Mechanism and dynamics of the CO-induced lifting of the Pt(100) surface reconstruction

The first atomistic simulations of the CO-induced lifting of the Pt(100)-hex reconstruction have been performed. During this phase transformation the surface changes back to bulk-terminated Pt(100)-(1 x 1), whereby the surface atom density decreases by approximately 20%. The simulations reveal a mechanism collective in nature, indicating that restructuring proceeds through ejection of chains of Pt atoms. These chains explain the anisotropy as seen in scanning tunneling microscopy experiments. The restructuring rate depends nonlinearly on the CO coverage, but the absence of local clustering of CO excludes an explanation in terms of elementary reaction kinetics as proposed previously.     

In-situ study of the catalytic oxidation of CO on a Pt(1 1 0) surface using ambient pressure X-ray photoelectron spectroscopy

CO and O₂ co-adsorption and the catalytic oxidation of CO on a Pt(1 1 0) surface under various pressures of CO and O₂ (up to 250 mTorr) are studied using ambient pressure X-ray photoelectron spectroscopy (APXPS) and mass spectrometry. There is no surface oxide formation on Pt under our reaction conditions. CO oxidation in this pressure (<500 mTorr), O₂ to CO ratio (<10), and temperature (150 °C) regime is consistent with the Langmuir-Hinshelwood reaction mechanism. Our findings provide in-situ surface chemical composition data of the catalytic oxidation of CO on Pt(1 1 0) at total pressures below 1 Torr.     

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465?K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513?K. A change in the front propagation mechanism is proposed to explain the observed effects.     

CO oxidation on Pt(110): scanning tunneling microscopy inside a high-pressure flow reactor

We have used a novel, high-pressure high-temperature scanning tunneling microscope, which is set up as a flow reactor, to determine simultaneously the surface structure and the reactivity of a Pt(110) model catalyst at semirealistic reaction conditions for CO oxidation. By controlled switching from a CO-rich to an O2-rich flow and vice versa, we can reversibly oxidize and reduce the platinum surface. The formation of the surface oxide has a dramatic effect on the CO2 production rate. Our results show that there is a strict one-to-one correspondence between the surface structure and the catalytic activity, and suggest a reaction mechanism which is not observed at low pressures.     

Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe₃O₄-supported Pd model catalyst by molecular beam (MB) methods, time resolved IR reflection absorption spectroscopy (TR-IRAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The well-shaped Pd particles were prepared in situ by metal evaporation and growth under ultrahigh vacuum (UHV) conditions on a well-ordered Fe₃O₄ film on Pt(1 1 1).It is found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (∼10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface, stabilized by the iron-oxide support. Their formation and reduction is fully reversible. Upon decomposition, oxygen is released which migrates back onto the metallic part of the Pd surface. In consequence, the Pd interface oxide layer acts as an oxygen reservoir, the capacity of which by far exceeds the amount of chemisorbed oxygen on the metallic surface.Additionally, Pd surface oxides can also be formed at temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature. This effect is addressed to different onset temperatures for oxidation of the particle facets and sites. It is shown that the presence of Pd surface oxides sensitively modifies the adsorption and reaction properties of the model catalyst, i.e. by lowering the CO adsorption energy and CO oxidation probability. Still, a complete reduction of the Pd surface oxides can be obtained by extended CO exposure, fully reestablishing the metallic Pd surface.     

Studies on self-sustained reaction-rate oscillations: II. The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum

CO oxidation on a platinum foil was studied in a high pressure flow cell (10²−10² Pa) and an UHV chamber (10⁻⁸ −5 × 10⁴ Pa) both interfaced to a surface infrared spectrometer. Real-time surface infrared and calorimetry experiments performed in the cell during oscillatory oxidation indicated a slow periodic variation (∼ 40%) in the number of active sites, the period of which was commensurate with that of the reaction-rate oscillations. Auger spectroscopy performed in the UHV chamber showed that surface carbon quantitatively accounted for the surface deactivation, as evidenced by the inverse correlation of the number of surface sites active towards CO adsorption with the surface carbon concentration and by the demonstration that, at the oscillation temperatures, carbon can diffuse from the bulk to the surface, oxygen can remove surface carbon and adsorbed CO can block carbon diffusion. Although silicon oxide was always detected on the surface with infrared spectroscopy, no periodic variation in it could be observed during the reaction-rate oscillations. Auger studies confirmed that the maximum and the variations in surface concentration of silicon oxide could not account for the variations in the number of active sites. A mechanism is therefore proposed in which carbon is driving the long-period self-sustained oscillations in the rate of CO oxidation on Pt.     

Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions

Experimental studies employing surface science methods indicate that kinetic oscillations, chaos, and pattern formation in heterogeneous catalytic reactions often result from the interplay of rapid chemical reaction steps and relatively slow complementary processes such as oxide formation or adsorbate-induced surface restructuring. In general, the latter processes should be analysed in terms of theory of phase transitions. Therefore, the conventional mean-field reaction–diffusion equations widely used to describe oscillations in homogeneous reactions are strictly speaking not applicable. Under such circumstances, application of the Monte Carlo method becomes almost inevitable. In this review, we discuss the advantages and limitations of employing this technique and show what can be achieved in this way. Attention is focused on Monte Carlo simulations of CO oxidation on (1 0 0) and (1 1 0) single-crystal Pt and polycrystal Pt, Pd and Ir surfaces and of NO reduction by CO and H₂ on Pt(1 0 0). CO oxidation on supported nanometre-sized catalyst particles and NO reduction on composite catalysts are also discussed. The results show that with current computer facilities the MC technique has become an effective tool for analysing temporal oscillations and pattern formation on the nanometre scale in catalytic reactions occurring on both single crystals and supported particles.     

Characterization of oxide impurities on Pt and their effect on the adsorption of CO and H2

The interactions between oxide support materials and Pt have been studied by incorporating silica, alumina, titania, and niobia into the surface of a clean Pt foil. Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of CO and H₂ were used for surface characterization. For all of these oxides, TPD indicated no change in the adsorption properties of CO and H₂. Peak temperatures were unaffected by the presence of oxide impurities. For silica and alumina, AES results indicated that suboxides could be formed after oxidation at 400 and 800 K respectively. Al₂O₃ and SiO₂ were formed at higher temperatures. Relatively large quantities of these oxides were required to substantially decrease the saturation coverages of CO and H₂, indicating that these oxides probably form clusters on the metal surface. For titania and niobia, AES indicated that these oxides dissolved into the Pt above 1300 K, but segregated back to the surface below 500 K. These segregated layers cover the Pt evenly and both oxides completely suppress H₂ and CO adsorption at an oxygen coverage of 1 × 10¹⁵/cm². These results are used to discuss the possible reasons for differences in the catalytic properties of Pt on these four oxide supports.     

使用金属掺杂SnO2(110)作为表面催化剂的电催化CO2还原反应：通过调控Sn:O原子比例来控制产物

电催化CO₂还原反应可以产生HCOOH和CO,目前该反应是将可再生电力转化为化学能存储在燃料中的最有前景的方法之一. SnO₂作为将CO₂转换为HCOOH和CO的良好催化剂,其反应发生的晶面可以是不同的. 其中(110)面的SnO₂非常稳定,易于合成. 通过改变SnO₂(110)的Sn:O原子比例,得到了两种典型的SnO₂薄膜：完全氧化型(符合化学计量)和部分还原型. 本文研究了不同金属(Fe、Co、Ni、Cu、Ru、Rh、Pd、Ag、Os、Ir、Pt和Au)掺杂的SnO₂(110),发现在CO₂还原反应中这些材料的催化活性和选择性是不同的. 所有这些变化都可以通过调控(110)表面中Sn:O原子的比例来控制. 结果表明,化学计量型和部分还原型Cu/Ag掺杂的SnO₂(110)对CO₂还原反应具有不同的选择性. 具体而言,化学计量型的Cu/Ag掺杂的SnO₂(110)倾向于产生CO(g),而部分还原型的表面倾向于产生HCOOH(g). 此外,本文还考虑了CO₂还原的竞争析氢反应. 其中Ru、Rh、Pd、Os、Ir和Pt掺杂的SnO₂(110)催化剂对析氢反应具有较高的活性,其他催化剂对CO₂还原反应具有良好的催化作用.     

Adsorption-induced step formation

Through an interplay between density functional calculations, Monte Carlo simulations and scanning tunneling microscopy experiments, we show that an intermediate coverage of CO on the Pt(110) surface gives rise to a new rough equilibrium structure with more than 50% step atoms. CO is shown to bind so strongly to low-coordinated Pt atoms that it can break Pt-Pt bonds and spontaneously form steps on the surface. It is argued that adsorption-induced step formation may be a general effect, in particular at high gas pressures and temperatures.     

One-dimensional PtO2 at Pt steps: formation and reaction with CO

Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) oxide structure forms at the steps of the Pt(332) surface after exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.     

Imaging surface reactions with a photoemission electron microscope

The photoemission electron microscope (PEEM), invented more than 60 years ago, began being utilized for imaging surface reactions less than 10 years ago to make observations of pattern formation during heterogeneous catalytic CO-oxidation on platinum. First investigations of adsorption and reaction of oxygen on a previously CO-covered Pt surface are discussed, showing the growth of isolated oxygen islands (O-islands) up to millimeter sizes. Features like plane wave-front propagation and a “memory effect” of islands have been observed. Next pattern formation during CO-oxidation on a Pt(110) surface is studied, where both of the reactants are supplied from the gas phase. By creating boundaries on the surface utilizing microlithography single spiral waves can be isolated. Here experiments with reactive boundaries of Pd are shown, where in addition to observations of common pattern formation with CO and oxygen on top of the surface, subsurface oxygen areas become an important feature. The latter also plays a dominant role for the oscillation cycles within a circular Pt(110) domain.     

The structure and stability of surface platinum oxide and of oxides of other noble metals

The formation and stability of surface layers of platinum oxides in platinum single crystals has been studied in ultrahigh vacuum. Low energy electron diffraction (LEED) was used to identify the ordered structures that formed on the surface of Pt(111), Pt(332), and Pt(110). It appears that these structures can be related to hexagonal planes of PtO₂. The cleanliness of the surface was monitored by Auger electron spectroscopy (AES). The presence of impurities like Ca and Si must be avoided as they oxidize preferentially to the Pt. It is shown that the Pt oxide layers are stabilized by the very slow kinetics of oxygen diffusion to the surface which is responsible for the observed long life of the oxide layers under most catalytic reactions that are carried out at temperatures below 500°C. The stability of other oxides of noble metals that have been observed in UHV studies is also reviewed.     

Monte Carlo simulations of FTIR studies during CO oxidation on a Pt/SiO2 catalyst

A Monte Carlo (MC) simulation of the reaction of CO with an oxygen covered Pt surface and oxygen with a CO covered Pt surface is presented in this paper. The effect of the adsorption, desorption, reaction, and surface migration rates on the formation of CO clusters is analyzed in terms of the CO frequency shift in the IR spectrum. The MC simulation calculates the CO frequency shifts according to a dipole-dipole interaction model. The IR frequency shifts predicted by the simulation depend on the value of the various kinetic processes considered. The simulation indicates that the CO migration on the surface is important at low pressure but is inhibited at high pressure. The IR frequency shifts predicted by the simulation agree qualitatively with experimental values obtained during CO oxidation on a Pt catalyst.     

An experimental setup combining a highly sensitive detector for reaction products with a mass-selected cluster source and a low-temperature STM for advanced nanocatalysis measurements

We report on a home-built detector for catalytic reaction measurements offering good gas isolation from the surrounding ultrahigh vacuum components, high sensitivity for reaction products and a fast response time of 10 ms enabling dynamic studies correlated to reactant gas pulses. The device is mounted in ultrahigh vacuum and combined with a low-temperature scanning tunneling microscope and a source for the deposition of mass-selected clusters. This combination allows for a direct correlation between surface morphology and catalytic properties of model catalysts. The performances of the new detector are illustrated by measurements on two model systems. Thermal desorption spectroscopy of CO carried out on morphologically well characterized Pt on TiO₂(110)-(1 × 1) reveals several desorption features, which can be attributed to different surface sites. Catalytic CO oxidation performed by alternatingly pulsing isotopic CO and O₂ on a Pt film on yttria stabilized zirconia reveals the CO or O rich temperature regimes. The CO₂ production rate correlated with either one of the reactants can perfectly be reproduced by a kinetic reaction model giving access to the respective adsorption energies.     

Mechanisms of the catalytic carbon monoxide oxidation on Pt (110)

The kinetics of the carbon monoxide oxidation on a clean Pt (110) crystal were investigated in an ultra-high vacuum system by utilizing Auger electron spectroscopy, low-energy electron diffraction and residual gas analysis. Two different catalytic reaction mechanisms were found to prevail for the experimental conditions chosen. In the temperature range, 100 < T ? 220°C, where essentially CO was preadsorbed on the Pt surface the subsequent adsorption of O₂ was competitive and the reaction exhibited the characteristics of a Langmuir-Hinshelwood mechanism. In this case the onset of the CO₂ formation was delayed by a characteristic time which depended strongly on temperature (“induction period”). A simple model for the Langmuir-Hinshelwood reaction was developed which permitted a more detailed evaluation of the kinetic curves yielding an activation energy for the catalytic reaction of 2.9 kcal/mole. On the other hand, when oxygen was preadsorbed on the Pt surface (T > 90°C) the subsequent reaction with CO occurred immediately and was temperature independent. This behavior was interpreted in terms of an Eley-Rideal mechanism. Both reactions were used for titration of the adsorbed species. From area measurements under the titration curves it was concluded that the saturation coverage for CO and oxygen on Pt(110) is approximately the same.     

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm⁻¹ for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.     

Oxide formation and reduction on rhodium surfaces

The growth of thin oxide layers on Rh and their reduction by CO has been investigated by imaging atom-probe mass spectroscopy and field-ion microscopy. Surface oxides were produced by heating Rh field-emitter tips in oxygen at pressures between 0.01 and 1.0 Torr and temperatures between 400 and 650 K. The oxidized samples were transferred under ultrahigh vacuum to an imaging atom-probe/field-ion microscope for compositional and structural analysis. Oxygen uptake was found to follow a logarithmic law with an initial activation energy of 3.1 kcal/mol. Imaging atom-probe analysis indicated that the oxide formed in 1 Torr O₂ was stoichiometric Rh₂O₃ for temperatures of 500 K and above. The onset pressure for oxidation at 500 K was found to be ≈0.01 Torr, with only a weak pressure dependence in the range from 0.01 to 1 Torr. Field-ion microscope images of the oxide showed ring structures suggestive of epitaxial growth above the (111) plane, and images of the substrate after removal of the oxide indicated that the oxide was thicker above the stepped regions of the surface than above the low-index planes. The oxide was quickly reduced in 1 Torr CO at temperatures above 420 K, and partially-reduced oxides were found to be substoichiometric throughout the oxide region. CO reduction exhibited a much stronger temperature dependence than surface oxide formation indicating a different rate-controlling step for the two processes. The time dependence for CO reduction at 418 K suggested that the COO surface reaction was rate-determining in the reduction process.     

The adsorption of ethylene oxide on Ag(110) and Pt(111): Relevance to selective olefin oxidation

The interactions of ethylene oxide (EtO) with the Ag(110) and Pt(111) surfaces have been studied using XPS, TDS, AES and EELS. On Ag(110), the interaction is very weak, with only molecular desorption observable. The heat of adsorption is ≈ 10.1 kcal mole⁻¹. In contrast, decomposition reactions strongly predominate on Pt(111) at low coverage. Molecular desorption is only seen at high coverages. The heat of adsorption decreases from > 11.9 to 10 kcal mole⁻¹ with increasing coverage. Condensed multilayers desorb at ≈ 140 K. Ultimate decomposition products on Pt(111) include H₂ and CO gas, and carbon residue on the surface. Evidence suggests that adsorbed decomposition intermediates may include atomic hydrogen, CO, acetyl and ethylidyne species, with at least one other, yet unidentified, species. These results imply that, if produced, adsorbed ethylene oxide would be unlikely to escape a reactor containing Pt catalyst without further decomposition reactions. This may help explain the uniqueness of Ag catalysts in ethylene epoxidation.