Noble metal alloy catalysts have been used for a number of applications [1-10]. In recent years, Pt-based alloys have been widely used in various fields to reduce the amount of Pt used. Among these alloys, the PtCu alloy is one of the most common alloy catalysts used in heterogeneous catalysis and electrocatalysis [2-10]. Adding Cu can improve the catalytic efficiency of Pt, leading to a reduction in the cost and enhancement in the catalytic performance. Most studies of PtCu alloy nanoparticles focus on tuning the size and morphology for improving their catalytic activities [11-14]. Different types of PtCu alloy nanoparticles including cubes, octahedrons, nanowires, rhombic dodecahedra, nanodendrites, and cubic cages have been successfully synthesized [15-25].

The morphology and surface composition are the most important factors that influence the catalytic performance [26, 27]. However, most industrial catalysts are supported on oxides to improve their efficiency and catalytic performance. Upon supporting nanoparticles onto oxide supports, their morphology, composition, and catalytic performance may change owing to their interaction with the support [28-35]. The so-called support effect often occurs at the metal/oxide interface by encapsulation [31] or high dispersion [32-35]. Among which, strong metal-support interaction (SMSI) is the most famous one [28-30]. Therefore, it is crucial to determine the dispersion, the surface composition as a function of the bulk composition, of an alloy catalyst.

In this paper, homogeneous PtCu alloy nanoparticles were successfully synthesized by a solvothermal method [36]. The obtained nanoparticles were loaded on common oxide supports. In situ X-ray photoelectron spectroscopy (XPS) [37-41] and high-sensitivity low-energy ion scattering spectroscopy (HS-LEIS) [42-44] were used to characterize the chemical states and compositions after different treatments. Phase diagrams of the surface composition as a function of the bulk composition were obtained and compared with those of unsupported PtCu alloy nanoparticles.

PtCu alloy: 4.0 mg of Pt(acac)₂ and 8.0 mg of Cu(acac)₂ were dissolved in 3.0 mL of DMF under ultrasonication. Subsequently, 10.0 mL of CTAC (0.01 mol/L in DMF) solution was injected under vigorous stirring. The mixed solution was further stirred for 30 min and then transferred into a 20-mL Teflon-lined stainless-steel autoclave. The sealed vessel was heated from room temperature to 160 ℃, maintained at this temperature for 10 h, and then naturally cooled to room temperature. The products were collected by centrifugation (9000 r/min for 5 min), and then washed several times with ethanol to remove any impurities.

PtCu/TiO₂ alloy nanocatalysts: The PtCu alloy nanoparticles were uniformly dispersed in ethanol under ultrasonication and loaded on the TiO₂ support (P25) using an impregnation method and then dried at 80 ℃ under vacuum overnight.

Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were conducted on a transmission electron microscope (JEM-2100) operated at 200 kV. High ang le annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray analysis spectroscopy (EDS) were performed using a FEI TECNAI F30 high-resolution transmission electron microscope operating at 300 kV. All samples subjected to TEM measurements were prepared by impregnating the diluted suspension in ethanol under ultrasonication with molybdenum grids (Beijing Xinxing Braim Technology Co. Ltd.). The Pt and Cu contents were analyzed quantitatively by an inductively coupled plasma-optical emission spectrometer (ICP-OES, IRIS Intrepid Ⅱ XSP). X-ray powder diffraction (XRD) measurements were recorded on a Rigaku Ultima Ⅳ X-ray diffractometer employing Cu K_α radiation at 35 kV and 15 mA. The surface compositions of the alloy were determined by XPS (Qtac-100 LEIS-XPS) using monochromatic Al K_α (1486.6 eV) radiation. The binding energies were calibrated with respect to the signal for Ti of the titania support (P25, Ti 2p_3/2 binding energy of 458.8 eV). The outermost surface compositions were detected by LEIS with Ne⁺. The related peak area sensitivities were calibrated by pure metal platinum and copper, TiO₂(110).

The representative HAADF-STEM images (Fig. 1(a, b)) reveal that PtCu nanoparticles were successfully prepared with a uniform size of ca. 13 nm. Both Pt and Cu were homogeneously distributed in each alloy particle, as indicated by the EDS elemental mapping images in Fig. 1(c, d) and the cross-sectional compositional line profiles in Fig. 1(g). A lattice distance of ca. 0.22 nm was observed in the high-magnification TEM image (Fig. 1(e)), which corresponds to the crystal lattice of the (111) planes of the face-centered-cubic PtCu alloy [36]. XRD pattern of the as-prepared nanoparticles only shows a group of peaks corresponding to PtCu alloy (JCPDS NO. 48-1549). No additional diffraction peaks from either Pt or Cu were detected, as shown in Fig. 1(f). The sharp peaks and perfect match between the diffraction peaks of the PtCu nanoparticles and standard PtCu alloy indicated the high crystallinity of the alloy PtCu phase. The EDS (Fig. 1(h)) and ICP-OES data both showed that each PtCu nanoparticle was composed of Pt and Cu with a close atomic ratio of 1:1 (Table 1). All the above information confirmed the formation of a homogeneous single-phase PtCu alloy nanoparticle.

The PtCu/TiO₂ sample was calcined at 250 ℃ in air for 2 h to remove the CTAC surfactant. XRD results (Fig. 2) show that the PtCu alloy phase is stable and still exists after calcining at 300 ℃, but shifts to a lower angle after it was calcined at higher temperature and reduced under H₂. This suggested the formation of Pt-rich PtCu_x alloys.

XPS analysis was performed to investigate the chemical state and composition for the PtCu/TiO₂ samples. The atomic percentages of the elements were calculated from the normalized peak areas. The binding energy of Ti 2p_3/2 at 458.8 eV was used as a reference. The C 1s peak at 284.6 eV is commonly used as a reference. However, for the PtCu/TiO₂ with successive redox process, the C 1s peak intensity was very weak and broad. Because TiO₂ is the support and is considerably stable during the redox process, Ti 2p_3/2 at 458.8 eV for TiO₂ was a better reference value. The XPS spectra of Pt 4f for the Pt/TiO₂ and PtCu/TiO₂ samples are presented in Fig. 3(a). The Pt 4f_7/2 for the Pt/TiO₂ is ca. 71.0 eV, corresponding to metallic Pt. A negative shift of 0.6 eV was observed for the PtCu/TiO₂ [45], consistent with the XRD result for the formation of PtCu alloy. The XPS spectra of Cu 2p_3/2 for the Cu/TiO₂ are shown in Fig. 3(b). The calcined sample shows a broad peak at 934.4 eV with a shoulder at 932.5 eV, characteristic of Cu²⁺ and Cu⁰/Cu⁺, respectively. The reduced sample shows a peak at 931.8 eV corresponding to Cu⁰ [46]. For the PtCu/TiO₂ sample, the Cu 2p_3/2 features are obviously different from those for the Cu/TiO₂. The peak for the alloy PtCu at 931.8 eV disappears quickly and the Cu²⁺ peak gradually increases during the oxidation process in Fig. 3(c). After the sample is reduced again, the Cu 2p_3/2 peak moves to 932.1 eV, which is slightly higher than that of 931.8 eV for the as-prepared alloy clusters in Fig. 3(d). This reveals that after the oxidation-reduction cycle, the alloy clusters become Pt rich, which may result from the high dispersion of Cu on the TiO₂ surface. The XRD (Fig. 2(b)) and TEM (not shown) analyses also confirm that the PtCu alloy phase cannot be fully recovered, and a Pt-rich PtCu_x alloy phase forms after the oxidation-reduction cycle.

The Cu 3p_3/2 peak is at approximately 75 eV, which overlaps with the Pt 4f peak. It is important to deconvolute the Cu 3p_3/2 peak from the Pt 4f peak to distinguish the surface Pt species. The Pt 4f can be fitted into three peaks (Fig. 4), 71.0‒71.1 eV for the metallic Pt, 71.8‒71.9 eV for Pt^δ+ that bonds with oxygen [47], and 70.4‒70.5 eV for Pt alloyed with Cu. After oxidation at 450 ℃, the peak for the alloying Pt disappears, whereas the peaks for metallic Pt and Pt^δ+ gradually increase. Finally, the peak for the metallic Pt becomes dominant. Oxidation of the pre-reduced sample (Fig. 4(b)), metallic and Pt^δ+ always coexist. The difference in the Pt species obtained by direct oxidation and oxidation of pre-reduction could result from the different dispersion of Pt. After reduction, Pt species interacted with Cu⁰to form an alloy phase again, with a small amount of Pt^δ+, probably owing to the interaction between Pt atoms and TiO₂ or CuO_x. The relative intensities of the different Pt species obtained from the peak area in the curve fitting as a function of the treating temperatures are summarized in Fig. 5.

The relative area ratios from both XPS and LEIS for the PtCu/TiO₂ sample during oxidation are shown in Fig. 6. After depositing PtCu nanoparticles onto the TiO₂ support and oxidation, both the Cu/Ti and Cu/(Cu+Pt) ratios increase significantly, but there is no significant change in the Pt/Ti ratios. The decrease of the Pt/(Cu+Pt) ratios is clearly evident and is caused by the increase in Cu content. These results indicate that de-alloying occurs with Cu spreading out predominately on the surface, and Pt clustering during the oxidation process [48]. In contrast, upon reduction of the pre-oxidized sample, the XPS Cu/Ti ratio decreased (Fig. 7(a)), whereas the XPS Pt/Ti ratio remained almost constant (Fig. 7(c)). Such tendencies are more apparent in the LEIS (Fig. 7(b, d)). Combined with the core-level binding energies from XPS, it can be concluded that part of Cu re-alloys with Pt during the reduction process. The surface Cu amount is much higher than that of the alloy surface as well as the bulk composition.

The above results show that the support has a significant effect on the dispersion and morphology of the alloy nanoparticles. It is important to understand the surface composition of the supported alloy nanoparticles. A phase diagram of the surface composition versus their bulk composition is the most intuitive to graphically display such alloy effects (Fig. 8). The Pt/total metal ratios obtained by both XPS and LEIS are significantly lower than in the bulk, which indicates that Cu dominates and disperses on the surface. For the unsupported PtCu_x samples, the surface Pt is also much lower than the bulk Pt under both the oxidation and reduction processes. The Pt ratio of the supported sample is higher when reduced at 400 ℃ but lower when oxidized at 400 ℃ than those for the unsupported samples. This demonstrates that the support plays a key role in determining the surface composition of an alloy sample.

The changes of the chemical state, morphology and composition of the PtCu/TiO₂ alloy nanoparticles are illustrated in Scheme 1. According to the core-level binding energy shift of Cu 2p and Pt 4f, the change of Cu⁰ to Cu²⁺ and Cu⁺ to Cu⁰ and small amount of Cu⁺ during the oxidation and reduction processes, respectively, are confirmed. From the elemental ratios determined by XPS and LEIS, oxidation results in a significant increase of the surface Cu amount but not Pt, which indicates that de-alloying occurs with Cu forming CuO wetting on the support surface. In contrast, reduction leads to a decrease in the amount of surface Cu, which indicates that de-wetting and re-alloying occurs.

In summary, homogeneous face-centered cubic PtCu alloy nanoparticles were successfully synthesized by the solvothermal method and dispersed onto a TiO₂ support. The surface composition and chemical states were investigated by in situ XPS and LEIS. Upon oxidation, de-alloying takes place with Cu being oxidized and well dispersed on the oxide support, whereas Pt being metallic and clustering. Reduction of the pre-oxidized PtCu/TiO₂ results in partial re-alloying of the surface Cu with Pt. The surface composition is significantly different from that of the bulk one, with a high Cu content on the surface. The support plays an important role in the dispersion of Cu.