MXene quantum dots decorated Ni nanoflowers for efficient Cr (VI) reduction
Graphical Abstract
Introduction
Chromium (Cr) is widely used in the chemical, pharmaceutical and textile industries (Pellerin and Booker, 2000, Ellis et al., 2002, Saha et al., 2011, Farooqi et al., 2021). Cr generally exists in two oxidation states of Cr (VI) and Cr (III). The toxicity of Cr (VI) is about one hundred times that of Cr (III), which has been proved to be harmful to humans beings and the environment (Stearns et al., 1995, Elliott and Zhang, 2001). However, Cr (III) is nontoxic, relatively stable, and is an essential nutrient for the human body (Giannakas et al., 2013, Josue et al., 2020). Therefore, it is highly expected to explore an effective and economical treatment process to reduce Cr (VI) in wastewater. Nowadays, the technologies for reducing Cr (VI) to Cr (III) include photocatalysis (Sharma et al., 2019b, Sharma et al., 2019a, Kumar et al., 2021), membrane separation (Hafez et al., 2002), biological process (Zakaria et al., 2007), precipitation (Almeida and Boaventura, 1998), and ion exchange (Kocaoba and Akcin, 2002). However, most of these approaches generally suffer from low efficiency, high cost and large chemical dosage. The chemical reduction method has recently gained a noticeable popularity in removing Cr (VI) due to its low cost and high efficiency (Dandapat et al., 2011). As a green chemical, HCOOH has a strong reducing ability and is a promising reductant for Cr (VI) reduction (Park et al., 2017, Islam et al., 2019). Nevertheless, there are still large kinetic obstacles in the reduction of Cr (VI) by HCOOH, which requires effective catalysts to promote the reaction kinetics.
Pd-based catalysts have been widely used for Cr (VI) reduction, but their large-scale commercial applications are plagued by the reserve scarcity, high cost and unsatisfactory stability. Alternatively, earth-abundant Ni-based catalysts have recently attracted considerable interest for catalytic Cr (VI) reduction due to their tunable morphology, favorable catalytic properties, high stability and ease of synthesis (Bhowmik et al., 2014, Park et al., 2019). Nevertheless, most reported Ni catalysts are shaped in nanoparticles (NPs) which are usually unstable and have a strong tendency to aggregate (Carenco et al., 2010, Ali et al., 2020), leading to a decrease in catalytic activity. A feasible way to address this issue is to decorate Ni NPs on various support materials, such as graphene (Bhowmik et al., 2014, Julkapli and Bagheri, 2015), polydopamine (Bakirci et al., 2017), metal-organic frameworks (MOF) (Meek et al., 2011) and carbon dots (de Medeiros et al., 2019), thereby avoiding NP agglomeration and boosting the catalytic activity. As a new generation of two-dimensional (2D) materials, MXenes have attracted a substantial attention due to their excellent conductivity, large surface area and plentiful surface hydrophilic groups (-OH, -O, or -F) (Xue et al., 2017, Shuck et al., 2020, Li et al., 2021). Impressively, MXene quantum dots (MQDs), a new class of QD materials derived from layered MXenes (Xue et al., 2017), have emerged as promising electrode materials for diverse applications of bioimaging, catalysis, sensing, and energy storage (Shuck et al., 2020, Manikandan et al., 2019, Neupane et al., 2021). MQDs not only inherit the intrinsic properties of MXenes, but also exhibit intriguing physicochemical properties owing to quantum confinement effects, rending MQDs promising support material for decorating metal NPs (Rafieerad et al., 2020, Rafieerad et al., 2019). However, the investigations on the synthesis of metal NPs@MQDs and their catalytic activities have been rarely reported.
In this work, we prepared novel flower-like Ni@MQD hybrids through a simple reduction process and examined Ni@MQDs as a catalyst to reduce Cr (VI) to Cr (III). The structures and catalytic properties of the Ni@MQDs were systematically studied. Furthermore, the catalytic Cr (VI) reduction mechanism over Ni@MQDs was profoundly investigated by density functional theory (DFT) calculations.
Section snippets
Synthesis of Ni@MQDs
MQDs were prepared according to the previous report (Xue et al., 2017), and the details are described in Section 1.4 of the Supporting information. The Ni@MQD hybrids were prepared through the chemical reduction method. Briefly, an amount of Nickel (Ⅱ) nitrate was dissolved directly in ethylene glycol. Then, 10 mL Ni (NO3)2·6H2O solution (45 mM) was mixed with 10 mL MQDs aqueous suspension in a 50 mL round bottom flask, followed by adding 1.125 g of hydrazine hydrate into the solution under
Characterizations of Ni@MQDs
The TEM images of MQDs shown in Figs. 1a–b and S1 indicate that the prepared MQDs are spherical and well dispersed on a carbon membrane. The HRTEM image of MQDs (Fig. 1b, inset) displays clear lattice fringes with an interplane spacing of 0.193 nm, which can be indexed to (101) plane of Ti3C2 (Neupane et al., 2021). The average size of MQDs is estimated to be 5.96 nm (Fig. 1c). Fig. 1d, e show the TEM images of Ni@MQDs at different magnifications. Visibly, Ni nanoflowers are uniformly dispersed
Conclusions
In summary, we have successfully prepared Ni@MQDs by a facile reduction reaction. The TEM investigations clearly revealed that Ni nanoflowers were uniformly coated with ultra-thin MQDs layers. On the other hand, Ni@MQDs had a lower activation energy (Ea = 18.9 kJ mol−1) and a higher kinetic constant (k = 0.4779 min−1), showing the excellent Cr (VI) reduction catalytic performance, which could be ascribed to their large surface areas and more active sites. DFT calculations further revealed that
CRediT authorship contribution statement
Yali Guo: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing. Yonghua Cheng: Validation, Resources, Data curation, Visualization, Writing – original draft, Writing – review & editing. Xingchuan Li: Resources, Investigation, Data curation, Visualization. Qingqing Li: Software, Data curation, Visualization. Xiaotian Li: Validation, Investigation, Resources. Ke Chu: Formal analysis, Writing – original draft, Writing – review &
Declaration of Competing Interest
We declare no competing financial interest.
Acknowledgements
This work was supported by the Youth Science Foundation of Lanzhou Jiaotong University (2019001), and Natural Science Foundation of Gansu Province (21JR1RA247, 20JR10RA241).
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