Nanoparticle-enabled integration of air capture and conversion of CO₂

Abstract

Integrating air capture and conversion of CO₂ is key to realizing energy sustainability. However, current integration approaches require high temperature and pressure, making them energy intensive. Here, we demonstrate a nanoparticle (NP) catalysis approach for the hydrogenation of alkyl carbonate, an intermediate obtained from the CO₂ capture process, to formate, achieving one-pot air capture and conversion of CO₂ under ambient conditions. The capture is realized in an ethylene glycol (EG) solution of KOH (EG–KOH) at room temperature, where CO₂ is selectively converted into HO–CH₂CH₂–O–COOK (∼100% conversion). This carbonate is then hydrogenated using ammonia borane (under ambient pressure and at 50 °C) to formate (HCOOK) (>90% yield) in the presence of a stable Pd NP catalyst with EG being regenerated. Atomistic simulations suggest that the CO₂ absorption process in the EG–KOH solution is energetically stable, and the catalyst surface provides the reaction site to break the C–O bond in the –O–COOK structure, enabling the hydrogenation of the alkyl carbonate to formate and the regeneration of EG. Our study provides a promising NP-catalysis approach for air capture and conversion of CO₂ into value-added chemicals/fuels under ambient conditions.

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Introduction

Energy-efficient air capture and conversion of CO₂ is a promising solution to combat climate change and build an energy-sustainable future.^1–5 The key to the success of this approach is the energy-efficient integration of the capture/conversion processes,^6,7 a goal that has been extremely challenging to achieve. Past approaches for this integration include CO₂ capture by an amine to form a carbamate, followed by catalytic hydrogenation of the carbamate to formate,^8–11 or by an alkaline (KOH) solution to form alkyl carbonate, followed by catalytic hydrogenation to methanol.^12–17 In all these studies, the hydrogenation step is performed under high pressure and temperature conditions and the conversion is catalyzed by homogeneous Ru-pincer hydride complexes.^18–23 To realize the energy-efficient integration of CO₂ capture and conversion, a stable and recyclable catalytic system should be present, which can activate and convert the alkyl carbonate into a valuable chemical or fuel under ambient conditions.

Nanoparticles (NPs) have been studied extensively for catalytic applications. Herein, we report a Pd nanoparticle (NP)-enabled approach for air capture and conversion of CO₂ under ambient conditions, as summarized in Scheme 1. The key idea of our approach is first to capture CO₂ in an ethylene glycol (EG) solution of KOH to convert CO₂ into alkyl carbonate and then to reduce this alkyl carbonate in situ to formate via Pd NP-catalyzed transfer hydrogenation of ammonia borane (NH₃BH₃, AB) with EG being regenerated. Previous studies have shown that pure CO₂ may be hydrogenated by a metal borohydride or AB in the presence of a special pincer complex as a catalyst,^24–28 but such a hydrogenation reaction is not suitable for the desired one-step air capture and conversion process. We found that Pd NPs were active in catalyzing the AB hydrogenation of alkyl carbonate to formate under ambient pressure. The reaction led to full conversion (100%) of alkyl carbonate and high yield (92%) formation of formate in a 6 h reaction period at 50 °C. The process can be readily applied to integrate air capture and conversion of CO₂ into formate. The Pd NPs are stable in the one-pot capture/conversion process and can be reused for the next capture and conversion step. This NP-catalysis enabled one-pot approach demonstrates a potential energy-efficient integration of air capture and conversion of CO₂ into value-added chemicals/fuels.

Scheme 1 Illustration of the integrated air capture and conversion of CO₂ into formate under ambient conditions.

Methods and materials

Materials

Palladium acetylacetonate (Pd(acac)₂, 99%) and hydrogen tetrachloroaurate(III) hydrate (HAuCl₄·3H₂O, 99.9%) were purchased from Strem Chemicals. Potassium hydroxide (99%), potassium bicarbonate (99%), potassium carbonate (99%), ammonia borane (90%), oleylamine (technical grade, 70%), borane-tert-butylamine (97%), triisopropylsilane (TIPS, 99%), ethylene glycol (anhydrous, 99.8%) and 1-methyl-2-pyrrolidinone (NMP, 99.5%) were purchased from Sigma Aldrich. d₆-DMSO was used as received. Hexane (98.5%), isopropanol (100%), ethanol (100%), N,N-dimethylformamide (99.8%), and acetic acid (98%) were purchased from Fisher Scientific. The platinum wire (0.5 mm diameter, Premion, 99.997%) and Toray carbon paper (TGP-H-60) were from Alfa Aesar. Polyvinylidene fluoride (PVDF) was from MTI Corporation. Deionized water was from a Millipore Autopure system. All chemicals were used without further purification. Nitrogen (99.99%) and carbon dioxide (99.9999%) were purchased from Corp Brothers, Inc.

Experimental procedures

Pd nanoparticle (NP) synthesis

5 nm Pd NPs were synthesized by modifying a previously reported method.²⁹ 0.150 g of Pd(acac)₂ and oleylamine (15 mL) were added into a three-necked flask and then degassed with nitrogen at 60 °C for 30 min. 0.300 g of borane tert-butylamine was dispersed in 4 mL of oleylamine by vortexing and shaking. The temperature of the Pd precursor solution was lowered to and maintained at 40 °C. The solution of borane-tert-butylamine was injected into the reaction flask quickly, and within one minute the color of the solution changed to dark brown-black. The temperature was raised to 90 °C at a ramp rate of 3–5 °C min⁻¹. The solution was maintained at 90 °C for 1 h. The NPs were precipitated by adding 100 mL of ethanol and separated by centrifugation at 9000 rpm for 8 min. The NPs were then washed twice with ∼75 mL of 1 : 9 hexane : ethanol (9000 rpm for 8 min). The NPs were redispersed in hexane for storage. The subsequent catalyst loading and the activation step are presented in the ESI.†

Direct air capture of CO₂ and in situ hydrogenation reaction

5 mmol of KOH was dissolved in 10 mL of EG. A water aspirator was used to create a partial vacuum in the reaction system so that ambient air (∼413 ppm CO₂) could continuously bubble through the capture solution. The flow rate was controlled to be 0.2 L min⁻¹. The solution pH was monitored during the capture process to determine the CO₂ saturation point. After 48 hours of bubbling, the solution volume increased by about 3 mL due to moisture in the air. The capture solution was then purged with N₂ to remove gas impurities. Next, 10 mg of Pd/C catalysts and 100 mg of ammonia borane were added to the solution, which was sealed using a balloon. The reaction mixture was maintained at 50 °C for 24 hours. Both gas and liquid products were subjected to GC and NMR analysis, respectively.

The hydrogenation reaction could also be performed under air without N₂ purging, yielding a similar reaction result.

All other electrochemical reaction procedures, characterization methods and computational details are given in the ESI.†

Results and discussion

To study the activation of alkyl carbonate, we first reacted pure CO₂ with an ethylene glycol (EG) solution of KOH to form HO–CH₂CH₂–O–COOK. This alkaline chemistry leading to efficient CO₂ capture has been well-known.^7,12,13 In this study, we bubbled CO₂ from its gas cylinder into the EG solution of KOH at room temperature. In the alkaline EG solution, EG can react with KOH to form a glycoxide anion and water, quickly reaching equilibrium. Upon exposure to CO₂ at room temperature, this equilibrium is shifted to the right, as CO₂ reacts with the glycoxide to form alkyl carbonate (Fig. 1A). This CO₂ absorption process reaches its saturation point once KOH is consumed. We monitored solution pH and product formation to determine the endpoint of the capture process as well as the CO₂ capture capacity. As shown in Fig. 1B, the initial pH of the EG solution of KOH (5 mmol KOH dissolved in 10 mL EG) is around 10.7. Upon CO₂ bubbling, the pH gradually decreased and reached a plateau of 7.8 after 40 min. The CO₂ capture capacity of this KOH solution at saturation was measured using the weight change of the solution before and after capture, which was determined to be 220 mg gravimetrically. The capture efficiency can reach nearly 100%, which corresponds to 1 mol CO₂ per mol KOH (Fig. 1C). The alkyl carbonate formed in the EG solution is stable after the capture process and can be quantitatively characterized by both ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, which is consistent with the weight change (Fig. S1†). In this capture process, the amount of KOH is the limiting factor, which determines the maximum amount of CO₂ the system can capture.

Fig. 1 (A) CO₂ capture by KOH–EG solution. Capture conditions: KOH (5 mmol), EG (10 mL), r.t., and t = 40 min. (B) pH change of capture solution during CO₂ bubbling. (C) The amount of captured CO₂ and capture efficiency during CO₂ bubbling.

The alkyl carbonate could not be activated and reduced by a common hydride agent (AB or sodium borohydride) under atmospheric pressure and at temperatures ranging from room temperature to 80 °C we screened. However, when we studied NP catalysis for transfer hydrogenation of AB, we found Pd NPs to be more active in catalyzing the hydrogenation of alkyl carbonate to formate at 50 °C. We prepared 5 nm Pd NPs via solution phase reduction of Pd(acac)₂ with borane t-butylamine in oleylamine.²⁹ Fig. S2A† shows the transmission electron microscopy (TEM) image of the 5 nm Pd NPs obtained from the synthesis. The NPs were then loaded onto carbon support, followed by an acetic acid wash to remove the surfactant oleylamine as reported.³⁰ The TEM image showed that the Pd NPs are well-dispersed on the carbon support after this treatment, showing no sign of aggregation (Fig. S2B†). The Pd NPs have a typical fcc structure as characterized by X-ray diffraction (XRD) (Fig. S2C†). We studied the reduction of alkyl carbonate by AB in the capture solution in the presence of a Pd NP catalyst (Fig. 2A and ESI†). We used a balloon to seal the reaction vessel to ensure that the reaction proceeded under ambient pressure. The conversion of alkyl carbonate was monitored by ¹H NMR using dimethylformamide as an internal standard in d₆-DMSO solvent. A small amount of aliquot was taken for analysis at each hour time mark. The alkyl carbonate conversion increased with reaction time and reached 100% after 6 h of reaction (Fig. 2B). In the solution phase, formate was the only product characterized by both ¹H and ¹³C NMR (Fig. S3†). The time-dependent formate yield during the reaction is shown in Fig. 2C, reaching its maximum of 92% after 6 h with a turnover frequency (TOF) of 0.059 s⁻¹. The rest of the products contained small amounts of CO and CH₄ as detected by gas chromatography, with CO at 6% and CH₄ at 2%, which might be the result of the deep reduction of the adsorbed formate on the Pd surface, similar to what has been reported for CO₂/formate reduction to CO/CH₄ on a Pd–Al₂O₃ surface.³¹ We also detected the formation of H₂ in the reaction system, which is the result of the competing reaction, alcoholysis of AB, under the transfer hydrogenation reaction conditions. Temperature-dependent studies showed that increasing the reaction temperature to 80 °C caused the formate yield to drop, which is due to the exothermic nature of the hydrogenation reaction,³² while decreasing the temperature to 20 °C also reduced the formate yield due to the slow kinetics (Fig. 2C). When the catalyst loading was doubled to 0.2 mol%, the formate yield did not change much under the same reaction conditions. Without the Pd NP catalyst, however, the formate yield was only 12% even after 48 h of reaction (Table S1†), suggesting that pure AB is not active enough to reduce alkyl carbonate. Our further control experiments showed that alkyl carbonate is stable in the reaction solution without AB and Pd NPs, even under heating at 120 °C (Fig. S4†). Additionally, without alkyl carbonate, the reaction of AB and Pd NPs in EG only yielded H₂ that is generated from the Pd-catalyzed solvolysis of AB in EG, similar to what has been extensively reported regarding NP-catalyzed hydrolysis/alcoholysis of AB to generate H₂.^33–35 We further investigated the Pd-catalyzed hydrogenation reaction in the presence of NaBH₄ and obtained a similar formate yield. However, atmospheric H₂ was not active enough for the hydrogenation process as the formate yield was only 7% (Table S2†). This indicates that under our current reaction conditions, a hydride intermediate, not a free H₂ molecule, is an active species in the Pd NP-catalyzed hydrogenation of alkyl carbonate to formate.

Fig. 2 (A) Integrated CO₂ capture and conversion scheme. (B) Time dependent alkyl carbonate hydrogenation reaction at 50 °C showing carbonate conversion. (C) Temperature effect on the alkyl carbonate hydrogenation showing formate yield. Reaction conditions: solutions from the capture step were directly used for hydrogenation. Alkyl carbonate: 2.0 mmol, ammonia borane (AB): 1.0 mmol, Pd nanocatalyst: 0.1 mol%, and ambient pressure. Formate yields were determined by ¹H NMR, and the average value was reported based on three independent experiments.

The formation of potassium carbonate (K₂CO₃) and/or bicarbonate (KHCO₃) is inevitable when water and KOH are present in the capture solution. We further studied how the formation of K₂CO₃/KHCO₃ affects the AB transfer hydrogenation of alkyl carbonate. We found that both K₂CO₃ and KHCO₃ could be dissolved in EG and further reduced by AB in the presence of Pd NPs to formate (87% and 91% yield, respectively) (Fig. 3). This finding is significant for the successful air capture and conversion of CO₂, as it makes the hydrogenation of K₂CO₃/KHCO₃ possible without high energy input (inorganic carbonate is generally considered inactive and can only be reduced under harsh conditions^36,37). Under our reaction conditions, EG plays an important role in activating K₂CO₃/KHCO₃ by forming alkyl carbonate, as confirmed by NMR of the precursor solution containing K₂CO₃/KHCO₃ and EG (Fig. S5†). This suggests that there exists an equilibrium between carbonate and alkyl carbonate. To further support this point, we performed K₂CO₃/KHCO₃ reduction in an aqueous solution without EG under the same conditions, and we could only detect a very small amount of formate (<6% yield). Therefore, we conclude that EG reacts with carbonates to form alkyl carbonate, reaching equilibrium. Under the Pd NP-catalyzed AB hydrogenation conditions, alkyl carbonate is reduced to formate, which shifts the equilibrium to the right until carbonates are consumed. This indicates that water in the EG/KOH solution only slows down the hydrogenation process, but does not affect the final reaction outcome, which is key to continuous air capture and conversion of CO₂.

Fig. 3 Investigation of inorganic carbonate/bicarbonate hydrogenation under mild conditions. Reaction conditions: carbonate/bicarbonate (2.0 mmol), EG (10 mL), AB (1.0 mmol), Pd nanocatalyst (0.1 mol%), ambient pressure, 50 °C and 6 h.

Our model study on pure CO₂ capture and conversion in EG–KOH solution as well as the EG role in activating K₂CO₃/KHCO₃ motivated us to further explore CO₂ capture from air, where moisture is also ubiquitously present. The capture and conversion system and the related reactions are outlined in Scheme 1. A water aspirator was used as a pump to draw ambient air into the capture solution, in which ultra-dilute CO₂ (∼413 ppm) was captured by KOH in EG.⁷ The solution pH was monitored throughout the capture process, and it took 48 h for the pH to stabilize (Fig. 4A). The ¹³C NMR analysis of the capture product shows an additional peak of CO₃²⁻ (165.70 ppm) in addition to the presence of alkyl carbonate (Fig. 4B and C). The CO₃²⁻ species must be formed from reactions among water (from the air), KOH, and CO₂. Based on the quantitative ¹³C NMR analysis, the ratio between alkyl carbonate and K₂CO₃ was about 1 : 1. When the air capture solution was hydrogenated by AB at 50 °C in the presence of the Pd NP catalyst for 24 h, all carbonates were converted into formate (91% yield). Compared with the result observed from the pure CO₂ capture and conversion demonstrated above, the hydrogenation of carbonates from air capture requires a longer reaction time due to the less efficient conversion of inorganic carbonate into alkyl carbonate in the presence of water. Another highlight of our reaction system is that the Pd NP-catalyzed transfer hydrogenation is O₂ tolerant – the reaction can proceed without the need to purge the reaction system with N₂ to remove O₂. As a comparison, O₂ must be removed from the reaction system under conventional hydrogenation reaction conditions.^2,7,12 Finally, the Pd NPs showed excellent stability under the catalytic transfer hydrogenation reaction conditions and maintained a formate yield of over 90% after 5 reaction cycles (Fig. 4D) (the same amounts of AB and KOH were added in each reaction cycle). The Pd NPs retain their crystal structure (Fig. S6A†) and NP size/dispersity (Fig. S6B†) as confirmed by the XRD and TEM analyses of the NPs before and after the catalytic reactions, indicating the long-term stability of the Pd NPs as an efficient catalyst for continuous air capture and conversion.

Fig. 4 (A) Capture solution pH change in the direct air capture process. (B) ¹³C NMR spectra of CO₂ captured from lab air in d₆-DMSO. (C) Enlargement of Fig. 4B showing carbonyl peaks for alkyl carbonate and CO₃²⁻. (D) Catalytic stability of formate yields after 5 consecutive catalytic cycles. Reaction conditions: KOH (5 mmol), EG (10 mL), AB (2.0 mmol), Pd nanocatalyst (0.1 mol%), ambient pressure, and 50 °C.

To understand why Pd NPs are efficient for the AB hydrogenation of alkyl carbonate, we used density functional theory (DFT) calculations to further investigate the reaction mechanism on Pd surfaces. We suggest a 5-step mechanism, starting with the incorporation of CO₂ into a solution of KOH in EG, as shown in Fig. 1. Given that the Pd NPs are nano-sized polyhedra (∼5 nm) (Fig. S2†), which means that they consist of many step and terrace sites, both the planar (111) and the stepped (211) crystal facets of Pd were examined using DFT calculations, as shown in Fig. S9.† This comparison shows a more facile route on the (211) step edge than on the (111) terrace, and we highlight the (211) results herein.

The overall suggested reaction mechanism and the corresponding Gibbs free energies are summarized in Fig. 5, where “*” represents an adsorbed species on the Pd surface. State 1 → state 2 represents the formation of an EG–CO₂ complex under the pH 7.8 environment; we considered the pH effect in state 1 because protons are involved in the reaction for forming the EG–CO₂ complex.³⁸ As expected from the experiments in which alkyl carbonate intermediates were formed immediately when CO₂ was purged, the reaction from state 1 to state 2 proceeds spontaneously with a downhill energy path. While the reaction from state 1 to state 2 takes place without a Pd catalyst, after state 2, the reaction takes place on the surface of the Pd catalyst. Accordingly, each Gibbs energy of activation (reaction energy barrier) from state 3 was calculated to investigate the rate-determining step. T1 is an abbreviation of transition state 1, and a total of 3 transition states (T1, T2, and T3) and each Gibbs energy of activation (, respectively) were calculated by the dynamic nudged elastic band (DyNEB) method.³⁹ We modeled the process of H₂ formation by AB as a process in which H* is adsorbed to Pd from H₂ (state 2 → state 3), and this process is a thermodynamically easy reaction with a downhill energy profile. Considering the consistency in the calculation environment, we started the reaction with an H₂ gas molecule represented by a black color bar, while also showing the reaction energy for generating H₂ from NH₃BH₃ in a gray color bar to account for the use of NH₃BH₃ in the actual experiments. The theoretical Gibbs energy of activation was calculated as the free energy change between state 3 (the lowest-energy state) and transition state 2 (the highest-energy barrier), which suggests that the rate-determining step of this reaction is the hydrogenation and release of EG (HOCH₂CH₂OCOO⁻ + H* → HOCH₂CH₂OH). Considering that the experimental turnover frequency (TOF) is 0.059 s⁻¹, we can estimate that (Gibbs energy for the experimental activation) is around 0.95 eV from the Eyring equation:

where κ is the transmission coefficient, which was assumed to be 1 in this study, ΔG^‡ is the Gibbs energy of activation, k_B is Boltzmann's constant, T is the temperature, h is Planck's constant, and R is the gas constant.

Fig. 5 The schemes at the top of the graph show the reaction process, with the red, gray, white, and cyan atoms in the schemes representing O, C, H, and Pd, respectively. Bottom shows the free energy diagram for formic acid (HCOOH) formation by EG and CO₂ at 50 °C and 1 atm. The y-axis represents the Gibbs free energy (G, eV), and the Gibbs free energy of activation is shown (red color) inside the graph. Free energy diagram with error estimation (blue gradient box for each state) obtained from the BEEF ensembles. The free energy required for H₂ production from NH₃BH₃ is shown in gray for state 1 and state 2.

Although the DFT-calculated barriers were close to the experimental barrier, we wanted to understand if the deviation was reasonable. We predicted errors from the ensemble-based Bayesian error estimation functional (BEEF) (Fig. 5, see the blue and red gradient box),⁴⁰ where the calculated was in the range between 0.80 eV and 0.95 eV, as shown in Fig. 5. Given the error in any experimental measurement, we consider the experimental and calculated barriers to be indistinguishable.

After the breakage of the intermediate product and adsorption on the Pd surface (state 4), a hydrogenation reaction proceeds to produce EG and HCOOH. State 4 → state 5 represents the production of recycled EG, and state 5 → state 6 is an HCOOH production step. As a result, it was confirmed that the CO₂ absorption process in the solution of KOH in EG is energetically stable (state 1 → state 2), and the major role of the Pd catalyst is to break the C–O bond of the alkyl carbonate intermediate (state 3 → state 4), as well as to provide a reservoir of hydride for product release.

We also tested if the direct air capture system can be coupled with electrocatalysis to reduce alkyl carbonate without the use of AB. In our preliminary studies, Pd NPs were active in catalyzing the electrochemical reduction of alkyl carbonate to formate, but the reduction at the catalyst surface produced formate at too low a concentration to detect. Instead of pursuing the reduction to a liquid product, we tested the reduction to a gaseous product that can be easily separated from the reaction system and characterized. Since Au nanowires are known to be active to electro-catalyze CO₂ to CO,⁴¹ we prepared 2 nm ultra-thin Au nanowires (Fig. S7†) and studied their catalysis for the electrochemical reduction of alkyl carbonate in the capture solution (EG/H₂O v/v ∼ 5/1) (Fig. S8†). At −1.4 V (vs. Ag/AgCl), the CO faradaic efficiency reached 21% with H₂ as the side product. An important point to note is that carbonate/bicarbonate cannot be reduced under common electrochemical CO₂ reduction conditions,⁴² and some studies suggest that the electrochemical conversion of carbonates from direct air capture solution could be attributed to the acidification of the carbonates to CO₂ at the bipolar membrane interface.⁴³ We are currently working on demonstrating nanocatalysts for efficient electro-reduction of alkyl carbonate as a new approach for the integration of direct air capture and conversion of CO₂ into value-added chemicals.

Conclusions

This report demonstrates an NP-catalyzed hydrogenation of carbonates to realize the integration of air capture and conversion of CO₂ into formate. The key concept is to fast capture CO₂ from the air via a strong alkaline solution of EG–KOH, converting the CO₂ into alkyl carbonate HOCH₂CH₂O–COOK. This carbonate is stable in the capture medium and can be characterized and quantified. More importantly, the alkyl carbonate can be further activated by Pd NPs and then hydrogenated by AB (100% conversion) to formate (>90% yield) under ambient pressure and 50 °C with EG being regenerated. The formation of carbonate/bicarbonate salts from the air (moisture) capture process only affects the reaction kinetics, not the overall reaction outcome due to the established equilibrium among carbonates, EG, and alkyl carbonate. Without EG, these carbonates cannot be easily hydrogenated, demonstrating the important role of EG in promoting the hydrogenation of carbonates. DFT calculations suggest that the (211) step edge exposure on the Pd NP surface is especially active for promoting the carbonate hydrogenation reaction. The Gibbs free energy diagram shows a 5-step reaction mechanism for the CO₂ capture and conversion process, and the rate-determining step is the hydrogenation and release of EG. After incorporating the solvation effect, the calculated theoretical activation barriers match well with the experimental ones. Our studies provide a promising approach for the continuous air capture of CO₂ and subsequent formate synthesis. Our preliminary tests also show that it is possible to use an NP catalyst to activate the alkyl carbonate electrochemically. We are working to develop a robust electrocatalyst system to selectively convert this alkyl carbonate into value-added chemicals/fuels and to realize energy-efficient integration of direct air capture and conversion of CO₂ for green and sustainable energy applications.

Data availability

The data supporting this article are available within the main article as well as its ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the NSF under Grants CHE-2102290 and CBET-2324345 and by Brown University's Office of Vice President for Research. Electronic structure calculations were carried out at the Brown University Center for Computation and Visualization (CCV).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr02925a

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