Photocatalysis has been known as a promising technique for environmental remediation and clean energy production. However, there are two weaknesses for photocatalyst in powder form applied in the gas-phase catalysis. One is the inadequate contact of gas pollutant as well as utilization of light energy and the other one is the intricate operations for recycle. Herein, we reported a new-type monolithic Pd-embedded g-C3N4/reduced graphene oxide aerogel (Pd-g-C3N4/RGOA) photocatalyst prepared through one-pot thermal reduction method. In this case, a synergistic effect was realized, in which g-C3N4 powder was uniformly dispersed on the surface of RGOA and connected with RGOA through the formation of 2D-2D pi-pi conjugated structure to enhance the contact of gas pollution and utilization of light energy as well as separation of charge carriers, while Pd acted as an electron sink to promote electron-hole separation and its unique electronic structure improved the gas adsorption. As a result, the obtained Pd-g-C3N4/RGOA revealed enhanced hydrogenation of CO2 to CH4 with a max CH4 evolution rate of 6.4 mu mol g(-1) h(-1), which is 12.8-fold enhancement than that of pure g-C3N4. This study provides a fresh insight for the design of efficient monolithic photocatalyst for gas-phase catalysis.