András Berkó

Adsorption and decomposition of methanol on Rh(111) studied by electron energy loss and thermal desorption spectroscopy

Abstract Methanol adsorbs readily on the Rh(111) surface at 100 K., with a high sticking probability, which decreases only slightly up to monolayer coverage. The adsorption occurs in a random fashion, as no long-range order was found by LEED measurements. Four adsorption states can be distinguished: a condensed layer which exhibits fractional kinetics ( E des = 37 kJ mol ), a physisorbed layer ( E des = 39 kJ mol ), and two chemisorption states. The methanol is initially adsorbed dissociatively to the accompaniment of the appearance of a loss feature at 13.7 eV in the EEL spectrum of Rh(111) in the electronic range. This process is followed by an associative chemisorption of methanol with a bonding energy of 48 kJ mol ; the corresponding loss feature in the EEL spectrum is at 12.1–11.2 eV. The surface concentration of chemisorbed methanol at monolayer formation is ~ 7 × 10 14 molecules cm 2 . The methoxy species is not stable on the Rh(111) surface; some of it reacts with adsorbed hydrogen and desorbs as methanol at 210–250 K, in a second-order process with an activation energy of 58 kJ mol . The remaining methoxy decomposes at around 200 K to produce CO and H on the surface. This process is indicated in the EEL spectra by the appearance of a loss at 13.1 eV due to chemisorbed CO. There are no indications of the formation of stable methoxy species.

STM study of rhodium deposition on the TiO2(110)-(1 × 2) surface

Abstract The structure of rhodium deposited on a well-ordered TiO2(110)-(1 × 2) surface in the submonolayer region and the effect of annealing were studied by scanning tunneling microscopy and Auger electron spectrometry. Deposition of Rh at low coverage gave nanosize (diameter of 1–3 nm) bumpy structures as a result of 3D particle formation. This process roughens the surface considerably. No further radical change in the surface morphology was observed at 0.10–1.0 ML (ML = monolayer). Following annealing of the Rh TiO 2 (110) -(1 × 2) system, three different processes can be distinguished: (i) the encapsulation of Rh particles in the temperature range 500–700 K as indicated by the decrease of the relative AES signal of Rh by 40%. This process is not accompanied by appreciable changes in STM images at higher Rh coverages, but results in an increase of the volume of outrising structures at very low Rh deposition (0.01 ML). (ii) An increase in the size of nanoparticles between 700 and 900 K, indicating their coalescence. The extent of this process depended on the amount of the deposited Rh. (iii) The separation of the 3–5 nm diameter and 3–5 atomiclayers thick Rh crystallites (with their (111) plane parallel to the substrate) from titania above 1100 K, which again exhibits a well-ordered (1 × 2) terrace structure. The evaluation of STM images at different Rh coverages led to the conclusion that this latter process is probably due to the de-encapsulation of the encapsulated Rh crystallites.

Coincident thermal desorption and salt formation in CO+K coadsorbed layers

Coadsorption of CO and K on metal surfaces leads to a mutual stabilization of both species and to their coincident thermal desorption. It is shown that the coincident desorption temperatures vary significantly with the nature of the metals and it increases in the sequence Cu-Pd-Ni-Ru-Pt-Rh-Fe: the difference between the lowest and highest value is about 239 K. It is concluded that neither salt formation nor the dissociation of CO, and hence the formation of a stable K-O species, could account for these features. It is proposed that a K8+-COδ surface complex is formed between CO and potassium which strongly interacts with the underlying metal. The stability of the complex on the surface is primarily determined by the bond strength of the CO with the host metal perturbed by potassium.

Adsorption and oxidation of HCN on oxygen-dosed Cu(111) surface studied by AES, ELS and TDS measurements

Abstract No detectable adsorption of HCN was observed on a clean Cu(111) surface at 300 K. The presence of adsorbed oxygen, however, exerted a dramatic influence on the interaction, and caused the dissociative adsorption of HCN with concomitant release of water. The adsorption of HCN on an oxygen-dosed Cu(111) surface produced two intensive losses at 10.4 and 12.5 eV in the electron energy loss spectra. At a low HCN exposure a weak loss at 13.4 eV was also noticed, which was attributed to the formation of NCO surface species. A reaction began in the co-adsorbed layer above 400 K with the evolution of CO2. The desorption of nitrogen started at 700 K, while the formation of C2N2 was observed above 800 K. The characteristic features of the formation of these species agreed well with those found in the reaction of NCO groups with adsorbed oxygen on this surface. It is proposed that the oxidation of CN groups on a Cu(111) surface occurs through the formation and oxidation of NCO species.

Adsorption and dissociation of CH3OH on clean and K-promoted Pd(100) surfaces

The adsorption and dissociation of methanol on clean and K-dosed Pd(100) surfaces were investigated in the temperature range 90–600 K. By means of photoelectron and thermal desorption spectroscopy, condensed layer, chemisorbed and dissociated methanol were distinguished. Methanol decomposes through the formation of a methoxy species. No adsorbed CH3 was detected on clean Pd(100) in the course of heating of the adsorbed layer from 90 K to high temperatures. The cleavage of a methanolic CO bond, and the formation of a small amount of adsorbed CH3 were observed only above 300 K, but during continuous dosing of the surface with CH3OH. Additive potassium markedly increased the surface concentration of chemisorbed methanol and the formation of methoxy species, but it did not enhance the methanolic C-O bond breaking. A significant stabilization of methoxy was also experienced: its complete decomposition occurred at 450–500 K. In the discussion of the data, a direct chemical interaction between methanol and potassium and the formation of a stable KOCH3 complex are proposed.

Effects of potassium on the chemisorption of CO2 and CO on the Pd(100) surface

Abstract It has been found that preadsorbed potassium dramatically affects the adsorption behaviour of CO2 and CO on Pd(100) surface. In increases the rate of adsorption, the binding energy of CO2 and CO and it induces the dissociation of CO2.

Structure and properties of potassium on Pd(100) surface

The adsorption of K on Pd(100) was investigated using LEED, EELS, work function and thermal desorption measurements. Adsorbed potassium on Pd(100) formed two well-ordered structures, p(2×2) at θ K =0.25, and c(2×2) at monolayer coverage. This latter corresponds to a surface concentration of 6.7×10 14 K atoms/cm 2 . The work function of Pd decreased to a minimum at a coverage of 2.7×10 14 K atoms/cm 2 and then increased toward the value of metallic potassium. The metallization began at∼1.5 monolayer. The activation energy of desorption decreased from 240 kJ/mol at zero coverage to 108 kJ/mol for monolayer coverage. Adsorption of K on Pd(100) produced four loss features (at 19.4, 18.4, 3.5 and 2.6 eV) in the EEL spectrum which were attributed to K(3p)→K(4s), K(3p)→Pd Fermi level, Pd d-band→K(4s) electron transitions and to a s-derived plasmon loss in metallic potassium.

Adsorption of CH3Cl on clean and Cl-dosed Pd(100) surfaces

Abstract The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH3Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH3Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH3Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH3Cl caused a significant work function decrease, Δϕ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH3Cl at lower concentration, prevented the adsorption of CH3Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.

Interaction of Carbon Monoxide with Au(111) Modified by Ion Bombardment: A Surface Spectroscopy Study under Elevated Pressure †

Gold based model systems exhibiting the structural versatility of nanoparticle ensembles and being accessible for surface spectroscopic investigations are expected to provide new information about the adsorption of carbon monoxide, a key process influencing the CO oxidation activity of this noble metal in nanoparticulate form. Accordingly, in the present work the interaction of CO is studied with an ion bombardment modified Au(111) surface by means of a combination of photoelectron spectroscopy (XPS and UPS), sum frequency generation vibrational spectroscopy (SFG), and scanning tunneling microscopy (STM). While no adsorption was found on intact Au(111), data collected on the ion bombarded surface at cryogenic temperatures indicated the presence of stable CO adsorbates below 190 K. A quantitative evaluation of the C 1s XPS spectra and the surface morphology explored by STM revealed that the step edge sites created by ion bombardment are responsible for CO adsorption. The identification of the CO binding sites was confirmed by density functional theory (DFT) calculations. Annealing experiments up to room temperature showed that at temperatures above 190 K unstable adsorbates are formed on the surface under dynamic exposure conditions that disappeared immediately when gaseous CO was removed from the system. Spectroscopic data as well as STM records revealed that prolonged CO exposure at higher pressures of up to 1 mbar around room temperature facilitates massive atomic movements on the roughened surface, leading to its strong reordering toward the structure of the intact Au(111) surface, accompanied by the loss of the CO binding capacity.

Effects of potassium on the adsorption and dissociation of CH3Cl on Pd(100)

Abstract The effects of potassium on the adsorption and dissociation of CH3Cl on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (in the electronic range EELS), temperature-programmed desorption (TPD) and work function change. In contrast to the clean surface, the adsorption of CH3Cl caused a significant work function increase, 0.9-1.4 eV, of potassium-dosed Pd. Preadsorbed K enhanced the binding energy of CH3C1 to the surface and induced the dissociation of adsorbed molecules. The extent of the dissociation increased almost linearly with the potassium content. The appearance of a new emission in the UPS spectrum at 9.2 eV, attributed to adsorbed CH3 species, and the low-temperature formation of ethane suggest that a fraction of adsorbed CH3Cl dissociates even at 115–125 K on potassium-dosed Pd(100). At the same time, a significant part of adsorbed CH3 radical is stabilized, the reaction of which occurs only at 250–300 K. By means of TPD measurements, H2, CH4, C2H6, C2H4, KCl and K were detected in the desorbing gases. The results are interpreted by assuming a through-metal electronic interaction at low potassium coverage and by a direct interaction of the Cl in the adsorbed CH3Cl with potassium at high potassium coverage. The latter proposal is supported by the electron excited Auger fine structure of the Cl signal and by the formation of KCl in the desorbing gases.