Greenhouse Gas Detection Using Metal Oxides: Experiments and Challenges
Abstract
The primary goal of this study has been to develop the mixed ion-electron conductors (MIECs) to illustrate their potential applications for real-time detection of CO2 and SO2 in ppm level. For this purpose, B-site substituted ordered perovskite-type MIECs Ba2(Ca0.66Nb1.34-xFex)O6-δ (x = 0, 0.34, 0.66, 1) (BCNF) and B-site disordered MIECs Ba(Mg0.33Nb0.67-xFex)O3-δ (x = 0, 0.17, 0.33, 0.50) (BMNF) were successfully synthesized by conventional solid state method in air at 1400 °C. Fe-substitution in BCNFs and BMNFs increased the total conductivity, while in-situ high temperature (25–800 °C) PXRD results under CO2 exhibited excellent chemical stability. The resistance of the MIEC ceramics significantly decreased upon exposure to CO2 gas in synthetic air. Response time (t90) of 4 min at 700 °C was obtained with higher Fe-content samples with thickness of 2 mm and diameter of 10 mm.
To develop a fundamental understanding of the sensing mechanism, Fe-doped BCNFs were investigated. Firstly, p-type bulk semiconducting nature of BCNF was explored using AC impedance spectroscope and DC measurements using different p(CO2) and p(O2). Secondly, BCNFs were further studied using SEM coupled with EDX, 57Fe Mössbauer spectroscopy, Raman spectroscopy, TPO-MS. Upon CO2 exposure, CO2 was found to be catalytically reduced to C and can be explained using oxidation of Fe3+ to Fe4+/ Fe5+. Thus, the increase in total conductivity upon CO2 exposure is interpreted by the deposition of conducting graphitic C, which was confirmed using Raman spectra and TPO-MS. Furthermore; SEM/EDX results displayed the deposited C due to the reduction of CO2.
For SO2 sensors, SnO2-TiO2 (S-T) composites with different molar ratios were also prepared at 700 °C in air to selectively monitor ppm level of SO2 in air. For S-T composites, the chemical stability were tested using PXRD, SEM and EDX, while the sensing performance was measured at different temperatures using various concentrations of SO2 (10-100 ppm). The highest sensitivity was obtained from a mixture of 25 mol.% of SnO2 and 75 mol.% ofTiO2 at 450 °C with t90 of ~5 mins. S-T composite sensors behave as a resistive-type SO2 sensor and sensing mechanism has been explained through the band structure model.
Description
Keywords
Chemistry, Chemistry--Inorganic, Chemistry--Physical, Energy, Engineering--Chemical, Materials Science
Citation
Mulmi, S. (2016). Greenhouse Gas Detection Using Metal Oxides: Experiments and Challenges (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/26322