Introduction

The unique synergetic effect between the two metal species in bimetallic catalysts have been studied for decades^[1-6]. Active sites with specific geometries can be regulated by the introduction of a second metal via the ensemble effect, which has increased interest in bimetallic catalytic systems in recent years^[7-10]. Goodman et al. first demonstrated that numerous factors can significantly affect the catalytic performance of Au catalysts^[11-12], including the addition of a second metal. Bimetallic catalysts based on Au-based bimetallic catalysts (Au-X, where X = Pd, Ag, Pt, etc.) have emerged as an attractive catalytic system for many reactions, particularly CO oxidation^[13-18]. For example, the addition of Pd to Au/SnO₂ and Au/TiO₂ systems enhances their catalytic activity in low-temperature CO oxidation reactions^[19-20]. In such systems, where Au atoms provide active sites for CO adsorption, contiguous Pd sites were responsible for O₂ dissociation^[21-23]. Despite the numerous investigations on bimetallic catalysts for CO oxidation published in the last decade, effective advanced bimetallic catalysts and their intrinsic mechanisms and synergetic effects are yet to be discovered and explored.

Bi is commonly used in liquid-phase alcohol generation^[23-26] and electrochemical applications^[27-29] owing to its low toxicity, low cost, and environmental sustainability. In bimetallic systems, it is generally accepted that Bi exists in a higher oxidation state and prevents the oxidation of noble metals^[30-34]. We previously demonstrated that the Bi-Cu interface exhibited higher catalytic activity for CO₂ activation and dissociation than pure Cu(111). The introduction of Bi atoms causes the dissociated O to migrate to the Cu-Bi sites, thereby freeing the Cu active sites at which CO₂ dissociation occurs and thus boosting the catalytic activity^[35]. Nan et al. recently reported the enhanced catalytic activity of Pt-Bi catalysts for CO oxidation^[36], in which the unique Pt-Bi interfaces provided superior activation of oxygen species at low temperatures and increased CO₂ production. An Au-Bi catalyst exhibited an interesting phenomenon during the hydrochlorination of acetylene, where electron transfer between Au and Bi led to the dispersion of Au nanoparticles in a higher-valence state, which enhanced C₂H₂ absorption^[37]. Based on these previous results, we decided to investigate whether similar effects could be achieved by the addition of Bi to Au-based catalysts for other typical catalytic reactions.

Recent advances in in situ characterization have enabled the exploration of surface segregation and reductive/oxidative evolution of model catalysts under near-ambient pressures. For example, near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is a powerful tool for elucidating reaction mechanisms because changes in the core-level orbitals of each reactant can be measured directly over the course of the reaction. Herein, NAP-XPS was employed to investigate the role of Bi in improving the catalytic performance of the Au-Bi/SiO₂ surface for CO oxidation. To achieve this goal, the catalytic activities of Au/SiO₂, Bi/SiO₂ and Au-Bi/SiO₂ catalysts with different coverages were evaluated using several separate in-situ-experiments. The Au 4f peaks in the NAP-XPS spectra of Au/SiO₂ catalysts did not significantly change during reactions under at both oxidative and reductive conditions, while the Bi sites in Au-Bi/SiO₂, facilitated the contiguous dissociation of O₂ with a further spillover of O species to Au, thus forming Au^δ+. Furthermore, strong interactions between Bi and CO were identified. Meanwhile, a reversible shift of the Au 4f peaks was observed, indicating the reversion of Au^δ+ to metallic Au, thus confirming the catalytic activity of Au-Bi surfaces in CO oxidation under the employed reaction conditions. This study illustrates the syntactic activity for CO oxidation of a Au-Bi/SiO₂ tandem surface in CO oxidation, which may facilitate the design of bimetallic catalysts containing Au species.

Materials and Methods

This study was conducted using Beamline 20U at the Shanghai Synchrotron Radiation Facility (SSRF). All NAP-XPS spectra were recorded with a Hipp-2 analyzer (Scienta Omicron) with a monochromated Al Kα radiation source (hν= 1486.6 eV, SPECS)^[38]. The ultrahigh vacuum (UHV) system was equipped with analysis and preparation chambers, with base pressures of 7 × 10⁻¹⁰ mbar and 5 × 10⁻¹⁰ mbar, respectively. The SiO₂ substrate (MaTeck) was mounted on a Ta sample holder and heated via laser heating from the rear. The surface was cleaned by repeated annealing at 1000 K in UHV until C species were no longer detected by C 1s XPS. The gas pipelines were purified using a mixture of alcohol and liquid nitrogen at approximately 173 K and monitored in situ using mass spectrometry to ensure no additional gas cross-contamination. The dosing gases were introduced into the analysis chamber using a leak valve and were equilibrated for at least 30 min prior to analysis. The XPS spectra were fitted with Gaussian-Lorentz peaks^[39] using Casa XPS software after subtraction of the Shirley background.

Results

The catalytic activities of Au/SiO₂, Bi/SiO₂, and bimetallic Au-Bi/SiO₂ in the oxidation of CO were compared by exposing these samples to O₂ and CO atmospheres. The Au 4f and O1s XPS spectra of a 0.4 monolayer (ML) Au/SiO₂ sample are shown in Fig. 1. The photoelectron spectra were recorded in the presence of O₂ (0.1 mbar) from room temperature (RT) to 373 K, and separately in the presence of CO (0.1 mbar)atmosphere from RT to 473 K. The Au 4f spectrum obtained prior to any catalytic reactions shows peaks corresponding to metallic Au at 84.0 eV (Au 4f7/2) and 87.7 eV (Au 4f5/2), demonstrating that only standard metallic Au species are present. Under these conditions, a single O peak characteristic of the SiO₂ substrate was observed at 531.7 eV. The shape and position of the Au 4f spectra remained essentially unchanged during the O₂/CO switching exposure process (Fig. 1a), indicating preservation of the metallic Au species. Upon exposure to O₂ (0.1 mbar) at RT, a peak at 531.7 eV (O_I) is observed, which increases slightly at 373 K, and thereafter remains essentially unchanged with increasing temperature (Fig. 1b). Greg et al. reported that the highest occupied molecular orbital (HOMO) in Au clusters and rough Au surfaces is localized, and its charge density distribution facilitates charge transfer into the π* orbital of O₂, which induces the binding of the molecule to gold. The HOMO of a flat face of a bulk surface tends be delocalized, which diminishes the ability of the surface to bind O₂ ^[40]. Conversely, the oxidation of the Au(111) surface increases with temperature ^[41], contrary to the results depicted in Fig. 1. The Au 4f peak (Fig. 1a) does not indicate the oxidation of Au; thus, we attributed the O_I component to the chemisorption of O₂ molecules on the Au species. At 373 K, the O₂ coverage of the surface, which is reflected by the peak intensity, approached the relative maximum. The intensity of the peak corresponding to O increased after dosing with CO (Fig. 1b, upper part), indicating CO adsorption on the surface at the same binding energy (BE). Metallic Au exhibits poor CO adsorption activity because adsorption requires negatively charged Au atoms, which are formed by back-donation from the Au d-electrons to the antibonding π* orbital of the adsorbed CO molecules^[42-44]. Thus, CO is most likely adsorbed on the SiO₂ surface in this case, as Au remains in the inert metallic state (Fig. S1). No BE shift of the Au 4f peaks was observed upon the adsorption of gas molecule, which was attributed to two primary reasons: (1) the sample surface exhibits poor activity under the experimental conditions, and (2) the concentration of adsorbed gas phase molecules is below the detection limit.

Fig. 1Full-screen View

(Color online) In-situ O₂/CO switching measurements on the Au/SiO₂ surface. (a) Au 4f and (b) O 1s spectra recorded after exposure to O₂ (0.1 mbar) and CO (01 mbar) under various experimental conditions.

Considering the role of bismuth in promoting the catalytic activity of bimetallic catalysts^[35], in situ AP-XPS spectra of the Bi/SiO₂ samples were also obtained to elucidate the effect of the O₂/CO atmosphere and annealing temperature (Fig. 2). To obtain a dense Bi surface, multiple layers of Bi were deposited on SiO₂ via vacuum sublimation of the Bi powder, followed by thermal annealing at 473 K to ensure a uniform distribution. The Bi 4f spectrum obtained under UHV at RT presents peaks characteristic of metallic Bi at 157.0 eV (Bi 4f7/2) and 162.3 eV (Bi 4f5/2). After introducing 0.1 mbar O₂ to the system at RT, immediate changes were found in the Bi 4f spectrum, where the pristine metallic Bi component was weakened, along with new sharp peaks appearing at 159.0 eV and 162.3 eV respectively, suggesting the formation of the bismuth oxidation state (Bi^δ+)^[46]. In addition to the O_I component, another peaks corresponding O emerges at 529.7 eV (Fig. 2b), which was assigned to oxygen-metal (O-M) bonding (denoted as O_II)^[46]. Thus, in this case, Bi-O bonding was induced by O species originating from O₂ dissociation ^[47-48]. In addition, the O 1s peak with a higher BE at 531.7 eV (the same BE as the adsorption of O₂ on SiO₂, Fig. 1) arises from the bonding between Bi and absorbed O₂^[48].

Fig. 2Full-screen View

(Color online) In situ XPS (a) Bi 4f and (b) O 1s spectra of the Bi/SiO₂ surface in O₂/CO atmospheres under various experimental conditions.

After evacuating the chamber to UHV and subsequently introducing CO (0.1 mbar) at RT, the signal corresponding metallic Bi (green shadow) is reduced in intensity, demonstrating the further oxidation of Bi. This behavior is similar to that observed under an O₂ atmosphere, and may result from the adsorption of CO and further dissociation on Bi/SiO₂ surface. To prove this hypothesis, a control experiment was conducted in which Bi/SiO₂ was directly exposed to CO (Fig. S2a and S2b). Similarly, CO adsorption resulted in the oxidation of Bi at RT, and the degree of oxidation increased with temperature, as reflected by the both the O_I (Bi-CO in red) and O_II (Bi-O in blue) components in the Bi 4f and O 1s spectra. This oxidized state is rather stable, and the Bi 4f and O 1s spectra remain essentially unchanged upon further introduction of a 0.1 mbar H₂ reductive atmosphere followed by annealing (Fig. S2c and S2d). Thus, we conclude that the weakened metallic Bi component in the spectra after the introduction of CO at RT (Fig. 2a) originates from CO adsorption and dissociation. After further thermal treatment, the intensity of the peak corresponding to f metallic Bi increased with temperature from 373 to 473 K (Fig. 2a) owing to the reduction of the oxidative surface. This may be explained by the oxidation of CO by the O species adsorbed on the surface, resulting in the formation of the CO₂ product. Without O₂ pretreatment, the Bi/SiO₂ surface underwent a continuous oxidation process, and the reduction reaction that prevailed after annealing in a CO atmosphere did not occur.

To explore the CO oxidation reaction on the bimetallic surface, a Au-Bi/SiO₂ sample was prepared by depositing 0.4 ML Au particles on a Bi/SiO₂ surface. The XPS spectra of the bimetallic surface are shown in Fig. 3. In contrast to the spectra of the Au/SiO₂ sample during O₂ exposure, wherein Au maintained a metallic state (Fig. 1), the Au 4f peaks in spectrum of the bimetallic sample shift upwards by 0.2 eV (4f5/2 at 87.9 eV and 4f7/2 at 84.2 eV) relative to the metallic Au peak in the presence of O₂ (0.1 mbar) at RT (Fig. 3a), indicating the existence of Au^δ+, which did not appear in the absence of Bi under equivalent experimental conditions. The formation of a Au-Bi alloy, which can also result in a Au BE shift^[6], can be excluded in this case as the Au BE in the spectrum of another sample of Au-Bi/SiO₂ remains unchanged under a CO atmosphere (Fig. S3). Therefore, the upward shift in the Au BE was reliably attributed to the Au oxidation state.

Fig. 3Full-screen View

(Color online) XPS (a) Au 4f and (b) Bi 4f and (c) O 1s spectra of Au-Bi/SiO₂ under O₂/CO atmospheres under various experimental conditions. O₂ and CO were present at a pressure of 0.1 mbar. The bottom spectra (labelled “Au-Bi/SiO₂”), was obtained under UHV.

Similarly, Bi^δ+ is also formed upon introduction of O₂, with Bi 4f peaks appearing at 159.0 eV and 162.3 eV (Fig. 3b), indicating the oxidation of Bi. Similarly, the O 1s peak located at a BE of 529.7 eV (Fig. 3c) increased compared with that observed at UHV, providing evidence of O₂ dissociation and the formation of O-M bonds. Similarly, the component at 531.7 eV (Fig. 3c) arising from O₂ adsorption on the surface also increases. Considering the strong role of adjacent Bi atoms in the capture and decomposition of O₂ molecules (Fig. 2), we concluded that the upward shift of the Au BE was caused by the migration of adsorbed O atoms (originating from O₂ dissociation) from the contiguous Bi sites. This conclusion is also consistent with Goodman's mechanism^[15-16], which suggests that O₂ dissociation occurs at the contiguous metal sites with further spillover of the adsorbed O species to Au atoms, which allows further CO oxidation. After annealing at 373 K, the fraction of metallic Bi was reduced, and the intensity of the two signals in the O 1s region increased, suggesting that the surface has a higher oxidation state owing to the intensification of the oxidation reaction.

After the addition CO gas followed by annealing to 473 K, the fraction of metallic bismuth was strengthened (upper part of Fig. 3b, the intensity ratio evolution of Bi^δ+/Si 2p is given in Fig. S4), which agrees with the phenomenon exhibited by the Bi/SiO₂ sample, in which the CO oxidation reaction occurs at the Bi sites. After annealing, the total amount of the O component decreased (Fig. 3c and Fig. S4), suggesting that the CO molecules reduced the surface oxides(Fig. 3c). Additionally, the Au 4f peak shifts returns to its original position under UHV (Fig. 3a) after annealing at 473 K in a CO atmosphere, indicating that Au^δ+ is reduced to metallic Au. Thus, we achieved reversible alternation of the Au state in oxidizing and reducing atmospheres by adding Bi to the Au/SiO₂ system. The XPS analysis shows the enhanced catalytic activity of the Bi-Au-SiO₂ interface in the oxidation of CO, relative to the case in Fig. 1. The universality of this phenomenon is further verified in Fig. S5) with lower Au coverage.

To clearly demonstrate the mechanism of the CO oxidation reaction on different samples, models illustrating the proposed mechanism are shown in Fig. 4. The original Au/SiO₂ surface was essentially inactive during both O₂ dissociation and CO adsorption, thereby blocking further reactions (Fig. 4a). The Bi/SiO₂ catalyst exhibited some activity for the CO oxidation reaction owing to the strong CO adsorption capacity and O₂ dissociation ability of Bi (process IV in Fig. 4b). The reversible shift of the Au 4f peaks in the spectrum of the Au-Bi/SiO₂ sample suggests that CO oxidation was indeed activated by the bimetallic surface. Based on these observations, a mechanism of CO oxidation on the Au-Bi/SiO₂ surface can be proposed (Fig. 4c). After exposure to O₂, the rapidly dissociated oxygen species on the Bi sites migrate to the neighboring Au atoms and form Au-O bonds (process III). The relatively strong interaction between Bi and CO enabled the surface to capture CO molecules (process IV); thus, the combination of adsorbed CO and O on both the Bi and Au sites occurred simultaneously with desorption of the CO₂ product from the surface (process V), resulting in the recovery of the Au valence state. The Au and Bi species, indicated by orange and dark blue spheres in Fig. 4, may be atoms or clusters. Thus, adjacent Bi sites facilitated the adsorption of O₂ and CO molecules on Au/SiO₂, and the tandem surface catalyzed the CO oxidation reaction. Because bismuth species are partly oxidized in O₂ at RT (Fig. 2a and Fig. 3b), the subsequent upshift in the Au BE observed under a CO atmosphere may be a consequence of Au-BiO_x formation and the enhanced catalytic activity of the interface. We also considered changing the sequence of introduced gas to explore the Au-BiO_x interface and CO adsorption state on the sample surface because the Bi atoms sites may be permanently occupied by the adsorption of CO molecules, in which case, no distinct difference in the chemical states of Bi would be distinguishable during subsequent processes. Therefore, NAP-XPS alone may not be sufficient to elucidate the role of the Au-BiO interface in enhancing the activity of the catalyst. Additional characterization methods may be required to provide a deeper insight into the role of the interface.

Fig. 4Full-screen View

(Color online) Schematic of the proposed mechanism of CO oxidation on the surface of (a) Au/SiO₂ (b) Bi/SiO₂, and (c)Au-Bi/SiO₂ catalysts. I Fresh sample surface. II Generation of Au^δ+ or Bi^δ+ after O₂ adsorption. III (b) Dissociation of O₂ molecules on Bi. (c) Oxidation of Au owing to spillover of adsorbed O from neighboring Bi sites. IV Bi-CO bonding resulting from strong adsorption. V Generation of CO₂ from adsorbed CO and O, (c) neutralization of Au and recovery of the catalyst.

Conclusion

AP-XPS was employed to investigate the surface evolution of Au-Bi/SiO₂ tandem catalysts for CO oxidation and to elucidate the origin of the enhanced catalytic activity. O₂ dissociation and CO adsorption do not readily occur on the Au particles adsorbed on the SiO₂ substrate; accordingly, the CO oxidation reaction was highly suppressed. Conversely, the introduction of Bi improves the O₂ dissociation ability of the surface, and the adsorbed O atoms subsequently spill over to adjacent Au atoms, resulting in the formation of Au^δ+. Further CO adsorption on Bi, followed by thermal treatment, facilitated the CO oxidation reaction at the Au-Bi interface, resulting in a reversible shift of the Au valence state to neutral. This study illustrates that the performance of Au/SiO₂ catalysts can be manipulated by Bi doping, which will shed new light on the design of bimetallic catalysts containing inert Au species.