Recent advances in integrated CO2 capture and utilization: a review

* a CO 2 is one of the most important greenhouse gases leading to severe environmental issues. The increase of CO 2 emissions from the consumption of fossil fuels has received much research attention. One promising solution to reduce the impact of CO 2 is to integrate CO 2 capture and utilization (ICCU), which shows many advantages compared to traditional separate CO 2 capture and utilization (CCU) processes. The ICCU process shortens the path of CO 2 utilization such as CO 2 transportation and storage, and further negates the need for puri ﬁ cation of products owing to the high conversion of CO 2 . As an emerging integrated process, the improvement of ICCU performance is crucial for future applications. This review analyses and discusses the in ﬂ uence of the key process parameters of ICCU such as temperature, the presence of O 2 or H 2 O in CO 2 , GHSV etc. , to provide guidance for future investigation. The development and application of dual functional materials (DFMs) in ICCU are investigated and the roles and in ﬂ uence of interaction between adsorbents and catalysts are discussed. CaO showed superiority as an adsorbent to combine with CO 2 utilization catalysts owing to its low cost and high CO 2 capture capacity. The DFM system has opportunities to retard the deactivation of CaO owing to the presence of catalysts and the formed interaction. Nevertheless, there are several considerations from the industrial application perspective such as the reduction of overall cost and the possible post-processing requirements.


Introduction
The signicant increase of CO 2 emissions, from 280 ppm in 1760 to 410 ppm in 2020, 4 has become a serious global warming problem, resulting in a series of severe climate and environmental changes. 5The emissions of CO 2 are mainly attributed to fossil fuel consumption, 6 in particular, the power generation sector emits the most CO 2 , followed by industrial and transportation vehicles. 6,7Therefore, there is an urgent requirement for CO 2 reduction which is recognised by the Intergovernmental Panel on Climate Change (IPCC) and the introduction of relevant policies and regulations. 8,9The measures to reduce carbon emissions include the improvement of fuel energy efficiency, 10 CO 2 capture, 11,12 carbon storage 13,14 and CO 2 conversion. 15CO 2 capture and storage (CCS) is a technology that decarbonises the use of fossil fuels in industries, such as power plants, steel works, cement kilns and oil reneries. 6CCS includes three main stages: (1) CO 2 capture; (2) transportation; (3) permanent CO 2 storage by mineralizing 14 or injecting CO 2 into the ground or deep ocean. 7Porous materials such as activated carbon and zeolites are possible adsorbents for carbon capture.However, the poor adsorption selectivity of physical carbon capture limits its industrial application.Chemical adsorption is more promising and has already been applied in the CO 2 capture process, mainly including aqueous monoethanolamine (MEA) adsorption 16 and calcium looping. 17The captured CO 2 can be released at a high concentration by temperature swing.As for the storage process, CCS could also be combined with the current fossil fuel extraction processes, for example, the underground injection of CO 2 for crude oil enhanced oil recovery. 16,18However, the high cost of separation, enrichment 19 and transportation of CO 2 (ref.20) limits the deployment of CCS.Moreover, physical storage methods, such as underground or ocean injection, have negative impacts on natural ecology. 21Therefore, in addition to the development of CCS, increasing attention is being paid to the application of the captured CO 2 as a feedstock to produce valuable chemicals or fuels. 2,22O 2 capture and utilization (CCU) is a more sustainable process that can partially close the carbon cycle (as shown in Fig. 1).It is attractive to store the excess and uncertain supply of energy from renewable sources as stable chemical energy (i.e.methane, syngas or liquid chemicals 23,24 ) by integrating with CCU. 25 Nowadays, there are many effective CO 2 utilization routes, including thermal-catalysis, photo-catalysis, electroniccatalysis, [26][27][28] plasma-catalysis, 29 etc.][33] However, considering the large scale application in industry, materials with high cost are not reasonable for reducing CO 2 emissions.This review focuses more attention on hightemperature CO 2 capture and in situ utilization technologies using low-cost adsorbents (i.e.MgO and CaO) and commonly used catalysts, such as Ni, Fe-based or low loading of noble metal-based materials.Although high-temperature applications will bring additional energy consumption, ICCU can be operated isothermally and continuously by swinging the inlet gas.Furthermore, rapid and efficient carbon capture can be achieved using low-cost CaO or MgO at high temperatures.Industrially applicable CO 2 hydrogenation can be easily realized using the most common catalysts (i.e.Ni, Fe or Cu-based catalysts).As an important C1 chemical, CO 2 has attracted much attention for various end-use applications including CO 2 methanation, 34,35 dry reforming of alkanes, 36,37 the reverse water-gas shi reaction, 38,39 etc. 15,31 However, CCU requires high CO 2 concentration for high conversion 40 and inevitable product purication before it can be utilized for the production of useful products.As shown in Fig. 1, the traditional CCU process includes several steps to achieve ideal carbon capture and utilization, and in which the process would be also accompanied by undesirable high costs.
In recent years, increasing studies have been carried out on integrated CO 2 capture and utilization (ICCU) to reduce the cost of the overall process by eliminating transportation and storage of CO 2 .As shown in Fig. 1, ICCU achieves in situ CO 2 adsorption, separation and conversion using dual-function materials (DFMs), which consist of CO 2 adsorbents and catalysts.First, DFMs can capture CO 2 from ue gas ($15 vol% CO 2 ) to effectively reduce carbon emissions.When the carbon capture process is completed, the feed gas is switched to a reducing agent (i.e.H 2 , CH 4 ) for the conversion of the adsorbed CO 2 accomplished with the regeneration of the adsorbents.The reduction of CO 2 in ICCU is carried out under reducing agentrich conditions, further avoiding the purication of products by signicantly improving the conversion of CO 2 .Compared with the traditional MEA adsorption and calcium looping process, the ICCU process avoids CO 2 desorption by temperature swing, which is considered as an energy-intensive process.What's more, the effective in situ CO 2 utilization accompanied by adsorbent regeneration signicantly simplies the overall CO 2 utilization process.
The CO 2 adsorption, desorption and catalytic performances are signicantly affected by the reaction temperature as well as the combination between catalysts and adsorbents.There are specic requirements for catalysts and adsorbents applied in DFMs, including the interaction between catalysts and adsorbents and the matching of adsorption/desorption efficiency and catalytic performance.ICCU has research gaps in both process optimization and catalytic mechanism investigation.In addition, the interactions and synergy effect between sorbents and catalysts of DFMs need signicant research for better DFM design.This review critically introduces recent literature on CO 2 capture integrated with methanation, dry reforming of methane (DRM) and the reverse waste gas shi reaction (RWGS) from the perspectives of ICCU performance (i.e.CO 2 conversion, product yield, selectivity and process parameter optimization), CO 2 adsorbents (i.e.adsorbent system and understanding of the synergistic effect between catalysis and adsorption) and catalysts (i.e.catalytic system, active sites and catalytic mechanism).CO 2 utilization requires a specic reaction temperature which limits the selection and design of adsorbent in DFMs.In contrast, the presence of adsorbents promotes the performance of catalysts by assisting catalyst dispersion and provides close contact with CO 2 and catalysts.The interactions between adsorbents and catalysts are believed to play multifunctional roles in ICCU including promoting the stability of DFMs, providing effective catalytic sites and affecting the optimal parameters of ICCU.Furthermore, this review proposes research directions by pointing out the shortcomings of existing research from the perspective of industrial applications.

Progress in integrated CO 2 capture and methanation
Methane is the simplest hydrocarbon with the lowest C/H ratio, and is widely used in transportation, domestic heating and power plants because of its high caloric value (55.7 kJ g À1 ), ready availability, low cost and clean combustion products.As shown in eqn (1), the conversion between CO 2 and CH 4 , representing two important chemicals in C1 chemistry, has attracted great attention.Integrating CO 2 capture and methanation (ICCU-methanation) has advantages in improving the process and energy efficiency.

Inuence of process parameters on ICCU-methanation
Temperature, reaction time, reducing agent parameters, the presence of O 2 and H 2 O, etc., have large inuences on the efficiency of ICCU-methanation which normally happens at an intermediate temperature ($300 C).Table 1 42 and Jo et al. 43 The matching of temperature between adsorption and methanation is also very important for ICCU.Generally, lower process temperature promotes CH 4 selectivity but decreases CO 2 conversion for ICCU-methanation.By comparing different temperatures (280-350 C), Zheng et al. 44 achieved the best ICCU performance at 320 C (32.41 ml CO 2 captured and 31.56 ml CH 4 generated).A higher temperature was found to decrease the CO 2 capture capacity and cause excessive oxidation of Ru for the processing of O 2 -containing ue gas (32.86 ml CO 2 captured and 29.73 ml CH 4 generated at 350 C), while a lower temperature signicantly limited the catalytic activity (41.21 ml CO 2 captured and 1.2 ml CH 4 generated at 280 C).

summarizes the
In addition to the effect of temperature, the reaction time of adsorption and conversion stages also affects the catalytic performance.Zheng et al. 44 investigated the inuence of adsorption time on ICCU-methanation.Increasing the reaction time of adsorption beneted the generation of CH 4 .However, it was found that the CO 2 adsorption rate dropped signicantly aer 20 min.In addition, a longer adsorption time resulted in the deactivation of catalysts by inducing excessive oxidation of catalysts in the ue gas.As listed in Table 1, most of the research used more than 20 min reaction time in the rst carbon capture stage.This could increase the overall CO 2 capture capacity.However, it might not be benecial to the overall ICCU-methanation process.Excessive CO 2 adsorption could produce more carbonates than active formate species which are mainly responsible for the formation of methane.For example, Zhou et al. 42 applied a fast adsorption and methanation process (25 s and 35 s for adsorption and methanation) and reported a nearly 100% CO 2 conversion.This was due to the formation of dominant formate species and few carbonates.
The concentration of H 2 is another important parameter that has a signicant impact on CH 4 yield.Wang et al. 48investigated the inuence of H 2 partial pressure (5% and 10% H 2 /N 2 ) and found that the presence of more sufficient H 2 could generate more CH 4 (58.66 ml and 131 ml CH 4 ).The authors reported that a higher H 2 concentration is important for the activation of Rubased DFMs when processing O 2 -containing ue gas.A higher H 2 concentration could also effectively improve the CO 2 conversion by positively promoting the equilibrium of CO 2 methanation. 43,53Apart from the gas concentration, GHSV also inuences the ICCU performance by altering the gas diffusion pathways within DFMs. 44Generally, a higher GHSV results in an enhanced CO 2 conversion and is accomplished with higher H 2 consumption.
In practice, it is known that contaminants in ue gas such as H 2 O and O 2 can affect CO 2 capture 55 and conversion. 56Martha et al. 50simulated a ue gas by mixing 4.5% O 2 and 15% H 2 O with 7.5% CO 2 /N 2 to investigate CH 4 formation using Ni-based DFMs.The reduction of ICCU performance in relation to CH 4 production was observed owing to the oxidation of active metals.In addition, the presence of H 2 O and O 2 in the feed gas decreases the capacity of CO 2 capture by competitive adsorption. 44Interestingly, the presence of H 2 O might not always play a negative role in ICCU.Miguel et al. 47 found that H 2 O can participate in CO 2 desorption, removing CO 2 and promoting CH 4 formation.The presence of H 2 O helped to dissolute K 2 CO 3 , which promoted the formation of bidentate carbonate in the CO 2 adsorption process.The H 2 O generated in CO 2 methanation was also suggested to be helpful for CO 2 desorption, especially in the regeneration of sorbent sites. 472 Development of DFMs for ICCU-methanation 2.2.1 Adsorbents in DFMs for ICCU-methanation.In this section, the role of sorbents in DFMs is reviewed and discussed for ICCU-methanation.As shown in Table 1 and Fig. 2, Na 2 O, K 2 O, MgO and CaO are the most common sorbents for this process.CaO is widely used as a CO 2 adsorbent owing to its  Mertha et al.50 found that Na 2 O showed the fastest kinetics towards CH 4 production compared to CaO, K 2 O and MgO.In addition, the Na 2 CO 3 -based DFMs showed acceptable stability (i.e.27% decrease aer 10 cycles) and the adsorption performance could be maintained by increasing the reduction time.48 The adsorption capacity of different alkali metal species varies as well.Bermejo-López et al. 52 investigated oxides (CaO or Na 2 O) and hydrated oxides (Ca(OH) 2 or NaOH) as CO 2 storage sites, and reported that the former oxides were more reactive for CO 2 adsorption.
Mg-based adsorbents are widely used in DFMs due to their low cost and medium adsorption temperature ($300 C), as shown in Table 1 and Fig. 2. In addition, MgO does not have serious sintering issues like CaO, and the use of adsorbent carriers can be avoided to achieve more abundant adsorption capacity.Miguel et al. 47 investigated the cycle stability of a Mg-Al hydrotalcite-based DFM.The authors obtained a decreased CO 2 capacity in cycles, from 0.52 mmol g À1 to 0.32 mmol g À1 , owing to the formation of irreversible bulk polydentate carbonate from unidentate and bridged carbonates.Thus promoters such as Li were used to stabilize MgO during the carbon capture stage of ICCU.Sun et al. 54 42 observed the formation of formate species in the CO 2 capture process, which was formed from the hydrogenation of carbonate and/or bicarbonate. 58These formed formate species are desirable for methanation. 59he loading of adsorbents in DFMs can directly affect the capacity of CO 2 capture.Melis et al. 46 reported a positive correlation between adsorbent loading and CO 2 capture capacity using dispersed K 2 CO 3 as an adsorbent.However, a higher loading of the adsorbent could not always result in a better CO 2 capture capacity when applying other alkali metalbased adsorbents.Melis et al. 46 found an optimal loading of 10% for Na 2 CO 3 and MgO.In addition to providing adsorption sites, the presence of sorbent could also promote catalytic performance in relation to the conversion of CO 2 .For example, Bermejo-López et al. 52 reported that the dispersion of Ru or Ni was improved with the increase of adsorbent loading.
Different adsorbents show various performances for the process temperature of ICCU-methanation.Bermejo-López et al. 52 observed that the performance of the CaO-based DFMs showed a signicantly more positive correlation with the increase of temperature, while the medium temperature (i.e.340 C) is optimal for the Na 2 CO 3 -based DFMs.In the case of using CaO, a higher temperature was attributed to the decomposition of stable carbonates, while the Na 2 CO 3 sorbent could release CO 2 at a relatively lower temperature.The CaO and Na 2 CO 3 -based DFMs with low adsorbent loadings (i.e. 5 wt%) possess relatively weak basicity, results in decreased CO 2 storage capacity and CH 4 production at higher temperature.
In addition, the carriers used to disperse the adsorbent affect the adsorption performance in ICCU-methanation.For example, Martha et al. 50studied carrier materials, including CeO 2 , CeO 2 /ZrO 2 , Nazeolite-X, H-mordenite zeolite, SiC, SiO 2 and ZrO 2 -Y and demonstrated that g-Al 2 O 3 was the most suitable carrier for DFMs.In addition to acting as a carrier, g-Al 2 O 3 could adsorb CO 2 over Al 2 O 3 -OH groups. 59Laura et al. 59 proposed that Al-O À -Na + species formed by the interaction between Na 2 O and Al 2 O 3 allowed CO 2 adsorption by the formation of bidentate carbonates over the sorbent surface using in situ DRIFTS (diffuse reectance infrared fourier transform Spectroscopy) characterization.
As shown in Fig. 2, sorbents for CO 2 capture could be dispersed into supports to enhance the stability of adsorbents, however, it could signicantly reduce the capacity of CO 2 capture owing to the reduction of adsorbent content.It is important to develop intermediate temperature adsorbents ($300 C) with excellent stability and CO 2 capture capacity for ICCU-methanation to increase the CO 2 throughput of DFMs.
2.2.2 Catalysts in DFMs for ICCU-methanation.The role of catalytic sites in DFMs for ICCU-methanation is discussed in this section.It is known that the reduction of CO 2 to CH 4 is an eight-electron complicated process with signicant kinetic limitations. 60Therefore, CO 2 methanation requires suitable catalysts to achieve acceptable activity and selectivity.Metalbased catalysts, such as Ni, Ru and Rh, have been widely investigated for this process.As shown in Table 1, Ni was chosen as the methanation catalytic site by many researchers due to its high catalytic activity and relatively low cost.The effect of Ni loading in DFMs on ICCU-methanation was studied by Bermejo-López et al. 41 In general, higher Ni loading promotes CO 2 capture and CH 4 generation owing to the closer contact between the adsorbents and metallic phase.Furthermore, increasing Ni loading could also slightly promote the decomposition of carbonates at lower temperatures.
Notably, the interaction between catalysts and adsorbents could positively affect ICCU-methanation.It was proposed that the reducibility of Ni species was enhanced in the presence of the adsorbent. 41The adsorbent impeded the close contact between Ni and Al 2 O 3 weakening the interaction between both phases and favoring the formation of reducible NiO species.
However, there are shortcomings of Ni-based catalysts in DFMs for ICCU-methanation.When processing O 2 -containing ue gas, Ni-based catalysts were oxidized and required much higher temperatures (i.e.600 C) to reduce. 51Other possible causes of the deactivation of Ni-based catalysts at low temperatures are the interaction of the metal particles with CO and the formation of mobile Ni subcarbonyls. 61  It is known that the loading of Ru is closely related to the catalytic performance.Bermejo-López et al. 52 proposed that a higher Ru loading in Ru-10%Na 2 CO 3 /Al 2 O 3 resulted in an acceptable ICCU performance even at a notably lower temperature (i.e.310 C).However, there is an optimum Ru loading for ICCU-methanation.For example, Sun et al. 54 reported that 10% Ru/CeO 2 -MgO showed poorer stability than 5% Ru/CeO 2 -MgO.It is mainly attributed to the presence of more oxygen vacancies that remained in 5% Ru/CeO 2 -MgO.Better dispersion of Ru also promotes ICCU-methanation.For example, Melis et al. 45 reported that increasing the weight ratio of CaO to Ru promoted CO 2 spillover from CaO to Ru sites and then increased the performance of ICCU-methanation.
The distance between the active metals and the adsorbents also affects the performance of ICCU-methanation.Melis et al. 45 compared physically mixed 10% Ru/g-Al 2 O 3 and 10% CaO/g-Al 2 O 3 with 10% Ru-10% CaO/g-Al 2 O 3 for ICCU-methanation.The physically mixed DFMs showed a poor ICCU performance (0.12 g-mol CH 4 /kg DFM) owing to the longer distance between active sites within the DFMs decreasing the effective CO 2 spillover.However, Sun et al.  both the capture capacity of CO 2 and activity of methanation were enhanced via doping with Li (nitrates), which could react with Al 2 O 3 to form a mixed spinel phase.
Investigating the catalytic mechanism helps to develop DFMs for ICCU-methanation.The mechanism and key intermediates of CH 4 formation from the ICCU process were studied by in situ diffuse reectance infrared fourier transform spectroscopy (DRIFTS). 59Proaño et al. 59 investigated the two steps of ICCU-methanation, including CO 2 adsorption and hydrogenation, over 5% Ru-6.1% Na 2 O/Al 2 O 3 DFMs.The authors found that CO 2 absorbed on the AlO À -Na + species formed bidentate carbonates (i.e.1st step in Fig. 3b), and then the adsorbed bicarbonates and bidentate carbonates spilled over onto the Rusupport interface during the CO 2 methanation process, with formates as reaction intermediates (i.e.2nd and 3rd steps in Fig. 3b), which is consistent with the results reported by Sun et al. 54 In addition to the adsorbent, Ru as the catalytic site also showed the capacity of CO 2 adsorption, 44,59 generating carbonyl groups. 54 lot of research has been conducted on the production of CH 4 by ICCU, but there is no valuable economic evaluation research in this eld yet.However, ICCU-methanation can be combined with renewable H 2 , and it still has a very broad prospect as integrated hydrogen storage and carbon-neutral solution.

Progress in integrated CO 2 capture and DRM
Compared to CO 2 , the greenhouse effect of CH 4 is 22 times higher.Currently, CH 4 is widely used to produce H 2 by chemical looping reforming 63 or steam methane reforming. 64Dry reforming of methane (DRM) utilizing these two major greenhouse gases, as shown in eqn (2), has received increasing attention in recent years. 36,65In addition to CH 4 , C 2 H 6 and other low-carbon alkanes could also be used for dry reforming.
Integrated CO 2 capture and dry reforming (ICCU-DRM) provides a promising solution for utilizing low carbon alkanes accompanied by the reduction of CO 2 emissions.Synthesizing liquid fuels or high-value hydrocarbons by Fischer-Tropsch synthesis using the products from the ICCU-DRM process could be a practical route for upgrading alkanes. (2)

Inuence of process parameters on ICCU-DRM
In this section, the inuences of process parameters on the performance of ICCU-DRM are reviewed and discussed.As shown in Fig. 4a, there are various side reactions in ICCU-DRM, including CH 4 decomposition and the reverse-water-gas shi reaction (RWGS), which would generate coke on the surface of DFMs and affect the ratio of H 2 /CO in the nal product.
Temperature is one of the most effective parameters to inuence the balance of DRM and side reactions.Molina-Ramírez et al. 66 studied different temperatures (600-700 C) of ICCU-DRM using a non-supported Ni-Ba bifunctional catalyst.The conversion of CH 4 increased with the increase of temperature (11.04% at 600 C and 18.57% at 700 C), while the selectivity to CO showed an opposite trend (14.89% at 600 C and 4.69% at 700 C).The side reactions, especially CH 4 decomposition, are favored at a higher temperature.The ratio of H 2 /CO increased from 5.7 at 600 C to 20.4 at 700 C, which was accompanied by a signicant carbon deposition on the surface of the DFMs.By applying dry reforming of ethane, Ahmed et al. 67 investigated the inuence of reaction temperature on Ni 20 @(K-Ca) 50 /(g-Al 2 O 3 ) 50 over ICCU and found that the yield of syngas continuously increased with increasing temperature.Ethane conversion was positively correlated with temperature and could be fully converted at a higher temperature (>650 C), while the CO 2 conversion equilibrated at $65% (>600 C).
For ICCU-DRM, a higher temperature is not always preferred.Molina-Ramírez et al. 66 reported that the optimal temperature of Ni-Ba DFMs for CO 2 adsorption is 650 C (0.37 mmol g À1 ), capturing 0.23 and 0.13 mmol g À1 CO 2 at 600 and 700 C, respectively.It is suggested that a higher temperature enables the adsorption to approach equilibrium faster, but reduces the adsorption capacity at equilibrium.In terms of the desorption temperature for ICCU-DRM, Kim et al. 1 proposed that a reaction temperature of around 720 C could steadily release CO 2 .Too fast desorption would cause excessive CO 2 release at the initial time of the conversion stage, while slow desorption would decrease the overall efficiency of ICCU-DRM.
Reaction time is another important parameter in ICCU-DRM.For example, Kim et al. 1 studied different stages of ICCU through continuous online gas analysis.As shown in Fig. 4b, three reaction stages were proposed: (1) in the prebreakthrough stage, CO 2 and CH 4 were almost fully converted, only 0.08% and 0.06% escaped, respectively.The produced syngas had a slightly higher H 2 /CO ratio ($1.06 : 1) than thermodynamic equilibrium (0.94 : 1) at 720 C, probably owing to the decomposition of CH 4 .(2) The CO yield was gradually decreased due to the reduction of CO 2 release, while the H 2 yield remained stable during the following $18 min, namely the breakthrough stage.( 3) The H 2 mole ow rate gradually decreased in the post-breakthrough stage due to the deactivation of the catalyst, which was caused by the deposition of carbon generated by CH 4 decomposition in the previous stages.Therefore, the ratio of H 2 /CO in the product can be controlled by adjusting the reaction time at different stages of ICCU-DRM.The presence of catalysts also affects the performance of adsorbents in ICCU-DRM.As shown in Fig. 5b, Tian et al. 2 reported that introducing catalysts in adsorbents signicantly promoted the decomposition of CaCO 3 ($3 times faster).A similar promotion effect of catalysts on adsorbents was also reported by Ahmed et al. 67 Other adsorbents were also applied in ICCU-DRM, however with poor adsorption performance.For example, Molina-Ramírez et al. 66 synthesized a Ni-Ba unsupported DFM and obtained 0.232 mmol g À1 capacity of CO 2 capture.In addition, MgO is not a suitable adsorbent for ICCU-DRM as Ahmed et al. 67 only achieved 0.22 mmol g À1 capture capacity of CO 2 using Ni 10 @(K-Mg) 25 /(g-Al 2 O 3 ) 75 DFMs at 650 C, which was much lower than that achieved by Ca-based adsorbents.
3.2.2.Catalysts in DFMs for ICCU-DRM.As DRM requires a large amount of energy to process owing to its thermodynamic properties, 68    In the 2nd stage, the interaction between Fe-based oxygen storage materials and CO 2 adsorbents accompanied by the CH 4 reforming of CaCO 3 (eqn ( 5) and eqn ( 6)) achieved pure syngas production.Fe 3 O 4 was gradually transformed into Fe and FeAl 2 O 4 with the grain size change in this stage.Along with further interactions, FeO rather than Fe was detected at the 3rd reaction stage.This might be attributed to the inuence of the oxygen sources, including CO 2 , lattice oxygen from the incompletely reduced Fe-based oxygen storage materials, or even lattice oxygen from the Al 2 O 3 support.In the nal stage, there was a reappearance of CaCO 3 , which suggested that when the spine FeAl 2 O 4 was reformed by methane, the unwanted byproduct CO 2 was recaptured by the carbonation reaction and thus utilized.In conclusion, the Ca-Fe DFM could restore its state and activity aer oxidation and carbonation in ue gas and showed acceptable cycle stability. (3) (5) Introducing specic additives promotes ICCU-DRM.Ahmed et al. 67 proposed that a K-introduced DFM showed better reducibility than a Na-introduced DFM by enhancing catalystsupport interactions.The promotion effect of Ni-Ca interactions was also veried by comparison with that of Ni-Mg DFMs.It was noted that with the introduction of alkaline metals, the ratio of H 2 /CO was affected.For example, the Kintroduced DFM showed a higher H 2 /CO ratio owing to the higher conversion of dry reforming ethane and other side reactions.

Progress in integrated CO 2 capture and RWGS
The reverse water gas shi (RWGS) reaction is another important reaction in C1-chemistry, which can connect with the production of valuable hydrocarbons through Fischer-Tropsch using the produced syngas. 69,70As shown in eqn (7), the RWGS reaction is an endothermic process with lower Gibbs energy than DRM.Although more expensive H 2 is used, the RWGS process has fewer side reactions than DRM.The integrated CO 2 capture and RWGS (ICCU-RWGS) process possesses the potential opportunity to directly produce pure syngas from ue gas.

Inuence of process parameters on ICCU-RWGS
In this section, several key parameters, including temperature, CO 2 concentration and the presence of contaminants, are reviewed for ICCU-RWGS.As shown in Fig. 7b, Luis et al. 71 reported that a higher temperature improved both CO 2 conversion and CO selectivity owing to the endothermic properties of reaction temperature with 5.8% CO 2 .The gas composition-capture phase: CO 2 diluted in nitrogen (ideal condition), CO 2 diluted in nitrogen saturated with 4% of water vapour (effect of water), CO 2 diluted in nitrogen with 4% of oxygen (effect of oxygen) and CO 2 diluted in nitrogen with 5% of oxygen and 4% of water (realistic condition).Gas hourly space velocity (GHSV)-1620 ml g cat À1 h À1 .Reduction phase: Pure hydrogen with a GHSV of 3900 ml g cat À1 h À1 .The CCR period length was 215 s, i.e. 107.5 s for the CO 2 capture and reduction phase. 71g. 6 Diagram of the reaction mechanism for the novel Ca-Fe chemicallooping reforming process. 3WGS.A similar trend in relation to the inuence of temperature was also reported by Jo et al. 43 using Ni/CaO DFMs.Shao et al. 72 analyzed the temperature-programmed desorption of ICCU-RWGS using a Fe x Co y Mg 10 CaO DFM and found that CO could be generated at 550 C, which was earlier than CO 2 release.Within a certain temperature range (<650 C), operating ICCU-RWGS at a higher temperature could generate more CO due to the release of more CO 2 .However, too fast CO 2 release rate at high temperatures could reduce its conversion to CO.
O 2 and H 2 O are present in ue gas and affect the performance of ICCU-RWGS.Luis et al. 71 investigated the inuence of the presence of O 2 or H 2 O on ICCU-RWGS using FeCrCu/K/ hydrotalcite DFMs (Fig. 7a).The realistic conditions resulted in a poorer ICCU-RWGS performance owing to the deactivation of active sites (e.g. by surface oxidation or adsorption).The presence of H 2 O could suppress ICCU-RWGS by competitively adsorbing CO 2 and affect RWGS equilibrium.Compared with the presence of H 2 O, the existing O 2 has a more negative impact on ICCU-RWGS.The presence of O 2 would signicantly decrease CO 2 conversion owing to the oxidation of catalytic sites.
The CO 2 concentration could affect not only the performance of CO 2 adsorption, but also the following RWGS process.With the increase of CO 2 concentration from 5.8% to 9.5%, the efficiency of CO 2 capture and CO 2 conversion slightly decreased at temperatures under 450 C, i.e. from 100% to 98% and from 80% to 67%, respectively.In contrast, CO selectivity was increased from 90% to 92%. 712 Development of DFM progress for ICCU-RWGS 4.2.1 Adsorbents in DFMs for ICCU-RWGS.CaO, a cheap and abundant adsorbent with excellent CO 2 capacity, has been widely used in ICCU-RWGS due to its mature application in calcium chemical looping.In order to alleviate the sintering of CaO in cyclic adsorption and desorption, constructing a porous structure or better dispersion helps to improve the cycle performance of the adsorbent.The sol-gel method is a simple and effective way to synthesize porous materials, and was applied by Shao et al. 72 and Sun et al. 73 In addition, introducing materials with high thermal stability also helps to limit excessive sintering of CaO.Sun et al. 73 reported that introducing CeO 2 as a physical barrier retarded the sintering of CaO.Similarly, Shao et al. 72 achieved impressive cycle stability (no decrease aer 10 cycles) by adding MgO into the DFMs.
In the existing ICCU-RWGS research work, transition metal-based catalysts have shown excellent catalytic performance.Shao et al. 72 applied bimetallic Fe 3+ /Fe 2+ and Co 3+ /Co 2+ redox couples in a hierarchical porous CaO/MgO composite and achieved an excellent ICCU-RWGS performance.The bimetallic couples signicantly lowered the electric potential difference of Fe 3+ /Fe 2+ through the newly formed Fermi level in Fe 5 Co 5 Mg 10 CaO, which made the electron spillover easier to improve the catalytic activity.It was suggested that the same content of Fe and Co could achieve optimal ICCU-RWGS performance.The authors also proposed that Fe 2+ was the active catalytic site, whereas Co acted as the catalytic promoter in

Progress in low temperature ICCU
The widely applied CO 2 capture (i.e.MEA adsorption and calcium looping) and utilization (i.e.CO 2 methanation and RWGS) consume a large amount of energy for sorbent regeneration and CO 2 conversion.To overcome the drawbacks of the above processes, many researchers paid attention to photo-, electronic-and plasma-promoted catalytic processes.The integrated process can achieve promising performance under more mild conditions (ambient temperature and pressure) with the introduction of photo, electronic or plasma energy.
The traditional CO 2 capture and utilization process can achieve excellent process efficiency.However, it needs either high temperature (>500 C) or high pressure (>20 bar).Furthermore, these processes required relatively expensive reducing agents (i.e.H 2 and CH 4 ) to convert CO 2 .Alternatively, photo-catalytic reduction of CO 2 with H 2 O has emerged as a promising option. 118Hybrid MgAl(LDO)/TiO 2 , 118 NH 2 -UiO-66/ TiO 2 , 119 Mg(OH) 2 /CuO/Cu 2 O, 120 etc. are applied in integrated CO 2 capture and utilization and achieved promising results.Imidazolium ionic liquids, imidazolylidene heterocyclic carbenes, and zeolitic imidazolate frameworks are also commonly used materials to realize this process. 33Integrated CO 2 capture and photo-catalytic conversion provide a sustainable solution for CO 2 emission reduction and utilization, which faces challenges in the improvement of catalytic efficiency.
][123] The CO 2 RR remains a huge challenge due to multiple proton and electron transfer processes and the chemical inertness of CO 2 molecules. 124Therefore, many researchers developed various nely designed materials to improve the efficiency of CO 2 conversion.For example, Jiang et al. 124 applied a wet chemistry and pyrolysis method to synthesize Sb SA/NC and achieved efficient formate production (faradaic efficiency of 94.0% at À0.8 V vs. RHE).Recently, Lee et al. 125 achieved an electrochemical upgrade of CO 2 from amine capture solution, promising progress in large-scale application of integrated CO 2 capture and electrocatalytic conversion.Electrocatalysis has a very high energy utilization efficiency.The possible drawbacks are the high requirements for the purity of the reactants to avoid the poisoning effect of impurities on the catalysts and equipment. 126on-thermal plasma (NTP) catalyzed CO 2 conversion has also become a promising method to signicantly reduce the reaction temperature because plasma can activate CO 2 at room temperatures and atmospheric pressure. 29,32In addition to DFMs containing adsorbents and catalysts, the membrane reactor is also gaining researchers' attention.Chen et al. 127 realized integrated CO 2 membrane separation and NTP CO 2 conversion with 91.8% carbon capture efficiency and 71.7% carbon utilization efficiency.This integrated process was also veried by Li et al. 128 ICCU with NTP can achieve excellent reaction efficiency and scalable CO 2 throughput.However, energy consumption related to NTP-ICCU needs to be particularly investigated.
Nowadays, low-temperature ICCU has gradually become a promising direction, and many nely designed materials are used in ICCU under mild conditions and have achieved excellent catalytic performance.CO 2 reduction and neutralization should deal with a concentrated CO 2 in industry (i.e.power plants and cement factories), which requires high CO 2 throughput.High material costs may not be suitable for largescale processing, and the resistance to impurities in the CO 2 sources will also determine the availability of related CCU technologies.

Conclusions and prospects
In this paper, we have critically reviewed the state-of-the-art progress in integrated high temperature CO 2 capture and catalytic conversion, including CO 2 methanation, DRM and RWGS, from the perspective of process parameters and catalytic materials.This process can not only decrease the overall CO 2 utilization cost by eliminating CO 2 enrichment and transportation, but also achieve outstanding CO 2 conversion performance owing to the reducing agent rich conditions.However, there are several research gaps in this eld that need to be addressed.For example, comprehensive considerations from the perspective of engineering are needed, including process design, and economic and technical analysis.The energy consumption for high temperature operations is noteworthy.Therefore, developing low-cost materials that can process ICCU at lower temperatures is promising.Furthermore, more research is needed for the development of dual functional catalysts for application in ICCU, including an in-depth understanding of the synergies between catalysts and adsorbents in addition to reaction intermediates.
For ICCU-methanation, an intermediate process temperature ($300 C) shows better catalytic activity and CH 4 selectivity.Ni-based DFMs can achieve excellent ICCU-methanation performance only in the absence of O 2 .The noble metalbased DFMs can obtain impressive ICCU performance, however, with inevitable high cost.The spillover of CO 2 from adsorbents to catalytic sites is a key step for ICCU-methanation, which can be promoted by the interactions between adsorbents and catalysts.More research on the interactions within DFMs is necessary for a better understanding of the ICCU-methanation process and effective catalyst design.Furthermore, an economic evaluation will guide the development of ICCUmethanation.
For ICCU-DRM, a high temperature (>500 C) is necessary to promote reactions.Therefore, both CaO and Ni are applicable and affordable.The reaction time is a critical parameter to control the coke formation in DFMs and optimize the H 2 /CO ratio of syngas in ICCU-DRM.The sintering of CaO and active metals at high temperatures is usually the main reason for the decrease of catalytic activity owing to the coverage of active sites and agglomeration of metals.The environmentally unfriendly CO generated from the coke in DFMs in the adsorption step is also worthy of attention.
ICCU-RWGS is a promising integrated process owing to its lower Gibbs energy and excellent selectivity of syngas.The ICCU-RWGS process shows great potential for surpassing the equilibrium limitation of CO 2 conversion in traditional RWGS.The interaction among different components within DFMs plays multifunctional roles in ICCU-RWGS, including prohibiting the sintering of adsorbents, dispersing active metals and providing active sites.Apart from the investigation on the interaction within DFMs, the H 2 /CO ratio of syngas from ICCU-RWGS also deserves more attention.
In conclusion, CO 2 capture and in situ catalytic conversion are still in their infancy.Reducing CO 2 emissions is not necessarily a high-cost industry.In contrast, it is possible to generate economic benets through industrial integration.ICCU provides an economic CO 2 utilization strategy, which can integrate the abundant research on CO 2 capture and conversion and provide a solution for an urgent environmental need.The integration of CO 2 capture and utilization can also be expanded to other processes, including producing methanol or other valuable C 2 + chemicals.By producing more valuable chemicals, ICCU has the potential to be protably accompanied by CO 2 emission reduction.

Fig. 1
Fig. 1 Schematic diagram of ICCU and its advantages over traditional CCU and CCS processes.
excellent theoretical adsorption capacity and low price.Due to sintering caused by volume change during the transformation between CaO and CaCO 3 , highly dispersed CaO could provide better stability than limestone derived CaO.Melis et al. 45 dispersed CaO onto g-Al 2 O 3 and reported an increased CH 4 yield, demonstrating that the spillover of CO 2 from CaO to active sites occurred within DFMs.Due to the temperature limitation of CO 2 methanation, Na 2 O and K 2 O are also popular for ICCU-CO 2 methanation.Martha et al. 50studied dispersed alkaline adsorbents, including Na 2 O, CaO, K 2 O and MgO, and obtained enhanced methanation performance using Na 2 O and CaO (0.614, 0.610, 0.466 and 0.213 mmol CH 4 g DFM À1 for Na 2 O, CaO, K 2 O and MgO, respectively).The doping of Na in g-Al 2 O 3 promotes the formation of Al-O À sites by promoting the formation of reversible bidentate and polydentate carbonates.57

Fig. 2
Fig. 2 Summary of the CO 2 adsorption capacity of DFMs applied in ICCU. 41-54.(orange zone: CaO application in ICCU; red zone: MgO application in ICCU; green zone: K 2 O and Na 2 O application in ICCU; black dotted zone: supported adsorbent application in ICCU).
Introducing other metals could help to decrease the reduction temperature of oxidized Ni-based DFMs in the step of CO 2 methanation.For example, Martha et al. 51 introduced noble metals (Pt, Pb or Ru # 1%) into Ni-based DFMs and obtained enhanced ICCU performance in the presence of H 2 O and O 2 .The authors 51 also reported stable capacity of CO 2 capture (0.52 mmol g cat À1 ) and CH 4 yield (0.38 mmol g cat À1 ) aer 20 cycles of capture and methanation using 1% Ru, 10% Ni, and 6.1%Na 2 O/Al 2 O 3 at 320 C, indicating excellent long term stability of the Rupromoted Ni-based DFM.Noble metals could not only be used as a promoter of Nibased DFMs, but also show impressive catalytic performance as the main active metal.Ru was the most promising one owing to its excellent catalytic activity of CO 2 methanation.Melis et al. 45 applied 5 wt% Ru/CaO/g-Al 2 O 3 DFMs in ICCUmethanation and obtained around 290 mmol g cat À1 CH 4 yield.
53 reported different conclusions by comparing physically mixed Ni/CeO 2 -CaO with impregnated Ni/CeCaO DFMs.The longer distance between Ni and CaO prevented the coverage of Ni from the formation of CaCO 3 .Furthermore, the introduction of a CeO 2 support would promote the dispersion of Ni and increase the performance of ICCU-methanation.The addition of additives could promote the catalytic performance of DFMs for ICCU-methanation.For example, Stefano et al.62 introduced various alkali metals (Li, Na, and K carbonates vs. nitrates) into 1% Ru/Al 2 O 3 DFMs and found that

Fig. 4
Fig.4(a)Gibbs free energy of related reactions in integrated CO 2 capture and DRM as a function of temperature. 2(b) Coupled CO 2 capture and conversion reactions: molar flow rate of the effluent gas in the first cycle of the coupled CO 2 capture-conversion process and schematic description of the main processes occurring in the reactor.1

3. 2
Development progress of DFMs for ICCU-DRM 3.2.1 Adsorbents in DFMs for ICCU-DRM.The current CO 2 adsorbents applied in ICCU-DRM are mainly alkali metal oxides.High-temperature adsorbents, especially CaO-based materials, are the most commonly applied adsorbents owing to the thermodynamic requirements of DRM.However, the sintering of CaO is the most severe problem in this process.Kim et al. 1 applied CaO as the adsorbent and observed a signicant decrease of CO 2 capture capacity aer several cycles of ICCU-DRM.What's more, the sintering of CaO would shorten the pre-breakthrough stage, as shown in Fig. 4b and 5a, which was suggested as the best stage for ICCU-DRM.
the introduction of catalysts such as Ni, Ru, Mo or Co-based catalysts can reduce the thermodynamic barrier and reduce the reaction temperature.The interaction between metals and adsorbents plays a key role in the ICCU-DRM process.Tian et al. 2 investigated two NiO forms in CaO-Ni bifunctional sorbent-catalysts by XPS and H 2 -TPR.The proportion of interacted NiO increased from 64.0 to 80.7 atomic % with the increase of the Ca/Ni ratio.It was suggested that the presence of interacted NiO enhanced the ICCU-DRM.The authors also proposed that the sintering of catalysts could be prohibited by strengthening the metal-support interaction and/ or the connement effect of the support in the material matrix.

Fig. 5
Fig. 5 (a): Molar flow rate and H 2 /CO ratio of the effluent gas as a function of cycle number: (-) 1st, (---) 5th, and (--) 10th cycles.The arrows highlight the trends with cycle number; 1 (b) comparison of decarbonation kinetics as a function of time at 1073 K between CaL methane reforming and separate CaL processes.2 Fe-based catalysts also showed a good performance for ICCU-DRM.For example, Zhao et al. 3 demonstrated a Ca-Fe chemical looping reforming process for ICCU.The authors observed the occurrence of the reaction between CH 4 and CaCO 3 and Fe 2 O 3 .As shown in Fig. 6b, in the 1st stage, the rapid full CH 4 oxidation reaction occurred and all Fe 2 O 3 was changed into Fe 3 O 4 (eqn (3) and eqn (4)) without CO generation.

Fig. 7
Fig.7CO 2 capture efficiency, CO 2 conversion, and CO selectivity as a function of (a) CO 2 concentration (5.8-9.5%) at 450 C and (b) reaction temperature with 5.8% CO 2 .The gas composition-capture phase: CO 2 diluted in nitrogen (ideal condition), CO 2 diluted in nitrogen saturated with 4% of water vapour (effect of water), CO 2 diluted in nitrogen with 4% of oxygen (effect of oxygen) and CO 2 diluted in nitrogen with 5% of oxygen and 4% of water (realistic condition).Gas hourly space velocity (GHSV)-1620 ml g cat À1 h À1 .Reduction phase: Pure hydrogen with a GHSV of 3900 ml g cat À1 h À1 .The CCR period length was 215 s, i.e. 107.5 s for the CO 2 capture and reduction phase.71 Fe 5 Co 5 Mg 10 CaO DFMs.The catalytic process is as follows: rstly, CO 2 was catalytically reduced to CO by magnetite (Fe 3 O 4 ).Secondly, hematite (Fe 2 O 3 ) was regenerated by H 2 with Co as the promoter.The well-dispersed Fe can ensure the continuous and efficient process of this catalytic process.Luis et al. 71 synthesized FeCrCu/K/MgO-Al 2 O 3 DFMs for ICCU-RWGS under realistic conditions, and the key functions of Cu and K were suggested for efficient CO 2 reduction and capture, respectively.Supports can also play key roles in the catalytic process by promoting and stabilizing active catalytic sites.Sun et al. 73 synthesized Ni-CeO 2 /CaO DFMs and achieved almost 100% CO selectivity, 51.8% CO 2 conversion and remarkable cycle stability aer 20 cycles of ICCU.The oxygen vacancies and interaction between Ni and CeO 2 were believed to play key roles in promoting ICCU-RWGS.It was suggested that the formed interaction would effectively retard the agglomeration of NiO.

Table 1
The catalyst system, performance and reaction condition details of reported integrated CO 2 capture and methanation research work 41 investigated the inuence of a wide temperature range, i.e. 200-600 C, on ICCU-methanation using Ni-Ca/Al 2 O 3 DFMs.The temperature showed a positive correlation with CO 2 capture capacity, CH 4 yield and CO yield.A similar trend of temperature related to CO 2 capture capacity was observed by Zhou et al.
DFM À1 CO 2 capacity aer 10 cycles at a temperature below 300 C. Attractively, Zhou et al. 42 synthesized 2D-layered Ni-MgO-Al 2 O 3 nano-sheets and obtained prolonged cycling performance at low temperatures (#250 C).However, the formation of difficult-to-reduce species, such as Ni-O-Mg and/ or Ni-O-Al, suppressed the reducibility of low Ni-loading catalysts.Zhou et al.
applied alkali metal (Li, Na, and K) promoted MgO in DFMs and obtained $4 mmol g 44 2 O 3 DFMs and obtained 0.35 mmol g cat À1 CH 4 yield aer 80 h operation.During the stability test, there was no loss of the BET surface area, CO 2 capture capacity and Ru dispersion.Ru showed acceptable performance in the presence of O 2 .For example, Zheng et al.44investigated the performance of Ru-CaO/Al 2 O 3 DFMs under simulated ue gas conditions and proposed that Ru-based catalysts could be easily reduced aer exposure to O 2 -containing CO 2 ue gas.As shown in Fig.3a, it is suggested that H 2 reduced oxidized Ru rst, followed by the spillover of CO 2 from CaO to Ru. Lastly, the dissolved CO 2 at Ru sites formed CH 4 with the assistance of H 2 .