Rapid Energy Exchange between In Situ Formed Bromine Vacancies and CO2 Molecules Enhances CO2 Photoreduction

Photocatalytic reduction of CO2 into fuels provides a prospective tactic for regulating the global carbon balance utilizing renewable solar energy. However, CO2 molecules are difficult to activate and reduce due to the thermodynamic stability and chemical inertness. In this work, we develop a novel strategy to promote the adsorption and activation of CO2 molecules via the rapid energy exchange between the photoinduced Br vacancies and CO2 molecules. Combining in situ continuous wave-electron paramagnetic resonance (cw-EPR) and pulsed EPR technologies, we observe that the spin–spin relaxation time (T2) of BiOBr is decreased by 198 ns during the CO2 photoreduction reaction, which is further confirmed by the broadened EPR linewidth. This result reveals that there is an energy exchange interaction between in situ formed Br vacancies and CO2 molecules, which promotes the formation of high-energy CO2 molecules to facilitate the subsequent reduction reaction. In addition, theoretical calculations indicate that the bended CO2 adsorption configuration on the surface of BiOBr with Br vacancies caused the decrease of the lowest unoccupied molecular orbital of the CO2 molecule, which makes it easier for CO2 molecules to acquire electrons and get activated. In situ diffuse reflectance infrared Fourier transform spectroscopy further shows that the activated CO2 molecules are favorably converted to key intermediates of COOH*, resulting in a CO generation rate of 9.1 μmol g−1 h−1 and a selectivity of 100%. This study elucidates the underlying mechanism of CO2 activation at active sites and deepens the understanding of CO2 photoreduction reaction.


Introduction
Economically and sustainably converting CO 2 and H 2 O by solar energy and photocatalysts can realize the production of solar fuels and valuable chemicals in a more eco-friendly manner [1][2][3][4].Unfortunately, the activation of the CO 2 molecule is extremely difficult, because a CO 2 molecule has 2 stable delocalized π 3 4 bonds with a high dissociation energy of 799 kJ mol −1 for C=O [5][6][7].The photocatalytic CO 2 reduction process is related to the active sites for the activation of the C=O bond, where the activation of the double bond is considered to be the rate-determining step for CO 2 reduction.Of note, the addition of a single electron induces a bending of the molecular structure, thereby decreasing the energy barrier of CO 2 activation [8][9][10].Consequently, it is crucial to promote the activation of CO 2 molecules by regulating the active sites with abundant localized electrons.
Recently, defect engineering is considered as a promising strategy to availably optimize the electronic structure, facilitating the adsorption and activation of CO 2 molecules, thereby improving the inducing activity of CO 2 molecules [11][12][13].As reported previously, the surface defects with abundant localized electrons can serve as highly active sites for catalytic reactions, which involve electron transfer between the catalytic surface and reactant molecules [14][15][16][17].However, the interactions between electrons and reactant molecules make the photophysical and photochemical processes difficult to understand and control owing to the lack of in situ characterization techniques [18][19][20][21].Thus, it is highly advisable, though challenging, to develop more accurate and advanced in situ characterization techniques to track the dynamic process of electrons and advance the understanding of the underlying CO 2 activation mechanism in the photocatalytic reduction process.
In this work, we utilize the tetragonal BiOBr (BOB) as a prototype catalyst to explore the interactions between CO 2 on the ground state and the excited state on catalyst sites, which is the key to elucidate the in-depth reaction mechanism.Multiple (quasi) in situ characterizations were employed to investigate the influence of the dynamic electronic behavior with structure evolution on the photocatalytic CO 2 activation process, including in situ electron paramagnetic resonance (EPR), in situ Raman, in situ x-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).The spin centers of excited state on photoinduced Br vacancies (PI-BrVs) could transfer energy to CO 2 molecules, generating the high-energy CO 2 molecules that are more likely to acquire electrons and be activated.Theoretical calculations further verify that the reduction of the lowest unoccupied molecular orbital (LUMO) of CO 2 molecules is decreased, which makes it easier to be adsorbed and activated, which is a key step for CO 2 activation but has been largely overlooked.The rapid energy exchange between PI-BrVs and CO 2 molecules enables a CO generation rate of 9.1 μmol g −1 h −1 and a product selectivity of 100%.Meanwhile, the conversion process of the activated CO 2 molecules is further investigated and revealed by in situ DRIFTS.This work provides a new perspective into the activation mechanism of CO 2 molecules and deepens the understanding of photocatalytic CO 2 reduction reaction.

The generation of in situ photoinduced Br vacancies
Catalysts are inherently dynamic during a reaction since they could change their local structure for responding to the environment [22][23][24][25].While the reconstruction of catalyst during the reaction is a recognized phenomenon, it can be disclosed via catalytic activity trends.Analyzing the state of reconstruction allows distinguishing real working catalytic sites, so as to design new catalysts [26][27][28].In this work, the dynamic surface reconstruction of BOB samples during photocatalytic CO 2 reduction and its influence on the catalytic reaction was studied.The x-ray diffraction pattern matches the tetragonal phase BOB (JCPDS 09-0393) in Fig. S1.The transmission electron microscopy (TEM) image showed 2D sheet-like morphology in Fig. S2A, while the high-resolution transmission electron microscopy (HRTEM) image in Fig. S2B revealed the high orientation along the [010] projection of tetragonal phase BOB, which has an open channel structure with the surface atomic alternation of [Bi 2 O 2 ] 2+ and Br − on the facet.More importantly, the g value of 2.003 in the EPR spectrum consists of a single line of typical Lorentzian shape, confirming the existence of vacancies after Xe light irradiation (Fig. 1A) [29].To investigate the types of vacancy generated on the BOB surface after light irradiation, the precipitation of Br − on the surface of BOB samples with different illumination times was determined by ion chromatography (IC).With the prolonged illumination time, the concentration of Br − gradually increased and reached equilibrium after 30 min in Fig. 1B.Therefore, we speculate that Br vacancies are generated on the BOB surface after irradiation.In addition, according to the following formula based on Hooke's law: where ω is Raman shift (cm −1 ), c is the velocity of light, and μ is the effective mass.The Raman shift of the Bi-Br bond (e.g., 162 cm −1 ) reduces after illumination, attributed to the decreased force constant (k) of the Bi-Br bond due to the introduced Br vacancy [30][31][32].This result further confirms the generation of Br vacancies during the light irradiation in Fig. 1C.
For surface structure analysis, quasi-in situ XPS was conducted.In the high-resolution XPS Br 3d spectra of the BOB sample in Fig. 1D, there are 2 peaks at 68.38 (Br 3d 3/2 ) and 68.34 eV (Br 3d 5/2 ) [33].After light irradiation of the surface of the BOB sample, the Br 3d shifted to lower binding energy, demonstrating the local chemical environment change resulting from the generation of PI-BrVs.The Bi 4f XPS spectra of BOB exhibit the characteristic spin-orbit doublet splitting center at 159.31 and 164.61 eV in Fig. 1E [34].In contrast, the Bi 4f spectrum of BOB-light shows a shift toward lower binding energy relative to the BOB, indicating the potential breakage of Bi-Br bonds for PI-BrVs formation.Besides, 3 oxygencontaining species in the O 1s XPS spectra can be assigned to the lattice oxygen atoms near the surface (about 530.14 eV), surface oxygen (about 532.05), and adsorbed oxygen on the sample surface (about 533.32) in Fig. 1F [35,36].After light irradiation, the O 1s XPS spectra have a slight shift toward lower binding energy, which is attributed to the introduction of Br vacancies.Con sequently, Br vacancies are generated on the surface of BOB during light irradiation.

The energy exchange mechanism between CO 2 molecules and Br vacancies
To investigate the electronic interactions between the PI-BrVs and CO 2 molecules, in situ EPR experiments were conducted under a CO 2 atmosphere.The in situ EPR visualization map of the BOB sample is shown in Fig. 2A.The BOB sample was illuminated for 2 h in a vacuum by a 300-W xenon lamp with an AM 1.5 G filter.The light-exposed BOB sample was transferred to a home-built in situ EPR catalytic reactor (Fig. S3).This in situ EPR device can continuously record the spectra under light irradiation, in gas atmosphere, or in a vacuum condition.Then, we recorded the in situ EPR spectra of the light-exposed BOB sample in a vacuum.Under light irradiation for 40 min, the EPR spectra have barely changed, indicating that the surface PI-BrVs have reached saturation after light irradiation of 2 h.Subsequently, EPR spectra intensity reduced with the injection of CO 2 molecules, demonstrating that electrons transfer from the surface of BOB to CO 2 molecules, which is a crucial step for CO 2 molecule activation.Furthermore, theoretical calculation suggests that the adsorbed CO 2 molecules on the BOB sample with Br vacancy have more obvious bending of the molecular structure than the BOB sample (Fig. S4A and B).The non-linear CO 2 molecules are more unstable than linear CO 2 molecules and exhibit a higher reactivity in CO 2 photoreduction.The energy level of the LUMO is lowered with the O=C=O bond angle reducing, thus decreasing the energy barrier for CO 2 activation (Fig. 2C).Therefore, the bent CO 2 molecules could be easily activated via electron transfer from Br vacancy to the unpopulated antibonding 1π u * orbital of a CO 2 molecule (Fig. S5).Of note, after the injection of CO 2 molecules, the EPR signal linewidth is wider as shown in Fig. 2B.According to Heisenberg's uncertainty principle, ∆E • ∆t ≈ ℏ can be described by the following equation: Of note, Δν(=γ e ∆B) (Hz) or ∆B (mT) means the linewidth of the EPR signal, and ∆t (s) stands for the lifetime of the spin state.A long-(or short-) lifetime state would produce a narrow (or wide) EPR signal.
The spin state lifetime (∆t) for α (Ms = + 1/2; spin up) or β (Ms = −1/2; spin down) is decided by relaxation times T 1e and T 2e : T 1e stands for the spin-lattice relaxation (SLR) time and T 2e stands for the spin-spin relaxation (SSR) time.SLR controls the energy exchange between the spin ensemble and the environment (1) (lattice), while SSR includes the interactions within the ensemble.In general, the relationships T 1e ≫ T 2e and 1/T 1e ≪ 1/T 2e generally hold, leading to Therefore, following the uncertainty principle, the linewidth is The EPR signal is typically expressed as absorption of the first derivative of A with respect to B as a function of B, dA/dB (Fig. 2E).The shape of A could be a Gaussian curve or a Lorentzian curve or be a suitable mixture of the two, where T 2e is multiplied by a function of T 2 1e , with T 2 2e either in the exponent (Gaussian) or in the denominator (Lorentzian).The eigen values are the maximum A max of A, the peak width of its half-height (A max /2) is ∆B 1/2 , and the peak-to-peak distance of the distance ∆B pp of the derivative curve dA/dB [37,38].Therefore, the SSR time T 2e decreases as linewidth broadens.Advanced in situ continuous wave-electron paramagnetic resonance (cw-EPR) and pulsed EPR technologies were applied to investigate the interactions between in situ generation Br vacancies and CO 2 molecules during the photoreduction process and reveal the mechanistic insights into the photocatalytic reduction of CO 2 , which provides technical support for the study of photocatalytic CO 2 reduction reaction from a fresh point.
In addition, the pulsed EPR measurement of the SSR time T 2e was performed on the X-band EPR spectrometer (CIQTEK, EPR-100) with a dry cooling system.The T 2 relaxation time before and after lighting in different atmospheres (Fig. 2D and Table S1) was compared.In the CO 2 atmosphere, the T 2 relaxation time after light irradiation is reduced by 198 ns compared with that in the dark condition, while it is only reduced by 23 ns in the Ar atmosphere.This result further confirms that CO 2 molecules can lead to the reduction of T 2 relaxation time.Of note, T 2 relaxation time represents the recovery process of the lateral component, which is called lateral relaxation time, also known as SSR time.This process is completed through the energy relay between spin centers as shown in Fig. 3A.Under the Ar atmosphere, a low-energy electron (|β> state) could absorb microwave energy hν and transition to a high-energy state (|α> state), which could transfer the energy to the next low-energy electron and then itself returns to the initial lowenergy state (|β> state).Under a CO 2 atmosphere, the excited high-energy electron (|α> state) transfers energy to the CO 2 molecule adsorbed on the surface; high-energy CO 2 molecules are more likely to be activated.The result is further verified by density functional theory calculation; the CO 2 molecule adsorbed on the Br vacancy displays more negative adsorption energy and more pronounced bond angle alteration than that on the BOB sample without Br vacancy, which indicates higher thermodynamics feasibility.Consequently, the EPR linewidth broadens and the relaxation time decreases due to the strong interaction between the spin center and CO 2 molecules, which is ascribed to the in situ generated surface Br vacancies.Subsequently, most of the CO 2 molecules were desorbed from the surface and the EPR signal recovered after vacuum treatment of the sample.Very few CO 2 molecules are strongly adsorbed or undergo chemical reactions, resulting in a small number of single electrons being consumed (Fig. S6).To further explore the interaction between CO 2 molecules and defective surface, the formation of reaction intermediates is monitored by in situ DRIFTS technology in Fig. 3B and C. When only CO 2 molecules flowed into the reactor, the IR peak at 1,421 cm −1 corresponding to •CO 2 − can be observed, indicating that adsorbed CO 2 molecules acquire electrons from the PI-BrVs.Additionally, the peak at 1,598 cm −1 and 1,396 cm −1 can be found, which could have contributed to the bidentate carbonate (b-CO 3 2−) group and monodentate carbonate (m-CO 3 2−) group, respectively [39].This carbonate most likely derived from the interaction between CO 2 and residual water vapor in the reactor, while the peak at 1,598 cm −1 disappeared under light irradiation, demonstrating that the dissociation of the bidentate carbonate occurs in the absence of water vapor.Subsequently, both CO 2 molecules and water vapor were simultaneously introduced into the reaction chamber.In addition, a new IR peak at 1,637 cm −1 was detected, which could be attributed to the COOH* species, considered as the critical active intermediate during photocatalytic CO 2 reduction [40].Meanwhile, the distinct peaks at 1,608 cm −1 and 3,100 to 3,750 cm −1 can be detected, which could be assigned to the adsorption of H 2 O molecules [34].This result further evidences that the in situ generated PI-BrVs act as active sites to promote the adsorption and activation of CO 2 molecules and H 2 O molecules.

Exploring the effects of photoinduced Br vacancies on H 2 O molecules
Reactant molecule adsorption and activation are an essential step in catalytic reaction, with the H 2 O molecule playing a critical role in photocatalytic CO 2 reduction.In general, H 2 O molecules can provide a proton source for the CO 2 hydrogenation process.To explore the adsorption and activation of H 2 O molecules on the PI-BrVs, an in situ EPR experiment in a moist Ar atmosphere (H 2 O/Ar) was carried out.As shown in Fig. 4A, after the generation of PI-BrVs under light irradiation, the EPR signal decreases with the injection of water vapor, indicating that H 2 O molecules adsorb on the PI-BrVs and interact with surface single electrons.Subsequently, the EPR signal is partially restored with the removal of H 2 O vapor in a vacuum (Fig. S7).This phenomenon demonstrates that, in a vacuum, the weakly adsorbed H 2 O molecules desorbed, while the strongly adsorbed or chemically reacted H 2 O molecules consumed electrons, leading to the part of the restoration of the EPR signal.Therefore, the H 2 O molecule could be adsorbed and activated by electron transfer on PI-BrV sites.The H 2 O molecule, as a proton donor during the reduction process of CO 2 , consumes electrons to generate the H* and OH − .Importantly, there is no significant change in the linewidth of the EPR spectrum after the injection of H 2 O molecules (Fig. 4B), which was different from the phenomenon of direct injection of CO 2 molecules, further indicating the special energy exchange between CO 2 molecules and the surface of PI-BrVs.
To study H 2 O molecules' adsorption and activation on the defect surface, in situ DRIFTS was performed in Fig. 4C and  D. Upon introducing H 2 O molecules into the reaction chamber, an obvious IR peak at 1,616 cm −1 emerged, which could be attributed to the deformation vibrations of adsorbed H 2 O molecules.In the meantime, the stretching vibrations of hydroxyl groups (3,050 to 3,700 cm −1 ) were observed and the peak strength evidently increased with the accumulation of adsorption time.These results suggest that the generation of Br vacancies greatly enhanced the adsorption of H 2 O molecules, which lays the foundation for further photocatalytic CO 2 reduction reactions.Subsequently, new peaks at 1,608 cm −1 , 1,421 cm −1 , and 1,396 cm −1 could be found when CO 2 molecules and H 2 O molecules were injected into the reaction chamber together, which could be ascribed to b-CO 3 2−, •CO 2 −, and m-CO 3 2−.More importantly, peaks at 1,635 cm −1 and 1,540 cm −1 were observed and corresponded to COOH*, an essential intermediate in CO 2 reduction to CO.After the light irradiation, the intensity of the peaks (at 1,616 cm −1 and 3,050 to 3,700 cm −1 ) associated with H 2 O molecules decreased dramatically, further evidencing the activation and consumption of H 2 O molecules, which could provide efficient proton for further photocatalytic CO 2 reduction reaction.

Investigating the impact of photoinduced Br vacancies on photocatalytic CO 2 reduction
In the above experiments, the adsorption and activation of CO 2 and H 2 O molecules on the PI-BrVs were investigated.To unravel the inherent reason for the CO 2 photoreduction properties, in situ EPR experiments in a mixture atmosphere (CO 2 /H 2 O) were further performed to explore the real reaction mechanism by simulating an actual photocatalytic environment.When the reactant molecules (CO 2 /H 2 O) were introduced into the reactor, the EPR signal reduction is attributed to the single-electron transfer from PI-BrVs to CO 2 and H 2 O molecules (Fig. 5A).The result indicates that the strong electronic interaction between Br vacancies and reactant molecules could facilitate CO 2 and H 2 O molecules' adsorption and activation.Subsequently, when the reactor is in a vacuum, some of the weakly adsorbed reactant molecules are desorbed from the defect sites, while the strongly adsorbed or chemically reactive molecules cannot desorb from the catalytic surface, resulting in the recovery of partial single-electron signals (Fig. S8).More importantly, the EPR signal line width broadened with the injection of the mixture (CO 2 and H 2 O), resulting in a shortened relaxation time.The shorter the relaxation time, the more efficient the energy transfer between spin and environmental, indicating that the reactant molecules interact more strongly with the PI-BrVs.In situ DRIFTS was used to probe reaction intermediates and elucidate the potential reaction pathways (Fig. 5D and E).A new peak at 1,419 cm −1 was observed, corresponding closely to the carboxylate (•CO 2 −) vibration frequency.Furthermore, CO 2 molecules and H 2 O (5) H 2 O + e − → H * + OH − molecules co-adsorbed on the Br vacancies, leading to the creation of the monodentate carbonate (m-CO 3 2−) group at 1,396 cm −1 and the asymmetric OCO stretches of the bidentate carbonate (b-CO 3 2−) group at 1,340 cm −1 [41].More importantly, the COOH* is regarded as one of the most crucial intermediates for converting CO 2 to CO, and its related peaks at 1,635 cm −1 and 1,540 cm −1 could be found [36,42].The most likely reduction pathway for photocatalytic CO 2 reduction is proposed as follows based on the above in situ DRIFTS and in situ EPR experiments: During the process of photoreduction of CO 2 , the BOB sample could produce 45.51 μmol g −1 of CO after 5 h of the photoreduction reaction (Fig. 5C).To assess its stability, the BOB sample was tested for an extended period with 10 h of light exposure.Figure S9 shows the linear growth of CO yields with increasing illumination time, providing strong evidence of good stability.In addition, the isotope probing method is employed to trace the source of C, where 13 CO 2 is used as the carbon feedstock (Fig. 5F).A peak at m/z = 29 was detected by mass spectroscopy, which was ascribed to the 13 CO.This is powerful evidence for CO generation through light-induced CO 2 .In addition, the absence of a 13 CH 4 peak in the isotopic labeling test is consistent with the absence of CH 4 or H 2 detected in Figs.S10 and S11.This result suggests 100% catalytic selectivity of stable photocatalytic CO 2 reduction reaction on the BOB sample.Compared with previously reported bismuth-based photocatalysts, BOB with PI-BrVs has a higher CO generation rate and product selectivity (Table S2).Overall, (6) the interaction between PI-BrVs and reactant molecules promotes the photocatalytic CO 2 reduction reaction.

Discussion
In this work, using the tetragonal BOB as a prototype, we reveal the dynamic behavior of active sites and elucidate the process of energy exchange between PI-BrVs and CO 2 molecules.In situ cw-EPR and pulsed EPR technologies reveal that the SSR time (T 2 ) of BOB is decreased by 198 ns during the photocatalytic CO 2 reduction reaction, which is also confirmed by the broadened EPR linewidth.This result demonstrates that the excited spin center of PI-BrV can transfer energy to the ground-state CO 2 molecules.The high-energy CO 2 molecules are more likely to be activated and proceed to the subsequent CO 2 reduction.In addition, theoretical calculations indicate that the bended CO 2 adsorption configuration on the surface of BOB with Br vacancies caused the decrease of the LUMO of the CO 2 molecule, which makes it easier for CO 2 molecules to acquire electrons and get activated.The activated CO 2 molecules can be converted into important COOH* intermediates, which would transform into the final product CO with a generation rate of 9.1 μmol g −1 h −1 and a selectivity of 100%.This work provides a new perspective for the activation mechanism of CO 2 molecules on photoinduced active sites.

Reagents
All chemicals used were of analytical purity and required no additional treatment.

Catalyst preparation
The BiOBr sample was prepared by a method of precipitation.Details of the experiment are shown in the Supplementary Materials.

In situ EPR test
The self-made in situ EPR reaction chamber is a quartz sample tube with an inner diameter of 8 mm.The type of EPR spectrometer is EMX nano (Bruker).Details of the experiment are shown in the Supplementary Materials.

In situ DRIFTS
In situ DRIFTS test was implemented using a TENSOR II FT-IR spectrometer (Bruker) equipped with an in situ diffuse-reflectance chamber (Harrick) and a high-temperature reaction chamber (HVC).Further experiment details are available in the Supplementary Materials.Foundation for Distinguished Scholars (2021JDJQ0006).Author contributions: Q.R. contributed to the synthesis, all the experiments, data analysis, theoretical DFT calculations, and the writing of the manuscript.Y.H. and Q.R. contributed to the synthesis and materials characterization.Y.S. and H.W. contributed to data analysis.F.D. designed and supervised the project, analyzed the data, and revised the manuscript.All authors contributed to the discussion during the whole project.Com peting interests: The authors declare that they have no competing interests.

Fig. 1 .
Fig. 1. (A) EPR spectra of BOB sample before and after light irradiation.(B) The Br − ion precipitation concentration with light exposure.(C) Raman spectra of BOB sample before and after light irradiation.(D) Br 3d of XPS spectra, (E) Bi 4f of XPS spectra, and (F) O 1s of XPS spectra.

Fig. 2 .
Fig. 2. (A) The visualization of in situ EPR spectra on BOB sample under light irradiation, in CO 2 atmosphere, and in a vacuum, respectively.(B) EPR spectra before and after CO 2 injection.(C) Highest occupied molecular orbital (HOMO) and LUMO energy of CO 2 molecules before and after adsorption.(D) Spin-spin relaxation time (T 2 ) was measured by pulse EPR under different conditions.(E) The absorption of EPR (A) and its first derivative (dA/dB) as a function of magnetic field strength (B).

Fig. 3 .
Fig. 3. (A) The energy transfer process under different atmospheres.(B and C) In situ DRIFTS tests for CO 2 interaction with BOB sample.

Fig. 4 .
Fig. 4. (A) In situ EPR visualization map of BOB sample under light irradiation, in H 2 O vapor atmosphere, and in a vacuum, respectively.(B) EPR spectra before and after the injection of H 2 O vapor.(C and D) In situ DRIFTS tests of the interaction between H 2 O molecules and BOB sample under light irradiation.
Photocatalyst (20 mg) was used in this test.The light source is a 300-W xenon lamp (PLS-SXE300UV, Beijing Perfectlight).The content of Br − was detected by an IC (Shimadzu Essentia LC-16i chromatograph).Details of the experiment are shown in the Supplementary Materials.

Fig. 5 .
Fig. 5. (A) In situ EPR visualization map of BOB sample under light irradiation, in a moist CO 2 atmosphere, and in a vacuum.(B) EPR spectra before and after moist CO 2 gas injection.(C) Photocatalytic CO 2 reduction reaction performance of BOB sample.(D and E) In situ DRIFTS tests for moist CO 2 molecules interaction with BOB sample under light irradiation.(F) The 13 CO 2 isotopic labeling experiment during photocatalytic reduction of 13 CO 2 .