Steady-state CO Research Articles

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.

Allocation of reserve‐derived and currently assimilated carbon and nitrogen in seedlings of Helianthus annuus under subambient and elevated CO2 growth conditions

Here, we analysed the transition from heterotrophic to autotrophic growth of the epigeal species sunflower (Helianthus annuus), and how transition is affected by CO(2). Growth analysis and steady-state (13)CO(2)/(12)CO(2) and (15)NO(3) (-)/(14)NO(3) (-) labelling were used to quantify reserve- and current assimilation-derived carbon (C) and nitrogen (N) allocation to shoots and roots in the presence of 200 and 1,000 micromol CO(2) mol(-1) air. Growth was not influenced by CO(2) until cotyledons unfolded. Then, C accumulation at elevated CO(2) increased to a rate 2-2.5 times higher than in sub-ambient CO(2) due to increased unit leaf rate (+120%) and leaf expansion (+60%). CO(2) had no effect on mobilization and allocation of reserve-derived C and N, even during the transition period. Export of autotrophic C from cotyledons began immediately following the onset of photosynthetic activity, serving roots and shoots near-simultaneously. Allocation of autotrophic C to shoots was increased at sub-ambient CO(2). The synchrony in transition from heterotrophic to autotrophic supply for different sinks in sunflower contrasts with the sequential transition reported for species with hypogeal germination.

Transient and steady state CO oxidation kinetics on nanolithographically prepared supported Pd model catalysts: Experiments and simulations

Applying molecular-beam methods to a nanolithographically prepared planar PdSiO2 model catalyst, we have performed a detailed study of the kinetics of CO oxidation. The model catalyst was prepared by electron-beam lithography, allowing individual control of particle size and position. The sample was structurally characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the kinetic experiments, the O-rich and CO-rich regimes were investigated systematically with respect to their transient and steady-state behaviors, both under bistable and monostable reaction conditions. Separate molecular beams were used in order to supply the reactants, allowing individual control over the reactant fluxes. The desorbing CO2 was detected by both angle-resolved and angle-integrated mass spectrometries. The experimental results were analyzed using different types of microkinetic models, including a detailed reaction-diffusion model, which takes into account the structural parameters of the catalyst as well as scattering of the reactants from the support. The model quantitatively reproduces the results as a function of the reactant fluxes and the surface temperature. Various kinetic effects observed are discussed in detail on the basis of the model. Specifically, it is shown that under conditions of limited oxygen mobility, the switching behavior between the kinetic regimes is largely driven by the surface mobility of CO.

Low temperature and low pressure CO oxidation on gold clusters supported on MgO(100)

CO oxidation has been investigated on Au/MgO(100) model catalysts at RT and low pressure. The gold particles prepared by UHV evaporation on clean MgO surfaces are characterized by HRTEM. The gold particles are FCC single crystals or multiple twins with five-fold symmetry. Infrared spectroscopy indicates that two types of adsorption sites are present which correspond to loosely and strongly bound CO. The equilibrium CO coverage for the strongly bound CO is smaller than 0.1 ML. CO titration experiments show that oxygen does not dissociate on the gold nanoparticles. The CO oxidation reaction is studied at RT by molecular beam methods. A steady state CO reaction probability up to 0.50 is measured, for the first time at low pressure, for gold particles with a mean size of 1.5 nm. A reaction mechanism is proposed in which CO adsorbed on low coordinated gold atoms reacts with oxygen adsorbed molecularly, possibly at the Au/MgO interface.

Structure-Activity Correlations for the Oxidation of CO over Polycrystalline RuO2 Powder Derived from Steady-State and Transient Kinetic Experiments

Abstract The oxidation of carbon monoxide was studied at atmospheric pressure in a plug-flow reactor over polycrystalline ruthenium dioxide powder in the temperature range from 363 to 453 K as a function of the pretreatment. Calcining RuO2 in flowing oxygen resulted in purified bulk RuO2, whereas reduction in hydrogen led to bulk Ru metal, which was partially oxidized again in flowing oxygen at increasing temperatures (T ox) up to 573 K to obtain RuO2/Ru shell-core particles with increasing RuO2 shell thickness. Using the TPR technique subsequent to steady-state CO oxidation to monitor the degree of oxidation, the most active and stable state of the unsupported ruthenium catalysts was identified as an ultra-thin RuO2 layer covering a metallic Ru core in agreement with the shell-core model established for supported Ru catalysts. Steady-state turnover frequencies (TOFs) obtained with the ultra-thin RuO2 films are in good agreement with TOFs reported for studies on Ru single crystal surfaces and with supported Ru catalysts. Only for RuO2 films thicker than 1 nm (T ox ≥ 473 K) and for fully oxidized RuO2 deactivation was observed, presumably due to the formation of inactive RuO2 surfaces such as the RuO2(100)-c(2×2) facet. Moreover, it was demonstrated that the presence of moisture in the reactant feed inhibits the oxidation of CO completely.

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.

Oxidation of CO in H 2–CO mixtures catalyzed by platinum: alkali effects on rates and selectivity

Turnover rates and selectivities for CO oxidation in H 2–CO–O 2 reactant mixtures are much greater on Pt clusters supported on Cs-modified SiO 2 than on clusters of similar size dispersed on SiO 2 or Al 2O 3 supports. Selectivity effects reflect the inhibition of spillover-mediated H 2 oxidation pathways on support surfaces. Infrared spectra showed that Cs, Rb, or Na titrates support hydroxyl groups required for these unselective pathways. These H 2 oxidation pathways are unaffected by competitive CO co-adsorption and can lead to significant selectivity losses during preferential CO oxidation in H 2-rich streams. CO oxidation selectivities on Pt are above 90%, even at H 2/CO ratios of ∼100; much lower selectivities reported previously reflect contributions from spillover-mediated H 2 oxidation pathways on supports, which are not inhibited by CO co-reactants. Cs and Rb also increased turnover rates for monofunctional oxidation of CO on Pt cluster surfaces, apparently by disrupting the growth of chemisorbed CO monolayers, which prevent activation of O 2 co-reactants. The addition of 1.6 Cs/nm 2 to SiO 2 led to CO oxidation turnover rates more than 10 times higher than those on Pt/SiO 2 catalysts with similar Pt dispersion. CO monolayer growth processes are evident from a gradual decrease in CO oxidation rates during the initial stages of H 2–CO–O 2 reactions, which occurs simultaneously with an increase in intensity for chemisorbed CO infrared bands measured during reaction. This densification of chemisorbed CO monolayers is consistent with the recovery of initial rates by thermal treatment in inert streams and with the observed monotonic increase in CO oxidation selectivity as catalysts gradually approach steady-state CO coverages during reaction. Alkali appears to disrupt CO monolayer growth by promoting the formation of unreactive chemisorbed carbon via CO dissociation and disproportionation reactions, the rates of which increase when alkali is added to Pt surfaces. Chemisorbed carbon blocks some active Pt surface atoms, but also introduces obstacles that inhibit the formation of dense CO monolayers, thus retaining exposed Pt atoms required for the activation of O 2 co-reactants. Rb and Cs cations, which are more electropositive than Na cations, increase CO oxidation turnover rates more strongly, consistent with the proposed mechanisms for the electronic promotion effects of alkali on CO dissociation.

Experimental and theoretical study on high temperature induced changes in chlorophyll a fluorescence oscillations in barley leaves upon 2 % CO<sub>2</sub>

Oscillations in many of photosynthetic quantities with a period of about 1 min can be routinely measured with higher plant leaves after perturbation of the steady state by sudden change in gas phase. Among all hypotheses suggested so far to explain the oscillations, an effect of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activation status to control the oscillations is highly probable, at least upon high temperature (HT) treatment when in vivo RuBPCO activity controlled by RuBPCO activase (RuBPCO-A) decreases. Therefore, we measured the oscillations in fluorescence signal coming from barley leaves (Hordeum vulgare L. cv. Akcent) after their exposure for various time intervals to different HTs in darkness. We also evaluated steady state fluorescence and CO2 exchange parameters to have an insight to functions of electron transport chain within thylakoid membrane and Calvin cycle before initiation of the oscillations. The changes in period of the oscillations induced by moderate HT (up to 43 °C) best correlated with changes in non-photochemical fluorescence quenching (qN) that in turn correlated with changes in gross photosynthetic rate (PG) and rate of RuBPCO activation (kact). Therefore, we suggest that changes in period of the oscillations caused by moderate HT are mainly controlled by RuBPCO activation status. For more severe HT (45 °C), the oscillations disappeared which was probably caused by an insufficient formation of NADPH by electron transport chain within thylakoid membrane as judged from a decrease in photochemical fluorescence quenching (qP). Suggestions made on the basis of experimental data were verified by theoretical simulations of the oscillations based on a model of Calvin cycle and by means of a control analysis of the model.

Soil properties differently influence estimates of soil CO2 efflux from three chamber-based measurement systems

Soil CO2 efflux is a major component of net ecosystem productivity (NEP) of forest systems. Combining data from multiple researchers for larger-scale modeling and assessment will only be valid if their methodologies provide directly comparable results. We conducted a series of laboratory and field tests to assess the presence and magnitude of soil CO2 efflux measurement system × environment interactions. Laboratory comparisons were made with a dynamic, steady-state CO2 flux generation apparatus, wherein gas diffusion drove flux without creating pressure differentials through three artificial soil media of varying air-filled porosity. Under these conditions, two closed systems (Li-6400-09 and SRC-1) exhibited errors that were dependent on physical properties of the artificial media. The open system (ACES) underestimated CO2 flux. However, unlike the two other systems, the ACES results could be corrected with a single calibration equation that was unaffected by physical differences in artificial media. Both scale and rank changes occurred among the measurement systems across four sites. Our work clearly shows that soil CO2 efflux measurement system × environment interactions do occur and can substantially impact estimates of soil CO2 efflux. Until reliable calibration techniques are developed and applied, such interactions make direct comparison of published rates, and C budgets estimated using such rates, difficult.

Unraveling the Surface Reactions during Liquid-Phase Oxidation of Benzyl Alcohol on Pd/Al2O3: an in Situ ATR−IR Study

The palladium-catalyzed liquid-phase reaction of benzyl alcohol to benzaldehyde was investigated in the presence and absence of oxygen by attenuated total reflection infrared (ATR-IR) spectroscopy. The 5 wt % Pd/Al2O3 catalyst was fixed in a flow-through ATR-IR cell serving as a continuous-flow reactor. The reaction conditions (cyclohexane solvent, 323 K, 1 bar) were set in the range commonly applied in the heterogeneous catalytic aerobic oxidation of alcohols. The in situ ATR-IR study of the solid-liquid interface revealed a complex reaction network, including dehydrogenation of benzyl alcohol to benzaldehyde, decarbonylation of benzaldehyde, oxidation of hydrogen and CO on Pd, and formation of benzoic acid catalyzed by both Pd and Al2O3. Continuous formation of CO and its oxidative removal by air resulted in significant steady-state CO coverage of Pd during oxidation of benzyl alcohol. Unexpectedly, benzoic acid formed already in the early stage of the reaction and adsorbed strongly (irreversibly) on the basic sites of Al2O3 and thus remained undetectable in the effluent. This observation questions the reliability of product distributions conventionally determined from the liquid phase. The occurrence of the hydrogenolysis of the C-O bond of benzyl alcohol and formation of toluene indicates that Pd was present in a reduced state (Pd0) even in the presence of oxygen, in agreement with the dehydrogenation mechanism of alcohol oxidation.

Integrated characterization of the human chemoreflex system controlling ventilation, using an equilibrium diagram.

The chemoreflex system controlling ventilation consists of two subsystems, i.e., the central controller (controlling element), and peripheral plant (controlled element). We developed an integral framework to quantitatively characterize individual ventilatory regulation by experimental determination of an equilibrium diagram using a modified metabolic hyperbola and the CO2 response curve. In 13 healthy males, the steady-state arterial CO2 pressure (P(a)CO2) and minute ventilation (V(E)) were measured. To characterize the central controller, we changed fraction of inspired CO2 (0, 3.5, 5 and 6% CO2 in 80% oxygen with nitrogen balance) and measured the P(a)CO2-V(E) relation. To characterize the peripheral plant, we altered V(E) by hyper- or hypoventilation using a visual feedback method, which made it possible to control both tidal volume and breathing frequency, and measured the VE-P(a)CO2 relation. The intersection between the two relationship lines gives the operating point. The relationship between P(a)CO2 and V(E) for the central controller was reasonably linear in each subject (r2 = 0.808-0.995). The peripheral plant approximated a modified metabolic hyperbolic curve (r = 0.962-0.996). The operating points of the system estimated from the two relationship lines were in good agreement with those measured under the closed-loop condition. The gain of the central controller was 1.9 (1.0) l min(-1) mmHg(-1) and that of the peripheral plant was 3.0 (0.5) mmHg l(-1) min(-1). The total loop gain, the product of the two gains, was 5.3 (2.5). We conclude that human ventilatory regulation by the respiratory chemoreflex system can be quantitatively characterized using an equilibrium diagram. This framework should be useful for understanding the mechanisms responsible for abnormal ventilation under various pathophysiological conditions.

Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O 2 concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature. As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO 2 production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO 2 peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO 2 production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO 2 production rate drops abruptly as the reaction “quenches”. Catalyst layer thickness was changed over the range 0.06–1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO 2 production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.

Vadose Zone Remediation of Carbon Dioxide Leakage from Geologic Carbon Dioxide Sequestration Sites

In the unlikely event that CO2 leakage from deep geologic CO2 sequestration sites reaches the vadose zone, remediation measures for removing the CO2 gas plume may have to be undertaken. Carbon dioxide leakage plumes are similar in many ways to volatile organic compound (VOC) vapor plumes, and the same remediation approaches are applicable. We present here numerical simulation results of passive and active remediation strategies for CO2 leakage plumes in the vadose zone. The starting time for the remediation scenarios is assumed to be after a steady-state CO2 leakage plume is established in the vadose zone, and the source of this plume has been cut off. We consider first passive remediation, both with and without barometric pumping. Next, we consider active methods involving extraction wells in both vertical and horizontal configurations. To compare the effectiveness of the various remediation strategies, we define a half-life of the CO2 plume as a convenient measure of the CO2 removal rate. For CO2 removal by passive remediation approaches such as barometric pumping, thicker vadose zones generally require longer remediation times. However, for the case of a thin vadose zone where a significant fraction of the CO2 plume mass resides within the high liquid saturation region near the water table, the half-life of the CO2 plume without barometric pumping is longer than for somewhat thicker vadose zones. As for active strategies, results show that a combination of horizontal and vertical wells is the most effective among the strategies investigated, as the performance of commonly used multiple vertical wells was not investigated.

Low-dose acetazolamide reduces the hypoxic ventilatory response in the anesthetized cat.

Low intravenous dose acetazolamide causes a decrease in steady-state CO(2) sensitivity of both the peripheral and central chemoreflex loops. The effect, however, on the steady-state hypoxic response is unknown. In the present study, we measured the effect of 4 mg x kg(-1) acetazolamide (i.v.) on the isocapnic steady-state hypoxic response in anesthetized cats. Before and after acetazolamide administration, the eucapnic steady-state hypoxic response in these animals was measured by varying inspiratory P(O2) levels to achieve steady-state Pa(O2) levels between hyperoxia Pa(O2) approximately 55 kPa, approximately 412 mmHg) and hypoxia (Pa(O2) approximately 7 kPa, approximately 53 mmHg). The hypoxic ventilatory response was described by the exponential function V(I) = G exp (-DP(o2) + A with an overall hypoxic sensitivity G, a shape parameter D and ventilation during hyperoxia A. Acetazolamide significantly reduced G from 3.057 +/- 1.616 to 1.573 +/- 0.8361 min(-1) (mean +/- S D). Parameter A increased from 0.903 +/- 0.257 to 1.193 +/- 0.321 min(-1), while D remained unchanged. The decrease in overall hypoxic sensitivity by acetazolamide is probably mediated by an inhibitory effect on the carotid bodies and may have clinical significance in the treatment of sleep apneas, particularly those cases that are associated with an increased ventilatory sensitivity to oxygen and/or carbon dioxide.

Modified quantification of cerebral blood flow and metabolic rate of oxygen with 15O-CO 2 and PET

Objectives: To shorten the scanning protocol for quantification of cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2), a new method has been developed using 15O-CO 2, CO, O 2 and PET. Methods: Fifteen patients with carotid occlusion or stenosis were employed. One-minute inhalation of 15O-CO and 3-min static scanning and blood sampling was performed to measure cerebral blood volume (CBV). With 2-min inhalation of 3 GBq 15O-CO 2, serial dynamic scanning and arterial blood sampling was performed for 6 min. With 15-min inhalation of 15O-O 2, steady state O 2 image was scanned for 5 min, and arterial blood and plasma were sampled to measure oxygen extraction fraction (OEF) and CMRO2. In addition, to compare with conventional method, steady state CO 2 image was scanned for 5 min with 15-min inhalation of 7.5 GBq 15O-CO 2 and arterial blood and plasma sampling. CBF and partition coefficient of water in the brain were estimated by non-linear curve fitting to the tissue time-activity curves. A new method was developed to measure OEF and CMRO2 using the known ratio of arterial H 2O and O 2 concentration in O 2 steady state. The method does not require measuring the plasma counts. For each subject, about 2000 regions of interest (ROIs) were positioned, estimates calculated from a new method (newCBF, newOEF, newCMRO2) were correlated with those from conventional steady state method (ssCBF, ssOEF, ssCMRO2). Results: newCBF, newOEF and newCMRO2 presented significant correlation with those from steady state method, respectively (newCBF (ml/min/100 g)=1.07 ssCBF (ml/min/100 g)+3.17, r=0.934, p<0.001; newOEF=0.77 ssOEF−0.01, r=0.707, p<0.001; newCMRO2 (ml/min/100 g)=0.95 ssCMRO2 (ml/min/100 g)+0.04, r=0.856, p<0.001). The length of the scanning period was reduced 20 min and cut to 35 min for each patient to perform the new protocol study to measure CBF, OEF and CMRO2 without arterial plasma sampling. New method was able to reduce 4.5 GBq of 15O-CO 2 inhalation compared with the steady state method. Conclusion: We have developed a new method that permits quantitative measurement of CBF, OEF and CMRO2 with a short period acquisition time without arterial plasma sampling. The method also reduces radiation exposure of patients. This protocol may be of great value in clinical brain PET using 15O gas.

Contribution of current carbon assimilation in supplying root exudates of Lolium perenne measured using steady-state C labelling.

Coupling growth of Lolium perenne L. in sterile solution culture with steady-state (13)CO(2) labelling allowed quantification of the contribution of C, assimilated either before or after a specific time point, both to plant compartments and root exudates. Plants were grown for 27 days in atmospheres containing CO(2) with delta(13)C signatures of either -13.5 or -36.1 per thousand. Air supplies to plants were then reciprocally switched to the opposing signature (day 0), plants were destructively harvested and root exudates collected over the next 8 days. Following the switch, C assimilated after day 0 and transported to the roots initially only appeared in root tips, later appearing in both tip and non-tip material. The delta(13)C signature of the root exudate changed exponentially with time. Assimilation pre- and post-day 0 contributed equally to exudate C at 4.5 days. Beyond day 8, assimilation pre-day 0 still contributed 41.7% of exudate C. Over all 8 days, a linear relationship existed between the delta(13)C signatures of root tips and exudate, suggesting that all newly assimilated C in the exudate was from root tips. Results imply pulse-labelling approaches to study root exudates are discriminative in the sources of exudates labelled and in the sites from which exudation occurs.

Carbon monoxide dehydrogenase from Rhodospirillum rubrum: effect of redox potential on catalysis.

The Ni-Fe-S-containing C-cluster of carbon monoxide dehydrogenases is the active site for catalyzing the reversible oxidation of CO to CO(2). This cluster can be stabilized in redox states designated C(ox), C(red1), C(int), and C(red2). What had until recently been the best-supported mechanism of catalysis involves a one-electron reductive activation of C(ox) to C(red1) and a catalytic cycle in which the C(red1) state binds and oxidizes CO, forming C(red2) and releasing CO(2). Recent experiments cast doubt on this mechanism, as they imply that activation requires reducing the C-cluster to a state more reduced than C(red1). In the current study, redox titration and stopped-flow kinetic experiments were performed to assess the previous results and conclusions. Problems in previous methods were identified, and related experiments for which such problems were eliminated or minimized afforded significantly different results. In contrast to the previous study, activation did not correlate with reduction of Fe-S clusters in the enzyme, suggesting that the potential required for activation was milder than that required to reduce these clusters (i.e., E(0)(act) > -420 mV vs SHE). Using enzyme preactivated in solutions that were poised at various potentials, lag phases were observed prior to reaching steady-state CO oxidation activities. Fits of the Nernst equation to the corresponding lag-vs-potential plot yielded a midpoint potential of -150 +/- 50 mV. This value probably reflects E degrees ' for the C(ox)/C(red1) couple, and it suggests that C(red1) is indeed active in catalysis.

CO 2 desorption dynamics on specified sites and surface phase transitions of Pt(110) in steady-state CO oxidation

The spatial and velocity distributions of desorbing product CO2 were studied in the steady-state CO oxidation on Pt(110) by cross-correlation time-of-flight techniques. The surface structure transformation was monitored by LEED in the course of the catalyzed reaction. In the active region, where the surface was highly reconstructed into the missing-row form, CO2 desorption split into two directional lobes collimated along 25° from the surface normal in the plane including the [001] direction, indicating the CO2 formation on inclined (111) terraces. The translational temperature was maximized at the collimation angle, reaching about 1900 K. On the other hand, CO2 desorption sharply collimated along the surface normal at CO pressures where (1×2) domains disappeared. The distribution change from an inclined desorption to a normally directed one was abrupt at the CO pressure where the half-order LEED spot already disappeared. This switching point was more sensitive than LEED towards the complete transformation from (1×2) to (1×1) and was then used to construct a surface phase diagram for working reaction sites in the pressure range from 1×10−7 Torr to 1×10−4 Torr of oxygen. The turnover frequency of CO2 formation was enhanced on (1×2) domains with increasing CO pressure.

II. Mechanisms of Elevated Temperature Methanol Electro-oxidation and Poisoning on Pt/C-Nafion Catalyst Layers

Half-cell measurements using fuel cell catalysts at elevated temperatures were made in a pulsed reactant electrochemical flow cell. Kinetics of methanol and the poisoning adlayer electro-oxidation were investigated on 20% Pt/Vulcan C-Nafion catalyst layers at 95°C. Transient measurement of the adlayer coverage during methanol electro-oxidation at 0.30 and 95°C showed that the surface reached a steady-state adlayer coverage after 30 s. Almost no change in adlayer coverage or current occurred from 30 to 300 s, indicating that steady-state conditions were maintained at 95°C. With increasing potential from 0.30-0.40 in 0.015 M methanol, the steady-state CO coverage decreased from 0.44 to 0.28 monolayers, while the steady-state current increased from 3.3 to 29 mA/mg Pt. The kinetics of CO adlayer oxidation were determined independently over this potential range for adlayers prepared by repeated, 60 s periods of methanol oxidation. The initial rates of adlayer electro-oxidation were 80-90% of the steady-state methanol electro-oxidation rates throughout the potential range. This indicates high selectivity through the CO adlayer at 95°C. Poisoning adlayer electro-oxidation occurred with faster kinetics on Pt(110) facets relative to Pt(100) facets. Dissociative adsorption of methanol at 0.05 and 95°C was also studied. The adlayer coverage reached 0.36 monolayers after a 60 s exposure to 0.015 M methanol, indicating that the Pt/C electrocatalyst reacts with methanol under these conditions. © 2003 The Electrochemical Society. All rights reserved.

Kinetic phase transitions in the reaction CO+O→CO2 on Ir(111) surfaces

The oxidation of CO on Ir(111) surfaces was investigated under UHV conditions in the temperature range 360 K to 700 K by CO2 rate measurements utilizing mass spectroscopy. Steady-state CO2 rates were measured at constant total CO+O2 gas flux and variable gas composition (YCO=Y, YO2=1−Y) using mass flow controllers which allowed changes in the CO/O2 gas composition down to 0.1%. Between 360 K and 450 K the CO2 rates initially increase proportional to Y (T&amp;lt;400 K) or to Y1.5 (420 K&amp;lt;T&amp;lt;450 K) and exhibit a sudden drop to a negligible value at a temperature-dependent critical value Y*. The rate drop indicates a kinetic phase transition induced by CO poisoning of the surface. This behavior is similar to the features described by the ZGB and more recently developed lattice gas (LG) models of the CO+O reaction on surfaces. However, in contrast to the ZGB model but in accordance with LG models and experimental results on other platinum metal surfaces, no oxygen poisoning was observed at small Y, i.e., the surface was reactive even at the lowest attainable values of Y. Between 450 K and 530 K the initial CO2 rates remain proportional to Y1.5 up to critical Y* values but the kinetic phase transition softens due to the onset of CO desorption. Accordingly, CO poisoning is not complete and the CO2 rates do not attain the zero level beyond the transition. Above 530 K a kinetic phase transition is no longer seen since substantial CO desorption prevents poisoning, in accordance with conclusions from LG modeling. The kinetic phase transitions, their dependence on Y and temperature, and the measured CO2 rates can be excellently reproduced by simple kinetic modeling.