Cutting Materials in Half: A Graph Theory Approach for Generating Crystal Surfaces and Its Prediction of 2D Zeolites

Scientific interest in two-dimensional (2D) materials, ranging from graphene and other single layer materials to atomically thin crystals, is quickly increasing for a large variety of technological applications. While in silico design approaches have made a large impact in the study of 3D crystals, algorithms designed to discover atomically thin 2D materials from their parent 3D materials are by comparison more sparse. We hypothesize that determining how to cut a 3D material in half (i.e., which Miller surface is formed) by severing a minimal number of bonds or a minimal amount of total bond energy per unit area can yield insight into preferred crystal faces. We answer this question by implementing a graph theory technique to mathematically formalize the enumeration of minimum cut surfaces of crystals. While the algorithm is generally applicable to different classes of materials, we focus on zeolitic materials due to their diverse structural topology and because 2D zeolites have promising catalytic and separation performance compared to their 3D counterparts. We report here a simple descriptor based only on structural information that predicts whether a zeolite is likely to be synthesizable in the 2D form and correctly identifies the expressed surface in known layered 2D zeolites. The discovery of this descriptor allows us to highlight other zeolites that may also be synthesized in the 2D form that have not been experimentally realized yet. Finally, our method is general since the mathematical formalism can be applied to find the minimum cut surfaces of other crystallographic materials such as metal–organic frameworks, covalent-organic frameworks, zeolitic-imidazolate frameworks, metal oxides, etc.

Photoluminescent, upconversion luminescent and nonlinear optical metal-organic frameworks: From fundamental photophysics to potential applications

Fatmah Ebrahim, Ngoc Tú Nguyên, Kyriakos Stylianou

Metal-organic frameworks (MOFs) are a class of porous materials prepared by the self-assembly of metal ions or clusters with organic ligands. The unique characteristics of MOFs, including structural tunability, high surface areas, low densities and tailored pore surface functionalization, have made them leading contenders as high-performance porous materials, alongside the established zeolites and activated carbons. Consequently, the permanent porosity of MOFs has been extensively exploited for gas capture and separation and catalysis. In recent years, the field has been expanded towards new applications and many studies of MOFs are venturing into unexplored avenues. A large number of studies have been focused on photoluminescent, upconversion luminescent, and nonlinear optical MOFs for applications in areas such as white-light emission, bioimaging, sensing, and photocatalysis. Within the first half of this tutorial review, we present the fundamental principles of luminescence, including detailed scientific discussions on the luminescence origin of different materials such as organic dyes, transition metal complexes, quantum dots, and lanthanide compounds. Principles and important parameters for the applications of luminescent MOFs are introduced, followed by a summary of recent interesting publications for each application. In the second half, we introduce nonlinear optical effects including second harmonic generation and two-photon absorption, and upconversion of luminescence, followed by detailed examples of MOFs that exhibit these phenomena. Finally, insights about the remaining challenges and future directions are discussed. (C) 2018 Elsevier B.V. All rights reserved.

2018

Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Kun Zhao

Renewable energy supply and energy storage in a closed materials cycle are the urgent global challenges of the 21st century. Carbon dioxide (CO2) hydrogenation over catalysts is a method to produce synthetic fuels from renewable energy in a CO2 neutral cycle. Numerous catalysts from pristine to novel materials have been investigated to achieve high CO2 conversion and selective hydrocarbon. Ahead of developing innovatively active catalyst with a high selectivity, it is crucial to understand the active sites and the related reaction mechanisms. Despite the fact that CO2 + H2 is a rather simple chemical equation, many reaction paths on a solid catalyst are possible. However, the proposed mechanisms from literatures have been controversial due to two main difficulties in 1) evidently identifying the reaction species, especially some vital transient species on the pathway of carbon hydrogenation and C-C coupling, and 2) proving the relation between surface structure of the catalysts and the reactivities. To address these difficulties, we built a new instrumental setup and developed an analysis program in order to investigate the catalyst surface and analyze the reaction mechanism in operando. We built up a diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy-gas chromatography (DRIFTS-MS-GC) instrument for an operando study of the surface, gas, and liquid products using DRIFTS, MS, GC, and ex situ NMR. A bilevel evolutionary Gaussi-an fitting (BEGF) program was developed for dataset treatment including peak deconvolution and kinetic plot. Standard chemicals like carbonates, bicarbonates, formic acid, acetic acid, methanol, and ethanol, and isotopic spectra using 13CO2 and D2 in the reactions were em-ployed for supporting peak assignments. Based on these infrastructures, CO2 adsorption and hydrogenation reactions on pristine metals (Fe, Co, Ni, and Cu), metal hydride alloy (La-Ni4Cu), and metal-oxide (Co-CoO) were studied. We found out pure Fe, Co, Ni, Cu and La-Ni4Cu metals are not efficient for CO2 hydrogenation, and have no detectable adsorption spe-cies on the surface. High conversion (> 90%) of CO2 on Co-CoO surface emphasized the im-portance of oxide for CO2 chemisorption and for the observations of the surface species. The following investigation on oxide supported metal catalyst Ru/Al2O3 via an in situ control of the individual formation and hydrogenation of each adsorption species demonstrates that the oxide assists CO2 initial activation. CO2 methanation starts at the interface of the metal and the oxide and passes through the steps of CO2 ï® HCO3-* (and/or HCOO-* in CO2+H2 co-adsorption condition) ï® CO* ï® CH4. We further explored the relationship between met-al/(metal+oxide) ratio and their reactivities. We synthesized Co/(Co+CoO) catalysts in the DRIFTS-MS-GC instrument, and investigated the CO2 hydrogenation reaction on site. The results reveal that high concentration of the oxide in the catalyst boosts CO2 methanation, be-cause the CO2 binding is moderate and the adsorption is enhanced when the oxide concentra-tion increased. In short, we established equipment for an operando study of CO2 hydrogenation on catalyst surfaces, unraveled the surface reaction mechanisms and the active sites, developed a highly active catalyst, as well as initiated an efficient data analysis program.

EPFL2020

Diversifying Databases of Metal Organic Frameworks for High-Throughput Computational Screening

Kevin Maik Jablonka, Sauradeep Majumdar, Seyedmohamad Moosavi, Daniele Ongari, Berend Smit

By combining metal nodes and organic linkers, an infinite number of metal organic frameworks (MOFs) can be designed in silico. Therefore, when making new databases of such hypothetical MOFs, we need to ensure that they not only contribute toward the growth of the count of structures but also add different chemistries to the existing databases. In this study, we designed a database of similar to 20,000 hypothetical MOFs, which are diverse in terms of their chemical design space-metal nodes, organic linkers, functional groups, and pore geometries. Using machine learning techniques, we visualized and quantified the diversity of these structures. We find that on adding the structures of our database, the overall diversity metrics of hypothetical databases improve, especially in terms of the chemistry of metal nodes. We then assessed the usefulness of diverse structures by evaluating their performance, using grand-canonical Monte Carlo simulations, in two important environmental applications-post-combustion carbon capture and hydrogen storage. We find that many of these structures perform better than widely used benchmark materials such as Zeolite-13X (for post-combustion carbon capture) and MOF-5 (for hydrogen storage). All the structures developed in this study, and their properties, are provided on the Materials Cloud to encourage further use of these materials for other applications.

AMER CHEMICAL SOC2021

Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Kun Zhao

EPFL2020