氮化硼负载的高分散Au-CuOx纳米颗粒用于低温CO氧化

负载型金催化剂显示出高的低温CO氧化活性,其催化性能与载体的性质密切相关.近年来,六方氮化硼作为一种新型催化材料引起了极大关注.已有研究表明,二维结构的氮化硼纳米片有利于传质扩散,并且暴露出大量的表面和边缘,作为新型非金属催化剂在烷烃氧化脱氢中表现出优异的活性.同时,CO氧化反应是强放热过程,氮化硼具有优良的导热性能,能够减少反应过程中热点的形成.然而氮化硼是非还原性载体,与活性组分金之间的相互作用较弱,需要通过改性来加强金与氮化硼载体间的相互作用.基于此,本文首先通过球磨处理来获得具有高比表面积和富缺陷的氮化硼纳米片载体,采用浸渍法在氮化硼纳米片上引入铜物种,实现对载体的改性,然后采用传统的沉积-沉淀法制备Au-CuO_x/BN催化剂.经氧化性气氛预处理后,Au-CuO_x/BN催化剂表现出良好的低温CO氧化活性,80℃下即可实现CO的完全转化.采用X射线衍射(XRD),高分辨透射电镜(HRTEM),氢气程序升温还原(H₂-TPR),X射线光电子能谱(XPS),CO吸附原位漫反射红外光谱(CO-DRIFT)等表征手段深入分析了Au-CuO_x/BN的结构与催化活性的关系.XRD测试结果未观察到明显的金和铜物种衍射峰,表明二者在氮化硼载体上高度分散.HRTEM和元素分析面扫描结果进一步表明,氧化铜主要分布于BN边缘的官能团和缺陷位上,金纳米粒子与铜物种的空间分布位置一致,表明BN通过稳定CuO_x物种进而实现了金纳米粒子(2.0 nm)的高分散,且反应后的金纳米粒子未发生明显团聚.H₂-TPR结果表明金和铜物种间的相互作用可促进铜物种的还原,XPS分析进一步证实了金和铜物种之间存在电子转移.CO-DRIFT结果表明,Au-CuO_x/BN催化剂对CO的吸附能力和提供活性氧物种的能力显著强于Au/BN催化剂,从而促进了CO氧化反应.综上,铜物种作为连接金和氮化硼载体之间的桥梁,促进了金纳米粒子在氮化硼载体上的分散和稳定,同时增强了CO的吸附和氧的活化.本文拓展了氮化硼在多相催化中的应用,为发展新型二维催化材料提供新的思路.

Highly dispersed boron-nitride/CuOx-supported Au nanoparticles for catalytic CO oxidation at low temperatures

Supported-Au catalysts show excellent activity in CO oxidation, where the nature of the support has a significant impact on catalytic activity. In this work, a hexagonal boron nitride (BN) support with a high surface area and adequately exposed edges was obtained by the ball-milling technique. There-after, impregnation of the BN support with Cu(NO3)2 followed by calcination under air at 400 ℃ yielded a CuO-modified support. After Au loading, the obtained Au-CuOx/BN catalyst exhibited high CO oxidation activity at low temperatures with a 50％CO conversion temperature (T50％) of 25 ℃ and a complete CO conversion temperature (T100％) of 80 ℃, well within the operational tempera-ture range of proton exchange membrane fuel cells. However, the CO oxidation activity of Au/BN, prepared without CuOx for comparison, was found to be relatively low. Our study reveals that BN alone disperses both Cu and Au nanoparticles well. However, Au nanoparticles on the surface of BN in the absence of CuO species tend to aggregate upon CO oxidation reactions. Conversely, Au nano-particles supported on the surface of CuO-modified BN remain small with an average size of～2.0 nm before and after CO oxidation. Moreover, electron transfer between Au and Cu species possibly favors the stabilization of highly dispersed Au nanoparticles on the BN surface and also enhances CO adsorption. Thus, our results demonstrate that thermally stable and conductive CuO-modified BN is an excellent support for the preparation of highly dispersed and stable Au catalysts.