Chapter 3: Experimental Methods
3.3 Catalysts Characterization
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sorption analysis are illustrated as an adsorption-desorption isotherm, which is a plot showing the quantity of molecules adsorbed in relation to relative pressure. In this research, surface area and porosity were measured on a TriStar II volumetric gas sorption instrument from Micrometrics using N2 sorption at 77 K. Before each
measurement, samples were degassed under vacuum at 200°C for 2 hrs. Surface areas were calculated using Brunauer-Emmett-Teller (BET) theory and pore size distributions were calculated using Barrett-Joyner-Halenda (BJH) model.
3.3.3 TEM Analysis
Transmission electron microscopy (TEM) is frequently utilized in characterizing the morphology, the particle sizes, and the structural phases of catalysts. A high-energy electron beam is used to blast the catalyst. Information on the phase type, particle size, and uniformity of the metal distribution is obtained through the electronic beam
interaction with the sample's crystalline lattice. In this research, transmission electron microscopy (TEM) images were generated by utilizing a JEOL-JEM 2100 electron microscope operated at an acceleration voltage of 200 kV.
3.3.4 Elemental Analysis
The elemental composition of materials can be determined using non-destructive
analytical method known as XRF (X-ray fluorescence). In order to assess the chemistry of a sample, XRF analyzers measure the fluorescence (or secondary) X-ray that the sample emits after being excited by a primary X-ray source. Elemental analysis of the catalysts in this work was obtained from wavelength dispersive X-ray fluorescence (WDXRF) measurements conducted on a Rigaku-ZSX Primus IV instrument.
3.3.5 XPS Analysis
X-ray photoelectron spectroscopy (XPS) is widely used to analyze the surface and few nm near the surface of solid substances. It is a renowned technique for its exceptional sensitivity to elements as well as their valence states. In addition to identifying the elemental binding states, X-ray photoelectron spectroscopy (XPS) offers a quantitative method for analyzing the surface elemental composition of a material. In this research,
23 X-ray photoelectron spectroscopy (XPS) was performed on a PHI VersaProbe III
spectrophotometer working under ultra-high vacuum.
3.3.6 H2-Temperature Programmed Reduction
H2-temperature programmed reduction (H2-TPR) is an efficient method for determining the reducibility of the oxided of the active metal on the surface of the support. It allows determining the temperature range at which the reduction takes place as well as the degree or the extent of reduction. Additionally, it provides information about how strongly the active phase interacts with the support. A metal oxide with poor metal- support interaction will be reduced under relatively low temperatures, while a species with a strong metal-support interaction will reduce under high temperatures.
In this research, H2-TPR studies were conducted on a Quantachrome ChemBET- TPR/TPD instrument equipped with a thermal conductivity detector (TCD) for the measurement of H2 uptake. A U-shaped quartz tube reactor was used where 50 mg of the sample under study was packed between two plugs of quartz wool. Before the reduction process, each sample was degassed in-situ at 350°C under a flow of N2 at 30 mL/min for 1 hr. Samples were then cooled to 25°C before allowing a flow of H2 (5%) in N2 at a rate of 50 mL/min while heating to 950°C at a rate of 10 deg./min. The H2 consumed
throughout the heating process was evaluated by the TCD detector which was pre- calibrated for H2.
3.3.7 H2 Pulse Chemisorption
The measurement of the active metal surface area, metal dispersion, and crystallite size are possible using the CO chemisorption technique. The ratio of total metallic atoms on the surface of the active metal particles accessible to the adsorbate species to the total atoms of the active metal is known as metal dispersion. This is measured by measuring the ratio between the total number of H2 molecules adsorbed and the total number of atoms of in the metal particles in the sample. Since H2 molecules only bind to the atoms of active metals, the quantity of molecules that have been adsorbed can be used to calculate the metallic surface area (m2/g metal) and the size of the active metal crystallite.
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In this research, Chemisorption of H2 was studied using a pulse titration technique on the same instrument used in H2-TPR. A quartz tube reactor was used where 50 mg of the sample in study was packed between two quartz wool plugs. The sample was degassed for 1 hr at 700°C under N2 flow at a rate of 50 mL/min before reduction at the same temperature for 2 hrs using H2 (5%) in N2 at a flow rate of 50 mL/min. The sample was then purged with N2 at 700°C for 20 min before cooling to 25°C under N2 flow. Pulse titration with pure H2 was then conducted using an inline sampling valve with 1mL sample loop. The amount of H2 chemisorbed on the surface of the metal particles was quantified by the signal of the TCD detector.
3.3.8 DRIFTS Analysis
In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is a very useful technique for studying functional groups and adsorbed species on the surface of solid catalysts. The technique is based on radiation scattering within the powder sample in study. When light strikes a sample, it may reflect only once (specular reflectance) or it may reflect several times, creating diffusely scattered light that covers a large region and is used in DRIFTS analyses.
In this research, In-situ DRIFT spectra were recorded during heating the sample under study at different temperatures using a Shimadzu IR spectrometer (Affinity-1) and a diffuse reflectance accessory from Pike Technology. The DRIFTS accessory included a ceramic sample holder placed in an electric furnace equipped with a temperature
controller. The sample holder and the furnace were housed in a sealed cell equipped with ZnSe window and a gas inlet and outlet allowing for in-situ analysis. Grinded powder samples (~200 mg, 2% in KBr) were used, and a prerecorded spectrum of KBr pretreated at 500°C under N2 (30 mL/min) was subtracted as a background from the samples’
spectra which were recorded collecting 64 scans at a 2 cm-1 resolution.
3.3.9 CO2 Temperature Programmed Desorption, CO2-TDP
The study of many chemical characteristics of solid surfaces, includes the acidic and the basic properties, is made possible by temperature programmed desorption (TPD) of probe molecules. The adsorbed molecules' desorption profiles provide details on the degree of adsorption and the degree of surface binding. The number of desorbed
25 molecules is frequently determined using a thermal conductivity detector. The surface basic sites are characterized using CO2 as a probe molecule. Because CO2 molecules are tiny, they can enter the sample's pores. After the solid sample has been completely saturated with the adsorbate molecules at room temperature, the amount of the desorbed gas is measured as a function of temperature. In CO2-TPD, desorption peaks at low temperatures correlate to weak basic sites, whereas those at high temperatures are associated with strong sites.
In this research, CO2-TPD was studied on the same chemisorption instrument used in the H2-TPR study. In each experiment, 120 mg of a pre-reduced sample was packed between quartz wool plugs in a U-shaped quartz tube reactor and was degassed under He flow (50 mL/min) for 1 hr at 350°C. Reduction of the sample was conducted again in situ at
750°C for 1 hr using a flow of H2 (5%) in N2 at a rate of 50 mL/min. After cooling to 30°C, CO2 was allowed to flow through the sample for 20 min at a rate of 40 mL/min followed by purging at the same temperature for 30 min before ramping to 850°C at a rate of 10 deg./min under a flow of He at 50 mL/min. The desorbed CO2 during the heating process was quantified using the TCD signal.
3.3.10 Thermogravimetric Analysis
Thermogravimetric analysis (TGA) is a technique that is based on measuring the change in the mass of a sample is as the temperature changes over time. It is a supplementary method used in conjunction with other characterization techniques to confirm a certain material's composition. It is used to describe composite materials, polymers, glasses, and ceramics. Additionally, it can be used to calculate the amount of water or remaining substance in a material by using evaporation and thermal degradation in air. Because there is a significant association between phase composition and heat stability, thermogravimetric investigation can support the results obtained by XRD about the phase compositions. The amount of weight loss during the heating process reveals details about the catalyst's temperature stability and phase composition. The technique of
preparation, the temperature of the calcination, and the interactions between the metal and the support often control these catalyst characteristics.
In this research, the amount of carbon deposits on the spent catalysts was measured by TGA using a TGA instrument from Mettler Toledo, model TGA-2 Star System. Around
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12 mg of each sample was equilibrated at 25°C for 5 min before heating to 950°C at a heating rate of 10°C/min under N2 flow of 50 mL/min. The mass loss was measured and reported versus temperature.
3.3.11 Raman Spectra
Raman spectroscopy is one of the effective vibrational spectroscopic techniques used for identifying chemical species. Information about both inorganic and organic chemical species can be obtained using Raman spectroscopy. One of its applications in catalysis related to methane reforming is the identification of carbon on spent catalysts.
In this research, Raman spectra of the spent catalysts were recorded on a spectrometer, from NOST Co., Ltd, equipped with a thermo-electrically cooled charge couple device (CCD) detector and an automated XY motorized stage. A laser of 532 nm was used as the excitation source and the instrument was configured in a 180° backscattering geometry. The average of three spectra recorded in different areas of a pallet of the sample was taken to ensure homogeneity.