-
[show abstract]
[hide abstract]
ABSTRACT: Surface science methods originating from analysis of noble metal catalysts are increasingly applied to metal oxides. These methods provide direct access to fundamental structural properties and phase equilibria governing the catalytic properties of metal oxide surfaces. However, no systematic way existed so far for transferring this knowledge to technical catalysts. The aim of this paper is to combine surface science with chemical engineering methods to bridge this gap. Styrene synthesis over pure and K-doped iron oxides is used as an example to develop and to explain the methodology. Single crystal films (SCF), grown epitaxially on a Pt-carrier are considered as ideal model surfaces. Comprehensive UHV analyses yield the structural properties of SCF as well as their interaction with relevant components of the reaction mixture. Their results are combined with conversion experiments to derive a mechanistic catalyst model along with quantitative information on the reaction rates. The activity of SCF as well as their phase transitions under reactive conditions can be described with a continuum model depending on the macroscopic properties of the system. This model forms the crucial link towards technical catalysts. It is shown that the behaviour of a powder catalyst can be described as a superposition of the above kinetic model and an appropriate porous model. In this paper we review the developed methodology and conclude with the evaluation of the concept.
Physical Chemistry Chemical Physics 08/2007; 9(27):3619-34. · 3.57 Impact Factor
-
Angewandte Chemie International Edition 01/2004; 42(46):5760-3. · 13.45 Impact Factor
-
-
-
-
[show abstract]
[hide abstract]
ABSTRACT: The pressure and materials gap between reactivity studies in UHV and real catalysis can only be overcome by application of in-situ methods for catalyst characterization and/or activity measurements under realistic pressure and temperature conditions. As a model reaction we study the economically important catalytic dehydrogenation of ethylbenzene (EB) to styrene (St) [1]. The technical catalyst consists of potassium-promoted iron oxides. We use single crystalline epitaxial layers of Fe2O3, Fe3O4 and K-Fe-oxides, characterized by surface science methods (LEED, STM, UPS, TDS, AES) concerning surface structure and composition [2]. They are transferred under vacuum into a stagnation point micro flow-reactor [3] where the EB®St conversion is studied with the EB pressure in the mbar region without or with H2O in excess as in the technical process. Also O2 can be admitted in order to study the oxidative dehydrogenation. Conversion yields are measured gas chromatographically. After interruption of the reaction, the sample can be transferred back into the UHV chamber for post-reaction surface analysis. The initial catalytic activity of Fe2O3 decreases after 20 to 40 min due to reduction to Fe3O4 [4]. A second activity decrease is related to coke deposition. This coke is still catalytically active and most likely responsible for the activity observed on unpromoted technical catalysts. Water limits coking and further reduction to metallic Fe. Reduction and coke deposition can be prevented by admission of a low partial pressure of O2 so that the initial high conversion rate is conserved. The activity of potassium promoted samples is similar to the initial high activity of unpromoted samples but the decrease by coking is much slower. Coke can be removed by stopping admission of EB while H2O admission is continued but this causes also K depletion. K thus prevents extensive coking but a thin coke layer is also needed to limit K depletion. High activity appears to result from the dynamic equilibrium between coke production and removal. It cannot be ruled out that the “right” coke produced in this way actually represents the catalytically active phase In conclusion, the combination of surface science for pre- and post-reaction analysis with studies of catalytic activity under realistic conditions demonstrates that the unpromoted iron oxide based catalysts for EB dehydrogenation can assume three different functional states. The initial high-yield state can be maintained by admission of oxygen. K on promoted catalysts prevents extensive coking. Yet, a thin coke layer is necessary to limit K depletion. Whether the thin coke layer is catalytically active has to be checked. [1] K. Kochloefl in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of heterogeneous catalysis, Wiley- VCH, Weinheim, vol. 5,(1997) p. 2151. [2] W. Weiss, W. Ranke, Progr. Surf. Sci. 70 (2002), 1. [3] C. Kuhrs. M. Swoboda, and W. Weiss, Top. Catal. 15 (2001) 13. [4] W.P. Addiego, C.A. Estrada , D.W. Goodman, and M.P. Rosynek, J. Catal. 146 (1994) 407.
-
[show abstract]
[hide abstract]
ABSTRACT: Dehydrogenation of ethylbenzene to styrene is usually run over potassium promoted iron oxide based catalysts at 870 K in presence of steam. Here we present conversion yield measurements on unpromoted single crystalline α-Fe2O3 (0001) model catalysts by combining surface science techniques with an in-situ micro flow reactor. The influence of H2O and O2 on the reaction was investigated by varying the composition of the feed. The initial conversion over Fe2O3 is always high (5-8%), independent of the type of the feed composition. Only the length of this period depends on the feed composition. In presence of O2 (EB:H2O:O2 = 2:20:1), the high yield period can be maintained, in absence of O2 (EB: H2O = 1:10) it decreases in two steps of about a factor of 2-3 each. The reaction was interrupted in the different yield regimes, and the sample structure and composition was analyzed. The high yield is related to Fe2O3 with almost no carbon deposits. O2 in the feed maintains this phase. Without O2, Fe2O3 is reduced to Fe3O4 and the yield drops to the intermediate region. The same yield is observed on clean Fe3O4. Carbon deposits increase but do not yet limit conversion. This happens at the transition to the low yield regime where a thick layer of carbon deposits is observed. With H2O in the feed, the oxide below the carbon deposits remains Fe3O4, without H2O, it is reduced to metallic Fe. We ascribe the low yield to catalysis by carbonaceous species. The study shows that the high yield is typical for Fe2O3 and can be maintained by proper admixture of O2 to the feed.
-
[show abstract]
[hide abstract]
ABSTRACT: Der Standardkatalysator für die Dehydrogenierung von Ethylbenzol (EB) zu Styrol (St) besteht aus Kalium-promotierten Eisenoxiden, aber auch unpromotierte Oxide sind aktiv. Um die tatsächliche Rolle des Eisenoxids zu verstehen, haben wir Untersuchungen an unpromotiertem Hämatit Fe2O3 wieder aufgenommen. In einem Versuch, die Lücke zwischen realen und idealen Systemen zu überbrücken, vergleichen wir Experimente auf Pulverpellets in einem Festbettreaktor unter technischen Bedingungen mit Modellkatalyse-Untersuchungen. Letztere schließen ein die Präparation und Charakterisierung von Einkristallproben um Ultrahochvakuum (UHV) und Konversionsmessungen auf diesen Proben unter realistischen Bedingungen in einem in-situ Mikroflussreaktor.
Angewandte Chemie, v.115, 5938-5941 (2003).
-
[show abstract]
[hide abstract]
ABSTRACT: An in-depth model catalysis study on a complex system is reviewed. Both unpromoted and potassium-promoted iron oxide model catalysts films of single crystalline quality are prepared and characterized in ultrahigh vacuum (UHV) using surface science methods. In order to bridge the pressure and material gap for the catalytic dehydrogenation of ethylbenzene to styrene in presence of steam, this reaction is studied at reactive gas pressures between 10-6 and 36 mbar. The samples are transferred under vacuum into an stagnation point micro-flow reactor where the reaction is studied, followed by post-reaction characterization in UHV. Clean hematite Fe2O3 is an excellent catalyst but deactivates quickly by reduction and by coking. Addition of H2O limits reduction to the oxidation state of magnetite Fe3O4 and counteracts coking. Both deactivation mechanisms can be avoided by addition of some O2 to the feed. Potassium has basically the same functions as O2. It does not seem to be involved in the catalytic dehydrogenation step but rather to block active sites if its concentration is high. Long-term deactivation occurs mainly by potassium removal in form of volatile KOH. Regeneration by “steaming” in pure H2O accelerates this process while ethylbenzene in the feed stabilizes potassium. This is ascribed to the formation of non-volatile K2CO3 which is an intermediate in potassium catalysed coke removal. The addition of O2 instead of K-promotion may be an alternative reaction route.
Recent Research Developments in Surface Science, 75-99 (2004).
-
[show abstract]
[hide abstract]
ABSTRACT: Styrene synthesis over iron oxide model catalysts was studied by combining UHV characterization methods with in situ conversion measurements in a microflow reactor under realistic reaction conditions. Both unpromoted Fe2O3 and K-promoted model catalysts show a similar high starting activity while that of Fe3O4 is clearly lower. Water limits and K promotion slows down deactivation by coking and oxide reduction. The deactivation can be prevented and the high initial yield preserved by adding a small amount of oxygen to the feed. Both the presence of Fe3+ and intermediate adsorption strength for ethylbenzene and styrene are essential for high conversion yields.
Journal of Catalysis.
