Unprotected and interconnected Ru ⁰ nano-chain networks: advantages of unprotected surfaces in catalysis and electrocatalysis†

Abstract

Surfactant- and support-free metallic, interconnected and unprotected Ru nano-chain networks are synthesized and screened for catalytic nitro arene hydrogenation and OER studies. Their excellent catalytic and electrocatalytic activities are due to the advantages of having unprotected Ru ⁰ surfaces.

Abstract

Seedless, surfactantless and support-free unprotected, metallic, interconnected nano-chain networks of ruthenium nanoparticles (NPs) were successfully synthesized via the reduction of ruthenium( iii) chloride (RuCl ₃) with sodium borohydride (NaBH ₄) at three different temperatures, viz. 30 °C, 45 °C and 60 °C. The molar ratio of RuCl ₃ solution and borohydride was optimized to be 1 : 1.5 to produce stable colloids with the optimum final solution pH of 9.7 ± 0.2. Average diameters of the interconnected nano-chain networks prepared at 30 °C (Ru-30), 45 °C (Ru-45) and 60 °C (Ru-60) were 3.5 ± 0.5 nm, 3.0 ± 0.2 nm and 2.6 ± 0.2 nm respectively. The morphology and composition dependent catalytic and electrocatalytic activities of these unprotected Ru nano-chain networks (Ru-30, Ru-45 and Ru-60) were studied in detail. The catalysis study was performed by investigating the transfer hydrogenation of several substituted aromatic nitro compounds. It was observed that Ru-60 was relatively more active compared to Ru-30 and Ru-45, which was reflected in their rate constant values. The electrocatalytic activities of Ru-30, Ru-45 and Ru-60 were screened for anodic water splitting in alkaline medium (0.1 M NaOH) and it was found that all of them showed almost the same activity which required an over-voltage of 308 ± 2 mV to obtain an anodic current density of 10 mA cm ^–2. The catalytic and electrocatalytic performances of these unprotected Ru ⁰ networks were compared with Ru ⁰ nanomaterials prepared under similar conditions with three different surfactants, viz. CTAB, SDS and TX-100, which revealed that unprotected Ru ⁰ networks are better catalysts than those stabilized with surfactants. The superior catalytic and electrocatalytic performance is due to the availability of unprotected Ru ⁰ surfaces. The present route may provide a new possibility of synthesizing other surfactant-free, unprotected metal colloids for enhanced catalytic and electrocatalytic applications.

Notes

†Electronic supplementary information (ESI) available: Information on reagents, instrumentation and analytical techniques employed are elaborated. Detailed calculation of conversion, selectivity, yield, TON, TOF and the rate constants for catalytic nitroarenes hydrogenation is given. TOF calculation for the electrocatalytic water splitting is explained. Calibration curves for finding the real concentrations of the nitro compounds as Fig. S1A–F and the corresponding concentration are provided as Table S1. Details on the construction of calibration curves are given. XRD and EDS spectra are given as Fig. S2 and S3A–C. The detailed concentration of nitroarenes taken for catalytic study and other reaction parameters are tabulated as Table S2. The comparative interpretation for the catalytic activity of unprotected Ru ⁰ nano-chain networks with the Ru catalysts in other forms and noble metal catalysts are provided as Tables S3 and S4. The LDPS particle size distribution is provided as Fig. S4A–C. The time dependent UV-Vis spectra and the corresponding first order kinetic plots for all nitroarenes except 4-NS are provided as Fig. S5A–M and N–Z. UV-Vis spectra, TEM micrographs and electron diffraction patterns of Ru-CTAB, Ru-SDS and Ru-TX-100 are provided as Fig. S6A–I. The time-dependent UV-Vis spectra for the hydrogenation of 4-NS by Ru-CTAB, Ru-SDS and Ru-TX-100 and their corresponding ln(conc.) vs. time plots are given as Fig. S7A–F. CV of Ru-30, Ru-45 and Ru-60 are given together as Fig. S8. Post-cycle CV for Ru-60 after 10 h of chronoamperometric analysis is given as Fig. S9. CV of Ru-60 modified GC with the potential window of –1 to 1 V for identifying the non-faradaic region is given as Fig. S10. See DOI: 10.1039/c5sc04714e