MnOx-CeO2 mixed oxides as the catalyst for NO-assisted soot oxidation: The key role of NO adsorption/desorption on catalytic activity

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X-Ray Diffraction (XRD)

The X-ray diffraction was performed on PANalytical-Empyrean diffractometer (Japan Science) with CuKα radiation (λ = 0.154056 nm) to obtain some information about internal lattice parameters of soot. The X-ray tube was operated at 40 kV and 40 mA. The XRD patterns of samples were recorded in the range of 10° ≤ 2θ ≤ 90°with a scanning step size of 0.02°. The stacking thickness of crystallites (D) and the crystalline length (L) as crystallite dimensions of soot samples can be calculated by applying the Scherrer equation to the bands 002 and 100, respectively, where K and K’ are the shape factors and β002 and β100 are the full widths at half maximum of the bands.

Temperature-Programmed Reduction (TPR)

The hydrogen temperature-programmed reduction (H2-TPR) profiles were obtained using a BELCAT-M (BEL-Japan) apparatus, equipped with a TCD. The materials were outgassed and activated at 400 °C for 1 h in Ar, then cooled down to room temperature and reduced until 700 °C in 5 vol.% H2/Ar at a heating rate of 8 °C/min.


30 mg of soot-catalyst mixture in tight contact was used for soot temperature-programmed reduction (soot-TPR). The sample was heated from 80 to 650 °C at a ramp rate of 5 °C/min in a pure Ar flow (100 ml/min). The consumption signals (COx concentration) of soot were monitored by a MS (mass spectrometry) detector (Figure 3-2).

Temperature-Programmed Desorption (TPD)

The catalyst (50 mg) was outgassed at 450 °C under O2/Ar flow for 30 min to clean the surface and then cooled down to 350 °C. NO and NO2 adsorptions in the catalyst were separately performed in a U-shaped quartz reactor by the exposure in 400 ppm NO/9% O2/Ar and 400 ppm NO2/9% O2/Ar for 1 h. After that, the sample was kept at 80 °C in pure Ar flow for 1 h for proper degassing. The desorption profiles of NOx and O2 were obtained by heating the sample from 80 to 550 °C at 10 °C/min in Ar flow (100 ml/min). The concentrations of desorption species were calculated by the m/z = 30 (NO), 32 (O2) and 46 (NO2) on a mass spectrometer (MS) (Figure 3-2).

NOx-TPD coupled to IR

The prepared MnOx-CeO2 mixed oxides were treated at 350 °C for 30 min in 600 ppm NO/10% O2/N2, and then cooled down to 100 °C, the gas mixture was replaced by pure N2 and the sample was maintained for 1 h to remove the weakly adsorbed NOx species. The NOx (NO and NO2) temperature-programmed desorption (NOx-TPD) tests were carried out in a fixed-bed reactor. 50 mg of pretreated sample was heated to 650 °C from 100 °C at a heating rate of 10 °C/min in N2 or 10% O2/N2 flow with a flow rate of 500 ml/min. The concentration of NOx desorbed from the catalyst was recorded on an IR detector.

Thermal Gravimetric Analysis (TGA)

The SOFs content of soot samples was evaluated by means of thermogravimetric oxidation in a SDT Q600 apparatus (TA Instruments), under air flow, heating from ambient temperature to 800 °C at a rate of 10 °C/min. The soot oxidation reactivity of the catalysts was also measured through means of thermogravimetric oxidation in a HCT-2 apparatus, in 10% O2/N2 flow (50 ml/min), heating from ambient temperature to 700 °C at a rate of 10 °C/min.

Transmission Electron Microscope (TEM)

High-resolution transmission electron microscopy (HR-TEM) (JEOL JEM 100CX) with a point resolution of 0.3 nm was utilized for an analysis of the internal structure of soot particles. The high-resolution images were acquired under 500,000 magnification. The carbon fringes lengths of different soot samples were also measured and analyzed by a software Image J in this work. The carbon fringe length distribution (f(x)) of soot was obtained by the following: 1 ( x )2 f (x) e 2 2 ,0  2. where µ is the mean value of fringe lengths, σ is the standard deviation and σ2 is the variance.

Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS)

Diffuse reflectance infrared Fourier transform spectra (DRIFTS) of soot were measured by using a Nicolet 6700 spectrometer with a DTGS detector and a high temperature cell. Soot sample was diluted by KBr at a soot/KBr ratio of 1/40, then the mixture was placed in a reaction cell and pretreated at 100 °C in pure N2 for 10 min. After that, the IR spectra were recorded.
The diffuse reflectance infrared Fourier transform spectra (DRIFTS) of catalyst were measured by using the same equipment. MnOx-CeO2 powder sample was diluted by KBr and placed in a reaction cell and then pretreated at 500 °C in pure N2, the background spectra were recorded at 250, 350 and 450 °C. The sample was finally exposed in 600 ppm NO/10% O2/N2 at each temperature for 30 min for DRIFTS measurements.

X-ray Photoelectron Spectra (XPS)

XPS measurement was conducted on an AXIS Ultra DLD spectrometer (Kratos, England) by using AlKα radiation as the excitation source (300 W). All binding energies (B.E.) were referenced to carbon C 1s line at 284.6 eV.

Reactivity evaluation by TPOs

The catalyzed and non-catalyzed oxidation activities of soot samples were studied by temperature program oxidation (TPO) tests under different reaction gases: 9% b.v. O2/Ar, 400 ppmv NO + 9% b.v. O2 in Ar, 400 ppmv NO2 + 9% b.v. O2 in Ar. It is well known that one of important functions of Diesel oxidation catalyst (DOC) placed the up-stream of DPF is to convert NO into NO2 and assist the DPF passive regeneration. Thus, NO2 as reaction gas was added into gas mixtures in order to simulate real DPF regeneration conditions.
Soot-TPOs were performed in a U-shaped quartz reactor (internal diameter 8 mm) with a porous frit as reaction bed (Figure 3-1). Each sample was placed in the reactor and heated by a thermally isolated furnace at a ramp rate of 10 °C. As shown in Figure 3-1, the reaction temperature was monitored by K-type thermocouple located in a thermowell in the reaction bed. The temperature program was controlled by a heating controller (Ɛ’EUROTHERM 2404). All gas flows were controlled with the mass flow meters (Brooks 5850S and Brooks Delta II). The gas mixtures passed through the reactor at a total flow rate of 15 Nl/h (GHSV ≈ 100,000 h-1). Concentrations of CO and CO2 (ppmv) in the outlet were measured by a Siemens Ultramat6 analyzer (Figure 3-2). Soot and MnOx-CeO2 catalyst powder were mixed by a spatula in an agate mortar for 2 min with a weight ratio of 1/10 (2.0 mg/20 mg) to obtain so-called loose-contact mixtures [105,109,113]. In order to compare the soot oxidation activity in different contact cases, the tight-contact and pressure-contact samples with same soot/catalyst ratio were prepared via using a pestle grinding for 2 min and loading about 60 bar pressure in a pneumatic press, respectively [105,113]. Tight contact was used in this work to study the intrinsic activity of soot catalytic combustion. In fact, in real catalytic wall-flow DPF, the formation of soot cake during soot filtration can lead to the increase of flow pressure, which promotes the interface contact between soot and catalyst. Base this case, pressure contact was selected to evaluate the catalytic reactivity of soot. To minimize the impact of hot spots, the soot-catalyst mixtures (22 mg) were diluted with 80 mg SiC pellets. In the obtained soot-TPO curves, Ti and Tm separately represent the ignition temperature at which the COx (CO + CO2) concentration reaches 100 ppm and the temperature of maximal soot oxidation rate. T5% and T50% separately represent the temperatures at which 5% and 50% of soot conversion.

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Table of contents :

1. Introduction
1.1 Overview
1.2 Objectives of Ph.D. work
2. Literature review on soot emission control
2.1 Diesel exhaust emissions, environment pollution and health impact
2.2 Diesel Particulate filter (DPF)
2.2.1 DPF filtration
2.2.2 DPF regeneration
2.3 Soot structure
2.4 Catalyst materials
2.4.1 Precious metal catalysts
2.4.2 Ce-based mixed oxide catalyst
2.5 Soot oxidation reaction mechanism
3. Materials and experimental methods
3.1 List of materials
3.1.1 Materials for soot preparation
3.1.2 Materials for catalyst preparation
3.2 Physicochemical characterization
3.2.1 Brunauer-Emmett-Teller (BET)
3.2.2 Laser Granulometry
3.2.3 X-Ray Diffraction (XRD)
3.2.4 Temperature-Programmed Reduction (TPR) H2-TPR Soot-TPR
3.2.5 Temperature-Programmed Desorption (TPD) NOx-TPD coupled to MS NOx-TPD coupled to IR
3.2.6 Thermal Gravimetric Analysis (TGA)
3.2.7 Transmission Electron Microscope (TEM)
3.2.8 Raman Spectroscopy (Raman)
3.2.9 Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS)
3.2.10 X-ray Photoelectron Spectra (XPS)
3.3 Reactivity evaluation by TPOs
3.2.1 Soot oxidation reactivity
3.3.2 NO oxidation reactivity (NO-TPO)
3.4 Isothermal experiment
4. MnOx-CeO2 mixed oxides as the catalyst for NO-assisted soot oxidation: The key role of NO adsorption/desorption on catalytic activity
4.1 Introduction
4.2 Materials preparation
4.3 Physicochemical properties
4.3.1 DRIFTS tests
4.3.2 XPS analysis
4.3.3 Raman analysis
4.3.4 TPD analysis with IR and MS
4.3.4 Soot-TPR tests coupled to MS
4.4 Soot oxidation activity
4.4.1 Soot-TPO
4.4.2 Isothermal reaction at 400 °C
4.5 Summary
5. Structure-reactivity study of model and Biodiesel soot in model DPF regeneration conditions
5.1 Introduction
5.2 Materials preparation
5.2.1 Soot samples preparation
5.2.2 Catalyst preparation
5.3 Physicochemical properties of soot
5.3.1 Laser granulometry
5.3.2 HRTEM analysis
5.3.3 XRD analysis
5.3.4 Raman spectra analysis
5.4 Soot oxidation activity analysis by TPOs
5.4.1 Non-catalytic soot oxidation TPOs in 9% b.v. O2/Ar TPOs in 400 ppmv NO2 + 9% b.v. O2 in Ar
5.4.2 Catalytic soot oxidation TPOs in 9% b.v. O2/Ar TPOs in 400 ppmv NO/NO2 + 9% b.v. O2 in Ar
5.4.3 Effect of soot-catalyst contact
5.5 Structure and reactivity correlation
5.6 Summary
6. Structure, surface and reactivity of activated carbon: From model soot to Bio Diesel soot
6.1 Introduction
6.2 Materials preparation
6.2.1 Soot samples preparation
6.2.1 Catalyst preparation
6.3 Reactivity analysis
6.3.1 TPO tests in O2
6.3.2 TPO tests in NO + O2
6.3.3 Isothermal reaction rate at 350 °C
6.4 Physicochemical properties of soot
6.4.1 Laser granulometry
6.4.2 BET analysis
6.4.3 Raman analysis
6.4.4 HRTEM analysis
6.4.5 XPS analysis
6.4.6 DRIFTS analysis
6.5 Correlation of structural and surface properties to reactivity
6.6 Summary
7. Effect of Biodiesel impurities (K, Na, P) on noncatalytic and catalytic activities of soot in model DPF regeneration conditions
7.1 Introduction
7.2 Materials preparation
7.2.1 Soot samples preparation
7.2.2 Catalyst preparation
7.3 Reactivity tests by TPOs
7.3.1 Soot oxidation under noncatalytic regeneration conditions Soot-TPOs in 9% b.v. O2/Ar + 5% H2O Soot-TPOs in 400 ppm NO2 + 9% b.v. O2/Ar + 5% H2O
7.3.2 Soot oxidation under catalytic regeneration conditions Soot-TPOs in 9% b.v. O2/Ar + 5% H2O Soot-TPOs in 400 ppm NO + 9% b.v. O2/Ar + 5% H2O Soot-TPOs in 400 ppm NO2 + 9% b.v. O2/Ar + 5% H2O
7.4 Physicochemical properties
7.4.1 Raman analysis
7.4.2 BET analysis
7.4.3 HRTEM analysis
7.4.4 XPS analysis
7.5 Summary
8. Conclusion


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