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OULU 2007 C 283 ACTA Virpi Kröger UNIVERSITATIS OULUENSIS C TECHNICA POISONING OF AUTOMOTIVE EXHAUST GAS CATALYST COMPONENTS THE ROLE OF PHOSPHORUS IN THE POISONING PHENOMENA FACULTY OF TECHNOLOGY, DEPARTMENT OF PROCESS AND ENVIRONMENTAL ENGINEERING, MASS AND HEAT TRANSFER PROCESS LABORATORY, UNIVERSITY OF OULU ACTA UNIVERSITATIS OULUENSIS C Technica 283 VIRPI KRÖGER POISONING OF AUTOMOTIVE EXHAUST GAS CATALYST COMPONENTS The role of phosphorus in the poisoning phenomena Academic dissertation to be presented, with the assent of the Faculty of Technology of the University of Oulu, for public defence in Kuusamonsali (Auditorium YB210), Linnanmaa, on November 10th, 2007, at 12 noon OULUN YLIOPISTO, OULU 2007 Copyright 2007 Acta Univ. Oul. C 283, 2007 Supervised by Professor Riitta L. Keiski Reviewed by Professor Edd Anders Blekkan Professor Jose Luis García Fierro ISBN (Paperback) ISBN (PDF) ISSN (Printed) ISSN (Online) Cover design Raimo Ahonen OULU UNIVERSITY PRESS OULU 2007 Kröger, Virpi, Poisoning of automotive exhaust gas catalyst components. The role of phosphorus in the poisoning phenomena Faculty of Technology, University of Oulu, P.O.Box 4000, FI University of Oulu, Finland, Department of Process and Environmental Engineering, Mass and Heat Transfer Process Laboratory, University of Oulu, P.O.Box 4300, FI University of Oulu, Finland Acta Univ. Oul. C 283, 2007 Oulu, Finland Abstract The aim of this thesis project was to gain new knowledge on the effect of phosphorus on the catalytic activity and characteristics of automotive exhaust gas catalyst components. The simultaneous roles of phosphorus and calcium were also studied. The first test series of powdery catalyst samples contained Rh and oxide (Test series 1) and the second, Pt and oxide or ZSM-5 (Test series 2). The catalysts were analyzed when fresh and after two ageing and phosphorus poisoning procedures developed in this work. The procedures consisted of adding poison via impregnation in an aqueous solution (for Test series 1) and in the gaseous phase under hydrothermal conditions (for Test series 2). The poison compounds formed and the changes in the washcoat were studied by using physisorption analyses, SEM, TEM, XRD, and FTIR-ATR. The poison content of the samples was determined by ICP-OES and XRF. Laboratory-scale activity measurements were done to investigate the catalytic activity. Thermodynamic calculations were used to obtain information about ageing conditions and phosphorus compounds formed during ageing. Phosphorus decreased the catalytic activity and the characteristic surface areas of the catalysts. Addition of calcium to a phosphorus-poisoned catalyst was found to have even a regenerating effect on the catalysts' activity. The poisoning methods developed in this study resulted in the same phosphorus compounds as can be found in vehicle-aged catalysts. Phosphorus was identified as cerium, zirconium, aluminium, and titanium phosphates. Phosphorus was detected in zeolites, but phosphorus-containing compounds were not observed. Phosphorus poisoning takes place in the gas phase at high operating temperatures and with high oxygen and water contents. It was also shown that the role of phosphorus poisoning was more pronounced than the role of hydrothermal ageing alone. Phosphorus poisoning mainly affects the oxide components used in this study, not the noble metals. The results can be utilized in the development of catalytic materials and catalyst compositions that can better tolerate phosphorus poisoning under hydrothermal conditions. The results can also be applied in evaluating the effects of phosphorus on different catalyst compositions and in estimating the age of commercial catalysts. Keywords: calcium, catalyst activity, deactivation, diesel catalysts, phosphorus, platinum, poisoning, rhodium, three-way catalysts, zeolite catalyst To my parents 6 Preface This study was carried out in the Department of Process and Environmental Engineering at the University of Oulu during the years During this time the author was also a student in the Graduate School on Functional Surfaces. In addition, the work was a part of the Catalytical Materials: Characterization and Control of the Surface Poisoning Phenomena (POISON) project funded by the Academy of Finland. The study was carried out in co-operation with Ecocat Oy, the Department of Chemistry at the University of Oulu, and the Institute of Materials Science at Tampere University of Technology. Part of the work was performed in the School of Chemical Engineering and Industrial Chemistry at the University of New South Wales, Australia, during a research visit in I cordially thank Prof. Riitta Keiski, the supervisor of this thesis project, for the opportunity to work in her research group in the Mass and Heat Transfer Process Laboratory. I also highly appreciate her expert knowledge on catalysis and the support she gave me during the work. My warm thanks are due to the advisor of the work, Prof. Ulla Lassi, whose help and encouragement were invaluable. M.Sc. Marko Hietikko, M.Sc. Tomi Kanerva, Dr. Minnamari Vippola, Prof. Toivo Lepistö, Prof. Risto Laitinen, Dr. Katariina Rahkamaa-Tolonen, M.Sc. Aslak Suopanki, Dr. Kauko Kallinen, Dr. Dennys Angove, Dr. David French, Lic.Tech. Juha Ahola, and M.Sc. Esa Turpeinen are acknowledged for their successful co-operation in the research work. I also express my thanks to Prof. David Trimm, Prof. Noel Cant, and Dr. Yun Lei for increasing my knowledge in the field of catalysis and for the unforgettable time I spent in Australia. I thank Mr. Jorma Penttinen and Ms. Hannele Nurminen for their invaluable help in developing the test procedures and in carrying out the experiments. I also thank M.Sc. Katri Kynkäänniemi for her assistance in the laboratory work. I present my gratitude to Prof. Edd Anders Blekkan and Prof. Jose Luis García Fierro, who reviewed the manuscript of my thesis. Mr. Keith Kosola is acknowledged for linguistic corrections. I warmly thank Dr. Tanja Kolli, Dr. Mika Huuhtanen, and Dr. Satu Ojala for the help and support they have given me during the past years. Thanks are also due to the other members of the Mass and Heat Transfer Process Laboratory staff. Special thanks are expressed to M.Sc. Riitta Raudaskoski for fruitful co-operation in both work and leisure activities. 7 Finally, I present loving thanks to my family. I thank my spouse Toni Kallio for bringing joy and a positive challenge into my life. I most cordially thank my mother Aira, father Heikki, sister Anne, and brother Vesa for their continuous care and support. This thesis is dedicated to my parents, who always encouraged me in my studies. The work was funded by the Graduate School on Functional Surfaces and the Academy of Finland. I also gratefully acknowledge the financial support given by the Faculty of Technology at the University of Oulu; Fortum Foundation; Oulu University Scholarship Foundation; Foundation of Technology; Jenny and Antti Wihuri Foundation; The Finnish Cultural Foundation, The Regional Fund of Northern Ostrobothnia; Finnish Konkordia Fund; and Finnish Catalysis Society. Ecocat Oy is acknowledged for donating catalysts for the laboratory tests. Oulu, August 2007 Virpi Kröger 8 List of abbreviations A/F ratio ATR BET BJH DOC EDS EDTA EGR EU FTIR HC ICP-OES OSC PGM PM SCR SEM SOF T 50 TEM TPD TWC XRD XRF ZDDP Air-to-fuel ratio Attenuated Total Reflection Brunauer-Emmet-Teller theory Barrett-Joyner-Halenda theory Diesel Oxidation Catalyst Energy Dispersive X-ray Spectrometer Ethylene Diamine Tetraacetic Acid Exhaust Gas Recirculation European Union Fourier Transform Infrared Spectroscopy Hydrocarbon Induced Coupled Plasma Optical Emission Spectroscopy Oxygen Storage Capacity Platinum Group Metal Particulate Matter Selective Catalytic Reduction Scanning Electron Microscopy Soluble Organic Fraction The temperature of 50% conversion of the feed component Transmission Electron Microscopy Temperature Programmed Desorption Three-way Catalyst X-ray Diffraction X-ray Fluorescence Zinc Dialkyldithiophosphate 9 10 List of original papers This thesis is based on the following publications, which are referred to in the text by their Roman numerals: I Kröger V, Hietikko M, Lassi U, Ahola J, Kallinen K, Laitinen R & Keiski RL (2004) Characterization of the effects of phosphorus and calcium on the activity of Rhcontaining catalyst powders. Topics in Catalysis 30/31: II Kröger V, Lassi U, Kynkäänniemi K, Suopanki A & Keiski RL (2006) Methodology development for laboratory-scale exhaust gas catalyst studies on phosphorus poisoning. Chemical Engineering Journal 120: III Kröger V, Hietikko M, Angove D, French D, Lassi U, Suopanki A, Laitinen R & Keiski RL (2007) Effect of phosphorus poisoning on catalytic activity of diesel exhaust gas catalyst components containing oxide and Pt. Topics in Catalysis 42 43: IV Kröger V, Kanerva T, Lassi U, Rahkamaa-Tolonen K, Vippola M & Keiski RL (2007) Characterization of phosphorus poisoning on diesel exhaust gas catalyst components containing oxide and Pt. Topics in Catalysis 45: V Kröger V, Kanerva T, Lassi U, Rahkamaa-Tolonen K, Lepistö T & Keiski RL (2007) Phosphorus poisoning of ZSM-5 and Pt/ZSM-5 zeolite catalysts in diesel exhaust gas conditions. Topics in Catalysis 42 43: VI Kanerva T, Kröger V, Rahkamaa-Tolonen K, Vippola M, Lepistö T & Keiski RL (2007) Structural changes in air aged and poisoned diesel catalysts. Topics in Catalysis 45: The manuscripts for the publications (Papers I V) were written by the author of this thesis. In Paper VI, the author was responsible for the chemical ageing procedure, activity measurements, and surface area studies. The manuscript was written in close collaboration with the first author. 11 12 Contents Abstract Preface 7 List of abbreviations 9 List of original papers 11 Contents 13 1 Introduction Background Purpose of the work Automotive exhaust gas catalysis General Three-way catalysis Diesel catalysis Oxidation catalysis NO x reduction Particulate filtration Catalyst deactivation Overview Chemical deactivation Poisoning by phosphates Poisoning by sulphur compounds Poisoning by other compounds Thermal deactivation Mechanical deactivation Catalyst regeneration Experimental Catalysts Phosphorus poisoning procedures Poisoning by impregnation Gaseous phase poisoning Catalyst characterisation techniques Physisorption analyses Scanning Electron Microscopy Transmission Electron Microscopy Chemical analyses X-ray diffraction 4.3.6 Fourier Transform Infrared-Attenuated Total Reflection X-ray Photoelectron Fluorescence Activity measurements Thermodynamic calculations Poisoning-induced changes in the catalyst components Loss in catalytic surface area Accumulation of phosphorus Phosphorus content Phosphorus compounds Loss of catalytic activity Effect of phosphorus Effect of phosphorus and calcium Changes in the active metal particles and washcoat grain size Evaluation of the poisoning procedures Integration of the results Evaluation and utilization of the results Suggestions for further research Summary and conclusions 75 References 77 Original papers 85 14 1 Introduction 1.1 Background Motor vehicles play a major role in urban air quality problems. Incomplete burning of petrol or diesel leads to formation of pollutants that have several harmful effects on the environment and human health. These pollutants include carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides (NO x ), and particulate matter (PM) (Kalantar Neyestanaki et al. 2004, Wallington et al. 2006). Catalysis is one of the key technologies used in controlling air pollution. Catalytic abatement of exhaust gas components has been used to reduce automobile emissions since the 1970s, and it has been a success story in the development of environmental catalysts. The increasing motor vehicle population and tightening emission standards create a major challenge for the development of more durable and effective automotive catalysts. Demand for a high performance catalyst and requirements for long catalyst life have increased the amount of research activity focused on automotive exhaust gas catalysis. (Greening 2001, Kašpar et al. 2003, Twigg 2007) Catalytic materials used in future three-way catalyst (TWC) and diesel catalyst applications will have to be more effective in controlling emissions. Thermal stability and good tolerance of poisons such as sulphur (S), which originates from fuel, and phosphorus (P), zinc (Zn), calcium (Ca), and magnesium (Mg), which originate from lubricating oils, are also needed. (Heck & Farrauto 1997, Farrauto & Heck 2000, Shelef & McCabe 2000, Lassi 2003) Many challenges remain before researchers and the catalyst industry can fulfil such demands. Robust research efforts are required to clarify the functioning of noble metals and washcoat components and to control deactivation phenomena. Further improvements are also needed in the efficiency and optimization of the entire system of engine, fuel, and exhaust gas after-treatment. (Wallington et al. 2006) Phosphorus and sulphur are present in aged catalytic converters in higher concentrations than other poisons, which increases the probability of their interaction with catalyst components such as aluminium oxide (Al 2 O 3 ) and cerium oxide (CeO 2 ) (Cabello Galisteo et al. 2004). The role of phosphorus in the deactivation of automotive catalysts has been discussed in several studies, but the mechanism of phosphorus poisoning is not yet completely known. 15 Nowadays, over 95% of the world s transportation fuel originates from fossil fuels (Wallington et al. 2006). However, due to the quest to reduce dependency on fossil fuels and to control climatic change, interest in forms of energy that would replace traditional crude oil-based fuels is growing (Hamelinck & Faaij 2006, Romm 2006, Mittelbach 1995, Sagar 1995). According to EU directive 2003/30/EC, the EU countries are supposed to replace the use of gasoline and diesel with a renewable form of energy to the extent that by the year 2010, 5.75% of energy, calculated according to energy content, would come from renewable sources (EU 2003). Therefore, production and use of biodiesel and other fuels derived from biomass is rapidly increasing (Mittelbach 1995). This emphasizes the role of phosphorus in exhaust gas catalysis. When using traditional fossil fuels, lubricating oils are normally a major source of phosphorus. In the case of biodiesel, phosphorus as well as potassium (K) and other possible poison compounds may be present in various concentrations, depending on the origin of the biomass (Grosser & Bowman 2006). Despite the increasing risk of phosphorus poisoning in automotive catalysis, the number of publications dealing with the effect of phosphorus on separate catalyst components is still very limited. Therefore, this study was carried out with simplified model catalysts in order to obtain detailed knowledge on the poisoning phenomena of catalyst components. Research work in this area is essential in order to meet emission limits in the future. Research on deactivation of exhaust gas catalysts is widely carried out by studying vehicle-aged, engine-aged, and oven-aged catalysts on the laboratory scale (Koltsakis & Stamatelos 1997, Xu et al. 2004, Uy et al. 2003, Cabello Galisteo et al. 2004). Vehicle- and engine-ageing methods provide realistic information about the overall deactivating effect caused by oil- and fuel-derived contaminants and high operating temperatures. Their disadvantages are that they are relatively expensive and time-consuming methods (Koltsakis & Stamatelos 1997). It may also be difficult to determine the relevance of a single mechanism of deactivation to changes in catalytic activity and characteristics. Therefore, procedures for studying chemical ageing on the laboratory scale were developed in this study. 1.2 Purpose of the work In the present work, chemical deactivation of catalyst powders was studied. The mechanism of phosphorus poisoning in automotive catalysts is not yet completely 16 known, and therefore, phosphorus was chosen to be the poisoning compound in ageing. The purpose of this thesis project was to examine the effect of phosphorus on the catalytic activity and characteristics of components widely used in automotive exhaust gas catalyst applications. As a special case, the combined effect of phosphorus and calcium was also studied. To understand poisoninginduced changes in the washcoat, alterations in the surface of powdered catalyst components, caused by phosphorus poisoning and hydrothermal ageing, were studied using several characterization techniques. In addition, differences between phosphorus poisoning of catalysts with and without a precious metal were investigated. The characteristics of aged samples containing different oxides were also compared. Vehicle-ageing and engine-ageing procedures are widely used in studying poisoning phenomena of exhaust gas catalysts. However, these methods are often relatively expensive and slow. In addition, studying the effects of a single deactivation mechanism on a decrease in catalytic activity may be complicated. The mechanism of phosphorus poisoning is not completely known, and therefore studies carried out with simplified model components are required. One objective of this study was to develop a methodology for accelerated chemical deactivation studies that can be carried out in a time-saving, affordable, safe, and controlled way on the laboratory scale. In this research, deactivation of automotive catalysts caused by phosphorus poisoning was studied by developing two different chemical ageing procedures on the laboratory scale. To obtain detailed knowledge about the mechanism of poisoning, the studies were carried out with simplified model components. Information about the formation and stability of poison compounds was gained by using thermodynamic calculations. These calculations were also used as a tool for planning ageing conditions. Another purpose of the work was to study the mechanism of phosphorus poisoning with different catalyst components used in present-day catalytic converters. The work also focused on obtaining information about the conditions in which poisoning takes place and finding out whether some components are more sensitive to phosphorus than others. A further purpose of the work was to differentiate between the role of thermal ageing and the role of phosphorus poisoning in catalyst deactivation. This thesis is structured as follows. Chapter 2 describes the composition and function of three-way and diesel catalysts used in automotives. In Chapter 3 the causes leading to deactivation of automotive catalysts, i.e. chemical, thermal and mechanical deactivation, are reviewed. Possible methods for regenerating 17 automotive catalysts are also discussed. Chapter 4 presents the experimental part of the work. The catalyst samples, ageing procedures, and characterization techniques are described. The research results of the work are presented in Chapter 5 and further integrated and evaluated in Chapter 6. Chapter 6 also discusses suggestions for further studies. Finally, the results are summarized and conclusions are drawn in Chapter 7. 18 2 Automotive exhaust gas catalysis 2.1 General Automotive exhaust catalysts were introduced in the 1970s. These early converters were used only to oxidize HC and CO emissions (Heck & Farrauto 2001). The first automotive catalysts to convert HC, CO, and NO x were dual-bed systems in which NO x was reduced in the first bed. Secondary air was injected between the beds, enabling oxidation of HC and CO in the second bed. The stricter NO x standards in the 1980s led to development of three-way catalysts that simultaneously catalyze three types of reactions: oxidation of CO and HC and reduction of NO x. (Gandhi et al. 2003) Modern automotive catalytic converters are very effecti
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