Microcalorimetric, infrared spectroscopic and DFT studies of ethylene adsorption on Ru, Ru/Sn and Ru/Cu catalysts

Microcalorimetric measurements of the adsorption of H₂, CO and C₂H₄ were conducted on silica-supported Ru, Ru/Sn, Ru/Cu and Cu catalysts; infrared spectroscopic measurements were made of adsorbed CO and C₂H₄. The adsorption of C₂H₄ leads to formation of di--adsorbed ethylene and ethylidyne species on Ru/SiO₂ at 300 K, with an initial heat of 160 kJ/mol. Ethylene adsorption at 203 K leads to the formation of di--adsorbed ethylene, ethylidyne species and weakly adsorbed -bonded ethylene. The initial heats are 110, 95 and 75 kJ/mol on Ru/SiO₂, 5Ru/Sn/SiO₂ and Ru/Cu/SiO₂, respectively. Lower heats of CO and C₂H₄ adsorption are measured on Ru/Cu/SiO₂, primarily as a result of these adsorbates binding on both Cu and Ru. Quantum chemical calculations employing density functional theory were performed using (0001) slabs of Ru and Ru/Sn. The results of these calculations indicate that Sn weakens the interaction of -bonded ethylene, di--bonded ethylene and ethylidyne species with Ru by 41, 23 and 15 kJ/mol, respectively. This behavior is in contrast to the effect of adding Sn to Pt and Pd, for which Sn preferentially weakens the bonding of ethylidyne species to the surface.     

Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

Density functional theory (DFT) and microcalorimetric investigations of CO adsorption on Pt clusters

Microcalorimetric measurements were conducted at 573 K of CO adsorption on Pt clusters supported in L-zeolite. The measured heat of CO adsorption is 175 kJ/mol, and the heat decreases to 90 kJ/mol near saturation coverage. Quantum chemical calculations were performed using density functional theory to study the interaction of CO with 10-atom Pt clusters. The heat of CO adsorption on atop-sites is calculated to be 209 kJ/mol, while a lower heat of 142 kJ/mol is calculated for CO on bridge-sites. These values decrease to 197 and 102 kJ/mol for population of two atop-sites and two bridge-sites, respectively, on the same Pt₁₀ cluster. The heat of adsorption decreases to 157 kJ/mol when six CO molecules adsorb on six atop-sites of the cluster. The calculated initial heat of CO adsorption on Pt₁₀ clusters is in agreement with experimental and theoretical values reported for CO adsorption on Pt single-crystal surfaces. The higher heat of CO adsorption at atop-sites may be caused by more σ-donation from CO to sp orbitals of Pt for atop-sites. The heat of CO adsorption on bridge-sites becomes higher on negatively charged platinum clusters. The calculated C-O stretching frequencies for charged and neutral platinum clusters agree with experimental data. This revised version was published online in August 2006 with corrections to the Cover Date.     

H2 adsorption and H/D exchange on Au/TS-1 and Au/S-1 catalysts

We report a combined quantum-mechanics/molecular-mechanics (QM/MM) analysis of H₂ dissociation and hydrogen–deuterium (H/D) exchange on four potential active sites inside the TS-1 pores: (1) Au₃/T6–Ti-non-defect, (2) Au₃/T6–Si-non-defect, (3) Au₃/T6–Ti-defect, and (4) Au₃/T6–Si-defect. We provide full kinetic and thermodynamic data calculated at standard conditions (298.15 K, 1 atm) for Eley–Rideal mechanisms on these sites. The H/D exchange on Au₃/TS-1 occurs in two steps: (1) first H₂ dissociation on Au₃/TS-1 to form H–Au₃–H species and (2) D₂ (or second H₂) attack on these H–Au₃–H species to form HD. The energetics of the first H₂ dissociation step is site-sensitive (with respect to Au sites), while that of the D₂ (or second H₂) addition step is not site-sensitive. We found that two different mechanisms for the second step are both kinetically and thermodynamically favorable. The most favorable mechanism (ΔE _act ~ 28 kcal/mol) involves an attack of D₂ on both the H atoms in the H–Au₃–H intermediate, and two HD molecules are formed simultaneously. The first H₂ dissociation step is almost thermoneutral and the D₂ (or second H₂) addition step is somewhat exothermic. A comparison of the pure QM and QM/MM calculations on Au₃/TS-1 suggests that the formation of the H–Au₃–H species inside the TS-1 pores becomes thermodynamically more favorable due to the long-range interactions. The activation energies for the first H₂ dissociation step (19–24 kcal/mol) are lower than those for the D₂ (or second H₂) addition step (28–31 kcal/mol). Therefore, the increase in the HD formation rate with temperature is likely to be stronger than the increase in the H–Au₃–H formation rate. On the basis of the calculated activation energies and the reaction thermochemistry, we predict a viable Eley–Rideal H/D exchange pathway that may operate at or above 573 K. We also found potential H/D exchange channels on bare TS-1 (without Au₃) where gas-phase D₂ (or second H₂) attacks the Ti–OH or Si–OH groups (of defect sites) and exchanges one of the D atoms to form HD and Ti–OD or Si–OD groups.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Structure sensitivity of CO oxidation over model Au/TiO2 2 catalysts

Model catalysts of Au clusters supported on TiO₂ thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO₂ catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)^-1 × s^-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO₂(001)/Mo(100) is structure sensitive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation between CO frequency and Pt coordination number. A DRIFT study on supported Pt catalysts

The infrared stretching frequencies of linear-bonded carbon monoxide on face, edge and corner atoms have been identified with four platinum catalysts. The metal particles were supported on different absorbentia and each having a different average particle size, ranging from about 10 to 200 Å. The values of the various absorption bands correlate linearly with the number of nearest neighbours of the different surface sites (n in C _n ). Effects of an interaction between Pt clusters and support on the linear-CO stretching frequency have not been observed. This empirical correlation between and C_n, and the advantages ofin situ Diffuse Reflectance Fourier transform Infrared Spectroscopy in combination with derivative spectrometry opens the possibility to monitor the dispersity of supported platinum in a simple and relatively quick way.     

Kinetics of ethylene oxidation on plane Pt/SiO2 catalysts in the viscous pressure regime: evidence of support activity

C₂H₄ oxidation on plane Pt/SiO₂ model catalysts with various Pt loadings was studied atT = 373–473 K and in the pressure ranges 10^–6–10² Torr C₂H₄ and 0.3–1500 Torr 02 (1 Torr = 133.3 Pa). Mass spectrometry combined with spatially resolved gas sampling enabled kinetic data to be collected far into the viscous pressure regime. Reaction orders and activation energies were similar to those of a macroscopic Pt surface. However, under fuel-lean conditions the global reaction rate decreases faster than the decrease in metal area. On the other hand, the global rate wasindependent of Pt loading and metal surface area in fuel-rich gas mixtures. This is interpreted in terms of a spillover effect.     

Rational design of gold catalysts with enhanced thermal stability: post modification of Au/TiO2 by amorphous SiO2 decoration

Au/TiO₂ is highly active for CO oxidation, but it often suffers from sintering in high-temperature environments. In this work, we report on a novel design of gold catalysts, in which pre-formed Au/TiO₂ catalysts were post decorated by amorphous SiO₂ to suppress the agglomeration of gold particles. Even after being aged in O₂–He at 700 °C, the SiO₂-decorated Au/TiO₂ was still active for CO oxidation at ambient temperature.     

The adsorption of Sn on Pt(1 1 1) and its influence on CO adsorption as studied by XPS and FTIR

The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5×10⁻⁵ M Sn²⁺ in 0.5 M H₂SO₄ has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of ?_max=0.35 (Sn to surface Pt atoms).Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm⁻¹ band associated with bridge-bonded CO and the 2070 cm⁻¹ band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm⁻¹ band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2×2)-3CO saturation structure on Pt(1 1 1). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV.     

The promoter function of molybdenum in Rh/Mo/SiO2 catalysts for CO hydrogenation

A series of Rh/Mo/SiO₂ catalysts with fixed Rh and different Mo contents were studied by FT-IR, chemisorption and CO hydrogenation. The FT-IR results at room temperature under CO atmosphere indicate that the addition of Mo to Rh/SiO₂ suppresses the linear and bridged CO species and promotes the twin CO species, which is consistent with the chemisorption results. It is suggested that the Mo promoter works via stabilization of Rh^1– ions and the coverage of Rh sites. The molybdenum promotes the formation of oxygenates and shifts the selectivity from hydrocarbons to oxygenates.     

碱金属离子交换Y型分子筛CO分子吸附量子化学研究

应用量子化学方法,从微观角度了解一氧化碳（CO）分子吸附在碱金属离子（Li⁺、Na^+、K⁺、Rb⁺、Cs⁺）交换Y型分子筛上的机理,详细研究了吸附时红外光谱的变化情况。研究发现,CO分子能与碱金属离子交换Y型分子筛发生多分子吸附,且其吸附行为和结构相类似。CO发生单分子和双分子吸附后,其红外光谱均会发生蓝移。当CO双分子吸附时,其红外光谱会出现两个新的吸附带,且其振动频率比单分子吸附时有所降低。当CO在Li-Y、Na-Y分子筛上发生双分子吸附时,其振动频率与单分子吸附时有较大差别;而CO在K-Y、Rb-Y、Cs-Y分子筛上发生双分子吸附时,其吸附前后的振动频率变化较小,且两个CO分子的振动频率很接近。能量计算结果显示,CO分子吸附强度会随着离子半径的增加而减弱。     

Metal-support effects in Pt/L-zeolite catalysts

A series of Pt/L-zeolites with different cations (varying acidity) were prepared and characterized by H₂ and CO chemisorption and competitive toluene/benzene hydrogenation. The ratio of adsorption equilibrium constants for toluene/benzene was extracted from rates of hydrogenation. The variation of this ratio across a series of alkaline-earth exchanged Pt/L-zeolites indicates that there is a varying support interaction with Pt clusters in the zeolite and suggests that the clusters interact electronically with the support.     

CO adsorption and oxidation on Au/TiO2

Preoxidized Au/TiO₂ showed no initial activity during a first heating stage up to 70°C, while prereduction yielded a high initial CO conversion at room temperature. With FTIRS, two different CO absorption bands were detected. One band is usually attributed to CO on an oxidic gold species (2151 cm^-1), the other one is characteristic of CO on metallic gold (2112 cm^-1). The presence of the first species appears to have a detrimental effect on the CO oxidation by O₂. The present results do not support a model in which the activity of supported gold catalysts in CO oxidation is ascribed to ionic Au particles. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reduction of NO by H2 and CO on Pt/TiO2/SiO2

Different catalysts based on platinum and a silica, titania or mixed titania/silica support were studied in NO reduction reactions by CO and H₂ in the temperature range of 25–400C. The mixed oxide catalysts showed considerably lower onset temperatures in NO/CO reactions but this coincided with a maximum in N₂O formation. In NO/H₂ reactions all titania containing catalysts produced more N₂O than silica supported platinum at low temperatures but were more selective to N₂ at high temperatures.     

Interaction of CO with planar Au/TiO2 model catalysts at elevated pressures

The interaction of CO with structurally well-defined, planar Au/TiO₂ model catalysts at elevated pressures (up to 50 mbar) was studied in-situ by polarization-modulated infrared reflection absorption spectroscopy and ex-situ by X-ray photoelectron spectroscopy performed before and after CO exposure. The results indicate a CO-induced partial reduction of the oxide surface, which is evidenced by a low frequency C–O vibration at 2060 cm⁻¹, combined with a spreading of the Au nanoparticles due to a modification of the Au-oxide interface energy. In a 2:1 CO:O₂ atmosphere, TiO₂ support reduction was not observed, and a pre-reduced surface was re-oxidized. The consequences of these results for the understanding of the CO oxidation mechanism on Au/TiO₂ (model) catalysts are discussed.     

Oscillations of CO and CO + H2 oxidation in air over supported and unsupported Pt catalysts

Oscillation behaviour of the oxidation of CO (0.2–2.2%) in air over Pt wire coils and over Pt/Al₂O₃ catalysts deposited onto the coils (pellistors) has been investigated. The waveforms differ considerably between the two catalytic systems. Over unsupported Pt at 240–260° C, regular oscillations were accompanied by slowly declining activity and by deposition of carbon. Over supported Pt at 110–180°C, relatively complex but sustained oscillation occurred by a different mechanism. This oscillation was greatly enhanced by H₂ (0.25–1.0%), and may involve fluctuations in the concentrations of CO and H₂ around the supported catalyst.     

CO oxidation and oxygen-assisted CO adsorption/desorption on Ag/MnOx catalysts

A series of Ag-doped manganese oxide catalyst were synthesized by the reflux method in an acid medium. The surface structure of the catalysts was characterized by N₂ adsorption, XRD and TEM experiments. The catalysts showed excellent catalytic activity for CO oxidation. The adsorption and oxidation of CO on a 1.0% Ag/MnO_x catalyst between 393 and 493 K were studied by means of single pulse experiments in a TAP reactor. The adsorption of CO was reversible at these temperatures and CO₂ was formed in an oxidation reaction of CO and lattice oxygen. Curve fitting to the experimental TAP response curves of the reactant and product was used to determine the kinetic parameters for the elementary steps. The activation energies were 83 kJ/mol for CO desorption, 31 kJ/mol for CO₂ desorption, and 116 kJ/mol for the surface CO oxidation by lattice oxygen. In addition, the effect of coadsorbed O₂ on CO adsorption was studied by the TAP technique. Below 353 K, there was a sharp increase, by about one order of magnitude, in the rate constant of CO adsorption promoted by the presence of coadsorbed O₂.     

Different support effect of M/ZrO2 and M/CeO2 (M = Pd and Pt) catalysts on CO adsorption: A periodic density functional study

CO adsorption over Pd₄ and Pt₄ cluster supported by c-ZrO₂(1 1 1) and CeO₂(1 1 1) catalyst systems was investigated using periodic density functional method in order to clarify the support effect on CO activation. We found that the support increases the CO activation for bridge and three-fold sites but decreases for the atop site. Moreover, it was found that the support changes the site preference for the CO adsorption. Bridge site on both the Pt₄/c-ZrO₂ and Pt₄/CeO₂ show larger CO adsorption energies than those on the other sites while the atop site is energetically preferable on isolated Pt₄ cluster. c-ZrO₂ supported Pd shows the largest CO activation with large charge transfer from the catalyst to the CO molecule. This reveals that ZrO₂ supported Pd can be a good catalyst for CO activation because of its higher probability to the three-fold site CO adsorption. We also found that positively charged M₄ clusters on the support keep their strong electron-donating properties and have enough charge density to contribute to the activation of an adsorbed CO molecule by a charge transfer.     

Comparison of the activity of Au/CeO2 and Au/Fe2O3 catalysts for the CO oxidation and the water-gas shift reactions

We compare the activity and relevant gold species of nanostructured gold–cerium oxide and gold–iron oxide catalysts for the CO oxidation by dioxygen and water. Well dispersed gold nanoparticles in reduced form provide the active sites for the CO oxidation reaction on both oxide supports. On the other hand, oxidized gold species, strongly bound on the support catalyze the water-gas shift reaction. Gold species weakly bound to ceria (doped with lanthana) or iron oxide can be removed by sodium cyanide at pH ≥12. Both parent and leached catalysts were investigated. The activity of the leached gold–iron oxide catalyst in CO oxidation is approximately two orders of magnitude lower than that of the parent material. However, after exposure to H₂ up to 400 °C gold diffuses out and is in reduced form on the surface, a process accompanied by a dramatic enhancement of the CO oxidation activity. Similar results were found with the gold–ceria catalysts. On the other hand, pre-reduction of the calcined leached catalyst samples did not promote their water-gas shift activity. UV–Vis, XANES and XPS were used to probe the oxidation state of the catalysts after various treatments.