- Origin of Electronic Modification of Platinum in a Pt 3 V Alloy and Its Consequences for Propane Dehydrogenation CatalysisPurdy, Stephen C., Ghanekar, Pushkar, Mitchell, Garrett, Kropf, A. Jeremy, Zemlyanov, Dmitry Y., Ren, Yang, Ribeiro, Fabio, Delgass, W. Nicholas, Greeley, Jeffrey, and Miller, Jeffrey T.ACS Applied Energy Materials 2020
We demonstrate the synthesis of a Pt3V alloy and Pt/Pt3V core/shell catalysts, which are highly selective for propane dehydrogenation. The selectivity is a result of the Pt3V intermetallic phase, which was characterized by in situ synchrotron XRD and XAS. Formation of a continuous alloy surface layer 2-3 atomic layers thick was sufficient to obtain identical catalytic properties between a core-shell and full alloy catalyst, which demonstrates the length scale over which electronic effects pertinent to dehydrogenation act. Electronic characterization of the alloy phase was investigated by using DFT, XPS, XANES, and RIXS, all of which show a change in the energy of the filled and unfilled Pt 5d states resulting from Pt-V bonding. The electronic modification leads to a change in the most stable binding site of hydrocarbon fragments, which bind to V containing ensembles despite the presence of 3-fold Pt ensembles in Pt3V. In addition, electronic modification destabilizes deeply dehydrogenated species thought to be responsible for hydrogenolysis and coke formation.
- Catalysis at Metal/Oxide Interfaces: Density Functional Theory and Microkinetic Modeling of Water Gas Shift at Pt/MgO BoundariesGhanekar, Pushkar, Kubal, Joseph, Cui, Yanran, Mitchell, Garrett, Delgass, W Nicholas, Ribeiro, Fabio, and Greeley, JeffreyTopics in Catalysis 2020
The impact of metal/oxide interfaces on the catalytic properties of oxide-supported metal nanoparticles is a topic of longstanding interest in the field of heterogeneous catalysis. The significance of the metal/oxide interaction has been shown to vary according to both the inherent reactivity of the metal nanoparticle and the properties of the oxide support, with effects such as the metal d-band center, the nanoparticle shape, and the reducibility of the oxide believed to contribute to the overall system reactivity. In recent years, the water gas shift (WGS) reaction, wherein carbon monoxide and water are converted to carbon dioxide and hydrogen, has emerged as a model chemistry to probe the molecular-level details of how catalysis can be promoted in such environments, and this reaction is the focus of the present contribution. Using a combination of periodic Density Functional Theory calculations and microkinetic modeling, we present a comprehensive analysis of the WGS mechanism at the interface between a quasi-one dimensional platinum nanowire and an irreducible MgO support. The nanowire is lattice matched to the MgO support to remove spurious strain at the metal/oxide interface, and reactions both on the nanowire and at the three-phase boundary itself are considered in the mechanistic analysis. Additionally, to elucidate the consequences of adsorbate–adsorbate interactions on the WGS chemistry, an ab-initio thermodynamic analysis of CO coverage is performed, and the impact of the higher coverage CO states on the reaction chemistry is explicitly evaluated. These results are combined with detailed calculations of adsorbate entropies and dual-site microkinetic modeling to determine the kinetically significant features of the WGS reaction network which are subsequently, validated through experimental measurements of apparent reaction orders and activation barrier. The analysis demonstrates the important role that the metal/oxide interface plays in the reaction, with the water dissociation step being facile at the interface compared to the pure metal or oxide surfaces. Further, explicit consideration of CO interactions with other adsorbates at the metal/oxide interface is found to be central to correctly determining reaction mechanisms, rate determining steps, reaction orders, and effective activation barriers. These results are captured in a closed-form Langmuir–Hinshelwood model, derived from a simplified version of the complete microkinetic analysis, which reveals, among other results, that the celebrated carboxyl mechanism of Mavrikakis and coworkers is the governing pathway when accounting for reaction-relevant CO coverages.
- Promoting a Safe Laboratory Environment Using the Reactive Hazard Evaluation and Analysis Compilation ToolTalpade, Abhijit D, Ghanekar, Pushkar, Ezenwa, Sopuruchukwu, Joshi, Ravi, Kravitz, Samuel, Tunga, Anirudh, Devaraj, Jayachandran, Ribeiro, Fabio H, and Mentzer, RayACS Chemical Health & Safety 2021
In the past several years, the U.S. Chemical Safety Board has found an increase in the frequency of laboratory accidents and injuries. An independent survey of industrial and academic laboratories by the authors indicated the shortage of documentation on best practices and lack of free and user-friendly risk assessment tools to be some of the key reasons for the occurrence of safety incidents. Thus, development of a framework to document, assess, and mitigate hazards is a critical starting point for ensuring safe laboratory practices. To address this requirement, Reactive Hazards Evaluation Analysis and Compilation Tool (RHEACT), an online platform to compile and scrutinize hazards-related information, was developed. When planning an experiment, the researchers provide RHEACT: (1) information about the chemicals involved in the reaction, in the form of Safety Data Sheets (SDS), and (2) operating parameters of the reaction. Through the user-supplied SDS, an operational hazard matrix and a chemical compatibility matrix are generated. In addition, adiabatic temperature rise of the reaction is estimated to ensure that the chemistry is within user-controlled bounds. The user is provided with a broad initial evaluation of potential hazards and is notified of safety concerns associated with the reaction before conducting the experiment. We believe that this user-friendly online tool will help engender a safer laboratory working environment.
- Adsorbate chemical environment-based machine learning framework for heterogeneous catalysisGhanekar, Pushkar, Deshpande, Siddharth, and Greeley, JeffreyChemRxiv 2021
Heterogeneous catalytic reactions are influenced by a subtle interplay of atomic-scale factors, ranging from the catalysts’ local morphology to the presence of high adsorbate coverages. Describing such phenomena via computational models requires generation and analysis of a large space of surface atomic configurations. To address this challenge, we present the Adsorbate Chemical Environment-based Graph Convolution Neural Network (ACE-GCN), a screening workflow that can account for atomistic configurations comprising diverse adsorbates, binding locations, coordination environments, and substrate morphologies. Using this workflow, we develop catalyst surface models for two illustrative systems: (i) NO adsorbed on a Pt3Sn(111) alloy surface, of interest for nitrate electroreduction processes, where high adsorbate coverages combine with the low symmetry of the alloy substrate to produce a large configurational space, and (ii) OH* adsorbed on a stepped Pt(221) facet, of relevance to the Oxygen Reduction Reaction, wherein the presence of irregular crystal surfaces, high adsorbate coverages, and directionally-dependent adsorbate-adsorbate interactions result in the configurational complexity. In both cases, the ACE-GCN model, having trained on a fraction ( 10%) of the total DFT-relaxed configurations, successfully ranks the relative stabilities of unrelaxed atomic configurations sampled from a large configurational space. This approach is expected to accelerate development of rigorous descriptions of catalyst surfaces under in-situ conditions.