LAYERED MATERIALS, PREPARATION OF NANOCOMPOSITESAND CATALYSIS UTILIZING NANOSTRUCTURED MATERIALS Flashcards
What do you understand with the term “layered material”?
A compound with a layered crystal structure is a compound that has anisotropy in the chemical bonding in xy direction relative to z-direction. Graphite, layered double hydroxides and Ruddlesden-Popper phases have layered crystal structures.
What is delamination, exfoliation and reconstruction?Explain in brief, aspects connected to chemical bonding in the 3D layered materials relative to their ability to delaminate/exfoliate.Give examplesof layered materials where the layers are held together by van der Waals forces, H-bonding, electrostatic forcesand polar covalent bonding.
The terms delamination and exfoliation is often used as synonyms. However, according to FRA HANDBOOK IN CLAY SCIENCE:
As long as there is a significant interaction between the two successive layers with some crystallographic orientation maintained, this separation is called delamination. When at some point, no further interaction occurs between the two delaminated units (isolated layers or stacking of few layers) which become independently mobile in the liquid phase, this phenomenon is called exfoliation. We have used the terms as synonyms, describing a process where the layers become independently mobile and there is no crystallographic orientation maintained.
Reconstruction is the reverse process–to repack the individual layers back to a 3D layered structure.
Graphite/MoS2; Layered double hydroxides; Ruddlesden-Popper phases
Chemically and mechanically, how canyou facilitate exfoliation?
i) intercalation (adding introducing particles between the layers), ii) exchange (exchanging already existing particles between the layers with new) and iii) sonication to disperse the sheets in a solution. I) and ii) are the chemical approaches. Iii) is a mechanical approach.
Explain in briefthe atomic arrangement in graphite, h-BN and h-MoS2.
- What do you think about these materials ability to delaminate/exfoliate?
- Suggest specific applications of the delaminated/exfoliated constituentsof these three compounds.
Graphite, h-BN and h-MoS2forms interconnected 6-rings, and forms therefore2D-layers interconnected by van der Waals forces. These kinds of layers are easy to separate with exfoliation, and therefore these materials are used widely as lubricants and many different ways. Because of their insulating properties, they are often used to increase heat resistance.
What is a layered double hydroxide, and how will you describe its crystal structure? Why is this class of compounds often found very interesting for applications?What do you think is “the driving force” of the delamination process of the layered double hydroxide Mg1-xAlx(OH)2(NO3)xnH2O in formamide?
An LDH is ionic solid materials with layers of metals (cations) with an OH-group (anions) above and below. In between these layers, there are anions or neutral molecules such as water. These materials are extremely interesting, due to their ability to easily exchange the anions in between the cationic layers. There are huge flexibility in tuning the chemical properties when changing the composition of the LDHs through synthesis. Therefore, these materials are excellent materials to use e.g. as catalysts and in batteries. The driving force of the delamination process of the stated LDH in formamide is the Hydrogen attached to formamide, which interacts strongly with the NO3-anions in between the layers.
Suggest why 2D nanosheets may be an excellent starting point to create nanostructures giving rise to materials with enhanced properties.
Starting from 2D nanosheets gives you very good control of the materials. Then you can tune the materials with respect to what you want them to do. You can for example make nanotubes and nanorods.You can get superparamagnetic nanoflakes.
Suggest an approach that would allow you to form metallic Ni nanoparticles anchored on an oxide composed of an Al-,Mg-mixed oxide, Ni/Mg(Al)O. Hint –you can do this by synthesizing layered double hydroxides(see e.g. the paper by Karthikeyan et al., 2015). Discuss key factors that would allow you to manipulate the chemical structuring in the LDH or the catalyst Ni/Mg(Al)O. (This question is also connected to questions in the “MAGNETISM” section.)
First you can delaminate LDHs consisting of MgAl, NiAl or MgNiAl, by mixing the materials with formamide and using ultrasonic treatment. Then; MgAl and Ni/Al or MgNiAl can be mixed in calculated ratios to get the target Ni wt% by mixing with K2CO3in the solution to obtain a restacking of the LDH sheets. Key factors that can help manipulate the chemical structuring is the chemical composition of the starting material as well as amount of Ni compared to the support (MgAl).
Which type of Bragg-reflections would you expect notto observe for the corresponding 2D nanoflakesof a LDH material?Why?What species do you believe is attached to the surface of the 2D-nanosheets?
I would not see the long rang order reflections of the super cell in the z direction. This is because when the material is only in 2D, it only has long range order in two directions. Layers must be charge neutral – cationic layers must have anions plus maybe somesolvent attached
Outline the principles behind preparing the RGO//NiFe-LDH nanocomposite (hint –look up lecture notes and the paper by Long et al., 2014). What is the main objective of forming the nanocomposite rather than mixing RGO and NiFe-LDH mechanically? At which length scale can you at best assume the mixing of the two constituents to be at, and which type of chemical analysis would you perform to document the nanostructuring. Justify your answer.
Starting from FeNi-CO3 LDH, it is possible to make FeNi-Cl LDH, as the latter is much easier to delaminate. Therefore, FeNi-CO3 LDH is decarbonated in a HCl and NaCl solution to obtain FeNi-Cl LDH. Thereafter, FeNi-Cl LDH is dispersed in a GO aqueous solution, and magnetically stirred for ten days, to ensure maximum delamination of FeNi-Cl LDH and reconstruction of FeNi-GO nanocomposites. The nanocomposites shows a much lower overpotential and has a high steady state current (catalytic activity) compared to a purely mixed material in the splitting of water to oxygen. The mixed layers (FeNi + GO) are about 1.5 nm thick. You can perform XRD to see shifts in the d-spacing due to different basal spacing in the c-direction when there are different anions between the layers. This will indicate whether you have CO3, Cl or GO. Then, you can use XPS to investigate the C 1s core level spectrum. Here you will see all the different electronic environments of C in FeNi-GO.
In the work of Leng et al. (2018), 2D hybrid perovskites are formed via delamination of Ruddlesden-Popper type phases. The optic properties of the compounds change when going from bulk to monolayers, see Figure 2 in the paper. Suggest an explanation to this. Additionally, would you expect amore “classic” Ruddlesden-Popper phase as La4Ni3O10(n = 3) to delaminate? Justify your answer.
The nanosheets have much more surface, and are therefore more flexible than the bulk material. This extra flexibility gives better control for tuning the band gap. At the surface, the materials also have more flexibility, so the increased surface in the atomically thin layers may help contribute to the reversible effect observed by the laser. I would not expect classic RP-oxides to delaminate easily, mainly due to their 3D character and strong covalent bonding in the materials.
What do we mean with the terms “homogenous catalysis” and “heterogeneous catalysis”?
Homogeneous catalysis is when the catalyst and the reactants are in the same phase (e.g. liquid-liquid). Heterogeneous catalysis is the opposite; here the catalyst has a different phase than the reactants (e.g. solid catalyst and reactants in gas phase).
Supported catalyst can be formed by classic impregnation routes or by depositing colloidal free-standing nanoparticles directly onto the support. Outline the latter approach, and explain potential benefits and issues in the preparation route. Suggest ways to document successful characterizationof the as-synthesized catalyst.
NP of a metal is coated with an organic surfactant having a hydrophobic and a hydrophilic end (e.g. oleic acid). This is immersed in a beaker with the support (e.g. Al2O3) dispersed in a solvent (e.g. hexane). Then, the solvent is evaporated, and we are left with surfactant-coated NP. Then, the NP is oxidized at high T to remove the organic ligands, and after a reduction again in H2, we have well-dispersed NP on the support. The synthesis approach is beneficial due to a much better control of the size and shape of the metal particles. The issue with this method is that it is complicated and time-consuming, and requires an organic precursor that need to be removed. To prove successful characterization, TEM images of the material would be a good start, and EDX to check the elemental analysis. Then, FT-IR spectroscopy is a good tool to see if there is any more organic material left on the material.
What do you understand with the term “metal-on-support interaction”? Elaborate on how this interface can affect catalytic performance and the stabilization of a metal particleon the support material. Suggest also why particles with different faceting may act differently as catalyst for a specific chemical reaction.
Metal-on-support interaction is when there are interesting properties arising from the interface between the particle and the surface. Varying the shape and size of the nanoparticle, as well as having different support materials can tune a reaction towards different products (change in selectivity). The amount of support interacting with the particle varies, as well as the bond strength and bond length between the support and the particle, when the faceting of the particle is changed. And in some reactions, it may be exactly the interface between support and particle that lowers the activation energy. Different metal-support interactions may induce different reaction pathways.
With reference to the lecture notes “Catalysis utilizing nanostructured materials” suggest explanations to why there is an optimal Pt-cluster size for the water-splitting active nanocomposite Pt/CdSe.
The reason there is an optimal Pt-cluster size for water-splitting active nanocomposite Pt/CdSe, because the electronic state of the Pt particle is size dependent. Meaning, it is more negatively charged when the size is small. The size of the Pt particle helps tune the LUMO band for the reduction process. At 1-3 nm, the band gap is at its optimum size for water splitting.
