Chapter 2 Experimental
2.2 Catalyst characterisation
38 metal nitrate solution was added drop-wise to the phosphate solution over a period of 1 hour with constant and vigorous stirring at room temperature. The pH was maintained at 11.0. A brown gelatinous precipitate that formed was heated to and maintained at 80 °C for 1.5 hours to ensure removal of NH4+ in solution. The precipitate was then filtered under vacuum and washed thoroughly to ensure the removal of excess unreacted precursor ions. The precipitate was then dried overnight in an oven at 120 °C and then powdered. The powdered catalyst material was calcined at a temperature of 550 °C at a ramp rate of 1.5 °C min-1 for 6 hours.
The 3 and 9 wt% iron substituted Hap catalysts were prepared in a similar manner.
39 TCU 750 temperature control unit. Measurements were taken over the temperature range of 100 – 750 °C at 50 °C increments under reducing and oxidising conditions. For reduction, a gaseous mixture of 5 % H2 in N2 was used and oxidation was carried out by flowing air through the reaction chamber.
2.2.3 Fourier transform infrared spectroscopy
Analyses were carried out at room temperature using a Perkin Elmer Spectrum 100 FTIR spectrometer fitted with a Universal Attenuated Total Reflectance (ATR) sampling accessory.
The data was processed using Spectrum® software. A small amount of the powdered catalyst was placed onto the ATR crystal and a pressure of 120 psi was applied to ensure contact between the crystal and the catalyst material.
2.2.4 Raman spectroscopy
Raman spectra were measured using a Delta Nu Advantage 532 instrument fitted with a 532 nm laser source (green) and operated by NuSpec® software. Laser intensity and the integration time of the scans were varied until each sample gave clear and reproducible spectra. All analyses were carried out at room temperature with the powdered sample loaded in quartz tubes.
2.2.5 Nitrogen physisorption analysis
Surface area, pore volume and N2 adsorption-desorption measurements were obtained by N2
physisorption isotherms at 77 K using the Micromeritics Tristar II Surface Area and Porosity Analyser. The powdered catalysts were degased at 200 °C under a flow of N2 for 16 hours prior to analysis using a Micromeritics Flow Prep 060 Instrument. Approximately 200 mg of powdered sample was measured into the sample holder. Analyses were carried out in the presence of liquid N2 at 153 K, over the pressure range, p/po = 30 – 790 mmHg.
40 2.2.6 Electron microscopy
Transmission electron microscopy
Transmission electron microscopy (TEM) was performed on a JEOL JEM 1010 Transmission Electron Microscope operated at a voltage of 100 μm. A small quantity of the powdered catalyst material was dispersed in a 50 % acetone in water solution by sonication for 2 minutes. Copper grids were immersed in these solutions and after drying were viewed under the microscope.
Scanning electron microscopy and energy dispersive X-ray spectroscopy
Surface morphology was investigated by this technique. Scanning electron microscopy (SEM) sample preparation involved the mounting of a powdered catalyst on an aluminium stub with the aid of double sided carbon tape. The samples were sputter coated with Au-Pd and transferred into the Zeiss ULTRA 55 (field emission tungsten hairpin filament with a ZrO reserve as an electron source) and secondary electron imaging was performed.
Sample preparation for energy dispersive X-ray spectroscopy (EDX) mapping involved the mixing of a small amount of the sample with resin in a plastic mould. After the curing period, the sample block was removed and the surface where the catalyst particles had settled was polished with diamond paste to obtain a smooth and flat surface. The polished sample was then coated with carbon and transferred directly into the microscope for analysis. The Zeiss EVO 40 (fitted with a tungsten filament) was used for the back scattered electron (BSE) imaging and EDX analysis. EDX data was interpreted using Bruker Esprit software. The EDX analysis was performed at accelerated voltages of 20 kV, a take-off angle of 35.0 ° and a sample tilt of 0.0 °. A working distance of 14 mm was used so that the EDX detector was directly in line with the point on the sample where the electron beam made contact.
41 2.2.7 Temperature programmed techniques
Temperature programmed techniques were performed using a Micromeritics 2920 Autochem II Chemisorption Analyser.
Temperature programmed reduction and oxidation
For the temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) experiments, approximately 50 mg of powdered sample was loaded into a quartz u- shaped reactor tube, sandwiched between two layers of quartz wool. Sample preparation involved heating the catalyst to 400 °C under a flow of Ar, followed by gradual cooling to 80
°C. For TPR analysis, the procedure involved the gradual heating of the sample to 950 °C at a rate of 10 °C min-1 under a steady flow of 5 % H2 in Ar. The reduced sample was cooled under a flow of Ar to 80 °C and then subjected to a TPO analysis with the same temperature program but under a steady flow of air. After the TPO analysis the sample was cooled to 80
°C under a flow of Ar and then subjected to a TPR analysis if required.
Temperature programmed desorption
CO2 temperature programmed desorption (CO2-TPD) experiments used approximately 30 mg of powdered sample. The samples were pre-treated at 350 °C under a flow of He for 1 hour.
The sample was then allowed to cool down to 80 °C. Thereafter a mixture of 5 % CO2 in He was passed over the catalyst at a flow rate of 30 mL min-1 for 1 hour. The excess CO2 was removed by purging with He for 30 minutes. The temperature was then gradually raised to 950 °C by ramping at 10 °C min-1 under a flow of helium and desorption data was recorded.