2.1.4 Preparation of phosphated zirconia
A preparation method similar to that reported by Osatiashtiani et al. [3] was followed for the impregnation of different phosphate loadings onto a zirconia catalyst surface. For this purpose, 5 g of zirconium (IV) hydroxide (MEL Chemicals, MELCat x20 1249/01), used as support, was impregnated with 50 cm3 of the desired molarity (0.01, 0.05, 0.1, 0.25 and 0.5M) of orthophosphoric acid (Fisher, >85%) and stirred for 5 hours at room temperature. Later, it was filtered and dried at 80ºC for 24 hours in a static oven. Catalysts with different acid loadings were divided into equal weights (~ 1.5 g) and subjected to different calcination temperatures (550, 650 and 750ºC) for 3 hours in
static air (ramp rate 5ºC·min-1) to follow any structural changes imparted with varying temperature. Finally, the obtained catalysts were stored at room temperature.
2.2 Catalyst characterization
2.2.1 Inductively coupled plasma – optical emission spectrometry
Solid samples required digestion prior to the ICP analysis, which in this project was performed using a CEM Discover SP-D 10/35 and Explorer 48 Sampler Automated Microwave Synthesis System. ICP measurements to determine the phosphorous content, the bulk concentration of atomic niobium present in the silica supported materials and in the bulk niobic acid samples were performed on a Thermo Scientific iCAP 7000 ICP-OES with charge injection device (CID) detector. A Niobium standard solution for ICP (Sigma Aldrich, 1000 ppm in nitric acid) and a phosphorous standard solution for ICP (Sigma Aldrich, 1000 ppm in nitric acid) were used for calibration. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a characterization technique used for elemental analysis of materials, offering improved sensitivity to low elemental concentrations in the range of ppb (0.2 ppb in the case of niobium) over conventional atomic absorption methods; exceptionally, ultralow concentrations (ppt) can be detected in the case of elements such as magnesium, iron or calcium[4]. However, ICP-OES also has detection limitations, as is the case of halogens, which require high excitation energy, and elements that either are used to produce the plasma torch (carbon, hydrogen, oxygen) or are present in the atmosphere surrounding the torch (nitrogen)[5]. In this technique, argon plasma (atoms in an ionised state) is employed as an excitation source at temperatures up to 8000 ºC. The sample to be analysed is injected in a liquid state into the instrument, and nebulised into small droplets which are transferred to the plasma where they are excited or ionised. The atoms in the sample emit monochromatic light of characteristic wavelength which is detected and quantified in an optical emission spectrometer. After amplification and comparison with calibration standards, calculation of elemental concentration is possible [6].
Typically, 20 mg of catalyst were dissolved in an acid mixture comprised of ammonium fluoride (200 mg), nitric acid (5 cm3), sulphuric acid (2 cm3) and deionised water (4 cm3)[7], and subjected to digestion in the microwave running at 300W for 20 minutes at 220 ºC with intense stirring using quartz tubes as the sample vessel with a PTFE liner. After the digestion period is completed, 1 cm3 of boric acid is added to the solution for neutralisation of formed hydrofluoric acid, and it is stirred in the same vessel for 5
minutes at 150 ºC in the microwave. The sample is diluted in a 1:10 ratio using an aqueous solution of 10% nitric acid, which is used also as mobile phase. Results are compared with a calibration curve acquired using the 309.418 nm signal realized prior to the analysis using the Nb standard solution in order to obtain the metal concentration in the samples (Figure 2.1).
Figure 2.1 Calibration curve for Nb quantification in ICP-OES
2.2.2 X-Ray photoelectron spectroscopy
XPS was performed using a Kratos Axis HSi photoelectron spectrometer equipped with a charge neutraliser, five channeltron detectors and monochromated Al Kα (hν = 1486.6 eV) X-ray source, using pass energy of 40 eV for high resolution and 160 eV for survey scans were employed. Samples were analysed at an operating pressure below 10-9 torr and X-ray emission used was 12 mA and high tension of 13 kV. The software utilized to process the results was CasaXPS Version 2.3.15. Apart from the atoms present in the samples, there is also a strong signal from the carbon 1s state, which is always present due to hydrocarbon contamination of the surface. This peak is used for calibrating the energy scale, which may be shifted due to charging effects; the nominal C 1s binding energy for C-C/C-H is 284.6 eV. A Shirley-type background was used to remove the background from the secondary electrons. Table 2.1 displays a list of the sought elements using XPS and the parameters fixed for the analysis. For the line shape GL (30) refers to a Gaussian-Lorentz 70% Gaussian and 30% Lorentzian, whilst DS indicates a Doniach-Sunjic modified Gaussian-Lorentz line shape, where 0.01 is the asymmetry parameter and 400 the convolution width.
Table 2.1 List of scanned elements in XPS
Element Orbital Start (eV) End (eV) Dwell (ms) Scans Line shape
O 1s 540 520 332 10 GL (30) C 1s 300 220 260 10 GL (30) Nb 3d 212 195 458 30 DS (0.01, 400) GL (30) Cl 2p 212 195 458 30 GL (30) Si 2p 110 90 397 10 GL (30) Zr 3d 190 175 458 10 GL (30) P 2p 138 130 458 30 GL (30)
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive analytical technique that relies on the photoelectric effect, and provides quantitative elemental and chemical information regarding the relative number of atoms of each type present on the surface. The kinetic energy of photo-emitted electrons is also sensitive to the oxidation state and chemical environment of element under investigation. Elements can be identified from the unique binding energy of electrons in each element’s orbitals, which act as a ‘fingerprint’. The reason why only electrons from surface atoms contribute to the photoelectron peak is because those emitted from the bulk are inelastically scattered. Low energy photoelectrons emitted have a short value of inelastic mean free path (IMFP), which is an index of how far an electron on average travels through a solid before losing energy, in the electron kinetic energy range of 10-1300 eV[8]. The IMFP is generally described by the Universal Escape Depth curve, which has a broad minimum at a kinetic energy of around 70-100 eV (Figure 2.2)[9].
Figure 2.2 Calculated IMFPs for a range of elements spanning Li-Bi as a function of
When the sample being analysed is irradiated with monochromatic soft X-rays, i.e. with energies 100 eV - 5 keV (10 - 0.1 nm wavelength), excitation of photoelectrons from the specimen with a kinetic energy characteristic of their initial atomic energy level and element leaves a core hole, which creates an excited ionic state (Figure 2.3)[10, 11]. The kinetic energy of ejected photoelectrons is related to the electron binding energy by Equation 2.3, and it can be measured by XPS to identify the elements present in the sample of study.
The core hole resulting from the photoemission process generally causes a rearrangement of the electronic levels, so that this vacancy can be filled by an electron from a higher energy level, which can lead to two possible final states having different binding energies, known as the spin orbit coupling effect which is manifested in form of a doublet (Equation 2.4)[12]. For example, analysis of the Nb 3d state will result in doublet peaks corresponding to 3d5/2 and 3d3/2 states.
Figure 2.3 Schematic representation of the photoemission process
𝐸𝑘 = ℎ𝜈 − 𝐸𝐵− 𝜙