Green Conversion of Spent HDS Catalysts Into Layered NiWO ₄ Nanosheets for High‐Performance Room Temperature NH₃ Sensing

Ammonia (NH 3 ) detection is critical for environmental protection and industrial safety; however, conventional sensors face challenges, such as high‐energy consumption and poor selectivity. In this study, a bifunctional breakthrough was achieved by repurposing a spent hydrodesulfurization (HDS) catalyst into a high‐performance room temperature NH 3 sensor. Taking advantage of the inherent composition of the precursors, a green synthesis route was developed to convert the spent Ni‐W/Al 2 O 3 catalysts into layered nickel tungstate (NiWO 4 ) nanosheets by melt leaching. The derived NiWO 4 sensor exhibits excellent NH₃ sensing performance at room temperature with a response value of 14.76 for 100 ppm of NH 3 , which greatly exceeds that of most reported metal semiconducting oxide (MOS) sensors. The improved sensitivity results from the synergistic effect of the material's lamellar morphology, abundant O V , and optimized charge transport pathways. In addition, the sensor exhibits remarkable selectivity and long‐term stability for NH 3 . This work not only establishes a sustainable pathway for the conversion of hazardous industrial wastes into functional nanomaterials but also promotes the development of energy‐efficient gas‐sensing platforms, in line with the principles of a circular economy and the goals of green chemistry.