Determination of diffusion in polycrystalline platinum thin films

Grain boundary diffusion of titanium through platinum thin films has been carried out in the temperature range from 200 to 600 °C. Five different platinum/titanium bilayer thicknesses, from 35 to 800 Å Pt, were annealed in 5% O2/95% N2.5%O2/95%N2. The accumulation of titanium at the platinum surface layer was measured by x-ray photoelectron spectroscopy (XPS) to determine the grain boundary diffusion coefficient (Db).(Db). Diffusivity values were calculated based on two different analysis methods assuming type C kinetics. For Pt layers thicker than 200 Å, the activation energy (Qb)(Qb) for titanium diffusion was found to be 118±15 kJ/mol (1.22±0.16 eV). For Pt layers thinner than 200 Å, there was a thickness dependence on the diffusion kinetics, resulting in activation energies as low as 20±4 kJ/mol (0.21±0.04 eV). XPS results gave no evidence for any Pt-Ti alloy formation in these layers. The suppression of alloy formation may be attributed to the presence of oxygen at the Pt/Ti interface during layer deposition. The quantitative analysis of titanium interdiffusion in platinum provides valuable information regarding Pt/Ti surface concentrations in thin-film chemical sensors, and for understanding changes in operational characteristics of platinum electrodes. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69829/2/JAPIAU-86-9-4902-1.pd

Bimetallic catalysts: Discoveries, concepts, and applications. By John H. Sinfelt, John Wiley & Sons, 1983. XI + 164 pp

No Abstract.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37393/1/690310823_ftp.pd

A mean‐field modeling study of the interaction between hydrogen and a palladium (110) single crystal

A new interpretation of the temperature programmed desorption (TPD) spectra of hydrogen on a Pd(110) single crystal surface is presented. The transient mean‐field model developed accounts for both adsorption on the surface and diffusion of hydrogen into the lattice of the crystal. A new approach for modeling TPD spectra is developed, in which both the isothermal exposure as well as the temperature ramp of the TPD experiment are modeled in a consistent way. We demonstrate that for systems with substantial diffusion of the adsorbate into the substrate’s lattice it is not sufficient to report exposure values. For such systems, exposure time has a far more pronounced effect than the exposure pressure has on the TPD spectra, because diffusion is a strongly time‐dependent process. The modeling and experimental results for the H2/Pd(110) system are in good agreement and suggest the existence of a distinct subsurface state, in addition to the surface and bulk states of hydrogen. Low frequency factors derived from the model for the processes connecting the subsurface with the surface and bulk state emphasize the restricted nature of the corresponding transition states. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70193/2/JCPSA6-105-18-8398-1.pd

FTIR study of bimetallic Pt-Sn/Al2O3 catalysts

A series of Pt-Sn/Al2O3 catalysts supported on Degussa nonporous alumina were characterized by infrared spectroscopy of adsorbed CO. The nominal platinum loading was kept constant in the catalysts at 1 wt% and the nominal amount of tin was varied from 0 to 5 wt%. At room temperature it was found that there was a general trend for the linearly adsorbed CO band to shift to lower frequencies with increasing amounts of tin in the catalyst, except for one of the catalysts (1.0 Pt-1.0 Sn/Al2O3) where a particle size effect seemed to be dominant. The amount of bridge-bonded CO species was very small and became undetectable in the case of large tin loadings (1 and 5 wt% Sn). Thermal desorption infrared spectroscopy was used to determine the vibration frequency of the isolated CO molecule and hence observe if there were any indications of electronic interactions between tin and platinum. For the monometallic 1.0 Pt/Al2O3 sample the singleton vibration frequency was 2041 cm-1. For all the Pt-Sn bimetallic samples it was found that the singleton vibration frequency lay within the range 2041 +/- 6 cm-1. This suggests that the electronic effect of Sn on Pt in our Pt-Sn/Al2O3 catalysts is not very large.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29705/1/0000037.pd

Neopentane reactions over bimetallic Pt---Sn/AI2O3 and Pt---Au/SiO2 catalysts

The reaction of 2,2-dimethylpropane (neopentane) with hydrogen was studied over two series of supported bimetallic catalysts containing platinum as the primary group VIII metal and either gold or tin as the second metallic component. In both catalyst series the platinum nominal loading was maintained constant at 1 wt%. The loading of the second metal component was varied. An effort has been made to better understand the reactivity trends in these catalysts, by bringing the neopentane reactivity data into context with previously obtained n-hexane reaction results and extensive catalyst characterization data. Small amounts of tin (1.0 Pt-0.1 Sn/Al2O3) caused the neopentane isomerization selectivity to drop in comparison to monometallic 1.0 Pt/Al2O3. However, further addition of tin up to 1.0 wt% caused the neopentane isomerization selectivity to increase above that of the monometallic 1.0 Pt/Al2O3 sample. On adding excess tin (5.0 wt%) the activity of the catalyst became prohibitively low. Hence, there seemed to be an optimum loading of tin giving desirable selectivity characteristics at reasonable activity levels. The differences in isomerization selectivity in the case of the Pt---Au/SiO2 catalysts were not as significant as those observed for the Pt---Sn/Al2O3 catalysts. The trends in catalytic behavior were consistent with previously measured n-hexane reaction trends and could be interpreted on the basis of microstructural differences between the bimetallic catalysts. The effect of gold appeared to be mainly a geometric one, disrupting larger Pt ensembles. On the other hand, the role of tin appeared to go beyond a simple geometric effect.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29012/1/0000041.pd

A chemisorption and XPS study of bimetallic Pt-Sn/Al2O3 catalysts

Bimetallic Pt-Sn/Al2O3 catalysts with nominally 1 wt% Pt and varying tin contents (0-3.25 wt%) prepared by coimpregnation of nonporous Degussa alumina were characterized by chemisorption of H2, O2, and CO at room temperature. The surface compositions and oxidation states of the reduced catalysts were tracked by XPS. Addition of tin to Pt resulted in significant differences in the gas uptake characteristics of the three adsorbates. Both H2 and CO showed an initial increase in gas uptake with addition of small amounts of tin, and then the chemisorbed amount tended to drop off with further addition of tin. In the case of O2 adsorption, there was a steady increase in gas uptake with increasing tin content. XPS of the reduced catalysts showed that in alumina-supported samples most of the tin was in a valence state of either Sn(II) or Sn(IV). On the other hand, large amounts of zero-valent tin were found in a SiO2-supported Pt-Sn catalyst which had been prepared from the same precursors and reduced under identical conditions. This supports the notion that interactions between the alumina support and tin prevent the complete reduction of tin.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29594/1/0000683.pd

Microstructure and reactivity of supported bimetallic platinum-gold catalysts

In this paper an attempt is made to correlate the microstructure of a supported bimetallic catalyst system with its activity and selectivity. A series of bimetallic platinum-gold catalysts supported on high-surface-area nonporous Aerosil was prepared by the incipient wetness technique. The platinum-gold system is known to be partially miscible in the bulk with the possibility of alloy formation at high and low Pt/Au atomic ratios. The reducibility of the various metal components in these catalysts was determined by temperature-programmed reduction. Microstructural characterization of the catalysts was performed by using analytical and high-resolution electron microscopic techniques. The combination of elemental analysis by EDX and lattice fringe imaging by HREM of individual metal particles was used to determine the presence and location of the various components of this bimetallic system. The kinetic behavior for the n-hexane conversion reaction and the product selectivities toward dehydrocyclization, isomerization, and hydrogenolytic cracking were measured. The product selectivity trends in n-hexane conversion can be attributed mainly to geometric effects related to the relative distribution and interdispersion of the platinum and gold atoms in the Catalysts.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27660/1/0000042.pd

Fischer-Tropsch synthesis on bimetallic ruthenium-gold catalysts

The effect of Au in the behavior of Ru as a Fischer-Tropsch catalyst was studied. Two series of catalysts were investigated, one supported on SiO2 and the other on MgO. Au did not seem to alter the product distribution on Ru at pressures up to 1 MPa and in the temperature range 490-570 K. However, the turnover frequencies for both CO hydrogenation and methanation showed a precipitous drop with the Au content in the SiO2-supported catalysts, whereas on the MgO series a maximum in activity was observed at an intermediate Au content. These activity patterns were correlated with extensive physical characterization placing major emphasis on analytical electron microscopy. The activity trends as a function of Au content were remarkably similar to those previously reported for the structure-sensitive ethane hydrogenolysis reaction. In both reactions, the effect of Au appears to be due to a dilution of the active Ru ensembles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25650/1/0000202.pd

The influence of hydrazine reduction on metal dispersion and support morphology in bimetallic Ru---Au/MgO catalysts

The influence of hydrazine reduction on metal dispersion, particle size distribution, and support morphology was investigated for a series of bimetallic Ru-Au/MgO catalysts. The catalysts were prepared by impregnation of MgO with chlorine-containing precursor salt solutions. Hydrazine reduction significantly lowered the residual chlorine content of the catalysts and allowed the formation of high-surface-area MgO. This, in turn, led to much higher Ru metal dispersions as compared to that obtained after H2 reduction alone. X-Ray energy dispersive spectroscopy (EDS) proved that bimetallic Ru-Au aggregates were confined to a particle size range of less than 5 nm. Particles larger than 10 nm contained without exception monometallic Au. The hydrazine-treated catalysts did not show the suppression in catalytic activity for CO hydrogenation that was observed in H2-reduced Ru/MgO catalysts. The increased metal dispersion and restored catalytic activity of Ru in the hydrazine-treated catalysts may be linked to the absence of chlorine contamination of the support.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26244/1/0000324.pd

Metal dispersion of bimetallic catalysts via stepwise chemisorption and surface titration : II. Ru---Au/MgO

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25587/1/0000131.pd