Visualizing reactant molecules induced lattice breath of catalyst and its role for activity enhancement


 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.


Introduction
Nowadays, it has been based on a "static" viewpoint for the study of heterogeneous catalysts during the dynamic and complex reactions 1-3 , 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 TEM 4,5 , people can directly observe the hyper ne surface reconstruction induced by the interaction between reactant molecules and catalysts [6][7][8][9] . For example, Yoshida 10 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 facets 11 . 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 hyper ne surface changes of the catalyst e.g the rearrangement of atoms and chemical bonds, its surface electronic and energy states would change accordingly 12,13 . 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-WO 3-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 WO 3-x

Results
Structure and active centers of the catalysts Single atomic Pt supported on defective WO 3-x and non-defective WO 3 nanosheets (samples denoted as Pt SA-WO 3-x and Pt SA-WO 3 ) were prepared as the model catalysts   14,15 . The high-angle annular dark-eld-scanning transmission electron microscope (HAADF-STEM) and energydispersive X-ray spectroscopy (EDXS) mapping pro les con rmed the individual Pt atoms (marked by the orange circles) dispersed on the surface of WO 3-x nanosheet ( Fig. 1a and Supplementary Fig. 6). The intensity pro le 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.  (Fig. 1b and 1c), the smooth and regular atomic arrangement in the crystal lattice of WO 3 became rougher and distorted after the introduction of defects 16 . Moreover, compared to the quantity of Pt loading on WO 3-x and WO 3 (Supplementary Table 2), it was inferred that existence of oxygen vacancy in the WO 3-x greatly improved the anchoring of isolated Pt atoms [17][18][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 WO 3-x were unstable and easily transformed into Pt clusters with diameters of <1 nm after reaction, which can be con rmed by HAADF-STEM (Fig. 1d, marked by the green circles). For comparison, the size of Pt nanoparticles supported by WO 3-x remained unchanged after reaction after the same time ( Supplementary Fig. 14) 20 . Further, the structure evolution of Pt species on the WO 3-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 Pt δ+ 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) con rmed 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 Pt δ+ (0 < δ < 2) in the Pt SA-WO 3-x 23 . 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 Pt 0 species, which was similar with that of supported Pt NP and indicated the formation of Pt clusters Therefore, the Pt 4 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  (Fig. 2a), the lattice fringes with spacing of 3.8 Å was assigned to a clean WO 3-x (020) surface under Ar. After the input of NO, we directly observed the lattice fringes expansion of WO 3-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 signi cantly leads to lattice expansion. Moreover, when the Pt SA-WO 3-x was exposed to the H 2 + NO mixture, it was found that the lattice fringe remain . Therefore, we con rm 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-O 25,26 . Then the exsitu Raman was used to characterize the chemical structure of the catalyst, there was no signi cant difference between pristine WO 3-x /WO 3 and Pt-loaded WO 3-x /WO 3 ( Supplementary Fig. 20), the bands at 815 cm -1 (~810 cm -1 for Pt SA-WO 3 ) 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 modes 25,27 . When carrying out the in situ Raman analysis, for the Pt SA-WO 3-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 owing 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 + H 2 (volume ratio NO:H 2 = 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-WO 3 ( 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 bene cial 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 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 H 2 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 WO 3-x and non-defective WO 3 supports exhibited very poor activity in the whole temperature range, although the defective WO 3-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-WO 3-x and Pt NP-WO 3-x exhibited much better deNO x catalytic activity than Pt SA-WO 3 and Pt NP-WO 3 in the low temperature of 50-250°C.
Speci cally, comparison of the Temperature programmed surface reaction (TPSR) results of NO + H 2 reaction on the Pt SA-WO 3-x and Pt NP-WO 3-x catalysts at 150°C ( Supplementary Fig. 25), both of them had excellent N 2 selectivity ( Supplementary Fig. 26), but Pt SA-WO 3-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 NO 30 .
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 di cult to control on WO 3-x and WO 3 . Based on the above considerations, an experiment was designed to further con rm the relationship of catalytic performance and lattice expansion. We used WO 3-x and WO 3 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-WO 3-x show better catalytic activity than Pt NP-WO 3 (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-WO 3-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 Pt 4 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 Pt 4 site to react with the activated H* due to a large energy barrier (Fig. 4d), and vice versa. The activation of H 2 and NO tend to occur only on Pt species. Therefore, we could infer that the enhanced catalytic activity of Pt NP-WO 3-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 H 2 on Pt 4 /WO 3-x model catalyst (Fig. 5a). The reaction path expression and reaction energy barrier for each step were shown in 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 atoms 28 . Therefore, we further performed the catalytic cycle for the Pt 4 cluster with a square structure, and found it could also complete the reduction of NO with H 2 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.  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-WO 3-x and Pt NP-WO 3 . The Pt particles supported on defective WO 3-x and nondefective WO 3 nanosheets were prepared by the same methods as that of Pt SA-WO 3-x and Pt SA-WO 3 .
Typically, the WO 3-x or WO 3 (0. Catalytic activity test. The catalytic activity measurements of series of Pt-W catalysts for H 2 -SCR were carried out in a xed-bed continuous ow 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 H 2 with Ar as balance gas. The total ow 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 - to remove the physically adsorbed species on the surface of the sample. Then the series of temperaturedependent 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 lm of TEM. For each experiment, the whole gas system was flushed with Ar for 30 min at atmospheric pressure, then the NO or H 2 was introduced and the temperature was increased by 30°C·min −1 to 150°C (423 K). The gases of pure NO (99.995%), H 2 (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 (OmniStar TM 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 ow 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 puri ed 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 ow 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 : H 2 = 1:4, 1 bar) were introduced into the reactor at the speci ed 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 radiation 41 . 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 : H 2 = 1:4, 1 bar) were introduced into the reactor at the speci ed 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 nd 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 packages 40 . The k 3 -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, k 3weighted χ (k) data of W L 3 -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 tting was performed using the ARTEMIS module of IFEFFIT software packages 41. Theoretical calculation. First-principles calculations were performed using density functional theory from CASTEP package with the plane-wave pseudopotential method 42 . The ion core and valence electron interaction were represented by Vanderbilt-type ultrasoft pseudopotential 43 . The exchange-correlation interactions were treated by the generalized-gradient approximation with spin-polarized Perdewn-Burke-Ernzerhof (PBE/GGA) scheme 44 . 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. WO 3 (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 de ned as: where E total is the total energy of adsorbed species on surface. E sub and E mol are the energy of isolated substrate and adsorbed species, respectively. The more negative the ΔE ads value, the more strongly adsorbed species binds on the surface.

Data availability.
The data that support the ndings of this study are available from the corresponding authors upon reasonable request.

Competing interests
The authors declare no competing interests in this work.   In situ TEM analysis of Pt SA-WO3-x and Pt SA-WO3. Time-lapse TEM images for Pt SA-WO3-x (a) and Pt SA-WO3 (b). Reaction conditions: after treatment of the Pt SA-WO3-x or Pt SA-WO3 with high purity Ar atmosphere at 150°C (423 K) for 30 min, then followed by switching on NO. The same position after NO was switched off, followed by purging with Ar, then switching the gas atmosphere to NO or a mixture of NO + H2 (volume ratio NO:H2 = 1:4). Scale bars: 1 nm. The change of outmost, sub and third lattice spacing of Pt SA-WO3-x or Pt SA-WO3 during the reaction (c, d).