Significant quantum effects in hydrogen activation

Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen, it is recognized that quantum effects are often important to its reactivity. However, understanding how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H2 up to ∼190 K and for D2 up to ∼140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H2 dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics, whereas the D2 dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation

Anisotropic Disorder and Thermal Stability of Silicane

Atomically thin silicon nanosheets (SiNSs), such as silicane, have potential for next-generation computing paradigms, such as integrated photonics, owing to their efficient photoluminescence emission and complementary-metal-oxide-semiconductor (CMOS) compatibility. To be considered as a viable material for next-generation photonics, the SiNSs must retain their structural and optical properties at operating temperatures. However, the intersheet disorder of SiNSs and their nanoscale structure makes structural characterization difficult. Here, we use synchrotron X-ray diffraction and atomic pair distribution function (PDF) analysis to characterize the anisotropic disorder within SiNSs, demonstrating they exhibit disorder within the intersheet spacing, but have little translational or rotational disorder among adjacent SiNSs. Furthermore, we identify changes in their structural, chemical, and optical properties after being heated in an inert atmosphere up to 475 °C. We characterized changes of the annealed SiNSs using synchrotron-based total X-ray scattering, infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, electron paramagnetic resonance, absorbance, photoluminescence, and excited-state lifetime. We find that the silicon framework is robust, with an onset of amorphization at ∼300 °C, which is well above the required operating temperatures of photonic devices. Above ∼300 °C, we demonstrate that the SiNSs begin to coalesce while keeping their translational alignment to yield amorphous silicon nanosheets. In addition, our DFT results provide information on the structure, energetics, band structures, and vibrational properties of 11 distinct oxygen-containing SiNSs. Overall, these results provide critical information for the implementation of atomically thin silicon nanosheets in next-generation CMOS-compatible integrated photonic devices.This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Nano, copyright © 2021 American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acsnano.1c04230. Posted with permission

Configurational Energies of Nanoparticles Based on Metal–Metal Coordination

Nanoparticle sintering remains a fundamental problem in heterogeneous catalysis, motivating mechanistic studies toward mitigating deactivation of precious metal catalysts. We present a model based on the local coordination environment of metal atoms that can be used to provide total energy estimates for metal nanoparticles in a space of generic configurations. All energies are based only on a small set of density functional theory calculations of single metal atom adsorption on metal slabs. A model that can provide accurate nanoparticle energies is an important step toward the goal of understanding their sintering behavior in practical catalytic contexts

Triacetic Acid Lactone and 4-Hydroxycoumarin as Bioprivileged Molecules for the Development of Performance-Advantaged Organic Corrosion Inhibitors

Biomass conversion, especially the development of bioprivileged molecules utilizing integration of biological and chemical processes, has shown potential to produce novel chemicals with enhanced properties. Here, organic corrosion inhibitors based on triacetic acid lactone (TAL) and 4-hydroxycoumarin (4HC) have been synthesized in good yield and tested for corrosion inhibition on mild steel in both sulfuric acid and hydrochloric acid. Sixteen novel corrosion inhibitors derived from TAL and 4HC were efficiently synthesized, and 12 of them showed high corrosion inhibition efficiency as confirmed by electrochemical impedance spectroscopy (giving values greater than 78%) and polarization analysis. While TAL-based compounds showed good corrosion inhibition performance, the 4HC-based compounds showed further improvement in corrosion inhibition performance. Scanning electron microscopy analysis of the mild steel coupon in the presence of selected inhibitors showed that corrosion was significantly diminished, with complementary X-ray photoelectron spectroscopy analysis suggesting that the inhibitor molecules strongly adsorbed on the steel surface. Quantum chemical calculations showed poor correlation between calculated parameters and the performance of these molecules. This large set of corrosion inhibitors containing a dozen promising corrosion inhibitors is useful for future rational design of corrosion inhibitors. The results demonstrate a new direction for the development of bioprivileged molecules.This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Sustainable Chemistry & Engineering, copyright © 2022 American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acssuschemeng.2c02940. Posted with permission

Deactivation and regeneration of carbon supported Pt and Ru catalysts in aqueous phase hydrogenation of 2-pentanone

Aqueous phase conversion of biomass-derived molecules requires development of catalysts and operating strategies that create viable operation for extended performance as necessitated for industrial applications. While structural collapse of the support and sintering/leaching of the supported metal particles have been reported in the literature as being the primary deactivation mechanisms, carbon deposition was found to be the dominant deactivation mode for carbon supported Pt catalysts in aqueous phase 2-pentanone hydrogenation reactions. A mild regeneration method involving air oxidation at 200 °C and H2 reduction at 180 °C led to full recovery of the catalytic activity. The regeneration method was also successfully applied to Ru catalysts leading to the full recovery of the activity. The mild regeneration method demonstrated likely has applicability to catalyst regeneration for additional aqueous phase reactions with oxygenated molecules.This is a manuscript of an article published as Huo, Jiajie, Hien N. Pham, Yan Cheng, Hsi-Hsin Lin, Luke T. Roling, Abhaya K. Datye, and Brent H. Shanks. "Deactivation and regeneration of carbon supported Pt and Ru catalysts in aqueous phase hydrogenation of 2-pentanone." Catalysis Science & Technology 10, no. 9 (2020): 3047-3056. DOI: 10.1039/D0CY00163E. Copyright 2020 The Royal Society of Chemistry. Posted with permission

Unraveling Electroreductive Mechanisms of Biomass-Derived Aldehydes via Tailoring Interfacial Environments

Electrochemical reduction of biomass-derived feedstocks holds great promise to produce value-added chemicals or fuels driven by renewable electricity. However, mechanistic understanding of the aldehyde reduction toward valuable products at the molecular level within the interfacial regions is still lacking. Herein, through tailoring the local environments, including H/D composition and local H3O+ and H2O content, we studied the furfural reduction on Pb electrodes under acid conditions and elucidated the pathways toward three key products: furfuryl alcohol (FA), 2-methylfuran (MF), and hydrofuroin. By combining isotopic labeling and incorporation studies, we revealed that the source of protons (H2O and H3O+) plays a critical role in the hydrogenation and hydrogenolysis pathways toward FA and MF, respectively. In particular, the product-selective kinetic isotopic effect of H/D and the surface-property-dependent hydrogenation/deuteration pathway strongly impacted the generation of FA but not MF, owing to their different rate-determining steps. Electrokinetic studies further suggested Langmuir–Hinshelwood and Eley–Rideal pathways in the formation of FA and MF, respectively. Through modifying the double layer by cations with large radii, we further correlated the product selectivity (FA and MF) with interfacial environments (local H3O+ and H2O contents, interfacial electric field, and differential capacitances). Finally, experimental and computational investigations suggested competitive pathways toward hydrofuroin and FA: hydrofuroin is favorably produced in the electrolyte through the self-coupling of ketyl radicals, which are formed from outer-sphere, single-electron transfer, while FA is generated from hydrogenation of the adsorbed furfural/ketyl radical on the electrode surface.This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Catalysis, copyright © 2022 American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acscatal.2c03163. Posted with permission

Atomistic insights into the nucleation and growth of platinum on palladium nanocrystals

Despite the large number of reports on colloidal nanocrystals, very little is known about the mechanistic details in terms of nucleation and growth at the atomistic level. Taking bimetallic core-shell nanocrystals as an example, here we integrate in situ liquid-cell transmission electron microscopy with first-principles calculations to shed light on the atomistic details involved in the nucleation and growth of Pt on Pd cubic seeds. We elucidate the roles played by key synthesis parameters, including capping agent and precursor concentration, in controlling the nucleation site, diffusion path, and growth pattern of the Pt atoms. When the faces of a cubic seed are capped by Br-, Pt atoms preferentially nucleate from corners and then diffuse to edges and faces for the creation of a uniform shell. The diffusion does not occur until the Pt deposited at the corner has reached a threshold thickness. At a high concentration of the precursor, self-nucleation takes place and the Pt clusters then randomly attach to the surface of a seed for the formation of a non-uniform shell. These atomistic insights offer a general guideline for the rational synthesis of nanocrystals with diverse compositions, structures, shapes, and related properties

Colloidally Engineered Pd and Pt Catalysts Distinguish Surface- and Vapor-Mediated Deactivation Mechanisms

Noble metal-based catalysts are ubiquitous because of their high activity and stability. However, they irreversibly deteriorate over time especially in high-temperature applications. In these conditions, sintering is the main reason for deactivation, and understanding how sintering occurs gives the opportunity to mitigate these detrimental processes. Previous studies successfully distinguished between two fundamental sintering modes, namely, particle migration and coalescence (PMC) and Ostwald ripening (OR). However, differentiation between surface- and vapor-mediated Ostwald ripening processes has not been demonstrated yet, even though it is crucial information to tune metal/support interactions and stabilize catalysts. Here, we demonstrate that surface- and vapor-mediated ripening occur in two distinct regimes of temperature with some overlap using Pt and Pd catalysts prepared from colloidal nanocrystals as precursors. By either co-impregnating the two metal nanocrystals on the same grain of alumina support or by physically mixing powders of the two distinct metal catalysts, we tune the intermetal particle distance between nanometers and micrometers. We then use methane complete oxidation as a reporter reaction that occurs at higher rates on pure Pd and lower rates on alloyed Pd/Pt catalysts to trace the movement of Pt in the system. Aging the catalysts at different temperatures allows us to reveal that Pt initially sinters by surface-mediated ripening until ∼750 °C, but at temperatures above 800 °C, vapor-mediated ripening by PtO2 becomes the main sintering mechanism. This work demonstrates how colloidal catalysts allow unique insights into the working and deactivation mechanisms of supported systems.ISSN:2155-543

Design of Chemoresponsive Liquid Crystals through Integration of Computational Chemistry and Experimental Studies

We report the use of computational chemistry methods to design a chemically responsive liquid crystal (LC). Specifically, we used electronic structure calculations to model the binding of nitrile-containing mesogens (4′-<i>n</i>-pentyl-4-biphenylcarbonitrile) to metal perchlorate salts (with explicit description of the perchlorate anion), which we call the coordinately saturated anion model (CSAM). The model results were validated against experimental data. We then used the CSAM to predict that selective fluorination can reduce the strength of binding of nitrile-containing nematic LCs to metal-salt-decorated surfaces and thus generate a faster reordering of the LC in response to competitive binding of dimethylmethylphosphonate (DMMP). We tested this prediction via synthesis of fluorinated compounds 3-fluoro-4′-pentyl­[1,1′-biphenyl]-4-carbonitrile and 4-fluoro-4′-pentyl-1,1′-biphenyl, and subsequent experimental measurements of the orientational response of LCs containing these compounds to DMMP. These experimental measurements confirmed the theoretical predictions, thus providing the first demonstration of a chemoresponsive LC system designed from computational chemistry