Irene M. N. Groot

Supersonic molecular beam studies of dissociative adsorption of H2 on Ru(0001)

We examined reactivity of H(2) on Ru(0001) using molecular beam techniques and we compared our results to experimental results for similar systems. The dissociative adsorption of H(2) on Ru(0001) is similar to that on Pt(111) and Ni(111), although on ruthenium nonactivated adsorption is strongly suggested. However, we find no clear signature of a steering- or precursor-based mechanism that favors nonactivated reaction paths at low kinetic energy. In comparison to Pd(111) and Rh(111) our results indicate that a universal mechanism enhancing reactivity at low energy does not have a mass dependence. In addition, we have compared our results to predictions of reactivity for H(2) on Ru(0001) from six-dimensional dynamical calculations using two different generalized gradient approximation functionals. It leads us to conclude that the PW91 functional yields a more accurate value for the minimum energy path but does not impose enough corrugation in the potential. The revised-Perdew-Burke-Ernzerhof (RPBE) functional appears to behave slightly better at higher energies, but we find significant quantitative disagreement. We show that the difference is not due to different energy resolutions between experiment and theory. However, it may be due to a dependence of the reactivity on rotational state or on omission of relevant dimensions in the theoretical description.

The Energy Dependence of the Ratio of Step and Terrace Reactivity for H2 Dissociation on Stepped Platinum

The fraction of dissociation processes fD for H2 on platinum that take place on low-coordinate sites of nanoparticles is strongly dependent on the gas temperature of the incoming molecules and on the diameter d of the nanoparticles (see picture). For high gas temperatures and large nanoparticles, dissociation occurs mostly on terraces. Therefore, assumptions that steps always dominate reaction in heterogeneous catalysis cannot be justified

Dynamics of hydrogen dissociation on stepped platinum

We have studied the reactivity of hydrogen on the Pt(211) stepped surface using supersonic molecular beam techniques. We observe an energy dependence that is indicative of indirect adsorption below 9 kJ mol(-1) and direct adsorption between 0 and 37 kJ mol(-1). Comparison of our results to predictions based on six-dimensional quantum dynamics calculations for Pt(211) [R. A. Olsen et al., J. Chem. Phys. 128, 194715 (2008)] yields reasonable agreement. Discrepancies between theory and our experiments at low kinetic energy strongly indicate that the wells in the used potential energy surface are too shallow. Discrepancies at high kinetic energy point toward neglect of degrees of freedom vital to capture the full dynamics.

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre-adsorbed CO on the dissociative adsorption of D(2) on Ru(0001) is studied by molecular-beam techniques. We determine the initial dissociation probability of D(2) as a function of its kinetic energy for various CO pre-coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D(2) dissociation and perturbs the local surface reactivity up to the nearest-neighbour Ru atoms. Non-activated sticking and dissociation become less important with increasing CO coverage, and vanish at theta(CO) approximately 0.33 ML. In addition, at high D(2) kinetic energy (>35 kJ mol(-1)) the site-blocking capability of CO decreases rapidly. These observations are attributed to a CO-induced activation barrier for D(2) dissociation in the vicinity of CO molecules.

Observing the oxidation of platinum

Despite its importance in oxidation catalysis, the active phase of Pt remains uncertain, even for the Pt(111) single-crystal surface. Here, using a ReactorSTM, the catalytically relevant structures are identified as two surface oxides, different from bulk α-PtO2, previously observed. They are constructed from expanded oxide rows with a lattice constant close to that of α-PtO2, either assembling into spoked wheels, 1–5 bar O2, or closely packed in parallel lines, above 2.2 bar. Both are only ordered at elevated temperatures (400–500 K). The triangular oxide can also form on the square lattice of Pt(100). Under NO and CO oxidation conditions, similar features are observed. Furthermore, both oxides are unstable outside the O2 atmosphere, indicating the presence of active O atoms, crucial for oxidation catalysts.Improving platinum as an oxidation catalyst requires understanding its structure under catalytic conditions. Here, the authors discover that catalytically important surface oxides form only when Pt is exposed to high pressure and temperature, highlighting the need to study catalysts in realistic environments.

From dull to shiny: A novel setup for reflectance difference analysis under catalytic conditions

We have developed an experimental setup for optically monitoring a catalytically active surface under reaction conditions. A flow reactor with optical access allows us to image the behavior of an active catalyst surface down to the millimeter length scale. We use reflectance difference measurements with 625 nm light to investigate CO oxidation on Pd(100) at 300 mbar and 320 °C. We conclude that the changes in visible contrast result from the formation of an oxide layer after surface oxidation.

Evidence of stable high-temperature Dx-CO intermediates on the Ru(0001) surface

We demonstrate the formation of complexes involving attractive interactions between D and CO on Ru(0001) that are stable at significantly higher temperatures than have previously been reported for such intermediate species on this surface. These complexes are evident by the appearance of new desorption features upon heating of the sample. They decompose in stages as the sample temperature is increased, with the most stable component desorbing at >500 K. The D:CO ratio remaining on the surface during the final stages of desorption tends towards 1:1. The new features are populated during normally incident molecular beam dosing of D(2) on to CO pre-covered Ru(0001) surfaces (180 K) when the CO coverage exceeds 50% of the saturation value. The amount of complex formed decreases somewhat with increasing CO pre-coverage. It is almost absent in the case of dosing on to the fully saturated surface. The results are interpreted in terms of both local and long-range rearrangements of the overlayer that give rise to the observed CO coverage dependence and limit the amount of complex that can be formed.

Combined scanning probe microscopy and x-ray scattering instrument for in situ catalysis investigations

We have developed a new instrument combining a scanning probe microscope (SPM) and an X-ray scattering platform for ambient-pressure catalysis studies. The two instruments are integrated with a flow reactor and an ultra-high vacuum system that can be mounted easily on the diffractometer at a synchrotron end station. This makes it possible to perform SPM and X-ray scattering experiments in the same instrument under identical conditions that are relevant for catalysis.

Fast and reliable pre-approach for scanning probe microscopes based. On tip-sample capacitance

Within the last three decades Scanning Probe Microscopy has been developed to a powerful tool for measuring surfaces and their properties on an atomic scale such that users can be found nowadays not only in academia but also in industry. This development is still pushed further by researchers, who continuously exploit new possibilities of this technique, as well as companies that focus mainly on the usability. However, although imaging has become significantly easier, the time required for a safe approach (without unwanted tip-sample contact) can be very time consuming, especially if the microscope is not equipped or suited for the observation of the tip-sample distance with an additional optical microscope. Here we show that the measurement of the absolute tip-sample capacitance provides an ideal solution for a fast and reliable pre-approach. The absolute tip-sample capacitance shows a generic behavior as a function of the distance, even though we measured it on several completely different setups. Insight into this behavior is gained via an analytical and computational analysis, from which two additional advantages arise: the capacitance measurement can be applied for observing, analyzing, and fine-tuning of the approach motor, as well as for the determination of the (effective) tip radius. The latter provides important information about the sharpness of the measured tip and can be used not only to characterize new (freshly etched) tips but also for the determination of the degradation after a tip-sample contact/crash.

Live Observations of Catalysts Using High-Pressure Scanning Probe Microscopy

Recently it has become clear that essential discrepancies exist between the behavior of catalysts under industrial conditions and the (ultra)high vacuum conditions of traditional laboratory experiments. Differences in structure, composition, reaction mechanism, activity, and selectivity have been observed. These differences indicated the presence of the “pressure gap”, and made it clear that meaningful results can only be obtained at high pressures and temperatures. This chapter focuses on the development of scanning probe microscopy for operando observations of active model catalysts . We have developed instrumentation that combines an ultrahigh vacuum environment for model catalyst preparation and characterization with a high-pressure flow reactor cell, integrated with either a scanning tunneling microscope or an atomic force microscope. We combine the structural observations obtained under high-pressure, high-temperature conditions with time-resolved mass spectrometry measurements on the gas mixture leaving the reactor. In this way, we can correlate structural changes of the catalyst due to the gas composition with its catalytic performance. This chapter provides an overview of the instruments we developed and illustrates their performance with results obtained for different model catalysts and reactions.