Tuned depositing Ag clusters on ZrO₂ nanocrystals from silver mirror reaction of silver–dodecylamine complexes

Abstract

We for the first time have synthesized Ag/ZrO₂ nanocomposites from silver mirror reaction in toluene, which refers to the reduction of silver–dodecylamine complexes by acetaldehyde (CH₃CHO) in the presence of ZrO₂ nanocrystals. The obtained nanocomposites show excellent catalytic activity in the successive reduction of p-nitrophenol (4-NP) by NaBH₄.

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Although platinum-group-metals (PGM) as catalysts exhibit excellent catalytic performance for various reactions, they have a limitation in worldwide application due to the shortage of resources as well as high cost.^1,2 For this reason, silver (Ag), as a less expensive noble metal, is regarded as a better candidate for the design of PGM-free catalysts if size control is successfully undertaken.³ During the past few decades, controlled synthesis of Ag nanoparticles with uniform size distribution and their corresponding catalytic properties and applications have been extensively studied.^4–7 Ag particles with size of several or several tens of nanometers are used as catalysts in some important reactions including selective oxidation of ethylene to ethylene oxide, methanol to formaldehyde and cyclohexene to adipic acid.^8–10 As the size of Ag particles further decreases down to sub-nanometer scale, they show some unique catalytic properties. For example, Ag trimers (Ag₃) show high activity and high selectivity for propylene epoxidation.¹¹ Also, the Ag catalyst with a mean silver particles size of 0.8–3 nm shows the highest activity for the direct C–C cross-coupling of secondary and primary alcohols.¹² Unfortunately, as the same of other metal clusters, Ag clusters are also susceptible to aggregation, resulting in dramatically losing their catalytic activity.¹³ Both theoretical and experimental studies have demonstrated that ZrO₂, CeO₂ and TiO₂ are effective supports in stabilizing Ag nanoparticles or clusters against aggregation through an enhanced metal-support interaction.^14–16 What is more, the synergistic interactions between metal nanoparticles or clusters and metal oxide can also raise the catalytic performance.^17,18 To date, only a photocatalytic process assisted by TiO₂ nanocrystals to synthesize Ag/TiO₂ nanocomposites has been reported.^19–21 However, the size of Ag particles on TiO₂ nanocrystals from this method is always 5 nm or above with a wide range of size-distribution as well as poor dispersion. Therefore, it is highly desirable to develop a reliable method to synthesis small Ag clusters with uniform size anchored on metal oxide nanocrystal.

Here, we describe a novel route to stabilize Ag clusters on ZrO₂ nanocrystals for a series of Ag/ZrO₂-x nanocomposites, where x (x = 5, 2.5, 1.5, 0.75) is the loading percentage of Ag by weight (wt%). In this approach, ZrO₂ nanocrystals are prepared by a two-phase solvothermal synthesis method as reported previously.^22–24 Transmission electron microscopy (TEM) image depicts the as-synthesized ZrO₂ nanocrystals with the size of about 4–6 nm (Fig. S1†). The result from X-ray diffraction (XRD) shows the mixture of monoclinic and tetragonal structures for ZrO₂ nanocrystals (Fig. S2†). The obtained ZrO₂ nanocrystals as well as Ag/ZrO₂ nanocomposite, highly dispersed in toluene, are stabilized by oleic acid, as confirmed from the FT-IR spectra (Fig. S3†). We for the first time synthesize Ag/ZrO₂ nanocomposites through a silver mirror reaction in toluene, since the reflective mirrors on tube wall can also appear in toluene as shown by the photograph in Fig. S4.† In this synthesis, firstly, Ag precursor (silver–dodecylamine complexes solution) was prepared by dissolving AgNO₃ and dodecylamine in ethanol and toluene mixture solution (volume ratio = 1 : 3). Then, a certain amount of Ag precursor was introduced to toluene containing the oleic acid capped ZrO₂ nanocrystals at 50 °C. The reduction was initiated after the subsequent addition of CH₃CHO toluene solution. A series of clear solutions containing Ag/ZrO₂-x nanocomposites were obtained as illustrated in the photographs (Scheme 1).

Scheme 1 Strategy for the synthesis of Ag/ZrO₂-x nanocomposites.

The present process succeeds in the finely controlled deposition of Ag atoms on ZrO₂ nanocrystals. The sharp absorption peak centered at 420 nm in Fig. 1a and S5† is assigned to the characteristic plasmonic resonance peak of Ag clusters.²⁵ With prolonging of reaction time, the transparent solution was converted to a deep purple solution (inset of Fig. 1a), and the increasing of absorbance was ascribed to the continuous deposition of Ag atoms on ZrO₂ nanocrystals. In the end, the absorbance ceased to increase, indicating the complete reduction. Moreover, the peak tail towards high wavelength, resulting from the interparticle coupling effect between the neighbouring Ag nanoparticles on supports or the scattering peak at 620 nm of large Ag particles,^25,26 is not found in this synthesis, indicating the form of smaller Ag clusters on ZrO₂ nanocrystals with a narrow size-distribution and good dispersion. The obtained Ag clusters on ZrO₂ nanocrystals are essentially maintained with the same size-distribution and uniformity with increasing Ag loading, as no red-shift of absorption peak is observed (Fig. 1b).^27,28 The kinetic curves for the formation of the corresponding Ag/ZrO₂ nanocomposites in Fig. 1c display that the reaction has a very short induction period, a stable reaction period, and reaction ending period. Interestingly, we can find that a linear evolution between A_max and t in a stable reaction period, suggesting that the population of Ag atoms deposited on ZrO₂ nanocrystals is proportional to the reaction time in the stable reaction period. At higher Ag loading, the reduction rate in the stable reaction period is higher, and the reduction ends in shorter time. These phenomena suggest that the reduction reaction is catalyzed by Ag clusters deposited on ZrO₂ nanocrystals. Thus, Ag/ZrO₂ nanocomposites with specific amount of Ag can be easily obtained through the controlled addition of Ag precursor.

Fig. 1 (a) Time-dependent UV-vis absorption spectra of Ag/ZrO₂-5 nanocomposite solution. The inset shows the corresponding photographs. (b) UV-vis absorption spectra of the finally obtained Ag/ZrO₂-x nanocomposite solutions (x = 5, 2.5, 1.5, 0.75). The inset shows the corresponding photographs. (c) Plots of A_max against the reaction time for Ag/ZrO₂-x nanocomposites. (d) Plots of A_max against the reaction time for Ag/ZrO₂-5 and Ag-5, respectively. The inset shows the reduction process of Ag precursor with or without the presence of ZrO₂ nanocrystals.

The key for the successful synthesis of nanocomposites in this work is the silver mirror reaction of silver–dodecylamine complexes in toluene. This can be illustrated by the synthesis of Ag nanocrystals in the same system (Fig. S6a†). Unlike the quick formation of large sized and irregular Ag particles in the conventional silver mirror reaction in water, in which nuclei formation and attaching growth of nuclei occurs concurrently,^29,30 the present reaction can be clearly separated into two periods of nuclei formation and crystal growth (Fig. S6b†). The formation of Ag nuclei takes place with a small absorbance increasing rate because it is an energy-consuming process.^31,32 As catalyzed by Ag nuclei, the crystal growth occurs with a high absorbance increasing rate. It is both dodecylamine and toluene that makes the reduction reaction and crystallization behaviours different from that in the aqueous system. On one hand, dodecylamine, as a complex agent, forms the stable complex with Ag⁺ ions. On another hand, dodecylamine can act as a surfactant agent to protect the formed Ag nuclei from aggregation to nanocrystals. Toluene as solvent for the replace of water can be facile for the stabilizing and dispersing of Ag nuclei as well as newly reduced Ag atoms. The introduction of ZrO₂ nanocrystals brings significant impact on reaction. The two periods of nuclei formation and crystal growth are merged into one upon the addition of ZrO₂ nanocrystals (Fig. 1d). Obviously, ZrO₂ nanocrystals act as nuclei and provide hetero-surfaces for Ag atom depositing, which replaces the energy-consuming self-nucleation process.³³ The emerging Ag atoms directly deposit on ZrO₂ nanocrystal surfaces with higher rate even at a low Ag precursor concentration (Fig. 1c). Compared with the same reaction system without the addition of ZrO₂ nanocrystals, the depositing rate on ZrO₂ nanocrystals is so high that reaction time decreases from 600 to 60 min (Fig. 1d). The high Ag depositing rate on ZrO₂ nanocrystals and the increase of depositing rate with Ag loading reflect the easy formation of Ag active sites on ZrO₂ nanocrystals and a greatly high amount of Ag active sites formed on ZrO₂ nanocrystals at the short induction period for catalyzing reduction reaction.

As indicated from the unique peak at 420 nm in the UV-vis spectra (Fig. 1a and b), the unprecedented small sized and uniform Ag clusters are formed on ZrO₂ nanocrystals from the present reaction system. The size of Ag clusters is so small that it is difficult to discriminate in both the TEM image and the Ultra-HRSEM image (Fig. 2a and S7†). The HRTEM image (Fig. 2b) discloses that Ag clusters (labelled by the white circles) with size of sub-1 nm are anchored on (−111) facet of ZrO₂ nanocrystals. Also, large sized, free-standing and irregular Ag particles are not found. This is consistent with the Ag depositing mechanism as discussed in the previous paragraphs that self-nucleation does not occur in the present system. We also choose a brighter region (probably containing Ag nanoparticles) from a scanning transmission electron microscopy (STEM) image for EDX analysis (Fig. 2c), the very weak signal from Ag element in the EDX spectra is mainly due to the Ag content below the detection limit and depicts that small size of Ag clusters are highly dispersed on ZrO₂ nanocrystals. We also perform XPS analysis to investigate the surface composition of Ag/ZrO₂ nanocomposites. As shown in Fig. S8a,† the Ag is detected on Ag/ZrO₂-5 nanocomposite surfaces, since the double peaks at around 370 eV are assigned to Ag 3d energy level. Moreover, the binding energy of Ag clusters on ZrO₂ nanocrystals in Fig. S8b† (Ag 3d peak at 367.2 eV and 373.2 eV) is lower than that of metallic Ag (Ag 3d peak at 368.2 eV and 374.2 eV), indicating that the tiny Ag clusters have formed a more strongly binding to the surface of ZrO₂ nanocrystals instead of a simple adsorption of metallic Ag particles on ZrO₂ nanocrystal surfaces.

Fig. 2 (a) TEM image of the Ag/ZrO₂-5 nanocomposite. (b) HRTEM image of Ag/ZrO₂-5 nanocomposite. Ag clusters are labelled by the white circles. (c) EDX spectra of Ag/ZrO₂-5 nanocomposites obtained from the square region in the STEM image of the inset.

The formation of highly dispersed and tiny Ag clusters on ZrO₂ nanocrystals can be related with the high affinity of Ag atoms to ZrO₂ nanocrystals and the strong interaction between ZrO₂ nanocrystals and Ag atoms. Both the mixture phases of XRD pattern in Fig. S2† and twinning structures of HRTEM image in Fig. 2b demonstrate that ZrO₂ nanocrystals can have more lattice distortion, steps and defects on their surfaces. The emerging Ag clusters can bind more strongly to the lattice distortion, steps and surface defects of ZrO₂ nanocrystals due to the large adhesion energy.¹⁵

We carried out the successive reduction of 4-NP by NaBH₄ to evaluate the catalytic performance of these nanocomposites. This reaction is known to be useful for the analysis of the catalytic activity of metal species, as pure ZrO₂ exhibits no hydrogenation activity (Fig. S9†).^34–39 Typically, the 4-NP exhibits an absorption peak at 400 nm in alkaline condition due to the formation of 4-NP ions, and the product, 4-AP, exhibits an absorption peak at 300 nm.^38,39 Fig. 3a shows time-dependent UV-vis absorption spectra of reaction solution in the five successive reduction of 4-NP. When the fifth reaction cycle is started, there is a rapid drop in intensity of the absorption peak at 400 nm, and the absorbance at 300 nm increased, which depicts the reduction of 4-NP to 4-AP (Fig. 3a).^35,36 In this reaction, NaBH₄ was added in great excess as compared to 4-NP so that the reduction rate can be evaluated by the pseudo-first-order rate with respect to 4-NP concentration. The plots of ln[C(t)/C(0)] against the reaction time with Ag/ZrO₂ nanocomposites display an almost linear evolution in Fig. 3b, where C(t) and C(0) correspond to the concentrations of 4-NP at time t and 0, respectively. Slope of the plot gives the apparent rate constant (k), calculated from the rate equation ln[C(t)/C(0)] = kt.^37–39

Fig. 3 (a) Time-dependent UV-vis absorption spectra of reaction solution in five successive reduction of 4-NP. For the previous four reaction cycles, the UV-vis spectrometer only records the absorption spectra of the product solution when the reaction cycle is finished. When the fifth cycle was started, there was a rapid drop in intensity of the absorption peak at 400 nm. This reaction cycle was finished in 180 s. (b) Plots of ln[C(t)/C(0)] against the reaction time with Ag/ZrO₂-x nanocomposites as catalysts. (c) The normalized k values of Ag/ZrO₂-x nanocomposites in successive reduction of 4-NP. (d) The normalized k values of Ag/ZrO₂-5 nanocomposite and pure Ag clusters in successive reduction of 4-NP.

Catalysts based on metal nanocrystals or clusters surfer from particles aggregation, low efficiency of separation and poisoning by the reaction product in liquid-phase reactions.^13,38 In this work, Ag/ZrO₂-x nanocomposites are not separated from the reaction system in the successive reduction of 4-NP. So no mass loss of Ag occurs. While the product of 4-NP in reaction system is continuously increasing all the time (Fig. 3a). Fig. 3c shows the changes of the k values in successive reduction of 4-NP with Ag/ZrO₂-x nanocomposites as catalysts. For comparison, we also prepared pure Ag clusters with size of ∼2 nm as shown in Fig. S10.† It is clear that all of Ag/ZrO₂-x nanocomposites almost maintain a steady catalytic activity with a slightly fluctuating k values (Fig. 3c and S11a–d†). This significantly enhanced resistance to aggregation is attributed to the strong metal-support interaction between Ag clusters and ZrO₂ nanocrystals. In mark contrast, the k value drops by about 85% for pure Ag (Fig. 3d and S11e†). This can be attributed to aggregation of Ag clusters or poisoning by the reaction product. The TOF values of Ag/ZrO₂-x nanocomposites vary from 14.2 to 37.5 min⁻¹, and the highest TOF value is attained on Ag/ZrO₂-2.5 nanocomposite. It can be speculated from the reaction mechanism that single Ag atom firstly deposits on ZrO₂ nanocrystals and Ag clusters are formed with further Ag atom depositing. High Ag loading can always form large size of Ag clusters, which leads to lower TOF value (Ag/ZrO₂-5). While low Ag loading can lead to more single Ag atom on ZrO₂ nanocrystals but also lower TOF value (Ag/ZrO₂-1.5). As reported in some articles, catalytic activity of Ag clusters¹¹ and Pt nanoparticles is higher than that of single atom in some reactions.⁴⁰ We can conclude that the reduction of 4-NP is mainly catalyzed by Ag clusters other than single Ag atom or large size of Ag clusters. However, further decreasing Ag loading from 1.5 wt% to 0.75 wt% can lead to a slightly higher TOF, which can be ascribed to aggregation of Ag atoms on ZrO₂ nanocrystal surfaces to high active Ag clusters through extending reaction time (Fig. S5b–5c†). In comparison with the recently reported results, the TOF of Ag/ZrO₂ nanocomposite catalyst (TOF = 37.5 min⁻¹) is more than twice as high as that of PGM catalysts (18 min⁻¹ of TOF for Pd/CNT-200) and even 50 times higher than previously reported Ag based catalysts (0.72 min⁻¹ of TOF for Ag/C).^34,41 These results firmly demonstrate that Ag/ZrO₂ nanocomposites exhibit excellent catalytic activity and stability in successive reduction of 4-NP by NaBH₄. During the reaction, Ag clusters deposited on ZrO₂ nanocrystals show better resistance against aggregation as well as poisoning by the reaction product (Table 1).

Table 1 Comparison of TOF of Ag/ZrO₂ nanocomposite catalysts and of some previously reported noble metal catalysts for the reduction of 4-NP by NaBH₄

Catalyst	Size of particles (nm)	TOF (min^−1)	Ref.
Ag/ZrO2-5	Below 1	19.1	This work
Ag/ZrO2-2.5	Below 1	37.5	This work
Ag/ZrO2-1.5	Below 1	14.2	This work
Ag/ZrO2-0.75	Below 1	17.4	This work
Pd/CNT-200	1.5	18	34
Pd/CNT	2.6	6.32	37
Pt@CeO2	3–5	1.5	38
Pt/ZrO2	—	12 ± 1	39
Ag/C	10.9 ± 6	0.72	41
Ag/FeOx	2–5	0.11	42

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Conclusions

In conclusion, a novel strategy to synthesize Ag/ZrO₂ nanocomposites has been developed. The synthesis is tuned by using silver–dodecylamine complexes in toluene. ZrO₂ nanocrystals are a good support for Ag depositing and also can maintain smaller Ag clusters on their surfaces. The superior catalytic activity and stability of Ag/ZrO₂ nanocomposites depicts that it can be promising materials for the design of PGM-free catalysts in industrially chemical reactions.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 21376238, 21476230, 21506208 and 21306189).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04947h

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