Modulating the electronic interaction between Au and nitrogen-rich porous organic polymers for enhanced CO₂ hydrogenation to formic acid

Abstract

The regulation of the electronic state of catalytic sites is essential to improve the intrinsic activity of catalysts. Herein, we modulated the metal state of Au species by varying their particle size on nitrogen-rich triazine-based porous organic polymer supports. Due to the different interface percentages between Au and nitrogen species in selected supports, the electronic state of the metal can be modulated. The catalyst with the smallest Au particle size presented the most negative metallic state and the highest surface energy, thus exposing more sites for H₂ activation and providing sufficient reactive H species to CO₂ hydrogenation absorbed on the adjacent N. The designed Au/Trz-TETA (1.23 nm) exhibited high catalytic activity for the CO₂ hydrogenation to formic acid and a turnover number (TON) up to 1687 over 10 h, which is higher than that of Au/Trz-DETA (2.24 nm) and Au/Trz-TEPA (1.96 nm) with a bigger metal particle size. This work shows a size-dependent CO₂ hydrogenation for various sizes of Au metal catalysts and provides a new way for regulating the metal electronic state.

---

Introduction

The drastically increased CO₂ emission attributed to the combustion of fossil fuels worldwide impacts the Earth's climatic conditions in an unprecedented fashion, leading to a realization that effective measures need to be taken to reduce CO₂ emissions.^1–4 In addition to CO₂ capture and storage, catalytic reduction of CO₂ into usable fuels and value-added chemicals has emerged as a crucial pathway to address the greenhouse effect and energy crisis. CO₂ catalysis generating formic acid is highly attractive because formic acid is a green energy carrier with high density, low toxicity and high hydrogen content.^5–8 Great efforts have been devoted in recent years to CO₂ to formic acid catalytic conversion. However, due to the high chemical inertness of C=O and the difficulty of H₂ activation, the conversion efficiency of CO₂ cannot meet the industrial application requirement.^9–11 Therefore, it is of great significance to develop novel catalysts and strategies to improve the catalytic efficiency of CO₂ reduction to formic acid.

As for H₂ activation, supported noble-metal catalysts such as Pd,^12–14 Ru,^15,16 Ir,^17–19 and Au (ref. 20 and 21) have been used extensively in hydrogenation catalysis due to their high efficiency and selectivity. In most cases, only the atoms on the surface of the nanoparticles (NPs) act as the active sites, while those inside the nanoparticles are spectators, which leads to a waste of the noble metal.²² Hence, regulation of metal electronic states to possess high surface energy and synthesis of highly dispersed noble metals will be effective ways to improve the intrinsic activity and their atomic efficiency. The maximum limit of the reduction of the size of metal particles will increase the metal–support interface percentages, thus maximizing the utilization of noble metals and lowering the cost of the catalysts.²³ However, when the particle size decreases, the surface energy will increase correspondingly, which leads to aggregation of the highly dispersed metal atoms. Due to thermodynamic instability and susceptibility to agglomeration, this phenomenon is particularly obvious for Au. Therefore, most of the typical supports have good catalytic activity with other noble metals such as Pd,^24,25 but the results are not satisfactory when loaded with Au.

To improve the catalytic activity of supported noble-metal catalysts, it is necessary to consider the active species MNP and the support materials.²⁶ Selecting suitable support materials and optimizing the pore size and surface properties of the support can improve the dispersion and utilization of noble metals, thus improving the cost-effectiveness of the catalysts. Given the high surface area and attractive chemical functionalization of porous organic polymers (POPs), it can be an excellent platform for anchoring and dispersing metals.^27,28 MNP can be effectively stabilized on the polymer surface and resistant to aggregate.²⁹ In particular, when organic amine functional groups are introduced into POPs, the abundant N atoms can be polydentate coordinated with metals, thus effectively solving the problem of poor metal dispersion.³⁰ In addition, N atoms with strong electron-donating properties not only adjust the electronic state of the active metal components^31,32 but also facilitate CO₂ enrichment and activation,^33–35 which is conducive to the follow-up of the catalytic reaction.

In this work, we achieved the modulation of the metal electronic state based on changing interface percentages between Au and nitrogen-rich triazine-based porous organic polymer supports. Specifically, three organic bases with different N contents were chosen for the synthesis of POPs to further achieve uniform dispersion of different metal sizes. The down-sized Au particles in Trz-DETA, Trz-TEPA, and Trz-TETA provided a model for us to study the size dependent CO₂ hydrogenation, and the smallest Au particle size presented the most negative metallic state and the highest surface energy, thus effectively promoting the adsorption and activation of CO₂ and the heterolytic dissociation of H₂.

Experimental

Materials

Cyanuric chloride (99%), diethylenetriamine (DETA) (99%), triethylenetetramine (TETA) (70%), tetraethylenepentamine (TEPA) (95%), KHCO₃ (99.5%) and NaHCO₃ (99.5%) were purchased from Macklin Reagent Company. NaBH₄ (98%), PdCl₂ (Pd 59–60%), RuCl₃·xH₂O (Ru ≥ 37%) and D₂O (99.9 atom% D) were purchased from Aladdin Corporation. Ethanol (≥99.7%) was obtained from Tianjin Fuyu Fine Chemical Co., Ltd. Methanol (99.5%), tetrahydrofuran (THF, 99.5%), triethylamine (TEA, 99%), dimethyl sulfoxide (DMSO, 99.5%) and 1,4-dioxane (99.5%) were purchased form Tianjin Kermel Chemical Reagent Development Center, China. HAuCl₄ (Au ≥ 47.8%) was purchased from Sinopharm Chemical Reagent Co., Ltd. The deionized water was obtained from a Milli-Q system (Millipore). All reagents were used without further purification.

Synthesis of the triazine-based N-rich porous organic polymers

The POPs were fabricated by reported methods with some modifications.³⁶ In order to synthesize the polymer, first 8 mmol of cyanuric chloride was dissolved in 100 mL of 1,4-dioxane (solution 1) in a 250 mL round-bottomed flask. Then 64 mmol of triethylenetetramine (TETA) was added to 50 mL of 1,4-dioxane (solution 2). Under vigorous stirring, solution 2 was added dropwise to solution 1. The resulting mixture was stirred at ambient temperature for 1 h, followed by refluxing at 80 °C for 5 h. The mixture was cooled at room temperature and a white precipitate was appeared. Finally, the Trz-TETA material was obtained after the precipitate was washed several times with deionized water, 1,4-dioxane and ethanol at pH = 7 and dried at 100 °C for 10 h. The Trz-DETA and Trz-TEPA products were synthesized by a similar method as that of Trz-TETA, and were obtained using the polymerization of cyanuric chloride with diethylenetriamine (DETA) and tetraethylenepentamine (TEPA), respectively.

Synthesis of Au/Trz-DETA, Au/Trz-TETA and Au/Trz-TEPA

The Au/Trz-TETA catalyst was prepared with a NaBH₄ wet chemical reduction method. Typically, 0.3 g of Trz-TETA was dispersed in 100 mL of ethanol and sonicated for 15 min. Then a solution of aqueous HAuCl₄ (0.9 g/50 mL, 0.35 mL) in 100 mL of ethanol was added rapidly to the above mixture at 80 °C, followed by stirring for 30 min. A 14-fold excess of NaBH₄ was added and the mixture was stirred continuously for 2 h at 80 °C. After the reaction, the product was collected by filtration and washed several times with water and ethanol. Finally, Au/Trz-TETA was obtained by vacuum drying at 60 °C. Au/Trz-DETA and Au/Trz-TEPA catalysts were also prepared using the same procedure.

General characterization

Powder X-ray diffraction (PXRD) was conducted on a Smartlab 9 kW (Rigaku Corporation) diffractometer at 240 kV and 50 mA Cu-Kα radiation with the scanning range was 5° to 80°. Thermogravimetric analysis (TGA) was performed using an STA 449F5 with a temperature range from 25 °C to 800 °C in a nitrogen atmosphere. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed using an Avio 220. Nitrogen adsorption–desorption isotherm and pore size distribution were taken using a BSD-PM1. Samples were treated at 150 °C for 4 h. X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ESCALAB250Xi (Thermo, USA) with Al Kα (hv = 1486.6 eV) as the exciting source. The binding energies of N 1s and Au 4f can be corrected with C 1s (284.8 eV). Transmission electron microscopy (TEM) images were performed with a JEM-F200 microscope at 200 KV and an energy-dispersive X-ray spectrometer (EDX) attached to a FESEM instrument was used to analyze the compositions of the samples. Fourier transform infrared (FT-IR) spectroscopy and in situ Fourier transform infrared (in situ FT-IR) spectroscopy were conducted with a Nicolet iS10 IR spectrometer equipped with an MCT/A detector. After the sample was placed in a vacuum at 100 °C for 40 min and cooled to room temperature, the background spectra were recorded at a resolution of 4 cm⁻¹, and then automatically subtracted from the subsequent spectra. Then the sample was exposed to CO₂ gas for 30 min, after which the CO₂ was desorbed. Finally, the sample was exposed to H₂ gas and heated to 100 °C, recording the spectra at intervals. ¹H NMR spectra were determined with a NEO 400 MHz NMR spectrometer.

General procedure for CO₂ hydrogenation to formic acid

The CO₂ hydrogenation to formic acid reaction was carried out using a 30 mL stainless steel autoclave reactor. In a typical experiment, 10 mg of catalyst was added to aqueous triethylamine solution, after which the reactor was sealed and the air was replaced three times with pure CO₂. The reactor was pressurized with CO₂ to 2.0 MPa and then pressurized with H₂ to 5 MPa, and subsequently heated to 100 °C and stirred for 10 h. When the reaction was finished, the reactor was cooled to room temperature and the pressure was slowly released. The catalyst was separated from the reaction solution by centrifugation. Formic acid concentrations were measured via the internal standard method of ¹H NMR, using DMSO as the internal standard and D₂O as the solvent.

Results and discussion

Synthesis and characterization of the catalysts

To adjust the particle size of Au, three organic bases DETA, TETA, and TEPA with different N contents were selected to synthesize POPs and coordinate with metals. The typical synthetic scheme was illustrated in Fig. 1a; three secondary amine-rich triazine-based porous organic polymer supports were synthesized by nucleophilic substitution reaction of cyanuric chloride with organic bases. Then, a simple impregnation method was used to deposit Au on the supports, and the composites were obtained by NaBH₄ reduced. The composites were named Au/Trz-TETA, Au/Trz-TEPA, and Au/Trz-DETA, respectively. The metal loadings were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis, and the results were similar to the theoretical value at ca.1 wt% (Table S1†).

Fig. 1 (a) Synthetic route of Au/Trz-DETA, Au/Trz-TETA and Au/Trz-TEPA. (b) FT-IR spectra and (d) TGA curves of Trz-DETA, Trz-TETA and Trz-TEPA. (c) XRD patterns of Au/Trz-DETA, Au/Trz-TETA and Au/Trz-TEPA.

The successful synthesis of the porous organic polymers was confirmed by the Fourier transform infrared spectroscopy (FT-IR) results. In the FT-IR spectrum of POPs (Fig. 1b), the peak at 3360 cm⁻¹ corresponded to the N–H stretching vibration. Notably, the vibrational bands at 2939, 2832, and 1424 cm⁻¹ were associated with the stretching and bending vibrations of the aliphatic C–H. Focusing on the region 1600–800 cm⁻¹, the peaks at 1540 and 1507 cm⁻¹ belonged to the C=N of the triazine ring, which appeared as double peaks due to the presence of tautomeric forms (Fig. S1†). The vibrational band ascribed to the out-of-plane vibration of the triazine ring was also clearly observed at 810 cm⁻¹.³⁷ Besides, the C–N stretching vibration that was not inside the ring can be seen at 1130 cm⁻.¹ However, the absence of a peak related to the C–Cl stretching at 850 cm⁻¹ was accordant with the fact that the chlorine of cyanuric chloride was completely substituted by the amino groups of the aminating agents.^38,39 The PXRD patterns (Fig. 1c) showed that the three samples had a broad peak spanning a 2θ range of ca. 21°, confirming that all the POP-based catalysts were amorphous. It is worth noting that due to the low content and high dispersity, there were no detectable peaks corresponding to Au nanoparticles. Thermogravimetric analysis (TGA) results (Fig. 1d) demonstrated the similar thermal stability of the three samples in a nitrogen atmosphere. The first weight loss before 100 °C was related to the loss of water from the materials. The second and third weight loss at around 400 and 500 °C were associated with the degradation of the amino groups and the decomposition of the triazine rings, respectively. Since the CO₂ hydrogenation reaction was carried out at 100 °C, the composites were able to maintain the structure for our subsequent catalyst-performance relationships analysis.

To reveal the morphological and structural features of the POP-based catalysts, transmission electron microscopy (TEM) was performed. It can be seen from Fig. 2a–c that all the catalysts exhibited porous morphology, which was favorable to the exposure of active sites. To measure the size of Au nanoparticles (NPs), aberration-corrected high-angle annular dark-field STEM (HAADF-STEM) was conducted (Fig. 2d–f). The Au NPs were uniformly distributed on the surface and in the pores of the POPs, with an average particle size of 1.23 nm, 1.96 nm, and 2.24 nm for Trz-TETA, Trz-TEPA and Trz-DETA respectively. It is noteworthy that the particle size of Au on Trz-TEPA was mainly concentrated at 1–2 nm, but a few Au particles larger than 4 nm appeared. This is because TEPA contains more N atoms compared with TETA and DETA; a large number of amine groups contained in the Trz-TEPA support have inherent weak reducibility. In the process of preparing Au/Trz-TEPA, the reducing property of Trz-TEPA can reduce some of the Au precursors to obtain the larger-sized Au particles before the addition of NaBH₄.⁴⁰ The EDS mapping image (Fig. 2g, S2†) clearly shows the presence of C, N, and Au atoms, further confirming the existence and uniform distribution of Au NPs. Based on the above results, we can conclude that both components and structural parameters in all composites were fixed to be similar, with the particle size of Au NPs being the only difference. Obviously, because of the downsize of Au NPs, the interface percentage between Au and the support will increase, thus modulating the electronic state of the metal by metal–support interactions, which may significantly improve catalytic performance.⁴¹

Fig. 2 TEM images of (a) Au/Trz-TETA, (b) Au/Trz-TEPA and (c) Au/Trz-DETA. HAADF-STEM images of (d) Au/Trz-TETA, (e) Au/Trz-TEPA and (f) Au/Trz-DETA. (g) EDX image of Au/Trz-TETA with the corresponding elemental mapping images of C, N and Au.

Even if Au was loaded in the POPs, the porous property of Trz-TETA was still retained, as revealed by nitrogen sorption isotherms. As shown in Fig. 3a, all sorption isotherm materials can be categorized as type I and IV according to the IUPAC,⁴² suggesting the existence of microporous and mesoporous structures, which is essential for CO₂ adsorption and activation. Notably, there was no significant change in the specific surface area and pore size between Au/Trz-TETA and Trz-TETA (Fig. 3a, Table S2†), indicating that Au NPs were uniformly dispersed in the catalyst support without blocking the pores, which was in agreement with the observation of TEM. However, the larger pore size distribution (Fig. 3b, Table S2†) observed for Trz-TEPA compared to that for Trz-TETA and Trz-DETA may also account for the bigger particle size of Au NPs. Besides, the appropriate specific surface area and pore size of the catalyst support can increase the loading points of the active components and expose more active sites to increase the reactivity, but too large pore sizes may lead to the inability of reactant molecules to remain on the inner surface of the catalyst for a long time.

Fig. 3 (a) N₂ adsorption desorption isotherms and (b) pore size distribution of Trz-DETA, Trz-TETA, Trz-TEPA and Au/Trz-TETA. (c and d) XPS spectra of N 1p and Au 4f of Au/Trz-DETA, Au/Trz-TETA and Au/Trz-TEPA.

To gain the electronic state of Au NPs and to understand the metal–support interactions, X-ray photoelectron spectroscopy (XPS) analysis was performed (Fig. S3b†). In the N 1s spectra (Fig. 3c), in addition to the two peaks at 399 eV (–NH–) and 397.8 eV (C=N of the triazine ring), the weak characteristic peak of the π–π* satellite at 404.2 eV suggested the formation of nitrogen-containing aromatic polymers, which may provide delocalized electrons for metal.⁴³ As expected, the values of the binding energy for the Au 4f line were found to decrease to different degrees compared to metallic Au (87.7 eV (4f_5/2) and 84.1 eV (4f_7/2)),^44,45 ascribing to the abundant N species on the POP surface (Fig. 3d). Among them, Au/Trz-TETA exhibited the most negative metallic state, because with decreasing Au particle size, the number of Au coordination atoms decreases significantly, and Au atoms with higher interfacial free energy can have stronger electronic interactions with amine groups and thus show stronger electronegativity. This electron-rich active center was beneficial for the heterolytic dissociation of H₂ and provides sufficient active H species for subsequent intermediate hydrogenation.

Catalytic CO₂ hydrogenation to formic acid performance

Given the property of CO₂ absorption and H₂ dissociation of catalysts, the catalytic performance was investigated for the preparation of formic acid by the CO₂ hydrogenation reaction. To explore the intrinsic activity of the metal and to circumvent the effect of particle size, we screened for optimal active sites by loading different metals on Trz-DETA (Fig. S5a†). The catalytic activity was conducted in triethylamine aqueous solution under a total pressure of 5.0 MPa (CO₂ : H₂ = 2 : 3) at 100 °C for 10 h. Compared with other metals, the POPs with Au NPs exhibited the highest catalytic activity and the TON achieved can be up to 502, and the catalytic activity of the support without metals loaded was negligible. Afterward, different Au loading amounts were also studied in Au/Trz-DETA. As shown in Fig. S5b,† the TON value of the CO₂ hydrogenation reached a maximum of 736 at 1.0 wt% Au, but decreased with further increase in metal content due to the Au agglomeration. The characteristic peak belonging to Au (111) that can be seen at 2θ = 38° in the PXRD pattern of Au/Trz-DETA at 2.0 wt% metal loading confirms the above conjecture (Fig. S3a†). Then, to explore the effect of the electronic state on CO₂ hydrogenation, the catalytic activity of different supports with 1.0 wt% Au was investigated. As revealed in Fig. 4a, the TON of Au/Trz-DETA and Au/Trz-TEPA with Au sizes of 2.24 nm and 1.96 nm was 736 and 978 respectively. Significantly, Au/Trz-TETA exhibited the highest catalytic activity (TON = 1687), which is 2.29 and 1.72 times as high as that of Au/Trz-DETA and Au/Trz-TEPA with a bigger metal particle size. Formic acid was obtained with >99% selectivity, and other products were not observed (Fig. S4†). Considering that the gap between the specific surface areas was much smaller than the gap between their activities, we can determine that the performance disparity stemmed from the intrinsic catalytic ability of different catalytic sites, in the sense that the high hydrogenation activity of Au/Trz-TETA was attributed to the more negative electronic state of Au. Moreover, the trend of the performance in CO₂ hydrogenation increases as the electron density in Au increases (observed in XPS), as well as the opposite tendency for the particle size (observed in TEM) confirm above results. Then, we performed cycling stability experiments on Au/Trz-TETA for further testing. From Fig. S6a,† we can see that the stability of Au/Trz-TETA was poor and the activity no longer decreased after four cycles, which could be attributed to the sintering of the Au nanoparticles. The growth of Au NPs from 1.23 nm to 3.94 nm can be seen in the HAADF-STEM image after Au/Trz-TETA cycling (Fig. S6b†), which is consistent with the observation in the above relationship between the performance and Au particle size. In conclusion, the smallest size and the most negative metallic state of Au in Au/Trz-TETA was beneficial for CO₂ reduction, which may be achieved by promoting H₂ heterolytic dissociation, the decisive step of the whole hydrogenation reaction (discussed later).

Fig. 4 (a) Catalytic activity of different catalysts. (b) Reactivity comparison of various solvents on Au/Trz-TETA. Reaction conditions: 10 mg of catalyst (1.0 wt% Au), 1 mL of triethylamine, 5 mL of H₂O, 2 MPa CO₂, 3 MPa H₂, 100 °C, 10 h. (c) Effect of partial pressure on CO₂ hydrogenation. (d) Arrhenius plot and (e) reaction orders of H₂ and CO₂ for Au/Trz-TETA. (f) Reactivity comparison of different carbon sources.

To further optimize the reaction conditions, control experiments were conducted. It has been demonstrated that the protonated solvent can facilitate the CO₂ reduction to formic acid. Solvents act as H-transfer reagents and participate in the reaction by transferring H radicals to CO₂, thus achieving the hydrogenation process.⁴⁶ Based on this (Fig. 4b), we investigated the effect of solvent polarity on the catalytic performance of Au/Trz-TETA. The results showed that with the increase of solvent polarity, the catalytic activity of the catalyst in THF, ethanol, methanol, and water increased successively, and similar patterns were also observed for Au/Trz-DETA and Au/Trz-TEPA (Fig. S7†). The higher the polarity of the solvent, the higher the yield of CO₂ hydrogenation to formic acid or formate.²⁰ This is due to the fact that the reaction intermediates generated by the interaction of CO₂ with nitrogen species on the catalyst surface have high polarity, so the polar solvent has a good stabilizing effect on these intermediate species.⁴⁷ However, the non-protonic solvent DMSO showed poor CO₂ hydrogenation activity despite its high polarity, which emphasizes the importance of the H-transfer solvent in our system. In addition, the vital role of H₂ pressure was also suggested by the observation of a decreased trend in formic acid yield as the H₂ partial pressure decreases (Fig. 4c).

Thermodynamic and kinetic studies were carried out to gain further insight into the catalytic performance of the catalyst. Based on Arrhenius plots as depicted in Fig. 4d, the apparent activation energy (E_a) of Au/Trz-TETA in CO₂ hydrogenation to formic acid was determined and the E_a value was 26.8 kJ mol⁻¹. Furthermore, the reaction kinetics of Au/Trz-TETA was also illustrated. The reaction rate (v) was expressed by v = k PH₂^m PCO₂ⁿ, where m and n were the reaction orders of PCO₂ and PH₂, respectively. It can be seen from Fig. 4e that the reaction orders of H₂ and CO₂ were 0.92 and 0.44, respectively, which indicates that H₂ is involved in the reaction and is the rate-controlling step of the reaction.

Possible reaction mechanism for the CO₂ hydrogenation

To further understand the reaction mechanism, we compared the catalytic performance of Au/Trz-DETA using KHCO₃ and NaHCO₃ as carbon sources, respectively. In Fig. 4f, the catalytic ability of the catalyst for hydrogenation of bicarbonate species was very weak, but the catalytic performance was greatly improved when 2 MPa CO₂ was added. Therefore, the bicarbonates in the system only act as base additives to promote the formation of formic acid. This fully indicated that Au/Trz-TETA catalyzed the direct hydrogenation of CO₂ to formic acid, which was different from the bicarbonate hydrogenation mechanism in conventional catalytic systems.⁴⁸ Then, the specific reaction mechanism of CO₂ hydrogenation to formic acid on Au/Trz-TETA was investigated by in situ Fourier-transform infrared (in situ FT-IR) experiments under the reaction conditions (Fig. 5a). Upon the introduction of CO₂ and H₂ on Au/Trz-TETA, the characteristic peaks of NH₂⁺ species (1633 and 1504 cm⁻¹),⁴⁹ COO⁻ species (symmetric stretching vibration peak at 1426 cm⁻¹ and asymmetric stretching vibration peak at 1565 cm⁻¹),^50,51 carbamate species (1666 cm⁻¹)⁵² and formate species (1393 and 1605 cm⁻¹)^53,54 can be observed. With increasing reaction time, the peak intensity of COO⁻ and carbamate species decreased continuously, while that of formic acid increased, proving that the hydrogenation reaction of CO₂ on the catalyst followed the path of carbamate hydrogenation.

Fig. 5 (a) In situ FT-IR spectra of Au/Trz-TETA. (b) Possible mechanism for the CO₂ hydrogenation to formic acid over Au/Trz-TETA.

Based on the above results, we propose a possible reaction mechanism of CO₂ hydrogenation on Au/Trz-TETA. As depicted in Fig. 5b, H₂ is activated and dissociated on Au NPs, resulting in the generation of reactive Au hydride species. At the same time, CO₂ is adsorbed and activated by the amine sites of Trz-TETA in the form of carbamate zwitterion species (RR′N⁺H⋯COO⁻) and concentrated near Au NPs. In this alkaline environment, the H atom of the unstable intermediate is immediately transferred to hydroxyl or neighboring amine groups to form carbamate species (RR'NCOO⁻ N⁺H₂RR′) (Fig. S8†). Then, the Au hydride species perform a nucleophilic attack to the C atom of the carbamate intermediate to yield a formate intermediate. In particular, due to the strong electron-donating ability of the abundant N atoms on Trz-TETA, the electron-rich Au NPs offer more negative hydride species, which is conducive to the nucleophilic attack to the C atom. Finally, formic acid is formed by the acid–base neutralization reaction, and the original Au/Trz-TETA catalyst is regenerated for the next catalytic cycle reaction.

Conclusions

In conclusion, we achieved the regulation of the metal electronic state by selecting different N contents of organic bases in the synthesized POPs to disperse Au in different sizes. The catalyst with the smallest Au particle size exhibited the highest performance of CO₂ hydrogenation due to the most negative metallic state and the highest surface energy. The abundant amine functional groups on the POPs not only facilitated the Au NPs to obtain more electrons for H₂ activation but also promoted the adsorption and activation of CO₂ in a near position of reactive H species, thus favoring efficient CO₂ hydrogenation. This work provides a simple and effective strategy for the modulation of the metal electronic state and offers insight into the construction of supported Au catalysts in the field of CO₂ hydrogenation catalysis.

Data availability

All relevant data are within the manuscript and its additional files.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21978032, 21676045, 22372023, and 22178040) and the Natural Science Foundation of Liaoning Province (2022-MS-142). The authors acknowledge the assistance of DUT Instrumental Analysis Center.

Notes and references

1. R. Fouquet and R. Hippe, Energy Res. Soc. Sci., 2022, 91, 102736.
2. S. Li, J. Zhang, H. Lin, Y. Ding, Y. Bai, Y. Zhou, B. Zhu and Z. Dai, Coal Sci. Technol., 2024, 52, 138–153.
3. Z. Zhang, T. Wang, M. J. Blunt, E. J. Anthony, A.-H. A. Park, R. W. Hughes, P. A. Webley and J. Yan, Appl. Energy, 2020, 278, 394–427.
4. S. Kar, M. Rahaman, V. Andrei, S. Bhattacharjee, S. Roy and E. Reisner, Joule, 2023, 7, 1496–1514.
5. K. Park, K. R. Lee, S. Ahn, C. Van Nguyen and K.-D. Jung, Appl. Catal., B, 2024, 346, 123751.
6. R. Sun, Y. Liao, S.-T. Bai, M. Zheng, C. Zhou, T. Zhang and B. F. Sels, Energy Environ. Sci., 2021, 14, 1247–1285.
7. H. Chai, C. Zhou, S. Li, R. Zhang, J. Yuan, J. Hu, Z. Liu, A. Duan, C. Xu and X. Wang, Fuel, 2024, 371, 131908.
8. Z. D. Feng, C. Z. Tang, P. F. Zhang, K. Li, G. N. Li, J. J. Wang, Z. C. Feng and C. Li, J. Am. Chem. Soc., 2023, 145, 12663–12672.
9. X. San, X. Li, L. Zhang, D. Meng, X. Chang and J. Qi, Process Saf. Environ. Prot., 2024, 187, 320–331.
10. W. Lyu, Y. Liu, J. Zhou, D. Chen, X. Zhao, R. Fang, F. Wang and Y. Li, Angew. Chem., Int. Ed., 2023, 62, e202310733.
11. H. Zhou, H. Zhang, S. Mu, W.-Z. Zhang, W.-M. Ren and X.-B. Lu, Green Chem., 2019, 21, 6335–6341.
12. P. Verma, S. Zhang, S. Song, K. Mori, Y. Kuwahara, M. Wen, H. Yamashita and T. An, J. CO2 Util., 2021, 54, 101765.
13. G. Ren, J. Sun, S. Zhai, L. Yang, T. Yu, L. Sun and W. Deng, Cell Rep. Phys. Sci., 2022, 3, 100705.
14. Z. Wang, D. Ren, Y. He, M. Hong, Y. Bai, A. Jia, X. Liu, C. Tang, P. Gong, X. Liu, W. Huang and Z. Zhang, ACS Catal., 2023, 13, 10056–10064.
15. J. Zhao, J. Wang, Y. Bai, H. Du, J. Yang, B. Yin, B. Jiang and H. Li, J. CO2 Util., 2023, 74, 102528.
16. K. R. Lee, A. Masudi, K. Park, S. Ahn, J. S. Lee, S. J. Sim and K.-D. Jung, Chem. Eng. J., 2024, 494, 152922.
17. X. Shao, X. Yang, J. Xu, S. Liu, S. Miao, X. Liu, X. Su, H. Duan, Y. Huang and T. Zhang, Chem, 2019, 5, 693–705.
18. R. Kanega, M. Z. Ertem, N. Onishi, D. J. Szalda, E. Fujita and Y. Himeda, Organometallics, 2020, 39, 1519–1531.
19. B. D. Bankar, K. Ravi, R. J. Tayade and A. V. Biradar, J. CO2 Util., 2023, 67, 102315.
20. G. A. Filonenko, W. L. Vrijburg, E. J. M. Hensen and E. A. Pidko, J. Catal., 2016, 343, 97–105.
21. Q. Liu, X. Yang, L. Li, S. Miao, Y. Li, Y. Li, X. Wang, Y. Huang and T. Zhang, Nat. Commun., 2017, 8, 1407.
22. K. Murata, E. Eleeda, J. Ohyama, Y. Yamamoto, S. Arai and A. Satsuma, Phys. Chem. Chem. Phys., 2019, 21, 18128–18137.
23. X. Sui, L. Zhang, J. Li, K. Doyle-Davis, R. Li, Z. Wang and X. Sun, Adv. Energy Mater., 2022, 12, 2102556.
24. K. Mori, H. Hata and H. Yamashita, Appl. Catal., B, 2023, 320, 122022.
25. S. Ali, R. Iqbal, A. Khan, S. U. Rehman, M. Haneef and L. Yin, ACS Appl. Nano Mater., 2021, 4, 6893–6902.
26. T. Parandhaman, M. D. Dey and S. K. Das, Green Chem., 2019, 21, 5469–5500.
27. K. S. Song, P. W. Fritz and A. Coskun, Chem. Soc. Rev., 2022, 51, 9831–9852.
28. B. Dziejarski, J. Serafin, K. Andersson and R. Krzyzynska, Mater. Today Sustain., 2023, 24, 100483.
29. S. Aggarwal and S. K. Awasthi, Dalton Trans., 2024, 53, 11601–11643.
30. D. Yadav, Subodh and S. K. Awasthi, Mater. Chem. Front., 2022, 6, 1574–1605.
31. B. Panic, T. Frey, M. Borovina, K. Konopka, M. Sambolec, I. Kodrin and I. Biljan, Polymers, 2023, 15, 229.
32. N. Gemo, F. Menegazzo, P. Biasi, A. Sarkar, A. Samikannu, D. G. Raut, K. Kordas, A.-R. Rautio, M. Mohl, D. Bostroem, A. Shchukarev and J.-P. Mikkola, RSC Adv., 2016, 6, 103311–103319.
33. M. Zheng, J. Yang, W. Fan and X. Zhao, Phys. Chem. Chem. Phys., 2021, 23, 24801–24813.
34. Y. Li, T. Ding, X. Xu, S. Chakir, Y. Ding, S. Zhu, J. Mei, H. Wang and X. Wang, J. Environ. Chem. Eng., 2024, 12, 113197.
35. T. Guo, Y. Zhang, Y. Geng, J. Chen, Z. Zhu, A. H. Bedane and Y. Du, Ind. Crops Prod., 2023, 206, 117582.
36. M. Niksefat, J. Rahimi, A. Maleki and A. S. Nia, Algal Res., 2023, 70, 103003.
37. M. J. Bojdys, J. Jeromenok, A. Thomas and M. Antonietti, Adv. Mater., 2010, 22, 2202–2205.
38. P. Puthiaraj and K. Pitchumani, Green Chem., 2014, 16, 4223–4233.
39. M. Sayin, M. Can, M. Imamoglu and M. Arslan, React. Funct. Polym., 2015, 88, 31–38.
40. Q. Liu, X. Wang, Y. Ren, X. Yang, Z. Wu, X. Liu, L. Li, S. Miao, Y. Su, Y. Li, C. Liang and Y. Huang, Chin. J. Chem., 2018, 36, 329–332.
41. W. Zhan, Y. Shu, Y. Sheng, H. Zhu, Y. Guo, L. Wang, Y. Guo, J. Zhang, G. Lu and S. Dai, Angew. Chem., Int. Ed., 2017, 56, 4494–4498.
42. M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol and K. S. W. Sing, Pure Appl. Chem., 2015, 87, 1051–1069.
43. J. A. Gardella, S. A. Ferguson and R. L. Chin, Appl. Spectrosc., 1986, 40, 224–232.
44. C. J. Powell, N. E. Erickson and T. Jach, J. Vac. Sci. Technol., 1982, 20, 625–625.
45. T. D. Thomas and P. Weightman, Phys. Rev. B: Condens. Matter Mater. Phys., 1986, 33, 5406–5413.
46. Z. Wang, Y. Kang, J. Hu, Q. Ji, Z. Lu, G. Xu, Y. Qi, M. Zhang, W. Zhang, R. Huang, L. Yu, Z.-q. Tian and D. Deng, Angew. Chem., Int. Ed., 2023, 62, e202307086.
47. W.-H. Wang, J. T. Muckerman, E. Fujita and Y. Himeda, ACS Catal., 2013, 3, 856–860.
48. K. Mori, T. Sano, H. Kobayashi and H. Yamashita, J. Am. Chem. Soc., 2018, 140, 8902–8909.
49. F. Baitalow, J. Baumann, G. Wolf, K. Jaenicke-Rößler and G. Leitner, Thermochim. Acta, 2002, 391, 159–168.
50. G. S. Foo, J. J. Lee, C.-H. Chen, S. E. Hayes, C. Sievers and C. W. Jones, ChemSusChem, 2017, 10, 266–276.
51. M. W. Hahn, J. Jelic, E. Berger, K. Reuter, A. Jentys and J. A. Lercher, J. Phys. Chem. B, 2016, 120, 1988–1995.
52. Y. Kuwahara, Y. Fujie, T. Mihogi and H. Yamashita, ACS Catal., 2020, 10, 6356–6366.
53. A. Karelovic and P. Ruiz, ACS Catal., 2013, 3, 2799–2812.
54. L. Fan, J. Zhang, K. Ma, Y. Zhang, Y.-M. Hu, L. Kong, A.-p. Jia, Z. Zhang, W. Huang and J.-Q. Lu, J. Catal., 2021, 397, 116–127.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy01151a

---