Characterization of platinum-based bimetallic catalysts

Transcription

1 Theses of PhD dissertation Characterization of platinum-based bimetallic catalysts Written by: Nóra Gy rffy Supervisor: Prof. Zoltán Paál HAS, Institute of Isotope Department of Catalysis and Tracer Studies University of Pannonia Material Sciences and Technologies PhD School Budapest

2 BACKGROUND Modern industrial technologies heavily utilize different catalytic processes. The challenge in the XXI. century is to produce catalysts that achieve very high selectivities (up to 100%) for catalyst-based chemical processes. This way it is possible to achieve clean manufacturing, almost without by-products. Our knowledge of selectivity is much poorer than our understanding of what controls the activity or the turnover rate of the catalysts. Gasoline reforming is one of the most important catalytic processes which is used to produce high octane number motor fuels from naphtha. The bimetallic heterogeneous catalysts (containing in certain cases even more then two metals) are widely used in these processes as well. Apparently the most important alloy catalysts are based on platinum. Deposition of a second metal modifies the activity, selectivity and the stability of the monometallic catalysts. The role and the effect of the second metal have not clarified yet. The interaction between the metals and the role of the position of the second metal are seriously disputed (geometric and electronic effects). I have investigated different bimetallic catalysts based on platinum. These catalysts were prepared by various methods, such as modifying the bulk and/or the surface. My aim was to determine the effect of the second (active or inactive) metal on the cataytic properties of platinum. I have chosen palladium and rhodium as active metals, germanium and tin as inactive metals. My purpose was to find appropriate test-reactions to examine the catalytic properties of the catalysts with different composition. EXPERIMENTAL The catalysts were tested in different reactions in a closed-loop glass reactor. The effects of pretreatments and reaction conditions (temperature and hydrogen excess) were investigated. The PdPt catalysts were tested in a conventional pulse-microreactor system. In addition, our catalysts were also characterized by XPS, TEM and 119 Sn-Mössbauerspectroscopy. NEW SCIENTIFIC ACHIEVEMENTS, THESIS 1. Platinum catalysts modified by adding different metals can be characterized by selective test reactions. a) Reactions with low activation energies are suitable for characterizing PtPd catalysts at low temperatures. The little differences between Pt and Pd can be tracked by the 6

3 hydrogenative ring opening reaction of cis- and trans-methyl-ethyl-cyclopropane (MECP) and in hydrogenation reactions of acrolein. b) Pt and Rh behaved differently in the hydrogenative splitting of C-C bond, therefore the hydrogenative ring opening of methylcyclopentane (MCP) was an appropriate reaction to characterize the PtRh catalysts. c) PtGe catalysts can be characterized by the reactions of C 6 -hydrocarbons. In the dissertation I showed the conversions of MCP and hexane. 2. Bimetallic active sites with properties different from those of its components are created when Pt is modified by active metals (Pd and Rh). These systems behave as true bimetallic catalysts. The activity is higher on PtPd in the reaction of MECP and acrolein, but lower on PtRh catalysts in the reaction of MCP. Accordingly, in the case of PtRh the selectivities of the reaction were different: the distribution of the products was more similar to platinum, but the ratio of the ring opening products was more rhodium-like. The two active metals form active surface species that contain different atoms from both components, and the catalytic properties of these differ from those of the metallic components. 3. The composition of the surface and the catalytic behavior of the bimetallic Pt catalysts strongly depend on the applied pre-treatment. The results are different in the following three cases: the test-reaction from the original state (without any treatment, as is ), after treatment in oxygen, and after treatment in hydrogen. In reducing atmosphere Pt, in oxidizing atmosphere, in turn, the other metal (Pd, Rh, Ge, Sn) becomes enriched on the surface. 4. Deposition of small amount of Ge modifies only the surface of the Pt: if the particles of the Pt are relatively large (low dispersion), the catalytic activity is increased, if the particles of the Pt are relatively small (high dispersion), the activity decreases. a) On supported Pt catalyst (D=53%), the surface hydrogen concentration decreased due to the electronic interaction of Pt and Ge, consequently the relative abundance of products formed from dehydrogenated surface intermediates increases. b) In the case of unsupported Pt (D=1%), partly oxidized Ge behaves as a support, forming PtGeO x adlineation sites, producing considerably amounts of saturated products. Further, by disrupting the continuous Pt surface, Ge decreases the formation of unsaturated products, as well as carbonization of the surface. 5. Pt and Sn form stable PtSn alloys. The Pt 3 Sn phase is inactive in the rupture of C-C bond but it shows a significant activity in hydrogenation of C=C double bond, exceeding even 7

4 the activity of pure Pt. Reactions involving C-C bond rupture are unsuitable for testing PtSn systems. PUBLICATIONS, PRESENTATIONS Published articles: 1. B. Veisz, L. Tóth, D. Teschner, Z. Paál, N. Gy rffy, U. Wild, R. Schlögl Palladium Platinum Powder Catalysts Manufactured by Colloid Synthesis I. Preparation and Characterization Journal of Molecular Catalysis A, 238 (2005) N. Gy rffy, L. Tóth, M. Bartók, J. Ocskó, U. Wild, R. Schlögl, D. Teschner, Z. Paál Palladium-Platinum Powder Catalysts Manufactured by Colloid Synthesis II. Characterization and Catalytic Tests after Oxidizing and Reducing Treatment Journal of Molecular Catalysis A, 238 (2005) A. Wootsch, Z. Paál, N. Gy rffy, S. Ello, I. Boghian, J. Leverd, L. Pirault-Roy Characterization and Catalytic Study of Pt-Ge/Al2O3 Catalysts prepared by organometallic grafting Journal of Catalysis, 238 (2006) Nóra Gy rffy, Zoltán Paál, Attila Wootsch, Sándor Szabó, István Bakos Study on the Pt-Rh interaction by methylcyclopentane ring opening reaction in Sampling Catalysis Research in the Pannonian Region (ed. Pálinkó I.) Magyar Zeolit Társaság, 2006, Szeged, ISBN: , p Z. Paál, N. Gy rffy, A. Wootsch, L. Tóth, I. Bakos, S. Szabó, U. Wild R. Schlögl Preparation, Physical Characterization and Catalytic Properties of Unsupported Pt-Rh Catalyst Journal of Catalysis, 250 (2007) N. Gy rffy, A. Wootsch, S. Szabó, I. Bakos, L. Tóth, Z. Paál Reactions of Methylcyclopentane on Rh-Pt Catalyst Prepared by Underpotential Deposition of Rh on a Pt/SiO2 Topics in Catalysis, 46 (2007) Oral and poster presentations: 1. N. Gy rffy, B. Veisz, L. Tóth, Z. Paál, D. Teschner, R. Schlögl, U. Wild, J. Ocskó, M. Bartók: Preparation, characterization of model Pd-Pt catalysts and its catalytic properties in the opening of the cyclopropane ring - POSTER Europacat-VI., augusztus 31.- szeptember 4., Innsbruck 2. Gy rffy Nóra: Modell Pt-Pd katalizátorok jellemzése és katalitikus vizsgálata metil-etilciklopropán izomerek hidrogenatív gy r felnyílási reakciójában - ORAL VII. Doktori Iskola, április 19., Tahi 8

5 3. A. Wootsch, N. Gy rffy, M. Chamam, L. Pirault-Roy, S. Szabó, I. Bakos, Z. Paál: Hydrocarbon Transformation as a Tool for the Disclosure of the Surface State on Metal Catalysts - ORAL 7th Pannonian International Symposium on Catalysis, 2004 szeptember , Srní 4. N. Gy rffy, A. Wootsch, S. Szabó, I. Bakos, Z. Paál: Characterization of Rh-Pt/SiO2 catalysts by methylcyclopentane ring opening reaction - POSTER Europacat-VII., augusztus 29 szeptember 2., Sofia 5. Gy rffy Nóra: Tesztreakciók alkalmazása kétfémes Pt katalizátorok jellemzésére -ORAL Katalízis Munkabizottsági ülés, PhD bemutató, március 7., Budapest 6. Gy rffy Nóra: Kétfémes Pt katalizátorok jellemzése tesztreakciókkal - ORAL Fiatal Kutatói Beszámoló, június 13., KK, Budapest 7. Nóra Gy rffy, Zoltán Paál, Attila Wootsch, Sándor Szabó, István Bakos: Study on the Pt- Rh interaction by methylcyclopentane ring opening reaction - ORAL 8th Pannonian International Symposium on Catalysis, július 4-7., Szeged 9