Effect of alkali and alkaline earth metal ions on benzyl alcohol oxidation activity of titanate nanotube-supported Au catalysts

Abstract

Sodium titanate nanotubes (NaTNTs) were prepared by alkali treatment of anatase titania. They were then ion-exchanged with alkali and alkaline earth metal ions to get ATNTs (A = Li⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺and Ba²⁺). Gold (1–5 wt%) was supported on these nanotubes by a deposition–precipitation method and investigated as a catalyst for the selective oxidation of benzyl alcohol with air/molecular oxygen (1 atm) under solvent- and alkali-free conditions. Detailed characterization by X-ray powder diffraction, high resolution transmission electron microscopy, N₂-physisorption, diffuse reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy and CO₂-temperature-programmed desorption techniques revealed that the basicity of the catalyst influences the uptake, mean particle size, electronic properties and oxidation activity of the supported gold. Benzaldehyde formed with a selectivity of about 99%. The catalytic activity (turnover frequency) was found to have a direct relationship with the basicity and an inverse relationship with the Au particle size. Among the catalysts investigated, Au/BaTNTs, having higher basicity, smaller Au particles and higher metal dispersion, showed enhanced catalytic activity than the other Au/ATNT catalysts. Pd addition to Au leading to Au–Pd/BaTNTs increased the activity (TOF) but lowered the selectivity for benzaldehyde (80 wt%). Titanate nanotubes donate electron density to Au particles, yielding electron rich Au ions, which are responsible for activating molecular oxygen and oxidizing benzyl alcohol. Au/BaTNTs, having higher basicity and lower size Au nanoparticles than the other Au/ATNT, activates molecular oxygen more easily and thereby enhances the catalytic activity.

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Introduction

Heterogeneous catalysis by gold is now a very fascinating research topic. Previously it was ignored for a long time as bulk gold is chemically inert. The field of catalysis by nano-gold was provoked by two contemporaneous discoveries made by Hutchings¹ and Haruta² for acetylene hydrochlorination and low-temperature CO oxidation, respectively. This led to several other explorations and exploitations of nano-gold and gold containing catalysts in oxidation, epoxidation, direct synthesis of hydrogen peroxide, hydrogenation, coupling and water–gas shift reactions through green chemistry practices.³ In particular, the selective liquid-phase oxidation of alcohols to the corresponding carbonyl compounds using gold catalysts under moderate conditions is highly desirable from economic and environmental perspectives.^3,4

The catalytic performance of supported gold nanoparticles (Au NPs) can be altered by controlling the particle size of Au, by choosing the right kind of support and by building strong contact of gold with the support (through the metal–support peripheral interface or via charge transfer from the support to gold).⁵ The effect of strong metal–support interactions (SMSI) was first observed in TiO₂-supported noble metal systems reported by Tauster et al.⁵ in 1978. One of the early hypotheses for this effect was that the alteration of the charge state of metal by electron transfer from or to a support leads to an improvement in the ability to activate reactants and thereby influencing the catalytic properties of the metal.⁶ This hypothesis was confirmed experimentally by several researchers. Bruix et al.⁷ successfully interpreted the enhanced activity of platinum particles supported on ceria in a water–gas shift reaction as the “electronic metal–support interaction” (EMSI), a term that was recently coined by Campbell.⁸ However, intrinsic metal effects, such as electronic quantum size effect⁹ and structure–sensitivity geometrical effect¹⁰ should be ruled out for a complete understanding of the EMSI effect. For supported metal particle/cluster catalysts, complete elimination of intrinsic metal effects is particularly difficult or even impossible. Milone et al.¹¹ reported that the catalytic properties of supported gold catalysts strongly depend on the type and structure of the support. From the comparative study of Fe₂O₃-, ZnO-, CaO-, and Al₂O₃-supported gold catalysts, they found that the basicity and lattice oxygen of supports have significant effects on the catalytic performance.

Recently, 1D nanoscale alkali titanates have attracted much interest in heterogeneous catalysis as catalyst supports. In particular, after the simple alkaline hydrothermal synthesis, sodium titanate nanotubes (NaTNTs) reported for the first time by Kasuga et al.¹² have emerged as a promising functional material. The NaTNTs have well-defined mesopores with a wall thickness of a few nanometers. They expose a large proportion of the Na⁺ counterions on the inner and outer surfaces of the nanotubes. These Na⁺ ions are ion-exchangeable.¹³ Ion-exchange has been established as an effective method for preparing highly dispersed supported metal catalysts with a narrow metal particle size distribution for use in heterogeneous catalysis.¹⁴ Moreover, the semiconducting properties of titanate nanotubes may lead to a strong electronic interaction between the support and metal, which could improve the catalytic performance in redox reactions. Specifically, titanate nanotubes have been used as a catalyst support for platinum catalyzed cyclohexene hydrogenation–dehydrogenation,¹⁵ iridium and cobalt catalyzed water splitting for hydrogen production,¹⁶ gold catalyzed water–gas shift reaction¹⁷ and CO oxidation^18,19 and bimetallic gold–palladium catalyzed direct synthesis of hydrogen peroxide.²⁰ Recently, we have reported the application of Au and Au–Pd supported on NaTNTs for the oxidation of primary and secondary alcohols.²¹ In continuation of this work, we report here a study of Au supported on various alkali and alkaline earth metal ion-exchanged titanate nanotubes for benzyl alcohol oxidation. The influence of the basicity of the support on the physicochemical characteristics and catalytic activity of gold is investigated.

Experimental

Catalyst preparation

Titanium dioxide (98% anatase TiO₂), sodium hydroxide (NaOH), and alkali and alkaline earth metal nitrates were obtained from Thomas Baker Chemicals Ltd. Chloroauric acid (HAuCl₄·3H₂O) was purchased from HiMedia Chemicals Ltd. and palladium acetate (Pd(OAc)₂) was procured from Aldrich Co. All reagents were used as received and without any further purification.

NaTNTs were prepared as reported by us earlier.²¹ To synthesize ion-exchanged titanate nanotubes, ATNTs (A = Li⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺), 2.5 g of dried NaTNTs was suspended in 120 mL of 1.0 M aqueous solution of alkali and alkaline earth metal nitrates. The suspension was stirred for 8 h while maintaining the temperature at 80 °C. The solid was separated and the ion-exchange procedure was repeated another two times. The solid isolated was washed with deionized water, filtered and dried at 110 °C overnight to obtain ATNTs. In the case of Cs⁺, Sr²⁺ and Ba²⁺ exchanged materials, the concentration of the nitrate solution used was 0.5 M instead of 1.0 M, as solubility is an issue with these salts.

In the preparation of 1, 3 and 5 wt% Au supported on ATNTs, an appropriate amount of 2 mM aqueous solution of HAuCl₄·3H₂O was added drop-wise to 1 g of ATNTs suspended in 100 mL of deionized water. The suspension was stirred vigorously for 12 h while maintaining the temperature at 80 °C. All of this operation was done in the dark or by covering the contents with aluminium foil. The solid obtained was filtered, washed with deionized water and dried at 110 °C for 20 h to obtain the gold loaded ATNTs. The materials were then reduced in flowing hydrogen (20 mL min⁻¹) at 250 °C for 2 h to yield the final catalysts.

Au–Pd (2 wt%, 1 : 1)/BaTNTs were prepared in the same manner as described above by simultaneously adding the required amounts of aqueous HAuCl₄·3H₂O (2 mM) and Pd(OAc)₂ (2 mM) solutions to a BaTNT suspension.

Catalyst characterization techniques

The metal loadings were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES; Spectro Arcos). Au/ATNTs were digested in aqua-regia. Standard solutions of known concentration were used for calibration. X-ray powder diffraction (XRD) patterns of the catalyst samples were recorded immediately after reduction in a H₂ flow (250 °C, 2 h) on a Philips X’Pert Pro diffractometer using Cu-Kα radiation (λ = 0.15406 nm) and a proportional counter detector with a scanning angle (2θ) of 5–80° at a scan speed of 4° min⁻¹. High resolution transmission electron microscopy (HRTEM) images were taken on a FEI Technai-F30 instrument with a 300 kV field emission gun. The specific surface area, pore volume and pore diameter of the catalysts were determined from N₂ adsorption–desorption isotherms measured at −196 °C using Quantachrome, USA (Autosorb-1C) equipment. Prior to N₂ adsorption, the samples were evacuated at 200 °C for 2 h. A reference alumina sample (supplied by Quantachrome, USA) was used to calibrate the instrument. Diffuse reflectance UV-visible (DRUV-vis) spectroscopic measurements of the powder samples were performed on a Shimadzu UV-2550 spectrophotometer equipped with an integrating sphere attachment (ISR 2200); BaSO₄ was used as the standard. Temperature-programmed desorption (TPD) studies were carried out on a Micromeritics Auto Chem 2910 instrument using CO₂ as the probe molecule to quantify the amount of basic sites. In a typical experiment, 0.1 g of the catalyst was taken in a U-shaped, flow-through, quartz sample tube. Prior to measurements, the catalyst was pre-treated in He (30 mL min⁻¹) at 250 °C for 1 h. It was then cooled to 25 °C and a mixture of CO₂ in He (10 vol%) was fed to the sample (30 mL min⁻¹) for 1 h. Then, the sample was flushed with He (30 mL min⁻¹) for 1 h at 100 °C. Before starting the desorption analysis, the baseline was checked for stability. The CO₂-TPD measurements were carried out in the temperature range of 100–500 °C. The area of the desorption peaks gave the amount of basic sites present in the catalyst. X-ray photoelectron spectra (XPS) of the samples were acquired on a VG Microtech Multilab ESCA 3000 with Al Kα radiation (hν = 1486.6 eV). The peak corresponding to carbon 1s (at 285 eV) was taken as the reference in estimating binding energy (BE) values of Au in the catalyst. The peak position was determined with a precision of ±0.2 eV.

Reaction procedure

Known quantities of catalyst (0.05 g) and benzyl alcohol (25 mmol) were charged into a triple necked, glass, round-bottom flask (25 mL) and placed in a temperature controlled oil bath. The reactor was connected with a water-cooled reflux condenser, a molecular oxygen/air-filled rubber balloon and a magnetic stirrer. The temperature of the reactor was raised to 80–120 °C and the reaction was conducted for a specific time. It was then cooled to 25 °C and the catalyst was separated by centrifugation. For identification and quantification of the products GC-MS (Varian CP-3800; CP-Sil8CB—30 m × 0.25 mm × 0.25 μm capillary column) and gas chromatography (Varian 3800; CP-8907—15 m × 0.25 mm × 0.25 μm column and a flame ionization detector) techniques were employed.

Results and discussion

Structural and textural characterization

NaTNTs showed XRD peaks at 2θ = 10.1, 24.3, 28.4 and 48.5° corresponding to reflections from the (200), (110), (211) and (020) planes, respectively (JCPDS files: 31-1329 and 41-0192). The ion-exchanged and Au loaded samples depicted similar XRD patterns indicating that they all have the same X-ray crystal structure as the NaTNTs. The diffraction peak for the (200) plane had, however, marginally shifted to a lower 2θ value (by 0.3°) when the cation in the ATNTs changed from Li⁺ to Ba²⁺. This shift in XRD peak position is due to an increase in the interplanar spacing with the increasing size of the counterion. But the positions of the other peaks remained nearly the same. Only representative XRD patterns of BaTNTs and Au (1–5 wt%)/BaTNTs are shown in Fig. 1, while the patterns for all the other catalysts are deposited in the ESI.† No additional peaks due to Au were detected in 1 wt% Au deposited samples (except for Au/CaTNTs) indicating that the size of the Au particles in all of those catalysts is below the detection limit of X-rays. However, at higher loadings (ca., 3 and 5 wt%), additional peaks at 38.2° and 44.6° corresponding to Au (111) and Au (200) were observed (Fig. 1) and decrease in the intensity of (200) [relative to (020)] was noted. This is due to the blockage of inter-planar spacing with Au nanoparticles or structural deformation as a consequence of strong interactions of Au with the support titanate nanotubes.

Fig. 1 XRD patterns of BaTNTs and Au/BaTNTs.

HRTEM images and Au particle size distribution curves of Au (1 wt%)/ATNTs are shown in Fig. 2. These images confirmed the presence of a hollow tubular multi-walled morphology of the support titanate nanotubes and the Au nanoparticles on the interior and outer surfaces of the nanotubes. Metal dispersions (D) were determined using eqn (1) and (2),

where V_m = volume occupied by a Au atom (1.69 × 10⁻²³ cc), a_m = area occupied by a Au atom (0.869 × 10⁻¹⁵ cm²), d_av = mean particle size of Au (HRTEM), V_i = volume of the i^th particle (HRTEM) and A_i = surface area of the i^th particle (HRTEM). The mean particle size and percentage dispersion of Au estimated from the HRTEM images are listed in Table 1. Wherever the characteristic peaks due to Au were observed in the XRD profiles, the average crystallite size of Au was determined using the Debye–Scherrer formula and reported in Table 1. The crystallite (XRD) and particle sizes (HRTEM) of Au (1 wt%)/CaTNTs are comparable. The value of d_av for different Au (1 wt%)/ATNTs is in the range 5.2–11.6 nm and the Au dispersion is between 10 and 20%.

Fig. 2 HRTEM images and particle size distribution histograms of Au (1 wt%)/ATNT catalysts.

Table 1 Chemical composition, textural properties and basicity of Au/ATNT catalysts

S. no.	Catalyst	Chemical composition^a (wt%)			Textural properties^b			Au particle/crystallite size^c (dav, nm)	Au dispersion^c (%)	Basicity^d (μmol m^−2)
		Au	Alkaline ion	Na^+	SBET (m^2 g^−1)	Pore volume (cc g^−1)	Pore diameter (nm)			Weak	Strong	Total
a ICP-OES. Alkaline ion means A^+ ion in ATNTs.b N2-physisorption.c HRTEM.d CO2-TPD; weak (100–300 °C) and strong (300–500 °C).e Crystallite size from XRD.f Value in parentheses is the Pd content.
1	Au (1 wt%)/LiTNTs	0.62	2.21	0.92	178	0.56	5.1	11.6	10	0.46	0.80	1.26
2	Au (1 wt%)/NaTNTs	0.65	—	7.2	182	0.50	5.6	7.2	16	0.43	0.87	1.29
3	Au (1 wt%)/KTNTs	0.55	9.90	0.61	153	0.45	5.5	6.3	18	0.43	1.05	1.70
4	Au (3 wt%)/KTNTs	1.25	9.32	0.36	135	0.41	5.3	7.3^e	—	0.40	1.13	1.53
5	Au (5 wt%)/KTNTs	3.33	8.80	0.22	122	0.38	5.1	15.4^e	—	0.37	0.98	1.35
6	Au (1 wt%)/CsTNTs	0.52	16.42	2.6	126	0.32	4.1	10.0	11	0.45	1.72	2.17
7	Au (1 wt%)/MgTNTs	0.90	2.50	0.62	165	0.41	5.6	9.9	12	0.21	2.12	2.19
8	Au (1 wt%)/CaTNTs	0.91	4.22	0.61	175	0.50	5.4	10.5 (10.0^e)	11	0.28	2.84	3.12
9	Au (1 wt%)/SrTNTs	0.72	9.21	0.30	173	0.44	5.5	5.4	19	0.25	2.82	2.75
10	Au (3 wt%)/SrTNTs	2.40	6.80	0.26	166	0.39	5.1	5.8^e	—	0.36	1.9	2.26
11	Au (5 wt%)/SrTNTs	3.30	5.35	0.27	152	0.36	5.0	8.7^e	—	0.41	1.78	2.19
12	Au (1 wt%)/BaTNTs	0.68	13.81	0.42	155	0.37	5.4	5.2	20	0.21	2.79	3.00
13	Au (3 wt%)/BaTNTs	2.80	12.00	0.36	146	0.36	5.3	5.5^e	—	0.32	2.07	2.39
14	Au (5 wt%)/BaTNTs	4.10	9.26	0.28	140	0.35	5.1	6.4^e	—	0.42	1.72	2.06
15	Au–Pd (1 : 1, 2 wt%)/BaTNTs	0.70 (0.82^f)	11.82	0.30	149	0.37	5.2	4.2	25	0.66	1.79	2.45

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The chemical composition (Au, alkali/alkaline earth metal ion and Na⁺ content) of the Au/ATNT catalysts was determined by ICP-OES (Table 1). The Na/Ti mole ratio of “bare” NaTNTs was 0.65, which is nearly equal to the theoretical value of 0.67 confirming the molecular formula of NaTNTs as Na₂Ti₃O₇. Ion-exchange with other alkali/alkaline earth metal ions occurs along the length of the nanotube. Na⁺ ions in the interlayer region were difficult to exchange and hence, remnants of non-exchangeable Na⁺ may be noted in the compositions of Au/ATNTs (Table 1).

Representative N₂ adsorption–desorption isotherms along with pore size distribution curves of BaTNTs and Au (1 wt%)/BaTNTs are shown in Fig. 3. These materials showed N₂ physisorption isotherms typical of type IV with a H₂-hysteresis loop. The isotherms represent mesoporosity with an open-ended pore structure. The samples showed broad pore size distribution curves. The size of the alkali/alkaline earth metal ion has an influence on the textural properties. The S_BET of Au/ATNTs was in the range 122–182 m² g⁻¹ and the average pore diameter was between 4.1 and 5.6 nm (Table 1). Bare supports have a S_BET between 132 and 196 m² g⁻¹ and an average pore diameter between 4.5 and 6.6 nm (ESI†). A marginal decrease in the specific surface area and average pore diameter is thus noted upon loading Au. A decrease in the total pore volume of Au/ATNTs compared to the supports was also observed.

Fig. 3 N₂-physisorption isotherms (top) and pore size distribution curves (bottom) of BaTNTs and Au (1 wt%)/BaTNTs.

Carbon dioxide was used as a probe molecule to determine the basicity of the Au/ATNT catalysts. The TPD profiles (ESI†) contained two desorption peaks, in the temperature regions 100–300 °C and 300–500 °C corresponding to weak and strong basic sites, respectively. The desorption temperature and amount of CO₂ desorbed refer to the strength and density of the basic sites. The basicity of the catalysts is reported in Table 1. In general, the alkaline earth metal ion exchanged titanates have higher basicity than the alkali metal ion exchanged catalysts. It appears that the amount of ions exchanged also determines the basicity value. The basicity of the catalysts influenced the uptake and particle size of Au (ESI†). Au (1 wt%)/NaTNTs showed an additional weak overlapping peak at 250–300 °C which is missing in the other ATNT samples.

Among alkali ion exchanged Au/ATNT catalysts the uptake of Au (Au content in the catalyst) decreased with increasing basicity of the catalyst. A similar observation was found also for alkaline earth metal ion exchanged Au/ATNT catalysts. In general, the percentage Au uptake (with respect to the input value) is higher for alkaline earth metal ion than alkali ion exchanged ATNTs. Also a decrease in the mean size (d_av) of Au with increasing basicity of the catalyst was observed (ESI†) with the values of Au supported on Cs⁺, Mg²⁺ and Ca²⁺-exchanged NaTNTs deviating from this relationship. The reason for this abnormal behaviour is not clear at this point of time. However, it appears that the strength of basicity and relative concentration of strong basic sites may influence the metal particle size.

Spectral characterization

DRUV-vis spectroscopy provided evidence for the presence of Au nanoparticles by showing a typical localized surface plasmon resonance (LSPR) band at 480–580 nm (ESI†). The peak position of the LSPR band (λ_max) is related to the Au mean particle size. It is related to the shape of the Au particle and dielectric function of the support material. As seen from Fig. 4, λ_max of the LSPR band increased with the increasing particle size of Au. We have already noted that the particle size of Au has an inverse relation with the basicity of the catalyst. In other words, the spectral/electronic properties of Au are greatly influenced by the nature of the support and Au particle size. Similar observations were made also by other researchers.²²

Fig. 4 Variation of LSPR band position (λ_max) with the mean particle size (d_av) of supported Au.

Au (1 wt%) supported on KTNTs, SrTNTs and BaTNTs showed a 4f_7/2 line in the XPS spectra at 83.2, 83.0 and 82.8 eV, respectively, metallic gold in gold foil shows this line at 84.0 eV.²³ This suggests that Au on ATNTs is rich in electron density. The formation of such electron rich Au species was attributed to occur through transfer of the electron density from the support.^23,24 Arrii et al.²⁵ have reported the dependence of Au 4f_7/2 binding energy on the nature of the support. On TiO₂ and Al₂O₃, the BE of Au 4f_7/2 showed a negative shift by −0.6 and −0.9 eV, respectively. A large shift up to −1.1 eV was observed for the spent Au/TiO₂ catalyst. When the cations were changed from K⁺ to Sr²⁺ and Ba²⁺ in the ATNTs, the BE of the Au signals shifted systematically downward (Table 2). This observation indicates that as the basicity of the support increased, the Fermi level of the metal particles shifted to a lower binding energy. Thus, the XPS spectra point out electron transfer from ATNTs to Au nanoparticles, the extent of which follows the order: BaTNTs > SrTNTs > KTNTs. Deconvolution of the XPS lines in the Au binding energy region (ESI†) revealed the presence of another set of signals at higher BE values but with lower intensity assignable to Au^δ+ ions. Pusztai et al.²³ reported that, on titanate nanowires, gold atoms can occupy ion exchange positions at a lower loading. They observed a higher portion of Au⁺/Au⁰ on the catalyst after reduction with H₂ compared to reduction with NaBH₄. This may be a reason for the presence of a small quantity of Au^δ+ species in ATNT samples. There are reports,^26,27 which argue that hydride transfer is involved in C–H bond activation, and the cationic gold is catalytically active. Abad et al.²⁷ have investigated the active sites on Au/CeO₂. Recently, Zhao et al.²⁸ have also reported the active sites on Au/Ni₂O₃ involved in the oxidation of alcohol. They showed a direct correlation between the concentration of Au⁺ or Au⁰ species and catalyst supports. The Au⁺/Au⁰ ratio in the present catalysts was determined (Table 2). The lower the BE value, the higher the catalytic activity was over the Au/ATNT catalysts (ESI†). For Au–Pd (2 wt%, 1 : 1)/BaTNTs, for the core level XPS spectrum the Au 4f_7/2 line appeared at 82.7 eV (ESI†). The weak intensity of these Au 4f lines is indicative of the location of Au inside the titanate nanotubes and masking of the Au surface by Pd particles. Smaller particle sizes can also lead to lowering in BE values. The spectral lines for Pd⁰ appeared at 334.8 eV (3d_5/2) and 340.0 eV (3d_3/2).

Table 2 XPS data (binding energy values in eV) of Au/ATNTs catalysts

Catalyst		Au^0		Au^δ+
		4f7/2	4f5/2	4f7/2	4f5/2
Au (1 wt%)/KTNTs	0.28	83.2	86.9	85.2	88.9
Au (1 wt%)/SrTNTs	0.27	83.0	86.7	85.2	88.9
Au (1 wt%)/BaTNTs	0.43	82.8	86.5	85.3	89.0
Au–Pd (1 : 1, 2 wt%)/BaTNTs	—	82.7	—	85.4	—

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Catalytic activity

The catalytic activity of Au supported on ATNTs for benzyl alcohol oxidation with molecular oxygen as the oxidant was studied at 120 °C and under solvent- and alkali-free conditions (Table 3). Benzaldehyde was the selective product (selectivity = 99 wt%). While the support had little effect on benzaldehyde selectivity, a remarkable influence of it on the benzyl alcohol conversion and turnover frequency (TOF) was observed (Table 3). Au (1 wt%) supported on Ba²⁺ ion-exchanged NaTNTs [Au (1 wt%)/BaTNTs] showed the highest catalytic activity [benzyl alcohol conversion = 43 wt% and TOF = 622 h⁻¹ (based on the total metal content) and 2237 h⁻¹ (based on the exposed surface metal atoms)]. This reaction occurred also with air instead of molecular oxygen over Au (1 wt%)/BaTNTs but then the conversion under the same reaction conditions was 24.6 wt% with a benzaldehyde selectivity of 96.5 wt%. The support (BaTNTs) alone could not catalyze this reaction to a notable extent (benzyl alcohol conversion = 2.8 wt%). The gold nanoparticles were smaller in size (d_av = 5.2 nm) and highly dispersed (metal dispersion = 20%) on BaTNTs than on other supports (Table 1). The smaller the particle size, the higher the number of exposed surface active sites and interaction with the support and the higher the catalytic activity. The catalytic activity of Au (1 wt%) supported on KTNTs, SrTNTs and BaTNTs with almost similar Au particle sizes followed the order: Au (1 wt%)/BaTNTs > Au (1 wt%)/SrTNTs > Au (1 wt%)/KTNTs (Table 3, run no. 3, 9 and 12a). This trend in catalytic activity parallels the variation in electron transfer from the support to the Au particles. With a view to see the influence of the addition of Pd on the catalytic activity of Au, we have prepared a Au–Pd (2 wt%, 1 : 1)/BaTNT composition. Addition of Pd decreased the mean particle size of Au from 5.2 to 4.2 nm and the metal dispersion increased from 20 to 25%. Au–Pd/BaTNTs showed a remarkably higher catalytic activity than the catalyst without Pd (benzyl alcohol conversion = 92.3 wt% as against 43 wt%). But the benzaldehyde selectivity dropped from 99 to 80.5 wt%. Note that a conversion of 92.3% was achieved on Au–Pd in just 5 h instead of 10 h over the Au catalysts. Hence, Au supported on NaTNTs exchanged with larger cations exhibited higher catalytic activity due to the higher amount of e⁻ density at Au in those catalysts, which could easily activate molecular oxygen and enable the oxidation of benzyl alcohol.²⁹ Hsu et al.¹⁴ observed such electronic effects on Pt catalysts with similar supports. Fig. 5 shows an interesting correlation between the basicity and mean particle size/catalytic activity (TOF). The particle size of Au decreased with increasing basicity (basic sites per unit surface area) of the catalyst while the TOF showed an increasing trend. In other words, the basicity of the support, mean particle size of Au and catalytic activity are inter-related to each other. The points related to Au (1 wt%) on CaTNTs and MgTNTs have fallen away from this relationship because there could be other factors like the strength of basicity that would influence the values of particle size and TOF. Fig. 6 depicts the variation of the oxidation activity of Au as a function of its particle size. Catalytic activity increased with decreasing Au particle size. It is interesting to note that all the catalysts have followed this trend unlike that observed in Fig. 5. This clarifies that other than the density, the strength of basicity plays a crucial role. With increasing Au content from 1 to 5 wt% on BaTNTs, the conversion of benzyl alcohol increased from 43 to 69.7 wt% but the selectivity for benzaldehyde decreased from 99 to 95.2 wt% (Table 3). An increase in activity with increasing Au content was observed also with KTNT and SrTNT supports.

Table 3 Catalytic activity data for the oxidation of benzyl alcohol over Au/ATNT catalysts^a

Run no.	Catalyst	Conversion (wt%)	TOF (h^−1)	Aldehyde selectivity^b (wt%)
a Reaction conditions: catalyst = 0.05 g, benzyl alcohol = 25 mmol, p (O2) = 1 atm, reaction temperature = 120 °C, and reaction time = 10 h, turnover frequency (TOF) = moles of benzyl alcohol converted per mole of Au in the catalyst (ICP-OES) per hour. TOF values in parentheses are those calculated based on moles of benzyl alcohol converted per mole of surface metal atoms (estimated from HRTEM) per hour. Run no. 12a is performed with molecular oxygen. Run no. 12b was conducted with air. For run no. 15, reaction time = 5 h and the products include benzaldehyde (80.5%), benzoic acid (1.0%), benzyl benzoate (10.2%), benzene (6.2%) and toluene (2.1%).b Balance selectivity is benzyl benzoate.
1	Au (1 wt%)/LiTNTs	7.2	114 (852)	99.0
2	Au (1 wt%)/NaTNTs	17.0	257 (1153)	99.0
3	Au (1 wt%)/KTNTs	21.9	392 (1583)	99.0
4	Au (3 wt%)/KTNTs	34.1	268	98.2
5	Au (5 wt%)/KTNTs	45.3	134	93.6
6	Au (1 wt%)/CsTNTs	8.8	166 (1068)	99.0
7	Au (1 wt%)/MgTNTs	11.6	127 (805)	99.0
8	Au (1 wt%)/CaTNTs	13.4	147 (976)	99.0
9	Au (1 wt%)/SrTNTs	37.8	516 (1791)	99.0
10	Au (3 wt%)/SrTNTs	46.3	190	96.2
11	Au (5 wt%)/SrTNTs	54.1	161	94.1
12a	Au (1 wt%)/BaTNTs	43.0	622 (2237)	99.0
12b	Au (1 wt%)/BaTNTs	24.6	356 (1280)	96.5
13	Au (3 wt%)/BaTNTs	62.5	220	97.0
14	Au (5 wt%)/BaTNTs	69.7	167	95.2
15	Au–Pd (1 : 1, 2 wt%)/BaTNTs	92.3	819 (4653)	80.5

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Fig. 5 Variation of the mean particle size and TOF with the basicity for Au (1 wt%)/ATNTs. Reaction conditions: catalyst = 0.05 g, benzyl alcohol = 25 mmol, p (O₂) = 1 atm, reaction temperature = 120 °C, and reaction time = 10 h.

Fig. 6 Correlation between the turnover frequency (TOF) and mean particle size of Au. Reaction conditions are the same as in Fig. 5.

Effect of the reaction conditions

The catalytic activity increased with increasing reaction time (ESI†). Only a marginal drop in benzaldehyde selectivity was noted at higher conversions. Also an increase in activity with the amount of catalyst was noted (ESI†). When the reaction temperature was raised from 80 to 100 and then to 120 °C, benzyl alcohol conversion on Au (1 wt%)/BaTNTs increased from 30.2 to 36.3 and then to 43 wt%. This increase was from 8.4 to 15.2 and then to 17 wt% over Au (1 wt%)/NaTNTs. The selectivity for benzaldehyde was 99 wt% at all of those conditions (ESI†).

Catalyst reusability

Fig. 7 displays the reusability of Au (1 wt%)/BaTNTs in benzyl alcohol oxidation. After the first run, the catalyst was separated from the reaction mixture by filtration, washed with water, dried at 110 °C for 4 h and then, reused in the next recycle conducted at the same conditions. Such recycles were done five times. The catalyst was recyclable. The XRD pattern of the spent catalyst is nearly the same as that of the fresh one (Fig. 1). ICP-OES analysis of the spent catalyst showed no loss of Au content but a little loss of Ba²⁺ ions (0.6 wt%) at the end of the 5^th recycle was detected. To confirm the heterogeneity of the oxidation reaction over Au (1 wt%)/BaTNTs, the reaction at 120 °C was stopped after 2 h (when the conversion was 17.2 wt%), the catalyst was taken out and the reaction was continued without the catalyst for another 8 h and the reaction mixture was analyzed by GC. No detectable change in benzyl alcohol conversion was observed (ESI†) suggesting that the reaction was heterogeneous in nature.

Fig. 7 Recyclability of Au (1 wt%)/BaTNTs in the oxidation of benzyl alcohol with molecular oxygen. Reaction conditions are the same as in Fig. 5.

The electrooxidation of ethanol over Au supported on glassy carbon requires a partial coverage of the Au surface with its oxide species. In CO oxidation, it was found that the oxidation activity increases with an increase in the concentration of the Au⁺ species; but the presence of metallic Au (electron rich) was essential. According to these reports, the active sites consist of an ensemble of metallic Au atoms and a cationic Au⁺ species.^30–32 In the present case, benzyl alcohol is activated on the support surface (as a consequence of its basic nature). Then the abstraction of a hydride ion is favoured at the metal–support interface (at coordinatively unsaturated sites/Au⁺ ions). The electron rich Au nanoparticles activate molecular oxygen to produce activated oxygen species, which help in removing H from AuH species to produce water as the by-product. Meanwhile, the benzyloxy cation releases a proton and forms the final product benzaldehyde.^33,34 A schematic representation of the reaction mechanism is shown in Fig. 8. The particle sizes of Au decreased with increasing basicity of the support while the catalytic activity had increased (Fig. 5). The higher the basicity, the higher the activation of benzyl alcohol and the higher the conversion. At the same time, the lower the particle size of Au, the higher the available surface area and support–Au interaction and the higher the activation of molecular oxygen and thereby, the higher the catalytic activity. Thus, basicity has a direct (by way of activating benzyl alcohol) and indirect (by way of support–metal interaction) influence on the catalytic activity. The mean particle size of Au on different supports decreased in the order: Au/BaTNTs < Au/SrTNTs < Au/KTNTs. The smaller the particle size the higher the SMSI effect, and there would be more peripheral Au that are in the δ+ state and hence, the higher the hydride ion abstraction and thereby, the higher the catalytic activity. This behavior is clearly evident from the XPS and catalytic activity data.

Fig. 8 Tentative mechanism for the oxidation of benzyl alcohol over Au/ATNTs.

Conclusions

Gold particles supported on alkali and alkaline earth metal ion-exchanged titanate nanotubes (ATNTs) were synthesized, characterized and their catalytic activities for benzyl alcohol oxidation with molecular oxygen/air were investigated. Gold particles on ATNTs have a mean diameter in the range of 5.2–11.6 nm and a Au dispersion in the range of 10–20%. The oxidation of benzyl alcohol formed benzaldehyde with a selectivity of 99 wt%. Alkali and alkaline metal ion exchange had a marked effect on the catalytic activity of titanate nanotubes. Among the catalysts investigated Au supported on BaTNTs showed the highest catalytic activity. The basicity of the support has an influence on the mean particle size and catalytic activity of Au. With increasing basicity, the particle size of Au had decreased while the activity of the catalyst had increased. This study teaches that the catalytic activity of Au can be enhanced by altering the titanate support through exchange with Ba²⁺ ions. Since Au draws electron density from the basic support, it can activate molecular oxygen more easily forming a superoxide like active oxygen species which in turn initiate the oxidation reaction.

Acknowledgements

D. N. acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, for the award of a Senior Research Fellowship.

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Footnote

† Electronic supplementary information (ESI) available: XRD, CO₂-TPD profiles, DRUV-vis spectra and XPS profiles of Au/ATNT, N₂ physisorption of supports, correlation profiles of basicity versus uptake and particle size of Au and B.E. values versus TOF and catalytic activity data. See DOI: 10.1039/c5ra06496a

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