A Simple Test Case for Convergence Order in Time and Energy Conservation of Black-Box Coupling Schemes

The most commonly used coupling schemes in partitioned multiphysics simulations suffer from a decrease in the order of convergence, specifically in the time domain; a phenomenon we call order degradation. This paper discusses when this issue arises and how it can be studied with a simple example. We present a simple mass-spring system of ordinary differential equations (ODEs) to analyze accuracy and energy conservation of different coupling schemes. The ability to restore higher order of convergence by using Strang splitting or waveform iterations is verified in the context of the presented example. This paper provides details on some aspects of the talk titled 'Design and evaluation of a waveform iteration­based approach for coupling heterogeneous time stepping methods via preCICE' given at WCCM-APCOM 2022

Mechanism of Photocatalytic Hydrogen Generation by a Polypyridyl-Based Cobalt Catalyst in Aqueous Solution

The mechanism of photocatalytic hydrogen production was studied with a three-component system consisting of fac-[Re(py)(CO)(3)bipy](+) (py = pyridine, bipy = 2,2'-bipyridine) as photosensitizer, [Co(TPY-OH)(OH2)](2+) (TPY-OH = 2-bis(2-pyridyl)(hydroxy)methyl-6-pyridylpyridine), a polypyridyl-based cobalt complex, as water reduction catalyst (WRC), and triethanolamine (TEOA) as sacrificial electron donor in aqueous solution. A detailed mechanistic picture is provided, which covers all processes from excited state quenching on the time scale of a few nanoseconds to hydrogen release taking place between seconds and minutes at moderately basic reaction conditions. Altogether these processes span 9 orders of magnitude in time. The following reaction sequence was found to be the dominant pathway for hydrogen generation: After reductive quenching by TEOA, the reduced photosensitizer (PS) transfers an electron to the Co-II-WRC. Protonation of CoI yields (CoH)-H-III which is reduced in the presence of excess CoI. (CoH)-H-II releases hydrogen after a second protonation step, which is detected time-resolved by a clark-type hydrogen electrode. Aside from these productive steps, the role of side and back reactions involving TEOA-derived species is assessed, which is particularly relevant in laser flash photolysis measurements with significantly larger transient concentrations of reactive species as compared to continuous photolysis experiments. Most notable is an equilibrium reaction involving Co-I, which is explained by a nucleophilic addition of Co-I to the oxidation product of TEOA, an electrophilic iminium ion. Quantum chemical calculations indicate that the reaction is energetically feasible. The calculated spectra of the adduct are consistent with the spectroscopic observations

Quinones as reversible electron relays in artificial photosynthesis

We explore the potential of various hydroquinone/quinone redox couples as electron relays in a homogenous water reduction system between a Re-based photosensitizer and a sacrificial electron donor [tris-(2-carboxyethyl)-phosphine, TCEP]. By using transient IR spectroscopy, flash photolysis as well as stopped-flow techniques covering timescales from picoseconds to 100 ms, we determine quenching rates and cage escape yields, the kinetics of the follow-up chemistry of the semiquinone, the recombination rates, as well as the re-reduction rates by TCEP. The overall quantum yield of hydrogen production is low, and we show that the limiting factors are the small cage escape yields and, more importantly, the slow regeneration rate by TCEP in comparison to the undesired charge recombination with the reduced water reduction catalyst

Mechanism of Photocatalytic Hydrogen Generation by a Polypyridyl-Based Cobalt Catalyst in Aqueous Solution

The mechanism of photocatalytic hydrogen production was studied with a three-component system consisting of <i>fac</i>-[Re­(py)­(CO)₃bipy]⁺ (py = pyridine, bipy = 2,2′-bipyridine) as photosensitizer, [Co­(TPY-OH)­(OH₂)]²⁺ (TPY-OH = 2-bis­(2-pyridyl)­(hydroxy)­methyl-6-pyridylpyridine), a polypyridyl-based cobalt complex, as water reduction catalyst (WRC), and triethanolamine (TEOA) as sacrificial electron donor in aqueous solution. A detailed mechanistic picture is provided, which covers all processes from excited state quenching on the time scale of a few nanoseconds to hydrogen release taking place between seconds and minutes at moderately basic reaction conditions. Altogether these processes span 9 orders of magnitude in time. The following reaction sequence was found to be the dominant pathway for hydrogen generation: After reductive quenching by TEOA, the reduced photosensitizer (PS) transfers an electron to the Co^II–WRC. Protonation of Co^I yields Co^IIIH which is reduced in the presence of excess Co^I. Co^IIH releases hydrogen after a second protonation step, which is detected time-resolved by a clark-type hydrogen electrode. Aside from these productive steps, the role of side and back reactions involving TEOA-derived species is assessed, which is particularly relevant in laser flash photolysis measurements with significantly larger transient concentrations of reactive species as compared to continuous photolysis experiments. Most notable is an equilibrium reaction involving Co^I, which is explained by a nucleophilic addition of Co^I to the oxidation product of TEOA, an electrophilic iminium ion. Quantum chemical calculations indicate that the reaction is energetically feasible. The calculated spectra of the adduct are consistent with the spectroscopic observations