Nature最新资讯：模型催化揭秘氢溢流现象
前言：
氢溢流通常指氢气在金属催化剂表面解离、活化，活化后氢原子转移到载体的现象。这一现象在多相催化的加氢反应中较为常见。如：氧化物（TiO2，Mn3O4等）负载的贵金属（Pt、Pd等）用于加氢反应。贵金属吸附活化氢气、氧化物吸附活化其他反应物。活化后的氢原子溢流到氧化物吸附活性位点实现加氢，如下图1所示：
图 1
不仅在多相催化中，氢溢流在储氢材料中也很有作用。通过贵金属的氢溢流作用可以实现对于H2的可控储存和释放。
最近来自苏黎世联邦理工学院的Bokhoven教授利用模型催化剂对氢溢流现象进行了深入研究。
正文：
图 2
前人研究表明氢溢流现象通常出现于贵金属和还原性载体组成的体系。对于非还原性载体是否存在溢流现象尚无定论。同时非接触体系（贵金属与氧化物非直接接触）是否存在氢溢流现象也不为所知。为了能够解决这些问题，Bokhoven教授利利用电子束刻蚀技术，在氧化物载体表面（还原性TiO2，非还原性Al2O3）构造出贵金属Pt、氧化铁的纳米对。Pt，氧化铁纳米对之间的距离可控，从0 nm到45 nm，如下图3所示。再结合原位、空间分辨X射线吸收谱检测每个纳米对中氧化铁的还原情况，从而判断氢溢流的发生与否以及程度。
图 3
首先他们以非还原性氧化铝作为载体。他们发现当Pt与氧化铁之间的距离超过15 nm，氧化铁完全没有被还原。15纳米之内，距离越近还原程度越高。
图 4
Figure 4 | Hydrogen spillover on the aluminium oxide support. a, XAS spectra at the Fe L3 edge measured in X-PEEM during hydrogen dosage at 1 × 10−5 mbar, demonstrating the degree of reduction of the iron oxide particles on the aluminium oxide support for three selected pairs (an overlapping pair of iron oxide and platinum particles, and the pairs with interparticle distances of 15 nm and 45 nm) and the iron oxide particle without platinum in its vicinity. The legend of the graph corresponds to the colour of the circles around the systems in the SEM image (inset).
b, Platinum–iron oxide pairs at varying distances d and a lone iron oxide particle on a single support, demonstrating the distance-dependent hydrogen spillover on the aluminium oxide support from the initial state in Fig. 1b. The percentage of the total composition of each particle at 343 K that is in the metallic iron phase, which forms owing to the reduction of the iron oxides, shows the extent of reduction at different distances from the platinum particle.
当载体由非还原性氧化铝换成还原性二氧化钛, 氢溢流强度大大增强。间距从0 nm到45 nm的“Pt-氧化铁”纳米对中所有的氧化铁都被还原（17 %）。同时载体TiO2整体也被还原。具体结果如下图5所示：
图 5
Figure 5 | Hydrogen spillover on the titanium oxide support. a, Fe L3 XAS spectra compares the extent of the reduction of iron oxide on a titanium oxide support in the pair ‘d3’ with an interparticle distance of 45 nm and the overlapping pair ‘a1’, circled in green and red, respectively, in the SEM image (inset). After hydrogen dosage at 1 × 10−5 mbar pressure in X-PEEM at 343 K, the initial state XAS (state 1) and the reduced state XAS (state 2) of both pairs overlap. b, The Ti L2,3 edge XAS of the initial state and after hydrogen spillover from platinum at 343 K. Equal surface reduction takes place around all 16 systems and in an isolated area far from the platinum (inset, SEM image). c, The Ti L2,3 edge XAS spectra of the initial state and after hydrogen dosage up to 425 K for a sample without platinum nanoparticles. There is no observable reduction of titanium oxide. d, Illustration of spillover on titanium oxide at 343 K from the initial state in Fig. 1b. All the iron oxide particles as well as the titanium oxide support reduce simultaneously, irrespective of their distance d from the platinum particles. A maximum reduction of 17% occurs for all iron oxide particles at this stage.
这一差异表明活化氢在还原性载体表面移动性更强。为了能够更好地解释这一差异，文中通过理论计算考察了H2在贵金属表面的解离、H从贵金属溢流到氧化物载体、以及H在载体间传输的能垒。研究发现活化H或者电子在TiO2载体表面的传输能垒要远小于其脱附的能垒，并且不受水分的影响。而在氧化铝的表面，H2O分子将与H2形成竞争吸附。同时H的吸附传输能垒要高于脱附能垒。所以H不易于在非还原性氧化铝表面传输，而倾向于解离（具体内容请参见原文）。
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