Synergistic Effects of GoldâPalladium Nanoalloys ... - ACS Publications

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Synergistic effects of Gold-Palladium nanoalloys and reducible supports on the catalytic reduction of 4-nitrophenol Ndzondelelo Bingwa, Rapelang Patala, Jihyang Noh, Matumuene Joe Ndolomingo, Siyamthanda Tetyana, Semakaleng Bewana, and Reinout Meijboom Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00903 • Publication Date (Web): 24 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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Synergistic effects of Gold-Palladium nanoalloys and reducible supports on the catalytic reduction of 4-nitrophenol Ndzondelelo Bingwa, Rapelang Patala, Ji-Hyang Noh, Matumuene J. Ndolomingo, Siyamthanda Tetyana, Semakaleng Bewana, Reinout Meijboom* Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa, [email protected]

Abstract Herein we report on the catalytic activity of mesoporous nickel, iron, cerium, cobalt and manganese oxides prepared using KIT-6 as a hard-template via evaporation assisted precipitation. The mesoporous metal oxides (MMOs) were characterized and used as heterogeneous catalysts in the reduction of 4-nitrophenol (4-Nip) by sodium borohydride (‫ܪܤ‬ସି ). Furthermore, polyamidoamide (PAMAM) dendrimers were used to synthesize Gold-Palladium nanoalloy particles. The size of AuPd/PAMAM was found to be 3.5 ± 0.8 nm in diameter before being immobilized on the aforementioned mesoporous metal oxides and used as catalysts in the reduction of 4-Nip. Prior to catalytic evaluation, the reduction profiles of the mesoporous metal oxides were investigated by hydrogen-temperature programmed reduction (H2-TPR), and showed that mesoporous metal oxides can be easily reduced at lower temperatures and that the immobilization of Gold-Palladium nanoalloy particles lowers their reduction temperatures. Mesoporous cobalt and manganese oxides showed catalytic activity towards 4-Nip reduction and the activity was enhanced after immobilization of the Gold-Palladium nanoalloys. Isolation of nanoparticles activity was achieved by immobilization of the Gold-Palladium nanoalloys on the inert silica support. From this we postulated an electron relay mechanism for the reduction of 4nitrophenol. With the use of power rate law we showed that 4-Nip reduction follows pseudo-first order kinetics. Keywords: Mesoporous metal oxides, dendrimers, 4-nitrophenol, electron relay mechanism, power rate law.

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1. Introduction Nitrophenols are classified as one of the leading environmental pollutants. The 4-Nip is one of the pollutants released in wastewater from agricultural and manufacturing industries.1 It attracted interest from academia because its reduction is regarded as a model reaction because of formation of only one product. Hence, this reaction can be easily executed and can be monitored with simple spectroscopic techniques. However, the reaction mechanism has been subject of investigation for some time. Ballauff group reported a surface reaction of adsorbed reactants on the reactive sites of nanoparticles embedded in polyelectrolyte brushes.2-4 Their results were confirmed by fitting of the experimental kinetic data using the Langmuir-Hinshelwood mechanism. We also showed that the reduction of 4-Nip by ‫ܪܤ‬ସି is a surface reaction with the use of the same Langmuir-Hinshelwood model, using Ru, Au, Ag, Pt, and Pd nanoparticles encapsulated in PAMAM dendrimer framework.5-8 The work of Mahmoud et al. proved that the reaction is purely heterogeneous. With the use of hollow Au nanotubes, they were able to demonstrate that there is no leaching of atoms or ions from the surface of the nanoparticles.9 Another study that proved 4-Nip reduction follows a heterogeneous-type mechanism was reported by our group using dendrimer-encapsulated Au nanoparticles.10 We reported that colloidal Au nanoparticles placed inside the dialysis membrane bag showed good catalytic activity even after the third catalytic run without any leaching, confirming a purely heterogeneous reaction. The available literature on 4-Nip reduction mostly details catalysis by metal nanoparticles in solution. Very few studies reports heterogeneous catalysis for this reaction. Even those studies that report 4-Nip reduction on heterogeneous catalysts lack in mechanistic details. Only studies with colloidal nanoparticles have be successful in detailing mechanistic information of the 4-Nip reduction on the catalyst surface. The use of catalytic supports makes it possible to study kinetics of nanoparticles in heterogeneous catalysis. This report looks at the effect of immobilizing GoldPalladium nanoalloy particles on catalytically active mesoporous metal oxides in the reduction of 4-Nip by ‫ܪܤ‬ସି as a model reaction. It is important to note that throughout this report the term support refers to the material used to immobilize nanoparticles. Palladium and Gold nanoparticles have been widely employed as catalysts for hydrogenation of nitrophenols. Many studies have employed these nanoparticles as nano-heterogenous catalysts.11-16 Our previous


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reports and reports from the Ballauff group have detailed in broader perspective that the reduction of 4-Nip takes place at the surface of the catalyst and thus can be modeled by the Langmuir-Hinshelwood kinetic model.3-7 We shift our focus to mesoporous metal oxides as potential catalysts and as catalytic supports for 4-Nip reduction. The ordered reducible mesoporous metal oxides have attracted interest in academia in the last few years. Their properties such as tunable surface area and pore volume, and nanocrystalline walls lead to a variety of applications in different fields such energy, separation, molecular sensing,17 and catalysis.18 These mesoporous metal oxides can be prepared mainly by two methods. The first method is the soft-template method and the second is the hard-template method, often referred to as nanocasting.19 The former resembles the synthesis of the first ordered mesoporous silica, and the latter is a templating method. A number of mesoporous metal oxides prepared by nanocasting were applied as catalysts and later as catalytic supports for Pt nanoparticles in carbon monoxide oxidation.20 The report showed that the mesoporous metal oxides were poor catalysts when compared to Pt nanoparticles, and that Pt nanoparticles supported on mesoporous metal oxides exhibit enhanced catalytic activity. These materials are not limited to oxidation reactions, they can also be applied as catalysts in hydrogenation reactions. Mandlimath et al. reported enhanced catalytic activity of Cu and Fe oxides when moving from bulk oxide to mesoporous oxide in the reduction of 4-Nip.21 Recently, Vivek et al. reported activity of NiO supported on CeO2 surface and the in-situ generation of Ni that results in enhanced reaction rates in 4-Nip reduction.22 It has been known for quite some time that the size and shape of nanoparticles play a crucial role in catalysis.23 The use of the 3-D poly(amidoamine), PAMAM, dendrimer helps obtain monodispersed and well defined nanoparticles.24 With the use of PAMAM dendrimer, the size of nanoparticles can be predetermined.25 Thus, one can aim to synthesize nanoparticles of desired size. Also the catalytically active surface atoms are less passivated when compared with nanoparticles synthesized by other methods that require stabilizers. Many stabilizers such as PVP have been reported to be ligating and this results in the majority of the surface atoms being passivated, and hence reduced catalytic activity. In this work, we make use of PAMAM dendrimers as templating and stabilizing agents for the synthesis of Gold-Palladium dendrimerencapsulated bimetallic nanoparticles (AuPd-DENs). In our previous reports, we successfully 3 ACS Paragon Plus Environment

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demonstrated the synthesis of monometallic Au and Pd nanoparticles using dendrimers as templates and stabilizing agents.5-6,

8

We now expand the synthesis of nanoparticles using

dendrimers to bimetallic systems, scheme 1.

Scheme 1. Illustration of the synthesis of dendrimer-encapsulated Gold-Palladium nanoalloys. There are a number of reports on the synthesis of bimetallic nanoparticles of Au and Pd using PAMAM dendrimers and other techniques.26-29 Ozturk et al. studied the decomposition pattern of PAMAM dendrimer and reported that temperatures as high as 400 °C are required for complete removal of dendrimers.30 Another study conducted by Lang et al. also showed using in-situ infrared spectroscopy that the complete removal of the dendrimer takes place at temperature above 300 °C.31 An interesting observation of increase nanoparticle size upon the removal of the dendrimer for supported catalysts has been reported.26, 32 This phenomenon results in decreased surface areas, thus, decrease in catalytic activity. The presence of the dendrimer in supported encapsulated nanoparticles plays a crucial role in retaining the size of nanoparticles as minimal change in particle diameters is often observed prior the removal of the dendrimer.32 However, the presence of a bulky dendrimer framework can significantly affect kinetic measurements of a catalytic reaction. The possibilities of diffusion controlled reaction cannot be ruled out. Although diffusion studies have been conducted in liquid-phase 4-Nip reduction using Pd55-DENs and showed no diffusion limitations,5 it is important to investigate this for heterogeneous catalysts as well if the dendrimer is not completely removed. Herein we report the effect of mesoporous metal oxides as catalysts and as catalytic supports for bimetallic Au and Pd nanoalloy particles in the reduction of 4-Nip by ‫ܪܤ‬ସି . We also investigate the synergistic effect of the mesoporous metal oxides and the immobilized nanoalloys.


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2. Experimental 2.1. Chemicals and instruments Generation 6 (5 wt.% in methanol) PAMAM dendrimers with hydroxyl terminal groups (G6 PAMAM-OH) were purchased from Dendritech Inc. Sodium hydroxide (NaOH) (99.8%) was purchased from Merck. Hydrochloric acid (HCl 32%) was purchased from Associated Chemical Enterprise (PTY) Ltd. Sodium borohydride (NaBH4) (≤99%) was purchased from Fluka. Potassium tetrachloropalladate(II) (K2PdCl4) (≤98%), chloroauric acid (HAuCl4) (99.9%), pluronic P-123 (EO20PO70EO20), n-butanol (BuOH), cobalt nitrate hexahydrate (99%), cerium nitrate (99%), manganese nitrate (99%) and tetraorthosilicate (TEOS) (≥99.0%), and toluene were all purchased from Sigma Aldrich. All chemicals were of analytical grade and used as received. Milli-Q (18 MΩ·cm) de-ionized water was used in all experiments. All pH measurements were performed using an ORION model 520A Schott pH blueline 25 electrode. All pH adjustments were performed using NaOH (0.1 M) and HCl (0.1 M). Catalytic monitoring was done by using a Shimadzu UV1800 UV-Vis spectrophotometer. 2.2. Preparation of AuPd-DENs nanoparticles Gold-Palladium bimetallic nanoparticles were synthesized using the method adapted from literature.33-34 Briefly, 10 µM G6-OH aqueous dendrimer solution was prepared by evaporating methanol from a predetermined volume of the PAMAM dendrimer stock solution, then sufficient amounts of deionized water were added. A 28 molar excess of Pd2+ (0.1 M) was added. Thereafter, 28 molar excess of Au3+ (0.1 M) was added and allowed to form a complex with the dendrimer. Thirdly, ten-fold excess of NaBH4 (0.1 M) was added to reduce the metal ions to nanoparticles. Finally, the nanoparticles were purified by dialysis against 3 liters deionized water. 2.3. Synthesis of mesoporous metal oxides KIT-6 was prepared using the method previously described by Lebed et al.35 Briefly, 9.0 g of Pluronic P-123 (EO20PO70EO20) was dissolved in 330 ml of de-ionized water and 17.5 ml of HCl under vigorous stirring. After Pluronic P-123 had dissolved, 9.0 ml of n-butanol was added dropwise. The resulting mixture was stirred at 308 K for an hour. Tetraorthosilicate, TEOS, 19.4 5 ACS Paragon Plus Environment

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ml was added and the resulting mixture was stirred at 308 K for 24 hours. The mixture was then aged at 353 K for 48 hours without stirring. The white powder was filtered and dried at 373 K. The pluronic P-123 surfactant was removed by calcination for 2 hours at 823 K. Prior to calcination, the white powder was dispersed in a mixture of ethanol and aqueous HCl (32%) (50/30 v/v). Mesoporous metal oxides were prepared by the hard-template method. The experimental procedure was adapted from the work of An et al.20 Briefly, 16 mmol of Co, Ni, Fe, Mn, and Ce nitrate salts were dissolved in 8 ml of de-ionized water in 25 ml Erlenmeyer flasks. The metal nitrate solutions were added to 300 ml beakers containing 4 g of KIT-6 in 50 ml toluene and stirred at 338 K. After all toluene evaporated, the resulting precipitate was dried at 373 K and further calcined at 823 K for 6 hours. To remove the template, the samples were washed with hot 2 M sodium hydroxide and dried thereafter. 2.4. Preparation of mesoporous metal oxide supported Gold-Palladium alloys The immobilization of dendrimer-encapsulated Gold-Palladium nanoalloys was achieved by dispersing 1.2 mL of the colloidal nanoalloys in water and mixing it with 0.5 g of the solid support. The slurry was then sonicated, separated by centrifuge and dried. Thereafter, these catalysts were dried at 100 °C. 2.5. Catalysts characterization Prior to catalytic evaluation, the catalysts together with the hard template were characterized with a Rigaku MiniFlex600 powder X-ray difractometer. The samples were dried and pulverized prior to analysis with Cu Kα1 radiation (λ = 0.1542 nm). High angle measurements were carried out between 10 and 90°, while low angle was carried out between 0.5 and 10°. Matching of the crystallographic information was performed using Match! 2 software.36 The TEM images were obtained from a Joel-Jem 2100F electron microscope operating at 200 kV coupled with INCA energy dispersive X-ray analysis (EDX). Prior to analysis the samples were dispersed in ethanol, sonicated for 30 minutes and deposited on a carbon coated copper grids and dried at room temperature. The size distribution of the nanoparticles was determined by imageJ software.37 Nitrogen adsorption-desorption measurements were carried out in a Micromeritics Tristar surface area and porosity analyzer. Samples of about 30 mg were weighed and degassed with nitrogen 6 ACS Paragon Plus Environment

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gas at 90 °C for 12 hours prior to analysis. The pore volume, pore diameter, and BET surface area measurements were carried out at -196 °C using nitrogen gas. Pore volume and pore diameter were evaluated from the adsorption-desorption branches using the BJH model. Temperature-programmed reduction and oxidation measurements were carried out in a Micromeritics Autochem II chemisorption analyzer. For TPR the measurements were carried out between 25 and 900 °C, with temperature ramp of 10 °C per minute using a mixture hydrogen and argon with a ratio of 1:9. The TPO measurements were carried out between 30 and 650 °C with temperature ramp of 5 °C per minute using a mixture of oxygen and helium with a ratio of 1:9. The metal loading of the immobilized Gold-Palladium nanoalloys was determined by Spectro Acros inductive coupled plasma-optical emission spectroscopy (ICP-OES) FSH 12 instrument. Various concentrations of Gold standard solutions used to obtain the calibration curve were diluted in aqua-rigia solution (1:3 v/v of HNO3 : HCl). Approximately 15 mg of the Gold containing catalysts were dissolved in 2.0 ml aqua-rigia solution and filled to 10 ml with deionized water. The specific surface area and size of the immobilized Gold-Palladium nanoalloys were determined by H2 chemisorption using a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. A Shimadzu ICPMS-2030 was used for catalyst leaching study. After the completion of the reaction, the supernatant was analyzed to determine the amount of Au and Pd leached from the catalyst. 2.6. Catalytic evaluation In a 200 ml round bottomed flask, 1.2 ml of 60 mM 4-Nip stock solution was added to 97.7 ml of de-ionized water and 2.085 ml of 0.1 M NaBH4 was added to the mixture with stirring. The final volume made by the reactants and water in the reactor was 100 ml. After stirring for a while the initial absorbance value was recorded. Thereafter, 10 mg of the mesoporous oxide catalysts were added and the change in absorbance was recorded in 5 minutes intervals using a Shimadzu UV1800 spectrophotometer. The same procedure was repeated for nanoparticles loaded mesoporous metal oxides catalysts. 3. Results and discussions 3.1. Powder X-ray diffraction


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The low angle diffraction pattern confirmed the highly crystalline nature of the templating KIT-6 and of the resulting mesoporous metal oxides. This is evidenced by small diffraction peaks observed below two degrees 2ϴ, figure 1(a). The wide angle diffraction peaks observed, figure 1(b), indicate that the mesoporous iron oxide is trigonal and takes a hematite form with space group R-3c (167) (JCPDS 96-900-9783), mesoporous cerium with cerianite form with space group Fm3m (225) in a cubic form (JCPDS 96-900-9009), mesoporous manganese oxide has a JCPDS 96-900-3477 and nickel oxide is cubic with a space group Fm-3m (225) (JCPDS 96-1010382). 3.2. Nitrogen sorption measurements The synthesized KIT-6 and all the synthesized mesoporous metal oxides showed type IV hysteresis loops, shown in figure 1(c). This type of isotherm shows the mesoporous nature of the materials. To ascertain that indeed the metal oxides were prepared inside the KIT-6, the pore diameter of the synthesized metal oxides appears to be larger than the pores of the templating KIT-6 due to the thicker wall structure of KIT-6. The pore volume of the resulting mesoporous metal oxides appeared to decrease due to the difficulty in filling the pores of templating KIT-6 during synthesis.19, 38 This is due to factors such as hydrogen bonding, Coulombic interations, coordination interactions, and the Van der Waals interactions.38 Table 1 gives the structural properties of the templating KIT-6 and its mesoporous metal oxide replicas obtained from nitrogen sorption measurements. Since the mesoporous metal oxide structure is interconnected by small bridges originating from the presence of disordered micropores between one dimensional mesopores of KIT-6, otherwise the structure would collapse and form nanowires, there might be different pore sizes in the material, see figure S1 in the supplementary information. The pore size distribution confirms the presence of different pores in the material. This is based on the fact that the pore size distribution does not correspond to the average pore size. To account for this and to simplify it, the cumulative pore distribution curve, figure 1(d), was used to get the correct average. This kind of analysis gives a good estimation of the average pore diameter by correlating the 50% of the pore volume to the pore diameter. From these results we can conclude that the nitrogen sorption results are valid. Table 1. Structural properties of KIT-6 and mesoporous metal oxides.


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Entry

SBET (m2/g)

Pore volume Vp (cm3/g)

Pore diameter dp (nm)

KIT-6

872

0.7

4.9

Co3O4

80

0.2

8.3

CeO2

130

0.4

9.8

MnO2

18

0.1

30.9

Fe2O3

109

0.4

12

NiO2

58

0.3

22.5

Figure 1. Powder XRD patterns for KIT-6 and corresponding mesoporous metal oxides. Graph (a) shows low angle measurements and (b) high angle measurements. Graphs (c and d) show type IV hysteresis loops of nitrogen sorption measurements and cumulative pore diameter plots, respectively.


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3.3.Temperature-programmed oxidation and Thermal analysis Investigation into the decomposition temperature of the dendrimer indicates that the temperature at which the catalysts were dried is not high enough for the removal of the dendrimer. Combustion in of the materials before and after immobilization of the dendrimer in TPO study suggests that the dendrimer is completely removed at temperatures above 300 °C. This result is complemented by analysis using thermal analysis (TGA). In the TGA analysis, pure Co3O4 oxide appears to be fairly stable at high temperatures (100-300 °C). These results are in agreement with those reported by Ozturk et al.30 Comparison of the thermograms, pure and AuPd-DENs, show the dendrimer composing around 280-350 °C and its complete removal above 500 °C. These two technique prove that the as-prepared catalysts have AuPd-DENs on the surface of the mesoporous metal oxide supports. The TPO and TGA plots are given in Figure S1 in the supplementary information. 3.4. Transmission electron microscopy The size revealed by TEM for the unsupported nanoalloy particles was 3.5 ± 0.8 nm in diameter. The particles’ narrow size distribution is shown in Figure 2(b), suggesting the importance of the dendrimer template. The mesoporous nature of the templating KIT-6 can also be seen from TEM image in Figure 2(c), evidenced by the pore structures and the channels when viewed from an angle. After obtaining mesoporous metal oxides by removal of the template using hot sodium hydroxide, the structures of all the mesoporous metal oxides resembled the templating KIT-6 with larger pore sizes, Fe2O3 is shown in figure 2(d) and the remaining mesoporous metal oxides can be seen in Figure S2 in the supplementary information. It can be observed from the TEM images that the templating KIT-6 has pores and the resulting mesoporous oxides are composed of interpenetrating bicontinuous structures taking the shape of the pores of the template. This is evidenced by channels observed in TEM images of the mesoporous metal oxides. However, the degree of order in the mesoporous metal oxides decreases after incorporation of the GoldPalladium nanoalloys, see Figure S3 in the supplementary information. One of the reasons for such can be the possibility of structure destruction during sonication when immobilizing the nanoparticles.


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Figure 2. TEM image of (a) Gold-Palladium alloy and (b) corresponding particle size distribution. (c) TEM image of the templating KIT-6 used in the synthesis of (d) mesoporous Fe2O3. (e) TEM image of Gold-Palladium alloys immobilized on mesoporous Fe2O3. (f) Illustration of hard-template approach for the synthesis of mesoporous metal oxides and immobilization of dendrimer-encapsulated nanoparticles. It is important to note the difficulty in observing and measuring the nanoparticles after immobilization on the mesoporous supports. Nanoparticles images suffer from low contrast due to decreasing electron transparency of the support. The difference in focal planes of the nanoparticles and the support when acquiring images adds to the difficulty of measuring the particles.39 The Gold-Palladium nanoalloys formed by the dendrimer-template approach are too small to be imaged properly on the mesoporous supports.


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In order to colerate the stability of the as prepared nanoparticles after immobilization on the support, H2 chemisorption method was used for AuPd/Co3O4 catalyst. Previous studies showed that H2 chemisorption is a proper method to determine the surface areas and the crystallite sizes of the supported metal nanoparticles.40-41 The isotherm for H2 chemisorption on AuPd/Co3O4 catalyst is shown in Figure S5. The surface areas of the immobilized Gold-Palladium nanoalloys were revealed to be 94.4 m2/g of metal and 2.5 m2/g of sample. The high surface areas may provide better dispersion of the active sites and easy diffusion of the reactants and, therefore, makes the as-prepared catalyst suitable for catalytic activity. This is in agreement with the catalytic results obtained in the reduction of 4-Nip. The size of the mono-dispersed Gold-Palladium nanoalloy particles for the AuPd/Co3O4 catalyst was found to be 3.6 nm in diameter using H2 chemisorption technique. Notwithstanding the fact that the specific surface area is the essential information acquired from the H2 chemisorption method, it is worth comparing the obtained immobilized particle sizes with those obtained by TEM before immobilization. A good agreement was found between the particle sizes of the Gold-Palladium nanoalloy obtained before and after immobilization. The sizes of the Gold-Palladium nanoalloy particles before immobilization as determined from TEM (3.5 nm) do not differ significantly with those determined by H2 chemisoprtion after immobilization (3.6 nm). Both particles sizes, before and after immobilization deviate in the order of 4.8%. This implies the stability of the GoldPalladium nanoalloy and also this can be attributed to the presence of the dendrimer in the final catalytic material. 3.5. Temperature-programmed reduction Characterization of the synthesized catalysts with hydrogen temperature-programmed reduction (H2-TPR) revealed that for spinel metal oxides, the first reduction signal is often assigned to the most electropositive cation. In the spinel cobalt oxide, the Co3+ ions are octahedraly coordinated, while the Co2+ ions are tetrahedraly coordinated. The Co3+ species is highly electropositive, thus it can be easily reduced, meaning its reduction can take place at lower temperatures as compared to its Co2+ counterpart which is less electropositive. The band just before 400 °C can be attributed to reduction of Co3+ species to Co2+ while the band around 620 °C can be attributed to 12 ACS Paragon Plus Environment

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the reduction of Co2+ to metallic cobalt. Manganese oxide showed one broad band at 360 °C with a shoulder at 390 °C, which can be seen as a band, corresponding reduction of MnO2 to Mn2O3 and Mn2O3 to MnO, respectively.42 Cerium oxide exhibited two reduction peaks, at 520 and 700 °C. The first peak can be assigned to the reduction of Ce4+ to Ce3+, with the last peak being further reduction of the Ce3+ species.43 Table 2. Respective reduction temperatures for all mesoporous metal oxides and the corresponding Gold-Palladium nanoalloy loaded mesoporous metal oxides. Entry

Reduction temperature (°C)

Reduction temperature (°C)

Empty MMOs

AuPd loaded MMOs

1st

2nd

1st

2nd

‫݋ܥ‬ଷ ܱସ

375

620

331

610

‫ܱ݊ܯ‬ଶ

360

390

311

386

‫ܱ݁ܥ‬ଶ

520

700

510

689

‫݁ܨ‬ଷ ܱସ

427

627

365

612

ܱܰ݅

369

363

It appears as if the presence of nanoparticles on the surface of the mesoporous metal oxide lower the reduction temperature for most mesoporous metal oxides. Figure 3 shows reduction profiles of the synthesized mesoporous manganese and iron oxides and Gold-Palladium nanoalloy loaded on them. The initial reduction peak at 350 °C shifts to lower temperature, 311 °C, after the immobilization of Gold-Palladium nanoalloys. Furthermore, the second peak which was initially at 400 °C shifted to 386 °C. The summary of the effect of immobilization of the alloys on all mesoporous metal oxides used can be seen in Table 2 and the corresponding TPR traces can be seen in Figure S6.


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Figure 3. H2-TPR results showing changes in reduction temperatures of a) mesoporous manganese oxide and b) iron oxide when Gold-Palladium nanoalloys are immobilized on them. 3.6. Catalytic reactions and mechanistic study The catalytic reduction of 4-nitrophenol was monitored at λ 400 nm. The original 4-nitrophenol peak is at λ 317 nm and it shifts to λ 400 nm upon addition of a base.13 The disappearance of the 4-nitrophenolate peak at λ 400 nm signifies that 4-nitrophenolate is being reduced and the appearance of 4-aminophenolate peak at λ 290 nm signifies its formation, Figure S7. This reaction is known to only take place in the presence of suitable catalyst and is believed to follow Langmuir-Hinshelwood kinetics.4 It has also been reported that in the catalyzed reduction of 4-Nip by ‫ܪܤ‬ସି the oxidation of ‫ܪܤ‬ସି takes place on the surface of the metal nanoparticles and thus transfers electron onto the metal surface for 4-Nip reduction. We made use of metal oxides that are known to be electron conductors to study their effect in enhancing catalytic activity when used as catalytic supports. Based on the results obtained experimentally, pure mesoporous cobalt and manganese oxides were catalytically active in 4-Nip reduction. While cerium, nickel and iron oxides showed no catalytic activity, Figure 4.


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Figure 4. Absorbance versus time plots showing the reduction of 4-Nip with time catalyzed by mesoporous metal oxides and Gold-Palladium nanoalloy loaded mesoporous metal oxides at 25 °C. With the two catalytically active mesoporous metal oxides, manganese oxide showed better catalytic activity compared to cobalt oxide. We also observed a synergistic effect of the mesoporous metal oxides and the nanoparticles. This is evidenced by increasing observed rates from Gold-Palladium nanoalloys immobilized on silica to Gold-Palladium nanoalloys immobilized on mesoporous metal oxides, shown in Table 3. Silica was used as a reference support because it is an inert support. To the best of our knowledge no investigation has reported the use of nanoparticles immobilized on active supports. Table 3. Results of catalytic reduction of 4-Nip showing observed rates, kobs, activity of the catalysts normalized to the moles of mesoporous metal oxides and Gold content in the nanoparticles, K, and comparison to literature values for 4-Nip reduction conducted at the same temperature, 25 °C. Catalyst

kobs (s-1) Without Au

kobs (s-1) Au containing catalysts

Au content (µmol)

K=kobs/mol MMOa (s-1molMMO)

K=kobs/mol Au b (s-1molAu-1)

Co3O4

0.004

0.056

1.8

90

31111


Reference

This work

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MnO2

0.020

0.060

1.6

1501

37500

This work

CeO2

0

0.114

1.1

0

103636

This work

Fe2O3

0

0.040

1.0

0

22380

This work

NiO

0

0.047

2.1

0

22381

This work

Sil100

0

0.016

0.9

0

17778

This work

‫ ݑܣ‬− ‫݁ܨ‬ଷ ܱସ

-

0.011

1.9

-

5789

44

‫݁ܨ‬ଷ ܱସ − ‫ݑܣ‬

-

1.7 × 10-4

0.4

-

425

45

‫ܱݎܼ@ݑܣ‬ଶ

-

0.008

284

-

29

46

@‫ݑܣ‬/‫ܱ݁ܥ‬ଶ

-

-

-

-

0.09c

47

‫ݑܣ‬ /‫ܱ݁ܥ‬ଶ @‫ܱ݁ܥ‬ଶ

-

0.013

91

-

141

48

a

Normalized rates, rate per mole of the mesoporous support. b normalized rates, rate per mole of Au. c value estimated by authors

in the cited reference.

The activity of nanoparticles on silica was found to be 17778 s-1·mol·Au-1. However, when the same nanoparticles are immobilized on mesoporous metal oxides, the activity increased by twofold to as high as six-fold. This proves that the reduction of 4-Nip by ‫ܪܤ‬ସି is purely heterogeneous. This is supported by the fact that the solid metal oxide support also determines the rate at which electrons are relayed from ‫ܪܤ‬ସି to the substrate. The electron relay mechanism is preceded by the adsorption of both the substrate and the reducing agent onto the surfaces of both the active support and the nanoparticles where the oxidation of ‫ܪܤ‬ସି takes place to form the borate ions and electrons that take part in the reduction of 4-Nip. The enhanced reaction rates observed when nanoparticles are immobilized on transition metal oxides, which most of them are classified as good electron conductors, is due to their ability to relay electrons to the substrate in additions to those relayed by nanoparticles. The proposed reaction pathway is similar to that postulated by Wang et al.49 and El-Bahly50 where both reports suggested electron relay via the metal nanoparticles from ‫ܪܤ‬ସି to 4-Nip. Similarly, Li et al. also postulated a similar reaction pathway where electrons deposited by ‫ܪܤ‬ସି are transferred to the substrate from the metal surface via a modified polymer.51 Also because the hydrolysis of ‫ܪܤ‬ସି is faster on the metal nanoparticles surface, the hydrogens generated in this process might spill over to the metal oxide surface and react with the adsorbed substrate, hence


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high reaction rates for mesoporous metal oxide immobilized nanoparticles. Scheme 2 illustrates the electron-transfer mechanism.

Scheme 2. Illustration of reaction pathway showing conduction of electrons by the metal oxide support based on Langmuir-Hinshelwood kinetic model and an electron relay mechanism. 3.7. Reaction kinetics Prior to investigating parameters relating to kinetics of the reaction, such as reaction orders, effect of substrate and reducing agent concentrations on reaction rates, it was important to establish whether the reaction is a catalytic reaction or not. Variation of the catalyst amount in the reaction was performed using AuPd/Meso-MnO2 oxide catalyst. The observed rate increased with increasing amount of catalyst, figure 5. Thus, it can be concluded that the reduction of 4Nip by ‫ܪܤ‬ସି is a catalytic reaction. Thereafter, the effect of substrate concentration was investigated. The results obtained show a significant decrease in the observed rate with increasing 4-Nip concentration. Wunder et al. reported the same trend in homogeneous phase 4Nip reduction using nanoparticles embedded in polyelectrolyte brushes, and argued that 4-Nip occupies more active sites at higher concentrations and starves hydrogen adsorption sites at the surface of the catalyst, resulting in slower reaction rates.4 While on the other hand, an increase in ‫ܪܤ‬ସି concentration resulted in increased rates. To support the above arguments on surface coverages of the substrate and reducing agent, we have previously reported on the adsorption parameters of this reaction. We reported that 4-Nip adsorbs more than ‫ܪܤ‬ସି at the surface of Ru,7 17 ACS Paragon Plus Environment

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Pd,5, 8 Ag,6 and Au 6 nanoparticles. Figure 5 also shows the effects of 4-Nip, ‫ܪܤ‬ସି , and catalyst concentrations on the reaction rates.

Figure 5. Dependence of observed rates on (a) 4-Nip concentration at constant catalyst and ‫ܪܤ‬ସି concentrations, (c) ‫ܪܤ‬ସି concentration at constant catalyst and 4-Nip concentrations, and (e)


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catalysts concentration at constant 4-Nip and ‫ܪܤ‬ସି concentrations. Graphs (b, d and f) are double logarithmic plots for determination of reaction order with respect to each species in the reaction. As reported in our previous reports and in reports from other groups,3-7 4-Nip reduction in the presence of excess ‫ܪܤ‬ସି as a reducing agent follows pseudo-first order kinetics. This is evidenced by the linear fit in the natural logarithm of absorbance versus time graph. Figure 6 shows the linear plots obtained from reduction using empty and AuPd loaded manganese oxide, and Figure S8 in the supplementary information shows first order plots of the remaining empty and nanoparticles filled mesoporous oxides.

Figure 6. Linear plots showing pseudo first order behavior of 4-Nip reduction by ‫ܪܤ‬ସି . These plots have been widely reported by different groups, including ours.5,

7-8

To further

ascertain that the reaction follows pseudo-first order kinetics, log-log plots in Figure 5 were obtained and showed close correlation with the results obtained in the ln versus time graphs. The log-log plots enabled the determination of rate law according to equation 1. ‫ݎ‬ൌ

−݀ሾܰ݅‫݌‬ሿ ݀‫ݐ‬


(1)

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Equation 1 can be simply expressed by introduction of concentrations of all reactants taking part in the reaction. Equation 2 shows equation 1 expressed by concentrations of all species present in the reaction. ‫ ݎ‬ൌ ݇ሾܰ݅‫݌‬ሿ௠ ሾ‫ܪܤ‬ସି ሿ௡ ሾܿܽ‫ݐ‬ሿ௢

(2)

Where [cat] is the concentration of the catalyst, and m, n, and o are the reaction orders with respect to 4-Nip, ‫ܪܤ‬ସି , and catalyst concentration, respectively. Analysis with the power rate law indicates that the reaction orders are -1.13 ± 0.23, 1.69 ± 0.33, and 0.85 ± 015 with respect to the concentration of 4-Nip, ‫ܪܤ‬ସି , and the catalyst concentrations, respectively. These values were obtained from the slopes of the log-log plots shown in figure 5. Thus, the complete rate law can be written as equation 3. ‫ ݎ‬ൌ ݇ሾܰ݅‫݌‬ሿିଵ.ଵ ሾ‫ܪܤ‬ସି ሿଵ.଻ ሾܿܽ‫ݐ‬ሿ଴.ଽ

(3)

It is important to note that the negative sign in the order of the reaction with respect of the 4-Nip concentration signifies the disappearance of the 4-nitrophenolate during the reaction. Taking into consideration the error associated with the slope values obtained, the orders of the reaction are first order with respect to each species in the reaction. The diffusion limitation study relating to the presence of the dendrimer on the final catalytic material was investigated using the second Damkhöler constant, DaII. This type of analysis takes into account the diffusion of reactants through the dendrimer in this case and the catalytic turnover on the active nanoparticles. Diffusion limitation occurs when the rate of reactants diffusion onto the active surface is slower than the catalytic turnover. The Damkhöler constant, DaII, is calculated using equation 4. ‫ ܫܫܽܦ‬ൌ

݇௔௣௣ ሾ4 − ܰ݅‫݌‬ሿ௡ିଵ ߚܽ

(4)

Where kapp, [4-Nip], n, β, and a are the apparent rate constant, 4-Nip concentration, the reaction order with respect to [4-Nip], the mass transport coefficient, and area of the interface, respectively. The term β is defined as the diffusion coefficient divided by the length, δ, in which diffusion takes place. The previously calculated diffusion constant for 4-Nip is 6.92×10-10 m2/s.52 Using the conditions set for the catalytic investigation at room temperature and the diameter of the PAMAM dendrimer, the DaII obtained are far less than unity and, thus, disqualifies the 20 ACS Paragon Plus Environment

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possibility of diffusion limited reactions. This then suggest that the dendrimer does not affect the kinetic of 4-Nip reduction when the as-synthesized catalysts are used. 3.8. Catalyst stability test The stability of the AuPd/Co3O4 catalyst was investigated by analyzing the reaction supernatant using ICP-MS. The was no detectable amount of metallic Au or Pd in the supernatant. This suggests good catalytic stability of the supported AuPd nanoalloys. It further indicates that the reduction of 4-Nip is not driven by leached active species, thus, it is purely a surface reaction.10 4. Conclusions We have successfully demonstrated the synthesis of various mesoporous metal oxides using the hard-template method. Mesoporous metal oxide exhibited large pore diameters compared to the hard templating KIT-6, owing to the thicker wall structure of KIT-6. Mesoporous cobalt and manganese oxides exhibited catalytic activity when employed as catalysts in the reduction of 4Nip by ‫ܪܤ‬ସି . Furthermore, we have successfully demonstrated the synthesis of bimetallic gold and palladium nanoparticles using the dendrimer-templating method. Nanoparticles synthesized by the dendrimer-templating method have narrow size distribution and are stable for a long period of time. The catalytic activity was enhanced by immobilization of Au and Pd alloys on the mesoporous metal oxides. An electron relay mechanism is proposed for the enhanced reaction rates when nanoparticles are immobilized on mesoporous metal oxides. The shifting of reduction temperatures to lower temperatures after the immobilization of nanoparticles supports the basis of an electron relay mechanism. 5. Acknowledgements This work is based on the research supported in part by the National Research Foundation of South Africa (Grant specific unique reference number (UID) 85386). We would like to thank the University of Johannesburg for funding and working space. We would also like to thank Dr. Meyer and Mr. Harris of Shimadzu South Africa for the use of their instruments. 6. References


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