The Pt function in Pt,Sn/Mg(Al)O and in Pt/Mg(Al)O catalysts has been studied by a combination of catalytic testing for ethane dehydrogenation at 450−650 °C under a C2H6/H2/CO2/N2/Ar = 10:1.6:6.8:5.9:75.7 flow and Fourier transform infrared spectroscopy (FT-IR), using CO as a probe molecule. The acid−base properties of the support were also investigated by FT-IR, using CH3CN as a probe molecule. CO adsorption experiments revealed the presence of Pt terraces, as well as Pt sites with low coordination number (steps, edges, corners, or defects) on Pt/Mg(Al)O. The same experiments on Pt,Sn/Mg(Al)O revealed that Sn covers the steps, corners, edges, and defects of the Pt particles, thus developing simultaneously a geometric and a chemical effect on the surface properties of the exposed Pt atoms. Accordingly, the ethane dehydrogenation reaction proceeds with a lower activation energy over Pt,Sn/Mg(Al)O compared to Pt/Mg(Al)O. Further, Sn addition leads to more selective and more stable ethane dehydrogenation catalysts. The higher dehydrogenation selectivity of Pt,Sn catalysts was correlated to the masking of low-coordinated Pt sites. The Pt/Mg(Al)O and Pt,Sn/Mg(Al)O catalysts were subjected to an activation procedure consisting of several test−regeneration cycles. Good correlation was found between the number of accessible Pt sites and the catalytic activity after each cycle.

A Cr/Al 2 O 3 alkane dehydrogenation catalyst exhibits a maximum in ethylene yield during an ethane dehydrogenation cycle. Isotopic labelling experiments with monolabelled 13 C-ethane and deuterium were used to elucidate whether the initial activity increase could be due to formation of an active, larger hydrocarbon intermediate on the surface. The results strongly indicate that this is not the case, and instead point to a traditional reaction cycle involving adsorption of ethane to form an ethyl species, followed by desorption of ethene and hydrogen. Transient kinetic data suggest that ethane adsorption is the rate-determining step of reaction.

Calcined hydrotalcite with or without added metal (Mg(Al)O, Pt/Mg(Al)O and Pt,Sn/Mg(Al)O) have been investigated with in situ X-ray Photoelectron Spectroscopy (XPS) during ethane dehydrogenation experiments. The temperature in the analysis chamber was 450ºC and the gas pressure was in the range 0.3 -1 mbar. Depth profiling of calcined hydrotalcite and platinum catalysts under reaction, oxidation and in hydrogenwater mixture was performed by varying the photon energy, covering an analysis depth of 10-21 Å. It was observed that the Mg/Al ratio in the Mg(Al)O crystallites does not vary significantly in the analysis depth range studied. This result indicates that Mg and Al are homogeneously distributed in the Mg(Al)O crystallites. Catalytic tests have shown that the initial activity of a Pt,Sn/Mg(Al)O catalyst increases during an activation period consisting of several cycles of reduction -dehydrogenation -oxidation. The Sn/Mg ratio in a Pt,Sn/Mg(Al)O catalyst was followed during several such cycles, and was found to increase during the activation period, probably due to a process where tin spreads over the carrier material and covers an increasing fraction of the Mg(Al)O surface. The results further indicate that spreading of tin occurs under reduction conditions. A PtSn 2 alloy was studied separately. The surface of the alloy was enriched in Sn during reduction and reaction conditions at 450°C. Binding energies were determined and indicated that Sn on the particle surface is predominantly in an oxidized state under reaction conditions, while Pt and a fraction of Sn is present as a reduced Pt-Sn alloy.

Mechanistic and kinetic information on the ethane dehydrogenation reaction over a semicommercial Pt,Sn/ Mg(Al)O catalyst has been elucidated from catalytic testing and isotopic labeling experiments under reaction conditions close to those used in the commercial dehydrogenation process (C 2 H 6 /H 2 /H 2 O/inert ) 10/1.5/2/32 or C 2 H 6 /H 2 /CO 2 /inert ) 10/2/5/83, 600-630°C reaction temperature, atmospheric pressure). From kinetic measurements, a negative dependence of the reaction rate in H 2 and C 2 H 4 partial pressures was observed, while the dependence on steam partial pressure was positive. Isotopic labeling experiments showed that the negative effect of H 2 and C 2 H 4 could not be attributed to the reverse reaction, but rather to competitive adsorption at the active sites for dehydrogenation. The observed reaction rate with respect to C 2 H 6 was close to first order. By fitting the experimental data to the rate equation derived from the elementary steps of ethane dehydrogenation, the observed deviation from the first order could be explained by partial occupation of the surface by adsorbed surface species. Methane and ethyne were the main byproducts of the dehydrogenation reaction. Cofeed experiments with 13 C 12 CH 6 / 12 C 2 H 4 indicated that both methane and ethyne are produced via ethene, and not directly from ethane. D 2 /C 2 H 6 cofeeding experiments revealed further that one H at the time is replaced with D in ethene, that addition of CO 2 does not affect the H-D distribution in ethene and ethyne and that practically no H-D exchange takes place over the support. The activation energy of the dehydrogenation of ethane over the present catalyst has in a previous study been determined to 116 kJ/mol [Virnovskaia, A.; Morandi, S.; Rytter, E.; Ghiotti, G.; Olsbye, U. Characterization of Pt,Sn/Mg(Al)O Catalysts for Light Alkane Dehydrogenation by FT-IR Spectroscopy and Catalytic Measurements. J. Phys. Chem. C 2007, 111, 14732]. The apparent activation energy for ethene hydrogenation over the same catalyst could be determined to -40 kJ/mol in the temperature range 497-596°C, indicating that in this temperature range the decrease in surface coverage with temperature overcompensates the increase of the rate constant of the rate determining step of the hydrogenation reaction.