Others

Besides the ongoing projects described above, we have during our tenure at UCR pursued a few additional directions of research in our laboratory in connection with problems in catalysis and materials science.  In one example related to catalysis, we have explored the possibility of alloying platinum catalysts with copper in order to tune their selectivity towards less dehydrogenation.  The composition of several supported Pt-Cu catalysts was characterized by infrared spectroscopy of adsorbed carbon monoxide.  The figure below displays representative data from those experiments.  They clearly show the preferential segregation of the copper to the surface: the 50% Pt:50% Cu catalyst was proven to behave very similarly to the copper-only sample.  Subtle effects were nevertheless seen during the oxidation and reduction of the different solids.

Transmission infrared spectra for carbon monoxide adsorbed on Pt-Cu mixed-metal particles dispersed on a high surface area silica support.  Three samples were studied, all at the same 2% loading: pure copper, pure platinum, and a 1:1 Pt:Cu mixture.  It is seen here that the alloy behaves in a similar way as the pure copper catalyst, a fact indicated by the weak nature of the CO adsorption (most of its IR signal disappears upon warming up to room temperature), and by the high frequency of the C–O bond (about 2130 cm-1).  This provides a clear indication for the preferential segregation of copper to the surface.

In an example related to materials science, we performed XPS investigations on the reactivity of aluminum surfaces with high-pressure gases in connection with gas storage tanks.  The inside surface of several aluminum gas cylinders were analyzed after different treatments.  Shown below are a couple of the C 1s XPS traces obtained for the treated surfaces in these studies.  This series of experiments was successful in determining the source of staining, which was traced back to the use of excessive lubricant in some parts during the extrusion of the cylinders.  This lead us to propose the implementation of some extra acid washing at the end of the manufacturing process, since the previous procedure proved insufficient on the lubricant and too much on the rest of the cylinder.  This project was carried out in conjunction with people at the Riverside factory of the Luxfer Group.

C 1s XPS data for samples taken from extruded aluminum cylinders before and after their acid washing treatment.  Clearly, the untreated cylinders display large amounts of surface carbon, but even the washed surfaces show some superficial carbon.  Changes in the cleaning procedure after extrusion were implemented as a result of these studies.

Picture of the beam line equipped with our fluorescence detection NEXAFS apparatus.  This system, installed at the National Synchrotron Light Source in the Brookhaven National Laboratory, is capable of detecting C, N, and O atoms of adsorbates on single-crystal surfaces under atmospheric pressures.

One of our long-term goals has been to understand the kinetics of reactions on solid surfaces.  To this end, our experimental measurements have been sometimes complemented with Monte Carlo computer simulation [F. Zaera and I. Rusinek, J. Comp. Chem. 2 (1981) 402; T. Nordmeyer and F. Zaera, J. Chem. Phys. 97 (1992) 9345].  One of our first studies in this direction was focused on the non-dissociative adsorption of gas-phase molecules onto a spatially homogeneous square lattice using a method which accounts for the existence of an extrinsic precursor state [J. Phys. Chem. Solids, 3, 95, 1957].  More recently, we have been working with Professor Giorgio Zgrablich to simulate the kinetics of NO reduction and molecular nitrogen formation on Rh(111) surfaces [V. Bustos, C. S. Gopinath, R. Uñac, F. Zaera and G. Zgrablich, J. Chem. Phys. 114 (2001) 10927; F. Zaera, S. Wehner, C. S. Gopinath, J. L. Sales, V. Gargiulo and G. Zgrablich, J. Phys. Chem. B 105 (2001) 7771; V. Bustos, C. S. Gopinath, R. Uñac, F. Zaera and G. Zgrablich, J. Chem. Phys., 114 (2001) 10927; V. Bustos, R. Uñac, F. Zaera and G. Zgrablich, J. Chem. Phys. 118 (2003) 9372; L. A. Avalos, V. Bustos, R. Uñac, F. Zaera and G. Zgrablich, J. Mol. Catal. A 228 (2005) 89].  These Monte Carlo simulations have proven particularly suited to the study of inhomogeneous systems.  One good example of this is the case of the nitrogen islands that appear to form on Rh(111) during the catalytic reduction of nitrogen oxide.  Our Monte Carlo simulations were used to explain the isotopic distributions observed by our molecular beam experiments (see appropriate web page) in terms of the formation of islands with the nitrogen isotopes distributed in an "onion" structure, the 14N atoms in a core surrounded by a 15N outer shell.  Appropriate kinetic parameters were extracted from these calculations (see figure below).

Results from Monte Carlo simulations on the desorption of molecular nitrogen from atomic-nitrogen surface islands.  These simulations were started with perfect hexagonal islands of varying sizes, ranging from 2 to 5 layers (from 7 to 61 atoms per island, from left to right).  Two sets of simulations were performed for each island size, with (bottom) and without (top) surface atom diffusion.  Plotted are molecular nitrogen yield fractions for all three possible isotopomers as a function of 15N fraction.  Fit of these data to our experimental results allow us to extract the appropriate kinetic parameters for the catalytic reduction of NO on Rh(111).

Last modified August 2. 2005