Characterization of metal organic frameworks of interest for gas adsorption/separation applications

Metal-Organic Frameworks (MOFs) are a class of porous materials that are applicable in many energy and environmentally relevant areas, due to their unique features including unprecedented internal surface areas and easy chemical tunability. Gas separation and storage are among the most important applications for which MOFs have been extensively studied. Considering the large number of potential MOFs that can be accessed, through a combination of numerous metal clusters and organic linkers, determining the relationship between their structural features (such as pore size and shape and chemical functionalities) and corresponding gas adsorption properties is of utmost importance. In this regard, in-situ characterization, especially diffraction, can be advantageous. MOFs can offer high crystallinity, and so their structure can readily be characterized via diffraction. In addition, due to the presence of metal ions, and predefined molecular building blocks, which exhibit multiple chemical moieties, the potential energy surface for a guest molecule within a MOF cavity possesses multiple minima that correspond to well-defined adsorption sites with varying binding energies. This makes diffraction the most direct way to probe static site-specific binding properties. Given this, the focus of this work is on understanding the structure-derived function of MOFs of interest in gas separation/storage applications. To do this, standard adsorption experiments are coupled with in-situ diffraction techniques. The latter is able to reveal the location and orientation of adsorbed guest species inside the framework, provide insights into the framework response to varying pressure, temperature, and atmosphere, and allow one to determine the relative differences in binding energy between neighboring adsorption sites. The aforementioned information can then be used to rationalize the relationship between different physiochemical features of a given framework and their performance in an adsorption process. In addition to establishing structure-property relations for the usage of MOFs, the obtained experimental results are used to corroborate those calculated by DFT methods, providing a stress test for computational methods aimed at predicting the structures and properties of hypothetical MOFs. The first chapter of this thesis, the introduction, offers a brief review of different characterization methods for studying gas adsorption/separation applications in MOFs. Although the focus of this chapter is on carbon dioxide capture as the application, these characterization methods are also used to study other adsorption processes. The next three chapters, chapters 2-4, present several prominent MOF families that are of interest in hydrogen storage and carbon dioxide capture applications. These case studies demonstrate how using in-situ diffraction techniques coupled with adsorption measurements and DFT calculations can be used to gain molecular level insight into how small molecules bind inside the selected MOF structures. Further, these structure-property correlation studies are able to help unveil how altering MOF building blocks, such as metals and ligands, gives rise to enhanced or diminished adsorption properties. The last chapter, chapter 5, is a study of the response of an activated MOF to varying temperature; in this chapter various structural motions, which are responsible for a large negative thermal expansion (NTE), are elucidated.

Design of Surface-Supported Bio-inspired Networks for CO2 and O2 Activation at Room Temperature

Daniel Eduardo Hurtado Salinas

The structure of a biological system defines its function. Therefore, nature has developed sophisticated systems that catalyze chemical reactions that are vital for life on earth. For instance, the photosynthesis is a biological process that is partially regulated by the metallo-enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), whose function is to catalyze the capture and transfer of CO2 into organic molecules. This carboxylation reaction occurs at the active site, where a magnesium ion (Mg2+) is bound to organic molecules with carboxylate (COO-) and amine (NH2) functional groups. In the last decades, supramolecular chemistry has proven to be a valid approach to replicate the structural characteristics of these natural metal-organic systems. Metal atoms and organic molecules are used as building blocks to form two-dimensional (2D) metal-organic networks (MONs) on metal surfaces, through a self-assembly process that occurs spontaneously. In these systems the molecules bind to the metal centers and control their oxidation states that, as found in metallo-enzymes, is crucial to perform a catalytic task. Therefore, these supramolecular assemblies show promising features to be explored for heterogeneous catalysis. The main objective of this thesis is to reverse the structure-function equation, that is: instead of finding specific functions of well-known 2D structures for future applications, 2D structures are designed to reproduce the specific functions of well-known bio-systems. Thus, inspired in the structure of the active site of RuBisCO, 2D Mg-based supramolecular assemblies are rationally designed on Cu(100) or Mg(0001) surfaces at room temperature (RT) as a platform for CO2 adsorption. This work is the first study of self-assembly of alkaline earth metal atoms (Mg) with organic molecules ((1,4-benzenedicarboxylic acid (TPA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)) on metal surfaces. The first part of this thesis investigates the self-assembly of TAPT molecules. Scanning Tunneling Microscopy (STM) and High Resolution X-ray Photoemission Spectroscopy (HR-XPS) demonstrate that the molecules typically semi-deprotonate to form MONs with the adatoms from the host substrates. The exposure of these structures to CO2 results in the carboxylation of the amino groups. The second part proves that the COOH moieties of TPA molecules fully-deprotonate to form homo-molecular structures. The incorporation of Mg adatoms result in ionic MONs, which reproduce the carboxylate environment of the active site of RuBiSCO. STM, XPS and Density Functional Theory (DFT) analyze the structural changes, the chemical reactions and charge transfer during either the CO2 or O2 adsorption on the Mg2+ centers. The last part focusses on the co-deposition of both molecules to form 2D metallic-hetero-molecular networks with Mg2+ centers. The role of the Mg2+ centers as active sites for molecular adsorption and model systems for gas storage is studied. STM reveals that CO2 partially modifies the network towards a configuration that resembles the active site of RuBisCO. The formation of 3D structures to enhance CO2 adsorption is also evaluated.

EPFL2018

An In-Situ Neutron Diffraction and DFT Study of Hydrogen Adsorption in a Sodalite-Type Metal-Organic Framework, Cu-BTTri

Mehrdad Asgari, Safak Bulut, Michele Ceriotti, Ilia Kochetygov, Wendy Lee Queen, Rocio Semino Barbaresi, Olga Trukhina, Shuliang Yang

Herein we present a detailed study of the hydrogen adsorption properties of Cu‐BTTri, a robust crystalline metal–organic framework containing open metal‐coordination sites. Diffraction techniques, carried out on the activated framework, reveal a structure that is different from what was previously reported. Further, combining standard hydrogen adsorption measurements with in‐situ neutron diffraction techniques provides molecular level insight into the hydrogen adsorption process. The diffraction experiments unveil the location of four D2 adsorption sites in Cu‐BTTri and shed light on the structural features that promote hydrogen adsorption in this material. Density functional theory (DFT), used to predict the location and strength of binding sites, corroborate the experimental findings. By decomposing binding energies in different sites in various energetic contributions, we show that van der Waals interactions play a crucial role, suggesting a possible route to enhancing the binding energy around open metal coordination sites.

2019

The design and synthesis of nanostructured hydrogen oxidation reaction electrocatalysts for hydroxide exchange membrane fuel cells

Weiyan Ni

Hydrogen fuel cell is a promising green technology that transforms the chemical energy in hydrogen into electricity. Currently, most of the efforts are focused on developing proton exchange membrane fuel cells (PEMFCs), however they are heavily dependent on rare and expensive platinum group metal (PGM) catalysts. In contrast, hydroxide exchange membrane fuel cells (HEMFCs) work in alkaline media, thus allow the use of cheaper and more abundant non-PGM catalysts. However, there is no materials found suitable to catalyze the anodic hydrogen oxidation reaction (HOR). Therefore, developing new, active and robust HOR catalysts in alkaline media is of paramount importance. Hereby, this dissertation presented my recent research works on preparation, characterization, and application of HOR catalysts for HEMFCs.Chapter 2 described the exploration of new earth-abundant HOR catalysts. A series of Ni-based compounds were synthesized and tested for HOR, among which Ni3N had the best activity. With further nano-engineering, the mass activity reached the highest at the time reported and the oxidation resistance of the supported version can be significantly improved. Spectroscopic evidence suggested that the downshift of the metal d band weakened the binding energies of the catalyst toward adsorbates, which accounted for the improved activity and stability of Ni3N/C.Chapter 3 demonstrated how to exploit strain-engineering for HOR catalysis. A series of Ni/C composites were prepared by pyrolyzing a Ni-based metal-organic framework (MOF). The optimal catalyst, Ni-H2-2%, has the highest mass activity (at the time reported) and abnormal high intrinsic activity. Characterizations using synchrotron XRD suggested that it was the distinct but relative optimal strain level of Ni-H2-2% that rendered it high activity.Chapter 4 is a complicated version of Chapter 3. The Ni catalysts were also prepared from the same Ni-based MOF with more complex synthesis conditions. The best catalyst, Ni-H2-NH3, possessed highest intrinsic activity among non-precious metals. Combining UPS, H2 chemisorption, isotopic studies, and electrochemical characterizations, the balanced hydrogen binding energy (HBE) and hydroxide binding energy (OHBE) is likely to be the origin of the high activity of Ni-H2-NH3. In the end, MEA measurements confirmed its excellent performance: for non-PGM MEA, the peak power density could reach 488 mW/cm2, 6.4 times higher than the previous record, which demonstrates the feasibility of efficient PGM-free HEMFCs.Chapter 5 exploited the interaction between PGMs and carbon support to improve their HOR activity. A porous N-doped carbon (pN-C) support was found to have prominent enhancement effect toward PGMs. Especially, the Pt0.25Ru0.75/pN-C had the up-to-date highest mass activity and intrinsic activity toward HOR. Using characterizations including XPS, UPS, H2 chemisorption, CO chemisorption, in situ surface-enhanced infrared adsorption spectroscopy (SEIRAS), and electrochemical characterizations, we concluded the superior HOR activity of Pt0.25Ru0.75/pN-C was a combination of interactions between Pt, Ru and the pN-C, with modulated the HBE and strengthened the H2O binding strength. The HEMFC using Pt0.25Ru0.75/pN-C also possessed great performance that far exceeded the U.S. Department of Energy (DOE) 2022 target for HEMFCs, even though the testing condition was harsher. This work demonstrated the feasibility of HEMFC replacing PEMFC.