May 16, 2006

2006 NSLS-CFN Joint Users' Meeting Workshop

Synchrotron Catalysis Consortium: New Opportunities for in situ XAFS Studies of Nanocatalysis

The active participation by over 70 attendees in a one-day workshop on in situ XAFS studies of nanocatalysts demonstrated that even though in situ XAFS studies of catalysts have been performed for over 30 years, the field is as vibrant and stimulating as ever. The workshop, held May 16, part of the 2006 Joint NSLS/CFN Users' Meeting, was co-organized by Simon Bare (UOP LLC) and Anatoly Frenkel (Yeshiva University). The format of the workshop was two overview presentations on the Synchrotron Catalysis Consortium, followed by nine technical presentations, and a poster session in conjunction with the main Users' Meeting. It is a pleasure to announce that the 1st place cash award for best student/postdoc poster was given to Minhua Shao (Stony Brook University) for his work on "Novel electrocatalysts for oxygen reduction reaction", and the 2nd place award was presented to Wen Wen (BNL - Chemistry) for his work on "In situ time-resolved characterization of novel Cu-MoO2 catalysts during the water gas shift reaction".

Participants of the Synchrotron Catalysis Consortium workshop

The day began with a presentation by Simon Bare (UOP LLC) on "Catalysis and synchrotrons: the need for a consortium", and described the history and reasoning behind the consortium. The acronym, SCC, for Synchrotron Catalysis Consortium, was given an alternate interpretation of Success via Collaboration and Cooperation. The premise is that there is a barrier for groups to conduct in situ XAFS studies due to the safety requirements, complexity, and perceived difficulty of the experiment and analysis. The idea is that the consortium encourages new users, in addition to providing enhanced support for existing users. Jingguang Chen (University of Delaware) followed by providing an overview of the SCC at the NSLS: the beamlines available, the staff, and the equipment, and after two cycles of beam time, the number of participating General Users. Overall the message was that the SCC is off to a great beginning, and that the members welcome feedback and suggestions for improvements.

The technical talks began with Laurent Menard (University of Illinois at Urbana-Champaign) who gave an excellent presentation on his thesis work of using in situ XAFS to probe the structure and dynamics of supported metal nanoparticles, using both ligand-protected and unprotected Au13 clusters as an example. He also presented some intriguing data implying that small Pt particles supported on gamma-Al2O3 exhibit unexpected thermal contraction.

Anatoly Frenkel then presented a detailed tutorial on the usefulness of EXAFS to determine the average particle size and shape of supported nanoparticles, both single phase and binary alloys. He highlighted both the advantages and potential pitfalls of EXAFS analysis of such systems. He illustrated his talk by using in situ EXAFS data of PtRu5 clusters supported on carbon.

Ian Drake (University of California, Berkeley, and now at Rohm and Haas) reported his thesis work on the in situ characterization of the local environment of Cu+ in Cu-Y zeolite, a catalyst active for the synthesis of dimethyl carbonate. He used a combination of both Cu K-edge XAFS and, by the design and use of a novel in situ cell, Al K-edge XAFS to define determine the location of the Cu+ inside the faujasite structure. The theme of his message was that by combining structural information from both absorption edges a more definitive answer could be obtained than by using the data from only one edge.

The structural determination of gold nanoparticles, a very active research area in catalysis, was somewhat of a unifying theme throughout several of the presentations. This was exemplified by the presentation by Viviane Schwartz (Oak Ridge National Laboratory) who described the detailed in situ XAFS studies she has conducted to investigate the Au particle size on functionalized amorphous silica supports in addition to titania as a support. In situ XANES at the Au L3-edge were used to correlate changes in the catalytic activity with Au particle size variations.

NOx storage/reduction (NSR) catalysts are critical for vehicle emission control, and Janos Szanyi (Pacific Northwest National Laboratory) discussed how, through a combination of in situ techniques, we are starting to understand the phases and chemical species present in the catalyst during both the storage and reduction cycles. He used time-resolved XRD and XANES to investigate the properties of BaO/Al2O3-based NSR materials. The crystalline phase changes associated with NOx uptake and release, the effect of moisture on the morphology, and the changes in Ba-containing crystalline phases upon reduction with hydrogen and CO were all studied. Szanyi presented a model that is consistent with all of his experimental data.

The feature lecture was by Jeroen van Bokhoven (ETH Zürich). In his talk he exemplified how in situ XAFS can be used to determine the structure of catalysts during pretreatment and reaction, and during deactivation. The specific examples he used were the structure of the metal sites involved in hydrocarbon conversion, the active sites in gold catalysts, and the electronic state of small Au particles. By the use of difference XANES spectra he showed how XANES, combined with theoretical calculations, could be used to determine the adsorption site of hydrocarbons and CO on small Pt clusters. Of particular note was the use of the new experimental methodology of high resolution fluorescence detected XAFS in order to better resolve features in the XANES region by removing core-hole lifetime broadening in the spectra. For the Au catalysis he concluded that metallic Au is the active species, and that independent of the support, it is the Au particle size that determines the overall catalytic activity due to changes in the d-band density of states.

The theme of emissions catalysts was further detailed by Chris Marshall (Argonne National Laboratory) who discussed the use of in situ time resolved XAFS for improved understanding of the reducibility of Cu and Ce in hydrocarbon selective catalyst reduction of NOx. He began by describing the experimental equipment that his group has developed, including the use of a six-sample holder enabling increased productivity in their in situ XAFS work. The heart of his talk was the effect of Ce on the activity of Cu-ZSM-5 for this reaction. The majority of the ceria is on the exterior of the zeolite crystals and promotes the oxidation of NO to NO2. A small fraction of the Ce enters the zeolite pores and lowers the reduction temperature of Cu(II) to Cu(I), which lowers the deNOx light-off temperature.

Yuri Tolmachev (Kent State University) illustrated how in situ XAFS is equally applicable to determining the structure of electrocatalysts as it is to heterogeneous catalysts. He detailed his work on the characterization of Os surface deposited on model Pt catalysts, and on Ru on Pt nanoparticles.

The detailed structure of both supported and unsupported molecular metal complexes was the topic of the presentation given by Susannah Scott (University of California at Santa Barbara). The approach her group takes is to graft organometallic coordination complexes onto oxide surfaces in order to obtain uniform, highly dispersed active catalytic sites. These are challenging materials to work with due to their sensitivity to both moisture and air. Nevertheless, she showed excellent XAFS data from numerous catalysts exemplified by supported metal alkyls and metal chlorides.

The successful workshop ended with a tour of the SCC facilities at the NSLS. The workshop was stimulating and thought provoking, and clearly showed the way forward to future developments at the SCC.

FOR MORE INFORMATION
Simon R. Bare
UOP LLC
Des Plaines, IL
Email: simon.bare@uop.com

Anatoly Frenkel
Yeshiva University
New York, NY
Email: frenkel@bnl.gov