Where NA is the Avogadro’s number and S is adsorption cross section. Evaluation of
the pore size distribution and mesopore volumes were calculated by means of the Barrett-Joyner-Halenda (BJH) method, which is based on a modification of the Kelvin equation (Equation 2.12), applied to the desorption branch of the isotherm[25]. The model is used to describe condensation of gases within a pore, here it has been used to relate the amount of adsorbate removed from the pores of the material, as the relative pressure (P/P0) is decreased from a high to low value, with the size of the
pores. Cylindrically shaped pores are assumed in this method and the radius of the capillary is estimated as equal to the sum of the Kelvin radius and the thickness of the
adsorbate film on the pore walls. The statistical thickness of the adsorbed layer during multilayer adsorption is determined by means of a t-plot, which may also be utilised for the estimation of micropore (< 2nm), surface area and volumes[26]. This method should be carefully interpreted in the micropore region since the Kelvin equation (basis of BJH) is reliable only for pores >2 nm.
𝑙𝑛
𝑃
𝑃
0=
−2 · 𝛾 · 𝑉
𝑚𝑟
𝑘· 𝑅 · 𝑇
· 𝑐𝑜𝑠𝜃
Equation 2.12 Kelvin equationWhere rk is the Kelvin radius which represents the radius of curvature of a
hemispherical meniscus, γ is surface tension of condensed phase, R is the universal gas constant, T is the temperature, and θ is contact angle of liquid with the pore wall. The method used for micropore analysis was the t-plot method, developed by Lippens and de Boer[27]. In this method, the amount of adsorbate was plotted against the multilayer thickness using the surface area calculated from the BET equation. The external surface area is calculated from the linear part of the t-plot, which takes place before the capillary condensation occurs. Since micropores do not experience multilayer adsorption, the micropore volume can be calculated from subtracting the external surface area from the total surface area obtained from the BET method[25].
2.2.5 Transmission electron microscopy
High resolution TEM images were recorded on a JEOL 2100F FEG STEM operating at 200 keV and equipped with a spherical aberration probe corrector (CEOS GmbH) and a Bruker XFlash 5030 EDX, with analysis carried out at the University of Birmingham. The sample was prepared by supporting a dispersion of the catalyst in ethanol onto a carbon Cu grid coated with a holey carbon support film (Agar Scientific Ltd) and images were analysed using the software ImageJ 1.41.
Transmission electron microscopy (TEM) is an analytical tool allowing high resolution images of specimens in the nanoscale realm. A TEM image is formed when a beam of electrons passes through the material object of study. The beam, of very short wavelength, is emitted from a tungsten filament at the top of a cylindrical column of about 2 m high, being able to achieve a resolution of 0.2 nm and magnifications up to 2.000.000x.
The electron beam produced at the top of the microscope travels through vacuum in the column of the microscope. Along the column, at specific intervals magnetic coils are placed in order to focus the electron beam like glass lenses do in a light microscope. The magnetic coils placed at specific intervals in the column acts as an electromagnetic condenser lens system[28]. Figure 2.6 depicts a schematic view of a TEM showing the different lenses and the pathway of the electrons.
The resultant beam contains some of the original free electrons that have not been changed in velocity or direction and some that have been changed either way or both. Depending on the density of the material present, some of the electrons are scattered and disappear from the beam. Since the electron image cannot be viewed directly by the eye, the image is projected onto a fluorescent screen at the bottom of the microscope, and then transmitted to a photographic plate or film, giving rise to a "shadow image" of the specimen, with its different parts displayed in varied darkness according to their density[29].
Figure 2.6 Schematic view of a TEM [30]
2.2.6 Thermogravimetric analysis
Thermogravimetric analysis (TGA) experiments were performed using a Mettler Toledo TGA/DSC1 Star System with a temperature range from ambient to 1100 ºC, autosampler and a thermostatted balance with fast cooling. Samples (5-7 mg) were
placed into an alumina crucible (70 µl) and placed in the autosampler positions. Nitrogen was used as a purge gas at a flow rate of 60 cm3/min. TGA is performed with a linear rate of 10ºC/min, ranging from 40ºC to 800ºC. This temperature is chosen so that all chemical reactions are completed (i.e., all of the carbon is burnt off leaving behind metal oxides). The sample’s mass was recorded as function of temperature, and analysed after subtraction of the corresponding mass variation of an empty crucible. Data was process using STARe Evaluation Software. TGA-MS analyses were performed in conjunction with a ThermoStar TM GSD 301 T3 mass spectrometer. TGA is an analytical technique used to determine a material’s thermal stability and its fraction of volatile components by monitoring the weight change that occurs as a function of temperature. It consists of a sample pan that is supported by a precision microbalance in a furnace that is heated during the experiment, monitoring its mass. In this thesis, the analysis is normally carried out in a nitrogen atmosphere or in a 20% oxygen in nitrogen atmosphere to simulate air conditions, although it can be performed under many gaseous environments, ideally ‘non-corrosive’, such as argon or helium. TGA in combination with differential scanning calorimetry (DSC) can be used to monitor the energy exchanged between the sample and the a reference holder, providing information regarding thermodynamics of sample decomposition and phase changes[31]. Thermal gravimetric analysis can also be interfaced with a mass spectrometer to measure the vapours generated, thus identifying the volatile components released.
2.2.7 Propylamine
chemisorption
and
temperature
programmed
desorption
Propylamine chemisorption and TPD analyses were performed in the previously mentioned Mettler Toledo used for TGA analysis in conjunction with a ThermoStar TM GSD 301 T3 mass spectrometer. Prior to analysis, the samples (5-7 mg) were impregnated with neat propylamine (Sigma Aldrich, ≥99%) (enough to soak the catalyst) and excess physisorbed propylamine removed in vacuo at 70ºC for 18 h. During the thermogravimetric analysis of propylamine impregnated samples, the temperature was increased up to 800ºC at a rate of 10ºC/min under nitrogen flow, and propene desorption was monitored by mass spectrometry (m/ z = 41). Moreover, mass channels of m/z = 17, 30, 35, 70, 79 and 95 were also controlled to inspect the presence of ammonia, unreacted propylamine, chloride, chlorine, phosphoryl and phosphate respectively. Since the niobium precursor was a chloride compound, chlorine species were investigated to track possible traces remaining from the
synthesis process, which could misleadingly enhance the activity of the catalysts. The loss of phosphate groups was monitored by following the desorption of PO32- and PO42-
. Acid sites densities were obtained after quantification of the propene signal obtained by the mass spectrometer.
Propylamine chemisorption and temperature programmed desorption was used to determine the number and strength of acid sites present in the samples. The method is based on the fact that Brønsted acids sites protonate the propylamine molecule forming propylammonium ion, which is decomposed to ammonia and propene by a reaction similar to the Hofmann-elimination (Equation 2.13)[32]. An unreactive carrier gas (nitrogen in this case) carries the reactively formed propene from the sample to the detector of the mass spectrometer as the temperature increases.
𝐶3𝐻7𝑁𝐻2+ 𝑍𝑂𝐻 → 𝐶3𝐻7𝑁𝐻3++··· 𝑍𝑂− 𝐶3𝐻7𝑁𝐻3++··· 𝑍𝑂−→ 𝐶3𝐻6+ 𝑁𝐻3+ 𝑍𝑂𝐻