Selective methane detection is essential for process safety in industries such as coal mining, where CO, NH3, and NO2 serve as interfering gases. A promising approach is to use metal oxide semiconductor (MOS)-based sensors, which are low-cost, highly sensitive, and easy to fabricate. However, the poor selectivity of MOS sensors due to nonselective surface reactions remains a significant challenge. In this study, we fabricated a ZnO/Pd@ZIF-7 core-shell structure–based gas sensor using a self-sacrificial method. The ZnO/Pd layer served as the sensitive layer to generate sensing signals, while the ZIF-7 shell acted as a filter. By manipulating gas diffusion, ZIF-7 significantly improved CH4 sensing selectivity against CO, NH3, and NO2. For NO2, which strongly interacts with ZIF-7, the diffusion through ZIF-7 was significantly hindered, resulting in a decreased response across all temperature ranges (110–250 °C). For CH4, CO, and NH3, which weakly interact with ZIF-7, the influence of ZIF-7 depended on temperature, as competition occurred between surface reactions and diffusion through ZIF-7. At low temperatures, ZIF-7 enriched gases and promoted the response of the three gases. At elevated temperatures, ZIF-7 separated gases according to their molecular polarity, where the diffusion of polar CO and NH3 was more hindered than nonpolar CH4. The excellent CH4 selectivity against CO, NH3, and NO2 was achieved at 210 °C, with fast response/recovery, good repeatability, and long-term stability. Our study not only provides a possible solution to enable sensing selectivity of MOS to CH4, but the insights into the effect of the ZIF-7 filter may also inspire the development of highly selective gas sensors.
This paper reports on the measurement of aerodynamic forces that act on a square cylinder which was downstream to an identical cylinder. When the two cylinders were in tandem formation, a critical spacing equal to about four times the side length D of the square cylinder, was found to exist. For L/D < ( L/D ) cri , where L is the streamwise centre to centre spacing between the two cylinders, the boundary layers which separate from the upstream cylinder reattach onto the downstream cylinder. Only the latter sheds vortices and it is subjected to a thrust rather than a drag force. For LID > ( L/D ) cri , both cylinders shed vortices and both are subjected to drag forces. When the two cylinders were in staggered formation, due to the presence of various lift generating mechanisms, the lift force that acts on the downstream cylinder can be either positive which points away from the wake centre line of the upstream cylinder or negative which points towards the wake centre line of the upstream cylinder. It was found that in the range 4 ⩽ LID ⩽ 12, an unstable regime exists in that the lift force generated pushes the downstream cylinder away from the upstream cylinder’s wake centre line at small dimensionless transverse spacing (Y/D) where Y is the transverse centre to centre spacing between the two cylinders and reverses its direction only at Y/D > 2. At L/D = 29, the largest streamwise spacing that can be investigated in the present study, the influence that the upstream cylinder has on the aerodynamic forces acting on the downstream cylinder was found to be still fairly appreciable.
Methane detection is important for the safety of production and life. Metal oxide semiconductor (MOS) methane detection is a mature and widely used technology but still experiences problems such as unsatisfying low-temperature sensing performances. In this study, ZnO/Pd with Pd nanoparticles of different diameters was prepared to study the influence of Pd dispersion on CH4 sensing properties. Results showed that CH4 sensing enhancements were positively correlated with the dispersity of Pd. Moreover, by galvanic replacement using Ag as the sacrificial template, a highly dispersive loading of Pd on ZnO was realized, and the CH4 sensing performance was further enhanced while the amount of Pd reduced from 1.35 wt% to 0.26 wt%. Experiments and DFT calculation indicated that improved CH4 sensing performance resulted from abundant catalytic sites induced by highly dispersed Pd NPs and the enhanced CH4 adsorption on positively charged Pds caused by electrons transferred from Pd to Ag. This study provides a strategy to achieve high dispersion of Pd to maximize the utilization of noble metal, which is promising for lowering the cost of the MOS-based CH4 sensors.
Arsenic (<i>As</i>) poisoning in water due to natural minerals or industrial pollution is a critical global problem that threatens the health and life of billions. Current arsenic removal techniques involving chemical reaction, ion exchange, or membrane processes can be expensive, inaccessible or infeasible for underdeveloped regions or remote areas. Here, we demonstrate that using a so-called directional solvent extraction (DSE) process, arsenic<i> </i>ions in water can be effectively removed without the need of a membrane or chemical reaction, and this process promises to utilize very low temperature heat (as low as 45 <sup>o</sup>C). We have tested feed water with different arsenic concentrations and arsenic ions in different forms (<i>As</i>-III and <i>As</i>-V) commonly found in nature. It is demonstrated that DSE using decanoic acid as the directional solvent can purify contaminated water to meet the drinking water standard (arsenic concentration < 10 parts per billion, ppb), and the arsenic removal efficiencies are higher than 91% for <i>As</i>-III and 97% for <i>As</i>-V. Moreover, DSE can remove <i>As</i>-III directly without the need of pre-oxidation, which is required in most of the state of art techniques. DSE can potentially lead to effective arsenic removal technologies with low resource settings that are suitable for remote and underdeveloped regions, which are impacted by arsenic poisoning the most.
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