Platinum catalysts surface studies

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Structure Sensitivity of Hydrocarbon Conversion Reactions on Platinum Surfaces How does the reaction rate depend on the atomic structure of the platinum catalyst surface To answer this question, reaction rate studies using flat, stepped, and kinked single-crystal surfaces with variable surface structure were very useful indeed. For the important aromatization reactions of n-hexane to benzene and Ai-heptane to toluene, it was discovered that the hexagonal platinum surface where each surface atom is surrounded by six nearest neighbors is three to seven times more active than the platinum surface with the square unit cell [155, 156]. Aromatization reaction rates increase further on stepped and kinked platinum surfaces. Maximum aromatization activity is achieved on stepped surfaces with terraces about five atoms wide with hexagonal orientation, as indicated by reaction rate studies over more than 10 different crystal surfaces with varied terrace orientation and step and kink concentrations (Figure 7.38). 

The choice of an appropriate support is of no less importance than that of active phase of a catalyst. We have focused our attention on the application of hydrophobic supports to prepare effective platinum catalysts for hydrosilylation since our preliminary experiments have shown that in a number of hydrosilylation reactions hydrophobic material-supported catalysts appeared to be superior to those based on hydrophilic supports such as alumina and silica. We have also aimed at selecting such supports which, in addition to their hydrophobicity, do not have acid centers on their surfaces, and due to this, they do not catalyze undesirable side reactions of isomerization. The supports selected for our study were styrene-divinylbenzene copolymer (SDB) and fluorinated carbon (FC), because nonfunctionalized SDB is free of acid sites and surface acidity of FC is extremely weak (H 9). The performance of SDB- and FC-supported platinum catalysts was studied in several reactions of hydrosilylation. 

It was seen when studying mixed systems Pt-WOj/C and Pt-Ti02/C that with increasing percentage of oxide in the substrate mix the working surface area of the platinum crystallites increases, and the catalytic activity for methanol oxidation increases accordingly. With a support of molybdenum oxide on carbon black, the activity of supported platinum catalyst for methanol oxidation comes close to that of the mixed platinum-ruthenium catalyst. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

We describe in some detail the techniques of nuclear magnetic resonance which are used for studying alumina-supported platinum catalysts. In particular, we describe the spin-echo technique from which the Pt lineshape can be obtained. We also discuss spin echo double resonance between surface Pt and chemisorbed molecules and show how the NMR resonance of the surface Pt can be separately studied. We present examples of experimental data and discuss their interpretation. 

The particular NMR properties of Pt caused an additional problem. Due to the presence of surfaces near most of the nuclei, the NMR line is very broad (approximately 4 kG wide). This means that only a small fraction of the nuclear spins can be excited by an rf pulse and thus contribute to any given NMR signal. Given these various constraints, our NMR studies of platinum catalysts required 1-gram samples containing 5-10% Pt by weight. 

Nickel-platinum bimetallic catalysts showed higher activity during ATR than nickel and platinum catalysts blended in the same bed. It was hypothesized that nickel catalyzes SR, whereas platinum catalyzes POX and, when they are added to the same support, the heat transfer between the two sites is enhanced [59, 60]. Advanced explanations were reported by Dias and Assaf [60] in a study on ATR of methane catalyzed by Ni/y-Al203 with the addition of small amounts of Pd, Pt or Ir. An increase in methane conversion was observed, ascribed to the increase in exposed Ni surface area favored by the noble metal under the reaction conditions. 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

Kinetic studies at short residence times first suggested the following reaction sequence ethylnaphthalene dehydrogenates to vinylnaphthalene vinylnaphthalene dehydrocyclizes to acenaphthylene and finally acenaphthylene is hydrogenated to acenaphthene. However, further work by Isagulyants and co-workers, using 14C-labeled 1-vinylnaphthylene, shows that over platinum on alumina at 470°C, acenaphthene and acenaphthylene are formed from both 1-ethylnaphthalene and 1-vinylnaphthalene. Vinylnaphthalene dehydrocyclizes about three times faster than ethylnaphthalene. The vinylnaphthalene intermediate remains adsorbed on the catalyst surface during the reaction (48). 

This and similar instruments (3,4) that allow one to study reaction rates and product distributions on small-area crystal and catalyst surfaces have been used in our studies of the mechanism of heterogeneous catalysis and the nature of active sites. These studies, which concentrated primarily on hydrocarbon reaction as catalyzed by platinum crystal surfaces, will be reviewed in the next section. 

We shall restrict most of our discussion to studies of platinum surfaces, which will serve as a model of surface studies of other catalysts. 

In a series of studies, the variation of the turnover number for the dehydrogenation reaction (the number of product molecules/platinum surface atoms/second) with the hydrogen to hydrocarbon ratio at a constant hydrocarbon pressure of 4 x 10"8 Torr was determined. The results are shown in Fig. 20 for the several stepped surfaces studied. The reaction rates increase with increasing hydrogen to hydrocarbon ratio. If no hydrogen is introduced into the reaction chamber, the catalyst behaves very differently. No benzene... 

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