Characterization of porous LaCoO3 prepared from wood powder template and its catalysis for diesel particulate matter

In this paper, LaCoO3 porous perovskite was synthesized using wood powder template combined with sol–gel process, while LaCoO3 prepared by traditional sol–gel method was used as a comparison. The catalysts prepared by different methods were characterized by X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), N2 adsorption–desorption, and the particulate matter (PM) trapping and catalytic oxidation tests were carried out by engine bench test and thermogravimetric analyzer (TGA). The results showed that the porous LaCoO3 successfully replicated the structural features of wood powder from macroscopic to tracheid, forming a porous structure. Porous LaCoO3 exhibits better catalytic activity than powdered LaCoO3 with similar specific surface area, this can be attributed to the porous structure of LaCoO3, which increases the contact area and the number of contact points between PM and catalyst.


Introduction
Diesel engines are widely used in passenger cars and engineering equipment due to their good fuel economy and high thermal efficiency [1,2]. However, the PM in diesel engine exhaust will cause harm to the atmospheric environment and human health [3,4].
Diesel catalytic traps, which combine trapping and catalytic oxidation, are one of the most promising after-treatment technologies for reducing particulate matter emissions [5].
The key to catalytic oxidation is to select a suitable catalyst to reduce the temperature of particle oxidation. Currently, the commonly used catalysts are mainly noble metal-based catalysts, perovskite oxides, and single-component metal oxides. Noble metal-based catalysts have outstanding performance for PM purification at the early stage, but their performance drops sharply in the late stage due to their thermal durability problems under high temperature and lean conditions [6]. Single-component metal oxide catalysts have a high specific surface area, good redox performance and low cost, but their biggest disadvantage is their low thermal stability and easy aggregation at high temperatures, resulting in a lower specific surface area [7]. In contrast, perovskite oxides have the advantages of low cost, good chemical stability, and high thermal stability [1,8,9], and are considered to be able to replace noble metals and metal oxides for catalytic oxidation of PM. Nunzio Russo et al. [10] prepared a series of La-Co nanostructured catalysts by combustion synthesis, deposited the catalysts on SiC wallflow traps, and conducted activity tests on a diesel engine bench. The results showed that the presence of the catalyst reduces the regeneration time of the trap and improves fuel economy. Kayode Akinlolu et al. [11] synthesized a series of La 1-x Ca x CoO 3 (x = 0, 0.2, 0.3, 0.4) doped perovskite catalysts using a sol-gel method and a calcination temperature of 750 °C, and tested the catalysts for PM. The results showed that after the introduction of Ca, the surface area of the catalyst was slightly increased and the catalytic activity of the catalyst was improved by about 30%. Zou et al. [12] synthesized Co 1-x La x O y catalysts by the citric acid complex method, and studied their catalytic activity for PM in the air, found that LaCoO 3 catalysts can promote the oxidation of PM. According to the different particle sizes, PM is mainly divided into three modes: nucleation mode (0.005 ~ 0.05 µm, the mass ratio is 1 ~ 20%), accumulation mode (0.03 ~ 1 µm) and coarse mode (> 1 µm, the mass ratio is 5 ~ 20%) [13]. In contrast, the pore size (< 10 nm) of powdered catalysts is smaller than that of PM [14,15], which leads to less contact between the inner surface of the catalyst and PM, limiting the catalytic efficiency of the catalyst for PM. To improve the contact conditions between the catalyst and PM, the pore structure of perovskite is often adjusted by improving the preparation method to obtain perovskite catalysts with higher catalytic efficiency [16]. Template method is an effective way to prepare porous structure catalysts. Li et al. [17] synthesized a series of highly active three-dimensional ordered macroporous (3DOM) La 1-x K x MnO 3 catalysts by the colloidal crystal template method. The structure can also improve the catalytic activity of the catalyst and reduce the ignition point of PM. Ma et al. [18] used the dip-sintering method to coat LaCoO 3 on the walls of 3DOM SiOC/cordierite and conducted catalytic oxidation studies of PM on the 3DOM samples, showing that the 3DOM structure provided the catalyst with lower PM than conventional catalysts combustion temperature, while LCO/3DOM SiOC/cordierite lowers the combustion temperature and reduces the back pressure. Zheng et al. [15] prepared 3DOM LaMn 1-x Fe x O 3 (x = 0, 0.05, 0.1, 0.15) with different pore sizes by using polymethyl methacrylate microspheres with different diameters as templates by colloidal crystal template method, the study showed that the 3DOM catalyst has good catalytic performance for diesel PM combustion, and its catalytic activity increases with the increase of pore size. However, the preparation process of 3DOM is very strict and complicated, and the collapse or loss of the three-dimensional porous structure may occur during the template removal process, which will lead to poor stability of the porous structure.
Bio-templates have shown good application prospects in the field of catalysis due to their natural pore structure, simple preparation process, low cost, clear morphology, and environmental friendliness [19]. For example, Song et al. [20] fabricated porous SnO 2 with pollen grains as templates, which displayed a fine hierarchical porous structure and exhibited excellent performance for gas molecule transport and sensory reactions. Zhao et al. [21] synthesized hierarchically porous LaFeO 3 perovskite by a simple process using pomelo peel as a biological template and tested the catalytic performance of the prepared samples for NO + CO, and the results showed that the NO conversion rate reached 95% at 324 °C, and the CO conversion reaches 94% at 350 °C.
Wood powder is a biomass organic polymer material, which is abundant in output as the leftover waste from wood processing. In terms of chemical composition, the wood powder is mainly composed of cellulose, hemicellulose and lignin, which contain a large number of functional groups such as hydroxyl and carboxyl groups [22], which have the effect of adsorbing metal ions [23]. In terms of physical structure, the special porous three-dimensional network structure of wood powder is conducive to the penetration of the precursor solution, and the tracheids contained in wood powder are conducive to the circulation of substances between adjacent cells [24]. Therefore, wood powder can be an ideal choice for bio-templated preparation of porous perovskites. Using wood powder as a template to prepare porous perovskite, its porous structure can play a role in spatial confinement for catalyst structure construction, providing more active sites and surface effects for the catalyst surface, and improving the catalytic performance of the material.
In this paper, porous LaCoO 3 was prepared by using wood powder and combined with the sol-gel method. The final LaCoO 3 not only retained the structure of wood powder, but also had multi-diameter pore structure. At the same time, powdered LaCoO 3 was prepared by the traditional sol-gel method, which was compared with porous LaCoO 3 . A series of characterizations were carried out on the prepared catalysts to determine the chemical composition and microscopic properties. The trapping and catalytic oxidation effects of catalysts on the particulate matter were investigated by engine bench test and thermogravimetric analyzer, respectively.

Catalyst preparation
The pretreatment of wood powder was carried out before the preparation of LaCoO 3 . Take 10 g of wood powder (mesh 20 ~ 40) and heat it in boiling 5% dilute ammonia water for 6 h. With this extraction method, wood extractive compounds such as gums, protocorms, fats, and fatty acids can be removed and the connectivity between pores and the affinity of cells for precursors enhanced [25]. The treated wood powder was washed with deionized water, and then placed in an electric drying oven for drying at 80 °C for 5 h.
Precursor solutions were prepared with La(NO 3 ) 3 ·6H 2 O, Co(NO 3 ) 2 ·6H 2 O and citric acid. Analytical grade 0.02 mol La(NO 3 ) 3 ·6H 2 O and 0.02 mol Co(NO 3 ) 2 ·6H 2 O were dissolved in 100 mL of deionized water with stirring until the nitrates were completely dissolved. Then, citric acid was added to the nitrate solution at a molar ratio of citric acid to total metal cations of 2:1 [26], and stirred until the citric acid was completely dissolved. The dried wood powder was immersed in a beaker of the precursor solution, stirred at room temperature for 1 h, and then the mixture was placed on a constant temperature magnetic stirrer at a constant temperature of 100 °C for stirring and heating, and the stirring was stopped after gel formation. The gel was dried in an electric blower drying oven at 100 °C for 10 h. Finally, the samples were placed in a tube furnace and heated from room temperature to 500 °C, 600 °C, 700 °C, and 800 °C in an air environment with a heating rate of 3 °C/min, and then calcined at a constant temperature for 5 h. After calcination, it was cooled to room temperature and taken out. The preparation process is shown in Fig. 1. The LaCoO 3 samples prepared by the template method are labeled as LCO b-t . In addition, LaCoO 3 prepared by the conventional sol-gel method does not need to add wood powder, and the sample is labeled as LCO s-g .

Characterization
The phase structure of the catalyst was determined using XRD (XRD-6100, Shimadzu, Japan). A closed tube X-ray light source with a power of 3 kW and a Cu target was used. The scanning angle is 20 ~ 80°. SEM (QUANTA 200, FEI, Valley City, ND, USA) was used to observe the catalyst and the micro-morphology of the catalyst after the bench test with PM attached.
FTIR (Spectrum 400, PerkinElmer, Waltham, MA, USA) was used to confirm the catalyst crystal structure and verify the XRD results.
Using a Micromeritics ASAP 2010 analyzer (USA), according to the N 2 desorption isotherm at -196 °C, the specific surface area (SSA) of the sample was measured according to the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was obtained by the Barrett-Joyner-Halenda (BJH) method. Before measurement, the samples were degassed at 200 °C for 3 h under vacuum to remove moisture.

Catalyst capture and activity evaluation
The capture of PM by LCO b-t and LCO s-g was investigated by bench test. The test was carried out in an environment of room temperature of 20 °C, relative air humidity of 50% ± 15%, and standard atmospheric pressure of 101.3 kPa. The test machine uses a Sparky XK 173F diesel engine. The main specifications of the engine are listed in Table 1. The diesel used in the test is 0# China VI standard diesel produced by CNPC, and relevant parameters are shown in Table 2. The diesel engine was tested after 20 min of stable operation. During the test, the diesel engine speed was 2200 r/min and the load was 50%. In order to avoid the effect of high temperature on PM oxidation, an engine mount with cooled exhaust is used. PM were collected after immobilizing LCO b-t and LCO s-g in cooling tubes for 5 h.
The catalysts with PM trapped through bench test were tested for catalytic activity on Q50 thermogravimetric analyzer. Heat the mixture from room temperature to 600 °C in  a flow of synthesis air (20% O 2 and 80% N 2 , 50 mL/min) at heating rates of 10 °C/min. Examine the quality change of the sample during this process. The particle conversion rate η is calculated by formula (1): where W i is the initial weight of the sample, W e is the weight after heating, and W p is the weight at each temperature point. Figure 2A shows the XRD patterns of LCO In addition, compared merged into one peak at 33.06°, which indicated that the catalyst showed high symmetry and the crystal structure of perovskite was transformed into highly symmetrical structure. The appearance of this structure facilitates the transport of oxygen vacancies, which can improve the catalytic ability of the catalyst [28]. With the increase of calcination temperature, the position of the diffraction peak does not change, indicating that the phase structure does not change. However, the diffraction peaks become sharper, indicating that the calcination temperature increases, the crystallization degree

XRD results and analysis
increases, the phase structure tends to be perfect, and the impurities are less and less. In addition, a relatively weak La 2 O 2 CO 3 diffraction peak still appears at 600 °C, and no Co 2 O 3 diffraction peak appears, indicating that cobalt oxides may exist in the product in an amorphous form.
The XRD patterns of LCO s-g calcined at different temperatures are shown in Fig. 2b. Compared with LCO b-t , the main phases of LCO s-g at 500 °C are also La 2 O 2 CO 3 and Co 3 O 4 , and no perovskite phase is formed. But from 600 °C, the perovskite phase is formed.

FT-IR results and analysis
Vibrational frequencies in the infrared region are the basis for determining crystal structure. The FT-IR spectra of the samples prepared by the template method and the sol-gel method in the wavenumber range of 2000-400 cm −1 are shown in Fig. 3a and Fig. 3b, respectively. It can be seen that LCO b-t and LCO s-g have three main vibration bands at 596 cm −1 , 560 cm −1 and 425 cm −1 , these bands are consistent with the two normal modes of vibrational frequencies reported by Couzi and Huong [29] for the perovskite-type structure. The vibrational bands at 560 and 596 cm −1 can be  . 2 The XRD patterns of (a) LCO b-t and (b) LCO s-g assigned to two kinds of Co-O stretching vibrations (ν 1 ) in the BO 6 octahedron, and the vibrational band at 560 cm −1 has the characteristics of a rhombohedral structure [30,31]. The 425 cm −1 is attributed to the bending vibration (ν 2 ) of the Co-O bond [29], which further proves that the prepared LaCoO 3 catalyst sample has a distinct perovskite oxide structure. The difference is that LCO b-t has a relatively weak absorption band at 668 cm −1 , and it decreases with the increase of calcination temperature, which may be caused by the vibration of the Co-O bond in the Co 3 O 4 structure [32,33], thus representing the existence of the Co 3 O 4 phase in the sample, a conclusion consistent with the XRD results. When the calcination temperature is 600 and 700℃, LCO b-t shows bands at 1457 and 1364 cm −1 , the vibrations at these two locations should belong to CO 3 2− [34], which indicates that carbonate is formed during the heating process, which is also consistent with the XRD results. With the increasing temperature, the intensity of the CO 3 2− absorption peak gradually weakened, and disappeared completely at 800 °C. In addition, both LCO b-t and LCO s-g exhibit a vibrant band at 1603 cm −1 , where the vibrational band corresponds to the carboxyl group (COO-) stretching vibration [35].

SEM results and analysis
The microscopic morphologies of LCO b-t are shown in Fig. 4. In Fig. 4a, c), there are mainly two types of external pore morphologies of porous LaCoO 3 , namely, tracheidshaped pores A and pores B generated by gas erosion of the catalyst. The formation process of pore A is that when the wood powder is immersed and stirred in the precursor, the tracheid channel inside is filled with the precursor solution. The sol formed after constant temperature heating at 120 °C encapsulates the tracheid, and after further drying at 100 °C, the water in the sol evaporates to form a gel attached to the surface of the tracheid. When calcined at high temperature, the wood powder template was removed and a tracheid-like LaCoO 3 catalyst was formed. In fact, there are a large number of A pores on each wood powder, but they cannot all be seen in SEM due to the difference in the spatial distribution of wood powder. Pore B is formed because the surface of the wood powder has different regular protrusions and depressions. When the wood powder is infiltrated in the precursor solution, La 3+ and Co 3+ are distributed on the surface of the wood powder and combined with the hydroxyl groups on the surface of the wood powder. During the sol formation process, the wood powder particles are wrapped and dried to form a gel. Finally, the wood powder disappears during the hightemperature calcination process, and a large amount of gas is released at the same time, and the gas erodes the surface of the catalyst and leaves pores. Figure 4d shows the internal structure of Pore A, which is mainly divided into two types of pores from a structural point of view. Firstly, LCO b-t retains the pit shape inside the tracheid while inheriting the tubular shape of the tracheid. Secondly, the inner surface of the tracheid leaves similar filamentous forms, which interweave into meshes of different diameters. The filamentous morphology of LCO b-t formation may be related to the original macrofibrils of wood powder, which may consist of variable numbers of microfibrils associated with their hydrophilic surfaces [36].
At different calcination temperatures, the pore sizes of the outer pores of LaCoO 3 prepared by the template method were distributed between 2.4 and 25 µm, and the pore sizes of the inner pores were between 0.2 and 4 µm. The size of the pore size is conducive to the attachment of PM to the surface of LaCoO 3 and the entry into the interior, which promotes the capture of PM by LaCoO 3 and the diffusion of PM inside the catalyst, thereby increasing the contact frequency and increasing the catalytic efficiency of LaCoO 3 for PM.
The microstructure of LCO s-g is shown in Fig. 5. In contrast, the surface of the catalyst prepared by the sol-gel method is smoother, and the surface pores of LCO s-g decrease with the increase in calcination temperature. The smooth surface is not conducive to the adhesion of PM, and will reduce the contact time between PM and LaCoO 3 under the action of airflow. On the other hand, the surface with fewer pores is not conducive to the entry of PM into the interior of LaCoO 3 , resulting in the reaction of PM only on the catalyst surface. Figure 6 shows nitrogen physisorption-desorption isotherms of LCO b-t and LCO s-g . The samples prepared by the two methods exhibited nearly identical isotherms and hysteresis loops at different temperatures. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, this type of isotherm is type IV. The isotherm shows an H 3 -type hysteresis loop, which is related to the filling and emptying of the mesopore by capillary condensation [20], this result indicates that both LCO b-t and LCO s-g prepared at different temperatures contain abundant mesopores.

BET and BJH results and analysis
The pore size distributions of LCO b-t and LCO s-g at different calcination temperatures are shown in Fig. 7. As shown in Fig. 7a, the pore size distribution of LCO b-t at 600 °C is centered at 20 nm, and the pore size distribution at 700 °C and 800 °C is centered at 50 nm. This indicates that the pore structures of LCO b-t calcined at different temperatures are different, with mesopores dominated at lower temperatures and macropores dominated at high temperatures, which may be due to the merger of mesopores and the enlargement of pores as the temperature increases. Meanwhile, the pore size change of the catalyst at 600 °C is more stable than that of the samples at 700 °C and 800 °C, which indicates that the pore size distribution of the catalyst is uniform and relatively concentrated at 600 °C. Therefore, the catalysts calcined at 600 °C have the best pore size distribution and mesopore quality. In Fig. 7b, the pore size distribution of LCO s-g is centered at 20 nm at 600 °C, 40 nm at the center at 700 °C, and 60 nm at the center at 800 °C. This phenomenon is the same as LCO b-t , that is, the pores of the catalyst gradually change from mesopores to macropores as the temperature increases.
The specific surface area and average pore diameter of LCO b-t and LCO s-g at different temperatures are summarized in Table 3. By comparison, it is found that the BET values of LCO b-t are slightly smaller than that of LCO s-g , and the average pore size of LCO s-g is smaller than that of LCO b-t . There may be two reasons: the first is that. LCO s-g contains a small number of pores smaller than 5 nm, which is beneficial to increase the specific surface area, the second is that the gas generated when the wood powder template is removed at a high temperature erodes the holes, resulting in a small number of holes collapsing.

Capture and catalytic activity test
The physico-chemical properties of the PM samples should be assessed before testing the PM trapping and catalytic performance of LaCoO 3 . The PM was sampled using a 50 mm diameter PTFE membrane (Cobetter, Zhejiang, China) with a pore size of 0.1 µm in a collection device. After sampling, the particle size distribution and elemental composition of the samples were determined using SEM and Energy-Dispersive X-Ray Spectroscopy (EDX). The soluble organic fraction (SOF) in PM was extracted by the Soxhlet method using dichloromethane as the extractant and the difference between the two results was the SOF contents [37,38].
The micromorphology of the collected PM is shown in Fig. 8 and the elemental and SOF contents is listed in Table 4. As can be seen from the SEM, the PM aggregates collected in this experiment appear to have a variety of shapes and sizes, exhibiting either cluster or chain morphology, and most are below 1 µm in diameter. In the elemental composition of PM, carbon accounts for the largest proportion, followed by oxygen. In addition to this there are small amounts of inorganic elements such as Mg, Al, Ca, Zn (Pt is sprayed during the SEM process and is not a component of the PM). Some studies have shown that inorganic elements come from lubricants, fuels and engine wear [39].
Due to the better distribution and average pore size of catalysts calcined at 600 °C, LCO b-t and LCO s-g calcined at 600 °C were selected as the objects for capturing and testing catalytic activity. The microscopic images of LCO b-t and LCO s-g after the collection are shown in Fig. 8. It can be seen from the figure that LCO s-g only contacts the PM on the surface (Fig. 9a), and PM with a larger particle size cannot enter the catalyst. In LCO b-t , pores A (Fig. 9b) and pores B (Fig. 9c) were observed with different particle sizes of PMs, both inside and outside the catalyst. This shows that the porous LaCoO 3 exhibits better adhesion, which facilitates the easy entry of PM into the inner pores of the catalyst, which is beneficial for the catalyst to trap PM with different  particle sizes, extend the reaction time of PM in the catalyst, increase the contact area between the catalyst and PM. Figure. 10 shows the catalytic oxidation curves of LCO b-t and LCO s-g for PM, respectively. The test values of T 10 , T 50 and T 90 are listed in Table 5. The data shows that the oxidation temperature of PM on LCO b-t is significantly lower than that of LCO s-g at the same conversion rate. This shows that the catalytic performance of the porous LaCoO 3 for PM is better than that of the powder when the specific surface area is similar. This indicates that the morphological structure of the catalyst is an important factor affecting the catalytic oxidation of PM. According to Teraoka's "triple contact point" mechanism [40], simultaneous removal of PM and O takes place at the point where the catalyst, solid particles and gaseous O come into contact. In this study, the porous LaCoO 3 increases the active sites and contact time of the catalyst with PM, so it is effective for PM oxidation.

Conclusion
In conclusion, LaCoO 3 was prepared by wood powder as a template combined with the sol-gel method and used to catalyze the oxidation of PM. Compared with the type of perovskite synthesized by the traditional sol-gel method, the template-based LaCoO 3 has a spatial structure composed of mesopores and macropores. From the results of the    topography analysis, the improved catalytic performance can be attributed to the significant increase in the number of contact points between the reactant PM and the catalyst surface, this is caused by the porous structure, which provides a larger contact area while accommodating particulate matter, which provides abundant attachment sites for PM. At the same time, when the specific surface area is close, the porous structure can promote the performance of the catalyst.