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Enhancing room-temperature CO oxidation via encapsulation of bimetallic PtNi clusters within mesoporous silicate-1 catalysts†

Ronghua Cui‡ ^a, Siyuan Yang‡^a, Lifeng Zhang^a, Xing Chen*^ab and Langli Luo*^ab
aInstitute of Molecular Plus, Department of Chemistry, Tianjin University, Tianjin 300072, China. E-mail: xing_chen@tju.edu.cn; luolangli@tju.edu.cn
^bHaihe Laboratory of Sustainable Chemical Transformations, Tianjin 300192, China

First published on 10th May 2025

Introduction

The CO oxidation reaction is a vital probe reaction to investigate model metal surfaces in fundamental studies of catalytic processes, and it also has a wide range of applications such as control of CO emission from industrial processes for the environmental sector,^8,9 CO removal in fuel cells,^10–12 and submarine and indoor CO poisoning prevention.^13,14 Numerous metal/oxide catalysts can effectively and stably oxidize CO at temperatures above 100 °C, including Pt/TiO₂,^15–17 Pt/CeO₂,^13,17,18 and Pt/SiO₂.^19–21 Nevertheless, only a limited number of catalysts can effectively oxidize CO at lower temperatures/room temperature. The Pt/CoNi@NC catalyst, i.e. graphene separated Pt from CoNi nanoparticles (Pt|CoNi), was used to efficiently catalyze the oxidation of CO under an oxygen-rich atmosphere and to achieve a room-temperature conversion, which was achieved by activating the Pt–graphene interface through the electron-penetration effect to achieve adsorption of O₂. In this process, Co and Ni are oxidized during the reaction, which needs to be restored to metallic states before it can be used again.²² The critical step in the CO oxidation reaction is the adsorption of CO and O₂ on the catalysts in an evenly matched manner, followed by the desorption of the product CO₂ in a timely manner.²³ However, noble metals such as Pt have a much stronger ability to adsorb CO than O₂, which cannot achieve balanced adsorption of two reactant gases. Alloying with another metal that more easily binds and dissociates O₂ molecules could achieve this balance, thus enhancing the activity for CO oxidation. For instance, incorporating a 3d transition metal (e.g., Cu,^24,25 Ni,^26,27 Co,^8,28 Fe,¹¹ etc.) into Pt showed enhanced performance for the CO oxidation reaction. The addition of a transition metal alters the electronic structure of the original metal surface, and sometimes, it introduces an additional interface such as Pt/MO_n (M = transition metal) in an O₂-rich environment of practical CO oxidation conditions.^25,29 This resembles the metal–support interaction (SMSI) concerning the interaction of Pt with reducible oxides during the reaction process.^24,30–35 However, the synergetic effect of alloy clusters in a confined space remains unclear, mainly due to the difficulty of identifying and correlating the encapsulated metal clusters’ atomic structure and chemical states to their catalytic performance

Herein, we prepared mesoscopic silicate-1 (S-1) zeolite catalysts encapsulating sub-nanometric PtNi atom clusters to investigate their geometric structure and chemical state in confined zeolite channels and the catalytic performance for the CO oxidation reaction. By comparing the zeolite catalysts encapsulating pure Pt and Ni atom clusters, we can reveal the synergetic effects on forming atom clusters and catalytic mechanisms for low-temperature CO oxidation reactions.

Results and discussion

Fig. 1a–c show typical SEM images of three catalysts presenting a scaly spherical morphology due to the packing of several faceted primary particles. The average size of secondary zeolite particles is a few micrometers, while the primary particle size is a few hundred nanometers. Fig. 1d–f show high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the cross-sectional view of zeolite particles prepared by ultramicrotomy, where dark-contrasted areas indicate the internal pores of the mesoscopic structure. The HAADF-STEM elemental analyses of the three samples show that the metals were uniformly distributed in the zeolites in Fig. S3 and 4.†

	Fig. 1 Structural characterization of Pt–Ni encapsulated S-1 zeolites. (a–c) SEM images of Pt@S-1, Ni@S-1, and PtNi@S-1. (d–f) STEM-HAADF images of Pt@S-1, Ni@S-1 and PtNi@S-1 show the porous structure of S-1 zeolite particles. (g–i) STEM-HAADF images of Pt@S-1, Ni@S-1, and PtNi@S-1 show the atom clusters encapsulated in the zeolites. (j–l) The size distribution charts of encapsulated atom clusters in PtNi@S-1, Pt@S-1, and Ni@S-1 (sample size > 500).

This hierarchical structure results in a large external specific surface area that improves metal dispersion, as shown in the HAADF-STEM images in Fig. 1g–i. The metal clusters can be directly visualized, and the size distribution is calculated as 0.85 ± 0.02, 1.1 ± 0.02, and 0.65 ± 0.02 nm (Fig. 1j–l). It was found that adding Ni reduces the average size of metal clusters from 0.85 nm to 0.65 nm. This indicates that the synergistic effect of Ni and Pt facilitates the migration of metals in zeolites. The higher diffusion rate of Ni leads to easy aggregation and growth of Ni nanoparticles by Ostwald ripening during high temperature reduction, while Ni²⁺ promotes Si–OH condensation and accelerates local nucleation of zeolites leading to the formation of S-1 macrocrystals. However, the metal Pt inhibits this process and leads to the formation of more dispersed nanoclusters.^25,36–38 High-magnification STEM images and the corresponding FFT patterns shown in Fig. S5† demonstrate different crystal orientations of primary particles in a secondary zeolite particle. Dark-contrasted areas in Fig. S6† give a direct morphological insight into the mesopores in the PtNi@S-1 catalyst.

The structure and chemical state of metallic species encapsulated in zeolites are vital to optimize the regioselectivity, activity, and stability of metal@zeolite catalysts. We previously identified the atomic structure of pure Pt species in S-1 zeolites as few-atom clusters encapsulated in channels, and increasing the size of Pt can be destructive to the S-1 framework.³⁹ Here, we investigate the atomic structure of encapsulated metallic species of PtNi by HAADF-STEM imaging. Fig. 2a shows a clear view along the 10-MR channels, where both single atoms (arrows) and few-atom clusters (circles) are visualized on the sidewalls of 10-MR. Larger atom clusters (square) across multiple 10-MR channels are also seen but are much less in number. As shown in Fig. 2b and c, the local STEM image of PtNi@S-1 with the corresponding linear EDS profiles for individual clusters show the overlapping of Pt and Ni intensity peaks, indicating that Pt and Ni are potentially in an alloy phase. Combined with the HAADF–STEM images and EDS analysis of PtNi@S-1, we propose that the PtNi species in S-1 include both PtNi alloy atom clusters and single atoms of Ni and Pt, as shown in the schematic of Fig. 2d.

	Fig. 2 Atomic structures of PtNi species in S-1 zeolites. (a) HAADF-STEM image of the PtNi@S-1 catalyst shows the single metal atoms and clusters. (b and c) HAADF-STEM image and the corresponding EDS line profiles show the alloy nature of Pt and Ni for encapsulated atom clusters. (d) Schematic of the PtNi species in zeolites.

Then, we evaluated the catalytic performance of Pt@S-1, Ni@S-1, and PtNi@S-1 catalysts for the CO oxidation reaction under a reaction gas mixture of 1% CO and 20% O₂ balanced in Ar with a space velocity of 60000 mL g⁻¹ h⁻¹. The conversion rate vs. reaction temperature plot is shown in Fig. 3a. The Pt@S-1 catalyst shows activity at a temperature as low as 20 °C and gradually ramps up to a nearly 100% conversion at 60 °C, while the Ni@S-1 catalyst shows almost no CO conversion below 60 °C. The PtNi@S-1 catalyst is also active at 20 °C but reaches a conversion rate of over 90% immediately at 30 °C. Fig. 3b shows the mass-specific activity and turnover frequency of these two samples. The PtNi@S-1 catalyst shows higher activity at every temperature point in the range from 293 to 323 K. For instance, the mass-specific activity of the PtNi@S-1 catalyst at 303 K (30 °C) was 78 μmol_CO g_cat⁻¹ s⁻¹, which was almost ten times higher than that of the Pt@S-1 catalyst (7.85 μmol_CO g_cat⁻¹ s⁻¹). Fig. 3c shows that the PtNi@S-1 catalyst has a smaller activation energy of 26.35 kJ mol⁻¹ compared to that of the Pt@S-1 catalyst (39.96 kJ mol⁻¹). These results demonstrate an enhancement in the catalytic performance of the Pt@S-1 catalyst by adding Ni species while maintaining the same Pt loading (Table S1†). In Fig. S8 and S9,† we explored zeolite catalysts with different content ratios of Pt:Ni and screened a PtNi@S-1 sample with a metal content of 3:1 for the best mass-specific activity and CO conversion among previously reported catalysts for CO oxidation, as shown in Fig. 3d and detailed in Table S3.† Meanwhile, we have checked the stability of the PtNi@S-1 catalyst by evaluating the CO conversion rate vs. reaction temperature before and after 10 h of reaction at 60 °C, which shows identical performance with no hysteresis (Fig. S10†). The catalyst is capable of stable 100% conversion of CO for more than 80 h (Fig. S11†). In addition, the Pt@S-1 samples were stable for more than 80 h at a temperature of 60 °C 100% CO conversion in Fig. S12.†

	Fig. 3 Catalytic performance for the CO oxidation reaction. (a) CO conversion of Pt@S-1, Ni@S-1, and PtNi@S-1 catalysts for the CO oxidation reaction (1% CO and 20% O2 balanced in Ar with a space velocity of 60000 mL g−1 h−1). (b) The turnover frequency (TOF) and the specific rate for Pt@S-1 and PtNi@S-1 catalysts measured when the conversion of CO is below 15% from 293 K to 343 K. (c) Arrhenius plots (turnover frequency vs. temperature) of Pt@ S-1, PtNi@S-1 and Ni@S-1. (d) Comparison of the reported catalysts for the CO oxidation reaction by mass-specific activity and CO conversion rate.

For Pt-based catalysts used for CO oxidation reactions, CO is more strongly adsorbed than O₂ on the bare Pt surface, which results in a much more difficult activation of O₂ on Pt due to the saturated coverage of CO.⁴⁰ It is noted that the Pt@S-1 catalyst shows a fluctuation in the conversion rate in the temperature range of 30–50 °C, as shown in Fig. S13.† This is possibly due to the CO adsorption being saturated on the Pt surface with increasing time, which limits the adsorption of O₂. This competitive adsorption has been alleviated by the addition of Ni species as shown above, and detailed mechanisms will be studied by ex situ and in situ spectroscopy techniques and theoretical calculations as follows.

To understand the electronic structure of Pt and Ni in the PtNi@S-1 catalyst, we performed X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses. From the normalized X-ray absorption near-edge structure (XANES) curves at the Pt-L₃ edge (Fig. 4a), the white line intensity of PtNi@S-1 slightly decreases compared to that of Pt@S-1, indicating a slight change in the oxidation state of Pt due to the alloying with Ni. However, the front absorption energy of the L-edge in PtNi@S-1 is close to that of Pt foil, which indicates the existence of large amounts of pure Pt clusters in PtNi@S-1. The front absorption energy of the Ni K-edge in PtNi@S-1 catalysts is close to that of NiO rather than that of Ni foil, indicating that Ni is in an oxidized chemical state (Fig. 4b). We also used the Fourier transform extended X-ray absorption fine structure (EXAFS) spectra (Fig. 4c and d) to clarify the local configurations of atomically dispersed Pt and Ni in PtNi atom clusters encapsulated in a zeolite. PtNi@S-1 and Pt@S-1 show the same weak peaks at 1.6 Å and 2.60 Å at the Pt L₃ edge, which correspond to the Pt–O for PtO₂ and Pt–Pt for Pt foil, respectively. This suggests a mixed oxidation state of Pt exists for both samples. The second coordination peak at 2.60 Å for PtNi@S-1 shifts to a higher distance than for the Pt foil and Pt@S-1. A change in the shell layer of neighboring metal atoms around the Ni atom is speculated, indicating the formation of Pt–Ni coordination. Besides, the Ni K-edge shows both Ni–O and Ni–Ni bonds at 1.6 Å and 2.17 Å, respectively, attributed to the abundant Si–O environment in zeolites. These findings support the proposed atomic structure of PtNi species in PtNi@S-1, shown in Fig. 2d, where the PtNi alloy atom clusters populate in the zeolite channels. Moreover, the XPS spectra in Fig. 4e show that the Pt⁰ species with a binding energy at 71.6 eV is found in Pt@S-1, while the Pt⁰ shifts to a much lower binding energy (71.0 eV) for PtNi@S-1, which further supports the above claims. The O 1s XPS spectra for all three samples show no difference as shown in Fig. S14.†

	Fig. 4 Chemical state and adsorption behaviors of the three catalysts examined by XAS and XPS. (a and b) XANES spectra at the Pt L3-edge (a) and Ni k-edge (b) of Pt@S-1 and PtNi@S-1. (c and d) FT-EXAFS spectra at the Pt L3-edge (c) and Ni k-edge (d) of Pt@S-1 and PtNi@S-1. (e) Pt 4f5/2 and 4f7/2 XPS spectra of PtNi@S-1 and Pt@S-1.

We also used temperature-programmed desorption (TPD) to investigate the difference in adsorption behaviors of Pt@S-1, Ni@S-1, and PtNi@S-1 catalyst surfaces. The CO-TPD curves in Fig. 5a show that the incorporation of Ni atoms into Pt changed the strength of the adsorption of CO molecules. After saturation at room temperature under 5% CO/He, the CO signal was collected under Ar gas purge at an elevated temperature of 700 °C. The desorption temperature of PtNi@S-1 was 111.3 °C, nearly 20 °C lower than that of Pt@S-1 (131.2 °C), indicating that the PtNi@S-1 catalyst can desorb CO more easily. The comparison of O₂-TPD curves shows that the PtNi@S-1 surface is more strongly adsorbed by O₂ and is more resistant to oxidation than Pt@S-1 (Fig. 5b). Therefore, the PtNi cluster formed by adding Ni changes the adsorption and desorption properties of the reaction gas (CO, O₂). It is worth noting that the coordination environment of such bimetallic clusters determines the synergistic effect. The PtNi@S-1 catalysts are not suitable for high temperature CO oxidation reactions in oxygen-rich environments, where the Ni metal has the potential to preferentially migrate, leading to selective sintering of the metal due to the low surface activation energy. This precise modulation of the catalyst to influence the reaction pathway is an effective way to improve the performance of the catalysts, and we need to further define the specific site of action of the species in the reaction process to analyze the actual role played by Ni in this bimetallic cluster catalyst.

	Fig. 5 Adsorption behaviors of Pt/Ni/PtNi@S-1 catalysts examined by TPD and in situ DRIFTS. (a and b) CO-TPD and O2-TPD curves of PtNi@S-1, Ni@S-1 and Pt@S-1. (c and d) In situ DRIFTS study of CO adsorption on the PtNi@S-1 (c) and Pt@S-1 surface (d) in a gas mixture of 2.5% CO/Ar at room temperature. (e) Schematic diagram of the adsorption relationship of PtNi@S-1.

For reactions in which CO is involved, especially as a reaction gas, in situ CO-DRIFTS is an effective means of tracing and exploring the dynamics of the catalyst's active site during the reaction process. The catalysts were pretreated in the in situ DRIFTS cells at 30 °C, and purged with air and water for 30 min under a N₂ flow. Then, in situ CO-DRIFTS experiments were conducted under reaction conditions (2.5% CO flow rate) at room temperature (30 °C). We collected and recorded the signals from 1 min to 20 min to verify the effect of the bimetallic system on the adsorption sites and strength of CO. As shown in Fig. 5c and d, sharp adsorption bands of CO appeared rapidly at 2052.85 and 2057.72 cm⁻¹, both of which are associated with Pt(δ+) ion-coordinated CO molecules and are attributed to the linear adsorption of CO on Pt, either Pt(0)-CO or Pt(δ)OH-CO, considering that little CO₂ is formed during this dry CO adsorption. The small packet peaks at 2168.90 and 2169.10 cm⁻¹ are attributed to the gaseous peaks of CO. It can be found that the CO adsorption intensity follows a trend of Pt@S-1 > PtNi@S-1 > Ni@S-1 as shown in Fig. S15,† which demonstrates that the addition of Ni weakened the CO adsorption on Pt and favored the equilibrium between CO and O₂ at the surface. The blue shift of linear CO on PtNi@S-1 compared to the Pt@S-1 catalyst (2052.85 cm⁻¹ to 2057.72 cm⁻¹) reflects the decreased back-donation of electrons from Pt to CO, which indicates a weakening of the adsorption strength of CO on the Pt clusters, which fits with the results in the CO-TPD curves. Meanwhile, we observed that the PtNi@S-1 catalyst reached the saturated adsorption state of CO at 6 min, while the Pt@S-1 catalyst reached this saturated adsorption state at 10 min. The saturated adsorption of PtNi@S-1 and Pt@S-1 catalysts at 20 min was further quantified, and it can be found that the PtNi@S-1 catalyst has lower saturated adsorption of CO (Fig. S15†), which facilitates co-adsorption with O₂ molecules. This demonstrates that adding Ni can effectively improve the gas adsorption–desorption equilibrium for the active sites. Furthermore, we noted that the Ni@S-1 catalyst had no absorption peak for CO (Fig. S16†). This indicates that the electronic interaction between Pt and Ni in bimetallic clusters plays a significant role in the synergistic effect.

We use density functional theory (DFT) calculations to further testify the surface adsorption energies of pure Pt and PtNi atom clusters encapsulated in S-1 zeolites. Previous studies^41–43 have shown that the CO molecule favors an end-on adsorption configuration, while the O₂ molecule adopts a side-on adsorption configuration (Fig. 6a–d). Therefore, we employed these preferred configurations to calculate the adsorption energies of CO and O₂ on Pt₁₃ and Pt₁₂Ni₁ clusters. The adsorption energies of CO on these two catalysts were nearly identical (−2.08 eV on Pt₁₃ and −2.11 eV on Pt₁₂Ni₁) (Fig. 6e). However, the adsorption of O₂ on Pt₁₂Ni₁ is stronger than that on pure Pt₁₃ by a difference of 0.23 eV (−1.52 eV on Pt₁₃ vs. −1.75 eV on Pt₁₂Ni₁). It can be inferred that the presence of Ni clusters significantly reduces the adsorption energy of O₂, thus balancing CO and O₂ adsorption, leading to an enhanced CO oxidation activity of the PtNi@S-1 catalyst in our experiments.

	Fig. 6 Calculated adsorption energies of CO and O2 on Pt vs. PtNi. (a–d) Atomic configurations of CO and O2 on Pt13 (a and c) and Pt13Ni1 (b and d), respectively. (e) Comparison of adsorption energies for CO and O2 on two clusters.

Conclusions

Data availability

Conflicts of interest

Acknowledgements

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