2.2.1 X-ray Diffraction
X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline inside catalyst by means of lattice structural parameters, and to obtain an indication of particle size.
metal oxides for the treatment of exhaust gases from internal combustion engine
The XRD pattern of a powdered sample was measured with a stationary X-ray source (usually Cu Kα) and a movable detector, which scans the intensity of the diffracted radiation as a function of the angle 2θ between the incoming and the diffracted beams.
In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that present in the catalyst. Because the technique is the based on the interference between reflecting X-ray from lattice planes, it requires samples that possess sufficient long-range order. Amorphous phases and small particles give either broad or weak diffraction at all, with the consequence that if catalysts contain particles with a large size distribution, XRD may only detect the larger ones. XRD at synchrotrons greatly improves the possibilities for the studies of small particles. Finally, the surface region is where catalytic activity resides, but this part of the catalyst is virtually invisible to XRD [98].
In this work, XRD patterns were mainly recorded using D8 Bruker Advanced diffactometer (Laboratory of the Petrochemical Refinering and Catalytic Materials, School of Chemical Engineering, Hanoi University of Science and Technology). The X-ray tube with a tungsten filament as the anode produces the X-rays. The monochromatic X-ray fall into the powder specimen, which is contained, on a specimen holder that can be turn an angle θ. The diffractometer has a Cu source with Cu Kα radiation (λ=0.154 nm). With this radiation, the XRD reflections of some single oxides can be visualized as seen in Table 2.2.
Table 2.2 Strong line of some metallic oxides
Phase 2θ, degrees (hkl)
β-MnO2 (Body- centered tetragonal)
28.8 (310)
37.2 (121)
43 (301)
56.8 (501) Co3O4 (Face-
centered cubic)
31.37 (220)
36.88 (311)
44.87 (400)
59.5 (511)
65.3 (440) CeO2 (Face-
centered cubic)
28.6 (111)
33.1 (200)
47.36 (220)
56.33 (311) NiO (Face-centered
cubic)
30.2 (122)
37.2 (222)
43.3 (400)
62.9 (440) SnO2
(Orthorhombic)
26.8 (112)
33.8 (006)
38.5 (122)
52.5 (118)
54.77 (133) CuO
(Monoclinic)
32.5 (110)
35.5 (-110)
39 (200)
46.3 (-112)
48.9 (-202)
53.4 (020)
61.51 (-113) ZnO
(Hexagonal)
31.82 (100)
34.3 (002)
36.5 (101)
47.6 (102)
57.2 (110)
63.2 (103) ZrO2 (Face-
centered cubic)
30.5 (111)
35.1 (200)
50.6 (220)
60.2 (311) γ-Al2O3 (Face-
centered cubic)
19.3 (111)
31.9 (220)
37.6 (311)
39.5 (222)
45.8 (400)
60.5 (511)
66.7 (440) V2O5
(Orthorhombic)
15.38 (200)
20.3 (001)
26.2 (110)
31 (300) BaO (Face-centered
cubic)
28 (111)
32.5 (200)
46.7 (220)
55.4 (311)
58 (222) WO3 (Monoclinic) 16
(011)
23.6 (020)
28.6 (-112)
36.9 (-103)
49.9 (400)
56 (402)
metal oxides for the treatment of exhaust gases from internal combustion engine 2.2.2 Scanning Electron Microscopy (SEM) and Transmission Electron
Microscopy (TEM)
Scanning electron microscopy (SEM) is carried out by rastering a narrow electron beam over the surface and detecting the yield of either secondary or backscattered electrons as a function of the position of the primary beam.
Emitted X-rays are characteristic of an element and allow for a determination of the chemical composition of a selected part of the sample. This technique refers to as energy dispersive X-ray analysis (EDX, EDAX).
Transmission Electron Microscopy (TEM) is a technique where an electron beam interacts and passes through a specimen. The electrons are emitted by a source and are focused and magnified by a system of magnetic lenses [98].
In this thesis, SEM images were obtained by using Hitachi S4800 instruments scanning electron microscope (Institute of Science Material, Vietnam academic of Science and Technology and National Institute of Hygiene and Epidemiology). TEM images and EDX results were collected from JEOL JEM.1010 Electron Microscope (National Institute of Hygiene and Epidemiology) and HRTEM Tecnai G2 F20 equipment (Electron Microscopy and Microanalysis Laboratory, Advance Institute of Science and Technology, Hanoi University of Science and Technology).
2.2.3 BET method for the determination of surface area
BET theory has been introduced by Brunauer, Emmett and Teller) aims to explain the physical adsorption of gas molecule on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of a material.
Surface area is determined based on BET plot made of P/V(P0 – P) against P/P0, which is a straight line with the volume of adsorbed gas is measured at a constant temperature as a function of the partial pressure. The linear relationship of this equation is maintained in the range of 0.05 < P/P0 < 0.30 [102].
The specific surface area of the samples in this thesis were measured at 77 K by the BET method using N2 adsorption apparatus on a ASAP 2010-Micromeritic (Laboratory of the Petrochemical Refinering and Catalytic Materials, School of Chemical Engineering) and Micromeritics VII 2390t (Laboratory of Environmental Friendly Material and Technology, Advance Institute of Science and Technology), Hanoi University of Science and Technology.
2.2.4 X-ray Photoelectron Spectroscopy (XPS)
XPS is among the most frequently used techniques in catalysis. It yields information on the elemental composition, the oxidation state of the elements and in favorable cases on the dispersion of one phase over another. When working with flat-layered samples, depth- selective information is obtained by varying the angle between sample surface and the analyzer. XPS is based on the photoelectric effect, in which an atom absorbs a photon of energy h, next a core or valence electron with binding energy Eb is ejected with kinetic energy [98].
In this thesis, X-ray photoelectron spectroscopy (XPS) measurements were carried out using a S-Probe monochromatized XPS spectrometer from Surface Science Instruments (VG) with an Al Kα X-ray (1486.6 eV) monochromatic source (Department of Inorganic and Physical Chemistry, Ghent University, Belgium). The measured surface was 250 μm by 1000 μm with a flood gun set to 3 eV. Experimental data were processed using the
metal oxides for the treatment of exhaust gases from internal combustion engine
software package CasaXPS (Casa Software Ltd., UK). All spectra were calibrated for a carbon 1s peak at 284.6 eV
The technique used to mould a sample for an analysis is pressing the powder into pellets. Atoms and their binding energy analyzed in the present work are as follows [102]:
Table 2.3 Binding energy of some atoms [102]
Atom Binding energy (eV)
O 1s 531.6
C 1s 284.6
Mn 2p3/2 650
Co 2p3/2 786
Ce 3d3/2 902
Ce 3d5/2 882.5
The binding energy of C1s (284.6 eV) was chosen as reference.
The atomic concentration (%) of each atom is calculated as an area of the peak times corresponding sensitivity factor. The atomic ratio of Mn/Co/Ce is calculated as the ratio amongst three corresponding atomic concentrations.
2.2.5 Thermal Analysis
Thermal analysis (TA) is a group of techniques in which a property of the sample is monitored against time or temperature while the temperature of the sample in a specified atmosphere, is programmed.
Thermal gravimetric analysis (TGA) is a method of thermal analysis in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate), or as a function of time (with constant temperature and/or constant mass loss).
In differential thermal analysis (DTA), the difference in temperature between the sample and an inert reference is measured during a programmed temperature change. The temperature difference between sample and reference should be the same until some thermal event, such as melting, decomposition or change in crystal structure, occurs, in which case the sample temperature either lags behind (if the change is endothermic) or leads (if the change is exothermic) the reference temperature.
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature [103].
TGA-DTA spectra in this thesis were recorded using NETZSCH STA 449F3 equipment (Department of Inorganic and Physical Chemistry, Ghent University, Belgium) and Perkin Elmer PYRIS Diamond apparatus (Vietnam Institute of Chemical Industry).
TGA-DSC spectra were obtained using NETZSCH STA 409PC instrument (Laboratory of the Petrochemical Refinering and Catalytic Materials, School of Chemical Engineering, Hanoi University of Science and Technology).
In order to determine the activity for soot oxidation of the catalyst, the mixture of soot and catalyst was heated from the room temperature (RT) to 700 oC, 800 oC or 1000 oC with the heating rate of 5 oC/min in the air flow of 20 ml/min.
2.2.6 Infrared Spectroscopy
Infrared spectroscopy can be considered as the first modern spectroscopic technique that has found general acceptance in catalysis. The most common application of infrared
metal oxides for the treatment of exhaust gases from internal combustion engine
spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed at the surface of the catalyst. In addition, the technique is useful in identifying phases that are present in precursor stages of catalyst during its preparation. Infrared spectroscopy is the most common form of vibration spectroscopy.
The sample consists typically of 10-100 mg of catalyst, pressed into a self-supporting disk of approximately 1 cm2 and a few tenths of a millimeter in thickness [102].
IR spectra in this thesis were obtained in KBr pellets using a Perkin Elmer RXI spectrometer (Laboratory of the Petrochemical Refinering and Catalytic Materials, School of Chemical Engineering, Hanoi University of Science and Technology).
Table 2.4 Specific wave number of some function group or compounds
Sample or group Wave number (cm-1) Literature
Co3O4 nano cubes 665 573 [111]
Co3O4 hollow sphere 666.24 581.80 [112]
Co3O4 nanorods 665.7 576.8 [113]
β-MnO2 708 539 481 417 [114]
CeO2 nanocubic 480 [115]
O-H (H2O) 3393 1652 [102]
C=O (CO2) 2350 1548 1417 [102]
CO 2143 [102]
2.2.7 Temperature Programmed Techniques
TPR-H2 is a technique in which a thermal conductivity detector (TCD) measures the hydrogen content of the gas mixture before and after reaction. With this type of apparatus, a TPR spectrum is a plot of the hydrogen consumption of a catalyst as a function of temperature.
TPD O2 is a technique in which offers the ability to investigate the temperature- dependence of desorption process of oxygen. The concentration of desorbing species is usually measured with a quadrupole mass spectrometer, but can also be determined with an ionization manometer or by monitoring the work function of the sample [104].
TPR-H2 and TPD O2 profiles of the catalysts were measured with a AutoChem 2920 II – Micromeritics device (Laboratory of the Petrochemical Refinering and Catalytic Materials, School of Chemical Engineering, Hanoi University of Science and Technology).
In TPD O2 characterization, about 0.1 g of the sample was heated from room temperature to 300 oC for 1 hour with the heating rate of 10 oC/minutes in gas flow of 10%
O2/He (40 ml/min). Afterward, the sample was cooled down to 120 oC in He flow. A flow 10% O2/He was flowed for 1 hour at 120 oC. Then, the temperature was increased to 700oC with the heating rate 10 oC/min and the sample was heated for 30 minutes. The adsorbed oxygen was calculated from the signal of TCD.
In TPR-H2 analysis, about 0.1 g of the sample was cleaned in a He flow for 60 min at 300 oC with a heating rate of 10oC/min. Afterward, the sample was cooled to room temperature. A flow of 10.2% H2/Ar was flowed through the sample while the temperature was increased to 750 oC with a heating rate of 10 oC/min. The sample was reduced by H2; the H2 concentration in the flow was detected using a TCD detector. The consumed H2 for the reduction was calculated based on the difference between the H2 signal before the reduction and after the reduction.