Robert J. Farrauto

Catalytic chemistry of supported palladium for combustion of methane

The high-temperature catalytic chemistry of supported palladium for methane oxidation has been studied. Palladium oxide supported on alumina decomposes in two distinct steps in air at one atmosphere. The first step occurs between 750 and 800 ° C and is believed to be a decomposition of palladium-oxygen species dispersed on bulk palladium metal designated (PdOx/Pd). The second decomposition is between 800 and 850 ° C and behaves like crystalline palladium oxide designated (PdO). To reform the oxide, the temperature must be decreased well below 650 ° C. Thus, there is a significant hysteresis between decomposition to palladium and re-formation of the oxide. Above 500 ° C, methane oxidation occurs readily when the catalyst contains PdO. However, when only palladium metal is present no oxygen adsorption occurs and no methane activity exists. One may conclude that the high temperature (> 500 ° C) activity of a supported palladium containing catalyst is due to the ability of palladium oxide to chemisorb oxygen. Palladium, as a metal, does not chemisorb oxygen above 650 ° C and thus, is completely inactive toward methane oxidation.

Automobile exhaust catalysts

It has now been over 25 years since the introduction of the catalytic converter to reduce emissions from the internal combustion engine. It is considered one of the greatest environmental successes of the 20th century, however, new emission control technologies are still being developed to meet ever more stringent mobile source (gasoline and diesel) emissions. This short review will discuss the basis for improvements and highlight technology area, which will require further improvements in emissions and fuel economy. Some of the issues related to fuel cells which some believe may replace the internal combustion engines for automobile applications is also be briefly discussed.

Catalytic converters: state of the art and perspectives

Abstract This paper gives an overview of the advanced technologies currently used for abating emissions from the gasoline and diesel internal combustion engines. The challenges towards the end of the 20th century into the 21st century will also be presented.

The application of monoliths for gas phase catalytic reactions

A general introductory review of the fundamental principles of monoliths as supports for catalytic gas phase reactions is presented. Monoliths are used because of low pressure drop and high mechanical strength required for the harsh conditions encountered in environmental applications. The chemical and physical properties of monoliths and the basics for mass transfer calculations and pressure drop are presented. Existing and emerging applications are briefly discussed. Reference citations are given for those requiring more depth.

Selective catalytic reduction of nitric oxide by hydrocarbons

Abstract There are several problems related to the application of currently available NO x control technologies (i.e. three-way catalysts and the selective catalytic reduction of NO by NH 3 ) to lean burn diesel engines, and thus a very significant market potential exists for an improved technology in this area. As a result, the selective catalytic reduction of NO by hydrocarbons derived from on board fuel has received a lot of attention in the past five years. Different forms of base and noble metal exchanged zeolites, as well as supported noble metals appear to be the most active catalysts for this family of reactions. The available literature in this area is summarized in this review, with emphasis placed on the performance of different zeolite and supported noble metal catalysts, the effect of the various exhaust gas components and the mechanistic implications.

Palladium catalyst performance for methane emissions abatement from lean burn natural gas vehicles

Abstract As little as 1 ppm SOx present in the exhaust of a lean burn natural gas engine strongly inhibits the oxidation of CH4 over a Pd containing catalyst. Non-methane emissions oxidation, such as C2H6, C3H8 and CO, are also inhibited by low SOx concentrations, but to a lesser extent than CH4 emissions. The mechanism for SOx inhibition indicates a 1 : 1 selective adsorption of SOx on PdO for palladium on a non-sulfating support such as SiO2. Deactivation is therefore very rapid. In contrast, palladium on sulfating supports, that is γ-Al2O3, deactivate more slowly and can tolerate more SOx because the SOx is also adsorbed onto the carrier. The activation energy for methane oxidation is dramatically increased after SOx poisoning for all Pd catalysts, while the Arrhenius pre-exponential term is relatively constant, indicating a transformation from very active PdO sites to less active PdO SOx sites. Platinum catalysts are considerably less active than Pd as evidenced by a much lower pre-exponential term, but are more resistant to deactivation by SOx. Non-methane hydrocarbon and particulate emissions standards for lean burn natural gas engines for the United States can be met with Pd catalysts. However, the non-enforced methane emissions standards are not met. For the European truck test cycle, methane emissions standards are met since the test cycle heavily weights the hotter modes where Pd SOx is sufficiently active.

Thermal decomposition and reformation of PdO catalysts; support effects

The thermal decomposition of PdO and the reformation of Pd to PdO is dependent on the support upon which they are dispersed indicating significant metal (oxide) support interactions. The study defines temperature regions of different stabilities for Pd:O species. The nature of the Pd:O structure is of great importance for a large number of environmental catalytic applications including the gasoline automobile catalytic converter, ozone decomposition catalysts, abating emissions from natural gas fueled vehicles, primary combustion catalysts, etc. The ZrO2 support shows the largest hysteresis effect between the temperature of decomposition and reformation. In contrast both TiO2 and CeO2 have small hysteresis effects because of a significant increase in the temperature for reformation of the PdO. There exists a large region of temperature stability of the PdO when dispersed on these two carriers. This is quite favorable for those catalytic reactions where the oxide state is important such as the primary catalytic combustion of natural gas. Further evidence of the importance of PdO for methane oxidation is presented.

Selective catalytic oxidation of CO in H2: structural study of Fe oxide-promoted Pt/alumina catalyst

Abstract An Fe-oxide promoted Pt/γ-alumina catalyst, highly active and selective for the oxidation of CO in H2, has been studied by a combination of electron microscopic and spectroscopic techniques. The goal of the study is to understand the role of Fe in promoting the activity of the catalyst for this reaction important for purifying the H2 for solid polymer electrolyte fuel cell applications. The results show that the promoter Fe oxide must be in intimate contact with the Pt to enhance high CO activity. The Fe oxide partially covers the Pt metal surface, interacts with the Pt particles and changes the electronic properties of the Pt metal particles. The Fe oxide provides oxygen to the CO adsorbed on the Pt thereby creating a dual site non-competitive mechanism for CO oxidation. The uniqueness of this arrangement enhances the catalytic activity for selective CO oxidation relative to Pt only catalysts in hydrogen streams.

Monolithic diesel oxidation catalysts

Abstract A flow-through ceramic monolithic catalyst, containing bulk CeO 2 as the catalytically active component, is reported for the catalytic oxidation of the liquid portion of the particulates present in diesel engine exhausts. This new technology has been successfully implemented into medium duty trucks in the U.S. beginning in 1994 meeting all required emission standards. Catalyst screening and engine durability/aging results and a proposed mechanism of operation are presented. The addition of a proprietary zeolite as a hydrocarbon trap and a small amount of Pt to the platform CeO 2 catalyst enables european emission standards for CO, HC and particulates to be met. Commercialization of this technology in Europe began in the first part of 1996.

Precious Metal Catalysts Supported on Ceramic and Metal Monolithic Structures for the Hydrogen Economy

Distributed hydrogen for the hydrogen economy will require new catalysts and processes. Existing large‐scale hydrogen plants can not simply be reduced in size to meet the economic, safety, and frequent duty cycle requirements for applications for fuel cells, hydrogen fueling stations, and industrial uses such as hydrogenation reactions, gas turbine cooling, metal processing, etc 1 2. Consequently, there is a need to completely reassess how hydrogen can be made for the emerging hydrogen economy. This article presents some of the technological advantages of precious metal monoliths over traditional base metal particulate catalysts for reforming hydrocarbons, such as natural gas, for the generation of distributed hydrogen.