2018
L. Reith, K. Lienau, D. S. Cook, R. Moré, R. I. Walton, G. R. Patzke
Monitoring the hydrothermal growth of cobalt spinel water oxidation catalysts: from preparative history to catalytic activity,
Chemistry – A European Journal 24 (69), 18424-18435
The hydrothermal growth of cobalt oxide spinel (Co3O4) nanocrystals from cobalt acetate precursors was monitored with in situ powder X-ray diffraction (PXRD) in combination with ex situ electron microscopy and vibrational spectroscopy. Kinetic data from in situ PXRD monitoring were analyzed using Sharp–Hancock and Gualtieri approaches, which both clearly indicate a change of the growth mechanism for reaction temperatures above 185 °C. This mechanistic transition goes hand in hand with morphology changes that notably influence the photocatalytic oxygen evolution activity. Complementary quenching investigations of conventional hydrothermal Co3O4 growth demonstrate that these insights derived from in situ PXRD data provide valuable synthetic guidelines for water oxidation catalyst production. Furthermore, the ex situ analyses of hydrothermal quenching experiments were essential to assess the influence of amorphous cobalt-containing phases arising from the acetate precursor on the catalytic activity. Thereby, the efficient combination of a single in situ technique with ex situ analyses paves the way to optimize parameter-sensitive hydrothermal production processes of key energy materials.
L. Reith, K. Lienau, D. S. Cook, R. Moré, R. I. Walton, G. R. Patzke
Frontispiece: Monitoring the Hydrothermal Growth of Cobalt Spinel Water Oxidation Catalysts: From Preparative History to Catalytic Activity,
Chemistry – A European Journal 24 (69)
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S. Yu, X.-B. Fan, X. Wang, J. Li, Q. Zhang, A. Xia, S. Wei, L.-Z. Wu, Y. Zhou, G. R. Patzke
Efficient photocatalytic hydrogen evolution with ligand engineered all-inorganic InP and InP/ZnS colloidal quantum dots,
Nature Communications 9 (1), 4009
Photocatalytic hydrogen evolution is a promising technique for the direct conversion of solar energy into chemical fuels. Colloidal quantum dots with tunable band gap and versatile surface properties remain among the most prominent targets in photocatalysis despite their frequent toxicity, which is detrimental for environmentally friendly technological implementations. In the present work, all-inorganic sulfide-capped InP and InP/ZnS quantum dots are introduced as competitive and far less toxic alternatives for photocatalytic hydrogen evolution in aqueous solution, reaching turnover numbers up to 128,000 based on quantum dots with a maximum internal quantum yield of 31%. In addition to the favorable band gap of InP quantum dots, in-depth studies show that the high efficiency also arises from successful ligand engineering with sulfide ions. Due to their small size and outstanding hole capture properties, sulfide ions effectively extract holes from quantum dots for exciton separation and decrease the physical and electrical barriers for charge transfer.
R. Müeller, I. Kuznetsov, Y. Arbelo, M. Trottmann, C. S. Menoni, J. J. Rocca, G. R. Patzke, D. Bleiner
Correction to Depth-Profiling Microanalysis of CoNCN Water-Oxidation Catalyst Using a λ= 46.9 nm Plasma-Laser for Nano-Ionization Mass Spectrometry,
Analytical Chemistry 90 (20), 12322-12322
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J. Soriano-López, F. Song, G. R. Patzke, J. R. Galan-Mascaros
Photoinduced oxygen evolution catalysis promoted by polyoxometalate salts of cationic photosensitizers,
Frontiers in Chemistry 6, 302
The insoluble salt Cs15K[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (CsCo9) is tested as heterogeneous oxygen evolution catalyst in light-induced experiments, when combined with the homogeneous photosensitizer [Ru(bpy)3]2+ and the oxidant Na2S2O8 in neutral pH. Oxygen evolution occurs in parallel to a solid transformation. Post-catalytic essays indicate that the CsCo9 salt is transformed into the corresponding [Ru(bpy)3]2+ salt, upon cesium loss. Remarkably, analogous photoactivated oxygen evolution experiments starting with the [Ru(bpy)3](5+x)K(6−2x)[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]·(39+x)H2O (RuCo9) salt demonstrate much higher efficiency and kinetics. The origin of this improved performance is at the cation-anion, photosensitizer-catalyst pairing in the solid state. This is beneficial for the electron transfer event, and for the long-term stability of the photosensitizer. The latter was confirmed as the limiting process during these oxygen evolution reactions, with the polyoxometalate catalyst exhibiting robust performance in multiple cycles, upon addition of photosensitizer, and/or oxidant to the reaction mixture.
R. Müller, I. Kuznetsov, Y. Arbelo, M. Trottmann, C. S. Menoni, J. J. Rocca, G. R. Patzke, D. Bleiner
Depth-profiling microanalysis of CoNCN water-oxidation catalyst using a λ= 46.9 nm plasma laser for nano-ionization mass spectrometry,
Analytical Chemistry 90 (15), 9234-9240
Nanoscale depth profiling analysis of a CoNCN-coated electrode for water oxidation catalysis was carried out using table-top extreme ultraviolet (XUV) laser ablation time-of-flight mass spectrometry. The self-developed laser operates at λ = 46.9 nm and represents factor of 4 reduction in wavelength with respect to the 193 nm excimer laser. The reduction of the wavelength is an alternative approach to the reduction of the pulse duration, to enhance the ablation characteristics and obtain smaller quasi-nondestructive ablation pits. Such a XUV-laser ablation method allowed distinguishing different composite components of the catalyst-Nafion blend, used to modify a screen-printed carbon electrode surface. Chemical information was extracted by fragment assignment and relative amplitude analysis of the mass spectrometry peaks. Pure Nafion and the exposed carbon substrate were compared as references. Material specific fragments were clearly identified by the detected nonoverlapping mass-to-charge peaks of Nafion and CoNCN. Three dimensional mapping of relevant mass peak amplitudes was used to determine the lateral distribution and to generate depth profiles from consecutive laser pulses. Evaluating the profiles of pristine electrodes gave insight into fragmentation behavior of the catalyst in a functional ionomer matrix and comparison of post-catalytic electrodes revealed spots of thin localized Co residues.
R. Jacot, J. M. Naik, R. Moré, R. Michalsky, A. Steinfeld, G. R. Patzke
Reactive stability of promising scalable doped ceria materials for thermochemical two-step CO 2 dissociation,
Journal of Materials Chemistry A 6 (14), 5807-5816
Metal-doped ceria (Ce1−xMxO2−δ) is an attractive redox-active material for thermo/electrochemical synthesis of renewable fuels due to its high mixed ionic/electronic conductivity and variable valence (Ce4+/Ce3+) and oxygen nonstoichiometry (δ) at high temperatures. Previously, we have investigated all 26 potentially tetravalent dopants for efficient thermochemical splitting of CO2. Here, we fine-tune the dopant activity (x = 0.10 Zr4+, 0.10 Hf4+, 0.07 Ta5+, and 0.05 Nb5+) of all thermally stable ceria materials with an oxygen exchange capacity (OEC) surpassing that of pristine ceria (CeO2−δ), and we employ thermogravimetric analysis to evaluate long-term stability of their OEC over 50 consecutive redox cycles. Each cycle swings between 40 min ceria oxidation with approximately 500 mbar CO2 at 1000 °C and 90 min ceria reduction in about 0.01 mbar O2 at 1500 °C. Along with analyses of phase purity and stability (PXRD), of composition and dopant concentration (EDX and ICP-MS), and of sintering via SEM, the cycling results show long-term stable OEC and kinetics of the oxygen exchange for Zr-, Hf-, and Nb-doped ceria, despite their distinctly sintered particle surfaces. This attractive performance is rationalized by characterizing oxidation states and oxygen vacancies and by excluding surface carbonation through Raman and FT-IR spectroscopy. Furthermore, we find that introducing stable oxygen vacancies in Ce0.95Hf0.05O2−δ by doping with additional 5% lower-valent Li+, Mg2+, Ca2+, Y3+, and Er3+ does not significantly accelerate the oxygen exchange kinetics. From this first comprehensive long-term stability study of systematically optimized ceria, we propose ceria co-doped with permutations of Hf4+, Zr4+, and Nb5+, yielding an optimal average dopant radius of 0.8 Å, as the benchmark redox material for thermochemical production of solar fuels.
R. J. Müller, J. Lan, K. Lienau, R. Moré, C. A. Triana, M. Iannuzzi, G. R. Patzke
Monitoring surface transformations of metal carbodiimide water oxidation catalysts by operando XAS and Raman spectroscopy,
Dalton Transactions 47 (31), 10759-10766
Transition metal carbodiimides MNCN (M = Co, Ni, Co0.9Ni0.1, Mn and Cu), were studied by simultaneous operando Raman and X-ray absorption spectroscopy (XAS) with focus on surface oxide detection during electrocatalytic water oxidation. As a proof of concept, easily modifiable screen-printed electrodes were used in this unified operando synchrotron setup for a trade-off between convenience of electrochemical anodization and spectroscopic data acquisition. Monitoring of chemical and structural transformations at the electrode surface during initial anodic electrode polarization shows stability for MNCN with M = Co, Ni, Co0.9Ni0.1 and Mn. While MnNCN is inactive, CoNCN emerges as the most active representative of the series. CuNCN displays pronounced side reactions and the formation of a surface copper oxide layer leading to lower current density attributed to water oxidation, as evident from an irreversible variation of the CuNCN redox behaviour in rotating ring-disc voltammetry. Furthermore, the accompanying structural and vibrational spectroscopy properties of the different MNCN compounds were explored with complementary ex situ analytical methods.
2017
R. Moré, M. Olah, S. E. Balaghi, P. Jäker, S. Siol, Y. Zhou, G. R. Patzke
Bi2O2CO3 Growth at Room Temperature: In Situ X-ray Diffraction Monitoring and Thermal Behavior,
ACS Omega 2 (11), 8213-8221
The room-temperature formation of bismuth oxycarbonate (Bi2O2CO3) from Bi2O3 in sodium carbonate buffer was investigated with in situ powder X-ray diffraction (PXRD) in combination with electron microscopy and vibrational spectroscopy. Time-resolved PXRD measurements indicate a pronounced and rather complex pH dependence of the reaction mechanism. Bi2O2CO3 formation proceeds within a narrow window between pH 8 and 10 via different mechanisms. Although a zero-dimensional nucleation model prevails around pH 8, higher pH values induce a change toward a diffusion-controlled model, followed by a transition to regular nucleation kinetics. Ex situ synthetic and spectroscopic studies confirm these trends and demonstrate that in situ monitoring affords vital parameter information for the controlled fabrication of Bi2O2CO3 materials. Furthermore, the β → α bismuth oxide transformation temperatures of Bi2O2CO3 precursors obtained from different synthetic routes differ notably (by min 50 °C) from commercially available bismuth oxide. Parameter studies suggest a stabilizing role of surface carbonate ions in the as-synthesized bismuth oxide sources. Our results reveal the crucial role of multiple preparative history parameters, especially of pH value and source materials, for the controlled access to bismuth oxide-based catalysts and related functional compounds.
F. Song, R. Moré, M. Schilling, G. Smolentsev, N. Azzaroli, T. Fox, S. Luber, G. R. Patzke
{Co4O4} and {CoxNi4–xO4} Cubane Water Oxidation Catalysts as Surface Cut-Outs of Cobalt Oxides,
Journal of the American Chemical Society 139 (40), 14198-14208
The future of artificial photosynthesis depends on economic and robust water oxidation catalysts (WOCs). Cobalt-based WOCs are especially promising for knowledge transfer between homogeneous and heterogeneous catalyst design. We introduce the active and stable {CoII4O4} cubane [CoII4(dpy{OH}O)4(OAc)2(H2O)2](ClO4)2 (Co4O4-dpk) as the first molecular WOC with the characteristic {H2O-Co2(OR)2-OH2} edge-site motif representing the sine qua non moiety of the most efficient heterogeneous Co-oxide WOCs. DFT-MD modelings as well as in situ EXAFS measurements indicate the stability of the cubane cage in solution. The stability of Co4O4-dpk under photocatalytic conditions ([Ru(bpy)3]2+/S2O82–) was underscored with a wide range of further analytical methods and recycling tests. FT-IR monitoring and HR-ESI-MS spectra point to a stable coordination of the acetate ligands, and DFT-MD simulations along with 1H/2H exchange experiments highlight a favorable intramolecular base functionality of the dpy{OH}O ligands. All three ligand types enhance proton mobility at the edge site through a unique bioinspired environment with multiple hydrogen-bonding interactions. In situ XANES experiments under photocatalytic conditions show that the {CoII4O4} core undergoes oxidation to Co(III) or higher valent states, which recover rather slowly to Co(II). Complementary ex situ chemical oxidation experiments with [Ru(bpy)3]3+ furthermore indicate that the oxidation of all Co(II) centers of Co4O4-dpk to Co(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the [CoIIxNi4–x(dpy{OH}O)4(OAc)2(H2O)2](ClO4)2 (CoxNi4–xO4-dpk) series as the first mixed Co/Ni-cubane WOCs. They newly bridge homogeneous and heterogeneous catalyst design through fine-tuned edge-site environments of the Co centers.
D. S. Cook, Y. Wu, K. Lienau, R. Moré, R. J. Kashtiban, O. V. Magdysyuk, G. R. Patzke, R. I. Walton
Time-resolved powder X-ray diffraction of the solvothermal crystallization of cobalt gallate spinel photocatalyst reveals transient layered double hydroxides,
Chemistry of Materials 29 (12), 5053-5057
Spinel oxides are an important family of materials studied for their electronic and magnetic properties and find use in many functional applications, 1 in particular catalysis 2 and photocatalysis. 3, 4 The metal deficient spinel γ-Ga2O3 has been applied for the photocatalytic degradation of volatile aromatics 5 and mixed-metal gallium oxide spinels, such as ZnGa2O4, have been employed for the photocatalytic degradation of organic pollutants. 6 The high temperatures required for the synthesis of many oxides usually prohibit the study of the mechanisms and reaction pathways of their formation. Such knowledge would be highly desirable to predict the outcome of future synthesis so to tailor materials with useful properties. Moorhouse et al. recently reported a time-resolved in situ powder X-ray diffraction (XRD) study of Bi5Ti3Fe0. 5Cr0. 5O15 crystallization from a molten salt reaction using high energy X-rays.
K. D. von Allmen, H. Grundmann, A. Linden, G. R. Patzke
Synthesis and Characterization of 0D–3D Copper-Containing Tungstobismuthates Obtained from the Lacunary Precursor Na9[B-α-BiW9O33],
Inorganic chemistry 56 (1), 327-335
The reaction of the lacunary polyoxometalate precursor Na9[B-α-BiW9O33]·19.5H2O with Cu(II) ions was explored in search of new economic ways to copper tungstobismuthates as interesting prototypes for water oxidation and reduction catalysts. The emerging series of new 0D–3D polyoxometalate architectures with distinct copper cores was structurally characterized. Na6Rb6[Cu3(H2O)3(BiW9O33)2] (Cu-4) and 3D-K6.56Cu0.43H2.20[(Cu3Cl)(K2.62Cu0.38(H2O)3)(B-α-BiW9O33)2]·13H2O (Cu-5) display a Cu3(H2O)3 core. The 2D representatives Na12[Cu2(H2O)4Cl2(BiW10O35)2] (Cu-1a), Na10[Cu2(H2O)6(BiW10O35)2] (Cu-1b), 2D-Na7K3Cu0.5Cl[Cu2(H2O)4(BiW10O35)2] (Cu-2), and 2D-Na5.5K2.5Cu[Cu2(H2O)4(BiW10O35)2] (Cu-3) contain Cu2(H2O)nW2O4 cores. Interestingly, the bismuth-free 1D paratungstate B Na4K4Cu[H2W12O42] (Cu-6) is formed through reassembly of the precursor. Cu-5 displays a disordered transition metal core, implying the presence of the polyanions [Cu4(H2O)4(BiW9O33)2]10– and [Cu5(H2O)5(BiW9O33)2]8–. The magnetic properties of Cu-5 as well as its activity as visible-light-driven H2 and O2 evolution catalyst were evaluated.
R. Jacot, R. Moré, R. Michalsky, A. Steinfeld, G. R. Patzke
Trends in the phase stability and thermochemical oxygen exchange of ceria doped with potentially tetravalent metals,
Journal of Materials Chemistry A 5 (37), 19901-19913
Ceria is among the most prominent materials for generating clean fuels through solar thermochemical CO2 reduction and water splitting. The main optimization parameter for ceria in solar reactors is the oxygen exchange capacity (OEC, Δδ), which can be notably improved through various dopant types. Among them, tetravalent dopants excel through the formation of active vacancies which lead to particularly high OEC values. We thus performed a comprehensive screening evaluation of all dopants in the periodic table which have been reported to adopt an oxidation state of +IV. All thermally stable doped ceria samples, M0.1Ce0.9O2−δ (M = Si, Ti, V, Cr, Zr, Nb, Rh, Hf, Ta, Nb, V, Pr, and Tb), were first analyzed for Δδ improvement with thermogravimetric analysis (TGA). Dopant solubility limits and behavior in the ceria host lattice was evaluated with scanning electron microscopy (SEM-EDX) and powder X-ray diffraction techniques. No indications for carbonate side product formation were found. Hf-, Zr-, and Ta-doped ceria display higher OEC than pristine ceria, and Raman spectroscopy indicated that their improved performance is accompanied by a higher versatility in the underlying vacancy formation processes. Furthermore, the effective dopant radii are close to an optimal dopant radius around 0.8 Å for maximum Δδ according to TGA cycling experiments. These experimentally derived trends for doped ceria were supported by density functional theory (DFT) calculations which analogously correlate Δδ with the partial electronic charge of the metal dopants.
J. Li, R. Güttinger, R. Moré, F. Song, W. Wan, G. R. Patzke
Frontiers of water oxidation: the quest for true catalysts,
Chemical Society Reviews 46 (20), 6124-6147
Development of efficient and economic water oxidation catalysts (WOCs) remains a crucial bottleneck on the way to artificial photosynthesis applications. Over the past few decades, WOC research has turned into a fascinating interdisciplinary field that ranges from bio-inspired molecular design over nanomaterials and thin films to solid materials tuning. Under the umbrella of WOC optimization, advanced in situ/operando analytical techniques are being developed as increasingly powerful tools to elucidate the controversial discussions about the molecular or nanoscale nature of many WOCs. More and more of these approaches also enable the monitoring of possible key intermediates as an essential prerequisite for proposing catalytic mechanisms. This review is organized in three main parts: first, recent highlights outline frontiers in WOC development, such as the benefits of connecting molecular WOCs with solids along with the introduction of molecular concepts into heterogeneous WOC research. Next, a brief overview of emerging in situ/operando approaches demonstrates new options for monitoring WOC transformations. Finally, selected monitoring studies over the entire WOC dimensionality spectrum illustrate interesting cases of catalytic border crossings as new input for WOC construction.
M. Hoes, C. L. Muhich, R. Jacot, G. R. Patzke, A. Steinfeld
Thermodynamics of paired charge-compensating doped ceria with superior redox performance for solar thermochemical splitting of H 2 O and CO 2,
Journal of Materials Chemistry A 5 (36), 19476-19484
Paired charge-compensating doped ceria (PCCD) using trivalent and pentavalent cations are evaluated as redox materials for the thermochemical splitting of H2O and CO2. The oxygen nonstoichiometries of PCCD materials with formulas of Ce0.9A0.05Nb0.05O2 (A = Y, La, Sc) and CexLa(1−x)/2Nb(1−x)/2O2 (x = 0.75, 0.95) were measured in a thermogravimetric analyzer over a range of temperatures (T = 1173–1773 K) and oxygen partial pressures (pO2 = 10−15–10−1 atm). Undoped and single element doped ceria (Ce0.9B0.1O2 where B = Y, La, Nb, Hf) served as a reference. At any given set of T and pO2, the relative reduction extent follows Ce0.9Hf0.1O2 > Ce0.9Sc0.05Nb0.05O2 > Ce0.9Y0.05Nb0.05O2 > CexLa(1−x)/2Nb(1−x)/2O2 > CeO2 > solely trivalent or pentavalent doped ceria. The partial molar reduction enthalpies were determined using Van't Hoff analysis coupled to defect modeling and range from 360 to 410 kJ mol−1. A system efficiency model predicts that these PCCD materials have the potential of achieving high solar-to-fuel energy conversion efficiencies because of their balanced reduction and oxidation properties. Ce0.9Y0.05Nb0.05O2 in particular can outperform undoped ceria and reach efficiency values of 31% and 28% for H2 and CO production, respectively.
2016
M. Croce, S. Conti, C. Maake, G. R. Patzke
Synthesis and screening of N-acyl thiolated chitosans for antibacterial applications,
Carbohydrate Polymers 151, 1184-1192
Low-molecular weight chitosan-thioglycolic acid has shown significant antibacterial properties against different microorganisms. In order to explore the potential and structure-activity relationships of newly synthesized alkyl thiomers, chitosan has been functionalized with a series of thio-acids with increasing alkyl chain length. All thiomers were characterized with special emphasis on the determination of their degree of deacetylation and substitution, as well as on their molecular weight and amount of thiol groups. The pre-screened chitosan-thiomers were further investigated with plate counting on Pseudomonas aeruginosa, Streptococcus sobrinus and Streptococcus mutans. Furthermore, LIVE/DEAD assays supported the efficiency of chitosan-thiomers against the above microorganisms. All fully characterized chitosan-thiomers showed comparable or enhanced antimicrobial activity compared to pristine chitosan. Our comprehensive approach paves the way to detailed explorations of much sought-after structure activity relationships in the complex chitosan parameter room, starting from correlations between alkyl chain length and antimicrobial activity.
K. D. Allmen, A. Linden, G. R. Patzke
From 0D to 3D-The structural diversity of polyoxometalate catalysts,
Acta Crystallographica Section A: Foundations and Advances 72, s365-s366
The reaction of the lacunary polyoxometalate precursor Na9[B-α-BiW9O33]·19.5H2O with Cu(II) ions was explored in search of new economic ways to copper tungstobismuthates as interesting prototypes for water oxidation and reduction catalysts. The emerging series of new 0D–3D polyoxometalate architectures with distinct copper cores was structurally characterized. Na6Rb6[Cu3(H2O)3(BiW9O33)2] (Cu-4) and 3D-K6.56Cu0.43H2.20[(Cu3Cl)(K2.62Cu0.38(H2O)3)(B-α-BiW9O33)2]·13H2O (Cu-5) display a Cu3(H2O)3 core. The 2D representatives Na12[Cu2(H2O)4Cl2(BiW10O35)2] (Cu-1a), Na10[Cu2(H2O)6(BiW10O35)2] (Cu-1b), 2D-Na7K3Cu0.5Cl[Cu2(H2O)4(BiW10O35)2] (Cu-2), and 2D-Na5.5K2.5Cu[Cu2(H2O)4(BiW10O35)2] (Cu-3) contain Cu2(H2O)nW2O4 cores. Interestingly, the bismuth-free 1D paratungstate B Na4K4Cu[H2W12O42] (Cu-6) is formed through reassembly of the precursor. Cu-5 displays a disordered transition metal core, implying the presence of the polyanions [Cu4(H2O)4(BiW9O33)2]10– and [Cu5(H2O)5(BiW9O33)2]8–. The magnetic properties of Cu-5 as well as its activity as visible-light-driven H2 and O2 evolution catalyst were evaluated.
M. Schilling, G. R. Patzke, J. Hutter, S. Luber
Computational investigation and design of cobalt aqua complexes for homogeneous water oxidation,
The Journal of Physical Chemistry C 120 (15), 7966-7975
We study the water oxidation mechanism of the cobalt aqua complex [Co(H2O)6]2+ in a photocatalytic setup by means of density functional theory. Assuming a water-nucleophilic-attack or radical coupling mechanism, we investigate how the oxidation state and spin configuration change during the catalytic cycle. In addition, different ligand environments are employed by substituting a water ligand with a halide, pyridine, or derivative thereof. This allows exploration of the effect of such ligands on the frontier orbitals and the thermodynamics of the water oxidation process. Moreover, the thermodynamically most promising water oxidation catalyst can be identified by comparing the computed free energy profiles to the one of an “ideal catalyst”. Examination of such simple (hypothetical) water oxidation catalysts provides a basis for the derivation of design guidelines, which are highly sought for the development of efficient homogeneous water oxidation catalysts.
G. R. Patzke, P. E. Car
Polyoxometalates,
MDPI - Multidisciplinary Digital Publishing Institute
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H. Liu, R. Moré, H. Grundmann, C. Cui, R. Erni, G. R. Patzke
Promoting photochemical water oxidation with metallic band structures,
Journal of the American Chemical Society 138 (5), 1527-1535
The development of economic water oxidation catalysts is a key step toward large-scale water splitting. However, their current exploration remains empirical to a large extent. Elucidating the correlations between electronic properties and catalytic activity is crucial for deriving general and straightforward catalyst design principles. Herein, strongly correlated electronic systems with abundant and easily tunable electronic properties, namely La1–xSrxBO3 perovskites and La2-xSrxBO4 layered perovskites (B = Fe, Co, Ni, or Mn), were employed as model systems to identify favorable electronic structures for water oxidation. We established a direct correlation between the enhancement of catalytic activity and the insulator to metal transition through tuning the electronic properties of the target perovskite families via the La3+/Sr2+ ratio. Their improved photochemical water oxidation performance was clearly linked to the increasingly metallic character. These electronic structure–activity relations provide a promising guideline for constructing efficient water oxidation catalysts.
A. S. Holmes‐Smith, J. Crisp, F. Hussain, G. R. Patzke, G. Hungerford
Use of Lanthanide‐Containing Polyoxometalates to Sensitise the Emission of Fluorescent Labelled Serum Albumin,
ChemPhysChem 17 (3), 418-424
Monitoring the interaction of biomolecules is important, and the use of energy transfer is a principal technique in elucidating nanoscale interactions. Lanthanide compounds are promising luminescent probes for biological samples as their emission is longer-lived than any native autofluorescence. Polyoxometalates (POMs) are interesting structural motifs to incorporate lanthanides, offering low toxicity and a size pertinent for biological applications. Here, we employ iso-structured POMs containing either terbium or europium and assess their interaction with serum albumin by sensitisation of a fluorescent tag on the protein via LRET (luminescence resonance energy transfer) by exciting the lanthanide. Time-resolved measurements showed energy transfer with an efficiency of over 90 % for the POM–protein systems. The Tb–POM results were relatively straightforward, while those with the iso-structured Eu–POM were complicated by the effect of protein shielding from the aqueous environment.
Y. Z. F. Wang, S. Wei, Z. Zhang, G. R. Patzke
Oxygen Vacancy as Active Site for H2S Dissociation on Rutile TiO2 (110) Surface: A First-Principles Study,
Physical Chemistry Chemical Physics 18, 6706
Spin-polarized DFT+U computations have been performed to investigate the role of oxygen vacancies in dissociating H2S on the rutile TiO2(110) surface. A bridged O2c atom is demonstrated to be the most energetically favorable oxygen vacancy site, which makes V(O2c) an electron donator center and induces an isolated defect level with narrowed band gaps. A H2S molecule is adsorbed dissociatively over V(O2c), but molecularly on the perfect surface. For H2S dissociation, the HS/H intermediate state reveals the best thermal stability on both defected and perfect surfaces. Moreover, potential energy surface analysis shows that V(O2c) reduces markedly the energy barriers for the paths along H2S dissociation. This indicates oxygen vacancies to be efficient trap centers for H2S dissociation, as evidenced by a significant interfacial charge transfer promoted by vacancies. This work could provide insights into the role of oxygen vacancies in facilitating the decomposition of H2S on rutile TiO2(110) surface.
2015
P. E. Car, G. R. Patzke
The fascination of polyoxometalate chemistry,
Inorganics 3 (4), 511-515
The room-temperature formation of bismuth oxycarbonate (Bi2O2CO3) from Bi2O3 in sodium carbonate buffer was investigated with in situ powder X-ray diffraction (PXRD) in combination with electron microscopy and vibrational spectroscopy. Time-resolved PXRD measurements indicate a pronounced and rather complex pH dependence of the reaction mechanism. Bi2O2CO3 formation proceeds within a narrow window between pH 8 and 10 via different mechanisms. Although a zero-dimensional nucleation model prevails around pH 8, higher pH values induce a change toward a diffusion-controlled model, followed by a transition to regular nucleation kinetics. Ex situ synthetic and spectroscopic studies confirm these trends and demonstrate that in situ monitoring affords vital parameter information for the controlled fabrication of Bi2O2CO3 materials. Furthermore, the β → α bismuth oxide transformation temperatures of Bi2O2CO3 precursors obtained from different synthetic routes differ notably (by min 50 °C) from commercially available bismuth oxide. Parameter studies suggest a stabilizing role of surface carbonate ions in the as-synthesized bismuth oxide sources. Our results reveal the crucial role of multiple preparative history parameters, especially of pH value and source materials, for the controlled access to bismuth oxide-based catalysts and related functional compounds.
T. Cooper, J. R. Scheffe, M. E. Galvez, R. Jacot, G. Patzke, A. Steinfeld
Lanthanum manganite perovskites with Ca/Sr A‐site and Al B‐site doping as effective oxygen exchange materials for solar thermochemical fuel production,
Energy Technology 3 (11), 1130-1142
Perovskite oxides have recently been proposed as promising redox intermediates for solar thermochemical splitting of H2O and CO2, offering the benefit of significantly reduced operating temperatures. We present a systematic experimental screening of doped lanthanum manganites within the composition space La1−x(Ca,Sr)xMn1−yAlyO3 and identify several promising redox materials. In particular, La0.6Sr0.4Mn0.6Al0.4O3 and La0.6Ca0.4Mn0.6Al0.4O3 boast a five- to thirteen-fold improvement in the reduction extent compared to the state-of-the-art material CeO2 in the temperature range 1200–1400 °C. The materials are shown to be capable of splitting CO2 into CO fuel when isothermally cycled between low-pO2 and high-pCO2 environments at 1240 °C and to approach full reoxidation in CO2 with temperature swings as low as 200 °C, with mass-specific fuel yields up to ten times that of CeO2. The underlying material thermodynamics are investigated and used to explain the favorable redox behavior.
H. Liu, M. Schilling, M. Yulikov, S. Luber, G. R. Patzke
Homogeneous Photochemical Water Oxidation with Cobalt Chloride in Acidic Media,
ACS Catalysis 5 (9), 4994-4999
The precise mechanisms of four-electron-transfer water oxidation processes remain to be further understood. Oxide-based precipitation from molecular catalysts as a frequent observation during water oxidation has raised extensive debates on the differentiation between homogeneous and heterogeneous catalysis. Although soluble cobalt salts are known to be active in water oxidation, CoOx species formed in situ were generally considered to be the true catalyst. Here we report on the possibility homogeneous water oxidation with cobalt chloride in acidic conditions, which prevent CoOx precipitation. Interestingly, both the buffer media and counteranions were found to significantly influence the oxygen evolution activity, and their roles in the water oxidation process were analyzed with various techniques. This study sheds new light on Co2+ ions in key transformation processes of homogeneous water oxidation catalysts.
F. Evangelisti, R. Moré, F. Hodel, S. Luber, G. R. Patzke
3d–4f {CoII3Ln(OR)4} Cubanes as Bio-Inspired Water Oxidation Catalysts,
Journal of the American Chemical Society 137 (34), 11076-11084
Although the {CaMn4O5} oxygen evolving complex (OEC) of photosystem II is a major paradigm for water oxidation catalyst (WOC) development, the comprehensive translation of its key features into active molecular WOCs remains challenging. The [CoII3Ln(hmp)4(OAc)5H2O] ({CoII3Ln(OR)4}; Ln = Ho–Yb, hmp = 2-(hydroxymethyl)pyridine) cubane WOC series is introduced as a new springboard to address crucial design parameters, ranging from nuclearity and redox-inactive promoters to operational stability and ligand exchange properties. The {CoII3Ln(OR)4} cubanes promote bioinspired WOC design by newly combining Ln3+ centers as redox-inactive Ca2+ analogues with flexible aqua-/acetate ligands into active and stable WOCs (max. TON/TOF values of 211/9 s–1). Furthermore, they open up the important family of 3d–4f complexes for photocatalytic applications. The stability of the {CoII3Ln(OR)4} WOCs under photocatalytic conditions is demonstrated with a comprehensive analytical strategy including trace metal analyses and solution-based X-ray absorption spectroscopy (XAS) investigations. The productive influence of the Ln3+ centers is linked to favorable ligand mobility, and the experimental trends are substantiated with Born–Oppenheimer molecular dynamics studies.
K. von Allmen, R. Moré, R. Müller, J. Soriano‐López, A. Linden, G. R. Patzke
Nickel‐Containing Keggin‐Type Polyoxometalates as Hydrogen Evolution Catalysts: Photochemical Structure–Activity Relationships,
ChemPlusChem 80 (9), 1389-1398
In search of structure–activity relationships for polyoxometalate (POM)-based water reduction catalysts, nickel-monosubstituted Keggin-type POMs ([Ni(H2O)XW11O39]n−; XP, Si, Ge) were compared with respect to their activity in photochemical hydrogen evolution. The title compound series was characterized by single-crystal X-ray diffraction methods and a wide range of spectroscopic and electrochemical techniques. Nickel substitution was identified as a crucial feature for catalytic activity through comparison with nickel-free reference POMs. Furthermore, turnover number (TON) and turnover frequency strongly depended on the heteroatom X, and the highest TON among the series was recorded for [Ni(H2O)GeW11O39]6−. Photochemical hydrogen evolution activity was compared with redox and onset potentials obtained from electrochemical analyses. Furthermore, activity trends were correlated with electronic structure properties derived from density functional theory calculations.
K. Schuh, W. Kleist, M. Høj, A. D. Jensen, P. Beato, G. R. Patzke, J. D. Grunwaldt
Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene,
Journal of Solid State Chemistry 228, 42-52
A variety of morphologically different α-MoO3 samples were prepared by hydrothermal synthesis and applied in the selective oxidation of propylene. Their catalytic performance was compared to α-MoO3 prepared by flame spray pyrolysis (FSP) and a classical synthesis route. Hydrothermal synthesis from ammonium heptamolybdate (AHM) and nitric acid at pH 1–2 led to ammonium containing molybdenum oxide phases that were completely transformed into α-MoO3 after calcination at 550 °C. A one-step synthesis of α-MoO3 rods was possible starting from MoO3·2H2O with acetic acid or nitric acid and from AHM with nitric acid at 180 °C. Particularly, if nitric acid was used during synthesis, the rod-like morphology of the samples could be stabilized during calcination at 550 °C and the following catalytic activity tests, which was beneficial for the catalytic performance in propylene oxidation. Characterization studies using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy showed that those samples, which retained their rod-like morphology during the activity tests, yielded the highest propylene conversion.
H. Liu, Y. Zhou, R. Moré, R. Müller, T. Fox, G. R. Patzke
Correlations among structure, electronic properties, and photochemical water oxidation: a case study on lithium cobalt oxides,
ACS Catalysis 5 (6), 3791-3800
Construction of {M4O4} motifs is an effective design paradigm for molecular polyoxometalate- and oxide-based water oxidation catalysts (WOCs). However, the mechanisms beneath this bioinspired design strategy remain a topic of intense debate. The two modifications of LiCoO2 with spinel-type and layer structures are exceptionally versatile model systems to explore the correlations among structure, electronic properties, and photochemical water oxidation. The electronic properties of both LiCoO2 modifications are tunable through delithiation while the basic structural frameworks are maintained. This provides a unique opportunity to assign the respective influence of structures and electronic properties on the water oxidation properties. While spinel-type LiCoO2 with {Co4O4} cubane motifs is active for photochemical water oxidation, the layered modification without cuboidal structural elements is nearly inactive. Here, we demonstrate that the water oxidation performance of both modifications can be significantly improved through chemical delithiation. A wide range of analytical methods were applied to investigate the transition of electronic properties upon delithiation, and a direct correlation between enhanced hole mobility and improved water oxidation activity was established. The difference in water oxidation activities between the two structural modifications was further linked to the role of {Co4O4} cubane motifs in constructing 3D Co–O–Co networks with expanded hole transfer paths. Thus, the promoting effects of both delithiation and {Co4O4} cubane motifs on water oxidation can be consistently explained by enhanced hole mobility.
F. Wang, K. Cao, Y. Wu, G. R. Patzke, Y. Zhou
Electronic and optical properties of N-doped Bi2O3 polymorphs for visible light-induced photocatalysis,
Journal of Molecular Modeling 21, 1-8
The effect of N doping on the crystal structure, electronic, and optical properties of α-Bi2O3 and β-Bi2O3 has been studied in detail based with first principle calculations. The crystallographic features of Bi2O3 polymorphs are not substantially changed through N doping, whereas charge transfer from Bi to N results in large variations of charge density distribution. N-doped β-Bi2O3 exhibits improved thermal stability due to stronger Bi-N covalent bonds and lower defect formation energy, and the convenient preparative access agrees well with experimental observations. Calculated band structures and optical properties indicate that N doping does not induce major band gap narrowing, but leads to the presence of isolated bands above the VBM induced by N 2p for both α-Bi2O3 and β-Bi2O3 which induce large red-shifts of their visible light absoprtion properties. These isolated bands act as acceptor levels and facilitate electron transition under visible light illumination through introduction of steps between VB and CB, thereby rendering the materials quite promising for photocatalytic applications.
D. Ressnig, M. Shalom, J. Patscheider, R. Moré, F. Evangelisti, M. Antonietti, G. R. Patzke
Photochemical and electrocatalytic water oxidation activity of cobalt carbodiimide,
Journal of Materials Chemistry A 3 (9), 5072-5082
Cobalt carbodiimide is introduced as a heterogeneous non-oxidic water oxidation catalyst prototype with dual photochemical and electrocatalytic activity in neutral and basic media. CoNCN exhibits higher initial turnover frequencies of (TOF/SBET: 2.1 × 10−1) for visible-light-driven oxygen evolution than cobalt oxide catalysts (TOF/SBET: 3.5 × 10−3) and a 18% higher oxygen yield (Ru-dye sensitized standard setup). Furthermore, CoNCN maintains stable current densities in electrolysis over 20 h, and structural tuning through cationic substitution revealed that mixed (Co, Ni)NCN catalysts with low Ni contents display higher current densities than pristine CoNCN. A wide range of bulk (XAFS/EXAFS, XRD, FTIR) and surface (XPS, EELS, HRTEM) analytical methods together with catalytic parameter variations and reference experiments were performed to confirm the stability of CoNCN under standard operational conditions. The carbodiimide matrix thus offers a straightforward structural alternative to oxide systems and a clear-cut starting point for optimization strategies and for mechanistic studies on the possible role of active carbon or nitrogen sites. This paves the way to metal carbodiimides as a novel catalyst design platform for heterogeneous energy conversion systems.
M. E. Gálvez, R. Jacot, J. Scheffe, T. Cooper, G. Patzke, A. Steinfeld
Physico-chemical changes in Ca, Sr and Al-doped La–Mn–O perovskites upon thermochemical splitting of CO 2 via redox cycling,
Physical Chemistry Chemical Physics 17 (9), 6629-6634
Thermochemical CO2-splitting via redox cycling of Ca, Sr and Al-doped La–Mn perovskites induces irreversible changes in the texture and chemical composition of these oxides. Though the crystal structure is mostly preserved after high-temperature redox cycling, the chemical stability is detrimentally affected by sintering and by the formation and eventual segregation of a carbonate phase during oxidation by CO2. Carbonation of the Ca and Sr phase was diminished by Al-substitution of the Mn-cation in the B-position.