Nickel & Molecular Sieve Purification

Purification of inert gases in the semiconductor industry has been used extensively starting in the 1970's, and the removal of gaseous impurities such as oxygen, carbon monoxide, carbon dioxide, moisture, methane, to ppb or ppt levels from inert streams bears a significant importance today. The process of trapping impurities can be physical or chemical adsorption.

Amongst the first methods of inert gases purification were various combinations of absorbers, reactants and catalysts. Typically molecular sieves have been widely used to remove moisture or carbon dioxide, while based metals such as copper or nickel are used to remove traces of carbon monoxide, oxygen, and even hydrogen. In the early 1980's, a new generation of adsorbent materials, or "non-evaporable getters" was introduced, through zirconium or titanium based alloys. They operate at 350 - 450°C and can lead to advanced inert gas purification. The technology is simple, requiring only one stage in most applications. Despite their adsorption capacity and range of impurities that can be removed through their employment, metal getters are expensive and require heat activation, therefore not attractive for point of use applications.

Nickel - based adsorption technology appears to attract interest in the purification of inert gas streams based on its ease of operation at room temperature. In the semiconductor industry nickel based materials have replaced the copper based compounds based on the concern that copper can poison the integrated circuit.

ADSORPTION ON NICKEL and NICKEL REACTION CHEMISTRY
The nature of adsorption of O2 or CO on an active (reduced) Ni surface nickel is chemisorption. Nickel is usually deposited on an inert support, such as alumina, silica, diatomaceous earths or other supports. The removal of impurities such as moisture, carbon dioxide or light hydrocarbons is limited and in some cases it can be due to adsorption on the support.

The adsorption capacity of a Ni adsorbent material used for O2 or CO depends on the number of Ni sites available for oxidation (Rxn (1) and (2)). The oxidation of Ni material takes place in several sequential regimes: dissociative chemisorption, oxide nucleation/coalescence, and in-depth oxide growth, however the latter may require temperature, therefore this step may not always be part of the room temperature gas purification process. In its reduced or oxide form, an adsorbent material based on supported Ni/ NiO can also trap hydrogen (Rxn (3) and (4)).
Ni + CO > NiCO (1)
2Ni + O2 > 2NiO (2)
NiO + H2 > Ni + H2O (3)
Ni + H2 > 2NiH (4)

Nickel can be periodically regenerated by heating the exhausted/ oxidized material in hydrogen or in a mixture of hydrogen and inert gas, and temperatures around 180°C usually suffice for the regeneration of a Ni trap that had been used for inert gas purification at room temperature. Regeneration is possible due to the reversible nature of adsorption on Ni, and it allows the revival of the sites oxidized by impurities. The regeneration can be performed in situ, an advantage for point of use applications. Numerous regenerations may be completed without significant loss in activity between regenerations.

The nickel regeneration reaction chemistry is shown below:
NiCO + 4H2 > Ni + CH4 + 2H2O (5)
2Ni + O2 > Ni + H2O (6)

DISADVANTAGES OF NICKEL:
Although Nickel offers various advantages for purification of inert gases to ppb/ ppt levels, it has its demerits, such as:

• Pyrophoric nature of reduced nickel, which makes it difficult to handle or ship.
• Lower overall capacity and range of impurities than for heated getters.
• Low capacity for the removal of carbon dioxide and light hydrocarbons such as methane.
• Long and tedious regeneration procedure, with hydrogen being required for regenerations (chemical regeneration).

Due to some limitations of Nickel for room temperature purification of inert gases, combinations of adsorbents can be used in a gas purification system: molecular sieves for removing moisture and carbon dioxide, Pt of Pt-Pd catalyst beds to convert the methane to carbon dioxide and moisture in presence of oxygen.

References:
1) Brian Warrick; Giovanni Carrea, "Inert gas purification to the ppt/ppq level", CleanRooms '96 West session, 602.
2) Donald.W.Breck; "Zeolite Molecular Sieves: Structure, Chemistry and Use", August 1984. Editor, Edition needed.
3) Kazutoshi Yagi-Watanabe, Yoshiko Ikeda, Yasuhiro Ishii, Tamami Inokuchi, and Hirohito Fukutani; "Reaction kinetics and mechanism of oxygen adsorption on the Ni (110) surface", Surface Science 2001 (482-485), 128-133.