Reactive Separations of CO/CO2 Mixtures over Ru-Co Single Atom Alloys

Developing catalysts to produce synthetic chemicals and fuels from captured CO₂ and renewable H₂ is critical for the development of a circular carbon economy. This process proceeds through a two-step thermo-catalytic pathway in which CO is produced first through the equilibrium limited reverse water-gas shift reaction. In the second step, CO further reacts with hydrogen and itself through Fischer-Tropch synthesis (FTS) to produce value added chemicals and fuels. To operate this process efficiently and safely, full CO₂ conversion during RWGS is unviable due to thermodynamic limitations which may lead to undesirable methane production during the second reaction step. Here, Ru-Co single atom alloy (SAA) catalysts are rigorously examined to determine their viability for FTS with a mixed CO/CO₂ reactant feed. Samples are synthesized using a 2D-layered double hydroxide confinement method. Reactor studies are performed on the Ru-Co SAA and a Co control catalyst at an industrial relevant gas hourly space velocity (GHSV) of 84,000 mL/g/h which showed activity at mild temperatures (200-225 ˚C) and pressure (300 psi) with a high selectivity towards value-added C_2-C₄ olefins (>15%) and C₅₊ products (>40%) with limited methane selectivity (<30%). Above 225 ˚C the Sabatier reaction dominates the kinetics, suggesting a rapid phase change to the Ru-Co SAA. To support these claims, aberration corrected transmission electron microscopy (AC-TEM) is used to demonstrate the formation of a single atom structure. Additional AC-TEM images of catalysts that endured conditions above 225 ˚C show the formation of a carbonaceous layer on the catalyst surface and evidence of a Co phase change. The role of Ru single atoms within the catalyst structure is also investigated as similar reactor performance is observed for both the Co control and Ru-Co SAA catalysts indicating that the Ru single atoms may have minimal effect on reaction performance. Further reactor experimentation indicates that the Ru dopants promote the reduction of the dominant Co species, which enables the catalyst to be active without a pre-reduction step, saving energy and time. This is further supported by the CO diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) which indicates the presence of more electron-rich cobalt centers located on the Ru-Co catalyst than on the Co control catalyst thus facilitating CO reduction and CO binding.