Bismuth Atom Tailoring of Indium Oxide Surface Frustrated Lewis Pairs Boosts Heterogeneous CO2 Photocatalytic Hydrogenation

doi:10.21203/rs.3.rs-49848/v1

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Bismuth Atom Tailoring of Indium Oxide Surface Frustrated Lewis Pairs Boosts Heterogeneous CO2 Photocatalytic Hydrogenation

https://doi.org/10.21203/rs.3.rs-49848/v1

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published 29 Nov, 2020

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The surface frustrated Lewis pairs (SFLPs) on defect-laden metal oxides provide catalytic sites to activate H2 and CO2 molecules and enable efficient gas-phase CO2 photocatalysis. Lattice engineering of metal oxides provides a useful strategy to tailor the reactivity of SFLPs. Herein a one-step solvothermal synthesis is developed that enables isomorphic replacement of In3+ ions in UV-absorbing In2O3 by single-site Bi3+ ions to generate a new class of full-spectrum UV-Vis-NIR absorbing BixIn2-xO3 materials. Through compositional tuning, these materials prove to be three orders of magnitude more photoactive for the reverse water gas shift reaction (i.e., CO2 + H2 CO + H2O) than In2O3 itself, while also exhibiting notable photoactivity towards methanol production from carbon dioxide (i.e., CO2 + 3H2 CH3OH + H2O). The defective form of In2O3 containing oxygen vacancy sites can create SFLPs involving In3+ nearby the oxygen vacancy, which function as Lewis acidic sites, while lattice oxide O2- act as Lewis basic sites. In this study, it is discovered that the reactivity of these SFLPs can be further enhanced by single-site Bi3+ ion isomorphic substitution of Lewis acidic site In3+, thereby enhancing the propensity to activate CO2 molecules. In addition, the increased solar absorption efficiency and efficient charge separation and transfer of BixIn2-xO3 also contribute to the improved photocatalytic performance. These traits lead to enhanced binding and activation of CO2, exemplifying the opportunities that exist for atom-scale engineering in heterogeneous CO2 photocatalysis, another step towards the vision of the solar CO2 refinery.

Physical sciences/Chemistry/Catalysis/Photocatalysis

Physical sciences/Energy science and technology/Renewable energy/Solar energy/Solar fuels

CO2 photocatalysis

surface frustrated Lewis pairs

Bismuth atom

Bi3

In3+ions

The increasing energy demands of civil society have accelerated the consumption of coal, oil and natural gas and associated greenhouse gas emissions. This situation is tipping the delicate balance of CO₂ in our atmosphere leading to global warming. To this end, the photocatalytic hydrogenation of CO₂ into value-added chemicals and fuels has attracted global attention, touted a promising means of achieving a carbon-neutral economy^1-3. Although materials such as Pd/Nb₂O₅⁴, Ru/Al₂O₃⁵, LDH nanosheets⁶ and [email protected]₂⁷ have been successfully employed as photocatalysts for CO₂ hydrogenation, a photocatalyst does not currently exist that can meet all the stringent requirements for practical application, including a broad solar response, high conversion efficiency, robust stability and low cost. This renders the design of a practical photocatalyst for CO₂ hydrogenation a challenge.

Besides catalyst modifications designed to broaden spectral response and improve charge transfer efficiency, it is also important to accelerate conversion rates of H₂ or CO₂ on specially designed surface sites to boost photon and energy efficiency. Recently, surface frustrated Lewis pairs (SFLPs) have shown a propensity towards H₂ dissociation and CO₂ activation, a key enabler for many catalytic reactions, including hydrogenation, hydroamination and CO₂ reduction^8-10. Generally speaking, SFLPs comprise proximal Lewis acidic and Lewis basic sites providing synergetic activation of reactant molecules. For example, SFLPs sites in the In₂O_3-x(OH)_y photocatalyst, comprised of a coordinately unsaturated Lewis acidic In atom, proximal to an oxygen vacancy and an adjacent Lewis basic hydroxide group, enable the heterolysis of H₂ and reaction with CO₂ to form either CO or CH₃OH^11-13. As well, SFLPs involving coordinately unsaturated surface cobalt sites adjacent to surface hydroxides in the CoGeO₂(OH)₂ photocatalyst form CH₄ from H₂O and CO₂¹⁴. In addition, SFLPs in oxygen vacancy laden CeO₂ bearing SFLPs catalyze the hydrogenation of alkenes and alkynes¹⁵. All of these cases utilize oxygen vacancies and hydroxides to engineer the catalytic activity of SFLPs. How to tailor the reactivity of the SFLPs themselves is rarely mentioned.

Indium sesquioxide (In₂O₃) is proving to be a promising catalyst for the thermal hydrogenation of CO₂ to CH₃OH or CO^16-18. Experimental and computational studies of CO₂ hydrogenation over oxygen vacancy laden In₂O₃ revealed that methanol formation was favored over the reverse water gas shift (RWGS) reaction^19,20. Methanol production was remarkably enhanced when In₂O₃ was supported on ZrO₂ arising from electronic support effects²¹. A bifunctional catalyst composed of partially reduced In₂O₃ supported on HZSM-5 could convert CO₂ directly into gasoline-range hydrocarbons with a 78.6% selectivity due to the synergistic effects of these two components²². By controlling the degree of non-stoichiometry in In₂O_3-x, a black indium oxide catalyst, which utilized the entire solar spectrum, facilitated the photothermal RWGS reaction under ambient conditions with 100% selectivity²³. Tailoring the electronic properties of In₂O₃ can also be achieved via replacement of an indium atom in the lattice with a H₂ spillover palladium atom, although the rarity and cost of palladium could prove an issue for its practical implementation²⁴.

Bismuth, regarded as a ‘green’ element, has a long and fascinating history²⁵. The Incas in sixteenth century South America, made corrosion resistant bronzes for their knives by mixing bismuth with tin²⁶. Paracelsus in fifteenth century Germany, recognized bismuth as a non-toxic brother to lead²⁷. Since, it is finding myriad eco-friendly uses from cosmetics and personal care products to medicine and lubricants. Most recently it has proved to be a serious contender for replacing toxic lead halide perovskite materials in solar cells with non-toxic bismuth oxyiodide, retaining a comparable energy conversion efficiency of 22%²⁸. Contextually, bismuth materials with layered structures and visible light absorption properties, exemplified by BiOX (X=Cl, Br, I), Bi₂MO₆ (M=Mo, W), BiVO₄ and Bi₂S₃, behave as photocatalysts to be applied in dye degradation, water and carbon dioxide reduction²⁹.

Described herein, we developed a facile solvothermal route to achieve atom-precise substitution of Bi³⁺ for In³⁺ sites in In₂O₃ and realize the tailor of the reactivity of SFLPs as well as the electronic properties of In₂O₃. To amplify, by substituting cheaper and safer bismuth for indium in UV absorbing In₂O₃, one can create the full-spectrum UV-Vis-NIR light absorber Bi_xIn_2-xO₃. Significantly, single-site Bi³⁺ substitution for In³⁺ provides strong Lewis acidic/basic Bi³⁺-O^2- pairs that enhance CO₂ adsorption and activation, while Bi 6s² lone-pair electrons create mid-gap energy states. Atom-precise lattice engineering of this kind, boosts the reactivity of SFLPs and the harvesting efficiency of solar photons by Bi_xIn_2-xO₃ compared to In₂O₃, which enables 1,000 times photoactivity enhancement of the RWGS reaction together with a noticeable increase in the production of solar methanol.

Structural characterizations of single-site Bi_xIn_2-xO₃. Bi_xIn_2-xO₃ nanocrystals were prepared via a one-step solvothermal route, in which the molar ratio of Bi could be controlled by adjusting the concentration of Bi(NO₃)₃ and In(NO₃)₃ precursors. The mole percent Bi content of each sample in the Bi_xIn_2-xO₃ series of nanocrystals was determined using inductively coupled plasma mass spectrometry (ICP-MS, Supplementary Table 1). Transmission electron microscopy (TEM) shows that the pristine In₂O₃ nanocrystals are flower-like agglomerates of small nanocrystals with an average size of 3.7 nm (Supplementary Fig. 1). Bi³⁺ substitution results in similarly sized Bi_xIn_2-xO₃ nanocrystals (3.5 nm) that show lattice fringes with a spacing of 2.92 Å, corresponding to the (222) plane of bcc In₂O₃ (Supplementary Fig. 2). The obtained SAED pattern shown no evidence of any metallic Bi or Bi₂O₃. Most significantly, spherical aberration-corrected scanning transmission electron microscopy (STEM) images provide an insightful and distinct result, in which atomically dispersed single-site Bi atoms are revealed under these high-resolution imaging conditions as bright dots (Fig. 1a and 1b). Energy-dispersive X-ray spectroscopy (EDS) line scans and elemental mapping (Fig. 1c and Supplementary Fig. 3) provided further evidence for the homogeneous distribution of elemental Bi in these Bi_xIn_2-xO₃ nanocrystals.

The phase structure of the obtained Bi_xIn_2-xO₃ nanocrystals was studied by powder X-ray diffraction (PXRD, Fig. 1d). All the Bi_xIn_2-xO₃ nanocrystals displayed nearly identical XRD patterns diagnostic of face-centered cubic In₂O₃ except that the diffraction peaks were shifted to lower 2q values relative to those of pristine In₂O₃. This result indicates In³⁺ were isomorphously substituted by Bi³⁺ which has a larger ionic radius than In³⁺ (i.e., 0.96 Å versus 0.81 Å). This conclusion is supported by the In 3d peaks in the corresponding X-ray photoelectron spectroscopy (XPS) spectra (Supplementary Fig. 4a), which exhibited a gradual positive energy shift for Bi_xIn_2-xO₃ nanocrystals relative to pure In₂O₃, that is attributed to the higher electronegativity of Bi³⁺ compared to In³⁺. The spin-orbit coupled doublet of Bi 4f XPS peaks at 158.6 eV and 163.9 eV define the oxidation state of bismuth as Bi³⁺ rather than Bi⁰ (Supplementary Fig. 4b), following the isomorphous substitution of In³⁺ by Bi³⁺. The electron paramagnetic resonance (EPR) spectra of Bi_xIn_2-xO₃ nanocrystals at both room temperature and 77 K revealed the absence of paramagnetic species, thereby providing further evidence for the lack of Bi⁰ (Supplementary Fig. 5).

Synchrotron radiation-based X-ray absorption spectroscopy (XAS) was further used to obtain information regarding the local structural environment of these distributed Bi sites. The Bi L₃-edge X-ray absorption near-edge structure (XANES) spectra in Fig. 1e reveal visible similarities between the 1.0% and 5.0% Bi_xIn_2-xO₃ sample spectra and that of the Bi₂O₃ reference. These similarity are to be expected, given that Bi atoms in both Bi₂O₃ and Bi_xIn_2-xO₃ lattices are expected to be octahedrally coordinated by oxygen atoms, leading to similarities in their structural and electronic properties. The Fourier-transformed Bi L₃-edge extended X-ray absorption fine structure (EXAFS) spectra are presented in Fig. 1f. The similar positions of the Bi–O peak positions suggest that these bonds lengths in the Bi_xIn_2-xO₃ samples are similar to those in the Bi₂O₃ reference. In stark contrast, though, the observed Bi–M peaks appear at distinctly different positions in the Bi_xIn_2-xO₃ spectra, revealing a distinct structural difference relative to the Bi₂O₃ reference. In order to more accurately quantify these differences in bond length and structure, the spectra were also fitted to extract key structural parameter values (Supplementary Fig. 6 and Supplementary Table 2). The resulting Bi–In bond lengths in the Bi_xIn_2-xO₃ samples are shorter than those found in the pristine Bi₂O₃ lattice, though slightly longer than those observed in the pristine In₂O₃ lattice. Relatively larger Debye-Waller coefficient values for the Bi–O peaks in Bi_xIn_2-xO₃ samples were also observed, reflecting a broader range of constituent Bi–O bond lengths.

CO₂ hydrogenation performance. The photocatalytic CO₂ hydrogenation activity of Bi_xIn_2-xO₃ nanocrystals was evaluated in a batch reactor under simulated solar light irradiation and using a 1:1 ratio of CO₂ and H₂ gases. In these experiments, the RWGS reaction (i.e., CO₂ + H₂ à CO + H₂O) led to CO being the sole product detected. The CO production rates revealed that 1.0% Bi_xIn_2-xO₃ was more active than In₂O₃ by approximately three orders of magnitude (Fig. 2a), with an impressive peak rate of 8000 mmol g^-1 h^-1 for its first run, as compared to just 35 mmol g^-1 h^-1 for pristine In₂O₃. The estimated turnover frequency (TOF) of In₂O₃ and 1.0% Bi_xIn_2-xO₃ is 0.42 h^-1 and 93.6 h^-1, respectively (Supplementary Note 1), implying that substituting Bi atoms into In₂O₃ nanocrystals significantly enhanced the photocatalytic activity towards CO₂ hydrogenation. Remarkably, this boost in catalytic activity was much more dramatic than that observed in analogous hydroxylated systems (i.e., Bi_xIn_2-xO₃(OH)_y)³⁰, thereby suggesting a distinct and potent mechanism of catalytic activity enhancement in Bi_xIn_2-xO₃. Furthermore, such a CO production rate is much higher than some of the most active noble metal decorated photocatalysts (Supplementary Table 3). In addition, the 1.0% Bi_xIn_2-xO₃ was very stable, exhibiting a CO production rate that was still roughly 600 times greater than the 11 mmol g^-1 h^-1 exhibited by pristine In₂O₃ under the same experimental conditions. In both cases, the CO production rate was observed to decrease over the course of five consecutive runs.

The actual bulk reaction temperature for In₂O₃ and Bi_xIn_2-xO₃ tested by IR camera is about 70 and 115 °C (Supplementary Fig. 7), respectively. The relatively low thermal energy supplied by solar light indicates the limited contribution of photothermal effect on the photocatalytic RWGS reaction. As an endothermic reaction, the rate of the RWGS is expected increase quite rapidly as a function of temperature. According to the Arrhenius Law, the rate should approximately double for every 10 °C increase in temperature. Thus, we would expect the reaction rate to increase by a factor of approximately 22.5, assuming that the bulk temperature of the catalyst accurately reflects the temperature of the catalytically active sites. Based on the rate of pristine In₂O₃ (i.e., 35 mmol g^-1 h^-1), this would result in a rate increase to about 788 mmol g^-1 h^-1 and account for about 10 % of the observed activity increase, thereby suggesting the existence of a significant photochemical effect. Notably, the marked increase in CO production rate upon substituting single-site Bi atoms into In₂O₃ was highly dependent on the concentration of Bi atoms (Fig. 2b). Meanwhile, isotopically labelled ¹³CO₂ experiment confirmed that CO was the unequivocal product from photocatalytic CO₂ hydrogenation (Fig. 2c).

We also investigated the dependence of the CO production rate on the wavelength of light, to demonstrate the CO₂ hydrogenation proceeds mainly through a photocatalytic process. As seen in the action spectra shown in Fig. 2d and Supplementary Fig. 8, the production rate of CO monotonically decreased with longer wavelengths of the light, which correlates with the optical absorption spectrum of pristine In₂O₃ and Bi_xIn_2-xO₃ catalysts. It should also be mentioned that the CO production rate remained at about 2000 mmol g^-1 h^-1 on 1.0% Bi_xIn_2-xO₃, even when a 500 nm cut-off filter was applied, thus implying that Bi_xIn_2-xO₃ nanocrystals can function as broadband, green photocatalysis for harvesting solar energy.

In light of the promising performance of Bi_xIn_2-xO₃ nanocrystals towards gas-phase CO₂ hydrogenation, this new catalyst was also studied for solar methanol production in a flow reactor at 230 °C, both with and without light irradiation. As shown in Supplementary Fig. 9, all catalysts exhibited similarly low CO and CH₃OH production rates under purely thermal conditions; however, a remarkable enhancement in the production rates of CO and CH₃OH was obtained on changing from dark to light conditions. In this case, pristine In₂O₃ exhibited CO and CH₃OH production rates of 312 and 82 mmol g^-1 h^-1, respectively (Fig. 3a and 3b). In comparison, the single-site Bi_xIn_2-xO₃ samples exhibited much better activities for both CO and CH₃OH production, with 1.0% Bi_xIn_2-xO₃ exhibiting the highest CO and CH₃OH production rates of 918 mmol g^-1 h^-1 and 158 mmol g^-1 h^-1, respectively. Overall, the measured activities of the Bi_xIn_2-xO₃ samples were highly dependent on Bi content and showed a volcano-shaped trend. The volcano trend of the activity towards CO₂ hydrogenation versus the extent of In³⁺ substitution by the larger, more electronegative, 6s² stereochemically active lone-pair containing Bi³⁺, can be attributed to a subtle interplay of numerous and competing intertwined properties: chemical effects (e.g., influence of surface Lewis acidity and basicity of In-O-In, In-O-Bi, Bi-O-Bi sites on CO₂-H₂ adsorption, activation, reaction processes) and physical effects (e.g., photogenerated electron and hole charge-separation and charge-trapping by bismuth and oxygen vacancy mid gap states).

The CH₃OH and CO production rates of Bi_xIn_2-xO₃ nanocrystals showed negligible deactivation, even after 50 h of continuous testing under light irradiation at 230 °C (Supplementary Fig. 10a), suggesting their excellent catalytic stability. The recorded XRD patterns, TEM images and XPS spectra (Supplementary Fig. 10b-d) for the spent Bi_xIn_2-xO₃ photocatalysts after 50 h of reaction demonstrate that, except for a slight increase in particle size, the phase and oxidation states of the nanocrystals were well maintained, confirming their favourable structural stability.

To obtain more information on the origin of the activity enhancement under flow reaction conditions, activity tests were also conducted at lower reaction temperatures, beginning where products can be observed (130, 195 and 210 °C), with and without light irradiation (Supplementary Fig. 11). The drastic activity difference between dark and light conditions lend further support confirmed the contribution of the photochemical effect on CO and CH₃OH production. Moreover, based on the Arrhenius plots for 1.0% Bi_xIn_2-xO₃, the apparent activation energy for the CO and CH₃OH photochemical processes are much lower than the thermochemical ones (Fig. 3c and 3d), reflecting the solar advantage for the excited-state reaction pathway relative to the ground state pathway³¹.

Photocatalytic reaction pathway. The photocatalytic CO₂ hydrogenation reaction involves photon-absorption, electron-hole separation, and CO₂ adsorption/activation processes. The first two steps are closely related to the intrinsic nature of the photocatalyst, while the third is highly dependent on the gas-solid interface. A significant red shift occurs in the absorption edge of UV-visible-NIR spectra (Fig. 4a), along with a broad absorption band in the spectral range of 400 to 800 nm, which grows with Bi³⁺ content and is accompanied by a change in color from cream to rust (Supplementary Fig. 12). Corresponding electronic band gaps of Bi_xIn_2-xO₃ reduce gradually from 2.77 to 2.45, 1.99, 1.46 and 1.27 eV as a result of Bi³⁺ substitution (Supplementary Fig. 13). This is consistent with the simulated band structures that show the introduction of Bi can decrease the band gap of In₂O₃ (Fig. 4b and 4c). The total density of states (DOS) and partial density of states (PDOS) (Supplementary Fig. 14) can further reveal that the substitution of Bi at the In site induces mid-gap energy states with Bi 6s states below the conduction band edge of In₂O₃ and is consistent with the reported results in this paper^32-34.

To understand the photogenerated charge transfer mechanism, the room temperature photoluminescence (PL) spectra of pristine In₂O₃ and 1.0% Bi_xIn_2-xO₃ nanocrystals are shown in Fig. 4d. The pristine In₂O₃ nanocrystals exhibited a strong green emission peak centered at ca. 440 nm, originating from the radiative recombination of photo-excited electrons trapped in mid-gap oxygen vacancy states with photogenerated holes in the valence band³⁵. The existence of oxygen vacancies is further evidenced by the O 1s core level XPS spectra (Supplementary Fig. 15). In contrast, the incorporation of single-site Bi into In₂O₃ leads to weakening and broadening of the PL emission peak. This can be explained in two ways. One is that the substitutional Bi³⁺ slightly decreases the concentration of oxygen vacancies (Supplementary Fig. 16), which would decrease the intensity due to electron-hole radiative recombination. Alternatively, the substituted Bi³⁺ states lying below the conduction band of In₂O₃ could also act as traps capturing photo-excited electrons and inhibiting electron-hole recombination emission, resulting in lower PL emission. Thus, as illustrated in Fig. 4e, the substituted Bi³⁺ sites (denoted as Bi’), oxygen vacancies [O], coordinately unsaturated indium In’ sites and oxygen O’ sites, exist as mid-gap defect states (comprising surface frustrated Lewis pairs, SFLPs) in the bandgap of Bi_xIn_2-xO₃, can function as traps for photogenerated electrons and holes enabling the reaction between CO₂/H₂^36,37. This results in the quenching of steady-state PL emission as well as a slight shortening of the average fluorescence lifetime from 120 to 110 ps, probed by time-resolved PL spectroscopy (Supplementary Fig. 17). Albeit small, this reduction of the fluorescence lifetime, suggests that, relative to In₂O₃, single-site Bi atoms can increase the occurrence of competitive non-radiative relaxation processes in Bi_xIn_2-xO₃^38-40.

Apart from its effect on the electronic structure and charge transfer, single-site Bi³⁺ substitution is also expected to strengthen the adsorption-bonding-activating ability of Bi_xIn_2-xO₃ toward CO₂. The surface area can be excluded as it maintains in the range of 32~34 m² g^-1 for the catalysts (Supplementary Table 4). With respect to the surface chemistry, CO₂ can bond through its carbon atom and oxygen atoms to either the surface oxygen atoms, metal sites, or directly with the oxygen vacancies of metal oxides (Supplementary Fig. 18)^41-43. To investigate the effect of Bi³⁺ substitution on the interaction between CO₂ and Bi_xIn_2-xO₃ or pristine In₂O₃ nanocrystals, CO₂ temperature-programmed desorption (CO₂-TPD) measurements were initially performed. As shown in Fig. 5a, one broad desorption peak at around 100 °C, corresponding to physically adsorbed CO₂, is observed for all Bi_xIn_2-xO₃ nanocrystals and pristine In₂O₃. A significant desorption peak is clearly observed at 256 °C for pristine In₂O₃ and can be attributed to the chemical desorption of CO₂ that is binding with oxygen vacancies to form bent CO₂^d^- species⁴⁴. Since Bi³⁺ substitution results in fewer oxygen vacancies, this peak intensity gradually decreases with increased Bi³⁺ doping of Bi_xIn_2-xO₃ nanocrystals and shifts slightly to higher temperatures (as high as 275 °C for 5.0 % Bi_xIn_2-xO₃), implying that the binding strength of CO₂ and oxygen vacancies is remarkably enhanced. Moreover, weak desorption peaks at higher temperatures (300 to 600 °C) were clearly observed for In₂O₃, and can be assigned to the decomposition of surface HCO₃^- and CO₃^2- species⁴⁵. After single-site Bi³⁺ substitution, typical desorption peaks can also be clearly identified and show a slight shift to higher temperatures, again indicating that these surface species are binding more strongly to the surface.

In-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) experiments were further carried out to identify surface species. Fig. 5b show the transient evolution of the surface species during CO₂ adsorption over 1.0% Bi_xIn_2-xO₃ nanocrystals. The bands at 1510 and 1372 cm^-1 are assigned to the asymmetric and symmetric OCO stretching modes of monodentate carbonates (m-CO₃^2-). The features at 1549 and 1330 cm^-1 are attributed to the asymmetric and symmetric OCO stretching modes of bidentate carbonates (b-CO₃^2-). The bent CO₂^d^- species adsorbed at oxygen vacancy sites can be identified by two bands at 1596 and 1348 cm^-1, corresponding to the asymmetric and symmetric stretching modes, respectively. The appearance of bands at 1625, 1437, 1390 and 1222 cm^-1 indicates the formation of bicarbonate species (HCO₃^-). Moreover, a small amount of a linearly-adsorbed CO₂ species with bands appearing between 1000 and 1100 cm^-1 can also be observed. Thus, via DRIFTS measurements, all the surface species observed during CO₂-TPD measurements, CO₃^2-, HCO₃^- and CO₂^d^-, were observed and identified on Bi_xIn_2-xO₃ nanocrystals. All these surface species can be observed on pristine In₂O₃; however, the peak intensities of these species are weaker than that on Bi_xIn_2-xO₃ nanocrystals, suggesting the improved CO₂ adsorption-bonding-activating capacity after Bi³⁺ substitution.

Density functional theory (DFT) slab calculations were further carried out to unravel the promotion effect of Bi³⁺ substitution on In₂O₃. The perfect In₂O₃ (110) surface was initially selected as it has proven to be most thermodynamically stable^20,46. The defective In₂O₃ (110) surface with an oxygen vacancy at the O₄ site was then created owing to the more favorable ability for CO₂ activation and hydrogenation¹⁹. Following then, we examined the possibility of Bi³⁺ substitution at the In site.

As shown in Fig. 6a, the perfect In₂O₃ (110) surface consists of chains of In and O atoms, with the numbering In and O atoms along the chain as repeating unit. The surface In and O in the chain are adjacent and contiguous to each other, forming a classic Lewis acid-base adjunct, whereas the unbonded In₃ and In₄ in the chain and O in the top layer show a distance of 4.106 and 4.312 Å, respectively, which may deliver SFLPs-like activity (Fig. 6d). However, the electronic interactions between In₃ or In₄ and its neighbouring O₄ will block the function of In₃-O or In₄-O pairs. Therefore, the removal of oxygen atom at the O₄ site is the prerequisite to construct a pair of unbonded Lewis acid and base sites. When the O₄ atom is removed, two In atoms (In₃ and In₄) are coordinatively unsaturated and one oxygen vacancy (O_V4) is produced (Fig. 6b). However, in this case, only one In atom (In₄) locates at the surface while the other one (In₃) moves to the inner atomic layer. The surface In₄ atom is found to be surrounded by two adjacent oxygen atoms, of which the In₄-O with a distance of 4.222 Å can construct a SFLPs site (Fig. 6e). We further investigated the effect of Bi³⁺ substitution at In₄ site on the configuration and charge population (Fig. 6c and f). As compared to In₄-O configuration, Bi₄-O shows a shorter distance (3.821 Å) but can still fall in the domain of solid SFLPs. On the other hand, the Bader charge calculations show that the related Lewis acid Bi³⁺ and Lewis base O^2- involve atomic local charges of + 1.500 e and − 0.910 e, respectively, which is higher than that of the In³⁺ and O^2- pair (+ 1.300 e and − 0.900 e). The larger charge difference between the Lewis acid and Lewis base pairs in the Bi_xIn_2-xO₃ compared with that of defective In₂O₃ would form more active Lewis acid-base pairs than the In₂O₃ pair can muster, and therefore could deliver a higher capability to activate CO₂ molecules, consistent well with the DRIFTs and CO₂-TPD results.

In the photo-excited state of a SFLPs system, the Lewis acidity and Lewis basicity have been shown to increase as compared with the ground state, thereby facilitating the photochemical CO₂ hydrogenation, with a decrease in activation energy. To get more insight into the improved activity from Bi³⁺ substitution, in-situ DRIFTS experiments were further performed under reaction conditions to detect the reaction intermediates and uncover the photocatalytic pathway in the CO₂ hydrogenation process. As shown in Fig. 5c, when the Bi_xIn_2-xO₃ nanocrystals were exposed to the mixture of CO₂ and H₂ gases, bidentate formate (*HCOO), methoxy (*H₃CO) and carboxylate (*CO₂)⁴⁷, were the three principal intermediates observed from the transformation of bicarbonate and carbonate species, as evidenced by the decrease and disappearance of characteristic bands at 1510, 1390 and 1222 cm^-1. The *HCOO species can be linked to fingerprint modes at 2973 and 2731 cm^-1, which correspond to a combination of the CH bending and OCO stretching modes^47,48. The bands at 1592 and 1370 cm^-1 can be assigned to the asymmetric and symmetric OCO stretching modes while that at 2868 cm^-1 is attributed to the CH stretching mode of the same species^47,49,50. The *H₃CO species is signalled by diagnostic modes at 2941 and 2838 cm^-1 that are assigned to the CH₃ stretching modes and the band at 1182 cm^-1 is attributed to the CO stretching mode of bridged methoxide species^47,48,51. In addition to *HCOO and *H₃CO, *CO₂ species were also observed, with bands at 1567 and 1379 cm^-1 that can be associated with the OCO stretching modes. Under light irradiation, all intermediates showed an increase in band intensity (Supplementary Fig. 19), thereby confirming the photochemical effect of CO₂ hydrogenation, and is consistent with the activity results. In the case of pristine In₂O₃ (Fig. 5c and Supplementary Fig. 20), *HCOO and *H₃CO species of virtually insignificant intensity were observed for CO₂ hydrogenation, and light irradiation resulted in much noisier peaks. This further indicates the moderate catalytic performance of pristine In₂O₃ and the significant promotion effect resulting from single-site Bi³⁺ substitution.

In summary, we have demonstrated a one-step solvothermal route towards atom-precise isomorphic substitution of In³⁺ in In₂O₃ by Bi³⁺ to generate Bi_xIn_2-xO₃ materials with full-spectrum UV-Vis-NIR absorption. The incorporation of single-site Bi atoms in the In₂O₃ host lattice provides strong Lewis acid-base Bi³⁺-O^2- pairs to enhance CO₂ adsorption and activation, resulting in distinctly enhanced reaction rates relative to those observed for pristine In₂O₃ and other indium oxide based catalysts. The Bi 6s² lone pairs create mid-gap energy states, which can increase the harvesting of solar photons and favour the generation and separation of photo-induced charge carriers. Remarkably, single-site Bi³⁺-substituted Bi_xIn_2-xO₃ proves to be a highly efficient and stable photocatalyst, achieving an impressive CO production rate three orders of magnitude greater than that of pristine In₂O₃, with notable photoactivity towards solar methanol. In addition to increased activity catalytic sites, the greening of indium oxide by single-site bismuth atom substitution represents a new approach to CO₂ photocatalyst engineering and is a further step towards the vision of a solar CO₂ refinery.

Synthesis of In₂O₃ and Bi_xIn_2-xO₃. Pristine In₂O₃ nanocrystals were prepared via a simple solvothermal route. In a typical synthesis, 0.3 g of In(NO₃)₃•4.5H₂O was dissolved in 17 mL anhydrous DMF solution. After stirring for 30 min, the obtained homogeneous solution was transferred into a Teflon-lined stainless steel autoclave and then heated at 150 °C for 24 hours. After being cooled to room temperature, the light-yellow product was collected through centrifugation, washed with ethanol and water, and finally dried at 60 °C in vacuum. Bi³⁺-substituted In₂O₃ nanocrystals were prepared using the same method employed for pristine In₂O₃, except that various amounts of Bi(NO₃)₃•5H₂O were added to the indium solution prior to solvothermal reaction.

Material characterizations. The content of Bi in Bi_xIn_2-xO₃ was determined using an inductively coupled plasma mass spectroscopy (ICP-MS) instrument (Optima 7300 DV). Powder X-ray diffraction (PXRD) was performed on a Bruker D2-Phaser X-ray diffractometer, using Cu Ka radiation at 30 kV. X-ray photoelectron spectroscopy (XPS) was performed using a PerkinElmer Phi 5500 ESCA spectrometer in an ultrahigh vacuum chamber with a base pressure of 1´10^-9 Torr. The spectrometer used an Al Ka X-ray source operating at 15 kV and 27 mA. The samples were coated onto carbon tape prior to analysis and all results were calibrated to C1s 284.5 eV. EPR spectra were obtained at room temperature and 77 K using a Bruker A-300-EPR X-band spectrometer. Transmission electron microscopy (TEM) measurements were conducted using a JEM–2010 microscope working at 200 kV. The double spherical aberration-corrected scanning transmission electron microscope (STEM) images were obtained on an FEI Themis Z instrument. X-ray absorption spectra were collected at the BL14W beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The storage ring of the SSRF was operated at 3.5 GeV with a stable current of 200 mA. Using a Si(111) double-crystal monochromator, the data collection was carried out in fluorescence mode using Lytle detector. All spectra were collected under ambient conditions. Diffuse reflectance spectra (DRS) of the powders were obtained for dry-pressed disk samples using a Cary 500 Scan Spectrophotometer (Varian, USA) over a range of 200 to 800 nm. Barium sulfate (BaSO₄) was used as a reflectance standard. Room-temperature photoluminescence (PL) spectra were measured on an FL/FS 920 (Edinburgh Instruments) system equipped with a 450 W Xe arc lamp as the excitation source and a red sensitive Peltier element-cooled Hamamatsu R2658 PMT as the detector. Time-resolved fluorescence decay spectra were recorded on the Delta Pro (HORIBA instruments) using a 357 nm laser as the excitation source. BET surface area analyses were performed on an ASAP2020 M apparatus (Micromeritics Instru-ment Corp., USA) with the samples degassing in vacuum at 110 °C for 10 h and then measuring at 77 K. The CO₂ temperature-programmed desorption (CO₂-TPD) measurements were carried out on AutoChem II 2920 Version. The density functional theory (DFT) calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP) computational codes. During the geometry optimization, lattice parameters and atomic positions were optimized simultaneously. Based on the experimental data, we replaced one In with Bi in the cell as the In₁₅BiO₂₄ model and deleted one of the oxygen atom that were coordinating with Bi to establish one In₁₅BiO₂₃(O_Vacancy) model. For calculating the electronic structures and density of states, the geometry optimization of In₂O₃ and In₁₅BiO₂₃(O_Vacancy) were calculated by the PBE method within Generalized Gradient-corrected Approximation (GGA), using the exchange-correlation potential. The Vanderbilt ultrasoft pseudopotential with a cutoff energy of 380 eV was used to ensure the precision of the results. Brillouin zone integration was represented using the K-point sampling scheme of 3×3×3 Monkhorst−Pack scheme. The convergence tolerance for geometry optimization was selected with the differences in total energy (5.0×10^-6 eV/atom), the maximal ionic Hellmann−Feynman force (1.0×10^-2 eV/Å), the stress tensor (2.0×10^-2 GPa), and the maximal displacement (5.0×10^-4 Å).

Gas phase CO₂ hydrogenation tests. Batch reactions were conducted in a custom-built 1.5 mL stainless steel batch reactor with a fused-silica viewport sealed with Viton O-rings. The reactor was evacuated using an Alcatel dry pump prior to being purged with the reactant high-purity H₂ reactant gas. After purging the reactor, it was filled with a 1:1 stoichiometric mixture of H₂ (99.9995%) and CO₂ (99.999%) until the total pressure reached 30 psi. The reactor were irradiated with a 300 W Xe lamp for a duration of 1 h without external heating. Product gases were analyzed using flame ionization and thermal conductivity detectors installed in a SRI-8610 gas chromatograph equipped with 3 in. Mole Sieve 13a and 6 in. Haysep D column. Isotopically labeled tracing experiments were performed using ¹³CO₂ (99.9 at%, Sigma-Aldrich). Isotope distributions in the product gases were measured using an Agilent 7890A gas chromatograph-mass spectrometer with a 60 m GS-carbon plot column leading to the mass spectrometer. Flow experiments were carried out in a fixed-bed tubular reactor with the catalyst material being packed into a quartz tube and immobilized at both ends with quartz wool. The quartz tube had an inner diameter of 2 mm with a wall thickness of 0.5 mm, and was placed into a groove carved out into a copper block. An OMEGA temperature controller was attached to two heating cartridges inserted into the copper block and a thermocouple was inserted into the quartz tube contacting the catalyst but covered by the quartz wool. A 300 W Xe arc lamp illuminated the catalyst plug at a measured intensity of 2 W cm^-2. CO₂ and H₂ were flowed through with a 1 : 3 ratio (1 sccm CO₂, 3 sccm H₂). The amounts of CO and CH₃OH produced were determined using gas chromatography-mass spectrometry (GC-MS, 7890B and 5977A, Agilent) using an He carrier gas.

In-situ DRIFT studies. The in-situ DRIFTS measurements were performed to detect the surface intermediates over pristine In₂O₃ and Bi_xIn_2-xO₃ nanocrystals under reaction conditions. The spectra were collected using a Fourier-transform infrared spectroscopy spectrometer (Thermo, Nicolet 6700) equipped with an MCT detector. Before measurement, the catalyst was purged with He at 250 °C for 2 h. The catalyst was subsequently cooled down to 230 °C. The background spectrum with a resolution of 4 cm⁻¹ was obtained at 230 °C in He flow. Then the catalyst was exposed to a mixture of CO₂, H₂, and He (1 sccm CO₂, 3 sccm H₂, and 16 sccm He, respectively) in dark and light conditions for different times. The in-situ DRIFT spectra were recorded by collecting 32 scans at 4 cm⁻¹ resolution.

Acknowledgments

T.Y. is thankful for financial support from the National Natural Science Foundation of China (21872081), Qingchuang Science and Technology of Shandong Province (2020KJC010). GAO acknowledges the financial support of the Ontario Ministry of Research and Innovation (MRI), the Ministry of Economic Development, Employment and Infrastructure (MEDI), the Ministry of the Environment and Climate Change’s (MOECC) and the Best in Science (BIS) Award. Also acknowledged is additional support from the Ontario Center of Excellence (OCE) Solutions 2030 Challenge Fund, the Low Carbon Innovation Fund (LCIF), Imperial Oil, the University of Toronto Connaught Innovation Fund (CIF), the Connaught Global Challenge (CGC) Fund, and the Natural Sciences and Engineering Research Council of Canada (NSERC). P.N.D. acknowledges personal funding providing by the NSERC PDF program.

Author contributions

T.Y. and G.A.O. conceived and designed the experiments. T.Y., N.L. and L.L.Wang prepared the materials for characterizations. T.Y. and N.T. carried out the batch and flow experiments for CO₂ methanation. W.R. and N.L. carried out the DFT calculation. T.Y. performed the in-situ DRIFTS study and CO₂-TPD test. L.Wang. performed the XPS characterization. M.X. and L.L.W. carried out the ICP-AES study. T.Y and L.L.Wan performed the MS test using ¹³C. P.D. performed the XAFS XANES experiments. T.Y., N.L. and G.A.O. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.

Additional information

Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints . Correspondence and requests for materials should be addressed to T.Y. and G.A.O.

Competing financial interests

The authors declare no competing financial interests.

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Supplementarymaterial.docx
Bismuth Atom Tailoring of Indium Oxide Surface Frustrated Lewis Pairs Boosts Heterogeneous CO2 Photocatalytic Hydrogenation
TableS1.png
The molar contents of Bi in the series of BixIn2-xO3 nanocrystals determined by ICP-MS.
TableS2.png
Fitting results of Bi L3-edge and In K-edge FT-EXAFS data.
TableS3.png
The compared CO production rate of different catalysts for photocatalytic CO2 hydrogenation.
TableS4.png
Specific surface area of pristine In2O3 and BixIn2-xO3 nanocrystals.
FigureS1.png
(a) TEM, (b) HRTEM, (c) size distribution, and (d) SAED of pristine In2O3. The highlighted yellow ring in (a) shows one flower-like agglomerate. The red arrows in (b) represent the single nanocrystals, and the inset in (b) shows the lattice fringe with a spacing of 2.92 Å.
FigureS2.jpg
(a) TEM, (b) HRTEM, (c) size distribution, and (d) SAED of 1.0% BixIn2-xO3 sample. (e) TEM and HRTEM of 3.0% BixIn2-xO3 sample. (f) TEM and HRTEM of 5.0% BixIn2-xO3 sample.
FigureS3.jpg
Elemental mapping profiles of 1.0% BixIn2-xO3.
FigureS4.jpg
In 3d and (b) Bi 4f XPS spectra of pure In2O3 and BixIn2-xO3 nanocrystals.
FigureS5.jpg
EPR spectra of 1.0% BixIn2-xO3 nanocrystals recorded at room temperature and 77 K.
FigureS6.jpg
Fitted Bi L3-edge and In K-edge FT-EXAFS spectra from (a) Bi2O3 and (b) In2O3 reference materials, as well as (c) 1.0% and (d) 5.0% BixIn2-xO3 samples.
FigureS7.png
The actual reaction temperature tested by IR camera on (a) pristine In2O3 and (b) 1.0% BixIn2-xO3 nanocrystals.
FigureS8.jpg
CO production rate as a function of absorption cut-off filter wavelength for pure In2O3 sample.
FigureS9.jpg
(a) CH3OH production and (b) CO production as function of reaction time on pristine In2O3 and BixIn2-xO3 nanocrystals in the flow reactor at 230 C, with and without light irradiation.
FIgureS10.jpg
(a) Plotted reaction rates demonstrating the long-term (50 h) catalytic stability of 1.0% BixIn2-xO3 nanocrystals during photocatalytic CO2 hydrogenation. (b) XRD patterns, (c) TEM image, and (d) Bi 4f XPS spectra of fresh and spent (i.e., after 50 h stability testing) 1.0% BixIn2-xO3 nanocrystals.
FigureS11.jpg
(a) CH3OH production rates and (b) CO production rates of the top-performing catalyst 1.0% BixIn2-xO3 as a function of reaction temperatures.
FigureS12.jpg
The color of pure In2O3 and substituted BixIn2-xO3 nanocrystals.
FigureS13.jpg
Inferred band gaps of pristine In2O3 and various BixIn2-xO3 nanocrystals.
FigureS14.jpg
Calculated DOS plots for (a) pristine In2O3 and (b) BixIn2-xO3 nanocrystals.
FigureS15.jpg
High-resolution O 1s core-level XPS spectra of 1.0% BixIn2-xO3 nanocrystals. The O 1s core level XPS spectra could be fitted into three peaks at 529.3 eV, 530.9 eV, and 532.8 eV, which are assigned to oxides (OI), oxygen vacancies (OII), and hydroxyl groups (OIII), respectively.
FigureS16.jpg
High-resolution O 1s core-level XPS spectra of pure In2O3 and BixIn2-xO3 nanocrystals.
FigureS17.jpg
Time-resolved PL spectra of pristine In2O3 and 1.0% BixIn2-xO3 nanocrystals.
FigureS18.jpg
Schematic of surface species contributing to the adsorption of CO2 on BixIn2-xO3 nanocrystals, as exemplified by monodentate carbonate-like species (m-CO32-), bidentate carbonate-like species (b-CO32-), bent adsorbed species (CO2-), and bicarbonate-like species (HCO3-).
FigureS19.jpg
In-situ DRIFTS spectra for CO2 hydrogenation under (a) dark and (b) light conditions on 1.0% BixIn2-xO3.
FigureS20.png
In-situ DRIFTS spectra for CO2 hydrogenation under (a) dark and (b) light conditions on pristine In2O3.

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Bismuth Atom Tailoring of Indium Oxide Surface Frustrated Lewis Pairs Boosts Heterogeneous CO2 Photocatalytic Hydrogenation

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