Microwave-Assisted Synthesis of Cu@IrO2 Core-Shell Nanowires for Low-Temperature Methane Conversion

A facile microwave-assisted synthesis was developed for the tunable fabrication of a Cu@IrO2 core@shell nanowire motif. Experimental parameters, such as (i) the reaction time, (ii) the method of addition of the Ir precursor, (iii) the capping agent, (iv) the reducing agent, and (v) the capping agent-to-reducing agent ratio, were subsequently optimized. The viability of other methods based on the previously reported literature, such as refluxing, stirring, and physical sonication, was studied and compared with our optimized microwave-assisted protocol in creating our as-prepared materials. It should be noted that the magnitude of the IrO2 shell could be tailored based on varying the Cu:Ir ratio coupled with judicious variations in the amounts of the capping agent and the reducing agent. Structural characterization techniques, such as XRD, XPS, and HRTEM (including HRTEM-EDS), were used to analyze our Cu@IrO2 motifs. Specifically, the shell could be reliably tailored from sizes of 10, 8, 6, and 3.5 nm with corresponding Cu:Ir ratios of 10:1, 15:1, 20:1, and 25:1, respectively. Moreover, the structural integrity of the motifs was probed and found to have been maintained after not only heat treatment but also the post-methane conversion process, indicative of an intrinsically high stability. Both components within the CuO-IrO2 interface were able to activate methane at temperatures between 400 and 500 K with a reduction of the associated metal cations (Cu2+ → Cu1+; Ir4+ → Ir3+) and the deposition of CHx fragments on the surface, as clearly observed in the ambient-pressure XPS results. Thus, on the basis of their stability and chemical activity, these core-shell materials could be very useful for the catalytic conversion of methane into "higher-value" chemicals.