Plasma Chemical Looping: Unlocking High-Efficiency CO2 Conversion to Clean CO at Mild Temperatures

We propose a plasma chemical looping CO2 splitting (PCLCS) approach that enables highly efficient CO2 conversion into O2-free CO at mild temperatures. PCLCS achieves an impressive 84% CO2 conversion and a 1.3 mmol g–1 CO yield, with no O2 detected. Crucially, this strategy significantly lowers the temperature required for conventional chemical looping processes from 650 to 1000 °C to only 320 °C, demonstrating a robust synergy between plasma and the Ce0.7Zr0.3O2 oxygen carrier (OC). Systematic experiments and density functional theory (DFT) calculations unveil the pivotal role of plasma in activating and partially decomposing CO2, yielding a mixture of CO, O2/O, and electronically/vibrationally excited CO2*. Notably, these excited CO2* species then efficiently decompose over the oxygen vacancies of the OCs, with a substantially reduced activation barrier (0.86 eV) compared to ground-state CO2 (1.63 eV), contributing to the synergy. This work offers a promising and energy-efficient pathway for producing O2-free CO from inert CO2 through the tailored interplay of plasma and OCs.


Table of contents
The atomic model of pristine CeO2 unit cell is shown in Figure S15 S3) and the most stable one (O2 site) is chosen for subsequent calculations.The computed lowest EO_Formation of Ce0.75Zr0.25O2-δ(111) is 2.43 eV, which is much lower than pristine CeO2 (2.99 eV).Upon the formation of one VO, all adjacent 3 O atoms are considered to migrate to the vacancy site, and the corresponding reaction energies are listed in the Table S4.Then the transition state calculations of the migration these oxygen atoms are performed as shown in Figure S17.The computed lowest migration barrier (0.34 eV) is lower than it is in CeO2 (0.46 eV), suggesting that the oxygen migration is more likely to occur in Ce0.75Zr0.25O2-δthan in CeO2.It is found that the O atom initially neighboring to three oxygen vacancies is most likely to migrate (O1 → VO2).Table S3.Computed formation energy of each oxygen vacancy in Ce0.75Zr0.25O2-δ(EO_Formation = Esubstrate -Esubstrate_vo + EO, where Esubstrate, Esubstrate_vo and EO represents the total energy of the entire system, the total energy of the system with O vacancy and the energy of O in free state vacuum).
Vacancy     We've calculated the adsorption energy of O2 and O to the active site (the oxygen vacancy) and found that both O2 and O are more prone to adsorption than CO2 (Table S8).

Figure S2 .
Figure S2.Schematic of the experimental system.

Figure S3 .
Figure S3.Time-resolved concentrations of the gas products in PCLCS experiments over reduced Ce 1- x Zr x O 2-δ with different Zr content.Figure S4.Time-resolved concentrations of the gas products in PCLCS experiments without OCs.

Figure S4 .
Time-resolved concentrations of the gas products in PCLCS experiments without OCs.

Figure S6 .
Figure S6.Redox stability of the PCLCS system over 10 cycles using reduced Ce 0.7 Zr 0.3 O 2-δ : (a) CO yield and CO 2 conversion; (b) CO purity in the gas product.

Figure S8 .
Figure S8.SEM images of the fresh (a) and cycled (b) Ce 0.7 Zr 0.3 O 2 oxygen carriers.

Figure S10 .
Figure S10.(a) H 2 -TPR profiles and (b) cumulative H 2 uptake of the prepared Ce 1-x Zr x O 2 OCs.

Figure S11 .
Figure S11.(a)O 1s and (b) Ce 3d XPS spectra of the fresh Ce 1-x Zr x O 2 oxygen carriers with different Zr content.

Figure S12 .
Figure S12.Plasma in situ gas sampling device.

Figure S14 .
Figure S14.O 1s XPS spectra of reduced Ce 0.7 Zr 0.3 O 2 before and after Ar plasma treatment.

Figure S15 .
Figure S15.Fraction of electron energy transferred to different channels of CO 2 excitation and ionization, as a function of the reduced electric field (E/n).

Figure S2 .
Figure S2.Schematic of the experimental system.

Figure S3 .
Figure S3.Time-resolved concentrations of the gas products in PCLCS experiments over reduced Ce1-xZrxO2-δ with different Zr contents.

Figure S4 .
Figure S4.Time-resolved concentrations of the gas products in PCLCS experiments without OCs.

Figure S11 .
Figure S11.(a) O 1s and (b) Ce 3d XPS spectra of the fresh Ce1-xZrxO2 oxygen carriers with different Zr content.

Figure S12 .
Figure S12.Plasma in-situ gas sampling device.The in-situ sampling set was an alumina ceramic tube (inner / outer diameter=1 mm / 2 mm) with a hole of 0.4 mm in diameter in the middle.During the sampling, carrier gas N2 with a flow rate of 1 L/min was injected into the tube, forming a fast flow (21 m/s) that can extract reactive gas from the chamber into the ceramic tube.In this way, the sampled gas can be in-situ diluted and cooled down, thereby "freezing" the chemical composition of the sampled gases by largely inhibiting the secondary reactions during sampling.

Figure S15 .
Figure S15.Fraction of electron energy transferred to different channels of CO2 excitation and ionization, as a function of the reduced electric field (E/n).The cross-sections of the electronimpact reactions for the investigated 95vol% Ar + 5vol% CO2 are obtained from 1 .The E/n of the RGA used in PCLCS is around 21-25 Td and is marked in the Figure S14.The E/n was evaluated using the following formulas.  ⁄ = ×× B × × 10 21 (1) where Td; U signifies reactor voltage, V; d denotes the discharge gap, m; and P represents atmospheric pressure, 1.01325×10 5 Pa; kB corresponds to Boltzmann constant,1.380649×10 -2 J K -1 ; T is the reaction temperature, K.The values of V, d and T used in this work are 290-292 V, 0.001 m and 593 K.
Figure S15(b).Due to symmetry, the selection of the Zr doping in the CeO2 unit cell is unique.
Figure S18(b).The energy potential surfaces of the migration pathways are further studied as shown in Figure S19.The most favorable pathway (Oc → O_D) has a very low barrier of 0.376 eV, which is much lower than CeO2 (111) surface (0.47 eV).

Table S1 .
Summary of the PCLCS experimental results over reduced Ce 0.7 Zr 0.3 O 2-δ OCs.

Table S2 .
Fresh CeO 2 and Ce 0.7 Zr 0.3 O 2 lattice parameters and refinement factors.

Table S3 .
Computed formation energy of each oxygen vacancy in Ce 0.75 Zr 0.25 O 2-δ .

Table S4 .
Computed migration barrier of each oxygen migration to O 2 in Ce 0.75 Zr 0.25 O 2-δ .

Table S5 .
Computed formation energy of each oxygen vacancy in Ce 0.75 Zr 0.25 O 2-δ .

Table S6 .
Computed migration barrier of each oxygen migration to O 2 in Ce 0.75 Zr 0.25 O 2-δ .

Table S7 .
Geometric Properties (explained in Figure6) of ground state and excited state CO 2 *dissolution.

Model construction, oxygen formation energy and migration barrier calculations of Ce 0.75 Zr 0.25 O 2-δ (111) surface
When it comes to the reaction substrate, the research of Skorodumova et al.3suggested that the surface energy of the CeO2(111) is low than that of CeO2(110) and CeO2(100) bottom layers are restricted while the top two are relaxed.In this model, there are four nonequivalent surface oxygen atom sites, namely O_A, O_B, O_C and O_D.The computed EO_Formation of all sites are lower than that of the pristine CeO2 (111) (2.56 eV).Then we move on to study the migration of adjacent sub-surface oxygens to O_D vacancy sites, which hold the lowest oxygen vacancy formation energy of 2.08 eV (see in TableS5).The corresponding reaction energies of all possible O migration pathways are listed in TableS6, which the location of Oa, Ob, and Oc shown in