Effects of Nano Particles of Ruthenium Oxide-Based Hybrid Catalyst on Sensitivity to Hydrogen Phosphate Ions
Authors: Kawaguchi Kenji , Sakamoto Akifumi , Tsukuma Takami , Morimitsu Masatsugu 
Published: May 2020

Hydrogen phosphate (HPO4
2-, HPO) is one of the target ions to detect and quantify phosphorus in environmental water, and the quantification is normally carried out by molybdenum blue method, in which a sample containing phosphate is mixed with an acid solution of Mo (VI) and the absorbance of the resulted PMo12O40
3− is measured. Our group has recently found a novel catalyst to oxidize HPO, of which the oxidation current measured with RDE at 60 s after applying the oxidation potential is linear to the HPO concentration, suggesting a possibility of the quantification of HPO in environmental water by the electrochemical sensor [1, 2]. The catalyst is the mixture of ruthenium oxide and tantalum oxide obtained by thermal decomposition method. Since the oxides are immiscible, the obtained catalyst is the hybrid system consisting of nano ruthenium oxide particles dispersed in amorphous tantalum oxide matrix. In this paper, the hybrid catalysts were prepared at different metal ratios and thermal decomposition temperatures to change the size and distribution of ruthenium oxide nano particles and the sensing properties of those catalysts to HPO were examined.
The precursor solution containing RuCl3 with and without TaCl5 in n-butanol was painted on the titanium disk (4 mm in diameter, 10 mm in thickness), which had been degreased and etched in 10 wt% oxalic acid solutions at 90 oC for 1 h, and then heated at a temperature ranging from 260 oC to 500 oC for 20 min. This process was repeated several times to obtain RuO2-Ta2O5 or RuO2 coated titanium disk. The Ru:Ta ratio was 30:70 mol% to 80:20 mol%. Characterization of the oxide coating was carried out by XRD, SEM, and EDX. The coated disk was mounted in RDE equipment and was used as the working electrode with the platinum counter electrode and the KCl saturated Ag/AgCl reference electrode. The electrolytes were based on 50 mmol/L KCl solutions, and Na2HPO4 was added into the solution. Cyclic voltammetry (CV) and chronoamperometry (CA) were performed, and the current transient in CA was measured at 1200 rpm. All measurements were carried out at 30 oC.
The XRD results of the RuO2-Ta2O5 coatings showed clear diffraction peaks of RuO2 when the coatings were prepared at 500 oC, and the peaks were weakened or disappeared when the thermal decomposition temperature was 260 oC. SEM observation of the coating surface also indicated that the diffraction peaks were seen with RuO2 particles of 100 nm or more, while the flat surface containing nano RuO2 particles dispersed in amorphous Ta2O5 matrix was observed for the coating prepared at 260 oC. The Ru:Ta ratio also influenced on the surface morphology; a higher tantalum ratio makes RuO2 smaller which means that Ta2O5 suppresses the crystal growth of RuO2 even at 500 oC. The oxidation current was measured by chronoamperometry, and the data at 60 s obtained with different concentrations of HPO4
2- were plotted, giving a linear relationship between the concentration and the oxidation current for a maximum concentration range from 10-6 mol/L to 10-2 mol/L. The slope of the plots was defined as the sensitivity to HPO and the results on the sensitivity at different thermal decomposition temperatures and different metal ratios revealed that a lower thermal decomposition temperature made a higher sensitivity and the sensitivity was maximum at the Ru ratio of 50 mol%. Low temperature thermal decomposition like at 260 oC produces nano RuO2 particles, while RuO2 is grown large with increasing temperature, so that the active surface area to HPO is high at low thermal decomposition temperature, resulting in a high sensitivity. The increase in the Ru ratio seems to make the sensitivity higher, since RuO2 is active to and Ta2O5 has no activity to HPO. However, the increase in the Ru ratio also means the decrease in Ta2O5 which has a role to suppress the growth of RuO2 as the matrix of the hybrid catalyst, so that the sensitivity may be a maximum at around Ru = 50 mol%. We will also show the effects of RuO2 particle size on the sensitivity which are obtained from the results with RuO2 coatings on titanium disk prepared at different thermal decomposition temperatures.
This work was supported by JSPS KAKENHI Grant Number JP19175698.
Reference
[1] M. Morimitsu, T. Tsukuma, Y. Shigeta, K. Kawaguchi, WET2019, Abst#3A-08, Osaka (2019).
[2] M. Morimitsu, C. Iketani, T. Tsukuma, K. Kawaguchi, ACCS2019, Abst#BO-006, Bali (2019).