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Sampaio, Renato N.

Publications and source records attributed to Sampaio, Renato N..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Catalytic Reduction of Carbon Monoxide to Liquid Fuels with Recyclable Hydride Donors

Solar light absorption and catalysis are physically separated processes in natural photosynthesis. Natural cofactors, such as nicotinamide adenine dinucleotides (NADH), transport electrons and hydrogen to regulate and activate enzymes at remote locations. The physical separation of light absorption from catalysis provides some inspiration for artificial photosynthesis. One rather extreme implementation is to use copper wires to transport carriers from photovoltaic cells to dark electrodes, where catalysis occurs. Indeed, with a futuristic electrical grid powered solely by photovoltaics, solar capture could be separated from catalysis by hundreds of miles. An alternative approach, that bares more similarity to natural photosynthesis, employs mobile NADH/NAD + -like species that shuttle between the light absorber and a proximate, yet unilluminated, location where catalysis occurs. Additionally, such a remote approach to solar photocatalysis was recently proposed for the reduction of carbon oxides, CO 2 and CO, to methanol by cascade catalysis. This developing artificial photosynthetic approach offers the promise of catalytic generation of methanol and oxygen gas with sunlight as the sole energy source and CO 2 and water as the only chemical feedstocks. This Viewpoint evaluates the strengths and weaknesses of this approach with an emphasis on CO reduction catalysis with photorecyclable hydride donors while looking forward to what might reasonably be achieved with continued research.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Methyl Termination of p-Type Silicon Enables Selective Photoelectrochemical CO 2 Reduction by a Molecular Ruthenium Catalyst

Methyl-terminated p-type silicon photoelectrodes selectively drive CO 2 reduction by a homogeneous [Ru(tpy)(Mebim-py)(NCCH 3 )] 2+ catalyst (tpy = 2,2′:6′,2″-terpyridine, Mebim-py = 1-methylbenzimidazol-2-ylidene-3-(2′-pyridine)). A 460 mV photovoltage is quantified for the photoelectrode. Under 1 sun illumination, this system achieves a Faradaic efficiency of 87% for CO at −1.7 V vs Fc +/0 , matching reports of the same catalyst at metallic electrodes operating at −2.1 V. When 5% water is introduced, the CH 3 -terminated Si photoelectrode remains stable, selectivity for CO is retained, and current density increases. Methyl termination suppresses the competitive hydrogen evolution observed for H-terminated Si photoelectrodes, which under the same conditions produce ca. 60% CO and 8% H 2 and have unstable performance. Furthermore, these results establish that a semiconductor photoelectrode can power a molecular CO 2 reduction catalyst without hydrogen evolution by the photoelectrode itself. Methyl termination of p-Si allows CO 2 reduction to kinetically outcompete proton reduction, revealing an important design principle for selective fuel formation.

Catalysts↗
DOE OSTI2024

Reduction of CO to Methanol with Recyclable Organic Hydrides

We report the reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H 2 generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of cis-[Ru(bpy) 2 (CO) 2 ] 2+ (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, cis-[Ru(bpy) 2 (CO)(CHO)] + , and the corresponding benzimidazolium on a seconds time scale. Kinetic experiments revealed a first-order dependence on the benzimidazole hydride concentration and an unusually large kinetic isotope effect, inconsistent with direct hydride transfer and more likely to occur by an electron transfer-proton-coupled electron transfer (EΤ-PCET) or related mechanism. Further reduction/protonation of cis-[Ru(bpy) 2 (CO)(CHO)] + with two equivalents of the organic hydride yields the hydroxymethyl complex cis-[Ru(bpy) 2 (CO)(CH 2 OH)] + . Visible light excitation of cis-[Ru(bpy) 2 (CO)(CH 2 OH)] + in the presence of excess organic hydride was shown to yield free methanol. Identification and quantification of methanol as the sole CO reduction product was confirmed by 1 H NMR spectroscopy and gas chromatography. The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO 2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature's NADPH/NADP + .

14 SOLAR ENERGY↗
DOE OSTI2024

Electronic and Electrochemical Control of Isostructural Ruthenium Hydricities and the Implications for Catalytic Overpotentials

Electronic tuning of metal hydrides enables precise control over potentials, mechanisms, selectivity, and rates of electrocatalytic reactions by regulating bond dissociation free energies such as the hydricity (Δ$G$ H- ° ) and $pK$ a of the catalyst. Here, we investigate a series of electronically tuned ruthenium hydrido complexes that are isostructural at the metal center: [Ru(4,4'-R 2 -bpy) 2 (CO)H] + (R = CF 3 , Cl, H, CH 3 , and CH 3 O; bpy = 2,2'-bipyridine) (denoted as (R)Ru-H+). A substantial 22 kcal mol -1 hydricity range is available across five complexes in three stable oxidation states: (R)Ru-H + , (R)Ru-H 0 , and (R)Ru-H - . Thermodynamic and mechanistic predictions of electrocatalytic proton reduction were tested experimentally by reducing protons from weak acids to H 2 . Two mechanisms are observed, depending on the acid strength and the catalyst hydricity. The rate constants for hydride transfer and protonation of the catalyst were, in some cases, extracted from the analysis of cyclic voltammetry data. A key finding is a 400 mV decrease in the catalytic overpotential for H 2 production by using a doubly reduced electron-poor metal hydride instead of a singly reduced electron-rich metal hydride. In conclusion, the former also exhibits a higher rate constant for hydride transfer, representing a strategy to disconnect rate and free energy relationships.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Discovery of a Hybrid System for Photocatalytic CO 2 Reduction via Attachment of a Molecular Cobalt-Quaterpyridine Complex to a Crystalline Carbon Nitride

While recent reports have demonstrated the attachment of molecular catalysts to amorphous, graphitic carbon nitrides (g-CN) for light-driven CO 2 reduction, approaches to the utilization of crystalline carbon nitrides have remained undiscovered. Herein, a functional hybrid photocatalyst system has been found using a crystalline carbon nitride semiconductor, poly(triazine imide) lithium chloride (PTI-LiCl), with a surface-attached CoCl 2 (qpy-Ph-COOH) catalyst for CO 2 reduction. The molecular catalyst attaches to PTI-LiCl at concentrations from 0.10 to 4.30 wt % and exhibits ∼96% selectivity for CO production in a CO 2 -saturated, aqueous 0.5 M KHCO 3 solution. Optimal loadings were found to be within 0.42–1.04 wt % with rates between 1,400 and 1,550 μmol CO/g·h at an irradiance of 172 mW/cm 2 (λ = 390 nm) and apparent quantum yields of ∼2%. This optimized loading is postulated to represent a balance between maximal turnover frequency (TOF; 300+ h –1 ) and excess catalyst that can limit excited-electron lifetimes, as probed via transient absorption spectroscopy. An increase in the incident irradiance yields a concomitant increase in the TOFs and CO rates only for the higher catalyst loadings, reaching up to 2,149 μmol CO/g·h with a more efficient use of the catalyst surface capacity. The lower catalyst loadings, by comparison, already function at maximal TOFs. Higher surface loadings are also found to help mitigate deactivation of the molecular catalysts during extended catalytic testing (>24 h) owing to the greater net surface capacity for CO 2 reduction, thus representing an effective strategy to extend lifetime. The hybrid particles can be deposited onto an FTO substrate to yield ∼60% Faradaic efficiency for photoelectrochemical CO production at −1.2 V vs Ag/AgCl bias. In conclusion, these results demonstrate the synergistic combination of a crystalline carbon nitride with a molecular catalyst that achieves among the highest known rates in carbon-nitride systems for the light-driven CO 2 reduction to CO in aqueous solution with >95% selectivity.

CO2 reduction↗
DOE OSTI2023

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions

Here, photovoltages for hydrogen terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH 3 ) 2 -4,4'-bipyridinium](PF 6 ) 2 , abbreviated MV 2+ , and [Ru(bpy) 3 ](PF 6 ) 2 , where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV 2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy) 3 ] 2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that were more positive than that measured with a degenerately doped n + -Si demonstrative of a photovoltage, Vph, that increased in the order: MV 2+/+ (260 mV) < MV +/0 (400 mV) < Ru 2+/+ (530 mV) ~ Ru +/0 (540 mV) ~ Ru 0/– (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well at the longest electron-hole pair lifetime and the highest surface electron concentration, occur when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.

14 SOLAR ENERGY↗
DOE OSTI2023

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO) 5 Cl to form complexes of the type Re(bpy)(CO) 3 Cl, which are related to known catalysts for CO 2 reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO 2 on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO 2 reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. So a conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

14 SOLAR ENERGY↗
DOE OSTI2021

Mechanistic investigation of a visible light mediated dehalogenation/cyclisation reaction using iron( iii ), iridium( iii ) and ruthenium( ii ) photosensitizers

The mechanism of a visible light-driven dehalogenation/cyclization reaction was investigated using ruthenium(II), iridium(III) and iron(III) photosensitizers by means of steady-state photoluminescence, time-resolved infrared spectroscopy, and nanosecond/femtosecond transient absorption spectroscopy. The nature of the photosensitizer was found to influence the product distribution such that the dehalogenated, non-cyclized products were only detected for the iron photosensitizer. Strikingly, with the iron photosensitizer, large catalytic yields required a low dielectric solvent such as dichloromethane, consistent with a previous publication. This low dielectric solvent allowed ultrafast charge-separation to outcompete geminate charge recombination and improved cage escape efficiency. Finally, the identification of reaction mechanisms unique to the iron, ruthenium, and iridium photosensitizer represents progress towards the long-sought goal of utilizing earth-abundant, first-row transition metals for emerging energy and environmental applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

Efficient excited-state electron transfer between an iron(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb) 2 ]+ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, that exhibited long-lived and luminescent ligand-to-metal charge transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with catalytic yieldsthat exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy) 3 ] 2+ . The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer that is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectoral and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, that include transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Proton-Coupled Group Transfer Enables Concerted Protonation Pathways Relevant to Small-Molecule Activation

The mechanistic identification of Nature's use of concerted reactions, in which all bond-breaking and bond-making occurs in a single step, has inspired rational designs for artificial synthetic transformations via pathways that bypass high energy intermediates that would otherwise be thermodynamically and kinetically inaccessible. Here, we electrochemically activate an organometallic Ruthenium(II) complex to show that, in acetonitrile solutions, the movement of protons from weak Brønsted acids, such as water and methanol, is coupled with the transfer of its negatively charged counterpart to carbon dioxide (CO 2 ) - a process termed proton-coupled group transfer - to stoichiometrically produce a metal-hydride complex and a carbonate species. These previously unidentified pathways have played key roles in CO 2 and proton reduction catalysis by enabling the generation of key intermediates such as hydrides and metallocarboxylic acids, while its applicability to carbon acids may provide alternative approaches in the electrosynthesis of chemical commodities via alkylation and carboxylation reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

Recent advances in our mechanistic understanding of dye-sensitized electron transfer reactions occurring at metal oxide interfaces are described. These advances were enabled by the advent of mesoporous thin films, comprised of anatase TiO 2 nanocrystallites, that are amenable to spectroscopic and electrochemical characterization in unprecedented molecular-level detail. The metal-to-ligand charge transfer (MLCT) excited states of Ru polypyridyl compounds serve as the dye sensitizers. Excited-state injection often occurs on ultrafast time scales with yields that can be tuned from unity to near zero through modification of the sensitizer or the electrolyte composition. Transport of the injected electron and the oxidized sensitizer (hole hopping) are both operative in the composite mechanism for charge recombination between the injected electron and the oxidized sensitizer. Sensitizers that contain a pendant electron donor, as well as core/shell SnO 2 /TiO 2 nanostructures, often prolong the lifetime of the injected electron and provide fundamental insights into adiabatic and nonadiabatic electron transfer mechanisms. Regeneration of the oxidized sensitizer by iodide is enhanced through halogen bonding, orbital pathways, and ion pairing. A substantial ~10 MV cm –1 electric field is created by electron injection into TiO 2 nanocrystallites that induces ion migration, reports on the sensitizer dipole orientation, and (in some cases) reorients or flips the sensitizer. Dye-sensitized conductive oxides also promote long-lived charge separation with bias dependent kinetics that provide insights into the reorganization energies associated with electron and proton-coupled electron transfer in the electric double layer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Spectroscopic characterization of a new Re(i) tricarbonyl complex with a thiosemicarbazone derivative: towards sensing and electrocatalytic applications

This work describes the preparation of a new thiosemicarbazone derivative, (Z)- N -ethyl-2-(6-oxo-1,10-phenanthrolin-5(6 H )-ylidene)hydrazinecarbothioamide (phet) and its respective Re( I ) tricarbonyl chloro complex, fac -[ReCl(CO) 3 (phet)]. The spectroscopic, photophysical and electrochemical properties of the new complex were fully investigated through steady state and time-resolved techniques along with computational calculations. In fac -[ReCl(CO) 3 (phet)], the new ligand is coordinated to the metal center through the pyridyl rings of the phenanthroline moiety. The unbound electron pairs in the S atom of the bending thiosemicarbazone group induce new low energy lying electronic transitions. Consequently, enhanced visible light absorption up to 550 nm is observed in acetonitrile due to the overlap between MLCT Re→phet and IL phet(n→π*) transitions. The absorption bands and emission quantum yields of fac -[ReCl(CO) 3 (phet)] are sensitive to proton concentration due to an acid-basic equilibrium in the N atoms of the thiosemicarbazone. Proton dissociation constants of 10.0 ± 0.1 and 11.4 ± 0.2 were determined respectively for the ground and excited states of the new complex. Spectral changes could also be observed in the presence of Zn 2+ cations which can be further explored for sensing applications. The electrochemical behavior of the new complex was studied in detail, revealing up to four one electron reduction processes in the range from 0 to -2.4 V vs. Fc + /Fc. With support of DFT calculations, the first three processes are ascribed to the reduction of the coordinated phet ligand followed by the Re I/0 reduction and consequent Cl - release. Furthermore, the new complex was able to act as an electrocatalyst for CO 2 reduction into CO ( E onset = -1.92 V vs. Fc + /Fc), with a turnover frequency of 2.81 s -1 and turnover number of 24 ± 1 in anhydrous acetonitrile, being the first Re( I ) tricarbonyl complex with a thiosemicarbazone derivative described for this goal. The detailed characterization carried out here can drive the development of new Re( I )-thiosemicarbazone derivatives for different applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗