Layered double hydroxide supported cobalt nanocluster: size control and the effect in catalytic hydrogen generation

Aishah Mahpudz,Hitoshi Inokawa,Katsuki Kusakabe,Siu Ling Lim,Ryuichi Tomoshige,A Nzihou,N.A Amran,B Abdullah,D.C.W Tsang,R Boopathy,M Nasef,S Yusup

doi:10.1051/e3sconf/202128702009

Abstract

Synthesizing metal nanoclusters with diameters smaller than 5nm is challenging, but desirable because of the high ratio of surface area to interior atom. However, in this report it was achieved by utilizing magnesium-aluminium layered double hydroxide (Mg/Al-LDH) as a host for cobalt citrate anion precursor, which was later reduced into cobalt nanoclusters (Co-NC). Size of the Co-NC was controlled by changing the concentration of cobalt-citrate (Co-citrate) precursor during anion exchange. XRD and FTIR showed that Co-citrate precursor was successfully intercalated on the LDH while nitrogen adsorption/desorption isotherms confirmed that mesopores in the sample were formed after chemical reduction. Furthermore, TEM/STEM observations confirmed the formation of Co-NC. It was also verified that reducing the concentration of Co-citrate from 4mM to 0.5mM resulted in a reduction in the size of Co-NC from 4.4 to 1.3 nm. However, catalytic hydrogen generation from sodium borohydride (NaBH4) hydrolysis experiment indicated that catalytic activity decreased as the size of Co-NC decreases. This is mainly attributed to the limitation in mass transport within the interlamellar space of the smaller cluster LDH compared to the bigger one. Overall, Co-NC-LDH is a promising catalyst for NaBH4 hydrolysis. However, an optimum Co-NC size is critical for enhanced catalytic activity.

Highlights

NaBH4 is considered as an important hydrogen (H2) storage chemical because of its high H2 storage capacity, high purity of H2 generation, and stability in alkaline solution [1]
The peak (0 0 3) appearing at 2θ =10° corresponds to a basal spacing d(003) of 0.88nm when calculated from Bragg’s equation. This value is in good agreement with the d(003) of a NO3¬ type of Mg/Al-layered double hydroxides (LDHs) as reported in literature [12]
Compared to the (NO3-)-LDH, after anion exchange a general trend is observed on all four cases where the peak (0 0 3) and (0 0 6) shifted to a lower 2θ angle which indicates an increase in basal spacing

Summary

Introduction

NaBH4 is considered as an important hydrogen (H2) storage chemical because of its high H2 storage capacity, high purity of H2 generation, and stability in alkaline solution [1]. For aqueous NaBH4 stored in alkaline solution usage of suitable catalyst is required to enable control of the hydrolysis reaction. This is because the rate of self-hydrolysis can be almost negligible. Among various metal catalysts that have been applied to facilitate this reaction, cobalt based catalyst is a good candidate because of its high activity [2]. Much work is still necessary to develop Co-based catalyst that is effective as well as stable for industrial use [6]

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Conclusion