Coadsorption Of Potassium Research Articles

Elucidating the Adsorption and Co-adsorption of Potassium and Oxygen on (110) MnO2, TiO2 and VO2 Surfaces

In metal air battery, oxygen reacts with lithium ions on the cathode side of the cell which makes it much lighter than conventional cathodes used in Li-ion batteries. Density functional theory (DFT) study is employed in order to investigate the surfaces of, (Rutile) R-MnO2, TiO2 and VO2 (MO2), which act as catalysts in metal-air batteries. Adsorption and co-adsorption of metal K and oxygen on (110) β-MO2 surface is investigated, which is important in the discharging and charging of K- air batteries. Only five values of (gamma) are possible due to the size of the supercell and assuming that oxygen atoms occupy bulk-like positions around the surface metal atoms. The manganyl, titanyl and vanadyl terminated surface are not the only surfaces that can be formed with Γ= +2, oxygen can be adsorbed also as peroxo species (O2)2-, with less electron transfer from the surface vanadium atoms to the adatoms than in the case of manganyl, titanyl or vanadyl formation. MnO2 promotes formation of KO2 for all configurations whereas TiO2 partially promote nucleation of KO2 whereas VO2 surfaces form very stable KO2 clusters, thus VO2 is not a good catalyst for the formation of KO2. The fundamental challenge that limits the use of metal air battery technology, however, is the ability to find a catalyst that will promote the formation and decomposition of discharge products during the charging and discharging cycle, i.e. oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).

Growth and evolution of tetracyanoquinodimethane and potassium coadsorption phases on Ag(111)

Alkali-doping is a very efficient way of tuning the electronic properties of active molecular layers in (opto-) electronic devices based on organic semiconductors. In this context, we report on the phase formation and evolution of charge transfer salts formed by 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) in coadsorption with potassium on a Ag(111) surface. Based on an in-situ study using low energy electron microscopy and diffraction we identify the structural properties of four phases with different stoichiometries, and follow their growth and inter-phase transitions. We label these four phases α to δ, with increasing K content, the last two of which (γ and δ-phases) have not been previously reported. During TCNQ deposition on a K-precovered Ag(111) surface we find a superior stability of δ-phase islands compared to the γ-phase; continued TCNQ deposition leads to a direct transition from the δ to the β-phase when the K : TCNQ ratio corresponding to this phase regime is reached, with no intermediate γ-phase formation. When, instead, K is deposited on a surface precovered with large islands of the low density commensurate (LDC) TCNQ phase that are surrounded by a TCNQ 2D-gas, we observe two different scenarios: on the one hand, in the 2D-gas phase regions, very small α-phase islands are formed (close to the resolution limit of the microscope, 10–15 nm), which transform to β-phase islands of similar size with increasing K deposition. On the other hand, the large (micrometer-sized) TCNQ islands transform directly to similarly large single-domain β-phase islands, the formation of the intermediate α-phase being suppressed. This frustration of the LDC-to-α transition can be lifted by performing the experiment at elevated temperature. In this sense, the morphology of the pure TCNQ submonolayer is conserved during phase transitions.

Photoemission of the K/W(100) system in the O2 atmosphere

Simultaneous adsorption of potassium and oxygen on W(100) is studied using the threshold photoemission spectroscopy. A metal potassium film is formed on W(100) in the course of coadsorption of potassium and oxygen at the first stage. At the second stage, the film is transformed into a dielectric layer with the K2O2 stoichiometry.

Alkali-induced hydrogenation of epitaxial graphene by water splitting at 100 K

The coadsorption of potassium with water at 100 K on graphene/Pt(111) has been studied by high-resolution electron energy loss spectroscopy. The adsorption of alkali metals induces water splitting and the formation of C-H and C-OH groups. Such finding is of great interest for tailoring graphene-Pt electro-catalysts. Furthermore, the alkali-promoted dissociation of water molecules offers the possibility to attain a partial hydrogenation of the graphene sheet even at low temperature.

Coadsorption of potassium and carbon monoxide on Ni(111): A density functional theory study

Corresponding to an experimental study with high-resolution electron energy loss spectroscopy [D. Pacil\'e et al., Chem. Phys. Lett. 413, 420 (2005)], three possible coadsorption structures of $\frac{1}{8}$ ML potassium with $p(2\ifmmode\times\else\texttimes\fi{}4)$ periodicity and $\frac{1}{2}$ ML carbon monoxide with $c(2\ifmmode\times\else\texttimes\fi{}4)$ periodicity on Ni(111) are calculated using density functional theory. The analysis of bond lengths and electronic structures of CO molecules with different distances to K shows that the K-induced effect is short ranged. The coadsorption structure formed by exposing CO after K is suggested to be the $p(2\ifmmode\times\else\texttimes\fi{}4)\text{\ensuremath{-}}\mathrm{K}∕4\mathrm{C}\mathrm{O}$ structure with K occupying the hollow site of the $c(2\ifmmode\times\else\texttimes\fi{}4)$ CO pattern. In addition, the possible reason for the observed difference in efficiencies of $\mathrm{C}{\mathrm{O}}_{2}$ formation with respect to different exposing orders of adsorbates is discussed.

Effect of alkali-metal adsorption on the conductance of a molecular device

We show by first-principles density functional calculations that the adsorption of potassium on the platinum electrodes of a benzene dithiolate molecular wire junction will modify the energy-level alignments of the device. But near the Fermi energy, the energy levels, which correspond to sulfur orbitals, are quite inert to the work function change of the electrode due to a strong terminal atom/metal bonding. Our results demonstrate unambiguously that while the coadsorption of potassium on the electrodes may influence the conductance of a molecular device, it cannot change the conductance in a drastic way as the transmission peak near the Fermi level shifts only slightly with the introduction of potassium atoms.

Spectroscopic investigations of the co-adsorption of potassium and oxygen on Ni(111) at room temperature

The electronic and vibrational properties of oxygen and potassium co-adsorbed on the Ni(1 1 1) surface at room temperature were studied by high resolution electron energy loss and X-ray photoelectron spectroscopies. The sample surface was prepared by means of two different procedures, namely by adsorbing oxygen on a pre-dosed potassium ad-layer and by reversing this sequential order. In both experiments we observed that a K coverage as high as 0.25 ML ( i.e. p (2 × 2) phase) is the condition necessary for measuring a weak K–O bond, while a K coverage greater than 0.30 ML (almost full layer), gives rise to a strong K–O bond which is evidenced by the loss feature at 27 meV. The loss spectrum of the K–O- c (4 × 2) phase shows that both species interact mainly with the substrate.

Spectroscopic investigations of the co-adsorption of potassium and oxygen on Ni(1 1 1) at room temperature

The electronic and vibrational properties of oxygen and potassium co-adsorbed on the Ni(1 1 1) surface at room temperature were studied by high resolution electron energy loss and X-ray photoelectron spectroscopies. The sample surface was prepared by means of two different procedures, namely by adsorbing oxygen on a pre-dosed potassium ad-layer and by reversing this sequential order. In both experiments we observed that a K coverage as high as 0.25 ML ( i.e. p(2 × 2) phase) is the condition necessary for measuring a weak K–O bond, while a K coverage greater than 0.30 ML (almost full layer), gives rise to a strong K–O bond which is evidenced by the loss feature at 27 meV. The loss spectrum of the K–O- c(4 × 2) phase shows that both species interact mainly with the substrate.

Theoretical study of the adsorption of oxygen on a Cu(100) surface and the coadsorption with alkali atoms

We performed a theoretical study of the adsorption of oxygen on a cluster model of the Cu(100) surface and also the surface coadsorbed with lithium and potassium atoms. The study showed that alkali coadsorption facilitates in a significant way the process of molecular adsorption, whereas the adsorption of atomic oxygen is only slightly modified. The alkali atoms on the copper surface produce an increase in the charge transfer toward the oxygen molecule, favoring the oxygen dissociation. The effect is greater for the potassium coadsorption. In addition, we found that the potassium coadsorption favored the dissociation and recombination processes by about 60 and 15%, respectively. In turn, the lithium coadsorption favored only the recombination process by about 50%. These results could be an important aspect for catalytic processes.

Temperature effects on the coadsorption of K–CO on Ni(100)

High-resolution electron energy-loss spectra of the coadsorption of potassium and CO on the Ni(100) surface show the role of temperature in driving chemical species along the surface towards the formation of ordered islands made of alkali atoms and CO species. At low potassium precoverages, the formation of K–CO islands occurs by heating the sample to 420 K. On the contrary, for a potassium coverage near 0.38 ML, K–CO patches of ordered phases grow spontaneously even at 220 K. The interpretation of the present data relies essentially on two literature works on the K–CO/Ni(100) system, one of them presenting calorimetric measurements and the other reoprting the theoretical temperature behaviour of the chemical species on the nickel surface.

Coadsorption of potassium at step edges on the Ni(100)(2 × 2)p4g-N reconstructed surface

High-resolution scanning tunnelling microscopy images show that low-coverage room temperature coadsorption of potassium on the Ni(100)(2 × 2)p4g-N surface occurs at step edges aligned preferentially along the 011 directions. The data indicate that the potassium atoms sit at the step edges a distance of 5.0±0.05 Å apart. A model is presented for the coadsorbed step structure.

A low-energy electron diffraction study of potassium - oxygen coadsorption phases on Ni(111)

The coadsorption of potassium and oxygen on a Ni(111) surface has been studied by low-energy electron diffraction (LEED). The initial potassium-induced structures on the bare and the oxygen-monolayer-covered surface exhibit similar LEED ring patterns, which may be attributed to the strong dipole - dipole repulsive interaction in both cases. On the other hand, a small amount of pre-adsorbed oxygen (θ0<0.1) prevents the formation of the potassium ring phase, suggesting an attractive K - O interaction. Once all of the pre-adsorbed oxygen is attached to the potassium adatoms, further potassium adsorption produces the same phases as on the oxygen-free surface. For intermediate O precoverages, a gradual structural transformation from a p(2 × 2) to a phase is observed upon K adsorption. Potassium reduces the critical oxygen coverage for the completion of this phase transformation, suggesting that (disordered) K mixes in the O layer. At higher O precoverages, coadsorption results in a new c(4 × 2) ordered phase in a structure which probably involves both K and O.

Further lowering of work function by oxygen adsorption on the K/Si(001) surface.

We have studied oxygen and potassium coadsorption on the Si(001) surface with first-principles molecular dynamics. The present calculation gives a strong support to the structural model based on double-layer K adsorption. Oxygen adsorption on the K/Si(001) surface reduces the work function by 0.7 eV and activates the negative electron affinity in accord with experimental results. The oxygen atoms are adsorbed slightly below the potassium atoms and accumulate a valence charge around the surface region, leading to large polarization of K atoms and further lowering of the work function.

A TDS study of oxygen, water and carbon dioxide coadsorption with potassium on graphite

The coadsorption of oxygen, water and carbon dioxide with potassium on the basal plane of graphite (HOPG) was studied by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES). Potassium depositions and gas exposures were performed at T = 160K. Concerted desorption of potassium and coadsorbed gas molecules is observed in all the studied coadsorption systems, indicating complex formation and decomposition. Oxygen adsorption on the potassium precovered graphite surface gives rise to desorption of K and CO 2, solely. All of the adsorbed oxygen thus reacts with carbon atoms in the graphite substrate to form CO 2. Most of the CO 2 desorption occurs around 800 K, simultaneously with K desorption. The CO 2 desorption shows features of zeroth order desorption, indicating reaction at defect sites. Most of the K desorption, in the presence of oxygen, occurs around 600 K. Exposure of the potassium covered graphite surface to water results in (i) formation of potassium hydroxide (KOH), which decomposes at 590 K, and (ii) desorption of potassium stabilized H 2O between 200 and 300 K. No reaction with the graphite surface was observed upon H 2O adsorption. Carbon dioxide and potassium coadsorption on graphite results in simultaneous K and CO 2 desorption around 780 K, which is attributed to formation and decomposition of potassium carbonate (K 2CO 3). Sequential pairwise exposures of the potassium covered graphite surface to O 2, H 2O and CO 2 indicate that water strongly inhibits, while carbon dioxide enhances, gasification of the graphite substrate.

A high resolution vibrational spectroscopic study of CO/K coadsorption on Pt{111}

The coadsorption of potassium and CO on Pt{111} has been investigated with infrared reflection-absorption spectroscopy (IRAS) and LEED by varying the individual coverages of both species over a wide range. The vibrational data show several C-O stretching bands with frequencies between 1390 and 1802 cm −1 each of which consists of several absorption lines: up to a K coverage of θ K = 0.2 additional vibrational frequencies were detected between 1830 and 2105 cm −1 which differ only slightly from those on the K-free surface. Five different ordered overlayers could be distinguished. The combination of IRAS and LEED results indicate that the favoured CO bonding site changes from on-top to bridge, in accordance with the results of recent calculations. The site change is confined to the immediate surrounding of the K atoms. The results can be explained by a dominant attractive short-range interaction between CO and K but also show the existence of a weaker long-range interaction.

UPS and HREELS investigation of ethylene oxide and potassium coadsorption on Ni(111)

The adsorption of ethylene oxide (EtO) in the presence of “metallic” K (θ(K) in the range of a first continuous overlayer) on Ni(111) is associated with a ring breaking reaction of EtO and the formation of an EtO + K reaction layer. The products are thermally stabilized to temperatures > 450 K. UPS spectra for EtO and “metallic” K demonstrate even at 100 K the formation of new surface species. Annealing to 450 K leads in the He I UPS spectrum to peaks positioned at 4.5, 8.8, 9.5 and 11.0 eV below EF. They are attributed to aldehyde-like species. This interpretation is supported by HREELS. For EtO and “metallic” K the ring breaking reaction is detected below 200 K with a vanishing of the 850 cm−1 loss feature typical for molecular EtO (C2O ring deformation mode). At 450–500 K annealing temperature new intense losses at 1140 and 1410 cm−1 and the splitting of the CH stretching mode into two losses (2720 and 3060 cm−1) are attributed to the formation of aldehyde-like radicals. For comparison, UPS and HREELS measurements have been performed for acetaldehyde and K on Ni(111) which support this interpretation.

The adsorption of H 2O on K precovered Ni(111) studied by ARUPS and TPD

The adsorption of H 2O on a Ni(111) surface precovered with various amounts of potassium has been studied by TPD, work-function measurements, and angle-resolved UPS using linearly polarized synchrotron radiation. Coadsorption of potassium leads to major changes in the desorption characterics of water which depend strongly on the amount of preadsorbed potassium. At potassium precoverages below θ K ≈ 0.14 ML water is molecularly adsorbed; for higher potassium precoverages dissociation of H 2O into an OH species as well as molecular adsorption is observed at 110 K. The molecularly bound species is significantly stabilized on the surface, as concluded from a high temperature desorption state (170–280 K) in TPD. At low potassium precoverages approximately three H 2O molecules are affected by one potassium atom. Using symmetry selection rules the symmetry of the adsorbed H 2O molecules is determined to C s(σ v, yz ), which is probably due to a tilting in the HOH plane. At high potassium precoverages H 2O decomposes upon adsorption forming an OH species; this OH species is stable up to temperatures above 400 K. The ratio of K to OH formed by dissociation is found to be about 1. From the angle and polarization dependences of the molecular orbitals a tilted orientation of the OH species is derived. The results are compared to those obtained for H 2O adsorption on clean and oxygen-precovered Ni(111) surfaces, and other alkali/metal systems.

The adsorption of H 2 on K precovered Ni(111) studied by ARUPS and TPD

The adsorption of H2O on a Ni(111) surface precovered with various amounts of potassium has been studied by TPD, work-function measurements, and angle-resolved UPS using linearly polarized synchrotron radiation. Coadsorption of potassium leads to major changes in the desorption characterics of water which depend strongly on the amount of preadsorbed potassium. At potassium precoverages below θK ≈ 0.14 ML water is molecularly adsorbed; for higher potassium precoverages dissociation of H2O into an OH species as well as molecular adsorption is observed at 110 K. The molecularly bound species is significantly stabilized on the surface, as concluded from a high temperature desorption state (170–280 K) in TPD. At low potassium precoverages approximately three H2O molecules are affected by one potassium atom. Using symmetry selection rules the symmetry of the adsorbed H2O molecules is determined to Cs(σv,yz), which is probably due to a tilting in the HOH plane. At high potassium precoverages H2O decomposes upon adsorption forming an OH species; this OH species is stable up to temperatures above 400 K. The ratio of K to OH formed by dissociation is found to be about 1. From the angle and polarization dependences of the molecular orbitals a tilted orientation of the OH species is derived. The results are compared to those obtained for H2O adsorption on clean and oxygen-precovered Ni(111) surfaces, and other alkali/metal systems.

Coadsorption of potassium and hydrogen on a Mo(100) surface

The coadsorption of potassium and hydrogen on a Mo(100) surface has been studied by thermal desorption measurements. A small amount of pre-adsorbed potassium (< 0.25 ML) promotes the uptake of hydrogen but does not affect the hydrogen desorption temperature. Higher potassium coverages induce a new hydrogen desorption state which shifts upwards in temperature with increasing coverage. The presence of hydrogen adsorbates also affects the thermal desorption of near-monolayer potassium, indicating a short range K-H interaction. The promotional effect on the hydrogen uptake distinguishes the H/K/Mo(100) system from most other alkali-metal-covered transition metal surfaces.

Coadsorption of potassium and hydrogen on a Mo(100) surface

Coadsorption of potassium and hydrogen on a Mo(100) surface