Education & Certifications
Master of Science, Stanford University, Chemical Engineering (2014)
Bachelor of Science, Northwestern University, Chemical Engineering (2012)
Understanding trends in C-H bond activation in heterogeneous catalysis.
While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Electronic structure calculations are playing an increasingly relevant role in this search, but large-scale materials screening efforts are hindered by computationally expensive transition state barrier calculations. The purpose of the present letter is twofold. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. Second, we combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates. Our model successfully rationalizes the available empirical data and lays the foundation for future catalyst design strategies that transcend different catalyst classes.
View details for DOI 10.1038/nmat4760
View details for PubMedID 27723737
Automated Discovery and Construction of Surface Phase Diagrams Using Machine Learning
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2016; 7 (19): 3931-3935
Surface phase diagrams are necessary for understanding surface chemistry in electrochemical catalysis, where a range of adsorbates and coverages exist at varying applied potentials. These diagrams are typically constructed using intuition, which risks missing complex coverages and configurations at potentials of interest. More accurate cluster expansion methods are often difficult to implement quickly for new surfaces. We adopt a machine learning approach to rectify both issues. Using a Gaussian process regression model, the free energy of all possible adsorbate coverages for surfaces is predicted for a finite number of adsorption sites. Our result demonstrates a rational, simple, and systematic approach for generating accurate free-energy diagrams with reduced computational resources. The Pourbaix diagram for the IrO2(110) surface (with nine coverages from fully hydrogenated to fully oxygenated surfaces) is reconstructed using just 20 electronic structure relaxations, compared to approximately 90 using typical search methods. Similar efficiency is demonstrated for the MoS2 surface.
View details for DOI 10.1021/acs.jpclett.6b01254
View details for Web of Science ID 000384966500036
View details for PubMedID 27558978
- Two-Dimensional Materials as Catalysts for Energy Conversion CATALYSIS LETTERS 2016; 146 (10): 1917-1921
- How Doped MoS2 Breaks Transition-Metal Scaling Relations for CO2 Electrochemical Reduction ACS CATALYSIS 2016; 6 (7): 4428-4437
- Direct Water Decomposition on Transition Metal Surfaces: Structural Dependence and Catalytic Screening CATALYSIS LETTERS 2016; 146 (4): 718-724
- Scaling Relationships for Binding Energies of Transition Metal Complexes CATALYSIS LETTERS 2016; 146 (2): 304-308
Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production.
2016; 10 (1): 624-632
Amorphous MoSx is a highly active, earth-abundant catalyst for the electrochemical hydrogen evolution reaction. Previous studies have revealed that this material initially has a composition of MoS3, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS2 in composition and chemical state. However, structural changes in the MoSx catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoSx catalysts activated under two hydrogen-rich conditions: ex situ in an electrochemical cell and in situ in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoSx catalyst after both activation procedures as well as spatially localized changes in the chemical state detected via electron energy loss spectroscopy. Using density functional theory calculations, we investigate the mechanisms for this phase transformation and find that the presence of hydrogen is critical for enabling the restructuring process. Our results suggest that the surface of the amorphous MoSx catalyst is dynamic: while the initial catalyst activation forms the primary active surface of amorphous MoS2, continued transformation to the crystalline phase during electrochemical operation could contribute to catalyst deactivation. These results have important implications for the application of this highly active electrocatalyst for sustainable H2 generation.
View details for DOI 10.1021/acsnano.5b05652
View details for PubMedID 26624225
- Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies NATURE MATERIALS 2016; 15 (1): 48-?
- Theoretical insights into the hydrogen evolution activity of layered transition metal dichalcogenides SURFACE SCIENCE 2015; 640: 133-140
- Predicting Promoter-Induced Bond Activation on Solid Catalysts Using Elementary Bond Orders JOURNAL OF PHYSICAL CHEMISTRY LETTERS 2015; 6 (18): 3670-3674
The Challenge of Electrochemical Ammonia Synthesis: A New Perspective on the Role of Nitrogen Scaling Relations
2015; 8 (13): 2180-2186
The electrochemical production of NH3 under ambient conditions represents an attractive prospect for sustainable agriculture, but electrocatalysts that selectively reduce N2 to NH3 remain elusive. In this work, we present insights from DFT calculations that describe limitations on the low-temperature electrocatalytic production of NH3 from N2 . In particular, we highlight the linear scaling relations of the adsorption energies of intermediates that can be used to model the overpotential requirements in this process. By using a two-variable description of the theoretical overpotential, we identify fundamental limitations on N2 reduction analogous to those present in processes such as oxygen evolution. Using these trends, we propose new strategies for catalyst design that may help guide the search for an electrocatalyst that can achieve selective N2 reduction.
View details for DOI 10.1002/cssc.201500322
View details for Web of Science ID 000357619000003
- Transition-metal doped edge sites in vertically aligned MoS2 catalysts for enhanced hydrogen evolution NANO RESEARCH 2015; 8 (2): 566-575
- Rational design of MoS2 catalysts: tuning the structure and activity via transition metal doping CATALYSIS SCIENCE & TECHNOLOGY 2015; 5 (1): 246-253
- Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends ENERGY & ENVIRONMENTAL SCIENCE 2015; 8 (10): 3022-3029
Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies
View details for DOI 10.1038/nmat4465
Predicting Promoter-Induced Bond Activation on Solid Catalysts Using Elementary Bond Orders
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2015; 6 (18): 3670–3674
View details for DOI 10.1021/acs.jpclett.5b01792
- Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction JOURNAL OF PHYSICAL CHEMISTRY C 2014; 118 (50): 29252-29259
- Understanding the Reactivity of Layered Transition-Metal Sulfides: A Single Electronic Descriptor for Structure and Adsorption JOURNAL OF PHYSICAL CHEMISTRY LETTERS 2014; 5 (21): 3884-3889
- Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction CHEMCATCHEM 2014; 6 (7): 1899-1905
Tuning the MoS2 Edge-Site Activity for Hydrogen Evolution via Support Interactions
2014; 14 (3): 1381-1387
The hydrogen evolution reaction (HER) on supported MoS2 catalysts is investigated using periodic density functional theory, employing the new BEEF-vdW functional that explicitly takes long-range van der Waals (vdW) forces into account. We find that the support interactions involving vdW forces leads to significant changes in the hydrogen binding energy, resulting in several orders of magnitude difference in HER activity. It is generally seen for the Mo-edge that strong adhesion of the catalyst onto the support leads to weakening in the hydrogen binding. This presents a way to optimally tune the hydrogen binding on MoS2 and explains the lower than expected exchange current densities of supported MoS2 in electrochemical H2 evolution studies.
View details for DOI 10.1021/nl404444k
View details for Web of Science ID 000335720300044
View details for PubMedID 24499163
Active edge sites in MoSe2 and WSe2 catalysts for the hydrogen evolution reaction: a density functional study
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2014; 16 (26): 13156-13164
MoSe2 and WSe2 nanofilms and nanosheets have recently been shown to be active for electrochemical H2 evolution (HER). In this work, we used periodic density functional theory to investigate the origin of the catalytic activity on these materials. We determined the relevant structures of the Mo/W-edges and the Se-edges under HER conditions and their differential hydrogen adsorption free energies. The Mo-edge on MoSe2 and the Se-edge on both MoSe2 and WSe2 are found to be the predominantly active facets for these catalysts, with activity predicted to be comparable to or better than MoS2. On the other hand, the (0001) basal planes are found to be inert. We further explain the enhanced activity at the edges in terms of localized edge states, which provide insight into the trends in HER activity seen between the two catalysts. Our results thus suggest that an optimal catalyst design should maximize the exposure of edge sites. Comparisons are also made between the transition metal selenide catalysts and their sulfide counterparts in order to understand the consequences of having either Mo/W or Se/S atoms. It is found that linear scaling relations describe the S/Se binding onto the edge and the H binding onto the S/Se.
View details for DOI 10.1039/c4cp01237b
View details for Web of Science ID 000337785400019
View details for PubMedID 24866567
- Synthesis of high-energy anatase nanorods via an intermediate nanotube morphology CHEMICAL PHYSICS LETTERS 2012; 546: 106-108
- DISTINGUISHING AMBER AND COPAL CLASSES BY PROTON MAGNETIC RESONANCE SPECTROSCOPY ARCHAEOMETRY 2012; 54: 332-348