EPR and X-Ray Spectroscopy Characterization of Reported Mono-Ruthenium Water Splitting Catalysts

Technology to convert Solar energy into fuel is being developed to search for alternate energy sources. This includes electrochemical cells based on artificial water splitting catalysts. By studying their water splitting mechanism we can hope to optimize the energy conversion process and also learn more natural water splitting complexes like Photosystem II. In this study, we analyzed two reported single site Ruthenium based catalysts, [RuII(bpy)(tpy)H2O]2+ and [RuII(bpy)(tpy)Cl]+. [RuII(bpy)(tpy)H2O]2+ is a widely studied water splitting catalyst for which [RuV(bpy)(tpy)= O]3+ has been detected as a rate limiting intermediate. However, using EPR, we see that this intermediate is not present when the sample is measured with CeIV in D2O or HNO3. Using, Ru-K edge EXAFs, we also see that the Ru=O distance measured after adding CeIV is more consistent with its assignment as a [RuIV(bpy)(tpy)=O]2+ intermediate and not with RuV=O.We also found that [RuII(bpy)(tpy)Cl]+ is not a catalyst and that its oxidation past RuIII is impeded by a lack of proton coupled electron transfer. Using L-edge and K-edge XANES spectra, we show that the edge position of the [RuII(bpy)(tpy)Cl]+ spectra is more consistent with the presence of [RuIII(bpy)(tpy)Cl]+. EXAFS analysis of [RuII(bpy)(tpy)Cl]+ also show shortening of the Ru-Cl bond (2.41 A to 2.34 A), implying a transition from RuII to RuIII. This is further tested in an Oxygen evolution assay where the sample is incubated in H2O for different time periods. We show that samples that are not incubated for long times show no activity, hence implying that a Cl- to H2O ligand exchange is necessary for [RuII(bpy)(tpy)Cl]+ to show catalytic activity. These studies give us crucial insight into the water splitting mechanism of these catalysts.