Ruthenium(IV) oxide

Ruthenium(IV) oxide (RuO2) is a black chemical compound containing the rare metal ruthenium and oxygen. The most often used O2 catalyst is ruthenium(IV) oxide, however care must be taken since hydrates of this oxide exist.

RuO2 is generally used as a catalyst in various industrial applications or an electrode in electrochemical processes. RuO2 is highly reactive with reducing agents, due to its oxidizing properties.

Structure and Physical Properties
Ruthenium(IV) oxide takes on the rutile crystal structure, similar to titanium dioxide and several other metal oxides. Due to its structure, ruthenium(IV) oxide easily forms hydrates.

Ruthenium(IV) oxide is a (nearly black) purple crystalline solid at room temperature. The hydrates of RuO2 have a blue color to them. RuO2 sublimes at 1200°C under standard conditions (and does not have boiling point). Ruthenium(IV) oxide has a density of about 6970 kg/m³ or 6.970 g/cm³.

Ruthenium oxide has great capacity to store charge when used in aqueous solutions. Average capacities of ruthenium(IV) oxide have reached 650 F/g when in H2SO4 solution and annealed at temperatures lower than 200oC. In attempts to optimise its capacitive properties, prior work has looked at the hydration of ruthenium oxide, its crystallinity and particle size.

The mass percent composition is as follows: Ruthenium 75.96%, Oxygen 24.04%

Ruthenium(IV) oxide is insoluble in water.

Preparation
There are various ways in preparing ruthenium(IV) oxide. The following processes described below are for preparing RuO2 as a film.

1. The chemical vapor deposition (CVD) of RuO2 from suitable voltile ruthenium compounds.

2. The pyrolysis, or heating of ruthenium halides, suitably deposited on the substrate by spraying on the heated substrate a solution of the halide. The most commonly used halide is ruthenium(III) chloride to form RuO2.

This technique has in fact been developed by Schafer for the preparation of nearly stoichiometric RuO2 single crystals.

Both process follow the same reaction mechanism:

Ru+(IV) + O2 (heat)→ RuO2

High temperature flashes of heat up to 1500oC can remove all oxides and contaminants, and form a new oxide layer on the ruthenium.

3. Another way to prepare RuO2 is through electroplating. Films can be electroplated from a solution of RuCl3.xH2O. Pt gauze was used as the counter electrode and Ag/AgCl as the reference electrode.

Uses
RuO2 is extensively used for the coating of titanium anodes for the electrolytic production of chlorine and for the preparation of resistors or integrated circuits.

Ruthenium(IV) oxide is a versatile catalyst and doping agent. hydrogen sulfide can be split by light by using a photocatalyst of CdS particles doped with ruthenium(IV) oxide loaded with ruthenium dioxide. This may be useful in the removal of H2S from oil refineries and from other industrial processes. The hydrogen produced could be used to synthesize ammonia, methanol, and possibly fuel a future hydrogen economy.

Ruthenium (IV) oxide is being used as the main component in the catalyst of the Deacon process which produces  chlorine by the oxidation of  hydrogen chloride.

Oxidative Catalyst
RuO2 by itself is a poor catalyst because without the presence of a hydrate its surface area is greatly decreased. To get pure ruthenium(IV) oxide, it needs to be annealed at 900oC. The best catalyst for electrochemical processes is to have some hydrate present, but not a completely hydrous one. RuO2 can be used as catalyst in multiple reactions. Such noteworthy reactions are the Fischer-Tropsch process and fuel cells.

Precautions
Use personal protection equipment when handling. Conditions and substances to avoid are: extreme heat, fire, acids, aqua regia, organic solvents.

Suppliers
RuO2 and its hydrates can be purchased commercially. Such suppliers are Alfa Aesar and American Elements.