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/lp/de-gruyter/catalytic-activity-of-gold-and-silver-nanoparticles-supported-on-zinc-i7yA7co2my
- Publisher
- de Gruyter
- Copyright
- Copyright © 2015 by the
- ISSN
- 2084-7246
- eISSN
- 2084-7246
- DOI
- 10.1515/supcat-2015-0001
- Publisher site
- See Article on Publisher Site
Abstract
Gold (AuNPs) and silver (AgNPs) nanoparticles have been prepared by "one-pot" synthetic method in the presence of poly(N-vinylpyrrolidone) (PVP). Absorption spectra, size, morphology, and structure of AuNPs and AgNPs were studied by UV-Vis spectroscopy, DLS, SEM, and TEM. According to DLS measurements the average sizes of metal nanoparticles stabilized by PVP in aqueous solution are varied from 10 to 25 nm for AuNPs and from 6.5 to 44 nm for AgNPs. The impregnation method is used to support AuNPs and AgNPs on the surface of ZnO. The amount of AuNPs and AgNPs immobilized on the surface of ZnO does not exceed 0.2 wt.%. The catalytic activity of AuNPs and AgNPs supported on the surface of ZnO was evaluated with respect to decomposition of hydrogen peroxide. The optimal conditions for H2O2 decomposition were found to be dependent on of the amount of the catalyst, concentration of substrate, molecular weight of PVP, and temperature. The activation energy of H2O2 decomposition is equal to 44.1 kJ×mol-1. The decomposition of H2O2 in the presence of AuNPs and AgNPs supported on ZnO surface is discussed in the context of supramolecular catalysis mimicking catalase-like behavior. The recyclability of AuNPs supported on ZnO surface was tested. Keywords: Gold and silver nanoparticles, poly(Nvinylpyrrolidone), zinc oxide, catalysis, hydrogen peroxide decomposition DOI 10.1515/supcat-2015-0001 Received November 17, 2014; accepted November 17, 2014 1 Introduction AuNPs and AgNPs attract significant attention of researchers due to their unique optical, electrical, biomedical and catalytic properties [1-5]. Hydrophilic polymers of nonionic [6-9], anionic [10], cationic [11,12] and amphoteric [13-15] nature are widely used for stabilization of AuNPs and AgNPs in solutions [16, 17]. The general principles and recent developments in the synthesis of gold nanoparticles (AuNPs) were recently reviewed [18]. Gold and silver catalysts represent rapidly growing area of interest due to their potential applicability to many reactions of both industrial and environmental importance [19,20]. Typical examples are the low-temperature catalytic combustion, partial oxidation of hydrocarbons, hydrogenation of carbon oxides and unsaturated hydrocarbons, reduction of nitrogen oxides, and so forth [21]. Recent review [22] describes the size-, shape-, structure- and composition-dependent behavior of AuNPs employed in alkylation, dehydrogenation, hydrogenation, and selective oxidation reactions for the conversion of hydrocarbons (with main emphasis on fossil resources) to fine chemicals. The perspectives of substituting platinum group metals for automobile emission control with gold were outlined [23]. Synthesis of gold and silver hydrosols was carried out in one-step process by reduction of aqueous solutions of metal salts using PVP [24]. Shape, size, and optical properties of the particles were tuned by changing the employed PVP/metal salt ratio. The size of PVP protected AuNPs ranging from 10 to 110 nm was easily controlled by varying the concentration of PVP in solution (0.01-10 g/dL) [6]. The catalytic activity of AuNPs deposited on different substrates such as ZnO, Al2O3 and MgO by a colloidal deposition method has been investigated on benzene *Corresponding author: Sarkyt E. Kudaibergenov: Laboratory of Engineering Profile, K.I. Satpayev Kazakh National Technical University, Almaty, 050013, Kazakhstan, E-mail: skudai@mail.ru Nurlykyz N. Yesmurzayeva, Bagadat S. Selenova: Department of Chemical Technology, K. I. Satpayev Kazakh National Technical University, Almaty, 050013, Kazakhstan Nurlykyz N. Yesmurzayeva, Zhanar A. Nurakhmetova, Gulnur S. Tatykhanova: Laboratory of Engineering Profile, K.I. Satpayev Kazakh National Technical University, Almaty, 050013, Kazakhstan Gulnur S. Tatykhanova , Sarkyt E. Kudaibergenov: Institute of Polymer Materials and Technology, Almaty, 050013, Kazakhstan © 2015 Nurlykyz N. Yesmurzayeva, et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. conversion and their activity are compared [25]. The catalytic activity of AuNPs/ZnO is much greater than that of AuNPs/Al2O3 and AuNPs/MgO. The high catalytic activity of the AuNPs/ZnO is attributed to the effects of strong metal-oxide interaction which is possibly originated from the small lattice parameter difference between Au {111} and ZnO {101} lattice planes. Presumably, the AuNPs deposited onto other metal oxides washes off from the substrate surface with a polymer solution during the synthesis, because of weak interaction between Au and metal oxides. The intrinsic enzyme-like activity of nanoparticles (NPs) has received a great deal of interest due to stability against denaturation, low cost and high resistance to high concentration of substrates compared with natural enzymes. Recent advances in NPs as enzyme mimetics and their analytical and environmental applications are reviewed in [26]. Authors [27] reported that AuNPs can catalyze rapid decomposition of hydrogen peroxide. It is demonstrated that AuNPs can act as superoxide dismutase (SOD) and catalase mimetics. Molecular recognition of AuNPs and AgNPs together with other noble metal nanoparticles with respect to DNA, proteins, nucleic acids and molecules from the family of supramolecular chemistry has been described in review [28]. A brief literature survey reveals that the amount of published papers on the topic of gold catalysis increases exponentially. The gold and silver nanoparticles belonging to nano-assembled structures can fall into the category of supramolecularly organized catalysts. Previously we synthesized AuNPs and AgNPs protected by PVP in both aqueous and organic solvents and deposited them on Al2O3 by impregnation method [29, 30]. Their catalytic activities are evaluated with respect to hydrogen peroxide decomposition [31]. In the present communication we stabilized AuNPs and AgNPs by PVP using "one-pot" synthetic protocol. After characterization of AuNPs and AgNPs in solution they were deposited on ZnO substrate and furthermore their catalase-like activity with respect to hydrogen peroxide decomposition was studied. substrate zinc oxide purchased from Sigma-Aldrich were used as received. Solutions of hydrogen peroxide were prepared by volumetric dilution of 50 wt.%H2O2 purchased from Sigma-Aldrich. 2.2 Methods Absorption spectra of AuNPs and AgNPs dispersions in water were recorded at room temperature with UV-Vis spectrophotometer (Specord 210 plus BU, Germany). The size of nanoparticles was determined with the help of Malvern Zetasizer Nano ZS90 (UK). The concentrations of Au and Ag in supernatant were determined by ioncoupled plasma atomic emission spectroscopy ICP-AES "Optima 5100DV" (Perkin Elmer, USA). Scanning electron microscopy (SEM) images were recorded using JEOL JSM 6490LA (Japan). Transmission electron microscope (TEM) measurement was carried out on JEM-1011 (JEOL, Japan). 2.3 Synthesis of AuNPs and AgNPs AuNPs stabilized with PVP were prepared by "one-pot" synthetic protocol [32]. For instance, aqueous solutions of HAuCl4 (5 mL), 0.5 M KOH (4 mL), and 4 wt.% PVP (5 mL) were mixed, stirred and heated up to 100 °C during several minutes. AgNPs stabilized by PVP were synthesized using the procedure described in [33]. For this 1×10-3 mol×L-1 aqueous solution of AgNO3 was added drop-wise to a freshly prepared and cooled to 0°C 2×10-3 mol×L-1 aqueous solution of NaBH4. 4 wt.% aqueous solution of PVP was used as a stabilizing agent. As a result the colored solutions of AuNPs (PVP-AuNPs) and AgNPs stabilized with PVP (PVP-AgNPs) were prepared as shown in Figure 1. 2.4 Depositing AuNPs and AgNPs on ZnO PVP-AuNPs and PVP-AgNPs were deposited on ZnO by impregnation method. For this 0.2 g of ZnO was added to 5 mL of PVP-AuNPs or PVP-AgNPs solutions and stirred during 5 hours. The precipitate was separated by preparative centrifugation using "Eppendorf 5810R" (Germany) at 10×103 rpm, then it was washed 5 times with distilled water. The content of Au and Ag in collected supernatants was determined by the ICP-AES. The amount of AuNPs and AgNPs immobilized on the surface of ZnO did not exceed 0.2 wt%. The precipitate was dried in vacuum oven at 50 °C. The powders of ZnO with supported PVP-AuNPs (ZnO/PVP-AuNPs) and PVP-AgNPs (ZnO/PVP-AgNPs) were used as catalysts for decomposition of H2O2. 2 experimental Section 2.1 Materials Standard aqueous solution of tetrachlorauric acid HAuCl4 with concentration 100 mg×L-1 and silver nitrate AgNO3 was purchased from Sigma-Aldrich. As polymeric stabilizing agent PVP with Mn= 10kD, 40kD, 350kD and as inorganic Figure 1: Samples of AuNPs (1-3) and AgNPs (a-c) stabilized by PVP with Mn = 10 kD (1, a), 40 kD (2, b) and 350 kD (3, c). 2.5 Decomposition of H2O2 Decomposition of H2O2 was carried out in thermostated glass reactor equipped with a magnetic stirrer (Figure 2). Powdered ZnO/PVP-AuNPs (or ZnO/PVP-AgNPs) were dispersed in 1 mL H2O2 and the volume of released oxygen at definite time interval was measured using a burette for measuring the volume of oxygen. The concentration of H2O2 varied from 10 to 40 wt.%. The recyclability of ZnO/ PVP-AuNPs in decomposition of H2O2 was tested at least 5 times. 3 Results and discussion 3.1 Formation and characterization of AuNPs and AgNPs protected with PVP and supported on ZnO Scheme 1 illustrates the synthetic pathway of AuNPs. Boiling of the mixtures of PVP, HAuCl4 and KOH in aqueous solution produces AuNPs. PVP plays the role of both the reducing and stabilizing agent. The detailed mechanism of HAuCl4 reduction in the presence of PVP was described [24]. It was indicated that AuNPs are formed due to the reduction of metal ions by the macromolecular chains. The speculative supramolecular interaction between functional groups of PVP and AuNPs with participation of chemisorbed lactame ring is suggested. The ZnO surfaces supply the anchor sites for the AuNPs. Figure 2: Catalytic reactor for studying of hydrogen peroxide decomposition. 1-thermostated glass reactor, 2-magnetic stirrer, 3- burette for measuring the volume of gases, 4-valve, 5-balancing funnel, 6-thermometer, 7-thermostate. CH N CH2 C O + H A u C l4 KOH 100 o C AuNPs N C S u p ram o l ecu l ar i n t erac ti on Scheme 1: Schematic representation of the supramolecular interaction between PVP and AuNPs. According to [25] the interplanar d spacing of Au {111} (~0.24 nm) is very close to that of ZnO {101} (~0.23 nm). The very small difference of lattice parameters between ZnO and AuNPs can significantly favor the formation of the metaloxide interfaces, which would play a crucial role in decomposition of H2O2. One of the main characteristics of AuNPs and AgNPs is the appearance of absorption spectra in visible region due to the so-called "plasmon resonance" phenomenon. Formation of AuNPs and AgNPs is easily detected due to the appearance of the bands at 520-550 nm and 410-415 nm, respectively (data not shown). Figures 3 and 4 represent the size distributions of AuNPs and AgNPs stabilized with PVP. Depending on the molecular weights of PVP the average sizes of PVPAuNPs increase in the following order: PVP-10kD (~10 nm) > PVP-40kD (~15nm) > PVP-350kD (~25 nm). The average sizes of PVP-AgNPs depend on the molecular weights of PVP and increase in the following order: PVP-10kD (~6.5 nm) > PVP-40kD (~12 nm) > PVP-350kD (~44 nm). Thus the optimal molecular weight of PVP leading to smaller size of AuNPs and AgNPs is PVP-10kD because the smaller metal nanoparticles due to larger surface area for chemical interactions promotes greater catalytic activity. The TEM images clearly show that AuNPs are deposited on ZnO surface (Figure 5). Gold nanoparticles on TEM images are represented by small dark particles while ZnO is shown as the larger particles with less intense color. On the surface of ZnO the shape of AuNPs is close to hemispherical indicating the occurrence of the strong metaloxide interaction effect, which would enhance the catalytic activity of the AuNPs/ ZnO catalyst. The average size of AuNPs deposited on ZnO is 8.2±0.8 nm and is in good agreement with dynamic light scattering (DLS) results (d = 10±2 nm). 3.2 Catalytic activity of ZnO/PVP-AuNPs in decomposition of hydrogen peroxide Kinetics and mechanisms of hydrogen peroxide decomposition in the presence of metal complexes are well described in literature [34]. According to ESR spectroscopy measurements [27] AuNPs generate reactive oxygen species along with formation of hydroxyl radicals at lower and evolution of O2 at higher pHs. We studied the influence of catalyst amount (mcat), concentration of substrate ([H2O2]) and temperature (T) to establish the optimal conditions of H2O2 decomposition in the presence of ZnO/PVP-AuNPs. It should be mentioned that ZnO itself, without immobilized AuNPs, decomposes only 10% of H2O2 during 4 h. At concentration of [H2O2] = 30 wt.% and T = 318K the decomposition rate of hydrogen peroxide increases with increasing of the mcat (Figure 6). As seen from Figure 6 the rate of decomposition of H2O2 at mcat = 30 mg is greater than at mcat = 50 mg. Thus the optimal amount of catalyst for H2O2 decomposition was considered to be 30 mg. Figure 3: Size distributions of AuNPs stabilized by PVP with Mn = 10kD (a) 40kD (b) and 350kD (c). Figure 4. Size distributions of AgNPs stabilized by PVP with Mn = 10kD (a) 40kD (b) and 350kD (c). Figure 5. TEM images of AuNPs protected by PVP-10kD (left) and deposited onto ZnO (right). At mcat = 30 mg, the decomposition rate of hydrogen peroxide gradually increases upon increase of temperature (Figure 7). However the rate of H2O2 decomposition at T = 328K is close to T = 318K, therefore the latter can be considered as an optimal temperature for this process. At T = 318 K and mcat = 30 mg the rate of H2O2 decomposition increases with increase in H2O2 concentration (Figure 8). However at [H2O2] = 40 wt.% a very fast decomposition of hydrogen peroxide taking place within several minutes is not effective for further oxidation of organic substrates. Therefore it is accepted that the optimal concentration of H2O2 is 30 wt.%. Thus the optimal conditions for H2O2 decomposition in the presence of ZnO/PVP-AuNPs were equal to mcat = 30 mg, T = 318K, and [H2O2] = 30 wt.%. The activation energy of H2O2 decomposition calculated from the curves of Figure7 was equal to 44.1 kJ×mol-1 that is lower than MnO2 (58 kJ×mol-1), iodide (56 kJ×mol-1) and the platinum metal catalysts (49 kJ×mol-1) [35] and higher than iron oxide (32.8 kJ×mol-1) [36]. The activation energy of the reaction is about 75 kJ×mol-1 in the absence of a catalyst and is about 7-8 kJ×mol-1 in the presence of catalase [35]. The efficiency of 1 ng of Au to decompose H2O2 was also calculated. Since the amount of AuNPs immobilized on the surface of ZnO was not exceeded 0.2 wt.%, the content of Au in 30 mg of ZnO/PVP-AuNPs catalyst was about 0.06 mg (or 6×104 ng). The maximal amount of released oxygen according to Figures 6-8 was 75-80 mL. Then the ability of 1 ng of Au to produce oxygen was about 1.25-1.50 µL. Figure 9 represents the recyclability of ZnO/PVPAuNPs in decomposition of hydrogen peroxide. The V (O2), mL 5mg 10 mg 30 mg 50 mg Figure 6. Decomposition rate of hydrogen peroxide on ZnO/AuNPsPVP-10kD in dependence of the mcat. T = 318K, [H2O2] = 30 wt.%. V(O 2), m L 90 120 Time, min Figure 7: Decomposition of hydrogen peroxide in the presence of ZnO/AuNPs-PVP-10kD at different temperatures. T = 298 (1), 308 (2), 318 (3), and 328 K (4). mcat = 30 mg, [H2O2] = 30 wt.%. Figure 8: Decomposition rate of hydrogen peroxide in dependence of the H2O2 concentration. The catalyst is ZnO/AuNPs-PVP-10kD. [H2O2] = 40 (1), 20 (2) and 10 wt.% (3). mcat = 30 mg, T = 318 K. Figure 9: The recyclability of ZnO/AuNPs-PVP-10 kD in decomposition of hydrogen peroxide during the first (1), second (2), third (3), fourth (4) and fifth (5) runs. [H2O2] = 30 wt.%, mcat = 30 mg, T = 318 K. achievement of stable catalytic performance is dependent on the nature of the support, interacting or not as well as to the morphology. The successive decomposition of hydrogen peroxide demonstrates the gradual loss of catalytic activity. In our mind either aggregation or leaching out of AuNPs in the course of hydrogen peroxide decomposition reaction is probably responsible for the observed deactivation during consecutive catalytic runs. Earlier [37] the same phenomenon was observed for PVP protected Pd nanoparticles immobilized within the gel matrix of polyacrylamide after hydrogenation of the successive portions of allyl alcohol. Moreover, an inward diffusion of gold nanoparticles within the porous structure of the support may also be responsible of the observed deactivation during consecutive catalytic runs. 298K 308K 318K 3.3 Catalytic activity of ZnO/PVP-AgNPs in decomposition of hydrogen peroxide As in the case with AuNPs, the AgNPs also generate either hydroxyl radicals at lower pH or oxygen molecules at higher pH [38]. The catalytic activity of ZnO/AgNPs-PVP-10kD was also checked with respect to H2O2 decomposition. The PVPAgNPs deposited on ZnO shows a lower catalytic activity than that of PVP-AuNPs. At similar conditions the volume of released oxygen in the presence of ZnO/AgNPs-PVP10kD is two times lower than ZnO/AuNPs-PVP-10kD. As seen from Figure 10 the hydrogen peroxide decomposition curves are slightly temperature-dependent. The catalytic activity of ZnO/AgNPs-PVP depends on the molecular weight of PVP and changes in the following order: ZnO/AgNPs-PVP-40kD > ZnO/AgNPs-PVP-350kD > ZnO/AgNPs-PVP-10kD (Figure 11). Decomposition of H2O2 Figure 10: Decomposition of hydrogen peroxide in presence of ZnO/ AgNPs-PVP-10kD at different temperatures. T = 298 (1), 308(2), and 318K (3). mcat = 30 mg, [H2O2] = 30 wt.%. Figure 11: Decomposition of hydrogen peroxide in presence of ZnO/ AgNPs-PVP at different molecular weight of PVP. Mn = 10kD (1), 350kD (2), 40kD (3). T = 318K, mcat = 30 mg, [H2O2] = 30 wt.%. is too slow and too fast in the presence of ZnO/AgNPsPVP-10kD and ZnO/AgNPs-PVP-40kD, respectively. The optimal molecular weight of PVP seems to be 350kD. As previously mentioned a very fast decomposition of hydrogen peroxide is not effective for further oxidation of organic substrates. 4 Conclusions Polymer-protected gold and silver nanoparticles supported on zinc oxide were used as nanosized supramolecular catalysts in decomposition of hydrogen peroxide. The DLS, SEM and TEM results reveal that the average size of AuNPs and AgNPs stabilized with PVP is varied from 10 to 25 nm and from 6.5 to 44 nm, respectively. It is established that the average sizes of AuNPs and AgNPs stabilized by PVP in aqueous solution increase with increasing molecular weight of PVP. The optimal conditions of H2O2 decomposition in the presence of AuNPs and AgNPs were established. The activation energy of H2O2 decomposition in the presence of ZnO/PVP-AuNPs is equal to 44.1 kJ×mol1 . The gradually loss of catalytic activity was observed in the course of successive decomposition of hydrogen peroxide. Acknowledgements: Authors thank the JSC "National Scientific-Technical Holding "Parasat" for financial support (Grant No.529). Proofreading of this manuscript by Prof. Vitaliy Khutoryanskiy at University of Reading, Reading, UK is greatly acknowledged.
Journal
Supramolecular Catalysis – de Gruyter
Published: Feb 2, 2015