Scientists Discover New Way To Make Water

ScienceDaily (Nov. 1, 2007) — In a familiar high-school chemistry demonstration, an instructor first uses electricity to split liquid water into its constituent gases, hydrogen and oxygen. Then, by combining the two gases and igniting them with a spark, the instructor changes the gases back into water with a loud pop.

Scientists at the University of Illinois have discovered a new way to make water, and without the pop. Not only can they make water from unlikely starting materials, such as alcohols, their work could also lead to better catalysts and less expensive fuel cells.

"We found that unconventional metal hydrides can be used for a chemical process called oxygen reduction, which is an essential part of the process of making water," said Zachariah Heiden, a doctoral student and lead author of a paper accepted for publication in the Journal of the American Chemical Society, and posted on its Web site.

A water molecule (formally known as dihydrogen monoxide) is composed of two hydrogen atoms and one oxygen atom. But you can't simply take two hydrogen atoms and stick them onto an oxygen atom. The actual reaction to make water is a bit more complicated: 2H₂ + O₂ = 2H₂O + Energy.

In English, the equation says: To produce two molecules of water (H₂O), two molecules of diatomic hydrogen (H₂) must be combined with one molecule of diatomic oxygen (O₂). Energy will be released in the process.

"This reaction (2H₂ + O₂ = 2H₂O + Energy) has been known for two centuries, but until now no one has made it work in a homogeneous solution," said Thomas Rauchfuss, a U. of I. professor of chemistry and the paper's corresponding author.

The well-known reaction also describes what happens inside a hydrogen fuel cell.

In a typical fuel cell, the diatomic hydrogen gas enters one side of the cell, diatomic oxygen gas enters the other side. The hydrogen molecules lose their electrons and become positively charged through a process called oxidation, while the oxygen molecules gain four electrons and become negatively charged through a process called reduction. The negatively charged oxygen ions combine with positively charged hydrogen ions to form water and release electrical energy.

The "difficult side" of the fuel cell is the oxygen reduction reaction, not the hydrogen oxidation reaction, Rauchfuss said. "We found, however, that new catalysts for oxygen reduction could also lead to new chemical means for hydrogen oxidation."

Rauchfuss and Heiden recently investigated a relatively new generation of transfer hydrogenation catalysts for use as unconventional metal hydrides for oxygen reduction.

In their JACS paper, the researchers focus exclusively on the oxidative reactivity of iridium-based transfer hydogenation catalysts in a homogenous, non-aqueous solution. They found the iridium complex effects both the oxidation of alcohols, and the reduction of the oxygen.

"Most compounds react with either hydrogen or oxygen, but this catalyst reacts with both," Heiden said. "It reacts with hydrogen to form a hydride, and then reacts with oxygen to make water; and it does this in a homogeneous, non-aqueous solvent."

The new catalysts could lead to eventual development of more efficient hydrogen fuel cells, substantially lowering their cost, Heiden said.

The work was funded by the U.S. Department of Energy.

Chemistry professor Thomas Rauchfuss, left, and graduate student Zachariah Heiden have devised a new way to make water. (Credit: Photo by L. Brian Stauffer)

据sciencedaily网站2007年11月1日报道，在我们熟悉的高中化学课中，老师首先把液态水电解成成分气体，即氢和氧。然后把两种气体混合在一起，用一个电火花点燃气体，在连续的砰砰声中老师又将气体还原成水。

伊利诺斯大学的科学家发明了一种新的造水方法，不需要点燃气体发出砰砰声。他们的新发明不仅可以从不稳定的原始材料，比如酒精中获取水，而且还可以制造出更好的催化剂和成本并不昂贵的燃料电池。撒迦利亚·赫登说，“我们发现非传统的金属氢化物可以用于一种称之为氧还原的化学反应过程。氧还原是制造水过程的基础部分”。撒迦利亚·赫登是一名博士生，他是该论文的主要撰写人。这篇新造水法论文刊登在了《美国化学会会刊》 (JACS)上，也上传到了该杂志的网站上。

水分子（二氢一氧化合物）由两个氢原子和一个氧原子组成。但是我们不能简单地将二个氢原子粘在一个氧原子上。造水的实际反应要更加复杂一些：2H2 + O2 = 2H2O +能量。

用英语来解释这个等式的意识就是：要制造水（H2O）的两个分子，就必须将两个双原氢与一个双原氧的结合在一起。这一化合过程中会释放出能量。

伊利诺斯大学化学教授托马斯·诺奇夫斯是本篇论文的撰写人之一。他说，“该反应（2H2＋O2＝2H2O＋能量）两个世纪前就为人们所知，但是直到现在也没有一个人能够找到类似的解决方案”。这一著名的反应同样在氢燃料电池中也会发生。

典型的燃料电池，双原氢气体进入电池的一边，而双原氧气体进入另一边。氢分子失去他们的电子，通过氧化过程变成带正电的氢原子，而氧分子获得四个电子，然后通过还原反应过程变成带负电的氧原子。带负电的氧离子与带正电的氢离子结合形成水，释放出电量。

诺奇夫斯说，“燃料电池的不同面发生的是氧还原反应而不是氢氧化反应。然而，我们发现氧还原新催化剂同时还可以引发新的氢氧化化学反应。”

诺奇和赫登最近对新型氢化催化剂进行了研究，该催化剂是用于氧还原的非常传统金属氢化物。在他们发表的论文中，研究人员采用了一种同质非水溶液，专门对铱氢化催化剂的氧化反应进行了研究。

赫登说：“大多数化合反应都是与氢或氧进行的，但是该催化反应却是氢和氧同时也进行化合反应。与氢的反应形成了一种氢化物，然后再与氧反应形成水。这些反应都是在同质非水溶液中进行的。”

赫登称新型催化剂可以促进更具效率的氢燃料电池的发展，充分地降低氢燃料电池的成本。该研究得到了美国能源部的资助。