Processes based on non-thermal plasma (NTP) for indoor air treatment inevitably lead to the formation of toxic by-products such as ozone (O3) and nitrogen oxides (NOx).Adding a step of heterogeneous catalysis in series with NTP could allow for the decomposition of the by-products.Therefore,different catalysts were developed based on transition metal oxides,such as NiOx,CoOx and MnOx with different weight percentage 1,5 and 10 wt.％,deposited on a γ-Al2O3 support.The O3 removal efficiency (ORE) and the NOx removal efficiency (NRE) were very encouraging in dry air:about 65％ and 80％,respectively,by using 2 g 5 wt.％ MnOx/Al2O3 catalyst under the experimental conditions.However,strongly negative effects of relative humidity (RH) on the catalytic decomposition performance were observed.To overcome this limitation,the catalyst surface was modified to make it hydrophobic using a cost-effective chemical grafting method.This treatment consisted in impregnating the 5 wt.％ MnOx/Al2O3 catalyst with different trichloro(alkyl)silanes (TCAS).The effects of different linker lengths and amounts of TCAS for the hydrophobicity and the decomposition performance of surface-modified catalysts under humid conditions were investigated.Our results show that the surface-modified catalyst with the shortest linker and 0.25 mmol/gcat of modifying agent represents the best catalytic decomposition performance for O3.Its ORE is 41％ at 60％ RH,which is twice that of the non-modified catalyst.