Platinum-Gallium (Pt-Ga) Intermetallic Alloys for Propane Dehydrogenation

Natural gas is a source of energy for the United States. The Center for Innovative Strategic Transformation of Alkane Resources (CISTAR) plans to use shale gas extracted from shale rock formations as a bridge fuel to replace coal and oil while the US transitions to renewable energy like solar and wind. After methane, the largest components in shale gas are light alkanes such as ethane and propane. These can be catalytically converted to olefins, which can be further reacted to produce fuels, for example. Olefins from alkanes can be accomplished by dehydrogenation by promoted platinum alloys. This study compares the structure and chemical properties of Pt-Ga alloys on silica (SiO2) and ceria (CeO2) supports to determine if the support plays an important role in this chemistry. The catalysts containing different Pt:Ga ratios were synthesized using incipient wetness impregnation. These catalysts were characterized by in situ X-ray diffraction (XRD) and X-ray adsorption spectroscopy (XAS) to determine if an alloy was formed, and if so, the structure of that alloy. Finally, the catalysts were tested in a fixed bed reactor, where it was found that the silica-supported Pt-Ga alloy has a selectivity of \u3e90% towards propylene. Understanding catalyst design can lead to higher catalytic conversion of substances and potentially an improved selectivity for the formation of preferred products. Pt-Ga on ceria is tested for comparison and there appears to behave differently from that on silica demonstrating the importance of the role of the support on these catalysts

Advanced Characterizations for the Identification of Catalyst Structures and Reaction Intermediates

In recent decades, alternatives to traditional coal and fossil fuels were utilized to reduce carbon emissions. Among these alternatives, natural gas is a cleaner fuel and is abundant globally. Shale gas, a form of natural gas that also contains light alkanes (C2-C4), is presently being employed to produce olefins, which can be upgraded to higher molecular weight hydrocarbons. This thesis describes efforts to develop new catalytic materials and characterizations for the conversion of shale gas to fuels. In the first half, silica supported Pt-Cr alloys containing varying compositions of Pt and Pt3Cr were used for propane dehydrogenation at 550°C. Although a change in selective performance was observed on catalysts with varying promoter compositions, the average nano-particle structures determined by in situ, synchrotron x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) were identical. Further, this work presents a method for the characterization of the catalytic surface by these methods to understand its relationship with olefin selectivity. From this, we can gain an atomically precise control of new alloys compositions with tunable surface structures. Once formed by dehydrogenation, the intermediate olefins are converted to fuel-range hydrocarbons. In the second half, previously unknown single site, main group Zn2+ and Ga3+ catalysts are shown to be effective for oligomerization and the resulting products follow a Schutlz Flory distribution. Mechanistic studies suggest these catalysts form metal hydride and metal alkyl reaction intermediates and are active for olefin insertion and -H elimination elementary steps, typical for the homogeneous, Cossee-Arlman oligomerization mechanism. Evidence of metal hydride and metal alkyl species were observed by XAS, Fourier transform infrared spectroscopy (FTIR), and H2/D2isotope exchange. Understanding the reaction intermediates and elementary steps is critical for identifying novel oligomerization catalysts with tunable product selectivity for targeted applications. Through controlled synthesis and atomic level in situcharacterizations, new catalysts compositions can be developed with high control over the resulting performance. An atomically precise control of the catalyst structure and understanding how it evolves under reaction conditions can help shed light on the fundamental principles required for rational catalyst design

First Principles Analysis of Ethylene Oligomerization on Single-site Ga3+ Catalysts Supported on Amorphous Silica

Amorphous, single site, silica-supported main group metal catalysts have recently been found to promote olefin oligomerization with high activity at moderate temperatures and pressures (~250°C and 1 atm). Herein, we explore the molecular-level relationship between active site structures and the associated oligomerization mechanisms by developing amorphous, silica-supported Ga3+ models from periodic, first-principles calculations. Representative Ga3+ sites, including three- and four-coordinated geometries, are tested for multiple ethylene oligomerization pathways. We show that the three-coordinated Ga3+ site promotes oligomerization through a facile initiation process that generates a Ga-alkyl intermediate, followed by a Ga-alkyl-centered Cossee-Arlman mechanism. The strained geometry of a three-coordinated site enables a favorable free energy landscape with a kinetically accessible ethylene insertion transition state (1.7 eV) and a previously unreported β-hydride transfer step (1.0 eV) to terminate further C-C bond formation. This result, in turn, suggests that Ga3+ does not favor polymerization chemistry, while microkinetic modeling confirms that ethylene insertion is the rate-determining step. The study demonstrates promising flexibility of main group ions for hydrocarbon transformations and, more generally, highlights the importance of the local geometry of metal ions on amorphous oxides in determining catalytic properties

Short contact time CH4 partial oxidation over Ni based catalyst at 1.5MPa

Gas-to-liquid technologies to produce Fischer\u2013Tropsch fuels are economically sustainable at very large scales\u2014 30\u2008000bbld 121. To achieve a viable process at a scale less than 100bbld 121 requires a compact design, like a short contact time reactor and mass manufacturing to reduce capital cost. We tested the activity of 2.25%Ni/0.1%Ru/CeO2 supported on FeCrAl gauze (Ni2510) to partially oxide methane at a contact time less than 0.050s. Besides, the very short contact time, an additional feature of this work is that the catalyst activated on-stream without a hydrogen pretreatment step. The reactor operated at 1.5MPa, 800\ub0C\ua0to\ua0950\ub0C, and a CH4/O2 ratio varying from 1.6 to 1.8 v/v. Methane partially oxidized carbon monoxide (direct mechanism) rather than combusting to CO2 followed by steam reforming to CO (indirect mechanism). The following phenomena support the direct mechanism hypothesis: (i) the selectivity improved when reducing residence time, (ii) the mass spectrometer detected both O2 and CO at the effluent (simultaneously), (iii) metallic Ni clusters on the Ni2510 were absent under reaction conditions based on in situ X-ray absorption spectroscopy. Loading Ni/Al2O3 powder downstream of the Ni2510 increased syngas yield, as this catalyst promoted steam and dry reforming. Soot forms upstream of the Ni2510 catalyst via a retro-propagation mechanism in which methyl radicals produced on the catalyst surface react with the incoming feed gas

A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum in fuel cells, but their active site structures are poorly understood. A leading postulate is that iron active sites in this class of materials exist in an Fe-N4 pyridinic ligation environment. Yet, molecular Fe-based catalysts for the oxygen reduction reaction (ORR) generally feature pyrrolic coordination and pyridinic Fe-N4 catalysts are, to the best of our knowledge, non-existent. We report the synthesis and characterization of a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for oxygen reduction to a prototypical Fe-N-C material, as well as iron phthalocyanine, (Pc)Fe, and iron octaethylporphyrin, (OEP)Fe, prototypical pyrrolic iron macrocycles. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (Pc)Fe and (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of both (Pc)Fe and (OEP)Fe, and are remarkably similar to those of Fe-N-C with compressed Fe–N bond lengths of 1.97 Å in (phen2N2)Fe that are close to the average 1.94 Å length in Fe-N-C. Electrochemical studies establish that both (Pc)Fe and (phen2N2)Fe have relatively high Fe(III/II) potentials at ~0.6 V, ~300 mV positive of (OEP)Fe. The ORR onset potential is found to directly correlate with the Fe(III/II) potential leading to a ~300 mV positive shift in the onset of ORR for (Pc)Fe and (phen2N2)Fe relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe and (Pc)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe and (Pc)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with 2O2 production, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data establish (phen2N2)Fe as a pyridinic iron macrocycle that effectively models Fe-N-C active sites, thereby providing a rich molecular platform for understanding this important class of catalytic materials.</p

High-Capacitance Pseudocapacitors from Li+ Ion Intercalation in Nonporous, Electrically Conductive 2D Coordination Polymers

© Electrochemical capacitors (ECs) have emerged as reliable and fast-charging electrochemical energy storage devices that offer high power densities. Their use is still limited, nevertheless, by their relatively low energy density. Because high specific surface area and electrical conductivity are widely seen as key metrics for improving the energy density and overall performance of ECs, materials that have excellent electrical conductivities but are otherwise nonporous, such as coordination polymers (CPs), are often overlooked. Here, we report a new nonporous CP, Ni3(benzenehexathiolate) (Ni3BHT), which exhibits high electrical conductivity of over 500 S/m. When used as an electrode, Ni3BHT delivers excellent specific capacitances of 245 F/g and 426 F/cm3 in nonaqueous electrolytes. Structural and electrochemical studies relate the favorable performance to pseudocapacitive intercalation of Li+ ions between the 2D layers of Ni3BHT, a charge-storage mechanism that has thus far been documented only in inorganic materials such as TiO2, Nb2O5, and MXenes. This first demonstration of pseudocapacitive ion intercalation in nonporous CPs, a class of materials comprising thousands of members with distinct structures and compositions, provides important motivation for exploring this vast family of materials for nontraditional, high-energy pseudocapacitors

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

© 2020, The Author(s). Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Iron- and nitrogen-doped carbon materials are effective catalysts for the oxygen reduction reaction whose active sites are poorly understood. Here, the authors establish a new pyridinic iron macrocycle complex as a more effective active site model relative to legacy pyrrolic model complexes