Mechanism of the Reduction of Lead Ions in Aqueous Solution (a Pulse Radiolysis Study)

Abstract

Elekrrochemie / Freie Radikale Reaktionskinetik / ~~r a~l e n c~e m~ePure lead is precipitated in the y-irradiation of aqueous solutions of lead perchlorate or lead acetate containing also an organic solute for scavengingOH radicals. The yields in the presence of the radical'scavenger are above 3 lead atoms/100eV for formate and aliphatic alcohols and about 0.3 for cyclopentane and t-butanol. Pulse radiolysis experiments showed that hydrated electrons and 1-hydroxy alkyl radicals reduce Pbz+ to form Pb'. Pb' can disproportionate to form Pb + Pb2+. Pb+ can also react with I-hydroxylalkyl radicals to form lead organic compounds that decompose to form Pb. The rate constants of these reactions were determined. Pb2+ solutions were also pulse irradiated under a hydrogen pressure of 50 atm. H atoms did not reduce Pb2' under our concentration conditions. They reacted with Pb+ to form the intermediate PbH+. Its spectrum was measured and the rate constant for its disappearance according to 2PbH' -+ Pb2+ + Pb + H2 was determined.The results are discussed in terms of the standard redox potentials of the systems Pb2+/Pb+ and Pb'/Pb in connection with the standard potentials of I-hydroxyalkyl radicals and hydrogen atoms. A comparison..of the lead reduction is also made with the reduction of Ni2+ and CdZ+. The intermediates of the reduction in the presence of H atoms may also be present in the dissolution of metals in water under HZ production. Reines Blei fallt bei y-Bestrahlung wadriger Losungen des Bleiperchlorats oder -acetats aus, wenn diese Losungen gleichzeitig eine organische Substanz als Fanger fur OH-Radikale enthalten. Die Ausbeute betragt mehr als drei Bleiatome pro 100 eV fur Formiat und aliphatische Alkohole als Fanger und etwa 0,3 fur Cyclopentan und t-Butanol. Die pulsradiolytischen Experimente zeigten, daO das hydratisierte Elektron und 1-Hydroxyalkylradikale Pb" zu Pb+ reduzieren. Pb+ kann zu Pb + Pbz+ disproportionieren. Pb+ kann auch mit I-Hydroxyalkylradikalen zu bleiorganischen Verbindungen kombinieren, die schlieBlich unter Bildung von Blei zerfallen. Die Geschwindigkeitskonstanten dieser Reaktionen wurden bestimmt. Pb2 +-Losungen wurden ebenfalls unter einem Wasserstoffdruck von 50 atm pulsbestrahlt. Das H-Atom konnte Pb2+ unter unseren Konzentrationsbedingungen nicht reduzieren. Ek reagierte aber mit Pb+ zu PbH+ . Das Spektrum dieser Teilchen und die Geschwindigkeitskonstante seines Verschwindens nach 2PbH+ --t Pb2+ + Pb + H2 wurden gemessen. Die Ergebnisse werden im Hinblick auf die Standard-Redoxpotentiale der Systeme Pb2 +/Pb+/Pb und die Standardpotentiale der Oxidation und Reduktion der I-Hydroxyalkyl-Radikale und das Wasserstoffatoms diskutiert. Ein Vergleich mit der Reduktion von NiZ 'und Cd2 '-Ionen wird ebenfalls angestellt. Die Zwischenprodukte der Reduktion in Gegenwart von H-Atomen spielen moglicherweise auch eine Rolle bei der Auflosung von Metallen in Wasser unter Entwicklung von Wasserstoff.

Reduction of CO2 under high pressure and high temperature on Pb-granule electrodes in a fixed-bed reactor in aqueous medium

“…However, the formation of CO 2 /CO 2 À pair needs more than À2.0 V (NHE) [25][26][27][28]. The reduction of CO 2 at more positive potentials than À2.0 V (NHE) can not be explained by the formulated mechanism in path 1.…”

“…HCOOH Or HCOO ad + e À ! HCOO À If we consider the applied potential values, such as À1.6, À1.7 V (SCE) in the present work in a detailed manner and compare these with the potentials formulated for CO 2 /CO 2 À formation in [26][27][28][29], a direct electron transfer onto CO 2 molecule appears to be impossible.…”

Ab initio molecular dynamics calculations of ion hydration free energies

“…The predicted value is significantly more endothermic than the Ϫ0.6 eV cited in the experimental radiolysis literature. 31,64,65 That Ϫ0.6 eV value was derived by estimating the Ni + ⌬G hyd using a simple Pauling ionic radius and a dielectric continuum approximation; 65 as the authors stressed, ligand field effects, which can be a fraction of an eV for first row transition metal ions in water, 66 were neglected. AIMD ⌬G hyd calculations, free from these assump-tions, should yield more accurate redox potentials for metal ions in unstable valence states encountered as transients in radiolysis experiments.…”

“…AIMD ⌬G hyd calculations, free from these assump-tions, should yield more accurate redox potentials for metal ions in unstable valence states encountered as transients in radiolysis experiments. 64,65,67 Finally, we note that the Ni 2+ ⌬G hyd depends on whether the DFT+ U approach is used in calculating ͗dH͑q͒ / q͘ q along the AIMD trajectory. Setting U =4͑6͒ eV already decreases the gas phase Ni 2+ -͑H 2 O͒ 6 cluster binding energy by ϳ0.5 eV ͑1.0 eV͒ without inducing noticeable changes in the geometry of the complex.…”

Reduction of ferricytochrome c, methemoglobin and metmyoglobin by hydroxyl and alcohol radicals

“…The reduction potential for methanol and ethanol radicals is E0(H + + CH20/CH2OH) = --0.97 V and E0(H ÷ + CH3CHO/CH3CHOH) = --1.1 V [18]. This can partly explain the higher rate constant for ethanol radicals (see Table II).…”