Macroporous TiO2 encapsulated Au@Pd bimetal nanoparticles for the photocatalytic oxidation of alcohols in water under visible-light

Xiaohui Wangab, Dahu Baiyilab and Xiaotian Li*a
aDepartment of Material Science and Engineering, Jilin University, Changchun, 130012, P. R. China. E-mail: xiaotianli@jlu.edu.cn
bCollege of Chemistry and Chemical Engineering, Inner Mongolia University for the Nationalities, Tongliao, 028000, P. R. China

Received 6th September 2016 , Accepted 20th October 2016

First published on 20th October 2016


Abstract

A novel photocatalyst composed of a macroporous TiO2 (anatase) scaffold and Au@Pd bimetal nanoparticles has been reported and showed excellent catalytic activity and stability in the aerobic oxidation of alcohols in water at ambient temperature when using air as the oxidizing agent under visible light irradiation.


Selective oxidation of alcohols to corresponding carbonyl compounds such as aldehydes, carboxylic acids and ketones is one of the most important and fundamental transformations in organic synthesis and chemical industry.1–3 Despite numerous catalytic systems having been established and giving remarkable results, heterogeneous catalytic oxidation has been considered as one of the most promising processes.4,5 Considering the increasing environmental concerns and energy crisis, developing green catalytic systems for the aerobic oxidation of alcohols has become an important mission.6,7 To achieve this goal, photocatalytic oxidation with O2/air via the simultaneous utilization of photogenerated electrons and holes has been widely explored over the past several years.8–11

Semiconductor oxides such as TiO2, a class of highly efficient photocatalysts for water splitting and dye degradation, were firstly explored for the aerobic oxidation of alcohols under light irradiation.12–20 For example, Palmisano's group demonstrated the selective oxidation of aromatic alcohols to corresponding aldehydes with O2 in solvent water by band gap excitation of rutile-type TiO2 under UV irradiation.18–20 Tanaka and co-workers even reported a TiO2–Nb2O5 nanocomposite for photocatalytic transformations of fatty alcohols under UV irradiation and the material showed enhanced performance with a higher selectivity than TiO2 and a higher activity than Nb2O5.21 However, due to the large bandgap, these metal oxides materials can be only responsive to the light in the UV region (about 5% of total solar energy), leading to a limited catalytic activity under the visible light irradiation. Therefore, visible-light-responding photocatalysts with high catalytic performances were further exploited. Metal nanoparticles with localized surface plasmon resonance (LSPR) decorating can not only expand the light absorption range of metal oxides, but also enhance the efficiently utilization of the photogenerated electrons/holes, which can improve the photocatalytic properties of the materials largely. Kominami's group designed and prepared Au/CeO2 materials with superior catalytic performances in the selective oxidation of alcohols to aldehydes or ketones in aqueous suspensions under irradiation by green light.22 They proved that the enhanced performances of Au/CeO2 were indeed resulted from the strong LSPR of the large Au nanoparticles. Au-nanorod@TiO2 yolk–shell nanostructures were also reported as an efficient visible-light-driven photocatalyst for oxidation of benzyl alcohol.23 Undoubtedly, metal nanoparticles loaded semiconductor oxides have been emerged as one of the most promising photocatalysts for green catalytic oxidation of alcohols.24–26

On the other hand, bimetal nanoparticles often show outstanding synergetic effect, and thus possess excellent catalytic properties in different catalytic reactions.27–33 Noble metal Au nanoparticles have strong LSPR effect, while metal Pd usually exhibits more superior catalytic activity in oxidation reactions than other metal such as Ag, Au and Rh. Obviously, Au–Pd bimetal nanoparticles will have excellent catalytic performances in the aerobic oxidation of alcohols under visible light irradiation.34,35 However, Au–Pd bimetal nanoparticles loaded semiconductor oxides has been rarely reported for photocatalytic oxidation of alcohols up to now.36–39

In this work, we reported a novel visible-light-responding photocatalyst composed of macroporous TiO2 scaffold and Au@Pd bimetal nanoparticles. The TiO2 scaffold consisted of uniform macropores and many anatase crystalline grains (8–10 nm). Moreover, because of the unique space-confined effect, the Au@Pd bimetal nanoparticles were highly dispersed in the macroporous TiO2 scaffold. The strong LSPR effect of bimetal Au@Pd nanoparticles endowed material superior photoabsorption properties ranging from UV light to visible light region. More importantly, this photocatalyst showed excellent catalytic activity and stability in the aerobic oxidation of different alcohols in water at ambient temperature when using air as the oxidizing agent under visible light irradiation.

Scheme 1 illustrates the general preparation process of the visible-light-driven photocatalyst Au@Pd@TiO2. Firstly, core–shell structured Au@SiO2 nanospheres with a diameter of 130 nm (Fig. S1) were prepared via a modified Stober method according to previous publications.6,40 And then the Au@SiO2 nanospheres were assembled to colloid templates by self-stacking for coating TiO2, and thus leading to the formation of Au@SiO2@TiO2 colloids. After selective removal of middle SiO2 layer via NaOH etching, macroporous Au@TiO2 scaffold with amorphous frameworks was obtained (Fig. S2). Finally, Au@TiO2 scaffold composed of Au nanospheres and anatase crystalline grains was produced by calcination at 500 °C for 2 hours under air atmosphere.


image file: c6ra22299d-s1.tif
Scheme 1 Schematic illustrating the preparation of the macroporous visible-light-driven photocatalyst Au@Pd@TiO2.

From the scanning electron microscopy (SEM) image in Fig. 1a, we can clearly observe abundant uniform macroporous structure of the Au@TiO2 sample. The high-magnification SEM image reveals that every Au nanoparticle directed by red arrows is loaded in the macropores of sample (Fig. 1b). The transmission electron microscopy (TEM) images (Fig. 1c and d) further confirm that the uniform macroporous structure is throughout the scaffold, and the 18–20 nm of Au nanoparticles are encapsulated in the uniform macropores of sample, respectively. The pore diameter of these macroporous is approximately 100 nm, a little less than the particle size of Au@SiO2 nanospheres, which is due to the structural shrinkage of the TiO2 framework during crystallization. It is worth mentioning that the Au nanoparticles are still highly dispersed in the macroporous TiO2 scaffold after high-temperature (500 °C) crystallization of amorphous TiO2, and any coking or aggregation are not observed, which should be attributed to the unique space-confined effect of macroporous TiO2. This strongly supports that highly dispersed Au@Pd bimetal nanoparticles with high catalytic activity can also be loaded in the macroporous TiO2 scaffold. Moreover, the high-magnification TEM images (Fig. 1e and f) indicate that the TiO2 scaffold is composed of many 8–10 nm of anatase crystalline grains. The lattice fringes with an interplanar distance of 0.35 nm corresponding to the (101) plane of anatase are clearly visible in the HRTEM image of Fig. 1f, suggesting a high crystallinity. The powder X-ray diffraction (PXRD) pattern shows a series of diffraction peaks (Fig. 3 red) at 2θ value of 25.3°, 38.4°, 48.0°, 54.0°, 55.0° and 62.7° directed by red circle, which can be assigned to the (101), (004), (200), (105), (211) and (204) reflections of anatase phase, further confirming the high crystallinity of TiO2 scaffold. All these results demonstrate that we have successfully prepared high-crystallized macroporous TiO2 scaffold containing well-dispersed Au nanoparticles via a colloid templated method.


image file: c6ra22299d-f1.tif
Fig. 1 SEM (a and b), TEM (c and d) and high-magnification TEM (e and f) images of the macroporous Au@TiO2 scaffold.

In order to obtain high photocatalytic power for oxidation of alcohols, we further introduced more active Pd into the macroporous Au@TiO2 scaffold through a seed-mediated overgrowth process that is a popular approach for constructing different bimetal nanoparticles.6,41 The TEM images (Fig. 2a and c) clearly indicate that as-obtained photocatalyst Au@Pd@TiO2 possesses identical structural properties as those of Au@TiO2 scaffold, including the uniform macroporous structure and the high-crystallized anatase grains. Importantly, the Pd nanoparticles epitaxially grow on the surface of Au nanoparticles, as shown in the high-magnification TEM image of Fig. 2b. The lattice fringes are clearly visible in the HRTEM image of Fig. 2d, which we assign to the (111) plane of palladium. The particle size distribution shows that the Au@Pd bimetal nanoparticles encapsulated in the macropores have an average size of 36 nm (Fig. 2e), much larger than that of Au nanoparticles (18 nm). Based on these results, we can conclude that the formed Au@Pd bimetal nanoparticles were consisted of an interior Au core and an external Pd shell. It is notable that the Au@Pd bimetal nanoparticles are still highly dispersed in the macroporous TiO2 material. The PXRD pattern (Fig. 3) also proves the presence of crystalline Pd and Au nanoparticles in the Au@Pd@TiO2 sample. When the diffraction peaks at 2θ value of 38.4°, 44.4°, 64.6° and 77.6° directed by blue square were assigned to the (111), (200), (220) and (311) reflections of gold, the diffraction peaks at 2θ value of 40.1° and 46.7° directed by green triangle were assigned to the (111) and (200) reflections of palladium, respectively.


image file: c6ra22299d-f2.tif
Fig. 2 TEM (a and b) and HRTEM images (c and d) of the visible-light-driven photocatalyst Au@Pd@TiO2. (e) Particle size distribution of Au@Pd bimetal nanoparticles.

image file: c6ra22299d-f3.tif
Fig. 3 PXRD patterns of the sample amorphous Au@TiO2 (black), crystallized Au@TiO2 (red) and the visible-light-driven photocatalyst Au@Pd@TiO2 (blue).

The photoabsorption properties of the samples were examined before testing the photocatalytic performances of Au@Pd@TiO2. As shown in the Fig. 4, the TiO2 sample only shows a strong absorption below 400 nm owing to the large bandgap of TiO2. Whereas, in addition to the similar absorption below 400 nm, the sample Au@TiO2 and Au@Pd@TiO2 also have a strong absorption in the range of 400–800 nm and the photoabsorption peak at 580 nm is distinctly broader than that of Au nanoparticles, which is probably resulted from the LSPR of Au and the strong interaction between Au nanoparticles and TiO2 support.10,22 Futhermore, in spite of the same amount of Au, sample Au@Pd@TiO2 exhibits photoabsorption in the range of 400–800 nm much stronger than that of sample Au@TiO2, which is ascribed to the synergistic effect of Au–Pd bimetal.39 Moreover, according to these UV-visible diffuse reflectance spectra, band gap narrowing can be observed from 3.30 eV to 3.22 eV. By immobilizing the bimetallic Au@Pd nanoparticles, the energy level of the valence bands of the photocatalysts was shifted to lower energy.42 This narrowed band gap resulted in the photocatalyst more absorption at longer wavelengths (400–800 nm), besides the LSPR absorption peaks (580 nm) of Au, and thus lead to a higher photocatalytic activity. Additionally, from the photograph in the inset in Fig. 4, it can be clearly identified that the sample Au@Pd@TiO2 is black blue while the sample TiO2 and Au@TiO2 is white and purple, respectively. Obviously, this experiment means that the sample Au@Pd@TiO2 has excellent ability, as our expected, to gain high intensity of light energy, and thus enhancing its catalytic performance when irradiated with visible light.


image file: c6ra22299d-f4.tif
Fig. 4 UV-visible diffuse reflectance spectra of the sample TiO2, Au@TiO2 and Au@Pd@TiO2.

The selective oxidation of 4-methoxybenzyl alcohol was choosed as the model reaction to evolute the catalytic activity of our prepared photocatalyst Au@Pd@TiO2, and the results are shown in Table 1. It was observed that Au@Pd@TiO2 showed a conversion of 70% with a selectivity of 95% at 40 °C (entry 4) in 3 h using air as an oxidizing agent under visible light irradiation, whereas the macroporous TiO2 scaffold containing only Au nanoparticles was inactive (entry 3). This activity was also higher than that of conventional heterogeneous catalyst Pd/MCM-41 (25%, entry 1) and novel nanoreactor Au@Pd@SiO2 (62%, entry 2). Moreover, a significant enhancement in catalytic activity (30%) could be observed under visible light irradiation in the presence of Au@Pd@TiO2, and this enhancement was much higher than that of Pd/MCM-41 (3%) and Au@Pd@SiO2 (14%), which should be ascribed to the strong LSPR effect of bimetal Au@Pd nanoparticles and the improved utilization of the photogenerated electrons/holes.22 In addition, the catalyst Au@Pd@TiO2 gave a conversion of 100% with a high selectivity of 93% after further reacting for 5 h, which is much higher than the activity of the reported Au nanorod@TiO2 yolk–shell nanostructures,23 and even comparable to the activity of Au–Pd alloy nanoparticles achieved using oxygen as an oxidizing agent and trifluorotoluene as an organic solvent.25 Furthermore, we found that Au@Pd@TiO2 also gave a high conversion of 91% coupled with a good selectivity of 84% (entry 5) when only increasing the reaction temperature from 40 °C to 80 °C. However, the selectivity of aldehyde was decreased to 84%, and the enhancement in catalytic activity resulted from the visible light irradiation was distinctly lower than the case of reacting at 40 °C (12% vs. 30%). This controlled experiment clearly indicated that the light irradiation could replace the heat energy to accelerate the oxidation reactions in current catalytic system, and thus leading to a higher selectivity towards corresponding aldehydes, which is very interesting for the industrial production of aldehydes via aerobic oxidations. In addition, the unique macropores structure (100–150 nm) not only can offer fast mass transfer for substrates and products, but also can enhance the light absorption and utilization efficiency of the material via multiple refractions within the continuous macropores,43 which is in favour of improving catalytic performances including activity and selectivity.

Table 1 Results of the selective oxidation of 4-methoxybenzyl alcohol over different catalystsa

image file: c6ra22299d-u1.tif

Entry Catalyst Incident light Conv.b (%) Sel.b (%)
a Reaction condition: 4-methoxybenzyl alcohol (0.5 mmol), K2CO3 (0.5 mmol, 1 equiv.), H2O (4 mL), catalyst (Pd 1.5 mol%), open air, 3 h.b Catalytic reaction products were analyzed and identified by GC-MS.c Reacting for 5 h.d Reacting at 80 °C for 3 h.e Selectivity of 4-methoxybenzoic acid.
1 Pd/MCM-41 Visible 25 98
Dark 22 98
2 Au@Pd@SiO2 Visible 62 88
Dark 48 97
3 Au@TiO2 Visible Trace
Dark Trace
4 Au@Pd@TiO2 Visible 70 95
Visiblec 100 93
Dark 40 97
5d Au@Pd@TiO2 Visible 91 84 (16)e
Dark 79 88 (12)e


The scope of the presented catalysis system was then subsequently extended to the aerobic oxidation of a large variety of alcohols. The results summarized in Table 2 show that Au@Pd@TiO2 was active and extremely selective for the oxidation of all substrates under visible light irradiation. Primary aromatic alcohols including benzyl alcohol, 4-methylbenzyl alcohol, 1-naphthylmethanol and cinnamyl alcohol could all be easily oxidized to the corresponding aldehydes with conversions of ≥67% and selectivities of ≥90% (entries 1–4). Secondary aromatic alcohols including 1-phenethyl alcohol and 1-(4-methoxyphenyl)ethanol were also effectively converted to their corresponding ketones with 100% selectivity (entries 5–6). Most importantly, a significant enhancement in catalytic activity resulted from the visible light irradiation could be also observed in all aerobic oxidation reactions. All these catalytic results indicate a high versatility of the current photocatalytic system.

Table 2 Aerobic oxidation of various alcohols over the catalyst Au@Pd@TiO2 under visible-light irradiationa

image file: c6ra22299d-u2.tif

Entry Substrate Product Conditions Conv./sel.b (%)
a Reaction condition: alcohol (0.5 mmol), K2CO3 (0.5 mmol, 1 equiv.), H2O (4 mL), catalyst (Pd 1.5 mol%), open air.b Catalytic reaction products were analyzed and identified by GC-MS.c Reacting at 80 °C.
1 image file: c6ra22299d-u3.tif image file: c6ra22299d-u4.tif Visible, 5 h 88/90
Dark, 5 h 48/95
2 image file: c6ra22299d-u5.tif image file: c6ra22299d-u6.tif Visible, 3 h 100/90
Dark, 3 h 76/92
3 image file: c6ra22299d-u7.tif image file: c6ra22299d-u8.tif Visible, 6 h 69/99
Dark, 6 h 43/99
4 image file: c6ra22299d-u9.tif image file: c6ra22299d-u10.tif Visible, 6 h 67/100
Dark, 6 h 39/100
5 image file: c6ra22299d-u11.tif image file: c6ra22299d-u12.tif Visible, 6 h 91/100
Dark, 6 h 50/100
Visible, 6 hc 100/100
Dark, 6 hc 93/100
6 image file: c6ra22299d-u13.tif image file: c6ra22299d-u14.tif Visible, 6 h 88/100
Dark, 6 h 46/100
Visible, 6 hc 100/100
Dark, 6 hc 95/100


Based on the photoabsorption property and the catalytic activity, a possible mechanism for the visible light-promoted aerobic oxidation of alcohols over the Au@Pd@TiO2 photocatalyst is proposed in Fig. 5. Firstly, incident photons are absorbed by Au nanoparticles through LSPR excitation, and the obtained hot electrons are transferred to Pd sites and anatase grains. These hot electrons can populate unoccupied orbitals of oxygen molecules, leading to the formation of a transient anion O–O species,24,44 which can cleave the O–H bond of the alcohol to present an alkoxide intermediate. And then this intermediate undergoes a rapid hydride transfer from C–H to the positively charged Au to form product aldehyde/ketone and an Au–H species.39,45 Finally, the formed O–OH and Au–H species are eliminated to liberate H2O2 (two-electron reduction step) and/or H2O (four-electron reduction step), and then the active sites are recovered for next recycle. Very high selectivity for aldehydes/ketones in the present system suggests that active oxygen species were removed from the reaction system.


image file: c6ra22299d-f5.tif
Fig. 5 Proposed mechanism for the aerobic oxidation of alcohols over the photocatalyst Au@Pd@TiO2 driven by visible light irradiation.

The catalytic stability of Au@Pd@TiO2 was further examined by using 4-methoxybenzyl alcohol as a model substrate. As shown in the Fig. 6, this novel photocatalyst still showed a very high activity with a conversion of ≥90% and a selectivity of ≥90% after ten recycling runs. Almost no dissolved Au and Pd species could be detected by inductively coupled plasma mass spectrometry in all recycled solutions. Obviously, this excellent catalytic stability was ascribed to the space-confined effect of its unique macroporous structure. Combined with all above results, we can attribute the excellent catalytic activity and stability observed in these experiments to the strong LSPR effect of bimetal Au@Pd nanoparticles and the strong interaction between noble metal nanoparticles and semiconductor oxides as well as the unique macroporous structure.


image file: c6ra22299d-f6.tif
Fig. 6 Recycling runs for the aerobic oxidation of 4-methoxybenzyl alcohol catalyzed by Au@Pd@TiO2 under visible light irradiation. Reaction condition: 4-methoxybenzyl alcohol (0.5 mmol), K2CO3 (0.5 mmol, 1 equiv.), 4 mL of H2O, catalyst (Pd 1.5 mol%), 40 °C, open air, 3 h.

Conclusions

In summary, we have developed a novel visible-light-driven photocatalyst Au@Pd@TiO2 composed of Au@Pd bimetal nanoparticles and macroporous TiO2 (anatase) scaffold via a colloid templated method. Owing to the strong LSPR effect and the unique macroporous structure as well as the strong interaction between noble metal nanoparticles and semiconductor oxides, the catalyst showed excellent catalytic activity and stability in aerobic oxidation of alcohols in water at ambient temperature using air as the oxidizing agent under visible light irradiation. A wide range of substrates, including different primary and secondary alcohols, can all be effectively oxidized to the corresponding aldehydes and ketones with high selectivities. Moreover, the catalyst was easily reusable at least ten times without appreciable reduction of catalytic efficiency. The green catalytic conditions and the outstanding catalytic performances make Au@Pd@TiO2 a potential material for practical applications in the selective aerobic oxidation of alcohols.

Acknowledgements

We thank the funding support from the National Natural Science Funding of China (21076094) and Science Foundation of Inner Mongolia University for the Nationalities (NMD1313).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22299d

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