3 research outputs found
Origins of the Photocatalytic NO<sub><i>x</i></sub> Oxidation and Storage Selectivity of Mixed Metal Oxide Photocatalysts: Prevalence of Electron-Mediated Routes, Surface Area, and Basicity
MgO, CaO, SrO, or BaO-promoted TiO2/Al2O3 was utilized in the photocatalytic
NOx oxidation and storage reaction. Photocatalytic
performance
was investigated as a function of catalyst formulation, calcination
temperature, and relative humidity. Onset of the photocatalytic activity
in TiO2/Al2O3 coincides with the
transition from the anatase to rutile phase and increasing number
of paramagnetic active centers and oxygen vacancies. Disordered AlOx domains enable the formation of oxygen vacancies
and paramagnetic centers on titania domains, hindering the nucleation
and growth of titania particles, as well as increasing specific surface
area (SSA) to store oxidized NOx species
away from titania active sites. Both e–- and h+-mediated pathways contribute to photocatalytic NO conversion.
Experiments performed using an e– scavenger (i.e., H2O2), suppressing
the e–-mediated route, attenuated the photocatalytic selectivity
by triggering NO2(g) release. Superior NOx storage selectivity of 7.0Ti/Al-700 as compared to other TiO2/Al2O3 systems in the literature was
attributed to an interplay between the presence of electrons trapped
at oxygen vacancies and superoxide species allowing a direct pathway
for the complete NO oxidation to HNO3/NO3– species, and the relatively large SSA of the photocatalyst
prevents the rapid saturation of the photocatalyst with oxidation
products. Longevity of the 7.0Ti/Al-700 was improved by the incorporation
of CaO, emphasizing the importance of the surface basicity of the
NOx storage site
Two-Dimensional Bimetallic Hydroxide Nanostructures for Catalyzing Low-Temperature Aerobic C–H Bond Activation in Alkylarene and Alcohol Partial Oxidation
Two-dimensional (2D)
bimetallic NixMn1–x(OH)y layered double
hydroxide (LDH) nanostructures were synthesized
and optimized as a remarkably active catalytic platform for low-temperature
aerobic C–H bond activation in alkylarenes and partial oxidation
of alcohols using a wide substrate (i.e., reactant)
and diverse solvent scope. The NixMn1–x(OH)y structure consists
of nonprecious and earth-abundant metals that can effectively operate
at low catalyst loadings, requiring only molecular oxygen as the stoichiometric
oxidant. Structurally diverse alkylarenes as well as primary and secondary
alcohols were shown to be competent substrates where oxidation products
were obtained in excellent yields (93–99%). Comprehensive catalyst
structural characterization via XRD, ATR-IR, TEM, EDX, XPS, BET, and
TGA indicated that the ultimately optimized Ni0.6Mn0.4(OH)y-9S catalyst
possessed not only particular rotational faults in its β-Ni0.6Mn0.4(OH)y domains but also distinct α/β-Ni0.6Mn0.4(OH)y interstratification
disorders, in addition to a relatively high specific surface area
of 125 m2/g, a 2D platelet morphology, and an average Mn
oxidation state of +3.5, suggesting the presence of both Mn3+ and Mn4+ species in its structure working in a synergistic
fashion with the Ni2+/x+ cations (the
latter is justified by the lack of catalytic activity in the monometallic
LDH catalysts Ni(OH)2 and Mn(OH)2). Kinetic
isotope effect studies carried out in the fluorene oxidation reaction
(kH/kD = 5.7)
revealed that the rate-determining step of the catalytic oxidation
reaction directly involved the scission of a C–H bond. Moreover,
the optimized catalyst was demonstrated to be reusable through the
application of a regeneration protocol, which can redeem the full
initial activity of the carbon-poisoned spent catalyst in the fluorene
oxidation reaction
Low-Pressure Deuterium Storage on Palladium-Coated Titanium Nanofilms: A Versatile Model System for Tritium-Based Betavoltaic Battery Applications
Deuterium (D2(g)) storage of Pd-coated Ti
ultra-thin
films at relatively low pressures is fine-tuned by systematically
controlling the thicknesses of the catalytic Pd overlayer, underlying
Ti ultra-thin film domain, D2(g) pressure (PD2), duration of D2(g) exposure, and the thin
film temperature. Structural properties of the Ti/Pd nanofilms are
investigated via XRD, XPS, AFM, SEM, and TPD to explore new structure-functionality
relationships. Ti/Pd thin film systems are deuterated to obtain a
D/Ti ratio of up to 1.53 forming crystallographically ordered titanium
deuteride (TiDx) phases with strong Tix+–Dy– electronic interactions and high thermal stability, where >90%
of
the stored D resides in the Ti component, thermally desorbing at >460
°C in the form of D2(g). Electronic interaction between
Pd and D is weak, yielding metallic (Pd0) states where
D storage occurs mostly on the Pd film surface (i.e., without forming
ordered bulk PdDx phases) leading to the
thermal desorption of primarily DOH(g) and D2O(g) at <265
°C. D-storage typically increases with increasing Ti film thickness, PD2, T, and t, whereas D-storage is found to be sensitive to the thickness and
the surface roughness of the catalytic Pd overlayer. Optimum Pd film
thickness is determined to be 10 nm providing sufficient surface coverage
for adequate wetting of the underlying Ti film while offering an appropriate
number of surface defects (roughness) for D immobilization and a relatively
short transport pathlength for efficient D diffusion from Pd to Ti.
The currently used D-storage optimization strategy is also extended
to a realistic tritium-based betavoltaic battery (BVB) device producing
promising β-particle emission yields of 164
mCi/cm2, an open circuit potential (VOC) of 2.04 V, and a short circuit current (ISC) of 7.2 nA