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Mesoporous nickel oxide and Pt@NiO nanocomposites were synthesized for the study of CO 2 methanation in the fixed bed reactor, atmospheric pressure, and photocatalytic degradation of methyl orange. All synthesized catalysts were characterized by XRD, N 2 isotherms, TEM, UV-Vis DRS, and ICP-MS analysis. NiO with 8 nm Pt nanoparticles was the most active catalyst in photocatalysis, however, the 2 nm Pt nanoparticles were most active in the thermal CO 2 activation reaction. High CO 2 consumption and CH 4 formation rates were obtained for Pt@NiO nanocomposites below 673 K compared with bare NiO by the formation of Pt/PtO x /Ni/NiO x interface in reaction condition. Pt2@NiO nanocomposite has shown a high CO 2 consumption rate because of high dispersion of 2 nm Pt nanoparticles within mesoporous NiO and high Pt to Ni surface area during the reaction. The photocatalytic degradation efficiency of Pt8@NiO nanocomposite was higher compared to that of bare NiO by a decrease in the recombination of electron-hole pair under UV-Vis irradiation.Keywords Size controlled pt nanoparticles • Mesoporous NiO • CO 2 methanation • Methyl orange