Stability investigation of a high number density Pt1/Fe2O3 single-atom catalyst under different gas environments by HAADF-STEM

Catalysis by supported single metal atoms has demonstrated tremendous potential for practical applications due to their unique catalytic properties. Unless they are strongly anchored to the support surfaces, supported single atoms, however, are thermodynamically unstable, which poses a major obstacle for broad applications of single-atom catalysts (SACs). In order to develop strategies to improve the stability of SACs, we need to understand the intrinsic nature of the sintering processes of supported single metal atoms, especially under various gas environments that are relevant to important catalytic reactions. We report on the synthesis of high number density Pt1/Fe2O3 SACs using a facial strong adsorption method and the study of the mobility of these supported Pt single atoms at 250 °C under various gas environments that are relevant to CO oxidation, water–gas shift, and hydrogenation reactions. Under the oxidative gas environment, Fe2O3 supported Pt single atoms are stable even at high temperatures. The presence of either CO or H2 molecules in the gas environment, however, facilitates the movement of the Pt atoms. The strong interaction between CO and Pt weakens the binding between the Pt atoms and the support, facilitating the movement of the Pt single atoms. The dissociation of H2 molecules on the Pt atoms and their subsequent interaction with the oxygen species of the support surfaces dislodge the surface oxygen anchored Pt atoms, resulting in the formation of Pt clusters. The addition of H2O molecules to the CO or H2 significantly accelerates the sintering of the Fe2O3 supported Pt single atoms. An anchoring-site determined sintering mechanism is further proposed, which is related to the metal–support interaction.


Introduction
Supported metal catalysts are widely used for both fundamental research and industrial applications because of their excellent activity, selectivity, and stability [1][2][3]. Supported Pt catalysts are extensively applied in many important chemical reactions including emission control of toxic gases, producing of chemicals, and energy conversion in fuel cells [4][5][6]. Both theoretical modeling and experimental research on the catalytic properties of Pt based catalysts have been extensively studied [7][8][9][10].
Metal-support interaction plays an important role in determining the catalytic performance of supported metal catalysts [11,12]. The nature of the metal-support interaction depends on many parameters including particle size, quality of the interfaces, the surface structure and chemistry of the support materials [13][14][15][16]. Among them, the metal nanoparticle size has proven to be crucial in modulating the metalsupport interaction. Many research studies have reported that the catalytic activities can be well regulated by controlling the metal particle size [15,17,18]. For example, in the Pt/Fe 2 O 3 system, the particle size of Pt colloid influenced the chemical states of Pt species and the strength of the metal-support interactions, and it is suggested that suitable metal-support interactions played a crucial role in determining the redox properties of the catalysts, thus being beneficial to the oxidation of CO at low temperature [19]. Additionally, for hydrotalcite supported Pt nanoparticles, the activity of propane conversion in the propane dehydrogenation process increases with the reduction of Pt nanoparticle size, while the selectivity to propylene performs in an inverse way [20]. Accordingly, downsizing the metal particle size in the supported metal catalysts has been one of the most important targets in the catalysis research field [21,22]. Single-atom catalysts (SACs), which reach the size limitation of supported metal catalysts and maximize the efficiency of the active metals, have been of increased interest for both fundamental study and industrial applications during the past few years [23][24][25]. As a pioneering work, a Pt 1 /FeO x SAC was practically fabricated by a coprecipitation method and exhibits very high activity and stability for both CO oxidation and preferential oxidation of CO in H 2 , attributed to the partially vacant 5d orbitals of positively charged, high-valent Pt atoms [24]. Much literatures has confirmed the outstanding performance of SACs compared to their nanoparticle or cluster counterparts. And it has also been verified that in some supported metal catalysts that consist of clusters or nanoparticles, the atomically dispersed metal atoms are the most important active sites during the catalytic reactions [23,26,27]. Many works have been devoted to the synthesis of SACs, among which the adsorption approach has proven to be an efficient way to anchor metal atoms onto the surface of the as-prepared supports, taking advantage of the metal-support interactions [28][29][30].
For supported metal catalysts, stability is another important factor that restricts their industrial application. Sintering of metal components during the heating process or catalytic reactions often leads a loss of activity or selectivity [31,32]. The sintering of supported metal catalysts has been intensively studied in recent years, and the strong metalsupport interaction has proven to be vital for the stability of heterogeneous catalysts [25,[33][34][35]. As for the SACs, due to the high surface-free energy and the low coordination number of metal single atoms, supported metal atoms are thermodynamically unstable and easy to sinter during catalytic reactions, especially at high reaction temperatures [36][37][38][39]. The investigation of the stability of supported single metal atoms under a specific gas environment can help us understand the sintering mechanisms of SACs during catalytic reactions and thus provide insights into developing stable SACs for practical applications. Researchers have used scanning tunneling microscopy to follow the evolution of supported Pt and Pd atoms in model catalysts under various environments illustrating the role of gas molecules in the anchoring and sintering of metal atoms [40][41][42]. However only a few reports have been published on this subject [41][42][43], and these are still somewhat limited, especially for the stability of SACs with a high number density studied by aberration-corrected (AC) electron microscopy, which is a convincing and intuitive approach to directly image the single metal atoms, clusters, and nanoparticles in the supported metal catalysts.
Here, a Pt 1 /Fe 2 O 3 SAC with a Pt 1 number density as high as 1.2 atom nm −2 was synthesized by a facial wet chemical method. We further investigated the mobility of high number density Pt single atoms in a Pt 1 /Fe 2 O 3 SAC at a moderate temperature under various gas environments that are relevant for CO oxidation, water-gas shift, and hydrogenation reactions.

Ex situ reactor
The stability of the as-prepared Pt 1 /Fe 2 O 3 SACs under various gas environments was evaluated in a fixed-bed reactor at 250°C for 2 h. The feed gas composition was 1 vol% of the target gas and balance helium with a flow rate of 33 ml min −1 . For each test, ∼20 mg Pt 1 /Fe 2 O 3 SAC was directly used without any pretreatment.

Characterization
AC high-angle annular dark-field (AC-HAADF) STEM was used to characterize the as-prepared and the gas-treated Pt 1 /Fe 2 O 3 SACs. Sub-ångström resolution AC-HAADF-STEM images were obtained on a JEOL JEM-ARM200F STEM/TEM, equipped with a CEOS CESCOR hexapole aberration corrector operated at 200 kV that enables an imaging resolution reaching 0.08 nm. The specimen used for the STEM investigation was prepared by dispersing the sample onto a Cu grid coated with a thin holey carbon film. X-ray photoelectron spectroscopy (XPS) characterization of the asprepared and the gas-treated Pt 1 /Fe 2 O 3 SACs was conducted on a Thermo Fisher ESCALAB 250Xi x-ray photoelectron spectrometer using Al Kα x-rays as the photon source.

Fresh Pt 1 /Fe 2 O 3 SAC
The as-prepared sample was first characterized with sub-ångström resolution AC-HAADF-STEM and was identified as a SAC with a high number density. The HAADF-STEM imaging method adopting the STEM method, where the image intensity is approximately proportional to the square of the atomic number, has proven to be an efficient tool to characterize heterogeneous catalysts [44][45][46]. When combined with the AC technique, atomic resolution could be obtained, and was applied for the direct observation of metal atoms on the support surface. We have done many studies on the synthesis and AC-STEM characterization of SACs composed with different metal atoms on various supports [47,48]. For the as-synthesized Pt 1 /Fe 2 O 3 , as shown in figures 1(a) and (b) and many other similar lowmagnification HAADF-STEM images, Pt particles or clusters were not present in the as-synthesized Pt 1 /Fe 2 O 3 SAC. The atomic-resolution images in figures 1(c) and (d) and the analyses of many similar images (figure S1 is available online at stacks. iop.org/NANO/29/204002/mmedia) reveal that in the as-synthesized Pt 1  , which is close to the Pt single atom density (1.0 atom nm −2 ) estimated by counting on many high-resolution HAADF-STEM images, and it has reached the density limitation synthesized with an adsorption method [49]. To the best of our knowledge, it is one of the SACs with the highest number density, which is beneficial to the study of metal atom stability under various gas environments [50][51][52]. For the SACs with low number density, it is reported that the metal atoms do not sinter into nanoparticles or clusters and keep a single atom nature during catalytic reactions. However, it cannot be unambiguously concluded that the metals atoms are stable, because the metal atoms may be moveable on the support surfaces but cannot collide with each other and form nanoparticles or clusters during the catalytic environment because of the low number density, which needs to be further confirmed using high number density SACs.

Anchoring sites of Pt atoms
The active center of the supported metal catalyst consists of metal atoms and their vicinal atoms of the support. The configuration of the active centers is determined by the anchoring sites of the metal atoms on the oxide support, and it is critical to the charge transfer between the metal-support interaction, influencing the catalytic performances of the SACs. For SACs with low metal loading levels, many previous works have indicated that the metal atoms are anchored to the cation positions of the metal oxide supports, where the surface cation vacancies serve as anchoring sites [24,29]. We have adopted AC-STEM images to confirm that the Pt, Ir, and Au atoms all occupied cation positions in the Pt 1 /FeO x [24,28], Ir 1 /FeO x [27], Pt 1 /ZnO [29], and Au 1 /ZnO [29] SACs. Density functional theory (DFT) simulations further verify that the three-fold hollow sites on the O 3 -terminated surface of Fe 2 O 3 (001), where the Pt atoms occupy the Fe cation positions, are the most stable anchoring sites [24,53]. Here, we experimentally confirm it using AC-HAADF-STEM on the SAC with ultra-low metal loading. The low metal loading (0.05 wt%) Pt 1 /Fe 2 O 3 (denoted as 0.05Pt 1 /Fe 2 O 3 ) was comparatively synthesized using a similar adsorption method by decreasing the usage of the Pt precursor. As shown in figure 3, the density of the isolated Pt 1 atoms in the 0.05Pt 1 /Fe 2 O 3 is much lower than that in the Pt 1 /Fe 2 O 3 , and the Pt atoms all occupied the positions of Fe atoms on the Fe 2 O 3 surfaces, which was confirmed by checking many atomic-resolution AC-STEM images. However, the cation vacancies on the Fe 2 O 3 surfaces are limited. Thus, the Pt atoms will be anchored to other site constitutions with the increase of the Pt loading level. Accordingly, the isolated Pt 1 atoms in the high metal loading SACs are not the same with different anchoring sites. Here, in our SAC with high metal loadings, Pt 1 single atoms anchored at different sites can also be directly confirmed by atomic resolution HAADF-STEM images. As shown in figure 2, when the metal loading was increased to 0.53 wt%, the anchoring sites of the Pt 1 atoms in the 0. Chen et al confirmed this using DFT computations [53]. Naoya Shibata et al show direct atomic-resolution images of individual Pt atoms adsorbed on TiO 2 (110) surfaces using AC STEM images with sub-ångström spatial resolution, identifying five different Pt atom adsorption sites on the TiO 2 (110) surface [54]. However, because of the irregular support surfaces and the high number density of Pt atoms in our Pt 1 /Fe 2 O 3 SAC, the accurate fraction of different anchoring sites cannot be directly calculated from our HAADF-STEM images.

Pt 1 /Fe 2 O 3 SAC under different gas environments
O 2 , CO, and H 2 broadly exist in many chemical reactions such as CO oxidation, methanol steam reforming, and watergas shift reaction. Investigation of the stability of SACs under these different gas environments provide insight into the stability of Pt atoms under reductive and oxidative environments, which is crucial to the deep understanding of catalyst activation and deactivation processes.
First, after annealing the as-prepared Pt 1 /Fe 2 O 3 SAC in 1 vol% O 2 at 250°C, all the Pt single atoms remained isolated, as shown in figure 4(b). Many similar high-resolution images and low-mag HAADF-STEM images (figure S2) confirm that the Pt 1 /Fe 2 O 3 SAC keeps the single atom nature after annealing in O 2 . The presence of O 2 molecules did not sinter the Pt single atoms at 250°C, confirming that the  Pt 1 /Fe 2 O 3 SAC is stable under the oxidative environment. In our research, we also found that the Pt 1 /Fe 2 O 3 SAC is stable in air at room temperature even after two years. This is different from previous reports on gas-assisted sintering in air for Pt/Fe 3 O 4 catalysts, where O 2 dissociates on the Pt clusters and spills over onto the support and then reacts with Fe from the bulk to create new Fe 3 O 4 (001) islands [40]. However, similar phenomena are not observed in our experiment. It is supposed that the calcination pretreatment is critical to the stability of Pt atoms under O 2 . In our calcined Pt 1 /Fe 2 O 3 SAC, Pt atom is interacted with support oxygen ions after calcination, forming (Pt 1 -O n -) moieties, which is verified by the XPS result. As shown in figure 5(a), the Pt 4f XPS peaks of the as-prepared Pt 1 /Fe 2 O 3 SAC consists of two pairs of doublets: the first doublet (75.0 and 78.4 eV) and the second doublet (72.6 and 76.0 eV) correspond to Pt 4+ and Pt 2+ , respectively, which reveals the presence of PtO 2 and PtO on the surface of the as-prepared Pt 1 /Fe 2 O 3 SAC. Peak deconvolution ( figure 5(b)) revealed the contribution from Pt 4+ (∼64 at.%) and Pt 2+ (∼32 at.%) and the presence of metallic Pt 0 is negligible in the fresh Pt 1 /Fe 2 O 3 SAC. The oxygen bonding with Pt atoms is saturated after calcination at 300°C, thus when exposed to O 2 steam under our test condition, the O 2 molecules were not absorbed on the oxygen-saturated Pt atoms, and thus the Pt atoms will maintain the single atom nature during the exposure to O 2 . This also confirms the strong metal-support interaction in our calcined Pt 1 /Fe 2 O 3 SAC.
Stability tests of Pt atoms under reductive atmosphere including CO (denoted as CO-Pt 1 /Fe 2 O 3 ) and H 2 (denoted as H 2 -Pt 1 /Fe 2 O 3 ) were further investigated. After annealing the as-prepared Pt 1 /Fe 2 O 3 SAC in 1 vol% H 2 at 250°C for 2 h, however, the initially isolated Pt single atoms agglomerated to form small clusters (figures 6(a) and S3), clearly suggesting that the H 2 reduction treatment facilitated the sintering of the Pt single atoms. Similarly, under the CO gas treatment, the Pt single atoms moved to form subnano Pt clusters (figures 6(b) and S4). The Pt atoms are reduced to metallic Pt 0 during the sintering process and the Pt 4+ species are negligible in the reduced samples, as evidenced by the Pt 4f XPS spectra ( figure 5). Compared to the H 2 treated sample, the size distribution of the CO treated samples is much narrower and the atomic-resolution HAADF images showed that the Pt clusters were highly disordered or amorphous-like. By counting large quantities of clusters and nanoparticles from many high resolution HAADF-STEM images, the size distributions of the gas-treated samples were obtained, as shown in figures 6(e) and (f). The size distribution of the catalyst treated with H 2 is about 1.0±0.3 nm, a little bigger than that of the catalyst treated with CO (0.8±0.2 nm), suggesting that the weakening effect of the (Pt 1 -O n -) bond by H 2 adsorption is stronger than that induced by CO adsorption. The gas induced metal sintering process is proposed to follow the procedure that the gas molecules firstly adsorbed onto the metal atoms, the adsorbed gas molecules then spilled over from the metal to the metal-support interaction and lastly the gas molecules at the metal-support interface extract substrate atoms at the metal-support interface thus breaking the metalsupport interaction and inducing the movement of metal atoms [40,41,43,50,[55][56][57][58]. In the case of CO, many reports have suggested that CO molecules strongly bond to Pt atoms, modifying the charge transfer between the metalsupport interaction. And CO could be delivered to the metalsupport interface and then react with lattice O atoms at the metal-support interface to create CO 2 , which weakens the metal-support interaction and induces the metal movement sintering into Pt clusters [40,59]. Under the H 2 atmosphere, adsorption and dissociation of H 2 molecules on the Pt atoms are followed by the spillover of H onto the metal-support interface to creates -OH groups [60,61]. The surface -OH group can diffuse over the oxide surface which are known to react with O lattice atoms, leading to the desorption of water [62,63]. This could be further verified by the O 1s XPS spectra of the H 2 -Pt 1 /Fe 2 O 3 sample (figure S5), where peak deconvolution confirmed the presence of surface -OH groups (531.3 eV, ∼28.7 at.%) and water species (533.1 eV, ∼8 at. %). The -OH groups and water species are negligible in the as-prepared Pt 1 /Fe 2 O 3 and CO-Pt 1 /Fe 2 O 3 samples. The diffusion of the -OH groups on the Fe 2 O 3 surface can accelerate the sintering of Pt atoms, thus the sintering process is faster and forms bigger Pt clusters or nanoparticles under H 2 compared to CO. However, the reduction of Fe 2 O 3 with CO by extraction of lattice oxygen is much weaker than that in the case of H 2 , which is confirmed by the Fe 2p XPS spectra in figure S6. The process of atom sintering is illustrated in figure 7. The effects of long-term gas treatment of supported Pt 1 SACs have also been investigated. When we extend the CO treatment at 250°C to 24 h, the Pt clusters grew into wellcrystallized Pt nanoparticles with sizes in the range of 5-10 nm, as shown in figure S7. From the high-resolution HAADF-STEM images shown in figure S7, there still exist some Pt single atoms that did not sinter into clusters, and they all occupied the Fe positions of the support surfaces. This is in accordance with previous reports that the three-fold hollow sites on the O 3 -terminated surface of Fe 2 O 3 (001), where the Pt atoms occupying the Fe cation positions are the most stable sites according to DFT calculations [24,53]. This is also in line with the reported results that single atoms are stable in SACs with low metal loading where the metal atoms occupied the cation positions [24,29,30]. So, increasing the number of cation vacancies on the substrate surfaces should help stabilize the single metal atoms during catalytic reactions, directing the synthesis of long-term stable SACs.
When water was added into the feed gas of CO and H 2 , the Pt sintering process was accelerated, and some larger Pt  This may result from the water species or -OH groups being associated with Pt 1 atoms forming ((OH) m -Pt 1 -O n -) entities, that induce the charge transfer between the metal atoms and the -OH functional groups and weaken the (Pt 1 -O n ) interaction, and thus facilitate the movement of the Pt atoms [28,[64][65][66]. Water splitting forming -OH groups at the Fe 2 O 3 surface can also be achieved, which have been proven by scanning tunneling microscopy and DFT calculations [63,67,68]. Thus, the diffusion of the -OH groups on the Fe 2 O 3 surface accelerate the sintering of Pt atoms, like that under pure H 2 .
The stability tests of Pt atoms under CO or H 2 at room temperature were further investigated. After annealing the asprepared Pt 1 /Fe 2 O 3 SAC in 1 vol% CO and H 2 at room temperature for 2 h, however, the initially isolated Pt single atoms agglomerated to form small clusters under CO (figure S10), while interestingly the Pt atoms keep the single atom nature under H 2 (figure S11). Contrary to the results conducted at 250°C, the sintering process is more efficient with CO at room temperature. This may come from the stronger adsorption of CO than that of H 2 at low temperature, which is proved by experimental result that H 2 conversion was strongly inhibited by the presence of CO in the gas phase during preferential oxidation of CO [69,70].
From these size distributions, we concluded that at 250°C  atoms keep the single atom nature during the treatment with a reductive environment. The extent of sintering can be well modulated by controlling the temperature, time, and gas composition, and thus could provide a protocol to obtained size controlled supported catalysts.

Conclusion
In summary, a Pt 1 /Fe 2 O 3 SAC with a high number density is synthesized using the modified adsorption method. The Pt atom density is as high as 1.2 atoms nm −2 , which reaches the number density limitation based on the adsorption mechanism. Investigation on the stability of Pt atoms under different environments suggests that the Pt atoms are stable under an oxidative atmosphere. Reductive gas including H 2 and CO weakens the metal-support interaction and induces the movement of Pt atoms, forming nanoparticles or clusters. The weakening effect of H 2 is stronger than that of CO and will be enhanced by the addition of water into the feed gas. These results will benefit the research on the stability of SACs during catalytic reactions and instruct the design of SACs with more stable anchoring sites. We also believe that with the help of AC environmental transmission electron microscopy, in situ characterization on the sintering process could provide us with more conclusive evidence and more detailed information in future work.