Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

Molybdenum-sulfur chemistry underpins one of the most important industrial catalysts, the MoS₂-based hydrodesulfurization catalyst used in removing sulfur compounds from petroleum by reaction with hydrogen and conversion to H₂S. I was introduced to these catalysts by E. R. Braithwaite of Climax Molybdenum Co. The synergic relationship with Climax continues now through A. W. Armour (who, while a graduate student with me, achieved the notable feat of determining the X-ray structure of ammonium dimolybdate, (NH₄)₂Mo₂O₇, with a crystal taken directly from the Climax manufacturing plant at Rotterdam).

In its lower oxidation states, molybdenum has extensive organometallic chemistry exemplified by the well-known hexacarbonyl [Mo⁰(CO)₆]. A feature of Mo(II) is strong Mo-Mo bonding, as in the acetate, Mo₂(CH₃CO₂)₄, and the so-called dichloride, Mo₆Cl₁₂.

The fundamental challenge of molybdenum chemistry, and the source of its continuing interest, is the subtle interplay of oxidation state, coordination number, and ligating atom, and their impact on structure and reactivity, as well as the potential for applications of molybdenum compounds.

Currently, I am involved in maintaining the Molybdenum Environmental Database for the International Molybdenum Association (http://www.imoa.info). The database is a primer for molybdenum biochemistry. MoS₂, MoO₃, and the molybdates have low toxicity. Molybdenum replaces more toxic elements in some applications--chromates in corrosion inhibitors and antimony in polyvinyl chloride smoke suppressants.

Philip C. H. Mitchell is Leverhulme Emeritus Fellow and formerly a reader in chemistry in the School of Chemistry at the University of Reading, England. He is a consultant for Climax Molybdenum Co. and the International Molybdenum Association.

MOLYBDENUM AT A GLANCE

Name: From the Greek molybdos, lead. Its primary ore was once confused with a lead compound.

Atomic mass: 95.94.

History: Discovered in 1778 by Swedish chemist Carl Welhelm Scheele.

Occurrence: Found primarily in the ore molybdenite (MoS2).

Appearance: Silvery white, hard metal.

Behavior: Molybdenum compounds have low toxicity.

Uses: Essential to life in trace amounts. Has a role in nitrogen fixation and in some enzymes. The metal is used as an alloy in stainless and other steels.

TUNGSTEN

RICK LOWDEN, OAK RIDGE NATIONAL LABORATORY

Tungsten is an incredible material. It is dense and hard, and it has the lowest vapor pressure and highest melting temperature of all metals. This combination of properties makes tungsten extremely valuable for a myriad of applications, while at the same time creates great challenges in the processing of the metal.

As a child, I was fascinated by how things work and spent a lot of time taking things apart. As with most budding engineers, I rarely reassembled them. Incandescent bulbs were one of my first quarries, carefully disassembled to reveal a hidden treasure: a tungsten filament. It was amazing that this tiny wire could be heated to white-hot temperatures to produce light.

Also at an early age, I was introduced to vacuum tubes, and to this day they are magical in my eyes. When a tungsten filament is heated in a vacuum, the electrons near the surface become energetic enough to be emitted into the surrounding space. Additional tungsten conductors, in the form of grids and plates, can be added to the bulb, and the electrons can then be manipulated to switch, rectify, and amplify. These electronic switches were crucial in the development of modern electronics.

Transistors and integrated circuits have almost entirely displaced tubes; however, some researchers are going "retro," exploring tungsten-containing miniaturized vacuum triodes or "nanotriodes" that may one day be used as miniature electronic switches. But one has to ask, "Do they glow?"

TAKING AIM Lowden prepares to test the form and function of 9-mm ammunition fabricated from a tungsten-containing composite replacement for lead.