CO Pre-coverage Research Articles

Influencing mechanism of carbon monoxide on the hydrogen absorption performance of Zr2Fe

Zr2Fe is considered as the promising hydrogen isotopes trapping material to control the quantity of tritium generated in deuterium–tritium fusion reaction. However, the vulnerability to poisoning by impurity gases such as carbon monoxide is the bottleneck for their applications. In this study, we show from our DFT calculations that the Zr2Fe(101) surface is saturated with 13CO molecules and the hollow site is the most stable CO adsorption site. The adsorption energy of CO is much lower than that of hydrogen, resulting in the preferential adsorption of CO on Zr2Fe(101). The dependence of the Gibbs free energies of CO on the temperatures under UHV conditions were also obtained. The climbing-image nudged elastic band results reveal that CO pre-coverage would block the hydrogen adsorption sites and affect the hydrogen diffusion. The Sieverts analysis demonstrates that even the low concentration CO is a fatal poison to the hydrogenation process of Zr2Fe at ambient temperature. Hydrogen absorption may occur at a higher temperature with a lower absorption rate and capacity, as CO starts to desorpt from Zr2Fe(101) with the increase of temperature and thus making H2 absorption possible, even though the H diffusion energy barriers are relatively high. Our results provide the basis for exploring the influence mechanisms of the impurity gas CO on the hydrogen adsorption properties of the Zr-based alloys, which would be beneficial for the applications of metal getter of tritium-containing waste gas.

Stable CO/H2 ratio on MoP surfaces under working condition: A DFT based thermodynamics study

By performing a systematic DFT calculation and applying the atomistic thermodynamics analysis, the adsorption configurations, stable concentrations of nCO + mH2 co-adsorption on three representative MoP surfaces [(101), (001)-Mo and (001)-P] were investigated. Our results show that CO adsorbs much stronger than dissociative H2 on (101) and (001)-Mo surfaces but competitively with dissociative H2 on the (001)-P surface, and the hydrogen saturation coverage decreases with increasing CO pre-coverage. Ab initio atomistic thermodynamics analysis indicates the quite different CO and H2 co-adsorption manners on three surfaces under syngas atmosphere, i.e., at the equilibrium co-adsorption state, the Mo/P-terminated (101) surface and the Mo-terminated (001) surface have more versatile surface CO and H2 ratios, which are entirely different from that in the gas phase. However, the P-terminated (001) surface has only hydrogen adsorption at a wide range of conditions, which plays a role of hydrogen reservoir. Such investigations reveal that surface CO/H2 ratio could be altered by manipulating the pressures of the gas phase and temperatures, which would be beneficial to modify the syngas conversion reactivity as well as different product distributions on solid catalyst surfaces.

Co-adsorption and mutual interaction of nCO + mH2 on the Fe(1 1 0) and Fe(1 1 1) surfaces

Spin-polarized density functional theory computations have been performed to investigate nCO+mH2 co-adsorption and mutual interaction on the Fe(110) and Fe(111) surfaces. It is found that CO pre-coverage affects hydrogen adsorption strongly, while H pre-coverage hardly affects CO adsorption. It is possible to have CO+H2 co-adsorption on Fe(110), while very strong preference of CO pre-coverage on Fe(111) has been found. On Fe(111), it is also found that each CO molecule blocks on average 2H adsorption sites at all CO pre-coverage. All these findings are consistent with available experimental results. Compared with CO/H2 ratios in gas phase, CO/H2 surface ratios are very different and much more sensitive to temperature than pressure. Our results provide the basis for exploring the mechanisms of iron-catalyzed conversion of synthesis gas.

Sticking Probability and Reactivity of Hyperthermal O2 Molecules Impinging on CO Pre-covered Pd(100): Effect of Rotational States with K > 1

Collisional alignment in the supersonic expansion of oxygen seeded in lighter carriers allows to produce rotationally aligned beams which were used to study the sticking probability, S, and the reactivity, \(P_{{co_{2} }}\), of helicoptering and cartwheeling O2 in the lowest rotational state impinging normally onto Pd(100). Molecules with different rotational alignment are thereby chosen by mechanically selecting either the slow or the fast speed tail of the beam velocity distribution. Here we show for O2 reacting with CO pre-covered Pd(100) that even a moderate increase in the nozzle temperature, TN, with respect to room temperature implies significant changes in both S and \(P_{{co_{2} }}\). Since TN affects mainly the rotational state distribution in the formed beam, we conclude that molecules in rotational states with K ≥ 3 behave very differently from those with K = 1. Data are presented for both small and intermediate CO pre-coverage.

Reduction of Nitric Oxide by Acetylene on Ir Surfaces with Different Morphologies: Comparison with Reduction of NO by CO

Reduction of nitric oxide (NO) by acetylene (C(2)H(2)) has been investigated by temperature-programmed desorption (TPD) on planar Ir(210) and faceted Ir(210) with tunable sizes of three-sided nanopyramids exposing (311), (311[overline]), and (110) faces. Upon adsorption, C(2)H(2) dissociates to form acetylide (CCH) and H species on the Ir surfaces at low C(2)H(2) precoverage. For adsorption of NO on C(2)H(2)-covered Ir, both planar and faceted Ir(210) exhibit high reactivity for reduction of NO with high selectivity to N(2) at low C(2)H(2) precoverage, although the reaction is completely inhibited at high C(2)H(2) precoverage. Coadsorbed C(2)H(2) significantly influences dissociation of NO. The N-, H-, and C-containing TPD products are dominated by N(2), H(2), CO, and CO(2) together with small amounts of H(2)O. For adsorption of NO on C-covered Ir(210) at fractional C precoverage, formation of CO(2) is promoted while production of CO is reduced. Reduction of NO by C(2)H(2) is structure sensitive on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in the reduction of NO by C(2)H(2) on faceted Ir(210) for average facet sizes of 5 nm and 14 nm. The results are compared with reduction of NO by CO on the same Ir surfaces. As for NO+C(2)H(2), the Ir surfaces are very active for reduction of NO by CO with high selectivity to N(2) and the reaction is structure sensitive, but clear evidence is found for size effects in the reduction of NO by CO on the nanometer scale. Furthermore, coadsorbed CO does not affect dissociation of NO at low CO precoverage whereas coadsorbed CO considerably influences dissociation of NO at high CO precoverage. The adsorption sites of CCH+H on Ir are characterized by density functional theory.

Consecutive N2 Adsorption on CO Precovered NaY Zeolite: FTIR Gives a Statistical Response of Surface Adsorbed Species

We analyze IR spectra recorded after the consecutive adsorption at 77 K of N2 ensuing from CO precoverage as molecular gas-probes on the microporous NaY zeolite. The study encompasses N2 loading ensuing from both medium and low CO precoverages. As a result, the corresponding IR spectra are thoroughly analyzed in connection with the adsorbed species that could be formed. Consequently, the formation conditions of polycarbonyls, polydinitrogen, and other species having N2 and CO as mixed ligands are discussed. Close analysis of the IR spectra recorded for each situation reveals an undeniable correlation between the statistical IR responses of the as-detected adsorbed species. This breakthrough result contradicts the widespread idea, which states that each IR peak should correspond to one specific adsorbed specie. Instead, IR response accounts for a statistical distribution of the various IR positions connected with as-adsorbed species. Thus, depending on the respective contribution and amounts of the different surface adsorbed species, large sets of IR responses and positions of IR bands are possible. These new findings relating on a statistical infrared response should provide useful advances for the understanding and assignments of some ambiguous IR spectra regarding surface adsorbed species.

Evidence of stable high-temperature Dx-CO intermediates on the Ru(0001) surface

We demonstrate the formation of complexes involving attractive interactions between D and CO on Ru(0001) that are stable at significantly higher temperatures than have previously been reported for such intermediate species on this surface. These complexes are evident by the appearance of new desorption features upon heating of the sample. They decompose in stages as the sample temperature is increased, with the most stable component desorbing at >500 K. The D:CO ratio remaining on the surface during the final stages of desorption tends towards 1:1. The new features are populated during normally incident molecular beam dosing of D(2) on to CO pre-covered Ru(0001) surfaces (180 K) when the CO coverage exceeds 50% of the saturation value. The amount of complex formed decreases somewhat with increasing CO pre-coverage. It is almost absent in the case of dosing on to the fully saturated surface. The results are interpreted in terms of both local and long-range rearrangements of the overlayer that give rise to the observed CO coverage dependence and limit the amount of complex that can be formed.

Stereoselectivity in catalytic reactions: CO oxidation on Pd(100) by rotationally aligned O2 molecules

We report on stereodynamical effects in heterogeneous catalytic reactions as measured by molecular beam-surface experiments. Specifically for CO oxidation on Pd(100) we find that the rotational alignment of the incoming O2 at low (Θ = 0.04 ML) and at intermediate (ΘCO = 0.17 ML) CO pre-coverage, causes a higher reactivity of molecules in high and in low helicity states, respectively (corresponding to helicoptering and cartwheeling motion of O2). In first approximation, at low CO pre-coverage the difference in reactivity is determined by the different location of the O atoms generated in the dissociation process by the different parent molecules, while at intermediate CO pre-coverage the reactivity is influenced also by the different ability of cartwheeling and helicoptering O2 to penetrate through the CO adlayer. In accord with this the total amount of CO2 produced is always largest for helicopters which generate supersurface O atoms at least in the low CO pre-coverage limit. A deeper inspection of the data indicates, however, that the dynamics is more complex, two different pathways being present for the reaction with O generated by helicopters and one for O generated by cartwheels. Moreover, cartwheels generated oxygen influences the reactivity of subsequently arriving helicopters.

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre-adsorbed CO on the dissociative adsorption of D(2) on Ru(0001) is studied by molecular-beam techniques. We determine the initial dissociation probability of D(2) as a function of its kinetic energy for various CO pre-coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D(2) dissociation and perturbs the local surface reactivity up to the nearest-neighbour Ru atoms. Non-activated sticking and dissociation become less important with increasing CO coverage, and vanish at theta(CO) approximately 0.33 ML. In addition, at high D(2) kinetic energy (>35 kJ mol(-1)) the site-blocking capability of CO decreases rapidly. These observations are attributed to a CO-induced activation barrier for D(2) dissociation in the vicinity of CO molecules.

Adsorption dynamics of CO2 on copper-precovered ZnO(0001)–Zn: A molecular-beam scattering and thermal-desorption spectroscopy study

Initial, S(0), as well as coverage-dependent adsorption probability measurements, S(Theta), have been conducted at normal impact angle and as a function of the impact energy of CO(2), E(i), adsorption temperature, T(s), and copper precoverage, Theta(Cu) (at 300 K). S(0), which decreased from approximately 0.4 exponentially to approximately 0.05 with E(i) was independent of Theta(Cu). Astonishingly, S(0) for Cu on ZnO(0001)-Zn is smaller than for the clean support which indicates a chemical modification of the support by the Cu deposits. S(Theta) curves consist of two regimes, a Kisliuk-type and Langmuirian-type section. The first is consistent with capture zone models; the second may indicate direct adsorption of CO(2) on the Cu cluster. The thermal-desorption (TDS) curves for Cu on ZnO(0001)-Zn consist of two structures with binding energies of 26 and approximately 40 kJmol (nu=1 x 10(13) ls). The TDS results indicate that CO(2) populates predominantly the Cu deposits and the rim along the Cu nanoparticles. No indications for CO(2) dissociation could be obtained with Auger electron spectroscopy.

CO oxidation on Pd(110): a high-resolution XPS and molecular beam study

We have employed synchrotron based high-resolution XPS and thermal molecular beams to investigate the interaction and reaction of CO and O 2 on Pd(110). CO and O 2 stick on this surface with high efficiency at room temperature, with initial sticking probabilities of 0.5 and 0.4, respectively. Precursor states play a strong role in the adsorption of CO; kinetic analysis indicates a greater precursor effect in comparison with that for CO adsorption on Rh(110). Temperature-programmed XPS of CO from Pd(110) shows a sharp shift in binding energy of both the C 1s and O 1s peaks at a coverage which is associated with the reconstruction of the surface to the (1×2) missing row. Similar experiments for O 2 adsorbed on Pd(110) show diffusion of oxygen into the subsurface region during the temperature ramp, at a temperature significantly below the onset of O 2 desorption. The CO oxidation reaction has been studied in a transient mode where CO/(O 2) is dosed onto an O 2/(CO) pre-dosed surface. At low temperatures, CO is immediately adsorbed with a long lifetime state on the O 2 pre-covered surface, in contrast to higher temperatures (>420 K) where there is no net uptake of CO at the beginning of the reaction owing to reactive loss of CO as CO 2. At 375 K, the CO 2 production curve shows two maxima in rate which we relate to the influence of surface reconstructions. In comparison, the reverse clean-off reaction where O 2 is dosed onto different CO pre-coverages shows that O 2 cannot adsorb on a CO-saturated surface. We have determined that there is a critical CO coverage of 0.60 (±0.06) ML of CO adsorbed above which oxygen dissociative adsorption is blocked. However, O 2 can adsorb on lower CO pre-coverages and there is fast reaction for CO coverages below 0.3 ML.

Real-time observation of coadsorption layers on Ru(001) using a temperature-programmed ESDIAD/TOF system

For the purpose of utilizing ESDIAD as a real-time probe for surface processes, we have developed an instrument which can measure ESDIAD images and time of flight (TOF) spectra of desorbing ions in temperature-programmed surface processes. TOF measurements are carried out to identify the mass and to determine the kinetic energy distribution of the desorbed ions. This temperature-programmed (TP-) ESDIAD/TOF system was used to observe coadsorption layers of methylamine and CO on Ru(001) which have been previously studied by our group using LEED, TPD and HREELS, also drawing upon a comparison of findings with the coadsorption system of CO and ammonia. ESDIAD images acquired for temperature-programmed surface processes in real time were found to provide new insight into the dynamic behaviour of the coadsorption layers. As to the pure adsorption of ammonia and methylamine, the second and the first (chemisorbed) layers can be easily discriminated in their different ESD detection efficiency due to the difference in neutralization rate. The intensity change of H + ions with temperature shows the process of the decomposition of methylamine to be dependent on CO coverage. The intensity of O + originating from CO changes due to the change of CO adsorption site in the reaction process. The angular distribution of H + ions which correspond to CH2NH…Ru species appears at 250–300 K in the presence of high CO pre-coverage.

Surface interactions between coadsorbed ammonia and carbon monoxide on the Pt(111) surface

Interactions between coadsorbed species were studied for the NH 3/CO/Pt(111) system using temperature-programmed desorption (TPD). Coadsorbed CO has a strong effect on the high-temperature ammonia desorption spectrum. For saturation CO precoverage, two distinct high-temperature peaks are observed at 260 and 325 K. In contrast, on a clean Pt(111) surface, ammonia desorption produces a broad, flat plateau from 200 to 375 K. For intermediate CO coverages, the two high-temperature peaks grow and merge into a single ammonia desorption peak at 295 K. CO and NH 3 compete for sites on the Pt(111) surface, with CO inhibiting ammonia-ammonia interactions. Coadsorbed CO does not have a pronounced effect on the low-temperature high-coverage desorption of ammonia. This hydrogen-bonded ammonia desorbs at the same temperatures from both clean and CO-modified Pt(111) surfaces; however, the saturation coverage is reduced. Ammonia does not displace adsorbed CO at 125 K, and no ammonia decomposition is observed.

Lateral interaction of H and CO coadsorbed on Cu 3Pt(111)

The coadsorption of hydrogen and CO on the ordered Cu 3Pt(111) surface has been studied using low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), UV-photoelectron spectroscopy (UPS), and high resolution electron energy loss spectroscopy (HREELS). The main feature of the ordered Cu 3Pt(111) surface is a 2 × 2 structure of the Pt atoms in the surface, hence, Pt atoms are completely surrounded by Cu atoms. At temperatures above 180 K CO adsorbs on top of the Pt atoms. A constant hydrogen desorption temperature regardless of the CO pre-coverage in TDS indicates a repulsive CO-H interaction. In this case the HREEL spectra of the CO are not different from the spectra of CO on the clean surface. Dosing CO on a hydrogen saturated surface at 130 K leads to a shift of the hydrogen TD spectra to lower temperatures. Eventually this shift will stop accompanied by the occurrence of a new higher coordination CO adsorption state (HREELS) that has not been found on the clean surface. The experiments suggest a short-range repulsive interaction between hydrogen and CO on the ordered Cu 3Pt(111) surface.

Interaction between CO and NH 3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (α-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia ( 15NH 3 or ND 3). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θ CO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θ CO = 0.5. The dissociation of ammonia in the β−(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the α−(2 × 2) structure. The HREEL spectra indicated that the C 3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β−(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH 3 interaction on metal surfaces is presented.

Interactions in the coadsorption system CO/acetonitrile on Pt(111): an IRAS investigation

The effect of coadsorbed acetonitrile and acetonitrile-d 3 on the high (2100 cm −1 ) and low energg (1850 cm −1 ) stretching vibrational band of CO on Pt(111) has been studied at 90 K in an uhv system using ir reflection-adsorption spectroscopy (IRAS). Successive increase of the acetonitrile dosage leads to a total suppression of the strong band of preadsorbed CO at 2100 cm −1 . Simultaneously the band shifts to lower frequency. At high CO precoverages a new band grows at about 2060 cm −1 (depending on precoverage). As a possible mechanism explaining the suppression we discuss the shielding of the dynamic dipole moment of the CO molecule and a change of adsorption sites induced by the coadsorbate. The adsorption site change can be monitored by observing the behaviour of the low energy band of CO at 1850 cm −1 . Supposing that chemical reactions do not occur and chemical interactions are of less importance, the growth of the new band at high CO precoverages can be understood in terms of formation of CO islands with high local coverage.

Coadsorption of NH 3 and CO on Ru(001): The ordering in mixed layers and the effect of intermolecular interactions on NH 3 dissociation

The coadsorption of CO and ammonia on Ru(001) and the dissociation of ammonia in the mixed adsorbed layer have been investigated by LEED, TPD and HREELS. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed according to the CO precoverage. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia ( 15NH 3 or ND 3). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased with increasing CO precoverage up to θ CO = 0.25, whereas a further increase of CO coverage promoted the ammonia dissociation. The HREEL spectra indicated that the molecular axis of C 3V ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the (2 × 2) phase with a high CO partial coverage.

Coadsorption of NH 3 and CO on Ru(001): The ordering in mixed layers and the effect of intermolecular interactions on NH 3 dissociation

The coadsorption of CO and ammonia on Ru(001) and the dissociation of ammonia in the mixed adsorbed layer have been investigated by LEED, TPD and HREELS. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed according to the CO precoverage. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia ( 15NH 3 or ND 3). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased with increasing CO precoverage up to θ CO = 0.25, whereas a further increase of CO coverage promoted the ammonia dissociation. The HREEL spectra indicated that the molecular axis of C 3V ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the (2 × 2) phase with a high CO partial coverage.

Lateral distribution of coadsorbed H and CO on Pt(111) studied by teas

Detailed information on the influence of the CO mobility on segregation in the coadsorption system H/CO/Pt(111) has been obtained by TEAS (thermal energy atom scattering) measurements. At a surface temperature of Ts=180 K the formation of large CO islands has been confirmed quantitatively; at CO precoverages up to at least θCO=0.18 and upon H saturation, the local density within the CO islands is θCOi=0.5. At Ts=80 K and precoverages θCO<0.04 the CO random distribution remains unchanged by H adsorption due to the lack of CO mobility; it is only for θCO>0.04 that a tendency towards CO clustering is observed. The presence of coadsorbed H appears to lower the temperature of the CO mobility onset by 20–30 K.

Formation of islands consisting of repelling adsorbates

Clear evidence of adsorbate island formation in a system with only repulsive adsorbate-adsorbate interactions is presented. This evidence is independently obtained by thermal energy atom scattering (TEAS) and LEED studies of the H−CO/Pt(111) coadsorption system. Specularly reflected He intensity at hydrogen saturation coverage on the CO precovered surface is up to an order of magnitude greater than in the absence of hydrogen. A c(4×2) CO/Pt(111) LEED pattern is observed at hydrogen saturation even for CO precoverages of less than 0.1 monolayer.