publications

2024

1. DFT+ML

   Accelerated Structural Optimization for the Supported Metal System Based on Hybrid Approach Combining Bayesian Optimization with Local Search

   Shinyoung Bae†, Dongjae Shin†, Haechang Kim, and 2 more authors

   Journal of Chemical Theory and Computation, 2024

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   Numerous systematic methods have been developed to search for the global minimum of the potential energy surface, which corresponds to the optimal atomic structure. However, the majority of them still demand a substantial computing load due to the relaxation process that is embedded as an inner step inside the algorithm. Here, we propose a hybrid approach that combines Bayesian optimization (BO) and a local search that circumvents the relaxation step and efficiently finds the optimum structure, particularly in supported metal systems. The hybridization strategy combining the capabilities of BO’s effective exploration and the local search’s fast convergence expedites structural search. In addition, the formulation of physical constraints regarding the materials system and the feature of screening structure similarity enhance the computational efficiency of the proposed method. The proposed algorithm is demonstrated in two supported metal systems, showing the potential of the proposed method in the field of structural optimization.

2023

1. 

   Change in the Electronic Environment of the VOx Active Center via Support Modification to Enhance Hg Oxidation Activity

   Woonsuk Yeo†, Dongjae Shin†, Moon Hyeon Kim, and 1 more author

   ACS Catalysis, 2023

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   Oxidation of elemental Hg (Hg0) is catalytically and economically an efficient way to remove the harmful Hg contained in the flue gas from coal combustion facilities. Thus, the development of a highly active V2O5/TiO2 catalyst, which is active to the Hg oxidation, is very essential. Support modification can change the reactivity of V2O5/TiO2 by affecting the V2O5 active center which is critical to the surface–reactant interaction, so understanding the effects of support tuning methods, e.g., crystallographic phase control and reduction treatment, on the Hg oxidation activity is valuable. Herein, density functional theory calculations were performed to mechanistically investigate the change of Hg oxidation reactivity by the support tuning methods and to elucidate the change of the electronic environment at the active site. The phase control to the TiO2 support was found to improve the Hg oxidation activity, but the reduction treatment decreased the activity, which is attributed to the change of the charge density at V2O5. Furthermore, the origin of the reactivity change was elucidated within a Sabatier-like principle that the interaction between the V site and the surface Cl critically contributes to the change of the Hg oxidation reactivity by balancing the competition between two key reaction steps of HCl dissociation and HgCl2 desorption. Our results provide guidance to improve the activity of the VOx/TiO2 catalyst for various reactions such as Hg oxidation and selective catalytic reduction of NOx and so on.

2. ML

   Surface segregation machine-learned with inexpensive numerical fingerprint for the design of alloy catalysts

   Dongjae Shin, Geonyeong Choi, Charmgil Hong, and 1 more author

   Molecular Catalysis, 2023

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   Metal alloy catalysts have been known to enhance many essential reactions. Surface-adsorbate interaction of an alloy catalyst depends on the composition and nanostructure of the surface, which can be changed by a phenomenon called surface segregation. Thus, thermodynamic information on the surface segregation is critical to tune the surface-adsorbate interaction, and thus the reactivity of alloy catalysts. Collected 1,366 density functional theory-calculated segregation energies (Esegr) from literature were featurized by 19-dimensional inexpensive numerical fingerprint, which represents facet-, site-, and elemental-dependencies. Deep neural network (DNN) model was constructed based on the data. Reasonable interpolative and extrapolative prediction by the DNN was clearly demonstrated using principal component (PC) analysis. On top of that, impact of each feature on the Esegr was analyzed by an explainable artificial intelligence approach, giving useful insights for alloy catalyst design based on surface segregation. As an example of the applications, the DNN model was used to explain the formation thermodynamics of site-selective nanosegregated Pt alloy catalyst, and was also used to perform elemental screening to find impurity-host metal combinations feasible to form the nanosegregated alloy catalyst. In addition, the guidance for data collection to improve the DNN model was given by analyzing PC space and performing the weight analysis of PCs.

2022

1. DFT+Expt.

   Role of an Interface for Hydrogen Production Reaction over Size-Controlled Supported Metal Catalysts

   Dongjae Shin†, Rui Huang†, Myeong Gon Jang, and 5 more authors

   ACS Catalysis, 2022

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   The water–gas shift reaction (WGSR) is important in industries because it can reduce the CO content of syngas to produce purified H2, which can be used as fuel or to make ammonia (NH3). Supported noble metal catalysts have been widely studied for the WGSR because they exhibit high reactivity. However, the role of a metal–support interface in the WGSR has not yet been revealed and remains elusive. Density functional theory (DFT) calculations were performed for a model system of Co3O4-supported Pd (Pd/Co3O4) catalysts. The presence of the interface was found to promote the H2O dissociation step, which is crucial for improving WGSR activity. Thus, the WGSR activity was predicted to be enhanced by an increased number of interfaces, which could be achieved by controlling the size of the supported Pd nanoparticles (NPs). Furthermore, electronic metal–support interactions (MSIs) were found to be a source of the promoted H2O dissociation at the interface. The DFT-predicted promotion of H2O dissociation was further experimentally validated using Pd/Co3O4 catalysts that were size-controlled with calcination temperatures, and the total length of the interface was shown to have a direct correlation with the WGSR rate. Theoretical insights into the role of the interface and the enhancement of WGSR activity due to increased interface sites, which can be achieved by size control, are believed to be useful in the design of efficient supported metal catalysts for the WGSR.

2. DFT+Expt.

   Boosting Support Reducibility and Metal Dispersion by Exposed Surface Atom Control for Highly Active Supported Metal Catalysts

   Myeong Gon Jang†, Sinmyung Yoon†, Dongjae Shin†, and 7 more authors

   ACS Catalysis, 2022

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   For oxide-supported metal catalysts, support reducibility and metal dispersion are the key factors to determine the activity and selectivity in many essential reactions involving redox process. Herein, we tuned the exposed surface atoms of the catalyst by facet control and doping methods, which were simultaneously applied to boost the reducibility and metal dispersion of an oxide support. Pd supported on Cu-doped CeO2 (Pd/CDC) for water–gas shift reaction (WGSR) was considered a model system; Cu was doped into the cubic and octahedral CeO2 enclosed with (100) and (111) facets, respectively. By a systematic combination of density functional theory calculations and experimental analyses, the WGSR activity of the Pd/CDC cube was verified to synergistically increase by more than just the sum of the morphology and Cu doping effects. The effect of each tuning method on the activity was further investigated from a mechanistic perspective. This work presents a rational design knowledge to enhance the catalytic activity that can be extended to a wide range of supported metal systems.

3. 

   Modulating the water gas shift reaction via strong interfacial interaction between a defective oxide matrix and exsolved metal nanoparticles

   Huijun Chen, Rui Huang, Myeong Gon Jang, and 6 more authors

   J. Mater. Chem. A, 2022

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   Perovskite oxides with exsolved metal nanoparticles have recently attracted great attention because of their outstanding activity and stability at elevated temperature. Despite many pioneering studies on catalyst development, the underlying mechanism for the high activity of exsolution materials is still not fully understood. Particularly, the role of an oxide–metal interface in determining the elemental reaction steps is still unrevealed. In this work, taking Pr0.4Sr0.6CoxFe0.9−xNb0.1O3−δ (PSCxFN, x = 0, 0.2, 0.7) as solid precursors, we synthesize layered perovskite oxide with metal nanoparticles on the surface by thermal reduction. The catalyst with a 20% Co doping level exhibits optimal high temperature water gas shift reaction (HT-WGSR) activity, which is noticeably better than that of commercial catalysts. The combination of an advanced spectroscopy technique and density functional theory calculations reveals that, by introducing oxygen vacancies in an oxide matrix, H2O adsorption and dissociation on the oxide–metal interface are effectively enhanced. Excessive oxygen vacancies, nevertheless, cause too strong binding of CO to the interfacial site, and a significantly high energy barrier for the carboxyl formation step, which is the rate determining step for the HT-WGSR. Our results provide critical insights into the role of the metal–oxide interface and can guide the rational design of exsolution materials for other high temperature thermal catalysis systems.

4. DFT+Expt.

   Alleviating inhibitory effect of H2 on low-temperature water-gas shift reaction activity of Pt/CeO2 catalyst by forming CeO2 nano-patches on Pt nano-particles

   Jaeha Lee, Dongjae Shin, Eunwon Lee, and 4 more authors

   Applied Catalysis B: Environmental, 2022

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   Pt/CeO2 has gained much attention for their high activity in low-temperature (LT) water-gas shift (WGS) reaction. However, the inclusion of H2 in the feed as in the practical reaction condition significantly degrades the LT-WGS activity of the Pt/CeO2 catalysts. In this contribution, the activity of Pt/CeO2 catalyst under the feed gas containing excess H2 (20 vol% of H2) was enhanced more than three times by forming CeO2 nano-patches on Pt nano-particles. Both in-situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculation results indicate that dissociated H2 on the Pt nano-particle inhibits the activity of the Pt/CeO2 catalysts by occupying the active sites (Pt nano-particle-CeO2 interface). On the other hand, thin CeO2 nano-patches on Pt nano-particle suppressed the H2 dissociation. As a result, the WGS reactivity of the active Pt nano-particle-CeO2 interface was less affected by H2, granting the catalysts the high activity under the practical reaction conditions.

5. DFT+ML

   Universally characterizing atomistic strain via simulation, statistics, and machine learning: Low-angle grain boundaries

   Matthew T. Curnan, Dongjae Shin, Wissam A. Saidi, and 2 more authors

   Acta Materialia, 2022

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   When applied to catalysis and related materials phenomena, grain boundary (GB) engineering optimizes over many currently disparately defined properties. Such properties include GB mobility, solute diffusivity, and catalytic footprints correlating current density with dislocation-induced strain. A recent universalizing framework has systematically classified low-Σ GBs in relation to analogous high-angle references, distinguishing them using footprints formed from the directional straining needed to reversibly yield bicrystals from their separated grains. Correlating the elastic work profiles derived from this thermodynamic process with matching changes in GB dislocations, strain footprints can comprehensively link formerly disparate catalytic properties and materials phenomena. This research investigates such structure-energy correlations to evaluate differences between low-angle (LAGBs) and high-angle (HAGBs) GBs, systematically delineating LAGB-HAGB transitions, explaining their origins, and connecting transitions to materials phenomena. A hierarchical statistical model, nesting GB degrees of freedom within one another, systematically detects such transitions via simplified strain footprints without failure of a single unique GB structure and material combination. A more comprehensive analysis of footprint directional components and discontinuities links transitions to catalytically relevant materials phenomena, describing thermal grooving, shear coupling, complexions, and defect migration under a single universal atomistic framework. With machine learning and spatially generalized strain footprints, this framework reconciles such phenomena via more comprehensive geometry-energy correlations.

2021

1. DFT

   Structure-activity relationship of VOx/TiO2 catalysts for mercury oxidation: A DFT study

   Dongjae Shin, Moon Hyeon Kim, and Jeong Woo Han

   Applied Surface Science, 2021

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   Elemental mercury (Hg0), mostly from coal combustion, poses a critical threat to ecosystems and the health of human beings, as it causes several fatal human diseases. Thus, there has been an effort to strengthen regulations around the world. Catalytic oxidation of Hg0 into HgCl2 is considered an economical and practical option, and selective catalytic reduction catalysts such as titania-supported vanadia (VOx/TiO2) have been shown to also oxidize Hg0 to Hg2+. Herein, based on density functional theory (DFT) calculations, we demonstrate the relationship between the coordinative environment of V in the VOx/TiO2 catalyst and the catalytic activity towards Hg0 oxidation, as well as the effect of hydroxylation. We mechanistically estimate the Hg0 oxidation activity of several VOx/TiO2 models using previously reported mechanisms. We also explore the temperature (T)- and pressure (p)-dependent thermodynamic stabilities of the VOx/TiO2 catalyst models by calculating the Gibbs free energy of formation (ΔGform). Finally, the thermodynamic (T, p) conditions that favor high activity of the VOx/TiO2 catalyst are suggested.

2. 

   Facet-Dependent Mn Doping on Shaped Co3O4 Crystals for Catalytic Oxidation

   Junemin Bae, Dongjae Shin, Hojin Jeong, and 4 more authors

   ACS Catalysis, 2021

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   Doping other metals is known as a facile strategy to improve the catalytic activity of metal oxide catalysts. However, the doping behavior heavily depends on the surface structure of the host metal oxide, possibly leading to different catalytic properties. Here, Mn was doped onto Co3O4 cubes or octahedra with different facets of (100) and (111), respectively. Mn could be successfully doped into (100) facets, whereas it was excessively accumulated on (111) facets, not incorporating into the lattice. For the simultaneous oxidation of CO, C3H6, and C3H8 with water vapor, Mn-doped Co3O4 cubes showed superior activity because the Mn doping could improve the amount of surface-adsorbed oxygen and the transfer of surface oxygen species. The elaborated Mn doping on Co3O4 cubes could minimize the metal oxide sintering, inducing superior durability. This non-precious-metal oxide catalyst can provide an efficient solution for low-cost automobile exhaust treatment.

2020

1. 

   Design of an Ultrastable and Highly Active Ceria Catalyst for CO Oxidation by Rare-Earth- and Transition-Metal Co-Doping

   Hyung Jun Kim†, Dongjae Shin†, Hojin Jeong, and 3 more authors

   ACS Catalysis, 2020

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   Catalyst design with good stability beyond simply having high activity is crucial for a variety of reactions. Here, we evaluate the ceria catalyst for CO oxidation as a model reaction to rationally design an ultrastable catalyst with high activity. The goal was achieved by co-doping with rare-earth (RE) and transition metals (TM) simultaneously. The RE dopant stabilized the catalyst by inhibiting sintering that could lead to catalyst deactivation. The TM dopant increased the activity by facilitating formation of surface defects. Consequently, ceria co-doped with RE (=La, Sm) and TM (=Cu) had increased catalytic activity as well as superior resistance to deactivation during 10 cycle measurement (1 cycle: 900 °C, 24 h → cooling at room temperature → target °C, 24 h) of ∼700 h, which is harsher than any other reported conditions. This approach will shed light on the design of ultrastable oxide materials for a wide range of catalytic reactions.

2. 

   Design of Ceria Catalysts for Low-Temperature CO Oxidation

   Hyung Jun Kim†, Myeong Gon Jang†, Dongjae Shin†, and 1 more author

   ChemCatChem, 2020

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   Abstract A catalyst active to carbon monoxide (CO) oxidation at low temperature is essential for environmental conservation, saving fuel and improvement of the quality of human life. Rational design of CO oxidation catalyst on the basis of comprehensive understanding of physicochemical properties of catalytic materials, rather than simply searching for the catalyst based on trial-and-error, is a promising approach to meet the increasingly stringent regulations. This review covers metal-doped and -loaded system based on CeO2 catalysts as strategies to significantly improve CO oxidation activity at low temperature. When incorporated into CeO2 lattice, active metals significantly lower the oxygen vacancy formation energy (Evf) of the catalyst surface, resulting in high catalytic activities at low temperature. When the active metals are loaded on the CeO2 surface, many active sites could be acquired by increasing the dispersion, and the catalytic activity can be dramatically improved by newly introducing the interfacial sites between the metals and the CeO2 support. Doping the support could further improve this loaded system in terms of specific surface area, oxygen vacancy formation, and spillover effects. In this review, based on this knowledge, we propose a rational design approach to a robust low-temperature CO oxidation catalysts. The desirable CO oxidation catalysts identified from the interplay between theoretical and experimental approaches would ultimately improve the quality of human life, and create potential economic benefits by alleviating air pollution.

3. DFT+Expt.

   Oxidative Methane Conversion to Ethane on Highly Oxidized Pd/CeO2 Catalysts Below 400 °C

   Gihun Kwon†, Dongjae Shin†, Hojin Jeong, and 7 more authors

   ChemSusChem, 2020

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   Abstract Methane upgrading into more valuable chemicals has received much attention. Herein, we report oxidative methane conversion to ethane using gaseous O2 at low temperatures (<400 °C) and atmospheric pressure in a continuous reactor. A highly oxidized Pd deposited on ceria could produce ethane with a productivity as high as 0.84 mmol gcat−1 h−1. The Pd−O−Pd sites, not Pd−O−Ce, were the active sites for the selective ethane production at low temperatures. Density functional theory calculations confirmed that the Pd−O−Pd site is energetically more advantageous for C−C coupling, whereas Pd−O−Ce promotes CH4 dehydrogenation. The ceria helped Pd maintain a highly oxidic state despite reductive CH4 flow. This work can provide new insight for methane upgrading into C2 species.

4. DFT+Expt.

   Controlling the Oxidation State of Pt Single Atoms for Maximizing Catalytic Activity

   Hojin Jeong†, Dongjae Shin†, Beom-Sik Kim, and 5 more authors

   Angewandte Chemie International Edition, 2020

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   Abstract Single-atom catalysts (SACs) have emerged as promising materials in heterogeneous catalysis. Previous studies reported controversial results about the relative level in activity for SACs and nanoparticles (NPs). These works have focused on the effect of metal atom arrangement, without considering the oxidation state of the SACs. Here, we immobilized Pt single atoms on defective ceria and controlled the oxidation state of Pt SACs, from highly oxidized (Pt0: 16.6 at %) to highly metallic states (Pt0: 83.8 at %). The Pt SACs with controlled oxidation states were then employed for oxidation of CO, CH4, or NO, and their activities compared with those of Pt NPs. The highly oxidized Pt SACs presented poorer activities than Pt NPs, whereas metallic Pt SACs showed higher activities. The Pt SAC reduced at 300 °C showed the highest activity for all the oxidations. The Pt SACs with controlled oxidation states revealed a crucial missing link between activity and SACs.

2019

1. DFT+Expt.

   Highly Water-Resistant La-Doped Co3O4 Catalyst for CO Oxidation

   Junemin Bae, Dongjae Shin, Hojin Jeong, and 3 more authors

   ACS Catalysis, 2019

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   Co3O4 is an attractive alternative to precious metal catalyst for CO oxidation due to its low cost and earth abundance, but its high catalytic activity is severely degraded in the presence of water. Here, we show that doping La into Co3O4 surfaces significantly enhances CO oxidation activity under moisture-rich condition. While bare Co3O4 is deactivated under moisture, the La-doped Co3O4 catalysts exhibit greatly improved activity and water resistance. The higher ratio of active Co3+ on the La-doped Co3O4 surface results in the enhanced activity. Especially, the La doping greatly reduces the formation of surface OH on the Co3O4 surface, which causes the poor water resistance. Density functional theory calculations reveal that the La doping suppresses the vacancy-assisted dissociative adsorption of H2O, resulting in less formation of surface OH and thereby mitigating water poisoning on the Co3O4 surface. This work can provide an insight into the surface restructuring for highly active and water-resistant Co3O4 catalysts.

2. DFT+Expt.

   Improved CO Oxidation via Surface Stabilization of Ceria Nanoparticles Induced by Rare-Earth Metal Dopants

   Kyung-Jong Noh, Kyeounghak Kim, Hyung Jun Kim, and 2 more authors

   ACS Applied Nano Materials, 2019

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   Rare earth (RE) metals have often been used as dopants to improve the catalytic activity of ceria. However, their exact role in the activity of ceria catalyst has not been clearly identified. Combining experimental and theoretical approaches, we extensively investigate CO oxidation as a model reaction on RE-doped ceria (REC). The apparent activity is linearly proportional to the specific surface area (AS), which is enlarged by RE dopants as a consequence of surface stabilization. To decouple the effect of each RE dopant on the surface inherent activity, we set AS of REC to be almost constant by adjusting the pH during synthesis. In this case, however, pure ceria shows higher activity than any REC. We therefore conclude that although the RE dopants have lower intrinsic activity than that of Ce, they have an important effect of increasing AS to a level that pure ceria can never attain synthetically, thereby increasing their catalytic activity.