NiO和Ni催化剂对苯甲酸热解机理的理论计算

doi:10.11949/j.issn.0438-1157.20181396

摘要/Abstract

摘要：

在煤热解过程中加入特定的催化剂可以改变煤结构中相关化学键的结合能，使热解在相对温和的条件下进行，促使更多的小分子从煤结构上解离成为产物释放，并调节产物的产率和组成，提高转化率及产物的品质。由于煤化学结构的复杂性，从分子水平研究煤的催化热解行为非常困难。基于此，以煤的催化热解为背景，采用煤模型化合物，借助密度泛函理论(DFT)，选取苯甲酸(C₆H₅COOH)为煤基模型，以NiO和Ni为催化剂，研究催化热解过程中催化剂价态改变对煤催化剂热解的作用。DFT结果显示，苯甲酸热解的主要路径为：C₆H₅COOH $→$ CO₂+C₆H₆和C₆H₅COOH $→$ C₆H₆COO $→$ CO₂+C₆H₆；在NiO上的分解路径为：C₆H₅COOH(g) $→$ *C₆H₅COO + *H $→$ *CO₂ + *C₆H₆ $→$ CO₂(g) + C₆H₆(g) ；在金属Ni上的分解路径为：C₆H₅COOH(g) $→$ *C₆H₅COOH $→$ *C₆H₅COO + *H $→$ *CO₂ + *C₆H₆ $→$ CO₂(g) + C₆H₆(g) 。Ni基催化剂的加入能够促进C₆H₅COOH的热解，同时改变了苯甲酸的热解路径，但是产物不变。当NiO被还原为金属Ni时，催化效果减弱。

关键词: 煤, 催化热解, 还原, 苯甲酸, 催化剂, 理论计算, 模型

Abstract:

The catalyst alters the binding energy of some chemical bonds in coal. It makes the conditions of pyrolysis milder, and adjusts the yield and composition of the product by promoting the micromolecules dissociation from coal. As a result, the conversion and the quality of product are increased. However, the chemical structure of coal is very complex, the catalytic pyrolysis mechanisms in coal are very difficult to study at the molecular level. Therefore, benzoic acid (C₆H₅COOH) is selected as the coal-based model compound to investigate the effect of valence states change of catalysts during catalytic pyrolysis by using density functional theory (DFT) method, in which NiO and Ni are selected as catalysts. The results showed that there are two reaction pathways for C₆H₅COOH pyrolysis. One is CO₂ and C₆H₆ are directly formed from C₆H₅COOH. The other is that C₆H₆COO is produced from H migration of C₆H₅COOH, then, C₆H₆ and CO₂ are obtained from C₆H₆COO decomposition. On NiO(100), the favorable reaction pathway is that *C₆H₅COO and *H are formed from C₆H₅COOH dissociative adsorption. Finally, *CO₂ and *C₆H₆ are produced from *C₆H₅COO decomposition with the assistance of *H. On Ni(111), the favorable reaction pathway is that *C₆H₅COO and *H are formation from *C₆H₅COOH decomposition, which C₆H₅COOH is nondissociative adsorption. Eventually, *CO₂ and *C₆H₆ are obtained. In general, Ni-based catalyst can promote C₆H₅COOH pyrolysis and change the pyrolysis pathways of benzoic acid, but the products are still the same. The catalytic effect decreases when NiO is reduced to metallic Ni.

Key words: coal, catalytic pyrolysis, reduction, benzoic acid, catalyst, theoretical calculation, model

中图分类号:

O 643.36

梁文胜, 刘江涛, 赵月, 黄伟, 左志军. NiO和Ni催化剂对苯甲酸热解机理的理论计算[J]. 化工学报, 2019, 70(4): 1429-1435.

Wensheng LIANG, Jiangtao LIU, Yue ZHAO, Wei HUANG, Zhijun ZUO. Theoretical calculation of effect of NiO and Ni catalysts for benzoic acid pyrolysis[J]. CIESC Journal, 2019, 70(4): 1429-1435.

图/表 11

图1 NiO(100)和Ni(111)的主视图和侧视图

Fig.1 Top and side views of NiO(100) and Ni(111) surfaces

图2 三种反应路径

Fig.2 Three types of pyrolysis pathways

图3 苯甲酸热解势能图

Fig.3 Potential energy diagrams of C6H5COOH pyrolysis

图4 苯甲酸热解过程中的初态、过渡态以及末态构型

Fig.4 Geometrical structures of initial states, transition states and final states for C6H5COOH pyrolysis

图5 NiO(100)面上C6H5COOH热解过程中各中间体最稳定吸附构型

Fig.5 The most stable adsorption structures of possible intermediates involved in pyrolysis of

表1 NiO(100)和Ni(111)面上各中间体吸附能及其到吸附位点的键长

Table 1 Adsorption energies and geometrical parameters for relevant species on NiO(100) and Ni(111) surfaces

Catalyst	Specie	Site	E _ads/eV	Bond length/nm
NiO(100)	C₆H₅COO	Ni_bri	－1.80	d _O—Ni =0.1980
	C₆H₅	O_top	－1.10	d _C—O =0.1386
	H	O_top	－1.20	d _H—O =0.0981
	C₆H₆	no bond	－0.01
	CO₂	Ni_top,O_top	－0.05	d _C—O=0.1458,d _O—Ni=0.2162
Ni (111)	C₆H₅COOH	top	－0.18	d _O—Ni =0.1987
	C₆H₆COO	bridge	－2.11	d _O—Ni =0.1956
	C₆H₅COO	bridge	－3.08	d _O—Ni=0.1949
	C₆H₅	top	－2.88	d _O—Ni =0.1871
	H	fcc	－2.35	d _H—Ni =0.1709
	C₆H₆	no bond	－0.05
	CO₂	no bond	－0.02

图6 NiO(100)上苯甲酸热解势能图

Fig.6 Potential energy diagrams of C6H5COOH pyrolysis on NiO(100) surface

图7 C6H5COOH在NiO(100)面上热解过程中的各基元反应的初态、过渡态以及末态构型

Fig.7 Geometrical structures of initial states, transition states, and final states for C6H5COOH pyrolysis on NiO(100) surface

图8 Ni(111)面上C6H5COOH热解过程中各中间体最稳定吸附构型

Fig.8 The most stable adsorption structures of possible intermediates involved in pyrolysis of C6H5COOH on Ni(111) surface

图9 Ni(111)上苯甲酸热解势能图

Fig.9 Potential energy diagrams of C6H5COOH pyrolysis on Ni(111) surface

图10 C6H5COOH在Ni(111)面上热解过程中的各基元反应的初态、过渡态以及末态构型

Fig.10 Geometrical structures of initial states, transition states and final states for C6H5COOH pyrolysis on Ni(111) surface

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