Adsorption and reaction of formaldehyde on thin-film cerium oxide

Formaldehyde adsorption and reaction have been studied on cerium oxide thin films that were vapor deposited on Ru(0 0 0 1). The formaldehyde behavior was examined as a function of temperature, exposure and Ce oxidation state. Formaldehyde chemisorbs on fully oxidized CeO₂ as dioxymethylene, CH₂O₂. The dioxymethylene decomposes and desorbs as formaldehyde between 200 K and 400 K. No other products are formed. On reduced ceria, formaldehyde also adsorbs as dioxymethylene. In addition to the formaldehyde desorption between 200 K and 400 K, a more strongly bound form of dioxymethylene is formed that produces formaldehyde at 440 K. Above 400 K, some of the dioxymethylene reacts to form formate and methoxy on the surface. These species decompose to produce H₂, CO and CH₂O above 500 K.     

A density functional theory study of formaldehyde adsorption and oxidation on CeO2(1 1 1) surface

The effects of different oxygen species and vacancies on the adsorption and oxidation of formaldehyde over CeO₂(1 1 1) surface were systematically investigated by using density functional theory (DFT) method. On the stoichiometric CeO₂(1 1 1) surface, the C-H bond rupture barriers of chemisorbed formaldehyde are much higher than that of formaldehyde desorption. On the reduced CeO₂(1 1 1) surface, the energy barriers of C-H bond ruptures are less than those on the stoichiometric CeO₂(1 1 1) surface. If the C-H bond rupture occurs, CO and H₂ form quickly with low energy barriers. When O₂ adsorbs on the reduced (1 1 1) surface (O₂/O_v species), the C-H bond rupture barriers of formaldehyde are greatly reduced in comparison with those on the stoichiometric CeO₂(1 1 1) surface. If O₂ adsorbs on oxygen vacancy at sub-layer surface, its oxidative roles on formaldehyde are much similar to that of O₂/O_v species.     

First principle calculations of benzotriazole adsorption onto clean Cu(1 1 1)

The adsorption of benzotriazole (BTAH or C₆N₃H₅) on a Cu(1 1 1) surface is investigated by using first principle density functional theory calculations (VASP). It is found that BTAH can be physisorbed (<0.1 eV) or weakly chemisorbed (∼0.43 eV) onto Cu(1 1 1), and the chemical bond is formed through nitrogen sp² lone pairs. The weak chemisorption can be stabilized by reaction with neighboring protonphilic radicals, like OH⁻. Furthermore, the geometries and associated energies of intermolecular hydrogen bonds between adsorbates on Cu(1 1 1) are also calculated. A model of the first layer of BTAH/BTA⁻ on Cu(1 1 1) surface is developed based on a hydrogen bond network structure.     

The adsorption and dissociation of O2 molecular precursors on Cu: the effect of steps

The adsorption and dissociation of dioxygen on Cu steps are studied using periodic self-consistent density functional theory (PW91-GGA) calculations. Cu steps are modeled with a Cu(2 1 1) surface. The results are compared with those on the flat Cu(1 1 1) surface. The adsorption of both atomic and molecular oxygen is enhanced on the stepped surface: the binding energy of atomic oxygen is −4.5 eV at its preferred site on the relaxed Cu(2 1 1) surface vs. −4.3 eV at its preferred site on the relaxed Cu(1 1 1) surface, and the binding energy of the molecular oxygen precursor is increased from ∼−0.6 to ∼−1.0 eV. Several possible O₂ dissociation paths at the edge of the Cu(2 1 1) step have been investigated. The activation energies range from 0.09 to 0.24 eV, comparable to a minimum activation energy of 0.20 eV found on Cu(1 1 1). However, compared to Cu(1 1 1) the paths on Cu(2 1 1) are stabilized in their entirety by the step by ∼0.5 eV in terms of initial state, transition state, and final state energies. The dissociation of O₂ precursors at the foot of the step is close to being barrier-less. Because of the small dissociation barrier on Cu(1 1 1), the effect of steps on O₂ dissociation is nevertheless not expected to be as pronounced as in other gas/metal systems.     

Adsorption of water on TiN (1 0 0), (1 1 0) and (1 1 1) surfaces: A first-principles study

We use first-principles density functional theory-based calculations in the analysis of the interaction of H₂O with (1 0 0), (1 1 0) and (1 1 1) surfaces of TiN, and develop understanding in terms of surface energies, polarity of the surface and chemistry of the cation, through comparison with H₂O adsorption on ZrN. While water molecule physisorbs preferentially at Ti site of (1 0 0) and (1 1 1) surfaces, it adsorbs dissociatively on (1 1 0) surface of TiN with binding stronger than almost 1.32 eV/molecule. Our analysis reveals the following general trends: (a) surfaces with higher energies typically lead to stronger adsorption, (b) dissociative adsorption of H₂O necessarily occurs on a charge neutral high energy surface and (c) lower symmetry of the (1 1 0) plane results in many configurations of comparable stability, as opposed to the higher symmetry (1 0 0) and (1 1 1) surfaces, which also consistently explain the results of H₂O adsorption on MgO available in literature. Finally, weaker adsorption of H₂O on TiN than on ZrN can be rationalized in terms of greater chemical stability of Ti arising from its ability to be in mixed valence.     

Adsorption and reaction of methanol on thin-film cerium oxide

Methanol adsorption and reaction has been studied on cerium oxide thin films that were vapor deposited on Ru(0 0 0 1). The methanol behavior was examined as a function of temperature and Ce oxidation state. Methanol reacts at low temperatures with fully oxidized CeO₂ to produce water at 200 K while formaldehyde and methanol desorb near 560 K. This leads to the reduction of the ceria. On reduced ceria, more methanol can be adsorbed and it undergoes more extensive decomposition producing CO and H₂ near 640 K in addition to formaldehyde and water. As the degree of ceria reduction increases, more H₂ and less H₂O are produced. TPD experiments using isotopically labeled CH₃OD show that deuterated water is produced from the oxidized surface at low temperatures, whereas the deuterium is stabilized on the reduced surface and is incorporated into the dihydrogen that desorbs near 600 K. High resolution C 1s and O 1s XPS and C k-edge NEXAFS measurements were performed to quantify the amount of methanol adsorbed and to identify the adsorbed species.     

Low energy Ar-ion bombardment effects on the CeO2 surface

The structure and electronic properties of epitaxial grown CeO₂(1 1 1) thin films before and after Ar⁺ bombardment have been comprehensively studied with synchrotron radiation photoemission spectroscopy (SRPES). Ar⁺ bombardment of the surface causes a new emission appearing at 1.6 eV above the Fermi edge which is related to the localized Ce 4f¹ orbital in the reduced oxidation state Ce³⁺. Under the condition of the energy of Ar ions being 1 keV and a constant current density of 0.5 μA/cm², the intensity of the reduced state Ce³⁺ increases with increasing time of sputtering and reaches a constant value after 15 min sputtering, which corresponds to the surface being exposed to 2.8 × 10¹⁵ ions/cm². The reduction of CeO₂ is attributed to a preferential sputtering of oxygen from the surface. As a result, Ar⁺ bombardment leads to a gradual buildup of an, approximately 0.69 nm thick, sputtering altered layer. Our studies have demonstrated that Ar⁺ bombardment is an effective method for reducing CeO₂ to CeO_2−x and the degree of the reduction is related to the energy and amount of Ar ions been exposed to the CeO₂ surface.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.     

Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.     

DFT study of NH3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH₃ molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH₃ molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH₃ incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH₃ molecule an energy barrier of ∼0.3 eV was calculated, yielding NH₂ on the adatom and H on the restatom. When two molecules are adsorbed, the NH₃-NH₃ interaction yields the weakening of a N-H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH₃ molecule at the adatom-restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).     

First-principles study on the catalytic role of Ag in the oxygen adsorption of LaMnO3(0 0 1) surface

In the present paper, the catalytic role of Ag in the oxygen adsorption of LaMnO₃(0 0 1) surface has been theoretically investigated using first-principles calculations based on the density functional theory (DFT) and pseudopotential method. The O₂ adsorption energy is larger for the vertical adsorption and the covalent bond was formed between O₂ molecule and surface Mn. The calculation of electronic properties of interaction between Ag atom and LaMnO₃(0 0 1) surface demonstrates that the most stable position for Ag adsorption is hollow site. The O₂ adsorption energy dramatically increased from 0.298 eV to 1.108 eV due to Ag pre-adsorbed. It is Ag pre-adsorbed that facilitates O₂ adsorption on surface. The bond length and bond population of O₂ molecule indicate that Ag atom facilitates O₂ molecule dissociative adsorption. The Ag atom strengthens LaMnO₃(0 0 1) substrate activity and activity center was formed on surface, which enhances the electrocatalytic activity of LaMnO₃ as solid oxide fuel cells cathode material at low temperature.     

Theoretical study on adsorption of thiophenethiolate molecule on Au(1 1 1) surface

We have theoretically studied the adsorption of a thiophenethiolate (C₄H₃S-S) molecule on the Au(1 1 1) surface by first-principles calculations. It is found that the bridge site is the most stable adsorption site with the adsorption energy of 1.02 eV. In the optimized adsorption geometry, the bond between the head S atom and the connected C atom in the tail thiophene molecule is tilted by 57.2° from the surface normal. In addition, the adsorption of thiophenethiolate induces large relaxations of the surface Au atoms around it. Furthermore, weak interactions between the S atom in the tail thiophene ring and the Au atoms also contribute to the adsorption on the Au surface.     

CO adsorption on oxygen-modified molybdenum surfaces

Structures of carbon monoxide layers on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces have been investigated by means of density-functional (DFT) calculations. It is found that CO molecules adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. The favorable positions for adsorption are shown to be near protrusions of electron density above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate; on the oxygen-saturated Mo(1 1 0) surface, the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at −19 eV (with respect to the Fermi level), while for the oxygen atom of an adsorbed CO molecule the related 3σ molecular orbital gives rise to a peak at −23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO, and therefore the position of the O 2s peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of the CO dissociation on Mo surfaces.     

Adsorption of 2-chlorophenol on Cu2O(1 1 1)-CuCUS: A first-principles density functional study

First-principles density functional theory and a periodic-slab model have been utilized to investigate the adsorption of a 2-chlorophenol molecule on a CuO(1 1 1) surface with a vacant Cu surface site, namely Cu₂O(1 1 1)-Cu_CUS. Several vertical and flat orientations have been studied. All of these molecular configurations interact very weakly with the Cu₂O(1 1 1)-Cu_CUS surface, an observation which also holds for clean copper surfaces and the Cu₂O(1 1 0):CuO surface. Hydroxyl-bond dissociation assisted by the surface was found to be endoergic by 0.42-1.72 eV, depending predominantly on the position of the isolated H on the surface. In addition, the corresponding adsorbed 2-chlorophenoxy moiety was found to be more stable than a vacuum 2-chlorophenoxy radical by about 0.76 eV. Despite these predicted endoergicities, however, we would predict the formation of 2-chlorophenoxy radicals from gaseous 2-chlorophenol over the copper (I) oxide Cu₂O(1 1 1)-Cu_CUS surface to be a feasible and important process in the formation of PCDD/Fs in the post-flame region where gas-phase routes are negligible.     

First principles study of H2S adsorption and dissociation on Fe(1 1 0)

We report first principles density functional theory (DFT) results of H₂S and HS adsorption and dissociation on the Fe(1 1 0) surface. We investigate the site preference of H₂S, HS, and S on Fe(1 1 0). H₂S is found to weakly adsorb on either the short bridge (SB) or long bridge (LB) site of Fe(1 1 0), with a binding energy of no more than 0.50 eV. The diffusion barrier from the LB site to the SB site is found to be small (∼0.10 eV). By contrast to H₂S, HS is predicted to be strongly chemisorbed on Fe(1 1 0), with the S atom in the LB site and the HS bond oriented perpendicular to the surface. Isolated S atoms also are predicted to bind strongly to the LB sites of Fe(1 1 0), where the SB is found to be a transition state for S surface hopping between neighboring LB sites. The minimum energy paths for H₂S and HS dehydrogenation involve rotating an H atom towards a nearby surface Fe atom, with the S-H bonds breaking on the top of one Fe atom. The barrier to break the first S-H bond in H₂S is low at 0.10 eV, and breaking the second S-H bond is barrierless, suggesting deposition of S on Fe(1 1 0) via H₂S is kinetically and thermodynamically facile.     

Oxygen adsorption on a Mo2C(0 0 0 1) surface: Angle-resolved photoemission study

Oxygen adsorption on a C-terminated α-Mo₂C(0 0 0 1) surface has been investigated with Auger electron spectroscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy utilizing synchrotron radiation. It is found that the oxygen atoms adsorb on the Mo atoms in the second layer forming a (1 × 1) orthorhombic periodicity. The oxygen adsorption induces a peculiar state around the Fermi level, which is observed at 0.4 eV in the normal-emission spectra. ARPES measurements show that the state is a partially occupied metallic state. The photoionization cross section of the state shows a maximum at the photon energy of 56 eV, which is assigned as originating from the resonance of the Mo 4d photoemission involving Mo 4p → 4d photoexcitation.     

The reaction of carbon monoxide with palladium supported on cerium oxide thin films

In this study we probe the reaction of carbon monoxide with Pd nanoparticles supported on cerium oxide thin films. With the use of soft X-ray photoelectron spectroscopy (sXPS), and temperature programmed desorption (TPD) the surface intermediates and pathways leading to reaction products of CO on Pd supported on ceria were investigated. When Pd is supported on the stoichiometric CeO₂ surface (Ce⁺⁴) only the molecular adsorption of CO on Pd is visible (286.4 eV). All of the CO desorbs below 520 K, however a small amount of O exchange between the CO and the ceria was indicated through the acquisition of labeled ¹⁸O from the substrate in the desorbed CO. The Pd nanoparticles are activated on partially reduced CeO_x to promote the dissociation of <10% of the CO as indicated by a C-Pd species (284.4 eV) in sXPS. The C recombines with O from the ceria and desorbs between 600 and 700 K. The majority of the CO does not dissociate, however, and the degree of dissociation does not increase with the degree of ceria reduction. This result is in contrast with Rh nanoparticles supported on ceria where the degree of dissociation increased with the degree of ceria reduction and nearly total dissociation was obtained when the ceria was highly reduced.     

Initial stages of H2O adsorption and hydroxylation of Fe-terminated α-Fe2O3(0 0 0 1) surface

The adsorption modes of H₂O on a Fe-terminated hematite(0 0 0 1) surface have been investigated by first principles Density Functional theory within a periodic slab model and the generalized gradient approximation. Molecular adsorption and dissociative adsorption in monolayer coverage, one H₂O per surface Fe, were both considered. Five plausible orientations were studied to determine the most favorable adsorption position. Molecular adsorption is shown to have a small effect on the underlying surface structure, while hydroxylation has a strong effect on the surface geometry. Electronic densities of state calculations reveal details of these different interactions. The heterolytic dissociation, which produces two types of surface hydroxyls, is the preferable adsorption mode, being slightly favored energetically over the molecular adsorption. Homolytic dissociative adsorption, forming a single hydroxyl on surface Fe, is energetically unfavored, even though strong binding interaction (∼3 eV) is found between the OH radical and surface. Dissociative adsorption on an oxidized ferryl site was also studied to investigate suggested local reactivity enhancement.     

Theoretical study of CO adsorption on the illuminated TiO2 (0 0 1) surface

A quantum modeling of the CO adsorption on illuminated anatase TiO₂ (0 0 1) is presented. The calculated adsorption energy and geometries of illuminated case are compared with the ground state case. The calculations were achieved by using DFT formalism and the BH and HLYP. Upon photoexcitation, an electron-hole pair is generated. Comparing of natural population in the ground state and the exited state, shows that an electron is trapped in a Ti⁴⁺ ion and a hole is localized in an oxygen ion. The photoelectron helps generation of a CO₂ molecule on the TiO₂ surface. As shown by optimization of these systems, the CO molecule adsorbed vertically on the TiO₂ (0 0 1) surface in the ground state case while the CO molecule made an angle of 134.3° to this surface at the excited state case. Based on the here used model the obtained adsorption energy was 0.36 eV which is in excellent agreement with the reported experimental value. In the present work the C-O stretch IR frequencies are calculated which are 1366.53 and 1423.16 cm⁻¹. These results are in good agreement with the earlier reported works for the surface carbonaceous compounds, and oxygenated carbon species.