M. K. Rose

Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies

During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, ‘spectators’ that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2—a central step in many industrially important catalytic processes—is generally assumed to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.

Subsurface impurities in Pd(111) studied by scanning tunneling microscopy

Low concentrations of three distinct impurity species beneath the Pd(111) surface are studied by STM. The subsurface impurities are distinguished by their image contrast, diffusion properties, and interactions with adsorbed molecules. Isolated subsurface impurities appear at low gap resistance (<≈ MΩ) as three-fold symmetric modulations of the Pd 1×1 surface corrugation. One impurity type is found to occupy substitutional sites in the layer below the surface. Based on Auger spectroscopy this species is identified as sulfur. The other two species are found to occupy octahedral interstitial sites immediately below the surface layer. Two-dimensional diffusion of the interstitial impurities occurs below room temperature. The onset temperature for diffusion is lowered dramatically in the presence of surface adsorbates. Quantitative measures of the diffusion barriers are consistent with surface facilitated diffusion of interstitial oxygen and carbon atoms. The mobile impurities interact with adsorbed atoms and ...

Scanning tunneling microscope with continuous flow cryostat sample cooling

We have constructed an ultrahigh vacuum scanning tunneling microscope (STM) for operation in the temperature range 20–300 K. The design consists of a vibration isolated sample holder mounted on a continuous flow cryostat. By rotation and linear motion of the cryostat, the sample can be positioned in front of various surface preparation and analysis instruments contained in a single vacuum chamber. A lightweight beetle-type STM head is lowered from the top onto the sample by a linear manipulator. To minimize helium convection in the cryostat, the entire vacuum system, including a liquid helium storage Dewar, can be tilted by a few degrees perpendicular to the cryostat axis, which improves the operation. The performance of the instrument is demonstrated by atomically resolved images of the Pd(111) surface and adsorbed CO molecules.

Method to characterize the vibrational response of a beetle type scanning tunneling microscope

We describe a method for analyzing the external vibrations and intrinsic mechanical resonances affecting scanning probe microscopes by using the microscope as an accelerometer. We show that clear correlations can be established between the frequencies of mechanical vibrational modes and the frequencies of peaks in the tunnel current noise power spectrum. When this method is applied to our “beetle” type scanning tunneling microscope (STM), we find unexpected low frequency “rattling resonances” in the 500–1700 Hz range that depend on the exact lateral position of the STM, in addition to the expected mechanical resonances of the STM above 4 kHz which are in good agreement with theoretical estimates. We believe that these rattling resonances may be a general problem for scanning probe microscopes that use some type of kinetic motion for coarse positioning.