The role of fluctuations in bistability and oscillations during the H2 + O2 reaction on nanosized rhodium crystals.

A combined experimental and theoretical study is presented of fluctuations observed by field ion microscopy in the catalytic reaction of water production on a rhodium tip. A stochastic approach is developed to provide a comprehensive understanding of the different phenomena observed in the experiment, including burst noise manifesting itself in a bistability regime, noisy oscillations, and nanopatterns with a cross-like oxidized zone separating the surface into four quadrants centered on the {111} facets. The study is based on a stochastic model numerically simulating the processes of adsorption, desorption, reaction, and transport. The surface diffusion of hydrogen is described as a percolation process dominated by large clusters corresponding to the four quadrants. The model reproduces the observed phenomena in the ranges of temperature, pressures, and electric field of the experiment.

Field emission techniques for studying surface reactions: applying them to NO-H2 interaction with Pd tips.

The adsorption of NO and its reaction with H(2) over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10(-3)Pa pressure range and at sample temperatures between 400 and 600K. By varying the H(2) partial pressure while keeping the other control parameters constant, the NO+H(2) reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H(2) pressures delimiting this hysteresis decrease as well. Abrupt transformations of the micrographs are observed by FIM from bright to dark patterns and vice versa. These transformations define the hysteresis region. The collected data allow establishing a novel kinetic phase diagram of the NO+H(2)/Pd system within the range of temperatures and pressures indicated. The observed features are correlated with a local chemical analysis by means of field pulses. NO(+) seems to be the dominating imaging species under all conditions. At high relative H(2) pressures (the "hydrogen-side"), H atoms seem to diffuse subsurface. This process is blocked at lower H(2) pressure (the "NO-side") due to NO(ad) and O(ad) accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO(2)(+) species in the mass spectra.

Catalytic reduction of NO2 with hydrogen on Pt field emitter tips: kinetic instabilities on the nanoscale.

The catalytic reduction of NO(2) with hydrogen on a Pt field emitter tip is investigated using both field electron microscopy (FEM) and field ion microscopy (FIM). A rich variety of nonlinear behavior and unusually high catalytic activity around the {012} facets are observed. Our FEM investigations reveal that the correlation function exhibits damped oscillations with a decaying envelope, showing that molecular noise will influence the dynamics of the oscillations. The dependence of the oscillatory period on the P(H(2))/P(NO(2)) pressure ratios is analyzed. Similar patterns are reported under FIM conditions. Corresponding density functional theory (DFT) calculations for the adsorption of NO(2) on Pt{012} in the presence of an external electric field are performed in order to gain an atomistic understanding of the underlying nonlinear phenomena.

C2H2 interaction with Ni nanocrystals: towards a better understanding of carbon nanotubes nucleation in CVD synthesis.

We present a study of the early stages of carbon nanotubes nucleation in CVD synthesis by combining field ion/electron emission microscopy (FIM/FEM) and atom-probe investigation (AP) of the nickel-carbon interaction. Acetylene decomposition on Ni tips at 873K is observed to induce additional step formation on an initially facetted (polyhedral) crystal. Carbon-enriched steps are then observed to act as preferential nucleation centers of graphene sheets formation. Atom-probe experiments reveal C(2) and C(3) species and frequency dependent studies demonstrate that the origin of these species is different from C(1). Experiments provide clear evidence for the crucial role of carbon-enriched steps as nucleation sites of graphene sheets on the Ni surface.

Oxygen adsorption on gold nanofacets and model clusters.

We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) m/bar) and temperatures (T = 300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O2+ intensities rather reflect ionization of O2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au10 clusters (Delta E = 0.023 eV) and comparatively stronger interaction with Au1/Au(100) model surfaces (Delta E = 0.25 eV). No binding is found on {210} facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra.

Field-induced CO adsorption and formation of carbonyl waves on gold nanotips.

We report a study of the adsorption and reaction of CO on a gold nanotip in high electrostatic fields. Field ion microscopy is used to investigate the emergence of a Au-carbonyl wave that is made visible with oxygen as the imaging gas. We set up a simple kinetic model that reproduces the adsorption wave and confirms that the presence of oxygen merely serves as an imaging gas and does not lead to field-induced oxidation of CO.