The role of negatively charged Au states in aerobic oxidation of alcohols over hydrotalcite supported AuPd nanoclusters

Abstract

The PVP-protected bimetallic gold–palladium nanoclusters (Au_xPd_y-PVP NCs) were prepared on the solid base hydrotalcite (HT) with various Au : Pd (x : y) molar ratios. Transmission electron microscopy showed narrow particle size distributions of Au_xPd_y-PVP NCs with a mean diameter in the range of 2.6–3.0 nm regardless of Pd content. Aerobic oxidations of 1-phenylethanol over the Au_xPd_y-PVP/HT catalysts showed that their catalytic activities were significantly affected by the Pd content. Correlations between charge transfer between Au and Pd and catalytic activity of the Au_xPd_y-PVP/HT catalysts were investigated with X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), Michaelis–Menten kinetic studies for alcohol oxidation, and other analytical techniques. The peaks of Au 4f in the XPS spectra were shifted to the lower energy side with increase of Pd content, indicating the electron transfer from Pd to Au atoms according to Pauling's electronegativity protocol. The electron densities in the Au 5d orbital in the Au_xPd_y-PVP/HT catalysts estimated by the Au L₃-XANES spectra correlated well with their catalytic activities. Moreover, the kinetic studies also proposed that the electron rich Au 5d states, resulting from the intermetallic electron transfer from Pd atoms, strongly contributed to the rate-determining step in the alcohol oxidation. It was concluded that the electronic negativity of the Au 5d states controlled by the Pd content accelerated the rate-determining step in alcohol oxidation through highly active radical-like intermediates.

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Introduction

Bimetallic nanoparticles (NPs) have attracted great interest and potential in advanced materials science because they achieved unique performances different from those of monometallic NPs.¹ Since the catalytic activities of nano-sized Au particles were reported by Haruta et al. in 1980s,^2,3 breakthroughs have been made in the development of a series of Au and incorporated second element NPs as catalysts such as Au clusters, AuPd, AuPt, AuAg and so on.^4–8 However, the contribution of these advanced catalysts to the reaction kinetics over the bimetallic catalyst is still questionable. In general, (i) each element promotes different elementary reaction steps (bifunction effect), (ii) the electron transfer among two elements improves reactivity (ligand effect), and (iii) the specific group of surface atoms developed by geometric growth (ensemble effect), were considered as driving forces for significant performances of heterometallic assemblies.^9–11 The difficulties in the synthesis of uniform bimetallic NPs with various mixing ratios and/or morphologies become a barrier to discuss and compare their performances in the reaction mechanism.

From the viewpoint of energy and environmental issues, aerobic oxidation of alcohols for the synthesis of fine chemicals over a highly active heterogeneous catalyst has been investigated.^12–18 It leads to environmentally-friendly synthesis routes compared to the stoichiometric oxidations over transition metal complexes. Additionally, the traditional process produces a large amount of undesirable salts and needs energy for separation of the product from the reaction mixture. Therefore, the notable protocols for the active sites synthesis on a nanoscale have a great impact on development of highly functionalized heterogeneous catalysts. In this regard, supported polymer capped highly dispersed bimetallic NP catalysts, e.g. Au@Pd-PVA/TiO₂,¹⁹ AuPt-PVA/MgO,²⁰ and AuPd-PVA/C,^21,22 have been extensively studied for alcohols oxidation into carbonyl compounds.

Herein, we examined the aerobic oxidation over the PVP-protected AuPd bimetallic (Au_xPd_y-PVP) NCs deposited onto the hydrotalcite (HT) catalysts with similar size distributions of Au_xPd_y-PVP NCs. The AuPd NPs are well-known as some of the most attractive active sites for the catalyst for various reactions such as not only alcohol oxidation,^19–24 but also hydrogenation of 1,3-cyclooctadiene,^25,26 the direct synthesis of H₂O₂,^27–29 acetoxylation of ethylene,³⁰ and oxygen reduction in electrodes.^31,32 The HT has been known as an effective support for various reactions such as deoxygenation, chemoselective reduction, and oxidation reactions because it exhibits tunable surface basic sites, achieving high catalytic performance through proton abstraction and uniform deposition of active metal on the surface.^33–39 It is supposed that the combination of AuPd NCs and HT becomes a significant heterogeneous catalyst for the alcohol oxidation. In addition, both the careful synthesis of bimetallic NCs and study of their reaction mechanism may give the clarification of their novelty in the reaction.

The effect of doping heteroatoms to Au nanoclusters (NCs) for catalytic activities has been widely focused by several researchers.^40–42 Notably, Toshima et al. reported that the degree of electron transfer in the bimetallic core–shell NCs is proportional to the visible-light-induced hydrogen generation from water in the EDTA/[Ru(bpy)₃]²⁺/MV²⁺/metal NC system. They clarified that the bimetallic NC can accept electrons more easily than the monometallic one, which accelerated the rate-determining step and production of methyl viologen cation radicals.⁴⁰ They also suggested that differences in the ionic potential between Pd and Pt could provide an uneven distribution of electrons, and the formed positive Pd shell in the Pt@Pd-PVP NCs favored the C=C double bond of the diene substrate and provided good catalytic activity.⁴¹ It has been said that the intermetallic electron transfer in the bimetallic NCs seems to play an important role in the acceleration of the rate-determining step.^9,43–45

In this study, we succeeded in the synthesis of extremely active Au₆₀Pd₄₀-PVP/HT possessing the highest turnover number (=395 700) for aerobic oxidation of 1-phenylethanol (250 mmol). Thereafter, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Michaelis–Menten kinetic studies for alcohol oxidation and other analytical techniques were employed in order to realize the novelty of the synthesized Au_xPd_y-PVP/HTs. By taking advantage of morphological and electronic analysis of AuPd-PVP NPs with kinetic studies in aerobic oxidation of alcohols with and without radical scavengers, it was also revealed that the remarkable activity of the Au_xPd_y-PVP/HT catalysts was strongly contributed by the electron rich Au 5d states, since the formed negative Au species induces the active radical-like peroxo-species formation which accelerated the rate-determining step of the alcohol oxidation.

Experimental

Chemicals and materials

Hydrogen tetrachloroaurate tetrahydrate (HAuCl₄·4H₂O), palladium chloride (PdCl₂), potassium chloride (KCl), ethylene glycol (EG), sodium carbonate decahydrate (Na₂CO₃·10H₂O) and benzyl alcohol were supplied by Wako Pure Chemical Ind., Ltd. Co. PVP (M.W. = 58 000) and MgAl HT (Mg/Al = 5) were purchased from Acros Organics Co. and Tomita Pharmaceutical Co., Ltd., respectively. 2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO) and 1-phenylethanol were provided by Tokyo Chemical Ind. Co. Ltd. 2,6-Di-tert-butyl-p-cresol, naphthalene and toluene were purchased from Kanto Chem. Co. Ltd.

Synthesis of the AuPd-PVP/HT catalyst

The Au_xPd_y-PVP NCs with various Pd content were prepared by the polyol reduction method according to the previous report⁴⁶ with some modifications. Briefly, an aqueous solution (50 ml) of PdCl₂ (y mmol) including KCl (0.1 g) and HAuCl₄·4H₂O (x mmol) were mixed with PVP (0.58 g) and EG (50 ml), then the obtained mixture was refluxed for 2 h. Thereafter, HT (1.0 g) was added to the formed colloidal dispersion to stabilize the formed Au_xPd_y-PVP NCs onto the surface of HT with stirring. The obtained precipitates were filtered, washed and dried in a vacuum overnight. The Pd content was varied in the range of 0 to 100, and the total amount of both metals in the mixed solution (x + y) was kept as 0.1 mmol; i.e. the prepared Au_xPd_y-PVP/HT catalysts contain 0.1 mmol metal per gram in stoichiometry.

Aerobic oxidation of alcohols

All reactants and solvents were purified before use. Oxidation reactions were carried out in the glass tube attached to a reflux condenser. In a general procedure, 2 mmol of alcohol in 5 ml of toluene and the Au_xPd_y-PVP/HT catalyst were added to the glass tube, and purged with O₂ flow before reaction under stirring (500 rpm). Subsequently, the mixture was stirred at desired temperature for a given time under O₂ flow (20 ml min⁻¹) at atmospheric pressure. After the reaction, resultant solution was filtered off with a Millex-LG 0.20 μm syringe. The products were analyzed by GC equipped with a DB-FFAP (30 m length, 0.25 mm id) or a DB-1 (30 m length, 0.25 mm i.d.) column with an FID detector using the internal standard curve method. The naphthalene was used as an internal standard to determine the conversion and yield.

Characterizations

Transmission electron microscopy (TEM) data were acquired using a Hitachi H-7650 operating at 100 kV. Energy-dispersive X-ray (EDS) and the scanning TEM-high angle annular dark field (STEM-HAADF) analyses were performed with a JEOL JEM-ARM200F operating at 200 kV. Ultraviolet and visible (UV-vis) spectra were measured by a Perkin-Elmer Lambda35 spectrometer at room temperature with a light path length of 1 cm. X-ray photoelectron spectroscopy (XPS) was measured on a Shimadzu Kratos AXIS-ULTRA DLD spectrometer using an Al target at 15 kV and 10 mA. The binding energies were calibrated with the C 1s level (284.8 eV) as the internal standard reference. Induced couple plasma spectroscopy (ICP) was recorded with a Shimadzu ICPS-7000 Ver.2. The contents of Pd and Au on the catalyst were estimated by the standard curve method. X-ray absorption near edge structure (XANES) measurement was performed at the BL01B1 in SPring-8 of Japan Synchrotron Radiation Research Institute (JASRI). The Au L₃-edge XANES spectra were recorded at room temperature using a Si(111) monochromator.

Results and discussion

Aerobic oxidations of 1-phenylethanol to acetophenone were carried out with Au_xPd_y-PVP/HTs with various Pd content. The results are summarized in Table 1. Size distributions of the Au_xPd_y-PVP/HTs were also listed in Table 1, and these values were similar among Au_xPd_y-PVP/HTs except for Au₁₀₀-PVP/HT (aggregation) (see Fig. S1, ESI†). Au₁₀₀-PVP/HT showed no activity (entry 1). The bimetallic Au_xPd_y-PVP/HTs exhibited different activities, and Au₆₀Pd₄₀-PVP/HT achieved the most significant activity among Au_xPd_y-PVP/HTs (entries 2–5). The excellent yields with Au₆₀Pd₄₀-PVP/HT were also obtained even at 300 K (>99% yield, entry 3^h) and under air conditions (>99 yield, entry 3ⁱ). It is likely that the Pd content in the Au_xPd_y-PVP/HTs has a strong influence on catalytic activity for aerobic oxidation of 1-phenylethanol. We also tested their reusability in aerobic oxidation of 1-phenylethanol, and found that they could be reused without significant loss of activity and selectivity after washing with acetone followed by a 10 wt% Na₂CO₃ aqueous solution and drying in a vacuum (Fig. S2, ESI†). The reaction over the Pd₁₀₀-PVP/HT catalyst was rarely proceeded regardless of the small particle size (2.6 nm) whereas the bare Au NPs stabilized on the HT (Au₁₀₀/HT) catalyst with a similar size distribution (2.6 nm) and metal amount (0.075 Au mmol g⁻¹) showed little activity (entry 7). It is supposed that the Au atoms in Au_xPd_y-PVP NPs mainly act as an active site for the alcohol oxidation. Furthermore, scope for Au₆₀Pd₄₀-PVP/HT in aerobic oxidation of various alcohols to the corresponding aldehydes or ketones under mild reaction conditions was also examined. Though the presence of substituents such as p-H₃CO– and p-Cl– on the aromatic ring affects the yield of the product (vide infra), the Au₆₀Pd₄₀-PVP/HT catalyst allows adaptive oxidation for various alcohol substrates (Table S1, ESI†).

Table 1 Aerobic oxidation of 1-phenylethanol under base free conditions using Au_xPd_y-PVP/HT catalysts with various Pd content^a

Entry	Catalyst	Conv.^b (%)	Yield^b (%)	Particle size (nm)^c	Metal amount (mmol g^−1)^d
					Au	Pd
a Reaction conditions: 1-phenylethanol (2 mmol), catalyst (0.2 g; Au + Pd = 0.02 mmol), mole ratio of alcohol/(Au + Pd) = 100 , toluene (5 ml), 313 K, 1 h, O2 flow (20 ml min^−1). b Analyzed by GC using an internal standard technique. c Determined by TEM measurement about 500 NPs for each sample. d Estimated with ICP analysis. e Reduced by KBH4. f 1-phenylethanol (4 mmol). g 300 K, 3 h. h Air purge, 12 h.
1	Au100-PVP/HT	2	0	Agglomerate	0.075	0
2	Au80Pd20-PVP/HT	100, 92^f	99, 92^f	3.1	0.115	0.034
3	Au60Pd40-PVP/HT	100, 100^f, 100^g, 100^h	>99, >99^f, >99^g, >99^h	2.6	0.054	0.042
4	Au40Pd60-PVP/HT	58	57	2.6	0.052	0.098
5	Au20Pd80-PVP/HT	19	19	2.6	0.023	0.135
6	Pd100-PVP/HT	2	0	2.6	0	0.154
7	Au100/HT^e	18	18	2.6	0.075	0

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The turnover number (TON) and turnover frequency (TOF) of the oxidation of 1-phenylethanol (250 mmol) into acetophenone were up to 395 700 and 69 100 h⁻¹, respectively, at 423 K for 24 h in the absence of solvents with 35% yield and 95% selectivity.^47,48 These values are comparable to the previous reports (detailed information is listed in Table S2, ESI†); i.e. Au/HT (93% yield, TON = 200 000, TOF = 8300 h⁻¹),³⁶ Au/CeO₂ (TON = 250 000 (after three recycles), TOF = 12 500 h⁻¹),^12,13 PdHAP (37.8% yield, TON = 236 000, TOF = 9800 h⁻¹),⁴⁹ and Au@Pd/TiO₂ (TOF = 269 000 h⁻¹).¹⁹ These results clearly show that the present Au₆₀Pd₄₀-PVP/HT catalyst is an excellent catalyst for oxidation of alcohols. Investigation of different supports with Au₆₀Pd₄₀-PVP NCs also provided the novelty of the combination of AuPd bimetallic NCs and HT as a catalyst for alcohol oxidation reaction (Table S3, ESI†).

To clarify the reaction pathway over the Au₆₀Pd₄₀-PVP/HT, investigations using radical scavengers were performed. From the results shown in Fig. 1, it was found that TEMPO moderately influenced the oxidation rate of 1-phenylethanol. It has been reported that the alcohol oxidation over the monometallic metal supported catalysts proceeds by the metal-alcohoxide formation mechanism, which is not affected by the radical scavengers.^{16–18,37,49} Interestingly, our finding is different from such previous studies over monometallic catalysts (vide infra). Additionally, if the dominant reaction mechanism involves the formation of carbon-centered free radical intermediates over the Au_xPd_y-PVP/HT catalyst, the yield of acetophenone should be unchanged by such a small amount of the TEMPO radical scavenger (0.0032 mmol). The 2,6-di-tert-butyl-p-cresol (0.0454 mmol) also influenced the oxidation rate of 1-phenylethanol (67% yield after 1 h reaction). These results implied that another radical intermediate was seemed to be formed during the reaction (vide infra).

Fig. 1 Time-course of aerobic oxidation of 1-phenyl ethanol in the absence (closed circle) or presence (open circle) of TEMPO. Reaction conditions: 1-phenylethanol (2 mmol), Au₆₀Pd₄₀-PVP/HT catalyst (0.2 g), TEMPO (0 or 0.5 mg), toluene 5 ml, 313 K, O₂ flow (20 ml min⁻¹).

From comparison of the reaction rate between primary and secondary alcohols, the oxidation of 1-phenylethanol (TOF = 755 h⁻¹) was faster than that of benzyl alcohol (TOF = 552 h⁻¹), individually (Fig. S3, ESI†). In contrast, benzyl alcohol (TOF = 315 h⁻¹) was oxidized faster than 1-phenylethanol (TOF = 69 h⁻¹) using an equimolar mixture of alcohols (intermolecular competitive oxidations) (Fig. S4, ESI†). These observations were also reported previously.^14,17 It is said that the faster oxidation of primary alcohol than secondary alcohol in competitive oxidation presumably supported the formation of metal-alcoholate intermediate species through the ligand exchange.^50–52 To determine the presence of a carbocationic character in the transition state of the reaction, the logarithm of the rate constants, log(k_X/k_H), against substituent constant (σ) reported by Hammet⁵³ was plotted for the aerobic oxidation of p-substituted benzyl alcohols. As shown in Fig. 2, there is a reasonable linearity between log(k_X/k_H) and the σ parameter in the following order of the reactivity; p-CH₃O > p-CH₃ > p-H > p-Cl > p-NO₂. The Hammet ρ value was −0.240 (R² = 0.96). The negative value indicates that a carbocationic character on the benzylic carbon is an intermediate species in the transition state of the alcohol oxidation over the Au_xPd_y-PVP/HT catalysts. Consequently, it is likely that the oxidation of alcohols over the Au_xPd_y-PVP/HT catalysts progressed via the alkoxide intermediate.

Fig. 2 Hammet plots for oxidation of benzyl alcohol and p-substituted benzyl alcohols. Reaction conditions: alcohol (0.5 mmol), toluene (5 ml), Au₆₀Pd₄₀-PVP/HT catalyst (10 mg), 313 K, 5 min, O₂ flow (20 ml min⁻¹).

In order to understand the superiority of the Au₆₀Pd₄₀-PVP/HTs for alcohols oxidation, their morphologies and electronic states were also investigated. Fig. 3 shows the STEM-HAADF image and STEM-EDS line analysis of the Au₆₀Pd₄₀-PVP NCs. Since the STEM-HAADF analysis technique offers enhanced contrast proportional to Z², the heavier Au atoms (atomic number; Z = 79) give rise to a brighter image than the lighter Pd atoms (Z = 46). However, the STEM-HAADF image of Au₆₀Pd₄₀-PVP NCs looks uniform contrast (Fig. 3(A)). As the result of quantitative analysis of the EDS profile, the Au₆₀Pd₄₀-PVP NCs were likely composed of homogeneously mixed 60.2% Au and 39.8% Pd atoms (Fig. 3(B)). These values agreed well with the stoichiometric ratio in the Au₆₀Pd₄₀ NC. Moreover, other components of Au_xPd_y-PVP NCs show similar images in the STEM-HAADF and the STEM-EDS line analyses (Fig. S5, ESI†), and the latter proposed that their Au/Pd ratios also suit the stoichiometric ratio of Au/Pd for each. The detailed morphologies of Au_xPd_y-PVP NCs could not be resolved due to the extremely small size of the NCs and the difficulty in obtaining electron beam diffraction. The UV-vis spectra of the Au_xPd_y-PVP NCs dispersed solutions show no specific surface plasmon (SPR) absorption of the Au NPs around 520–580 nm⁵⁴ whereas the Au₁₀₀-PVP NPs show the SPR feature at 527 nm (Fig. S6, ESI†). As mentioned above, the unimodels and narrow size distributions among Au_xPd_y-PVP NCs with various Pd content were observed (Fig. S1, ESI†). If the alloy is not formed, the size related to the isolated Au NPs will also be obtained. Therefore, distribution would lead to a broad or bimodal one due to different growth rates for the two metal NPs and the SPR phenomena. According to these results, it was suggested that the compositions of the Au_xPd_y-PVP NCs were different from that of Au or Pd mother clusters and the mixtures of isolated Au and Pd NCs, which indicates that each particle contains both Au and Pd elements in the form of alloy phase. It seems to be possible to form the alloy via the spontaneous alloying mechanism even at the low temperature.^55–57

Fig. 3 (A) STEM-HAADF image and (B) STEM-EDS line analysis of the Au₆₀Pd₄₀-PVP NCs. The blue and green lines are corresponding to the presence of Au and Pd elements, respectively.

The XPS spectra of Au_xPd_y-PVP NCs were measured in order to discuss the charge transfer between two metals. The peaks around Au 4f components are shown in Fig. 4(A). The humped peaks were attributed to Au 4f_7/2 (at around 83 eV) and 4f_5/2 (at around 87 eV). All Au_xPd_y-PVP NCs exhibited negative shifts in the both Au 4f binding energies compared to that of pure Au foil (84.0 and 88.0 eV).⁵⁸ Importantly, increasing the Pd content induced more shifts in binding energy to the lower energy side (Fig. 4(B)). Thus, it is supposed that the charge transfer from Pd to Au atoms was facilitated by increase in Pd content of Au_xPd_y-PVP NCs. The largest negative shift in Au 4f_7/2 was −1.96 eV in the case of Au₂₀Pd₈₀-PVP NCs. The negative shifts of binding energy in the Au 4f_7/2 peak were also reported in the crown-jewel-structured Au₁₂/Pd₁₄₇-PVP NCs (−1.5 eV),⁹ Au₉₀-Ag₁₀-PVP alloy NCs (−1.4 eV),⁴³ Au₁@Pt₄-Rh₂₀-PVP NCs (−0.15 eV),⁴⁴ and Au₉₀Pt₅Ag₅-PVP NCs (−1.4 eV).⁴⁵ The large number of negative shift in the Au 4f peak supposed that the Au_xPd_y-PVP NCs have a lot of agglutinations between Au and Pd atoms, which increased the opportunity for electron transfer from Pd to Au atoms. While, the XPS peaks in Pd 3d were hard to analyze because of weak intensities and energy overlapping at 335.0 eV corresponding to Pd 3d_5/2 and Au 4d_5/2 (Fig. S7, ESI†).⁵⁸ It was reported that the Pd NCs with a diameter less than 2.0 nm have an amorphous structure whereas those with a diameter more than 2.5 nm have crystalline structures.⁵⁹ It is indicated that the phase transfer of Pd NCs was found between 2.5 and 4.0 nm. In our case, the average size of Au_xPd_y-PVP NCs was below 2.7 nm. Therefore, the Pd atoms in the Au_xPd_y cluster supposedly have the amorphous structure of Pd aggregates and/or the less ordered AuPd structure, which makes the Pd related peaks broaden. From XPS analysis, the electron transfer from Pd to Au was elucidated, however, the correlation between the electron transfer and their catalysis still remains unclear.

Fig. 4 (A) XPS spectra of (a) Au₂₀Pd₈₀-PVP, (b) Au₄₀Pd₆₀-PVP, (c) Au₆₀Pd₄₀-PVP and (d) Au₈₀Pd₂₀-PVP NCs around Au 4f components, and (B) plots of the peak positions of Au 4f_7/2 as a function of Pd content.

To confirm the presence of the electronegative Au atoms in Au_xPd_y-PVP NCs, Au L₃-edge XANES spectra were also measured for AuPd-PVP/HTs, as shown in Fig. 5(A). The white-line (WL) feature in the L₃-edge spectrum is related to the transition of 2p electron to unoccupied 5d electron states. Though the unperturbed Au atom possesses no holes in the 5d orbital (electron configuration [Xe]6s¹⁴f¹⁴5d¹⁰), the Au bulk L₃-edge XANES spectra exhibit the WL feature due to the s–p–d hybridization which leads to the small amount of electron transformation from 5d to s–p states.^60–62 The WL features around 11.925 eV in the Au_xPd_y-PVP/HTs indicated lower intensities than Au foil (Fig. 5(A)). Therefore, it was supposed that the Au_xPd_y-PVP/HTs contained more 5d electrons than Au foil. To further elucidate the differences in the 5d electron densities among Au_xPd_y-PVP/HTs, the areas of WL features in Au_xPd_y-PVP/HTs were calculated. Because the width in the WL feature depends on the lifetime of core holes, the dipole-transition matrix element, and the distribution of the density of states of the unoccupied d band at the Fermi level of the element,⁶⁰ the range from 10 eV below the X-ray absorption edge (E_o) to 13 eV above E_o was applied for calculation.^63,64 Here, μ₃ = 107.7 cm² g⁻¹ and ρ = 19.32 g cm⁻³ are the X-ray absorption cross section at L₃-edge jump and the density of Au, respectively. The areas as a function of Pd content were plotted with the yield of acetophenone in Fig. 5(B). The area of Au L₃-edge XANES spectrum was attributed to the amount of 5d holes in Au atoms. In other words, the area decreased with increasing the 5d electrons. Therefore, it was clarified that Au₆₀Pd₄₀-PVP/HT possessed the largest number of the 5d electron-rich Au atoms among Au_xPd_y-PVP/HTs. Relationships between the areas and yield of acetophenone as a function of Pd content implied that the electron density of 5d states in Au atoms strongly contributed to the catalytic activity for aerobic oxidation (Fig. 5(B)). One other important feature is that the increase of intensities with increasing Pd content at around 11.935 keV was also observed in the Au L₃-edge XANES spectra (Fig. 5(A)). It seemed to be evidence of the homogeneously-mixed AuPd alloy formation because these features were relatively sensitive to the interatomic redistribution of charge when Au and Pd atoms come closer together or further apart in AuPd NPs.^65–68

Fig. 5 (A) The changes in Au L₃-edge XANES spectra and (B) correlations between the area in the range of −10 eV < E_o < 13 eV among Au_xPd_y-PVP/HT catalysts with various Pd content.

We proposed the catalytic cycle for alcohol oxidation over the Au_xPd_y-PVP/HT catalysts in Fig. 6, which proceeds via alkoxide intermediates and radical-like peroxo-species. Firstly, an O₂ molecule adsorbed onto the negatively charged Au site on Au_xPd_y NCs, then the adsorbed molecular O₂ is dissociated through the electron donation from Au 5d to the antibonding 2π* orbital of O₂ (AuO²⁻ peroxo- and/or AuO₂²⁻ superoxo-species formation). Activating the adsorbed molecules (e.g. CO and O₂) on anionic and/or neutral Au clusters was also suggested in the literature based on the experiment and theoretical studies.^69–73 This proposed step agrees well with Au-PVP NC catalysts for aerobic oxidation of alcohol.⁷⁴ Secondary, an alcohol molecule is dissociatively adsorbed on Au and/or Pd affording [metal-alkoxide]⁻ (oxidative addition). The HT basicity promotes this process owing to abstraction of proton from alcohol involving [H–HT]⁺ formation. Third, the H atom on the α-carbon of the adsorbed alkoxide is transformed into the Au-peroxo- (and/or superoxo-) species (β-hydrogen elimination), then the corresponding carbonyl compound and Au-hydroperoxide species were formed.⁷⁵ Finally, another O₂ attacked and removed the hydroperoxide species from the Au surface with the formation of H₂O, thereby, the catalytic cycle was completed.⁷⁶ The Au_xPd_y NCs seem to have key factors in this step since the β-hydrogen elimination is considered as the rate-determining step in the alcohol oxidation via alkoxide formation in the previous reports.^14,18

Fig. 6 Proposed reaction mechanism of alcohol oxidation over Au_xPd_y-PVP/HT catalysts.

According to the these reaction mechanisms, both the [RCH(O)CH₃–(AuPd)–OO^δ−] type of Michaelis complex and Michaels–Menten-type kinetics [eqn (1)] were assumed in the 1-phenylethanol oxidation over Au_xPd_y-PVP/HT catalysts. Because the Lineweaver–Burk plots in the oxidation of 1-phenyl ethanol over the Au₆₀Pd₄₀-PVP/HT catalyst showed good linear correlation between the inverse of initial rate (1/R_o) and the inverse of substrate concentration (1/substrate) (Fig. 7), the oxidation of 1-phenylethanol follows the Michaels–Menten-type kinetics. This is because the primary alcohol oxidation over the bare Au supported catalyst also follows Michaels–Menten-type kinetics [eqn (1)]. The alcohol oxidation over the bare Au₁₀₀/HT catalyst also agrees well with the mechanism via the similar [RCH₂O–Au] intermediate.^14,37

Fig. 7 Lineweaver–Burk plot and Michaelis–Menten kinetics in the aerobic oxidation of 1-phenyl ethanol over the Au₆₀Pd₄₀-PVP/HT (solid line) and bare Au₁₀₀/HT (dashed line) catalysts. Reaction conditions: 1-phenylethanol (0.25–3 mmol), catalyst (10 mg), toluene 5 mL, 313 K, 5 min, O₂ flow (20 ml min⁻¹).

In the case of the Au₆₀Pd₄₀-PVP/HT catalyst, K_M and k₂ were calculated to be 3228 mM and 0.152 mM s⁻¹, respectively, whereas over the bare Au₁₀₀/HT catalyst, they were 1112 mM and 0.033 mM s⁻¹. It was indicated that the rate of β-hydrogen elimination was drastically facilitated in the bimetallic AuPd-PVP/HT catalyst than that in the monometallic Au₁₀₀/HT catalyst. To further investigate the reaction mechanism, the results of Lineweaver–Burk plot and Michaelis–Menten kinetics in the aerobic oxidation of 1-phenylethanol over the Au₆₀Pd₄₀-PVP/HT catalyst were compared in the presence and absence of TEMPO as a radical scavenger (Fig. 8). In the presence of TEMPO, both K_M and k₂ were decreased to 475 mM and 0.023 mM s⁻¹, respectively, whereas the slopes in the Lineweaver–Burk plot (K_m/V_max) were similar irrespective of TEMPO. Therefore, TEMPO affected the reaction pathway by the uncompetitive inhibition mechanism; i.e. TEMPO is not combined with the active site but intermediate species. Since TEMPO cannot combine with the alkoxide intermediate,^17,49 the prospective radical-like peroxo- species were protected by TEMPO, which makes the reaction slower. These results suggested that the remarkable activity of Au_xPd_y-PVP/HT catalysts for aerobic oxidation of alcohols might be originated from the negatively charged Au (5d states) generating the peroxo- and/or superoxo- species which strongly enhances the reaction rate in the β-hydrogen elimination of the alkoxide intermediates.

Fig. 8 Lineweaver–Burk plot and Michaelis–Menten kinetics in the aerobic oxidation of 1-phenyl ethanol over the Au₆₀Pd₄₀-PVP/HT catalyst in the presence (solid line) and absence (dashed line) of the TEMPO radical scavenger. Reaction conditions: 1-phenylethanol (0.25–3 mmol), catalyst (10 mg), TEMPO (0.0032 mmol), toluene 5 ml, 313 K, 5 min, O₂ flow (20 ml min⁻¹).

Conclusions

The effect of intermetallic charge transfer in bimetallic nanoparticles was examined by using Au_xPd_y-PVP NCs supported onto HT catalysts (Au_xPd_y-PVP/HT) with narrow particle size distributions. The Pd content in Au_xPd_y-PVP/HTs strongly influenced the activities for the aerobic oxidation of alcohols. The XPS analysis suggested that the electron transfer from Pd to Au atoms was facilitated by increase in Pd content of Au_xPd_y-PVP NCs. The close correlations between the electron density in the Au 5d state and reaction activity was obtained by the Au L₃-edge XANES analysis of the Au_xPd_y-PVP/HTs. Moreover, the Michaels–Menten-type kinetic studies clearly indicated that the electron rich Au 5d states strongly accelerated the rate-determining step in the alcohol oxidation. These findings pointed out that design of electron transfer using heterometallic catalysts may form a guiding principle for further endeavors in the field of bimetallic nanocatalysis, and atomic-scale design of nanoparticles with remarkable catalytic activities.

Abbreviations

PVP	poly(N-vinyl-2-pyrrolidone)
PVA	poly(vinyl alcohol)
X@Y	X-core Y-shell (X, Y; element)

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Footnote

† Electronic supplementary information (ESI) available: UV-vis, XPS, STEM-EDS, and results of alcohol oxidations. See DOI: 10.1039/c2cy20244a

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