From Wikipedia, the free encyclopedia

Adams's catalyst
IUPAC name[hide] Platinum(IV) oxide
Other names[hide] platinum dioxide, platinic oxide
Identifiers
CAS number	1314-15-4
ChemSpider	306130
Jmol-3D images	Image 1
SMILES [show]
InChI [show]
Properties
Molecular formula	PtO2
Molar mass	227.08 g/mol
Appearance	black solid
Density	10.2 g/cm³
Melting point	450 °C, 723 K, 842 °F
Solubility inwater	insoluble
Solubility	insoluble in alcohol, acid, aqua regia soluble in caustic potash solution
(verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Adams's catalyst, also known as platinum dioxide, is usually represented as platinum(IV) oxide hydrate, PtO₂-H₂O. It is a catalyst for hydrogenation and hydrogenolysis in organic synthesis. 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.

Contents [hide] 1 Preparation 2 Uses 3 Development 4 Safety 5 References 6 External links

[edit]Preparation

Adams's catalyst is prepared from chloroplatinic acid H₂PtCl₆ or ammonium chloroplatinate, (NH₄)₂PtCl₆, by fusion with sodium nitrate. The first published preparation was reported by V. Voorhees and Roger Adams.^[1] The procedure involves first preparing a platinum nitrate which is then heated to expel nitrogen oxides.^[2]

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.

[edit]Uses

Adams's catalyst is used for many applications. It has shown to be valuable for hydrogenation and hydrogenolysis, dehydrogenation, and oxidation reactions. During the reaction, platinum metal (platinum black) is formed which has been cited to be the active catalyst.^[3] [4]Hydrogenation occurs with syn stereochemstry when used on an alkyne resulting in a cis-alkene. Some of the most important transformations include the hydrogenation of nitro compounds to amines and ketones to alcohols. Although, 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.

PtO₂ + 4 H₂ → Pt + 2 H₂O

[edit]Development

Prior to the development of Adams's catalyst, organic reductions were carried out using colloidal palladium, colloidal plati