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Ultra-fine platinum species supported on niobium pentoxide for CO oxidation

Platinum oxide supported on a Lewis acid niobium oxide (Nb2O5) support has been used for various heterogeneous and homogeneous catalysts. In this work, we used urea as a precipitating agent to obtain crystallized Nb2O5 with high surface area via a hydrothermal route. Nb2O5-supported Pt catalysts were subsequently synthesized via an incipient wetness impregnation approach. Multiple characterizations including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption confirmed the identical structural and textural properties of the Nb2O5 support before and after the impregnation process. Furthermore, the X-ray absorption fine structure technique (XAFS) results with related data analysis indicate that the platinum species in the fresh and H2-pretreated samples were in the form of single atoms or ultrafine clusters. In addition, the decrease in coordination number (CN) of the first-shell Pt–O bond, as well as the formation of Pt–Pt contribution with very low CN, after H2-pretreatment was verified, which corresponds to the decrease of oxidation state for Pt species on the surface of supports. Thus, the ultrafine-clustered metallic Pt species are considered to be more active than the oxidized Pt single ions. The current results will be of great significance in controllable synthesis of active Pt-based catalysts for other catalytic oxidation reactions.


Introduction
Platinum-based catalysts have been widely used in industry to catalyse a large number of important chemical reactions because of their superior activity and stability. [1][2][3][4] On the other hand, highly dispersed metal atoms on redox supports normally show excellent catalytic reactivity in various reactions by means of maximizing the efficiency of active metal atoms. [5][6][7] Therefore, more and more research groups have shown that sub-nanosized clusters or single atoms exhibit better catalytic performance than nanosized metal or metal oxide particles. [8][9][10][11][12][13][14] Judai et al. conrmed that the cluster catalysts consisting of only a few Pd atoms are active at much lower temperature than the bulk analogues for the NO reduction by CO. 9 The Zhang group synthesized isolated single Pt atoms anchored onto iron oxide (FeO x ), which exhibited very high activity and stability for both CO oxidation and preferential oxidation (PROX) of CO. 11 Other research results have shown that increasing the surface area of the supports can effectively improve dispersion to optimize activity. 15 The pretreatment of the catalysts by specic gas can also disperse the active metal particles thereby increasing catalytic activity. For instance, The Qin group used HCl steam to re-disperse Pt nanoparticles in process of propane dehydrogenation for better catalytic activity. 13 Original largesized Mo oxide on carbon support was calcined in an H 2 atmosphere for obtaining small-sized Mo species. 16 Aer vacuum calcination and H 2 reduction, thermal treatment of 0.2 wt% Pt/SiO 2 was used to synthesize Pt particles less than 1 nm that are uniformly dispersed on the SiO 2 . 17 At present, some researchers on the evolution of single-atom catalysts (SACs) during activation and catalytic reactions show that ultrane cluster is more active than pure single atoms. Recently, Piccolo Group demonstrated the importance of Pt clusters for CO oxidation reaction by means of in situ characterization methods. 18 Pt-O-Pt sites, represented by a Pt 8 O 14 model cluster were built on ceria for low-temperature CO oxidation reaction, which is 100-1000 times more active than their single atom Pt 1 / CeO 2 parent. 19 Many oxides ( In this work, we used urea as a precipitating agent to obtain pure-phase Nb 2 O 5 with high specic surface area by hydrothermal method using niobium(V) oxalate hydrate as a precursor and loaded different amounts of Pt species via incipient wetness impregnation subsequently. The catalytic activity of catalysts were tested via the CO oxidation reaction which is a typical benchmark reaction for estimating catalysts and plays a pivotal role in catalysis for both fundamental research and industrial applications, 34,39-41 including purication of polluted gases, preferential removal of CO in proton exchange membrane fuel cells under hydrogen-rich conditions, CO gas sensors and CO elimination of automotive exhaust gases. Our results indicate that the method of preparing Nb 2 O 5 can effectively disperse and anchor Pt atoms due to high specic surface area (127 m 2 g À1 ) of the Nb 2 O 5 compared to that of other reports (10 m 2 g À1 -65 m 2 g À1 ). [32][33][34]36 In the case of Ptbased catalysts, the pretreatment with H 2 of the catalysts was to reduce the platinum species for obtaining reduced platinum which is active species for CO oxidation. But X-ray absorption ne structure (XAFS) analysis results of this work indicated the increase of catalyst activity aer hydrogen pretreatment is not only due to the decrease of the oxidation state for Pt species but also to the appearance of Pt ne-clusters on the surface of supports.

Catalyst preparation
Preparation of Nb 2 O 5 supports. 2.402 AE 0.001 g of urea and 4.304 AE 0.001 g of niobium(V) oxalate hydrate powder (molar ratio, CO(NH 2 ) 2 : Nb ¼ 10 : 1) was dissolved in 20 mL of deionized and 60 mL of deionized water at room temperature, respectively. The mixed solution was transferred to a Teonlined stainless-steel autoclave (inner volume: 100 mL) and then heated at 80 C for 8 hours in vessel. And the temperature was sequentially elevated up to 200 C and kept for two days. Aer the hydrothermal process nished and the vessel was cooled down to room temperature, the resulting precipitate was isolated by centrifugation, washed repeatedly with deionized water until neutral. The as-washed products were dried at 80 C overnight and then calcined at 400 C for 4 h at muffle furnace. Aer grinding and sieving (100 mesh) the calcined samples, the Nb 2 O 5 fresh powder was obtained with the white colour.
Preparation of Pt/Nb 2 O 5 samples. The deposition of platinum onto the Nb 2 O 5 supports was obtained by incipient wetness impregnation (IMP). The atomic percentage of Pt was designed to be 0.05, 0.1, 0.3, 0.5 and 1.0 at%, which was denoted as 0.05 Pt/Nb 2 O 5 , 0.1 Pt/Nb 2 O 5 , and 0.3 Pt/Nb 2 O 5 , 0.5 Pt/Nb 2 O 5 and 1.0 Pt/Nb 2 O 5 , respectively. Aer the impregnation step, the catalysts were dried at 80 C for 12 h and then calcined at 400 C for 4 h under static air in a muffle furnace.

Characterization
Transmission electron microscope (TEM). All the tested samples were suspended fully in ethanol and then dropped on an ultrathin carbon stainless-steel Mo grid. The prepared samples were dried naturally under ambient conditions before placed on the TEM sample holder. The transmission electron microscopy, high-resolution transmission electron microscopy (HRTEM) images were obtained from a FEI Tecnai G2 F20 microscope (FEI Co., Hillsboro, TX, USA) operating at 200 kV with a Genesis XM2 accessory (EDAX Inc., Mahwah, NJ, USA).
X-ray diffraction. The grinded catalyst was placed inside a quartz-glass sample holder and attened before each testing. X-ray diffraction was carried out on a Bruker AXS D2 PHASER, using Cu Ka radiation (l ¼ 0.15406 nm). The diffraction patterns were collected from 10 to 90 with a step of 0.02 . The 2q angles were calibrated with a micrometer-scale alumina disc.
X-ray absorption ne structure (XAFS). The XAFS spectra at Pt L3-edge (E 0 ¼ 11 564.0 eV) were performed at BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) operated at 3.5 GeV under "top-up" mode with a constant current of 220 mA. The sample mixing LiF uniformly was pressed into a sheet, then sealed with Kapton tape before the XAFS test. The XAFS data were recorded under uorescence mode with a standard Lytle ion chamber. The energy was calibrated with Pt foil as a reference. Athena and Artemis codes were used to extract the data and t the proles. For the X-ray absorption near-edge spectroscopy (XANES) part, the experimental absorption coefficients as function of energies m(E) were processed by background subtraction and normalization procedures and reported as "normalized absorption". Based on the normalized XANES proles, both the molar proportion of Pt 4+ /Pt 0 and the oxidation state of Pt can be determined by the linear combination t with the help of various references (Pt foil for Pt 0 and PtO 2 for Pt 4+ ). For the extended X-ray absorption ne structure (EXAFS) part, the Fourier transformed (FT) data in R space were analysed by applying PtO 2 and metallic Pt model for Pt-O and Pt-Pt contributions. The passive electron factors, S 0 2 , were determined by tting the experimental data on Pt foils and setting the coordination number (CN) of Pt-Pt to be 12, and then xed for further analysis of the measured samples. The parameters describing the electronic properties (e.g., correction to the photoelectron energy origin, E 0 ) and local structure environment including CN, bond distance (R), and Debye-Waller factor around the absorbing atoms (s 2 ) were allowed to vary during the t process. The tted ranges for K and R spaces were selected to be k ¼ 2. Nitrogen adsorption-desorption. For testing the BET specic surface area and pore size distribution of the samples, the nitrogen adsorption-desorption characterization was performed on an ASAP2020-HD88 analyzer (Micromeritics Co., Ltd., Atlanta, GA, USA) at 77 K. The fresh powders were degassed at 250 C under vacuum over 4 h and then analysed under N 2 atmosphere cooling with liquid nitrogen. The BET specic surface areas (S BET ) were obtained from data of the relative pressure between 0.05 and 0.20.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES). The platinum concentrations were examined by ICP-AES on Optima 5300DV (PerkinElmer corp.) H 2 -temperature programmed reduction (H 2 -TPR). H 2 -TPR measurement of the samples was performed on micromeritics AutoChem 2920 equipped with a thermal conductivity detector (TCD) to detect H 2 consumption. Before measurement, the fresh sample (50 mg, 20-40 mesh) was pretreated in 5% O 2 /Ar (30 mL min À1 ) at 300 C for 30 min. Aer cooling down, the measured sample was heated from room temperature to 600 C with a step of 10 C min À1 in continuous 5% H 2 with He as balance gas.

Catalytic test
CO oxidation reactions were carried out in micromeritics AutoChem 2920. The detection processes of catalyst activity were as follows: The sieved sample (40-60 mesh, 50 mg) was loaded into a U-shaped quartz tube, and then preactivated at 5% H 2 /95% He or 5% O 2 /95% N 2 for 30 min with a ow rate of 50 mL min À1 before the reaction. Aer the reactor cooled to room temperature, pure He gas was used to purge H 2 or O 2 for 20 minutes. Aerwards, CO oxidation activities of Pt/Nb 2 O 5 catalysts were measured in a stoichiometric gas mixture of 2% CO/ 1% O 2 /97% He at a ow rate of 30 mL min À1 , corresponding to a gas hourly space velocity (GHSV) of 36 000 mL h À1 g cat À1 . The reactant temperature began from 30 C to 300 C with the heating rate of 10 C min À1 and the outlet gas compositions of CO and CO 2 were recorded on-line at 30 43 In addition, no Pt-containing peaks were identied for all fresh catalysts shown in Fig. S1a. † 1 Pt and 0.3 Pt catalysts were selected for hydrogen pretreatment and further XRD testing (Fig. S1b †). There are no Pt peaks on H 2 -pretreated catalysts, which preliminarily indicates the high dispersion or small size of Pt species. As for used samples, no Pt species are found for all samples (see Fig. S2a and b †). The average crystallite size calculated by the Scherrer equation for pure Nb 2 O 5 is 11.9 nm (see Table 2). Crystallite sizes of niobium pentoxide with hexagonal structure synthesized using solvent evaporation method were 61.4 nm in Ziolek' work. 32 (100) peaks of the XRD of the used samples were selected to plot the intensity seen in Fig. S3. † There is no obvious evidence to prove the regular development of materials with hydrogen pretreatment or oxygen pretreatment, as Summarizing the results of XRD analysis, the fresh catalysts, the H 2 -pretreated samples and the used samples are not observed peaks related to platinum structure due to the ultra-ne sizes or high dispersion of platinum species.
The ICP-AES results (see Table 1) show that the experimental Pt content is close to the target value for each catalyst. N 2 adsorption-desorption results are present in Fig. S4 † and summarized in Table 1. Both Nb 2 O 5 support and Pt-Nb 2 O 5 catalysts show type IV adsorption isotherm and H1-type hysteresis loop with the BJH pore volumes (Vp) between 0.280-0.296 cm 3 g À1 . Also, the surface area was almost the same aer Pt deposition (114-121 m 2 g À1 ), indicating the good support of nanosized Nb 2 O 5 for platinum. Furthermore, a particularly high BET specic surface area (127 m 2 g À1 ) was determined for Nb 2 O 5 , which provides a matrix for Pt loading in this work. All these data indicate that the introduction of Pt species has no signicant effect on the textural properties of Nb 2 O 5 , and Pt-Nb 2 O 5 catalysts were stable aer air-calcination at 400 C.
For further analysis, the Nb 2 O 5 , 0.3 Pt/Nb 2 O 5 and 1 Pt/Nb 2 O 5 samples were subjected to TEM characterization. The TEM images of pure Nb 2 O 5 support in Fig. 2a show a worm-like aggregation with a primary particle size of about 11 nm (see Table 2), which is consistent with the XRD results. The HRTEM image in Fig. 2b clearly displays an interplanar spacing of Table 1 Pt concentration, BET specific surface areas (S BET ), BJH pore volume (V p ) of Nb 2 O 5 supports, temperature for 50% CO conversion (T 1/2 )

Sample
Pt loading a (at%)   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 12326-12333 | 12329 0.392 nm, corresponding to the (001) lattice fringe of pseudohexagonal Nb 2 O 5 . No metallic Pt particles but the only structure of Nb 2 O 5 was observed in HRTEM for both 0.3 Pt/Nb 2 O 5 (Fig. 3a) and 1 Pt/Nb 2 O 5 (Fig. 3b). Similarly, the HRTEM images of the H 2 -pretreated samples and used samples shown in Fig. 3c-f Fig. S5, † which indicates that Pt is uniformly dispersed on the support, but size and form of Pt are still unclear. Consulting other groups' work about Pt/Nb 2 O 5 , Pt species were loaded on Nb 2 O 5 with size of particles of 2.7-17.9 nm, 30,31,36,44 while the Pt species of calcined samples in this work cannot be identied on HRTEM images, suggesting that ultrane Pt species supported in Nb 2 O 5 obtained by the urea-based deposition-precipitation approach, proving the superiority of nanosized Nb 2 O 5 in this work.
The results of this part are briey summarized: Nb 2 O 5 nanocrystalline obtained by the urea-based deposition-precipitation approach is thermally stable. XRD and HRTEM characterization revealed that the deposited Pt species on Nb 2 O 5 are extremely small. Fig. 4 shows the H 2 -TPR proles of the as-calcined samples. On the one hand, the TPR of the Nb 2 O 5 support results in no reduction peak between 30 and 500 C, conrming that no surface reduction of the support takes place without Pt. On the other hand, the two small reduction peaks for Nb 2 O 5 -supported Pt samples were observed with the peaks at 61 C and 331 C. The former reduction peak suggested a reduction of surface Pt oxide to metallic platinum. 36,44 And the latter reduction peak showed that the reduction of surface lattice oxygen related to the metal oxide support interacting with Pt species. 36,45,46 Since no peaks of Pt species could be observed from XRD and about the size and dispersion of Pt species could not be obtained from HRTEM. For further research, XAFS was used to study the electronic and the local coordination structure of the 1 Pt/Nb 2 O 5 and 0.3 Pt/Nb 2 O 5 . The XANES data for Pt L3-edge in Fig. 5a and b clearly demonstrate that the white line peaks of samples are all between Pt foil and PtO 2 but closer to Pt foil, indicating that the low valence state of Pt in fresh samples and H 2 -pretreatment samples. By means of the linear combination Pt/Nb 2 O 5 fresh sample and the 1 Pt/Nb 2 O 5 fresh sample are 1.82 (Table 3). However, the valence states of Pt in the H 2 -pretreated     Table 3.

Reducibility and active species of Pt/Nb 2 O 5 catalysts
The related EXAFS spectra of Pt L3-edge exhibit a single prominent peak at ca. 1.98 A originating from the rst shell of Pt-O contribution with a coordination number (CN) of around 4.1 (see Table 3) for both of 0.3 Pt/Nb 2 O 5 fresh sample (Fig. 5c) and 1 Pt/Nb 2 O 5 fresh sample (Fig. 5d), which strongly indicate that Pt species exist with the form of single atoms for calcined samples. Aer hydrogen pretreatment, according to the results of the tting, only the Pt-O shell was found without the peak of the Pt-Pt shell because the second shell is too small to tted. But the existence of ultrane clusters cannot be denied and neglected because there are very small peak corresponding to the Pt-Pt shell in Fig. 5c and d Fig. 5c and d, which show that bigger Pt clusters were formed on used samples.

Discussion
There are no characteristic peaks of Pt species on the XRD patterns ( Fig. S1 and S2 †) of the all Pt-based catalysts. 0.3 Pt-Nb 2 O 5 and 1 Pt-Nb 2 O 5 samples aer CO oxidation with H 2pretreatment were chosen to be characterized by TEM. The TEM images are not available for obtaining the information of Pt species of the used samples because the size of Pt species is less than the limit of detection of the device, which indicates that no Pt nanoparticles were formed aer the reaction. TEM also shows no structural transformation on supports for H 2pretreatment and used catalysts compared with fresh samples. All the above analysis shows that catalyst supports have no obvious deformation or aggregation during reaction test, so the catalysts are very stable during the CO reaction.
H 2 -TPR conrmed the interaction between the Pt species and Nb 2 O 5 because the pure support did not exhibit the reduction peak of the surface lattice oxygen in 331 C in the test temperature range, while it appeared aer the loading of Pt. It should be noted that since the low-content catalysts do not easily exhibit peaks in H 2 -TPR, only the spectrum of the 1 Pt catalyst is shown here.
The most important information about explanation of the above phenomenon is derived from XAFS results undoubtedly. Comparing the XAFS spectrum and analytical results of the H 2pretreated catalysts and the as-prepared catalysts, the H 2 -pretreated samples possess a tiny peak belonging to the Pt-Pt shell that is absent for fresh catalysts. The only one Pt-O shell for fresh catalysts sufficiently indicate that Pt species exist in the form of single atoms, which illustrates the success of the synthetic method of Nb 2 O 5 . The lower valence of Pt aer H 2pretreatment accounts for higher activity of CO conversion corresponding to the "light-off" curves of the CO oxidation reaction, which conrmed by the works of other research groups. 19,42 Then, decrease of coordination number and slight extension of Pt-O bond aer H 2 -pretreatment and the appearance of Pt-Pt shell, which indicates H 2 -pretreatment process promotes a few of single atoms to form ultrane Pt clusters on the Nb 2 O 5 surface resulting in higher activity than O 2 -pretreatment catalysts. We hypothesize the similar local coordination structure for O 2 -pretreated catalysts and the fresh catalysts because the fresh catalysts were obtained by calcination in air. Therefore, the above conclusions obtained by analysing and comparing the XAFS results of fresh catalysts and H 2 -pretreated catalysts are reliable and reasonable. XAFS reveals more details about the used catalysts. The used samples produce Pt-O shell and Pt-Pt shell according to the tting results (see Fig. 5 and Table 3). At the same time, the coordination number of Pt-O shell for used samples decreased (3.9-1.2 A for 1 Pt-Nb 2 O 5 and 4.2-1.6 A for 0.3 Pt-Nb 2 O 5 ) compared to H 2 -pretreat samples. It indicates that the coordination environment of Pt species undergone dramatic changes during the reaction. In other words, ultrane Pt clusters form bigger Pt clusters during CO

Conclusions
Nb 2 O 5 with a high surface area was obtained by the hydrothermal method using urea as a precipitating agent and different content Pt/Nb 2 O 5 catalysts were synthesized via incipient wetness impregnation. XAFS, a powerful tool for exploring the relationship between catalyst structure and structure-activity, indicates that Pt species present in fresh and H 2 -pretreated catalysts with the form of single atoms and ne clusters even when the content of Pt is 1%. The reasons for higher catalyst activity aer H 2 -pretreated catalysts than O 2pretreated is not only due to the reduction of the valence state of Pt species but also to the appearance of Pt ne-clusters on the surface of supports according to XAFS spectrum and its corresponding analysis results. The current results will be of great signicance in controllable synthesis of active ultrane clusters or single atom Pt-based catalysts for other catalytic oxidation reactions.

Conflicts of interest
There are no conicts to declare.