Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

Summary

Presented here is a protocol for the synthesis of silver-palladium (Ag-Pd) alloy nanoparticles (NPs) supported on ZrO₂ (Ag-Pd/ZrO₂). This system allows for harvesting energy from visible light irradiation to accelerate and control molecular transformations. This is illustrated by nitrobenzene reduction under light irradiation catalyzed by Ag-Pd/ZrO₂ NPs.

Abstract

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles (NPs) can accelerate and control the selectivity of a variety of molecular transformations. This opens possibilities for the use of visible or near-IR light as a sustainable input to drive and control reactions when plasmonic nanoparticles supporting LSPR excitation in these ranges are employed as catalysts. Unfortunately, this is not the case for several catalytic metals such as palladium (Pd). One strategy to overcome this limitation is to employ bimetallic NPs containing plasmonic and catalytic metals. In this case, the LSPR excitation in the plasmonic metal can contribute to accelerate and control transformations driven by the catalytic component. The method reported herein focuses on the synthesis of bimetallic silver-palladium (Ag-Pd) NPs supported on ZrO₂ (Ag-Pd/ZrO₂) that acts as a plasmonic-catalytic system. The NPs were prepared by co-impregnation of corresponding metal precursors on the ZrO₂ support followed by simultaneous reduction leading to the formation of bimetallic NPs directly on the ZrO₂ support. The Ag-Pd/ZrO₂ NPs were then used as plasmonic catalysts for the reduction of nitrobenzene under 425 nm illumination by LED lamps. Using gas chromatography (GC), the conversion and selectivity of the reduction reaction under the dark and light irradiation conditions can be monitored, demonstrating the enhanced catalytic performance and control over selectivity under LSPR excitation after alloying non-plasmonic Pd with plasmonic metal Ag. This technique can be adapted to a wide range of molecular transformations and NPs compositions, making it useful for the characterization of the plasmonic catalytic activity of different types of catalysis in terms of conversion and selectivity.

Introduction

Among the several applications of metal nanoparticles (NPs), catalysis deserves special attention. Catalysis plays a central role in a sustainable future, contributing to less energy consumption, better utilization of raw materials, and enabling cleaner reaction conditions^1,2,3,4. Thus, progress in catalysis can provide tools for enhancing the atomic efficiency of chemical processes, making them cleaner, more economically viable, and more environmentally friendly. Metal NPs encompassing silver (Ag), gold (Au) or copper (Cu) can display interesting optical properties in the visible range that arise from the unique way these systems interact with light at the nanoscale via the localized surface plasmon resonance (LSPR) excitation^5,6,7,8. In these NPs, referred to as plasmonic NPs, the LSPR comprises the resonant interaction between the incident photons (from an incoming electromagnetic wave) with the collective motion of electrons^5,6,7,8. This phenomenon takes place at a characteristic frequency which is dependent on the size, shape, composition, and dielectric constant of the environment^9,10,11. For example, for Ag, Au, and Cu, these frequencies can range from the visible to the near-IR, opening up possibilities for the utilization of solar energy to excite their LSPR^{5,6,7,8,12,13}.

Recently, it has been demonstrated that the LSPR excitation in plasmonic NPs can contribute to accelerate the rates and control the selectivity of molecular transformations^{5,14,15,16,17,18,19}. This gave birth to a field called plasmonic catalysis, which focus on using energy from light to accelerate, drive, and/or control chemical transformations^{5,14,15,16,17,18,19}. In this context, it has been established that the LSPR excitation in plasmonic NPs can lead to the formation of energetic hot electrons and holes, referred to as LSPR-excited hot carriers. These carriers can interact with adsorbed species through electronic or vibrational activation^15,16. In addition to increased reaction rates, this process can also provide alternative reaction pathways not accessible via traditional thermochemically-driven processes, opening up new avenues for the control over reaction selectivity^{20,21,22,23,24,25}. Importantly, it is worth noting that the plasmon decay can also lead to thermal dissipation, leading to a temperature increase in the vicinity of the NPs which can also contribute to speed up reaction rates^15,16.

Due to these interesting features, plasmonic catalysis has been successfully employed towards a variety of molecular transformations¹⁸. Nevertheless, an important challenge remains. While plasmonic NPs such as Ag and Au display excellent optical properties in the visible and near-IR ranges, their catalytic properties are limited in terms of the scope of transformations. In other words, they do not display good catalytic properties for several of transformations. Additionally, metals that are important in catalysis, such as palladium (Pd) and platinum (Pt), do not support LSPR excitation in the visible or near-IR ranges. To bridge this gap, bimetallic NPs containing a plasmonic and catalytic metal represents an effective strategy^{20,26,27,28,29}. In these systems, the plasmonic metal can be employed as an antenna to harvest energy from the light excitation through the LSPR, which is then used to drive, accelerate, and control molecular transformations at the catalytic metal. Therefore, this strategy enables us to extend plasmonic catalysis beyond traditional plasmonic metal NPs^{20,26,27,28,29}.

This protocol describes the facile synthesis of bimetallic silver-palladium (Ag-Pd) alloyed NPs supported on ZrO₂ (Ag-Pd/ZrO₂) that can act as a plasmonic-catalytic system for plasmonic catalysis. The Ag-Pd/ZrO₂ NPs were prepared by co-impregnation of the corresponding metal precursors on the ZrO₂ support followed by simultaneous reduction³⁰. This approach led to the formation of bimetallic NPs around 10 nm in size (diameter) directly at the surface of the ZrO₂ support. The NPs were composed of 1 mol% of Pd to minimize the utilization of the catalytic metal while maximizing the optical properties of the resulting Ag-Pd NPs. A protocol for the application of the Ag-Pd/ZrO₂ NPs in plasmonic catalysis was demonstrated for the reduction of nitrobenzene. We employed 425 nm LED illumination for the LSPR excitation. Gas chromatography was performed to monitor the conversion and selectivity of the reduction reaction under the dark and light irradiation conditions. LSPR excitation led to enhanced catalytic performance and control over selectivity in Ag-Pd/ZrO₂ NPs relative to purely thermally driven conditions. The method described in this protocol is based on a simple photocatalytic reaction setup coupled with gas chromatography and can be adapted to a wide range of molecular transformations and NPs compositions. Thus, this method makes possible the characterization of photocatalytic activity, in terms of conversion and reaction selectivity, of different NPs and for a myriad of liquid-phase transformations. We believe this article will provide important guidelines and insights to both newcomers and more experienced scientists in the field.

Protocol

1. Synthesis of Ag-Pd/ZrO₂ NPs

NOTE: In this procedure, the Pd mol% in Ag-Pd corresponded to 1%, and the Ag-Pd loading on ZrO₂ corresponded to 3 wt.%.

1. Place 1 g of ZrO₂ powder in a 250 mL beaker.
2. Add 50 mL of an AgNO₃ (aq) (0.0059 mol/L) and 9.71 mL of a K₂PdCl₄ (aq) (0.00031 mol/L) solutions to the beaker under vigorous magnetic stirring (500 rpm) at room temperature.
3. Add 10 mL of lysine (0.53 M) aqueous solution.
4. Keep the mixture under vigorous stirring (500 rpm) for 20 min.
5. After 20 min, use a pipette to add to the suspension 10 mL of a freshly prepared NaBH₄ (aq) (0.035 M) solution dropwise, at a rate of 1 mL/min. Keep the suspension under stirring (500 rpm) throughout the process.
6. Let the mixture stir for 30 min at room temperature.

2. Separation and purification of the catalyst

1. Transfer the suspension to centrifuge tubes and separate the solids from the mixture by centrifugation at 3,260 x g for 10 min.
2. Carefully remove the liquid phase with a pipette and add 15 mL deionized water to the tubes.
   1. Shake vigorously until dispersion of the solid is obtained. If good dispersion is not achieved, place the tubes in an ultrasonic bath for 5 min.
   2. Centrifuge the dispersion at 3,260 x g for 10 min.
3. Repeat the washing steps (2.2. to 2.2.2.) two more times using deionized water, then once using ethanol instead of water.
4. Remove the ethanol and dry the solid in an oven at 60 °C for 12 h.
5. Characterize the prepared Ag-Pd/ZrO₂ NPs by a variety of microscopy, elemental, and spectroscopic techniques.

3. Synthesis of Ag/ZrO₂ NPs

NOTE: In this procedure, Ag loading on ZrO₂ corresponded to 3 wt.%.

1. Place 1 g of ZrO₂ powder in a 250 mL beaker.
2. Add 50 mL of an AgNO₃ (aq) (0.0059 mol/L) solution to the beaker under vigorous magnetic stirring (500 rpm) at room temperature.
3. Add 10 mL of lysine (0.53 M) aqueous solution.
4. Keep the mixture under vigorous stirring (500 rpm) for 20 min.
5. After 20 min, use a pipette to add to the suspension 10 mL of a freshly prepared NaBH₄ (aq) (0.035 M) solution dropwise, at a rate of 1 mL/min. Keep the suspension under stirring (500 rpm) throughout the process.
6. Let the mixture stir for 30 min under room temperature.

4. Separation and purification of the catalyst

1. Transfer the suspension to centrifuge tubes and separate the solids from the mixture by centrifugation at 3,260 x g for 10 min.
2. Carefully remove the liquid phase with a pipette and add 15 mL deionized water to the tubes.
   1. Shake vigorously until the dispersion of the solid is observed. If good dispersion is not achieved, place the tubes in an ultrasonic bath for 5 min.
   2. Centrifuge the dispersion at 3,260 x g for 10 min.
3. Repeat the washing steps (4.2. to 4.2.2.) two more times using deionized water, then once using ethanol instead of water.
4. Remove the ethanol and dry the solid in an oven at 60 °C for 12 h.
5. The prepared Ag/ZrO₂ NPs can then be characterized by a variety of microscopy, elemental, and spectroscopic techniques.

5. Investigation of plasmonic catalytic performance towards the nitrobenzene reduction under LSPR excitation (light illumination)

1. Place 30 mg of catalyst in a 25 mL round-bottom flask along with a magnetic stirring bar.
2. Add 5 mL of a solution of nitrobenzene (0.03 mol/L) in isopropyl alcohol (IPA) to the reactor.
3. Then, add 11.22 mg of KOH powder (0.0002 mol).
4. Purge the reactor by bubbling the suspension with an argon flow for 1 min. Immediately after purging, seal the flask.
5. Place the reactor in an oil bath heated at 70 °C above a temperature-controlled magnetic stirrer (500 rpm).
6. Irradiate the tube using 4 LED lamps with a wavelength of 425 nm as the light source, and a light intensity of 0.5 W/cm². The distance from the lamps to the reaction flask should be 7 cm.
7. Let the reaction proceed for 2.5 h at 70 °C under vigorous magnetic stirring (500 rpm).
8. Then, turn the light off, open the reactor and use a syringe and a needle to collect a 1 mL sample. Filter it through a 0.45 μm filter, to remove the catalyst particulates, into a gas chromatography vial.

6. Reaction in the absence of LSPR excitation (dark conditions)

1. Follow the same steps as described in 5, but without light irradiation. Wrap the reaction tube with aluminium foil to prevent any light exposure.

7. Gas chromatography (GC) analysis preparation

1. Prepare an IPA solution containing approximately 30 mmol/L nitrobenzene (NB), 30 mmol/L of aniline (AN), and 30 mmol/L of azobenzene (AB).
2. Run a GC analysis of the solution using a suitable method. Different methods can be tested by varying the column temperature and gas flow programs. The selected method should be able to separate the peaks corresponding to IPA, NB, AN, and AB in the minimum period of retention time.
3. Once the method has been selected, prepare a set of solutions of 50 mM, 25 mM, 10 mM, 5 mM and 2.5 mM NB in IPA, and another set of solutions of AN and AB in IPA with the same concentrations.
4. Run a GC analysis of the prepared solutions. Each chromatogram should present 2 peaks: the higher one corresponds to IPA and the lower one corresponds to NB, AN, or AB. For each chromatogram, note down the retention time and peak area of all the peaks.
5. Trace the calibration curves of NB, AN, and AB by plotting the concentration versus peak area of each sample.

8. GC analysis

1. Run a GC analysis on the samples collected in steps 5. and 6. with the same method used for steps 7.2. and 7.4.
2. For each chromatogram, note down the retention time and peak area and use the calibration curves plotted previously to determine the concentration of NB, AN, and AB in the samples.
3. Calculate the nitrobenzene conversion as well as the aniline and azobenzene selectivity using the equations:
   
   
   
   Where is the initial NB concentration (0.03 mol/L), and C_NB, C_AN, C_AB  correspond to the NB, AN, and AB concentrations, respectively, after 2.5 hours reaction by the GC analysis.

Results

Figure 1A shows digital photographs of the solid samples containing the pure ZrO₂ oxide (left) and the Ag-Pd/ZrO₂ NPs (right). This change in color from white (in ZrO₂) to brown (Ag-Pd/ZrO₂) provides the initial qualitative evidence on the deposition of Ag-Pd NPs at the ZrO₂ surface. Figure 1B shows the UV-visible absorption spectra from the Ag-Pd/ZrO₂ NPs (blue trace) as well as ZrO₂ (...

Discussion

The findings described in this method demonstrate that the intrinsic catalytic activity of Pd (or other catalytic but not plasmonic metal) can be significantly enhanced by LSPR excitation via visible-light irradiation in bimetallic alloyed NPs³⁵. In this case, Ag (or another plasmonic metal) is capable of harvesting energy from visible-light irradiation via LSPR excitation. The LSPR excitation leads to the formation of hot charge carriers (hot electrons and holes) and localized heating...

Materials

Name	Company	Catalog Number	Comments
2-Propanol (anhydrous, 99.5%)	Sigma-Aldrich	278475	CAS Number 67-63-0
Aniline (for synthesis)	Sigma-Aldrich	8.22256	CAS Number 62-53-3
Azobenzene (98%)	Sigma-Aldrich	424633	CAS Number 103-33-3
Ethanol	Honeywell	32221	CAS Number 64-17-5
Hydrochloric acid (37%)	VWR	PRLSMC310066	CAS Number 7647-01-0
L-Lysine (crystallized, ≥98.0% (NT))	Sigma-Aldrich	62840	CAS Number 56-87-1
Nitric acid (65%)	Merck	100456	CAS Number 7697-37-2
Nitrobenzene	Sigma-Aldrich	8.06770	CAS Number 98-95-3
Potassium hydroxide	Fisher	10448990	CAS Number 1310-58-3
Potassium tetrachloropalladate (II) (98%)	Sigma-Aldrich	205796	CAS Number 10025-98-6
Silver nitrate (ACS reagent, ≥99.0%)	Sigma-Aldrich	209139	CAS Number 7761-88-8
Sodium borohydride (fine granular for synthesis)	Sigma-Aldrich	8.06373	CAS Number 16940-66-2
Zirconium (IV) oxide (nanopowder, <100 nm particle size (TEM))	Sigma-Aldrich	544760	CAS Number 1314-23-4