More Like this

1. Modeling Current-Potential Responses of Homogeneous-Heterogeneous Photocathodes

   Wadsworth, B. L.; Beiler, A. M; Khusnutdinova, D.; Moore, G. F. (April 2019, Abstract of Papers, Materials and Research Society Spring Meeting and Exhibit)

   Chemical modification of semiconductor surfaces with molecular electrocatalysts provides a strategy for developing integrated homogeneous-heterogeneous materials capable of converting sunlight to fuels and other value-added products, but their development is hampered by an incomplete understanding of the factors limiting their performance. Although kinetic models have been separately developed to describe photoelectrochemical or homogeneous electrocatalytic reactions, related modeling for molecular-modified hybrid photoelectrodes has not been as extensively elaborated. This presentation addresses the interplay between light absorption, charge transfer, and catalytic activity during photoelectrosynthetic transformations at a molecular-modified semiconductor surface. The analysis provides opportunities to better understand the principles governing these hierarchal constructs and develop improved photocatalytic assemblies. 

   more » « less
2. Analysis of multi-electron, multi-step homogeneous catalysis by rotating disc electrode voltammetry: theory, application, and obstacles

   https://doi.org/10.1039/C9AN02192B  

   Lee, Katherine J.; Gruninger, Cole T.; Lodaya, Kunal M.; Qadeer, Saad; Griffith, Boyce E.; Dempsey, Jillian L. (February 2020, The Analyst)

   Rotating disc electrode (RDE) voltammetry has been widely adopted for the study of heterogenized molecular electrocatalysts for multi-step fuel-forming reactions but this tool has never been comprehensively applied to their homogeneous analogues. Here, the utility and limitations of RDE techniques for mechanistic and kinetic analysis of homogeneous molecular catalysts that mediate multi-electron, multi-substrate redox transformations are explored. Using the ECEC′ reaction mechanism as a case study, two theoretical models are derived based on the Nernst diffusion layer model and the Hale transformation. Current–potential curves generated by these computational strategies are compared under a variety of limiting conditions to identify conditions under which the more minimalist Nernst Diffusion Layer approach can be applied. Based on this theoretical treatment, strategies for extracting kinetic information from the plateau current and the foot of the catalytic wave are derived. RDEV is applied to a cobaloxime hydrogen evolution reaction (HER) catalyst under non-aqueous conditions in order to experimentally validate this theoretical framework and explore the feasibility of RDE as a tool for studying homogeneous catalysts. Crucially, analysis of the foot-of-the-wave via this theoretical framework provides rate constants for elementary reaction steps that agree with those extracted from stationary voltammetric methods, supporting the application of RDE to study homogeneous fuel-forming catalysts. Finally, obstacles encountered during the kinetic analysis of cobaloxime, along with the voltammetric signatures used to diagnose this reactivity, are discussed with the goal of guiding groups working to improve RDE set-ups and help researchers avoid misinterpretation of RDE data. 

   more » « less
3. Heterogeneous Nature of Electrocatalytic CO/CO ₂ Reduction by Cobalt Phthalocyanines

   https://doi.org/10.1002/cssc.202001396  

   Wu, Yueshen; Hu, Gongfang; Rooney, Conor_L; Brudvig, Gary_W; Wang, Hailiang (July 2020, ChemSusChem)

   Abstract Molecular catalysts for electrochemical CO₂reduction have traditionally been studied in their dissolved states. However, the heterogenization of molecular catalysts has the potential to deliver much higher reaction rates and enable the reduction of CO₂by more than two electrons. In light of the recently discovered reactivity of heterogenized cobalt phthalocyanine molecules to catalyze CO₂reduction into methanol, direct comparison is needed to uncover the distinct catalytic activity and selectivity in homogeneous catalysis versus heterogeneous catalysis. Herein, soluble cobalt phthalocyanine derivatives were synthesized, and their catalytic activities in the homogeneous solutions were evaluated. The results show that the observed catalytic activities for both CO₂‐to‐CO and CO‐to‐methanol conversions in aqueous solutions of the cobalt phthalocyanines are predominantly heterogeneous in nature through the adsorbed species on the electrode. 

   more » « less
4. Modeling the roles of rigidity and dopants in single-atom methane-to-methanol catalysts

   https://doi.org/10.1039/D1TA08502F  

   Jia, Haojun; Nandy, Aditya; Liu, Mingjie; Kulik, Heather J. (January 2022, Journal of Materials Chemistry A)

   Doped graphitic single-atom catalysts (SACs) with isolated iron sites have similarities to natural enzymes and molecular biomimetics that can convert methane to methanol via a radical rebound mechanism with high-valent Fe( iv )O intermediates. To understand the relationship of SACs to these homogeneous analogues, we use range-separated hybrid density functional theory (DFT) to compare the energetics and structure of the direct metal-coordinating environment in the presence of 2p ( i.e. , N or O) and 3p ( i.e. , P or S) dopants and with increasing finite graphene model flake size to mimic differences in local rigidity. While metal–ligand bond lengths in SACs are significantly shorter than those in transition-metal complexes, they remain longer than SAC mimic macrocyclic complexes. In SACs or the macrocyclic complexes, this compressed metal–ligand environment induces metal distortion out of the plane, especially when reactive species are bound to iron. As a result of this modified metal-coordination environment, we observe SACs to simultaneously favor the formation of the metal–oxo while also allowing for methanol release. This reactivity is different from what has been observed for large sets of square planar model homogeneous catalysts. Overall, our calculations recommend broader consideration of dopants ( e.g. , P or S) and processing conditions that allow for local distortion around the metal site in graphitic SACs. 

   more » « less
5. Stability analysis for electromagnetic waveguides. Part 2: non-homogeneous waveguides

   https://doi.org/10.1007/s10444-024-10130-x  

   Demkowicz, Leszek; Melenk, Jens M; Badger, Jacob; Henneking, Stefan (April 2024, Advances in Computational Mathematics)

   This paper is a continuation of Melenk et al., "Stability analysis for electromagnetic waveguides. Part 1: acoustic and homogeneous electromagnetic waveguides" (2023), extending the stability results for homogeneous electromagnetic (EM) waveguides to the non-homogeneous case. The analysis is done using perturbation techniques for self-adjoint operators eigenproblems. We show that the non-homogeneous EM waveguide problem is well-posed with the stability constant scaling linearly with waveguide length L. The results provide a basis for proving convergence of a Discontinuous Petrov-Galerkin (DPG) discretization based on a full envelope ansatz, and the ultraweak variational formulation for the resulting modified system of Maxwell equations, see Part 1. 

   more » « less