Platinum in Zeolites

The extreme case of an ultrananoparticle consists of one single atom. Such mono-atomic Pt sites have been realized in Pt/H-zeolites that are prepared at low temperature. They are identified by their unique selectivity in catalyzing the U/D exchange between cyclopentane and D2. Numerous data in the literature show that over conventional Pt 

FIGURE 9.4. Distribution patterns of deuteron cyclopentanes Multiple exchange after H/D exchange over conventional R catalysts with Ptn particles. Abscissa Value of x inC5H(io-x)D c fraction ordinate rel. abundance. 

It is not exactly known how large a Pt ensemble must be which can catalyze the multiple H/D exchange with D2 of alkanes such as cyclopentane, but it stands to reason that at least two adjacent Pt atoms are required (probably more). It follows that a catalyst which has its Pt atoms predominantly isolated from each other should NOT show this product pattern, but give a product distribution typical of stepwise exchange. Such a product should follow the binomial law i.e. no predominant peak at C5H5D5 the concentrations of the CsHio- iD products at low exchange should show a monotonous decrease with x. 

To test this hypothesis a Pt/mordenite catalyst was prepared from H-mordenite by exchanging a small amount of Pt and reducing at low temperature, so that Pt-H adducts could be formed and most Pt atoms were located in the side pockets of the 

FIGURE 9.5. H/D stepwise exchange over Pti in H-MOR. a. Cyclopentane b. Neopentane. 

---

Although by 1972 Dalla Betta and Boudart (168) and later Foger and Anderson (169) showed that electron-deficient platinum (in zeolites Y LaY, CaY, and NaY) is much more active than Pt/Si02 (169) and Pt/Al203... 

A succession of experiments characterizing platinum in zeolite Y with essentially all the available techniques, summarized by Boudart et al. [31] and by Samant and Boudart [32], has led to estimates of the average number of platinum atoms per cluster ranging from 6 to about 40. It is difficult to provide precise statements of cluster sizes when the clusters are present in mixtures and when the clusters contain more than a few atoms. 

---