DFT Investigation of Hydrogenation and Dehydrogenation Reactions on Binary Metal Alloys: Effect of Surface Ensembles and Composition

Meeting Program – March 2015

Fuat E Celik
Department of Chemical and Biomolecular Engineering
Rutgers, The State University of New Jersey

Fuat Celik
Abstract
In supported metal catalysts, the tradeoff between activity and selectivity presents an important challenge for catalyst design. By allowing two dissimilar metals, we can attempt to tune the selectivity of the catalyst by enhancing bond-formation and desorption rates through the addition of a less-reactive element, while maintain high bond dissociation activity from the more active metal. The resulting catalyst properties depend strongly on the catalyst composition and ratio of the two metals (electronic effect), but may also depend on the local structure of surface ensembles of the alloy components (geometric effect). In this talk we will explore two examples of binary alloys where surface composition and geometry play an important role in determining the selectivity of the catalyst through density functional theory (DFT).

In the first example, we have examined the effect of platinum tin alloy structure and composition on the kinetics and thermodynamics of dehydrogenation and coke formation pathways during light alkane dehydrogenation. Light alkane dehydrogenation to olefins can add significant value to hydrocarbon processes that generate ethane and propane by converting low value commodity fuels to high-value chemical and polymer precursors. Supported Pt catalysts are known to be active but show significant coke formation and deactivation, which can be alleviated by alloying with Sn and other main group elements. We aim to understand how the structure and composition of these alloys affect their ability to suppress coke formation. We investigate the potential energy surfaces from ethane along the desired pathway to ethene, and along the undesired pathways towards surface carbon/coke. The effect of Pt/Sn ratio and surface geometry is investigated. As compared to pure Pt, bond scission is more difficult on the alloys and desorption is more facile, and both effects are enhanced as three-fold hollow sites consisting of only Pt atoms are eliminated.

In the second example, we evaluate Au/Ni near-surface alloys as potential oxygen reduction catalysts for the direct synthesis of hydrogen peroxide from O2 and H2, thereby avoiding the current anthraquinone process. While Au may have higher O-H bond formation activity, it is a poor O2-dissociation catalyst, and likewise Ni is very effective at O2-dissociation but not oxygen hydrogenation. Alloying Au with Ni(111) lowers H2 dissociation barrier while keeping the O2 dissociation barrier large relative to O2 hydrogenation. Desorption of H2O2 is similarly competitive with H2O2 dissociation on alloy surfaces. However, the selectivity for the OOH radical remains a challenge, with barrierless O-O bond dissociation and large (1.3 eV) hydrogenation barriers. We further investigate how the Au/Ni surface may rearrange itself to regenerate three-fold hollows of Ni atoms in the presence of strongly adsorbing surface species.

Methane Conversion to Methanol on Copper Containing Small Pore Zeolites

Meeting Program – February 2015

Bahar Ipek
Department of Chemical and Biomolecular Engineering
University of Delaware

Bahar Ipek
Abstract
Methanotrophic bacteria containing particular methane monooxygenase (pMMO), a Cu-containing enzyme, or soluble methane monooxygenase (sMMO), an iron-metalloenzyme can oxidize methane to methanol selectively at ambient conditions 1. The zeolite Cu-ZSM-5 was reported to activate the methane C-H bond—with a homolytic bond dissociation energy of 104 kcal/mol— at temperatures as low as 120 °C 2 after pretreatment in O2 3. The reactive copper species are believed to contain extra-lattice oxygen, and in the case of Cu-ZSM-5, to be a mono-μ-oxo-dicopper complex ([Cu—O—Cu]2+) 4. Although a correlation was found between the concentration of mono-μ-oxo-dicopper species and the amount of methanol produced by Cu-ZSM-5 5, no such correlation was found for other zeolites that produce methanol such as Cu-mordenite and Cu-ferrierite 2. We have recently showed methanol production on copper (II) exchanged small pore zeolites including SSZ-13 (CHA), SSZ-16 (AFX) and SSZ-39 (AEI) with yields as high as 39 μmol CH3OH/g and CH3OH/Cu ratios up to 0.09 (the largest reported to date).6 Here, copper species in these small pore zeolites were investigated with UV–vis and Raman spectroscopy after O2-treatment at a temperature of 450 °C. No evidence of mono-μ-oxo-dicopper species was found in the spectra of Cu-SSZ-13,Cu-SSZ-16 and Cu-SSZ-39 6, however Cu—Oextralattice vibrations at 574 cm-1 were detected in Raman spectra of Cu-SSZ-13 and Cu-SSZ-39 zeolites which is indicative of a different CuxOy active species responsible for methanol production in small pore zeolites.

References
1. Hanson, R. S.; Hanson, T. E., Methanotrophic Bacteria. Microbiological Reviews
1996, 60, 439-471.
2. Smeets, P. J.; Groothaert, M. H.; Schoonheydt, R. A., Cu based zeolites: A UV–vis
study of the active site in the selective methane oxidation at low temperatures.
Catal. Today 2005, 110 (3-4), 303-309.
3. Groothaert, M. H.; Smeets, P. J.; Sels, B. F.; Jacobs, P. A.; Schoonheydt, R. A.,
Selective Oxidation of Methane by the Bis(mu-oxo)dicopper Core Stabilized on
ZSM-5 and Mordenite Zeolites. Journal of American Chemical Society 2005, 127,
1394-1395.
4. Woertink, J. S.; Smeets, P. J.; Groothaert, M. H.; Vance, M. A.; Sels, B. F.;
Schoonheydt, R. A.; Solomon, E. I., A [Cu2O]2+ core in Cu-ZSM-5, the active site in
the oxidation of methane to methanol. Proceedings of the National Academy of
Sciences of the United States of America 2009, 106 (45), 18908-13.
5. Beznis, N. V.; Weckhuysen, B. M.; Bitter, J. H., Cu-ZSM-5 Zeolites for the Formation
of Methanol from Methane and Oxygen: Probing the Active Sites and Spectator
Species. Catal. Lett. 2010, 138 (1-2), 14-22.
6. Wulfers, M. J.; Teketel, S.; Ipek, B.; Lobo, R. F., Conversion of Methane to Methanol
on Copper Containing Small Pore Zeolites and Zeotypes. Chem Commun 2015, xx,
xx-xx.

Bridging Heterogeneous Catalysis and Electro-catalysis: Catalytic Reactions Involving Oxygen

Meeting Program – February 2015

Dr. Umit S. Ozkan
Department of Chemical and Biomolecular Engineering
The Ohio State University

Umit Ozkan
Abstract
Catalytic reactions that involve oxygen can be found in a large number of processes, including those in energy-related applications, in emission control and in processes important for the chemical industry. Whether the catalytic reaction is an oxygen insertion step as in a selective oxidation reaction, or an oxygen removal step as in a hydrodeoxygenation reaction, oxygen has proven to be a very challenging component, often determining the selectivity of the reaction. Some examples from our laboratories that bridge catalysis and electro-catalysis will be discussed, ranging from oxidative dehydrogenation of alkanes to oxygen reduction reaction in fuel cells.

Challenges and Advances in Catalytic Fast Pyrolysis of Biomass using Zeolites

Meeting Program – January 2015

Dr. Julia Valla
Chemical and Biomolecular Engineering Department
University of Connecticut, Storrs, CT

Abstract
Thermochemical conversion of biomass to energy, fuels and chemicals is an attractive technology for the transition from fossil resources to a renewable-based economy. Catalytic Fast Pyrolysis (CFP) of biomass is a particularly interesting technology for biomass conversion considering the already extensive infrastructure for hydrocarbons production. However, many challenges remain unsolved before the deployment of the biomass CFP can be realized, including: a) char and coke formation, which causes rapid catalyst deactivation; and b) high oxygen content in the bio-oil, which makes it incompatible with today’s hydrocarbon fuels. With respect to the first challenge, it is imperative to first understand the origin and the formation of char and coke during CFP. Considering the second challenge, it is important to understand which catalyst properties can enhance the deoxygenation reactions and increase the bio-oil selectivity to hydrocarbons. ZSM-5 zeolites have been recognized as one of the most promising zeolites for CFP due to their shape selectivity and their deoxygenation ability. However, their micropore structure can limit the accessibility of heavy compounds to the active sites of their framework. Modifying the zeolite pore architecture to create hierarchical structures could provide a solution to this challenge. Furthermore, the CFP process design itself (in situ or ex situ) can alter the product yield and selectivity and, thus, the bio-oil quality. During this presentation we will discuss how the zeolite properties and location within the CFP process (in situ or ex situ) can affect the coke/char formation and the deoxygenation reactions for enhanced bio-oil quality.
Julia Valla
Biography
Ioulia (Julia) Valla is an Assistant Professor in the Chemical & Biomolecular Engineering Department at the University of Connecticut. She received her PhD in the field of the development of new zeolites for the decomposition of sulfur compounds in naphtha and the production of environmental gasoline from the Aristotle University of Thessaloniki in Greece. She has served in a leadership role with Rive Technology, Inc. on the commercialization of a novel zeolite with ordered mesoporous structure for refinery applications. Dr. Valla’s research focuses on the modification of zeolites structure and their application in catalysis, adsorption and energy. She is the author/co-author of 9 papers in peer-reviewed journals, 1 book chapter and 2 patents. Dr. Valla is the recipient of the European Award “RUCADI, Recovery and Utilization of Carbon Dioxide” for her study on the role of CO2 on the reforming of natural gas for the production of methanol. At the University of Connecticut, Dr. Valla received an award sponsored by the National Science Foundation for the study “Turning Tars into Energy: Zeolites with Hierarchical Pore Structure for the Catalytic Removal of Tars”. The study is focused on a novel application of hierarchically structured mesoporous bifunctional catalysts for the thermochemical upgrading of undesirable tars from biomass pyrolysis or gasification to valuable hydrocarbons.

Challenges and Solutions in Developing Zeolite Supported Transition Metal Catalysts for Lean-Burn NOx Emission Control

Meeting Program – September 2014

 
Hai-Ying Chen
Emission Control Technologies
Johnson Matthey Inc.
Wayne, PA

chenh@jmusa.com
 
Abstract – Reduction of NOx emissions from lean-burn engine exhaust has been a main topic of environmental catalysis in the past 20 years. The challenge is the selective conversion of a low concentration of NOx (~100 ppm) in the presence of large excess of O2 (~10%). Although zeolite supported transition metal catalysts were identified in early 1990s as promising catalysts, such a technology was not implemented till recently.

Early studies mainly focused on the development of zeolite supported transition metal, primarily Cu and Fe, catalysts for the selective catalytic reduction of NOx with hydrocarbons (HC-SCR). Even though the HC-SCR technology has been considered as the “holy grail” of automotive catalysis, technical challenges on the activity, selective and durability of the catalysts were recognized to be difficult to overcome for the technology to be implemented into real world applications. However, the vast amount of research work, especially the fundamental studies on the reaction and the catalyst deactivation mechanisms, demonstrated that the activity and selectivity of this type of catalysts can be drastically improved if an alternative reductant, NH3, is available in the feed.

Extensive investigations on the selective catalytic reduction of NOx with NH3 (NH3-SCR) began in the middle 2000s aimed to enable diesel powered vehicles to meet the US EPA 2007/2010 emission regulations. Both Cu and Fe catalysts were considered. Zeolite supported Cu SCR catalysts are more active at low temperature, thus more attractive for applications with low exhaust temperature. The conventional medium-pore zeolite (10-ring, such as ZSM-5) or large-pore zeolite (12-ring, such as beta) supported Cu catalysts, however, cannot meet the long-term durability requirements. To overcome this major technical hurdle, small-pore zeolite (8-ring) supported Cu catalysts were invented. On the other hand, zeolite supported Fe SCR catalysts are more selective in utilizing NH3 for NOx reduction at high temperatures but show a strong dependence on the NO to NO2 ratio in the feed gas at low temperatures. System approaches were developed to enhance the low temperature SCR activity of the Fe SCR catalysts. As such, both Cu and Fe SCR catalysts were successfully commercialized and applied on lean-burn diesel vehicles meeting the stringent US EPA 2010 emission standards.

Hai-Ying ChenBiography – Dr. Hai-Ying Chen is a Scientific and Product Development Manager at Johnson Matthey, where he leads a team of scientists to develop advanced emission control catalysts and technologies for both gasoline engine and diesel engine powered vehicles to meet the government emission regulations.

Dr. Chen received his Ph.D. in Chemistry from Fudan University, China. He has published more than 50 technical papers in peer-reviewed journals and holds 14 US/international patents. He received the Top Cited Article Award by Catalysis Today for articles published in 1998, and was a recipient of the American Chemical Society Award for Team Innovation in 2009. He was named as the 2014 Herman Pines Award in Catalysis by the Chicago Catalysis Club and the 2014 Catalysis Club of Philadelphia Award by the Catalysis Club of Philadelphia.

Intrinsic Deactivation in Cobalt-Catalyzed Fischer-Tropsch Synthesis

Meeting Program – April 2014

 
Gabor Kiss, Stuart Soled, Chris Kliewer
ExxonMobil Res. and Eng. Co.
Annandale, NJ

gabor.kiss@exxonmobil.com
 
Abstract – In this paper, we describe three intrinsic deactivation modes observed in experimental cobalt Fischer-Tropsch synthesis catalysts: cobalt oxidation reversible by mild hydrogen treatment, cobalt agglomeration, and cobalt-support mixed oxide formation. All three mechanisms involve redox transformation of the catalytically active cobalt metal.
 
Gabor_KissBiography – Gabor Kiss received his M.Sc. in chemical engineering from the University of Veszprem (now Pannon University), Hungary, in 1981. He worked in the Hungarian oil industry for eight years before enrolling the graduate school at the University of Miami. After receiving his Ph.D. in chemistry in 1993, he accepted a position at Exxon’s (now ExxonMobil) Corporate Research Laboratories in Clinton, NJ, where he is currently a Sr. Scientific Associate. His research interests include the kinetics, thermodynamics, and mechanism of both homogeneous and heterogeneous catalytic processes. He has published 26 peer-reviewed papers and has 38 patents.