Self-induced Electrochemical Promotion of Noble Metal Nanoparticles for Environmentally Important Reaction Systems

Title: Self-induced Electrochemical Promotion of Noble Metal Nanoparticles for Environmentally Important Reaction Systems
Authors: Isaifan, Rima
Date: 2014
Abstract: Volatile organic compounds (VOCs) and carbon monoxide are considered the main greenhouse gas pollutants from either automotive engines or stationary sources. The increased concentration of these pollutants in air severely affects human health and causes changes in earth climate and vegetation growth rates. Ethylene is one of the VOCs closely related with photocatalytic pollution when it reacts with nitrogen oxides in the presence of sun light to form ground-level ozone. It is also responsible for quick repining of fruits and vegetables. Carbon monoxide, on the other hand, is a poisonous gas mainly released by vehicle emissions, and when inhaled in high concentrations, it causes severe health problems related to the respiratory system leading to significant rates of deaths annually in Europe and North America. Globally, The World Health Organization (WHO) estimates that seven million people die yearly due to poor air quality-related reasons which urges current and future stringent regulations to control air pollution emissions. In the past four decades, several equipment modifications and processes have been studied for reducing these emissions. Among them is the phenomenon of Electrochemical Promotion of Catalysis (EPOC) which was first reported in the early 1980s. EPOC has been successfully shown to convert automotive, indoor and industrial air pollutants such as VOCs, CO and nitrogen oxides (NOx) to harmless gases. It involves reversible changes in the catalytic properties of catalysts deposited on solid electrolytes when a small electric current or potential is applied. More recently, it was demonstrated that EPOC can be thermally induced without any electrical polarization, in analogy to the well-known phenomenon of metal-support interaction, by using noble metal nanocatalysts supported on ionically conducting materials such as yttria-stabilized zirconia (YSZ). The objective of this research is to gain deeper understanding of the factors affecting metal-support interaction between the active metal and the support to enhance their catalytic activity for environmentally-important reaction systems; specifically, ethylene and carbon monoxide oxidation as well as hydrogen fuel purification by carbon monoxide methanation. First, the activity of platinum nanoparticles deposited on carbon black, which is a conventional support used in catalysis, is studied. The effect of particle size of four Pt/C nanoparticles synthesized using a modified reduction method for ethylene (C2H4) complete catalytic oxidation is investigated. These catalysts show high activity towards C2H4 oxidation which is found to be a strongly size-dependent reaction. Full conversion of 1000 ppm C2H4 is achieved over the smallest nanoparticles (1.5 nm) at 100oC while higher temperature 170oC is required to completely oxidize ethylene over the largest nanoparticle (6.3 nm). The second stage of this research compares the catalytic activity of platinum and ruthenium nanoparticles when deposited on ionic or mixed ionic conductive vs. non ionic conductive supports for CO and VOCs oxidation. The Pt and Ru nanoparticles are deposited on yttria-stabilized zirconia (8% Y2O3-stabilised ZrO2), cerium (IV) oxide (CeO2), samarium-doped ceria (SDC), gamma-alumina (γ-Al2O3), carbon black and on novel perovskite group Sm1-xCexFeO3 (x = 0, 1, 5) resulting in ≤ 1 wt. (weight) % of Pt and Ru on each support. It is found that the nanocatalysts deposited on ionic conductive or mixed ionic conductive supports outperformed the catalysts deposited on non ionic conductors due to strong metal-support interaction that greatly affects the electronic and catalytic properties of the catalysts. The enhanced catalytic activity towards CO and C2H4 oxidation reactions is shown by earlier catalytic activity and complete conversion, lower activation energies, greater turnover frequencies and higher intrinsic rates per active surface area. To further investigate the effect of ionic conductivity of the supports and the exchange of O2- (oxygen vacancy) between the support and the catalyst surface, complete oxidation of pollutants is studied in the absence of oxygen in the gas phase. For the first time, complete oxidation of CO and C2H4 in an oxygen-free environment at low temperatures (< 250oC) is achieved, which represents the main novel finding in this research. The idea of pollutant removal in the absence of oxygen is extended to a practical reaction for fuel cells application which is hydrogen fuel purification from CO impurities at temperatures < 100oC. Moreover, the effect of particle size, pollutant concentration, operating conditions and support nature in the absence of oxygen in the gas feed is studied. It is proposed that the metal nanoparticles and the solid electrolyte form local nano-galvanic cells at the vicinity of the three-phase boundary where the anodic reaction is CO or C2H4 oxidation and the cathodic reaction is the surface partial reduction of the support. A systematic catalyst reactivation process is suggested and the catalytic activity of these nano-catalysts is studied which can be further investigated for air pollution control applications such as in vehicle catalytic converters, indoor air quality units and power plant emissions.
CollectionThèses, 2011 - // Theses, 2011 -