Recently, a few in situ observations showed that reactant molecules can induce the hyper-fine surface change of catalyst under realistic conditions. However, hardly any report further illustrated that whether and how could this phenomenon affect the catalytic performance. Herein, using in situ TEM, we visually captured the reversible lattice expansion of Pt-WO3-x catalyst induced by NO in exemplified reduction of NO with H2. Results showed that the NO could adsorb on the oxygen vacancy sites and energetic favorably induced the reversible stretching of W-O-W bonds. 20% enhancement of catalytic activity was then correlated with this lattice expansion. Moreover, DFT calculations showed that the lattice expansion can reduce the adsorption energy of NO on Pt4 centers and also the energy barrier of the rate-determining step.
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Visualizing reactant molecules induced lattice breath of catalyst and its role for activity enhancement
https://doi.org/10.21203/rs.3.rs-114909/v1
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Nowadays, it has been based on a “static” viewpoint for the study of heterogeneous catalysts during the dynamic and complex reactions1-3, due to the limitation of powerful in-situ methods. Therefore, some crucial dynamic information of catalyst e.g. its real active site or transient surface states would be missed for further understanding the nature of catalysis. Recently, with the development of in situ characterization techniques such as in situ TEM4,5, people can directly observe the hyperfine surface reconstruction induced by the interaction between reactant molecules and catalysts6-9. For example, Yoshida10 found that after adsorption of CO molecules, the distance between the {200} and the adjacent {100} crystal planes of supported Au particles could change from 0.29 nm and 0.20 nm to 0.25 nm, respectively. Also, it is observed that the adsorbed CO molecules on the supported Pt nanoparticles could cause the transformation of exposed {100} facets into vicinal stepped high Miller index facets11. These progresses have brought further advances in understanding the precise dynamic change of catalyst in the reaction environment. Considering the complexity of the heterogeneous catalysis, more surface dynamic information in new model reactions is urgent to be collected. Moreover, when the reactant molecules cause these hyperfine surface changes of the catalyst e.g the rearrangement of atoms and chemical bonds, its surface electronic and energy states would change accordingly12,13. This would probably affect the performance of the catalyst. However, up to now, hardly any study has been conducted in this regard.
Herein, the regular lattice expansion of Pt-WO3-x catalyst induced by NO in a new model reaction of NO reduction was directly captured using in situ TEM. After the adsorption of NO, the lattice fringes spacing expanded from 3.8 Å to 4.1 Å, and then recovered by purging with Ar. This phenomenon was then explained by in-situ Raman spectroscopy and density functional theory (DFT) calculations, which showed that the adsorption of NO on the O vacancy sites of WO3-x would induce the stretching of W-O-W bonds, exhibiting the lattice expansion. Thereafter, 20% enhancement of catalytic activity was correlated with the lattice expansion effect. Moreover, the DFT calculations demonstrated the stretching of W-O-W bonds (lattice expansion) could improve the catalytic activity by reducing the adsorption energy ( -2.44 eV vs. -3.56 eV) of NO on Pt centers and activation energy barrier (1.37 eV vs. 1.51 eV) of the rate-determining step.
Structure and active centers of the catalysts
Single atomic Pt supported on defective WO3-x and non-defective WO3 nanosheets (samples denoted as Pt SA-WO3-x and Pt SA-WO3) were prepared as the model catalysts (Supplementary Figs. 1-5)14,15. The high-angle annular dark-field-scanning transmission electron microscope (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDXS) mapping profiles confirmed the individual Pt atoms (marked by the orange circles) dispersed on the surface of WO3-x nanosheet (Fig. 1a and Supplementary Fig. 6). The intensity profile taken along the orange line in the Supplementary Fig. 7, the intensity of the brighter spot (marked with the orange circle) was higher than that of the surrounding W atomic column, corresponding to the existence of Pt single atoms. The characterization of XPS, EPR, H2-TPR and O2-TPD clearly indicated that the rich surface oxygen vacancies on the WO3-x surface compared with WO3 (Supplementary Figs. 8-11), despite they showed the same phase, similar morphology and close specific surface areas (Supplementary Figs. 2 and 12, Supplementary Table 1). This was consistent with the analysis of HAADF-STEM and corresponding Fast Fourier Transform (FFT) (Fig. 1b and 1c), the smooth and regular atomic arrangement in the crystal lattice of WO3 became rougher and distorted after the introduction of defects16. Moreover, compared to the quantity of Pt loading on WO3-x and WO3 (Supplementary Table 2), it was inferred that existence of oxygen vacancy in the WO3-x greatly improved the anchoring of isolated Pt atoms17-19. Besides, XAS was also carried to study the coordination environment of isolated Pt. However, we could not get any valid information of coordination environment of Pt species because of the overlap of the absorption energy range of Pt and W elements (Supplementary Fig. 13).
Notably, the Pt single atoms on the WO3-x were unstable and easily transformed into Pt clusters with diameters of <1 nm after reaction, which can be confirmed by HAADF-STEM (Fig. 1d, marked by the green circles). For comparison, the size of Pt nanoparticles supported by WO3-x remained unchanged after reaction after the same time (Supplementary Fig. 14)20. Further, the structure evolution of Pt species on the WO3-x was further investigated by CO adsorption DRIFTS. The adsorption of CO bands centered at 2120 cm-1 was ascribed to linearly bonded CO on Ptδ+ single-atom sites with a top geometry and the peak appearing at 2084 cm-1 was assigned as adsorbed CO on Pt nanoparticles (Supplementary Fig. 15)21,22. It was noteworthy that the redshift of the IR peak from 2120 cm-1 to 2106 cm-1 after the reaction (Fig. 1e), indicating Pt clusters were formed during the reaction. Moreover, the in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) confirmed the transformation of Pt SA to Pt clusters during the reaction (Fig. 1f). In the ultra-high-vacuum (Supplementary Fig. 16a), the binding energy of 75.43 eV and 72.03 eV were assigned to the Ptδ+ (0 < δ < 2) in the Pt SA-WO3-x23. While at reaction conditions (Supplementary Fig. 16b), additional Pt peaks at 74.08 eV and 70.58 eV could be found and assigned to the Pt0 species, which was similar with that of supported Pt NP and indicated the formation of Pt clusters (Supplementary Figs. 17 and 18)24. This was consistent with the results of HAADF-STEM and in situ CO-DRIFTS. Moreover, by a comprehensive consideration of experimental and theoretical results, the size of Pt4 cluster is basically consistent with that of the Pt clusters observed under HAADF-STEM, and that the Pt4 cluster is the most proper model for the clusters that can stably exist on the surface of WO3-x. Therefore, the Pt4 cluster was selected as a model for the real active centers of the catalysts (Fig. 1g).
Visualizing and analyzing the lattice expansion
To unravel the interaction between NO molecules and the surface of catalysts at the real reaction condition, we carried out in situ high-resolution transmission electron microscopy (in situ TEM, experimental details in supporting information). The NO or H2 or a mixture of NO + H2 (volume ratio NO:H2 = 1:4, 1 bar) was introduced to the microscope column at 150°C (423 K). Then, the real-time structural change on (020) surface edge of the Pt SA-WO3-x and Pt SA-WO3 were visualized by time-resolved TEM images as shown in Fig. 2. For Pt SA-WO3-x (Fig. 2a), the lattice fringes with spacing of 3.8 Å was assigned to a clean WO3-x (020) surface under Ar. After the input of NO, we directly observed the lattice fringes expansion of WO3-x. After 380s input of NO, the lattice fringes spacing expanded from 3.8 Å to 4.1 Å. Then followed by purging with Ar, the lattice fringes shrank to 3.8 Å. While exposed to the NO again, the lattice fringe expanded to 4.1 Å again, showing a reversible process, just like the breath. The corresponding dynamic microscopic evolution of catalyst was also recorded in the Supplementary movie 1, which clearly showed that the NO adsorption significantly leads to lattice expansion. Moreover, when the Pt SA-WO3-x was exposed to the H2 + NO mixture, it was found that the lattice fringe remain expansion throughout the reaction (Fig. 2a and Supplementary movie 1). While for the Pt SA-WO3, no lattice expansion or shrinkage was observed (Fig. 2b and Supplementary Movie 2). Furthermore, from the detailed change of outmost, sub and third lattice spacing of Pt SA-WO3-x and Pt SA-WO3 during the reaction in the Fig. 2c and 2d, we could discover lattice expansion of WO3-x induced by NO occurred mainly at the outmost lattice layer. In addition, to rule out the electron beam effects on the lattice expansion during NO reduction, we performed continuous electron-beam irradiation experiments on Pt-WO3-x without reactant NO and H2. The results clearly indicate that the outmost edge of WO3-x surface is truly stable, and the 10-minute irradiation of the electron beam at 200 keV did not induce obvious lattice fringes spacing changes at the outmost edge of surface (see more details in the Supplementary Fig. 19 and Supplementary Movie 3). Therefore, we confirm that the lattice expansion was caused by the reactant molecules, rather than the electron beam irradiation.
To further reveal the chemical nature of lattice expansion during the reaction, in situ Raman spectroscopy was then performed (Fig. 3). As shown in Fig. 3a, according to the rules of Raman spectra, the stretching of the bond length of W-O could cause the decrease of the vibration frequency of W-O25,26. Then the ex-situ Raman was used to characterize the chemical structure of the catalyst, there was no significant difference between pristine WO3-x/WO3 and Pt-loaded WO3-x/WO3 (Supplementary Fig. 20), the bands at ~815 cm-1 (~810 cm-1 for Pt SA-WO3) and ~703 cm-1 (~715 cm-1) were assigned to W-O-W symmetric and asymmetric stretching frequencies, respectively. The bands at ~266 cm-1 (~269 cm-1) and ~328 cm-1 (~324 cm-1) were related to the W-O-W bending modes25,27. When carrying out the in situ Raman analysis, for the Pt SA-WO3-x, all vibrational characteristic peaks (815 cm-1, 703 cm-1, 328 cm-1 and 266 cm-1) of W-O-W bonds were red shifted (809 cm-1, 695 cm-1, 325 cm-1 and 261 cm-1) after flowing NO at 150°C (423 K) for 30 min (Fig. 3b). This was due to the bond length of W-O was stretched after the adsorption of NO, resulting in the decrease of the vibration frequency of W-O. While after switching the gas atmosphere to NO + H2 (volume ratio NO:H2 = 1:4), there was no further shift of the strong peak at ~809 cm-1 corresponds to the W-O-W (Fig. 3c). And all the peaks could restore the original position after purging with Ar. As a comparison, no obvious peak shift was observed on the Pt SA-WO3 (Fig. 3d and 3e). These results were consistent with those of in situ TEM observations and further elucidated that the NO induced lattice expansion could be ascribed to the stretching of W-O-W bonds. Also, the existence of oxygen vacancies was beneficial for the adsorption of NO and the stretching of W-O-W bonds.
Moreover, to further demonstrate the lattice expansion at molecular level, the DFT calculations were conducted. As shown in Supplementary Figs. 21-23, three aspects were considered for building the models: 1) the adsorption of NO on the (010) surface of non-defective WO3 and defective WO3-x; 2) the role of oxygen vacancies on defective WO3-x; 3) the adsorption of NO on Pt4 cluster supported on defective WO3-x28. Results indicated that Pt cluster had the strongest adsorption capacity for NO, and the adsorption energy of one NO on non-defective WO3 was lower than those of adsorption on vacancy sites of defective WO3-x and on Pt4 centers (-0.51 eV vs. -2.71 eV vs. -3.56 eV). However, the strength of lattice expansion of W-O-W did not follow this order. The length of surface W-O-W bond (equal to lattice spacing) expanded from 3.79 Å to 3.91 Å after adsorption of one NO on the oxygen vacancy site, while the strength for adsorption on Pt4/WO3-x was very weak (from 3.83 Å to 3.85 Å) and there was no change in lattice spacing of non-defective WO3 (from 3.73 Å to 3.73 Å). Besides. we also calculated and found that the lattice expansion was positively correlated with the number of oxygen vacancies (Supplementary Table 3). The above theoretical calculations revealed that the lattice expansion was mainly caused by the adsorption of NO on the oxygen vacancy sites, which induced the stretching of W-O-W bonds.
Correlation of activity and lattice expansion
To elucidate the relationship of catalytic performance and lattice expansion, we also chose reduction of NO with H2 as the model reaction (Fig. 4)29. Before designing of proper experiments, several tests about catalytic performance have been performed concerning the effect of supports and the active centers (Fig. 4a and 4b). Firstly, both defective WO3-x and non-defective WO3 supports exhibited very poor activity in the whole temperature range, although the defective WO3-x had a strong adsorption capacity for NO (see the results of NO-TPD in Supplementary Fig. 24). While introducing Pt species on the catalysts could dramatically improve the catalytic activity. Moreover, both of Pt SA-WO3-x and Pt NP-WO3-x exhibited much better deNOx catalytic activity than Pt SA-WO3 and Pt NP-WO3 in the low temperature of 50-250°C. Specifically, comparison of the Temperature programmed surface reaction (TPSR) results of NO + H2 reaction on the Pt SA-WO3-x and Pt NP-WO3-x catalysts at 150°C (Supplementary Fig. 25), both of them had excellent N2 selectivity (Supplementary Fig. 26), but Pt SA-WO3-x exhibited better catalytic performance. These results proved that Pt was the main active center for NO adsorption and activation and the oxygen vacancy mainly exhibited as the adsorption site of NO30.
Accordingly, the lattice expansion was mainly related to oxygen vacancies of support and had no relation to the loading of Pt atoms or particles on support. Besides, the accurate loading amounts of single atom were quite difficult to control on WO3-x and WO3. Based on the above considerations, an experiment was designed to further confirm the relationship of catalytic performance and lattice expansion. We used WO3-x and WO3 to load the same amount of Pt nanoparticles (0.3 wt%), and there was no obviously difference in the dispersion state of Pt particles observed by TEM (Supplementary Fig. 27), which ensured that there was almost no difference in the Pt active centers between the two catalysts. The result indicated that Pt NP-WO3-x show better catalytic activity than Pt NP-WO3 (ratios of differ by 20%, Fig. 4c). In this case, by excluding the effect of active centers and support, we can further correlated the activity difference with three reasons: 1) lattice expansion of Pt NP-WO3-x induced by the adsorption of NO, 2) the pre-adsorbed NO on the oxygen vacancy could easily transferred to Pt active sites for activation, 3) the activated H* on Pt species spills over the adsorbed NO on the oxygen vacancy for further reaction. Then DFT calculations were carried out to evaluate these viewpoints, we created an oxygen vacancy in the ortho position of Pt4 cluster, and then adsorbed a NO molecule on the oxygen vacancy. Then the results show that the adsorbed NO could not be transferred to the Pt4 site to react with the activated H* due to a large energy barrier (Fig. 4d), and vice versa. The activation of H2 and NO tend to occur only on Pt species. Therefore, we could infer that the enhanced catalytic activity of Pt NP-WO3-x was mainly correlated with the lattice expansion effect.
Elucidation the effect of lattice expansion
To further evaluate the role of surface lattice expansion for enhancing the catalytic activity, comprehensive DFT calculations were then conducted (Fig. 5). Firstly, we proposed a catalytic cycle for reduction of NO with H2 on Pt4/WO3-x model catalyst (Fig. 5a). The reaction path expression and reaction energy barrier for each step were shown in Table S4. The cycle was initiated by adsorption of NO and H2 on the Pt4 site. The results showed H2 could dissociate into two adsorbed H* directly and generally considered to be barrier less31,32. Then one adsorbed H* could be transferred to the adsorbed NO* to form NOH* (NO* + H* = NOH*) with an activation barrier of 1.19 eV (TS1). Thereafter, the other proton from dissociation of H2 would react with the NOH* (NOH* + H* = N* + H2O) to generate H2O via TS2 (1.03 eV). Subsequently, the other NO could adsorb on the N site and produce N-NO, and then activated and dissociated the second H2 (step Ⅵ). Similarly, the intermediate N-NOH was generated from the HN-NO, which was produced from the combination of H* and N-NO with an energy barrier of 0.94 eV (TS3). However, results showed that the other H* from dissociation of second H2 could not be transferred to the adsorbed N-NOH to form the second H2O due to a large energy barrier. In our experiments, the H2 in the actual reaction process was in excess amount. Thus, we introduced the third H2 molecule, and the N-NOH could react with the active H* to generate another H2O (N-NOH* + H* = N-N* + H2O) via TS4 (1.50 eV), which was the rate-determining step. Finally, the adsorbed N-N could form N2, and the produced H2O and N2 desorbed from the surface and complete the catalytic cycle. However, note that the structure of Pt4 changed from tetrahedral to square after the reaction (Supplementary Fig. 28). Previous DFT studies had shown that higher reactivity of NO decomposition was obtained on Pt cluster, which had a square structure composed of four Pt atoms28. Therefore, we further performed the catalytic cycle for the Pt4 cluster with a square structure, and found it could also complete the reduction of NO with H2 cycle, and follow the same reaction path and keep the stable square structure (Supplementary Figs. 29 and 30, Supplementary Table 5). These results also reveal that the whole reaction mechanism and path was reasonable via both thermodynamic and kinetic pathways.
Furthermore, considering that the lattice expansion was a metastable process, we have established a special model of Pt4/WO3-x (Supplementary Fig. 31) to investigate its effect toward catalytic activity, which artificially stretched the surface W-O-W lattice spacing from 3.8 to 4.0 Å. Then the initial step (adsorption of NO on Pt) and rate-determining step (generation of the second H2O) of the reaction were evaluated. The results showed that when the lattice expansion happened, the adsorption energy of NO and the barrier energy of the rate-determining step decreased from -3.56 eV and 1.52 eV to -2.44 eV and 1.37 eV, respectively (Fig. 5b and 5c, Supplementary Table 6). Therefore, the moderate adsorption energy of NO and the decreased barrier energy would enhance the catalytic activity33. This result further proved the lattice expansion of W-O-W occurring on the interface of the NO-Pt4/WO3-x was beneficial to the activation of NO and promotes the reaction.
In summary, choosing the Pt species supported WO3-x for reduction of NO with H2 as a model, we have focused on reactant molecules interacting with the surface of catalyst via in situ techniques and theory. The reversible surface lattice expansion of the Pt SA-WO3-x in reaction condition were visualized by in situ TEM, then further elucidated by in situ Raman spectroscopy and DFT calculations, respectively. Results showed that lattice expansion can be ascribed to the W-O-W stretching, which was induced by the adsorption of NO on the oxygen vacancy sites of defective WO3-x. Thereafter, the control experiments correlated 20% activity enhancement with the lattice expansion effect. Accordingly, DFT calculations further demonstrated the lattice expansion could reduce the adsorption energy of NO and activation energy barrier of the rate-determining step, thus promoting the catalytic activity. This work presented new insight that the reactant molecules could reversely affect the catalytic performance of catalyst by inducing the hyper-fine surface change of the catalyst. Moreover, it could also inspire more brilliant studies on rebuilding the dynamic and real working mechanism of heterogeneous catalyst.
Chemicals and materials. All chemicals were directly used without any further purification.
Synthesis of WO3 and defective WO3-x nanosheets. The WO3 and defective WO3-x nanosheets were synthesized according to a facile hydrothermal method16. For a typical preparation of defective WO3-x, 2.0 mmol of Na2WO4·2H2O (Aladdin, ≥99% purity) and 3.0 mmol of citric acid (Aladdin, ≥99% purity) were added into 60 mL of ultrapure water (18.25 MΩ) at room temperature under vigorous stirring. Then, 20 mmol of glucose (Aladdin, ≥99% purity) was added into the solution as a reductant. After vigorous stirring for 0.5 h, 6 mL of HCl (6 M) was dropped into the mixed solution and continued stirring for 0.5 h. The mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 393 K for 24 h. After cooling down to room temperature naturally, the obtained products were separated by centrifugation, washed with ultrapure water and anhydrous ethanol (Aladdin, ≥99% purity) thoroughly and dried at 333 K in vacuum overnight, theWO3-x was obtained. The synthesis method of WO3 nanosheets is the same as that of defective WO3-x except the absence of glucose.
Synthesis of Pt SA-WO3-x and Pt SA-WO3. The single atom Pt supported on defective WO3-x and non-defective WO3 nanosheets were prepared by a wet chemical method14,34-36. Typically, the WO3-x or WO3 (0.3 g) was dispersed in 25 mL ultrapure water by sonicating for 10min, and then an appropriate amount of H2PtCl6.6H2O (2 mg·mL-1) was added dropwise into the solution, followed by stirring at 343 K for 4h. The obtained products were separated by centrifugation, washed with ultrapure water and anhydrous ethanol thoroughly. After drying (333 K, in vacuum, 12 h) and further calcination (573 K, in air, 4 h). the Pt SA-WO3-x and Pt SA-WO3 were obtained.
Synthesis of Pt nanoparticles. The Pt nanoparticles were synthesized according to the reported method20,37. Typically, 50.75 mg of H2PtCl6.6H2O and 222 mg of polyvinylpyrrolidone (PVP, Aladdin, M=50, 000) were added into 20 mL of ethylene glycol (≥99% purity) at room temperature under vigorous stirring, and then the mixture solution was heated at 453 K in oil bath for 10 min. The obtained products were separated by centrifugation, washed with acetone (≥99% purity) and hexane (≥99% purity) to remove excess free PVP, the Pt nanoparticles were obtained and dispersed in ultrapure water for further use.
Synthesis of Pt NP-WO3-x and Pt NP-WO3. The Pt particles supported on defective WO3-x and non-defective WO3 nanosheets were prepared by the same methods as that of Pt SA-WO3-x and Pt SA-WO3. Typically, the WO3-x or WO3 (0.3 g) was dispersed in 25 mL ultrapure water by sonicating for 10min, and then accurate amount of Pt particles (the amount of Pt particles was same as the Pt content in Pt SA-WO3-x and Pt SA-WO3, as determined by ICP-MS) was added dropwise into the solution, followed by stirring at 343 K for 4h. The obtained products were separated by centrifugation, and then dried at 333 K in a vacuum for 12 h and further calcination (573 K, in air, 4 h), the Pt NP-WO3-x and Pt NP-WO3 were obtained.
Catalytic activity test. The catalytic activity measurements of series of Pt-W catalysts for H2-SCR were carried out in a fixed-bed continuous flow quartz microreactor (inner diameter = 10 mm). Select 0.1g catalyst of 40-60 mesh for each test. The reactant gas was controlled as follows: 1000 ppm NO, 4000 ppm H2 with Ar as balance gas. The total flow rate of the reactant gas was 100 mL·min−1, and the corresponding gas hourly space velocity (GHSV) was about 60, 000 mL·g-1·h-1. The component (NO, N2O and NH3) and concentration in the inlet gas and outlet gas were monitored by OPTIMA7 flue gas analyzer (MRU, GER) and FTIR spectrometer (MultiGasTM 2030 FTIR Continuous Gas Analyzer). The data for steady-state activity of the catalysts were measured after approximately 30 min at target temperature. The NOx conversion was obtained using the following equations: [NOx]out and [NOx]in denote the concentrations of NOx in the inlet and outlet, respectively. The NOx and N2 selectivity were defined as:
where [NOx] = [NO] + [NO2], the subscript “in” and “out” showed the inlet concentration and outlet concentration of corresponding gas at steady state respectively.
Material Characterizations. The powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX 2500v/PC X-ray diffractometer with Cu-Ka radiation (λ=1.5418 Å), operating at 40 kV and 40 mA and ranging from 10 to 80° with 5° step. The nitrogen adsorption–desorption isotherms were carried out on a Micromeritics ASAP 2460 at -196°C (77 K). The specific surface area was calculated based on the Brunauer-Emmett-Teller (BET) equation and the pore size distribution was calculated by a cylindrical pore model (BJH method). TEM and HRTEM were carried out on the FEI Talos F200X microscope operating at an accelerating voltage of 200 kV. The HAADF images and EDS-mapping results were obtained using the FEI Titan Themis G1 with double aberration correctors. Temperature programmed surface reaction (TPSR), Temperature Program Reduction by H2 (H2-TPR) and Temperature Program Desorption by O2 (O2-TPD) were performed on the Micromeritics Autochem 2920 instrument with a thermal conductivity detector (TCD). Prior to test, the sample was pretreated at 200°C (473 K) for 1 h under a He (30 mL·min-1) to remove the physically adsorbed species on the surface of the sample. Then the series of temperature-dependent spectra (10°C/min up to 1000°C (1273 K)) were sequentially collected. The X-ray photoelectron spectroscopy (XPS) analysis was carried out a surface analysis system (Thermal ESCALAB 250 spectrometer) equipped with Al Ka monochromatized radiation. The C 1s peak at 284.6 eV was chosen as a reference. Electron Paramagnetic Resonance (EPR) was carried out a Bruker E500 spectrometer.
In situ transmission electron microscopy (in situ TEM). In situ TEM was carried out on the FEI Talos F200X with an in situ TEM gas-phase holder (Climate S3+, DENSsolutions)38. Prior to experiments, the sample was encapsulated into a DENSsolutions chip, whose specified range of error for temperature is ≤ 5%. The holder was then inserted into Talos F200X and check the air tightness of the system. The whole in situ experiment was at an accelerating voltage of 200 kV and using a high-speed camera to record image and film of TEM. For each experiment, the whole gas system was flushed with Ar for 30 min at atmospheric pressure, then the NO or H2 was introduced and the temperature was increased by 30°C·min−1 to 150°C (423 K). The gases of pure NO (99.995%), H2 (99.995%) and Ar (99.999%) were purchased from Beijing Zhaoge Gas Technology Co., Ltd (Beijing, China). The production gas was detected by the on-line mass spectrometer (OmniStarTM GSD 320 O1 from Pfeiffer Vacuum, Germany).
In situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS). In situ DRIFTS were collected using a Thermo Nicolet 6700 spectrometer equipped with a mercury cadmium telluride (MCT) detector and in situ heating reaction cell (Harrick). The spectra were collected 64 scans at a resolution of 4 cm−1. Prior to test, the sample was placed in a reaction chamber and pretreated at 200°C (473 K) in a flow of He for 30 min (30 mL·min-1), then cooled down to 50°C (323 K). In situ DRIFT spectra were obtained to detect the CO adsorption on the samples, while the gas was switched to 1% CO/Ar (30mL·min-1), then purified with Ar.
In situ Raman spectra. All of the in-situ and ex-situ Raman spectra were carried out on a confocal Raman spectrometer (TEO SR-500I-A, Hong Kong) with an in situ heating reaction cell, using the excitation laser wavelength of 532 nm. Spectral resolution (FWHM) ≤ 1 cm-1. Spectral repeatability ≤ ± 0.2 cm-1. For the in situ Raman spectra, prior to test, the sample was placed in a reaction chamber and pretreated at 200°C (473 K) in a flow of He for 30 min (50 mL/min), then cooled down to reaction temperature. In the current work, high purity Ar gas or reactant mixed gas (NO : H2 = 1:4, 1 bar) were introduced into the reactor at the specified temperature for 0.5 h, then the Raman spectrum was measured.
In situ ambient pressure X-ray photoelectron spectroscopy (in situ AP-XPS). All of the in-situ and ex-situ XPS analysis were carried out a surface analysis system (Thermal ESCALAB 250 spectrometer) equipped with Al Ka monochromatized radiation41. The C 1s peak at 284.6 eV was chosen as a reference. For the in situ AP-XPS, high purity Ar gas or reactant mixed gas (NO : H2 = 1:4, 1 bar) were introduced into the reactor at the specified temperature for 0.5 h, followed by transferring to the analysis chamber for XPS measurement in an ultra-high vacuum system.
The X-ray absorption fine structure (XAFS). The X-ray absorption find structure spectra experiment was carried out on the beamline 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). All the data were collected at room temperature using ionization chamber in transmission mode. The acquired EXAFS date was analyzed and processed using the ATHENA module implemented in the IFEFFIT software packages40. The k3-weighted EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, k3-weighted χ(k) data of W L3-edge were Fourier transformed to real (R) space using a hanning windows (dk=1.0 Å-1) to separate the EXAFS contributions from different coordination shells. To obtain the quantitative structural parameters around central atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of IFEFFIT software packages41.
Theoretical calculation. First-principles calculations were performed using density functional theory from CASTEP package with the plane-wave pseudopotential method42. The ion core and valence electron interaction were represented by Vanderbilt-type ultrasoft pseudopotential43. The exchange-correlation interactions were treated by the generalized-gradient approximation with spin-polarized Perdewn-Burke-Ernzerhof (PBE/GGA) scheme44. The kinetic energy cutoff was set to 400 eV. The convergence thresholds between optimization cycles for energy change and maximum force were set as 10-5 eV/atom and 0.03 eV/Å, respectively. WO3 (010) surface was constructed using slab models with a vacuum thickness of 15 Å to avoid the interaction between periodic images. The Monkhorst-Pack grid of 2×2×1 was used to carry out the interface calculations. The transition states were searched by the generalized synchronous transit (LST/QST) method.
The adsorb energies of adsorbed species are defined as:
ΔEads = Etotal – Esub – Emol
where Etotal is the total energy of adsorbed species on surface. Esub and Emol are the energy of isolated substrate and adsorbed species, respectively. The more negative the ΔEads value, the more strongly adsorbed species binds on the surface.
Data availability.
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
Competing interests
The authors declare no competing interests in this work.
Author contributions
Z. Hao and G. Liu conducted catalyst syntheses and performance tests. H. Zhang participated in catalyst characterization. W. Sun performed DFT calculations. L. Zheng performed the XAFS measurements and analysed the data. X. Mi performed the TEM characterization. Y. Li, H. Wang and P. Wang provided helpful discussions. Z. Shen. and S. Zhan directed and supervised the project. Z. Hao, Z. Shen and S. Zhan prepared the manuscript. All authors discussed the results and commented on the manuscript.
Acknowledgments
The authors gratefully acknowledge the financially support by the Natural Science Foundation of China (grant Nos. 21722702 and 21872102), the Tianjin Commission of Science and Technology as key technologies R&D projects (grant Nos. 18YFZCSF00730, 18YFZCSF00770,18ZXSZSF00230 and 19YFZCSF00740) and National Key Basic Research Program of China (2017YFA0403402). We acknowledge Dr. W. Xi at Tianjin University of Technology for the in situ TEM characterization and helpful discussions.
- Turner, M. et al. Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters. Nature 454, 981–983 (2008).
- Valden, M., Lai, X. & Goodman D.W. Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties. Science 281, 1647–1650 (1998).
- Chen, M.S. & Goodman, D.W. The structure of catalytically active gold on titania. Science 306, 252–255 (2004).
- Fan, Z. et al. In situ transmission electron microscopy for energy materials and devices. Adv. Mater. 31, 1900608 (2019).
- Jinschek, J.R. Advances in the environmental transmission electron microscope (ETEM) for nanoscale in situ studies of gas-solid interactions. Chem. Commun. 50, 2696–2706 (2014).
- Tao, F.F. & Crozier, P.A. Atomic-scale observations of catalyst structures under reaction conditions and during catalysis. Chem. Rev. 116, 3487–3539 (2016).
- Böller, B., Durner, K.M. & Wintterlin, J. The active sites of a working Fischer–Tropsch catalyst revealed by operando scanning tunnelling microscopy. Nat. Catal. 2, 1027–1034 (2019).
- Therrien, A.J. et al. An atomic-scale view of single-site Pt catalysis for low-temperature CO oxidation. Nat. Catal. 1, 192–198 (2018).
- Vendelbo, S.B. et al. Visualization of oscillatory behaviour of Pt nanoparticles catalysing CO oxidation. Nat. Mater. 13, 884–890 (2014).
- Yoshida, H. et al. Visualizing gas molecules interacting with supported nanoparticulate catalysts at reaction conditions. Science 335, 317–319 (2012).
- Avanesian, T. et al. Quantitative and atomic-scale view of CO-Induced Pt nanoparticle surface reconstruction at saturation coverage via DFT calculations coupled with in situ TEM and IR. J. Am. Chem. Soc. 139, 4551–4558 (2017).
- Nolte, P. et al. Shape changes of supported Rh nanoparticles during oxidation and reduction cycles. Science 321, 1654–1658 (2008).
- Rodriguez, J.A., Ma, S., Liu, P., Hrbek, J., Evans, J. & Pérez, M. Activity of CeOx and TiOx nanoparticles grown on Au(111) in the water-gas shift reaction. Science 318, 1757–1760 (2007).
- Qiao, B. et al. Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 3, 634–641 (2011).
- Zhang, H., Liu, G., Shi L. & Ye, J. Single-atom catalysts: emerging multifunctional materials in heterogeneous catalysis. Adv. Energy Mater. 8, 1701343 (2018).
- Zhang, N. et al. Defective tungsten oxide hydrate nanosheets for boosting aerobic coupling of amines: synergistic catalysis by oxygen vacancies and bronsted acid sites. Small 13, 1701354 (2017).
- Wang, J. et al. Hydrogenolysis of glycerol to 1,3-propanediol under low hydrogen pressure over WOx-supported single/pseudo-single atom Pt catalyst. ChemSusChem 9, 784–790 (2016).
- Yang, J. et al. Single atomic vacancy catalysis. ACS Nano 13, 9958–9964 (2019).
- Tian, H. et al. Oxygen vacancy-assisted hydrogen evolution reaction of the Pt/WO3 electrocatalyst. J. Mater. Chem. A 7, 6285–6293 (2019).
- Fang, X. et al. Single Pt atoms confined into a metal-organic framework for efficient photocatalysis. Adv. Mater. 30, 1705112 (2018).
- Ding, K. et al. Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts. Science 350, 189–192 (2015).
- Zhang, Q. et al. Isolated Platinum atoms stabilized by amorphous tungstenic acid: metal-support interaction for synergistic oxygen activation. Angew. Chem. Int. Ed. 57, 9351–9356 (2018).
- Park, J. et al. Investigation of the support effect in atomically dispersed Pt on WO3-x for utilization of Pt in the hydrogen evolution reaction. Angew. Chem. Int. Ed. 58, 16038–16042 (2019).
- Gu, F., et al. Atomically dispersed Pt (II) on WO3 for highly selective sensing and catalytic oxidation of triethylamine. Appl. Catal. B: Environ. 256, 117809 (2019).
- Cazzanelli, E., Vinegoni, C., Mariotto, G., Kuzmin, A. & Purans, J. Raman study of the phase transitions sequence in pure WO3 at high temperature and in HxWO3 with variable hydrogen content. Solid State Ion. 123, 67–74 (1999).
- Vidya, S., Solomon, S. & Thomas, J.K. Synthesis and characterisation of MoO3 and WO3 nanorods for low temperature co-fired ceramic and optical applications. J. Mater. Sci. Mater. Electron. 26, 3243–3255 (2015).
- Wang, Y. et al. Rational construction of oxygen vacancies onto tungsten trioxide to improve visible light photocatalytic water oxidation reaction. Appl. Catal. B: Environ. 239, 398–407 (2018).
- Fernandez, E. et al. Low-temperature catalytic NO reduction with CO by subnanometric Pt clusters. ACS Catal. 9, 11530–11541 (2019).
- Wang, L. et al. Catalysis and in situ studies of Rh1/Co3O4 nanorods in reduction of NO with H2. ACS Catal. 3, 1011–1019 (2013).
- Nguyen, L. et al. Reduction of nitric oxide with hydrogen on catalysts of singly dispersed bimetallic sites Pt1Com and Pd1Con. ACS Catal. 6, 840–850 (2016).
- Zhang, J. et al. Origin of synergistic effects in bicomponent cobalt oxide-platinum catalysts for selective hydrogenation reaction. Nat. Commun. 10, 4166 (2019).
- Zhang, B. et al. Atomically dispersed Pt1-polyoxometalate catalysts: how does metal-support interaction affect stability and hydrogenation activity? J. Am. Chem. Soc. 141, 8185–8197 (2019).
- Cui, X. et al. Room-temperature electrochemical water–gas shift reaction for high purity hydrogen production. Nat. Commun. 10, 86 (2019).
- Wu, X. et al. Surface step decoration of isolated atom as electron pumping: atomic-level insights into visible-light hydrogen evolution. Nano Energy 45, 109–117 (2018).
- Zhang, Z. et al. The simplest construction of single-site catalysts by the synergism of micropore trapping and nitrogen anchoring. Nat. Commun. 10, 1657 (2019).
- Spezzati, G. et al. Atomically dispersed Pd-O species on CeO2(111) as highly active sites for low-temperature CO oxidation. ACS Catal. 7, 6887–6891 (2017).
- Teranishi, T., Hosoe, M., Tanaka, T. & Miyake, M. Size control of monodispersed Pt nanoparticles and their 2D organization by electrophoretic deposition. J. Phys. Chem. B 103, 3818–3827 (1999).
- Xi, W. et al. Dynamic co-catalysis of Au single atoms and nanoporous Au for methane pyrolysis. Nat. Commun. 11, 1919 (2020).
- Zhang, S. et al. Catalysis on singly dispersed bimetallic sites. Nat. Commun. 6, 7938 (2015).
- Ravela, B. & Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFITJ. Synchrotron Rad. 12, 537–541 (2005).
- Rehr, J. J. & Albers, R. C. Theoretical approaches to x-ray absorption fine structure. Rev. Mod. Phys. 72, 621–654 (2000).
- Clark, S.J., Segall, M. D., Pickard, C.J., Hasnip, P.J. & Probert, M.I.J. First principles methods using CASTEP. Z. Krist-Cryst. Mater. 220, 567–570 (2005).
- Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 41, 7892–7895 (1990).
- Perdew, J.P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).
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- Supplementaryinformation.docx
- SupplementaryMovie1.mp4
Supplementary Movie 1
- SupplementaryMovie2.mp4
Supplementary Movie 2
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Supplementary Movie 3