Promoting effects of barium substitution on the catalytic performances

Subscriber access provided by TUFTS UNIV

Kinetics, Catalysis, and Reaction Engineering

Promoting effects of barium substitution on the catalytic performances of FeCeO2-# for soot oxidation Bin Guan, Yong Huang, He Lin, and Zhen Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01005 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Promoting effects of barium substitution on the catalytic performances of FeCeO2-δ for soot oxidation

Bin Guan*, Yong Huang, He Lin, and Zhen Huang Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China

*Corresponding author: Bin Guan Dongchuan Road No.800, Min Hang District, Shanghai, P.R.China 200240 Tel.: +86 21 34206859; fax: +86 21 34205553. E-mail: [email protected]

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In this work, the promoting effects of the introduction of barium to FeCeO2-δ on soot oxidation were thoroughly studied. A series of FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ were prepared by self-propagating high-temperature synthesis method. The physiochemical properties of catalysts were investigated using BET, XRD, TEM, NO-TPO, and NOx-TPD. Ba(10%)FeCe displays the best catalytic performance and possesses the lowest soot ignition temperature of 282 °C in the presence of NO. However, FeCeO2-δ was the most active one showing the lowest soot ignition temperature of 342 °C in the absence of NO. NO-TPD demonstrates that barium loading in FeCeO2-δ dramatically improves the NOx adsorption and desorption capacity. In addition, in situ DRIFTS indicates that NO can be adsorbed on catalysts’ surface to form nitrite and nitrate species, and that the corresponding nitrates species over BaFeCeO2-δ are stable at high temperatures, which plays a crucial role and ensures the excellent performance of soot oxidation. Keywords: Barium; SHS; soot combustion; NOx adsorption and desorption; in situ DRIFTS; mechanism

1. INTRODUCTION Vehicle emission pollutants have become the main fine particulate matter source in atmosphere by the fine particulate matter (PM) source apportionment, especially as the primary and secondary particulates from the diesel vehicles.1,2 In Combination with the research and application both domestic and overseas, the diesel oxidation catalyst (DOC) and the diesel particulate filter (DPF) are together installed in the diesel vehicle. Moreover, in order to further promote the purifying capacity and the DPF carrier regeneration, the surface of the DPF carrier is coated with catalysts, namely, the catalyzed DPF 2


Page 2 of 36

Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(CDPF). CDPF is the most promising method in the engine development to remove the soot particles efficiently, whereas the key challenge of CDPF is to find effective catalysts supported on the particulate filters, which are required for the excellent low temperature activity of soot oxidation and the good chemical stability.3-5 The traditional catalysts supported on the CDPF carrier, such as precious metals, ceria-based materials, alkaline or alkaline earth oxides, and perovskite or perovskite-like oxides, have all received considerable attention for this purpose. The precious metal catalysts are known to possess excellent catalytic activity; however, owing to poor thermal stability, sulfur poisoning, and high price, they are limited in practical applications.6,7 As one potential alternative to precious metal catalysts, the CeO2 and ceria-based mixed oxides play a significant role on the utilization of the diesel soot combustion. In particular, the existence of the oxygen vacancy defects and the repeatedly reduced/oxidized states (Ce4+/Ce3+) redox cycle confer cerium to possess the outstanding redox ability and the elevated oxygen transport capacity.8-10 The use of ceria alone as the catalyst is not appropriate for direct applications. Much attention has been devoted to the modification of CeO2 with different metal ions to enhance the low temperature catalytic activity and stabilize the structure against thermal sintering.11 Many previous investigations pointed out that the incorporation of some transition metal oxides such as Fe, Cu, Mn, and Co cations into the ceria lattice may achieve higher catalytic performance of soot combustion and improve the capacity towards the NO oxidation.12-16 The effect of the introduction of alkali (Li, Na, and K) and alkaline-earth metal (Ba, Mg, and Ca) for soot oxidation has also been reported.17-20 A very important performance was revealed that alkali and alkaline-earth metal could effectively improve the capacity for storing NOx and decrease the maximum oxidation temperature.21,22 For example, the standard NOx storage reduction (NSR) catalysts consisting of alkali or alkaline-earth metal and 3



precious metals have been used widely to accomplish the removal of soot and NOx effectively.

In several studies, barium was found to play the role of NOx adsorbent in the reaction process and to positively affect the soot oxidation performance of the catalyst. In present study, the key role of NOx storage capacity was taken into account. Since previous studies mainly concentrated on the catalytic activity of the combination with previous metal and alkaline-earth metal catalysts, such as Pt-Ba-Ce or Pt-K-Ce,23,24,25 the information available on the effect of the introduction of barium to the transition metal and the cerium system towards the soot combustion and NOx storage capacity is limited. Based on the aspects above, the current study aims to investigate the adsorption/desorption of NOx and the catalytic activity for soot oxidation with the introduction of different barium contents to FeCeO2-δ. For this purpose, a series of FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts were synthesized by the SHS method under the same preparation conditions. The physicochemical properties were studied by the X-ray diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), NO-temperature programmed oxidation (NO-TPO), and NOx-temperature programmed desorption (NOx-TPD) methods, respectively, whereas the soot catalytic performance was evaluated by the TPO experiments. Furthermore, in order to explore the evolution of the species adsorbed on the surface of the FeCeO2-δ and BaFeCeO2-δ catalysts, we performed the in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of NO-TPO experiments at different temperatures. The reaction mechanism for catalyzing the soot combustion over the barium substitution FeCeO2-δ was proposed accordingly.

2. EXPERIMENTAL SECTION 4


Page 4 of 36

Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.1. Catalyst Preparation. A series of FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ mixed oxides samples were prepared via the self-propagating high-temperature synthesis (SHS) method. All reagents (AR grades) used in all experiments were from Aldrich and were used as received, without further purification. The nitrate precursors Ce(NO3)3·6H2O, Fe(NO3)3·9H2O, and Ba(NO3) were dissolved and mixed in deionized water according to the molar ratio of Fe/Ce = 1: 9 and Ba/(Fe + Ce) = 0, 0.05, 0.1, and 0.2. The glycine (CH2NH2COOH) in appropriate amounts was added into the precursor mixture as the liquid phase combustion fuel. After being sufficiently stirred and heated at 80 °C for 2 h, a homogeneous solution was obtained. Then this solution was introduced into a closed electrical furnace and calcined at 450 °C for 5 h. The obtained powders were cooled to the room temperature in the furnace and then submitted for physical and chemical characterization. These catalyst samples prepared would be generically marked as FeCe, Ba(5%)FeCe, Ba(10%)FeCe, Ba(20%)FeCe, and Ba(10%)Ce, respectively. 2.2. Catalyst Characterization. The specific surface areas of the prepared samples were measured by the nitrogen adsorption/desorption at -196 °C on an automatic surface analyzer (Quantachrome NOVA 2000e). The specific surface areas were obtained from the Brunauer-Emmett-Teller (BET) method at relative pressures within the range of 0.05-0.3. Prior to the N2 adsorption, all samples were degassed at 250 °C overnight. The X-ray diffraction (XRD) patterns of powders were measured using a D/max2200PC instrument employing Cu Kα radiation (λ = 0.15418 nm). The X-ray diffractogram was recorded at 0.02° intervals with a scanning rate of 5 °/min, and the scanning range of 2θ was from 20° to 80°. The XRD patterns of the catalysts were compared with those found in the JCPDS reference database for phase 5



identification. The average crystallite size of the catalysts was calculated from the Debye-Scherrer equation and the lattice constant was estimated with Cohen’s method. The transmission electron microscope (TEM) images of the powders were obtained via a JEOL-2010 instrument with an acceleration voltage of 200 kV. After the samples were dispersed into the ethyl alcohol with ultrasonic treatment for 10 min, suspensions were placed on a copper grid and dried in air for the TEM observation. The NO temperature-programmed oxidation (NO-TPO) experiments were carried out to study the capacity of the NOx oxidation and the NOx adsorption/desorption over the catalysts. 100 mg of the catalyst powders were heated from the room temperature to 650 °C at a constant rate of 10 °C/min in a quartz tube. The inlet gas mixture consisting of 10% O2, 1,000 ppm NO, and N2 balanced gas was introduced into the quartz tube at a total flow rate of 500 ml/min. The concentration of the outlet gas was continuously monitored by a FTIR spectrometer (Thermo NICOLET 6700 with quantification software OMNIC). The NO temperature-programmed desorption (NO-TPD) experiments were performed in the same apparatus with the outlet gas monitored by an infrared spectrometer (Thermo Nicolet 6700). Prior to the NO-TPD test, 100 mg of the catalyst powders were pretreated under a flow of 1,000 ppm NO/10% O2/N2 stream (500 ml/min) at 350 °C for 60 min, and were then cooled down to the room temperature and flushed by N2. Afterwards, the desorption was conducted by heating the catalysts from the room temperature to 650 °C at a heating rate of 5 °C/min under a flow of 10% O2/N2 stream (200 ml/min). The concentration of NO and NO2 was monitored by the 6700 spectrometer. The in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was performed on a Nicolet 6700 apparatus equipped with a MCT detector and a heating chamber. The 6


Page 6 of 36

Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst powders were purged in situ and pretreated with 10% O2/N2 (100 ml/min) at 450 °C for 30 min, and were then cooled down to 20 °C in order to remove the contaminants from the catalyst surface. During the cooling down process to 20 °C, the treated catalyst was taken as background at 50 °C, 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, and 450 °C, respectively. Then the in situ experiments were carried out by feeding 1,000 ppm NO/10% O2/ N2 at a flow rate of 100 ml/min. All spectra were recorded by 64 scans accumulation and 4 cm-1 resolution in succession, and the heating rate is 10 °C /min. 2.3. Catalytic Activity Test. The catalytic activity of the prepared catalysts was evaluated by the soot temperature-programmed oxidation (soot-TPO) experiments and the test system is presented in Figure 1. The carbon black from Degussa (Printex-U) was employed in the soot-TPO tests, which is widely used as the model diesel soot for laboratory experiments. The particle size of soot was 25 nm and the specific area was 100 m2/g. Before the reaction, the soot-catalyst mixture was mixed by a spatula in an agate mortar at a mass ratio of 1: 10. The contact manner of the soot-catalyst mixture is defined as “loose contact” considering the actual contact in practical applications. In the soot-TPO experiments, 110 mg of the above mixture was heated at a constant rate of 5 °C/min from the room temperature to 650 °C in a quartz reactor. The feeding gas was O2/N2 (10% O2, balance N2; total flow 500 ml/min), NO/O2/N2 (1,000 ppm, 10% O2, and balance N2; total flow 500 ml/min), and NO/O2/N2 (2,000 ppm, 10%O2, and balance N2; total flow 500 ml/min). The gas hourly space velocity (GHSV) was 30,000 h-1. The concentration of the outlet CO2, CO, NO2, and NO was continuously measured by the Thermo NICOLET 6700 equipment. The temperature when the concentration of CO2 reaches 0.01% is defined as the ignition temperature (Ti) of the soot oxidation and Tm represents the maximum soot oxidation rate temperature. The characteristic 7



temperature T10, T50, and T90 are defined as the temperatures at which 10%, 50%, and 90% of the soot is converted, respectively. The molar ratio of CO2/(CO2 + CO) in the outlet gas is referred as the selectivity to CO2 ( s CO2 ) in the products. The Ozawa method was also employed to determine the apparent activation energy (Ea) during the soot combustion.

Figure 1. Schematic of the experimental setup for the temperature programmed oxidation (TPO) reaction test.

3. RESULTS AND DISCUSSIONS 3.1. Solid Properties. The XRD patterns of the BaFeCeO2-δ catalysts with different barium contents are shown in Figure 2. The results show that the main diffraction peaks of all catalysts are observed at 2θ of 28.53°, 33.05°, 47.48°, 56.33°, 59.08°, 69.38°, 76.66°, and 79.05°, which can be attributed to the cubic fluorite structure of CeO2.26,27 There are no diffraction peaks of iron oxide phase observed in all catalysts, indicating that the iron oxide species are in an amorphous state or with a relatively high dispersion on 8


Page 8 of 36

Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the ceria crystallites.28 For the BaFeCeO2-δ catalysts, the diffraction peaks of BaCO3 can be detected at 2θ of 23.88° when the barium content reaches 10% of the (Fe + Ce) mole number. The intensity of the diffraction peaks of BaCO3 heightens with the increase of the barium content.

Figure 2. XRD patterns of the FeCe, Ba(5%)FeCe, Ba(10%)FeCe，Ba(20%)FeCe, and Ba(10%)Ce samples.

The BET surface areas of the catalyst samples and the average crystallite and lattice constant of ceria are listed in Table 1. FeCeO2-δ has the largest surface area of 55.6 m2/g. However, with the increase of the barium content, the BaFeCeO2-δ catalyst samples gradually decrease on the BET surface area, which is due to the blocking of the support pores caused by the effect of the barium loading.21,29 The mean crystallite size of CeO2 in various samples seems not to be affected by the barium content. Table. 1 also shows that the lattice constant sizes of the barium-containing or iron-containing catalysts are very close to 0.5388 nm. Although the iron and barium nitrates were mixed in the precursor mixture as the liquid phase combustion fuel, the barium and iron may be limited on the surface, not significantly 9



Page 10 of 36

incorporated into the fluorite structure.

(a)

(b)

Figure 3. N2 adsorption-desorption isotherms (a) and BJH desorption pore size distributions (b) based on the N2 adsorption-desorption isotherms of FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts.

Figure 3 (a) presents the N2 adsorption-desorption isotherms of the BaFeCeO2-δ catalysts measured at the liquid nitrogen temperature. All samples show Type-IV isotherms with a H3 hysteresis loop at a high relative pressure. This suggests that the catalyst has a broad pore size distribution in a “large” mesoporous region. The BJH desorption pore size distributions derived from the desorption branches of the N2 adsorption-desorption isotherms are shown in Figure 3 (b), revealing that the active crystallite phases formed in the above catalysts mainly possess micropores and mesopores. As can be noticed from Table 1, the average pore diameter and the lattice constant decrease gradually with the barium substitution to the FeCeO2-δ catalyst, which indicates the blocking of the pores of the catalysts. Table 1. Structural Properties of the Catalysts Catalysts

BET surface

Pore volume

Average pore

Lattice

Average

area

(cm3/g)

diameter

constant

crystallite size

(nm)

(nm)

(nm)

(m2/g)

10


Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FeCe

55.6

0.134

12.0

0.5388

11.3

Ba(5%)FeCe

47.2

0.108

11.4

0.5389

11.2

Ba(10%)FeCe

33.2

0.085

10.9

0.5389

11.4

Ba(20%)FeCe

19.4

0.072

9.5

0.5391

11.4

Ba(10%)Ce

11.6

0.056

8.1

0.5388

13.2

TEM was carried out to observe the actual surface morphology of different catalysts. The

representative TEM images of the FeCeO2-δ and BaFeCeO2-δ catalysts and the corresponding particle size distribution are displayed in Figure 4. From the view of the particle size distribution, the FeCeO2-δ and BaFeCeO2-δ catalysts synthesized by the SHS method attained the nano-levels. As shown in Figure 4, the FeCeO2-δ catalyst is well dispersed and the average size is 12.845 nm, relatively smaller than that of the BaFeCeO2-δ catalyst. The particle size of the as-prepared catalyst samples increases with the content of the barium substitution. The average size of the BaFeCeO2-δ catalyst grows up to 13.206 nm and these differences can be explained by the barium substitution. (a)

(b)

11



(c)

(d)

Figure 4. TEM images (a) and particle size distribution (b) of FeCe catalyst sample, TEM images (c) and particle size distribution (d) of Ba(10%)FeCe.

3.2. NO-TPO. It is known that NO2 as a powerful oxidant can greatly promote the soot oxidation process. The NO2 production capacity is affected by the oxidation of gaseous NO and the decomposition capacity of the nitrates stored at low temperatures.30-32 Figure 5 (a) shows the NO2 production profiles of the FeCeO2-δ and BaFeCeO2-δ catalysts obtained in the NO-TPO experiments. In order to analyze the effect of the barium loading on the NO oxidation to NO2, we compared the catalyst samples with different barium contents. The NO2 production onset temperature of FeCeO2-δ is 234 °C, far below that of the BaFeCeO2-δ catalysts, indicating that FeCeO2-δ exhibits much better activity for NO oxidation than the BaFeCeO2-δ catalysts. For the FeCeO2-δ catalyst, the maximum NO conversion to NO2 of approximate 39% is at 345 °C. The BaFeCeO2-δ catalysts present the similar NO2 profiles, among which the Ba(10%)FeCe catalyst sample shows the best performance with NO2 formation at 258 °C and a maximum NO2 concentration level at 397 °C. It can be observed that both the temperature of the NO2 production and the maximum concentration NO2 level increase with the content of the barium loading. This phenomenon can be explained by the active sites blocking of the BaFeCeO2-δ catalysts. Figure 5 (b) 12


Page 12 of 36

Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shows the total NOx profiles of the FeCeO2-δ and BaFeCeO2-δ catalysts. The amount of the adsorbed NO and the desorbed NOx species of the studied catalysts is different. It can be found that the FeCeO2-δ catalyst has strong capacity to store NO at low temperatures; however, no obvious NOx desorption can be observed when the temperature exceeds 200 °C. For the BaFeCeO2-δ catalysts, the introduction of different barium contents to FeCeO2-δ greatly improves the NOx storage and desorption capacity at a high temperature range from 250 °C to 550 °C.

(b)

(a)

Figure 5. Evolutions of NO2 (a) and NOx (b) during the NO-TPO tests in the gas mixture consisting of 10% O2, 1,000 ppm NO, and N2 balanced gas.

In order to mitigate the adsorption and desorption phenomena, which are unavoidable in the ramp test, the NO-TPO experiments through isothermal steps are carried out. Figure 6 (a) shows the NO2 production profiles of the FeCeO2-δ and BaFeCeO2-δ catalysts obtained in the NO-TPO experiments under the isothermal conditions, in which the concentration of the components at each temperature was obtained after 0.5 h of steady state reaction. The FeCeO2-δ catalyst exhibits much better activity for NO oxidation than the BaFeCeO2-δ catalysts. The onset production of NO2 is nearly at 250 °C and the maximum concentration of NO2 is at 350 °C. Figure 6 (b) shows the total NOx profiles of the FeCeO2-δ and BaFeCeO2-δ catalysts. The barium substitution to the FeCeO2-δ catalyst largely improves the NOx 13



Page 14 of 36

storage capacity, which is consistent with the results from the experiments through the non-isothermal steps. (a)

(b)

Figure 6. Evolutions of NO2 (a) and NOx (b) during the NO-TPO tests in the gas mixture consisting of 10% O2, 1,000 ppm NO, and N2 balanced gas under the isothermal conditions.

3.3. NO-TPD. In order to study the stability as well as the composition of the adsorbed NOx species, we carried out the NO-TPD experiments. Figure 7 (a) and Figure 7 (b) show the results obtained from the NO-TPD experiments on the FeCeO2-δ and BaFeCeO2-δ catalyst samples, which are the concentration profiles of the NO2 and NO products. In the case of the FeCeO2-δ catalyst samples (Figure 7 (a)), the strong NO2 desorption peak is observed, showing a maximum value of 338 ppm. The decomposition of the stored NO2 occurs at a relatively slow temperature of 228 °C and shows a significant release of NO2 in the range of 228 °C to 400 °C. For the BaFeO2-δ catalyst samples (Figure 7 (a)), the production of NO2 starts at approximately 270 °C and the concentration of NO2 shows a very slow increase. The Ba(5%)FeCe reaches the maximum NO2 value of 150 ppm. Similar results have been obtained in the case of Ba(10%)FeCe and Ba(20%)FeCe, and the maximum NO2 value is 105 ppm and 89 ppm, respectively. It can be observed that the FeCeO2-δ catalyst exhibits much better activity for the NO 14


Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidation than the BaFeCeO2-δ catalyst. This is in line with the previous results, indicating that the barium loading on the FeCeO2-δ catalyst decreases the number of the contact points between the catalyst particles and the adsorbed NOx, especially on the surface oxidant sites. These sites are reduced when different contents of barium are loaded, thus decreasing the overall reaction rate of the NO oxidation to NO2. At 450 °C, the NO2 decomposition of all catalyst samples is completed, while Ba(10%)Ce shows the worst activity. Figure 7 (b) presents the desorption profiles of NO at 50-600 °C. In the range of 50 °C to 200 °C, NO is the major decomposition product, and no detectable amounts of NO2 can been found. The increase of the NO concentration observed at the beginning of the TPD profiles may result from the desorption of the weakly adsorbed NOx. In fact, the nitrogen oxides adsorbed on the catalyst surface -

-

form the unstable species of NO3 and NO2 ions, which can decompose a large amount of NO at low temperatures. For the BaFeCeO2-δ catalyst samples, the decomposition of the stored NO is also observed, but it is clearly apparent that the introduction of barium to FeCeO2-δ greatly reduces the weak NOx adsorption. In the range of 200 °C to 300 °C, there is no obvious NO observed in the TPD profiles. It is worth noting that the barium loading in the FeCeO2-δ catalyst dramatically affects its ability to absorb and decompose the trapped NOx above 300 °C. The NO concentration of the BaFeCeO2-δ catalyst samples rapidly increases, showing a maximum value of 380 ppm near 400 °C, and then decreases till the heating ramp at 500 °C. That is due to the formation of the Ba(NO3)2 species whose decomposition needs a relatively high temperature.33

15



(a)

Page 16 of 36

(b)

Figure 7. Evolutions of NO2 (a) and NO (b) during the NO-TPD tests in the gas mixture consisting of 10% O2, 1,000 ppm NO, and N2 balanced gas.

3.4. Soot-TPO. The aims of the soot-TPO experiments are: first, to study the catalytic performance of the barium loading in the FeCeO2-δ catalyst samples towards the soot combustion reaction; second, to discuss the effect of the NOx assistance on promoting the soot combustion. In order to achieve the results above, we employed three reaction modes with NO concentrations of the gas flow from 0 to 2,000 ppm. The soot-TPO profiles and the CO2 conversion curves during the catalytic tests on the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalyst samples are plotted as a function of temperature in Figure 8. The derived values of the ignition temperature (Ti), the maximum soot oxidation rate temperature (Tm), 10% soot conversion (T10) , 50% soot conversion (T50), and 90% soot conversion (T90) under different reaction conditions are listed in Table 2. In an attempt to conduct a proper comparison, we employed the same amount of catalyst (90 mg) and soot (10 mg) in all experiments. To evaluate the catalytic activities of the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts in the absence of NO, the soot-TPO experiments carried out with 10% O2/balanced N2 are shown in Figure 8 (a) and Figure 8 (b). The soot-TPO curve corresponding to the FeCeO2-δ catalyst was the most active 16


Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst that showed the soot ignition temperature of 342 °C and the maximum soot oxidation temperature of 504 °C. In all investigated samples of the BaFeCeO2-δ catalysts, a slight decrease of the soot ignition temperature is achieved when the barium adding is 5%, 10%, and 20%. This fact is in accordance with the previous results that the BaFeCeO2-δ catalysts are less efficient as NO2 producers than the FeCeO2-δ catalysts. Therefore, the FeCeO2-δ catalyst exhibits better soot catalytic activity than the BaFeCeO2-δ catalyst in the absence of NO due to the blocking of the active sites in the BaFeCeO2-δ catalyst. Among the BaFeCeO2-δ catalysts, the Ba(10%)FeCe catalyst displays the best catalytic performance. Ti of the Ba(10%)FeCe catalyst is 386 °C and Tm is 524 °C. The CO2 selectivity of the Ba(10%)FeCe catalyst reaches a high level (92.2%). The BaCeO2-δ catalyst shows the worst soot oxidation activity in view of its Ti (around 430 °C) and Tm (around 555 °C), and the CO2 selectivity is low (78.9%) due to less NO2 formation. Different from the soot combustion behaviors when only fed with O2 and N2, much higher catalytic soot combustion activity is observed when 1,000 ppm NO is introduced as shown in Figure 8 (c) and Figure 8 (d). All soot combustion profiles and conversion curves show a significant shift towards lower temperatures in the presence of NO, especially for the BaFeCeO2-δ catalyst. It should be noted that there are dramatic differences in the soot oxidation process when the feeding gas contains NO. The promotion effect of NO on the soot combustion is more obvious on the BaFeCeO2-δ catalysts. The soot ignition temperatures of the Ba(5%)FeCe, Ba(10%)FeCe, and Ba(20%)FeCe catalysts decrease to 296 °C, 284 °C, and 299 °C with high CO2 selectivity of 95.4%, 96.1%, and 95.2%, respectively, which is superior to the catalytic performance of the FeCeO2-δ catalyst with the soot ignition temperature of 305 °C and the CO2 selectivity of 95.0%. Even though the FeCeO2-δ catalyst generates a much higher amount of NO2 than the BaFeCeO2-δ catalysts in the 1,000 ppm NO/10% O2/N2 atmosphere, the 17



Page 18 of 36

FeCeO2-δ catalyst is less efficient in the soot combustion compared with whatever BaFeCeO2-δ catalysts selected for this study. On one hand, the high catalytic activity for the soot oxidation in the 1,000 ppm NO/10% O2/N2 atmosphere is related to the production of NO2 from the NO oxidation. NO2 is a stronger oxidant than O2 and plays crucial roles for the soot combustion in the NOx atmosphere. On the other hand, it has also to be mentioned that the Ba-containing catalyst has much better NOx storage and desorption capacity. The Ba(NO3)2 species is one of the intermediates in the soot-TPO reaction and it can decompose a great deal of NO2 to assist the soot oxidization at temperatures up to 300 °C. Figure 8 (e) and Figure 8 (f) show the soot combustion profiles and the soot conversion curves obtained under 2,000 ppm/10% O2/N2, as the O2 concentration remained constant (10 vol% O2). It should be remarked that the results obtained when NO concentration is 2,000 ppm are quite similar to those obtained in Figure 8 (c) and Figure 8 (d). Ti values over the FeCeO2-δ, Ba(5%)FeCeO2-δ, Ba(10%)FeCeO2-δ, Ba(20%)FeCeO2-δ, and BaCeO2-δ catalysts are 304 °C, 290 °C, 282 °C, 288 °C, and 372 °C, and Tm values are 426 °C, 414 °C,405 °C, 460 °C, and 514 °C, respectively, which are slightly lower than the Ti and Tm obtained in the 1,000 ppm NO atmosphere. The CO2 selectivity of the FeCeO2-δ, Ba(5%)FeCeO2-δ, Ba(10%)FeCeO2-δ, Ba(20%)FeCeO2-δ, and BaCeO2-δ catalysts are 95.8%, 96.2%, 96.8%, 96.6%, and 79.9%, respectively. That is to say, the soot catalytic combustion can be slightly improved when the NO concentration is up to 2,000 ppm.

(b)

(a)

18


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

(d)

(e)

(f)

Figure 8. CO2 and CO concentration evolution (a) and soot conversion (b) in the gas mixture consisting of 10% O2, and N2 balanced gas, CO2 and CO concentration evolution (c) and soot conversion (d) in the gas mixture consisting of 1,000 ppm NO, 10% O2, and N2 balanced gas, CO2 and CO concentration evolution (e) and soot conversion (f) in the gas mixture consisting of 2,000 ppm NO, 10% O2, and N2 balanced gas during soot-TPO tests.

Subsequently, the soot oxidation activity of the above catalysts in the isothermal conditions was tested, and the results are shown in Figure 9. The oxidation rate of the particles reaches maximum at a regeneration temperature of 450 °C with 10% barium substitution of FeCeO2-δ, consistent with the result achieved in the non-isothermal conditions. Similarly, it can be observed that the Ba(5%)FeCe, Ba(10%)FeCe, Ba(20%)FeCe catalysts show better low temperature oxidation activity than the FeCeO2-δ and BaCeO2-δ catalysts, indicating that a certain proportion of barium contributes to better 19



Page 20 of 36

soot oxidation activity.

(b)

(a)

Figure 9. CO2 and CO concentration evolution (a) and soot conversion (b) over the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts in the gas mixture consisting of 1,000 ppm NO, 10% O2, and N2 balanced gas in the isothermal conditions.

Furthermore, in order to estimate the parameters based on the intrinsic kinetics, we performed the soot oxidation of the catalysts evaluated at tight contact with soot. The corresponding soot-TPO curves and the characteristic temperature and kinetic parameter of the soot combustion over the FeCe, BaFeCeO2-δ, and BaCeO2-δ catalysts are shown in Figure 10 and Table 3, respectively. A further promotion of the soot combustion performance could be observed due to the tight contact. The soot-TPO curves show that the Ba(10%)FeCe catalyst was the most active catalyst that showed the soot ignition temperature of 314 °C and the maximum soot oxidation temperature of 484 °C.

(b)

(a)

20


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. CO2 and CO concentration evolution (a) and soot conversion (b) over the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts in the gas mixture consisting of 10% O2 and N2 balanced gas in the tight conditions.

To ensure that the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts are solidified, we evaluated the soot-TPO experiments on the catalysts calcined at 650 °C. The corresponding soot-TPO curves and the characteristic temperature and kinetic parameter of the soot combustion over the FeCe, BaFeCeO2-δ, and BaCeO2-δ catalysts are shown in Figure 11 and Table 4, respectively. The soot catalytic ignition temperature for soot is slightly improved. The best result has been attained with Ba(10%)FeCeO2-δ over which the maximum soot oxidation rate temperature improved to 452 °C and the soot ignition temperature was 293 °C. The properties of the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts calcined at 650 °C were inferior to those of the samples calcined at 450 °C. However, the overall catalytic performance is almost the same with each other.

(a)

(b)

Figure 11. CO2 and CO concentration evolution (a) and soot conversion (b) over the FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts calcined at 650 °C in the gas mixture consisting of 1,000 ppm NO, 10% O2, and N2 balanced gas.

21



Page 22 of 36

Table 2. Characteristic Temperature and Kinetic Parameter of the Soot Combustion over the FeCe, BaFeCeO2-δ, and BaCeO2-δ Catalysts Calcined at 450 °C

Feed gas

10% O2/N2

1,000 ppm NO/10% O2/N2

2,000 ppm NO/10% O2/N2

Ti

Tm

T10

T50

T90

Ea

(°C)

(°C)

(°C)

(°C)

(°C)

(kJ/mol)

FeCe

332

504

408

497

568

124.3

Ba(5%)FeCe

373

528

436

524

598

138.4

Ba(10%)FeCe

394

538

442

528

600

140.8

Ba(20%)FeCe

400

547

448

536

604

147.2

Ba(10%)Ce

432

558

457

548

610

154.2

FeCe

305

480

373

468

540

109.1

Ba(5%)FeCe

288

448

356

445

525

97.4

Ba(10%)FeCe

282

439

340

428

506

92.6

Ba(20%)FeCe

302

460

362

453

529

102.7

Ba(10%)Ce

379

516

414

510

570

128.8

FeCe

296

462

358

450

529

98.5

Ba(5%)FeCe

282

336

424

508

90.6

Ba(10%)FeCe

276

419

324

408

486

84.9

Ba(20%)FeCe

286

438

343

434

515

94.1

Ba(10%)Ce

370

514

419

510

570

129.4

Samples

433

Table 3. Characteristic Temperature and Kinetic Parameter of the Soot Combustion over the 22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FeCe, BaFeCeO2-δ, and BaCeO2-δ Catalysts Calcined at 450 °C under Tight Contact

Feed gas

10% O2/N2

Ti

Tm

T10

T50

T90

Ea

(°C)

(°C)

(°C)

(°C)

(°C)

(kJ/mol)

FeCe

314

484

389

477

523

130.5

Ba(5%)FeCe

351

506

424

503

554

142.9

Ba(10%)FeCe

373

519

423

509

562

144.9

Ba(20%)FeCe

385

525

425

519

586

149.3

Ba(10%)Ce

419

536

437

525

575

155.6

Samples

Table 4. Characteristic Temperature and Kinetic Parameter of the Soot Combustion over the FeCe, BaFeCeO2-δ, and BaCeO2-δ Catalysts Calcined at 650 °C

Feed gas

1,000 ppm NO/10% O2/N2

Ti

Tm

T10

T50

T90

Ea

(°C)

(°C)

(°C)

(°C)

(°C)

(kJ/mol)

FeCe

313

487

380

474

545

104.2

Ba(5%)FeCe

295

456

366

454

530

91.7

Ba(10%)FeCe

293

452

352

441

520

87.5

Ba(20%)FeCe

312

472

375

464

542

96.5

Ba(10%)Ce

393

530

428

522

583

123.2

Samples

To summarize this section and further interpret the intrinsic mechanism of the soot combustion over the catalysts, we listed some characteristic temperatures of Ti, Tm, T10, T50, and T90 from the soot-TPO curves and the values of the apparent activation energy by kinetic studies in Table 2. For the 23



BaFeCeO2-δ catalysts, the ability of the soot oxidation in the atmosphere of 1,000 ppm NO/10% O2/N2 is significantly different from that in the absence of NO in the feed gas. Among the catalysts applied, Ba(10%)FeCe is found to exhibit the best catalytic ability for the soot combustion. However, when the NO concentration shifted to 2,000 ppm, the catalytic behavior of the soot oxidation was not drastically improved for the BaFeCeO2-δ catalysts. The apparent activation energy (Ea) for the soot combustion was measured for comparison as well. It can been seen that the loading of barium may cover some of the active sites on the FeCeO2-δ catalysts and result in a rise of the activation energy for the supported barium catalysts in the absence of NO. Under the 1,000 ppm NO/O2/N2 atmosphere, the calculated activation energy of all catalysts dramatically dropped. It is also worth noting that the Ba(10%)FeCe catalyst exhibits the lowest activation energy (92.6 kJ/mol), which represents a significant drop of the activation energy, further supporting the considerable enhancement of the catalytic activity. These results are well correlated with the previous experiment data, where the barium loading is revealed as crucial for promoting the soot combustion34. The detailed mechanism of catalyzing the soot combustion over the BaFeCeO2-δ catalysts will be discussed subsequently.

3.5. In Situ DRIFTS. To take a close look at the adsorbed NOx species on the surface of the FeCeO2-δ and BaFeCeO2-δ catalysts, we carried out the in situ FTIR experiments, where the gas mixtures of 1,000 ppm NO/10% O2/N2 were fed at a flow rate of 100 ml/min. The DRIFT spectra of the stored NOx species on the FeCeO2-δ surface as a function of temperature are shown in Figure 12 (a). FeCeO2-δ mainly exhibits bands corresponding to nitrites (1204 cm-1 for bidentate nitrite and 1475 cm-1 for monodentate nitrite) and nitrates (999 cm-1 and 1025 cm-1 for bridging nitrate, 1349 cm-1 for monodentate nitrate, 1557 cm-1 24


Page 24 of 36

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for chelated nitrate, 1592 cm-1 for bidentate nitrate, and 1630 cm-1 for bridged nitrate).35-38 With the increase of the temperature, all of the bands assigned to nitrites decline in intensity. Then the bands at 1194 cm-1 and 1475 cm-1 disappear at 200 °C, whereas a transformation of nitrites to nitrates occurs. All of the bands assigned to bridging nitrate, monodentate nitrate, chelated nitrate, and bridged nitrate remain almost unchanged as the temperature increases from 50 °C to 300 °C. Then at higher temperatures over 300 °C, the bands drop intensely in intensity and almost disappear at 450 °C. Considering the previous NO-TPO experiments, the trend of the NO2 concentration as a function of temperature accords well with the in situ DRIFTS results. The addition of barium into the FeCeO2-δ catalysts may change the state of the adsorbed NOx species and the in situ DRIFT spectra over the Ba(10%)FeCe catalysts are exhibited in Figure. 12 (b). The obtained results confirmed the formation of nitrites (1495 cm-1 for monodentate nitrite and 1204 cm-1 for bidentate nitrite) and nitrates (1025 cm-1 for bridging nitrate, 1359 cm-1 for monodentate nitrate, 1569 cm-1 for chelated nitrate, 1597 cm-1 for bidentate nitrate, and 1642 cm-1 for bridged nitrate) on the catalyst surface.39 All of the bands assigned to the nitrites on the surface of the BaFeCeO2-δ catalysts are relatively stable, compared to those on the surface of the FeCeO2-δ catalysts, and are transformed to nitrates at temperatures over 350 °C, which demonstrates that the oxidation capacity of BaFeCeO2-δ is weaker than that of FeCeO2-δ. It is interesting to find that the adsorption band at 1521 cm-1 and 1172 cm-1 arises in the temperature range from 350 °C to 450 °C. This phenomenon can be attributed to the Ba(NO3)2 species formed on the BaFeCeO2-δ catalyst surface.40,41 Furthermore, the two adsorption bands exist till the temperature reaches 450 °C.

25



(a)

(b)

Figure 12. DRIFT spectra obtained over FeCe (a) and Ba(10%)FeCe (b) in 1,000 ppm NO/10% O2/N2 as a function of temperature.

3.6. Mechanism of Catalyzing Soot Combustion Reaction. On the basis of the in situ characterization results and the evaluation of the catalytic performance over the catalysts, the mechanism of catalyzing the soot combustion reaction on the surface of the 26


Page 26 of 36

Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BaFeCeO2-δ catalyst in the presence of NOx can be proposed.

Figure 13. Scheme of the whole reaction process of catalyzing the soot combustion.

There are several reaction pathways during the soot oxidation over the BaFeCeO2-δ catalysts. The whole process is summarized in the scheme of Figure 13. According to previous studies,9,42-44 the active oxygen species of O- and O2- on the surface of the ceria-based catalysts can react with soot particles and release CO and CO2 (Eq. (1-2)). -

2C + 3O → CO + CO2

(1)

-

4C + 3O2 → 2CO + 2CO2

(2)

Furthermore, NO can be readily transferred to NO2 (Eq. 3) and the oxidability of NO2 toward soot is much higher than that of O2 and NO, which can initiate soot combustion (Eq. (4-5)). 2NO + O2 → 2NO2

(3)

C + 2NO2 → 2NO + CO2

(4)

C + NO2 → NO + CO

(5) 27



Page 28 of 36

It can be observed from the DRIFTS experiments that a large amount of surface nitrite and nitrate species was generated in the process of soot oxidation. NO and NO2 react with O2- and O- in the temperature range from 50 °C to 350 °C according to the equations (Eq. (6-8)). Such surface nitrite and nitrate species directly react with soot and are converted into N2 (Eq. (9-10)). -

2NO + O2 → 2NO2-

(6)

-

NO2 + O → NO3 -

(7)

-

NO + O → NO2

(8)

-

2NO3 + 2C → N2 + CO + CO2 + 3O -

-

(9)

-

2NO2 + 2C → N2 + CO + CO2 + O

(10)

Then at temperatures over 350 °C, the Ba(NO3)2 species were found to be generated on the surface of the BaFeCe2-δ catalyst, which demonstrated that the adsorption of NOx species is also a crucial step in promoting the soot combustion. The reaction to form Ba(NO3)2 species is shown by the equations (Eq. (11-12)). Then in this reaction pathway, the decomposition of such Ba(NO3)2 species produces a great deal of NO2, which is active toward the soot combustion (Eq. (13-14)). 4NO + 3O2 + 2BaO → 2Ba(NO3)2

(11)

4NO2 + O2 + 2BaO → 2Ba(NO3)2

(12)

2Ba(NO3)2 → 2BaO + 4NO + 3O2

(13)

2Ba(NO3)2 → 2BaO + 4NO2 + O2

(14)

Besides, the direct contribution of the continuous supplement of atmospheric O2 cannot be ruled out and O2 can initiate the soot combustion at a high temperature. C + O2 → CO2

(15)

2C + O2 → 2CO

(16) 28


Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. CONCLUSIONS In this study, a series of FeCeO2-δ, BaFeCeO2-δ, and BaCeO2-δ catalysts prepared by the SHS method were characterized and studied for the soot oxidation in the presence and absence of NO. Special attention has been paid to the introduction of barium to the FeCeO2-δ catalyst on the diesel soot combustion and to the capacity of the NOx adsorption and desorption. For the FeCeO2-δ catalysts, the highest BET surface area can be observed and FeCeO2-δ presents superior activity of soot oxidation than the catalysts with different contents of barium in the absence of NOx, which is due to the active sites blocking when barium is doped onto the FeCeO2-δ catalysts. However, a prominent enhancement effect on the soot oxidation is observed in the presence of NOx. All BaFeCeO2-δ catalysts tested are more effective for soot combustion than previous FeCeO2-δ, and the highest soot catalytic performance with Tm of 284 °C is obtained for the Ba(10%)FeCe sample. In spite of the reduced NO oxidation activity, the capacity of NOx adsorption and desorption is largely improved by the barium loading, which plays a crucial role in the process of soot combustion. The in situ DRIFTS characterization results were consistent with the NO-TPD results. NOx can be adsorbed over the catalyst surface to form nitrite and nitrate species and the excellent performance of the soot oxidation is closely related to the formation and decomposition of nitrates. Furthermore, several surface reactions for catalyzing the soot combustion reaction on the surface of the BaFeCeO2-δ catalysts have been proposed by previous studies. First, the active oxygen species of Oand O2- on the surface of the ceria-based catalysts can not only act as an oxidant for the NO oxidation to

promote the soot oxidation, but also play a role of efficient mobile oxidizing agent to oxidize soot directly. Second, the surface nitrite and nitrate species generated in the process of soot oxidation 29



directly react with soot and are converted into N2 in the temperature range from 50 °C to 350 °C. Third, the decomposition of Ba(NO3)2 species produces a large amount of NO2 to promote the soot combustion.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2017A07), the National Natural Science Foundation of China (51176118), the National Natural Science Foundation of China for Young Scientists (51306115), the National Natural Science Foundation of China (51676127), the National key research and development plan (2016YFC0205200), the National key research and development plan (2016YFC0208000), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry.

REFERENCES (1) Wang, Y. J.; Li, L.; Chen, C. H.; Huang, C.; Huang, H. Y.; Feng J. L.; Wang, S. X.; Wang, H. L.; Zhang, J. F.; Zhou, M.; Cheng, P.; Wu M. H.; Sheng, G. Y.; Fu, J. M.; Hu Y. T.; Russell, A. G.; Wumaer A. Source Apportionment of Fine Particulate Matter During Autumn Haze Episodes in Shanghai, China. J. Geophys. Res.-Atmos. 2014, 119, 1903-1914. (2) Tian, S L.; Pan, Y. P.; Wang, Y. S. Size-resolved Source Apportionment of Particulate Matter in 30


Page 30 of 36

Page 31 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Urban Beijing During Haze and Non-haze Episodes. Atmos. Chem. Phys. 2016, 16, 9405-9443. (3) Fang, Y.; Lou, D.; Hu, Z.; Tan, P. Effects of DOC + DPF on Physical and Chemical Characteristics of PM Emitted from Diesel Engine Fueled with Biodiesel Blends. Transactions of Csice. 2016, 34, 142-146. (4) Choi, B. C.; Foster, D. E. Overview of the Effect of Catalyst Formulation and Exhaust Gas Compositions on Soot Oxidation in DPF. J. Mech. Sci. Technol. 2006. 20, 1-12. (5) Fino, D.; Bensaid, S.; Piumetti, M. A Review on the Catalytic Combustion of Soot in Diesel Particulate Filters for Automotive Applications: From Powder Catalysts to Structured Reactors. Appl. Catal. A-Gen. 2016, 509, 75-96. (6) Lim, C. B.; Kusaba, H.; Einaga H.; Teraoka, Y. Catalytic Performance of Supported Precious Metal Catalysts for the Combustion of Diesel Particulate Matter. Catal. Today. 2011, 175, 106-111. (7) Matsuoka, K.; Orikasa, H.; Itoh, Y.; Chambrion, P.; Tomita, A. Reaction of NO with Soot over Pt-loaded Catalyst in the Presence of Oxygen. Appl. Catal. B-Environ. 2000, 26, 89-99. (8) Setiabudi, A.; Chen, J.; Mul, G.; Makkee, M.; Moulijn, J. A. CeO2, Catalysed Soot Oxidation: The Role of Active Oxygen to Accelerate the Oxidation Conversion. Appl. Catal. B-Environ. 2004, 51, 9-19. (9) Bueno-López, A.; Krishna, K.; Makkee, M.; Moulijn J. A. Active Oxygen from CeO2, and its Role in Catalysed Soot Oxidation. Catal. Lett. 2005, 99, 203-205. (10) Zhou, K.; Wang, X.; Sun, X.; Peng, Q.; Li, Y. Enhanced Catalytic Activity of Ceria Nanorods from Well-defined Reactive Crystal Planes. J. Catal. 2005, 229, 206-212. (11) Atribak, I.; Bueno-López, A.; García-García, A.; Navarro, P.; Frías, D. Catalytic Activity for Soot Combustion of Birnessite and Cryptomelane. Appl. Catal. B-Environ. 2010, 93, 267-273. 31



(12) Shen, Q.; Lu, G.; Du, C. Guo, Y.; Wang, Y. Role and Reduction of NOx in the Catalytic Combustion of Soot over Iron-ceria Mixed oxide Catalyst. Chem. Eng. J. 2013, 218, 164-172. (13) Liu, S.; Wu, X. D.; Wen, D.; Ran, R. Ceria-based Catalysts for Soot Oxidation: A Review. J. Rare. Earth. 2015, 33, 567-590. (14) Wang, Y.; Wang, J.; Chen, H.; Yao, M.; Li, Y. Preparation and NOx-assisted Soot Oxidation Activity of a CuO-CeO2 Mixed Oxide Catalyst. Chem. Eng. Sci. 2015, 135, 294-300. (15) Zhang, H.; Gu, F.; Liu, Q.; Gao, J.; Jia, L. MnO-CeO Supported On a Three-dimensional and Networked SBA-15 Monolith for NO-assisted Soot Combustion. Rsc Adv. 2014, 4, 14879-14889. (16) Ye, K. H.; Zhou, S. A.; Zhu, X. C.; Xu, C. W.; Shen, P. K. Stability Analysis of Oxide (CeO2, NiO, Co3O4, and Mn3O4) Effect on Pd/C for Methanol Oxidation in Alkaline Medium. Electrochim. Acta. 2013, 90, 108-111. (17) Aneggi, E.; Leitenburg, C. D.; Dolcetti, G.; Trovarelli, A. Diesel Soot Combustion Activity of Ceria Promoted with Alkali Metals. Catal. Today. 2008, 136, 3-10. (18) An, H.; Kilroy, C.; Mcginn, P. J. Combinatorial Synthesis and Characterization of Alkali Metal Doped Oxides for Diesel Soot Combustion. Catal. Today. 2004, 98, 423-429. (19) Jiménez, R.; García, X.; López, T.; Gordon, A. L. Catalytic combustion of Soot. Effects of Added Alkali Metals on CaO-MgO Physical Mixtures. Fuel. Process. Technol. 2008, 89, 1160-1168. (20) Peralta, M. A.; Zanuttini, M. S.; Querini, C. A. Activity and Stability of BaKCo/CeO2 Catalysts for Diesel Soot Oxidation. Appl. Catal. B-Environ. 2011, 110, 90-98. (21) Wu, X.; Liu, S.; Lin, F.; Weng D. Nitrate Storage Behavior of Ba/MnOx-CeO2 Catalyst and its Activity for Soot Oxidation with Heat Transfer Limitations. J. Hazard. Mater. 2010, 181, 722-728. (22) Alinezhadchamazketi, A.; Khodadadi, A. A.; Mortazavi, Y.; Nemati, A. Catalytic Evaluation of 32


Page 32 of 36

Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Promoted CeO2-ZrO2 by Transition, Alkali, and Alkaline-earth Metal Oxides for Diesel Soot Oxidation. J. Environ. Sci-China. 2013, 25, 2498-2506. (23) Pereda-Ayo, B.; Torre, U. D. L.; González-Marcos, M. P.; González-Velasco, J. R. Influence of Ceria Loading on the NOx, Storage and Reduction Performance of Model Pt-Ba/Al2O3, NSR Catalyst. Catal. Today. 2016, 267, 119-129. (24) Matarrese, R.; Aneggi, E.; Castoldi, L.; Llorca, J.; Trovarelli, A. Simultaneous Removal of Soot and NOx, over K- and Ba-doped Ruthenium Supported Catalysts. Catal. Today. 2016, 267, 119-129. (25) Marcano, S. J. C.; Bensaid, S.; Deorsola, F. A.; Russo, N.; Fino, D. Multifunctional Catalyst Based on BaO/Pt/CeO2, for NO2-Assisted Soot Abatement and NOx Storage. Fuel. 2015, 149, 78-84. (26) Atribak, I.; Azambre, B.; López, A.B.; García-García, A. Effect of NOx, Adsorption/Desorption over Ceria-zirconia Catalysts on the Catalytic Combustion of Model Soot. Appl. Catal. B-Environ. 2009, 92, 126-137. (27) Bueno-López, A.; Krishna, K.; Makkee, M.; Moulijn, J. A. Enhanced Soot Oxidation by Lattice Oxygen via La3+-doped CeO2. J. Catal. 2005, 230, 237-248. (28) Damyanova, S.; Perez, C. A.; Schmal, M.; Bueno, J. M. C. Characterization of Ceria-coated Alumina Carrier. Appl. Catal. A-Gen. 2002. 234, 271-282. (29) Zhang, Z.; Han, D.; Wei, S.; Zhang, Y. Determination of Active Site Densities and Mechanisms for Soot Combustion with O2, on Fe-doped CeO2 Mixed Oxides. J. Catal. 2010, 276, 16-23. (30) Guillén-Hurtado, N.; Bueno-López, A.; García-García, A. Catalytic Performances of Ceria and Ceria-zirconia Materials for the Combustion of Diesel Soot under NOx/O2, and O2. Importance of the Cerium Precursor Salt. Appl. Catal. A-Gen. 2012, 437-438, 166-172. (31) Milt, V. G.; Querini, C. A.; Miró, E. E.; Ulla, M. A. Abatement of Diesel Exhaust Pollutants: NOx 33



Adsorption on Co, Ba, K/CeO2 Catalysts. J. Catal. 2003, 220, 424-432. (32) Radlik, M.; Adamowska, M.; Łamacz, A.; Krztoń A.; Costa, P. D. Study of the Surface Evolution of Nitrogen Species on CuO/CeZrO2 Catalysts. React. Kinet. Mech. Cat. 2013, 109, 43-56. (33) Zhou, G.; Luo, T.; Gorte, R. J. An Investigation of NOx, Storage on Pt-BaO-Al2O3. Appl. Catal. B-Environ. 2006, 64, 88-95. (34) Sahoo, T. R.; Armandi, M.; Arletti, R.; Piumetti, M.; Bensaid, S.; Manzoli, M.; Panda, S. R.; Bonelli, B. Pure and Fe-doped CeO2 Nanoparticles Obtained by Microwave Assisted Combustion Synthesis: Physico-chemical Properties Ruling Their Catalytic Activity towards CO Oxidation and Soot Combustion. Appl. Catal. B-Environ. 2017, 211, 31-45. (35) Hadjiivanov, K. Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy. Catal. Rev. 2000, 42, 71-144. (36) Shen, Q.; Lu, G.; Du, C.; Guo Y.; Wang, Y. Role and Reduction of NOx in the Catalytic Combustion of Soot over Iron-ceria Mixed Oxide Catalyst. Chem. Eng. J. 2013, 218, 164-172. (37) Kayhan, E.; Andonova, S. M.; Şentürk, G. S.; Chusuei, C. C.; Ozensoy, E. Fe Promoted NOx Storage Materials: Structural Properties and NOx Uptake. J. Phys. Chem. C. 2010, 114, 122-138. (38) Wang, L.; Ran, R.; Wu, X. D. In situ DRIFTS study of NOx adsorption behavior on Ba/CeO2 catalysts. J. Rare. Earth. 2013, 31, 1074-1080. (39) Setiabudi, A.; Chen, J.; Mul, G.; Makkee M.; Moulijn J. A. CeO2 Catalysed Soot Oxidation: The Role of Active Oxygen to Accelerate the Oxidation Conversion. Appl. Catal. B-Environ. 2004, 51, 9-19. (40) Lin, F.; Wu, X.; Weng, D. Effect of Barium Loading on CuOx-CeO2, Catalysts: NOx, Storage Capacity, NO Oxidation Ability and Soot Oxidation Activity. Catal. Today. 2011, 175, 124-132. 34


Page 34 of 36

Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Banús, E. D.; Milt, V. G.; Miró, E. E.; Milt, V. G. Co, Ba, K/ZrO2, Coated onto Metallic Foam (AISI 314) as a Structured Catalyst for Soot Combustion: Coating Preparation and Characterization. Appl. Catal. A-Gen. 2010, 379, 95-104. (42) Li, Z.; Meng, M.; Li, Q.; Xie, Y.; Hu T. Fe-substituted Nanometric La0.9K0.1Co1-xFexO3-δ, Perovskite Catalysts Used for Soot Combustion, NOx, Storage and Simultaneous Catalytic Removal of Soot and NOx. Chem. Eng. 2010, 164, 98-105. (43) Bueno-López, A. Diesel Soot Combustion Ceria Catalysts. Appl. Catal. B-Environ. 2014, 146, 1-11. (44) Liang, Q.; Wu, X.; Weng, D.; Xu, H. Oxygen Activation on Cu/Mn-Ce Mixed Oxides and the Role in Diesel Soot Oxidation. Catal. Today. 2008, 139, 113-118.

35



For Table of Contents Only

36


Page 36 of 36

Promoting effects of barium substitution on the catalytic performances

Recommend Documents