Selective catalytic oxidation (SCO) of NH3 to harmless N2 and H2O is an ideal technology for its removal. To develop air purification systems for a living environment, catalysts that can work at room temperature with high selectivities to N2 are required. However, it has been a technical challenge because the reported catalysts either needed high operating temperatures or showed low selectivities to N2. In this study, we first demonstrated that acidic metal-oxide-supported gold catalysts showed good N2 selectivities compared with that of other metal-oxide-supported gold catalysts. A gold catalyst with niobium oxide synthesized by the hydrothermal method as a support showed high catalytic activity and high selectivity to N2 at low temperatures (18% NH3 conversion with 100% N2 selectivity at 25 °C) and at high temperatures (100% NH3 conversion with 95% N2 selectivity at 245 °C). Important roles of Brønsted acid sites and formation of active oxygen sites in improving N2 selectivity were revealed in this study. To the best of our knowledge, this is the first report of efficient catalysts that presented high NH3 conversion with high N2 selectivity at 25 °C which will offer great scopes for commercial applications related to control of odors. In addition, this breakthrough finding that acid sites would greatly affect N2 selectivity and catalytic activity will provide a new trend in designing efficient catalysts not only for SCO of NH3 but also for the other selective catalytic oxidation.

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Epoxidation of propylene into propylene oxide (PO) in the gas phase is a highly challenging reaction. Cu-based catalysts have been active for this reaction, but the state of Cu as an active species is still debatable. In this paper, we report the propylene epoxidation activity of solution combustion synthesized Cu/CeO2 catalysts with the CO + O2 mixture at low temperatures (50–100 °C) peaking at ∼80 °C. The highest PO yield was obtained with 20–25% Cu loading in CeO2. In contrast, the reaction over the catalyst containing nonreducible support such as Cu/SiO2 occurred above 170 °C. Detailed structural characterization indicated two types of Cu species such as Cu2+ partly (∼3%) dissolved in CeO2 forming a Cu x Ce1–x O2−δ phase and the remaining amount formed highly dispersed CuO as a separate phase. Thus, the highest activity relates to the optimum presence of CuO along with Ce1–x Cu x O2−δ. The reducibility of the Cu species in two phases was significantly shifted toward lower temperatures, indicating strong electronic interaction between the two phases. The substituted Cu2+ was reduced first, and then, the bulk CuO reduction was initiated. In situ spectroscopic studies showed Cu+ formation from Cu2+ over Cu/CeO2 catalysts even at room temperature unlike in CeO2 or CuO + CeO2 physical mixtures, indicating strong electronic interaction between Ce1–x Cu x O2−δ and CuO phases on CO adsorption in the Cu/CeO2 catalyst. It is proposed that substituted Cu2+ along with Ce4+ is reduced easily, and then, Ce3+ promotes the reduction of the interfacial CuO phase that might donate active oxygen species for epoxidation reaction.

Ag-based catalysts, especially Ag/Al2O3, show high NH3 oxidation activity in NH3-selective catalytic oxidation (NH3–SCO). However, the low N2 selectivity limits their further application. To improve N2 selectivity, herein, a series of Ag/Al2O3-X catalysts (X = calcination temperature) are designed by simple calcination. Only Ag nanoparticles (Ag NPs) were observed on the Ag/Al2O3-400 catalyst, while highly dispersed Ag species (Ag HDs) were dominated on the Ag/Al2O3-800 catalyst. NH3–SCO results showed that Ag NPs/Al2O3 forms much N2O by-products, whereas Ag HDs/Al2O3 achieves >99% N2 selectivity over the Ag/Al2O3-800 catalyst at <200 °C. More importantly, various intermediates (NO3 ads, NH2 ads, NN–M, and NN–O–M) and their internal transformations were detected, and the reaction pathways for the formation of N2O and N2 are evidenced by in situ NH3-Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). This study not only provides a convenient and effective approach to obtain excellent N2 selectivity in NH3–SCO but also affords a systematic insight into the reaction pathways over Ag/Al2O3 catalysts.

The article describes the synthesis of bismuth nanosheets (BiNSs) in the presence of a small quantity of graphene oxide (GO) which is helpful for the formation of twodimensional BiNSs and improves dispersity. The material, when placed on a glassy carbon electrode (GCE), is shown to enable catalytic stripping voltammetric determination of total dissolved iron without the need for adding a complexing agent. The average thickness and length of the BiNSs are 3 to 4 nm and 100 to 200 nm, respectively. The unique nanostructure of the BiNSs, the ability of Bi to form alloys with metal, and the current amplification of the catalytic system make the modified GCE an excellent choice for electrochemical determination of Fe(III). Under the optimal conditions, the electrode has a linear response to Fe(III) in the 0.01 to 20 μM concentrations range, with a lower detection limit of 2.3 nM.The electrode was successfully applied to the sensitive determination of Fe(III) in coastal waters.

Co 3 O 4 with spinel structure shows CO oxidation activity at very low temperature under dry conditions. This study aims at finding the origin of the unique catalytic activity of Co species in Co 3 O 4 based oxides. Although, octahedral site Co 3+ species have been reported to be active in Co 3 O 4 based catalysts, there is no solid explanation as to why Co is so special as compared with other metals like Fe having similar redox states. In this study, mainly, three model spinel catalysts including MnCo 2 O 4 , MnFe 2 O 4 , and CoCr 2 O 4 have been chosen. A detailed analysis of bulk and crystal surface structure, surface properties of the catalysts, and redox properties of the active metals has been performed to understand the unusual catalytic activity. Low-temperature CO oxidation activity decreases in the following order: MnCo 2 O 4 ≫ MnFe 2 O 4 > CoCr 2 O 4 . It indicates that the Co 2+ species in a tetrahedral site (in CoCr 2 O 4 ) remains inactive for lowtemperature catalytic activity, while Co 3+ in an octahedral site (in MnCo 2 O 4 ) is active in Co 3 O 4 based catalysts. This result is corroborated with CoFe 2 O 4 which shows a higher activity than CoCr 2 O 4 , as it has partial occupation of the octahedral site. Fe, being a weak redox metal, does not show low-temperature activity, although crystallite facets of MnCo 2 O 4 and MnFe 2 O 4 catalysts are predominantly exposed in the (100) and (110) lattice planes, which contain quite similar concentrations of Co 3+ and Fe 3+ species in both. The intensity of the redox peak for CO oxidation involving a Co 3+ /Co 2+ couple in MnCo 2 O 4 indicates a highly favorable reaction, while a nonresponsive behavior of Co species is observed in CoCr 2 O 4 . As expected, MnFe 2 O 4 is proven to be weak, giving a much lower intensity of electrochemical CO oxidation. Both CO-and H 2 -TPR indicate a much higher reducibility of Co species in MnCo 2 O 4 as compared with Co species in CoCr 2 O 4 or Fe in MnFe 2 O 4 .

a b s t r a c tAn electrochemical sensor based on reduced graphene oxide (rGO) and gold nanoparticles (AuNPs) modified electrode was utilized for determination of trace iron in coastal waters with the aid of the iron complexing ligand 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP). 5-Br-PADAP reacted with iron in short chelating reaction time (<3 min). rGO as a support provided large specific surface area for AuNPs, which would facilitate the electrochemical reduction of Fe(III)-5-Br-PADAP. Under the optimized conditions, the response of Fe(III) at this resulting sensor was linear in the range of 30 nM to 3 M with a detection limit of 3.5 nM. This sensor also had excellent reproducibility and repeatability. Additionally, the modified electrode had been successfully applied to the determination of Fe(III) in real coastal waters.

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