Structural characterization of bismuth-doped platinum samples. A serial of silica-supported platinum and platinum-bismuth samples were prepared by a co-incipient wetness impregnation method. The bulk concentrations of platinum and bismuth (Pt: 0.8, 0.9 and 0.9 wt.%; Bi: 2.3 and 6.1wt.% for 1Pt-SiO2, 1Pt2Bi-SiO2, 1Pt5Bi-SiO2 respectively) are close to these designed numbers, indicating the disposition of bismuth species have no effect on the loading of platinum (Supplementary Table 1). Furthermore, the Bi-free and Bi-doped samples have the similar textural properties, such as SBET values and the type of adsorption-desorption isotherms (Supplementary Table 1 and Supplementary Fig. 1), in which we can exclude the physical effect on the following catalytic performance.
Small-size oxide species were extremely stable on silica surface in Bi-promoted samples in aberration-corrected high-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) images (Fig. 1a,b), even after heat treatment in air. Only ultrafine clusters of 1.7±0.4 nm with narrow size-distribution were created in 1Pt2Bi-SiO2 (Supplementary Fig. 2e,f and 3a), without any crystallized platinum or bismuth metal/oxide particles. However, after calcination in air, huge metallic Pt particles of ~ 100 nm (Supplementary Fig. 2a) and platinum oxide clusters (1.6±0.5 nm) simultaneously appeared in 1Pt-SiO2 (Supplementary Fig. 2b,c and 3b). It illustrates that the addition of bismuth element could suppress the growth of metal/oxide particles, similar to the promotion by alkali ions9 and silica support shows poor ability to stabilize platinum species. Furthermore, the corresponding aberration-corrected energy dispersive spectroscopy (EDS) mapping results of 1Pt2Bi-SiO2 (Fig. 1c) show that platinum and bismuth elements distribute uniformly at the cluster level within the same areas on the surface of SiO2 without evident separation. Xie et al. also reported that bismuth species was deposited selectively on the Pt particles rather than the carbon support22. Meanwhile, the X-ray diffraction (XRD) also confirmed the promotion of bismuth species, in which no obvious Pt/PtO/PtO2/Bi/Bi2O3 phase was detected in 1Pt2Bi-SiO2, even in a “slow-scan” mode (Fig. 1d and Supplementary Fig. 4). When bismuth oxide species were deposited on silica separately (2Bi-SiO2), it also stabilized in small-size without any diffraction peaks of Bi/Bi2O3 in XRD profiles (Fig. 1d). However, the observation of sharp Pt peaks (39.7º and 46.2º) verifies the formation of huge Pt particle in 1Pt-SiO2. As shown in Fig. 1c, the dopant of bismuth species reaches optimization at 2 wt.% and the formation of broad diffraction peak of Bi2O3 for 1Pt5Bi-SiO2. Therefore, bismuth oxide species as a promoter could improve the anti-sintering ability for platinum dispersed on an inert support.
The local coordination structure of platinum and bismuth species. According to aberration-corrected HAADF-STEM images, we can observe the existence of platinum-bismuth oxide clusters. X-ray absorption fine structure (XAFS) technique was applied to clarify the local structure of this cluster in Bi-promoted samples. The X-ray absorption near-edge structure (XANES) of Pt-L3 edge profiles (Fig. 2a, Supplementary Table 2) showed platinum is in a low valence state (+ 1.8), due to the formation of huge metallic Pt particles in 1Pt-SiO2. As the increasing of bismuth-dopant, the average oxidation state of platinum arises from 1.8 to 3.5, may due to bismuth oxide species could make interaction with platinum to stabilize abundant oxygen around Pt atom and thus enhancing average valance state of platinum. The extended X-ray absorption fine structure (EXAFS) fitting results (Fig. 2b and Supplementary Table 2) indicated that the major Pt − O (R ≈ 2.0 Å, CN ≈ 5) shell plus an apparent Pt − Bi (R ≈ 3.0 Å, CN ≈ 4) component originated from the Pt−[O]x−Bi structure, further demonstrating the formation of homogeneous oxidized PtxBiyOz cluster (Fig. 2e). Meanwhile, the XANES profiles of Bi−L3 edge (Fig. 2c) and X-ray photoelectron spectroscopy (XPS) spectra display that bismuth species were in the pure Bi3+ state for Bi-promoted samples (Supplementary Fig. 5), which could exclude the possibility of PtBi alloy. While for 1Pt-SiO2, without strong interaction between Pt and SiO2, a dominant Pt − Pt metallic shell (R ≈ 2.8 Å, CN ≈ 6.4) from metallic Pt particle and a minor Pt − O−Pt shell (R ≈ 3.1 Å, CN ≈ 2.7) from small-size PtxOz cluster could be determined. In addition, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were frequently employed to detect the existence form of platinum species (single atoms, clusters or particles). As shown in Fig. 2d and Supplementary Fig. 6, only gases CO peaks was detected for 1Pt2Bi-SiO2. However, for 1Pt-SiO2-400 with almost identical oxide cluster size and loading amounts of platinum species (Supplementary Fig. 7) with 1Pt2BI-SiO2, two CO adsorption peaks on Ptδ+ single atom (2093 cm− 1), oxide cluster (2075 cm− 1)23 were determined. It further evidences the formation of oxidized PtxBiyOz cluster, which possesses totally different structural composition and adsorption capacity in Fig. 2e. To our best knowledge, it is the first time to observe the formation of uniform platinum-bismuth oxide clusters to suppress the aggregation of Pt species.
Catalytic performance of Pt/PtBi-SiO 2 catalysts in CO oxidation. CO oxidation was applied as a model reaction to investigate the role of bismuth-dopant. When the catalysts were pretreated at 300 °C under air, Bi-free and Bi-promoted samples shows almost same CO oxidation activity with complete CO conversion at ~ 220 °C (Supplementary Fig. 8), may due to poor ability to adsorb CO or overhigh valence of platinum species24. However, we found that hydrogen reduction significantly enhanced CO oxidation activity for Bi-promoted catalysts (Fig. 3a). The temperature of 50% CO conversion dropped off from 165 to 85 °C as the reduction temperature increasing from 0 to 210 °C. Interestingly, a platform appeared as hydrogen reduction at 150 and 180 °C, indicating a structure transformation of active site occurred during the hydrogen reduction compared with fresh Bi-doped catalysts. On the basis of the CO oxidation activity (Fig. 3b), a remarkable promotion to platinum-silica catalysts was observed by the addition of Bi oxide species with similar Pt loading (0.9 wt.%). The catalytic performance reaches the maximum at the dopant of 2 wt.% (Supplementary Fig. 9), may due to overmuch bismuth oxide species hindering CO adsorption or covering platinum active site. For comparison, pure Bi catalysts (2Bi-SiO2) shows no CO oxidation activity below 160 °C (Fig. 3b), demonstrating bismuth species are not active site just as secondary dopant to modification platinum active site. Furthermore, we collected the kinetic data to compare the inherent catalytic activity. The specific activity normalized by the platinum amount for 1Pt2Bi-SiO2 was 487 µmolCO gPt−1 s− 1 at 110 °C, as active as the reported Pt/CeO2 catalysts (103−518 µmolCO gPt−1 s− 1 at 80−130 °C, see Table 1), as well as ten times higher than that of pure Pt catalyst (Supplementary Table 3). For eliminating size-effect on active site, 1Pt-SiO2-400 was similarly inactive for CO oxidation at 100 °C reaction (Supplementary Fig. 9b), though possessing similar cluster size (1.2 ± 0.1 nm) to 1Pt2Bi-SiO2 (Supplementary Fig. 7). It indicates that the dispersion of platinum species does not dominantly determines the CO oxidation activity. In another hand, the apparent activation energy (Ea) of Bi-promoted platinum catalysts (∼52 kJ mol− 1) is similar to that of CeO2-supported Pt catalyst (40 − 50 kJ mol− 1)2,25, and distinctly lower than that of pure 1Pt-SiO2 (∼70 kJ mol− 1) and other inert support platinum catalysts5,26 (Fig. 3c). This may give a hint on the totally different reaction mechanism or active site for our Bi-promoted Pt-SiO2 catalysts. Additionally, 1Pt2Bi-SiO2 showed remarkable thermo-stability (Fig. 3d) and catalytic stability under the extremely high space velocity (300,000 mL·gcat−1·h− 1, Supplementary Fig. 9c).
Structural characterization of used Pt/PtBi-SiO 2 catalysts. After CO oxidation, we employed a comprehensive characterization to determine the actual structure of active site. The aberration-corrected HAADF-STEM images (Fig. 4a-c and Supplementary Fig. 10) showed that the platinum species were still in the formation of cluster (~ 2 nm) without any visible lattice fringes of crystal Pt/PtO/PtO2 component after CO oxidation. Meanwhile, the related STEM-EDS mapping results indicated that the Pt and Bi elements still distribute together (Fig. 4d and Supplementary Fig. 11). Thus, the bismuth element around platinum cluster still make an interaction with platinum species to prevent aggregation of clusters into huge particles, even after hydrogen reduction. The EXAFS fitting results in Supplementary Fig. 12 indicated the Pt−[O]x−Bi can be maintained after hydrogen pretreatment at 210 °C without air exposure during the whole test process. In another hand, there is an obvious aggregation of platinum cluster (~ 3.0 nm) compared to fresh 1Pt-SiO2 (~ 1.7 nm) in Supplementary Fig. 10c. Mahmudov and co-workers found obvious aggregation of platinum particles on activated carbon after hydrogen reduction27. The XRD results in Fig. 4e also confirmed the huge metallic platinum particles were still maintained in 1Pt-SiO2 with sharp diffraction peaks at 39.7º and 46.2º. In contrast, no obvious diffraction peaks of Pt/PtO/PtO2 phases were detected in Bi-promoted samples, further confirming the high dispersion of platinum species. There was a broad peak of Bi2O3 in 1Pt5Bi-SiO2, due to the aggregation of bismuth species.
Furthermore, we employed XAFS technique to detect the valance state and local coordination structure of active sites. XANES data in Fig. 4f indicated platinum species are at low valence state after CO oxidation: 1Pt5Bi-SiO2 (1.3) > 1Pt2Bi-SiO2 (0.7) > 1Pt-SiO2 (0.4) (Supplementary Table 2) due to hydrogen reduction, indicating lower oxidized state of Pt species are appropriate for lower temperature CO oxidation24,28. In order to require more reliable local coordination structure for used Bi-promoted samples, we conducted the EXAFS fitting process with or without Pt−O−Bi shell in Supplementary Fig. 13. Obviously, Pt−O−Bi shell is an essential composition to acquire the best fitting results. The EXAFS fitting results for Pt − Pt shell with CN ≈ 8 also confirmed the average grain size of platinum cluster was ~ 2 nm for 1Pt2Bi-SiO229 as observed in HAADF-STEM images (Fig. 4a-c). In addition, minor Pt − O (R ≈ 2.0 Å, CN ≈ 1.2 − 2.2) plus Pt − O−Bi (R ≈ 3.0 Å, CN ≈ 2.2) composition was detected (Fig. 4e, Supplementary Table 2), due to the existence of Pt−[O]x−Bi structure. Thus, the clusters in 1Pt2Bi-SiO2 observed in aberration-corrected HAADF-STEM images (Fig. 4a-c) was metallic platinum cluster. In another hand, the XANES and XPS profiles in Fig. 4h,i indicated that bismuth species were still in oxidative state even after hydrogen reduction and CO oxidation, which can exclude the formation of PtBi alloy. For pure platinum catalyst, only a metallic Pt − Pt shell (R ≈ 2.76 Å, CN ≈ 10) was fitted for 1Pt-SiO2 without apparent Pt − O shell, which may result in low activity due to no surface-active oxygen to participate in CO oxidation30. As a reference, 1Pt-SiO2-400 exhibited low activity in CO oxidation, even though possessing similar local coordination structure for Pt − O and Pt − Pt shell to the used 1Pt2Bi-SiO2 (Supplementary Fig. 14b). Thus, we can draw a conclude that the surface Pt−[O]x−Bi structure plays a key role in low temperature CO oxidation reaction rather than oxidized PtxOz cluster.
The reducibility and active oxygen for Pt/PtBi-SiO 2 . As we known, the reducibility of catalysts is crucial in various redox reactions31–33. For fresh samples, a main reduction peak located at ~ 100 °C appeared on profiles of H2−temperature programmed reduction (H2−TPR) in Fig. 5a for 1Pt-SiO2 contributed by the reduction of PtxOz clusters34. However, for Bi-doped samples, the first broad reduction peak was shifted to 162 °C (1Pt2Bi-SiO2) and 197 °C (1Pt5Bi-SiO2) in Supplementary Fig. 15a, due to the strong interaction of Pt − O−Bi35, as confirmed by EXAFS fitting results. The hydrogen consumption values (Supplementary Table 5) increased for 107 µmol/g (1Pt-SiO2) to 185 µmol/g (1Pt2Bi-SiO2). Although the doping of bismuth oxide species can enhance the surface oxygen, there is no promotion in CO oxidation due to over strong interaction (Pt − O−Bi) cannot release oxygen atom to take part in reaction. CO−temperature programmed reduction (CO−TPR) results in Supplementary Fig. 16 also demonstrated the surface-active oxygen for 1Pt2Bi-SiO2 (~ 75 µmol/g) is almost two times than that (~ 40 µmol/g) of 1Pt-SiO2. However, these oxygen species only reacted with CO molecule above 100 °C, well consistent with low activity in CO oxidation with oxidative pretreatment (Supplementary Fig. 8).
We also carried out the H2−TPR and CO−TPR experiments for used Pt/PtBi-SiO2 samples after CO oxidation without air exposure absolutely to detect reducibility and active oxygen of active site. For H2−TPR experiments, no reduction peak below 200 °C appeared in 1Pt-SiO2 due to the use of hydrogen reduction treatment before reaction. However, a broad reduction peak starting from 50 °C still existed in Bi-doped samples, indicating the existence of Pt−[O]x−Bi structure. It indicates the new active site of platinum cluster with surface Pt−[O]x−Bi structure exhibits more reducible property. Additionally, CO−TPR results of used 1Pt2Bi-SiO2 also evidenced these surface oxygen atoms in Pt−[O]x−Bi structure was superior active to react with CO molecule generating CO2 from ~ 50 °C (Fig. 5b), well consistent with the CO oxidation “light off” temperature (Fig. 3b). However, 1Pt-SiO2 did not activate the surface hydroxyls to produce CO2 (water-gas shift reaction) until 195 °C9. This highly reducible oxygen species may motivate initial CO oxidation (~ 50 °C) through Mars-van Krevelen (MvK) mechanism36 and show a strong correlation between reaction rate and active oxygen amount in Fig. 5c. Therefore, the Pt−[O]x−Bi structure with low valance state of Ptδ+ (0<δ<2) species in the used Bi-promoted samples provides superior active oxygen species to catalyze preliminary stage of CO oxidation.
The CO adsorption on Pt−[O] x −Bi active site. Despite the identification of active site structure in silica-supported platinum-bismuth catalysts being important, the adsorption of reaction gas is a more key factor for catalytic behavior. We further intend to investigate the CO adsorption on Pt−[O]x−Bi structure for Bi-promoted catalysts. The in-situ DRIFTS experiments displayed that CO adsorption intensity of Bi-promoted sample is moderate compared to that of pure platinum sample (Fig. 6a), indicating the surface Pt−[O]x−Bi structure could prevent oversaturated adsorption of CO molecule (CO poison) on Pt clusters or nanoparticles, just like the reported alkali-doped Pt catalysts9. The CO adsorption reached saturation rapidly for 1Pt-SiO2 at 2062 cm− 1 attributed to linear CO adsorbed on Pt0 sites (Pt0–CO)37 (Fig. 6a and Supplementary Fig. 17), resulting in no other Pt sites dissociating gases oxygen into active oxygen species38. However, the peak for CO adsorption over 1Pt2Bi-SiO2 was detected at 2043 cm− 1, which was absolutely different linear CO adsorbed on Pt0 sites. This red-shift phenomenon can be eliminated the possibility of either size-effect of Pt species or CO adsorption on pure Bi species. In one hand, 1Pt-SiO2-400 with total platinum cluster also showed the peak around 2060 cm− 1 (Supplementary Fig. 18) similar with the peak (2062 cm− 1) in 1Pt-SiO2. In another hand, there is no CO adsorption peak for 2Bi-SiO2 except for gases CO peaks (Supplementary Fig. 19), no matter with oxidative or reduced pretreatment. Therefore, the low-frequency band at 2043 cm− 1 is attributed to CO molecules adsorbed on the new active site (Pt−[O]x−Bi), even less doping of bismuth species (Supplementary Fig. 20), and this may result from the unique local coordination structure of Pt−[O]x−Bi structure or electron transfer from platinum atom to CO molecule39,40. Panagiotopoulou and co-workers reported that the alkali additives to Pt/TiO2 catalyst generate a low-frequency shoulder band (~ 2030 cm− 1) resulting in strengthen of Pt − CO bond40. Furthermore, when the oxygen was introduced, the activated CO molecule adsorbed on Pt−[O]x−Bi structure could be converted to CO2 quickly (∼135 s) and completely (Fig. 6b). Our density functional theory (DFT) calculation builds up various simulation model for Pt@PtOx without and with Bi dopant (Supplementary Fig. 21) and the corresponding calculated vibrational frequencies of a CO molecule adsorbed on different sites. The vibration peaks at 2062 cm− 1 and 2043 cm− 1 in the experiments were attributed to the configurations in Supplementary Figs. 21c,f respectively. One can see that upon the doping of the Bi atoms, the number of the Pt-O bonds had decreased. It is not surprising since Bi atoms are oxophilic and can seize O atoms from Pt. In Supplementary Fig. 22, we presented the density of states (DOS) for the d electrons of the Pt atom (Supplementary Figs. 21c,f) on which the CO molecule adsorbs. As the coordination number of the Pt atom decreases, the center of the d band becomes closer to the Fermi level, corresponding to strengthened activity. Thus, due to the unique structure of Pt−[O]x−Bi, more electron was transformed from Pt atom to CO molecules to activated carbon monoxide, resulting in the red-shift on CO adsorption band in DRIFTS experiments.
Additionally, we, employed in-situ DRIFTS experiments with a mode: “CO adsorption → reaction conditions (1 vol.% CO/20 vol.% O2/N2 flow) at 200 °C → CO adsorption” to detect the stability of active site. For 1Pt2Bi-SiO2, the Pt−[O]x−Bi structure could be maintained after CO oxidation (2042 cm− 1), well consistent with the band after hydrogen pretreatment at 210 °C (Fig. 6c). For 1Pt-SiO2, after CO oxidation, the CO adsorption band occurred a blue shift (2072 cm− 1) compared to the band (2062 cm− 1) after hydrogen reduction at 210 °C (Supplementary Fig. 23), due to the surface oxidation of metallic Pt cluster.