Novel catalytic combustion-type CO gas sensors were successfully devised by applying precious metal-free catalysts composed of the p- or n-type semiconducting Ln2CuO4-loaded CeO2-ZrO2-ZnO (Ln2CuO4/CZZn; Ln = La, Nd, and Sm). Complete CO oxidation using the p-type semiconducting La2CuO4/CZZn catalyst occurred at 350 °C, while the n-type semiconducting Nd2CuO4/CZZn and Sm2CuO4/CZZn catalysts completely oxidized CO gas at 220 °C and 200 °C, respectively. All the fabricated sensors exhibited quantitative detection of CO gas over 250 °C. The La2CuO4/CZZn catalyst has the smallest heat capacity, which dramatically increases the Pt coil temperature compared with the other catalysts, resulting in the highest sensor signal. However, the sensor applying Nd2CuO4/CZZn and Sm2CuO4/CZZn catalysts exhibited faster response time to CO gas than the sensor employing La2CuO4/CZZn catalyst due to their superior CO oxidation activity.

The new nanoporous framework [Gd 4 (di-nitro-BPDC) 4 (NO 2 ) 3 (OH)(H 2 O) 5 ]•(solvent) (I; di-nitro-BPDC 2− = 2,2′-dinitrobiphenyl-4,4′-dicarboxylate) has been designed and synthesized through a simple one-pot reaction. In addition to its exceptional thermal and water stabilities, I exhibited multifunctional properties. A sudden CO 2 uptake to a maximum of 4.51 mmol g −1 (195 K and 1 bar) with notable selectivity over O 2 and N 2 (CO 2 /O 2 = 39 at 195 K and 0.10 bar, CO 2 /N 2 = 46 at 195 K and 0.10 bar) and an isosteric adsorption enthalpy of 20.7(4) kJ mol −1 have been revealed. Depending on the temperature and humidity, I also showed distinguished superprotonic conductivities with a maximum value and activation energy of 6.17 × 10 −2 S cm −1 (55 °C, 99 RH%, and 1 V AC voltage) and 0.43 eV, respectively. With respect to the linear dependence of conductivities on both temperature (25−55 °C at 99 RH%) and humidity (55−99 RH% at 25 °C), the potential of I in temperature and humidity sensing was evaluated, disclosing an excellent sensing resolution and exceptional accuracy, precision, and repeatability for the measurements.

Catalytic combustion-type CO gas sensors employing the mixture of Pt-loaded CeO 2 ZrO 2 ZnO catalyst and artificial diamond with high thermal conductive nature were successfully fabricated. While the sensor employing only Pt-loaded CeO 2 ZrO 2 ZnO catalyst exhibited CO sensing performance over 100°C with high electrical noise, the sensor with the mixture of artificial diamond and the catalyst could operate even at 80°C and the electrical noise was also reduced. At the same operating temperature at 130°C at which both sensors showed the highest sensitivity, the 50% response time toward CO gas concentration change was accelerated from 103 s for the sensor without artificial diamond to 76 s by using the sensor with artificial diamond. Furthermore, the sensitivity toward CO gas was also increased.