AVS2013 Session NS+AS+EN+SS-TuA: Nanoscale Catalysis and Surface Chemistry

Sluggish kinetics of the electrochemical reduction of molecular oxygen (ORR) in polymer electrolyte membrane fuel cells (PEM-FCs) requires a continued search for advanced catalysts. Our work combines synthesis and characterization of well-defined, multimetallic model catalysts in UHV and direct evaluation of their catalytic performance in an electrochemical cell accessible by a multifunctional ambient-pressure antechamber and sample transfer system. Surface structures and electronic properties [#] [#] of these electrocatalysts were characterized by LEED, AES, LEIS, XPS, and STM. We found previously that Pd₃Fe(111) annealed at high temperature is highly active for ORR, but may lose activity due to slow dissolution of Fe. To address this issue, we developed a method to improve the stability while maintaining high activity of the catalyst involving a submonolayer amount of Au at the surface. While bulk gold is catalytically inactive, the surface prepared by submonolayer amounts of Au deposited on a Pd/Pd₃Fe(111) substrate was discovered to be highly active for the ORR, more active than Pt(111). The activity was strongly dependent on the Au coverage, with the highest activity found near 0.5-monolayer Au. The high activity and durability of this Au-modified Pd₃Fe(111) surface is associated with formation of 2D nanostructures at the step edges at the surface, small nano-islands, which were revealed by STM. DFT calculations of O₂ adsorption on these 2D Au islands and of Au diffusion barriers to the top of Au terraces with and without O and O₂ were used to provide a more thorough understanding of the origin of the high catalytic activity of this system. We found that adsorbed oxygen atoms can enhance diffusion in Au protrusions, resulting in a 3D Au nanostructure that can lead to O₂ dissociation. In this work, we not only discovered a potential candidate for non-Pt catalysts to replace Pt cathode catalysts for the ORR, but also identified conditions for activating gold for use in practical electrocatalysts.

B.E.K. acknowledges support by NSF Grant No. CHE-1129417. R.W. acknowledges support by NSF Grant No. CHE-0802913 and computing time at XSEDE.

Hydrogenation reactions are central to the petrochemical, fine chemical, pharmaceutical, and food industries and are of increasing interest in energy production and storage technologies. Typical heterogeneous catalysts often involve noble metals and alloys based on platinum, palladium, rhodium and ruthenium. While these metals are active at modest temperature and pressure, they are not always completely selective and are expensive. We have demonstrated that single palladium atoms can convert the otherwise catalytically inert surface of an inexpensive metal into an ultraselective catalyst. We used high resolution imaging to characterize the active sites and temperature programmed reaction spectroscopy to probe the chemistry. The mechanism involves facile dissociation of molecular hydrogen at individual palladium atoms followed by spillover onto the copper surface, where ultraselective catalysis occurs by virtue of weak binding. The reaction selectivity is in fact much higher than that measured on palladium alone, illustrating the unique synergy of the system.

Tungsten carbide has garnered considerable interest for its catalytic activity and it is uniquely suited as an electrode/catalyst in microbial fuel cells. One of the bottlenecks in the use of W-carbide materials is their susceptibility to oxidation and concomitant loss in activity, albeit some surface processes benefit from the presence of surface oxide. The challenge is to establish the optimum oxide-carbide composition for a specific reaction and to stabilize this surface.

The W-carbide layers are synthesized from C₆₀ and W, which are deposited MgO(001) by electron and beam and thermal evaporation. This affords control of film composition, and lead us to an in-depth study of W-C₆₀ interaction, and discovery of a unique carbide nanosphere phase. The experiments are performed in UHV, and film growth and reactivity are studied with Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), which delivers conductivity maps illustrating the progression of oxidation and variation in surface electronic structure.

Our work includes study of C₆₀ - W interaction and carbide thin film growth using deposition of C₆₀ on W and vice versa, and co-deposition between 300-1000 K, and the study of oxidation of carbide films with a C/W ratio between 50/50 and 75/50, spanning the range from the line compound WC to carbon enriched films. The W/C 75/50 films have distinct graphite layers at the surface, while the W/C 60/40 surface presents mostly graphene patches. We will discuss the formation of carbide films, which retain some granularity defined by the C₆₀ molecule, in terms of thermodynamic and kinetic limitations. The progression of oxidation on the bare carbide surface, and competition with graphite/graphene etching define the overall reactivity. Conductivity maps yield a nano-scale view of the oxidation reaction as a function of temperature and p(O₂), which will be presented in the framework of a preliminary model.

We focus the discussion of W-C₆₀ interaction on the two major discoveries: (1) the intercalation of W into the C₆₀ matrix, and (2) the formation of carbide nanospheres.

The deposition of W on C₆₀ leads to the intercalation of W, and the cessation of molecule rotation due to the formation of W-C₆₀ complexes. The formation of exohedrally W-doped C₆₀ is described by a competition between complex formation and W-cluster growth. The inverted experiment, where C₆₀ is deposited on a W-surface and annealed at 600 (700)K leads to formation of carbide nanospheres, which are highly ordered with a narrow size distribution centered at 1.3 nm diameter. These nanospheres are metallic, and presumably form through the reaction between W and C₆₀ , which acts as scaffold.

1. A. Pittrof et al., Acta Biomaterialia 7 (2011)

2. K. N. Chappanda et al., Nanotechnology 23 (2012)

Biotin-avidin technology is a widely explored interaction in bioscience. Biotin’s affinity for the protein avidin, makes it ideal for protein and nucleic acid detection or purification methods. This strong interaction if often used in pretargeting strategies for cancer treatment. In most cases a probe molecule (antibody) is connected to a marker molecule (fluorophore or nanoparticle) through the biotin-avidin bridge. Biotinylated nanoparticles can play a role in improving this interaction and creating an electronic or optical detection method. Polyaniline is a polymer which can be easily functionalized to be specific for various biomolecules and has ideal sensor characteristics. In this study we will design a process to functionalize polyaniline with biotin to create a biotin-avidin biosensor. We began with a 2-aminophenol which is a hydroxyl substituted aniline monomer. This monomer undergoes polymerization to yield 2-hydroxy polyaniline. The polymer’s hydroxyl group was functionalized by Steglich esterification which refluxes a carboxylic acid with an alcohol. This esterification drives the reaction and dehydrates the products shifting the equilibrium towards the product. In this reaction DCC (dicyclohexylcarbodiimide) activates the carboxylic acid of biotin to further reaction and DMAP (4-dimethlyaminopyridine) acts as the acyl transfer catalyst. The biotinylated polyaniline derivative was characterized using FT-IR spectroscopy, ¹H NMR spectroscopy, UV-VIS spectroscopy, and Scanning Electron Microscopy.