Catalysis

The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane The expression reforming is misleading since it is used also for the well-knowm process for improvement of the octane number of gasoline [1]. In the gas industry, reforming has generally been used for “the changing by heat treatment of a hydrocarbon with high heating value into a gaseous mixture of lower heating value” [2].

Catalysts have been widely used to lower the emissions of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust of automobiles in the United States since the introduction of 1975 models in the fall of 1974. These catalysts, contained in so-called catalytic converters in the exhaust system of automobiles, promote the oxidation of CO and HC to CO₂ and H₂O under net oxidizing conditions (e.g. A/F > 14.6¹): Until 1978, emission control requirements for nitrogen oxide (NO _x ) emissions were met through noncatalyst technology, primarily exhaust gas recirculation (EGR) [1, 2]. Starting with some vehicles sold in California in 1977, NO _x emissions from gasoline engines have been subject to catalytic control. The catalyst here has the additional function to promote the reduction of NO to N₂ via reaction of NO with hydrogen or CO. Catalyst systems designed to reduce NO _x are considerably more complex than the earlier control systems. For example, the control system introduced by General Motors on some 1978 model year cars has closed-loop air-fuel ratio control (closed-loop fuel metering system, exhaust gas oxygen sensor, and an electronic control unit) as well as a three-way catalyst which simultaneously promotes the conversion of HC, CO, and NO _x [3]. Stringent federally mandated emission control requirements of 1 gram per mile (g mi⁻¹) for NO _x have led to the further application of three-way catalysts. This review will emphasize the state-of-the-art of catalytic control of automobile exhaust emissions since 1978, specifically three-way catalysts. A recent review by J. Kummer covers part of this period and earlier years [4]. Other reviews of this subject are listed in the reference section [5–14].

After nearly thirty years of intensive application, infrared spectroscopy remains the most widely used, and usually most effective, spectroscopic method for characterization of the surface chemistry of high-area heterogeneous catalysts and of many other high-area solids of major commercial importance. Starting from the work of Terenin and other Russian scientists in the 1940’s and early 1950’s [1] and important pioneering work by Eischens and coworkers [2, 3] in the 1950’s on supported metal catalysts, the use of infrared spectroscopy in surface studies generally, and in catalytic research particularly, has grown rapidly. Hundreds of articles, reviews [3–11], and three books [12–14] presently attest to its popularity. Infrared spectroscopy [5] and closely related vibrational spectroscopies of other kinds [15–17] have also played, and continue to play, a very important role in research on the surface chemistry of low-area solids of many kinds.

X-ray diffraction (XRD) was developed during the first half of the century and was soon applied to identify the solid phases involved in heterogeneous catalysts. Identification was based on the Bragg law giving the lattice spacings which are the fingerprints of any crystalline phase. Soon, this powerful analytical tool was complemented by line broadening analysis (LBA) giving the average crystallite size in the directions perpendicular to the reflecting plane, thus providing a direct evaluation of the state of division of the solid phases. The development of small angle X-ray scattering (SAXS) brought another method to determine the particle size and the specific surface area of catalysts. Finally, the theory of X-ray scattering by amorphous matter made possible the determination of the interatomic distances between nearest neighbor atoms in materials which do not even exhibit any diffraction lines.