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- PMC11097354
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ACS Omega. 2024 May 14; 9(19): 21082–21088.
Published online 2024 May 1. doi:10.1021/acsomega.4c00743
PMCID: PMC11097354
Zhongze Bai,† Xi Zhuo Jiang,
*‡ and Kai H. Luo*†
Author information Article notes Copyright and License information PMC Disclaimer
Abstract
Chemical looping combustion (CLC) is a promising andnovel technologyfor carbon dioxide (CO2) capture with a relatively lowenergy consumption and cost. CuO, one of the most attractive oxygencarriers (OCs) for carbon dioxide (CO) oxidation, suffers from sinteringand agglomeration during the reduction process. Applying an electricfield (EF) may promote the CO oxidation process on the CuO surface,which could mitigate sintering and agglomeration by decreasing operatingtemperatures with negligible combustion efficiency loss. This studyperforms density functional theory (DFT) simulations to investigatethe effects of EF on the oxidation of CO on the CuO (111) surface.The results indicate that both the orientation and strength of theEF can significantly affect the oxidation characteristics of CO onthe CuO (111) surface such as total reaction energy, energy barriersof reactions, CO adsorption, and CO2 desorption. For thefirst time, this study reveals the role of EF in enhancing CO oxidationthrough CLC processes via first-principle calculations. Such findingscould provide new strategies to improve the performance of CLC processes.
1. Introduction
Chemical looping combustion(CLC) is regarded as a promising andnovel technology for CO2 capture during fuel combustionwith relatively low energy consumption and cost, which could helpaddress global warming issues.1−4 During such a process, the oxygen carrier (OC) isused to transfer oxygen for fuel combustion, which could avoid thedirect contact between fuel and air and obtain high purity CO2 without the mixture with N2. OCs, the key componentfor fuel combustion performance in the CLC process, have been synthesizeddiversely, while improvements on reactivity, thermal stability, resistanceto agglomeration, and sintering are still needed.5,6
Applying an external electric field (EF) to the CLC process couldbe an effective approach to enhancing the behaviors of OCs. Duringthis process, EF can rearrange the electronic orbitals of intermediates,altering the binding energies and reaction mechanisms.7 Therefore, the exploration of EF influence on the CLC processis of great importance. Among the numerous materials, CuO is one ofthe suitable OCs because of its high reactivity and oxygen transportcapacity, suitable equilibrium partial pressure of oxygen under combustiontemperature, stable recyclability of oxygen release and uptake; however,CuO suffers from sintering and agglomeration during the reductionprocess.6 There have been many effortsin the past to improve the performance of OCs by reducing CO oxidationtemperatures during CLC in synthesizing nanomaterials, alloys, etc.8−13 For instance, the CO oxidation on CuO-CeO2 catalystswas explored by a series of experiments.8,10−12 Varghese and co-workers investigated the CO oxidation on CuO-Co3O4 catalyst and found that CuO-Co3O4 catalyst exhibits superior catalytic properties over pureCo3O4.9 Zedan andco-workers improved the reducibility and stability of CuO in the generationof CuO nanoparticles.13 In the presentwork, we chose CO (the main component for carbon-containing fuels)oxidation on the CuO surface as a representation to study the influenceof EF on the CLC process. Hopefully, EF would promote CO oxidationprocess on CuO surface, which can effectively mitigate sintering andagglomeration by decreasing operating temperatures while maintainingthe high combustion efficiency of the CLC process.6
Recently, Cu-based OC has attracted much attentionfrom researchers.Three types of mechanisms occur in a CLC process, including the Mars–vanKrevelen (MvK) mechanism,14,15 Eley–Rideal(ER) mechanism16,17 and Langmuir–Hinshelwood(LH) mechanism,16,17 respectively. Wu and co-workersinvestigated the reaction mechanisms of CO and O2 overthe CuO (111) surface through density functional theory (DFT) calculations,18 and found that the reactions between CO andlattice O of CuO (111) surface by the MvK mechanism were less activethan those between CO and adsorbed oxygen-containing species (O andO2). Zheng and co-workers explored the NOx removal behaviorsduring a CLC process by studying microscopic reactions between HCNheterogeneous reactions on CuO surface by DFT calculations.19 The effects of sulfur-containing species (H2S, HS and S) on CO oxidation over CuO surface were revealedby Zheng and Zhao through DFT simulations.20 Although the CO oxidation mechanisms over CuO surfaces under variousconditions were reported in previous studies, the exploration of EFinfluence on CO combustion over CuO surfaces was rarely seen, andit is worth exploring the effects of EF on a CLC process.
Inthe present study, the role of EF in the reaction of CO oxidationon CuO surfaces, a widely accepted route for fuels oxidation by metaloxide materials,21 is investigated followingthe MvK mechanism. In such a process, CO is adsorbed on the CuO surfacesand reacts with lattice O forming adsorbed CO2; the desorbedCO2 detaches from the CuO surfaces in a gas phase and leavean O vacancy subsequently.18 The effectsof EF on every individual step of CO oxidation over CuO surfaces areexplored via DFT calculations in the present study, including theEF influence on the adsorption and desorption processes of CO andCO2, and the chemical processes from CO to CO2.
2. Methods
All DFT calculations were carriedout using the Vienna Ab initioSimulation Package (VASP) package22,23 with generalizedgradient approximation (GGA) and Perdew–Bruke–Ernzerh(PBE).24 Plane wave energy cutoff and theconvergence criteria for total energy and forces were set to 500 eV,1.0 × 10–5 eV and 0.03 eV/Å, respectively.The DFT-D3 method with Becke–Johnson damping was used to considervan der Waals interaction.25,26 GGA + U with the valueof 7.5 eV was adopted to consider the strong electron correlationsfor Cu atoms.4
The selection of CuOunit cell, as shown in Figure Figure11a, (a = 4.631 Å, b =3.418 Å, c = 5.079 Å andβ = 100.01°) agrees well with experimental parameters (a = 4.682 Å, b = 3.424 Å, c = 5.127 Å and β = 99.42°) with an averageerror of only 0.4%.27 A three-layer P (2× 2) CuO (111) slab with 15 Å vacuum space, which is themost used model surface because it has the lowest surface energy,28 was constructed, as shown in Figure Figure11b. Four kinds of top siteson the CuO (111) surface were constructed including the saturated4-fold copper site (CuCSS), the unsaturated 3-fold coppersite (CuCUS), the saturated 4-fold oxygen site (OCSS) and the unsaturated 3-fold oxygen site (OCUS). Monkhorst–Packk-point grids of 7 × 9 × 6 and 3 × 2 × 1 wereused for CuO bulk cell and CuO slab, respectively. To determine reactionbarriers, the climbing image nudged elastic band (CI-NEB) method wascarried out to search for transition state (TS).29 The calculations were assisted by the VASPKIT30 and QVASP31 code.The influence of EF on CO oxidation on CuO (111) was investigatedby imposing EF ranging from −1 to 1 V/Å along the Z axis,where positive and negative values of the EF were imposed along the+ Z and -Z directions, respectively.
Figure 1
(a) CuO unit cell; (b) three-layer CuO(111) slab. Cu and O atomsare represented in blue and red, respectively.
To facilitate the study, the adsorption energy(Eads), desorption energy (Edes), energy barrier (Eb)and overall reactionenergy (Eall) are calculated as eqs 1–4, respectively.
1
2
3
4
where, E(AB), E(A), E(B), E(TS), E(IS) and E(FS) are the energies of adsorption structure,substrate, adsorbate, transition state, initial state, and final state,respectively. Lower values of Eads and Edes indicate higher adsorption and desorptionabilities of the molecules.
3. Results
3.1. Effects of Electric Field on CO Adsorptionon CuO (111) Surface
The most stable adsorption structuresof CO on CuO (111) under varying EF strengths are illustrated in Figure Figure22a after calculationof the adsorption energies of CO at all adsorption sites, includingthe top, bridge, and hollow sites. Under the EF-free case, Cucus is the most stable adsorption site for CO, and the bondlengths of C–O as well as Cu–C are 1.145 and 1.869Å, respectively. Those adsorption parameters are in good agreementwith previous simulation results.18 AlthoughEF has no effect on the adsorption sites of CO, the direction andmagnitude of EF could alter other CO adsorption behaviors significantly.Specifically, the Cu–C and C–O bond lengths decreaseand increase, respectively, with E increasing from−1 to 0.75 V/Å. In the E = 1 V/Åcase, the bond length of Cu–C is longer than that in the E = 0.75 V/Å case; for the C–O bond, it staysthe same in the E = 0.75 V/Å conditions. Figure Figure22b presents the adsorptionenergy of CO on the CuO (111) surface under varying EF values. Whenthe EF strength ranges from −1 to 0 V/Å, the CO adsorptionenergy presents an upward trend with E rising. When E is greater than 0, the adsorption energy of CO decreasesin a fluctuating manner with the increase in EF strength.
Figure 2
(a) Adsorptionstructures of CO on the CuO(111) surface under varyingEF values. Cu and O atoms are represented in blue and red, respectively.The numbers in figure represent bond length (Å). (b) Effectsof EF on adsorption energy of CO on CuO (111) surface.
To further explain EF influence on CO adsorptioncharacteristicsat the electronic level on the CuO (111) surface, the Bader chargeand partial density of states (PDOS) were analyzed for the adsorptionprocess. Table 1 summarizesthe charge transfers of C, Cu and O atoms as well as band centersof Cu and C atoms during CO adsorption on the CuO (111) surface with E = −1 to 1 V/Å. For PDOS analysis, the selectionof the energy window, −12 to 2 eV, refers to previous work.18 For bonding orbitals, 3d and 2p are chosen forCu and C atoms to characterize the strength of interatomic interactions,respectively.
Table 1
Charge Transfers of Cu, O and C Atomsas well as Band Centers of Cu and C Atoms during CO Adsorbs on CuO(111) Surface with E Ranging from −1 to 1V/Å
| E(V/Å) | –1 | –0.75 | –0.5 | –0.25 | 0 | 0.25 | 0.5 | 0.75 | 1 |
|---|---|---|---|---|---|---|---|---|---|
| Chargetransfer(|e|) | |||||||||
| C → Cu | 0.120 | 0.102 | 0.082 | 0.062 | 0.048 | 0.027 | –0.002 | –0.024 | –0.019 |
| C → O | 0.977 | 0.991 | 1.035 | 1.039 | 1.058 | 1.064 | 1.085 | 1.099 | 1.096 |
| Total C loss charge | 1.097 | 1.093 | 1.117 | 1.101 | 1.106 | 1.091 | 1.083 | 1.076 | 1.077 |
| Bond center (eV) | |||||||||
| Cu-3d | –3.367 | –3.386 | –3.441 | –3.445 | –3.481 | –3.509 | –3.515 | –3.499 | –3.519 |
| C-2p | –5.810 | –6.013 | –6.250 | –6.458 | –6.495 | –6.375 | –6.172 | –5.033 | –5.596 |
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In E = −1 to 0.25 V/Åcases, electronstransfer from C to Cu atoms on CuO (111) with CO-adsorbed structures,while electrons will transfer from Cu to C atoms when E is higher than 0.25 V/Å. Overall, the increasing EF strengthinhibits electron transfer from C to Cu atoms with E values of −1 to 0.75 V/Å. When E is1 V/Å, EF promotes electron transfer from the C to Cu atoms.For O atoms, EF enhances electron transfer from C to O atoms until E = 0.75 V/Å. The total C loss electrons are less sensitiveto the applied external EF, which indicates that the EF alters theelectron distribution from Cu atoms to O atoms. During the adsorptionprocess, the O and C atoms are connected by a covalent bond, and theO atom is in saturated status. By contrast, the Cu atoms in CuCUS are unsaturated. Thus, fewer charge transfers from C tothe O and Cu atoms will benefit O and Cu atoms reaching stable statesand forming more stable bonds with shorter bond lengths. Based onthe above analysis of charge transfers, consequently, the bond lengthsof Cu–C and C–O present a downward trend and an upwardtrend until E = 0.75 V/Å.
To better understandthe effects of EF on the CO adsorption energyon the CuO(111) surface, we also carried out PDOS analysis as shownin Table 1. The bondcenters of Cu-3d decrease from −3.367 eV to −3.519 eVwith EF strength ranging from −1 V/Å to 1 V/Å, whichmeans the Cu-3d bond center leaves the Fermi energy level (0 eV) andthe interactions of Cu with adsorbates decrease correspondingly. TheC-2p bond center decreases with E ranging from −1to 0 V/Å and shows an opposite trend when E ishigher than 0 V/Å. Both negative and positive EF can enhancethe reactivity of C atoms. The absorption energy decreases becausethe interactions of Cu and C atoms are inhibited by the increasing E values when E is lower than 0 V/Å.In the E = 0 to 1 V/Å conditions, the increasein EF strength promotes bonding of C atoms more than it inhibits bondingof Cu atoms, leading to a decrease in the adsorption energy of COon the Cu (111) surface.
3.2. Effects of Electric Field on CO2 Desorption on CuO (111) Surface with O Vacancy
Subsequently,we explored the CO2 adsorption configurations on the CuO(111) surface with an O vacancy under all EF conditions as shown in Figure Figure33a. The adsorptionsites on the CuO surface of CO2 are the same as those ofCO adsorption and remain unchanged in all cases with and without EF.Different from the CO adsorption configurations, the adsorption sitesin CO2 are O (lattice) atoms. The bond lengths of C–Oand Cu–O (lattice) follow the same trend: the bond lengthsincrease when E ranges from −1 to 0.75 V/Åand decrease in the E = 1 V/Å case. The EF influenceon C–O (lattice) bond lengths shows an opposite trend to C–Oand Cu–O (lattice), and the increase in EF strength shortensthe C–O (lattice) lengths until E = 0.75 V/Å. Figure Figure33b illustrates theeffects of EF on the desorption energy of CO2 on the CuO(111) surface with an O vacancy. The rising EF decreases the desorptionenergy of CO2, favoring the departure of adsorbed CO2 from the CuO (111) surface.
Figure 3
(a) Adsorption structures of CO2 on the CuO(111) surfacewith an O vacancy under varying EF values. Cu and O atoms are representedin blue and red, respectively. The numbers in the figure representbond length (Å). (b) Effects of EF on the desorption energy ofCO2 on the CuO(111) surface with an O vacancy.
The effects of EF on CO2 desorptionbehaviors from theCuO (111) surface with an O vacancy at the electronic level were furtherexplored. Bader charge and PDOS analyses were performed for the CO2 desorption process, respectively, as shown in Table 2. For C and O (lattice) atoms,the total C loss charge and O (lattice) obtained charge fluctuatein E = −1 to 1 V/ Å. When E ranges from −1 to 1 V/Å, the EF enhances the chargetransfers from C to O. The EF inhibits electron transfers from O (lattice)to Cu and C to O (lattice). Based on the analysis in Section 3.1, O (lattice)&C andCu atoms are in saturated and unsaturated states, respectively. Thus,the enhancement by rising EF for charge transfers from C to O andCu to O (lattice) decreases the stability of C–O and Cu–O(lattice), and the corresponding bond length increase. By contract,the bond length of C–O (lattice) shows a downward trend withthe increase of EF strength because of the inhibition influence ofEF on electron transfers from C to O (lattice). The change in bondlengths is inconsistent with that of charge transfer in the case of E = 1 V/Å. When E is 1 V/Å, thechanges in electron transfer for C → O, C → O (lattice),and O (lattice) → Cu are significantly higher than other conditions(over 10 times). The electrons transfer from C to O and O to Cu risegreatly and increase the shared electrons between atoms. More sharedelectrons could enhance the bond stability and shorten bond lengths.Similarly, in the E = 1 V/Å case, the bond lengthof C–O (lattice) decreases because of the reduction of sharedelectrons between C and O (lattice) atoms.
Table 2
Charge Transfers and Band Centersof Cu, O and C Atoms during CO2 Adsorbs on the CuO (111)Surface with an O Vacancy with E Ranging from −1to 1 V/Å
| E(V/Å) | –1 | –0.75 | –0.5 | –0.25 | 0 | 0.25 | 0.5 | 0.75 | 1 |
|---|---|---|---|---|---|---|---|---|---|
| Chargetransfer(|e|) | |||||||||
| C →O | 0.951 | 0.979 | 0.979 | 0.996 | 1.024 | 1.024 | 1.051 | 1.063 | 1.331 |
| C → O (lattice) | 1.177 | 1.164 | 1.131 | 1.104 | 1.118 | 1.082 | 1.050 | 1.040 | 0.773 |
| O(lattice)→Cu | 0.057 | 0.045 | 0.034 | 0.021 | 0.014 | –0.001 | –0.013 | –0.025 | –0.293 |
| Total C loss charge | 2.127 | 2.143 | 2.110 | 2. 100 | 2.142 | 2.106 | 2.101 | 2.103 | 2.105 |
| Total O (lattice) obtain charge | 1.120 | 1.118 | 1.097 | 1.083 | 1.104 | 1.083 | 1.063 | 1.065 | 1.067 |
| Bondcenter (eV) | |||||||||
| Cu-3d | –2.560 | –2.574 | –2.626 | –2.661 | –2.675 | –2.702 | –2.702 | –2.735 | –2.738 |
| O (lattice)-2p | –6.208 | –6.544 | –6.816 | –7.131 | –7.449 | –7.755 | –7.994 | –8.280 | –8.166 |
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To further understand the effects of EF on CO2 desorptionbehaviors from the CuO (111) surface with an O vacancy at the electroniclevel, Bader charge and PDOS analysis were performed for the CO2 desorption process, respectively, as shown in Table2. For C and O (lattice) atoms,the total C loss charge and O (lattice) obtained charge fluctuatein E = −1 to 1 V/ Å. When E ranges from −1 to 1 V/Å, EF enhances the charge transfersfrom C to O, but it inhibits electron transfers from the O (lattice)to Cu and C to the O (lattice). Based on the analysis in Section 3.1, O (lattice)&Cand Cu atoms are in saturated and unsaturated states, respectively.Thus, the enhancement by rising EF for charge transfers from C toO and Cu to O (lattice) decreases the stability of C–O andCu–O (lattice), and the bond length of them will also increasecorrespondingly. By contract, the bond length of C–O (lattice)shows a downward trend with the increase of EF strength because ofthe inhibition influence of EF on electron transfers from C to O (lattice).However, the change in bond lengths is inconsistent with that of chargetransfer in the case of E = 1 V/Å. When E is 1 V/Å, the changes in electron transfer for C→ O, C → O (lattice) and O (lattice) → Cu aresignificantly higher than other conditions (over 10 times). A sharpvariation in the shared electrons between atoms plays a key role inthe bond lengths. Specifically, the electrons transferred from C toO and from O to Cu rise greatly and also increase the shared electronsbetween atoms. More shared electrons could enhance the bond stabilityand shorten the bond lengths. Similarly, in the E = 1 V/Å case, the bond length of C–O (lattice) decreasesbecause of the reduction of shared electrons between C and O (lattice)atoms.
PDOS analysis was conducted for Cu-3d and the O (lattice)-2porbitsto better reveal the EF influence on the CO2 desorptionenergy. As illustrated in Table 2, both the Cu-3d and the O (lattice)-2p bond centerspresent a downward trend with E ranging from −1to 0.75 V/Å, indicating that the interactions between Cu andthe O (lattice) are weakened by rising EF strength. Thus, the desorptionabilities of CO2 from the CuO surface become stronger,and the desorption energy decreases. When E is 1V/Å, the Cu-3d bond center continues to move away from the Fermienergy level (0 eV); however, the bond center of the O (lattice)-2pbond increases. Considering the decrease in desorption energy in E = 1 V/Å, it can be concluded that the promotion influenceof the Cu-3d bond center desorption plays a dominant role in CO2.
3.3. Effects of Electric Field on the Reactionsof CO on CuO (111) Surface
According to the MvK mechanism,adsorbed CO will react with lattice O to form adsorbed CO2 on the CuO (111) surface with an O vacancy. In this section, thechemical process from CO to CO2 on the CuO (111) surfaceis investigated in the E = 0 case. Figure Figure44a shows the energy profileand structures of the conversion from CO to CO2 on theCuO (111) surface following the MvK mechanism. The absorbed CO andCO2 are the initial state (IS) and final state (FS) ofthe reaction process, respectively. A transition state (TS) betweenthe IS and FS is a critical state with the highest energy. Energybarrier (Eb) and the energy differencebetween TS and IS, is the key indicator for reaction rates. Eall represents the total energy change duringthe reaction, which is the energy difference between the FS and IS. Eb and Eall are 0.950eV and −0.952 eV, respectively, under EF-free conditions.
Figure 4
(a) Energyprofile and structures of the conversion from CO toCO2 on the CuO (111) surface without EF applied via theMvK mechanism. Cu and O atoms are represented in blue and red, respectively.(b) The influence of EF on the overall reaction energy (Eall) and energy barrier (Eb) during CO oxidation on the CuO (111) surface.
The overall reaction energy (Eall)and energy barrier (Eb) in the CO combustionon the CuO (111) surface under different EF strengths are shown in Figure Figure44b. Eall increases from −1.186 to −0.805 eV withEF strength ranging from −1 to 0.75 V/Å, which means thatthe rising EF inhibits heat release in the CO oxidation on the CuOsurface. EF promotes heat release in the CO combustion in the E = 1 V/Å case. Regarding reaction barriers, positiveEF has a greater influence on Eb thannegative EF. Specifically, when EF strength is larger than 0 V/Å, Eb presents a parabolic trend and reaches itslowest point, 0.727 eV, with an E of 0.5 V/Å.In the E = −1 to 0 V/Å cases, Eb first decreases until E =−0.25 V/Å with the increase in EF strength. The EF canreduce the energy barrier during CO oxidation on the CuO surface byabout 23% and accelerate the CO combustion, correspondingly.
4. Discussion
In this study, we systematicallyexplored the behaviors of CO oxidationon the CuO (111) surface with and without an EF. The effects of EFon CO adsorption, overall reaction energy, energy barrier, and CO2 desorption were investigated and analyzed during CO combustionon the CuO (111) surface.
According to the DFT results, boththe magnitude and directionof the EF have significant effects on the oxidation characteristicsof CO on the CuO (111) surface. EF along the −Z direction cansignificantly benefit CO adsorption and overall reaction energy; itslightly lowers the reaction energy barrier, however, inhibiting CO2 desorption. For EF along the +Z direction, it can promoteCO adsorption, CO2 desorption, and reaction rates by reducingenergy barriers, but it hinders the exothermic heat of CO oxidation.Such findings demonstrate that the application of EF could be an effectivemethod to mitigate sintering and agglomeration of CuO by decreasingoperating temperatures with negligible loss of combustion efficiency.The choice of electric field strength and direction should be basedon the practical requirements. Further evaluation of kinetic characteristicsfor CO adsorption, CO combustion, and CO2 desorption ratesis required.
In the present study, the MvK mechanism is considered,and theinfluence of EF on other mechanisms such as ER and LH can be exploredfor future work. The application of EF to other OC materials and fuelsmay also be an effective way to improve the CLC performance, whichwarrants further investigation.
5. Conclusions
CO oxidation on the CuO(111) surface following the MvK mechanismwas investigated under different EF strengths via DFT calculations.The EF influence on CO adsorption and CO2 desorption onthe CuO (111) was clarified at the electron level for the first time.The effects of EF on the reaction barriers and overall reaction energywere revealed. Results indicate that both positive and negative EFcan enhance the CO adsorption on the CuO (111) surface, and the negativeEF presents a better promotion influence on CO adsorption than positiveEF. For CO2 desorption, positive EF lowers the desorptionenergy of CO2 and benefits CO2 desorption fromthe CuO (111) surface. The negative and positive EFs present promotionand inhibition effects on heat release of CO oxidation on the CuO(111) surface, respectively. Finally, both negative and positive EFcan lower the energy barrier for CO oxidation on the CuO (111) surface.Specifically, positive EF shows better performance on energy barrierreduction, which can decrease the energy barrier by 23%. This studydemonstrates that the EF can promote the CLC process and can be usedto control CLC behaviors. For future work, application of EF in otherOC materials and fuels deserves further study, which can help expandthe application area and improve performance of the CLC process.
Acknowledgments
The workis supported by UK Engineering and Physical SciencesResearch Council (EPSRC) under Grant No. EP/T015233/1. ARCHER2 supercomputingresources provided by the EPSRC under the project “UK Consortiumon Mesoscale Engineering Sciences (UKCOMES)” (Grant No. EP/X035875/1)are also acknowledged. This work made use of computational supportby CoSeC, the Computational Science Centre for Research Communities,through UKCOMES.
Notes
The authorsdeclare no competing financial interest.
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