A potential solution for the rise in CO₂ emissions is to convert CO₂ into valuable chemical compounds and fuels utilizing renewable energy. Due to their high reactivity, Pt-based nanoparticles are interesting candidates as catalysts for fuel cells, CO₂ hydrogenation, oxygen reduction reaction, etc. Important catalysts transformations that can occur during a chemical reaction may lead to catalyst deactivation (coalescence of particles, surface poisoning, etc). This study utilizes an environmental cell TEM holder to follow the materials microstructure evolution under controlled gaseous environment, high temperature and pressure, providing a better understanding of the catalyst behavior during the reaction and its relation to the catalytic reaction products. Pt-Co hollow nanospheres (HNS) with mean outer diameter of 15 nm±2nm and a shell thickness of 2-3nm have been synthesized using a well-defined ratio of Pt salts (H₂PtCl₆) and Co NPs as precursors. The electron tomography (ET) essays confirmed the complex 3D structure of the HNS with the shell constituted by small nanoparticles (NPs) with diameters of 2 nm. Moreover, the ET allowed one to identify the presence of pores within the shell, which is an indication of an augmentation of the active surface area as the accessibility to the inner surface of the nanosphere is ensured. The catalysts morphological evolution was monitored using a closed cell experimental device (Protochips: Atmospehere) under CO₂ and H₂ mixture (H₂/CO₂=4 instead of a ratio of 3 as most ex situ experiments, since increasing the H₂/CO₂ input ratio would boost the conversion at low temperatures) at the atmospheric pressure and by varying the temperature with a rate of 20°C/min. In the same time, the evolution of the reaction products (HCO₂H, CH₃OH and H₂O) was monitored. As expected, the growth of the shell NPs via Ostwald ripening mechanism was identified, such that at 240°C, a continuous shell is formed. Although the accessibility to the inner surface is blocked the reaction was still occurring on the outer surface of the nanospheres up to 450°C. Beyond this temperature, the nanospherical microstructure starts collapsing, process accompanied by the increase of the reaction products. This suggests that a part of the reaction products is encapsulated within the HNS inner volume and released upon their collapse.