M. N. Vargaftik

A novel giant palladium cluster

Electron microscopy, electron diffraction, EXAFS, and ultracentrifuging data show that a new catalytically active cluster prepared by reduction of Pd(OAc)2 by H2 in the presence of L = 1,10-phenanthroline, or 2,2′-bipyridine, followed by O2 treatment, contains a close-packed metal nucleus (570 ± 30 Pd atoms) bearing 60 ± 3 co-ordinated L and 180 ± 10 OAc– in the outer sphere of the cluster.

Pd–Cu catalysts from acetate complexes in liquid-phase diphenylacetylene hydrogenation

Properties of Pd–Cu/Al2O3 catalysts prepared using PdCu(CH3CO2)4 acetate heteronuclear complexes as precursors in the liquid-phase diphenylacetylene (DPA) hydrogenation have been studied. It has been established that the reaction over the Pd–Cu/Al2O3 catalyst proceeds more selectively than over the commercial Lindlar catalyst; in addition, high activity is achieved at a substantially lower palladium content. The maximum selectivity of DPA hydrogenation is observed with the catalyst reduced in a hydrogen atmosphere without any intermediate calcination that can result in the destruction of the bimetallic acetate complex. FTIR spectroscopy data for adsorbed CO show that the high selectivity of hydrogenation is due to the formation of homogeneous Pd–Cu particles and to the absence of monometallic palladium particles. This can be explained by the retention of the initial complex structure at all of the catalyst preparation stages until the formation of bimetallic particles during hydrogenation.

Unexpected participation of nucleophiles in the reaction of palladium(II) acetate with divalent 3d metals

The kinetics of reactions of palladium(II) acetate with cobalt(II), nickel(II), and copper(II) acetates were studied by spectrophotometry. These reactions produce heterobimetallic complexes PdII(μ-OOCMe)4MII(OH2)(HOOCMe)2, where M = Co, Ni, or Cu. These reactions are very slow in carefully dehydrated (<0.01% H2O) acetic acid, but are considerably enhanced by water or acetonitrile. Our data indicate that the activation of the kinetically inert ring structure of the initial palladium complex Pd3(μ-OOCMe)6 by means of the nucleophilic attack of an H2O or acetonitrile molecule is the key step of the reaction mechanism.

Supported catalysts based on Pd–In nanoparticles for the liquid-phase hydrogenation of terminal and internal alkynes: 1. formation and structure

The formation of Pd–In catalysts synthesized from the heteronuclear acetate complex PdIn(CH3COO)5 was studied by temperature-programmed reduction, electron microscopy, IR spectroscopy of adsorbed CO and hydrogen temperature-programmed desorption (H2-TPD). IR spectroscopy of adsorbed CO and H2-TPD confirmed the formation of bimetallic Pd–In nanoparticles. It was found that the Pd–In nanoparticle surface contains predominantly Pd atoms separated from one another by indium atoms, which is evidenced by the disappearance of the CO band shift resulting from the lateral dipole–dipole interaction between adsorbed CO molecules and by a significant decrease in the band intensity of CO adsorbed in bridged form. Almost complete inhibition of palladium hydride (PdHx) provides additional evidence of the formation of Pd–In bimetallic particles.

Supported catalysts based on Pd–In nanoparticles for the liquid- phase hydrogenation of terminal and internal alkynes: 2. catalytic properties

Pd–In/Al2O3 and Pd–In/MgAl2O4 catalysts prepared from dinuclear Pd–In acetate complexes were studied in the hydrogenation of alkyne compounds with different structures. The Pd–In catalysts demonstrate high selectivity in the hydrogenation of internal alkynes comparable with that of the Lindlar catalyst. Similar activity/selectivity characteristics are reached at a significantly lower Pd content. For terminal alkynes, the favorable effect of Indium introduction is considerably less pronounced. An analysis of the In effect on the selectivity and the ratio between the rates of the first and second hydrogenation steps suggests that the reaction selectivity is determined to a large extent by a thermodynamic factor (adsorption–desorption equilibrium between the reactants and the reaction products).

Formation of Pd–Ag nanoparticles in supported catalysts based on the heterobimetallic complex PdAg2(OAc)4(HOAc)4

The formation of Pd–Ag nanoparticles deposited from the heterobimetallic acetate complex PdAg2(OAc)4(HOAc)4 on α-Al2O3, γ-Al2O3, and MgAl2O4 has been investigated by high-resolution trans-mission electron microscopy, temperature-programmed reduction, and IR spectroscopy of adsorbed CO. The reduction of PdAg2(OAc)4(HOAc)4 supported on γ-Al2O3 and MgAl2O4 takes place in two steps (at 15–245 and 290–550°C) and yields Pd–Ag particles whose average size is 6–7 nm. The reduction of the Pd–Ag catalyst supported on α-Al2O3 occurs in a much narrower temperature range (15–200°C) and yields larger nanoparticles (~10–20 nm). The formation of Pd–Ag alloy nanoparticles in all of the samples is demonstrated by IR spectroscopy of adsorbed CO, which indicates a marked weakening of the absorption band of the bridged form of adsorbed carbon monoxide and a >30-cm–1 bathochromic shift of the linear adsorbed CO band. IR spectroscopic data for PdAg2/α-Al2O3 suggest that Pd in this sample occurs as isolated atoms on the surface of bimetallic nanoparticles, as is indicated by the almost complete absence of bridged adsorbed CO bands and by a significant weakening of the Pd–CO bond relative to the same bond in the bimetallic samples based on γ-Al2O3 and MgAl2O4 and in the monometallic reference sample Pd/γ-Al2O3.

Catalytic properties of nanostructured Pd–Ag catalysts in the liquid-phase hydrogenation of terminal and internal alkynes

A comparative catalytic study of Pd–Ag bimetallic catalysts and the commercial Lindlar catalyst (Pd–Pb/CaCO3) has been carried out in the hydrogenation of phenylacetylene (PA) and diphenylacetylene (DPA). The Pd–Ag catalysts have been prepared using the heterobimetallic complex PdAg2(OAc)4(HOAc)4 supported on MgAl2O4 and aluminas (α-Al2O3 and γ-Al2O3). Physicochemical studies have demonstrated that the reduction of supported Pd–Ag complex with hydrogen results in homogeneous Pd–Ag nanoparticles. Equal in selectivity to the Lindlar catalyst, the Pd–Ag catalysts are more active in DPA hydrogenation. The synthesized Pd–Ag catalysts are active and selective in PA hydrogenation as well, but the unfavorable ratio of the rates of the first and second stages of the process makes it difficult to kinetically control the reaction. The most promising results have been obtained for the Pd–Ag2/α-Al2O3 catalyst. Although this catalyst is less active, it is very selective and allows efficient kinetic control of the process to be carried out owing to the fact that, with this catalyst, the rate of hydrogenation of the resulting styrene is much lower than the rate of hydrogenation of the initial PA.

Role of the V(V)/ 1O2Complex in Oxidative Reactions in the H2O2/V(V)/AcOH System: Oxidation of Alkenes and Anthracenes

Anthracene and its alkyl derivatives undergo oxidation in the V(V)/H2O2/AcOH system via a nonradical mechanism through the intermediate formation of the vanadium(V) complex with singlet dioxygen as a ligand. The 1O2molecule is transferred from this complex to an unsaturated substrate. The free singlet dioxygen 1O2(1Δg) is almost inactive toward anthracene in AcOH solution. Consequently, the vanadium(V) complex with singlet dioxygen is the only oxidant species active in the reaction. The ratio between the rate constant of the reaction of this complex with 2-ethylanthracene and the rate constant of its deactivation is an order of magnitude greater than the ratio between the rate constant of the reaction of dissolved free singlet dioxygen with the same substrate and the rate constant of its deactivation (physical quenching).

Highly selective catalysts for liquid-phase hydrogenation of substituted alkynes based on Pd—Cu bimetallic nanoparticles

Catalytic properties of Pd—Cu bimetallic catalysts supported on SiO2 and Al2O3 were studied in a model reaction of selective hydrogenation of diphenylacetylene. Application of PdCu2(AcO)6 heterobimetallic acetate complex as a precursor made it possible to obtain homogeneous Pd—Cu bimetallic nanoparticles. This result was supported by the data of IR spectroscopy of adsorbed CO. The Pd-Cu catalysts showed considerably higher selectivity than monometallic samples. Moreover, the introduction of copper decreases the hydrogenation rate of diphenylethylene (DPE) to diphenylethane. As a result, the maximum yield of the target product, DPE, increased from 78 to 93% in the presence of Pd—Cu catalysts.

Bimetallic Pd-M (M = Co, Ni, Zn, Ag) nanoparticles containing transition metals: Synthesis, characterization, and catalytic performance

The reductive thermolysis of Pd(OOCMe)4M(OH2) (M = NiII, CoII, ZnII) and Pd(OOCMe)4Ag2(HOOCMe)4 molecular complexes results in the generation of bimetallic Pd-based Pd-M (M = Co, Ni, Zn, Ag) nanoparticles. The composition and morphology of nanoparticles and the electron state of metal atoms were characterized using electron microscopy, elemental ICP analysis, X-ray diffraction, and XAFS (XANES/EXAFS) techniques. The catalytic performance of nanoparticles was studied using the example of reactions of catalytic hydrazine decomposition and U(VI) reduction to U(IV) by hydrazine and formic acid. The catalytic performance of Pd-Ni nanoparticles is superior to that of the standard supported Pd/SiO2 catalyst containing a similar amount of Pd atoms, while Pd-Co, Pd-Zn, and Pd-Ag nanoparticles do not catalyze the studied reactions.