Atomic Dispersion of Pt Clusters Encapsulated Within ZSM‐5 Depending on Aluminum Sites and Calcination Temperature

Atomically dispersed Pt atoms encapsulated inside ZSM‐5 zeolites are synthesized via thermally induced dispersion under oxygen atmosphere. Dispersion of Pt clusters within ZSM‐5 and silicate‐1 is compared to investigate its dependence on the support and calcination temperature. Detailed density functional theory (DFT) calculations are applied to rationalize the dispersion process, which includes the detachment of Pt species from the bulk and the migration of volatile PtO x species, both of which are accelerated by thermal treatment at high temperature. DFT results also indicate that Al atoms within zeolites provide the anchoring sites to retain the dominated PtO species, though their interaction with the support is weakened with the increase in temperature. Our findings suggest that successful dispersion depends on the strong metal–support interaction and thermal treatment at moderate temperature (500 °C) which is crucial to balance the detachment and fixation of Pt atoms. This study paves the way to further understand the dispersion mechanism and assist in the development of stable single‐atom catalysts.


Introduction
Oxide-supported precious metal nanoparticles (NPs) as heterogeneous catalysts are widely used in industrial processes. In order to decrease the cost of catalysts, developing synthetic methods to reduce the size of precious metal NPs and stabilize the dispersed species is crucial. [1] Singleatom catalysts (SACs) offer the most effective way to utilize all atoms of the precious metal. [2] However, SACs suffer from difficulty in synthesizing homogeneously and stabilizing the dispersed single atoms because the surface free energy of metals increases largely with decreasing particle size, which promotes aggregation of the dispersed metal species.
Zeolite offers the opportunity to encapsulate metal NPs in its channels, which would inhibit the aggregation of metal atoms and improve stability. A few different noble metal NPs (e.g., Pt, Pd, Ru, Rh) have been successfully encapsulated within zeolites, such as SOD, GIS, ANA, CHA, and LTA, via hydrothermal synthesis. [3] The dispersion of metal clusters into subnanometer or even single atom by reversing the thermal sintering process was reported. [4] Using a similar strategy, surface inducements (i.e., -OH or H þ ) can trigger the dispersion of metal clusters within zeolites. [5] Gate's group reported that surface proton H þ could promote the dispersion of Rh clusters to single atoms in the Rh@HY catalyst for water-gas shift reaction. [5b] This dispersion technique is promising to stabilize the dispersed single atoms within zeolites and prevent aggregation, thus resulting in highly catalytic ability.
The dispersion process can be explained by the "strain" model, where the newly formed metal oxide at the support-metal interface induces a metal strain energy that is relaxed in part by the fracture of large particles. [6] It was reported that various metal-oxide interactions, including metal vacancies, [7] oxygen vacancies, [8] covalent metal-support interaction, [9] and electronic metal-support interactions, [10] are responsible for the dispersion of metal clusters on metal oxides. Meanwhile, thermal treatment at high temperature is considered to provide the driving force for dispersion, while the optimal temperature is still under debate. Moreover, metal NPs splitting into single atoms at high (800°C) or low temperature (210°C) were reported. [11] Thus, three factors DOI: 10.1002/sstr.202200115 Atomically dispersed Pt atoms encapsulated inside ZSM-5 zeolites are synthesized via thermally induced dispersion under oxygen atmosphere. Dispersion of Pt clusters within ZSM-5 and silicate-1 is compared to investigate its dependence on the support and calcination temperature. Detailed density functional theory (DFT) calculations are applied to rationalize the dispersion process, which includes the detachment of Pt species from the bulk and the migration of volatile PtO x species, both of which are accelerated by thermal treatment at high temperature. DFT results also indicate that Al atoms within zeolites provide the anchoring sites to retain the dominated PtO species, though their interaction with the support is weakened with the increase in temperature. Our findings suggest that successful dispersion depends on the strong metal-support interaction and thermal treatment at moderate temperature (500°C) which is crucial to balance the detachment and fixation of Pt atoms. This study paves the way to further understand the dispersion mechanism and assist in the development of stable single-atom catalysts.
including thermal treatment, oxygen stream, and metal-support interaction are considered to strongly relate to the dispersion process. Although there are many studies on metal dispersion, understanding dispersion mechanism and its dependence on the support and calcination temperature is deficient. [5b,12] Unlike the complicated environment on the metal oxide surface, the encapsulation of metal clusters within zeolites occurs under homogeneous environment, which helps to investigate the effect of support on the dispersion behavior. In this work, the dispersion process of Pt clusters encapsulated within ZSM-5 and silicate-1 (S-1) under different calcination temperatures is studied to illustrate the dependence of metal atom dispersion on the support and thermal treatment.

Results and Discussion
Via hydrothermal synthesis, Pt NPs were encapsulated in situ in silicate-1 (Pt@S-1) and ZSM-5 with Si/Al ratios of %160, 100, and 60 (named Pt@Z1, Pt@Z2, and Pt@Z3, respectively) with similar Pt loading amounts (Table S1, Supporting Information). All the catalysts maintain the morphology of MFI with Pt dispersion ( Figure S1, Supporting Information). After hydrogen treatment at 400°C, Pt NPs appeared inside all three Pt@Zx-H/400 catalysts (x ¼ 1-3, Figure 1a-c). With the decrease of Si/Al ratios, the average size of Pt NPs slightly increased from 2 to 4 nm due to declining crystallinity ( Figure S1, Supporting Information), which reduced the confinement effect and resulted in agglomeration of Pt NPs. After oxygen treatment at 500°C subsequently, most Pt NPs disappeared for all three Pt@Zx-H/400-O/500 catalysts ( Figure 1d-f ), indicating that dispersion was induced under oxygen atmosphere. The images in Figure 1d-f obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM) in the high-angle-annular dark-field (HAADF) imaging mode confirmed the existence of atomically dispersed Pt (marked with red circles). The results suggest a successful transformation of Pt clusters to isolated atoms.
Diffuse reflection infrared Fourier-transformation of CO spectroscopy (CO-DRIFT) was used to study the existence form of Pt in Pt@Z1-H/400 and Pt@Z1-H/400-O/500. There are two main CO absorption peaks near 2115 and 2075 cm À1 for Pt@Z1-H/ 400 (Figure 2a), among which the absorption peak at 2075 cm À1 is generally regarded as CO adsorbed on Pt NPs. The peak at 2115 cm À1 is the absorption peak of CO on isolated Pt atoms. [13] After oxygen treatment, the peak at 2075 cm À1 in Pt@Z1-H/400-O/500 disappeared and the intensity of absorption peak at 2115 cm À1 increased, indicating that oxygen treatment can induce the dispersion of Pt NPs to isolated atoms ( Figure 2a). Energy-dispersive spectroscopy (EDS) with elemental mappings also confirmed the existence of atomically dispersed Pt (Figure 2b H 2 -temperature-programmed reduction (H 2 -TPR) was conducted to explore the interaction between Pt and zeolite. As shown in Figure S4, Supporting Information, the reduction www.advancedsciencenews.com www.small-structures.com peak at higher temperature was dominant in Pt@Z1; however, more H 2 consumption occurred (i.e., large reduction peak area) at low temperature in Pt@S-1. The results indicated that Al sites provided stronger interaction to anchor Pt atoms inside ZSM-5. Pt 4f X-ray photoelectron spectroscopy (XPS) (Figure 3a and Table S2, Supporting Information) shows the proportion of Pt 2þ that is dominated, which increased with decreasing Si/Al ratios from Z1 to Z3. This indicates that the increase of Al sites in zeolites is beneficial to forming Pt 2þ species. Compared with Pt@Z1, a shift to higher binding energies of Pt 2þ was detected for Pt@Z2 and Pt@Z3, indicating the increasing interaction of PtO x species with supports. [14] The decreasing intensity of Al 2p from Pt@Z1 to Pt@Z3 may be attributed to the reconstruction of zeolite and dealumination. [15] Fourier transform infrared spectroscopy of Pt@Z1 showed a strong hydroxyl peak at 3610 cm À1 after reduction ( Figure S5, Supporting Information). The reason is that agglomeration of Pt atoms decreases their occupancy on hydroxyl sites. [16] After oxidation, the dispersion of Pt atoms results in generating more isolated Pt-O x -(OH) y species, and some hydroxy groups may disappear by replacing the hydroxyl hydrogen with Pt 2þ to form H 2 O, which is verified by our DFT calculations ( Figure S6 and S7, Supporting Information). This also explains the descending hydroxyl peak shown Figure S5, Supporting Information (i.e., blue line). The dehydration resulting in the reconstruction of zeolite is in accordance with the decreasing crystallinity of ZSM-5 ( Figure S2, Supporting Information). Density functional theory (DFT) calculations were performed to understand the effect of Si/Al ratios and O 2 . Dispersion of Pt clusters includes the detachment of Pt species and migration of volatile PtO x species. For a dissociation process, models of Pt clusters (Pt 1 -Pt 4 ) inside silicate-1 and ZSM-5 (Si/Al % 100, 50) were constructed and the most stable configurations were determined ( Figure S8 and S9, Supporting Information). The detachment energy of one Pt atom from clusters decreases with an increasing number of Al substitutions (Figure 3b), indicating that more Al sites are beneficial to the detachment of Pt atoms. Interestingly, the Pt 3 cluster shows higher stability among the three zeolites due to the transfer of the Brønsted proton to  Pt 3 . Similar results were observed when -OH was adsorbed on Rh clusters. [5a] When oxygen is introduced into the models, the detachment energy of PtO x from Pt n O clusters displays a concurrent descent trend with increasing Al sites over these three types of samples. It is noteworthy that a sharp decline in dissociation energies of Pt n O was observed compared with Pt n in Pt n @ZSM-5. This indicates that the oxidation of Pt n clusters promotes the dissociation of Pt atoms from bulk clusters. XPS confirmed that stable Pt single atoms exist within zeolites in the form of PtO or PtO 2 , which was ejected by a thermal driving force and then trapped by the support. Only strong interaction between PtO x and the support can contain stable single atoms; otherwise, scattered single atoms would agglomerate and grow again. The binding energies between PtO x (x ¼ 0-2) and zeolites were also calculated (Figure 3c), which represent the interaction between PtO x species and the support. The most stable configurations are shown in Figure S9 and S10, Supporting Information. The binding energy increases significantly with the increase in Al substitutions for both Pt and PtO x species, suggesting that Al sites could improve the interaction between PtO x and the support, which also is verified by H 2 -TPR ( Figure S4, Supporting Information). The notably higher binding energy of PtO species than that of PtO 2 indicated that PtO possesses stronger interaction with the support, which is in accordance with XPS results (Figure 3a). The binding energy of PtO x located at T8-T8 inside the straight channel shows a similar trend ( Figure S10, Supporting Information). Based on the above analysis, the dispersion of Pt clusters within zeolites proceeds via detachment of Pt species and fixation of volatile The Pt@Z1-H/400 catalyst was treated at temperatures ranging from 250 to 650°C in O 2 . As shown in Figure 4a, Pt NPs existed after calcination at 250°C for 4 h and most Pt NPs can be observed with the temperature increasing to 350°C (Figure 4b). Well-dispersed Pt atoms were achieved at 500°C (Figure 4c), which must be very close to the optimal calcination temperature as Pt NPs (marked with red circles) reappeared at higher temperature (650°C) in Figure 4d. As temperature increases, the decomposition pressure of PtO x will approach the partial pressure of PtO x . [17] Once higher than the critical temperature, the volatile PtO x species are emitted from Pt clusters. [11a] The detachment of Pt atoms promoted by increasing temperature was verified by the decrease of dissociation energy with the increase of temperature (Figure 4e). For the stabilization process of PtO x species, decreasing binding energy between PtO x and zeolites with the increase in temperature indicates that the interaction between volatile species and supports is weakened at high temperature (Figure 4f and S11, Supporting Information). The above analysis suggests that higher temperature displays deteriorated effect on the fixation of Pt atoms.
Environmental TEM (ETEM) was used to visualize the dynamic process of Pt NPs in Pt@ZSM-5 catalysts under  2.6 mbar of O 2 conditions. HAADF-STEM imaging on the specific area of Pt@ZSM-5 catalysts shows Pt NPs with a brighter contrast due to enhanced Z contrast (marked with green circles), which shows similar particle size from 100 to 600°C on ZSM-5 ( Figure 5a-d, f-i). Dispersion of Pt NPs was observed when further increasing the temperature to 700°C in O 2 (Figure 5e,j). The absence of Pt contrast in HADDF-STEM imaging suggests the disintegration of NPs into highly dispersed Pt species. The higher temperature of dispersion compared with ex situ experiments is due to the lower oxygen partial pressure in ETEM. [18] Due to high surface energy, Pt atoms tend to aggregate in nature. Heat treatment at high temperature under oxygen atmosphere triggers the detachment of PtO x from Pt clusters, however, the interaction with supports becomes weak which promotes the migration of PtO x . Thus, both strong metal-support interaction (SMSI) and suitable thermal driving force are required to balance the migration and immobilization of volatile PtO x species, which in turn results in the successful dispersion of Pt atoms. Moreover, the property of supports and coordination environment of metal clusters play a significant role. As shown in Figure 4, the driving force is insufficient at low temperature (250°C) so that PtO x is difficult to be detached. However, the interaction with supports becomes weak at higher temperature (650°C), resulting in the reaggregation of dispersed Pt atoms.  Thus, thermal treatment at moderate temperature (500°C) not only provides suitable driving force to detach PtO x from bulk, but also keeps the strong interaction with supports to trap Pt atoms.
In order to further verify the anchoring effect of Al sites, the states of Al species in Pt@Z1 samples suffering from calcination under different atmospheres were investigated by 27 Al magicangle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The peaks around 54 ppm are attributed to the framework aluminum in the form of four-coordinate aluminum (Figure 6a). [19] After hydrogen reduction, decreasing peak intensity of the framework aluminum suggests partial extraction of framework Al atoms to extraframework positions. [20] The deficiency of framework aluminum atoms affords fewer anchoring sites, resulting in the aggregation of mobile Pt atoms, which is in accordance with results shown in Figure 1a-c. The peak intensity of framework aluminum increased again after subsequent calcination under oxygen atmosphere. It is   speculated that some extra-framework Al species might migrate into the ZSM-5 zeolite framework during calcination. [21] The increasing Al species provides anchoring sites and promotes dispersion of Pt clusters. Moreover, it is noteworthy that calcination under nitrogen atmosphere fails to increase the intensity of framework Al species in Pt@Z1-H/400-O/500. [22] The result also explains the unsuccessful dispersion of Pt clusters ( Figure S3, Supporting Information). The effect of oxygen-induced dispersion on the catalytic performance of partial oxidation of methane (POM) reaction was studied over Pt@Z1. Pt@Z1-H/400-O/500 showed higher conversion rate and selectivity for H 2 and CO under hightemperature reaction conditions for 24 h than that of Pt@Z1-H/400 due to the formation of highly dispersed and stable Pt single atoms (Figure 6b-d). Our results demonstrate welldispersed Pt single atoms anchored on Al sites in the form of Pt-O x (OH) y , which resulted in outstanding catalytic performance and stability of the POM reaction. [23]

Conclusion
Dispersion of Pt clusters encapsulated within ZSM-5 and S-1 was achieved via thermal induction at high temperature under oxygen atmosphere. The role of Al sites and calcination temperature on the dispersion process was investigated. The resultant Pt@ZSM-5 displayed better catalytic ability and stability than Pt clusters due to the atomically dispersed Pt species and SMSI. Thermal treatment in O 2 triggers the detachment of Pt atoms from the bulk to form PtO x (PtO, PtO 2 ), leading to the migration of PtO x species. The detachment energy of PtO x decreases at higher temperature, which promotes the detachment process. Strong binding energy with Al sites offers the opportunity to trap and anchor PtO x , while it becomes weak at over-high calcination temperature. More Al sites in zeolites are conducive to the dispersion of Pt clusters under O 2 , and moderate temperature (i.e. 500°C) is required to balance the detachment and mobility of Pt atoms.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.