Reactivity and Characterization of Intermetallic Alloy Catalysts for Alkane Dehydrogenation

As the United States works towards energy independence shale gas has become an attractive domestic recourse for use as a feedstock to produce fuels. One potential approach to utilize shale gas is to first convert the C2 and C3 paraffinic components into olefins, valuable chemical building blocks, by catalytic dehydrogenation. The goal of this dissertation is to study how the geometric and electronic changes to a metal upon alloying influence its selectivity for light alkane dehydrogenation. In the first three projects bimetallic catalysts comprising of either Pd or Pt and a post-transition metal known to promote olefin selectivity were investigated. In all the systems studied the bimetallic catalysts were found to be more selective for ethane dehydrogenation than the monometallic analogue. In situ characterizations revealed the formation of intermetallic compounds (IMC) which contained either small ensembles of or completely isolated active atoms in the bimetallic catalysts. It is believed that these geometric changes to the active metal are the dominant factor leading to improved dehydrogenation selectivities. From a study performed on Pd-In catalysts it was proposed that IMC structures similar to the active metal are preferentially formed. In a separate study, two distinct IMC structures were formed in Pt-In catalysts with different In:Pt atomic ratios and the two phases were found to have different turnover rates (TOR) and apparent activation energies. These results showed that the catalytic properties of metals could be altered by forming different IMC structures. Lastly, a study on Pt-Zn catalysts revealed changes in energy of the 5d states of Pt upon IMC formation. The observed energy change is believed to be responsible for increases in dehydrogenation TOR. In the fourth project Pt-Fe bimetallic catalysts were investigated as an extrapolation of the findings of the first set of studies. Pt and Fe were found to form three IMC structures as the Fe:Pt atomic ratio was varied. All three structures contained Pt atoms with local geometries identical to the catalysts selective for ethane dehydrogenation. When tested for propane dehydrogenation the IMC catalysts were found to be highly selective for propylene. Although Pt and Fe are both catalytic, the much higher activity of the former results in the latter behaving as an inert diluent. This results in the small ensembles of and isolated Pt atoms in the IMC structures being highly selective for dehydrogenation. Electronic structure measurements and calculations showed small changes in the average energies of the 5d states of Pt as the Fe content of the IMC changed. Associated with the valance energy shifts were changes in metal-adsorbate bond strengths which were believed to be the cause of increased dehydrogenation TOR. These results demonstrated that it is possible to change the electronic structure of metals by forming IMCs with different promoters or stoichiometries. While electronic effects play a secondary role in alkane dehydrogenation, this insight could provide useful for other catalytic chemistries

Electrophilic Organoiridiunn(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization

Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous catalysis while offering opportunities to investigate the impact of metal–support interactions on reactivity. We report a (dmPhebox)Ir(III) (dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported species. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analytical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting onto sulfated zirconia, imbuing high levels of activity in electrophilic C–H bond functionalization reactions such as the stoichiometric dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for β-H elimination. Catalytic hydrogenation of olefins was also facilitated by the sulfated zirconia-supported (dmPhebox)Ir(III) complex, while the homologous complex on silica was inactive under comparable conditions

Atomically dispersed single iron sites for promoting Pt and Pt3Co fuel cell catalysts: performance and durability improvements

Significantly reducing platinum group metal (PGM) loading while improving catalytic performance and durability is critical to accelerating proton-exchange membrane fuel cells (PEMFCs) for transportation. Here we report an effective strategy to boost PGM catalysts through integrating PGM-free atomically-dispersed single metal active sites in the carbon support toward the cathode oxygen reduction reaction (ORR). We achieved uniform and fine Pt nanoparticle (NP) (∼2 nm) dispersion on an already highly ORR-active FeN4 site-rich carbon (FeN4–C). Furthermore, we developed an effective approach to preparing a well-dispersed and highly ordered L12 Pt3Co intermetallic nanoparticle catalyst on the FeN4–C support. DFT calculations predicted a synergistic interaction between Pt clusters and surrounding FeN4 sites through weakening O2 adsorption by 0.15 eV on Pt sites and reducing activation energy to break O–O bonds, thereby enhancing the intrinsic activity of Pt. Experimentally, we verified the synergistic effect between Pt or Pt3Co NPs and FeN4 sites, leading to significantly enhanced ORR activity and stability. Especially in a membrane electrode assembly (MEA) with a low cathode Pt loading (0.1 mgPt cm−2), the Pt/FeN4–C catalyst achieved a mass activity of 0.451 A mgPt−1 and retained 80% of the initial values after 30 000 voltage cycles (0.60 to 0.95 V), exceeding DOE 2020 targets. Furthermore, the Pt3Co/FeN4 catalyst achieved significantly enhanced performance and durability concerning initial mass activity (0.72 A mgPt−1), power density (824 mW cm−2 at 0.67 V), and stability (23 mV loss at 1.0 A cm−2). The approach to exploring the synergy between PGM and PGM-free Fe–N–C catalysts provides a new direction to design advanced catalysts for hydrogen fuel cells and various electrocatalysis processes

Dehydrogenative Transformations of Imines Using a Heterogeneous Photocatalyst

Heterogeneous semiconductors are underexploited as photoredox catalysts in organic synthesis relative to their homogeneous, molecular counterparts. Here, we report the use of metal/TiO₂ particles as catalysts for light-induced dehydrogenative imine transformations. The highly oxophilic nature of the TiO₂ surface promotes the selective binding and dehydrogenation of alcohols in the presence of other oxidizable and Lewis basic functional groups. This feature enables the clean photogeneration of aldehyde equivalents that can be utilized in multicomponent couplings

Rapid Electrochemical Methane Functionalization Involves Pd–Pd Bonded Intermediates

High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of PdII to a critical dinuclear PdIII intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a PdIII dimer with a Pd-Pd bond and a 5-fold O atom coordination by HxSO4(x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd2II,III species as a key intermediate during the PdII-to-PdIII2 oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as <-4.5 kcal/mol for Pd2II,III and <-9.1 kcal/mol for PdIII2. The structural and thermochemical data suggest that the aggregate effect of metal-metal and axial metal-ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of PdII to a M-M bonded PdIII dimer and provides a foundation for understanding its rapid methane functionalization reactivity

Mechanistic Aspects of a Surface Organovanadium(III) Catalyst for Hydrocarbon Hydrogenation and Dehydrogenation

Understanding the mechanisms of action for base metal catalysis of transformations typically associated with precious metals is essential for the design of technologies for a sustainable energy economy. Isolated transition-metal and post-transition-metal catalysts on oxides such as silica are generally proposed to effect hydrogenation and dehydrogenation by a mechanism featuring either sigma-bond metathesis or heterolytic bond cleavage as the key bond activation step. In this work, an organovanadium(III) complex on silica, which is a precatalyst for both olefin hydrogenation and alkane dehydrogenation, is interrogated by a series of reaction kinetics and isotopic labeling studies in order to shed light on the operant mechanism for hydrogenation. The kinetic dependencies of the reaction components are potentially consistent with both the sigma-bond metathesis and the heterolytic bond activation mechanisms; however, a key deuterium incorporation experiment definitively excludes the simple sigma-bond metathesis mechanism. Alternatively, a two electron redox cycle, rarely invoked for homologous catalyst systems, is also consistent with experimental observations. Evidence supporting the formation of a persistent vanadium(III) hydride upon hydrogen treatment of the as-prepared material is also presented

Reactivity of a Carbon-Supported Single-Site Molybdenum Dioxo Catalyst for Biodiesel Synthesis

A single-site molybdenum dioxo catalyst, <b>(O</b>_<b>c</b><b>)</b>_<b>2</b><b>Mo­(</b><b>O)</b>_<b>2</b><b>@C</b>, was prepared via direct grafting of MoO₂Cl₂(dme) (dme = 1,2-dimethoxyethane) on high-surface-area activated carbon. The physicochemical and chemical properties of this catalyst were fully characterized by N₂ physisorption, ICP-AES/OES, PXRD, STEM, XPS, XAS, temperature-programmed reduction with H₂ (TPR-H₂), and temperature-programmed NH₃ desorption (TPD-NH₃). The single-site nature of the Mo species is corroborated by XPS and TPR-H₂ data, and it exhibits the lowest reported MoO_<i>x</i> <i>T</i>_max of reduction reported to date, suggesting a highly reactive Mo^VI center. <b>(O</b>_<b>c</b><b>)</b>_<b>2</b><b>Mo­(</b><b>O)</b>_<b>2</b><b>@C</b> catalyzes the transesterification of a variety of esters and triglycerides with ethanol, exhibiting high activity at moderate temperatures (60–90 °C) and with negligible deactivation. <b>(O</b>_<b>c</b><b>)</b>_<b>2</b><b>Mo­(</b><b>O)</b>_<b>2</b><b>@C</b> is resistant to water and can be recycled at least three times with no loss of activity. The transesterification reaction is determined experimentally to be first order in [ethanol] and first order in [Mo] with Δ<i>H</i>^⧧ = 10.5(8) kcal mol^–1 and Δ<i>S</i>^⧧ = −32(2) eu. The low energy of activation is consistent with the moderate conditions needed to achieve rapid turnover. This highly active carbon-supported single-site molybdenum dioxo species is thus an efficient, robust, and low-cost catalyst with significant potential for transesterification processes

Electrophilic Organoiridium(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization

Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous catalysis while offering opportunities to investigate the impact of metal–support interactions on reactivity. We report a (dmPhebox)Ir(III) (dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported species. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analytical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting onto sulfated zirconia, imbuing high levels of activity in electrophilic C–H bond functionalization reactions such as the stoichiometric dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for β-H elimination. Catalytic hydrogenation of olefins was also facilitated by the sulfated zirconia-supported (dmPhebox)Ir(III) complex, while the homologous complex on silica was inactive under comparable conditions