Synthesis gas,composed of H2 and CO,is an important fuel which serves as feedstock for industrially relevant processes,such as methanol or ammonia synthesis.The efficiency of these reactions depends on the H2∶ CO ratio,which can be controlled by a careful choice of reactants and catalyst surface chemistry.Here,using a combination of environmental scanning electron microscopy (ESEM) and online mass spectrometry,direct visualization of the surface chemistry of a Ni catalyst during the production of synthesis gas was achieved for the first time.The insertion of a homebuilt quartz tube reactor in the modified ESEM chamber was key to success of the setup.The nature of chemical dynamics was revealed in the form of reversible oxide-metal phase transitions and surface transformations which occurred on the performing catalyst.The oxide-metal phase transitions were found to control the production of synthesis gas in the temperature regime between 700 and 900 ℃ in an atmosphere relevant for dry reforming of methane (DRM,CO2∶ CH4 =0.75).This was confirmed using high resolution transmission electron microscopy imaging,electron energy loss spectroscopy,thermal analysis,and C18O2 labelled experiments.Our dedicated operando approach of simultaneously studying the surface processes of a catalyst and its activity allowed to uncover how phase transitions can steer catalytic reactions.