2.2 Temperature programmed desorption
2.2.1 TPD–MS
A previously described3 homebuilt TPD rig coupled to a mass spectrometer was used to
investigate the desorption properties of samples. The sample was heated under a continuous argon gas flow. The outlet flow of the apparatus can be connected to a mass spectrometer in order to directly analyse the desorption products.
A sample of mass approximately 0.15g was prepared inside an argon-filled glove box and loaded into a quartz reaction tube (7 mm O/D, 4 mm I/D) which was sealed at one end. The quartz tube was then loaded and sealed inside a steel reaction chamber incorporating a thermocouple, before being removed from the glove box and placed into the TPD equipment. The TPD rig had previously been repeatedly evacuated and refilled with argon.
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An argon flow of 100 ml min−1 was established and regulated using a mass flow controller
(Hastings 200 Series, Teledyne). A barrel heater was located around the reaction vessel and a thermocouple which was in contact with the sample was used to monitor the temperature, allowing the observation of any exo- or endo-thermic events. Evolved gases were monitored using a quadrupole mass spectrometer (HPR-20, Hiden Analytical) fitted with a Faraday cup (m/z >20) and secondary electron multiplier for lighter ions (m/z < 20).
2.2.2 Calibration
As NH3 and H2 levels were of specific interest in these experiments, the partial pressure of the NH2•+ fragment was also monitored. As the NH
3•+ fragment has the same m/z ratio as OH•+
which can exaggerate readings of ammonia levels even in a high vacuum system. The NH2•+
fragment has an intensity of ~80% of the NH3•+ fragment and thus allows the determination of the
correct levels of ammonia released. Carrying out calibration with standardised amounts of NH3 and H2 in the argon flow allows the exact ratio of detected gases to be ascertained.
To determine the sensitivity of the mass spectrometer to these gases, a standardised calibration gas (BOC Speciality Gases, 4736 ppm H2, 4898 ppm NH3, balance Ar) was used. The calibration gas was flowed through the TPD rig in the same way as the argon carrier gas used in
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experiments, but at a flow rate of 65 ml min–1. Partial pressures of mass channels (m/z) of 2 (H
2•+),
16 (NH2•+), 17 (NH
3•+/OH•+), 28 (N2•+), 32(O2•+) and 40 (Ar•+) were monitored until a consistent
signal was achieved. A comparable data set was collected for the argon carrier gas, to allow any necessary background corrections to be made, accounting for residual H2 or NH3 in the argon supply before signal calibration.
To determine relative sensitivity values (Equation 2-2) for the mass spectrometer, the partial pressure channels of interest (m/z = 2 and 16), H2+ (𝑃𝐻2) and NH2+ (𝑃𝑁𝐻2) were first converted to fractions (𝑥𝐻2 and 𝑥𝑁𝐻2respectively) of the observed argon signal (𝑃𝐴𝑟).
𝑥𝐻2 =
𝑃𝐻2
𝑃𝐴𝑟 𝑥𝑁𝐻2=
𝑃𝑁𝐻2
𝑃𝐴𝑟 Equation 2-1
The background fractional amounts of H2+ (𝑥𝑏 𝐻2) and NH2+ (𝑥𝑏 𝑁𝐻2) in the carrier gas
normally used (Ar) were subtracted from the fractional signals observed for H2+ (𝑥𝑐 𝐻2) and NH2+
(𝑥𝑐 𝑁𝐻2) in the calibration gas (473.6ppm and 498.8ppm, respectively). The relative sensitivity
(𝑅𝐻2and 𝑅𝑁𝐻2) of each was found by dividing the background corrected fractional signals by the defined molar fraction of both in the calibration gas. By dividing by these values, the true values of hydrogen and ammonia released by the sample can be calculated.
R
H2=
xc H2−xb H24.736 ×10−3
R
NH2=
xc NH2−xb NH2
4.898 ×10−3 Equation 2-2
When experimental data were collected in TPD–MS experiments a background correction was also required to produce accurate results. Data were collected before the start of each heating regime until the background signals of H2+ (𝑥𝑏 𝐻2) and NH2+ (𝑥𝑏 𝑁𝐻2) stabilised to give a
background value. This value was deducted from the fractional signal amounts (
𝑥
𝐻2 and𝑥
𝑁𝐻2)
observed in the experiment and divided by the relative sensitivity values giving corrected molar fractions for the two channels of interest, (
𝑥
∗𝐻2 and
𝑥
∗31
x
∗H2=
xH2−xb H2 RH2x
∗ NH2=
xNH2−xb NH2 RNH2 Equation 2-32.2.3 TGA–MS
A Thermogravimetic analyser (TGA, Netzsch 209 TGA) consists of a thermo-microbalance which measures mass change as a function of time or temperature under an inert controlled atmosphere. The temperature of the furnace is “sample-controlled” which allows for more accurate temperature logging. The gas flows vertically from the bottom to the top of the “hot-zone” where the sample is placed, carrying any desorbed gases to the outlet. The gas outlet is connected to a mass spectrometer (Hiden Analytical, HPR20) providing gas desorption data about the sample in question (TGA–MS).
Calibration of the TGA was carried out by measuring the melting points of metal standards before baseline measurements were collected. Melting points of several metals were determined across the temperature range required for these experiments and compared to literature values, providing a multi-point calibration of measured vs. expected temperatures. Recording baseline measurements is essential to ensure the accuracy of TGA results. Baseline measurements were recorded for all heating processes using an empty alumina crucible to account for buoyancy phenomenon, and are automatically subtracted from any data produced. Several factors such as gas flow drag and velocity effects, air buoyancy and temperature gradients contribute to a buoyancy effect, where the empty crucible and lid appear to gain mass under heating. The TGA was located within a flowing argon glove box and approximately 15 mg of sample was loaded into an alumina crucible and placed on a thermo-microbalance and top-loaded vertically into the TGA. The samples were heated at 1, 2, 5, 10 and 15°C min−1 to 400°C under 100 ml min−1 argon and
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2.3 Mass spectrometry
Mass spectrometry is an analytical method which measures quantitatively the mass to charge ratios (m/z) of various charged gaseous ions. A mass spectrometer can scan across a range of undefined m/z values or be pre-set to select specific m/z values, to determine the amounts of each. The m/z values which of specific interest to hydrogen storage work are 2 (H2•+), 17 (NH
3•+)
and 16(NH2•+). The mass spectrometer was used in multiple ion detection (MID) mode to
maximise the amount of data collected. The analyte entered the mass spectrometer in an argon carrier gas through a heated capillary with a small diameter to restrict flow rate. This is necessary to maintain a low-pressure vacuum to avoid ion collisions inside the spectrometer. The three main components (processes) in a mass spectrometer are an ion source (ionisation), mass analyser (separation) and a detector (detection).
2.3.1 Ionisation
Samples must be ionised or charged prior to analysis. The method of ionisation usually depends on physico-chemical properties of the analyte and if ions of the molecular species or fragments are required. As analysis in this work was focused on the gas phase, only electron ionisation (Electron impact) was suitable. This technique frequently induces extensive fragmentation. In this source, a heated filament produces electrons by thermionic emission that are accelerated, and via energy transfer, an electron can be expelled from the gaseous analyte to create a singly charged molecule. By tuning the energy of the electrons, multiple ionisations can mostly be avoided. As the physical property, the mass-to-charge ratio (m/z), of an ion is measured, not the mass, and so the presence of singly charged species is important. Positively charged ions are then accelerated by a repeller electrode and focussed towards the mass analyser. In this work the fragmentation of NH3 into NH2+ was taken into account by scanning both (m/z) = 17 and (m/z)
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2.3.2 Separation
In this work a quadrupole mass analyser was used. A quadrupole mass analyser works on the principle of trajectory stability in an oscillating electric field to separate cations according to the m/z values. A quadrupole analyser consists of four parallel rods, ideally hyperbolic in shape, where each opposing pair is electrically connected. By oscillating the potentials, the cation moves towards a negatively charged rod and as the potential switches the ion changes direction. Specific combinations of potentials and frequencies will produce a stable trajectory for certain ions (with a specific (m/z) ratio) to reach the detector (Figure 2-2). By varying the oscillating radio field, a range of preselected m/z values can be measured.
2.3.3 Detection
The detector transforms the number of incident ions into an electric current proportional to the abundance of the (m/z) ratio. In a Faraday cup detector, a current is produced through electron transfer when an ion is neutralised on the surface. The current produced is proportional to the number of ions detected for each (m/z) value. This method of detection has low sensitivity, so a secondary electron multiplier (EM) may also be used. Positively charged ions are accelerated into a negatively charged conversion dynode at high potential, causing several negative secondary particles (e.g. electrons) to be released. By amplification of electrons through additional discrete or continuous dynodes, a cascade of electrons is created and the resulting current is measured.
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However, the accuracy of an EM is affected by the nature (impact velocity, mass etc.) of the ions, so the precision is not as high when compared to ion detection in Faraday cups.
2.4 Hydrogenation
A crude hydrogenation method was used to determine the potential reversibility of a sample. By leaving a sample under a high hydrogen pressure and heating to a fixed temperature for between 12 and 24 hours it could usually be seen whether hydrogenation was possible. Ideally once it had been determined that a sample had the capacity to absorb hydrogen, refined hydrogenation experiments using a manometric gas sorption analyser would have been carried out. This would have allowed hydrogen absorption to be measured quantitatively as a function of pressure and/or temperature. Using a range of temperatures, a series of pressure-composition- isotherms (PCI’s) could have been recorded to form a pressure-composition-temperature (PCT) diagram. Unfortunately, due to lack of sufficient equipment time, although this work was planned, it was not possible to carry out these measurements.
2.4.1 Crude hydrogenation
A previously described3 homebuilt stainless steel hydrogenation reactor vessel and gas
manifold set-up were used in order to study the rough hydrogenation properties of samples. Samples of ~0.15g were prepared and loaded in quartz reaction tubes (7 mm O/D, 4 mm I/D) and sealed at one end. The quartz tubes were loaded into the bottom of the hydrogenation vessel, sealed up to 60 Nm2 in the glove box and further sealed up to 90 Nm2 on removal, using an
adjustable torque wrench. The reactor was placed in a vertical tube furnace, then exposed to an argon-filled manifold system. The whole system (manifold and vessel) was evacuated and purged with argon several times before undergoing the same routine with low pressure hydrogen. A pressure release valve was used to vent any excess pressure build up.
A hydrogen pressure of around 80 bar was then set to allow for pressure build up due to increased temperature. As the pressure gauge was on the gas manifold system, whilst heating took
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place the vessel and manifold were sealed together to allow monitoring of the hydrogen pressure. Leak checks on the whole system were carried out at different stages in the process using hydrogen monitors. On completion of the heating regime, the hydrogen pressure was vented and the system evacuated and purged again to refill it with argon before returning it to the glove box.
2.5 Raman spectroscopy
Raman spectroscopy is a technique used to measure vibrational, rotational and other low frequency phonon modes of atoms, bonds and molecules in crystals. Raman spectroscopy plots the intensity of photon scattering as a function of wavenumber. For a frequency mode to be Raman active, the thermal motion must be accompanied by a change in the direction or degree of polarisability of the molecule. Symmetric stretches are typically accompanied by the biggest change in polarisability, giving rise to the most intense bands in the spectrum whilst asymmetric movements are weaker. Whether a vibration is Raman active is governed by group theory, based on the molecules’ symmetry.
Inelastic or Raman scattering of monochromatic light (with a wavelength within infrared, visible or ultraviolet) must be observed upon irradiation of the sample to observe a Raman spectrum. Inelastic scattering occurs when the molecule is shifted by one vibrational energy unit, as depicted in Figure 2-3. When the sample is irradiated the energy from the photon is absorbed
Virtual energy states Vibrational energy states Rayleigh
scattering scattering Stokes Anti- Stokes scattering
Figure 2-3: Schematic diagram of Raman and Rayleigh scattering showing the states and transitions involved.
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by the molecule, causing a polarisation of the electron cloud around the nuclei and this forms a higher (virtual) energy state. The energy of this is dependent on the wavelength of the incident radiation. Relaxation of the molecule from the unstable short lived energy state back to the vibrational energy states is accompanied by the emission of a photon. For the majority of transitions, the energy of the emitted and absorbed photons is very similar as scattering by electrons is very slight, which leads to elastic or Rayleigh scattering.
When the energy of the emitted photon is different to that of the absorbed photon, caused by inelastic interactions, this is either Stokes or Anti-Stokes scattering. When inelastic scattering is observed, induced nuclear motion may transfer some energy to the photon from the molecule or vice-versa. This can result in the molecule returning to an excited state (Stokes), or returning to the ground state from a previously excited state (Anti-Stokes). The occurrence of spontaneous Raman scattering is often very weak, around 1 in 106-108 photons. The observation of Stokes
scattering will be dominant due to the nature of conducting experiments at close to or at room temperature, where most molecules reside in the ground vibrational state. The exchange of energy during these scattering events is depicted in Figure 2-3.
Ex-situ Raman spectra were collected using a Renishaw InVia Raman microscope using a Helium-Neon 633nm laser coupled to a grating with 1200 lines/mm. Samples were loaded and sealed into a THMS 600 cell inside an argon filled glove box before being transferred to the Raman microscope. In-situ spectra were collected using an Argon 488nm laser and an Instec HCS621V cell under 1 bar flowing argon, heated at 2 °C min-1. Auto-focus was completed after every 10
spectra, which were recorded directly after one another.