As a new frontier in catalysis, atomically dispersed catalysts [1, 2] have attracted widespread interest in diverse research areas such as chemistry, material science and environmental science. By using atomically dispersed catalysts, several new catalytic mechanisms have been discovered, thereby enriching our understanding on the most basic subject in nature and chemical industry: catalysis [3-11]. Though conventional preparation methods (e.g., co-deposition, impregnation) have proven effective for preparing various atomically dispersed catalysts, many new methods are emerging for creating atomically dispersed catalysts that are difficult to synthesize by traditional methods. For instance, mass-selected and soft-landing techniques allow the preparation of catalysts based on single metal atoms or clusters of fixed metal atoms [12, 13]. Atomic layer deposition was successfully used in anchoring Pd and Pt on graphene [14, 15]. UV-based deposition followed by sodium cyanide leaching was successfully employed to prepare atomically dispersed Au on TiO₂ [10, 16]. Recently, an effective photochemical route has been developed to prepare highly active and stable Pd₁/TiO₂ catalysts with a high Pd loading content [17]. The key point of the photochemical route is the UV-induced formation of ethyl glycol (EG) radicals that facilitate the removal of Cl^- and ensure the stabilization of Pd atoms via the formation of more Pd-O bonds. However, it is yet to be determined whether the photochemical method is universal for different TiO₂ supports.

Atomically dispersed catalysts provide an ideal system for investigating complex interface phenomena in catalysis. For example, though the catalytic oxidation of CO is a widely studied model reaction, the reaction mechanism remains unresolved [18-22]. To investigate the mechanism, Pt₁ and Pd₁ catalysts synthesized through different methods have been studied. However, quite controversial results have been reported [2, 11-13, 23, 24]. Some studies have claimed that Pt₁ and Pd₁ catalysts are highly active even at 303 K [2, 24], whereas others have shown that the atomically dispersed catalysts only show decent catalytic activity at high temperatures e.g., above 500 K [11, 12, 23]. The different performance of the catalysts could be caused by the different coordination environments around the single-atom metal centers [25], similarly to the so-called "support effect" in traditional heterogeneous catalysis. Hence, it is important to evaluate the effectiveness of different methods in preparing atomically dispersed catalysts on various supports or even similar supports of different surface properties.

In the present study, we report the application of the photochemical method to prepare atomically dispersed Pd catalysts on two different TiO₂ supports, (001)-exposed anatase nanocrystals (NCs) and commercial TiO₂ (P25). The 0.1% Pd₁/TiO₂ catalysts, which are prepared on both TiO₂ supports, show higher activity and stability in the hydrogenation of styrene when compared with catalysts prepared by other methods. Moreover, the catalysts show a low-temperature activity in the catalytic oxidation of CO that is the highest among the best Pd-based catalysts reported to date [24].

Titanium(Ⅳ) fluoride (TiF₄, 99%), 1-octadecene (1-ODE, 90%) and styrene were purchased from Acros Organics. Oleylamine (OLAM, 70%) and oleic acid (OLAC, 90%) were purchased from Sigma-Aldrich. P25 was purchased from Degussa. Palladium chloride (PdCl₂, 59.8%) was purchased from Kunming Institute of Precious Metals. Pd/C was purchased from Alfa Aesar. Hydrochloric acid (HCl, ~35%), ethanol, toluene, hexanes, acetone, methanol, N, N-dimethylformamide (DMF) and 2-propanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). H₂ (99.999%) was purchased from Linde Gas (Xiamen Corporation Ltd, China). The water used in all experiments was ultrapure (18.2 MΩ). All chemicals were used as received without further purification.

Preparation of the TiF₄ stock solutions was achieved by following the procedure reported therein [26]. The synthesis was undertaken in a glovebox under a nitrogen atmosphere. After mixing 0.2 mol/L TiF₄ and 1.0 mol/L OLAC in 1-ODE, the mixture was stirred in a water bath at 353 K to promote dissolution of TiF₄. Once dissolved, the TiF₄ stock solution was orange-brown.

The synthesis was performed using standard Schlenk line techniques under a nitrogen atmosphere. In a 125-mL flask, 30 mmol OLAM, 10.2 mL 1-ODE, and 0.48 mL (1.5 mmol) OLAC were combined, and the reaction mixture was degassed at 393 K for 1 h and cooled to 333 K under N₂ atmosphere. Then, 1.5 mL TiF₄ stock solution was added, and the resulting mixture was promptly heated to 563 K and held for 10 min to allow the formation of seeds. Then, 8 mL TiF₄ stock solution was injected into the flask at 0.3 mL/min with a syringe pump. The heating mantle was then removed to allow the flask to cool naturally to room temperature. Then, the reaction contents were diluted with 5 mL toluene and centrifuged at 6000 r/min for 5 min to separate the NCs. The NCs were then redispersed in toluene via sonication, precipitated by 2-propanol and methanol and recovered by centrifugation. This washing procedure was repeated twice.

Prior to using the NCs as a support for the catalyst preparation, they were treated with NaOH solution [27]. Briefly, 50 mg NCs was dispersed in 5 mL NaOH aqueous solution (H₂O/ethanol = 1:1) and stirred in the dark for 8 h to remove surface fluoride.

The general synthesis of the Pd/TiO₂ catalysts was performed by dispersing either TiO₂ NCs or P25 (100 mg) in 10 mL water. Then, 0.2 mL H₂PdCl₄ solution (5 mmol/L) was added to the dispersion under stirring. The dispersion was then subjected to UV treatment at room temperature under stirring. The UV treatments were conducted on a xenon-lamp parallel light source system (CHF-XM35-500W, Beijing Trusttech Co. Ltd.) equipped with an optical filter that only allowed the transmission of 365 nm UV. All UV treatments were performed at an operating current of 10 A. The power density of the UV light was 1.94 mW/cm², as measured by a radiometer (FZ-A, Beijing Normal University photoelectric instrument factory). After 10-min irradiation, the light grey 0.1% Pd/TiO₂ product was collected via centrifugation and subsequently washed with water. The reference sample 1% Pd/TiO₂ was prepared using the same procedure, however, using 2 mL H₂PdCl₄ solution. Subsequently, 0.1% Pd/P25 was calcined at 623 K for 4 h.

Transmission electron microscopy (TEM) studies were performed on a TECNAI F-30 transmission electron microscope operating at 300 kV. For the analysis, the samples were ultrasonically dispersed in ethanol, and droplets of the suspension were deposited on a copper grid coated with a holey carbon film, followed by drying under ambient conditions.

X-ray absorption spectroscopy (XAS) of the samples was performed by recording the X-ray absorption spectra at the Pd K-edge of the samples at the XAFS station (BL14W1) of the Shanghai Synchrotron Radiation Facility. The electron storage ring was operated at 3.5 GeV. Using Si (311) double-crystal monochromator, data collection was performed in transmission mode for Pd XAS. All spectra were collected in air at room temperature. By using the third ionization chamber, standard compounds, including Pd foil, PdO standard were measured simultaneously for the respective edges and used for energy calibration. The beam size was limited by the horizontal and vertical slits, achieving a measurement area of 1 × 4 mm². The XAS data were processed using WinXAS version 3.11.

X-ray absorption fine structure (XAFS) simulations were conducted using the ab initio self-consistent field code FEFF8.2 [28]. The model of bulk Pd was built with Diamond program for the FEFF input file, and the coordination number of Pd-Pd was fixed at 12 to determine the amplitude reduction factor (S₀²). All data were processed using a consistent methodology of background subtraction, k2-weighting and a k-range of ~3.5-10.5 Å^-1 for one-shell fits to FT R-space. Reliable parameter values, such as bond distances and coordination numbers, were determined via single-shell R-space fitting of Pd spectra.

For the catalysis studies, the catalysts were first dispersed in ethanol. For styrene hydrogenation, 5 × 10^-3 μmol Pd was diluted in 10 mL ethanol in a glass pressure vessel to achieve a Pd/styrene mole ratio of 1:10000. The catalyst dispersion was then sonicated and mixed with 50 μmol styrene by a magnetic stirring bar. The vessel was then charged with H₂ to 0.1 MPa and maintained at 303 K in a water bath during stirring. The reaction was stopped after the desired time (every 10 min) to extract 100-μL aliquots of the reaction mixture for gas chromatographic analysis and conversion determination.

For CO oxidation, the performance of the catalysts was investigated using a fixed-bed glass reactor (internal diameter 10 mm). CO conversion was monitored by an online gas chromatograph. The gas mixture of air (40 mL/min) and CO (10 mL/min, 5 vol % in N₂) was regulated by mass flow controllers at atmospheric pressure and passed through the catalysts with a space velocity of 3750 L/(g·h). The reactor was heated, and the temperature of the catalyst was measured with a K-type thermocouple. The temperature of the reactor was incrementally ramped from room temperature to 473 K. CO conversion (X_CO) was calculated as follows:

The (001)-exposed TiO₂ NCs were prepared through a seeded growth technique, following the synthetic method reported by Murray et al. [26]. Small TiO₂ crystal seeds were formed by rapidly heating the 1-ODE solution of TiF₄ with excess surfactants OLAC and OLAM. Then, the stock solution was slowly added via a syringe pump to prevent further nucleation. Preferential adsorption of F^- on anatase (001) facet allowed the formation of uniform anatase NCs with a high percentage of (001) facets [6], as shown in Fig. 1(a). Before photodeposition of Pd on TiO₂ NCs, the surface F^- was substituted by OH^- by treating the surface with NaOH. The photochemical method was then applied to anchor the Pd atoms to the surface of TiO₂ NCs. By treating the water dispersion of TiO₂ NCs and H₂PdCl₄ with UV light, two catalysts with Pd loading contents of 0.1% and 1% (weight ratio, m_Pd/m_catalysts) were prepared for comparison. As observed in the high-resolution TEM (HRTEM) image in Fig. 1(b), Pd nanoparticles were not detected in 0.1% Pd/TiO₂.

To further demonstrate the atomic dispersion of Pd, XANES and EXAFS of 0.1% Pd/TiO₂ were examined. The shape of the XANES spectrum of 0.1% Pd/TiO₂ was identical to that of Pd oxide, but significantly different from that of Pd foil [29, 30], thereby suggesting that all Pd species are positively charged. The Fourier transform (FT) of k2-weighted EXAFS data of 0.1% Pd/TiO₂ revealed the absence of peaks attributed to Pd-Pd bonds that in contrast were observed in the spectrum of Pd foil at ~2.5 Å. However, an apparent peak attributed to Pd-Pd bonds was observed in the FT-EXAFS profile of 1% Pd/TiO₂, thus indicating the formation of Pd clusters or nanoparticles.

To conduct fittings of the experimental data, simulations of EXAFS scattering paths were performed using the ab initio self-consistent field computational code FEFF8.2. As shown in Table 1, the coordination number for Pd-O was ~4 for 0.1% Pd/TiO₂ and 3 for 1% Pd/TiO₂. No Pd-Pd path was observed in the fitting result of 0.1% Pd/TiO₂, thereby confirming the sole presence of atomically dispersed Pd species in 0.1% Pd/TiO₂.

In our previous study, Pd atoms were photochemically deposited on EG-capped TiO₂ nanosheets owing to the UV-induced formation of EG radicals that assisted the removal of Cl^- from Pd atoms [17]. To investigate the formation mechanism of 0.1% single-atom Pd catalyst in the present study, a sample was prepared following the same procedure used for preparing 0.1% Pd/TiO₂, however, in the absence of UV treatment. By simply impregnating TiO₂ NCs with H₂PdCl₄, the sample was collected through centrifugation and washed with water twice. The EXAFS data revealed that both Pd-O and Pd-Cl paths were detected in the first-shell coordination environment of the Pd atoms (Fig. 2). The coordination numbers for Pd-O and Pd-Cl were ~2.0 and ~1.9, which were quite similar to our previously reported result for the EG-capped TiO₂ nanosheets system. After UV treatment, Cl^- atoms were completely removed from Pd. In the literature, it is well documented that the production of reactive oxygen species, such as hydroxyl radicals (·OH), on TiO₂ is enhanced by the presence of anatase (001) facets owing to the selective migration of photo-generated holes to the (001) facets [31-34]. We believe that the UV-assisted removal of Cl^- is highly related to the photochemical reactions occurring on TiO₂ NCs. The developed photochemical route is applicable to prepare single-atom Pd catalysts on other TiO₂ systems.

The as-prepared 0.1% Pd/TiO₂ on TiO₂ NCs was highly active in the hydrogenation of styrene. The turn-over frequency (TOF) was calculated to be 6000 h^-1, which is considerably higher than that of commercial Pd/C catalyst (TOF = 970 h^-1). More importantly, 0.1% Pd/TiO₂ prepared through photochemical method was notably more stable than catalysts prepared by chemical reduction methods. After impregnating TiO₂ NCs with H₂PdCl₄, either NaBH₄ or H₂ was used as a reducing agent to prepare reference samples. As shown in Fig. 3(b), the conversion-time curves of both reference samples were not straight lines during the entire reaction period, indicating activity loss during catalytic reaction. As styrene hydrogenation is a zero-order reaction with respect to styrene [35, 36], the concentration of styrene has no effect on the reaction rate. Thus, the declines in the reaction rates of the reference samples can be attributed to changes in the state of the catalysts. Such changes might involve the aggregation of Pd atoms into small clusters or nanoparticles. For the catalyst 0.1% Pd/TiO₂ prepared though photochemical method, the UV-assisted removal of Cl^-, previously adsorbed on Pd atoms, at room temperature and easy formation of four-oxygen-coordinated Pd atoms on TiO₂ might be the key for the high stability of the catalyst.

With the current understanding, we extended the use of the photochemical method to the synthesis of atomically dispersed Pd catalyst on commercial TiO₂ (P25). As shown in Fig. 4(a) and (b), the TEM image and FT-EXAFS R-space spectrum of 0.1% Pd/P25 clearly demonstrated the sole presence of Pd as single atoms. Neither the formation of Pd nanoparticles nor Pd-Pd scattering was observed in the TEM and XAS data, respectively.

Because CO oxidation is highly sensitive to the chemical environment around the metal centers, it was chosen as a model reaction to investigate the difference among catalysts prepared by the photochemical method. As shown in Fig. 4(c), the 0.1% Pd deposited on TiO₂ NCs and P25 catalysts showed high activities toward CO oxidation even at temperatures below 373 K. Various investigations on the catalytic performances of single-atom Pd catalysts toward CO oxidation have been conducted by several groups. Pd₁/MgO prepared by mass-selected techniques could convert CO into CO₂ if the catalyst was pre-adsorbed by O₂ [37]. In contrast, Pd₁/TiO₂ prepared using the same method showed no activity even at 500 K [12]. To date, the most active Pd-based catalyst for CO oxidation is Pd/La-doped alumina [24], which shows a low-temperature activity with an onset temperature of 313 K. The TOF of Pd/La-doped alumina at 343 K was reported to be 1.9 × 10^-3 s^-1.

In the present work, the activity of 0.1% Pd/TiO₂ in CO oxidation was much higher than that of Pd/La-doped alumina. As shown in Fig. 4(a), both 0.1% Pd₁/TiO₂ catalysts showed activity at room temperature with an onset temperature as low as 303 K. Over 96% conversion was achieved at 373 K. When compared with the highest TOF achieved by Pd catalysts (Pd/La-doped alumina) reported in the literature, the measured TOF of 0.1% Pd/TiO₂(NC)catalyst at 303 K was 6.7 × 10^-3 s^-1, which represents a 3.3-fold increase. Such an enhancement could be attributed to the support effect of TiO₂. Moreover, though 0.1% Pd/P25 also showed a high activity at low temperature, the calculated apparent activation energy of 71.2 kJ/mol was much higher than that of 0.1% Pd/TiO₂. It should be noted that the formation of Pd nanoparticles was not detected in either catalyst (0.1% Pd/P25 and 0.1% Pd/TiO₂) after the catalytic reaction. This result reveals that the catalytic performance of single-atom catalysts is highly influenced by the surface state of support. Though the atomically dispersed species was the same in both 0.1% Pd/TiO₂ and 0.1% Pd/P25, the catalytic oxidation of CO on these two catalysts proceeded under different mechanisms, which needs further investigation. In contrast, through the pre-exponential factor for 0.1% Pd/TiO₂ was much higher, the activation energies for 0.1% and 1% Pd/TiO₂ were similar, thereby indicating that the reaction center of both catalysts might be similar.

In the present study, we demonstrated that the photochemical method could be extended to the preparation of atomically dispersed Pd catalysts on various TiO₂ supports. Using (001) facets-exposed TiO₂ nanocrystals and commercial TiO₂ P25 as supports, 0.1% Pd/TiO₂ and 0.1% Pd/P25 atomically dispersed catalysts were prepared by simply mixing with H₂PdCl₄, followed by UV irradiation. UV irradiation was essential to remove Cl^- adsorbed on Pd atoms to obtain the catalysts at room temperature. However, in the absence of surface passivation of organic ligands (e.g., ethylene glycolate) on TiO₂, Pd loading was limited. Increasing the loading of Pd up to 1% led to the formation of small Pd clusters or nanoparticles. The as-prepared 0.1% Pd/TiO₂ catalysts exhibited excellent catalytic activities and stabilities in both the hydrogenation of styrene and oxidation of CO. A low-temperature activity in CO oxidation was observed for the 0.1% Pd/TiO₂ catalysts prepared using (001) facets-exposed TiO₂ nanocrystals and P25 as supports. Their TOFs were the highest among all Pd-based catalysts reported to date.

Acknowledgments

We thank the XAFS station (BL14W1) of the Shanghai Synchrotron Radiation Facility (SSRF). Dr. Pengxin Liu thanks the National postdoctoral program for innovative talents (BX201600093).