New Developments in Charge Transfer Multiplet Calculations: Projection Operations, Mixed-Spin States and pi-Bonding

This paper presents a number of new additions to the charge transfer multiplet calculations as used in the calculation of L edge X-ray absorption spectra of 3d and 4d transition metal systems, both oxides and coordination compounds. The focus of the paper is on the consequences of the optimized spectral simulations for the ground state, where we make use of a recently developed projection technique. This method is also used to develop the concept of a mixed-spin ground state, i.e. a state that is a mixture of a high-spin and low-spin state due to spin-orbit coupling combined with strong covalency. The charge transfer mechanism to describe {pi}-bonding uses the mixing of the metal-to-ligand charge transfer (MLCT) channel in addition to the normal CT channel and allows for the accurate simulation of {pi}-bonding systems, for example cyanides

Tuning the Coordination Structure of Cu-N-C Single Atom Catalysts for Simultaneous Electrochemical Reduction of CO2 and NO3 - to Urea

Closing both the carbon and nitrogen loops is a critical venture to support the establishment of the circular, net-zero carbon economy. Although single atom catalysts (SACs) have gained interest for the electrochemical reduction reactions of both carbon dioxide (CO₂RR) and nitrate (NO₃RR), the structure–activity relationship for Cu SAC coordination for these reactions remains unclear and should be explored such that a fundamental understanding is developed. To this end, the role of the Cu coordination structure is investigated in dictating the activity and selectivity for the CO₂RR and NO3RR. In agreement with the density functional theory calculations, it is revealed that Cu-N₄ sites exhibit higher intrinsic activity toward the CO₂RR, whilst both Cu-N₄ and Cu-N₄−x-Cx sites are active toward the NO3RR. Leveraging these findings, CO₂RR and NO₃RR are coupled for the formation of urea on Cu SACs, revealing the importance of *COOH binding as a critical parameter determining the catalytic activity for urea production. To the best of the authors’ knowledge, this is the first report employing SACs for electrochemical urea synthesis from CO₂RR and NO₃RR, which achieves a Faradaic efficiency of 28% for urea production with a current density of −27 mA cm–2 at −0.9 V versus the reversible hydrogen electrode.Josh Leverett, Thanh Tran-Phu, Jodie A. Yuwono, Priyank Kumar, Changmin Kim, Qingfeng Zhai, Chen Han, Jiangtao Qu, Julie Cairney, Alexandr N. Simonov, Rosalie K. Hocking, Liming Dai, Rahman Daiyan, and Rose Ama

Unraveling the structure-activity-selectivity relationships in furfuryl alcohol photoreforming to H2 and hydrofuroin over ZnxIn2S3+x photocatalysts

ZnxIn2S3+x has emerged as a promising candidate for alcohol photoreforming based on C-H activation and C-C coupling. However, the underlying structure-activity-selectivity relationships remain unclear. Here we report on ZnxIn2S3+x with varying Zn:In:S ratios for visible-light-driven furfuryl alcohol reforming into H2 and hydrofuroin, a jet fuel precursor, via C-H activation and C-C coupling. S-• radicals are directly identified as the catalytically active sites responsible for C-H activation in furfuryl alcohol, promoting selectivity toward H2 and hydrofuroin. The optimum ZnxIn2S3+x activity derives from a trade-off between enhanced carrier dynamics and diminished visible light absorption as the x value in ZnxIn2S3+x increases. Further, a higher Zn-S:In-S layer ratio prolongs the S-• lifetime in the Zn-S layer, promoting C-H activation and delivering a higher C-C coupling product selectivity. The findings represent a step toward further establishing sulfide-based photocatalysts for sustainable H2 production via organic photoreforming.Denny Gunawan, Jodie A. Yuwono, Priyank V. Kumar, Akasha Kaleem, Michael P. Nielsen, Murad J.Y. Tayebjee, Louis Oppong-Antwi, Haotian Wen, Inga Kuschnerus, Shery L.Y. Chang, Yu Wang, Rosalie K. Hocking, Ting-Shan Chan, Cui Ying Toe, Jason Scott, Rose Ama

Insight into pH Dependent Formation of Mn Oxide Phases in Electrodeposited Catalytic Films Probed by Soft X Ray Absorption Spectroscopy

MnOx films electrodeposited under basic, neutral, and acidic conditions from an ionic liquid were investigated by means of X ray absorption spectroscopy at the manganese L2,3 edges and the oxygen K edge. Such films can serve as catalysts for the water oxidation reaction. Previous studies showed that the catalytic activity could be controlled by varying the deposition parameters, which influence the formation of MnOx phases and the film composition. Herein the film compositions are investigated in detail, indicating different ratios of MnOx structural phases in the films. All films in the series predominately consist of varying proportions of three MnOx phases Mn2O3, Mn3O4, and birnessite, while an increase of the average Mn oxidation state in the film is identified when going from basic to acidic conditions during electrodeposition. The contribution of these three phases shows a systematic dependency on the pH during electrodeposition. While no specific single MnOx phase was found to dominate the composition of samples that were previously found to show high catalytic activity, the X ray spectroscopic results revealed the compositions of those samples prepared under close to neutral conditions to be most sensitive to changes in p

Water Oxidation Catalysis by Nanoparticulate MnOx Thin Films Probing the Effect of the Manganese Precursors

Nanoparticulate MnOx, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyse water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnOx present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn II species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn Me3TACN OMe 3] and [ Me3TACN 2MnIII2 amp; 956; O amp; 956; CH3COO 2]2 Me3TACN N,N ,N trimethyl 1,4,7 triazacyclononane revealed that highly dispersed MnOx nanoparticles 10 20nm were generated in the Nafion film. In contrast, the use of [Mn OH2 6]2 resulted in both higher manganese oxide loading and aggregated nanoparticles with 50nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies ca. 10 fold higher per Mn center were measured for the MnOx derived from molecular precursors than for the material formed from [Mn OH2 6]2 . Thus, the catalytic activity is dependent on the ability to generate well defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnOx and to further evaluate the origin of differences in catalytic activit

Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d-Electrons into the Porphyrin Ligand

Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin π->π* transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P->Fe σ and π-donation and Fe->P π back-bonding. We find that π-donation to Fe(III) is much larger than back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry

Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d-Electrons into the Porphyrin Ligand

Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin π->π* transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P->Fe σ and π-donation and Fe->P π back-bonding. We find that π-donation to Fe(III) is much larger than back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry

Evolution of Oxygen Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X Ray Spectroscopy

Manganese oxide (MnOx) electrocatalysts are examined herein by in situ soft X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L‐edge XAS data with previous mechanistic studies indicates MnIV is the highest oxidation state involved in the catalytic mechanism. MnOx is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn−O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K‐edge provide complementary evidence for such a transition. This step is crucial for the formation of O2 from water

Impurity Tolerance of Unsaturated Ni-N-C Active Sites for Practical Electrochemical CO2 Reduction

Demonstrating the potential of the electrochemical carbon dioxide reduction reaction (CO2RR) in industrially relevant conditions is a promising route for achieving net-zero emissions through decarbonization. This requires a catalyst system that displays not only high activity and stability but also the capacity to deliver a consistent performance in the presence of waste stream impurities. To explore these opportunities, we investigate the role that the Ni coordination structure plays on the impurity tolerance of highly active single-atom catalysts (SACs) during CO2RR. The as-synthesized materials are highly active for CO2RR to CO, achieving a current density of 470 mA cm−2 and a CO selectivity of 99% in a CO2 electrolyzer. We demonstrate, through high-temperature pyrolysis, that a higher concentration of “unsaturated” Ni-N4‑x-Cx sites significantly improves the tolerance to NOx, SOx, volatile organic compounds, and SCN− impurities in aqueous electrolyte, paving the way for SACs capable of CO2RR in industrial conditions.Josh Leverett, Jodie A. Yuwono, Priyank Kumar, Thanh Tran-Phu, Jiangtao Qu, Julie Cairney, Xichu Wang, Alexandr N. Simonov, Rosalie K. Hocking, Bernt Johannessen, Liming Dai, Rahman Daiyan, and Rose Ama