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Ye J.,University of Pittsburgh | Johnson J.K.,University of Pittsburgh | Johnson J.K.,Pittsburgh Quantum Institute
ACS Catalysis | Year: 2015

The capture and reuse of CO2 as a liquid fuel could reduce the overall anthropogenic carbon footprint but requires a catalytic pathway for CO2 hydrogenation under mild conditions, coupled with a renewable source of H2 or another reducing agent. We have computationally designed eight functional groups having both Lewis acid and base sites for inclusion inside a porous metal-organic framework (MOF) and have evaluated these functionalized MOFs for their catalytic activity toward CO2 hydrogenation. We have used density functional theory to compute reaction energies, barriers, and geometries for the elementary steps of CO2 reduction. The reaction pathways involve two elementary steps for each of the eight functional groups, consisting of heterolytic dissociation of H2 on the Lewis acid and base sites followed by concerted addition of a hydride and a proton to CO2 in a single step. Our analysis of the reaction energetics reveals that the reaction barrier for hydrogen dissociation can be correlated as a function of the chemical hardness of the Lewis acid site. Furthermore, we have identified a Brønsted-Evans-Polanyi relationship relating the barrier for the second step, CO2 hydrogenation, with the H2 adsorption energy on the Lewis sites. Surprisingly, this linear relationship also holds for correlating the hydrogenation barrier with the hydride attachment energy for the gas-phase Lewis acid site. These correlations provide a computationally efficient method for screening functional groups for their catalytic activity toward CO2 hydrogenation. These relationships are further utilized to carry out a Sabatier analysis on a simplified model of the reaction to generate contour plots of the Sabatier activity that can be used to identify properties of the functional groups for maximizing the reaction rate. © 2015 American Chemical Society.

Bi F.,University of Pittsburgh | Bi F.,Pittsburgh Quantum Institute | Huang M.,University of Pittsburgh | Huang M.,Pittsburgh Quantum Institute | And 6 more authors.
Journal of Applied Physics | Year: 2016

LaAlO3/SrTiO3 heterostructures are known to exhibit a sharp, hysteretic metal-insulator transition (MIT) with large enhanced capacitance near depletion. To understand the physical origin of this behavior, the electromechanical response of top-gated LaAlO3/SrTiO3 heterostructures is probed using two simultaneous measurement techniques: piezoresponse force microscopy (PFM) and capacitance spectroscopy. The observed hysteretic PFM responses show strong correlation with the capacitance signals, suggesting an interfacial carrier-mediated structural distortion associated with the gate-tuned MIT. In addition, the frequency dependence of the capacitance enhancement in LaAlO3/SrTiO3 is found to be well-matched to local PFM measurements. Our experimental results provide a fuller understanding of the top-gate tuned MIT in oxide heterostructure, which could be helpful for the development of future oxide-based nanoelectronics. © 2016 AIP Publishing LLC.

Ewing C.S.,University of Pittsburgh | Ewing C.S.,Pittsburgh Quantum Institute | Veser G.,University of Pittsburgh | Lambrecht D.S.,Pittsburgh Quantum Institute | And 2 more authors.
Surface Science | Year: 2016

Metal-support interactions significantly affect the stability and activity of supported catalytic nanoparticles (NPs), yet there is no simple and reliable method for estimating NP-support interactions, especially for amorphous supports. We present an approach for rapid prediction of catalyst-support interactions between Pt NPs and amorphous silica supports for NPs of various sizes and shapes. We use density functional theory calculations of 13. atom Pt clusters on model amorphous silica supports to determine linear correlations relating catalyst properties to NP-support interactions. We show that these correlations can be combined with fast discrete element method simulations to predict adhesion energy and NP net charge for NPs of larger sizes and different shapes. Furthermore, we demonstrate that this approach can be successfully transferred to Pd, Au, Ni, and Fe NPs. This approach can be used to quickly screen stability and net charge transfer and leads to a better fundamental understanding of catalyst-support interactions. © 2016 Elsevier B.V.

Bi F.,University of Pittsburgh | Bi F.,Pittsburgh Quantum Institute | Huang M.,University of Pittsburgh | Huang M.,Pittsburgh Quantum Institute | And 6 more authors.
Applied Physics Letters | Year: 2015

Complex-oxide heterostructures exhibit rich physical behavior such as emergent conductivity, superconductivity, and magnetism that are intriguing for scientific reasons as well as for potential technological applications. It was recently discovered that in-plane magnetism at the LaAlO3/SrTiO3 (LAO/STO) interface can be electronically controlled at room temperature. Here, we employ magnetic force microscopy to investigate electronically controlled ferromagnetism at the LAO/STO interface with LAO thickness t varied from 4 unit cell (u.c.) to 40 u.c. Magnetic signatures are observed only within a thickness window 8 u.c. ≤ t ≤ 25 u.c. Within this window, the device capacitance corresponds well to the expected geometric value, while for thicknesses outside this window, the capacitance is strongly suppressed. The ability to modulate electronic and magnetic properties of LAO/STO devices depends on the ability to control carrier density, which is in turn constrained by intrinsic tunneling mechanisms. © 2015 AIP Publishing LLC.

Ewing C.S.,University of Pittsburgh | Ewing C.S.,Pittsburgh Quantum Institute | Veser G.,University of Pittsburgh | McCarthy J.J.,University of Pittsburgh | And 4 more authors.
Journal of Physical Chemistry C | Year: 2015

The interaction between catalytic nanoparticles (NPs) and their supports, which are often amorphous oxides, has not been well characterized at the atomic level, although it is known that, in some cases, NP-support interactions dominate the catalytic activity of the system. Furthermore, there is a lack of understanding of how support preparation affects both the stability of the NP (resistance to sintering) and the catalytic activity. We present first-principles density functional theory (DFT) calculations on amorphous silica supported Pt NPs of various sizes. Our calculations predict that support preparation methods that lead to higher hydroxyl density when NPs are deposited on the support will lead to higher resistance to sintering. We find that the total charge on supported NPs, which can affect catalyst activity, depends linearly on the number of Pt-silica bonds formed during NP deposition. The number of bonds between an NP of a known geometry and the silica support with a known hydroxyl density can be estimated from very fast discrete element method simulations, enabling the prediction of both the net charge and the adhesion energy of the particle from a linear fit correlation derived from DFT calculations of a series of differently sized Pt clusters. This work quantifies interactions between Pt NPs and amorphous silica supports and demonstrates a new method for rapid estimation of NP-support interactions on amorphous supports. (Figure Presented). © 2015 American Chemical Society.

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