Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects

We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed on TiO₂(110). First, the nucleation and growth of vapor-deposited gold on this surface was experimentally examined using x-ray photoelectron spectroscopy and low energy ion scattering. Gold initially grows as two-dimensional islands up to a critical coverage, θ _cr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using a hot filament technique. The desorption energy of O_a was examined using temperature programmed desorption (TPD). The O_a is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this O_a to make CO₂, probably via adsorbed CO. The reactivity of O_a with CO increases with increasing particle size, as expected based on Br?nsted relations. Propene adsorption leads to TPD peaks for three different molecularly adsorbed states on Au/TiO₂(110), corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands, with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using temperature-programmed low-energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to 700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for 1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid solutions can result in very narrow particle size distributions.     

The kinetics of CO oxidation by adsorbed oxygen on well‐defined gold particles on TiO2(110)

Very tiny Au particles on TiO₂ show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO₂(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO₂(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO₂, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O₂ desorption temperatures (741 K) from ultrathin gold particles on TiO₂(110) than from thicker particles (545 K). This implies that O_a bonds much more strongly to ultrathin islands of Au. Thus from Brønsted relations, ultrathin gold particles should be able to dissociatively adsorb O₂ more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO₂) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (CO_g + O_a → CO_2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Brønsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).     

The chemisorption of methanol on Cu films on ZnO(000¯1)-O

The interactions of methanol with well-defined Cu films on the oxygen-terminated ZnO(000¯1)-O surface have been studied, mainly using temperature programmed desorption (TPD). The Cu films, which were from submonolayer to multilayer in coverage, had been structurally characterized in previous studies using XPS, LEIS, ARXPS, LEED and work function measurements, and by CO, H₂O and formic acid adsorption. On clean Cu films methanol is adsorbed reversibly, desorbing at 200–260 K from atom-thick Cu islands, and at 155 K from multilayer islands preannealed to 550 K. In this respect, the atom-thin islands resemble Cu(110) sites and multilayer islands resemble Cu(111), consistent with behavior of other adsorbates. On oxygen-predosed multilayer films (preannealed to 600 K), methanol reacts to form methoxy species which decompose at 395 K to yield formaldehyde and hydrogen in TPD, also like Cu(111). Multilayer films preannealed to >750 K show a decrease in the peak area for methoxy decomposition which correlates with the loss of Cu area due to severe clustering. Oxygen-predosed Cu islands which are but one Cu atom thick show no clear evidence for a methoxy state in TPD. This suggests that oxygen atoms on such atom-thin Cu islands are poor Brønsted bases relative to O_a on bulk Cu surfaces, consistent with results for adsorbed water. Results on high-area Cu/ZnO catalysts are discussed in the light of these new results.     

Imaging gold clusters on TiO2(110) at elevated pressures and temperatures

Variable temperature scanning tunneling microscopy (STM) has been used to image oxide-supported nanoclusters of Au at temperatures from 300 to 450 K and oxygen pressures from 10^–10 to 4 Torr. Oxygen-induced morphological changes of the TiO₂(1×2) reconstruction are apparent at room temperature and prolonged exposure (3×10³ L (langmuir)) at 10^–4 Torr oxygen. Gold clusters with diameters smaller than 4 nm are unstable toward sintering at ca. 450 K and oxygen pressures >10^–1 Torr. Oxygen at pressures >10^–4 Torr weakens the interaction between the gold cluster and the titania support. Increasing the sample temperature to >300 K facilitates disruption of the cluster–support interaction.     

Modeling heterogeneous catalysts: metal clusters on planar oxide supports

Model catalysts consisting of Au and Ag clusters of varying size have been prepared on single crystal TiO₂(110) and ultra-thin films of TiO₂, SiO₂ and Al₂O₃. The morphology, electronic structure, and catalytic properties of these Au and Ag clusters have been investigated using low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) and spectroscopy (STS) with emphasis on the unique properties of clusters <5.0 nm in size. Motivating this work is the recent literature report that gold supported on TiO₂ is active for various reactions including low-temperature CO oxidation and the selective oxidation of propylene. These studies illustrate the novel and unique physical and chemical properties of nanosized supported metal clusters.     

The Temperature-Programmed Desorption of Oxygen from an Alumina-Supported Silver Catalyst

The associative desorption kinetics of O₂ from a 15 wt% Ag/-Al₂O₃ atalyst were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments. Saturation with chemisorbed atomic oxygen (O*) was achieved by dosing O₂ for 1 h at 523 K and at atmospheric pressure followed by cooling in O₂ to room temperature. The TPD spectra showed almost symmetric O₂ peaks centred above 500 K, indicating associative desorption of O₂ from Ag metal surface sites. By varying the heating rates from 2 to 20 K min^-1, the O₂ TPD peak maxima were found to shift from 508 to 542 K, respectively. A microkinetic analysis of these TPD traces yielded an activation energy for desorption of 149±2 kJ mol^-1 and a corresponding pre-exponential factor of 2×10¹²±1×10¹² s^-1 in good agreement with the kinetic parameters reported for O₂ desorption under UHV conditions from Ag(111) and Ag(110) single crystal surfaces.     

Growth of Ag and Au Nanoparticles on Reduced and Oxidized Rutile TiO2(110) Surfaces

The nucleation and growth of Au and Ag nanoparticles on rutile TiO₂(110)–(1 × 1) surfaces in different oxidation states is studied by means of photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM). Au and Ag nanoparticles were found to bind much more strongly to oxidized TiO₂(110) model supports than to reduced TiO₂(110) surfaces, as directly revealed by STM. Detailed PES studies addressing small Au and Ag particles complete this picture and show that the PES core level spectra acquired on Au/TiO₂(110) and Ag/TiO₂(110) can be best described by fitting with two binding energy (BE) components. Particularly for coverages in the sub-monolayer regime and for depositions at low temperatures (100 K) the PES core level spectra must be fitted with at least two BE components. The higher BE component is attributed to atoms at the interface between the metal clusters and the TiO₂(110) support. For Au/TiO₂(110), the two BE components were evident in the core level spectra for higher coverage than for Ag/TiO₂(110), consistent with different growth modes for Au (initially 2D) and Ag (3D) on TiO₂(110). Finally, strong evidence for charge transfer from Ag nanoparticles to the TiO₂(110) support is presented, whereas the charge transfer between Au nanoparticles and the TiO₂(110) support is very small.     

Reproducibility of highly active Au/TiO2 catalyst preparation and conditioning

We present results of a comparative study on the reproducibility of Au/TiO₂ catalyst preparation and conditioning. Comparison of their physical characteristics and their catalytic activity for CO oxidation shows that highly active catalysts can indeed be prepared in a very well-defined and reproducible way, with particle sizes of 2 nm (conditioning via reductive or redox treatment) or 3 nm (conditioning via calcination).     

A study of the electronic structure and reactivity of V/TiO2(110) with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS)

The interaction of vanadium with TiO₂(110) surface was studied with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) under ultrahigh vacuum (UHV) conditions. A strong interaction between vanadium and TiO₂ is observed with the addition of the first vanadium layer. Adsorption and reaction of ethanol to ethylene were investigated with MIES, UPS and temperature programmed desorption (TPD) and found to be sensitive to the surface electronic structure. In particular the temperature of ethylene evolution decreased by ~100 K with a change in the vanadium coverage from 0.5 to 5 ML.     

Photo-enhanced catalytic decomposition of isopropanol on V2O5

The effect of UV-visible photo irradiation has been investigated on the decomposition of isopropanol over V₂O₅. A 2.5-fold photo-enhancement in the rate of the dehydration (-H₂O) reaction was observed under UV-visible photo irradiation compared to thermal heating of the catalyst. The increase in propene yield under photo irradiation is believed to be initiated from photo excitation (photo-reduction) of the V₂O₅ complex to [-V⁴⁺-O^–] * thus, increasing surface concentration of V⁴⁺ species. The V⁴⁺ active surface species is assumed to serve as the center for adsorption of isopropanol. The observed increase in water formation along with the formation of propene suggests that the rate determining step for the dehydration reaction is desorption of water. Selectivity toward the dehydrogenation (-H₂) reaction has also been measured to be 1% under UV-visible photo irradiation as compared to 15% under thermal heating of the catalyst.     

The temperature-programmed desorption of hydrogen from copper surfaces

The desorption kinetics of H₂ from a Cu/ZnO/Al₂O₃ catalyst for methanol synthesis were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments after various pretreatments of the catalyst. Complete saturation with adsorbed atomic hydrogen was obtained by dosing highly purified H₂ for 1 h at 240 K and at a pressure of 15 bar. The TPD spectra showed symmetric H₂ peaks centered at around 300 K caused by associative desorption of H₂ from Cu metal surface sites. H₂ TPD experiments performed with different initial coverages resulted in peak maxima shifting to higher temperatures with lower initial coverages indicating that the desorption of H₂ from Cu is of second order. The microkinetic analysis of the TPD traces obtained with different heating rates yielded an activation energy of desorption of 78 kJ mol^–1 and a corresponding frequency factor of desorption of 3×10¹¹ s^–1> in good agreement with the kinetic parameters obtained with Cu(111) under UHV conditions.     

Thermal decomposition of C2H5I on Ag(111)

The decomposition of C₂H₅I on Ag(111) has been studied using temperature programmed desorption (TPD), work-function change measurements () and X-ray photoelectron spectroscopy (XPS). Adsorption of C₂H₅I at 100 K is mostly molecular with little dissociation. C-I bond cleavage starts around 110 K. Below 1 monolayer coverage, all adsorbed C₂H₅I(a) dissociates to C₂H₅(a) and I(a) during TPD. C₂H₅(a) undergoes only recombination, producing gas phase butane (C₄H₁₀) around 190 K. No C-H or C-C bond cleavage takes place. On D/Ag(111), hydrogenation of C₂H₅(a) to C₂H₅(g) occurs readily between 150 and 220 K.     

Effect of Carbon Deposits on Reactivity of Supported Pd Model Catalysts

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al₂O₃/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at 550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di- bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.     

An innovative approach to electro-oxidation of dopamine on titanium dioxide nanotubes electrode modified by gold particles

Au/TiO₂/Ti electrodes were prepared by galvanic deposition of gold particles from an acidic bath containing KAu(CN)₂ in the presence of a citrate buffer onto TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride-containing electrolyte. The morphology and surface characteristics of Au/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 40–80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti electrodes for the dopamine electro-oxidation was studied by cyclic voltammetry and differential pulse voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electro-catalytic activity toward the oxidation of dopamine. The catalytic oxidation peak current showed a linear dependence on dopamine concentration and a linear calibration curve was obtained in the concentration range of 0.5–2.5 mM of dopamine.     

Adsorption and decomposition of ethanol on supported Au catalysts

The adsorption and reactions of ethanol are investigated on Au nanoparticles supported by various oxides and carbon Norit. The catalysts are characterized by means of XPS. Infrared spectroscopic studies reveal the dissociation of ethanol to ethoxy species at 300 K on all the oxidic supports. The role of Au is manifested in the enhanced formation of ethoxy species on Au/SiO₂, and in increased amounts of desorbed products in the TPD spectra. The supported Au particles mainly catalyse the dehydrogenation of ethanol, to produce hydrogen and acetaldehyde. An exception is Au/Al₂O₃, where the main process is dehydration to yield ethylene and dimethyl ether. C–C bond cleavage occurs to only a limited extent on all samples. As regards to the production of hydrogen, the most effective catalyst is Au/CeO₂, followed by Au/SiO₂, Au/Norit, Au/TiO₂ and Au/MgO. A fraction of acetaldehyde formed in the primary process on Au/CeO₂ is converted above 623 K into 2-pentanone and 3-penten-2-one. The decomposition of ethanol on Au/CeO₂ follows first-order kinetics. The activation energy of this process is 57.0 kJ/mol. No deactivation of Au/CeO₂ is observed during 10 h at 623 K. It is assumed that the interface between Au and partially reduced CeO₂ is responsible for the high activity of the Au/CeO₂ catalyst.     

TiO2 Supported Nano—Au Catalysts Prepared via Solvated Metal Atom Impregnation for Low-Temperature CO Oxidation

TiO₂ supported nano-Au catalysts were prepared by solvated metal atom impregnation (SMAI) method. The catalysts were characterized by means of AAS, TPD, H₂ reduction desorption (H₂-RD), XRD, TEM, XPS and tested for low-temperature CO oxidation. XRD and TEM results showed that the pretreatment temperature had an influence on the particle size of Au/TiO₂catalysts. The average particle size increased with the increase in pretreatment temperature. XPS indicated that gold in the catalysts was presented in the form of metallic state clusters. Catalytic studies showed these catalysts were very active and stable in low-temperature CO oxidation. The CO oxidation activity of the catalysts increased as the Au particle size decreased. The measurement results of AAS, TPD and H₂-RD revealed that there were some organic fragments on the surface of Au particles which might be responsible for the high stability of the Au/TiO₂ catalysts.     

FTIR study of NO, C3H6 and O2 adsorption and interaction on gold modified MCM-41 materials

This paper is devoted to the detailed FTIR study of the adsorption, co-adsorption, and interaction of all the reagents used in NO HC-SCR process addressed to lean-burn engines with the surface of new gold catalysts based on ordered mesoporous materials. Gold was introduced into silicate and niobiosilicate matrices by the impregnation (Au/MCM-41 and Au/NbMCM-41, respectively) and via co-precipitation with siliceous and niobium sources (AuNbMCM-41). The in situ FTIR study allowed the estimation of the possible chemisorption of the reagents and their interaction towards intermediates, depending on the chemical composition of the catalyst and the way of gold introduction. It has been found that propene is chemisorbed, but not, NO, on gold species at room temperature. Chemisorbed C₃H₆ interacts with NO only in the presence of oxygen excess. Oxygen oxidizes NO to NO₂, the latter interacts with chemisorbed propene towards carboxylates (1570 cm⁻¹) and NO₂ is reduced to N₂O. At higher temperatures carboxylates interact with gaseous NO to carbonate, N₂O, CO and CO₂. The presence of niobium in the NbMCM-41 matrix enhances the oxidative properties of the catalysts and as a consequence the interaction between intermediates in NO reduction with propene in the oxygen excess. The co-precipitated AuNbMCM-41 exhibits higher NOx storage properties than the impregnated one.     

Comparison of the reactivity of bulk and surface oxides on Pd(100)

The reactivity of bulk PdO clusters produced by plasma oxidation of Pd(100) towards propene oxidation was characterized using temperature programmed desorption (TPD) and isothermal oxygen titration. The TPD results were dominated by simultaneous CO₂ and water desorption in a peak at 490 K. The only other product observed was a small amount of CO near saturation propene coverages that also desorbed at 490 K. The propene coverage saturated at exposures between 0.5–1 l, indicating a sticking coefficient close to one. In the titration experiments, CO₂ production peaked almost immediately upon exposure to propene, indicating that the propene oxidation rate fell as the surface was reduced. Above 450 K, virtually all of the propene was completely oxidized to CO₂ and water, while at lower temperatures small amounts of CO were observed and unreacted propene fragments accumulated on the surface. In comparison, previous results for a well-ordered surface oxide on Pd(100) were similar in that CO₂ and water also desorbed simultaneously indicating a similar mechanism, but different in that the sticking coefficient on the surface oxide was a factor of 20 lower, and the desorption peaked 60 K lower. These differences cause the bulk oxide to be far more active at higher temperatures than the surface oxide, but the surface oxide displays some activity down to lower temperatures where propene simply accumulates on the bulk oxide surface.     

The Surface Chemistry of 1-Iodopropane Adsorbed on Pt(111)

On Pt(111) at 110 K, 1-iodopropane, C₃H₇I, adsorbs molecularly, but for doses below 1.7 × 10¹⁴ molecules cm⁻², only H₂ and I appear in thermal desorption. C–I bond cleavage occurs between 160 and 220 K, forming adsorbed n-propyl, C_(a)H₂CH₂CH₃, and atomic iodine, based on temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). n-Propyl undergoes β-hydride elimination forming propylene, with desorption peaks at 185 and 240 K. At 240 K, hydrogenation to propane also occurs. Some di-σ bonded propylene, C_(a)H₂C_(a)HCH₃, remains at 240 K and it rearranges to propylidyne near 300 K. Atomic H, bound to Pt, recombines and desorbs at ca. 260 K. Further desorption of H₂ is limited by C–H bond breaking and occurs over a broad temperature range with local maxima at ca. 280, 320, and 420 K, typical of propylidyne fragments on Pt. Atomic iodine desorbs in a broad feature at 825 K.     

Adsorbed Atoms and Molecules Destined for a Reaction

The idea of an activation complex is popular for explaining reaction rates, but the characteristics of reactions and catalysis may not be explained in this way. A predestined state for each reaction composed of surface atoms and adsorbed species is responsible for these features. Two single Sn atoms trapped in adjacent half-unit cells of an Si(111) 7 × 7 surface is an example of a predestined state. An isolated Sn atom in a half-unit cell does not migrate to other half-unit cells at room temperature, but when two single Sn atoms are in adjacent half-unit cells they undergo rapid combination to form an Sn₂ dimer. In addition, these two single Sn atoms replace the center Si adatoms and an Si₄ cluster is formed. The spatial distribution of molecules desorbing from surfaces may reflect the predestined states for the desorption processes. The spatial distribution in the temperature-programmed desorption (TPD) of NO on Pd(110) and Pd(211) surfaces and that in the temperature-programmed reaction (TPR) of NO + H₂ were studied. N₂ desorbing from Pd(110) by the recombination of N atoms obeys cos⁶ – cos⁷ but the N₂ produced by a catalytic reaction of NO with H₂ obeys cos. In contrast, the N₂ desorbing with NO at 490 K in the TPD of Pd(110) shows a sharp off-normal distribution expressed by cos⁴⁶( – 38). The adsorption of NO on Pd(211) predominantly occurs on the (111) terrace but the spatial distribution suggests that the predestined states for the reaction and desorption are formed on both the (111) terrace and (100) step surfaces.