Towards a standardization of biomethane potential tests

Production of biogas from different organic materials is a most interesting source of renewable energy. The biomethane potential (BMP) of these materials has to be determined to get insight in design parameters for anaerobic digesters. Although several norms and guidelines for BMP tests exist, inter-laboratory tests regularly show high variability of BMPs for the same substrate. A workshop was held in June 2015, in Leysin, Switzerland, with over 40 attendees from 30 laboratories around the world, to agree on common solutions to the conundrum of inconsistent BMP test results. This paper presents the consensus of the intense roundtable discussions and cross-comparison of methodologies used in respective laboratories. Compulsory elements for the validation of BMP results were defined. They include the minimal number of replicates, the request to carry out blank and positive control assays, a criterion for the test duration, details on BMP calculation, and last but not least criteria for rejection of the BMP tests. Finally, recommendations on items that strongly influence the outcome of BMP tests such as inoculum characteristics, substrate preparation, test setup, and data analysis are presented to increase the probability of obtaining validated and reproducible results.info:eu-repo/semantics/publishedVersio

Cobalt Complexes of Polypyridyl Ligands for the Photocatalytic Hydrogen Evolution Reaction

The reductive part of artificial photosynthesis, the reduction of protons into H2, is a two electron two proton process. It corresponds basically to the reactions occurring in natural photosystem I. We show in this review a selection of involved processes and components which are mandatory for making this light-driven reaction possible at all. The design and the performances of the water reduction catalysts is a main focus together with the question about electron relays or sacrificial electron donors. It is shown how an original catalyst is developed into better ones and what it needs to move from purely academic homogeneous processes to heterogeneous systems. The importance of detailed mechanistic knowledge obtained from kinetic data is emphasized

Influence of Hetero-Biaryl Ligands on the Photo-Electrochemical Properties of [ReINCS(N∩N)(CO)3]-Type Photosensitizers

A complete set of the symmetrical bidiazine (bdz) and asymmetrical pyridyldiazine (pydz) ligands, along with the known 2,2′‐bipyridyl (bpy) ligand, and their respective rhenium(I)‐tricarbonyl‐bromo and ‐thiocyonato complexes are presented. A bathochromic shift is observed with an increasing number of nitrogen atoms, caused by a stabilization of the diimine based LUMO. As expected from the energy gap law, this results in an increase in the non‐radiative decay constant (knr) along the line bpy‐pydz‐bdz. Interestingly, the increase in knr for both pyridazine ligands (2‐(pyridin‐2‐yl)pyridazine (pypdz) and 3,3′‐bipyridazine (bpdz)) is much less pronounced as compared to the other pydz resp. bdz ligands. This results in overall longer lifetime and quantum yield, at the increased spectral overlap with sunlight – both properties sought for in the creation of light‐harvesting schemes in photochemistry

Shedding Light on the Molecular Surface Assembly at the Nanoscale Level: Dynamics of a Re(I) Carbonyl Photosensitizer with a Coadsorbed Cobalt Tetrapyridyl Water Reduction Catalyst on ZrO2

We present systematic kinetic studies of the interaction of a rhenium-based photosensitizer with a cobalt(II) tetrapyridyl water reduction catalyst coadsorbed on ZrO2 by transient IR and visible spectroscopies. The study focuses on the competition between the reduction of the excited photosensitizer by an electron donor in solution and nonproductive quenching between the photosensitizer and the catalyst, either by Dexter energy transfer or by electron transfer followed by ultrafast geminate recombination. The implications of both interactions for the charge transfer reactions on the surface are investigated. We find that the kinetics of the system as a whole and the achievable yield of reduced photosensitizer are determined by the inhomogeneous distribution of next neighbor distances between photosensitizers and the water reduction catalysts at the nanoscale level. This provides insight for rational design of heterogeneous water splitting systems with coimmobilized photosensitizers and catalysts

Cobalt Complexes of Polypyridyl Ligands for the Photocatalytic Hydrogen Evolution Reaction

The reductive part of artificial photosynthesis, the reduction of protons into H2, is a two electron two proton process. It corresponds basically to the reactions occurring in natural photosystem I. We show in this review a selection of involved processes and components which are mandatory for making this light-driven reaction possible at all. The design and the performances of the water reduction catalysts is a main focus together with the question about electron relays or sacrificial electron donors. It is shown how an original catalyst is developed into better ones and what it needs to move from purely academic homogeneous processes to heterogeneous systems. The importance of detailed mechanistic knowledge obtained from kinetic data is emphasized

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

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air

Comparative Study of the Different Anchoring of Organometallic Dyes on Ultrathin Alumina

Significant improvements in incident photon-to-current efficiencies can be obtained by covering inorganic semiconductors with ultrathin alumina films and sensitizing them with adsorbed dye molecules. The anchoring mode of the latter to the substrate affects the charge transport between the dye and the electrode via tunneling, and consequently, the device efficiency. In this work, we employ X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) for a comparative study of the adsorption of three rhenium and two ruthenium organometallic dyes, one of each being ionic, with different anchoring modes on single-crystalline ultrathin alumina films. Molecular monolayers were prepared by self-assembly from solution. Quantitative XPS analysis reveals higher surface densities for the Re dyes. Nearly stoichiometric coadsorption of counterions is observed for the ionic dyes. Density functional theory (DFT) calculations for the Re dyes show that the most stable adsorption configurations exhibit the expected bonding via the dedicated anchoring groups (carboxyls or methylphosphonic acid), with an additional sulfur–aluminum bond for the dyes containing a thiocyanate ligand. The alignment of the occupied molecular levels with respect to the alumina valence band maximum, obtained for these geometries, follows the experimental trend in the UPS data and places the lowest unoccupied molecular orbitals (LUMOs) close to the Fermi level of the systems, far inside the alumina band gap. Dynamical charge screening is found to be important for this type of system when comparing UPS and DFT results. This work provides a general guideline for the systematic characterization of related molecules on surfaces

Structure-Activity and Stability Relationships for Cobalt Polypyridyl-Based Hydrogen-Evolving Catalysts in Water

A series of eight new and three known cobalt polypyridyl-based hydrogen-evolving catalysts (HECs) with distinct electronic and structural differences are benchmarked in photocatalytic runs in water. Methylene-bridged bis-bipyridyl is the preferred scaffold, both in terms of stability and rate. For a cobalt complex of the tetradentate methanol-bridged bispyridyl–bipyridyl complex [CoIIBr(tpy)]Br, a detailed mechanistic picture is obtained by combining electrochemistry, spectroscopy, and photocatalysis. In the acidic branch, a proton-coupled electron transfer, assigned to formation of CoIII−H, is found upon reduction of CoII, in line with a pKa(CoIII−H) of approximately 7.25. Subsequent reduction (−0.94 V vs. NHE) and protonation close the catalytic cycle. Methoxy substitution on the bipyridyl scaffold results in the expected cathodic shift of the reduction, but fails to change the pKa(CoIII−H). An analysis of the outcome of the benchmarking in view of this postulated mechanism is given along with an outlook for design criteria for new generations of catalysts