Platinum black

Platinum black (Pt black) is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color.

Platinum black is widely used as a thin film covering solid platinum metal, forming platinum electrodes for applications in electrochemistry. The process of covering platinum electrodes with such a layer of platinum black is called "platinization of platinum". The platinized platinum has a true surface area much higher than the geometrical surface area of the electrode and, therefore, exhibits action superior to that of shiny platinum.

Platinum black powder is used as a catalyst in proton exchange membrane fuel cells. In common practice, the platinum black is either sprayed using an ultrasonic nozzle or hot pressed onto the membrane or gas diffusion layer. A suspension of platinum black and carbon powder in ethanol-water solutions serves to optimize the uniformity of the coating, electrical conductivity, and in the case of application to the membrane, to prevent dehydration of the membrane during the application.

Historically many "self-lighting" gas lamps, ovens, and stove burners used platinum black to catalyze the oxidation of a small amount of gas, lighting the device without a match or spark. This works particularly well for producer gas, town gas, and wood gas which contain a substantial fraction of hydrogen gas (H₂) which is particularly well catalyzed by platinum black.

Platinum black powder can be manufactured from ammonium chloroplatinate by heating at 500 °C in molten sodium nitrate for 30 minutes, followed by pouring the melt into water, boiling, washing, and reduction of the brown powder (believed to be platinum dioxide) with gaseous hydrogen to platinum black.

Before platinization, the platinum surface is cleaned by immersion in aqua regia (50% solution, i.e., 3 volumes of 12 mol/kg of HCl, 1 volume of 16 mol/kg HNO₃, 4 volumes of water).

Platinization is often conducted from water solution of 0.072 mol/kg of chloroplatinic acid and 0.00013 mol/kg of lead acetate, at a current density of 30 mA/cm² for up to 10 minutes. The process evolves chlorine at the anode; the interaction of the chlorine with the cathode is prevented by employing a suitable separation (e.g., a glass frit).

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