Oxygen Sponge May Also Function as Hydrogen Sponge

Researchers at the Department of Energy’s Oak Ridge National Laboratory have discovered that the “oxygen sponge” catalyst found in vehicle exhaust systems could also be used as a “hydrogen sponge.” This discovery is important because it could pave the way for more effective catalysts for selective hydrogenation reactions, which are vital to producing double-bonded alkenes needed for plastics and fuel production. 

The oxygen sponge

In vehicle engines, oxygen is needed for hydrocarbon fuel to burn. The exhaust that is generated contains deadly carbon monoxide and unburned hydrocarbons. In the catalytic converter, the catalyst cerium oxide grabs oxygen from air and adds it to carbon monoxide and hydrocarbons to turn them into carbon dioxide, which is nonlethal. The finding that cerium oxide may grab hydrogen as well as oxygen is promising for efforts to engineer it to catalyze both reactions that cause electron gain (“reduction” of a reactant) and electron loss (“oxidation”).

Two mechanisms have been proposed to explain the interaction between molecular hydrogen and cerium oxide. One suggests both hydrogen atoms associate only with oxygen atoms to produce the same product (two hydroxyl species, or OH chemical groups) on the surface. In the other mechanism posited, one hydrogen atom associates with an oxygen atom to make OH and the other hydrogen atom associates with a cerium atom to make cerium hydride (CeH). The former mechanism is termed “homolytic,” and the latter is called “heterolytic.”

While theory predicted the heterolytic reaction, it had not been observed.

Proving theory through observation

The researchers made nanoscale crystalline rods of cerium oxide with well-defined surface structure to facilitate an understanding of catalytic reactions that would be difficult with commercial, normally spherical particles of cerium oxide. The nanoscale rods allowed them to differentiate hydrogen in the bulk from hydrogen on the surface, where catalysis was presumed to happen. The first observation of hydrides both on the surface and in the bulk of ceria was important because it established that the bulk of the material also can participate in chemical reactions.

In addition, the researchers performed in situ experiments using infrared and Raman spectroscopies, which scatter photons to create spectra that give “fingerprints” of atomic vibrations. Unfortunately, these optical techniques “see” only vibrating oxygen–hydrogen bonds (from stretching between oxygen and hydrogen bonds); they are blind to hydride species on ceria. To see the hydrogen interactions directly, the researchers turned to inelastic neutron scattering.

For more information, see the researchers' published findings.