No abstract

M1 phase MoVTeNb mixed oxides exhibit unique catalytic properties that lead to high C2H4 yields in oxidative conversion of C2H6 at moderate temperatures. The role of the heptagonal channel micropores of the M1 phase in regulating reactivity and selectivity is assessed here using reactant size-dependent kinetic probes and density functional theory (DFT) treatments for C2H6 and cyclohexane (C6H12) activations inside and outside the micropores. The sizes of C2H6 and the micropores suggest a tight guest–host fit, but C6H12 cannot access intrapore sites. Measured C2H6 to C6H12 activation rate ratios on MoVTeNbO are much higher than those measured on nonmicroporous vanadium oxides (VO x /SiO2) and estimated by DFT on external surfaces, suggesting that most C2H6 activations on MoVTeNbO occur inside the micropores under typical conditions. C2H6 exhibits higher activation energy than C6H12 on VO x /SiO2, consistent with the corresponding C–H bond strengths; the activation energy difference between C2H6 and C6H12 is lower on MoVTeNbO because micropores stabilize C–H activation transition states through van der Waals interactions. Product selectivities for C2H6 and C6H12 suggest that the ability of VO x /SiO2 to activate C–H bonds and resist O-insertion in products is similar to the external surfaces of MoVTeNbO, but the micropores in the latter oxides are more selective for C–H activation. DFT calculations show that the tight confinement in micropores hinders the C–O contact necessary for O-insertion. These insights provide guidance for utilizing shapes and sizes of confining voids to mitigate selectivity limitations dictated by thermodynamics of sequential oxidation reactions and electronic properties of redox catalysts.

Faceted ZnO nanorods with different aspect ratios were synthesized by a solvothermal method by tuning the reaction time. Increased reaction leads to the formation of high aspect ratio ZnO nanorods largely bound by the prism planes. The high aspect ratio rods showed significantly higher visible light photocatalytic activity when compared to the lower aspect ratio structures. It is proposed that the higher activity is due to better charge separation in the elongated 1D structure. In addition, the fraction of unsaturated Zn(2+) sites is higher on the {1010} facets, leading to better adsorption of oxygen-containing species. These species enhance the production of reactive radicals that are responsible for photodegradation. The photocurrent for these ZnO nanostructures under solar light was measured and a direct correlation between photocurrent and aspect ratio was observed. Since the underlying mechanisms for photodegradation and photocurrent generation are directly related to the efficiency of electron-hole creation and separation, this observation corroborates that the charge separation processes are indeed enhanced in the high aspect ratio structures. The efficiency of photoconduction (electron-hole pair separation) could be further improved by attaching Au nanoparticles on ZnO, which can act as a sink for the electrons. This heterostructure exhibits a high chemisorption of oxygen, which facilitates the production of highly reactive radicals contributing to the high photoreactivity. The suggested mechanisms are applicable to other n-type semiconductor nanostructures with important implications for applications relating to energy and the environment.

Oxidation of small organic molecules in a fuel cell is a viable method for energy production. However, the key issue is the development of suitable catalysts that exhibit high efficiencies and remain stable during operation. Here, we demonstrate that amine-modified ZnO nanorods on which ultrathin Au nanowires are grown act as an excellent catalyst for the oxidation of ethanol. We show that the modification of the ZnO nanorods with oleylamine not only modifies the electronic structure favorably but also serves to anchor the Au nanowires on the nanorods. The adsorption of OH(-) species on the Au nanowires that is essential for ethanol oxidation is facilitated at much lower potentials as compared to bare Au nanowires leading to high activity. While ZnO shows negligible electrocatalytic activity under normal conditions, there is significant enhancement in the activity under light irradiation. We demonstrate a synergistic enhancement in the photoelectrocatalytic activity of the ZnO/Au nanowire hybrid and provide mechanistic explanation for this enhancement based on both electronic as well as geometric effects. The principles developed are applicable for tuning the properties of other metal/semiconductor hybrids with potentially interesting applications beyond the fuel cell application demonstrated here.

Quantum clusters (QCs) of silver such as Ag7(H2MSA)7, Ag8(H2MSA)8 (H2MSA, mercaptosuccinic acid) were synthesized by the interfacial etching of Ag nanoparticle precursors and were loaded on metal oxide supports to prepare active catalysts. The supported clusters were characterized using high resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and laser desorption ionization mass spectrometry. We used the conversion of nitro group to amino group as a model reaction to study the catalytic reduction activity of the QCs. Various aromatic nitro compounds, namely, 3-nitrophenol (3-np), 4-nitrophenol (4-np), 3-nitroaniline (3-na), and 4-nitroaniline (4-na) were used as substrates. Products were confirmed using UV-visible spectroscopy and electrospray ionization mass spectrometry. The supported QCs remained active and were reused several times after separation. The rate constant suggested that the reaction followed pseudo-first-order kinetics. The turn-over frequency was 1.87 s-1 per cluster for the reduction of 4-np at 35°C. Among the substrates investigated, the kinetics followed the order, SiO2 > TiO2 > Fe2O3 > Al2O3.