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New Technology for Olefin Oxidation to Carbonyls Using a Palladium and Polyoxoanion Catalyst System
Published in John R. Kosak, Thomas A. Johnson, Catalysis of Organic Reactions, 2020
John H. Grate, David R. Hamm, Suresh Mahajan
Two ways chloride might kinetically accelerate palladium metal oxidation are to increase the palladium metal surface area accessible for oxidation and to participate in the transition state for palladium(0) oxidation, lowering the activation energy. One mechanism by which chloride accelerates the corrosion of weakly oxophilic metals is by disrupting or dissolving insoluble metal oxide or hydroxide that passivates the metal surface, by complexing the metal cations, thereby exposing metal to oxidation. In our acidic phosphomolybdovanadate solutions, however, palladium metal surface should not be passivated. Palladous oxide and hydroxide, which may passivate the surface of palladium metal in less acidic environments, are solubilized to Pd(H2O)42+ at pH<2 [9,11].
Porous Inorganic Nanoarchitectures for Catalysts
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Qingmin Ji, Jiao Sun, Shenmin Zhu
Shi et al. synthesized a carbon-free catalyst, Pt loaded on mesoporous WO3, by a template-replicating method [69]. The parent mesoporous silica with cubic Ia3d symmetry (designated as KIT-6) was used as a hard template for mesoporous WO3. Controllable amount of Pt is loaded into the mesoporous WO3. WO3 can form a hydrogen tungsten bronzes compound (HxWO3) in acidic electrolyte. This tungsten bronzes compound can make the dehydrogenation of methanol molecules be adsorbed on Pt surface more effective, since the spillover of hydrogen onto the surface of hydrogen tungsten bronze can free these Pt sites for further chemisorption of methanol molecules [70]. In addition, the oxophilic nature of the oxide also helps in removing the adsorbed intermediates during the methanol oxidation [71]. This carbon-free mesostructrued Pt/WO3 composite shows high electrocatalytic activity toward methanol oxidation and good electrochemical stability, and its overall activities are also significantly higher than that of commercial Pt/C catalysts with the same Pt loading amount. Both the assistant catalytic effect and mesoporous structure of WO3 support play a large role for the enhanced electrocatalytic activity.
NMR and EPR Spectroscopy in the Study of the Mechanisms of Metallocene and Post-Metallocene Polymerization and Oligomerization of α-Olefins
Published in Evgenii Talsi, Konstantin Bryliakov, Applications of EPR and NMR Spectroscopy in Homogeneous Catalysis, 2017
Evgenii Talsi, Konstantin Bryliakov
The majority of transition metal–based olefin polymerization catalysts are deactivated by polar compounds due to their high oxophilicity. This problem can be solved by using late transition metal catalysts that are less oxophilic [128,129]. By varying the catalyst structures and polymerization conditions, the microstructures of the resulting PE or oligoethylene can be varied from strictly linear to highly branched (as a result of a chain walking mechanism) [128,130]. Neutral κ2-(N,O)-salicylaldiminato NiII complexes are an example of catalysts of this type: they tolerate the presence of polar, oxygen-containing compounds and can conduct polymerizations even in aqueous media [131–133].
Electrodeposition of metallic molybdenum and its alloys – a review
Published in Canadian Metallurgical Quarterly, 2019
Siti Nur Hasan, Min Xu, Edouard Asselin
Various studies [37,79] were done in the past to investigate the influence of molybdate concentration on the deposit. They observed that the concentration of molybdate could control the chemical composition of the deposit and alter the morphology as well. Beltowska-Lehman and Iyndika [37] observed that, at higher molybdate concentration, a large amount of molybdate ions reached the cathode surface but only some were reduced to metallic Mo. Most were deposited as multivalent oxides that progressively blocked the cathode surface. Increasing bath Mo concentration caused a decrease in deposition current density of Ni and Mo. As discussed earlier, due to the oxophilic nature of the molybdate ion, the cathode surface was covered by an intermediate product of the incomplete reduction of the citrate complex of molybdate to Mo oxide, according to the following equilibrium [25]:
Intermetallic compounds in catalysis – a versatile class of materials meets interesting challenges
Published in Science and Technology of Advanced Materials, 2020
Intermetallic compounds can be relevant for catalysis in three ways (Figure 3). First, they can be used as such, either in an unsupported or supported state, and are stable under reaction conditions (case 1). In this case, the intermetallic compound is synthesised and structurally characterised at least before and after studying the catalytic properties. Exchanging the supporting material allows addressing the contribution of the support material to the catalytic properties. Intermetallic compounds can also be precursors and decomposed to the catalytically active species by (partial) oxidation or leaching (case 2). Raney catalysts are a prominent example where caustic leaching of Al-Ni or Ni-Si intermetallic compounds leads to nickel with high specific surface area, which is then applied as hydrogenation catalyst[45]. A trickier example is the formation of intermetallic hydrides under reaction conditions[46]. The hydride may only be stable under reaction conditions, so the structural characterisation before and after the reaction results in the detection of the intermetallic compound while actually the hydride is the catalytically active species. Rather often, the oxidation of intermetallic compounds by one of the reactants under reaction conditions has been described. An early example is the work of Wallace who explored the methanation of carbon monoxide using MNi5 (M = La, Ce, Pr and Th) intermetallic compounds[47]. Since the rare-earth metals are strongly oxophilic and even in intermetallic compounds often easy to oxidise, the materials decompose under reaction conditions to elemental nickel supported on the corresponding rare-earth oxide. Decomposition is not restricted to hydride or oxide formation, the formation of intermetallic nitrides has also been reported[48]. Thus, in case 2 the observed catalytic properties can not be assigned to the intermetallic compound but are due to the reaction products (elements, hydrides, oxides, …). Last but not least, intermetallic compounds can be formed under reaction conditions (un)intentionally (moving from case 3 to case 1). The ingredients needed are metal particles, e.g. Pd, supported on an oxide, which can be (partially) reduced under reaction conditions, e.g. In2O3, and the corresponding reducing conditions, e.g. hydrogen-containing atmosphere (either as reactants or product of the catalytic reaction) and elevated temperature. If the temperature is sufficient, the hydrogen is activated by the metal and reduces the oxide near the metal particle. Subsequently, the resulting metal atoms diffuse inside the metal particle and form the phases corresponding to the binary phase diagram, i.e. substitutional alloys or intermetallic compounds. This effect is the so-called reactive metal-support interaction for which an extensive review is available[49].