孤独的原子CBA联赛重启：升降级制度或成为新赛季焦点话题，幸福的团聚

　　在磁铁矿表面铂原子的神奇行为CBA联赛重启：升降级制度或成为新赛季焦点话题，使其成为更好的催化剂。现在，维也纳工业大学的科学家们能够说明，铂原子在一氧化碳的帮助下如何形成配对的原因。

　　乍一看，磁铁矿似乎是一个不起眼的灰色矿物。但是，在原子级上，它却具有显著的性质CBA联赛重启：升降级制度或成为新赛季焦点话题：在磁铁矿上，单个金属原子被固定在某一地点，或者使它们在磁铁矿表面移动。有时候，几个金属原子在磁铁矿上形成小簇。这种现象可以极大地改变材料的化学活性。磁铁矿表面上的原子运动过程，决定了某些金属原子作为化学反应的催化剂到底有多大的作为。

　　维也纳工业大学的科学家与荷兰乌得勒支大学的同事合作，观察单个铂原子形成小原子簇。在这个过程中一氧化碳扮演了双重角色：它让单个铂原子移动形成配对，然后它让这些成双成对的原子长时间不分离。只有通过升高温度，才能把这些成对的铂原子强行拆散。

　　孤独的原子

　　这是一个心酸的爱情故事：“两个铂原子真心希望在一起，但是磁铁矿却棒打鸳鸯，”维也纳工业大学的罗兰 布利姆说。与加雷斯 帕金森教授，乌尔力克 德波尔德教授和CBA联赛重启：升降级制度或成为新赛季焦点话题他们的同事一道，罗兰 布利姆用扫描隧穿显微镜分析了铂原子的行为。

　　“当铂原子遇到磁铁矿表面，磁铁矿里面的氧原子使其保持在固定位置上。铂原子就在这里终老。在另一面，铂原子想要成双成对，但是磁铁矿不答应，” 罗兰 布利姆说。铂原子只能呆在磁铁矿晶体的特定位置上，如果没有外界的帮助，它是无法离开的。

　　然而，随着一氧化碳的出现，这种状况被彻底改观了：“一个一氧化碳分子可以和一个铂原子贴在一起，并把它托举起来”，加雷斯 帕金森说。“我们称之为天棚效应”。升降过程将被磁铁矿紧紧束缚的原子解放出来，然后，一氧化碳分子带着铂原子在磁铁矿表面四处闲逛。

　　当一个自由移动的铂原子发现了另一个原子，它们就形成了一个化学键（只要它们两个都是被一氧化碳解救出来的），从而减少了磁铁矿对它们的影响。

　　当温度升高到250°C时，一氧化碳的法力失效，与铂原子分离，键断裂。两个铂原子必须再次在磁铁矿表面上找到各自的位置。这种效应开启了将原子簇转化成单个原子的策略，这个重要的过程被称为“单原子催化剂”。有时候，形成几个原子的团簇。然而，这些较大的团簇不能被分解，即使在高温下也不能被分解。

　　原子分辨率影像

　　“在我们的扫描隧穿显微镜下，我们可以将磁铁矿表面的同一个部位反复成像，这样我们就创建了一个电影，展示原子的连续动作” 罗兰 布利姆说。“这对于理解磁铁矿表面到底发生了什么至关重要，当它们在磁铁矿表面游荡或结合在一起的时候，我们可以观察单个原子。如果我们只能获取最终的结果照片，我们就不能肯定地说，一个特定结构是不是由一个、两个或更多的原子组成的，只有了解原子运动的时间演化过程，我们才知道哪种解释是正确的。”布利姆不仅进行了实验操作，还从事了复杂的理论计算，以解释在量子力学层次上，铂原子的奇特行为。

　　对于化学催化作用，这样的研究成果起着非常重要的作用。“金属如铂经常被用作催化剂，” 加雷斯 帕金森说。“但是许多金属原子组成的原子簇可能与磁铁矿表面上的单个金属原子具有完全不同的化学性质。当我们想要优化催化剂时，我们必须要了解和控制原子的行为。这项工作朝着这个目标迈进了一大步。”

　　“英文原文”

　　Lonely atoms, happily reunited

　　The remarkable behaviour of platinum atoms on magnetite surfaces could lead to better catalysts. Scientists at TU Wien (Vienna) can now explain how platinum atoms can form pairs with the help of carbon monoxide.

　　At first glance, magnetite appears to be a rather inconspicuous grey mineral. But on an atomic scale, it has remarkable properties: on magnetite, single metal atoms are held in place, or they can be made to move across the surface. Sometimes several metal atoms on magnetite form small clusters. Such phenomena can dramatically change the chemical activity of the material. Atomic processes on the magnetite surface determine how well certain metal atoms can serve as catalysts for chemical reactions.

　　Scientists at TU Wien (Vienna), together with colleagues from Utrecht University, can now watch single platinum atoms form tiny clusters. Carbon monoxide plays a dual role in this process: It allows single platinum atoms to move and form pairs, and then it holds these pairs together for a long time. Only by increasing the temperature can the pair-bonds between platinum atoms can be broken.

　　Lonely Atoms

　　It sounds a bit like an unhappy love story: "Two platinum atoms would actually like to be together, but the magnetite surface keeps them apart", says Roland Bliem (TU Wien). Together with Professor Gareth Parkinson, Professor Ulrike Diebold and their colleagues, he analysed the behaviour of platinum atoms using a scanning tunnelling microscope.

　　"When a platinum atom hits the magnetite surface, it is kept in place by the oxygen atoms in the magnetite. The atoms always end up alone. On other surfaces, pair formation would be favoured, but magnetite does not allow that", says Roland Bliem. The platinum atoms sit on specific places on the magnetite crystal and cannot get away without outside help.

　　However, with the appearance of carbon monoxide, the situation changes completely: "A carbon monoxide molecule can attach to a platinum atom and lift it up", says Gareth Parkinson. "We call that the skyhook effect." The lifting process frees the atom from the tight grip of the magnetite, and together, the molecule and the platinum atom can start moving around randomly across the magnetite surface.

　　When one mobilized platinum atom finds another, they can form a bond – as long as both of them are being lifted up by carbon monoxide, diminishing the influence of the magnetite below.

　　When the temperature is increased to 250°C, the carbon monoxide separates from the platinum atom and the bond breaks up. The two platinum atoms must once again find separate places on the magnetite surface. This effect opens up a strategy to turn clusters into single atoms – an important process in so called "single-atom catalysts". Sometimes clusters of several atoms are formed. These larger clusters, however, cannot be broken up, even at high temperatures.

　　Movies with Atomic Resolution

　　"In our scanning tunnelling microscope, we can image the same part of the surface again and again, so that we can create a movie, showing the dancing atoms", says Roland Bliem. "This is crucial for understanding what really happens on the magnetite surface. We can watch single atoms as they wander across the magnetite surface or bond with each other. If we only had a picture of the end result, we could not say with certainty, whether one specific structure consists of one, two or more atoms. Only by following the time evolution of the atomic motion, we know which interpretation is correct." Bliem did not only conduct the experiments, he also performed complex theoretical calculations to explain the peculiar behaviour of the platinum atoms on a quantum mechanical level.

　　For chemical catalysis, such findings play an important role. "Metals such as platinum are frequently used as catalysts", says Gareth Parkinson. "But a large cluster of many metal atoms may have completely different chemical properties than single metal atoms sitting separately on a surface. When we want to optimize catalysts, so we must be able to understand and control the behaviour of the atoms. This work is one step further towards that goal."