In situ XPS investigation of Pt(Sn)/Mg(Al)O catalysts during ethane dehydrogenation experiments

Abstract

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.

CO oxidation and oxygen reduction activity of bimetallic Sn–Pt electrocatalysts on carbon: effect of the microstructure and the exclusive formation of the Pt3Sn alloy

“…After exposure to 300 mbar H 2 at RT for 2 h the Pt become fully metallic and about 60 % of the tin content was reduced to Sn 0 showing a Sn 3d 5/2 binding energy of 485.3 eV. This value is somewhat high for bulk metallic Sn (485.0 eV or below), but corresponds well to those reported for Pt-Sn alloys [48][49][50]. The somewhat low Sn M 4 N 45 N 45 Auger kinetic energy (436.0 eV giving an Auger parameter of 921.5 eV) is still compatible with a metallic environment; the shift from the bulk metallic value is again indication of Sn-Pt alloy formation.…”

“…The smaller than expected Pt/Sn ratio indicates that metallic tin still remains a surface species. According to the literature, surface segregation of tin in Pt-Sn alloys is a known property [50,51]; the nature of the segregation seems to be also related to the Pt particle size [52]. As it is shown in Fig.…”

The influences of Mg/Al molar ratio on the properties of PtIn/Mg(Al)O- x catalysts for propane dehydrogenation reaction

“…Previous studies showed that metal oxide supports including K-L zeolite, spinels, alkali-doped alumina, calcined hydrotalcite greatly suppressed coke formation due to the free of acid sites and low alkene adsorption [32][33][34][35][36][37]. Among these supports, the studies about the calcined hydrotalcite (Mg(Al)O) or hydrotalcite-like materials were especially attractive due to their moderate basic characteristics and high thermal stability.…”

Zinc influence on the formation and properties of Pt/Mg(Zn)AlO catalysts synthesized from layered hydroxides

“…The main advantages of LDH as catalyst precursors list the possibility to use various cationic pairs in the formation of a layered structure and hence the synthesis of materials with different controllable properties; the distribution of cations in the LDH structure at the atomic level, which prevents their significant segregation during further treatments; and the formation of mixed oxides with a developed surface area upon decomposition of LDH. The application of mixed oxides produced by thermal treatment of magnesium-aluminum LDH as non-acid supports for platinum catalysts ensures the high stability of the catalysts under the coking conditions and during the redox treatment [11,12]. Thus, one of the promising applications of such catalysts is the hightemperature dehydrogenation of alkanes [13][14][15][16][17][18][19][20][21].…”