Metal dependent catalytic hydrogenation of nitroarenes over water-soluble glutathione capped metal nanoparticles

Highlights

• 2.4 nm glutathione capped Pd, Pt, Au, and Ag NPs were synthesized.

• Catalytic hydrogenation of nitroarenes over these NPs found to metal dependent.

• Metal dependence attributed to different rates of H₂ adsorption/dissociation.

• Less dense packed glutathione molecules render reactants to access NP surface.

Abstract

The water soluble glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) with size 2.4 ± 0.2 nm were synthesized by borohydride reduction of metal ions in the presence of glutathione as capping ligand and used as catalyst for the hydrogenation of nitroaniline in aqueous phase. The rate of catalytic hydrogenation was dependent on metal type and the trend of catalytic activity over these M-GS nanoparticles was found to be Pd-GS (k_app = 0.0227 (±3 × 10⁻⁴)) s⁻¹ ≫ Pt-GS (k_app = 0.0043 (±1 × 10⁻⁴)) s⁻¹ > Au-GS (k_app = 0.0015 (±0.2 × 10⁻⁴)) s⁻¹ > Ag-GS (k_app = 0.0008 (±0.2 × 10⁻⁴)) s⁻¹. The similar trend of catalytic activity was found for the hydrogenation of nitrobenzene. Our experimental results, along taking into account the theoretical calculations done by other research groups, suggest that the observed catalytic activity trend is attributed to the “different rates of H₂ molecule adsorption and dissociation” on the M-GS nanoparticles. The “high rate of H₂ molecule adsorption” and “highly oxidized surface” make Pd-GS nanoparticles an ideal candidate for the rapid hydrogenation. On the basis of our experimental results, we proposed that small gaps between less densely packed branched thiol “glutathione molecules” provide the access to metal nanoparticle surface for the hydrogenation reaction.

Introduction

Noble metal nanoparticles have drawn immense research interest owing to their importance in catalysis [1] , sensing [2], surface-enhanced Raman scattering (SERS) [3] and biomedical applications [4]. The large surface area of ultrafine noble metal nanoparticles makes them an ideal candidate for catalyzing various reactions. The hydrogenation of nitroarenes to aniline derivatives catalyzed by noble metal nanoparticles is an important reaction from industrial, synthetic and environmental point of view. The functionalized aniline derivatives are important intermediates for pharmaceutical polymers, dyes, pigments and other fine agrochemicals [5], [6], [7], [8], [9]. The nitroarenes are also found as contaminants in industrial waste water and hence, hazardous to the environment. Therefore, it becomes necessary to reduce them to industrial chemicals and intermediates in order to clean the environment.

The hydrogenation of nitroarenes to corresponding amino benzenes catalyzed by nanoparticles immobilized on oxide supports such as AuTiO₂/Fe₂O₃/MgO [10], [11], AgTiO₂ [12] and AgrGO [13] has been studied vastly in the literature. However, the hydrogenation of nitroarenes over ligand capped fine metal nanoclusters and nanoparticles are scantily reported. Recently, ligand capped novel metal clusters [14], [15] have been explored as catalyst for a number of important reactions. Very recently, Fenger and co-workers [16] studied size dependent catalysis of Au nanoparticles, respectively for the hydrogenation of 4-nitrophenol. However, most of the literature on the catalytic hydrogenation of nitroarenes is still limited to Au nanoparticles and to the best of our knowledge, there is no report yet on the metal dependent catalysis by ligand capped metal nanoparticles. Thus, it becomes important to gain better understanding of metal dependence catalysis by ligand stabilized metal nanoparticles.

In present work, firstly, glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) of same size were synthesized and then, their catalytic activity toward the hydrogenation of nitroarenes was compared. Herein, we report the metal dependent catalytic activity of water soluble glutathione (GSH) capped Pd, Pt, Au and Ag nanoparticles for the hydrogenation of nitroarenes. The trend of catalytic activity of the glutathione capped metal (M-GS) nanoparticles was found to be following: Pd-GS ≫ Pt-GS > Au-GS > Ag-GS. Our experimental results suggest that the observed catalytic activity trend is attributed to different rates of H₂ molecule adsorption and dissociation. However, high rate of H₂ adsorption along with high amount of surface oxidation on Pd-GS nanoparticles make them ideal catalyst for rapid hydrogenation of nitroarenes.