CO Surface Coverage Research Articles

Screening effects in surface infrared spectra of CO adsorbed on electrodes: application to isothermal desorption measurements

Infrared spectra of CO adsorbed on platinum electrodes were examined for effects of dielectric screening. Screening of the incident infrared radiation field by the adlayer polarization typically results in non-linear relationships between adsorbed CO vibrational band intensities and CO surface coverages. It is shown that the solvent in the electrochemical environment can reduce the strong Beer’s law deviations observed for adsorbed CO in vacuum by maintaining the adlayer polarizability more constant with changes in CO surface coverage. The screening analysis was applied to correct adsorbed CO infrared peak absorbances in measurements of rate constants for CO desorption from electrodes.

Macroscopic and mesoscopic characterization of a bistable reaction system: CO oxidation on Pt(111) surface

The catalytic oxidation of CO by oxygen on a platinum (111) single-crystal surface in a gas-flow reactor follows the Langmuir–Hinshelwood reaction mechanism. It exhibits two macroscopic stable steady states (low reactivity: CO-covered surface; high reactivity: O-covered surface), as determined by mass spectrometry. Unlike other Pt and Pd surface orientations no temporal and spatiotemporal oscillations are formed. Accordingly, CO+O/Pt(111) can be considered as one of the least complicated heterogeneous reaction systems. We measured both the macroscopic and mesoscopic reaction behavior by mass spectrometry and photoelectron emission microscopy (PEEM), respectively, and explored especially the region of the phase transition between low and high reactivity. We followed the rate-dependent width of an observed hysteresis in the reactivity and the kinetics of nucleation and growth of individual oxygen and CO islands using the PEEM technique. We were able to adjust conditions of the external control parameters which totally inhibited the motion of the reaction/diffusion front. By systematic variation of these conditions we could pinpoint a whole region of external control parameters in which the reaction/diffusion front does not move. Parallel model calculations suggest that the front is actually pinned by surface defects. In summary, our experiments and simulation reveal the existence of an “experimental” bistable region inside the “computed” bistable region of the reactivity diagram (S-shaped curve) leading to a novel dollar ($)-shaped curve.

In situ detection and surface coverage measurements of CO during CO oxidation on polycrystalline platinum using sum frequency generation

Optical infrared-visible sum-frequency generation (SFG) surface vibrational spectroscopy was used for in situ detection of CO during heterogeneous CO oxidation on a polycrystalline platinum foil. SFG spectra were recorded in the substrate temperature range 300- 700 K at a total pres- sure of 20 mbar. A least-squares fit procedure was employed to determine the temperature dependence of the vibrationally nonresonant contribution and the frequency, amplitude and halfwidth of the CO vibrationally resonant contribution from the SFG spectra. Thermal desorption measurements were car- ried out to investigate the relationship between the SFG signal and the instantaneous CO surface coverage. After these cal- ibration measurements, it was possible to derive CO surface coverages from the SFG spectra measured under laminar flow conditions during heterogeneous CO oxidation.

Imaging surface reactions with a photoemission electron microscope

Abstract 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.

A photoemission electron microscopy (PEEM) study of the effect of surface acoustic waves on catalytic CO oxidation over Pt{110}

A photoemission electron microscope (PEEM) was used to study the influence of surface acoustic waves (SAWs) on the reactive behaviour of adsorbate structures during CO oxidation over Pt{110} at temperatures around 485 K and CO and O 2 pressures around 10 −5 and 10 −4 mbar, respectively. It was found that surface acoustic wave excitation can suppress and reverse the process of CO poisoning of the catalyst, and thus bring the system back into the highly reactive regime. From work function measurements of the CO-covered surface during acoustic excitation it is concluded that the observed effect is due to acoustically induced CO desorption.

Reactive scattering of Cs + from chemisorbed molecules on a Ni(100) surface. Secondary neutral mass spectrometry with a hyperthermal ion beam

Reactive scattering of hyperthermal Cs + ions is studied from a Ni(100) surface chemisorbed with CO, OH, and benzene. The products of reactive scattering, CsCO +, CSOH +, and CsC 6H 6 +, reflect molecular identity of the chemisorbed species. It is demonstrated that this method can be used to monitor adsorption kinetics of the adsorbates in real-time. The reaction dynamics is investigated through measurement of reaction yields as a function of adsorbate surface coverage, Cs + beam exposure, and beam incidence angle. Cs + collision induces desorption of CO from a CO-covered surface, with a desorption cross section of 2.4(± 1.4) × 10 −15 cm 2 molecule −1 for a 30 eV beam incident at 45°. The desorbing CO molecules are combined to the scattered Cs + ions with an association probability of 1.0(± 0.6) × 10 −3. This number indicates that a hyperthermal Cs + ion provides an efficient means of cationizing a collisionally desorbed neutral molecule. From angular dependence study, it is suggested that Cs + ions scatter from a Ni(100) surface to a specular direction with a sharp angular distribution. The desorbing CO molecules have an angular distribution that is peaked more probably toward a specular direction than the surface normal.

Chemical Evolution in Preprotostellar and Protostellar Cores

The chemistry of developing and collapsing low-mass protostellar cores is followed using a chemical code with a time-varying density. Two evolutionary scenarios are represented, gravitational collapse in the presence of magnetic fields and the slow core growth by accretion near equilibrium. The chemical code includes gas-phase reactions and depletion onto grains with both CO and H2O ice mantles. We find that various species will selectively deplete from the gas phase at times that correspond to the middle to late stages of dynamical evolution when the densities are highest. These depletions do not depend in detail on the dynamical solution and should exist for any centrally condensed density profile. Sulfur-bearing molecules are particularly sensitive to the density increase: CS, SO, and C2S show significant depletions both on a strongly bound water mantle and on the weakly bound CO-covered grain surface. In contrast, CO and HCO+ show large depletions only for an H2O grain mantle and remain in the gas phase for models with CO grain mantles. Two species, NH3 and N2H+, do not deplete from the gas phase for the densities considered in our models. We also find that for very high densities, nH2>106 cm-3, depletion becomes important for all molecules. The effects of coupling chemistry and dynamics on the resulting physical evolution are discussed. We compare our results with current high-resolution observations of preprotostellar cores and to more evolved objects and suggest that ratios of the abundances of few species can be used in concert with our models as sensitive discriminators between different stages of core and star formation.

Catalytic ignition in the CO[sbnd]O2 reaction on platinum: experiment and simulations

Catalytic ignition temperatures for the CO+O2 reaction on a Pt wire, and the corresponding transient ignition curves, have been measured and analyzed theoretically for nonflammable reactant mixtures in Ar and He at atmospheric pressure. At very low relative CO concentrations (< 0.02%), the experimental ignition temperature rises rapidly with increasing concentration, and then more slowly at higher concentrations. These data are explained by employing a kinetic model based on the standard three-step mechanism for CO oxidation on Pt(111), combined with time-dependent mass and heat transport. The simulations reproduce both the dependence of ignition temperatures on the reactant concentrations and the transient behavior after ignition without any kinetic fitting parameters. The model predicts that ignition at intermediate CO concentrations is entirely governed by the interplay between the reaction kinetics on the CO-covered surface and the heat transport. At very low CO/O2 ratios, CO concentration gradients are established in the pre-ignition regime, which facilitates ignition. After ignition, but before the mass-transport controlled steady-state is established, a kinetic phase transition occurs from a CO-covered to an oxygen-covered surface.

Adsorbed Species and Reaction Rates for NO–CO over Rh(111)

We have studied the reaction of NO with CO over a Rh(111) catalyst by monitoring the infrared (IR) intensity of surface CO and NO at various partial pressures of NO (PNO), CO (PCO), and sample temperatures (T). Reaction rates for the products (CO2, N2O, and N2) were previously measured at the same conditions in our apparatus [J. Phys. Chem.99, 16344 (1995)]. Surface coverages were observed both atPNO=8 Torr, where the reaction yields mostly N2O and the selectivity is insensitive to eitherPCO(1 to 40 Torr) orT(<673 K), and atPNO=0.8 Torr, where the major product changes from N2O below 635 K to N2above 635 K. Changes in the surface coverages of NO and CO correlate well with the observed changes in N2O selectivity. Below 635 K, where N2O formation is favored, NO dominates the surface. Above 635 K, where N2formation is preferred, CO is the majority surface species. Our IR data support a model in which product N2O and N2are formed as adsorbed N reacts with either adsorbed NO or N, respectively. In an additional kinetic experiment, we used isotopically labeled N2O to show that gas phase N2O is not an intermediate to N2formation at 648 K—this helps to rule out an alternative model. In our IR experiments, two types of spectra were obtained with a Fourier transform IR spectrometer. Polarization spectra were obtained using two fixed polarizers, one before the sample, oriented to pass approximately equal amounts ofs- andp-polarized light, and one after the sample that selecteds- orp-polarized light. Spectra obtained withs- andp-polarized light were ratioed. We also obtained conventional reflectance absorbance IR spectra. In a separate calibration experiment with NO on Rh(111) in ultrahigh vacuum we observed five IR bands at 1448, 1530, 1590, 1643, and 1693 cm−1. We attribute all of these to bridging NO, most likely at threefold hollow sites. As NO coverage increases up to saturation, the distribution of population among these bands (and their individual frequencies) gradually changes—the average vibrational frequency of the NO increases from ∼1440 to ∼1650cm−1. From the linear dependence of the integrated IR absorption on NO coverage we find that the vibrational polarizability of the adsorbed NO is 0.20±0.02 Å3, a factor 6.5 larger than for free NO. This increase, proportionally larger than for CO on transition metal surfaces, is explained by the facile charge transfer that accompanies donor–acceptor bonding between NO and Rh(111).

In situ Optical Second Harmonic Generation Study of Electrochemical Oxidation of Formaldehyde on a Polycrystalline Platinum Electrode

Abstract In situ optical second harmonic generation(SHG) technique was employed to investigate the electrochemical oxidation of formaldehyde on a polycrystalline Pt electrode. At 532 nm excitation, the SH signal generated at a Pt surface seems to be enhanced by the existence of adsorbed CO and can be related to the surface coverage of CO(a) which cannot be determined by the electrochemical methods. Thus, the potential dependence of the SH signal provides useful information to understand the mechanism of this reaction.

Surface diffusion of carbon monoxide and potassium coadsorbed on Ru(001): Confirmation of a 1:1 CO:K trapping interaction

The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, ΘCO, was less than the potassium coverage, ΘK. The CO surface mobility increased dramatically when ΘCO≳ΘK. At ΘK=0.10 ML and 315 K, the CO diffusion coefficient was DCO&amp;lt;4×10−10 cm2/s for ΘCO&amp;lt;0.10 ML and increased to DCO=2.5×10−7 cm2/s for ΘCO≳0.10 ML. At ΘK=0.25 ML and 315 K, the CO surface diffusion coefficient was DCO&amp;lt;4×10−10 cm2/s for ΘCO&amp;lt;0.25 ML and increased to DCO=5×10−9 cm2/s for ΘCO≳0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both ΘK=0.10 ML and ΘK=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) techniques. At ΘK=0.10 ML and CO coverages ranging from ΘCO=0.06 ML to ΘCO=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with ΘK=0.10 ML and ΘK=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO–K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results.

Dissociative adsorption dynamics of formaldehyde on a platinum electrode surface; one-dimensional domino?

When a potentiostatically controlled polycrystalline platinum electrode is dipped into a sulfuric acid aqueous solution containing formaldehyde, the surface is covered by CO, which is formed as a result of the dissociative adsorption of formaldehyde. The kinetics of this dissociative adsorption process is investigated by the combination of the electrochemical and the laser desorption methods. The surface CO coverage is linearly increased as a function of time. This zeroth order kinetics is explained by the domino mechanism.

Synergistic alloying behaviour of Pd 50Cu 50 single crystals upon adsorption and co-adsorption of CO and NO

We have studied the surface reactivity of the Pd 50Cu 50 (111) and (110) single crystals in the adsorption and the co-adsorption of CO and NO at low temperature. The bond strengths of CO and No on Pd and Cu in the alloy are significantly different from those observed on pure metals. Carbon monoxide and nitric oxide are more tightly bound to Cu and less tightly bound to Pd than to the pure components. The calculated NO coverages on PdCu samples are approximately four times lower than for CO due to the dissociation of a fraction of NO on the surfaces. However, the CO or NO adsorption did not suggest an important difference of reactivity between the two surfaces studied. In the co-adsorption experiments, the total surface coverage of CO and NO on Pd 50Cu 50 (111) and (110) appears quite dependent on the order in which the gases are adsorbed in the successive adsorptions. The pre-saturation with NO leads to molecular adsorbed states and irreversible dissociated species which are not removed upon an exposure to CO. The subsequent CO co-adsorption induced only a slight replacement of molecular NO by CO and the amount of adsorbed CO is quite low ( θ CO=0.06 ML) as compared to the coverage measured with CO alone ( θ CO=0.40 ML). The pre-saturation with CO gives a different behaviour, the subsequent co-adsorption of NO leads to the replacement of CO by NO up to the saturation coverage found for NO alone ( θ NO=0.10 ML). Finally, when NO is adsorbed on a partially CO covered surface, NO displaces CO up to around 0.15 ML but the total amount of surface species (0.30 ML) is lower than the amount (0.55 ML) of pre-adsorbed CO, indicating a strong displacement of CO by NO. The competitive adsorption of CO and NO on PdCu surfaces is discussed in terms of dissociation of NO at low temperature which partially inhibits the further adsorption of CO on the PdCu alloy surfaces.

Terrace width effect on adsorbate vibrations: a comparison of Pt(335) and Pt(112) for chemisorption of CO

The adsorption of CO on the stepped Pt single crystals Pt(335) (four-atom wide terraces) and Pt(112) (three atom wide terraces) has been studied by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Pt(335) and Pt(112) have similar terrace and step site structures (terrace-(111); step-(100)). Similar CO adsorption and desorption kinetics related to step and terrace sites were observed in the TPD studies from the two crystals. In contrast, IRAS shows different CO adsorption behavior. On Pt(335), four absorbance bands of CO were observed at high coverages. They are terminal CO on the step and terrace sites (νCO ≈ 1900cm−1), and bridge CO on the step and terrace sites (νCO ≈ 1800cm−1). In contrast, on Pt(112), step bound and terrace bound terminal CO can not be resolved easily by infrared spectroscopy. In addition, on Pt(112), based on relative IRAS intensities, only a small surface coverage of bridge CO develops compared to Pt(335). Rather different properties of Pt(335) and Pt(112) single crystals which expose four and three-atom wide terraces, respectively, are thus revealed by IRAS studies using CO as a probe. These differences result from a number of factors including the electrostatic field difference, the terrace width difference, and the differences in average CO ... CO coupling distances for CO molecules on step and terrace sites. The Pt(335) crystal with the wider terraces more closely resembles Pt(111) than the more highly stepped Pt(112) crystal, insofar as CO chemisorption is concerned. The results give insight into the properties of the step defect sites and the effect of step spacing on adsorbates.

Effects of Carrier Doping on Kinetic Parameters of CO2 Hydrogenation on Supported Rhodium Catalysts

A study of CO2 hydrogenation on Rh/TiO2 catalysts and of the influence of doping the TiO2 carrier with W6+ cations on kinetic parameters has been conducted. It is found that while the reaction order with respect to H2 and CO2 partial pressures does not significantly change, the specific activity of CO2 methanation is enhanced by up to 24 times upon doping the TiO2 support with less than 1 at.% W6+ cations. A reduction of the apparent activation energy by ca. 25-34 kJ/mol and improvement of the stability of the catalytic performance are also observed by carrier doping. The greatly increased activity caused by doping is found not to be related to the SMSI phenomenon but to enhanced hydrogen adsorption capacity and weakening of the Rh-CO bond, as revealed by H2 chemisorption measurements and infrared spectroscopic studies. The rate of gaseous CO formation is also found to be affected by carrier doping to a significant extent, via the alteration of surface CO coverage and of the strength of the MCO bond. The intrinsic reason for the dependence of catalytic parameters upon doping of the carrier is discussed with respect to alterations of the electronic structure of the support upon doping, and electronic interactions between rhodium crystallites and the doped carrier.

Semi-empirical calculations of the vibrational frequency of carbon monoxide adsorbed on noble metal single-crystal surfaces

In this work we perform a theoretical analysis of the shift of the observed IR reflection—absorption spectroscopy (IRRAS) band of CO adsorbed on Pt(111), Rh(111) and Pt(100) single crystals as a function of both the degree of CO surface coverage and the electric potential applied at either the metal—vacuum or the metal—solution interface. The wavefunctions obtained using a modified extended Hückel molecular orbital (EHMO) method are used to predict the vibrational frequency data. The Pt(110), Rh(111) and Pt(111) single crystals are modelled by bilayer clusters of 25 and 22 atoms respectively. A theoretical description of the observed IRRAS shift is analysed by means of a population analysis of CO molecular orbitals, which confirms the donation—back-donation model.

Solitary-wave phenomena in an excitable surface reaction.

A simple reaction-diffusion model for the CO oxidation on Pt(110), exhibiting among other features bistability and oscillations, is studied in an excitable parameter region where traveling solitary pulses with high O coverage may be formed on an otherwise CO-covered surface. If it is assumed that small patches are structurally imperfect, effects of wave splitting of a pulse, as well as partial annihilation and solitonlike behavior, result upon collision of two pulses, in agreement with experimental observations.

Reactivity of CO on vanadium-promoted rhodium catalysts as studied with transient techniques

The effect of vanadium promotion on the kinetics of CO methanation on silica-based rhodium catalysts was probed. Transient model experiments employing temperature-programmed surface reaction spectroscopy and pulse surface reaction rate analysis were used to unravel changes in rates of elementary reaction steps. CO dissociation is the rate-limiting step in the overall methanation reaction. The rate of CO dissociation is found to be enhanced by vanadium. The activation energy is lowered from 90 kJ/mol for Rh/SiO 2 to 65 kJ/mol for the promoted catalyst. The activation energy for CO dissociation is not dependent on the hydrogen partial pressure, while it is a strong function of the CO surface coverage. CO adsorption at temperatures above 250°C results in very reactive surface carbonaceous intermediates. Even at −15°C methane formation was observed upon hydrogenation. This reactive surface carbon can be incorporated in higher hydrocarbons. This process is favoured by vanadium promotion.

Kinetics of carbon monoxide adsorption on nickel(110) studied in real time by ion-stimulated desorption of neutral molecules

The adsorption of CO on Ni(110) has been studied in situ using a low-current pulsed Ar + beam, followed by laser ionization of the desorbed neutral CO species and reflecting time-of-flight mass spectrometry of the resulting CO + photoions. Sincethe fractional surface coverage of CO is monitored in real time, adsorption kinetics are conveniently studied even at temperatures where adsorption and desorption occur concurrently and equilibrium coverages are below saturation

Kinetic study of gas-surface molecular exchange between CO/CO on metals

Isotopic switching technique has been used to study the exchange of gaseous CO* with chemisorbed CO molecules on metal surface. The kinetic curves observed on Re(0001) exhibit a nonlinear dependence of desorption rate on the surface coverage of CO. However, the desorption rate for CO on Rh(111) in the same temperature range is strictly linear with the coverage of CO. A concerted adsorption-desorption mechanism is postulated which emphasizes that the desorption of CO in enhanced by the pressure of gas phase CO* on the one hand, and may be retarded by the screening effect of the CO* molecule chemisorbed on the surface on the other hand. It is pointed out that the adsorption of CO* (or CO) and the desorption of CO (or CO*) on a surface at high coverage, i.e. the molecular exchange processes, are primarily taking place simultaneously and concertedly.