CO-induced structural changes of Pd particle surfaces

The interaction of CO with Pd particle surfaces has been studied by means by field ion microscopy (FIM). CO-induced structural changes ranging from well defined reconstructions of individual surface planes to a reshaping of the apex crystal have been made visible with atomic resolution. For example, after reaction with CO at 1 hPa and at 300 K, the {011} and {113} surface planes, still covered by CO_ad, are found to exhibit the (1×2) missing row structure in which every second chain of atoms is removed. Other high index planes in the transition region between the low index {111} and {001} planes have dissolved in the presence of adsorbed CO and form facets, the terraces of which again contain the densely packed planes. The adsorbate-induced changes are explained on the basis of earlier mass spectrometric and kinetic investigations by means of atom-probe techniques according to which adsorbed subcarbonyl entities, Pd(CO)_2,3, liberated from kink site positions, diffuse across the surface until they decompose at favourable sites.     

Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

Methane combustion over supported palladium catalysts I. Reactivity and active phase

The performance of Al₂O₃, ZrO₂ and ZrO₂ stabilized with SiO₂ (ZrO₂-s) supported palladium catalysts for the methane combustion was studied between 473 and 873 K. The nature of the surface species of palladium catalysts under reaction conditions were detected by FT-IR and microcalorimetry of CO adsorbed. The different behavior of palladium catalysts under reaction conditions is attributed to support effects associated to differences in thermal conductivity and oxygen mobility of supports. Prereduction of the catalysts enhances their activity. Under reaction conditions, the prereduced sample becomes partially oxidized by preferential adsorption/reaction of oxygen both on Pd (1 1 1) planes and on the sites that can multibondedly adsorb CO. The reconstruction of the metallic particles and the formation of PdO_x (0<x≤1) phase were directly observed by FT-IR and microcalorimetry of adsorbed CO. Combination of different characterization techniques with reaction results suggests that a mixed phase, Pd⁰/PdO_x, is the most active phase for methane combustion, and that a redox mechanism may occur on this phase.     

The role of water in propylene partial oxidation: thermal desorption studies on Ag(110)

The desorption and reactions of propylene and propylene oxide adsorbed on atomic oxygen covered and hydroxyl covered Ag(110) were investigated to elucidate the effect of water on the oxidation of propylene over silver catalysts. Previous studies clearly indicate enhancement of propylene partial oxidation by the addition of water to reactor feed streams. Propylene combustion by oxygen adatoms on Ag(110) is completely passivated by water coadsorption on the oxygen atom covered surface (water adsorption on O-Ag(110) results in hydroxyl groups). The desorption activation energy of propylene and propylene oxide is increased by up to 30% by adsorbed oxygen atoms on Ag(110). The desorption activation energy for propylene and propylene oxide is reduced on the hydroxyl covered surface relative to desorption from atomic oxygen covered Ag(110). These results suggest that the inhibition of deep oxidation plays an important role in the previously observed water enhancement. In addition, the decreased desorption activation energies for both propylene, the reactant, and propylene oxide, the desired product, may influence the selectivity of this complex reaction system. Potential changes in catalytic reactivity and selectivity caused by water addition are discussed in terms of a general catalytic reaction rate law. This revised version was published online in July 2006 with corrections to the Cover Date.     

The dynamics of CO oxidation on Pt(110) studied by infrared chemiluminescence of the product CO2: effect of CO coverage

The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on a Pt(110)(1×2) surface. The vibrational and the rotational states of CO₂ formed on the reconstructed Pt(110)(1×2) surface are more excited than those on the terrace Pt(111) surface. The vibrational state of the product CO₂ strongly depends on the CO coverage: the vibrational temperature (T_V) of the product CO₂ becomes higher, as the coverage of CO increases. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure sensitivity of alcohol reactions on (110) and (111) palladium surfaces

A temperature programmed reaction/desorption (TPD) study of decomposition pathways of methanol, ethanol, 1-propanol and 2-propanol was conducted on the clean Pd(110) surface under ultra-high vacuum conditions. No alcohol underwent C-O scission. Alcohols appear to react on this clean surface via the same dehydrogenation and decarbonylation steps observed on the Pd(111) surface. In contrast to previous reports noting substantial differences in methanol chemistry on the Pt(110) and (111) surfaces, the reactions of methanol and ethanol were found to be the same on the Pd(110) and (111) surfaces, giving rise to H₂ plus CO from methanol, and H₂, CO, and CH₄ from ethanol. The C₃ alcohols, 1- and 2-propanol, did produce somewhat different products on the Pd(110) and (111) surfaces, but these differences can be accounted for by differences in the chemistry of intermediate reaction products, rather than different reaction pathways of the parent alcohols.     

Effect of Carbon Deposits on Reactivity of Supported Pd Model Catalysts

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al₂O₃/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at 550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di- bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.     

Favorable synergetic effects between CuO and the reactive planes of ceria nanorods

High-energy, more reactive {001} and {110} planes of CeO₂ nanorods were found to generate favorable synergetic effects between CuO and ceria, resulting in significant enhancement of the copper catalyst performance for CO oxidation.     

Elucidation of CO2 Formation Mechanism in CO + NO Reaction on Pd(111) and Pd(110) Surfaces Using IR Chemiluminescence Method

The infrared (IR) chemiluminescence technique was applied to steady-state CO oxidation by NO on Pd(111) and Pd(110). From a comparison of IR emission spectra of CO₂ between the CO + NO and CO + O₂ reactions, it was found that the vibrational energy states of CO₂ in the CO + NO reaction were similar to those in the CO + O₂ reaction. This indicates that the reaction path of CO₂ formation in CO + NO is the same as that in CO + O₂, although the vibrational states are very dependent on the surface structure.     

A comparison of the NO–CO reaction over Rh(100), Rh(110) and Rh(111)

Reaction rates and product selectivities were measured over the Rh(100) surface as a function of temperature, and CO and NO partial pressures. These results are compared with our prior studies of the NO–CO reaction on the Rh(111) and Rh(110) surfaces. The only products detected for all three surfaces were CO₂, N₂O, and N₂. Furthermore, for the Rh(100) surface we have found a significant change in the apparent activation energy (E _a) with reaction temperature. For the Rh(100) surface it was found that the E _a can change by a factor of 2.3 in the temperature range investigated here, from 528 to 700 K, with the lower values obtained at higher temperatures. In contrast, E _a's were found to remain constant over the same temperature range for the Rh(110) and Rh(111) surfaces. The results observed for the Rh(100) surface suggest that reaction kinetics are dominated by variations in NO coverages. At low temperatures, the surface is fully saturated with NO, and dissociation is limited by the availability of vacancy sites through NO desorption. At high temperatures, the surface is still primarily covered with NO, however, the number of vacancy sites has increased substantially. In this case, we propose that the apparent activation energy is now reflecting NO dissociation kinetics rather than those for NO desorption. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics and mechanism of NO reduction with CO on Ir surfaces

The adsorption and thermal reactivity of NO and CO and the kinetics of the NO reduction with CO on Ir surfaces were studied using X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, and temperature programmed desorption. The NO adsorption and dissociation activity was strongly dependent on the Ir surface structure. The NO dissociation activity of the Ir planes decreased in the order (100) > (211) ? (111). In contrast, the type of the CO adsorption site was independent of the Ir surface structure. The activity of Ir(111) for N₂ and CO₂ production from the NO + CO reaction was low compared with the activities of Ir(100) and Ir(211). The kinetic data for an Ir/SiO₂ powder catalyst were similar to data obtained for Ir(211). The order of the turnover frequencies for N₂ and CO₂ formation for the Ir planes was in good agreement with the order for NO dissociation activity, and this agreement indicates that the catalytic activity for NO reduction was dependent on NO dissociation. A kinetic study of the elementary steps indicated that the rate-limiting step for NO reduction with CO was the NO dissociation step.     

Cation-Aided Joining of Surfaces of β-Silicon Nitride: Structural and Electronic Aspects

An atomic-scale approach has been applied to the examination of both the physical and electronic structures of stable surfaces of β-Si₃N₄. Sterical constraints prevent the (001) surface from effective chemical reaction with the interface. The theoretical surface-to-surface bonding is investigated by using a periodical tight-binding approach. Based on the interpretation of the density of states, the balance of the number of states and electrons is performed for stoichiometric Si₃N₄, ideal N-terminated (110) surfaces, oxygen-overlayered (110) slabs, and the metal monolayer with which the slabs are brought into contact. The stable electronic configuration, which is attained when the cation binds to the interface, represents the electronic driving force behind the diffusion of the additive and/or impurity atoms toward grain boundaries. The different bonding propensities of the (001) and (110) surfaces imply that effective bonding of the planes parallel to the c -direction to the interphase restrains the crystal from growth in the lateral direction. Conversely, geometry-constrained bonding of the (001) planes allows the crystal growth that produces the rod-shaped β-grains.     

Vibrationally Excited CO2 Formed in CO + NO Reaction on Pd(110) Surface in High Surface Temperature Range

The infrared chemiluminescence spectra of CO₂ formed during steady-state CO+NO reaction over Pd(110) indicated that the temperature of the bending vibrational mode was much higher than that of the antisymmetric one at higher surface temperatures such as 800–850 K. Especially, in the high temperature range, more vibrationally excited CO₂ was formed from CO+NO reaction than CO+O₂ reaction. On the basis of the result, we propose the model structure of reaction intermediates for CO₂ formation in CO+NO reaction, which is different from that in CO+O₂ reaction.     

In situ FTIR studies of methanol adsorption and dehydrogenation over Cu/SiO2 catalyst

In situ FTIR spectroscopy was used to identify the adsorbed species and the intermediates during methanol dehydrogenation over Cu/SiO₂ catalyst, and a schematic reaction network was proposed. Methoxy species on copper, which were derived from adsorbed methanol, dehydrogenated into formaldehyde. Then several competitive pathways took place. The adsorbed formaldehyde could desorb to the gas phase, or react with another adsorbed methoxy group to form methyl formate, and/or undergo further dehydrogenation to CO and H₂. Carbon monoxide formed from the decomposition first adsorbed on high-index planes of copper, and then on low-index planes as the reaction progressed. With the increase of temperature, the concentration of formaldehyde and CO in gas phase increased, and that of methyl formate decreased.     

Fracture Initiation in the Directionally Solidified NiO-CaO Eutectic

Initiation of fracture in the directionally solidified, lamellar NiO-CaO eutectic was examined using the indentation fracture technique. Fracture could not be induced along the interphase boundary on transverse sections of the directionally solidified eutectic. Instead, radial cracks evolved at angles of approximately 35° and 55° with respect to the lamellar interface and were consistent with median/radial crack formation on (TlO) and (001) planes, respectively. Indentation of single-crystal NiO resulted in fracture only along {110} planes. Crack initiation on {110} planes in both materials was attributed to a dislocation coalescence model proposed by Keh et al. while crack formation on (001) planes in the eutectic was believed to be initiated by a Stroh-type mechanism involving dislocation pile-ups. Variations in the interlamellar spacing resulted in a Hall-Petch-type behavior for the hardness but had little effect on the fracture toughness of the eutectic.     

Acetate formation and explosive decomposition during ethanol oxidation on Rh

The adsorption and reaction of ethanol with the Rh(110) surface has been studied using a thermal molecular beam system and temperature programmed desorption. On the clean surface, ethanol shows a very simple dehydrogenation, producing hydrogen in the gas phase, adsorbed CO (which is desorbed by heating to 550 K) and carbon. Since in alcohol synthesis reactions it is likely that the surface will be partially oxidised, the reaction with predosed oxygen was also investigated. The reaction pathway then becomes much more complex. The main changes are (i) CH₄ and H₂O evolution during adsorption, and (ii)Acetate formation by oxygen insertion in the molecule. The acetate shows very unusual decomposition kinetics — a surface explosion with a very narrow peak-yielding CO₂ and H₂ in the gas phase and adsorbed C. The acetate is always seen on Rh catalysts which are selective for alcohol synthesis from CO and H₂, and it is proposed that oxidic promoters such as vanadia may act to stabilise this intermediate.     

Catalytic CO Oxidation on Individual (110) Domains of a Polycrystalline Pt Foil: Local Reaction Kinetics by PEEM

Abstract  

The reaction kinetics of catalytic CO oxidation on individual grains of a polycrystalline Pt foil has been studied simultaneously by photoemission electron microscopy (PEEM) and mass spectroscopy (MS), in the pressure range ~10⁻⁵ mbar. By processing the video-PEEM images of ongoing catalytic reaction, the kinetic transitions were tracked for individual [110]-oriented domains. The obtained local kinetic phase diagrams were contrasted to those obtained from global MS activity measurements. These data and the observation of reaction front propagation on different Pt(110) domains indicate a quasi-independent behaviour of the crystallographic domains. The observed front propagation velocities and the degree of their anisotropy on Pt foil corroborate earlier observations on Pt(110) single crystals, confirming our concept of using Pt foil to monitor and compare different surface terminations in parallel.     

Kinetics and Active Surfaces for CO Oxidation on Pt-Group Metals Under Oxygen Rich Conditions

This mini review summaries recent works on identifying the active surfaces for CO oxidation on Pd, Pt, and Rh under oxygen rich conditions. A significantly high reaction rate for CO oxidation under oxygen rich conditions has been observed. Results using in situ characterization methods of ambient scanning tunneling microscope, surface X-ray diffraction, ambient pressure X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and infrared reflection adsorption spectroscopy (IRAS), were included. Most X-ray related methods reveal that the achievements of the high reaction rates for CO oxidation on Pd, Pt, and Rh under oxygen rich conditions are accompanied with the appearance of oxides on the surface, leading to that the oxide phase is considered to be the active surface. In contrast, recent in situ IRAS results conclude that a chemisorbed oxygen covered metallic surface is the active surface. Kinetic data support that the reaction on the metallic surfaces can reach the high rate, e.g. a mass-transfer limit turnover frequency, without the necessity of the presence of oxide. Therefore, we point out that the appearance of oxides on Pt-group metals during CO oxidation is possibly due to the transfer-limit of CO gas, resulting in exposing the catalyst surface to an ambient atmosphere much richer in oxygen and thus building-up the oxide. Moreover, photons in X-ray related experiments may aid to overcome the formation barrier of oxide on a chemisorbed oxygen covered metallic surface. The formation of oxide is also affected by the mass-transfer properties of the in situ reaction cells. If the amount of incoming CO molecules under the mass-transfer limit of CO is high enough, the build-up of oxide may be precluded being consumed by reacting with CO.