Infrared chemiluminescence study of CO2 formation in CO + NO reaction on Pd(110) and Pd(111) surfaces

Infrared (IR) chemiluminescence studies of CO2 formed during steady-state CO + NO reaction over Pd(110) and Pd(111) surfaces were carried out. Kinetics of the CO + NO reaction were studied over Pd(110) using a molecular-beam reaction system in the pressure range of 10-2-10-1 Torr. The activity of the CO + NO reaction on Pd(110) was much higher than that of Pd(111), which was quite different from the result of other experiments under a higher pressure range. On the basis of the experimental data on the dependence of the reaction rate on CO and NO pressures and the reaction rate constants obtained by using a reaction model, the coverage of NO, CO, N, and O was calculated under various flux conditions. From the analysis of IR emission spectra in the CO + O2 reaction on Pd(110) and Pd(111), the antisymmetric vibrational temperature (TVAS) was seen to be higher than the bending vibrational temperature (TVB) on Pd(110). In contrast, TVB was higher than TVAS on Pd(111). These behaviors suggest that the activated complex for CO2 formation is more bent on Pd(111) than that on Pd(110), which is reflected by the surface structure. Both TVB and TVAS for the CO + O2 reaction on Pd(110) and Pd(111) increased gradually with increasing surface temperature (TS). On the other hand, in the case of the CO + NO reaction on Pd(110) and Pd(111), TVAS decreased and TVB increased significantly with increasing TS. TVB was lower than TVAS at lower TS, while TVB was higher than TVAS at higher TS. Comparison of the data obtained for the two reactions indicates that TVB in the CO + NO reaction on Pd(110) at TS = 800 and 850 K is much higher than that in the CO + O2 reaction on Pd(110).     

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.     

Studies of CO adsorption on Pt(100), Pt(410), and Pt(110) surfaces using density functional theory

Adsorption of CO on Pt(100), Pt(410), and Pt(110) surfaces has been investigated by density functional theory (DFT) method (periodic DMol(3)) with full geometry optimization and without symmetry restriction. Adsorption energies, structures, and vibrational frequencies of CO on these surfaces are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The same site preference as inferred experiments is obtained for all the surfaces. For Pt(100), CO adsorbs at the bridge site at low coverage, but the atop site becomes most favorable for the c(2 x 2) structure at 1/2 monolayer. For Pt(410) (stepped surface with (100) terrace and (110) step), CO adsorbs preferentially at the atop site on the step edge at 1/4 monolayer, but CO populates also at other atop and bridge sites on the (100) terrace at 1/2 monolayer. The multiple possible adsorption sites probably correspond to the multiple states in the temperature-programmed desorption spectra for CO desorption. For Pt(110), CO adsorbs preferentially at the atop site on the edge for both the reconstructed (1 x 2) and the un-reconstructed (1 x 1) surfaces. When adjacent sites along the edge row begin to be occupied, the CO molecules tilt alternately by ca. 20 degrees from the surface normal in opposite directions for both the (1 x 2) and (1 x 1) surfaces.     

DFT investigation of CO adsorption on Pt(211) and Pt(311) surfaces from low to high coverage

Adsorption of CO on Pt(211) and Pt(311) surfaces has been investigated by the density functional theory (DFT) method (periodic DMol3) with full geometry optimization. Adsorption energies, structures, and C-O stretching vibrational frequencies are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The calculated C-O stretching frequencies agree well with the experimental ones, and precise determination of adsorption sites can be carried out. For Pt(211), CO adsorbs at the atop site on the step edge at low coverage, but CO adsorbs at the atop and bridge sites simultaneously on both the step edge and the terrace with further increasing CO coverage. The present results interpret the reflection adsorption infrared (RAIR) spectra of Brown and co-workers very well from low to high coverage. For Pt(311), CO adsorbs also at the atop site on the step edge at low coverage. The lifting of reconstruction by CO adsorption occurs also for Pt(311), whereas the energy gain for lifting the reconstruction of the Pt(311) surface is smaller than that for Pt(110). The largest difference between the stepped Pt(211)/Pt(311) and Pt(110) surfaces is the occupation on the edge sites at higher coverage. For the stepped surfaces, the bridge site begins to be occupied at higher coverage, whereas the atop site is always occupied for the Pt(110) surface.     

Pt（111）表面上一氧化碳的吸附与氧化反应

Ｐｔ（１１１）表面上一氧化碳的吸附与氧化反应１）刘金尧（清华大学一碳化工国家重点实验室北京１０００８４）ＸｕＭＺａｅｒａＦ（ＤｅｐａｒｔｍｅｎｔｏｆＣｈｅｍｉｓｔｒｙＵｎｉｖｅｒｓｉｔｙｏｆＣａｌｉｆｏｒｎｉａＲｉｖｅｒｓｉｄｅＣＡ９２５２１）关键词...     

CO oxidation on nanostructured SnO(x)/Pt(111) surfaces: unique properties of reduced SnO(x)

We have investigated surface CO oxidation on "inverse catalysts" composed of SnO(x) nanostructures supported on Pt(111) using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS) and temperature-programmed desorption (TPD). Nanostructures of SnO(x) were prepared by depositing Sn on Pt(111) pre-covered by NO(2) layers at low temperatures. XPS data show that the SnO(x) nanoparticles are highly reduced with Sn(II)O being the dominant oxide species, but the relative concentration of Sn(II) in the SnO(x) nanoparticles decreases with increasing Sn coverage. We find that the most active SnO(x)/Pt(111) surface for CO oxidation has smallest SnO(x) coverage. Increasing the surface coverage of SnO(x) reduces CO oxidation activity and eventually suppresses it altogether. The study suggests that reduced Sn(II)O, rather than Sn(IV)O(2), is responsible for surface CO oxidation. The occurrence of a non-CO oxidation reaction path involving reduced Sn(II)O species at higher SnO(x) coverages accounts for the decreased CO oxidation activity. From these results, we conclude that the efficacy of CO oxidation is strongly dependent on the availability of reduced tin oxide sites at the Pt-SnO(x) interface, as well as unique chemical properties of the SnO(x) nanoparticles.     

Surface chemistry on bimetallic alloy surfaces: adsorption of anions and oxidation of CO on Pt3Sn(111)

The microscopic structure of the Pt(3)Sn(111) surface in an electrochemical environment has been studied by a combination of ex situ low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) and in situ surface X-ray scattering (SXS) and Fourier transform infrared (FTIR) spectroscopy. In ultrahigh vacuum (UHV) the clean-annealed surface produces a p(2 x 2) LEED pattern consistent with the surface composition, determined by LEIS, of 25 at. % Sn. SXS results show that the p(2 x 2) structure can be "transferred" from UHV into 0.5 M H(2)SO(4) and that the surface structure remains stable from 0.05 to 0.8 V. At 0.05 V the expansion of Pt surface atoms, ca. +2% from the bulk lattice spacing, is induced by adsorption of underpotential-deposited (UPD) hydrogen. At 0.5 V, where Pt atoms are covered by (bi)sulfate anions, the topmost layer is contracted relative to 0.05 V, although Sn atoms expand significantly, ca. 8.5%. The p(2 x 2) structure is stable even in solutions containing CO. In contrast to the Pt(111)-CO system, no ordered structures of CO are formed on the Pt(3)Sn(111) surface and the topmost layer expands relatively little (ca. 1.5%) from the bulk lattice spacing upon the adsorption of CO. The binding site geometry of CO on Pt(3)Sn(111) is determined by FTIR. In contrast to the near invariant band shape of a-top CO on Pt(111), changes in band morphology (splitting of the band) and vibrational properties (increase in the frequency mode) are clearly visible on the Pt(3)Sn(111) surface. To explain the line shape of the CO bands, we suggest that in addition to alloying effects other factors, such as intermolecular repulsion between coadsorbed CO and OH species, are controlling segregation of CO into cluster domains where the local CO coverage is different from the coverage expected for the CO-CO interaction on an unmodified Pt(111) surface.     

Mode-dependent coupling between vibration and translation of product CO(2) in CO oxidation on Pd(111)

The vibrational temperatures of product CO(2) were measured in CO oxidation on Pd(111) as a function of the desorption angle by means of infrared chemiluminescence. The antisymmetric vibration temperature was separately determined from the other vibrational modes from the normalized chemiluminescence intensity. The product CO(2) desorption is sharply collimated along the surface normal. The antisymmetric vibrational temperature increased from 1300 to 1600 K as the desorption angle increased from 0 degrees to 30 degrees , whereas the averaged vibrational temperature over bending and symmetric modes decreased from 2450 to 1530 K. From these angle dependences, an energy partitioning model in repulsive desorption is proposed.     

CO and methanol adsorption on (2 × 1)Pt(110) and ion‐eroded Pt(111) model catalysts

Methanol adsorption on ion‐sputtered Pt(111) surface exhibiting high concentration of vacancy islands and on (2 × 1)Pt(110) single crystal were investigated by means of photoelectron spectroscopy (PES) and thermal desorption spectroscopy. The measurements showed that methanol adsorbed at low temperature on sputtered Pt(111) and on (2 × 1)Pt(110) surfaces decomposed upon heating. The PES data of methanol adsorption were compared to the data of CO adsorbed on the same Pt single crystal surfaces. In the case of the sputtered Pt(111) surface, the dehydrogenation of H_xCO intermediates is followed by the CO bond breakage. On the (2 × 1)Pt(110) surface, carbon monoxide, as product of methanol decomposition, desorbed molecularly without appearance of any traces of atomic carbon. By comparing both platinum surfaces we conclude that methanol decomposition occurs at higher temperature on sputtered Pt(111) than on (2 × 1)Pt(110). Copyright © 2010 John Wiley & Sons, Ltd.     

Isotopic exchange of CO adsorbed on Pt(111)

The isotopic exchange of CO adsorbed on Pt(111) was studied using polarization modulation IR reflection absorption spectroscopy (PM-IRRAS) and temperature programmed desorption. It was found that the rate constants for the exchange reaction are much higher than would be expected from previous investigations of CO adsorbed on Pt nanoparticles. The adsorption of CO on Pt(111) under elevated pressures of CO and H(2) was also studied using PM-IRRAS. It was seen that CO pressures above 1 mbar lead to a shift in the absorption peak arising from CO adsorbed on a bridge site from 1850 to 1875 cm(-1). Exposing the CO-covered Pt(111) surface to 1000 mbar H(2) did not lead to any significant desorption of CO at room temperature, whereas at 363 K H(2) exposure did lead to a significant desorption of CO, due to the increased chemical potential of H(2). In a mixture of CO and H(2) with partial pressures of 0.01 mbar and 1000 mbar, respectively, no significant effect of H(2) on the PM-IRRAS spectrum was seen at temperatures below 423 K.     

Angular dependence of rotational and vibrational energies of product CO2 in CO oxidation on Pd(110)

The rotational and vibrational energies of product CO(2) in the CO oxidation on Pd(110) surfaces were measured as functions of desorption angles. The antisymmetric vibrational temperature (T(a)) was separately determined from the other vibrational modes from the normalized chemiluminescence intensity. The rotational temperature (T(rot)) and vibrational temperature averaged over the symmetric and bending modes (T(sb)) were then determined by the position and width of spectra by comparison with simulated spectra. On Pd(110)-(1x1), with increases in the desorption angle, T(a), T(sb) and T(rot) decreased in the [001] direction but remained constant in [11[combining macron]0]. However, such anisotropy disappeared when the ratio of exposure of O(2) to that of CO decreased, resulting in a gradual decrease of the three temperatures with increases in the desorption angle. On Pd(110) with missing rows, the three temperatures increased in [001] but decreased in [11[combining macron]0], indicating that the transition state changes with the geometry of the substrate. On Pd(110) with missing rows, T(a) was significantly lower than T(sb), although T(a) was close to or higher than T(sb) on Pd(110)-(1x1). However, there was no significant difference in the angular dependence between T(a), T(sb) and T(rot).     

Kinetics of CO oxidation on high-concentration phases of atomic oxygen on Pt(111)

Temperature-programmed reaction spectroscopy (TPRS) and direct, isothermal reaction-rate measurements were employed to investigate the oxidation of CO on Pt(111) covered with high concentrations of atomic oxygen. The TPRS results show that oxygen atoms chemisorbed on Pt(111) at coverages just above 0.25 ML (monolayers) are reactive toward coadsorbed CO, producing CO(2) at about 295 K. The uptake of CO on Pt(111) is found to decrease with increasing oxygen coverage beyond 0.25 ML and becomes immeasurable at a surface temperature of 100 K when Pt(111) is partially covered with Pt oxide domains at oxygen coverages above 1.5 ML. The rate of CO oxidation measured as a function of CO beam exposure to the surface exhibits a nearly linear increase toward a maximum for initial oxygen coverages between 0.25 and 0.50 ML and constant surface temperatures between 300 and 500 K. At a fixed CO incident flux, the time required to reach the maximum reaction rate increases as the initial oxygen coverage is increased to 0.50 ML. A time lag prior to the reaction-rate maximum is also observed when Pt oxide domains are present on the surface, but the reaction rate increases more slowly with CO exposure and much longer time lags are observed, indicating that the oxide phase is less reactive toward CO than are chemisorbed oxygen atoms on Pt(111). On the partially oxidized surface, the CO exposure needed to reach the rate maximum increases significantly with increases in both the initial oxygen coverage and the surface temperature. A kinetic model is developed that reproduces the qualitative dependence of the CO oxidation rate on the atomic oxygen coverage and the surface temperature. The model assumes that CO chemisorption and reaction occur only on regions of the surface covered by chemisorbed oxygen atoms and describes the CO chemisorption probability as a decreasing function of the atomic oxygen coverage in the chemisorbed phase. The model also takes into account the migration of oxygen atoms from oxide domains to domains with chemisorbed oxygen atoms. According to the model, the reaction rate initially increases with the CO exposure because the rate of CO chemisorption is enhanced as the coverage of chemisorbed oxygen atoms decreases during reaction. Longer rate delays are predicted for the partially oxidized surface because oxygen migration from the oxide phase maintains high oxygen coverages in the coexisting chemisorbed oxygen phase that hinder CO chemisorption. It is shown that the time evolution of the CO oxidation rate is determined by the relative rates of CO chemisorption and oxygen migration, R(ad) and R(m), respectively, with an increase in the relative rate of oxygen migration acting to inhibit the reaction. We find that the time lag in the reaction rate increases nearly exponentially with the initial oxygen coverage [O](i) (tot) when [O](i) (tot) exceeds a critical value, which is defined as the coverage above which R(ad)R(m) is less than unity at fixed CO incident flux and surface temperature. These results demonstrate that the kinetics for CO oxidation on oxidized Pt(111) is governed by the sensitivity of CO binding and chemisorption on the atomic oxygen coverage and the distribution of surface oxygen phases.     

A site-selective in situ study of CO adsorption and desorption on Pt(355)

Using time-dependent high-resolution x-ray photoelectron spectroscopy at BESSY II, the adsorption and desorption processes of CO on stepped Pt(355) = Pt[5(111) x (111)] were investigated. From a quantitative analysis of C 1s data, the distribution of CO on the various adsorption sites can be determined continuously during adsorption and desorption. These unique data show that the terrace sites are only occupied when the step sites are almost saturated, even at temperatures as low as 130 K. The coverage-dependent occupation of on-top and bridge adsorption sites on the (111) terraces of Pt(355) is found to differ from that on Pt(111), which is attributed to the finite width of the terraces and changes in adsorbate-adsorbate interactions. In particular, no long-range order of the adsorbate layer could be observed by low-energy electron diffraction. Further details are derived from sticking coefficient measurements using the method devised by King and Wells [Proc. R. Soc. London, Ser. A 339, 245 (1974)] and temperature-programmed desorption. The CO saturation coverage is found to be slightly smaller on the stepped surface as compared to that on Pt(111). The initial sticking coefficient has the same high value of 0.91 for both surfaces.     

In situ infrared reflection absorption spectroscopy of carbon monoxide adsorbed on Pt(S)-[n(100)x(110)] electrodes

Structural effects on the adsorption of CO have been studied using infrared reflection absorption spectroscopy (IRAS) on Pt(S)-[n(100)x(110)] surfaces (n = 2, 5, 9) that have densely packed kink atoms in the step. Coverage and potential dependence of the IRAS spectra are scrutinized. On-top and bridge-bonded CO are found on all of the surfaces examined. CO is adsorbed on only kink at low coverage (thetaCO < or = 0.2). Adsorbed CO on kink gives an IR band at lower frequency than that on step. CO is adsorbed on both kink and terrace at 0.3 < or = thetaCO. Water is adsorbed on the terrace of Pt(510) n = 5 and Pt(910) n = 9 at low CO coverage, but water is not found on Pt(210) n = 2 of which the first layer is composed of only kink atoms. It is suggested that coadsorbed water on the terrace enhances the activity for the oxidation of adsorbed CO on the kink remarkably.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

DRIFTS investigation and DFT calculation of the adsorption of CO on Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2

Molecular structures and vibrational spectra of the CO species adsorbed on the Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2 have been investigated by means of density functional theory (DFT) calculation and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The geometrical structures and vibrational frequencies were obtained at the MPW1PW91/SDD level. Theoretical calculation shows that the calculated IR spectra were in good agreement with the experimental results. The calculated results clarify the assignment of the adsorbed CO species on the surface of Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2.     

Kinetic studies of adsorbed CO electrochemical oxidation on Pt(335) at full and sub-saturation coverages

Electrochemical measurements were performed to characterize the kinetics of adsorbed CO oxidation on the surface of the stepped Pt(s)-[4(111)x(100)][triple bond, length half m-dash]Pt(335) single crystal electrode. For CO adsorbed to full coverage at 0.1 V (versus the reversible hydrogen electrode, RHE) in 0.5 M H(2)SO(4) at ambient temperature (23 degrees C), oxidation of the layer gave 7.6 x 10(14) +/- 0.3 CO/cm(2) as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 10(14) +/- 0.6 CO/cm(2)). In potential step measurements carried out between 0.75 and 0.9 V, the peak region in the current-time transient was consistent with the surface reaction between adsorbed CO and adsorbed oxide as rate limiting. Plotting the log of the rate constant for the surface reaction versus potential gave a Tafel slope of 79 mV per decade, consistent with responses for CO electrochemical oxidation on structurally related stepped Pt electrodes. For CO coverages below saturation, current-time transients were more stable in 0.05 M H(2)SO(4) than in the higher concentration electrolyte. Numerically solving the rate equations to the Langmuir-Hinshelwood model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients measured at 0.7 V in 0.05 M H(2)SO(4) for sub-saturation CO coverages. The results provide new insights into CO oxidation on Pt at sub-saturation coverage and confirm that anions play a role in CO surface chemistry.     

Mechanism of CO oxidation on Pt(111) in alkaline media

Electrochemical techniques, coupled with in situ scanning tunneling microscopy, have been used to examine the mechanism of CO oxidation and the role of surface structure in promoting CO oxidation on well-ordered and disordered Pt(111) in aqueous NaOH solutions. Oxidation of CO occurs in two distinct potential regions: the prepeak (0.25-0.70 V) and the main peak (0.70 V and higher). The mechanism of reaction is Langmuir-Hinshelwood in both regions, but the OH adsorption site is different. In the prepeak, CO oxidation occurs through reaction with OH that is strongly adsorbed at defect sites. Adsorption of OH on defects at low potentials has been verified using charge displacement measurements. Not all CO can be oxidized in the prepeak, since the Pt-CO bond strength increases as the CO coverage decreases. Below theta(CO) = 0.2 monolayer, CO is too strongly bound to react with defect-bound OH. Oxidation of CO at low coverage occurs in the main peak through reaction with OH adsorbed on (111) terraces, where the Pt-OH bond is weaker than on defects. The enhanced oxidation of CO in alkaline media is attributed to the higher affinity of the Pt(111) surface for adsorption of OH at low potentials in alkaline media as compared with acidic media.     

Surface structural and chemical characterization of Pt/Ru composite electrodes: a combined study by XPS, STM and IR spectroscopy

Electrochemical Ru deposits on Pt(111) surfaces are investigated by STM; the images of the Ru-modified surfaces show islands of monoatomic height and between 2–5 nm in diameter. The density of islands on the surface depends on the Ru deposition potential (observed by STM and XRSD) and the cyclic voltammograms indicate an increasing Ru coverage for lower deposition potentials. The Ru surface coverage is determined by ex-situ XPS measurements and a linear dependence of the Ru coverage on the deposition potential is demonstrated. IR spectra of a monolayer of adsorbed CO on the Ru-modified Pt(111) surfaces show distinct bands for CO adsorbed on Pt and on Ru. For the integrated band intensity of the CO/Ru vibration a linear dependence on deposition potential is found indicating that lateral dipole interactions between CO adsorbed on Pt and Ru are unimportant and that the CO coverage on the Ru islands is constant for the Ru coverages investigated. The possibility of using adsorbate vibrational bands for the determination of the coverage of deposits is discussed. Received: 24 June 1996 / Revised: 6 December 1996 / Accepted: 12 December 1996     

CO在Pt(111)和Pt-M(111)(M=Ni,Mg)表面吸附的理论研究

采用密度泛函理论与周期性平板模型相结合的方法,对CO在Pt(111)表面top,fcc,hcp和bridge 4个吸附位和Pt-M(111)(M=Ni,Mg)表面h-top,M-top,Pt(M)Pt-bridge,Pt(M)M-bridge,Pt(Pt)M-bridge,M(Pt)M-bridge,Pt1M2-hcp...