The design of the good-performance materials for toxic formaldehyde (CH₂O)-gas-detection is critical for environmental preservation and human health. In this work, density functional theory (DFT) calculations were employed to investigate the adsorption behavior and electronic properties of CH₂O on transition metal (TM)-doped phthalocyanine monolayers. Our results prove that PdPc and RuPc monolayers are thermodynamically stable. Analysis of the adsorption energy showed that the CH₂O gas molecule was chemisorbed on the RuPc monolayer, while it was physisorbed on the PdPc nanosheet. The microcosmic interaction mechanism within the gas-adsorbent system was revealed by analyzing the density of states, the charge-density difference, the electron-density distribution, and the Hirshfeld charge transfer. Additionally, the RuPc monolayer was highly sensitive to CH₂O due to the obvious changes in electrical conductivity, and the recovery time of CH₂O molecule was predicted to be 2427 s at room temperature. Therefore, the RuPc monolayer can be regarded as a promising gas-sensing material for CH₂O detection.

The identification and mitigation of SF₆ decomposition gases (SO₂, H₂S, HF, SOF₂, and SO₂F₂) are critical for environmental protection and human health. In this study, we utilized density functional theory (DFT) to systematically investigate the adsorption characteristics and sensing properties of these gases on pristine nickel-porphyrin (NiPr) and nonmetal-atom-modified nickel-porphyrin (NiNPr, N = B/O) monolayers. The studied results indicate that the NiPr monolayer is not suitable as a gas-sensitive material for the detection of SF₆ decomposition gas molecules due to its weak adsorption ability and low sensitivity. However, the incorporation of nonmetallic B and O atoms into the NiPr monolayer significantly enhances the binding energy, ensuring the structural stability of the resulting NiBPr and NiOPr monolayers. The bandgap values of these doped monolayers are reduced to 0.326 eV (NiBPr) and 0.057 eV (NiOPr) compared to 0.985 eV for the pristine NiPr monolayer. Notably, the NiBPr and NiOPr monolayers demonstrate enhanced adsorption energy for SO₂, H₂S, SOF₂, and HF gases (−0.56 eV to −1.08 eV), indicating chemical adsorption. A comprehensive analysis of the total electron density distribution, charge density difference, and density of states was conducted to elucidate the interaction mechanisms between the gases and the monolayers. Additionally, significant changes in work function and bandgap values confirm the high sensitivity of these monolayers. Recovery time assessments for gases including SO₂, H₂S, HF, SOF₂, and SO₂F₂ reveal that the NiBPr monolayer is viable as a reusable gas sensor for SO₂, H₂S, HF, and SOF₂, while the NiOPr monolayer is apt for sensing H₂S, HF, and SOF₂ gases. These findings are significant in highlighting the potential of NiBPr and NiOPr monolayers as innovative sensors for the SF₆ decomposition gases.