Prediction of morphological changes of catalyst materials under reaction conditions by combined <i>ab initio</i> thermodynamics and microkinetic modelling

Cheula, Raffaele,Soon, Aloysius,Maestri, Matteo Royal Society of Chemistry 2018 Catalysis science & technology Vol.8 No.14

<▼1><P>Microkinetic modeling, <I>ab initio</I> thermodynamics and Wulff–Kaishew construction are used to predict catalyst structural changes under reaction conditions.</P></▼1><▼2><P>In this article, we couple microkinetic modelling, <I>ab initio</I> thermodynamics and Wulff–Kaishew construction to describe the structural variation of catalyst materials as a function of the chemical potential in the reactor. We focus specifically on experiments of catalytic partial oxidation (CPO) of methane on Rh/α-Al<SUB>2</SUB>O<SUB>3</SUB>. We employ a detailed structureless microkinetic model to calculate the profiles of the gaseous species molar fractions along the reactor coordinate and to select the most abundant reaction intermediates (MARIs) populating the catalyst surfaces in different zones of the reactor. Then, we calculate the most stable bulk and surface structures of the catalyst under different conditions of the reaction environment with density functional theory (DFT) calculations and <I>ab initio</I> thermodynamics, considering the presence of the MARIs on the catalyst surface in thermodynamic equilibrium with the partial pressures of their reservoirs in the gas phase surrounding the catalyst. Finally, we exploit the Wulff–Kaishew construction method to estimate the three-dimensional shape of the catalyst nanoparticles and the distribution of the active sites along the reactor coordinate. We find that the catalyst drastically modifies its morphology during CPO reaction by undergoing phase transition, in agreement with spectroscopy studies reported in the literature. The framework is also successfully applied for the analysis and interpretation of chemisorption experiments for catalyst characterization. These results demonstrate the crucial importance of rigorously accounting for the structural effect in microkinetic modeling simulations and pave the way towards the development of structure-dependent microkinetic analysis of catalytic processes.</P></▼2>

Nonstoichiometric Nucleation and Growth of Multicomponent Nanocrystals in Solution

Min, Yuho,Kwak, Junghyeok,Soon, Aloysius,Jeong, Unyong American Chemical Society 2014 Accounts of chemical research Vol.47 No.10

<title>Conspectus</title><P>The ability to assemble nanoscale functional building blocks is a useful and modular way for scientists to design valuable materials with specific physical and chemical properties. Chemists expect multicomponent, heterostructured nanocrystals to show unique electrical, thermal, and optical properties not seen in homogeneous, single-phase nanocrystals. Although researchers have made remarkable advances in heterogeneous nucleation and growth, design of synthetic conditions for obtaining nanocrystals with a target composition and shape is still a big challenge.</P><P>There are several outstanding issues that chemists need to address before they can successfully carry out the design-based synthesis of multicomponent nanocrystals. For instance, small changes in the reaction parameters, such as the precursor, solvent, surfactant, reducing agent, and the reaction temperature, often result in changes in the structure and chemical composition of the final product. Although scientists do not fully understand the mechanisms underlying the nucleation and growth processes involved in the synthesis of these multicomponent nanocrystals, recent progress in understanding of the thermodynamic and kinetic factors have improved our control over their final structure and chemical composition. In this Account, we summarize our recent advances in understanding of the nucleation and growth mechanisms involved in the solution-based synthesis of multicomponent nanocrystals. We also discuss the various challenges encountered in their synthesis, emphasizing what still needs special consideration.</P><P>We first discuss the three different nucleation paths from a thermodynamics perspective: amorphous nucleation, crystalline nucleation, and two-step nucleation. Amorphous nucleation and two-step nucleation involve the generation of nonstoichiometric nuclei. We initiate this process mainly by introducing an imbalance in the concentrations of the reduced elements. When the nonstoichiometric nuclei grow, we can add secondary elements to the growing nonstoichiometric nuclei. This leads to either the physical deposition or atomic mixture formation through the diffusion and rearrangement of constituents.</P><P>The processes of mixture formation and the physical deposition of the secondary constituent element also compete and determine the shape and chemical composition of the final product. If the free energy change by mixture formation is positive (Δ<I>G</I><SUB><I>AB</I></SUB> ≥ 0), physical deposition takes place predominantly, and the spreading coefficient (<I>S</I>) determines the structure of the nanocrystals. However, when mixture formation is highly spontaneous (Δ<I>G</I><SUB><I>AB</I></SUB> < −ξ), the chemical composition of the final product is usually stoichiometric, and its shape then depends on the size of the primary nanocrystals. When the mixture formation and physical deposition are in competition (−ξ ≤ Δ<I>G</I><SUB><I>AB</I></SUB> < 0), as commonly seen for many nanoalloy systems, both the chemical composition and the structure are determined by the size of the primary nanocrystals as well as the degree of mixture formation at the interface of the constituent components. Finally, we discuss the challenges and caveats that one needs to take into account when synthesizing multicomponent nanocrystals.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/achre4/2014/achre4.2014.47.issue-10/ar500133w/production/images/medium/ar-2014-00133w_0007.gif'></P>

Remarkably low-energy one-dimensional fault line defects in single-layered phosphorene

Jang, Woosun,Kang, Kisung,Soon, Aloysius The Royal Society of Chemistry 2015 Nanoscale Vol.7 No.45

Understanding the Enhancement of Ionic Transport in Heterogeneously Doped Zirconia by Heterointerface Engineering

Kilic, Mehmet Emin,Soon, Aloysius American Chemical Society 2018 The Journal of Physical Chemistry Part C Vol.122 No.39

<P>Zirconia-based ceramics have been the most promising oxide electrolyte material with high ionic conductivity for solid oxide fuel cell (SOFC) applications. Even though yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) are typically used for the SOFC at high temperatures, their performance is not optimal at operating temperatures with respect to their ionic conductivity and stability. The literature has focused largely on ionic diffusion dynamics in bulk YSZ and ScSZ, whereas their heterogeneously doped alloy and heterolayered superlattices are less investigated. In this work, using molecular dynamics simulations and diffusion dynamics analysis, we examine and consider five main mechanisms that may contribute to the enhancement of the overall ionic conductivity of these doped zirconia, namely, the influence of cation size, concentration, distribution, the crystal orientation and direction, and lastly, the degree of atomic roughness at the interface in the heterolayered structures. Our results support that heterointerface engineering at the atomic scale greatly reduces local lattice distortions (commonly seen in the bulk phases) while inducing an in-plane strain and thus leading to an overall enhancement of the ionic conductivity and stability for SOFC applications.</P> [FIG OMISSION]</BR>

In<SUB>0.53</SUB>Ga<SUB>0.47</SUB>As나노필름의 열전도도와 음향자 구속 효과

김훈,Metmet Emin Kilic,Aloysius Soon,김우철 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.5

Acute mechano-electronic responses in twisted phosphorene nanoribbons

Jang, Woosun,Kang, Kisung,Soon, Aloysius Royal Society of Chemistry 2016 Nanoscale Vol.8 No.31

<P>Many different forms of mechanical and structural deformations have been employed to alter the electronic structure of various modern two-dimensional (2D) nanomaterials. Given the recent interest in the new class of 2D nanomaterials - phosphorene, here we investigate how the rotational strain-dependent electronic properties of low-dimensional phosphorene may be exploited for technological gain. Here, using first-principles density-functional theory, we investigate the mechanical stability of twisted one-dimensional phosphorene nanoribbons (TPNR) by measuring their critical twist angle (theta(c)) and shear modulus as a function of the applied mechanical torque. We find a strong anisotropic, chirality-dependent mechano-electronic response in the hydrogen-passivated TPNRs upon vortical deformation, resulting in a striking difference in the change in the carrier effective mass as a function of torque angle (and thus, the corresponding change in carrier mobility) between the zigzag and armchair directions in these TPNRs. The accompanied tunable band-gap energies for the hydrogen-passivated zigzag TPNRs may then be exploited for various key opto-electronic nanodevices.</P>

Polytypism in Hexagonal Tungsten Trioxide: Insights from Ab Initio Molecular Dynamics Simulations

Lee, Yonghyuk,Lee, Taehun,Soon, Aloysius American Chemical Society 2018 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.122 No.37

<P>Temperature-dependent microstructural evolution of hexagonal WO<SUB>3</SUB> (<I>h</I>-WO<SUB>3</SUB>) polytypes is explored via ab initio molecular dynamics calculations within the density-functional theory framework. We present simulated finite temperature radial distribution function and X-ray diffraction patterns to reinterpret recent experimental pair distribution function analysis. This work clearly demonstrates that after a more careful analysis of the finite temperature structural properties of <I>h</I>-WO<SUB>3</SUB>, an intermediate H1-like structure is predicted at higher temperatures, while the more stable H4 polytype (and not the experimentally suggested H2 polytype) is obtained nearer ambient temperatures. This is further corroborated by our electronic structure analysis which shows that the electronic band gap energy of the ambient temperature H4-like structure agrees much better with the experimentally reported band gap energies.</P> [FIG OMISSION]</BR>

Unraveling the Intercalation Chemistry of Hexagonal Tungsten Bronze and Its Optical Responses

Lee, Yonghyuk,Lee, Taehun,Jang, Woosun,Soon, Aloysius American Chemical Society 2016 Chemistry of materials Vol.28 No.13

<P>In an attempt to promote energy saving through the clever control of varying amounts of visible light and solar energy in modern buildings, there has been a surge of interest in the novel design of multifunctional glass windows otherwise known as smart windows. The use of chromogenic materials (e.g., tungsten oxides and their alloys) is widespread in this cooling energy technology, and for the case of hexagonal tungsten oxide (h-WO3)-based systems, the overall efficiency is often hindered by the lack of a systematic and fundamental understanding of the interplay of intrinsic charge transfer between the alkali-metal ions and the host h-WO3. In this work, we present a first-principles hybrid density-functional theory investigation of bulk hexagonal tungsten bronzes (i.e., alkali-metal-intercalated h-WO3) and examine the influence of the intercalation chemistry on their thermodynamic stability as well as optoelectronic properties. We find that the introduction of the alkali-metal ion induces a persistent n-type electronic conductivity, and dramatically reduces the optical transmittance (down to similar to 28%) for infrared wavelengths while maintaining fair optical transparency for next-generation electrochromic devices in very energy efficient chromogenic device technology.</P>

In search of non-conventional surface oxidic motifs of Cu on Au(111)

Lee, Taehun,Lee, Yonghyuk,Kang, Kisung,Soon, Aloysius The Royal Society of Chemistry 2016 Physical chemistry chemical physics Vol.18 No.10

<P>Growing ultrathin oxide layers on metal surfaces presents a new class of low-dimensional nanomaterials with exceptional chemical and physical properties. These 'new oxides'' can be used in many niche technologies and applications such as nanoscale electronics and heterogeneous nanocatalysis. In this work, we study the formation of surface oxidic structures and motifs of Cu, supported on the Au(111) substrate, using first-principles density-functional theory calculations in conjunction with an ab initio atomistic thermodynamics model. In particular, we systematically examine and analyze the detailed atomic structure and surface energetics of various oxidic motifs of Cu on Au(111), in particular, p2, p2s, p2(6q6) and the newly suggested metastable p2(6q6) + O-3, in comparison to both the binary O/Cu(111) and O/Au(111) systems. Depending on the oxygen atmosphere and the type of surface defects introduced in the oxidic layer, various non-conventional, non-hexagonal surface oxidic motifs of Cu could be obtained. Our theoretical results agree with recent scanning tunneling microscopy (STM) experiments and we propose that metastable non-hexagonal surface motifs may pave a way to pursue further studies of these interesting complex surface oxidic layers on various metal supports.</P>

Assessing the influence of van der Waals corrected exchange-correlation functionals on the anisotropic mechanical properties of coinage metals

Lee, Ji-Hwan,Park, Jong-Hun,Soon, Aloysius American Physical Society 2016 Physical Review B Vol.94 No.2

<P>Current materials-related calculations employ density-functional theory (DFT), commonly using the (semi-) local-density approximations for the exchange-correlation (xc) functional. The difficulties in arriving at a reasonable description of van der Waals (vdW) interactions by DFT-based models is to date a big challenge. In this work, we use various flavors of vdW-corrected DFT xc functionals-ranging from the quasiempirical force-field add-on vdW corrections to self-consistent nonlocal correlation functionals-to study the bulk lattice and mechanical properties (including the elastic constants and anisotropic indices) of the coinage metals (copper, silver, and gold). We critically assess the reliability of the different vdW-corrected DFT methods in describing their anisotropic mechanical properties which have been less reported in the literature. In the context of this work, we regard that our results reiterate the fact that advocating a so-called perfect vdW-inclusive xc functional for describing the general physics and chemistry of these coinage metals could be a little premature. These challenges to modern-day functionals for anisotropically strained coinage metals (e.g., at the faceted surfaces of nanostructures) may well be relevant to other strained material systems.</P>