Model catalysts ideal crystals

Vickerman and Ertl (1983) have studied H2 and CO chemisorption on model Cu-on-Ru systems, where the Cu is deposited on single-crystal (0001) Ru, monitoring the process using LEED/Auger methods. However, the applicability of these studies carried out on idealized systems to real catalyst systems has not been established. Significant variations in the electronic structure near the Eermi level of Cu are thought to occur when the Cu monolayer is deposited on Ru. This implies electron transfer from Ru to Cu. Chemical thermodynamics can be used to predict the nature of surface segregation in real bimetallic catalyst systems. 

With differences in activity between crystal faces of only a factor of five or less for oxygen reduction in perchloric acid, the particle models of Kinoshita indicate that there would be little or no crystallite size effect for the ORR in a nonadsorbing acid, such as trifluoromethane sulfonic acid or closely related derivatives [44]. In KOH, because the (111) face is the most active, an increase in activity with the smallest particles (e.g., 1 nm)—ideally (uniform size, perfect geometry) would be predicted by the Vt hkl) data—would be by about a factor of five. With real catalysts, where all particles are neither uniformly sized nor perfectly facetted, the effect might be much less, perhaps only a factor of two or so. Unfortunately, we do not know of any studies of crystallite size effects for supported Pt catalysts in either KOH or... 

The structure gap concept derives from the difficulty of knowing to what extent idealized catalysts are representative of the results obtained with real-life catalysts. The most idealized catalysts expose only one well-defined single crystal plane with surface areas of the order of 1 cm and are most often studied under UHV conditions. In contrast to such simple single crystal model systems, real-life catalysts normally consist of small-supported nanoparticles buried in a porous support material. For example, in emission cleaning... 

The porosity of catalyst is correlative with the geometric accmnulative ways between the particles in pellet. If the particles accumulate in the ideal waj of crystal structure, the changes of the porosity with particles size are given by Graton-Fraser model (Table 7.8). 

A common approach to study heterogeneous catalyst materials is by means of surface science techniques, in particular spectroscopy [4], These techniques allow one to characterize and investigate surfaces and interfaces (routinely) and improved their understanding significantly [5, 6]. However, the techniques of surface science are mainly restricted to UH V pressure conditions, thus are in general only applicable under ideal conditions far from real catalyst environments. Further, for application of i.e. spectroscopy, usually model systems with reduced complexity and trying to mimic real catalysts are used in order to understand catalytic phenomena [7]. In the early days of surface science these systems were predominantly single crystal surfaces [8], that evolved into supported particles [9, 10], which still often lack the complexity of real catalyst materials. 

There is a third aspect of spatial effects in catalytic processes which researchers with a bent toward surface science will claim is the most important. This is the fact that spatial uniformity of a catalyst surface is an idealization. In reality, no surface is truly uniform, and there is reason to believe that the natural spatial variations present in all catalytic systems are important in the understanding of behavioral features. Figure 11 is a view taken from a scanning electron microscope of a polycrystalline foil of the type used in the experiments described previously. Various crystal orientations are evident and the scale of these variations is probably too large to justify a general assumption of spatial uniformity in mathematical models, particularly in the unsteady state. Studies with single crystal surfaces have pointed to the possibility that... 

---