Au supported on γ-AlOOH and γ-Al₂O₃ for low temperature oxidation of CO and aromatic alcohols

Abstract

0.5%Au supported on γ-AlOOH and γ-Al₂O₃ was used for low temperature oxidation of CO and aromatic alcohols. Various characterization techniques, including X-ray diffraction, N₂ adsorption, FT-IR spectroscopy, XPS, TEM, CO₂-TPD and solid-state MAS NMR, were employed to characterize these catalysts. The Au/γ-AlOOH sample has abundant hydroxyl groups and basic sites on its surface, exhibiting strong adsorption for aromatic alcohols, and outstanding activity for low-temperature oxidation of aromatic alcohols. The Au/γ-Al₂O₃ sample shows sufficient Au⁺ sites leading to high performance for low-temperature CO oxidation.

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1 Introduction

Small Au nanoparticles have attracted considerable attention in the last few decades due to their high activity for catalytic oxidation of various reactions.^1,2 Low temperature oxidation of carbon monoxide is the classical example of gold catalysis, as first reported by Haruta and co-workers in 1987.^3,4 Recently, low temperature oxidation of alcohol on Au nanoparticles using molecular oxygen as an oxidant to the corresponding aldehydes, ketones and acids has met the demand of green synthesis of organic products.^5–9

The support can influence catalyst activity by participating in the reaction. For example, the synergetic catalysts of Au nanoparticles and alkaline metal oxide support promote the oxidation activity of both alcohol and CO. Costa et al. found that using MgO as a support for preparing gold catalysts enables the effective oxidation of a wide range of alcohols with molecular oxygen as the sole oxidant.¹⁰ Schüth and his coworkers synthesized gold supported on MgO, which exhibited unprecedented oxidation activity for CO even at −89 °C.¹¹ The support can also influence the catalytic activity as it affects parameters, such as the Au particle size, Au oxidation state, and metal–support interaction.¹² Reducible oxides such as TiO₂, ZrO₂, Fe₂O₃, CeO₂ and Co₃O₄ are considered most active because of their excellent ability to provide reactive oxygen to the active gold sites.^1,13 TiO₂ has been reported to reduce the size of Au nanoparticles, which results in high oxidation activity of CO.^14–16 The metal–support interaction between Au and the CeO₂ support has also been reported, which indicated high oxidation activity of alcohol and CO.^17–21 The ionic states of supported gold were stabilized by the cerium oxide and Au⁺ cations were claimed as the active sites in the partial oxidation of alcohols.^17,18 Corma and coworkers prepared a nanocrystalline CeO₂ support, which enhances the activity of Au for CO oxidation by two orders of magnitude compared to the catalysts prepared using conventional CeO₂ support, by influencing the surface electronic properties and, consequently, the gold–support interaction.²¹

Hydroxy groups and alkalinity of support are also known to influence the stability and activity of catalytic Au nanoparticles.^22–24 γ-Al₂O₃ is the most commonly used noble metal catalyst carrier due to its large specific surface area, which is usually obtained by interlayer dehydroxylation of γ-AlOOH at temperatures of 450–750 °C.²⁵ γ-Al₂O₃ and γ-AlOOH carriers have significant differences in hydroxyl groups, surface Al³⁺, acidity as well as alkalinity. Huang et al. claimed that the strong alkaline sites on the surface of γ-AlOOH enable Au/γ-AlOOH to exhibit high activity in the oxidation of α,ω-diols, despite its gold particle size being much larger than that of Au/γ-Al₂O₃.²⁴ The morphology of Al₂O₃ support also affects the activity of catalytic Au nanoparticles. Recently, the thin porous Al₂O₃ sheets were reported as exceptional catalyst supports for Au nanoparticles, exhibiting high activity for low-temperature CO oxidation and stabilizing Au nanoparticles at annealing temperatures up to 900 °C.²⁶ The Au particle size and water vapor in the atmosphere is crucial for the low temperature CO oxidation performance of Au/Al₂O₃ catalysts,^27–29 Moroz et al. discovered that the Au/Al₂O₃ catalysts containing Au particles with a diameter of ≤5 nm demonstrate remarkable catalytic activity for the oxidation of CO when water vapor is present under near-ambient conditions.²⁹

Here we present an unexpected result for CO and aromatic alcohols oxidation with Au/γ-AlOOH and Au/γ-Al₂O₃ as catalysts. The Au/γ-AlOOH exhibits higher aromatic alcohols oxidation activity than that of Au/γ-Al₂O₃ catalyst, while Au/γ-Al₂O₃ exhibits higher CO oxidation activity. We also investigated the effects of support and Au oxidation state on the oxidation of CO and aromatic alcohols.

2 Experimental

2.1 Chemicals

NaAlO₂ (Al₂O₃, 54.09 wt%; Na₂O, 40.44 wt%) were purchased from J&K Chemical Ltd. Sodium bicarbonate, ammonium sulphate, ethanol (EtOH), HAuCl₄ and KBH₄ all in AR grade, were supplied by Sinopharm Chemical Reagent Co. Ltd. The chemicals were used as received, without further purification.

2.2 Preparation of boehmite (γ-AlOOH) and γ-alumina (γ-Al₂O₃)

The hierarchically structured boehmite and γ-alumina supports with a flower-like morphology were synthesized taking bayerite as the starting raw materials following the reference.³⁰

Typical synthesis of bayerite was as follows: 20.1 g NaAlO₂ was dissolved in 134.0 g of hot water, stirred and then cooled to room temperature. After that, 190 mL of 1 mol L⁻¹ NaHCO₃ solution was added drop-wise to the above solution. A white suspension was formed. The suspension was agitated for about 6 h, then filtered and washed thoroughly with hot water. The resulting filter cake was then dried at 80 °C overnight.

Typical syntheses of hierarchically structured boehmite and alumina were as follows: A mixture of 0.8 g bayerite, 0.6 g (NH₄)₂SO₄, 2.7 g H₂O and 2.3 g EtOH was placed in a 25 mL Teflon-lined stainless-steel autoclave, heated to 175 °C, maintained at the temperature for 12 h, and then cooled down naturally. The obtained white solid was recovered by filtration, washed with hot water, and dried at 80 °C overnight. The product was denoted as γ-AlOOH. The γ-Al₂O₃ sample was obtained by calcination of the γ-AlOOH at 550 °C for 6 h with a heating rate of 2 °C min⁻¹.

2.3 Supporting 0.5%Au on the γ-AlOOH and γ-Al₂O₃

The Au/γ-AlOOH or Au/γ-Al₂O₃ was prepared as follows:

10 g γ-AlOOH or γ-Al₂O₃ was added into a vessel (50 mL) containing 25 mL of 0.01 mol L⁻¹ HAuCl₄ and stirred for 2 min. Subsequently, a 5% NH₃·H₂O solution was added dropwise to adjust the pH of the suspension to 8–9. The suspension was further stirred for 2 h. Then, 0.27 g of KBH₄ was added to reduce the Au³⁺ species. The obtained solid was filtered, washed with deionized water, and dried under vacuum overnight.

2.4 CO oxidation testing

The CO oxidation reaction was performed in a continuous flow fixed-bed quartz reactor under atmospheric pressure. A 0.2 g (40–60 mesh) sample was loaded and pre-treated with a 20 vol% O₂/He mixture (50 mL min⁻¹) at 100 °C for 30 min. After cooling down to 20 °C, a gas mixture with 1.0 vol% CO/2.5 vol% O₂/He (50 mL min⁻¹) was introduced. Moisture was controlled by passing the feed gas through a soda lime tube. The concentrations of CO, CO₂ and O₂ in the outlet streams were measured using an on-line gas chromatograph.

2.5 1-Phenylethanol, benzyl alcohol and 4-methoxybenzyl alcohol oxidation testing

The oxidations of aromatic alcohols (1-phenylethanol, benzyl alcohol, or 4-methoxybenzyl alcohol) were carried out in a 25 mL glass reactor equipped with a reflux condenser and a magnetic stirrer. In a typical reaction, 100 mg of catalyst, 1 mmol of aromatic alcohols and 5 mL of tetradecane were introduced into the reactor. The air flow rate was 60 mL min⁻¹, the string rate was 350 rpm, and the mixture was heated to 40∼70 °C and maintained at this temperature for 6∼8 h. 0.5 mmol 1,3,5-triisopropylbenzene was used as an internal standard. The liquid samples were analyzed using a Shimadzu GC-2014 gas chromatograph equipped with a 30 m DB-Wax capillary column and an FID detector.

2.6 Characterization methods

Powder X-ray diffraction (XRD) patterns were collected on a Rigaku-Ultima diffractometer using a Cu Kα radiation source (λ = 0.15432 nm) in the 2θ range from 5° to 80°. Transmission electron microscopic images were conducted on TECNAI G2 F30 operating at 300 kV after the specimens were dispersed in ethanol and deposited on holey copper grids. N₂ adsorption–desorption isotherms were measured at −196 °C on a Quanta chrome Autosorb-3B volumetric adsorption analyzer. Before the measurements, the samples were outgassed in the degas port of the adsorption apparatus at 150 °C for 6 h. BET specific surface area was calculated using adsorption data acquired at a relative pressure (P/P₀) range of 0.05–0.30 and the total pore volume determined from the amount adsorbed at P/P₀ of about 0.99. Pore size distribution (PSD) curves were calculated from the adsorption isotherm branches using the Barrett–Joyner–Halenda (BJH) algorithm. Au contents were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. Surface electronic state of Au was also evaluated using X-ray photoelectron spectroscopy (XPS) measurements with a Thermo Fisher Scientific ESCALAB 250Xi spectrometer with Al Kα radiation (1486.6 eV) as incident beam with a monochromator. All the spectra were obtained at room temperature, and the binding energies of elements were referenced to the adventitious C1s peak at 284.8 eV. ²⁷Al MAS NMR spectra were measured on a VARIAN VNMRS 400WB NMR spectrometer.

Temperature-programmed desorption of CO₂ (CO₂-TPD) testing was performed using a TP-5080 chemisorption instrument (Xianquan Co., Ltd, Tianjin, China) with a thermal conductivity detector (TCD). After pretreatment of each sample (100 mg) at 200 °C under flowing helium (25 mL min⁻¹) for 2 h, the sample was cooled to 50 °C, and then adsorbed to saturation by introducing 10 vol% CO₂/He (25 mL min⁻¹) mixture for 10 min. CO₂ physically adsorbed on the catalyst was removed by flushing the sample with helium (25 mL min⁻¹) for 20 min. Thermal desorption of CO₂ was carried out in the temperature range of 50∼200 °C increasing at a rate of 10 °C min⁻¹.

Infrared (IR) spectra were recorded on a Nicolet Fourier transform infrared spectrometer (NEXUS 670). The self-supported wafers (about 10 mg, Φ 1 cm) were pretreated in the IR cell under vacuum at the temperature from 20 °C to 550 °C for 10 min and then the IR spectra of OH group were recorded. For IR spectra of absorbed 1-phenylethanol, the self-supported wafers were pretreated in the IR cell under vacuum at 200 °C for 10 min, after the samples were cooled down to room temperature, 1-phenylethanol vapor dozed into the IR cell. IR spectra of absorbed 1-phenylethanol were recorded under vacuum at the temperature from 20 °C to 200 °C for 10 min.

3 Results and discussions

3.1 Characterization of Au/γ-AlOOH and Au/γ-Al₂O₃ samples

The wide-angle XRD patterns of the Au/γ-AlOOH and Au/γ-Al₂O₃ are shown in Fig. 1. The diffraction peaks at 14.48°, 28.18°, 38.34° and 48.93° are related to boehmite phase (JCPDS no. 21-1307).³¹ The γ-Al₂O₃ structure (JCPDS no. 10-0425) is formed by calcining γ-AlOOH at 550 °C.³² No peaks related to the Au phase were observed, suggesting low Au concentration and small particle size, with the Au diffraction peaks overlapping with the strong peaks of γ-AlOOH and γ-Al₂O₃.

Fig. 1 Wide-angle XRD patterns of the as-synthesized γ-AlOOH (A, dotted line), Au/γ-AlOOH (a, solid line), γ-Al₂O₃ (B, dotted line) and Au/γ-Al₂O₃ (b, solid line). The arrows mark reflexes from Au particles.

Fig. 2 shows the TEM images of Au/γ-AlOOH (left) and Au/γ-Al₂O₃ (right) samples. The shape of primary nanosheet (1–5 nm thick) particles of γ-AlOOH is preserved after conversion to γ-Al₂O₃ by calcination. Au nanoparticle can be observed in both samples, the average particle sizes of Au/γ-AlOOH sample and Au/γ-Al₂O₃ sample are 4.6 nm and 4.4 nm, respectively.

Fig. 2 TEM images (left) and Au particle size distribution (right) of the as-synthesized Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b).

Fig. 3 presents the N₂ adsorption/desorption isotherms and pore size distribution curves of the Au/γ-AlOOH and Au/γ-Al₂O₃ samples. Both Au/γ-AlOOH and Au/γ-Al₂O₃ samples show typical characteristics of type IV isotherms with the hysteresis loop, where the adsorbed amount increases continuously with rising P/P₀.³³

Fig. 3 N₂ adsorption/desorption isotherms and pore size distribution curves (insert) of the as-synthesized Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b) samples. The curves (b) are offset a little for clarity.

Table 1 summarizes the surface areas, total pore volumes and the pore diameters of the Au/γ-AlOOH and Au/γ-Al₂O₃ samples. The surface area of the Au/γ-AlOOH sample is 217 m² g⁻¹ and the total pore volume is 0.48 cm³ g⁻¹. The surface area of the Au/γ-Al₂O₃ is 279 m² g⁻¹, and the total pore volume is 0.58 cm³ g⁻¹. Both the surface area and pore volume of Au/γ-Al₂O₃ are higher than those of the Au/γ-AlOOH. Both the samples exhibit bimodal pore size distribution.³⁴ The Au/γ-AlOOH sample has the peaks centered at 3.4 nm and 15.9 nm, respectively and the Au/γ-Al₂O₃ sample has those at 6.3 nm and 18.9 nm, respectively.

Table 1 Textual properties of the Au/γ-AlOOH and Au/γ-Al₂O₃ samples

No.	Sample	SBET (m^2 g^−1)	Vtotal (cm^3 g^−1)	dBJH^a (nm)
a The pore size is derived from the adsorption isotherm by BJH method.
1	Au/γ-AlOOH	217	0.48	3.4/15.9
2	Au/γ-Al2O3	279	0.58	6.3/18.9

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Fig. 4 gives ²⁷Al NMR spectra of as-synthesized γ-AlOOH, Au/γ-AlOOH, γ-Al₂O₃ and Au/γ-Al₂O₃ samples. The resonance at ∼10 ppm is associated with octahedrally coordinated aluminum. Additional peaks at ∼68 ppm and ∼38 ppm are observed in the spectra of γ-Al₂O₃ and Au/γ-Al₂O₃ samples, indicating generation of tetrahedrally and pentahedrally coordinated aluminum after calcination of boehmite.³⁵ Similar spectra were observed between the support and Au-loaded samples, suggesting that the addition of Au does not alter the support's structure.

Fig. 4 ²⁷Al NMR spectra of as-synthesized γ-AlOOH (A, dotted line), Au/γ-AlOOH (a, solid line), γ-Al₂O₃ (B, dotted line) and Au/γ-Al₂O₃ (b, solid line).

The valences of Au species in Au/γ-AlOOH and Au/γ-Al₂O₃ samples are investigated by using XPS. As shown in Fig. 5, Au 4f peaks are chosen to compare the valence of Au in Au/γ-AlOOH and Al–Al₂O₃ samples. Both the Au/γ-AlOOH and Au/γ-Al₂O₃ samples exhibit the presence of Au⁰ species on the catalyst surface.³⁶ The Au/γ-Al₂O₃ sample shows particular Au⁺ species, compared with the Au/γ-AlOOH sample. Casaletto et al. claimed that presence of Au⁺ species is the main requisite for achievement of the highest CO conversion at the low temperatures.¹⁷

Fig. 5 Au 4f XPS spectra of Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b).

The alkalinity properties and total amounts of basic sites are investigated by using CO₂ temperature-programmed desorption (CO₂-TPD) technique. As shown in Fig. 6, Au/γ-AlOOH and Au/γ-Al₂O₃ samples display similar CO₂ desorption at 110 °C, which is due to the desorption of CO₂ on the weak basic sites.²⁴ According to the peak areas of CO₂ desorption in the two samples, the total basic sites of Au/γ-AlOOH are 1.6 times higher than that of Au/γ-Al₂O₃. Ide et al. proposed that adsorbed hydroxyl groups can activate O–H and C–H bonds, thereby increasing the oxidation rate of glycerol, even bulk gold powder becomes an active oxidation catalyst in alkaline water.³⁷ Tang et al. found that alkali metal dopants were found to significantly delay total oxidation of CO over Co₃O₄ nano-catalyst,³⁸ Gluhoi et al. found that alkali (earth) metal oxide additives act as structural promoters, which may contribute to the activation of O₂ during the low-temperature oxidation of CO.³⁹

Fig. 6 CO₂-TPD profiles of Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b).

Fig. 7 displays the OH stretching vibrations between 2800 cm⁻¹ and 4000 cm⁻¹ for the Au/γ-AlOOH and Au/γ-Al₂O₃ samples. Two characteristic absorption bands of OH groups of Au/γ-AlOOH were observed at 3673 cm⁻¹ and 3668 cm⁻¹. A broad flat peak attributed to the skeletal Al–OH of γ-AlOOH were observed at 2960∼3480 cm⁻¹. The Au/γ-Al₂O₃ showed three bands at 3721 cm⁻¹, 3671 cm⁻¹, 3578 cm⁻¹, which attributes to the absorption bands of OH groups of γ-Al₂O₃. Under vacuum condition of 20 °C and 50 °C, the absorption bands of OH groups in Au/γ-AlOOH are much higher than that in Au/γ-Al₂O₃. As the temperature increases, γ-AlOOH gradually transforms into γ-Al₂O₃, and the surface Al–OH in γ-AlOOH gradually disappears. After vacuum treatment of 550 °C, the IR spectrum (OH region) of the Au/γ-AlOOH and Au/γ-Al₂O₃ is basically the same, which is due to the conversion of γ-AlOOH to γ-Al₂O₃.⁴⁰

Fig. 7 Change of IR spectra (OH region) of the Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b) as a function of temperature.

The surface properties of the Au/γ-AlOOH and Au/γ-Al₂O₃ samples are further examined by the 1-phenylethanol adsorption. Fig. 8 displays IR spectra of 1-phenylethanol adsorbed on the Au/γ-AlOOH and Au/γ-Al₂O₃ samples. Both Au/γ-Al₂O₃ and Au/γ-AlOOH samples exhibit the peaks at 1400–1650 cm⁻¹, which is attributed to the aromatic C=C stretching vibrations from the benzene ring and α-deformation (bending vibrations) of the CH₃ group.^41–43 In addition, the Au/γ-AlOOH sample shows more 1-phenylethanol absorption peak than the Au/γ-Al₂O₃ sample, as shown in Table 2, which is consistent with the IR spectra (OH region) results. At 50 °C, the area of 1-phenylethanol absorption peak at 1443 cm⁻¹ of Au/γ-AlOOH is 3.32 times that of Au/γ-Al₂O₃. Higher alcohol adsorption capacity helps to enhance alcohol oxidation activity.

Fig. 8 IR spectra of 1-phenylethanol absorbed on Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b), all samples were normalized by subtracting the peak of the vacuum adsorption sample at 200 °C.

Table 2 The area of 1-phenylethanol absorption peak at 1443 cm⁻¹ for the samples Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b)

Desorption temp. (°C)	Peak area (1443 cm^−1)
	Au/γ-AlOOH	Au/γ-Al2O3
20	8.7	5.0
50	6.3	1.9
100	5.2	1.7
150	4.3	1.4
200	2.8	1.0

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3.2 Catalytic properties of Au/γ-AlOOH and Au/γ-Al₂O₃ samples in CO oxidation

The Au/γ-AlOOH and Au/γ-Al₂O₃ samples are tested for CO oxidation at 20 °C under a reaction stream with a gas composition of 1.0 vol% CO/2.5 vol% O₂/He mixture. The conversion vs. reaction time is shown in Fig. 9. The initial CO conversions of the Au/γ-Al₂O₃ and Au/γ-AlOOH samples are 82% and 26%, respectively. The CO oxidation performance of the Au/γ-Al₂O₃ sample is much superior than that of the Au/γ-AlOOH sample. XPS spectra suggest that the Au/γ-Al₂O₃ sample contains more Au⁺ species than the Au/γ-AlOOH sample, which is favorable for CO₂ forming. XPS spectra suggest that the Au/γ-Al₂O₃ sample has more Au⁺ species than those the Au/γ-AlOOH sample has, which is favorable for CO₂ forming. These results lead to the outstanding CO oxidation performance of Au/γ-Al₂O₃, as shown in Scheme 1. Then, the catalytic performance of these samples tends to reduce, CO conversions of the Au/γ-Al₂O₃ and Au/γ-AlOOH samples decreased to about 50% and 8% after 450 hours, respectively. This result is in good agreement with the literature data,²⁹ they hypothesized that the CO + O₂ reaction occurs by inserting adsorbed CO molecules into the Au^x+–OH complex to form carboxylate species, and the presence of water vapor is crucial for maintaining sufficient concentrations of Au^x+–OH species.^29,44 Therefore, the absence of water vapor will lead to a decrease in CO activity over time.

Fig. 9 The CO conversion of Au/γ-AlOOH (a) and Au/γ-Al₂O₃ (b) samples.

Scheme 1 Proposed reaction mechanism for the oxidation of CO and alcohol over Au/γ-AlOOH and Au/γ-Al₂O₃ samples.

3.3 Catalytic properties of Au/γ-AlOOH and Au/γ-Al₂O₃ samples in oxidation of alcohols

The selective oxidation of alcohols is one of the most important and fundamental transformations in organic synthesis, acted as the versatile intermediates of valuable compounds such as pharmaceuticals, agricultural chemicals, and fine chemicals. We use 1-phenylethanol, benzyl alcohol and 4-methoxybenzyl alcohol oxidation as the probe reaction of aromatic alcohols oxidation. Table 3 gives the conversion of 1-phenylethanol, benzyl alcohol and 4-methoxybenzyl alcohol oxidation on the Au/γ-AlOOH and Au/γ-Al₂O₃ samples, all reactions display about 100% selectivity, no other by-products can be detected. Au/γ-AlOOH sample display a conversion of 73.0% for 1-phenylethanol oxidation at 40 °C for 6 h. Chen et al. claimed a conversion of 82% and selectivity of 90% for 1-phenylethanol oxidation over Pd/SiO₂–Al₂O₃–H₂ catalyst at 150 °C for 24 h,⁴⁵ and Yamaguchi et al. reported a conversion of >99% and selectivity of >90% over Ru/Al₂O₃ at 83 °C for 1 h.⁴⁶ Herein, 1-phenylethanol can be almost completely oxidized over Au/γ-AlOOH at 40 °C for 6 h, which must be energy saving and eco-friendly. This result shows some advantages of flower-like AlOOH as support catalysis in the alcohols oxidation. Au/γ-AlOOH sample is also suitable for the oxidation of benzyl alcohol and 4-methoxybenzyl alcohol.

Table 3 The conversion of 1-phenylethanol, benzyl alcohol and 4-methoxybenzyl alcohol oxidation of Au/γ-AlOOH and Au/γ-Al₂O₃ samples

Entry	Substrate	Product	Temp. (°C)	Time (h)	Conv. (%)
					Au/γ-AlOOH	Au/γ-Al2O3
1			40	6	73.0	32.1
2			70	8	32.6	16.9
3			70	6	25.4	14.8

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The carrier morphology of Au/γ-AlOOH and Au/γ-Al₂O₃ samples is similar, and both samples exhibit a low Au concentration and small Au particle size. The Au/γ-AlOOH sample demonstrates much higher 1-phenylethanol, benzyl alcohol and 4-methoxybenzyl alcohol oxidation conversion than the Au/γ-Al₂O₃ sample does. This result shows some advantages of the Au/γ-AlOOH sample in the alcohols oxidation. IR spectra of OH regions and 1-phenylethanol adsorption indicate the abundant hydroxyl groups and strong adsorption capacity for alcohols of the Au/γ-AlOOH sample. Jiang et al. speculated through theoretical calculations that the –OH groups promote chemisorption of alcohol molecules on a gold cluster surface as the initial step of their oxidation by a gold cluster,⁴⁷ which is confirmed by our experimental results.

3.4 Influence of support and Au oxidation state on catalytic properties

Scheme 1 exhibits the proposed reaction mechanism for the oxidation of CO and alcohol over Au/γ-AlOOH and Au/γ-Al₂O₃ samples. Au/γ-AlOOH and Au/γ-Al₂O₃ samples exhibit a low Au concentration and small Au particle size, and both samples can be used for low-temperature oxidation of CO and aromatic alcohols. During alcohol oxidation, both the carrier and gold significantly influence the reaction. The hydroxyl groups and basic sites in the carrier sometimes have a greater impact on alcohol oxidation than Au. Ide et al. proposed that bulk gold powder acts as an active catalyst for alcohol oxidation in alkaline medium.³⁷ The Au/γ-AlOOH sample contains a large amount of OH and alkalinity, and has strong adsorption capacity for aromatic alcohols, thus exhibiting excellent low temperature oxidation activity for aromatic alcohols. Au/γ-Al₂O₃ possesses a large amount of Au⁺ which is effective in promoting the low temperature CO oxidation.¹⁷

4 Conclusions

Au/γ-AlOOH and Au/γ-Al₂O₃ samples with low Au concentrations and small Au particle sizes were synthesized for low-temperature oxidation of CO and aromatic alcohols. Under low temperature conditions, the CO oxidation conversion by the Au/γ-Al₂O₃ sample is about three times that of the Au/γ-AlOOH sample, while the oxidation conversions of 1-phenylethanol, benzyl alcohol, and 4-methoxybenzyl alcohol by the Au/γ-Al₂O₃ sample are only about half of those by the Au/γ-AlOOH sample.

The high oxidation activity of Au/γ-AlOOH sample towards 1-phenylethanol, benzyl alcohol, and 4-methoxybenzyl alcohol is mainly due to its rich content of hydroxyl groups and basic sites, which facilitate the adsorption of aromatic alcohols. The high CO oxidation activity of the Au/γ-Al₂O₃ sample is primarily attributed to the efficient Au⁺ sites.

Data availability

All relevant data are within the main article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Panxi Experimental Zone Key Scientific and Technological Project, and National Key Research and Development Program Nanotechnology Specific Project (No. 2020YFA0210900). We are very grateful to Dr Xinsheng Liu and Dr Yimeng Wang for their professional knowledge and insightful comments, which have made significant contributions to the success of this study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra00527b

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