Investigation of Adsorption Characteristics and Inuencing Factors for Diesel Engine Exhaust Particles

: The adsorption process of diesel exhaust particles has a great influence on 9 the particles. A single-cylinder diesel exhaust particle collection system was established 10 to collect particle samples with different adsorption environment conditions, and the 11 adsorption capacity of the samples was analyzed and characterized by an isothermal 12 adsorption test; the change patterns of particle characteristics were investigated by the 13 scanning electron microscope and thermogravimetric analyzer, correlation analysis of 14 the factors influencing the adsorption process was performed. The results show that the 15 particles have adsorption capacity and belong to the type of multilayer adsorption on 16 porous media; the pore diameter is continuously distributed in the range of 8-80 nm 17 with multiple peaks, which is the category of mesoporous and macroporous; the 18 adsorption capacity of the sample particles increase with the increase of engine speed. 19 Compared with the particles at the inlet of the exhaust pipe, the box-counting dimension 20 (D B ) of particles at the outlet increased, the content of water and soluble organic fraction (SOF) increased, the activation energy (E) decreased. Among the parameters affecting 22 the adsorption process, the increase of hydrocarbons concentration contribute to the 23 increase of particle adsorption and E reduction; the increase of the average temperature 24 of the exhaust pipe inhibit the increase of the D B , and the increase of the temperature 25 difference between the inlet and outlet facilitate the adsorption of water and SOF by the 26 particles; the decrease of the adsorption time is one of the main reasons for the 27 slowdown of the increase in D B ; the average pore diameter had the greatest positive 28 correlation with the amount of variation in the D B ; the increase in specific surface area 29 and pore volume of the sample particles was the dominant cause of the increase in 30 adsorption capacity as well as the decrease in E. 31

(SOF) increased, the activation energy (E) decreased. Among the parameters affecting 22 the adsorption process, the increase of hydrocarbons concentration contribute to the 23 increase of particle adsorption and E reduction; the increase of the average temperature

65
Adsorption behavior of particles is one of the main ways of exhaust particle 66 surface growth. Many studies have been conducted by domestic and foreign scholars 67 on the evolution of particle characteristics during the exhaust process. XinLing and 68 Zhen (2012) study argued that the exhaust dilution process of diesel engines causes 69 volatile organic compounds to condense and adsorb on the surface of existing particles, 70 and that temperature is an important factor affecting the gas-particle conversion process.  The results indicated that as the exhaust length increased and the exhaust temperature 73 decreased, gaseous hydrocarbons and sulfates were adsorbed on the particle surface,    at the inlet and outlet of the exhaust pipe was measured, and the environmental 120 parameters of the adsorption process were obtained as shown in Table 2. To satisfy the different requirements for particle morphology and component 123 analysis, two particle collection methodologies were used as shown in Fig.1 properties of the particles.

141
A commercial carbon black Printex U from Degussa was employed and a specific  Table 3. Under isothermal conditions, nitrogen is   injection flow rate of the thermogravimetric analyzer was set to 50 mL·min -1 , and the 166 reaction gas used was O2 and the protective gas was N2. Before the test, the gas was 167 introduced to discharge the air, and 3 mg of the particulate matter was taken for testing.

168
Thermogravimetric analysis of the particles was performed at a heating rate of 169 10°C·min -1 , and the temperature in the heating furnace was heated from 40°C to 900°C 170 to obtain a temperature dependence curve of the mass of the particles. The measured 171 accuracy and uncertainty of each instrument are given in The adsorption between particles and HC substances occurs on the surface of the The latter term in equation (5) was regarded as a constant, and equation (5) was 234 reduced to a one-primary equation (6), where 1/T is the independent variable x; and ln (-235 ln(1-c)/T 2 is the Dependent variable y.
The activation energy of the particles is derived by fitting the TG curve.  The specific surface area contains the particle surface area and the pore wall area,  The adsorption capacity of the adsorbent also depends on its morphological 299 structure (Tang 2020). Fig. 4 shows the SEM images of the sample particles. Be 300 observed as shown in Fig. 4, the diesel exhaust particle agglomerates have certain pore

307
It also is seen from Figure 4 that with the increase of diesel engine speed, the basic 308 carbon particles of the sample particles have a smaller particle size, looser structure, 309 more complex pore structure and finer pores. This is explained by the fact that under of the characteristics for the three groups of sample particles is shown in Fig. 5. 327 As is seen in Fig. 5, the DB of sample particles 1, 2 and 3 at the exhaust outlet 328 increased by 0.0379, 0.0381 and 0.0274, respectively, compared to the particles at the 329 exhaust inlet, which indicates that the particle gap was gradually filled by adsorbate as performance parameters) were considered as independent variables, and the parameters 360 of the independent variables of the adsorption process are shown in Table 6. Pearson 361 correlation analysis was performed on the independent and dependent variables. The correlation heat map of various adsorption process influencing factors on the 364 amount of change in particle characteristics is shown in Fig.6. 365 As is shown from Fig.6, ΔDB is negatively correlated with the exhaust pipe 366 temperature and HC concentration, and positively correlated with the adsorption time.

367
The increase of the average temperature in the exhaust pipe is not conducive to the  between the factors influencing the adsorption process and the changes in particle 420 characteristics was investigated. The research conclusions are as follows:

421
(1) Diesel exhaust particles and carbon black have comparable adsorption capacity, 422 and both belong to the type of multi-layer adsorption of porous medium for nitrogen 423 gas, and the pore sizes present a continuous distribution of multi-peaks, which 424 correspond to medium and large holes category. 100% load, the diesel engine speed 425 increased from 1500 r·min -1 to 3600 r·min -1 , the specific surface area of sample 426 particles 1, 2 and 3 increased, the average pore diameter decreased, the pore volume 427 increased, and the pore structure increased. The contact probability between the 428 particles and the adsorbate increased, the amount of adsorbent that could be 429 accommodated on the particle surface and in the pores increased, and the adsorption 430 capacity was enhanced.

431
(2) The adsorption behavior changes the physicochemical properties of the particles 432 significantly. Compared with the particles at the inlet of the exhaust pipe, the box-433 counting dimension of samples 1, 2 and 3 at the outlet of the exhaust pipe increased by 434 2.03%, 1.98% and 1.42%, respectively; The mass fraction of water in the particles 435 increased by 0.99%, 1.24% , 1.32%, and the mass fraction of SOF increased by 4.84%, 436 9.12%, and 10.52%, respectively; the energy of samples 1, 2, and 3 were reduced by 437 34.77 kJ•mol -1 , 38.88 kJ•mol -1 , and 47.43 kJ•mol -1 , respectively.

448
(4) The analysis of the factors influencing the adsorption process reveals that when 449 the diesel engine particles with large specific surface area and abundant pore structure 450 are emitted into the atmosphere with the exhaust gas, the decrease in ambient 451 temperature will cause particles to inevitably adsorb volatile components from the air.      Schematic diagram of the test setup and particle collection  Correlation heat map of the factors in uencing the adsorption process and changes in particle characteristics