Amorphous Oxide Catalysts Research Articles

In Situ Stimulated Raman Spectroscopy Reveals the Phosphate Network in the Amorphous Cobalt Oxide Catalyst and Its Role in the Catalyst Formation

Amorphous oxides are one of the most active catalysts for the oxygen evolution reaction (OER). However, very little is known about the structure of the amorphous oxide catalyst during OER, especial...

Effects of Oxide Composition of Amorphous RuO2-Ta2O5 Catalyst on Electrochemical Sensing to Hydrogen Carbonate Ion

Hydrogen phosphate (HPO4 2-) and hydrogen carbonate (HCO3 -) ions are major in environmental water and blood, and the quantification is normally carried out by molybdenum blue method for HPO4 2- and by titration method or the gas analysis of blood for HCO3 -, while no quantitative method by electrochemical sensing has been developed. We have recently reported that these ions can be detected and quantified by electrochemical oxidation with the novel catalyst, amorphous RuO2-Ta2O5 mixed oxide, prepared by thermal decomposition [1,2]. For practical uses of the amorphous oxide catalyst for the detecting electrode, it is important to know the effects of the oxide composition of the catalyst to the sensitivity and selectivity to HCO3 - and HPO4 2-. In this paper, we studied the sensing properties to these ions with RuO2-Ta2O5 catalysts prepared at different compositions. RuO2-Ta2O5 catalysts were prepared by thermal decomposition of a precursor solution containing Ru (III) and Ta (V) painted on a titanium disk. The Ru:Ta mole ratio was 20:80, 30:70, 50:50, or 80:20, and thermal decomposition was carried out at 260 oC. The characterization of the oxide coating was done with XRD, SEM, and EDX. Electrochemical measurements were performed with a conventional three-electrode cell with KCl-saturated Ag/AgCl reference electrode and 50 mmol/L KCl solutions with or without various concentrations of KHCO3 or Na2HPO4. Cyclic voltammetry was carried out to analyze the oxidation reactions of these ions, and chronoamperometry was also used to measure the oxidation current at various concentrations of HCO3 - or HPO4 2-. All measurements were done at 30 oC. XRD and SEM measurements revealed that the obtained RuO2-Ta2O5 catalysts were amorphous, irrespective of the oxide composition. The cyclic voltammograms presented an oxidation wave for each of HCO3 - and HPO4 2-, and the oxidation current density increased with increasing concentration of each ion, indicating that HCO3 - and HPO4 2- are oxidized on the catalyst prior to oxygen evolution. The onset potential and peak potential of the oxidation of HCO3 - or HPO4 2- shifted negatively with increasing Ru mole ratio of the catalyst, in which the onset potential of HCO3 - was 0.1 V nobler than that of HPO4 2-. The oxidation current at different ion concentrations measured by chronoamperometry revealed that the concentration of HCO3 - or HPO4 2- was linear to the oxidation current density for each ion at all oxide compositions from Ru = 20 mol% to 80 mol%. Among them, the sensitivity for HCO3 - or HPO4 2- was highest at Ru = 50 mol%. More detailed results will be shown in this paper. This work was supported by “Kyoto Area Super Cluster Program” of Japan Science and Technology Agency (JST).

Effects of Temperature on Electrochemical Sensing of HPO4 2- with Amorphous RuO2-Ta2O5 Catalyst

Our research group has recently revealed that a novel sensing catalyst, amorphous RuO2-Ta2O5 mixed oxide, can detect and quantify hydrogen phosphate (HPO4 2-) by electrochemical oxidation, of which the current density is linear to the concentration in the wide range from 10-6 mol/L to 10-2 mol/L [1], while no other catalyst has been found for electrochemical sensing of HPO4 2-. This new catalyst and sensing method can be applied to measure the concentrations of hydrogen phosphate or phosphorus in environmental water, blood, foods, medicines, and agricultural products. For practical uses of the amorphous oxide catalyst for the detecting electrode, it is important to know the effects of the temperature of the test solution on the sensitivity to HPO4 2-, especially for continuous monitoring of phosphorus in environmental water. In this paper, we report the effects of temperature on the onset potential, the oxidation current density, and the sensitivity of the oxidation of HPO4 2- examined by cyclic voltammetry and chronoamperometry using rotating disk electrode (RDE). The amorphous RuO2-Ta2O5 catalyst was prepared on a titanium disk for RDE by calcination of the precursor solution containing Ru (III) and Ta (V), in which the Ru mole ratio was 50 mol% and thermal decomposition was carried out at 260 oC. Electrochemical measurements were performed using a conventional three-electrode cell and 50 mmol/L KCl solutions with or without various concentrations of Na2HPO4 at a temperature from 3 oC to 50 oC. The cyclic voltammograms showed the oxidation wave or plateau of HPO4 2-, and the onset potential shifted negatively and the maximum current density increased with increasing temperature of the test solutions. From the results, the potential to measure the oxidation current by chronoamperometry was determined at each temperature, so that the applied potential was more positive than that at which the maximum oxidation current in the cyclic voltammograms was observed and was less than the onset potential of oxygen evolution. The relationship between the oxidation current density and the concentrations of HPO4 2- obtained at 3 oC, 10 oC, 20 oC, 30 oC, 40 oC, or 50 oC revealed that the linear relationship was observed at all the temperatures and the slope increased with increasing temperature, suggesting that the sensitivity is enhanced. However, the linear region was seen in the concentration range from 0.4 mmol/L to 10 mmol/L, which was independent of temperature. These results are reasonable, because chronoamperometry measured the diffusion-limited current of HPO4 2- oxidation with RDE and the diffusion co-efficient becomes large at higher temperature. It was also found that the sensitivity was proportional to the temperature, which implies that it is easy to calibrate the sensitivity in practical uses. More detailed results will be shown in this paper. This work was supported by “Kyoto Area Super Cluster Program” of Japan Science and Technology Agency (JST). Reference [1] T. Tsukuma and M. Morimitsu, The 66th Annual Meeting of International Society of Electrochemistry, Abs# s01-014, Taipei, Taiwan (2015).

A Novel Sensing Method for HPO42− and HCO3− by Electrochemical Oxidation with Amorphous Mixed Oxide Catalyst

Amorphous RuO2-Ta2O5 catalysts prepared by thermal decomposition of the precursor solution were investigated to develop a novel sensing method for HPO42− and HCO3− by electrochemical oxidation, and the catalyst formed on a titanium substrate was used as the detecting electrode. Cyclic voltammetry showed that the oxidation of both anions occurred and the onset potential of the oxidation of HPO42− was less noble than that of HCO3-, suggesting that these anions can be detected separately by changing the electrode's potential, and chronoamperometry indicated that the oxidation current density was linear to the concentration of HPO42− or HCO3−. The results revealed that the quantification of both anions is possible with this novel amorphous oxide catalyst.

Oxyanion induced variations in domain structure for amorphous cobalt oxide oxygen evolving catalysts, resolved by X-ray pair distribution function analysis.

Amorphous thin film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling photoanode materials in solar-driven, photoelectrochemical `artificial leaf' devices. This report demonstrates the ability to use high-energy X-ray scattering and atomic pair distribution function analysis, PDF, to resolve structure in amorphous metal oxide catalyst films. The analysis is applied here to resolve domain structure differences induced by oxyanion substitution during the electrochemical assembly of amorphous cobalt oxide catalyst films, Co-OEC. PDF patterns for Co-OEC films formed using phosphate, Pi, methylphosphate, MPi, and borate, Bi, electrolyte buffers show that the resulting domains vary in size following the sequence Pi < MPi < Bi. The increases in domain size for CoMPi and CoBi were found to be correlated with increases in the contributions from bilayer and trilayer stacked domains having structures intermediate between those of the LiCoOO and CoO(OH) mineral forms. The lattice structures and offset stacking of adjacent layers in the partially stacked CoMPi and CoBi domains were best matched to those in the LiCoOO layered structure. The results demonstrate the ability of PDF analysis to elucidate features of domain size, structure, defect content and mesoscale organization for amorphous metal oxide catalysts that are not readily accessed by other X-ray techniques. PDF structure analysis is shown to provide a way to characterize domain structures in different forms of amorphous oxide catalysts, and hence provide an opportunity to investigate correlations between domain structure and catalytic activity.

Local environment of vanadium in V/Al/O-mixed oxide catalyst for propane ammoxidation: Characterization by in situ valence-to-core X-ray emission spectroscopy and X-ray absorption spectroscopy

The local environment of vanadium in V/Al/O amorphous oxide catalyst has been studied under propane ammoxidation reaction conditions. In situ valence-to-core X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) at the V K-edge showed that acrylonitrile production starts when vanadium in the bulk structure of the oxide material changes its formal oxidation state from +4.8 to +3.8 ± 0.1. Exposure of the catalyst to pure NH 3 at 500 °C leads to further reduction of vanadium. Valence-to-core XES has also proved that the level of bulk nitridation of vanadium in the active catalyst was rather small. XPS analysis performed ex situ after the catalytic tests confirmed the presence of significant amounts of N 3−, NH x , and –NN– species on the catalyst surface. It can be concluded that activation of V/Al/O catalyst on-stream is mainly associated with reduction of vanadium in the bulk structure of the material and nitridation of vanadium atoms on the surface.

Amorphous oxide as a novel efficient catalyst for direct selective oxidation of methanol to dimethoxymethane

We report for the first time the use of an amorphous oxide catalyst for the selective oxidation of methanol in the gas phase, leading at 553 K to the production of dimethoxymethane with a selectivity as high as 90% at high methanol conversion (68%).

Spectroscopic characterization of magnesium vanadate catalysts. Part 2.—FTIR study of the surface properties of pure and mixed-phase powders

The surface chemistry of the three stable monophasic powders magnesium orthovanadate Mg3(VO4)2, magnesium pyrovanadate Mg2V2O7 and magnesium metavandate MgV2O6, as well as of an amorphous oxide catalyst with an Mg : V atomic ratio of 3 : 2, has been investigated by FTIR spectroscopy. The bulk structure of the amorphous catalyst has been characterized by FT-FIR and Raman spectroscopies. The nature of the surface hydroxy groups, as well as the adsorption of molecular probes such as pyridine, acetonitrile and CO2, has been investigated. The samples show weak Lewis acidity and no Bronsted acidity. Moreover, they show significant but not extreme basic character. They weakly adsorb oxygenate compounds like alcohols and ketones and interact only at high temperatures with alkanes and alkenes. These materials, active as alkane oxydehydrogenation catalysts, are much less reactive than other vanadia-based selective oxidation catalysts, like V2O5–TiO2 and (VO)2P2O7, towards organic molecules. This is attributed to the basic environment generated by the MgO component in catalysts belonging to the MgO–V2O5 system, which causes a parallel decrease of the Lewis acidity and of the oxidizing powder of the active vanadium ions.