Kanwal Iqbal

A new Ce-doped MgAl-LDH@Au nanocatalyst for highly efficient reductive degradation of organic contaminants

A new type of convenient, environmentally friendly, and recyclable nanocatalyst (abbreviated as MgAlCe-LDH@Au) was designed and successfully assembled by loading Au nanoparticles (Au NPs; ∼3 nm average diameter) on a MgAlCe-LDH support through an in situ reduction of HAuCl4 by NaBH4. The MgAlCe-LDH support was prepared by doping the Magnesium–Aluminum Layered Double Hydroxide (MgAl-LDH) with cerium ions. The obtained MgAlCe-LDH@Au nanocatalyst was fully characterized by conventional methods and possesses excellent properties, such as a narrow size distribution, a high structural stability, a large specific surface area, and a good distribution of the Au NPs. Besides, this nanocatalyst displays a very remarkable activity in the reductive degradation of 4-nitrophenol by NaBH4 with a rate constant (kapp) of 0.041 s−1 and a catalyst turnover frequency (TOF) of 1.2 × 106 h−1; the reactions proceed in aqueous medium at room temperature and atmospheric pressure. The MgAlCe-LDH@Au nanocatalyst can also be recycled, maintaining its original activity even after seven consecutive reaction cycles. Additionally, MgAlCe-LDH@Au is a highly efficient catalyst for the reductive degradation (discoloration) of common organic dyes, including methylene blue, methyl orange, Congo red, rhodamine B, and rhodamine 6G, resulting in up to 3.2 × 104 h−1 values of TOFs. For comparative purposes, a related Ce-free MgAl-LDH@Au material was assembled and tested as the catalyst. The superior activity of MgAlCe-LDH@Au over MgAl-LDH@Au or MgAlCe-LDH can be explained by the following factors: (1) LDH itself can act as a co-catalyst and the doping of MgAl-LDH with cerium ions increases the charge separation efficiency of surface electrons; (2) Ce ions can strongly interact with Au atoms, modifying their electronic structure, stabilizing the oxidation states, and enhancing the fixation of Au NPs and their dispersion. Furthermore, the achieved catalytic activity of the MgAlCe-LDH@Au nanocatalyst is significantly superior when compared with other state-of-the-art systems for the degradation of similar types of organic contaminants.

A new multicomponent CDs/Ag@Mg–Al–Ce-LDH nanocatalyst for highly efficient degradation of organic water pollutants

A novel type of multicomponent and recyclable nanocatalyst CDs/Ag@Mg–Al–Ce-LDH was successfully assembled by functionalizing a magnesium–aluminum layered double hydroxide support doped with Ce (denoted as Mg–Al–Ce-LDH) with carbon dots (CDs) and silver nanoparticles (Ag NPs). The selection of CDs was governed by their ability to act as both the stabilizing and the reducing agent, allowing accomplishing an in situ reduction of Ag+ ions to Ag NPs at the surface of the Mg–Al–Ce-LDH matrix. The obtained CDs/Ag@Mg–Al–Ce-LDH nanocatalyst and the related CDs@Mg–Al–Ce-LDH and Ag@Mg–Al–Ce-LDH materials were fully characterized by standard methods. The catalytic activities of all these materials for the reduction of 4-nitrophenol (4-NP) and discoloration of common organic water pollutants were investigated in detail. Among the tested heterogeneous catalysts, the multicomponent CDs/Ag@Mg–Al–Ce-LDH nanocatalyst revealed an excellent catalytic performance. All the reactions are very quick and proceed in aqueous medium under ambient conditions. Such a high activity of CDs/Ag@Mg–Al–Ce-LDH could be explained by a significant synergic effect between CDs and Ag NPs, while the Mg–Al–Ce-LDH carrier itself acts as a co-catalyst wherein cerium ions also increase charge separation efficiency between surface electrons. In addition, the CDs/Ag@Mg–Al–Ce-LDH nanocatalyst can also be easily recovered and successfully reused in several consecutive reaction cycles. This CDs/Ag@Mg–Al–Ce-LDH nanocatalyst thus represents one of the most efficient and recyclable systems so far reported for the reduction of similar kinds of organic water pollutants.

Hybrid Metal–Organic-Framework/Inorganic Nanocatalyst toward Highly Efficient Discoloration of Organic Dyes in Aqueous Medium

Nanoscale metal-organic frameworks (NMOFs) represent a unique class of solids with superior adsorption, mass transport, and catalytic properties. In this study, a facile and novel approach was developed for the generation of hybrid Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst through in situ self-assembly and solvothermal synthesis of a 2D Cu-NMOF, [Cu2(μ-OH)(μ4-btc)(phen)2] n·5 nH2O {H3btc, trimesic acid; phen, 1,10-phenanthroline}, on a cerium-doped Mg-Al layered double hydroxide (Ce-doped-Mg-Al-LDH) matrix. Self-assembly between Cu-NMOF nanocrystals and exfoliated LDH led to their nanoscale mixing and prevented the formation of aggregated Cu-NMOF nanoparticles. In the resulting hybrid nanostructure, Cu-NMOF nanocrystals (∼10-20 nm particle size) are anchored uniformly on a Ce-doped-Mg-Al-LDHs surface, possessing a dimension of several hundred nanometers. Catalytic activity of Cu-NMOF/Ce-doped-Mg-Al-LDH and Cu-NMOF was evaluated under ambient conditions in the reductive degradation (discoloration) of aqueous solutions of 4-nitrophenol (4-NP, model substrate) and a series of commercial organic dyes by applying sodium borohydride as a reducing agent. The Cu-NMOF/Ce-doped-Mg-Al-LDH nanocatalyst exhibited an outstanding catalytic activity toward degradation of 4-NP, with kapp (rate constant) of 0.03 and a catalyst TOF (turnover frequency) up to 7.1 × 103 h-1. Full and very quick discoloration of organic dyes {rhodamine B (RhB), methylene blue (MB), Congo red (CR), methyl orange (MO), and rhodamine 6G (R6G)} was also achieved with TOF values of up to 1.4 × 105/h. A superior activity of the hybrid nanocatalyst over Cu-NMOF can be regarded as a synergic effect among Cu-NMOF and Ce-doped-Mg-Al-LDH components, while the Ce-doped-Mg-Al-LDH carrier acts as a cocatalyst. The hybrid nanocatalyst can easily be recovered and reused successfully for the five consecutive reaction runs with the same catalytic performance. This study also shows that NMOFs can be easily incorporated onto conventional catalyst supports, resulting in hybrid nanocatalysts with a highly uniform structural architecture, controlled chemical composition, and excellent catalytic function.