Amine functionalized surface frustrated Lewis pairs boost CO 2 photocatalysis †‡

The archetype surface frustrated Lewis pair (SFLP) that facilitates CO 2 photocatalytic hydrogenation to methanol and carbon monoxide, is an InOH (cid:2)(cid:2)(cid:2) In site positioned in the surface of a nanoscale indium oxide hydroxide, denoted In 2 O 3 (cid:3) x (OH) y . Proximal Lewis acid In( III ) and Lewis base InOH of this genre serve as surface active sites that enable the photochemical heterolytic H 2 dissociation and reduction of CO 2 to the mentioned products. The conversion rate enabled by light has been found to far exceed that enabled by heat. Eﬀorts to enhance the CO 2 photocatalytic performance of the SFLP have involved modifications of the Lewis acidity and basicity through isomorphic substitution of In( III ) with Bi( III ) and changes in the population of oxygen vacancies through control of oxide non-stoichiometry. Replacement of the Lewis base hydroxide InOH by the stronger Lewis base amine InNH 2 heretofore remains unexplored. The strategy described herein to explore this opportunity begins with the synthesis of In 2 O 3 (cid:3) x (EDA) y . This new material is proven to contain an InNH 2 (cid:2)(cid:2)(cid:2) In SFLP and its CO 2 photocatalytic performance is demonstrated to outperform that of its In 2 O 3 (cid:3) x (OH) y progenitor. Tailored Lewis acidity and basicity surfaces bring CO 2 photocatalysis another step closer to the vision of solar CO 2 refineries


Introduction
Photocatalytic conversion of CO 2 into valuable chemical fuels has emerged as a potential solution to address both energy and environmental concerns.However, breaking the highly stable CQO bonds in CO 2 molecules remains a challenging task.2][3] The thermodynamically metastable O Vs , which are formed through hydrogen reduction or hydroxyl dehydration, facilitate the incorporation of CO 2 oxygen atoms into the oxide sublattice, enabling subsequent CO 2 hydrogenation.More recently, heterogeneous catalysts with solid surface frustrated Lewis pair (SFLP) have been developed for various significant catalytic reactions, including CO 2 hydrogenation, hydroamination, and methane conversion. 4,5FLP consists of Lewis acidic and Lewis basic sites in close proximity, preventing their interaction and forming Lewis acidbase pair that synergistically activates small molecules such as H 2 , CO 2 , and CH 4 .][8][9][10][11] Isolated O Vs systems, represented by CeO 2Àx , exhibit a high density of surface O Vs , which leads to adjacent metal atoms with unsaturated coordination; thus, creating independent Lewis acidic and basic sites. 12On the other hand, OH-O Vs systems typically involve a surface metal site with unsaturated coordination (Lewis acid) located near oxygen vacancies and a surface hydroxide site (Lewis base).Examples of such systems include defect-laden hydroxylated oxides like In 2 O 3Àx (OH) y , TiO 2Àx (OH) y , and CoGeO 2 (OH) 2 . 7,13,14However, achieving a balanced concentration between O Vs and hydroxide sites in this manner is challenging.Although our recent research demonstrated that the reactivity of Lewis acidic sites can be controlled through the replacement of In 2 O 3 with single-site Bi 3+ ion, maximizing the reactivity of SFLP by simultaneously creating high-level Lewis acidic and basic sites remains a significant challenge. 15ncorporating amine functional groups has been widely employed to enhance CO 2 capture efficiency in CO 2 photocatalysis.][18] The adsorbed amine groups on photocatalysts can activate CO 2 molecules by forming carbamate intermediates, resulting in a significant distortion of the original linear CQO bond and reducing the energy barrier for C-O bond formation. 19The previous work by Lu et al. demonstrated that amine groups can act as Lewis basic sites, coupling with Lewis acidic sites (e.g., Au surface) to form solid-molecule SFLP.This SFLP configuration enables the activation of H 2 and subsequent hydrogenation of imines and nitriles. 20In contrast to the aforementioned SFLP types, amine-functionalized SFLP systems circumvent the reliance on hydroxyl groups to generate O Vs , potentially maximizing the reactivity of SFLP.Moreover, the amine group possesses higher electronegativity than the hydroxyl group, suggesting its potential as a more active Lewis basic site.Consequently, grafting amine groups onto defect-laden oxides by replacing hydroxyl groups could result in the formation of NH 2 -O Vs systems, offering enhanced reactivity and providing strong Lewis acid and base sites to facilitate CO 2 adsorption and activation, thereby improving photocatalytic CO 2 reduction performance.
In this study, we present a one-pot solvothermal approach for synthesizing amine-functionalized In 2 O 3 catalysts for gasphase CO 2 hydrogenation photocatalysis.Extensive characterization techniques, coupled with density functional theory (DFT) calculations, are employed to gain valuable insights into the formation of InNH 2 Á Á ÁIn SFLP systems, the surface and electronic nature of the active sites, and the identification of key intermediates during photocatalytic CO 2 hydrogenation.The findings of this study provide a foundation for sustainable chemical fuel production through the engineering of moleculefunctionalized metal oxide-based heterogeneous CO 2 photocatalysis.

Results and discussion
A facile one-pot solvothermal method was employed to synthesize amine-functionalized In 2 O 3 nanocrystals (E-IO) using In(NO 3 ) 3 as the indium precursor and ethylenediamine (EDA) as both the reaction medium and functionalizing agent.As illustrated in Fig. 1A, EDA initially chelates with indium ions in the precursor solution, forming In n (EDA) m 3+ complexes.These complexes then react with OH À ions, derived from the hydrolysis of EDA, resulting in the generation of In n (EDA) mÀx (OH) (3Àx)+ x intermediates.These intermediates gradually dissociate and condense, forming small In 2 O 3 nuclei with EDA attached to the surface through hydrogen bonding by single coordination according to the previous reports. 21,22The newly formed In 2 O 3 nuclei tend to rapidly attach together, minimizing surface energy.Subsequently, they undergo further rearrangement and crystallization through a ''self-assembly'' aggregation growth mechanism. 23,24SEM and TEM images of the amine-functionalized In 2 O 3 nanocrystals are shown in Fig. 1B and C, respectively.These images reveal a predominantly spherical morphology with an average diameter of approximately 30 nm.The In 2 O 3 nanospheres are composed of small agglomerated nanoparticles with average diameters around 6.50 nm (Fig. 1D).HRTEM analysis in Fig. 1E shows well-defined lattice fringes with an interplanar distance of 0.293 nm, corresponding to the (222) facet of the cubic structure of In 2 O 3 .The SAED pattern displayed in Fig. 1F exhibits multiple rings composed of individual spots, indicating the polycrystalline nature of the amine-functionalized In 2 O 3 nanocrystals.EDS mapping of the nanospheres confirms the homogeneous distribution of In, O, and N elements in the In 2 O 3 nanocrystals (Fig. 1G).The atomic content of N in the E-IO sample was determined to be approximately 5.51 wt% (Fig. S1, ESI ‡).
To gain a deeper understanding of the role played by amine groups in the active In 2 O 3 -based photocatalysts, we synthesized indium hydroxide via a hydrothermal method (Scheme S1, ESI ‡), where only water was used as the reactant solvent.The resulting indium hydroxide was then calcined to produce In 2 O 3 with a fresh surface. 25TEM images of the hydrothermally synthesized In 2 O 3 sample (H-IO) (Fig. S2A-C, ESI ‡) reveal that it is a cube-shaped nanoparticle with an edge length exceeding 400 nm.The significant difference in particle size between E-IO and H-IO makes it challenging to determine whether the variation in photocatalytic activity is attributable to particle size or surface amine groups.To directly compare the In 2 O 3 samples with and without amine functionalization, we further synthesized In 2 O 3 (Si-IO) with smaller particle size using SiO 2 nanospheres as templates in an aqueous solution.The resulting In 2 O 3 sample consists of small agglomerated nanorods with a diameter of approximately 10 nm (Fig. S2D-F, ESI ‡), which is comparable in particle size to the E-IO sample.Although a larger surface area may facilitate CO 2 adsorption, the subsequent photocatalytic results indicate that surface area is not the primary factor contributing to the improved photocatalytic activity in CO 2 hydrogenation.
To confirm the successful functionalization of amine groups on the surface of In 2 O 3 nanocrystals, several characterizations, including FTIR, TG-MS, XPS, and NMR were conducted on the collected samples.In the FTIR spectra of the three In 2 O 3 samples (Fig. S5, ESI ‡), four intense peaks centered at 602, 565, 538, and 428 cm À1 are observed, corresponding to the vibration peaks of In-O bonds in cubic In 2 O 3 . 26The peak at 1383 cm À1 corresponds to the stretching vibration of OH bonds, with the H-IO sample exhibiting the strongest OH peak due to the abundance of surface OH groups.Notably, the E-IO sample displays several new peaks centered at 3241, 2947, 2890, 1561, 1495, 1343, 1270, 1178 and 1113 cm À1 (Fig. 2A).0][31][32] Additionally, peaks centered at 2947, 2890 and 1495 cm À1 can be assigned to CH 2 vibrational bands. 27,28,32TG-MS analysis was further performed to confirm the presence of amine groups in the E-IO samples.It is evident that E-IO exhibits significant weight loss during thermal treatment in the temperature range of 30 to 520 1C, compared to the other two samples (Fig. S6, ESI ‡).This weight loss can be attributed to the release of ethylenediamine molecules bonded to the surface of E-IO upon thermal treatment.The TG-MS data (Fig. 2B) reveal that the E-IO sample has a substantial amount of physically and chemically absorbed H 2 O on the surface, which is desorbed at around 150 and 280 1C, respectively.Physically absorbed CO 2 is completely removed at approximately 200 1C.4][35] Of note, the additional Raman (Fig. S7, ESI ‡) and wide-range N 1s XPS (Fig. S8, ESI ‡) results can exclude the possibility of the generation of NO deriving from salt anion (NO 3 À ).Furthermore, according to the previous reports shown in Table S2 (ESI ‡), the N 1s XPS spectrum of E-IO exhibits three distinct peaks: O-In-N bonding at 399.55 eV, 36 C-N bonding at 398.30 eV and coordinately-bound NH groups at 400.39 eV (Fig. 2C). 37,38What's more, as shown in for the E-IO presents the characteristic carbon resonance signals at 39.1 ppm, which can be assigned to the C atom in CH 2 groups arising from EDA molecular.3][44] Collectively, all these results confirm the successful grafting of amine groups onto the surface of E-IO nanoparticles during the solvothermal process.Ethylenediamine was employed as both an appropriate amine-modifying reagent and a facilitator for generating a significant quantity of oxygen vacancies during the synthesis of amine-functionalized In 2 O 3 nanocrystals.The presence and semi-quantitative analysis of oxygen vacancies were determined using XPS and EPR techniques.Fig. S11A-C (ESI ‡) shows the high-resolution XPS spectra of the O 1s region in the H-IO, Si-IO, and E-IO samples.The spectra exhibit three distinct peaks at binding energies of 529.35, 530.86, and 531.81 eV, corresponding to lattice oxides, oxygen vacancies, and hydroxyl groups, respectively. 9,45Notably, E-IO displays a significantly higher oxygen vacancy concentration of 34.43% compared to H-IO (20.68%) and Si-IO (18.15%).Additionally, the binding energy of the In 3d core level in E-IO exhibits a noticeable shift toward lower binding energy (Fig. S11D, ESI ‡), indicating an unsaturated state of indium in the amine-functionalized In 2 O 3 nanocrystals and a higher surrounding electron density.This observation further confirms the presence of a substantial number of oxygen vacancies in the amine-functionalized In 2 O 3 sample.EPR spectroscopy is an effective semi-quantitative characterization method for evaluating oxygen vacancies.As depicted in Fig. S12 (ESI ‡), all three samples exhibit an EPR signal related to oxygen vacancies, appearing at the g-value of 2.003. 46Notably, E-IO displays the strongest EPR signal, indicating that the presence of ethylenediamine promotes the formation of O Vs during the solvothermal synthesis.Consequently, based on the XPS and EPR findings, it can be concluded that the amine-functionalized In 2 O 3 sample possesses a substantial number of surface oxygen vacancies.As a higher population of oxygen vacancies corresponds to a lower oxygen coordination number around the In(III) sites, the coordinately unsaturated In(III) becomes more Lewis acidic.Furthermore, the substitution of hydroxyl groups with amine groups, which possess stronger electronegativity in the amine-functionalized In 2 O 3 nanocrystals, makes them more Lewis basic.Consequently, the amine-functionalized In 2 O 3 (E-IO) should have intensified Lewis acid and Lewis base sites, showing promising potential in enhancing photocatalytic performance for CO 2 reduction.
Density functional theory (DFT) slab calculations were conducted to validate the SFLP property achieved by substituting EDA for hydroxide groups on the In 2 O 3Àx (OH) y surface.Previous studies have demonstrated that SFLP, consisting of an unsaturated In atom as the Lewis acid site and a hydroxide group as the Lewis base site, can be constructed by removing a surface oxygen atom and introducing a hydroxide group on the (110) facet of cubic In 2 O 3 . 15,47,48Herein, we investigate the optimized structure of the substitution of EDA at the hydroxide group site.As shown in Fig. 2E, two kinds of surface configurations, In2-N1 and In2-N2, are identified on the aminefunctionalized surface.The In2-N1 and In2-N2 with respective distances of 4.556 Å and 3.378 Å fall in the domain of solid SFLP.The Bader charge calculations indicate that the related In2, N1 and N2 pairs have atomic local charges of +1.27e, À0.84e and À0.84e, respectively.Compared with In2-N1 configuration, In2-N2 with a shorter distance may deliver a higher capability to activate reactant molecules. 49he existence of unsaturated In atom as Lewis acid site is essential for the construction of InNH 2 Á Á ÁIn SFLP, where the NH 2 in the InNH 2 Á Á ÁIn SFLP systems refers to the noncoordinated NH 2 group of EDA, while the other NH 2 group being coordinated with the In atom in In 2 O 3 .To prove this, in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) of adsorbed CO and CO 2 at 298 K were investigated, respectively.After thermal treatment of the E-IO at 423 K for 2 h, saturated doses of CO or CO 2 were introduced into IR cells.1][52] In addition, in the adsorption spectrum of CO 2 (Fig. 2F, left), the diagnostic peak at 2359 cm À1 can be clearly observed and this peak is usually ascribed to the linear adduct of CO 2 coordinated to Lewis acidic sites. 53,54Moreover, the diagnostic peaks at 2120 and 2342 cm À1 can be attributed to the physical adsorption of CO and CO 2 , respectively.These two spectroscopic characterizations clearly highlight that CO/CO 2 can be able to interact with unsaturated In atoms that act as Lewis acid sites, providing strong evidence for the presence of grafting state InNH 2 Á Á ÁIn.
The catalytic performance of CO 2 hydrogenation was assessed in a flow reactor under reaction temperatures of 200, 250, and 300 1C with and without simulated solar light irradiation, using a gas mixture of CO 2 and H 2 in a 1 : 3 ratio, within a miniaturized photothermal catalytic microreactor system (Fig. S13, ESI ‡).All three In 2 O 3 samples (E-IO, Si-IO, H-IO) exhibited catalytic activity for the reverse water gas shift (RWGS) reaction to produce CO and were capable of synthesizing CH 3 OH, although the production rate and selectivity of the products varied among the catalysts.At 200 1C, regardless of light irradiation, all In 2 O 3 samples showed negligible CO activity, while the E-IO sample exhibited significantly higher CH 3 OH activity compared to the other catalysts (Fig. S14 and S15, ESI ‡).When the reaction temperature was increased to 250 1C, the Si-IO and H-IO catalysts displayed poor CO and CH 3 OH production rates similar to those observed at 200 1C (Fig. 3A and B).Remarkably, the amine-functionalized E-IO catalyst showed a substantial improvement in both CO and CH 3 OH formation under these reaction conditions.Specifically, at this temperature, E-IO demonstrated CO production rates of 248.07 mmol g cat À1 h À1 in the dark and 390.74 mmol g cat À1 h À1 under light irradiation, with a photoenhancement rate of 158%.A significant photo-enhancement rate of approximately 217% was also observed for CH 3 OH on E-IO.The solar-powered CH 3 OH production rate of 59.02 mmol g cat À1 h À1 was more than 15.4 higher than that of the reference cubic In 2 O 3 catalyst (Si-IO).This photoenhancement activity can be attributed to the lower activation energy of the photocatalytic process compared to the thermochemical process. 15Furthermore, when the reaction temperature was further increased to 300 1C (Fig. S13, ESI ‡), although both H-IO and Si-IO samples exhibited improved activities for CO and CH 3 OH production, the E-IO sample still exhibited the best catalytic performance among the three catalysts.
In particular, under light irradiation, the CO production rate on E-IO reached a high value of 1814.73 mmol g cat À1 h À1 , which is nearly 2.3 times higher than that of H-IO and Si-IO, and comparable to the reported rhombohedral polymorph of In 2 O 3 . 15However, due to the exothermic nature of the CH 3 OH production reaction, the production rate of CH 3 OH on E-IO decreased to 37. Based on the catalytic results, it can be concluded that the amine functionalization of In 2 O 3 significantly enhances the performance of CO 2 hydrogenation, exhibiting remarkable photo-enhancement in both CO and CH 3 OH production.In light of the superior catalytic performance of E-IO, we conducted long-term stability tests under atmospheric pressure conditions at 200, 250, and 300 1C.Prior to the long-term light irradiation testing, a 4 hour thermal catalytic process was performed to establish a steady-state activity.As depicted in Fig. 3C and D, the E-IO catalyst demonstrated excellent photocatalytic stability for both CO and CH 3 OH production.Notably, optimal CO production and CH 3 OH formation were achieved at 300 and 250 1C, respectively.After 20 h of continuous light irradiation testing, no significant decrease in production rate was observed, with a CO rate of 1787.spent E-IO catalyst after 20 h of reactions at 250 1C showed no significant changes in crystal phase, surface chemical groups, and chemical oxidation states, confirming its structural stability (Fig. S17 and S18, ESI ‡).It should also be noted that when the E-IO catalyst suffered from catalytic reaction at a higher temperature (300 1C), the surface coordinated EDA molecules disappeared (Fig. 2B and Fig. S18c, ESI ‡) and the InNH 2 Á Á ÁIn SFLP was destroyed, which then contributes to a remarkably decreased photocatalytic activity.
To substantiate that the CO and CH 3 OH products were from the photocatalytic reaction, we conducted two parallel control experiments: one was using high-purity Ar instead of CO 2 , and the other was using carbon-13-labeled carbon dioxide ( 13 CO 2 ).Fig. 3E demonstrated that no product was observed in the Ar condition, indicating that both carbon products were exclusively derived from CO 2 reduction.When the reaction atmosphere changed into 13 CO 2 and H 2 , obvious M/S signals of 13 CO (m/z = 29) and 13 CH 3 OH (m/z = 33) products were detected, supporting the conclusion that the products were generated via CO 2 reduction.As compared, under 12 CO 2 and H 2 atmosphere, unequivocal signals of 12 CO and 12 CH 3 OH products were detected (Fig. 3F).All these results collectively confirm that the carbon source in the CO 2 hydrogenation originates from the CO 2 , rather than the decomposition of EDA molecules grafted on the catalyst surface.To further reflect the strong chemical interaction of EDA molecule grafting on the In 2 O 3 surface, we also prepared amine-modified In 2 O 3 samples via the impregnation method.As shown in FTIR spectra (Fig. S19, ESI ‡), there exhibited nearly absent characteristic absorption peaks related to EDA on the catalyst surface, indicating efficient removal of surface-grafted EDA molecules during the washing process.To maintain the residual of the EDA molecules, the impregnated catalyst without washing was also prepared and the characteristic absorption peaks related to EDA can be clearly detected.However, the photocatalytic CO 2 hydrogenation performance of all the impregnated amine-modified In 2 O 3 showed almost the same as the pristine In 2 O 3 samples (Fig. S20, ESI ‡), suggesting the minimal effect of physical impregnation.
Fig. 4A illustrates the UV-visible DRS spectra and corresponding bandgaps of the H-IO, Si-IO, and E-IO catalysts.All samples exhibit solar light absorption in the ultraviolet region (wavelengths below 440 nm).Notably, E-IO demonstrates significantly stronger UV light absorption, approximately double that of the other two samples.This observation suggests an increase in surface charge of the indium oxide material due to the introduction of surface amine groups.Furthermore, by employing the Kubelka-Munk equation and Mott-Schottky analysis (Fig. S21, ESI ‡), we calculated the band gap (E g ) and conduction band (CB) of the samples, resulting in the band diagrams (Fig. 4B). 55It is important to note that In 2 O 3Àx (EDA) y (E-IO) exhibits a more moderate band gap energy with a value of 3.04 eV, compared to In 2 O 3Àx (OH) y (H-IO with a band gap energy of 2.80 eV and Si-IO with an E g of 2.90 eV), as well as pristine cubic indium oxide (with an E g of 3.6 eV as a direct semiconductor). 56This indicates that E-IO enhances light absorption while avoiding excessive recombination of electron-hole pairs caused by a narrow band gap, thereby improving the transfer efficiency of photogenerated charge carriers.Moreover, E-IO exhibits a more negative conduction band at approximately À0.759 eV compared to the other two samples, indicating a better reduction potential that facilitates CO 2 photocatalytic reduction.
To investigate the photogenerated charge transfer mechanism, the room-temperature photoluminescence (PL) spectra of the H-IO, Si-IO, and E-IO samples are presented in Fig. S22A (ESI ‡).All three samples display a green emission peak centered around 460 nm.However, E-IO exhibits a weaker PL emission peak compared to the other two samples.This can be attributed to the fact that amine groups and oxygen vacancies, similar to hydroxide and oxygen vacancies, act as traps, creating more moderate mid-gap states.These mid-gap states effectively capture photo-excited electrons and inhibit electronhole recombination, leading to a reduction in PL emission intensity.Furthermore, Fig. S22B and C (ESI ‡) demonstrate that E-IO exhibits the highest photocurrent density and lowest impedance, indicating superior charge separation and transfer efficiency.These photoelectrochemical characterizations substantiate that the construction of InNH 2 Á Á ÁIn SFLP can enhance the transport of photogenerated charges, thereby improving the photocatalytic performance of amine-functionalized In 2 O 3 .
To gain a deeper understanding of how surface-grafted NH 2 groups and oxygen vacancies affect the electronic band structure of amine-functionalized In 2 O 3 , we conducted density of states (DOS) calculations and partial density of states (PDOS) for four types of surfaces: pristine In 2 O 3 , In 2 O 3Àx , In 2 O 3Àx (OH) y , and In 2 O 3Àx (EDA) y , using hybrid DFT calculations (Fig. 4C).On the surface of In 2 O 3Àx with only oxygen vacancies, new states near the original conduction band were observed, closer to the Fermi level compared to the defect-free In 2 O 3 surface.These states were fully occupied, indicating an increase in electron surface charge due to the donor properties of oxygen vacancies.However, the introduction of oxygen vacancies also pushed the valence band further away from the Fermi level, somewhat diminishing the overall advantage of introducing an oxygen vacancy.Fortunately, these drawbacks can be effectively mitigated when oxygen vacancies are introduced together with amine or hydroxyl groups.Similar to the hydroxyl group, the amine group induced the accumulation of p electrons at the edge of the valence band and shifted the Fermi level toward the edge, exhibiting typical acceptor properties (Fig. S23, ESI ‡).The simplified schematic in Fig. 4D illustrates that in the In 2 O 3Àx (EDA) y specimen, oxygen vacancies create shallow donor states below the conduction band edge, while surface amine groups act as shallow acceptor states above the valence band edge.The presence of surface amine groups and oxygen vacancies alters the relaxation dynamics of charge carriers, enhancing non-radiative relaxation processes and facilitating the separation of electron-hole pairs, thereby improving the catalytic efficiency of photogenerated electrons in reduction reactions.These findings align with the results obtained from the DRS and PL experiments discussed earlier.
Besides the electronic structure and charge transfer efficiency, the enhanced Lewis acid-base pairs in amine-functionalized In 2 O 3 are also expected to improve the adsorption and activation capacity for CO 2 .To investigate the CO 2 uptake ability, we recorded the CO 2 adsorption profiles on H-IO, Si-IO, and E-IO catalysts.As shown in Fig. 5A, E-IO exhibits the highest adsorption capacity of 5.72 cm 3 g À1 for CO 2 , which is 2.44 and 2.86 times higher than that of H-IO and Si-IO, respectively.It is evident that the superior CO 2 adsorption capacity of E-IO cannot be attributed solely to its surface area, as E-IO has a moderate surface area (50.54 m 2 g À1 ) compared to the other catalysts.Considering the significant enhancement of Lewis acid-base pairs due to NH 2 substitution, the improved CO 2 adsorption capacity can be mainly attributed to the strong chemical interaction between CO 2 molecules and Lewis basic NH 2 sites, as well as the abundance of oxygen vacancies.To gain further insight into the effect of NH 2 substitution on the interaction between CO 2 and the catalysts, we conducted CO 2 temperature-programmed desorption (CO 2 -TPD) tests.As depicted in Fig. 5B, the TPD peak intensity is higher for the amine-functionalized In 2 O 3 compared to the other catalysts.Notably, the TPD profile of E-IO can be divided into three regions: a low-temperature region (100-200 1C), a mediumtemperature region (200-360 1C), and a high-temperature region (360-550 1C).The weak desorption peak at 146 1C corresponds to physically adsorbed CO 2 .The significant desorption peak observed at 289 1C is attributed to the desorption of CO 2 that is bound to NH 2 groups, forming carbamate (H 2 NCOO À ) species. 57,58The additional peak at 324 1C can be associated with the chemical desorption of bent CO 2 d À species, which is related to oxygen vacancies. 59,60rthermore, the desorption peaks at higher temperatures (449 and 495 1C) are primarily attributed to the decomposition of surface carbonates and bicarbonates. 15,61n situ DRIFTS experiments were conducted to investigate the adsorption of CO 2 .It is evident that E-IO exhibits superior adsorption capacity for CO 2 compared to H-IO and Si-IO, resulting in the formation of a greater number of intermediate species through bonding interactions (Fig. S24, ESI ‡).The transient evolution of surface species during CO 2 adsorption on H-IO, Si-IO, and E-IO samples is depicted in Fig. 5C.In the case of E-IO, the observed features at 3394 cm À1 can be attributed to the NCOO À skeletal vibration and N-H stretching of carbamate formation, indicating that surface amine groups adsorb and activate CO 2 by forming carbamates. 62,63The peaks at 1716, 1513, 1370, and 1314 cm À1 correspond to the asymmetric and symmetric OCO stretching modes of carbonates (CO 3

2À
), and the appearance of four peaks at 3726, 3626, 2901, 1620 and 1450 cm À1 indicates the formation of bicarbonate species (HCO 3 À ). 15,64,65 the asymmetric and symmetric stretching modes, respectively, and can be attributed to the bent CO 2 dÀ species adsorbed at oxygen vacancy sites.Peaks at 3705 and 3595 cm À1 correspond to gaseous CO 2 fingerprint modes. 66Additionally, peaks at 2980, 2869, and 1268 cm À1 can be attributed to the asymmetric and symmetric OCO stretching vibrations of adsorbed bidentate formate (HCOO À ) species. 45,67,68Although some formate species may also be observed on H-IO and Si-IO, their peak intensities are weaker compared to E-IO.These collective results indicate that amine-functionalized In 2 O 3 exhibits strong CO 2 adsorption, bonding, and activation abilities due to the presence of grafted NH 2 groups and abundant oxygen vacancies.
To gain a deeper understanding of the enhanced reactivity facilitated by InNH 2 Á Á ÁIn SFLP.In situ DRIFTS experiments were conducted under reaction conditions (CO 2 + H 2 , 250 1C) to identify reaction intermediates and elucidate the photocatalytic mechanism involved in the CO 2 hydrogenation process.Fig. 5D shows the identified surface species on E-IO under reaction conditions with different irradiation times.Vibration absorption peaks of HCOO À were detected at 2995, 2878, 1482, and 1282 cm À1 , which have been previously identified as intermediate species in the generation of CO and CH 3 OH. 62,67The vibration absorption peak of H 3 CO À species, another key intermediate in methanol formation, was indicated by the diagnostic mode at 2825 cm À1 , which corresponds to the CH 3 stretching modes. 15,68Additionally, the peaks at 1204 and 1123 cm À1 correspond to the CQO stretching mode of bridged methoxide species.The peak at 2266 cm À1 is attributed to CO, indicating the formation of CO on the surface of E-IO. 69n contrast, only vibration absorption peaks of CO and a small amount of HCOO À were detected on the surfaces of H-IO and Si-IO (Fig. S25, ESI ‡).These results demonstrate that In 2 O 3Àx (EDA) y catalysts can effectively promote the formation of intermediate species HCOO À and H 3 CO À during the photocatalytic hydrogenation reaction, leading to an enhanced production of CH 3 OH and CO.
Based on the comprehensive analysis, a plausible catalytic mechanism for CO 2 hydrogenation on In 2 O 3Àx (EDA) y with robust InNH 2 Á Á ÁIn SFLP can be proposed in Fig. 5E.The traditional InOHÁ Á ÁIn SFLP with moderate Lewis acid and base sites, can facilitate the heterolysis of H 2 , in which the proton bound to the hydroxide Lewis base and hydride bound to the coordinately unsaturated indium, and subsequently react with CO 2 to generate CO and CH 3 OH.As for InNH 2 Á Á ÁIn SFLP, the amine group Lewis base provides a robust adsorption site for CO 2 molecules, preferentially favouring the activation of CO 2 into carbamate (NHCOO À ) intermediates.Moreover, the unsaturated In atom acting as Lewis acid can also provide an active site to activate and bond with CO 2 molecules.This synergetic Lewis acid-base enhancement in the activation ability towards CO 2 molecules effectively promotes a continuous multi-electron hydrogenation and reduction process, leading to the formation of crucial intermediates and ultimately the production of CO and CH 3 OH.

Conclusions
In summary, this study successfully synthesized amine-functionalized In 2 O 3 (In 2 O 3Àx (EDA) y ) nanoparticles, which possess robust InNH 2 Á Á ÁIn SFLP, using a one-step solvothermal method.In comparison to conventional InOHÁ Á ÁIn, the InNH 2 Á Á ÁIn SFLP demonstrates superior performance in the photocatalytic hydrogenation of CO 2 .This enhanced activity can be attributed to the strong adsorption, binding, and activation capabilities of the Lewis acid-base pairs present in InNH 2 Á Á ÁIn SFLP.Additionally, the existence of moderate shallow states facilitates the separation of electron-hole pairs, thereby enhancing the reactivity of the Lewis acid-base sites.By leveraging the unique properties of surface InNH 2 Á Á ÁIn SFLP, it becomes possible to design efficient and sustainable indium oxide catalysts for the conversion of CO 2 into valuable products.

Fig. S3 (
Fig. S3 (ESI ‡) presents the XRD patterns of the In 2 O 3 samples with and without amine functionalization.The patterns of all three samples exhibit similar diffraction peaks that can be assigned to the cubic phase of indium oxide (c-In 2 O 3 ) (JCPDS no.71-2195).The crystallite sizes estimated using the Scherrer equation are 6.41 nm for E-IO, 29.95 nm for H-IO, and 9.47 nm for Si-IO (Table S1, ESI ‡).Nitrogen adsorption/desorption isotherms of the three samples are shown in Fig. S4 (ESI ‡), revealing the presence of mesopores in all In 2 O 3 samples.The specific surface area follows the order of Si-IO (94.97 m 2 g À1 ) 4 E-IO (50.54 m 2 g À1 ) 4 H-IO (15.67 m 2 g À1 ).Although a larger surface area may facilitate CO 2 adsorption, the subsequent photocatalytic results indicate that surface area is not the primary factor contributing to the improved photocatalytic activity in CO 2 hydrogenation.To confirm the successful functionalization of amine groups on the surface of In 2 O 3 nanocrystals, several characterizations,
Fig. S9 and S10 (ESI ‡), the three peaks detected in the C 1s XPS spectrum of E-IO can be attributed to the C-C bonding (284.60 eV), C-N bonding (285.98 eV) and O-CQO bonding (288.83eV), respectively, 39,40 and the In 3d 5/2 and In 3d 1/2 peaks of E-IO are conventionally deconvoluted to In-N bonding (443.59eV and 251.18 eV), and In-O bonding (444.17eV and 451.73 eV). 41Through these results, the existence state of EDA on the surface of E-IO can be confirmed: the amine group at one end of EDA is coordinated with the indium atom of In 2 O 3 , while the amine group at the other end exists in an uncoordinated NH 2 group state.Solid-state NMR experiment can further provide strong evidence for the successful grafting of EDA molecular onto the catalyst surface.As shown in Fig. 2D, the 13 C magic angle spinning nuclear magnetic resonance ( 13 C MAS NMR) spectrum

Fig. 2
Fig. 2 Characterization of samples (A) FTIR spectra of H-IO, Si-IO and E-IO.(B) TGA-MS curves of E-IO in air condition.(C) High resolution N 1s XPS spectra of E-IO.(D) 13 C MAS NMR spectra of E-IO at room temperature.(E) Optimized structure of the grafting state InNH 2 Á Á ÁIn.(F) In situ DRIFTS spectra of the adsorption of CO 2 (left) and CO (right) on the E-IO surface at 298 K.
57 and 41.06 mmol g cat À1 h À1 under dark and light conditions, respectively, showing almost no photoenhancement at this high reaction temperature.The estimated turnover number (TON) of the three samples shows a similar activity trend towards CO and CH 3 OH production (Fig. S15, ESI ‡), further demonstrating that the amine functionalization of In 2 O 3 can significantly enhance the performance of photocatalytic CO 2 hydrogenation.

Fig. 3
Fig. 3 CO 2 hydrogenation performance (A) CO production rate at 250 1C with and without solar irradiation.(B) CH 3 OH production rate at 250 1C with and without solar irradiation.(C) Long-term (20 h) CO production over E-IO at different reaction temperatures with solar irradiation.(D) Long-term (20 h) CH 3 OH production over E-IO at different reaction temperatures with solar irradiation.(E) GC-MS data from Ar and 13 CO 2 /H 2 atmosphere under irradiation condition at 250 1C.(F) 13 CH 3 OH and 13 CO obtained from 13 CO 2 hydrogenation, and 12 CH 3 OH and 12 CO as a reference.

Fig. 4
Fig. 4 Photoelectrochemical characterizations (A) DRS and corresponding Tauc plots (inset) of H-IO, Si-IO, and E-IO.(B) Band diagrams.(C) The total DOS for In 2 O 3 , In 2 O 3Àx , In 2 O 3Àx (OH) y , and In 2 O 3Àx (EDA) y (110) surface, expanded view of the DOS in the conduction band region (inset).(D) A schematic illustration of charge carrier recombination pathways in In 2 O 3Àx (EDA) y nanoparticles.