- Open Access
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- Authors : Mayuranath Suresh Kumar , Mohammed Haasheem. M, Niyaz Ahmed. I
- Paper ID : IJERTV7IS100054
- Volume & Issue : Volume 07, Issue 10 (October – 2018)
- Published (First Online): 05-01-2019
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License: This work is licensed under a Creative Commons Attribution 4.0 International License
Study on Three-Way Catalytic Converters and the use of Oxides as Reduction and Oxidation
Study on Three-Way Catalytic Converters and the use of Oxides as Reduction and Oxidation
Mayuranath Suresh Kumar
Department of Mechanical
Sri Ramakrishna Engineering College
Mohammed Haasheem. M
Department of Mechanical
Sri Ramakrishna Engineering College
Niyaz Ahmed. I
Department of Mechanical
Sri Ramakrishna Engineering College
Abstract-This is a study on catalytic converter to change the materials used in a conventional catalytic converter to replace noble metals and to introduce oxides as a catalyst, fusing oxides it is stated that emissions can be reduced. Since oxides act as both reduction and oxidation, conventional three-way catalytic converter can be reduced in steps to make a single way catalytic converter. Thus, this reduces size and cost
Automobiles predominantly use internal combustion petrol or diesel engines which burn hydrocarbons imperfectly and emit significant amounts of unburned HCs, CO and NOx. These pollutants, in presence of sunlight and atmospheric water vapours react with each other to form secondary pollutants such as ozone, polyaromatic hydrocarbons (PAH), and photochemical smog, which is a mixture of complex organic substances, including peroxyacetyle nitrite (PAN). These emissions cause numerous health issues such as eye irritation, damage to breathing passage and lungs, reduces the oxygen carrying capacity of blood, causes premature death, increases infant mortality, and even causes damage at the ecosystem scale by causing acid rain.
In 2010, automotive emissions around the world caused about 180,000 deaths worldwide mainly by ischemic heart disease (91,000) and respiratory distress (34,000). Vehicular air pollution drew attention around the world in the 1950s and 1960s, as higher concentration of air pollution noticed in large cities like Los Angeles and Tokyo. As a remedy approach, catalytic converter was first introduced in USA in 1975 for curbing automotive emissions with great success. Subsequently, it was introduced in Japan, and later on Europe adopted the technology at industrial-scale in 1986. Catalytic converters reduce unburned hydrocarbons (HCs), carbon monoxide (CO) and nitrogen oxides (NOx) into less toxic water vapor, carbon dioxide (CO2), and nitrogen (N2) to minimize negative impacts on health and environment. Regulatory agencies, particularly in USA, Japan, and Europe, enacted stringent emission standards for HCs, CO and NOx around 1970 to control vehicular emissions.
In USA, catalytic converters significantly lowered the automobile originated HCs, NOx and CO emissions over the period 1970 to 1990. Global annual sales of catalytic converter also rose from just over 9 million units in 2006 to around 19 million units in 2012, and it will be around 140
million units by 2019. Evidently, catalytic converters are playing a significant role in saving human lives. On the other hand, catalytic converters pose substantial risks to lives as they contain platinum group metals (PGMs) which involves harmful mining and processing. In addition, risks on health and lives are also posed by the end-of-life recycling, and disposal of catalytic converters due to their PGM contents. Assessment of sustainability of a product, process or technology should encompass social, environmental, and economic wellbeing in order to avoid problem-shifting in the product system.
Life cycle assessment (LCA) is a sustainability assessment tool that considers both the reduction of emission by catalytic converters in their use and accounts for the negative impacts associated with the entire product life cycle of raw materials extraction to end-of-life disposal. According to a 2001 LCA study, catalytic converters reduce exhaust gas emissions during use phase, while they increase environmental burdens during production. Their study lacked in addressing the social impact in terms of human health of the catalytic converters throughout their life cycle. This journal deals with the numerous possibilities of variation of oxides which can be implemented in the catalytic converter.
The monodispersed polymethyl methacrylate (PMMA) microspheres with the average diameter of 300 nm were prepared following a previously reported procedure. Three-dimensionally ordered macroporous (3DOM) Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) mixed oxide catalysts were synthesized by carboxy-modified colloidal crystal templating (CMCCT). The template synthesis was conducted at room temperature. Ce(NO3)36H2O (99.5%), ZrOCl28H2O (98%), Mn(NO3)2 (49.051.0%), Co(NO3)26H2O (>98.5%) and Ni (NO3)26H2O (>98%) were
used as the metal precursors. Ce(NO3)36H2O, ZrOCl28H2O, and M(NO3)xnH2O (M = Mn, Co, Ni) were dissolved in the commixture of distilled water, ethylene glycol (EG) and methanol (EOH) (with the volume ratio of 10:7:3) under vigorous stirring for 40 min to obtain the precursor solution. After the stirring, a PMMA template was added into the precursor solution for 12 h to achieve full impregnation.
Subsequently, the solution was vacuum filtrated to remove the excess precursor solution and precipitate was dried at 50 C in the vacuum oven, followed by calcination in air for 10 h heating from room temperature to 550 C with the ramp rate of 1 C/min, thus generating 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) mixed-oxide catalyst.
X-ray diffraction (XRD) was conducted with a powder X-ray diffractometer (Shimadzu XRD 6000) using Cu Ka radiation with a Nickel filter and the operating voltage and current of 40 kV and 10 mA, respectively. Nitrogen adsorption-desorption isotherms were performed at 196 C using an automatic TriStarII 3020 instrument. Scanning electron microscopic (SEM) images were obtained on a Quanta 200F instrument using accelerating voltages of 5 kV. The transmission electron microscope (TEM) images were obtained using a JEOl JEM 2100 electron microscope and 200 kV of accelerating voltage was used. Raman spectra of the samples were obtained in the anti-Stokes range of 1002000 cm1 using an in Via Reflex-Renishaw spectrometer. X-ray photoelectron spectra (XPS) were obtained using on a PerkinElmer PHI-1600 ESCA spectrometer with Mg Ka (hv = 1253.6 eV) as the X-ray source. Temperature programmed desorption of ammonia (NH3-TPD) was performed using a Tianjin XQ TP-5076 conventional flow apparatus and was monitored by a thermal conductivity detector (TCD). Prior to the TPD measurements, each sample was degassed at 500 C for 1 h and then cooled down to 60 C and was then saturated with high purity anhydrous ammonia at 60 C for 1 h and flushed at the same temperature for 1 h in order to remove physiosorbed ammonium. Temperature-programmed reduction with H2 (H2-TPR) measurements were carried out on a chemical adsorption apparatus. Prior to H2-TPR analysis, a 100 mg sample was pre-treated at 300 C in Ar for 1 h in order to remove the adsorbed water and carbon dioxide and was then cooled to room temperature. Then, 10% H2/Ar flow (40 ml/min) was exposed to the catalyst bed and the hydrogen consumption signal was monitored on-line by a thermal conductivity detector (TCD).
In-situ diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded by a Thermo Nicolet Is50 spectrometer equipped with an MCT detector cooled by liquid nitrogen and a heating chamber. The catalyst was loaded in the Harrick IR cell and pretreated at 400 C under N2 at the total flow rate of 100 ml/min for 60 min in orer to remove the adsorbed impurities. A background spectrum was collected under a flowing through N2 atmosphere and was subtracted from the sample spectrem. DRIFT spectra were recorded by accumulating 32 scans with the resolution of 4 cm1.
The measurements of the simultaneous removal of PM and NOx over 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni)
catalysts were performed in a fixed bed quartz reactor (id = 10 mm). The composition of the feed gas was 1000 ppm NH3, 1000 ppm NO, 3% O2 and N2 as the balance gas with the total flow of 100 ml/min. The performance of the optimum catalyst was also evaluated at a higher O2
concentration (10%). Printex-U were used as model PM. Prior to each activity test, 100 mg catalyst and 10 mg PM were mixed with a spoon in order to obtain the loose contact that is similar to the circumstances of an actual diesel engine exhaust. The quartz wool was packed into the quartz tube to fix the mixture and avoid the axial diffusion. Each activity test was carried out from 30 to 600 C at the heating rate of 3 C/min. The concentration of the outlet gases, such as NH3, NO, NO2, N2O, CO2 and CO inclusive were monitored at the outlet of the catalyst by an online infrared spectrometer (Thermo Is50 FTIR) equipped with a 2.4 m gas cell. A fine and robust IR quantification method for multiple gaseous components were constructed (Qin and Cadet, 1997; Sinelli et al., 2010; Stec et al., 2011; Valencia et al., 2009). The standard gases with diverse intensity of different components were employed to draw the standard curves. The obtained standard curves were finally analysed by the TQ Analyst software. Precisions of the measurements have been guaranteed by successive refining the wavenumber ranges, correcting baselines, non-linear manners and components interactions. Prior to each measurement, the sample was swept by 100 ml/min N2 approximately 45 min for the collection of the background spectrum. Then, the IR spectrum was recorded after purging with 1000 ppm NH3, 1000 ppm NO, 3% O2 and N2 for 30 min in order to obtain a steady state. The data acquisition was performed at 3 C intervals from room temperature to 600 C. After the reaction, the capability of the catalyst for the oxidation of PM was represented by the PM combustion temperatures, and the temperature at the maximum combustion rate is denoted as Tm. The capability of the catalyst for NOx reduction was evaluated by the maximal conversion of NO to N2, and NO conversion was defined as
Â¼ Â½NOinletÂ½ ———- Â½NOoutlet 100%;
NO Conversion NO inlet.
The activate energy (Ea) of the catalytic reaction was measured. To fit the activation energy using the Arrhenius method, Tm was detected from room temperature to 873 K with the heating rates of 3, 5, 10 and 15 K/min. We can measure the change of the mass and temperature under non-isothermal condition, and estimate the value of the kinetic parameters from the plots displaying data of ln(a/T2 ) versus 1/T to estimate the Ea for the reaction.
3.1 Catalytic activity
3.1.1 De-NOx performance of SCRPF catalysts
It is observed that the partial substitution of Ce with transition metal elements could indeed influence the SCR activity of the Ce0.8Zr0.2O2 catalyst. Under identical operating conditions, among all 3DOM Ce0.8M0.1Zr0.1O2 (M
= Mn, Co, Ni) catalysts, Ce0.8Mn0.1Zr0.1O2 sample exhibits the best performance with 100% NO conversion in the 374 512 C range. However, after the substitution by Co and Ni, 3DOM Ce0.8Co0.1Zr0.1O2 and 3DOM Ce0.8Ni0.1Zr0.1O2
catalysts seem to inhibit the NO conversion and show very narrow operation temperature windows. Both of the maximum NO conversion cannot reach 100%, obtaining
55% and 70%. These results suggest that different dopants give rise to various effects on the Ce0.8Zr0.2O2 catalyst.
NH3-SCR performance for the particle Ce0.8Zr0.2O2 and Ce0.8Mn0.1Zr0.1O2 was also tested. Particle Ce0.8Zr0.2O2 shows the maximal NO conversion of approximately 75% at 400 C, whereas 90% NO conversion was observed at 450 C for the particle Ce0.8Mn0.1Zr0.1O2.
3.1.2. PM oxidation over SCRPF catalysts
In practical applications, since the rate of PM oxidation at low temperatures is more important and cost- efficient for the entire purification process, we focus especially on the excellent catalytic activity at low Tm temperature. Table 1 lists Tm temperatures of PM and the temperature range of 100% NO conversion over 3DOM. Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts under loose contact condition. For the unsubstituted sample 3DOM Ce0.8Zr0.2O2 in Table 1, Tm is 435 C for the catalytic combustion of PM; however, after the substitution by the transition metals, the catalytic activity values for these 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts are different. In the case of 3DOM Ce0.8Mn0.1Zr0.1O2 catalysts, the Mn dopant remarkably enhances the catalytic activity for the removal of PM under loose contact conditions between the catalyst and PM, showing Tm value of 402 C. Whereas for 3DOM Ce0.8Co0.1Zr0.1O2 and Ce0.8Ni0.1Zr0.1O2 catalysts, the maximum catalytic PM combustion temperatures are approximately 433 and 429 C, respectively.
The particle Ce0.8Zr0.2O2 and Ce0.8Mn0.1Zr0.1O2 exhibit remarkably high combustion temperature for PM oxidation. It is obvious that 3DOM Ce0.8Mn0.1Zr0.1O2 sample not only displays the lowest Tm temperature but also the widest NO conversion temperature range. Moreover, this temperature range is similar to the temperature of the exhaust pollutants emitted from the diesel engines. The catalytic activity for the simultaneous PM and NOx abatement of the Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) samples are different due to the presence of different dopant. The Ce0.8Zr0.2O2 catalyst required progressively higher temperatures to reach the maximal conversion for PM combustion and NO conversion (Shang et al., 2015).
Figure 1: Temperature vs CO2 and NO concentration
Figure 1 shows NO conversion and CO2 concentration as a function of temperature for the simultaneous removal of PM and NOx over 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) and particle catalysts in a gas feed containing 1000 ppm NH3, 1000 ppm NO, 3% O2 and balance N2 at a gas hourly space velocity of 25,000 p.The experiment was performed for the simultaneous removal reaction under a high O2 concentration (10%). Fig. S1 shows the catalytic activity over 3DOM Ce0.8Mn0.1Zr0.1O2 catalyst. The figure describes about the practical applications, since the rate of PM oxidation at low temperatures is more important and cost-efficient for the entire purification process, we focus especially on the excellent catalytic activity at low Tm temperature. Table 1 lists Tm temperatures of PM and the temperature range of 100% NO conversion over 3DOM. Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts under loose contact condition.
The following observation was observed for the above mentioned experimental procedures. The table shown below details the temperature for maximum concentration CO2 (oC), the range of catalysts obtained and the achieved 100% NO conversion range.
Table 1: Effect induced by Various catalyst.
The temperature for maximum concentration CO2 (oC)
CO2 maximal concentration (ppm)
Achieved 100% NO
conversion temperature range (C)
Figure 2. Arrhenius plot of simultaneous removal of PM and NO conversion over exposure of 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts loosely mixed with PM with a different rate of 3 K/min, 5 K/min, 10 K/min, 15 K/min. (gas feed contains 1000 ppm NH3, 1000 ppm NO, 3% O2 and balance N2 at a gas hourly space velocity of 25,000 p).
The finding is observed due to the unrestricted monolayer- multilayer adsorption,whereas the appearance of the H3 hysteresis loop is an indication of the macroporous structure (Li et al., 2013; Wang et al., 2013). In the case of 3DOM Ce0.8Co0.1Zr0.1O2 and Ce0.8Ni0.1Zr0.1O2 catalysts, there are weak adsorption peaks at approximately 220 and
From the Table it is clear that the increase of the
that could be assigned to the CoAO and NiAO
O2 concentration resulted in the decrease of the NH3-SCR performance. 3DOM Ce0.8Mn0.1Zr0.1O2 catalyst showed rather high NOx conversions of 100% within the 374512 C operation temperature window under the 3% O2 atmosphere. NO conversion above 80% could be found at 378498 C under the 10% O2 atmosphere, and the maximal NO conversion is 94%. Additionally, the maximum temperatures for PM combustion are 402 and
415 C, respectively. The promotion of the catalytic performance under high O2 concentration should be addressed in future work.
3.2. Characterization of SCRPF catalysts
The patterns of 3DOM Ce0.8Zr0.2O2 and various Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) samples are shown in Fig. S2 (in the supporting information). All main characteristic peaks are consistent with the face-centered cubic fluorite-like structure of CeO2 (JCPDS PDF# 43- 1002). 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts
exhibit typical diffraction peaks belonging to CeO2, indicating that the dopant does not change the cubic structure of the catalysts. These lattice planes assigned to (111), (200), (220), (311), (222), (400), (331), (420) and
(422) could be detected.
Fig. S3 shows the type II characteristic isotherm accompanied with H3 hysteresis loop in the relative high pressure (P/P0) range and a nearly linear correlation in the low-pressure range.
Figure 2: Arrhenius plot
stretching vibrations in the samples. This suggested that Co
and Ni species should be present on the Ce0.8Zr0.2O2 surface. On the other hand, no Raman lines due to MnAO are observed for Ce0.8Mn0.1Zr0.1O2. Therefore, Mn is probably inserted into the Ce0.8Zr0.2O2 lattice to form solid solutions as shown in Fig. S4.
XPS is a powerful technique for the exploration of the electronic properties of surface element components, metal oxides states and adsorbed oxygen species of the catalysts. Fig. 4 depicts Ce 3d, O 1s XPS spectra of 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts, and the corresponding quantitative analytic results are summarized in Table 2. It can be observed that the Ce3d spectra are split into eight peaks labeled V (881.9 eV), V0 (884.1 eV), V00(888.6 eV), V000 (897.8 eV), U (900.4 eV), U0 (902.7
eV), U00 (907.2 eV), U000 (916.3 eV). The decomposed peaks centered at V, V00, V000, U, U00, U000 are attributed to the 3d104f0 state of Ce4+ species, and the peaks assigned to V0 and U0 correspond to the 3d104f1 state of Ce3+ species (Trovarelli, 1999). In Table 2, we observe a minor discrepancy in the surface Ce3+/Ce4+ ratio of these 3DOM Ce0.8M0.1Zr0.1O2.
Figure 3: Images of Honey comb structure under SEM
Figure. 3. SEM images of 3DOM catalysts (a) Ce0.8Zr0.2O2 (b) Ce0.8Mn0.1Zr0.1O2 (c)
Ce0.8Co0.1Zr0.1O2 (d) Ce0.8Ni0.1Zr0.1O2 (e) particle Ce0.8Zr0.2O2 (f) particle Ce0.8Mn0.1Zr0.1O2. It shows
the representative SEM images of 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) and particle samples. It is observed that after calcination at 550 C, all samples (ad) display a high quality 3DOM structure possessing a skeleton surrounding a uniform periodic arrangement of voids with the average diameter of 300 Â± 20 nm and the wall thickness of 25 nm, which is smaller than the 400 nm of the PMMA spheres. For 3DOM samples, all catalysts have the uniform pore size, windows and wall thickness, and these macrospores are highly periodically arrayed and interconnected with the small windows. Obviously, the next layer is also visible in SEM image and is strongly interconnected through the opening windows. According to SEM result shown in Figure. 3, it is clear that 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts show the similar 3DOM architecture with 3DOM Ce0.8Zr0.2O2, indicating that the dopant does not induce significant changes in 3DOM architecture. However, the particle samples show the irregular structure.
Figure 4: XPS result linking binding energy and intensity Table 2: Surface composition and oxidation state of Ce 3d
and O1s over 3DOM catalyst by XPS analysis.
It can be deduced that the interaction between cerium and the dopant may lead to transfer of the lattice oxygen in the ceria over the surface, maintaining the low- valent state of cerium. Among 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) samples, 3DOM Ce0.8Mn0.1Zr0.1O2 sample exhibits the highest surface Ce3+/Ce4+ ratio (the value of Rb). The increase of the Ce3+ content is due to the interaction between cerium and the surrounding atoms that leads to an increase in the oxygen vacancies. However, after the substitution by Co and Ni, it leads to a drop of the surface Ce3+/Ce4+ ratio. This evidence is mainly due to the formation of CoOx and NiOx on the surface of 3DOM Ce0.8Co0.1Zr0.1O2 and 3DOM Ce0.8Ni0.1Zr0.1O2
samples, further implying that the interaction between the dopant and CeO2 varies. Concurrently, it is reported that a
rise in Ce3+ favors the increase in the number of the oxygen vacancies and the relatively high mobility of the bulk oxygen species (Du et al., 2012; Krishna et al., 2007; Wei et al., 2012).
Ce0.8M0.1Zr0.1O2 catalysts. According to the curve fitting methods, O1s can be decomposed to three dominant peaks a(529.1 eV), b (530.7 eV), c(532.2 eV), which ascribes to the lattice oxygen (O2) and chemical adsorbed oxygen (O, O2 ), respectively. As is known, the adsorbed oxygen is viewed as the dominant active oxygen species and plays a dominant role in the redox reaction. In other words, the Oads/Olatt ratio could reflect the content of active oxygen species of the reaction. Table 2 shows that 3DOM Ce0.8- Mn0.1Zr0.1O2 sample possesses the highest Oads/Olatt ratio and Ce3+ concentration. A Ce-based catalyst with abundant oxygen vacancies and defects is an effective catalyst. The reduction of Ce4+ to Ce3+ in ceria facilitates the formation of oxygen vacancies, further promoting the oxygen escaping from the lattice sites and leaving vacant sites with two electrons at each vacancy. (Arandiyan et al., 2015) Furthermore, the abundant active oxygen is beneficial for the oxidation of PM.
H2-TPR analysis is an ideal tool for measuring the redction behaviour of the catalysts. The reducibility of the metallic ion from high to low valence, and the nature of taking up oxygen as well as the species of the absorbed oxygen or activated could be reflected in this analysis. H2- TPR profiles of 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts are shown in Fig. 5. It can be observed that distinct reduction peaks centered at 565 C are found for 3DOM Ce0.8Zr0.2O2 sample. The low temperature reduction peak is probably due to the reduction of Ce4+ to Ce3+ ions in the outermost layers as well as the reduction of the surface capping oxygen of CeO2. Interestingly, after the introduction of metal and Zr ions into CeO2, the reduction peaks shift to the low temperature range, indicating that the mobility of surface lattice oxygen species is generally improved by the dopant. The improvement is mainly the result of the synergistic effect between the dopants and CeO2. For 3DOM Ce0.8Mn0.1Zr0.1O2 sample, the initial reduction peak centered at 486 C is due to the reduction of surface adsorption oxygen species, while the high reduction peak is the result of the reduction of lattice oxygen and the inner Ce4+
Figure 5: H2-TPR of 3DOM catalysts
The obvious reduction attributing to Mn could be detected, further confirming the formation of a CeAMnAZrAO solid solution (TÃ¶pfer and Goodenough, 1997). However, relatively low temperature reduction peaks appear in 3DOM Ce0.8Co0.1Zr0.1O2 and 3DOM Ce0.8Ni0.1Zr0.1O2 catalysts, which is in accordance with the reduction of CoOx and NiOx. Considering the results of Raman and TEM analyses, the appearance of low temperature reduction peaks may be ascribed to the reduction of CoOx and NiOx, respectively. This phenomenon indicates that CoOx and NiOx are present on the surface of the catalyst. It is generally accepted that the formation of a CeO2-based solid solution could improve the reducibility of the materials. Using Mn doped into the CeAZr mixed oxidation, CeAMnAZrAO solid solutions can be easily obtained. Nevertheless, the doping of Co and Ni may cause strong sintering of the CoOx and NiOx nanoparticles, which would result in the shift of the reduction peaks to low temperatures (Li et al., 2013; Shang et al., 2015; Zhou et al., 2016). It is noted that among these catalysts, 3DOM Ce0.8Mn0.1Zr0.1O2 catalyst display better reacting behaviors toward H2 consumption at the lower temperatures. Evidently, the introduction of Mn with molar concentration of 0.1 promotes the reducibility of the catalysts.
It can be speculated that the formation of a more homogeneous of CeAMnAZrAO tenerary solid solution may give better catalytic performances for the simultaneous removal of PM and NOx from diesel engine exhausts. Ce0.8Zr0.2O2 and Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts with the results shown in Fig. 6. Interestingly, it is obvious that each catalyst shows a strong peak centered below 200 C, characteristic of the weakly physical desorption of NH3 or the ammonium species adsorbed at weak acid sites (Cheng et al., 2015; Martins et al., 2008; Onida et al., 1996). The peak in 250 450 C is due to the ammonia adsorbed at mid-strong acid sites. The peak above 470 C may belong to strong acid sites arising from the interaction between CeO2 and the doped element. Analyzing the amounts of NH3 desorption, we conclude that the introduction of a dopant plays a positive role for providing the acid sites. NH3TPD peak of 3DOM Ce0.8Zr0.2O2 is much lower than those of 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) samples. In addition, it is generally accepted that the peak location is closely related to the adsorption strength and the peak area corresponds well to the amount of adsorbed NH3. Therefore, the modification of Ce0.8Zr0.2O2 by transition metals could enhance the surface acidity of Ce0.8Zr0.2O2. Moreover, the stronger acidity of 3DOM Ce0.8M0.1Zr0.1O2 increases its NH3 adsorption capacity. This is favorable to NH3-SCR reaction. Doping with Mn promotes the formation of active acid sites and is beneficial for the adsorption and activation of NH3.
In-situ DRIFT experiments were also employed to explore the NH3 desorption behavior on the catalysts. In-situ DRIFT of NH3 adsorption was conducted to examine the change of acidity on the catalyst with the results presented in Fig. 7. The bands at 1159 and 1313 cm1 are related to the coordinated NH3 bound to Lewis acid sites. The band at approximately 1341 cm1 is characteristic for the ANH2 species. The band at 1443 cm1 is attributed to the NH4+
species on the BrÃ¸nsted acid, which is due to the asymmetric bending vibration of the NAH bond in the ANH3+ group, generated from the decomposition of the NH4+ chemisorbed on a BrÃ¸nsted acid site. For 3DOM Ce0.8Zr0.2O2 catalysts, only the bands at 1159, 1313 and 1341 cm1 could be detected, which are due to the NH3 adsorption on the Lewis acid sites and ANH2 species, respectively. For 3DOM Ce0.8Mn0.1Zr0.1O2 catalysts, obvious bands at 1065, 1159, 1313, 1341, 1443 and 1539 could be detected, characteristic for the Lewis acid sites, BrÃ¸nsted acid as well as ANH3+ group and ANH2 species. Compared with 3DOM Ce0.8Co0.1Zr0.1O2 and Ce0.8Ni0.1Zr0.1O2 samples, the bands at 1159 and 1341 cm1 ascribed to Lewis acid sites and ANH2 species can be detected. This differences show that the dopants of different transition metal into Ce0.8Zr0.2O2 deeply affect the NH3 adsorption and activation. More acid sites could be detected on the Mn-doped samples. The adsorption and activation of NH3 promotes the formation of active acid sites. After the Mn dopant, Ce-Zr mixed oxides is prone to produce more ANH3+ group that can easily combine with NO to produce NO3 , which is beneficial for PM combustion. In-situ DRIFT spectra over 3DOM catalysts at 400
C in a flow of 1000 ppm NH3+ 1000 ppm NO+ 3% O2. For 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) samples, several bands at 1159, 1341, 1443, 1536 cm1 could be observed. The band at 1159 cm1 is related to the coordinated NH3 bound to Lewis acid sites, and the bands at 1443 cm1 is attributed to the NH4+ species on BrÃ¸nsted acid. The band at 1536 cm1 is attributed to the asymmetric bending vibration of the NAH bond in the ANH3+ group, generated from the decomposition of NH4+ chemisorbed on BrÃ¸nsted acid site. The bands at 1341 cm1 are related to the scissoring and wagging vibrations of the ANH2 species formed by hydrogen abstraction from the NH3 coordinated to Lewis acid sites. Whereas for 3DOM Ce0.8Zr0.2O2 samples, only bands at 1159, 1341, 1445 cm1 could be observed. For 3DOM Ce0.8Mn0.1Zr0.1O2 catalysts, obvious bands at 1159, 1289, 1341, 1443 and 1536 cm1 could be detected, characteristic for monodentate nitrate, bidentate nitrate, respectively.
Only band at 1289 and 1443 cm1 were observed for 3DOM Ce0.8Co0.1Zr0.1O2 and 1159, 1289 and 1443 cm1 for 3DOM Ce0.8Ni0.1Zr0.1O2 samples. It is evident that there are many bands ascribed to NH4+ and ANH2 species on 3DOM Ce0.8M0.1Zr0.1O2 catalysts, promoting PM oxidation and NO reduction.
Influence of the structure on the performance of 3DOM The reaction of PM combustion is a typical heterogeneous catalysis reaction that occurs at the three- phase boundary among a solid catalyst, a solid reactant (PM), and gaseous reactants (O2, NO). Thus, a vital issue strongly influencing the catalytic activity of a catalyst for the solid-solid reaction is the contact conditions between PM and the catalyst. As revealed by many studies, the catalytic performance of catalysts for PM combustion is very excellent under tight condition between PM and the catalyst due to their good contact condition. However, this cannot meet the practical contact condition because catalysts on the
surface of the filter and PM are in loose contact (Oi- Uchisawa et al., 2003), weakening the catalytic effect between catalysts and PM. Thus, it is significantly necessary to study and design the active catalysts for PM combustion under loose contact conditions. Even though numerous nanoparticle catalysts show remarkable catalytic activities for PM combustion (Liu e al., 2009), the catalytic activities of these catalysts are still decreased considering that their small pore sizes limit the transport of PM into their inner pore, making less full use of the inner active site. Thus, the efficiency of a catalyst is strongly influenced by the contact between PM and the catalyst. The contact between catalyst active sites and PM must be improved for process intensification. In this work, 3DOM catalysts possess special large pore size and ordered interconnected macrosporous tunnels structure, and the window size is approximately 300 nm, which are much larger than the size of PM (approximately 25 nm, Fig. S6). Moreover, this special structure can permit PM to diffuse easily into the inner 3DOM catalysts by the airflow in the reaction and shows less resistance for transport through the catalyst structure. The catalyst is in close proximity with PM and more catalyst surface is exposed to reaction atmosphere. As shown in
Influence of NH3 and different dopant
Even though there are many reports on the simultaneous abatement of PM and NOx from diesel engines (Milt et al., 2008; Wang et al., 2014), few investigation have been performed regarding the concentration of the reducing gas and almost no reducing gas was used. Thus, these catalysts exhibited low NO conversion. In general, it is challenging to reduce NOx under the oxygen-rich conditions. To improve the efficiency of NO conversion, different dopants are also added into the CeAZr mixed oxides. Based on NH3-TPD analysis, Ce0.8M0.1Zr0.1O2 shows a high acid amount and acid strength than those of the Ce0.8Zr0.2O2 sample, indicating that the dopant is beneficial for increasing the acid amount and strength of 3DOM Ce0.8Zr0.2O2. The bands at 1539 and 1341 cm1 are also observed. The latter is related to scissoring and wagging vibrations of the ANH2 species, formed by hydrogen abstraction from NH3 coordinated to a Lewis acid site. For the Ce0.8Mn0.1Zr0.1O2 catalyst, the bands at 1065, 1159, 1341, 1443 and 1539 cm1 could be obviously observed, while for 3DOM Ce0.8Co0.1Zr0.1O2 and Ce0.8Ni0.1Zr0.1O2 catalyst, the band at 1443 cm1 attributed to the NH4+ species on BrÃ¸nsted acid is absent. NH3+ as an intermediate product is prone to react with NO to produce N2, which is beneficial for the conversion of NO to N2 and promotes the NH3-SCR reaction. This indicates that more NH3 will be adsorbed and activated over Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalyst. The addition of Mn can enhance NH3 adsorption and activation. Moreover, the active acid sites easily absorb NH3 molecules and produce more NH4+ species, which is also beneficial for the improvement of the catalytic activity for NO reduction.
The new SCRPF combining catalyst technology is an efficient and economical approach for purification of PM
and NOx from diesel engine exhaust. For the simultaneous removal of PM and NO, a series of highly ordered monolithic 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts are successfully prepared by a colloidal crystal template method. The diffusion and combustion reaction of PM and the reduction of NOx are process intensified by the unique 3DOM structure and the doping metal ions. Thus the following conclusions were made from the various references involved. 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) sample possesses high-quality 3DOM architecture exhibiting better low temperature reducibility than 3DOM Ce0.8Zr0.2O2.
The Ce3+/Ce4+ ratio and the surface-active oxygen species on 3DOM Ce0.8Mn0.1Zr0.1O2 are much higher than those on the other 3DOM Ce0.8M0.1Zr0.1O2 (M = Mn, Co, Ni) catalysts. 3DOM Ce0.8Mn0.1Zr0.1O2 sample presented the most pronounced catalytic activity with the maximum concentration of CO2 at 402 C and nearly a 100% NO conversion in the 374512 C range, achieving the purposes of the removal of PM and NOx within the same temperature range. 3DOM Ce0.8Mn0.1Zr0.1O2 exhibits the lowest Ea value (86.4 kJ/mol), while the lower active energy remarkably reduces the reaction energy barrier. This is more conductive for the catalytic reaction to proceed smoothly.
Hence, various oxides can be used to follow out the oxidation and reduction process as the suitable catalyst chemicals. Of these oxide, depending on the cost, time taken for conversion of harmful gases to less harmful ones and affecting temperature, the required oxides can be selected and used. Thus this journal deals with the numerous possibilities of variation of oxides which can be implemented in the catalytic converter.
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