Title: ETH_Sem6 / ACIV / Section3

Q: Silica main synthesis methods (3)
A: 1. Precipitation: Silica precursor dissolved in water and precipitation induced via pH change (add acid) -> highly hydrophilic, amorphous silica\[\ce{Na2SiO3 + H2SO4 -> SiO2 + Na2SO4 + H2O}\]
2. Flame Pyrolysis: Produces dense, amorphous, hydrophobic silica\[\ce{SiCl4 + 2 H2 + O2 -> SiO2 + 4HCl}\]
3. Sol-Gel: Convenient approach in the Lab.\[\begin{aligned}\ce{Si(OEt)4 + H2O &-> Si(OH)4 + 4EtOH} \\ \ce{[Si(OH2)4]^4+ &<=> [Si(OH)_h(OH2)_{(n-h)}]^{(4-h)+} + h H+} \end{aligned}\]\(\ce{Si(OH)4}\) is part of a dynamic equilibrium of various aquo, hydroxo, and oxo species. The position of this equilibrium is governed by the pH of the solution.

Q: Easily identify coordination of silica sites
A: \(^{29}\ce{Si}\) NMR

Q: Silica OH IR bands after temperature treatment
A: The surface OH are more isolated, leading to stronger (i.e. more distinct) OH-bands (there is a smaller distribution of different OH sites as only the very strongly bonding sites still contain OH at high T; therefore we get narrow peaks)

Q: Silica surface area
A: Up to \(200 \, \mathrm{m^2/g}\), can decrease upon temperature treatment but relatively constant up to \(700 \, \mathrm{^{\circ} C}\).

Q: Dominant species of silicate at different pH
A: pH | Species
\(\leqslant 1\) | \(\ce{[Si(OH)3(OH2)]+ }\)
\(1-6\) | \(\ce{[Si(OH)4]}\)
\(6-13\) | \(\ce{[Si(O)(OH)3]−}\)
\(\geqslant 13\) | \(\ce{[Si(O)2(OH)2]^{2-}}\)

Q: Silica can be made using sol-gel method only in a pH range from 1 to 9 -> Why?
A: The neutral precursor \(\ce{Si(OH)4}\) has to be dominant for repeated condensation to occur, otherwise there would be charge accumulation.

Q: For silica the sol-gel kinetics are faster at pH 7-8 and pH 2, while slow at intermediate pH -> Why?
A: As silica is formed via oxolation, acid or base catalysis accelerate the reaction. At low pH, \(\ce{[Si(OH2)(OH)3]+}\) and at higher pH, \(\ce{[SiO(OH)3]-}\) serves as cat.

Q: When synthezising silica from \(\ce{Si(OEt)4}\) via sol-gel, at low pH linear, while at high pH branched oligomers are observed
A: The terminal hydroxyl groups are more nucleophilic compared to the internal/bridging ones and vice versa for the Si-atoms.
At low pH the catalytic species is electrophilic \(\Rightarrow\) reacts with the most nucleophilic group of the polymer which are the terminal hydroxile groups \(\Rightarrow\) linear polymer, poor cross-linking of particles.
At high pH it's the other way around and the cat reacts with the most electrophilic species of the polymer, being the internal Si groups \(\Rightarrow\) branching polymer.

Q: Synthesis method, precursor to make very dense, hydrophobic silica.
A: Flame spray pyrolysis, using \(\ce{SiCl4}\) as a precursor.
If hydrolytic sol-gel was used, high hydroxyl coverage of the silica and therefore a very hydrophilic material would be obtained.

Q: How to synthesize \(\ce{Si}\) NPs coated with \(\ce{Al2O3}\) and how to control the particle size and thickness of coating?
A: Silica NPs (nanoparticles) can be synthesized via sol-gel, with the gelation period and the drying fine-tuned to prevent agglomeration of the particles to control size. To achieve a uniform distribution, the nucleation should be as short as possible, which can be achieved by tuning the hydrolysis rate and rate of nucleation via the choice of precursor.
The Alumina coating can subsequently be done via ALD, controlling the thickness by the number of cycles.

Q: Schematic mechanism for the dehydroxylation reaction between two vicinal silanols. Can this happen if there are no vicinal silanols shown spectroscopically?
A: This can happen if no vicinal silanols are present initially, as at elevated temperature the OH groups become mobile.

Q: Which types of silanols are present on the silica surface after dehydroxylation at the following temperatures?
A: \(\ce{SiO_{2-700}}\) mostly contains isolated, but also \(5-10\%\) geminal silanol groups.
In addition to isolated and geminal silanols, \(\ce{SiO_{2-300}}, \ce{SiO_{2-500}}\) also contain vicinal silanols.

Q: Alumina applications (7)
A: - Abrasive for hard materials,
- jewelry,
- lasers,
- electric insulators,
- catalyst support,
- catalyst
- Also, pure aluminum is synthesized from electrolytic splitting of alumina

Q: Alumina bulk structure
A: Variety of polymorphs, that differ in their primary building blocks and connectivity.
- Hydroxides \(\ce{Al(OH)3}\)
- Oxohydroxides \(\ce{Al(O)(OH)}\)
- Oxides \(\ce{Al2O3}\)

Q: IR spectra CO on the surface of Alumina (different pressure)
Explain why the middle peak is red-shifted at higher partial pressure.
A: The peak is actually less blue-shifted, due to interaction between surface CO at high coverage.

Q: Comparison \(\ce{CO}\) adsorbed at \(60\, \mathrm{K}\) vs. \(\ce{H2}\) at \(20 \, \mathrm{K}\). Why do they look almost like mirror images?
A: For \(\ce{H2}\) the \(\sigma\)-bond acts as a lewis base \(\Rightarrow\) stronger LA leads to a weaker bond and thus red-shift. For \(\ce{CO}\) stronger LA leads to more triple bond character (since orbital from which CO gives electron density to coordinate has anti-bonding character) \(\Rightarrow\) blue-shift.

Q: Why can \(\ce{Al2O3}\) not be synthesized directly via precipitation?
A: \(\ce{Al^{3+}}\) condenses via olation \(\Rightarrow\) only the hydroxide \(\ce{Al(OH)3}\) can be obtained, which precipitates as Bayerite or Gibbsite.

Q: What types of materials are Boehmite and Diaspore? How can they be obtained?
A: Aluminium-oxohydroxides \(\ce{Al(O)(OH)}\).
Boehmite can be obtained via dehydroxylation (calcination) of Bayerite or Gibbsite at ca. \(150 \, \mathrm{^{\circ}C}\).
Diaspore can be obtained via hydrothermal synthesis, where the desired material is precipitated from aq. solution at elevated \(T, p\).

Q: The \(^{29}\ce{Al}\) NMR Spectrum of Boehmite shows a single peak. After heating to \(500 \, \mathrm{^{\circ}C}\) two broader peaks, heating to \(1000 \, \mathrm{^{\circ}C}\) again single peak. Explain! Also why do the peaks not follow the expected Lorentzian shape?
A: Boehmite only contains \(\ce{Al}\) in octahedral environments \(\Rightarrow\) single peak. Upon heating it transforms into \(\gamma\)-alumina which contains octahedral and tetrahedral \(\ce{Al}\). Further heating gives \(\alpha\)-alumina which again only contains octahedral \(\ce{Al}\).
In all materials, there are multiple distinct \(O_h/T_d\) sites, leading to a distortion of the peaks from the expected lineshape. Additionally as \(\ce{Al}\) is a quadrupolar nucleus, the asymmetric environment leads to a distortion.

Q: Wulff construction
A: Predicts the equilibrium shape of a crystal of fixed volume. To minimize the surface energy, the most stable surfaces are exposed.
The surface energy is proportional to the length of a vector drawn normal to a crystal face to the center of the crystal (for a spherical crystal this would be the radius). For each surface, the proportionality factor \(\hat h\) has to be provided. The vectors are drawn from the center with that length and the crystal shape will correspond to the enclosed volume:
The total free energy of the crystal is the sum over all surfaces \(i \) of the product of surface energy \(\gamma_i\) and surface area \(A_i\).
\(\Delta G= \sum_i \gamma_i A_i\)

Q: Consider the (100) and the (110) facet of \(\gamma\)-alumina, which facet has the higher surface energy and why?
A: As a rule of thumb, planes with larger (hkl) tend to have higher energies, as fewer atoms lie directly on the plane, meaning that more defects are caused by cutting the crystal along that facet.

Q: Weakness of the Wulff construction
A: It does not take into account the adsorption of water (which can change the surface energy of sites), which is inevitable when synthesizing something under aqueous conditions. Also kinetic effects are ignored.

Q: Why do too low and too high pretreatment temperatures quench the ability of \(\gamma\)-alumina to dissociate \(\ce{H-H}\) and \(\ce{C-H}\) bonds?
A: \(\ce{Al_{III}}\) facets of sufficient LA have to be present on the (100) facet. At low temperatures \(\leqslant 400 \, \mathrm{^{\circ} C}\), the water coverage is too high, quenching the LA. At high temperatures \(\geqslant 800 \, \mathrm{^{\circ} C}\), phase transitions to \(\gamma \rightarrow \delta \rightarrow \theta \rightarrow \alpha\)-alumina occur leading to a loss in specific surface area shutting down the reactivity.

Schematics of Alumina Synthesis (different phases)

Q: Alumina synthesis (3)
A: 1. Precipitation: \(\ce{NaAlO2 -> AlO2 (acidic)}\) or \(\ce{Al2(SO4)3 -> Al(OH2)6^{3+} (basic)}\)
2. Flame spray pyrolysis: from \(\ce{Al(acac)3}\)
3. Sol-Gel: \(\ce{Al(OR)3}\): Condensation from \(\ce{Al(OH2)6^{3+}}\) in basic or \(\ce{Al(OH)4-}\) in acidic media. Precursors: \(\ce{NaAlO2}\) in basic and \(\ce{Al2(SO4)3}\) in acidic media

Q: Influence of pH on \(\ce{CN(Al)}\) (5)
A: - Positively charged and neutral \(\ce{Al}\) species: \(CN = 6 \Rightarrow \ce{[Al(OH2)6]^{3+}}\) or \(\ce{[Al(OH)3(OH2)3]}\)
- Negatively charged \(\ce{Al}\) species: \(CN=4 \Rightarrow \ce{Al(OH)4-}\)
- \(\delta(\ce{H2O}) > 0\) for hexacoordinated neutral and posivitely charged species and < 0 for negatively charged species, which would mean \(\ce{e-}\) transfer from the metal cation to water ligand (unrealistic) \(\Rightarrow\) negatively charged species is tetracoordinated (i.e. not coordinated to water).
- Hydrolysis ratio: \(\ce{Al2O3}\) precipitates in large pH range \((6-11)\) in which neutral species \((h=3)\) dominates
- Stability of \(\ce{Al2O3}\): \(6 < \ce{pH} < 10\)

Q: Alumina: Effect of hydration level on presence of specific sites on (110)
A: - S0: No water present: LA decreases acc. to \(\ce{Al_{III} > Al_{IVb} > Al_{IVa} }\)
- S1: \(\ce{1H2O}\)/unit cell: \(\ce{H2O}\) adsorbed to \(\ce{Al_{III}}\) and split on \(\ce{Al_{III}-O}\) bond so that \(\ce{2 OH}\) groups form on surface; next most stable configuration: \(\ce{H2O}\) adsorbed to \(\ce{2 Al_{IVa}}\) sites as \(\mu_2 - \ce{OH}\) bridge is better than having water chemisorbed on single (stronger) \(\ce{Al_{IVb}}\) site. \(\Delta E\) between \(\ce{Al_{III}}\) and \(\ce{Al_{IVa}}\) adsorption small \(\Rightarrow\) still have some \(\ce{Al_{III}}\) sites in S1
- S2: \(\ce{2H2O}\)/unit cell: Three possible configurations of water dissociative adsorption equally probable (same adsorption energy) \(\Rightarrow\) more probable to find \(\ce{Al_{III}}\) sites than for S1, but adsorption of \(\ce{2H2O}\) causes reorganization of surface: \(\ce{Al_{IVb}}\) now tetrahedral and former \(\ce{Al_{III}}\) coordinated by sub-lattice \(\ce{O}\) making it a less LA tetra-coord. \(\ce{Al_{IV}}\)
- S3: \(\ce{3H2O}\)/unit cell: All LA sites must coordinate at least \(\ce{1 H2O}\) \(\Rightarrow\) \(\ce{Al_{III}}\) can no longer exist.

Q: Zeolites applications (6)
A: - Synthetic zeolites for detergents
- Catalysis
- Desiccants/adsorbents/molecular sieves (release \(\ce{H2O}\) upon heating)
- Gas separation
- Ion exchange, purification
- Fuel-cracking of petroleum (high natural occurrence of zeolites)

Q: Aluminosilicates (6)
A: - Layered structures of interconnected \(\ce{Si}\) tetrahedra with tetra- and penta- or hexa-coord. \(\ce{Al}\)
- Structures require cations to compensate for excess charge of \(\ce{Al}\)
- Simple AS: Silinamite, Andalusite, Kyanite (nat. occuring)
- Complex AS with additional ions: Muscovite \((\ce{K+})\), clay materials
- Zeolites: Crystalline AS of only \(\ce{Si}\) tetrahedra and \(\ce{Al}\) sites (high natural abundance)
- Amorphous AS as bulk of surface modified alumina or silica

Q: Zeolite Structure
A: - Microporous 3D crystalline aluminosilicates with high SA (mostly internal in micropores)
- Corner sharing \(\ce{Si}\) and \(\ce{Al}\) tetrahedra
- Negative charge on \(\ce{Al}\) compensated by cations like \(\ce{Li+, Na+, K+, Mg^{2+}, H+}\)
- Löwenstein rule: Never two \(\ce{Al}\) next to each other due to charge repulsion \(\Rightarrow\) max ratio \(\ce{Si:Al} = 1 : 1\)
The structure of zeolites is built from secondary building units and constructed of \(\ce{SiO4}\) and \(\ce{AlO4}\) tetrahedra:

Q: Mesostructured oxides
A: Mesoporous material (silica, alumina, titania, aluminosilicates) with long-range 3D order of porous network with mono-disperse pore size distribution, synthesized by sol-gel in presence of SDA.

Q: Composites
A: Non-homogeneous simple mixture of inorganic and organic parts, no strong interaction (covalent bonds), preparation via sol-gel where inorganic parts condense around organic parts.

Q: Metal organic frameworks MOFs
A: - Hybrid materials, based on inorganic SBUs connected by organic linkers
- Crystalline coordination polymer with defined microporosity (similar to zeolites) and ordered inner surface
- SBUs: often metal clusters with well-defined coordination geometry that add rigidity to network
- Linkers: often carboxylates (balance + charge of SBU)
- Highly charged SBUs and short rigid linkers give more stable structures
- Synthesis: In-situ crystal growth and subsequent solvent removal; Formation of network controlled by slow generation of carboxylate from carboxylic acid via deprotonation by slow diffusion of base into precursors solution
- Most often solvothermal synthesis: Solution of carboxylic acid and metal precursor in DMF heated in autoclave (DMF reacts with residual water to form dimethylamine (base): \(\ce{DMF + H2O -> Me2NH}\))
- Reversible interaction between organic and inorganic parts makes MOFs very dynamic in solution
- Application: Gas storage and separation via interactions (VdW, electrostatic, LA-LB, \(\pi\)-backbonding)
- Covalent organic frameworks (COFs): linked via covalent bonds

Q: Glasses (based on metal oxides) applications (3)
A: - Bottles
- Self-cleaning windows (use titania to generate OH radicals or O^2- superoxide that degrade organic matter)
- Smartphone screens

Q: Glasses (based on chalcogens) application
A: Includes glasses with S, Se, Te
Photocopy machines

Q: Zachariasens Rules for glass formation
A: \[\frac{CN(\ce{M})}{charge(\ce M)} = \frac{CN(\ce O)}{charge(\ce{O})}\]
- \(\ce{O}\) linked to max two other atoms: \(CN(\ce{O}) \leqslant 2\)
- Number of \(\ce{O}\) surrounding a metal must be small: \(CN(\ce{M}) = 3, 4\)
- \(\ce{O}\) polyhedra share corners with each other (no edges or faces)
- For 3D network: at least 3 corners in each \(\ce{O}\) polyhedron must be shared

Q: Glass formers
A: Elements that form glasses without any additives \(\ce{(B, Si, Ge, As, Sb) \Rightarrow (SiO2 , B2O3 , GeO2 , As2O3 , Sb3O5 )}\)
Elements with covalent oxides

Q: Conditional glass formers
A: \(\ce{Al, Ga, Se, Te, Bi}\): only form glasses with other non-glass forming oxides \(\ce{(CaO-Al2O3)}\)

Q: Network modifiers
A: Mono- or divalent cations that do not form glasses at all, used to modify structure of glass (\(\ce{Li+ , Na+ , K+ , Mg^{2+} , Ca^{2+}}\) used to break up structure of \(\ce{SiO2}\) glass to lower m.p. and simplify synthesis), not part of e.g. silicate structure; as some silica \(T_d\) are broken there are anionic terminal \(\ce{O}\) that are no longer \(\mu_2\) and are charge-balanced by a cation.

Glass formers, Conditional glass formers, Network modifiers in PTE

Q: Synthesis of glasses
A: Metastable products: The collapse to the thermodynamic product has to be avoided.
From melt: Cool liquid at high rate so that under-cooled liquid at melting \(T_m\) is obtained instead of crystalline solid. Lowering \(T\) further below the glass transition \(T_g\) transforms liquid into glass. The high viscosity of the glass at low \(T\) ensures that the transition from the glass into a crystalline material is no longer possible (ion mobility too much decreased).
Viscosity is an important parameter in the synthesis of glasses.
At high \(T\), there is high mobility and sufficient energy to make and break bonds. Below \(T_g\), there is no longer sufficient energy to break the covalent bonds of e.g. \(\ce{SiO2}\) and reorganization is not possible, making the glass "stable". Ionic bonds on the other hand can be made and broken easily at lower \(T\) which is why ionic substances do not form glasses.

Q: Vitreous Silica
A: Pure \(\ce{SiO2}\) glass without any additives, high \(T_g\) (1200 °C) \(\Rightarrow\) expensive and energy-intense. Main disadvantage of pure \(\ce{SiO2} \) is that due to the high m.p., one has to work at high \(T\) for fabricating pure \(\ce{SiO2}\) glasses. Therefore one prefers to work with eutectic mixtures that allow working at lower \(T.\)

Q: Vycor glass
A: \(\ce{Na2O (10\%), B2O3 (30\%), SiO2 (60\%)}\)
Cooling melt of this composition separates into \(\ce{SiO2.Na2O}\) phase (96%) and \(\ce{B2O3}\) rich phase which is removed by acid dissolution \(\Rightarrow\) highly porous honeycomb structure that is annealed at \(1000 \, ^{\circ} \mathrm{C}\) to provide non-porous \(\ce{SiO2}\) glass.

Q: Pyrex glass
A: Obtained from \(\ce{Na2O (4\%), B2O3 (16\%), SiO2 (80\%)}\)
Melt phase separates upon cooling as immiscible sodium-borate-rich droplets \((10-20\%)\) are formed and remain trapped into silica glass, providing high durability.

Q: Glass ceramics (Description, Application, Synthesis)
A: - Combine properties of glass (ease of synthesis, synthesize under hermetic sealing) and ceramic (mechanical strength, low or negative thermal expansion coeff.)
- Applications: Mirror, cooktop, cookware, reflectors, telescopes
- Synthesis: Heat treatment (=annealing) of glass leading to homo- or heterogeneous nucleation and growth of small crystals within glassy matrix \((10^{12} - 10^{15} \, \mathrm{crystal nuc./cm^3})\)

Q: Titania: Applications (5)
A: - White paints
- Photocatalysis (environmental de-pollution, water treatments, e.g. self-cleaning windows or at train station)
- Additive (food, toothpaste, cosmetics, sunscreen)
- Ceramics
- Electronic devices

Q: Cement: Definition and examples
A: Substance of \(\ce{SiO2, Al2O3, CaO}\) that sets and hardens due to chemical reactions upon addition of water.
Egyptians: Gypsum \((\ce{CaSO4.H2O})\), Romans: Limestone \((\ce{CaCO3})\), Portland cement (modern cement) \((\ce{Ca2SiO4, Ca3SiO5})\)

Q: Cement: Making of Portland cement
A: 1. Mining of limestone and clay \(\Rightarrow\) large blocks are crushed, grounded and mixed with other minerals, then heated in rotary kiln (= Drehofen) at high \(T (1500 \, \mathrm{^{\circ}C})\) and transformed into small granules called clinker
2. Clinker is grounded with gypsum (to prevent flash setting) to a fine powder = cement
3. Formation under high \(T:\) Heating in rotary kiln at \(1500 \, \mathrm{^{\circ}C}\)
4. Heat induces formation of \(\ce{CO2}\) and solid-state rxns occur due to partial melting
5. Oxidizing atmosphere allows \(\ce{Fe}\) to remain as \(\ce{Fe^{III}}\) \(\Rightarrow\) formation of black lumps (called clinker) that are discharged from bottom of kiln and cooled rapidly by air blast and crushed
6. Gypsum added to prevent flash setting
7. When water (and appropriate aggregate) added: Hydrates to form set cements or concrete
8. Addition of sucrose: Slows down cement setting
9. Problem of cement production: Freeing of \(\ce{CO2}\) due to high energy demand

Q: Titania: Bulk Structure
A: Phases: Anatase, Rutile or \(\ce{TiO2-II}\)
Rutile: tetragonal crystal system, neighboring \(O_h\) share corners along \(⟨110⟩\) direction and are stacked with their long axis alternating by \(90°\)
Anatase: tetragonal crystal system, corner-sharing \(O_h\) form \((001)\) planes and are connected via edges with plane of \(O_h\) below

Q: Critical micelle concentration? How can it be measured? Why is it usually higher for ionic surfactants? What is dynamic light scattering?
A: Concentration of surfactant above which micelles form and all additional surfactants added to the system go to micelles.
Measured by finding sharp discontinuity and concentration dependence in the region of self-aggregation (micellization):
Another method is to employ dynamic light scattering: The intensity of the scattered light increases drastically once the CMC has been reached and micelles have formed.
Micellation of ionic surfactants ultimately leads to charge accumulation and thus generally occurs at higher conc. compared to non-ionic surfactants, where there is no (or at least comparatively little) electrostatic repulsion between the polar head groups of the surfactant.

Q: \(\ce{TiO2}\) homologues (2)
A: - \(\ce{IrO2}:\) Only oxide stable at very low pH to carry out electrooxidation of water to \(\ce{O2}\) (OER)
- \(\ce{RuO2}:\) More active than \(\ce{IrO2}\) but not stable (dissolves)

Q: Water splitting with \(\ce{TiO2}\)
A: \[\ce{2H2O -> 2H2 + O2} \quad \quad \Delta G = +240 \, \mathrm{kJ/mol} \]
- Difficult, as unfavorable thermodynamics and multi-electron oxidation
- Red: \(\ce{2.[2H+ + 2e- -> H2]} \quad E^{\circ} = 0\)
- Ox: \(\ce{2H2O -> 4H+ + 4e- + O2} \quad E^{\circ}=1.23 \, \mathrm{V}\)
- \(\Rightarrow\) requirement: \(\mathrm{CB} < 0 \, \mathrm{V}, \mathrm{VB} > 1.23 \, \mathrm{V}\)
- \(\ce{TiO2}\) as possible photocat: \(\ce{e}\) and \(\ce{h}\) generated upon photoexcitation have sufficient redox potentials to catalyze reduction and oxidation rxn
- Problem: high waste of energy, as energetic position of VB and CB not ideal: huge potential difference between VB \((2.9 \, \mathrm{V})\) and redox potential of OER \((1.23 \, \mathrm{V})\)
- For efficient process: \(\ce{e}\) and \(\ce{h}\) must be separated and diffuse to surface without recombining for rxn to take place \(\Rightarrow\) efficient photocat mus facilitate \(\ce{e/h}\) separation and avoid recombination. These conditions are fulfilled by materials with low number of defects, minimum grain boundaries and minimum diffusion length -> i.e crystalline materials with small particles -> self-excluding
- Parameters to increase efficiency: Increase crystallinity, decrease particle size (both fulfilled for P25 -> Z-scheme), tune BG
- Co-catalysts: Facilitate \(\ce{h/e}\) separation by trapping \(\ce{e}\) with \(\ce{Pt}\) and \(\ce{Au}\) and \(\ce{h}\) with \(\ce{NiO}\) and \(\ce{RuO2}\)

Q: Photoactivity and surface chemistry of titania
A: Absorption of UV photon generates \(\ce{e}\) in CB and \(\ce{h}\) in VB: \(\ce{TiO2 -> e- + h+}.\) The \(\ce{e}\) and \(\ce{h}\) migrate to the surface where they participate in ox and red with surface species \((\ce{H2O, OH}),\) generating highly reactive \(\ce{OH}\)-radicals or super-oxides that further react with adsorbed molecules via radical chain processes (used in de-pollution applications):
\[\begin{aligned} \ce{H2O + h^+_ &-> HO^. + H+} \\ \ce{O2 + e- &-> O2^{-.}} \end{aligned}\]

Q: Tune BG of \(\ce{TiO2}\)
A: Problem with \(\ce{TiO2}:\) Requires UV light for photoactivity \(\Rightarrow\) Lower BG to achieve with VIS light (we want to have a lower BG because we waste a lot of energy if the bandgap is much larger than the 1.23 V potential difference required for the water splitting reaction. Also UV is expensive and using VIS we have "for free"):
1. CB \(\downarrow\): Doping metals, e.g. perovskites \(\ce{SrTiO3, KTiO3}\)
2. VB \(\uparrow\): Doping oxides with \(\ce{N}\) or moving to sulfides or nitrides. Problems: Photooxidation \((\ce{CdS + 2h+ -> Cd^{2+} + S})\) and corrosion \((\ce{ZnO + 2h+ -> Zn^{2+} + 0.5O2})\)
3. Z-scheme: Use two materials with small BGs and electron shuttle between them in Z-scheme (e.g. in P25); need two photons, but they can have lower energy. P25 is a more efficient photocatalyst than pure rutile or anatase as it leads to more reactive holes and electrons. However, the band gap is not modified.

\(\ce{TiO2}:\) Decouple Ox and Red

Q: Surfactant/inorganic self-assembly
A: The SDA and the inorganic precursor self assemble in a cooperative manner. They interact with each other in solution and simultaneously form the superstructure with the lowest free energy.

Q: True liquid-crystal templating
A: The micelles first self-assemble into a superstructure (liquid crystal), then the inorganic-material forms around this superstructure, e.g. via sol-gel condensation. The organic template is removed during calcination, leaving an inorganic material whose pore network has the shape of the organic template.

Q: Nanocasting
A: Soft templating methods are not sufficient for some non-siliceous analogues whose precursors have different properties. In the hard templating approach, one uses a mesostructured silica-material such as SBA-15 or MCM-48 as a template instead of an organic SDA. The precursor is then introduced and the silica template is removed.

Q: Why SBA-15 (from \(\ce{PEO_{20}PPO_{70}PEO_{20}}\), \(\ce{PEO = polyethylene oxide}\), \(\ce{PPO = polypropylene oxide}\)) has parallel pores arranged in a hexagonal network while SBA-16 (from \(\ce{PEO_{106}PPO_{70}PEO_{106}}\)) has a structure of spherical body-centered nanocages in a cubic arrangement.
A: The difference arises from the use of different surfactants in the synthesis. The packing parameter \[g = \frac{V}{a \cdot L}, \begin{cases} g < \frac{1}{3} & \Rightarrow \text{spherical} \\ g > \frac{1}{3} & \Rightarrow \text{cylindrical} \end{cases}\]with \(V\) the volume, \(a\) the area of the hydrophilic part (between 0 and 1) and \(L\) the chain length of the hydrophobic part of the surfactant. As the polymer for SBA-15 has a significantly smaller hydrophilic part, than the one for SBA-16, we found the reason.

Q: In material a) the organic moieties have random spacings while in material b) they obey a monodisperse distribution over the silica surface. Describe a suitable synthesis method to arrive at situations a) and b) and come up with applications for which a monodisperse distribution is important.
A: a) Such a material can be obtained by grafting functionalized organosilane precursors onto oxide supports.
b) Mesoporous hybrid materials are commonly synthesized using sol-gel techniques in the presence of an SDA. When a silicon-precursor that contains a non-polar organic group is added during the synthesis, the organic moiety preferably interacts with the non-polar tails of the surfactant that are located on the inside of the micelles and therefore end up in the pores after calcination of the organic template. As the free energy of the overall assembly of the precursors and micelles is minimized during the synthesis, the organic parts are uniformly distributed.
A uniform distribution is important for various applications such as catalysis but also for DNP applications.

Q: Key aspect that distinguishes a crystal from a glass
A: Absence of long range periodicity and symmetry in the network of a glass.

Q: Difference crystal / glass according to Zachariasens model
A: Using the language of coordination polyhedra, both a crystal and a glass consist of the same oxygen polyhedra, however in crystals, the relative orientation of these polyhedra will be the same throughout the entire lattice, while the relative orientation of the polyhedra varies in a glass.

Q: Based on Zachariasens rules, evaluate whether \(\ce{SiO2}, \ce{MgO}, \ce{Al2O3}\) are glass forming oxides.
A: Zachariasens rules are:
1. The \(CN\) of the cation is small \(CN(\ce{M}) = 3, 4.\) The preferred coordination number of the metal can be estimated by taking the ratio of the radii of \(\ce{M}\) and \(\ce{O}\) (Paulings packing rule).
2. Oxygen atoms are linked to no more than two other metals \(CN(\ce{O}) \leqslant 2.\) The \(CN\) of \(\ce{O}\) can be calculated if the \(CN\) of \(\ce{M}\) is known using \[\frac{charge(\ce{M})}{CN(\ce{M})} = \frac{charge(\ce{O})}{CN(\ce{O})}\]
3. Coordination polyhedra share corners, but no faces or edges.
4. Polyhedra are linked together to form a 3D network.
\(\ce{SiO2}:\) \(\ce{Si}\) prefers tetrahedral bonding, the \(\ce{O}\) \(CN \) is 2, and the crystal structure can be described by corner sharing tetrahedra. All of Zachariasens rules are fulfilled and \(\ce{SiO2}\) may form a glass under appropriate synthesis conditions.
\(\ce{MgO}:\) The \(\ce{Mg-O}\) bond is rather ionic and \(\ce{Mg}\) prefers octahedral bonding \(CN(\ce{Mg}) = 6.\) The \(CN\) of \(\ce{O}\) is 6, as the crystal structure consists of edge-sharing octahedra. \(\ce{MgO}\) does not satisfy Zachariasens rules and therefore does not form glasses.
\(\ce{Al2O3}: \ce{Al}\) is mostly in an octahedral environment \(CN(\ce{Al}) = 6,\) leading to \(CN(\ce{O}) = 4.\) Therefore, \(\ce{Al2O3}\) does not form a glass. However in aluminosilicates, where \(\ce{Al}\) is fourfold coordinated, the \(CN\) of \(\ce{O}\) is reduced to 2, and aluminosilicate glasses are possible.

Q: Predict whether the following compounds are monomers, clusters, infinite chains or 3D networks:
1. \(\ce{Na2Si3O7}\)
2. \(\ce{CaAl2Si2O8}\)
A: 1. \(\ce{M:O} = 1:2.33\) \(\Rightarrow\) the structure is in between infinite sheets and a 3D network.
2. \(\ce{M:O} = 1:2\) \(\Rightarrow\) infinite 3D framework.

Q: Will a \(1:2\) ratio of \(\ce{Na2O}\) and \(\ce{SiO2}\) make a glass?
A: Final stoichiometry \(\ce{Na2Si2O5},\) which corresponds to \(\ce{M:O} =1:2.5\) \(\Rightarrow\) Infinite sheets, not a 3D network.

Q: Main disadvantage of pure \(\ce{SiO2} \) glasses and how to avoid.
A: Due to the high m.p., one has to work at very high \(T.\) This can be circumvented by using eutectic mixtures.

Q: XRD of \(\ce{CaO-Al2O3-SiO2}\) system with anorthite composition \(\ce{CaAl2Si2O8}\) prior to annealing and after heat treatment at two different \(T.\) How does it change upon annealing?
A: During annealing, crystallization occurs and the glassy material transforms into a glass ceramic. After annealing at \(930 \, \mathrm{^{\circ}C},\) impurities of \(\ce{SiO2}\) and \(\ce{CaSiO3}\) are present. When increasing \(T\) to \(1100 \, \mathrm{^{\circ}C},\) these impurities dissolve in the main matrix and a single phase \(\ce{CaAl2Si2O8}\) compound is produced.

Q: Main disadvantage of using oxide additives for nucleation in the synthesis of glass ceramics.
A: Especially when substantial amounts of these additives are required for nucleation to occur, they are integrated as significant impurities in the final material and may alter its properties.

Q: Other Metal Oxides and Materials (4)
A: 1. Perovskites \(\ce{ABX3, (A^{2+}B^{4+}X^{2-}}):\) stability determined by Goldschmidt tolerance factor:\[0.9 \leqslant t = \frac{r_{\ce{A}}+r_{\ce{O}}}{\sqrt{2}(r_{\ce{B}}+r_{\ce{O}})} \leqslant 1\]
2. Pyrochlores: \(\ce{A2B2O7 / A2B2O6X_x}\)
3. Spinel: \(\ce{AB2X4}\) with \(\ce{X=O}\) or \(\ce{S, A^{2+}} T_d\) and \(B^{3+} O_h\) e.g. \(\ce{MgAl2O4}\)
4. Metal chlorides, sulfides, carbides, nitrides, phosphides, borides, ...

Q: Precursor to synthesize \(\ce{TiO2}\) via a hydrolytic sol-gel approach.
A: Alkoxides such as \(\ce{Ti(OEt)4}.\)

Q: Expected condensation mechanism for sol-gel \(\ce{TiO2}\) synthesis.
A: As \(\ce{Ti(IV)}\) is a hard cation, condensation via oxolation is predicted.

Q: Sol-gel synthesis of \(\ce{TiO2}:\) Species present at \(\ce{pH} = 3, 9.\) Partial charges and decision whether they can undergo condensation.
A: Using\[h = \frac{1}{\frac{\chi_{\ce{S}} - \chi^*_{\ce{H}}}{\sqrt{\chi^*_{\ce{H}}}} - k} \left[\frac{\chi_{\ce{S}} - \chi^*_{\ce{M}}}{\sqrt{\chi^*_{\ce{M}}}} + N\left(\frac{\chi_{\ce{S}} - \chi^*_{\ce{M}}}{\sqrt{\chi^*_{\ce{O}}}} + 2 \frac{\chi_{\ce{S}} - \chi^*_{\ce{H}}}{\sqrt{\chi^*_{\ce{H}}}} \right) - k z \right]\]with \(N = 6, z = 4\) and \(\chi^*_{\ce{M}} = \chi^*_{\ce{Ti}} = 1.32,\) we find a hydrolysis ratio of \(h \approx 3.5\) for \(\ce{pH = 3}.\) Therefore \(\ce{Ti(OH)3(OH2)3+}\) and \(\ce{Ti(OH)4(OH2)2}\) are expected to be the dominant species in solution. At \(\ce{pH = 9},\) \(h \approx 4.5\) and therefore we predict \(\ce{Ti(OH)4(OH2)2}\) and \(\ce{Ti(OH)5(OH2)-}\) to be dominant. The partial charges on \(\ce{TiO2}\) and \(\ce{OH}\) are as follows:
All species listed satisfy the empirical criteria for condensation: \(\delta_{\ce{OH}} < 0\) and \(\delta_{\ce{M}} > +0.3.\) Furthermore, at both pH there is a large portion of neutral precursor in the solution and we can therefore expect unlimited condensation and subsequent precipitation to be possible.

Q: Anatase crystals commonly expose \((001)\) and \((101) \) facets. How do they differ in terms of their behavior concerning water adsorption? Can you achieve a complete change in crystal structure just by varying the amount of adsorbed water?
A: While the \((001)\) facets adsorb and split water at a broad \(T\) range, \((101)\) facets readily desorb water above \(450 \, \mathrm{K}\) and water is only adsorbed but not split. However, different from \(\ce{Al2O3},\) the shape of the crystal stays more or less the same independent of the water coverage, as the \((101)\) remains the most stable facet at all times.

Q: Expected relative surface energies for \(\ce{TiO2} \) (Anatase) for \((x, y)\) or \(z\) normal plane.
A: The \(\ce{Ti-O}\) bonds in the \(x\) and \(y\) directions are shorter and higher in \(E\) compared to the \(\ce{Ti-O}\) bonds in \(z\) direction. This is in line with the high reactivity of the \((001)\) facets, which are obtained by cutting parallel to the \(xy\) plane.

Q: Considering the energies of the \((001)\) and \((101)\) facets of anatase surrounded by \(\ce{X}\) atoms, strategy to obtain anatase crystal that predominantly expose \((001)\) facets.
A: Using e.g. \(\ce{HF}\) as reaction medium or other \(F\) containing reagents, crystals that expose \((001)\) facets predominantly can be obtained.
We see that under these conditions, the surface energy \(\gamma\) is smaller for (001) compared to (101).

Q: Main advantage of using microwave-assisted synthesis for \(\ce{TiO2}\) (anatase) and expected change of exposure of the \((001)\) facets by increasing the concentration of ionic liquid.
A: Microwaves can provide rapid and uniform heating of reagents and solvents and therefore accelerate the hydrolysis of the precursors and the nucleation of anatase crystals, leading to a narrow size distribution and high crystalliniity. With an increasing amount of ionic liquid, the \((001)\) facets are exposed to a higher extend.