Electrochemical dissolution of platinum in an ionic liquid

Recycling platinum is a difficult, complicated process. The first step is the dissolution of the used platinum. Because platinum is a very special precious metal, this isn’t so easy. The solvents used for this are usually highly corrosive aqua regia, a mixture of nitric and hydrochloric acids, or a highly oxidizing mixture of sulfuric acid and hydrogen peroxide known as piranha. There are also electrochemical recycling processes, but these mostly require highly toxic electrolytes or corrosive media, or they release toxic gases. They also suffer from insufficient current densities and passivation of the electrodes.

Huang and Chen have now developed a novel process that avoids all of these disadvantages. In this procedure, platinum is electrochemically dissolved in a mixture of zinc chloride and a special ionic liquid. An ionic liquid is an organic salt that is in a melted state at temperatures below 100 °C. Ionic liquids are considered environmentally friendly solvents because they have very low vapor pressures and are very thermally stable, so they do not release any toxic substances. They also have high ionic conductivity, which makes them very useful in electrochemical applications.

The used platinum is introduced in the form of an electrode, a voltage is applied, and the surrounding ionic liquid heated to about 100 °C. The platinum then dissolves surprisingly fast. The dissolved platinum can then be removed on a carrier electrode, either as the pure metal or as a zinc alloy, without prior treatment of the solution. The scientists are optimistic that this process can also be adapted for other precious metals.

Says Huang: “We are doing our best to solve the problem about the effective use of precious metals. The recycling of precious metals is a possible strategy. Even now, we do not think we have found the best process. We will continuously modify the process in order to extend its applications or look for a much better one”.

More information: Angewandte Chemie International Edition, DOI: 10.1002/anie.201107997

Provided by Wiley

Large or small, platinum clusters provide new insights

Using Environmental Molecular Sciences Laboratory's high-performance supercomputing capabilities, scientists helped resolve longstanding controversies about the effect of platinum cluster size on some emissions-reducing reactions in automobile catalysts.

The research team included scientists from the University of California, Nanostellar, Inc., the University of Virginia and Lawrence Berkeley National Laboratory.

Carbon monoxide (CO) removal from combustion exhaust, typically carried out by oxidation on three-way catalysts containing platinum, is critically important for cleaner-burning engines.

The researchers used EMSL’s Chinook supercomputer to carry out detailed ab initio quantum mechanical calculations on very large (201-atom) platinum clusters to model the environment of platinum nanoparticles fully covered with CO.

The team integrated rigorous kinetic, isotopic, and in-situ spectroscopy studies of platinum clusters with theoretical simulations of CO oxidation catalysis at conditions prevalent in many industrial applications to gain a better understanding of catalytic activity on platinum clusters.

When a platinum catalyst covered with CO is at low temperatures, the size of the platinum cluster makes little difference in the rate of oxidation.

For other reactions catalyzed by platinum clusters in automobile exhaust, larger clusters will oxidize nitric oxide or dimethyl ether much faster.

CO oxidation is also a critical step in the production of pure hydrogen streams for use in fuels cells and in many chemical processes.

More information: Allian AD, et al. 2011. "Chemisorption of CO and Mechanism of CO Oxidation on Supported Platinum Nanoclusters." J. Am. Chem. Soc., 2011, 133 (12), pp 4498–4517 DOI: 10.1021/ja110073u

Provided by Environmental Molecular Sciences Laboratory

Paperweight for platinum: Bracing catalyst in material makes fuel cell component work better and last longer

A new combination of nanoparticles and graphene results in a more durable catalytic material for fuel cells, according to work published online at the Journal of the American Chemical Society. The catalytic material is not only hardier but more chemically active as well. The researchers are confident the results will help improve fuel cell design.

"Fuel cells are an important area of energy technology, but cost and durability are big challenges," said chemist Jun Liu. "The unique structure of this material provides much needed stability, good electrical conductivity and other desired properties."

Liu and his colleagues at the Department of Energy's Pacific Northwest National Laboratory, Princeton University in Princeton, N.J., and Washington State University in Pullman, Wash., combined graphene, a one-atom-thick honeycomb of carbon with handy electrical and structural properties, with metal oxide nanoparticles to stabilize a fuel cell catalyst and make it better available to do its job.

"This material has great potential to make fuel cells cheaper and last longer," said catalytic chemist Yong Wang, who has a joint appointment with PNNL and WSU. "The work may also provide lessons for improving the performance of other carbon-based catalysts for a broad range of industrial applications."

Muscle Metal Oxide

Fuel cells work by chemically breaking down oxygen and hydrogen gases to create an electrical current, producing water and heat in the process. The centerpiece of the fuel cell is the chemical catalyst -- usually a metal such as platinum -- sitting on a support that is often made of carbon. A good supporting material spreads the platinum evenly over its surface to maximize the surface area with which it can attack gas molecules. It is also electrically conductive.

Fuel cell developers most commonly use black carbon -- think pencil lead -- but platinum atoms tend to clump on such carbon. In addition, water can degrade the carbon away. Another support option is metal oxides -- think rust -- but what metal oxides make up for in stability and catalyst dispersion, they lose in conductivity and ease of synthesis. Other researchers have begun to explore metal oxides in conjunction with carbon materials to get the best of both worlds.

As a carbon support, Liu and his colleagues thought graphene intriguing. The honeycomb lattice of graphene is porous, electrically conductive and affords a lot of room for platinum atoms to work. First, the team crystallized nanoparticles of the metal oxide known as indium tin oxide -- or ITO -- directly onto specially treated graphene. Then they added platinum nanoparticles to the graphene-ITO and tested the materials.

Platinumweight

The team viewed the materials under high-resolution microscopes at EMSL, DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. The images showed that without ITO, platinum atoms clumped up on the graphene surface. But with ITO, the platinum spread out nicely. Those images also showed catalytic platinum wedged between the nanoparticles and the graphene surface, with the nanoparticles partially sitting on the platinum like a paperweight.

To see how stable this arrangement was, the team performed theoretical calculations of molecular interactions between the graphene, platinum and ITO. This number-crunching on EMSL's Chinook supercomputer showed that the threesome was more stable than the metal oxide alone on graphene or the catalyst alone on graphene.

But stability makes no difference if the catalyst doesn't work. In tests for how well the materials break down oxygen as they would in a fuel cell, the triple-threat packed about 40% more of a wallop than the catalyst alone on graphene or the catalyst alone on other carbon-based supports such as activated carbon.

Last, the team tested how well the new material stands up to repeated usage by artificially aging it. After aging, the tripartite material proved to be three times as durable as the lone catalyst on graphene and twice as durable as on commonly used activated carbon. Corrosion tests revealed that the triple threat was more resistant than the other materials tested as well.

The team is now incorporating the platinum-ITO-graphene material into experimental fuel cells to determine how well it works under real world conditions and how long it lasts.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Pacific Northwest National Laboratory.

Journal Reference:

Rong Kou, Yuyan Shao, Donghai Mei, Zimin Nie, Donghai Wang, Chongmin Wang, Vilayanur V Viswanathan, Sehkyu Park, Ilhan A. Aksay, Yuehe Lin, Yong Wang, Jun Liu. Stabilization of Electrocatalytic Metal Nanoparticles at Metal-Metal Oxide-Graphene Triple Junction Points. Journal of the American Chemical Society, 2011; 110208101227051 DOI: 10.1021/ja107719u

Note: If no author is given, the source is cited instead.

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