Noble Metals - Introduction

Introduction

Palladium, platinum, gold and mercury can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, but iridium and silver cannot. (Silver can dissolve in nitric acid, though.) Ruthenium can be dissolved in aqua regia only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Niobium and tantalum are resistant to acids, including aqua regia.

This term can also be used in a relative sense, considering "noble" as an adjective for the word "metal". A "galvanic series" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows designers to see at a glance how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.

In physics, the definition of a noble metal is even more strict. It is required that the d-bands of the electronic structure are filled. Taking this into account, only copper, silver and gold are noble metals, as all d-like bands are filled and do not cross the Fermi level. For platinum two d-bands cross the Fermi level, changing its chemical behaviour; it is used as a catalyst. The different reactivity can easily be seen during the preparation of clean metal surfaces in an ultra-high vacuum; surfaces of "physically defined" noble metals (e.g., gold) are easy to clean and keep clean for a long time, while those of platinum or palladium, for example, are covered by carbon monoxide very quickly.