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Hexavalent chromium, Cr(VI), is a highly toxic carcinogen occurring in natural and industrial environments. Pathways to economical reduction to the more benign trivalent form, Cr(III), are necessary for treatment of contaminated groundwater. Magnetite’s (Fe₃O₄) mixture of Fe(II) and Fe(III) make it a promising material for remediation. This study investigated the mechanisms for reduction of Cr(VI) catalyzed by Fe₃O₄as a redox mediator in the presence of oxalic acid in HClO₄and SO₄²⁻solutions, a system where the interactions among these species are not fully understood. The reduction of Cr(VI) in different anion environments is first measured on an Au rotating disk electrode. SO₄²⁻inhibits the formation of a passivation layer and Cl^-partially inhibits passivation. The reduction of Cr(VI) on Fe₃O₄is limited by the availability of Fe(II) surface sites. Addition of oxalic acid works synergistically through liberation of Fe(II)-oxalate and soluble Cr(III)-oxalate products. A combination of Fe₃O₄activated by exposure to oxalic acid and use of an oxalic acid solution as a medium for reduction of Cr(VI) produces over 97% removal of Cr(VI). These results provide relevant insights regarding interactions of Fe₃O₄with organic acids and the anion environment which lead to the effective reduction of Cr(VI). 

Perovskite materials are used for high temperature electrochemical applications such as solid oxide fuel cells (SOFC) and electrolyzers due to their tunable conductivity and catalytic activity. However, high temperature operation poses significant challenges in both fabrication and durable operation that is further complicated by the operating environment. We studied barium niobates with various A and B site dopants. These doped niobates showed enhanced thermochemical stability in SOFC relevant conditions and catalytic activity towards methane activation. The redox behavior of the Nb^4+/5+couple seem to be at a key reason behind this redox stability while the size and electronegativity of the dopants affect the electrical properties. The chemical stability was analyzed by TGA measurements followed by analysis of the perovskite powders using PXRD measurements. Impedance measurements were utilized to analyze their electrical conductivity. Our results demonstrate doped barium niobates as a promising candidate for stable operation in high temperature electrochemical applications. 

Doped perovskite metal oxide catalysts of the form A(B_xM_1-x)O_3-δhave been instrumental in the development of solid oxide electrolyzers/fuel cells. In addition, this material class has also been demonstrated to be effective as a heterogeneous catalyst. Co-doped barium niobate perovskites have shown remarkable stability in highly acidic CO₂sensing measurements/environments (1). However, the reason for their chemical stability is not well understood. Doping with transition metal cations for B site cations often leads to exsolution under reducing conditions. Many perovskites used for the oxidative coupling of methane (OCM) or the electrochemical oxidative coupling of methane (E-OCM) either lack long term stability, or catalytic activity within these highly reducing methane environments. The Mg and Fe co-doped barium niobate BaMg_0.33Nb_0.67-xFe_xO_3-δshown activity in E-OCM reactors over long periods (2) (>100 hrs) with no iron metal exsolution observed by diffraction or STEM EDX measurements. In contrast, iron decorated BaMg_0.33Nb_0.67O₃showed little C2 conversion activity. 

Abstract Efficient conversion of methane to value-added products such as olefins and aromatics has been in pursuit for the past few decades. The demand has increased further due to the recent discoveries of shale gas reserves. Oxidative and non-oxidative coupling of methane (OCM and NOCM) have been actively researched, although catalysts with commercially viable conversion rates are not yet available. Recently,$${{{{{{{\mathrm{Sr}}}}}}}}_2Fe_{1.5 + 0.075}Mo_{0.5}O_{6 - \delta }$$ ${\mathrm{Sr}}_{2}F{e}_{1.5+0.075}M{o}_{0.5}{O}_{6-\delta }$(SFMO-075Fe) has been reported to activate methane in an electrochemical OCM (EC-OCM) set up with a C2 selectivity of 82.2%¹. However, alkaline earth metal-based materials are known to suffer chemical instability in carbon-rich environments. Hence, here we evaluated the chemical stability of SFMO in carbon-rich conditions with varying oxygen concentrations at temperatures relevant for EC-OCM. SFMO-075Fe showed good methane activation properties especially at low overpotentials but suffered poor chemical stability as observed via thermogravimetric, powder XRD, and XPS measurements where SrCO₃was observed to be a major decomposition product along with SrMoO₃and MoC. Nevertheless, our study demonstrates that electrochemical methods could be used to selectively activate methane towards partial oxidation products such as ethylene at low overpotentials while higher applied biases result in the complete oxidation of methane to carbon dioxide and water.