Kinetics and mechanism of hydrogen peroxide decomposition in the presence of the tetraaquapalladium(II) complex

Abstract

Kinetics of hydrogen peroxide decomposition in the presence of the tetraaquapalladium(II) complex in an aqueous solution at 40-70 °C was studied. The reaction rate is the first order with respect to the concentration of both Pd II and H 2 O 2 and the negative first order with respect to perchloric acid. Using free radicals traps, the reaction mechanism was found to differ from the traditional free radical mechanism known for d metal aqua ions and proceeds with out generation of hydroxyl radicals. The kinetic data obtained suggest a mechanism involving the formation of an intermediate palladium complex with oxygen.

Catalytic Role of Multinuclear Palladium–Oxygen Intermediates in Aerobic Oxidation Followed by Hydrogen Peroxide Disproportionation

“…As oxygen is a four-electron oxidant, aerobic oxidations mediated by Pd complexes are generally proposed to generate hydrogen peroxide (H 2 O 2 ), ,, as shown in Schemes and . In some cases, H 2 O 2 has been detected during catalytic oxidations with Pd, and when H 2 O 2 is not detected, its rapid disproportionation to H 2 O and O 2 has been proposed. ,,,, While the conversion of LPd 0 to LPd II and catalytic production of H 2 O 2 by Pd is well studied ( vide supra ), mechanistic and kinetic studies of the disproportionation of H 2 O 2 by homogeneous complexes of Pd are rare, − particularly for ligand environments common to the well-studied palladium–acetate aerobic oxidation catalysts.…”

“…These studies demonstrate that 1 and 2 are both active hydrogen peroxide disproportionation catalysts and shed light on the role of multinuclear Pd intermediates in the activation of O 2 and the catalytic disproportionation of H 2 O 2 . In contrast to the rich variety of multinuclear metalloenzymes and coordination compounds in biology and chemistry which reduce molecular oxygen, to date there are relatively few reports ,− of multinuclear activation of oxygen by palladium(0) species. The kinetics, reactivity, isotope labeling, ESI-MS, and NMR results highlight a complex catalytic reaction network for 1 in the reduction of O 2 to H 2 O and the disproportionation of H 2 O 2 into O 2 and water.…”

“…70 A possible candidate for this intermediate is a dinuclear dicationic Pd bridging peroxo complex 8, which we had previously proposed 50 as an intermediate in the formation of 2 and which others had proposed as an intermediate in hydrogen peroxide disproportionation. 40 While the precise structure of the cationic dinuclear peroxo complex 8 is not clear, the fact that the ion The kinetics for the conversion of the trinuclear complex 2b to the cationic Pd−acetate 1b was studied by 1 H NMR under pseudo-first-order conditions with [2b] = 1 mM, excess 7b (8− 20 mM), and excess HOAc (37−62 mM). The decrease in [2b] corresponded to the rate law shown in eq 2 (see Supporting Information for details).…”

“…A related species has been proposed as a key intermediate in H 2 O 2 disproportionation by tetraaqua Pd II salts. 40 Neutral Pd 2 O 2 peroxo compounds are known, 53,73−76 and their chemistry has been investigated. Their activity as oxygen centered bases has been well characterized, 74−78 and some formal O atom transfers have been studied where the O 2 moiety appears to react well with electrophilic reagents, but sluggishly with nucleophilic substrates.…”

Microchannel Reactor System Design & Demonstration For On-Site H2O2 Production by Controlled H2/O2 Reaction

“…Decomposition of hydrogen peroxide is a widely studied reaction and a number of studies were done on decomposition catalyzed by palladium on a variety of supports, for example (Beena et al, 1996;Barteri et al, 1978;Potekhin et al, 2005;Salem et al, 2000;Choudhary and Gaikwad, 2003). In the literature, the ranges of H 2 O 2 concentration covered were typically less than 1 mol/l and the heterogeneously catalyzed decomposition reaction was always found to follow first-order kinetics with respect to H 2 O 2 .…”