Water formation from O<sub>2</sub> and H<sub>2</sub> on Rh tips: studies by field ion microscopy and pulsed field desorption mass spectrometry

Proc. Natl. Acad. Sci. U.S.A.

et al. 2009

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Field ion microscopy combined with video techniques and chemical probing reveals the existence of catalytic oscillatory patterns at the nanoscale. This is the case when a rhodium nanosized crystalconditioned as a field emitter tip-is exposed to hydrogen and oxygen. Here, we show that these nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the nanofacets that are simultaneously exposed at the tip's surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction-diffusion mechanism, plays a major role in determining the self-organizational behavior of multifaceted nanostructured surfaces. Surprisingly, this nanoreactor, composed of the tip crystal and a constant molecular flow of reactants, is large enough for the emergence of regular oscillations from the molecular fluctuations.field ion microscopy ͉ heterogeneous catalysis ͉ nanopatterns ͉ nonequilibrium oscillations T he Belousov-Zhabotinskii reaction is probably the most famous example of an oscillating chemical reaction in the liquid phase (1, 2). However, studies of oscillations in heterogeneous catalysis begun in the 1970s (3, 4). One decade later, Ertl et al. demonstrated for the first time (5, 6) that oscillatory surface reactions are associated with the occurrence of specific pattern formations ranging from tens to hundreds of micrometers. More recently, oscillations have been discovered on the nanoscale in field electron and field ion microscopes by using video techniques (FEM and FIM, respectively) (7-10). At this length scale of tens of nanometers, self-sustained oscillatory patterns are observed about which little is known. This is certainly the case for the catalytic water production when exposing oxygen and hydrogen to a rhodium field emitter tip (11,12). Our purpose in the present article is to show that this nonequilibrium self-organizational behavior can be understood by taking into account the structural anisotropy of the crystalline tip that results in different catalytic properties on the various nanofacets. Moreover, we demonstrate that the external electric field, as applied in a FIM (of the order of 10 V/nm), promotes surface oxidation thus giving way to a feedback mechanism to explain self-sustained rate oscillations in the H 2 /O 2 /Rh system.The understanding of self-sustained oscillations at the macroscale was pioneered by Prigogine who showed that such phenomena are consistent with thermodynamics as long as the system is open and far from equilibrium (13). Such selforganization phenomena are understood in terms of reactiondiffusion processes, which also explains the mesoscopic patterns observed on oriented metal single-crystal surfaces (14, 15). However, the nanopatterns observed during a catalytic reaction in a FIM have a spatial scale smaller than typical diffusion lengths. Moreover, one may wonder how the molecular fluctuations that manifest themselves in such small systems (16, 17) affect the oscillations. Indeed, their regularity disappears...

“…1C. Previous atom-probe studies during the ongoing reaction have shown that this is due to the presence of Rh x O y indicating the formation of a surface oxide (12).…”

Section: Experimental Observationsmentioning

confidence: 85%

Section: Experimental Observationsmentioning

confidence: 87%

mentioning

confidence: 86%

Section: Oscillatory Nanopatternsmentioning

confidence: 99%

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Nanometric chemical clocks

McEwen

Gaspard

Proc. Natl. Acad. Sci. U.S.A.

et al. 2009

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“…For most conditions the NO + intensities are high and therefore contribute strongly, if not exclusively, to the image formation. In contrast to the H 2 -O 2 /Rh and Pt systems, where the produced water acts as the imaging species [28,29], only low amounts of water are detected under the experimental conditions corresponding to the H 2 -and NO-sides of the system (Fig. 4).…”

Section: Pulsed Field Desorption Mass Spectrometrymentioning

confidence: 94%

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

Kruse

2011

Ultramicroscopy

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a b s t r a c tThe 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 À 3 Pa pressure range and at sample temperatures between 400 and 600 K. 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.

Heterogeneous Catalysis and High Electric Fields

Kruse

Handbook of Heterogeneous Catalysis

2008

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Introduction The relationships between high electric fields and heterogeneous catalysis are twofold: First, high electric fields are applied in field emission and field ion microscopes and related methods. These techniques can provide detailed in situ information on catalytic surface processes with a lateral resolution on the atomic scale. Second, high electric fields can affect the surface properties of solids. Electronic and structural parameters are frequently field dependent and influence chemisorption processes of molecules and also catalytic activity and reaction pathways of surface reactions.High electric fields, defined as fields of the order of some tens of volts per nanometer, are what valence electrons in atoms and molecules experience and what these electrons are exposed to on adsorption at solid surfaces. Thus, these fields are ubiquitous in chemistry and form, in the context of this contribution, an intimate part of heterogeneous catalysis. Quantum mechanical aspects of chemical bonding of solid surfaces distinguish between shortrange covalent and long-range electrostatic contributions. These long-range effects create an electrostatic Coulomb potential which causes changes in ionization potentials and electron affinities of adsorbed molecules. As a consequence, catalytic surface reactions may be altered. However, in real catalysts it is sometimes difficult to distinguish between contributions of local binding and long-range electrostatic fields.Long-range fields of the order of 10 V nm −1 are generated in a controlled manner in the field ion microscope, originally developed by Muller [1] for imaging * Corresponding author.