Room Temperature Adsorption Research Articles

Calorimetric and spectroscopic study of the coordinative unsaturation of copper(I) and silver(I) cations in ZSM-5 zeolite: Room temperature adsorption of NH 3

The room temperature (RT) adsorption of NH 3 was used to probe the coordinative unsaturation of Cu(I) and Ag(I) cations highly dispersed in ZSM-5 zeolites. Adsorption microcalorimetry allowed to characterise both quantitatively and energetically the amino complexes formed at the cationic sites, as revealed by IR spectroscopy. Copper(I) sites were found to form tetra-amino [Cu(NH 3) 4] + adducts, whereas silver(I) ones do form only di-amino [Ag(NH 3) 2] + species. Both results are in agreement with the homogeneous chemistry of Cu(I) and Ag(I) cations. The heat of formation of the different amino-complexes was found to be comprised in the 130–50 kJ/mol interval for both kind of Me(I) sites, according to the number of ligands progressively bound and in spite of the different stoichiometry of the species formed. The adducts were found to be only partially reversible upon outgassing in the adopted conditions (303 K and p≈10 −5 Torr). Two distinct, nearly equally populated families of sites were revealed in both Me(I)-ZSM-5 samples, corresponding to different coordinative unsaturations of the metal cations. EXAFS data, used to check the local environment of noble metal cations, confirmed the proposed model. In the case of Cu(I) sites, the formation of mixed amino-carbonyl complexes was also studied. The thermodynamic stability of amino- and carbonyl-like species was found not to be the same.

Microcalorimetric and IR-spectroscopic study of the room temperature adsorption of CO 2 on pure and sulphated t-ZrO 2

The room temperature adsorption of CO 2 on pure and sulphated tetragonal ZrO 2 (t-ZrO 2) was studied, in order to assess how and to what extent the acid–base properties of ZrO 2 are modified by the presence of surface sulphate groups. The formation of different species, as well as their stability upon outgassing, was monitored by IR spectroscopy (2000–1000 and 2400–2300 cm −1 regions), whereas the population and the distribution of sites of different kind was quantitatively and energetically characterised by adsorption microcalorimetry. The formation of carbonate-like species was found to be a highly energetic ( q ads>100 kJ/mol) and irreversible process, occurring only on the (at least partially) dehydrated surface of pure t-ZrO 2. Bicarbonate-like (hydrogen-carbonate) species (either of stable type) were found to form on both pure and sulphated t-ZrO 2 specimens, and to be accompanied by a heat of adsorption variable in the 100–40 kJ/mol range. The presence at the surface of sulphate groups causes the basicity of the t-ZrO 2 matrix to be dramatically depressed, the most basic OH − and/or coordinatively unsaturated (CUS) O 2− ions being preferentially consumed by the sulphation process. Linear coordinated CO 2 complexes were also found to form on CUS Zr 4+ cations, the acidic strength of which increases with the extent of surface dehydration and as a consequence of the presence of charge-withdrawing surface sulphate groups.

From oxygen adsorption to the growth of thin oxides on silicon surfaces

Oxidation of silicon surfaces at relatively low temperatures is shown to go through several activated steps, in the form of configurations inert to further uptake of oxygen. Starting from room temperature adsorption, different configurations of oxygen atoms adsorbed on and in the Si(1 1 1) and Si(0 0 1) surfaces are found, with history and/or coverage dependent energy barriers connecting them. From well below to slightly above an effective oxide coverage of a monolayer, clustering of up to three oxygen atoms around one single silicon atom has been predicted for the Si(0 0 1) surface to represent one such energy minimum; this model is confirmed here experimentally. These and other clusters are shown to agglomerate into silicon dioxide islands before coalescing into a contiguous, inert layer upon higher oxygen supplies. Another problem addressed here is the presence of molecular adsorbates in the oxidation reaction path, an issue which is still debated in the literature. For the Si(1 1 1) surface a molecular, charged oxygen species has earlier been found at temperatures up to room temperature, but not for the Si(0 0 1) surface. This is confirmed in the present experiments, and new data for this state shows that it is highly mobile until quenched at a critical oxygen coverage. It is not the initial state of oxygen on silicon, and therefore not the precursor for atomic insertion of oxygen; rather, it is found to co-exists with atomic oxygen inserted in back-bonds, at a certain, low coverage regime in which parts of the Si(1 1 1) surface are still ordered.

Adsorption of chlorine on the Si(0 0 1)-2×1 surface

It is shown that the room-temperature dissociative adsorption of Cl 2 on the Si(0 0 1)-2×1 surface results in the bonding of Cl atoms to both the up- and down-dimers which then become symmetrical. At saturation coverage the Si 2p core-level spectra exhibit only a single Cl-induced component with emission twice as strong as that from the up-dimer of the clean surface and with a core-level shift of +930±8 meV . These results were obtained using an analysis in which the intensities of the core-level spectra from the individual Si layers are constrained by an escape depth. It facilitates the identification of components down to the second subsurface layer and yields an inelastic mean free path of 3.4±0.2 Å at a photon energy of 140 eV.

Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon

Amorphous materials are usually characterized using nitrogen adsorption isotherms at 77 K taken at pressures up to 1 bar to obtain pore size distributions. Activated carbons are amorphous microporous graphitic materials containing pores which can range from nanometers to microns in width and which can, in principle, be tailored to adsorb specific molecules or classes of molecule by changing the method of preparation (the activation process). For the physical chemist, they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. In this paper, we compare pore size distributions of an ultrahigh surface area activated carbon (AX21) determined from nitrogen adsorption measurements up to 0.6 bar at 77 K with those determined from carbon dioxide adsorption measurements up to 20 bar at 298 K. Our analysis employs grand canonical and Gibbs ensemble Monte Carlo simulations together with accurate site−site interaction models of the adsorbates. We find that the calculated pore size distributions for each adsorbate are quite different, and the adsorption of one gas can be estimated from the adsorption of the other gas to within an error of 25% at the highest pressures only. At lower pressures, we speculate that large errors are due to the behavior of nitrogen in carbon micropores in which diffusion is severely limited. To substantiate this speculation, we have calculated the self-diffusion coefficient for nitrogen at 77 K and carbon dioxide at 298 K in carbon slit pores using equilibrium molecular dynamics. The results suggest that nitrogen is diffusionally limited, and possibly frozen, in such pores whereas carbon dioxide remains mobile. We conclude that room-temperature carbon dioxide adsorption isotherms up to the saturation pressure could provide a more accurate characterization of carbon microstructure than nitrogen isotherms at 77 K up to 1 bar.

Reactions of methylamines at the Si(100)-2×1 surface

We have investigated the room temperature adsorption of methylamine, dimethylamine and trimethylamine using density functional theory (DFT) and multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy. It was found that the reaction pathways of the amines resemble the precursor-mediated dissociative chemisorption of ammonia. Our calculations showed that although dissociation involving N–C bond cleavage is thermodynamically more favorable than the N–H dissociation pathway, the activation barrier for N–CH3 dissociation is significantly higher than that for N–H dissociation. This leads to selective cleavage of N–H bonds in the surface reactions of methylamine and dimethylamine, while trapping trimethylamine in its molecularly chemisorbed state through the formation of a Si–N dative bond. We also identified the products of the reactions of the amines on the Si(100)-2×1 surface by surface IR studies, confirming the theoretical predictions. The selectivity observed in the surface chemistry of simple model amines is briefly discussed in the context of organic chemistry at semiconductor surfaces.

Interaction of organophosphorous compounds with TiO 2 and WO 3 surfaces probed by vibrational spectroscopy

The interaction of organophosphorous compounds with TiO 2 and WO 3 high surface area powders has been studied by thin film infrared spectroscopy. Room temperature adsorption of dimethyl methyl phosphonate (DMMP), dimethyl hydrogen phosphonate (DMHP), and trimethyl methyl phosphonate (TMP) is through hydrogen bonding of the PO functional group to the hydroxyl groups of the metal oxide surface. At higher reaction temperatures, these hydrogen bonded organophosphorous compounds dissociate and form covalently attached species. Above 200°C, the methoxy groups desorb from the surface while the methyl groups remain stable. Above 300°C, a stable phosphate surface complex is formed. The presence of the phosphate complex may be responsible for poisoning effects observed during DMMP gas exposure of chemiresistive sensors operating in this temperature range.

Thermal chemistry of toluene and benzene on Si(1 0 0)2×1 and modified surfaces

The effects of methyl substitution on the room-temperature (RT) adsorption and reactivity of toluene on Si(1 0 0)2×1 have been investigated by thermal desorption spectrometry (TDS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The similarities in the exposure dependence of the molecular desorption profiles of d 8-toluene and those of d 6-benzene suggest that both molecules have similar adsorption configurations on the (2×1) surface. In particular, three molecular desorption features are found and can be attributed to adsorption on the single-dimer, double-dimer, and defect sites. However, the higher coverage and much lower molecular desorption of toluene than those of benzene suggest that the majority of adsorbed toluene has undergone other surface reactions during the TDS experiment. In addition to the weak molecular desorption, strong recombinative hydrogen desorption at 820 K, characteristic of hydrogen coming from mono-hydride (Si–H) surface species, is observed for toluene. The lack of a correspondingly strong D 2 desorption peak for the RT exposure of d 6-benzene to Si(1 0 0)2×1 indicates that the deposition of D atoms results from hydrogen abstraction from the methyl group of d 8-toluene. Since a RT post-exposure of atomic hydrogen to Si(1 0 0)2×1 saturated with d 8-toluene does not appear to increase molecular desorption, the observed hydrogen abstraction therefore occurs upon adsorption and appears to be an irreversible process at RT. The difference in the D 2 desorption profiles between toluene and D 2 on Si(1 0 0)2×1 suggests, in addition to hydrogen desorption, other surface processes including condensation polymerization and/or dissociative desorption that occur at 750–950 K. The Si(1 0 0)2×1 surface is found to be more active in hydrogen abstraction and toluene dissociation than Si(1 1 1)7×7. For a RT exposure of d 8-toluene (d 6-benzene) to a sputtered Si(1 0 0) surface, the presence of an additional broad mass 4 desorption band at ∼600 K suggests the presence of a range of pathways by which D 2 evolves directly from the dissociation of adsorbed d 8-toluene. A RT exposure of O 2 to Si(1 0 0)2×1 pre-adsorbed with d 8-toluene (d 6-benzene) is found to induce a substantial increase in the mass 4 desorption. The increase in the dissociation can be attributed to either surface-mediated oxidation or an O 2-induced process that stabilizes the adsorbed molecules to a higher temperature at which dehydrogenation and dissociation occur.

Effect of a Methyl-Protecting Group on the Adsorption of Pyrrolidine on Si(100)-2 × 1

The room-temperature adsorption of pyrrolidine and its methyl-protected analogue, N-methylpyrrolidine, on the Si(100)-2 × 1 surface has been investigated using multiple internal reflection Fourier transform infrared spectroscopy and ab initio quantum chemistry calculations. For both compounds, initial adsorption occurs by barrierless formation of a dative bond between the nitrogen lone pair and the electrophilic atom of the Si dimer. However, while pyrrolidine proceeds to chemisorb dissociatively through N−H bond cleavage, methylpyrrolidine is shown to be trapped in its dative-bonded precursor state at room temperature due to a substantial barrier for N−CH3 cleavage. Additionally, the saturation coverage of methylpyrrolidine on Si is seen to be significantly less than that of pyrrolidine, due likely to both steric factors and charge-transfer effects.

Room temperature adsorption of Na on Si(1 1 1)-7×7 as studied by resonant optical second harmonic generation

Resonantly enhanced optical second harmonic generation at a pump laser energy of 1.17 eV was applied to investigate the Na-adsorption process on the 7×7 reconstructed Si(1 1 1) surface at room temperature. The change of second harmonic (SH) intensity as a result of Na deposition at normal incident pump radiation varies significantly from that measured at 45° angle of incidence. The observed difference can be explained in terms of electronic transitions from the rest-atom state to the adatom state of the 7×7 reconstructed Si(1 1 1) surface as well as of the features of Na growth on the silicon surface. By measuring the SH polarization dependence for different stages of Na deposition, a change of the interface symmetry in both directions, namely, parallel and perpendicular to the (1 1 1) plane, was clearly observed at the very beginning of the adsorption process.

Scanning tunneling microscopy studies of oxygen adsorption on Cu(111)

The adsorption of O2 on Cu(1 1 1) at room temperature has been investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Adsorption of oxygen leads to formation of a surface oxide by incorporation of Cu atoms from step edges and terraces. This process is most rapid along the close packed direction of the surface and leads to mesoscopic changes in surface morphology. Three characteristic features were observed during the initial stage of adsorption; dark fringes along the Cu(1 1 1) step edges, dark domains within the Cu(1 1 1) terrace, and rather mobile light patches on top of the Cu terraces. Within these regions atomic scale features could be imaged and the structure related to that of Cu2O. The dark fringes and dark domains grew slowly in oxygen, whereas the bright patches only became visible when gas-phase O2 was evacuated. STM observation at elevated temperatures indicates mobilization and rearrangement of surface features formed during room temperature adsorption. O/Cu(1 1 1) surfaces annealed at 473–623 K indicated a well ordered (73R5.8°×21R−10.9°) lattice structure (‘44’-structure) on the terraces which was also confirmed by LEED. Annealing up to 723 K exhibited slight disordering of this lattice structure presumably due to inter-diffusion of O and Cu atoms with the bulk. The mesoscopic process of oxygen adsorption onto the clean Cu(1 1 1) surface was also investigated at elevated temperature by STM. The reaction of the surface maintained at temperatures between 373 and 773 K was investigated for oxygen pressures in the 10−5–10−3 Pa range. The oxidized surface formed at 373 K was largely disordered with only small areas of the ‘44’-structure (√73R5.8°×√21R−10.9°). Well-ordered adlattices of the ‘44’-structure were formed upon O2 exposure at 473 K. Such surfaces could be converted to the ‘29’-structure (√13R46.1°×7R21.8°) by annealing at 673 K in vacuum. On the basis of the STM images we propose a new model for the ‘29’ structure. Annealing at higher temperatures (∼773 K) retrieved the ‘44’-structure in small domains on a disordered background. A disordered oxide surface which was produced at room temperature and annealed at 723 K can be ordered into the ‘44’-structure by O2 exposure and heating at 623 K.

C 60 adsorption on the quasicrystalline surface of Al 70Pd 21Mn 9

Room temperature adsorption of C 60 on the flat quasicrystalline surface of Al 70Pd 21Mn 9 has been investigated using scanning tunnelling microscopy. A dispersed overlayer is formed at low coverage, with avoidance of step-edges. There is no evidence of island formation or clustering. As the coverage is increased, a higher density layer is formed with no evidence of the formation of hexagonal ordered adsorbate structures seen on other substrates. This is followed by the onset of second layer formation. A range of bonding sites for C 60 molecules is implied from measurements of apparent molecular heights and from thermal effects. Detailed analysis of the surface at a low coverage (∼0.065 ML) provides evidence of adsorbate local order, with Fibonacci ( τ-scaling) relationships between the C 60 molecules. Where this occurs, the preferred adsorption site is tentatively identified as the pentagonal hollow. These local correlations however are not found to extend over larger regions of the surface.

The electronic properties at the maleic anhydride/Si(1 0 0)-2×1 interface

The formation of electronic states in the fundamental gap at the maleic anhydride/Si(1 0 0)-2×1 interface has been studied with photoluminescence (PL) measurements and high resolution electron energy loss spectroscopy (HREELS). We observe that the room temperature adsorption of maleic anhydride on Si(1 0 0)-2×1, where the organic molecules are mainly in a di-σ coordination, results in a gap state density D it =1.8(2)×10 12 eV −1 cm −2 which is slightly lower than determined for the clean Si(1 0 0)-2×1 surface. After heating maleic anhydride/Si(1 0 0)-2×1 to 1115 K, the gap state density has increased to 6.4(6)×10 12 eV −1 cm −2 which we relate to the formation of silicon carbide structures on the silicon surface.

Molecular and dissociative adsorption of 2-bromo-1-chloropropane on Cu(111)

The molecular adsorption and subsequent reaction of 2-bromo-1-chloropropane with Cu(111) has been studied using primarily ultraviolet photoelectron spectroscopy and line of sight temperature programmed desorption. The molecule reacts by the following two steps: ClCH 2CHBrCH 3(g)→ClCH 2CHBrCH 3(phys) ClCH 2CHBrCH 3(phys)→Cl(chem)+Br(chem)+CH 2CHCH 3(g)↑ At room temperature these reactions go to completion with emission of propene and formation of a ( 3 × 3 )R30°-Cl/Br surface. At 100 K adsorption of the first layer of BCP occurs molecularly, but with the adsorption of a second and subsequent layers, the activation energy for decomposition of the first layer is lowered, such that it decomposes at 100 K. On heating a multilayer surface of BCP formed at 100 K, the second and third layers of BCP are also observed to decompose with emission of propene and formation of the same ( 3 × 3 )R30°-Cl/Br surface formed by room temperature adsorption. It was found that a single layer of BCP is stabilised by adsorption to this halogenated surface, only desorbing at the relatively high temperature of 180 K.

Role of hydrogen prepairing in the hydrogen desorption kinetics from Si(100)-2×1: effects of hydrogenating-gas and thermal history

Hydrogen desorption kinetics from Si(100)-2×1:H has been systematically investigated using temperature-programmed desorption (TPD) on several hydrogenating gases and thermal conditions. As a result, the desorption kinetic order with the hydrogen coverage was found to increase in the order: atomic hydrogen<disilane<silane and room-temperature adsorption<high-temperature adsorption<post-annealing. These variations in kinetic order, depicted as a TPD peak shift at low hydrogen coverages, are universally described with a single surface parameter, γ 0, the fractional coverage of unpaired hydrogen atoms. Fitting with obtained TPD spectra demonstrates that γ 0 is a delicate function of the hydrogenating gas and thermal history.

Silver Ion-Exchanged Zeolites Y, X, and Low-Silica X: Observations of Thermally Induced Cation/Cluster Migration and the Resulting Effects on the Equilibrium Adsorption of Nitrogen

Silver is known to strongly affect the adsorptive properties of some zeolites. It is also known that thermal vacuum dehydration of some argentiferous zeolites leads to the formation of charged silver clusters within the zeolite. In this work we have synthesized silver zeolites of the types Y, X, and low-silica X. The zeolites were treated in such a way as to promote the formation of intracrystalline charged silver clusters. Equilibrium room-temperature isotherms were measured for adsorption of nitrogen for each of the zeolites after various heat treatments and dehydration. These materials were structurally characterized via Rietveld refinement using neutron powder diffraction data. Color changes upon heat treatment and subsequent X-ray photoemission spectroscopy confirmed some reduction of Ag+ → Ag0. The effects of various dehydration conditions, including the time, temperature, and atmosphere, on the room-temperature adsorption of nitrogen are discussed. Structural characterization, along with valence bond calculations, revealed the presence of cations in site II*, which are more active in Ag−LSX samples that were vacuum dehydrated at 450 °C as compared to those that were vacuum dehydrated at 350 °C.

Si 2H 6 adsorption and hydrogen desorption on Si(100) investigated by infrared spectroscopy

The adsorption and decomposition of disilane on Si(100)(2×1) was investigated using in-situ infrared (IR) absorption spectroscopy. The IR data demonstrate that upon room-temperature adsorption, disilane dissociatively adsorbs on the unsaturated dangling bonds of dimers with the dimer bonds unbroken, to produce mono-, di-, and tri-hydride species. At low coverages, dissociative adsorption without breaking of the SiSi bond of Si 2H 6 is favored. Thermal annealing following room-temperature disilane adsorption produces the doubly-occupied adatom dimers (DOD, HSiSiH) and isolated monohydride species. These hydride species are generated via the rupture of dimer bonds of the substrate. Hydrogen desorption from the isolated monohydride site occurs at lower temperatures than from the DOD site.

Adsorption and decomposition of methylsilanes on Si(100)

We have investigated in-situ the adsorption and thermal decomposition of methylsilanes, SiH x (CH 3) 4− x ( x=1–3), on Si(100)(2×1), using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry. IRAS spectra revealed that at initial stages of adsorption, monohydride (–SiH) and CH 3-substituted hydride species (–SiH x (CH 3) 3− x ) are generated with monohydride species being dominant. We suggest that upon room temperature adsorption of methylsilanes, breaking of the SiH bonds of methylsilane is favored over that of the SiC bonds. It is found that the dissociative adsorption of SiH 3(CH 3) exhibits the second-order kinetics. Due to thermal annealing, surface species –SiH x (CH 3) 3− x are thermally decomposed to generate surface SiH and SiC bonds, and subsequently H 2 desorption from the SiH bonds occurs.

Cs Adsorption on ZrC(111): Photoemission Spectroscopy Study

Room-temperature adsorption of Cs on ZrC(111) surface has been studied by core-level photoemission spectroscopy. The work function of the substrate surface decreases monotonically with Cs adsorption and approaches the Cs metal value without showing a clear work function minimum. Cs 4d core-level lineshape analysis reveals that the loss peaks due to overlayer plasmon excitation are already formed in the initial stages of adsorption. These results suggest that the adsorbed Cs atoms are in a metallic state which is brought about by the formation of Cs islands at low coverages and that adsorption proceeds via growth of these islands.

Infrared study of adsorption and thermal decomposition of Si2H6 on Si(100)

Sumario We have investigated the adsorption and thermal decomposition of disilane on Si(100) (2×1) using infrared absorption spectroscopy. We demonstrate that at room temperature, disilane dissociatively adsorbs onto the surface with the dimer bond unbroken, to produce mono-, di-, and tri-hydride species. At low coverages, the disilane molecule adsorbs on the surface without breaking the Si–Si bond of the molecule. Thermal annealing following room-temperature adsorption of disilane produces a hydrogen-terminated adatom dimer (HSi–SiH) and an isolated monohydride species. We suggest that these hydride species are generated through the rupture of the substrate dimer bonds. Hydrogen desorption from the isolated monohydride occurs at much lower temperature than that from the adatom dimer.