Distinguishing homogeneous and heterogeneous water oxidation catalysis when beginning with cobalt polyoxometalates

2013 Fall.Includes bibliographical references.Development of energy storage technologies is required prior to broad implementation of renewable energy sources such as wind or solar power. One of the leading proposals is to store this energy by splitting water into hydrogen and oxygen--that is, to store energy in chemical bonds. A major obstacle en route to this overall goal is the development of efficient, cost-effective water oxidation catalysts (WOCs). Due to the highly oxidizing environment needed to drive this reaction, one question which has arisen when dealing with homogeneous precatalysts is whether these precursors remain as intact, homogeneous WOCs, or whether they are transformed into heterogeneous metal-oxide catalysts. This problem, reviewed in Chapter II, addresses the methods and literature studies related to distinguishing homogeneous and heterogeneous water oxidation catalysts. Chapters III through V further develop the methodology for distinguishing homogeneous and heterogeneous water oxidation catalysis when beginning with the cobalt polyoxometalate [Co4(H2O)2(PW9O34)2]10- (Co4POM). In Chapter III, the investigation of Co4POM using electrochemical oxidation at a glassy carbon electrode reveals that under the conditions therein, an in-situ formed, heterogeneous cobalt-oxo-hydroxo (CoOx) material is the dominant catalyst and is formed from Co2+ leached from the Co4POM. In Chapter IV, investigation of whether the intact Co4POM could be a catalyst under other, more forcing conditions of higher electrochemical potentials and lower Co4POM concentrations is reported. Although the Co4POM shows different electrochemical properties relative to CoOx controls, the possibility that the Co4POM is being transformed into a meta-stable heterogeneous catalyst cannot be ruled out since the Co4POM degrades during the experiment. Lastly, Chapter V presents a kinetic and mechanistic study of the Co4POM when using a ruthenium(III)tris(2,2'-bipyridine) (Ru(III)(bpy)33+) chemical oxidant to drive the water oxidation reaction (i.e., rather than electrochemically driven oxidation). In this study, it was found that Co4POM catalyzes the oxidation of water as well as oxidation of the 2,2'-bipyridine ligand. In contrast, controls with in-situ formed CoOx catalysts more selectively promote the catalytic oxidation of water. The difference in reactivity and kinetics between the Co4POM and CoOx systems indicates that the active catalysts are fundamentally different when a chemical oxidant is employed. Overall, these studies demonstrate the need for careful experimental controls and highlight the importance which reaction conditions--in particular the source and electrochemical potential of the oxidant--can play in determining the active oxidation catalyst in water oxidation reactions

Water Oxidation Catalysis Beginning with 2.5 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–: Investigation of the True Electrochemically Driven Catalyst at ≥600 mV Overpotential at a Glassy Carbon Electrode

Evidence for the true water oxidation catalyst (WOC) when beginning with the cobalt polyoxometalate [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– (Co₄–POM) is investigated at deliberately chosen low polyoxometalate concentrations (2.5 μM) and high electrochemical potentials (≥1.3 V vs Ag/AgCl) in pH 5.8 and 8.0 sodium phosphate electrolyte at a glassy carbon working electrodeconditions which ostensibly favor Co₄–POM catalysis if present. Multiple experiments argue against the dominant catalyst being CoO_<i>x</i> formed exclusively from Co²⁺ dissociated from the parent POM. Measurement of [Co²⁺] in the Co₄–POM solution and catalytic controls with the corresponding amount of Co­(NO₃)₂ cannot account for the O₂ generated from 2.5 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– solutions. This result contrasts with our prior investigation of Co₄–POM under higher concentration and lower potential conditions (i.e., 500 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–, 1.1 V vs Ag/AgCl, as described in Stracke, J. J.; Finke, R. G. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 14872) and <i>highlights the importance of</i> <i>reaction</i> <i>conditions in governing the identity of the true, active WOC.</i> Although electrochemical studies are consistent with Co₄–POM being oxidized at the glassy carbon electrode, it is not yet possible to distinguish a Co₄–POM catalyst from a CoO_<i>x</i> catalyst formed via decomposition of Co₄–POM. Controls with authentic CoO_<i>x</i> indicate conversion of only 3.4% or 8.3% (at pH 8.0 and 5.8) of Co₄–POM into a CoO_<i>x</i> catalyst could account for the O₂-generating activity, and HPLC quantification of the Co₄–POM stability shows the postreaction Co₄–POM concentration decreases by 2.7 ± 7.6% and 9.4 ± 5.1% at pH 8.0 and 5.8. Additionally, the [Co²⁺] in a 2.5 μM Co₄–POM solution increases by 0.55 μM during 3 min of electrolysisfurther evidence of the <i>Co</i>_<i>4</i><i>-POM instability under oxidizing conditions</i>. Overall, this study demonstrates the challenges of identifying the true WOC when examining micromolar amounts of a partially stable material and when <i>nanomolar</i> heterogeneous metal-oxide will account for the observed O₂-generating activity

Water Oxidation Catalysis Beginning with Co₄(H₂O)₂(PW₉O₃₄)₂^10– When Driven by the Chemical Oxidant Ruthenium(III)tris(2,2′-bipyridine): Stoichiometry, Kinetic, and Mechanistic Studies en Route to Identifying the True Catalyst

Stoichiometry and kinetics are reported for catalytic water oxidation to O₂ beginning with the cobalt polyoxometalate Co₄(H₂O)₂(PW₉O₃₄)₂^10– (Co₄POM) and the chemical oxidant ruthenium­(III)­tris­(2,2′-bipyridine) (Ru­(III)­(bpy)₃³⁺). This specific water oxidation system was first reported in a 2010 <i>Science</i> paper (Yin et al. <i>Science</i> <b>2010</b>, <i>328</i>, 342). Under standard conditions employed herein of 1.0 μM Co₄POM, 500 μM Ru­(III)­(bpy)₃³⁺, 100 μM Ru­(II)­(bpy)₃²⁺, pH 7.2, and 0.03 M sodium phosphate buffer, the highest O₂ yields of 22% observed herein are seen when Ru­(II)­(bpy)₃²⁺ is added prior to the Ru­(III)­(bpy)₃³⁺ oxidant; hence, those conditions are employed in the present study. Measurement of the initial O₂ evolution and Ru­(III)­(bpy)₃³⁺ reduction rates while varying the initial pH, [Ru­(III)­(bpy)₃³⁺], [Ru­(II)­(bpy)₃²⁺], and [Co₄POM] indicate that the reaction follows the empirical rate law: −d­[Ru­(III)­(bpy)₃³⁺]/d<i>t</i> = (<i>k</i>₁ + <i>k</i>₂)­[Co₄POM]_soluble[Ru­(III)­(bpy)₃³⁺]/[H⁺], where the rate constants <i>k</i>₁ ∼ 0.0014 s^–1 and <i>k</i>₂ ∼ 0.0044 s^–1 correspond to the water oxidation and ligand oxidation reactions, and for O₂ evolution, d­[O₂]/d<i>t</i> = (<i>k</i>₁/4)­[Co₄POM]_soluble[Ru­(III)­(bpy)₃³⁺]/[H⁺]. Overall, at least seven important insights result from the present studies: (i) Parallel WOC and Ru­(III)­(bpy)₃³⁺ self-oxidation reactions well documented in the prior literature limit the desired WOC and selectivity to O₂ in the present system to ≤28%. (ii) The formation of a precipitate from ∼2 Ru­(II)­(bpy)₃²⁺/3 Co₄(H₂O)₂(PW₉O₃₄)₂^10– with a <i>K</i>_sp = (8 ± 7) × 10^–25 (M⁵) greatly complicates the reaction and interpretation of the observed kinetics, but (iii) the best O₂ yields are still when Ru­(II)­(bpy)₃²⁺ is preadded. (iv) CoO_<i>x</i> is 2–11 times more active than Co₄POM under the reaction conditions, but (v) Co₄POM is still the dominant WOC under the Co₄POM/Ru­(III)­(bpy)₃³⁺ and other reaction conditions employed. The present studies also (vi) confirm that the specific conditions matter greatly in determining the true WOC and (vii) allow one to begin to construct a plausible WOC mechanism for the Co₄POM/Ru­(III)­(bpy)₃³⁺ system

The UV-B photoreceptor UVR8 promotes photosynthetic efficiency in Arabidopsis thaliana exposed to elevated levels of UV-B

The UV-B photoreceptor UVR8 regulates expression of genes in response to UV-B, some encoding chloroplast proteins, but the importance of UVR8 in maintaining photosynthetic competence is unknown. The maximum quantum yield of PSII (F v/F m) and the operating efficiency of PSII (Φ PSII) were measured in wild-type and uvr8 mutant Arabidopsis thaliana. The importance of specific UVR8-regulated genes in maintaining photosynthetic competence was examined using mutants. Both F v/F m and Φ PSII decreased when plants were exposed to elevated UV-B, in general more so in uvr8 mutant plants than wild-type. UV-B increased the level of psbD-BLRP (blue light responsive promoter) transcripts, encoding the PSII D2 protein. This increase was mediated by the UVR8-regulated chloroplast RNA polymerase sigma factor SIG5, but SIG5 was not required to maintain photosynthetic efficiency at elevated UV-B. Levels of the D1 protein of PSII decreased markedly when plants were exposed to elevated UV-B, but there was no significant difference between wild-type and uvr8 under conditions where the mutant showed increased photoinhibition. The results show that UVR8 promotes photosynthetic efficiency at elevated levels of UV-B. Loss of the DI polypeptide is probably important in causing photoinhibition, but does not entirely explain the reduced photosynthetic efficiency of the uvr8 mutant compared to wild-type