Browsing by Subject "Palladium"
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Item Catalytic chemistry of Pd−Au bimetallic surfaces(2015-08) Yu, Wen-Yueh; Mullins, C. B.; Henkelman, Graeme; Hwang, Gyeong S.; Korgel, Brian A.; Sitz, Greg O.Catalyst development is important to the contemporary world as suitable catalysts can allow chemical processes to proceed with reduced energy consumption and waste production. In order to design catalysts with improved performance, the fundamental studies that correlate catalytic properties with surface structures are essential as they can provide mechanistic insights into the reaction mechanism. Pd−Au bimetallic catalysts have shown exceptional performance for a number of chemical reactions, however, the interplay between the reactive species and surface properties are still unclear at the molecular level. In this dissertation, the catalytic chemistry of Pd−Au surfaces was investigated via model catalyst studies under ultrahigh vacuum conditions. A range of Pd−Au model surfaces were generated by annealing Pd/Au(111) surfaces and characterized/tested by surface science techniques. The findings in this dissertation may prove useful to enhance the fundamental understanding of structure-reactivity relation of Pd−Au catalysts in associated reactions.Item Cathode catalysts for low-temperature fuel cells : analysis of surface phenomena(2013-12) Mathew, Preethi; Manthiram, Arumugam; Goodenough, John B.The electrochemical oxygen reduction reaction (ORR) steps on a noble metal catalyst in an acidic aqueous electrolyte depend on the nature of the catalytic surface with which the O₂ molecule interacts. It has been assumed that the O₂ molecules interact directly with a bare noble-metal surface. By studying the nature of chemisorbed species on the surface of a metal catalyst as a function of the voltage on the anodic and cathodic sweeps, it is shown here that the O₂ reacts with a surface covered with oxide species extracted from the aqueous electrolyte and not from the O₂ molecules; the ORR is more active when the surface species are OH rather than O. Moreover, the strength of the chemical bond of the adsorbed species was shown to depend on the relative strengths of the metal-metal versus metal-oxide bonds. The Pt-Pt bonds are stronger than the Pd-Pd bonds, and the relative Pd-O bonds are stronger than the relative Pt-O bonds. As a result, the chemisorbed O species is stable to lower anodic potentials on Pd. CO oxidation to CO₂ occurs at a higher potential on Pd than on Pt, which is why Pd (not Pt) is tolerant to methanol. Experiments with alloys show the following: (1) methanol tolerance decreases with the increase of Pt in the Pd-Pt alloys with Pd₃Pt/C showing an initial tolerance that decreases with cycling; (2) OH is formed on Pt₃Co/C and core-shell Pt-Cu/C, which results in a higher activity and durability for the ORR on these catalysts; (3) a 300°C anneal is needed to stabilize the Pd₃Au/C catalyst that forms an O adsorbate; and (4) OH is formed on Pd₃Co/C and Pd₃CoNi/C. These studies provide a perspective on possible pathways of the ORR on oxide-coated noble-metal alloy catalysts.Item First-principles investigation of the surface reactivity of Pd-based alloys for fuel cell catalyst applications(2011-12) Ham, Hyung Chul; Hwang, Gyeong S.; Ekerdt, John G.; Mullins, Buddie C.; Arumugam, Manthiram; Ferreira, Paulo J.In recent years, palladium (Pd) has been extensively studied for a possible alternative for Pt that has been most commonly used as a catalyst in fuel cells. However, Pd shows lower activity than Pt towards the cathodic oxygen reduction reaction (ORR) and also exhibits poor tolerance toward carbon monoxide (CO) poisoning occurring in the anode process. To improve its performance, alloying Pd with other transition metals has been suggested as one of promising solutions as the Pd-based alloys have been found to boost the ORR activity and yield significant improvement in the CO tolerance. However, a detailed understanding of the alloying effects is still lacking, despite its importance in designing and developing new and more cost effective fuel cell catalysts. This is in large part due to the difficulty of direct characterization. Alternatively, computational approaches based on quantum mechanics have emerged as a powerful and flexible means to unravel the complex alloying effects in multimetallic catalysts; such first principles-based computational studies have provided many invaluable insights into the mechanisms of catalytic reactions occurring on the alloy surfaces. Using first-principles density-functional theory calculations, we have examined the surface reactivity of Pd-based bimetallic catalysts with the aim of better understanding the alloying effects in association with atomic arrangement, facet, local strain, ligand interaction, and effective atomic coordination number at the surface. More specifically, this thesis work has focused on examining the following topics: Role of Pd ensembles in selective H₂O₂ formation on AuPd alloys; Effect of local strain and low-coordination number at the surface on the performance of Pd monomer in selective H₂O₂ formation; Different facet effects on the activity of Pd ensembles towards ORR; Structure of ternary Pd-Ir-Co alloys and its reactivity towards ORR; Pd ensembles effects on CO oxidation on CO-precovered Pd ensembles; Role of ligand and ensembles in determining CO chemisorptions on AuPd and AuPt. Our first principles-based theoretical investigation of bimetallic alloys offers some insights into the rational design and development of alloyed catalysts.Item Rhodium porphyrin alkylations with ammonium and quinolinium salts and cyclic ether formation via a palladium catalyzed dehydrogenative annulation(2016-05) Thompson, Samuel John; Dong, Guangbin; Krische, Michael J; Sessler, Jonathan L; Liu, Hung-Wen; Rose, Michael JThe formation and study of metal–carbon σ-bonds can help unveil unique reactivities of organometallic complexes and provide support for further catalytic transformations. Rhodium porphyrins have shown exceptional reactivity through radical- type transformations, attracting significant attention towards understanding these metalloradical-mediated mechanisms. The stability and selectivity of rhodium porphyrins are promising for catalytic transformations, however, strong rhodium–carbon bonds frequently limit catalyst turnover. To gain a better understanding of Rh–C bonds in the porphyrin system, the synthesis of alkyl rhodium porphyrins through a C–N bond dealkylation of ammonium and quinolinium salts was conducted. The organometallic complexes were formed under air and with water, serving as a convenient method to prepare Rh–C bonds. Mechanistic studies support rhodium(I), rhodium(II), and rhodium(III) porphyrin intermediates operating in the alkylation, with a SN2-like reaction in the Rh–C bond forming step. A directed sp3 C–H bond functionalization strategy was also investigated to accomplish cyclic ether formation via an intramolecular alkoxylation reaction. An oxime vii directing group provided chemoselective activation at β-methyl positions, forming annulated products from the addition of tethered alcohol nucleophiles. Four- to seven- membered rings could be accessed through this dehydrogenative annulation pathway. Tethered primary, secondary, and tertiary free hydroxyl groups can all react to give the corresponding cyclized products. Protected silyl and benzyl alcohols were also compatible nucleophiles for the coupling. Preliminary mechanistic analysis supports an sp3 C–H activation/intramolecular SN2 pathway.Item Synthetic studies on the pluramycin family of antitumor antibiotics : the total synthesis of isokidamycin(2010-12) O'Keefe, Brian Michael; Martin, Stephen F.; Krische, Michael J.; Bielawski, Christopher W.; Laude, David A.; Whitman, Christian P.A total synthesis of the complex C-aryl glycoside isokidamycin was achieved during an effort to construct the natural product kidamycin, a member of the pluramycin family of antitumor antibiotics. The angolosamine carbohydrate was appended, along with annelation of a benzene ring by the implementation of the Martin group's silicon tether-directed, intramolecular aryne-furan cycloaddition strategy. The vancosamine-derived carbohydrate was then installed by an O -> C-glycoside rearrangement. A novel protocol for the carbonylative cross-coupling of ortho-disubstituted aryl iodides with aryl boronic acids and alkynyl zinc reagents was also discovered during efforts to introduce the pyranone ring of kidamycin. The reaction proved general, as a variety of electron-rich and electron-poor aryl iodides, boronic acids, and alkynyl-zinc reagents participated in the cross-coupling.Item Synthetic, spectroscopic and mechanistic studies of Pd(II)-carbon bonding in chloranilatopalladium(II) complexes(Texas Tech University, 1988-08) Jeong, Woo-yeongThe organometallic chemistry of palladium(II) has slowly built momentum. While the early work focused on coordination of organic molecules to palladium, the evolution of industrial catalytic applications of palladium(II) compounds stimulated a search for reactions mediated by palladium. The growth is reflected in the appearance of several books I'2'^ devoted to the chemistry of palladium and about 30 reviews in the last 15 years. However, the complexities and subdeties of organopalladium chemistry have hampered the understanding and adoption of palladium mediated reactions in organic synthesis. For example, dramatic changes in chemoselectivity, regioselectivity and stereocontrol can frequendy be accomplished by minor modifications of experimental conditions, especially choice of ligands. These effects are mainly due to structural and/or kinetic requirements of Pd(II) intermediates involved in the specific reaction. To increase both predictability and apphcabiUty of palladium mediated reactions in organic synthesis, it is of great importance to investigate the structural and kinetic natures of Pd(Il) intermediates.Item The development of metal-organic frameworks using palladium metal complexes for catalysis and the utilization of a dihaloimidazolidinedione for easy acid chloride generation(2016-08) Nguyen, An Ngoc-Michael; Humphrey, Simon M.; Jones, Richard APart 1. Metal-Organic Frameworks (MOFs) have been extensively studied due to their functional versatility and well-defined, porous structures. It is, however, exceptionally rare to find MOFs that use late transition metal complexes as a building block. Incorporation of these complexes would have immediate applications in catalysis and enhanced gas adsorption/storage. Furthermore, because of the MOF’s well-defined structures, heterogeneous catalysis properties can be more readily studied and improved. Herein, 1,2-bis(bi(para-carboxyl)phenylphosphino)benzene palladium dichlo-ride’s (PdCl2(BBCB)) catalytic ability is studied using the Mizoroki-Heck reaction, and 1,2-bis(bi(4-carboxy-biphenyl-4’-phosphino))ethane palladium dichloride (PdCl2(BBCE-1L)) is used to make a MOF with an enlarged pore size and greater catalytic ability. BBCB, a tetra-para-carboxylic acid derivative of dppb, is used to form a palladium dichloride bis(phosphine) complex. This complex was found to perform a Mizoroki-Heck reaction in decent yields despite formation of palladium black. BBCE-1L, a tetra-4-carboxylic acid biphenyl derivative of dppe, was then developed to deter palladium black formation and enlarge the pore size of the MOF. A MOF was made using a Zn(II) paddlewheel as the metal node and PdCl2(BBCE-1L) as the organic linker. This MOF was found to have moderate thermal stability and potentially high pore volume. Part 2. Acid chloride generation is a widely researched field for the synthesis of amides and esters, motifs found throughout nature and pharmaceuticals. Recently, we found that dihaloimidazolidinediones could substitute a variety of alcohols with halogens through an amide-stabilized carbocation. As an extension of this chemistry, we were able to activate most carboxylic acids into the corresponding acid chloride with near quantitative yields. Mechanistic studies of electronic and steric effects suggest that the rate-determining step is the nucleophilic attack of a free chloride ion on the carbonyl center to produce the corresponding acid chloride. Electronic effects of ring size on electrophilicity was also observed and explained through analysis of a simplified Walsh diagram.Item The synthesis and spectroscopic characterization of bis(acetylacetonato) palladium (II) phosphine complexes(Texas Tech University, 1989-08) Childress, Steven RayNot availableItem The Transannular bis-Michael Reaction in the Synthetic Studies of Celastrol and the Development of Novel Palladium-Catalyzed Reactions(2013-11-11) Kaiser, Thomas MaxwellPharmaceutical R&D is currently undergoing a productivity crisis. Also, the loss of activity of established medicines continues to reduce the pool of agents capable of treating infectious diseases. Small molecule synthesis and synthetic methodology development continue to be essential scientific endeavors due to the ability of synthetic chemistry to create new starting points for the development of medicines. Therefore, in order to increase the ability to discover new medicines, more efficient synthetic strategies and transformations capable of generating structurally complex drug-like molecules are required. This work explored the transannular bis-Michael reaction (TMR) as a potential method to access polycyclic natural products in an efficient manner. We sought to develop an expedient route to an all-carbon Z,E macrocyclic precursor to the TMR and we then evaluated whether the Z,E isomer would follow our proposed model for the TMR. Our strategy relied on a 1,3-dipolar cycloaddition to access the TMR precursor. However, this 1,3-dipolar synthetic route had a low synthetic efficiency. Consistent with our other studies, this Z,E-macrocyclic bis-enone was found to be inactive in the transannular bis-Michael reaction cascade for the conditions evaluated. En route, we also discovered that our 1,3-dipolar cycloaddition gave a rare 3,4-disubstituted isoxazole under kinetic reaction conditions. We also demonstrated that the dipolar cycloaddition is reversible and the thermodynamic 3,5-disubstituted isoxazole can be obtained through isomerization of its 3,4-disubstituted isomer under elevated temperature. Our initial mechanistic studies support the role of hydrogen-bonding in accelerating the isomerization process. Our work in developing new palladium-catalyzed reactions resulted in a novel palladium-catalyzed enamine Heck reaction. This reaction is capable of generating ?,?-unsaturated ketones directly from aldehydes and vinyl iodides. However, the limitations of scope in both vinyl iodide and aldehyde severely limit the synthetic utility of the reaction described herein. Also, our work clearly demonstrated a novel enantioconvergent approach to 3-allyl-3-alykl-indolenines through the use of a chiral palladium/trialkylborane dual catalyst system. We suggest a greater role of trialkyl borane beyond allylic alcohol activation in previous allylation examples employing R_(3)B/allyl alcohol as the allyl source. Finally, we extended or understanding of the role of Et_(3)B to the Tamaru allylation. The Lewis acid, Et3B, facilitates enolization and behaves as a co-catalyst to effect the allylation of aldehydes. We have also begun developing an enantioselective version of this reaction that suffers from low enantioselectivity. This reaction was shown to be selective for aldehydes as ketones did not react under the described conditions.Item Theoretical Studies of Structures and Mechanisms in Organometallic and Bioinorganic Chemistry: Heck Reaction with Palladium Phosphines, Active Sites of Superoxide Reductase and Cytochrome P450 Monooxygenase, and Tetrairon Hexathiolate Hydrogenase Model(2010-07-14) Surawatanawong, PanidaThe electronic structures and reaction mechanisms of transition-metal complexes can be calculated accurately by density functional theory (DFT) in cooperation with the continuum solvation model. The palladium catalyzed Heck reaction, iron-model complexes for cytochrome P450 and superoxide reductase (SOR), and tetrairon hexathiolate hydrogenase model were investigated. The DFT calculations on the catalytic Heck reaction (between phenyl-bromide and ethylene to form the styrene product), catalyzed by palladium diphosphine indicate a four-step mechanism: oxidative addition of C6H5Br, migratory insertion of C6H5 to C2H4, b-hydride transfer/olefin elimination of styrene product, and catalyst regeneration by removal of HBr. For the oxidative addition, the rate-determining step, the reaction through monophosphinopalladium complex is more favorable than that through either the diphosphinopalladium or ethylene-bound monophosphinopalladium. In further study, for a steric phosphine, PtBu3, the oxidative-addition barrier is lower on monopalladium monophosphine than dipalladium diphosphine whereas for a small phosphine, PMe3, the oxidative addition proceeds more easily via dipalladium diphosphine. Of the phosphine-free palladium complexes examined: free-Pd, PdBr-, and Pd(h2-C2H4), the olefin-coordinated intermediate has the lowest barrier for the oxidativeaddition. P450 and SOR have the same first-coordination-sphere, Fe[N4S], at their active sites but proceed through different reaction paths. The different ground spin states of the intermediate FeIII(OOH)(SCH3)(L) model {L = porphyrin for P450 and four imidazoles for SOR} produce geometric and electronic structures that assist i) the protonation on distal oxygen for P450, which leads to O-O bond cleavage and formation of (FeIV=O)(SCH3)(L) H2O, and ii) the protonation on proximal oxygen for SOR, which leads to (FeIII-HOOH)(SCH3)(L) formation before the Fe-O bond cleavage and H2O2 production. The hydrogen bonding from explicit waters also stabilizes FeIII-HOOH over FeIV=O H2O products in SOR. The electrochemical hydrogen production by Fe4[MeC(CH2S)3]2(CO)8 (1) with 2,6-dimethylpyridinium (LutH ) were studied by the DFT calculations of proton-transfer free energies relative to LutH and reduction potentials (vs. Fc/Fc ) of possible intermediates. In hydrogen production by 1, the second, more highly reductive, applied potential (-1.58 V) has the advantage over the first applied potential (-1.22 V) in that the more highly reduced intermediates can more easily add protons to produce H2.