摘要
氨是重要的化肥原料,也是颇具潜力的氢能源载体,对于可再生能源的储存、运输和终端利用至关重要.然而,传统Haber-Bosch工艺合成氨的反应条件苛刻,需要高温高压条件,并消耗大量化石能源及排放大量二氧化碳.可再生能源电解水制氢耦合温和合成氨新技术(eHB),不仅能实现可再生能源电力的"消纳和调峰",而且可进行低成本、跨地域长距离存储运输,并可将"绿氨"与氢能产业相结合.然而,现有的高温高压合成氨催化剂与eHB工艺相不匹配,因此,迫切需要开发温和条件下高效合成氨催化剂技术,以实现可再生能源电力电解制氢体系和合成氨技术互补融合.目前,虽然助剂对于Ru基纳米簇(≥l nm)合成氨催化剂的影响规律已得到了广泛研究,但它们对于Ru原子簇催化剂的作用机制尚不清楚,需要进一步揭示.本文考察了Ba及Ce助剂对Ru原子簇催化剂的影响规律,并分析了其作用机制.首先,通过简单的浸渍法将Ba和/或Ce物种掺杂到Ru原子簇催化剂(2 wt％Ru ACCs),制得Ba/Ce/2 wt％Ru ACCs催化剂;然后,通过一系列实验考察了这些催化剂的合成氨性能,并利用多种表征手段对其进行了深入分析.合成氨性能测试结果表明,添加Ba和Ce助剂后,2 wt％Ru ACCs催化剂的合成氨速率明显提高,在400℃和1 MPa下,Ba/Ce/2 wt％Ru ACCs催化剂的合成氨反应速率达到56.2 mmolNH3 gcat-1 h1,是2 wt％Ru ACCs的7.5倍,且催化剂表现出较好的稳定性,在稳定运行140 h后活性未见明显降低.球差校正电子显微镜和X射线吸收精细结构谱结果表明,负载Ba和/或Ce后,Ru以Ru3原子簇形式存在.X射线吸收近边结构谱和X射线光电子能谱结果表明,Ru与Ba及Ce物种之间存在较强的簇-金属氧化物助剂电子相互作用,可促进电子转移到Ru物种,形成富电子状态的Ru,进而促使电子转移到N2的π*反键轨道,提高温和条件下合成氨反应速率.利用25％N2+75％D2气氛下的原位红外光谱研究催化剂的合成氨反应机理,结果表明,在Ba/Ce/2 wt％Ru ACCs催化剂表面同时检测到N2D2物种和N2Dx物种的振动吸收峰,说明添加Ba和/或Ce物种没有改变Ru原子簇催化剂活化N2的方式,N2仍是通过加氢的路径合成氨.综上,本文考察了助剂对Ru原子簇的影响规律,揭示了其作用机制,为设计高效的温和条件合成氨催化剂提供参考.
Abstract
Ammonia(NH3)is an excellent candidate for hydrogen storage and transport.However,producing NH3 under mild conditions is a long-term,arduous task.Atomic cluster catalysts(ACCs)have been shown to be effective for catalytic N2-to-NH3 conversion,opening the door to the development of efficient catalysts under mild conditions.Still,ACC formation with thermally stable catalytic sites remains a challenge because of their high surface free energy.Herein,we report anchoring Ba and/or Ce onto Ru ACCs(2 wt％Ru atomic clusters supported on N-doped carbon)to form so-called clusters-metal oxide promoters electronic interaction(CMEI)to stabilize the Ru atomic clusters.The resulting Ba/Ce/Ru ACCs significantly boost the NH3 synthesis rate to 56.2 mmolNH3 geat-1 h-1 at 400 ℃ and 1 MPa,which is 7.5-fold higher than that of Ru ACC.The strengthened CMEI between the Ba/Ce and Ru atomic clusters across the Ba/Ce/Ru ACC enables electron transfer from Ba and/or Ce to Ru atomic clusters.As such,the electron-enriched Ru atom could facilitate electron transfer to N≡N bond π*orbitals,which would weaken the N=N bond and drive the eventual conversion of N2 to NH3.This study offers insight into the role of CMEI in Ru ACCs and provides an effective approach for designing stable atomic cluster catalysts for NH3 synthesis.
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