Recent efforts to design selective catalysts for multi‐step reactions, such as hydrodeoxygenation (HDO), have emphasized the preparation of active sites at the interface between two materials having ...different properties. However, achieving precise control over interfacial properties, and thus reaction selectivity, has remained a challenge. Here, we encapsulated Pd nanoparticles (NPs) with TiO2 films of regulated porosity to gain a new level of control over catalyst performance, resulting in essentially 100 % HDO selectivity for two biomass‐derived alcohols. This catalyst also showed exceptional reaction specificity in HDO of furfural and m‐cresol. In addition to improving HDO activity by maximizing the interfacial contact between the metal and metal oxide sites, encapsulation by the nanoporous oxide film provided a significant selectivity boost by restricting the accessible conformations of aromatics on the surface.
Die Nanomorphologie legt die Ausrichtung der Reaktantmoleküle in den aktiven Zentren eines Pd/TiO2‐Katalysators fest. Das Resultat ist eine beispiellose Selektivität für die Hydrodeoxygenierung in der katalytischen Umwandlung von aus Biomasse stammenden aromatischen Alkoholen/Aldehyden und Phenolen.
The main objective of this dissertation was to utilize a molecular approach combining DFT calculations and numerous experimental techniques to develop carbon tolerant electrocatalysts for solid oxide ...fuel cell (SOFC) anodes. SOFCs are solid-state electrochemical devices that can convert the chemical energy of hydrogen, CO, and hydrocarbons into electrical energy. One of the main issues associated with the direct operation of SOFCs using hydrocarbons is the deactivation of the conventional anode electrocatalysts, such as Ni on yttria-stabilized zirconia (YSZ) due to the formation of carbon deposits. To tackle the problem of carbon-induced deactivation of Ni electrocatalysts, we have utilized DFT calculations to identify the chemical transformations that govern carbon poisoning of Ni. We found that the processes of C-C and C-O bond formation as well as carbon nucleation played an important role in the carbon-induced deactivation of the Ni catalysts. These insights led to the identification of the Ni surface alloys (i.e. Sn/Ni, Au/Ni...) as promising carbon tolerant catalysts. Electrocatalysts containing Sn/Ni and Ni supported on YSZ were synthesized. Extensive characterization of electronic and geometric characteristics of the Sn/Ni electrocatalysts suggested the formation of a Sn/Ni surface alloy. The carbon tolerance of the Sn/Ni surface alloy and monometallic Ni was tested using packed-bed reactor experiments and electrochemical SOFC studies. We found that the Sn/Ni surface alloy exhibited a significantly improved carbon tolerance compared to monometallic Ni in hydrocarbons steam reforming reactions and as SOFC anode electrocatalyst. To advance our understanding of the chemistry that occurs on the Sn/Ni surface alloy, we have also performed detailed kinetic studies and isotope labeling experiments in combination with DFT calculations to identify the critical elementary steps that govern the performance of the Ni and Sn/Ni electrocatalysts. Furthermore, we have utilized electron microscopy and spectroscopy experiments to measure the electronic structure (i.e electronic states just above and below the Fermi Level) of the supported nonmodel catalysts (i.e. Ni and Ni alloys) and have related that to their chemical and catalytic performance. These detailed atomistic studies allowed us to derive a very general set of principles that allow us to identify novel carbon tolerant alloy catalysts for hydrocarbon reforming and electro-oxidation. The work described in this desertion presents a rare example where DFT calculations combined with various experimental techniques led to the bottom-up (based on molecular insights rather than on empirical trial and error testing) identification of improved electrocatalysts.
Alkali metal–O2 batteries (i.e., Li/Na–O2) with high specific energies are promising alternatives to state-of-the-art metal-ion batteries. However, they are plagued by challenges arising from the ...underlying redox chemistry, resulting in reduced efficiencies. These challenges for Li/Na–O2 batteries stem from the nature of the interface between solid discharge product(s) and either (i) the aprotic electrolyte or (ii) the solid cathode. In the former, the reactive nature of the solid/liquid interface leads to chemical disproportionation of the discharge product(s) and the electrolyte, while in the latter, the presence/lack of atomistic interactions at the solid–solid interface leads to large overpotential losses (>1 V) during charging. Approaches to overcome these challenges would involve decoupling these factors. For instance, the use of inert aprotic electrolytes would facilitate catalytically driven, surface-mediated discharge product(s) growth, providing avenues to use cathode surface modifications as levers to enhance voltaic efficiency and discharge product stability, resulting in improved performance.
We have carried out equilibrium dissociation vapor pressure measurements on dimethonium bromide dihydrate and pentamethonium chloride and bromide dihydrates. None of these salts forms a monohydrate. ...Surprisingly the thermodynamic parameters for the two pentamethonium hydrates are nearly identical, although the properties of these hydrates are quite different. This is explained by a larger negative differential lattice enthalpy for the chloride dihydrate dissociation, which lowers the observed enthalpy of H
2O removal, and greater stabilization of saturated solution by chloride ion which makes the chloride dihydrate deliquescent while the bromide dihydrate is efflorescent. Infrared comparison suggests that tetramethonium chloride dihydrate and pentamethonium chloride and bromide dihydrates have the ladder type halide–water structure determined by X-ray analysis in tetramethonium bromide dihydrate.
Admixture of equal parts of liquid tropone and solid hydroxytropylium perchlorate gives a quantitative yield of a homogeneous white crystalline material; infrared spectra demonstrate that neither ...starting material is present in the new substance in its original form. Ab initio molecular orbital treatment supports a formulation as the perchlorate salt of a proton bridged cationic dimer of tropone with a nearly symmetrical three-center OHO bond and a attraction energy of about −30
kcal
mol
−1.
We have used HF/3-21G(*) geometry optimization to determine the relative energies, structures, symmetries, and nature of frontier orbitals for the seven isomeric 7.7.10
x,
y
ousenes in which two ...cationic tropyliumyl (C
7H
6
+–) rings are substituted on the cage of the B
10H
10
2− anion. The
x,
y=(2,7) isomer is the most stable. Energetically the remaining fall into two groups: (1,10), (2,4), and (1,6) which differ from (2,7) by less than 2
kcal
mol
−1, and (2,6),(2,3), and (1,2) which differ by about 10
kcal
mol
−1. The lower stability of the latter group is attributed to repulsive interactions between substituent rings rather than electronic effects. Three ousenes (1,2), (1,10), and (2,3) have lower symmetry than expected for a simple disubstituted B
10H
10
2− cage as a result of steric effects. Examination of frontier orbitals demonstrates that all seven species should show charge transfer excitations from cage to both rings similar to that previously experimentally observed for the 7.10
2 hemiousenide ion.
We have used ab initio (HF/3-21G(∗) and HF/6-31+G
∗) single point energy calculations on model structures to investigate the nature of methyl C–H to anion interactions in tetramethylammonium ...tetrahydroborate. The preferred structure has four cations about the anion in a
D
2
d
arrangement, each of which forms a CHHB dihydrogen bond to anion. This arrangement is in good accordance with previous infrared spectral studies. The alternative
D
2
d
arrangement with cation C–H directed to faces of the BH
4
− tetrahedron is 6
kcal
mol
−1 higher in energy than the dihydrogen bonded model.
First-principles methods can be utilized to obtain elementary step mechanisms for chemical reactions on model systems. In this chapter, we will illustrate how this molecular information can be ...employed to motivate novel heterogeneous catalyst formulations. We will discuss a few examples where first-principles studies on idealized model systems were utilized, along with various experimental tools, to identify alloy catalysts that exhibit improved performance in a number of catalytic processes. We will emphasize the role of molecular approaches in the formulation of these catalysts.
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