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摘要: 为了更深入地了解微孔生物质焦的孔隙结构特征，在水蒸气气氛下制备椰壳焦（CSCs），并且采用了不同分子探针、计算模型和校准步骤对其进行表征。结果表明，椰壳焦有较高的碳含量和比较丰富的孔隙度，适合进一步活化以制备活性炭。表征椰壳焦较为合适的方法是：以Ar为分子探针，并采用非定域密度泛函（NLDFT）模型。当校准步骤优先进行时，以N2和Ar为分子探针的吸附测试结果如孔径分布（PSD）和吸附等温线会受到孔隙阻塞的影响，从而错误地描述椰壳焦的孔隙结构。实验结果还表明，273 K下仪器的真空处理可以去除绝大部分残留的He，降低孔隙阻塞的影响。Abstract: To get more insight into the pore structure characterization of nanoporous biomass chars, different probe molecules, models, and calibration steps were used and compared. The coconut shell chars (CSCs) were prepared under a steam atmosphere and characterized using N2, Ar, and CO2 adsorption. The results show that coconut shell chars are suitable for further activation, due to the high carbon content and abundant porosity. Ar adsorption with application of Non-Local Density Functional Theory (NLDFT) model can more accurately characterize the pore structure of CSC. When the calibration step is performed before adsorption measurement, the important results of N2 and Ar adsorption, such as pores size distribution (PSD) and isotherm, are affected by pore blocking, leading to the erroneous understanding of CSC in special applications. Vacuum treatment at 273 K for 1 h after He calibration is enough to remove He, which could reduce effect of pore blocking.
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Key words:
- adsorption /
- pore structure characterization /
- coconut shell char /
- He calibration /
- pore blocking
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Figure 1 N2(a) and Ar (b) adsorption/desorption isotherms performed with He calibration before and after the adsorption process
Figure 2 PSD of N2(a), Ar (b), and CO2(c) adsorption performed with He calibration before and after the adsorption process
Figure 3 Comparison of N2, CO2, and Ar adsorption isotherms performed with He calibration after the adsorption process
Figure 5 N2(a), Ar (b), and CO2 (c) adsorption isotherms in logarithmic scale performed with He calibration before and after the adsorption process
Figure 6 CO2 adsorption isotherms performed with He calibration before and after the adsorption process
Figure 7 CO2 (a) and Ar (b) adsorption isotherms on CSC performed with He calibration before the adsorption process
Table 1 Proximate and ultimate analyses of CSC
Sample Proximate analysis w/% Ultimate analysis w/% Ad Vdaf FCdaf Cdaf Hdaf Ndaf St, d CSC 1.06 3.47 96.53 96.63 0.46 < 0.01 < 0.01 A: ash; V: volatile matter; FC: fixed carbon; C: carbon; H: hydrogen; N: nitrogen; St: total sulfur; d: dry basis;
daf: dry ash-free basis
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Table 2 SA obtained after application of BET model and NLDFT model (assuming slit-shape pores)
Molecular probe p/p0 Vacuum temperature T/Ka ABET /(m2·g-1) ANLDFT /(m2·g-1) N2 0-0.995 77.4 before 473 593 after 473 596 Ar 0-0.995 77.4 before 453 525 after 459 604 CO2 0-0.03 273 before 438 553 after 438 556 a: samples were evacuated after the He calibration;
before: the He calibration was performed before the adsorption measurement;
after: the He calibration was performed after the adsorption measurement
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