Although solar energy is considered as one of the ideal abundant sources of renewable energy to be integrated into the catalytic processing, photonic and thermal excitations were perceived till recently as independent strategies in overcoming Arrhenius-type activation energy barriers in catalytic reactions. However, this dualmode excitation of catalysts is receiving a growing interest nowadays due to promising evidence of synergy effects. A Ru/TiO2 photo-thermo- catalyst was prepared according to a UV-A light photo-assisted synthesis method allowing a fine control of the Ru nanoparticle size distribution, and the influence of a combined photonic (UV-A) and thermal activation on its performances in the gas phase decomposition of formic acid into hydrogen was studied. The results showed that a dual photonic/thermal excitation in a one-pot operation allows to increase the formic acid conversion and consequently the production of hydrogen vs. the reaction in the dark, the enhancement being all the more pronounced as the irradiance is high. The combined excitation allowed to drive the reaction at lower temperatures upon irradiation while maintaining a similar conversion level. This lowtemperature shift was all the more marked as the irradiance was high, and was accompanied by a strong increase in the selectivity to hydrogen. The change in both conversion and selectivity patterns suggested the implication of the light-excited electrons in the non-plasmonic Ru nanoparticles for conducting the reaction through an alternative low-energy transition state, with a lowering of the apparent energy activation, rather than a mechanism of localized-heat delivery.