High CO Coverage Research Articles

Dissociative Adsorption of O2 on Clean and CO-Precovered Pt Surfaces

ABSTRACTIt is generally accepted that CO oxidation on transition metals follows a Langmuir-Hinshelwood mechanism. The oxidation reaction takes place in two sequential steps where the oxygen molecule first dissociates into atomic oxygen and then reacts with an adsorbed CO to form CO2. One critical question concerning the reaction kinetics under high pressure is the probability of oxygen dissociation on a highly CO covered surface. On bare transition metal surfaces, molecularly adsorbed oxygen readily dissociates with little or no apparent activation barrier. In industrial diesel engine catalysis, the metal surface is initially packed with CO. Subsequent reactions such as oxygen dissociation must take place on a CO covered surface. In this paper, we performed density functional theory (DFT) calculations for O2 dissociation on Pt(111) in the presence of different CO adsorption environments. While several stable O2 molecular precursor states (top-bridge-top, top-fcc-bridge, and top-hcp-bridge) exist on a clean Pt(111) surface, these precursors become endothermic beyond a critical CO coverage of ∼0.44 ML. Furthermore, the reaction path for CO oxidation via dissociated atomic oxygen becomes less favorable at higher CO coverage, primarily due to competitive adsorption and lateral repulsion. It was found that the oxygen dissociation barrier and the binding energies of atomic oxygen are well correlated via the Evans-Polanyi relationship.

Theoretical study of pattern formation during the catalytic oxidation of CO on Pt{100} at low pressures

Theoretical studies have thus far been unable to model pattern formation during the reaction in this system on physically feasible length and time scales. In this paper, we derive a computational reaction-diffusion model for this system in which most of the input parameters have been determined experimentally. We model the surface on a mesoscopic scale intermediate between the microscopic size of CO islands and the macroscopic length scale of pattern formation. In agreement with experimental investigations [M. Eiswirth et al., Z. Phys. Chem., Neue Folge 144, 59 (1985)], the results from our model divide the CO and O(2) partial pressure parameter space into three regions defined by the level of CO coverage or the presence of sustained oscillations. We see CO fronts moving into oxygen-covered regions, with the 1 x 1 to hex phase change occurring at the leading edge. There are also traveling waves consisting of successive oxygen and CO fronts that move into areas of relatively high CO coverage, and in this case, the phase change is more gradual and of lower amplitude. The propagation speed of these reaction waves is similar to those observed experimentally for CO and oxygen fronts [H. H. Rotermund et al., J. Chem. Phys. 91, 4942 (1989); H. H. Rotermund et al., Nature (London) 343, 355 (1990); J. Lauterbach and H. H. Rotermund, Surf. Sci. 311, 231 (1994)]. In the two-dimensional version of our model, the traveling waves take the form of target patterns emitted from surface inhomogeneities.

Lifting of Ir{100} reconstruction by CO adsorption: An ab initio study

The adsorption of CO on unreconstructed and reconstructed Ir{100} has been studied, using a combination of density functional theory and thermodynamics, to determine the relative stability of the two phases as a function of CO coverage, temperature, and pressure. We obtain good agreement with experimental data. At zero temperature, the (5 x 1) reconstruction becomes less stable than the unreconstructed (1 x 1) surface when the CO coverage exceeds a critical value of 0.09 ML. The interaction between CO molecules is found to be weakly repulsive on the reconstructed surface but attractive on the unreconstructed, explaining the experimental observation of high CO coverage on growing (1 x 1) islands. At all temperatures and pressures, we find only two possible stable states: 0.5 ML CO c(2 x 2) overlayer on the (1 x 1) substrate and the clean (5 x 1) reconstructed surface.

Kinetics of the water–gas shift reaction on Pt catalysts supported on alumina and ceria

We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO2, 22% H2O, 37.3% H2, and 25.4% Ar) at a total pressure of 1atm and in the temperature range of 180–345°C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate (r) on Pt/alumina catalysts as a function of the forward rate (rf) was found to be: r=rf(1−β), where rf=kf[CO]0.1[H2O]1.0[CO2]−0.1[H2]−0.5, kf is the forward rate constant, β=([CO2][H2])/(Keq[CO][H2O]) is the approach to equilibrium, and Keq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO2 and H2. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H2 and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study.

Response of a model of CO oxidation with CO desorption and diffusion to a periodic external CO pressure

We present a study of the dynamical behavior of a Ziff–Gulari–Barshad model with CO desorption and lateral diffusion. Depending on the values of the desorption and diffusion parameters, the system presents a discontinuous phase transition between low and high CO coverage phases. We calculate several points on the coexistence curve between these phases. Inclusion of the diffusion term produces a significant increase in the CO 2 production rate. We further applied a square-wave periodic pressure variation of the partial CO pressure with parameters that can be tuned to modify the catalytic activity. Contrary to the diffusion-free case, this driven system does not present a further enhancement of the catalytic activity, beyond the increase induced by the diffusion under constant CO pressure.

IR Chemiluminescence Study of CO2 Formed during Steady-State CO Oxidation on Pt(110) and Pt(111) Surfaces

The infrared (IR) chemiluminescence spectra of CO2 molecules were measured during the steady-state CO oxidation 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 (θCO<0.2) or at high surface temperatures (TS≥700 K). This can be related to the activated complex of CO2 formation in a more bent form on the declined (111) terraces of the Pt(110)(1×2) structure. On the other hand, at high CO coverage (θCO≥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)(1×2) surface to the (1×1) form with high CO coverage.

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.

Structure of Activated Complex of CO2 Formation in a CO + O2 Reaction on Pd(110) and Pd(111)

Examinations of CO2 formed during steady-state CO oxidation reactions were performed using infrared (IR) chemiluminescence. The CO2 was formed using a molecular-beam method over Pd(110) and Pd(111). The CO2 formation rate is temperature dependent under various partial pressure conditions. The temperature of the maximum formation rate is denoted as TSmax. Analyses of IR emission spectra at surface temperatures higher than TSmax showed that the average vibrational temperature (TVAV) is higher for Pd(111) than for Pd(110). The antisymmetric vibrational temperature (TVAS) is almost equal on both surfaces. These results suggest that the activated CO2 complex is more bent on Pd(111) and straighter on Pd(110). Furthermore, the difference in the TVAV value was small for surface temperatures less than TSmax. The TVAS value was much higher than TVAV on both surfaces. These phenomena were observed only when the surface temperature was lower than TSmax: they became more pronounced at lower temperatures, suggesting that the activated complex of CO2 formation is much straighter on both Pd surfaces than that observed at higher surface temperatures. Combined with kinetic results, the higher CO coverage at the lower surface temperatures is inferred to be related to the linear activated complex of CO2 formation.

Infrared chemiluminescence study of CO + O 2 reaction on Pd(1 1 0): Activated complex of CO 2 formation at high CO coverage

The infrared chemiluminescence study of CO 2 formed during steady-state CO + O 2 reaction over Pd(1 1 0) under various CO/O 2 pressure ratios was carried out. Highly excited antisymmetric vibrational mode compared to bending vibrational mode was observed under reaction conditions at high CO coverage. This can be related to the activated complex of CO 2 formation in more linear form.

Decay of metastable phases in a model for the catalytic oxidation of CO

We study by kinetic Monte Carlo simulations the dynamic behavior of a Ziff-Gulari-Barshad model with CO desorption for the reaction CO + O-->CO(2) on a catalytic surface. Finite-size scaling analysis of the fluctuations and the fourth-order order-parameter cumulant show that below a critical CO desorption rate, the model exhibits a nonequilibrium first-order phase transition between low and high CO coverage phases. We calculate several points on the coexistence curve. We also measure the metastable lifetimes associated with the transition from the low CO coverage phase to the high CO coverage phase, and vice versa. Our results indicate that the transition process follows a mechanism very similar to the decay of metastable phases associated with equilibrium first-order phase transitions and can be described by the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by nucleation and growth. In the present case, the desorption parameter plays the role of temperature, and the distance to the coexistence curve plays the role of an external field or supersaturation. We identify two distinct regimes, depending on whether the system is far from or close to the coexistence curve, in which the statistical properties and the system-size dependence of the lifetimes are different, corresponding to multidroplet or single-droplet decay, respectively. The crossover between the two regimes approaches the coexistence curve logarithmically with system size, analogous to the behavior of the crossover between multidroplet and single-droplet metastable decay near an equilibrium first-order phase transition.

Desorption of products in 193 nm photo-induced reactions in (O2+ CO) adlayers on Pt(112)

The spatial distributions of desorbing products were examined in 193 nm photo-induced reactions in O2 + CO adlayers on stepped Pt(112) = [(s)3(111) x (001)]. At high coverage of O2(a) and CO(a), both O2 and CO2 desorption collimated closely along the (111) terrace normal. The results were compared with those in thermal CO oxidation, and the origin of the collimation angle shift in the latter is discussed. On the other hand, at low CO(a) coverage, O2 and CO2 desorption collimated in inclined ways in the plane along the surface trough. At these collimation positions, the kinetic energy of desorbing 02 and CO2 was maximal, confirming the hot-atom collision mechanism.

Defect Enhanced Carbon Monoxide Oxidation at Elevated Oxygen Pressures on a Pt/Al2O3 Thin Film

Carbon monoxide oxidation has been studied, using in situ soft X-ray techniques and ultrahigh vacuum (UHV) temperature-programmed reaction spectroscopy (TPRS), on a well-characterized supported platinum thin film over the100-1000 K temperature range and in pressures ranging from UHV to 0.01 Torr. In high oxygen pressures the reaction is enhanced by oxidation at defect sites. Temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) studies of preadsorbed CO coverages heated in pressures of flowing oxygen, ranging from I x 10 - 5 to I x 10 - 2 Torr, have been performed. Unlike with the Pt(111) surface, the onset temperature for oxidation of a saturated coverage of CO on the platinum thin film decreases dramatically with increasing oxygen pressure. The CO oxidation onset temperature decreased from 340 K in 1 x 10 - 5 Torr of flowing oxygen to 230 K in 1 x 10 - 2 Torr of flowing oxygen. Therefore, oxidation is not limited by CO desorption for high CO coverages, and reactions at defect sites control the rate of CO oxidation at high oxygen pressures. Isothermal oxidation experiments in 0.002 Torr of flowing oxygen have yielded an activation energy of 8.5 kcal/mol. TPRS of coadsorbed carbon monoxide and oxygen has indicated a primary oxidation channel at 350 K corresponding to the oxidation of CO on Pt( 111) terrace sites and minor oxidation channels above 400 K. The thin Pt/Al 2 O 3 film has been characterized using both chemical and structural methods. CO and O 2 temperature-programmed desorption (TPD) experiments indicate the Pt film surface is similar to Pt(111), but with a higher defect site concentration. Plane-view and cross-section transmission electron microscopy (TEM) experiments have confirmed a 100 A Pt film consisting of ∼450 A, well-ordered grains, as indicated by local area electron diffraction patterns, with the (111) orientation parallel to the substrate.

Spatial Distributions of Desorbing Products in 193 nm Photo-induced O2+CO Reaction on Pt(112)

The angular distributions of desorbing products were examined in 193 nm photo-induced reactions of the O2 + CO adlayers on stepped Pt(112) = [(s) 3(111)×(001)]. At high coverage of O2(a) and CO(a), the product (CO2) desorption collimated closely the (111) terrace normal, where the translational temperature was maximized to 3240 K. On the other hand, at low coverage, the CO2 desorption collimated in highly inclined ways in the plane along the surface troughs, where the translational temperature was enhanced, confirming the hot atom mechanism. These results were compared with those in thermal CO oxidation and the origin of the collimation angle shift in the latter was discussed.

A vibrational spectroscopic investigation of the CO+O2 reaction on Pt{110}

The CO coverage of a Pt{110} surface in both the high and low reaction rate branches of the bistable CO oxidation reaction has been determined by Infrared Reflection-Absorption Spectroscopy (IRAS), first performing extensive calibration experiments on the various factors determining the absorbance and frequency associated with the C–O vibrational stretching mode. The same two states of the surface are shown to be present under steady-state low and high reaction rates and when the surface is undergoing pattern formation and homogeneous reaction rate oscillations. Using the CO coverages determined by IRAS, the intensities observed in a series of photoelectron emission microscopy images have been used to elucidate the oxygen coverage in both coadsorption states. The low reaction rate branch is found to be associated with a high CO coverage (0.5±0.1 ML) and very low O coverage (0.03±0.01 ML) consistent with the (1×1) unreconstructed phase. In the high rate branch the surface has a low CO coverage (0.05±0.03 ML) and O coverages in the range 0.3–0.7 ML [(1×2) reconstructed phase]. No evidence for bridged CO, oxide, or subsurface oxygen, variously proposed to play a role in the reaction rate bistability, was found under the conditions measured. These findings are consistent with the site blocking and reconstruction model. Coadsorption experiments of CO and oxygen under nonreactive conditions, performed as part of the IRAS calibration process, demonstrate that CO and O can occupy a mixed adlayer and identify two different chemical environments for CO adsorption.

Sum frequency generation vibrational spectroscopy at solid–gas interfaces: CO adsorption on Pd model catalysts at ambient pressure

Carbon monoxide adsorption on Pd(1 1 1) and Pd nanoparticles supported by Al 2O 3/NiAl(1 1 0) was examined by vibrational sum frequency generation spectroscopy from 10 −8 to 1000 mbar, and from 100 to 400 K. Identical CO saturation structures were observed on Pd(1 1 1) under ultrahigh vacuum (∼10 −7 mbar, 95 K) and at high pressure (e.g. ⩾1 mbar, 190 K) with no indications of pressure-induced surface rearrangements. Special attention was paid to experimental artifacts that may occur under elevated pressure and may be misinterpreted as “high pressure effects”. Vibrational spectra of CO on defect-rich Pd(1 1 1) exhibited an additional peak that originated from CO bound to defect (step or edge) sites. The CO adsorbate structure on supported Pd nanoparticles was different from Pd(1 1 1) but more similar to stepped Pd(1 1 1). At low pressure (10 −7 mbar CO) the adsorbate structure depended strongly on the Pd morphology revealing specific differences in the adsorption properties of supported nanoparticles and single crystal surfaces. At high pressure (e.g. 200 mbar CO) these differences were even more pronounced. Prominent high coverage CO structures on Pd(1 1 1) could not be established on Pd particles. However, in spite of structural differences between well faceted and rough Pd nanoparticles nearly identical adsorption site occupancies were observed in both cases at 200 mbar CO. Initial tests of the catalytic activity of Pd/Al 2O 3/NiAl(1 1 0) for ethylene hydrogenation at 1 bar revealed a remarkable activity and stability of the model system with catalytic properties similar to impregnated catalysts.

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NOx by hydrogen under lean burn conditions over Pt/Al2O3 is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NOx conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al2O3. In this case, although pure H2 and pure CO are ineffective for NOx reduction under lean burn conditions, H2/CO mixtures are very effective. With a realistic (1:3) H2:CO ratio, typical of actual exhaust gas, Pd/Al2O3 is significantly more active than Pt/Al2O3, delivering 45% NOx conversion at 160°C, compared to >15% for Pt/Al2O3 under identical conditions. The nature of the support is also critically important, with Pd/Al2O3 being much more active than Pd/SiO2. Possible mechanisms for the improved performance of Pd/Al2O3 in the presence of H2+CO are discussed.

CO dissociation on Mo(1 1 0) studied by high-resolution core-level spectroscopy

We have studied the CO/Mo(1 1 0) adsorption system at different sample temperatures by high-resolution core-level spectroscopy and near-edge X-ray-absorption fine-structure measurements. We demonstrate that CO molecules having different tilt angles with respect to the surface normal may be assigned to different components of the C 1s core-level spectra. The tilt of the CO molecules is shown to depend on the CO coverage. The thermally induced dissociation of CO is studied and is shown to differ at low and high initial CO coverages. At low coverage CO dissociation starts already at temperatures as low as 125 K whereas at saturation CO coverage no dissociation occurs below 280 K. The reasons for this difference in dissociation behavior are discussed and it is concluded that steric effects play a large role.

Reaction kinetics on supported model catalysts: Molecular beam/in situtime-resolved infrared reflection absorption spectroscopy study of the CO oxidation on alumina supported Pd particles

By combining molecular beam techniques and time-resolved infrared reflection absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation under ultrahigh vacuum conditions onto a well-ordered alumina film, prepared on a NiAl(110) single crystal. Particle size, density, and structure of the Pd deposits have been characterized in previous studies. In the low temperature region, transient and steady-state experiments have been performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beams on the sample surface. We determine the steady-state CO2 production rate as a function of the CO fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites under steady-state conditions is monitored by in situ infrared spectroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 production after switching off the CO beam. For the model catalyst investigated, detailed reaction rate measurements in combination with TR-IRAS show that the origin of the particular transient behavior of the supported model system is not due to the presence of specific adsorption sites on small particles, as has been proposed previously. Instead, we suggest that the transient behavior is a consequence of the inhibition of the dissociative adsorption of O2 at high CO coverage. Additionally, the inherent heterogeneity of the supported particle system can enhance the observed effect.

Chemical crosstalk between heated gas microsensor elements operating in close proximity

Gas microsensor arrays often have closely-spaced elements, typically separated by hundreds of microns. For such devices, crosstalk between elements operated within a gaseous environment is a concern because sensing materials held at elevated temperatures have an increased probability for disrupting gas flows and activating gas-film interactions that can consume analytes or evolve reaction products. To explore such effects in a microarray, microhotplate array platforms were used to sense carbon monoxide. Carbon monoxide sensing was chosen as a model system because the oscillatory kinetics of CO oxidation on Pt films are known to exhibit sensitive gas-phase coupling. Under proper conditions, a Pt/SnO 2 microsensor was observed to oscillate between two stable CO/O 2 coverage ratios. The high CO coverage state results in higher film conductance, and the oscillation frequency is extremely sensitive to gas-phase CO concentration. Crosstalk was observed between adjacent microsensors with Pt particles supported on SnO 2 films, as evidenced by synchronization of sensor response and mixed-mode oscillations (similar to those observed in CO oxidation on Pt films). The range of crosstalk effects was studied by operating a single device in an oscillatory CO sensing mode and heating neighboring elements in an array. The operation of nearby sensors is believed to produce reactions that effectively lower the CO partial pressure at the monitored device, thereby reducing the period of oscillation. The magnitude of the effect was calculated from the frequency change using a CO concentration calibration, and the effect is greater than 10% when operating 15 neighboring sensors. The effects of heating neighboring microsensors have also been studied for hydrogen and methanol sensing on both Pt/SnO 2 and bare SnO 2 microsensor arrays. While crosstalk effects are observed for these gases, the effects we observe on Pt/SnO 2 sensors are less pronounced than in the case of CO sensing. A less than 1% effect occurs for methanol sensing and the largest effect for H 2 sensing was an apparent concentration decrease of ≈4% when heating 15 neighboring devices in 10 μmol/mol (10 ppm) of the analyte. On the bare SnO 2 devices, we observe an apparent increase in concentration of methanol and H 2 of approximately 5 and 15%, respectively. While crosstalk is only measured for the case of conductometric sensing in this work, similar phenomena are likely to occur for sensors that utilize other detection principles.

Formation of Ca2+(CO)3 Complexes during Low-Temperature CO Adsorption on CaNaY Zeolite

Adsorption of CO on CaNaY at 85 K results in formation of three types of calcium carbonyl complexes. At high CO coverage (equilibrium CO pressures above ca. 100 Pa) Ca2+(CO)3 species, characterized...