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Photoluminescence (PL) spectroscopy was used to infer that oxygen adsorption changes the band bending of the anatase phase of TiO2 within P25 nanopowder in different ways. On the one hand, oxygen can adsorb through irreversible reaction with defects which reduces the intrinsic upward band bending at the TiO2 surface and results in increased PL emission. On the other hand, oxygen exposure also leads to molecular chemisorption that yields an outermost negative charge at the surface which increases the upward band bending of TiO2 and decreases the PL emission. Since band bending plays an active role in directing charge carrier migration to the surface, the finding that oxygen adsorption can have two different, and quite opposite, effects on the band bending of TiO2 provides a new perspective on how oxygen may influence photocatalytic reaction efficiencies.

Hydrogen adsorption on TiO 2 is of importance in chemical and photochemical reduction processes. Using several surface science methods, we clearly distinguish two kinds of H species on the surface of rutile TiO 2 (110)-1 × 1. In contrast with the well-studied bridge-bonded OH species (α-H) originating from H 2 O dissociation on the surface oxygen vacancy site on TiO 2 (110), atomic H adsorption on the TiO 2 (110) (denoted as β-H) exhibits special high sensitivity to the electronic excitation of the TiO 2 (110) by either electrons or UV photons. The formation of molecular H 2 gas by photoexcitation of β-H/TiO 2 (110) surfaces has been observed, which may shed light on the basic understanding of the processes of photocatalytic H 2 production by splitting water.

In this paper, we observed physically adsorbed CO molecules on C60 surface being displaced by impinging noble gas atoms (He, Ne, Ar, Kr), either through a dynamic displacement process or an exothermic replacement process, depending on their adsorption energies. This displacement mechanism could shift from one to the other depending on the surface coverage and temperature. Furthermore, rotational energy of the impinging molecules may also contribute to the dynamic displacement process by supplying additional energy.

The partial oxidation of model C2–C4 (acetic, propionic, and butyric) carboxylic acids on Au/TiO2 catalysts consisting of Au particles ∼3 nm in size was investigated using transmission infrared spectroscopy and density functional theory. All three acids readily undergo oxidative dehydrogenation on Au/TiO2. Propionic and butyric acid dehydrogenate at the C2–C3 positions, whereas acetic acid dehydrogenates at the C1–C2 position. The resulting acrylate and crotonate intermediates are subsequently oxidized to form β-keto acids that decarboxylate. All three acids form a gold ketenylidene intermediate, Au2C═C═O, along the way to their full oxidation to form CO2. Infrared measurements of Au2C═C═O formation as a function of time provides a surface spectroscopic probe of the kinetics for the activation and oxidative dehydrogenation of the alkyl groups in the carboxylate intermediates that form. The reaction proceeds via the dissociative adsorption of the acid onto TiO2, the adsorption and activation of O2 at the d...

Photoluminescence (PL) spectroscopy was employed to study the effect of embedded single-walled carbon nanotubes (SWNTs) on charge transport in powdered TiO2. It was found that 1–5 wt % SWNTs mixed with TiO2 accept electrons from photoexcited TiO2 and quench the PL intensity from TiO2. The PL quenching efficiency by SWNTs is proportional to the fractional occupancy of TiO2 particles which experience electrical contact with SWNTs. Ultraviolet light was used to cause surface charging of the SWNT/TiO2 sample, and the charging/discharging rate was measured using PL. The PL charging/discharging rate of all SWNT/TiO2 mixtures is identical to that of pure TiO2, indicating that SWNTs only accept photoexcited electron from TiO2 but do not transport electrons under conditions of the experiment. This is due to the effect of positively charged TiO2 particles which immobilize the photoexcited electrons transferred to SWNTs at the interface between first-layer TiO2 and SWNTs, inhibiting charge transport through SWNT cha...

Photoluminescence spectroscopy was employed to study the photoinduced charge transfer between TiO2 nanoparticles and Au nanoparticles under vacuum. We found that small coverages of 3 nm Au nanoparticles deposited on TiO2 significantly diminish the 540 nm (2.3 eV) photoluminescence emission from TiO2 because of the redistribution of the photoexcited charge to Au nanoparticles that are capable of accepting negative charges behind the Schottky barrier. The lack of development of the photoluminescence emission of Au/TiO2 during continuous UV irradiation occurs because of a short circuit established through Au nanoparticles in which transferred electrons in Au recombine nonradiatively with holes in TiO2. The photoexcited electron transfer from TiO2 to the Au nanoparticles occurs beyond the Au particle perimeter over a distance of at least 4 nm. The quenching of photoluminescence by resonance energy transfer from TiO2 to Au nanoparticles is unimportant as Au plasmonic absorption is not observed.

The oxidative-dehydrogenation of carboxylic acids to selectively produce unsaturated acids at the second and third carbons regardless of alkyl chain length was found to occur on a Au/TiO2 catalyst. Using transmission infrared spectroscopy (IR) and density functional theory (DFT), unsaturated acrylate (H2C═CHCOO) and crotonate (CH3CH═CHCOO) were observed to form from propionic acid (H3CCH2COOH) and butyric acid (H3CCH2CH2COOH), respectively, on a catalyst with ∼3 nm diameter Au particles on TiO2 at 400 K. Desorption experiments also show gas phase acrylic acid is produced. Isotopically labeled (13)C and (12)C propionic acid experiments along with DFT calculated frequency shifts confirm the formation of acrylate and crotonate. Experiments on pure TiO2 confirmed that the unsaturated acids were not produced on the TiO2 support alone, providing evidence that the sites for catalytic activity are at the dual Au-Ti(4+) sites at the nanometer Au particles' perimeter. The DFT calculated energy barriers between 0.3 and 0.5 eV for the reaction pathway are consistent with the reaction occurring at 400 K on Au/TiO2.

Gold (Au) nanoparticles supported on reducible oxides such as TiO2 demonstrate exceptional catalytic activity for a wide range of gas phase oxidation reactions such as CO oxidation, olefin epoxidation, and water gas shift catalysis. Scientists have recently shifted their hypotheses on the origin of the reactivity of these materials from the unique electronic properties and under-coordinated Au sites on nanometer-sized particles to bifunctional sites at the Au-support interface. In this Account, we summarize our recent experimental and theoretical results to provide insights into the active sites and pathways that control oxidation over Au/TiO2 catalysts. We provide transmission IR spectroscopic data that show the direct involvement of the Au-Ti(4+) dual perimeter sites, and density functional theory results that connect the electronic properties at these sites to their reactivity and to plausible reaction mechanisms. We also show the importance of interfacial Au-Ti(4+) sites in adsorbing and activating O2 as a result of charge transfer from the Au into antibonding states on O2 causing di-σ interactions with interfacial Au-Ti(4+) sites. This results in apparent activation energies for O2 activation of 0.16-0.60 eV thus allowing these materials to operate over a wide range of temperatures (110-420 K) and offering the ability also to control H-H, C-H, and C-O bond scission. At low temperatures (100-130 K), adsorbed O2 directly reacts with co-adsorbed CO or H2. In addition, we observe the specific consumption of CO adsorbed on TiO2. The more strongly held CO/Au species do not react at ∼120 K due to high diffusion barriers that prevent them from reaching active interfacial sites. At higher temperatures, O2 directly dissociates to form active oxygen adatoms (O*) on Au and TiO2. These readily react with bound hydrocarbon intermediates via base-catalyzed nucleophilic attack on unsaturated C═O and C═C bonds or via activation of weakly acidic C-H or O-H bonds. We demonstrate that when the active Au-Ti(4+) sites are pre-occupied by O*, the low temperature CO oxidation rate is reduced by a factor 22. We observe similar site blocking for H2 oxidation by O2, where the reaction at 210 K is quenched by ice formation. At higher temperatures (400-420 K), the O* generated at the perimeter sites is able to diffuse onto the Au particles, which then activate weakly acidic C-H bonds and assist in C-O bond scission. These sites allow for active conversion of adsorbed acetate intermediates on TiO2 (CH3COO/TiO2) to a gold ketenylidene species (Au2═C═C═O). The consecutive C-H bond scission steps appear to proceed by the reaction with basic O* or OH* on the Au sites and C-O bond activation occurs at the Au-Ti(4+) dual perimeter sites. There is a bound-intermediate transfer from the TiO2 support to the Au sites during the course of reaction as the reactant (CH3COO/TiO2) and the product (Au2═C═C═O) are bound to different sites. We demonstrate that IR spectroscopy is a powerful tool to follow surface catalytic reactions and provide kinetic information, while theory provides atomic scale insights into the mechanisms and the active sites that control catalytic oxidation.

Photoluminescence spectroscopy was employed to observe electron transport between TiO2 particles. Ultraviolet (UV) irradiation (3.88 eV) was shown to positively enhance the photovoltage of TiO2 particles at the powder surface, causing an enhancement of their photoluminescence (PL) at 530 nm. The charging of the TiO2 particles on the powder surface by UV irradiation partially discharges in the dark, where the displaced bulk negative charge diffuses back toward the TiO2 surface. This charge flow partially restores upward band bending, causing the PL intensity to decrease. The rate of the discharging process was used to estimate the electron migration mobility (∼10–10 m2 V–1 s–1 at 300 K) between TiO2 particles in the TiO2 matrix. Electron migration between TiO2 particles is temperature-dependent with an activation energy of 0.015 ± 0.008 eV. In addition, it was found that the adsorption of an immobile electron-donor molecule (NH3), attracts negative charge on the TiO2 surface which does not exhibit mobility...

We report the photodeposition of a carbonaceous layer grown on a TiO2 thin film by extreme ultraviolet (EUV)-induced chemistry of adsorbed n-tetradecane and the subsequent photo-oxidation of this film. Chemical analysis of the carbonaceous layer indicates that irradiation by 92 eV photons converts a fraction of the 14-carbon-atom alkane into two polycyclic aromatic hydrocarbon (PAH) molecules, anthracene and phenanthrene, both also containing 14 carbon atoms. Under continuing irradiation, EUV-induced dehydrogenation and cross-linking of PAHs and precursor alkane fragments form a complex network of carbon bonds with the inferred structure of an sp2-carbon rich film. Exposure of the carbonaceous film to five oxygen-containing molecules has little or no effect in the dark at 300 K. However, in the presence of EUV photons, the ability of the molecules to etch the carbonaceous film increases significantly. Their relative photo-oxidation activity follows the trend H2O2 ≈ O3 > NO ≈ O2 > H2O. The rate of the phot...

The partial oxidation of ethylene to form adsorbed acetate on a Au/TiO2 catalyst at temperatures as low as 370 K is reported here using Fourier transform infrared (FTIR) spectroscopy, gas chromatography-mass spectrometry (GC–MS) and density functional theory (DFT) calculations. Ethylene reacts with oxygen on Au/TiO2 to produce acetate on the TiO2 support as determined by the comparison with a blank TiO2 and Au/SiO2 catalyst. As shown by DFT calculations, O2 dissociation occurs at the dual-perimeter Au–Ti4+ sites of Au/TiO2 catalysts. Surprising, no ethylene oxide on the catalyst surface or in the gas phase is detected by either FTIR or GC–MS techniques at temperatures up to 673 K. The reaction pathway to ethylene oxide involves a higher barrier (~1.0–1.5 eV) than the pathway for acetate formation (~0.1–0.6 eV). The rate-limiting step to form adsorbed acetate was found to be the protonation of the H2C*C(OH)O* intermediate to produce the bound acetic acid. The theoretical initial deuterium kinetic isotope effect is ~3 which is consistent with the experimental data.

Single-walled carbon nanotubes (SWNTs) exhibit high surface areas and precisely defined pores, making them potentially useful materials for gas adsorption and purification. A thorough understanding of the interactions between adsorbates and SWNTs is therefore critical to predicting adsorption isotherms and selectivities. Metallic (M-) and semiconducting (S-) SWNTs have extremely different polarizabilities that might be expected to significantly affect the adsorption energies of molecules. We experimentally and theoretically show that this expectation is contradicted, for both a long chain molecule (n-heptane) and atoms (Ar, Kr, and Xe). Temperature-programmed desorption experiments are combined with van der Waals corrected density functional theory, examining adsorption on interior and exterior sites of the SWNTs. Our calculations show a clear dependence of the adsorption energy on nanotube diameter but not on whether the tubes are conducting or insulating. We find no significant experimental or theoretical difference in adsorption energies for molecules adsorbed on M- and S-SWNTs having the same diameter. Hence, we conclude that the differences in polarizabilities between M- and S-SWNTs have a negligible influence on gas adsorption for spherical molecules as well as for highly anisotropic molecules such as n-heptane. We expect this conclusion to apply to all types of adsorbed molecules where van der Waals interactions govern the molecular interaction with the SWNT.

Using scanning tunneling microscopy we observed reaction products of two chemisorbed thiolate species, methylthiolate and phenylthiolate, on the Au(111) surface. Despite the apparent stability, organometallic complexes of methyl- and phenylthiolate with the gold-adatom (RS-Au-SR, with R as the hydrocarbon group) undergo a stoichiometric exchange reaction, forming hybridized CH3S-Au-SPh complexes. Complementary density functional theory calculations suggest that the reaction is most likely mediated by a monothiolate RS-Au complex bonded to the gold surface, which forms a trithiolate RS-Au-(SR)-Au-SR complex as a key intermediate. This work therefore reveals the novel chemical reactivity of the low-coverage "striped" phase of alkanethiols on gold and strongly points to the involvement of monoadatom thiolate intermediates in this reaction. By extension, such intermediates may be involved in the self-assembly process itself, shedding new light on this long-standing problem.

The dissociative adsorption of water at oxygen-vacancy defect sites on the TiO2(110) surface spatially redistributes the defect electron density originally present at subsurface sites near the defect sites. This redistribution of defect-electrons makes them more accessible to Ti(4+) ions surrounding the defects. The redistribution of electron density decreases the O(+) desorption yield from surface lattice O(2-) ions in TiO2, as excited by electron-stimulated desorption (ESD). A model in which OH formation on defect sites redistributes defect electrons to neighboring Ti(4+) sites is proposed. This switches off the Knotek-Feibelman mechanism for ESD of O(+) ions from lattice sites. Conversely, enhanced O(+) reneutralization could also be induced by redistribution of defect electrons. The redistribution of surface electrons by adsorption is further verified by the use of donor and acceptor molecules that add or remove electron density.

We report experimental as well as theoretical evidence that suggests Au-CO complex formation upon the exposure of CO to active sites (step edges and threading dislocations) on a Au(111) surface. Room-temperature scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, transmission infrared spectroscopy, and density functional theory calculations point to Au-CO complex formation and migration. Room-temperature STM of the Au(111) surface at CO pressures in the range from 10(-8) to 10(-4) Torr (dosage up to 10(6) langmuir) indicates Au atom extraction from dislocation sites of the herringbone reconstruction, mobile Au-CO complex formation and diffusion, and Au adatom cluster formation on both elbows and step edges on the Au surface. The formation and mobility of the Au-CO complex result from the reduced Au-Au bonding at elbows and step edges leading to stronger Au-CO bonding and to the formation of a more positively charged CO (CO(δ+)) on Au. Our studies indicate that the mobile Au-CO complex is involved in the Au nanoparticle formation and reactivity, and that the positive charge on CO increases due to the stronger adsorption of CO at Au sites with lower coordination numbers.

The diffusion of dilute metallic lithium across the surface and into the bulk of atomically clean highly oriented pyrolytic graphite in ultrahigh vacuum is reported. Auger electron spectroscopy and a surface-dependent chemical reaction were utilized to monitor the coverage, oxidation state, and diffusion of Li. A very small diffusion activation energy (0.16 ± 0.02 eV; 15.4 ± 1.8 kJ/mol) is found for Li surface diffusion across the graphite basal plane. A model involving diffusion-rate-limited Li atom transport across the basal plane of graphite through step edge sites into the interior is proposed. These measurements indicate that the diffusion coefficient, D, for Li is very large (D(300 K) ≈ 5 × 10–6 cm2 s–1).

The reaction of submonolayer Li atoms with CH3Cl at 100 K on a highly oriented pyrolytic graphite (HOPG) surface has been studied under ultrahigh vacuum. We exploit the low defect density of the high quality HOPG used here (∼109 defects cm–2) to eliminate the effects of step edges and defects on the graphite surface chemistry. Li causes C–Cl bond scission in CH3Cl, liberating CH3 radicals below 130 K. Ordinarily, two CH3 species would couple to form products such as C2H6, but in the presence of graphite, CH3 preferentially adsorbs on the flat basal plane of Li-treated graphite. A C–CH3 bond of 1.2 eV is formed, which is enhanced relative to CH3 binding to clean graphite (0.52 eV) due to donation of electrons from Li into the graphite and back-donation from graphite to CH3. A low yield of C1, C2, and C3 hydrocarbon products above 330 K is found along with a low yield of H2. The low yield of these products indicates that the majority of the CH3 groups are irreversibly bound to the basal plane of graphite, a...

Chemisorbed acetate species derived from the adsorption of acetic acid have been oxidized on a nano-Au/TiO(2) (∼3 nm diameter Au) catalyst at 400 K in the presence of O(2)(g). It was found that partial oxidation occurs to produce gold ketenylidene species, Au(2)═C═C═O. The reactive acetate intermediates are bound at the TiO(2) perimeter sites of the supported Au/TiO(2) catalyst. The ketenylidene species is identified by its measured characteristic stretching frequency ν(CO) = 2040 cm(-1) and by (13)C and (18)O isotopic substitution comparing to calculated frequencies found from density functional theory. The involvement of dual catalytic Ti(4+) and Au perimeter sites is postulated on the basis of the absence of reaction on a similar nano-Au/SiO(2) catalyst. This observation excludes low coordination number Au sites as being active alone in the reaction. Upon raising the temperature to 473 K, the production of CO(2) and H(2)O is observed as both acetate and ketenylidene species are further oxidized by O(2)(g). The results show that partial oxidation of adsorbed acetate to adsorbed ketenylidyne can be cleanly carried out over Au/TiO(2) catalysts by control of temperature.

We report the first measurements of the kinetics of adsorption on the very outermost surface sites of a porous material compared to measurements made of adsorption on the interior sites. NH(3) and CO were employed in this study as representative of slow diffusion and fast diffusion, respectively, through porous TiO(2). Adsorption of NH(3) at 200 K occurs mainly at the very near surface (~20 nm) region as observed by photoluminescence (PL) spectroscopy, and its distribution by surface diffusion through the powder is highly retarded as judged by transmission IR spectroscopy. In contrast, the adsorption of CO in the near-surface region at 120 K is followed by the fast distribution of CO by surface diffusion into TiO(2) powder, causing the near-surface CO coverage to lag behind the coverage in the bulk. In the desorption process, the near-surface region delivers adsorbed CO molecules into the gas phase, accompanied by the supply of diffusing CO molecules from the interior. As a result, the adsorption/desorption processes for CO in the near-surface region of porous TiO(2) show a pronounced hysteresis effect. As surface diffusion is retarded at lower temperatures, the hysteresis effect gradually disappears.

The influence of charge transfer from single Au atoms to TiO2(110) on the photostimulated desorption (PSD) of 18O2 has been studied using a measurement of the rate of hole transport which mediates PSD. Band bending effects observed experimentally and theoretically by density functional theory indicate that Auδ+ atoms are present. The presence of atomically dispersed Au on the TiO2(110) surface with a constant coverage of preadsorbed 18O2 depresses the photoinduced hole transport rate from the bulk to the TiO2 surface and decreases the 18O2 PSD yield. This indicates that single Au atoms donate a fraction of an electron to the surface, causing downward band bending. DFT calculations show that ∼0.2 electron transfers from single Au atoms to the O2/TiO2(110) surface and the valence and conduction bands bend ∼0.6 eV downward. With increasing Au coverage, the positive charge per Au atom decreases due to the formation of small Au clusters.

Transmission Fourier transform infrared spectroscopy, coupled with CO adsorption, was used to study how the oxidation state of Au surface sites change during H2 dissociation, migration, and electronic excitation of TiO2 in a nanoparticulate Au/TiO2 catalyst. Data reveals that atomic H, produced by H2 dissociation on Au particles, readily hydroxylates the Au–O–Ti linkages around the periphery of the gold particles. Following passivation of peripheral sites, the H atoms diffuse into the bulk of the titania where they protonate the semiconductor, while donating an electron to shallow trapped (ST) states just below the conduction band (CB). The donated electrons effectively n-dope the semiconductor. By simultaneously monitoring changes in the infrared absorbance due to transitions involving ST electrons, free CB electrons, and the fundamental stretching mode of CO(a), we have found that the most active sites for hydrogen dissociation are Au0 sites at the free step edges or other defect sites in the Au nanopar...

Graphite has been subjected to surface damage by Ar+ ion bombardment. It has been found by scanning tunneling microscopy (STM) that deposited Cs atoms preferentially cluster at the artificially-produced vacancy clusters in the basal plane at 300 K. Density functional theory calculations show that Cs is more strongly bound at these defect sites than at basal plane step edges, providing a plausible explanation for the experimental observation. STM reveals that Cs intercalation into graphite occurs by diffusion through these vacancy clusters, acting as entry ports to the interior, at 700 K. DFT calculations show that these defects must consist of more than four contiguous atoms in order for Cs intercalation to be energetically easy. Experimentally, Cs atom nucleation at defect sites seems to culminate at Cs cluster heights near 1 nm and cluster diameters near 5 nm. Intercalated Cs produces coherent single Cs islands in the galleries as well as layered structures caused by superposition of overlapping individual islands. Oxygen exposure to graphite containing Ar+-produced defects does not influence Cs clustering at the defects or Cs entry into galleries beneath the defect. However, large oxygen exposures these clusters results in diminution of Cs intercalation, probably due to surface oxidation of Cs clusters.

The prevailing view of CO oxidation on gold-titanium oxide (Au/TiO 2 ) catalysts is that the reaction occurs on metal sites at the Au/TiO 2 interface. We observed dual catalytic sites at the perimeter of 3-nanometer Au particles supported on TiO 2 during CO oxidation. Infrared-kinetic measurements indicate that O-O bond scission is activated by the formation of a CO-O 2 complex at dual Ti-Au sites at the Au/TiO 2 interface. Density functional theory calculations, which provide the activation barriers for the formation and bond scission of the CO-O 2 complex, confirm this model as well as the measured apparent activation energy of 0.16 electron volt. The observation of sequential delivery and reaction of CO first from TiO 2 sites and then from Au sites indicates that catalytic activity occurs at the perimeter of Au nanoparticles.

The interaction between CO and O adsorbed on TiO2(110) has been investigated using scanning tunneling microscopy (STM), electron stimulated desorption (ESD), temperature programmed desorption (TPD), and density functional theory (DFT). Coadsorption of CO and O produces CO−O and CO−O−CO surface complexes via weak attractive interaction as revealed by STM and DFT. The effect of the adsorbates interaction can also be observed in two ensemble-averaged techniques, ESD and TPD, strongly supporting the STM and DFT results. The CO molecules adsorbed near chemisorbed O cause a ∼70% decrease in the O+ ion yield in ESD. The interaction between CO and O is considered to be electrostatic in nature due to the charge rearrangement upon chemisorption.

This special issue on surface chemistry is introduced with a brief history of the field, a summary of the importance of surface chemistry in technological applications, a brief overview of some of the most important recent developments in this field, and a look forward to some of its most exciting future directions. This collection of invited articles is intended to provide a snapshot of current developments in the field, exemplify the state of the art in fundamental research in surface chemistry, and highlight some possibilities in the future. Here, we show how those articles fit together in the bigger picture of this field.

The chemisorption and surface diffusion/desorption of pyridine from TiO2 powder (P25) have been measured as a function of temperature using transmission IR spectroscopy. Two classes of diffusion have been measured with activation energies of 36 kJ/mol (fast) and 90 kJ/mol (slow). By comparing density functional theory (DFT) calculations of bonding energies on TiO2 rutile (110) and anatase (101), the dominant crystal planes expected in P25 TiO2, it is found that fast diffusion would be expected on the rutile phase and that much slower diffusion would be expected on the anatase phase. In addition, the presence of oxygen defect sites will produce more strongly bound pyridine than either of the model crystal planes selected for investigation. To both types of TiO2 surfaces, pyridine bonding occurs to coordinatively unsaturated Ti cation sites through the N lone pair of the pyridine molecule. More than 85% of the molecules exhibit slow surface diffusion, and this is attributed to diffusion on the dominant anat...

The role of electrons and holes in the electronically excited oxidation of adsorbed CO on TiO(2)(110) has been investigated by tuning the surface electron and hole availability by the adsorption of Cl(2) or O(2). The presence of an electron acceptor (Cl(2) or O(2)) on the TiO(2)(110) surface causes upward band bending, increasing the excited hole availability and decreasing the excited electron availability in the near surface region. This enhances O(2) desorption and depresses CO(2) production during electronic excitation. This result gives clear evidence for the first time that the electronically excited CO oxidation reaction is caused by an electron-mediated process in contrast to O(2) desorption which is mediated by holes.

The doping of single-walled carbon nanotubes (SWNTs) under ultrahigh vacuum by Li atoms has been explored experimentally and theoretically. The chemical effect of Li in breaking the C−Cl bond in chloromethane has been observed. Temperature programmed desorption (TPD) experiments show that at low coverage CH3Cl is physisorbed to the undoped SWNT sample, exhibiting a desorption process near 178 K. The CH3Cl desorption peak shifts to about 240 K for lithiated SWNTs, indicating an increase in binding energy of about 0.16 eV. More importantly, the integrated intensity of the CH3Cl desorption peak is dramatically reduced in the lithiated SWNT case, and CH3, C2H6, or related species are not observed in significant quantities in the TPD experiments up to a temperature of 500 K. This strongly indicates that CH3Cl reacts on lithiated SWNTs to produce an irreversibly bound species. Products LiCl and Li2Cl2 are observed to desorb near 700 K. Density functional theory calculations present possible reaction mechanisms ...

The role of band bending on the efficiency of charge transfer across the TiO2(110) single crystal surface has been measured in ultrahigh vacuum, in the absence of a wide range of surface site inhomogeneities, surface impurities and solvent effects, and particle size effects. The adsorption of the Cl2 (electron acceptor) molecule and the O2 (electron acceptor) molecule have been found to enhance hole transport from TiO2 to 18O2 molecules adsorbed on oxygen vacancy sites, increasing the rate of electron stimulated desorption (ESD) of 18O2. This confirms that O2-ESD is hole mediated. Conversely, adsorption of CH3OH, a donor molecule, reduces the transfer rate for holes to the adsorbed O2, reducing its rate of ESD to near zero. The maximum effect of donor and acceptor molecules occurs near 1 monolayer coverage.

The adsorption of O2 from air is an essential step in the use of TiO2 as a photo-oxidation catalyst. This work explores the properties of a weakly bound O2 species which is probably the precursor to “active” O2 needed for the photo-oxidation of molecules by TiO2. Adsorbed molecular oxygen was observed via FTIR for the first time on nanosized TiO2 particles. The observed O−O stretching mode frequency at 1550 cm−1 is identical to the Raman displacement of O2 gas, showing that the interaction between the IR active O2 molecules and TiO2 is relatively weak. It is noteworthy that weak adsorption of O2 on TiO2 is accompanied by the development of a weak IR absorption band due to the production of a small dipole in the molecule. In addition, the constant νO2 observed for increasing coverage indicates that the O2 molecules exhibit only small interactions with each other in the adsorbed layer. The adsorption enthalpy of O2 on TiO2 is measured to be between −16 to −21 kJ mol−1. The O2− species formed by O2 adsorbing...

Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.

The kinetics of surface and bulk electron−hole pair recombination have been measured separately on TiO2(110) in ultrahigh vacuum under well-controlled surface conditions for the first time. Using 100 eV incident electrons, excitation of electron−hole pairs occurs within ∼6 A of the surface, and the rate of stimulated desorption of O2 was measured as a means of determining the charge carrier recombination kinetics, which are found to be mediated by the surface and to be first-order in charge carrier concentration. Comparison with a previous O2 photodesorption experiment, where excitation by 3.4 eV photons was used for electron−hole pair generation in a deeper region below the surface (∼100 A), shows that bulk recombination in TiO2 occurs by second-order kinetics in charge carrier concentration.

The diffusion of interstitial Ti3+ species (Tii3+) in rutile TiO2(110) from the bulk to the surface has been studied utilizing two experimental techniques. Electron-stimulated desorption of O+ ions was employed to kinetically monitor the reaction between oxygen adatoms with Tii3+ species at temperatures between 360 and 400 K. Scanning tunneling microscopy was also used to measure the Tii3+ diffusion rate. Both methods yield a rate constant kTii3+ = 5 × 10−4 s−1 at 393 K. The activation energy as measured by the rate dependence on temperature is ∼1.0 eV.

Photochemistry from TiO 2 surfaces is described for two cases: The UV-induced photodesorption of O 2 from TiO 2 (1 1 0) – 1 × 1; and the hydrophilic effect caused by UV irradiation on TiO 2 . In both cases fundamental information about how these processes occur has been found. In the case of the O 2 photodesorption kinetics, it has been found that the rate of the process is proportional to the square root of the UV flux, showing that second-order electron–hole pair recombination is dominant in governing the photodesorption rate. In addition these measurements provide an estimate of the concentration of hole traps in the TiO 2 crystal. In other measurements of the UV-induced hydrophilicity, starting with the atomically-clean TiO 2 surface, it has been shown that the effect occurs suddenly at a critical point during irradiation as a result of photooxidation of a monolayer of hydrocarbon ( n -hexane) at equilibrium with ppm concentration of n -hexane in O 2 at 1 atmosphere pressure.

Doping of opened single-walled carbon nanotubes (SWNTs) with metallic Li has been carried out under ultrahigh vacuum. Li atoms enter into the interior sites of the SWNTs by surface migration with an activation energy of 0.3 ± 0.04 eV. Density functional theory calculations indicate that the Li atoms ionize inside the SWNTs to produce Li+ ions. The Li-doped SWNTs exhibit an ∼10% enhancement of the van der Waals bonding for n-heptane molecules, a model for volatile organic compounds, within the interior due to polarization of the alkane molecules in the strong electrostatic field around the Li+ ions. The investigations reveal the utility of temperature-programmed desorption from SWNTs for understanding the energetics of confined molecule adsorption in pristine and doped SWNTs. Statistical mechanical simulations using a polarizable potential for alkanes give increases in isosteric heats of adsorption at high alkane loading that are in excellent agreement with experiments. Our simulations predict that the bin...

The influence of water vapor on the surface diffusion behavior of pyridine adsorbed on powdered MgO surfaces has been studied by Fourier transform IR (FTIR) absorption spectroscopy. It has been found that the introduction of water vapor significantly increases the pyridine surface diffusion coefficient. FTIR spectra showed that water vapor converted Lewis acid Mg2+ sites to Bronsted Mg−OH sites. These measurements also detected the change in surface bonding of pyridine to the two types of sites. The activation energy for escape of chemisorbed pyridine into a mobile precursor state is lower for pyridine bound to Mg−OH sites than for binding to Mg2+ sites, causing the hydroxylated MgO to exhibit a higher diffusivity than that found for dry MgO containing no surface hydroxyl groups. This effect was confirmed by DFT calculations of the binding energy of pyridine to MgO(100) sites and to defect sites on MgO(100), where hydroxylation decreases the binding energy by ∼30 kJ mol−1 on each type of site.

The molecular transport of pyridine through nanosized MgO particles has been investigated by Fourier transform IR spectroscopy. Two regimes of chemisorbed pyridine diffusion are observed. A fast di...

The formation of hydroxyl chains from water dissociation on the Cu(110) surface has been studied by using a combination of scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and density functional theory (DFT) calculations. Annealing the D2O-covered surface to a temperature of ∼200 K leads to desorption of D2O molecules and produces a zigzag structure due to adsorbed OD groups with a periodicity of 5 A along the direction in the STM image. Coadsorption of O2 promotes the water dissociation reaction and produces hydroxyl chains with much higher coverage. ESDIAD measurements show a two-beam pattern consistent with OD(a) species inclined ∼40° with respect to the surface normal and orientated along the azimuth. The calculations reveal the existence of stable chain structures comprised solely of hydroxyl groups as well as of interacting water and hydroxyl groups that are consistent with the observed STM image.

The effect of triethylenediamine (TEDA) (also named 1,4-diazabicyclo [2.2.2]octane, DABCO) on the adsorption of ClCN on a γ-Al2O3 absorbent has been investigated. Both Fourier transform infrared (FTIR) and theoretical studies indicate that no direct interaction between amine groups of TEDA and ClCN molecules takes place. Instead, we found that TEDA competes with ClCN for active surface sites on γ-Al2O3. In addition, the adsorption behavior of cyanogen chloride (ClCN) on a clean γ-Al2O3 surface has been studied. The sequence of the thermally activated processes of diffusion, adsorption, desorption, and decomposition of ClCN molecules on the clean γ-Al2O3 surface following icelike ClCN layer formation at lower temperature was observed. One of the decomposition products, Al−NCO, was assigned by using an Al−18OH labeled surface for reaction with ClCN. In addition, Al−CN and Al2−OCN species were also detected upon ClCN decomposition. Good correlation of the calculated vibrational frequencies for the adsorbed s...

We present a direct side-by-side comparison of the interaction of Li atoms and N 2 molecules on the atomically stepped Ru(1 0 9) single crystal surface and on the atomically smooth Ru(0 0 1) single crystal surface using infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). At low adsorbate coverages there is spectroscopic evidence for the formation of a Li x (N 2 ) y complex on the Ru(1 0 9) surface, whereas no such complex is observed on the Ru(0 0 1) surface. This complex is due to local interactions between an adsorbed Li atom and N 2 adsorbed on the atomic steps of Ru(1 0 9). The short range interaction near the atomic steps is characterized by the development of several highly red-shifted ν (N 2 ) modes in the region of ∼2130 cm −1 in the IR spectra. Adsorbed N 2 molecules on both Ru(1 0 9) and Ru(0 0 1) also are influenced by the long range electrostatic field produced by Li adsorbate atoms, causing a red shift in the uncomplexed N 2 species, which monotonically increases as the Li coverage in increased. On the Ru(0 0 1) surface, small coverages of N 2 influenced by the long range effect of Li are initially chemisorbed parallel to the surface resulting in the absence of infrared activity. In addition we have also found that Li does not cause N–N bond scission on Ru(0 0 1) below 250 K.

We present a direct side-by-side comparison of the adsorption and desorption of nitrogen on the atomically-stepped Ru(1 0 9) surface and the atomically-flat Ru(0 0 1) surface. Both infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) are employed in this study, along with density functional theory (DFT). We find that the chemisorptive terminal binding of N2 is stronger on the atomic step sites than on the terrace sites of Ru(1 0 9) as indicated by TPD and by a reduction of the singleton vibrational frequency, ν(N2), by 9 cm-1, comparing steps to terraces. In addition, we find that metal–metal compression effects on the terrace sites of Ru(1 0 9) cause stronger binding of N2 than found on the Ru(0 0 1) surface, as indicated by a reduction of the terrace-N2 singleton vibrational frequency by 11 cm-1 when compared to the singleton N2 mode on Ru(0 0 1). These spectroscopic results, comparing compressed terrace sites to Ru(0 0 1) sites and confirmed by TPD and DFT, indicate that N2 bonds primarily as a σ-donor to Ru. Using equimolar 15N2 and 14N2, it is found that dynamic dipole coupling effects present at higher N2 coverages may be partially eliminated bymore » isotopically detuning neighbor oscillators. These experiments, considered together, indicate that the order of the bonding strength for terminal-N2 on Ru is: atomic steps > atomic terraces > Ru(0 0 1). DFT calculations also show that 4-fold coordinated N2 may be stabilized in several structures on the double-atom wide steps of Ru(1 0 9) and that this form of bonding produces substantial decreases in the N2 vibrational frequency and increases in the binding energy, compared to terminally-bound N2. These highly coordinated N2 species are not observed by IRAS.« less

Sequential stages of formation of a self-assembled monolayer of flat-lying 2,6-dimethylpyridine molecules on a single crystal Cu(1 1 0) surface have been observed by low-temperature scanning tunneling microscopy (LT-STM). At an adsorption temperature of 10 K, all of the molecules are randomly distributed at low coverage upon adsorption. The isolated molecules align their molecular axes parallel to the 〈0 0 1〉 azimuth of the Cu lattice. The nitrogen atom in the molecule is located at the four-fold hollow site. Upon annealing to 100 K, the molecules associate to form head-to-head dimers. The dimer units involve a pair of weak hydrogen bonds between methyl group-hydrogen atoms and N moieties on adjacent molecules, forming a core structure for further growth. In a later stage of self-assembly, single head-to-tail weak hydrogen bonds between ring C–H bonds and N moieties form in chains on the periphery of the central cores, leading to larger domains with a c(6 × 2) overlayer structure.

FTIR spectroscopy has been used to monitor the transport of CO to the Pt cores of Pt@CoO nanoparticles forming CO/Pt species. It was found that external Pt sites are not present on the outer surfaces of the approximately 10 nm diameter nanostructures and that CO transports to Pt adsorption sites by an activated surface diffusion process through the CoO shells surrounding approximately 2 nm diameter Pt cores. The CO transport process is not due to gas-phase transport below 300 K. The weakly bound adsorbed CO/CoO species responsible for transport was directly observed at approximately 2147 cm(-1) during transport through the CoO shells.

We discuss our own studies of molecular adsorption on and inside of single-wall carbon nanotubes in the broader context of important theoretical and experimental developments in the field. We show that adsorption in the nanotube interior sites as well as in the groove and exterior sites may be resolved by various experimental methods. In addition, the changes that the adsorbate phases undergo due to confinement in the nanotube interior are discussed, particularly focusing on confined molecules of water, alkanes, and an alkene. Attention is also devoted to the use of oxidizing agents such as ozone to open the ends and walls of nanotubes for interior adsorption.

It has been demonstrated that the confinement of physisorbed 1-heptene molecules inside of carbon single walled nanotubes (SWNTs) results in lowering of their reactivity to atomic hydrogen compared to 1-heptene molecules adsorbed on external SWNT sites.

The adsorption and vibrational properties of chemisorbed HCN on Lewis acid sites, Lewis base sites, and Bronsted Al-OH acid sites on a partially hydroxylated [gamma]-Al2O3 surface have been obtained by a combination of FTIR and density functional theory studies. The vibrational modes from the molecular and dissociative adsorption of HCN were assigned by using deuterium and 13C-labeled D13CN molecules at 170 K. In addition, [eta]2(C, N)-HCN bonding is also found from the [nu](CdN) vibrational spectra. Good correlation of the calculated vibrational frequencies for the adsorbed species with experimental data is found. The effect of triethylenediamine (TEDA) (also called 1, 4-diazabicyclo [2.2.2]octane, DABCO) on the adsorption of hydrogen cyanide (HCN) on the high area [gamma]-Al2O3 surface has been investigated using transmission FTIR spectroscopy. During HCN adsorption on TEDA-functionalized surfaces, there is no spectral change or emerging feature in either the TEDA or HCN spectral regions, indicating that no direct interaction occurs between these two molecules. Instead, we found that TEDA competes with HCN for the active sites on [gamma]-Al2O3. The observed [nu](C [identical with] N) mode on a TEDA-precovered surface is due to the HCN adsorption on Lewis base sites (Al-O-Al) which are less affected by TEDA preadsorption.

We have experimentally measured long-range self-diffusivity of normal heptane adsorbed on single-walled carbon nanotubes at 275 K. This was done by observing the displacement kinetics of heptane by a deuterium-labeled molecule, 1-deuteroheptane. We also performed MD simulations of heptane self-diffusion inside an individual nanotube under full loading. The experimentally measured diffusion coefficient (D = 7.7 × 10-12 cm2 s-1) was 8 orders of magnitude lower than the diffusion coefficient inside an individual nanotube from the simulation (D = 8.2 × 10-4 cm2 s-1). Because heptane only adsorbs in the nanotube interior at 275 K, we conclude that the experimentally observed long-range transport is rate-limited by a very slow exchange of heptane between different nanotubes in the bulk SWNT sample. At the same time, the mixing of heptane inside individual nanotubes is fast. The reasons for slow diffusion between different tubes could be the potential energy barriers at the entry points into the nanotubes, as we...