Facet-associated ZnO/TiO2 interfaces for photocatalytic purification of trace methane from air under humid conditions

Photocatalytic removal of trace methane at room temperature is hindered by low partial pressure, weak adsorption, and sluggish C H bond activation. Here, anatase T(100), T(101), and their ZnO-modified counterparts were evaluated in a closed CH 4 /air system containing approximately 400 ppm CH 4 . Under the standard humidification protocol, the measured inlet water-vapor concentration was 30,900 ± 600 ppmv (3.09 ± 0.06 vol%). ZnO modification enhanced apparent CH 4 removal, reaching 91.70 ± 0.49% for 10%ZnO-T(101) after 1 h of UV irradiation. The 5%ZnO samples were selected for mechanistic comparison because they retained a more pronounced facet-associated difference. HRTEM and UV-Raman confirm the presence of nanoscale crystalline ZnO domains in intimate contact with anatase TiO 2 . PL, transient photocurrent, and EIS indicate improved charge separation and interfacial transport for 5%ZnO-T(101), while TPD, XPS O 1 s, and EPR support weaker retention of CO 2 /carbonate-related species and stronger light-induced ·OH and ·O 2 − signals. At approximately 1000 ppm CH 4 , 10%ZnO-T(101) achieved a CH 4 conversion of 78.46 ± 0.57%, a CO 2 mineralization yield of 60.78 ± 0.50%, and a gas-phase carbon closure of 85.43 ± 0.50%. CO 2 accounted for 77.47 ± 0.68% of converted carbon and CO for 3.96 ± 0.31%; the unclosed 14.57% carbon fraction may include low-abundance oxygenated intermediates, surface-associated carbon species, and analytically unresolved trace products. Four humid-state cycles retained 97.56 ± 0.24% of the initial activity. These results highlight facet-associated ZnO/TiO 2 contact effects involving improved charge utilization, H 2 O/O 2 activation, and product-species regulation.