Production of high purity hydrogen from fossil fuels or renewable feedstocks requires efficient water-gas shift (WGS) catalysts to remove traces of carbon monoxide, a catalyst poison for, for example, proton-exchange membrane fuel cells (PEMFCs) and ammonia catalysts. [1] State of the art heterogeneous WGS catalysis is performed in a combination of high-temperature shift (HTS; Fe 2 O 3 /Cr 2 O 3 , T = 450 8C, P = 3 MPa) and low-temperature shift (LTS, CuO/ZnO/Al 2 O 3 , T = 200 8C), reducing the CO level to 0.1-0.3 wt %. [2] This level is still higher than acceptable for most direct applications of the obtained hydrogen and therefore additional purification steps, such as selective oxidation, methanization, or adsorption, are required. To reduce the equilibrium CO level in the exothermic WGS reaction further, catalysts are required that efficiently operate at temperatures below 180 8C (ULT, ultra-low temperature), a goal that could never be achieved with the known commercial heterogeneous catalytic systems. [3] A promising alternative to heterogeneous WGS catalysts is the clever application of homogeneous transition metal complexes in multiphase systems. Some examples of homogeneous WGS catalysis, such as, for example, [Ru 3 (CO) 12 ] in trimethylamine/water by Pettit et al. or the pyridine-modified ruthenium systems in the group of Pakkanen, have been reported. [4] However, moderate activities, the requirement for high total pressures (typically 2.5 MPa) and catalyst recycling problems have been major drawbacks of these systems. For future hydrogen production scenarios based on decentralized biogas conversion, high WGS activity at atmospheric pressure is mandatory.Recently, we have shown that WGS catalysts based on supported ionic liquid phase (SILP) technology [5] are active under very mild reaction conditions below 160 8C and 0.1 MPa. [6] The SILP WGS catalyst previously reported by our group consisted of a RuCl 3 catalyst precursor dissolved in the ionic liquid (IL) 1butyl-2,3-dimethylimidazolium trifluoromethanesulfonate [BMMIM][OTf], highly dispersed as a thin film on silica gel. [6] Using a model feedstock of H 2 O and CO in a continuous screening rig, an activity of 3 mol H 2 mol À1 Ru h À1 was attained at 160 8C and 0.
