摘  要

加快煤炭绿色高值化利用、促进CO₂资源化转化被认为是实现碳中和目标的重要途径。从化学组成来看，煤炭是一种天然高碳矿物材料，结合其有机大分子芳香结构特征，开发新型功能碳纳米材料，并用于发展可再生能源驱动CO₂转化合成碳基燃料或化学品的新技术，具有一举多得之效果，在能源环境可持续发展领域展现出巨大的应用前景。光催化技术，能够利用太阳能将CO₂转化为高能量密度清洁碳基燃料（如CH₄和CH₃OH等），被认为是一种绿色温和的转化策略。但由于CO₂分子在热力学上十分稳定，其转化反应涉及复杂的多电子/质子耦合过程，且催化剂光激发电子-空穴对极易复合，导致CO₂活化能势垒高、催化反应动力学缓慢和产物选择性低等问题。因此，构建具有丰富催化活性位点和快速载流子转移通道的高效光催化体系，对于实现CO₂高活性、高选择性转化至关重要。基于此，本文提出以煤为原料，制备系列煤基石墨烯材料，并聚焦于提高CO₂光催化反应动力学和产物选择性，以煤基氧化石墨烯为载体，引入具有可见光响应的铜基光催化剂活性组分，通过表界面结构设计和微纳织构调控等策略，可控构筑了系列多尺度、高性能的煤基石墨烯复合铜基纳米光催化剂材料，分别研究并揭示了其在CO₂光催化转化过程中对于“光捕获-载流子分离-表面催化反应”的增强机制。具体研究结果如下：

（1）以两种不同变质程度的煤种（太西无烟煤和神府烟煤）为前驱体，采用高温石墨化-化学氧化剥离-热还原策略，制备得到煤基石墨烯材料。并借助谱学测试手段对原煤及过程产物进行表征，探究了不同煤种在制备煤基石墨烯过程中化学组成和结构的演变。研究表明：经高温热处理可以显著提高原煤的芳碳率和石墨化度，其中太西无烟煤基石墨的芳碳率和石墨化度较高，分别达83.7%和76.0%。通过氧化剥离获得的煤基氧化石墨烯主要以C、O元素为主，sp² (C=C)/sp³ (C-C) 比例显著减小，O含量达 ~31.6 at.%，表面含氧官能团以环氧基为主，羟基和羰基次之，还含有少量羧基。通过热还原制备的煤基石墨烯呈少层透明纳米片状，层数达2~5层，其表面sp² (C=C)/sp³ (C-C) 比例增大，部分sp² C结构得到恢复，同时O含量降至7 at.%以下，残留的少量含氧官能团以羟基和羰基为主。由于神府烟煤中较多的五元杂环（造成碳层扭曲或空位）等结构，导致其制备的煤基氧化石墨烯和石墨烯中存在较多sp³ C晶格缺陷。

（2）以无烟煤基氧化石墨烯（A-CGO）纳米片为载体，具有含氮配体的Cu-ZIF为铜基光催化剂前体，通过原位自组装-煅烧策略制备得到N掺杂Cu₂O@煤基石墨烯复合材料（N-Cu₂O@NG），并考察了其光催化还原CO₂性能。实验表明，当煅烧温度为450℃时，所获得的N-Cu₂O@NG-450光催化还原CO₂活性最高，产物CH₃OH产率达到510.7 μmol·g^-1·h^-1，选择性为93.1%，且参与CO₂还原反应的有效转移电子数可达3571.1 μmol·g^-1·h^-1。通过结构表征和DFT计算表明，煤基石墨烯纳米片和Cu₂O纳米晶表面原位N掺杂形成的强路易斯碱性活性位点，增强了CO₂分子的吸附和活化；界面内建电场的形成，能够诱导Cu₂O光激发电子向煤基石墨烯纳米片的转移，从而提高光生电荷分离效率，促进CO₂多电子还原为CH₃OH。

（3）以A-CGO纳米片为载体，采用溶剂热自组装策略负载Cu-BTC纳米八面体前体，并通过受限环境下热解制备得到3D多孔碳限域Cu/Cu₂O纳米簇@煤基石墨烯复合光催化剂（Cu/Cu₂O-NPC-G），实现了CO₂高效光还原为CH₃OH。实验表明，当热解温度为300℃时，所制备的Cu/Cu₂O-NPC-G-300复合材料光催化还原CO₂为CH₃OH的产率最高，达到1886.7 μmol·g^-1·h^-1，且CH₃OH选择性达95.0%，参与CO₂还原反应的有效转移电子数高达12272.6 μmol·g^-1·h^-1，约为N-Cu₂O@NG-450的3.4倍。光电化学性质表征和DFT计算表明，Cu/Cu₂O-NPC-G-300增强的光催化性能归因于Cu/Cu₂O纳米簇界面电场的形成，加速了Cu₂O光激发电子向Cu的定向传输，增强了光生电荷分离动力学；同时3D多孔碳基质和煤基石墨烯纳米片促进了CO₂分子的吸附，并作为电子受体，使催化剂表面电子富集，有助于CO₂多电子还原生成CH₃OH。

（4）采用简单的溶液超声策略，以A-CGO纳米片为基体，通过π-π相互作用实现Cu-NH₂-BDC在A-CGO表面原位自组装，获得一种新型2D/2D Cu-NH₂-BDC/CGO异质结复合光催化剂，并考察了其光催化还原CO₂为CH₃OH性能。实验表明，在纯水反应体系中Cu-NH₂-BDC/CGO表现出优异的光催化还原CO₂活性，目标产物CH₃OH的产率高达1697.5 μmol·g^-1·h^-1，约为相同条件下块体Cu-NH₂-BDC的3.1倍。通过结构表征和光电化学性质测试表明，Cu-NH₂-BDC/CGO优异的光催化活性主要归因于其增强的可见光吸收、丰富可及的表面活性中心和紧密的二维异质界面，显著提高了复合材料的CO₂吸附能力和光生载流子分离效率。同时，氨基化配体赋予Cu-NH₂-BDC更负的导带位置，使Cu-NH₂-BDC/CGO在热力学上有利于CO₂还原为CH₃OH。

ABSTRACT

Low-carbon and high-value utilization of coal and effective conversion of CO₂ resources are considered to be important ways to accelerate the realization of carbon neutrality goal. Considering that coal is essentially a natural carbon-rich mineral material, and its organic macromolecular structure consists of numerous condensed aromatic nuclei units, which is similar to carbon materials. Therefore, using coal as a carbon source to prepare advanced functional carbon nanomaterials, and developing new technologies based on it for the conversion of CO₂ into valuable fuels or chemicals (such as CH₄ and CH₃OH), is a promising strategy for promoting sustainable energy and environmental development. Photocatalytic reduction of CO₂ into clean fuels with high calorific value can directly convert solar energy into chemical energy, which is regarded as an ideal solution. However, the stable C=O covalent double bonds in CO₂ molecule leads to high activation energy barrier, and the reduction process of CO₂ involves multiple consecutive proton-coupled electron transfer reactions resulting in slow reaction kinetics and inferior product selectivity. Concurrently, severe electron/hole recombination limits the photocatalytic performance. Hence, constructing excellent photocatalytic systems with abundant catalytic active sites and fast carrier transfer channels is essential to achieve high activity and highly selective conversion of CO₂. In this paper, coal-based graphene materials were prepared by graphitization-chemical oxidation-heat reduction method with different rank coal as precursor. Then, in order to improve the reaction kinetics and product selectivity of CO₂ photoreduction, using coal-based graphene oxide nanosheets as catalyst carrier, and copper-based photocatalyst with visible light absorption ability as active components, a series of highly active and multi-dimensional coal-based graphene functional photocatalysts were constructed through surface and interface structure design and micro-nano scale control. Furthermore, the enhancement mechanism of photocatalysts for “light harvesting ability-carrier separation-surface catalytic reaction” in the photoreduction of CO₂ was investigated. The main research contents are as follows:

(1) Coal-based graphene (rCGO) materials were prepared by a high-temperature graphitization-chemical oxidation exfoliation-thermal reduction strategy using two different rank coals (Taixi anthracite and Shenfu bituminous coal) as precursors. The evolution of chemical composition and structure of raw coal and process products in the preparation of rCGO was investigated by spectroscopic tests. The results show that the aromaticity and graphitization of raw coal can be significantly improved by high temperature heat treatment, and the aromaticity and graphitization of Taixi anthracite-based graphite is higher than that of Shenfu bituminous coal based graphite, reaching 83.7% and 76.0%, respectively. The coal-based graphene oxide (CGO) prepared by chemical oxidation exfoliation is mainly composed of C and O elements, in which the ratio of sp² (C=C)/sp³ (C-C) is significantly reduced, and the O content reaches ~31.6 at.%. The oxygen-containing functional groups on the surface of CGO are dominated by epoxy groups, followed by hydroxyl and carbonyl groups, and also contain a small amount of carboxyl groups. The rCGO obtained by thermal reduction has a transparent thin layer, and the number of exfoliated layers reaches 2 to 5 layers. Moreover, the sp² (C=C)/sp³ (C-C) ratio of rCGO increases, and the sp² C structure is partially restored, while the O content decreased to below 7 at.%, in which the residual oxygen-containing functional groups are mainly hydroxyl and carbonyl groups. In comparison, due to the existence of structures such as five-membered heterocycles (which cause distortion or vacancies in the carbon layer) inherent in bituminous coal, there are more sp³ C defects in the CGO and rCGO prepared from Shenfu bituminous coal.

(2) Using anthracite-based graphene oxide (A-CGO) nanosheets as the carrier, and Cu-ZIF with nitrogen-containing ligand as the copper-based photocatalyst precursor, N-doped Cu₂O@coal-based graphene composites (N-Cu₂O@NG) were prepared by in situ self-assembly and calcination strategies, and their performance for photocatalytic reduction of CO₂ was evaluated. Notably, when the calcination temperature is 450℃, the obtained N-Cu₂O@NG-450 showed the highest photocatalytic activity, with a CH₃OH generation rate of 510.7 μmol·g^-1·h^-1 and a selectivity of 93.1%, and the number of effective transfer electrons involved in CO₂ reduction reaches 3571.1 μmol·g^-1·h^-1. Detailed characterization and theoretical calculations revealed that the strong Lewis basic active sites formed by in situ N-doping on the surfaces of coal-based graphene and Cu₂O nanocrystals enhance the adsorption and activation of CO₂ molecules. Moreover, the formation of a built-in electric field between Cu₂O and coal-based graphene nanosheets improves the efficiency of photogenerated charge separation, which facilitates the multi-electron reduction of CO₂ to CH₃OH.

(3) Using A-CGO nanosheets as the carrier, and Cu-BTC nano-octahedral as the copper-based photocatalyst precursor, N-doped porous carbon confined Cu/Cu₂O nanoclusters@coal-based graphene composite photocatalyst (Cu/Cu₂O-NPC-G) was obtained by pyrolysis in a controlled environment, which realized efficient photoreduction of CO₂ to CH₃OH. When the pyrolysis temperature is 300℃, the prepared Cu/Cu₂O-NPC-G-300 catalyst possessed the highest formation rate (1886.7 μmol·g^-1·h^-1) and selectivity (95.0%) of CH₃OH product. Especially, the number of effective transfer electrons involved in CO₂ reduction reaction was as high as 12272.6 μmol·g^-1·h^-1, which was about 3.4 times that of N-Cu₂O@NG-450. Photoelectrochemical characterization and DFT calculations showed that the outstanding photocatalytic performance can be due to the formation of interfacial electric field of Cu/Cu₂O nanoclusters, which enhanced the kinetics of photogenerated charge separation. Meanwhile, the N-doped porous carbon and coal-based graphene nanosheets promoted the adsorption of CO₂ molecules and acted as electron acceptors, resulting in electron enrichment on their surfaces, which contributes to the multi-electron reduction of CO₂ to CH₃OH.

(4) A novel 2D/2D Cu-NH₂-BDC/CGO heterojunction was successfully constructed by coupling a 2D copper-based metal-organic framework (Cu-NH₂-BDC) and A-CGO nanosheets through on π-π interaction. The resultant Cu-NH₂-BDC/CGO composites could efficiently photo-reduce CO₂ to CH₃OH without any assistance of a sacrificial agent and photosensitizer. The yield of CH₃OH was as high as  1697.5 μmol·g^-1·h^-1, which is about 3.1 times that of pristine Cu-NH₂-BDC. Structural and photoelectrochemical tests showed that the excellent catalytic activity of Cu-NH₂-BDC/CGO can be ascribed to the enhanced light harvesting, abundant accessible surface active centers, and intimate 2D heterostructure, which significantly improve the CO₂ adsorption and the photogenerated charge separation and enrichment abilities of composites. Meanwhile, the aminated ligand endows Cu-NH₂-BDC with a more negative conduction band position, making Cu-NH₂-BDC/CGO thermodynamically favorable for the reduction of CO₂ to CH₃OH.