CO-induced restructuring of Pt(110)-(1×2): Bridging the pressure gap with high-pressure scanning tunneling microscopy

High-Pressure Scanning Tunneling Microscopy

This is a Review of recent studies on surface structures of crystalline materials in the presence of gases in the mTorr to atmospheric pressure range, which brings surface science into a brand new direction. Surface structure is not only a property of the material but also depends on the environment surrounding it. This Review emphasizes that high/ambient pressure goes hand-in-hand with ambient temperature, because weakly interacting species can be densely covering surfaces at room temperature only when in equilibrium with a sufficiently high gas pressure. At the same time, ambient temperatures help overcome activation barriers that impede diffusion and reactions. Even species with weak binding energy can have residence lifetimes on the surface that allow them to trigger reconstructions of the atomic structure. The consequences of this are far from trivial because under ambient conditions the structure of the surface dynamically adapts to its environment and as a result completely new structures are often formed. This new era of surface science emerged and spread rapidly after the retooling of characterization techniques that happened in the last two decades. This Review is focused on the new surface structures enabled particularly by one of the new tools: high-pressure scanning tunneling microscopy. We will cover several important surfaces that have been intensely scrutinized, including transition metals, oxides, and alloys.

Self-assembly of acetate adsorbates drives atomic rearrangement on the Au(110) surface

Weak inter-adsorbate interactions are shown to play a crucial role in determining surface structure, with major implications for its catalytic reactivity. This is exemplified here in the case of acetate bound to Au(110), where the small extra energy of the van der Waals interactions among the surface-bound groups drives massive restructuring of the underlying Au. Acetate is a key intermediate in electro-oxidation of CO₂ and a poison in partial oxidation reactions. Metal atom migration originates at surface defects and is likely facilitated by weakened Au–Au interactions due to bonding with the acetate. Even though the acetate is a relatively small molecule, weak intermolecular interaction provides the energy required for molecular self-assembly and reorganization of the metal surface.

The efficiency of a catalyst relies on the stability of intermediates on its surface. Here, the authors find that van der Waals interactions between acetate adsorbates on Au(110) provide a small but necessary energy contribution to stabilize the acetate and drive restructuring of the Au surface.

Structural and surface coverage effects on CO oxidation reaction over carbon-supported Pt nanoparticles studied by quadrupole mass spectrometry and diffuse reflectance FTIR spectroscopy

CO oxidation on Pt nanoparticles (average size of 2.8 to 7.7 nm) depends on the adlayer and surface structure.

Powder diffraction in studies of nanocrystal surfaces: chemisorption on Pt

Atoms at the surface of nanocrystals contribute appreciably to the X-ray diffraction pattern. Phenomena like chemisorption, affecting the displacement of surface atoms with respect to their positions in the perfect crystallographic structure, cause diffraction peak shifts and intensity changes. These effects are easily measurable for small nanocrystals up to 10 nm size. This article reports diffraction effects of chemisorption of adsorbing gases H2, O2, CO and NO for a series ofin situpowder diffraction experiments on nanocrystalline Pt supported on silica. On the basis of previous diffraction observation of Pt surface reconstruction during hydrogen desorption, it was possible to quantify this effectversuscrystallite size and rationalize the observed diffraction peak shift for the other adsorbing species. This enabled the surface reconstruction to be distinguished from the surface relaxation effect, the latter depending monotonically on the adsorption energy. Even if no phase transition occurs, monitoring of a peak's position, intensity, width and gas composition (viamass spectrometry) during a carefully designed physicochemical process (including surface chemical reaction) enables insight into and understanding of the surface structure evolution (e.g.amorphization, relaxation, reconstruction or changes in the overall morphology). The proposed technique can be used as a surface science tool, allowing studies of nanocrystals under high pressure.

Quick low temperature coalescence of Pt nanocrystals on silica exposed to NO – the case of reconstruction driven growth?

We report an operando XRD/MS experiment on nanocrystalline Pt supported on silica, monitoring quick, low temperature coalescence of Pt in an NO atmosphere accompanied by surface reconstruction deduced from an apparent lattice parameter (ALP) evolution.

Orthogonal Intermolecular Interactions of CO Molecules on a One-Dimensional Substrate

By low-temperature scanning tunneling microscopy, we study CO molecule chemisorption on a quasi–one-dimensional Cu(110)-(2×1)-O surface. Atom-resolved images reveal how the interaction of CO with the surface Cu-O- chains gives rise to orthogonal attractive and repulsive intermolecular interactions. First-principles calculations show that CO molecules induce unprecedented lifting of the host Cu atoms by 1 Å from the Cu-O- chains, enabling the Cu-CO unit to tilt by 45° from the surface normal. Contrary to the behavior of CO on metal surfaces, this structural distortion enables unprecedented, orthogonal, short-range intermolecular dipole-dipole attraction and long-range, surface-mediated repulsion. These interactions lead to self-assembly into molecular nanograting structures consisting of arrays of single-molecule-wide CO rows. The origin of the novel behavior of CO molecules in the electronic and geometrical properties of the quasi–one-dimensional substrate suggests that similar molecule-molecule and molecule-substrate interactions could play an important role at catalytic sites on reactive surfaces.

Formation of nanometer-sized surface platinum oxide clusters on a stepped Pt(557) single crystal surface induced by oxygen: a high-pressure STM and ambient-pressure XPS study

We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O(2) pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O(2) and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O(2) to 10(-8) Torr. Changes in the surface oxide concentration in response to variations in the O(2) gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.

Orthogonal interactions of CO molecules on a one-dimensional substrate

We investigate the chemisorption structure of CO molecules on the quasi-one-dimensional Cu(110)-(2 × 1)-O surface by low-temperature scanning tunneling microscopy and density functional theory. Contrary to flat metal surfaces, where CO molecules adsorb in an upright geometry and interact through repulsive intermolecular interactions, we find the most stable adsorption structure of single CO molecules to be at Cu atoms of substrate Cu-O- chains with the Cu-CO unit bent by ~±45° in two equivalent structures at low coverages. At higher coverages, CO molecules combine in the same structure into highly ordered single-molecule-wide rows perpendicular to the substrate chains in an approximately 8 × 1 full monolayer structure. First-principles calculations attribute the unprecedented chemisorption behavior of CO molecules to lifting of the host Cu atoms by 1 Å from the surface Cu-O- chains, in order to optimize the bonding and reduce the repulsive interactions with the substrate. This structural distortion enables short-range intermolecular dipole-dipole attraction and creates orthogonal long-range surface-mediated repulsion leading to unusual self-assembly of CO molecules into coherent nanometer scale molecular grating structures.

Colloidally prepared Pt nanowires versus impregnated Pt nanoparticles: comparison of adsorption and reaction properties

Ligand-capped Pt nanowires, prepared by colloidal synthesis and deposited on a high surface area γ-Al(2)O(3) support, were subjected to surface characterization by electron microscopy and FTIR spectroscopy using CO as a probe molecule. The structural, adsorption, and catalytic reaction properties of the colloidal Pt nanowires were compared to those of conventional, impregnated Pt nanoparticles on the same Al(2)O(3) support. In situ FTIR spectroscopy indicated ligand effects on the CO resonance frequency, irreversible CO-induced surface roughening upon CO adsorption, and a higher resistance of colloidal catalysts toward oxidation (both in oxygen and during CO oxidation), suggesting that the organic ligands might protect the Pt surface. Elevated temperature induced a transformation of Pt nanowires to faceted Pt nanoparticles. The colloidal catalyst was active for hydrodechlorination of trichloroethylene (TCE), but no ligand effect on selectivity was obtained.

In situ STM observation of the CO adlayer on a Pt(110) electrode in 0.1 M HClO4 solution

We have obtained the first in situ STM molecular image of a CO adlayer on a Pt(110)-(1 x 1) electrode surface in 0.1 M HClO(4) solution. The observed CO adlayer formed an ordered (2 x 1)-2CO structure at saturated coverage. The CO molecules were found to adsorb on top of each Pt surface atom; however, they were tilted with a zigzag configuration along the atomic rows because of the dipole-dipole repulsion of neighboring CO molecules. The high activity of the Pt(110) electrode for surface CO oxidation can be attributed to the low packing density of the adsorbed CO molecules as well as their tilted orientation.

Epitaxial growth of single-crystalline Al(2)O(3) films on Cr(2)O(3)(0001)

Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 A. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 A, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.

Bridging the pressure gap in model systems for heterogeneous catalysis with high-pressure scanning tunneling microscopy

A high-pressure scanning tunneling microscope (HP-STM) enabling imaging with atomic resolution over the entire pressure range from ultrahigh vacuum (UHV) to one bar has been developed. By means of this HP-STM we have studied the adsorption of hydrogen on Cu(110), CO on Pt(110) and Pt(111), and NO on Pd(111) at high pressures. For all of these adsorption systems we find that the adsorption structures formed at high pressures are identical to high-coverage structures formed at lower pressures and temperatures. We thus conclude that for these systems the so-called pressure gap can be bridged, i.e. the results obtained under conventional surface science conditions can be extrapolated to higher pressures. Finally, we use the HP-STM to image the CO-induced phase separation of a Au/Ni(111) surface alloy in real time, whereby demonstrating the importance of catalyst stability in the study of bimetallic systems.

IR Chemiluminescence Probe of the Vibrational Energy Distribution of CO2 Formed during Steady-State CO Oxidation on Pt(111) and Pt(110) Surfaces

The infrared (IR) chemiluminescence spectra of CO2 were measured during the steady-state CO + O2 reaction over Pt(110) and Pt(111) surfaces. Analysis of the IR emission spectra indicates that the bending vibrational temperature (TVB), as well as the antisymmetric vibrational temperature (TVAS), was higher on Pt(110) than on Pt(111). On the Pt(110) surface, the highly excited bending vibrational mode compared to the antisymmetric vibrational mode was observed under reaction conditions at low CO coverage (theta(CO) < 0.2) or at high surface temperatures (TS > or = 700 K). This can be related to the activated complex of CO2 formation in a more bent form on the inclined (111) terraces of the Pt(110)(1 x 2) structure. On the other hand, at high CO coverage (theta(CO) > 0.2) or at low surface temperatures (TS < 650 K), TVAS was higher than TVB, which can be caused by the reconstruction of the Pt(110)(1 x 2) surface to the (1 x 1) form with high CO coverage.

Adsorbate-induced alloy phase separation: a direct view by high-pressure scanning tunneling microscopy

The influence of high pressures of carbon monoxide (CO) on the stability of a Au/Ni(111) surface alloy has been studied by high-pressure scanning tunneling microscopy. We show that CO induces a phase separation of the surface alloy at high pressures, and by means of time-lapsed STM movies we find that Ni atoms are removed from the surface layer during the process. Density functional theory calculations reveal the thermodynamic driving force for the phase separation to be the Au-induced compression of the CO overlayer with a resulting CO-CO repulsion. Furthermore, the atomistic mechanism of the process is shown to be kink-site carbonyl formation and evaporation which is found to be enhanced by the presence of Au.

The Adsorption Structure of NO on Pd(111) at High Pressures Studied by STM and DFT

Using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we study the adsorption structure of NO on Pd(111) at pressures of up to 720 Torr. From atomically resolved STM images, we identify, at high pressures, only the (2 x 2)-3NO structure, which is identical with the highest NO-coverage structure found at low pressure and low temperature. DFT calculations confirm that the (2 x 2)-3NO structure is indeed the most stable adsorption structure at high pressures. Contrary to recent suggestions in the literature, we therefore conclude that we find no evidence for a (3 x 3)-7NO structure on Pd(111) at high NO pressure.

Realistic kinetic Monte Carlo study of the surface phase reconstruction

Reversible 1 x 1 <==> 1 x 2 reconstruction of Pt(110) surface is studied with the help of a kinetic Monte Carlo model. Activation energies of the allowed atomic steps are estimated using available computational and experimental data, and some discrepancies are reconciled to fit macroscopic data on surface reconstruction. Both energies of the various atomic configurations and activation energies depend on CO coverage and are estimated with the account of Pt-CO binding energies and repulsive interactions on adjacent sites. The model well reproduces both scanning tunneling microscopy results obtained for a clean 1 x 2 surface and available macroscopic data on 1 x 1 <==> 1 x 2 reconstruction.

A realistic kinetic Monte Carlo simulation of the faceting of a Pt(110) surface under reaction conditions

The faceting process on Pt(110) is studied with the help of a kinetic Monte Carlo model taking into account realistic Pt-Pt, Pt-CO, and Pt-O interactions. The activation energies of the allowed atomic steps are estimated using available computational and experimental data. The model well reproduces the region in the parameter space where faceting occurs. Under kinetic instability conditions, the simulated faceted pattern forms a periodic hill and valley structure with a lateral periodicity of approximately 140-170 A, which is comparable with experimental data. The simulations reproduce the development of faceting on a realistic time scale.