Morphology and structure characterization

To investigate the morphology, surface and bulk properties of the materials several techniques were applied and reported in this work. Following, a short description of the methods and related experimental conditions are reported.

- N2-sorption

Surface area and porosity are important parameters for heterogeneous catalysis. Gas adsorption (particularly nitrogen) is one the most common method used for investigation of surface area and porosity of carbon-based materials.

To carry out the N2-sorption measurements a Quantachrome autosorb 3-B instrument was used.

Before the sorption analysis, it is necessary to remove any gas and vapor that may have been adsorbed on the surface in contact with air. Therefore, the sample (known mass, m) was fed into a tube and heated to 200° C where it was held in vacuum of ca. 0.15 mbar for 12 hours for degassing process. Then, the tube was filled with a controlled flow of nitrogen. The nitrogen

adsorption on the surface leads to pressure reduction lower than the pressure of the reference cell connected to the setup.

The contribution of Ioanna Martinaiou, Stephen Paul and W. David Z. Wallace is gratefully acknowledged. [Measurement and data analysis]

Transmission Electron Microscopy (TEM)

This technique involves a high voltage electron beam emitted by a cathode through an ultra-thin specimen. The electron interacts with this specimen when it passes through. The transmitted electrons form an image containing information about the structure of the specimen. Then, the formed image is magnified by a series of lenses and can be recorded by focusing on a fluorescence or CCD camera (light sensitive camera, transferable to a computer).

In order to perform TEM experiments, a small amount of the sample was dispersed in ethanol by placing the solution in the sonication bath for 30 seconds. Then, the suspension was kept for settlement of the particles using a magnet, to remove the large magnetic particles. A drop was cast on holey carbon grid (Plano) and allowed to dry. The grid was coated with carbon (Baltec MED010) to avoid charging under the incident electron beam.

Transmission electron microscopy (TEM) characterization was performed with a FEI CM20STEM (Eindhoven, The Netherlands) microscope equipped with a LaB6 cathode and a Gatan double tilt holder at a nominal acceleration voltage of 200 kV. EDS were recorded using an Oxford X-MAX 80 silicon drift detector (Oxford Instruments Nanoanalysis, High Wycombe, United Kingdom) attached to the CM20. Spectra were quantified using the internal Cliff-Lorime sensitivity factors from Oxford Instruments INCA Ver. 4.15.

The contribution of Markus Kübler and Stefan Lauterbach is gratefully acknowledged.

[Measurement and data analysis]

- X-ray Diffraction (XRD)

XRD is widely used to study the crystal structures and atomic spacing. The X-rays are generated by a cathode ray tube and directed to bombard the sample. Due to the interaction of the incident beam with the sample, a diffracted ray is produced. Then, this diffracted beam is detected for analysis based on Bragg’s law. For the MOF project, measurements were done in transmission with a STADIP (STOE & Cie GmbH, Darmstadt) diffractometer in Debye−Scherrer geometry with a position-sensitive detector using either Mo Kα1 radiation (λ = 0.70930 Å) [Ge(111)

monochromatic] or Cu Kα1 radiation (λ = 1.54056 Å). Samples were prepared on an acetate foil tape that gives low background intensity in the diffractograms. Patterns were collected three times with a step size of 0.5° and a collection time of 30 s step⁻¹ and then overlaid three times with a step size of 0.5° and a collection time of 30 s step⁻¹ and then overlaid.

For the PANI project, XRD measurements were performed using a Bruker D8 Advance in Bragg−Brentano geometry with Cu Kα radiation and a VANTEC detector. Data were recorded in an angular range between 5° and 50° (2θ) for a total measurement time of 1h using a step size of ∼0.007°, a step time of 0.5 step s^-1 and a fixed divergence slit of 0.3°.

The contribution of Stebastian klemenz, Stephanie Dolique and Mohammad Ali Nowroozi is gratefully acknowledged. [Measurement and data analysis]

- Raman Spectroscopy

Raman spectroscopy is a nondestructive technique which involves scattering of electromagnetic radiation by atoms or molecules. Thereof, vibrational, rotational and other low frequency modes of molecules are examined by interaction of the exposed light. During the experiment, the sample is illuminated by a monochromatic light source (e.g. laser). The electric field of the incident light can interact with the sample through the polarization of the molecule and results in molecular vibrations. Therefore, in the case of in elastic scattering, the energy lost or gained after interaction with the molecule can be detected from scattered light. The Raman spectra demonstrates the intensity of the light versus for each energy of incident light.

Raman spectroscopy has become a key technology particularly for the characterization of carbon-based materials. This technique offers detailed information regarding, carbon structural disorder, state of dispersion and orientation.[133] Moreover, the low-wave length analysis of the Raman spectra would provide useful information regarding the existence of transition metals composites (metal oxides/sulfides). Hence, this technique provides crucial information about multi-heteroatom doped carbon-based catalysts (Me-N-C) structure.

In this thesis, an alpha 300R confical Raman microscope from WiTec with a grid of 600 lines mm^-1 was used. A laser with the power of 1 mW with excitation of 532.2 nm was applied to obtain the Raman spectra. The measurement range of spectra was from 0 to 4000 cm^-1. The spectra were obtained by overlapping 10 scans, and with an integration time of 10 seconds per scan. The measurements were repeated for at least 3 positions for each sample and an average of the spectra was reported.

The contribution of Ioanna Martinaiou [measurement and data analysis] and Ling mei Ni [measurement] is gratefully acknowledged.

- X-ray Photoelectron Spectroscopy (XPS)

Photoelectron spectroscopy (PES) is a surface-sensitive technique that irradiates a solid with photons and analyzes the energy of emitted electrons, based on the photoelectric effect. With respect to the photon energy range provided by the source, different experiments and analyses could be performed. Figure 3.5 illustrates different types of experiments involved in PES. In order to study the core level electron, the X-ray regime of 100 eV to > 1000 eV is required.

Irradiation by an X-ray beam energy larger than the core level electron binding energy, results in escaping of the electron and emitting out of the surface.

Figure 3.5 Sketch of PES experiments

Therefore, the emitted electrons have measured kinetic energy (Ek) given by:

𝐸_𝑘 = ℎ𝑣 − 𝐵𝐸 − ∅_𝑠

Where hv is the incident photon energy, BE is the binding energy of the electrons (generally in solids referred to the Fermi level) and ∅_𝑠 is the spectrometer work function. Figure 3.6 shows the energy-level diagram in a metal, where the Fermi level is higher than the valence band. This means that the photoelectron can be detected with kinetic energy (EK) in the vacuum if the absorption is happening in the core level with binding energy of BE. It should be noted that the term ∅_𝑠 related to the work function can be determined experimentally. In this way, the Fermi level of a clean metal surface would be measured and applied for future calibration. The emitted electrons are detected by an electron spectrometer, based upon their kinetic energy and sent to the analyzer. Thereof, scanning for different energies is obtained by applying a retardation potential (0 to > hv) before the analyzer. Then, the number of electrons for a given detection time and energy is counted and displayed as a function of the binding energy.

Figure 3.6 Schematic illustration of energy levels in a solid (metal) in the presence of incident photon XPS is a popular technique to study the chemical and physical changes applied for heterogeneous catalysts in which the active sites are located on the surface. Particularly for Me-N-Cs with a complex structure, the surface analysis is remarkably informative in order to understand the elemental compositions, local environmental changes of an atom.

Me-N-Cs are the heat treated analogous to MeN4 macrocycles in which it was reported that the N 1s region analysis provides more useful information regarding charge distribution than Me 2p region. In the previous studies, no change in the metal ion binding energy with small change in the charge distribution between the metal orbitals and macrocycle was detected. Hence N 1s binding energies are more sensitive to the local changes, since nitrogen is the link between ∂-bonding orbitals of metal-nitrogen and 𝜋-∂-bonding orbitals of carbon-nitrogen.[38] Therefore, the N 1s analysis were used to study the local changes on metal atoms via Me-N binding energies in this thesis. The schematic illustration of suggested nitrogen coordination for Me-N-C catalysts are reported in Table 7. Moreover, an exemplary N 1s spectrum of Co-N-C (Cat. A chapter 4.2.1) is depicted in Figure 3.7.

Figure 3.7. It demonstrates different proposed nitrogen species present in Me-N-C catalysts.

Table 7 the model constrains for N 1s analysis used in this work adapted from Jaouen et al. [39]

Figure 3.7 Examplary deconvolution of N 1s spectrum of Co-N-C catalyst with the suggested model

Moreover, the model constrains used for the analysis of O 1s and S 2p spectra with an exemplary plot (Cat. A chapter 4.2.1) are depicted in this part.

Table 8 The model constrains for O 1s analysis used in this work based on Lindberg et al. [134]

N 1s Pyridinic Me-N Pyrrolic Graphitic oxidized

No. I II III IV, V VI

Binding energy 398 – 399.5 399 – 400.5 400.2 – 400.9 401 - 403 402 - 405

FWHM 1.1-1.3 1-1.1 1-1.1 G2 (1-1.1)

G3 (1-2.5) 1-4

O 1s Me-O Me-OH C-O C=O / S=O

No. I II III IV

Binding energy 529.5-530.1 531-532 532-533 533-534

FWHM 1-1.2 1-1.2 1-1.2 1-2

Figure 3.8 Examplary deconvolution of O 1s spectrum of Co-N-C catalyst with the suggested model

Table 9 The model constrains for O 1s analysis used in this work based on Biesinger and Alstrup et al.

[135, 136]

Figure 3.9 Examplary deconvolution of S 2p spectrum of Co-N-C catalyst with the suggested model

In this work, the measurements were performed with a Specs Phoibos 150 hemispherical analyzer and a Specs XR50M Al Kα X-ray source (E = 1486.7 eV) at Daisy Fun belonged to Prof.

Jeagermann’s group. For sample preparation, the catalyst powder was squeezed on an indium foil and mounted on the sample holder. For all the survey scans and the high-resolution scans,

S 2p

∆ =1.18 eV MeSx S-C S-O-C S-O

No. I II III IV

Binding energy 161.9-162.7 163.6-163.7 165.7-165.8 168.2-168.3

FWHM 1-1.2 1-1.2 1-1.2 1-2

in order to analyze the data. A Shirley background was used, and fits were made allowing for both Gaussian and Lorentzian (GL30) contributions to the peaks.

Table 10 XPS experimental parameters for MOF-project

region Survey C 1s N 1s O 1s Me 2p Zn 2p

No. scans 2 10 100 10 5 5

Table 11 XPS experimental parameters for PANI-project

region Survey C 1s N 1s O 1s Me 2p S 2p

No. scans 2 15 50 15 15 20

The partial contribution of Natascha Weidler [Data analysis] is gratefully acknowledged. The possibility to do X-ray-induced photoelectron spectroscopy at the DAISY-FUN system of Wolfram Jaegermann’s group at TU Darmstadt is gratefully acknowledged.