Adams' catalyst, also known as platinum dioxide, is usually represented as platinum(IV)

oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

, PtO₂•H₂O. It is a catalyst for hydrogenation and hydrogenolysis in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

. This dark brown powder is commercially available. The oxide itself is not an active catalyst, but it becomes active after exposure to hydrogen whereupon it converts to platinum black, which is responsible for reactions.

Preparation

Adams' catalyst is prepared from chloroplatinic acid H₂PtCl₆ or ammonium chloroplatinate, (NH₄)₂PtCl₆, by fusion with

sodium nitrate Sodium nitrate is the chemical compound with the formula . This alkali metal nitrate salt is also known as Chile saltpeter (large deposits of which were historically mined in Chile) to distinguish it from ordinary saltpeter, potassium nitrate. T ...

. The first published preparation was reported by V. Voorhees and Roger Adams. The procedure involves first preparing a platinum nitrate which is then heated to expel nitrogen oxides. :H₂PtCl₆ + 6 NaNO₃ → Pt(NO₃)₄ + 6 NaCl _(aq) + 2 HNO₃ :Pt(NO₃)₄ → PtO₂ + 4 NO₂ + O₂ The resulting brown cake is washed with water to free it from nitrates. The catalyst can either be used as is or dried and stored in a desiccator for later use. Platinum can be recovered from spent catalyst by conversion to ammonium chloroplatinate using aqua regia followed by ammonia.

Uses

Adams' catalyst is used for many applications. It has shown to be valuable for hydrogenation, hydrogenolysis, dehydrogenation, and oxidation reactions. During the reaction, platinum metal ( platinum black) is formed which has been cited to be the active catalyst. Hydrogenation occurs with syn stereochemistry when used on an alkyne resulting in a cis-alkene. Some of the most important transformations include the hydrogenation of ketones to alcohols or ethers (the latter product forming in the presence of alcohols and acids) and the reduction of nitro compounds to amines. However, reductions of alkenes can be performed with Adam's catalyst in the presence of nitro groups without reducing the nitro group. When reducing nitro compounds to amines, platinum catalysts are preferred over palladium catalysts to minimize hydrogenolysis. The catalyst is also used for the hydrogenolysis of phenyl phosphate esters, a reaction that does not occur with palladium catalysts. The pH of the solvent significantly affects the reaction course, and reactions of the catalyst are often enhanced by conducting the reduction in neat acetic acid, or solutions of acetic acid in other solvents.

Development

Before development of Adams' catalyst, organic reductions were carried out using colloidal platinum or platinum black. The colloidal catalysts were more active but posed difficulties in isolating reaction products. This led to more widespread use of platinum black. In Adams' own words:

"...Several of the problems I assigned my students involved catalytic reduction. For this purpose we were using as a catalyst platinum black made by the generally accepted best method known at the time. The students had much trouble with the catalyst they obtained in that frequently it proved to be inactive even though prepared by the same detailed procedure which resulted occasionally in an active product. I therefore initiated a research to find conditions for preparing this catalyst with uniform activity."

Safety

Little precaution is necessary with the oxide but, after exposure to H₂, the resulting platinum black can be pyrophoric. Therefore, it should not be allowed to dry and all exposure to oxygen should be minimized.

References

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