Selective aerobic oxidation of organic molecules (including alcohols, glucoses, hydrocarbons, and CO) under mild conditions is a great fundamental and practical issue in modern chemistry [1-3]. Because the ground state of molecular oxygen (O₂) is a triplet state, reactions between organic molecules (mainly in a singlet state) and ground-state O₂ to produce new singlet compounds are generally forbidden by the Wigners spin selection rules [4, 5]. Therefore, efficiently activating O₂ under mild reaction conditions is considered as a critical step and the greatest challenge.

In principle, organometallic catalysts, like metal alkoxides, show significant advantages in the oxidation of alcohols, owing to their defined coordination geometry, uniform active metal center, and ligands. However, their application is largely limited by their poor separation and recovery performance [6]. Since Zhang et al. [7] reported a Pt single-atom catalyst (SAC) exhibiting extremely high activity for CO oxidation and proposed the concept of SACs in 2011, atomically dispersed catalysts have attracted enormous attention in the catalysis field [7-11]. Because individual metal atoms in SACs are usually stabilized by covalent coordination or ionic interactions with neighboring surface atoms like homogeneous catalysts, single-atom catalysis has been regarded as a bridge to connect homogeneous and heterogeneous catalysis [12-14]. For these reasons, currently, it has become highly desirable to develop heterogeneous SACs with reactivity and atom efficiency equivalent to those of homogeneous catalysts.

An increasing number of researchers have been studying SACs in oxidation reactions with O₂ as the oxidant. Li et al. [15] reported that ceria-supported Au and Pt SACs showed high activity and selectivity for benzyl alcohol oxidation owing to the restricted geometries of the active sites, which was because the lattice oxygen activated at the interfacial sites exhibited higher selectivity than the O₂ activated on the metal surfaces. Hackett et al. [16] reported that atomically dispersed Pd demonstrated exceptional conversion in the aerobic selective oxidation (SELOX) of allylic alcohols, and the highest activity or the maximum turnover frequency (TOF) value could be achieved on the Pd SACs according to their correlation of activity with Pd size. Although some promising results have been obtained on atomically dispersed catalysts, the activation mechanism of O₂ and the reaction route of SELOX of allylic alcohols are still ambiguous.

In this work, atomic and nano-dispersed Pd catalysts (labeled as Pd₁/Al₂O₃ and Pd/Al₂O₃) were facilely prepared with the same loading amount (0.5 wt%) of Pd anchored onto an Al₂O₃ support rich in penta-coordinate Al³⁺ (Al_penta³⁺) centers. The Pd₁/Al₂O₃ catalyst displayed extremely high activity for the aerobic oxidation of cinnamyl alcohol with a conversion of 92% at 80 ℃ after an 8-h reaction. Its TOF was 15.5 fold of that over nano Pd/Al₂O₃ and superior or comparable to other reported catalysts with similar metal loading under the same reaction conditions. The specific O₂ activation mechanism and the alcohol oxidation route were also proposed.

The morphologies of the two catalysts were determined with aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and extended X-ray absorption fine structure (EXAFS) spectrometry. In the HAADF-STEM images in Figs. 1(a) and (b), Pd₁/Al₂O₃ clearly displays bright dots of single atoms marked with red circles. No nano Pd particles were observed in Fig. S1, and its X-ray diffraction (XRD) pattern in Fig. S2 only showed typical peaks of Al₂O₃ (JCPDS No. 10-0425). By comparison, Pd/Al₂O₃ shows nanoparticles with sizes of 3–7 nm, marked by yellow boxes in Figs. 1(c) and (d), and no Pd single atoms were observed in Fig. S3, in consistent with its XRD pattern (Fig. S2) in which a small peak of Pd (JCPDS No. 46-1043) arose. The detailed structure properties of all the samples are listed in Table S1.

In the case of EXAFS spectra, only one notable peak at 1.58 Å attributed to Pd–O coordination [17, 18] was detected in Pd₁/Al₂O₃, as presented in Fig. 2, whereas no appreciable Pd–Pd coordination peak was identified, again verifying the existence of Pd single atoms. For Pd/Al₂O₃, only one main peak at 2.55 Å was detected, which was corresponding to the coordination of Pd–Pd, whereas no peak related to Pd–O bonds was measured. These results indicated the characteristics of the two catalysts, i.e., they are either entirely isolated Pd single atoms or completely Pd nanoparticles, with the same Pd loading amount on the Al₂O₃ support.

Cinnamyl alcohol was then chosen to evaluate the aerobic SELOX performance of the two catalysts. Pd₁/Al₂O₃ demonstrated extremely high activity relative to Pd/Al₂O₃. Approximately 92% conversion of cinnamyl alcohol was obtained on Pd₁/Al₂O₃ at 80 ℃ under 1 atm after an 8-h reaction (Fig. 3(a)), together with excellent selectivity of 91% to cinnamaldehyde. In comparison on Pd/Al₂O₃, only 29% conversion with 89% selectivity was achieved under the same reaction conditions. Lowering the Pd loading to 0.05% in weight, for which the atomically dispersed HAADF-STEM images are shown in Fig. S4, also obtained a higher cinnamyl alcohol conversion of 44% (Table S2), better than that of the nano Pd/Al₂O₃ catalyst with 0.5 wt% loading. For the low conversion below 30%, the TOF value of Pd₁/Al₂O₃ was calculated as 775 h^–1 (Fig. 3(b)), which was 15.5 times higher than that of Pd/Al₂O₃ (50 h^–1). This performance is superior to many other catalysts with similar metal loadings reported in the literature (Table S3). High activity in SELOX of another primary alcohol, i.e., benzyl alcohol, was also achieved on Pd₁/Al₂O₃, as displayed in Fig. 3(a). The apparent barrier energy (E_app) of Pd₁/Al₂O₃ derived from the Arrhenius plot (Fig. S5) was fairly low (34.1 ± 5.7 kJ mol^–1) compared to those of Pd/Al₂O₃ and similar catalysts [16, 19], which reflects its high activity from another aspect. We further tested the reusability of the Pd₁/Al₂O₃ sample and observed no significant changes in either the conversion or the selectivity over five cycles (Fig. 3(c)).

To elaborate the high catalytic performance of the Pd₁/Al₂O₃ SAC, the electronic states of the Pd species were examined in detail with X-ray photoelectron spectroscopy (XPS), normalized X-ray absorption near-edge structure (XANES), and diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFT). As shown in the Pd 3d XPS spectra in Fig. 4(a), the peaks at 336.7 and 336.9 eV of the unreduced Pd/Al₂O₃ and Pd₁/Al₂O₃ could be ascribed to PdO [20-22]. After reduction at 400 ℃, the Pd 3d_5/2 peak of Pd/Al₂O₃ shifted to a lower binding energy (BE) of 335.0 eV, which is awarded to the Pd⁰ species. In comparison, Pd₁/Al₂O₃ shifted the Pd 3d_5/2 peak to 336.6 eV, corresponding to a partially reduced Pd^δ+ state [21]. The normalized XANES spectra shown in Fig. 4(b) could confirm the above analysis. The white-line intensity of the Pd species of Pd₁/Al₂O₃ was much higher than those of Pd/Al₂O₃ and Pd foil, indicating that the Pd species were positively charged. The CO adsorption behavior could further attest to this. As illustrated by the CO-DRIFT in Fig. 4(c), two CO stretching vibration bands were observed at 1987 and 2079 cm^–1 on Pd/Al₂O₃. The former is attributed to bridge-adsorbed CO on the step sites of the Pd⁰ nanoparticles, and the latter is assigned to the linear-adsorbed CO on the Pd⁰ (111) facets [23-25]. For Pd₁/Al₂O₃, only one adsorption band at 2073 cm^–1 was observed, ascribed to the CO adsorbed on the Pd^δ+ atoms in a top configuration [26, 27]. In correlation with the catalytic performance, the partially reduced Pd atoms could be proposed as active sites for Pd₁/Al₂O₃, well consistent with previous reports [28, 29]. On the other hand, one may notice that the peak position of the linear CO on Pd₁/Al₂O₃ was slightly red-shifted in comparison to that on Pd/Al₂O₃. This suggested that the electrons of Pd may easily transfer from the atomically dispersed samples.

Based on the above, the Pd₁/Al₂O₃ SAC possesses a unique electronic state. It has been reported that the interaction between the metal and support affects the physiochemical property and catalytic performance of a catalyst severely [30-32]. Herein, the different preparation methods may result in different metal–support interactions (MSIs) between Pd and Al₂O₃, thereby leading to a distinct electronic state of the Pd species. To testify this, ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was employed. Three characteristic peaks centered at 5, 35, and 65 (δ) were observed for all the samples (Fig. 5), which are attributed to Al³⁺ cations in octahedral (Al_octa³⁺), pentahedral (Al_penta³⁺), and tetrahedral (Al_tetra³⁺) coordination, respectively [33]. Among them, the Al_penta³⁺ sites are coordinately unsaturated, apt for binding to metal atoms through oxygen bridges; thus, the intensity change of Al_penta³⁺ could reflect the interaction between Pd and Al₂O₃. As shown in the spectra, Pd₁/Al₂O₃ presented a substantial drop in the Al_penta³⁺ intensity compared to pure Al₂O₃, indicating that strong interactions occurred between Al_penta³⁺ and Pd atoms during calcination and reduction. In comparison for Pd/Al₂O₃, the number of Al_penta³⁺ sites remained almost unchanged under the same treatment, revealing a weak MSI. This is probably the consequence of the physical barrier of the inverse nano micelles-encapsulated Pd precursors, which are formed by surfactant P123 during the nucleation and calcination process. In addition, a strong MSI could also explain the higher BE of Pd 3d_5/2 in the unreduced Pd₁/Al₂O₃ (336.9 eV) than that in Pd/Al₂O₃ (336.7 eV), as the PdO species in Pd₁/Al₂O₃ are mostly located on the Al_penta³⁺ sites.

It is well known that O₂ activation is extremely critical to oxidation reactions. According to the Wigners spin selection rule, high-energy oxygen species (e.g., O₂^-, ^·OH, and ¹O₂) activated from ground-state O₂ are highly desirable for the oxidation reaction. To decode the detailed activation process and reaction mechanism, in-situ UV-vis spectra were collected in this study. As presented in Fig. 6(a), the cinnamyl alcohol showed an absorption peak at 281.8 nm and a shoulder peak at 293.0 nm with or without Pd/Al₂O₃, assigned to the π–π* transition of the conjugated bonds between the benzene ring and C=C. Contrastingly, a remarkable red shift from 281.8 to 284.2 nm occurred on contact with Pd₁/Al₂O₃, owing to the effective expansion of the π-conjugation induced by the charge or energy transfer between the adsorbed cinnamyl alcohol and Pd single atoms on Pd₁/Al₂O₃ [34]. This indicates that cinnamyl alcohol molecules adsorb over the single-atom Pd sites and form partially dehydrogenated intermediates. To detect the activated state of O₂, 2, 2, 6, 6-tetramethyl-4-piperidone (4-oxo-TMP) was used as a probe molecule for electron spin resonance (ESR) examination in view of its preferable ability to trap singlet oxygen species (¹O₂) [35]. As shown in Fig. 6(b), no ESR signals were perceived for the Al₂O₃ support, whereas triplet ESR signals were observed for Pd₁/Al₂O₃ and Pd/Al₂O₃, suggesting that an active oxygen species behaving chemically like singlet-O₂ was formed during the O₂ activation process. Compared to Pd/Al₂O₃, Pd₁/Al₂O₃ showed an enhanced signal of the singlet O₂ species, presenting a stronger ability for O₂ activation. In addition, 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) was employed as a probing molecule to further identify other oxygen species. No DMPO/^·OH (1:2:2:1 quartet signal) or DMPO/^·O₂^- (1:1:1:1:1:1 6-line signal) signal was detected [4, 36], suggesting that no superoxide (O₂^-) and hydroxyl radicals (^·OH) species are produced in these systems (Fig. S6).

Based on the above results, a stepwise reaction process was proposed for aerobic SELOX of allylic alcohols over the Pd₁/Al₂O₃ catalyst. First, partial dehydrogenation occurs through a charge transfer between the adsorbed allylic alcohols and the Pd atoms. Concurrently, active oxygen species behaving chemically like singlet-O₂ are generated on the interaction of O₂ with Pd₁/Al₂O₃. Then, the active oxygen species oxidize the partially dehydrogenated intermediates to the desired alkenyl aldehyde.

In summary, atomic and nano-dispersed catalysts are prepared with the same Pd loading on an Al₂O₃ support. Because of the different strengths of the MSI, the single-atom Pd₁/Al₂O₃ catalyst exhibits much higher catalytic activity than the nano Pd/Al₂O₃ catalyst for the aerobic SELOX of allylic alcohols under mild conditions. The partially positive electronic properties of the single-atom Pd sites are expected to lead to the more efficient activation of allylic alcohols and O₂. This work offers a promising path for the design and development of highly active catalysts for aerobic oxidation reactions.

We are grateful to the BL 14W beamline at the Shanghai Synchrotron Radiation Facility (SSRF).