlling the potential applied to each catalyst within a single reactor, which highlights the importance of transport and device engineering considerations when designing these tandem systems^30,31. Hybrid catalysts have also been explored to achieve more intimate contact between two catalysts, and Fig. 3 shows an example of single-atom Ni anchored on nitrogen assembly carbon promoting CO₂ → CO conversion, leading to enhanced C₂H₄ production over the adjacent Cu nanowires³². CO₂RR reduction to C₂H₄ was also demonstrated with up to 55% C₂H₄ Faradaic efficiency on a core@shell catalyst composed of Cu and Ni-coordinated nitrogen-doped carbon (NiNC)²⁴, which is still notably higher than the approximately 30% Faradaic efficiency that is typical of pure Cu.

Contrary to their widespread adoption in thermocatalytic applications, zeolites and metal–organic frameworks (MOFs) are relatively underexplored in the context of electrocatalysis. Preliminary work has shown that Cu-based MOFs grown on conductive electrodes can be used to catalyze CO₂ conversion to CH₄ and C₂H₄ while suppressing CO production³³. This demonstrates the viability of using MOFs for electrocatalysis, and more work should be done to elucidate electrocatalytic reaction mechanisms within the MOF framework to develop new multifunctional electrocatalysts for CO₂ conversion with tunable product distributions. Moreover, improving the electrical conductivity and electrochemical stability of MOFs is necessary for widespread use of these materials. Zeolites have been demonstrated to have excellent electrochemical stability, so they present a promising avenue for developing multifunctional electrocatalysts and should be investigated alongside MOFs in the context of tandem electrocatalysis^15,34,35.

Tandem reactors

Efforts to develop tandem electrocatalytic reactors have focused almost exclusively on electrochemical CO₂RR to produce CO followed by electrochemical upgrading of CO to value-added products. The alkaline environment required to achieve high reaction rates for CO₂RR results in large amounts of (bi)carbonate production, which has limited CO₂RR to <50% selectivity for C₂₊ products³⁶. CO₂RR to CO, however, can be carried out in a non-alkaline environment with >90% CO selectivity on a Ag catalyst, and this electrochemically produced CO can then be reacted in a second electrochemical reactor to produce valuable C₂₊ products without producing (bi)carbonate³⁷. Because of this, electrochemical COR has emerged as a downstream reaction that logically and easily couples with CO₂RR to produce a myriad of products^37,38. Coupling CO₂RR with COR in this manner also overcomes the selectivity and product complexity issues associated with direct electrochemical CO₂RR. For example, as shown in the bottom panel of Fig. 3, the production of acetate using tandem CO₂RR and COR reactors has been demonstrated with Cu catalysts under alkaline conditions^38,39,40. This tandem approach is also desirable because it allows for the two reactors to be operated at different pH values, thereby enabling more precise control of the product distribution from each reactor.

The tandem reactor strategy also allows more complex chemistries by co-reacting CO with another molecule in the second reactor. For example, electrochemical co-reduction of CO with NH₃ can produce acetamide over a Cu catalyst at commercially relevant production rates⁴¹. Overall, tandem electrochemical reactor schemes involving CO₂RR conversion to CO followed by CO reduction or reaction have shown promise for converting CO₂ to more complex products than can be produced by direct CO₂RR, and future work should investigate the scalability and durability of these processes. At present, very little has been done to develop tandem electrocatalytic reactors beyond the CO₂RR–COR scheme, which should be explored for electrochemically synthesizing chemicals from CO₂.

Hybrid tandem processes

In the tandem EC–TC configuration, electrochemical CO₂RR products can be subsequently upgraded in a thermochemical reactor. Although CO₂RR has been widely studied as a method for sustainable CO₂ conversion, it often suffers from low single-pass conversion, low production rates of oxygenated products (for example, methanol and ethanol) and complex products (such as aromatics), and low multi-carbon (C₂₊) product selectivities^20,36. However, using the EC–TC tandem strategy, it is possible to achieve high selectivities for desirable products at more commercially relevant production rates. Many well-understood thermocatalytic reactions are compatible with a mixture of simple CO₂RR products (such as CO, H₂, C₂H₄) as input, so the CO₂RR product stream can be immediately used as the feed for a thermochemical reactor without any intervening product separation. Thus, this paradigm of tandem EC–TC reactors can overcome the limitations of direct electrochemical CO₂ conversion to operate more efficiently and produce molecules that are more complex than can be produced by direct electrocatalytic or thermocatalytic CO₂ conversion. Another advantage of utilizing tandem EC–TC is to avoid the energy-intensive separation of liquid products (for example, oxygenates) from the aqueous electrolyte employed in CO₂RR.

Tandem EC–TC reactors

The CO product stream from an electrochemical CO₂RR reactor can be used as a feed for thermocatalytic processes⁴². Suppressing H₂ production is often viewed as a desirable trait for CO₂RR electrocatalysts, but co-production of CO and H₂ results in a product stream of synthesis gas (syngas), which is a feedstock for many thermochemical processes, including methanol synthesis and the Fischer–Tropsch process for long-chain liquid hydrocarbon synthesis^42,43. CO and H₂ can be co-produced in an electrochemical CO₂RR reactor using many catalysts and optimizing the CO:H₂ ratio for the downstream thermochemical reactor allows for CO₂ to be converted into valuable products with high conversion and selectivities.

For example, BTEX aromatics are used to manufacture a wide range of products, including paints, adhesives and pharmaceuticals, but these molecules are too complex to be produced directly by any electrochemical processes explored so far. Using the tandem strategy, electrochemical CO₂RR with a Cu catalyst can be coupled with thermochemical C₂H₄ aromatization over zeolite catalysts to produce BTEX^11,44,45. In the tandem EC–TC reaction scheme, as depicted in Fig. 4 (top), CO₂ is electrochemically reduced to produce C₂H₄, which is fed to thermochemical reactor to produce a mixture of BTEX. This tandem EC–TC paradigm has also been demonstrated for producing C₃ oxygenates⁴⁶ and butane⁴⁷. In all three cases, the ultimate products are very difficult to produce directly from CO₂ in a single electrochemical or thermochemical reactor, so coupling EC–TC reactors in this way opens previously inaccessible pathways for direct CO₂ conversion to complex products. There are also opportunities for coupling CO₂RR with organic synthesis methods to directly produce complex polymers⁴⁸. When engineering tandem EC–TC processes, the relative contributions of the competing CO₂RR and HER must be carefully tuned such that the product ratio from the electrochemical reactor is suitable for the subsequent thermochemical reactions toward the target products.

Fig. 4: Tandem EC–TC and TC–EC reactors for CO₂ conversion.

Top: tandem EC–TC reactor scheme for CO₂ conversion. In the first reactor, Cu catalyzes electrochemical CO₂ reduction to produce C₂H₄, which undergoes thermochemical aromatization over Ga/ZSM-5/P to produce benzene in the second reactor. Bottom: tandem TC–EC reactor scheme for CO₂ conversion. Ni/γ-Al₂O₃ catalyzes thermochemical dry reforming of CH₄ with CO₂ to produce CO, which is subsequently reduced using a Ag–Cu electrocatalyst in the second reactor to produce n-propanol.

Tandem TC–EC reactors

To the best of our knowledge, there has been no demonstration so far of a tandem reactor scheme wherein an electrochemical reactor is implemented downstream of a thermochemical reactor; however, there are noteworthy opportunities for coupling tandem TC–EC for CO₂ conversion. Dry reforming of CH₄ is an attractive route for syngas production because it simultaneously reduces both CH₄ and CO₂ emissions^49,50. Although this process is usually discussed in the context as means of generating feedstocks for existing thermochemical reactions, it stands to reason that thermochemical dry reforming of CH₄ can be implemented upstream of a CO electrolyzer in a TC–EC tandem reactor system to produce specialty chemicals using renewable electricity.

Outlook

As demonstrated in the examples discussed above, tandem reaction schemes for CO₂ valorization provide advantages in catalytic activity and selectivity. Figure 5 summarizes strategies involving TC–TC, EC–EC and TC–EC, using either one-pot tandem catalysis or sequentially coupled tandem reactors, to enable CO₂ conversion into many products. Such tandem processes can potentially offer opportunities to convert CO₂ into value-added products that cannot be conventionally achieved.

Fig. 5: An illustration of progress in and opportunities for CO₂ upgrading by single reactor tandem catalysis and tandem reactors.

A gradient directly from orange to blue represents tandem reactions within a single reactor, whereas orange and blue regions separated by a gray region represent tandem reactors. a, Tandem thermocatalytic reactors and reactions. b, Tandem electrocatalytic reactors and reactions. c, Tandem EC–TC reactors. d, Plasma-assisted thermocatalytic reactions. e, Tandem EC–biocatalytic reactors.

However, more research efforts are needed to take advantage of tandem processes. Understanding the effects of by-products and unreacted CO₂ from the first reaction for tandem catalysis, or from the first reactor for tandem reactors, on the overall catalytic performance is crucial. Mechanistic studies that include these potentially undesired molecules will be needed to develop catalysts that are stable under these mixed-feed conditions. Understanding the interplay and coordination of multiple catalytic active sites, either in the single reactor or the tandem reactor configuration, will require density functional theory calculations in conjunction with in situ catalyst characterization. Developing a thorough understanding of the fundamental principles of catalysis in the context of these tandem systems will be required to advance this underexplored field.

To ensure fair comparisons between tandem processes, it is essential to report performance data and reaction conditions using standardized and consistent metrics. This includes reporting selectivity and yield based on total consumption, and reporting product formation based on total CO₂ conversion. Factors such as productivity based on catalyst or metal weight, as well as turnover frequency based on number of active sites, alongside stability evaluation at the same space velocity and/or the same number of active sites, should also be reported. In addition, any inert species used as a carrier gas or diluent should be clearly stated to better assess the practical performance when the reaction is scaled to a commercially relevant process. By adhering to these uniform metrics, researchers can enhance transparency, maintain consistency and facilitate comparability in tandem process studies, leading to a more reliable understanding of their potential advantages over individual processes.

A thorough analysis of tandem processes in terms of CO₂ footprint and energy cost can help reveal the potential for enhancing both environmental and economic benefits compared with the conventional individual processes. The interconnected nature of tandem processes streamlines synthesis, with the main, side or waste products of one process becoming valuable inputs for another, enhancing overall atom economy and promoting sustainability. In evaluating the economic considerations and scale-up potential of tandem versus individual processes across various product classes, a balance must be achieved between the cost of an additional reactor and the advantages of independently optimizing reaction conditions. Although the initial investment in tandem processes may be higher, the ability to fine-tune each reaction step can greatly improve reaction rates and selectivity. Simple calculations such as cost per unit of product or raw material usage should provide valuable insights into the practical viability of tandem processes. In addition, the evaluation of scale-up potential depends on more comprehensive factors such as catalyst performance, ease of integration and operation, heat- and mass-transfer efficiency, raw material availability and cost, economic analysis, market value, waste management, and compliance with safety regulations. It is also necessary to perform energy cost and CO₂ footprint analyses to ensure that tandem processes can reduce net CO₂ emissions, such as analyses conducted for CO₂ conversion to methanol⁵ and to C₃ oxygenates⁴ that used commercial processes as a benchmark.

The effect of implementing a separation step between two tandem reactors should also be considered when analyzing the effectiveness of a tandem process. If water and CO₂ lead to deactivation of a thermocatalyst, using a water trap and a scrubber, respectively, between the two reactors can be beneficial. When separation units are required, the energy requirements, costs and environmental impacts of these separation units should be carefully considered to fully understand how the tandem process compares with the conventional reactors.

Among the tandem processes shown in Fig. 5, the EC–TC strategy is not well explored and can lead to products that cannot be achieved by direct TC or EC processes. Coupling electrochemical CO₂ reduction with subsequent thermochemical upgrading in this way leverages decades of research in thermocatalysis and enables the production of chemicals that are too complex to be produced via electrochemical processes alone. The modularity of this tandem reaction framework also allows for reactors to be combined and upgraded as needed to meet the demands of the chemical industry and evolve alongside advances in sustainable catalysis. There are many opportunities for tandem EC–TC reactors that have yet to be demonstrated. For example, the CO₂RR → syngas → complex products pathway represents a powerful paradigm for CO₂ valorization. The well-understood TC reactions of Fischer–Tropsch, methanol synthesis, methanol-to-olefins and methanol-to-aromatics reactions, among others, are very well-characterized reactions that can be potentially coupled with electrochemical CO₂ conversion to syngas. In a recent work, tandem EC–TC reactors were demonstrated for the production of solid carbon nanofibers by electrochemical CO₂RR to produce syngas followed by thermochemical fixation of CO into carbon nanofibers⁵¹. This process represents a new pathway for long-term sequestration of CO₂ into useful solid materials and enables carbon storage for longer durations than can be achieved by converting CO₂ into typical commodity chemicals or fuels. Matching the reactor temperatures of the EC–TC scheme as closely as possible should improve overall energy efficiency. To this end, high-temperature electrochemical cells have the potential to be integrated with thermochemical reactors and bridge the temperature gap. CO, for example, can be produced at high temperatures in a solid oxide electrolysis cell (SOEC)^52,53. Although these devices are more complex and require more energy, SOECs exhibit higher CO₂ conversion, higher CO production rates and selectivity, and longer lifetimes than low-temperature devices^52,53. SOECs can also operate at similar temperatures to the thermochemical reactor. If high-temperature electrochemical reactors are used, H₂ can be produced either in situ with CO or ex situ in a steam electrolyzer to create a syngas stream for input into the thermochemical reactor⁵⁴. SOECs are a mature technology owing to decades of research on high-temperature electrochemical device components for solid oxide fuel cells, so these devices are potentially well poised for rapid scale-up and deployment for EC–TC tandem processes^52,53.

This Perspective focuses on single reactor tandem catalysis and tandem reactors for CO₂ conversion by thermocatalysis and electrocatalysis, but there are other notable opportunities for developing tandem processes. For example, tandem catalysis strategies employing photocatalysis as the initial CO₂ activation step are also beginning to show promise. Photoreduction of CO₂ can be coupled with organic synthesis within a single reactor by using an organic substrate to convert CO₂ into complex organic molecules⁵⁵. Photocatalytic CO₂ reduction to CO can also be coupled with a downstream CO upgrading reaction, and this tandem reactor configuration has been demonstrated for carbonylation reactions⁵⁶. Tandem photochemical–thermochemical reactors have also been demonstrated for the simultaneous upgrading of H₂O and CO₂ to produce CH₄ (ref. ⁵⁷). These initial demonstrations have been promising; however, photocatalysis is a more nascent field of study relative to thermocatalysis and electrocatalysis, and studying photocatalysis in the context of these tandem reaction systems presents an exciting new and underexplored pathway for sustainable CO₂ conversion.

Tandem electrocatalytic–biocatalytic processes have been investigated, including as CO₂RR to produce acetate followed by biocatalytic acetate fermentation⁵⁸. Carbon-neutral or carbon-negative acetate can be selectively produced by various means^59,60, including CO electroreduction⁶¹. Acetate fermentation has been explored for the production of a wide variety of bio-based chemicals^62,63,64, but the toxicity of acetate to many species of bacteria limits these processes⁶³. Electrochemical CO₂ reduction to produce syngas has been coupled with fermentation of the mixture of CO, H₂ and unreacted CO₂ to produce hexanol and butanol^65,66. Bioelectrochemical systems, which involve the transfer of electrons directly to certain species of bacteria, represent another emerging area that combines electrocatalysis and biocatalysis in a single reactor⁶⁷.

Plasma catalysis has garnered recent interest for CO₂ valorization^4,68, and plasma technologies are well suited to integration with renewable energy because of the short start-up and shut-down times⁶⁹. Plasma-assisted thermocatalysis has been demonstrated successfully for the production of C₃ oxygenates⁷⁰ and formaldehyde⁷¹; however, these processes are limited by low selectivities. The tandem processes combining plasma with electrocatalysis should also be explored for CO₂ conversion, although more investigations will be needed to understand how the plasma-produced energetic species diffuse toward the electrodes in aqueous electrolytes.

Further work should be done to understand the best use cases for each technique discussed in this section, as well as to investigate how these different categories of catalytic processes can be effectively coupled with each other to sustainably convert CO₂ into value-added products that cannot be efficiently produced by a single conventional catalytic method.