Structure, Optical, and Catalytic Properties of Novel Hexagonal Metastable h-MoO3 Nano- and Microrods Synthesized with Modified Liquid-Phase Processes

Single crystalline h-MoO3 nano- and microrods were successfully synthesized using modified liquid-phase processes with concentrated HNO3 and H2SO4. Their X-ray powder diffraction (XRD) data were unambiguously indexed based on a hexagonal structure with the lattice constants a ≈ 10.57 and c ≈ 3.72 Å instead of a = 10.53 and c = 14.98 Å (JCPDS 21-0569) usually adopted. Rietveld refinements of the XRD data were pioneeringly performed based on the (Na·2H2O)Mo5.33[H4.5]0.67O18 structure with the space group of P63/m regardless of H+ and Na+. Nanorods synthesized under different conditions show different sizes and aspect ratios. Annealing at 300 °C for 3 h significantly improves the crystallinity and phase purity of as-synthesized h-MoO3 rods, which is evidenced by sharpening of peaks in micro-Raman spectra with no shift. An irreversible transition from h-MoO3 to α-MoO3 occurring between 413 and 436 °C can be triggered by irradiation of either electrons or laser with high energies or powers as well. The turning points on both differential thermal analysis (DTA) and thermogravimetry (TG) curves show presence of water molecules interacted differently with the lattice which escape at different temperatures. h-MoO3 rods reduce the temperatures of soot oxidation to 482−490 °C, much higher than its structural transition temperatures. This makes it simply suitable for catalyzing reactions taking place at temperatures lower than the transition temperatures, say, as the catalyst of the selective oxidation of methanol.