Enantioselective chemisorption on a chirally modified surface in ultrahigh vacuum: adsorption of propylene oxide on 2-butoxide-covered palladium(111)

The enantioselective chemisorption of (S)- and (R)-propylene oxide is measured on a Pd(111) surface chirally modified using (S)- and (R)-2-butanol. Reflection-absorption infrared spectroscopic (RAIRS) data suggest that adsorbed 2-butanol forms 2-butoxide species when heated to approximately 150 K and converts to a ketone with a concomitant loss in chirality at 200 K. Methyl ethyl ketone, ethylene, methane, CO, and hydrogen are found as products in temperature-programmed desorption (TPD). Propylene oxide adsorbs reversibly on Pd(111) at 80 K without undergoing any thermal decomposition, thus providing an ideal probe of surface chirality. The coverage of (R)-propylene oxide adsorbing on an (R)-2-butoxide-covered surface, ratioed to that on one covered by (S)-2-butoxide, reaches a maximum value of approximately 2 at a relative 2-butoxide coverage of approximately 25% of saturation and decreases to unity at a coverage of approximately 50% of saturation. This implies that the enantioselectivity depends critically on coverage and arises due to chiral "pockets" formed on the surface.     

Probing enantioselectivity on chirally modified Cu(110), Cu(100), and Cu(111) surfaces

Temperature programmed desorption methods have been used to probe the enantioselectivity of achiral Cu(100), Cu(110), and Cu(111) single crystal surfaces modified by chiral organic molecules including amino acids, alcohols, alkoxides, and amino-alcohols. The following combinations of chiral probes and chiral modifiers on Cu surfaces were included in this study: propylene oxide (PO) on L-alanine modified Cu(110), PO on L-alaninol modified Cu(111), PO on 2-butanol modified Cu(111), PO on 2-butoxide modified Cu(100), PO on 2-butoxide modified Cu(111), R-3-methylcyclohexanone (R-3-MCHO) on 2-butoxide modified Cu(100), and R-3-MCHO on 2-butoxide modified Cu(111). In contrast with the fact that these and other chiral probe/modifier systems have exhibited enantioselectivity on Pd(111) and Pt(111) surfaces, none of these probe/modifier/Cu systems exhibit enantioselectivity at either low or high modifier coverages. The nature of the underlying substrate plays a significant role in the mechanism of hydrogen-bonding interactions and could be critical to observing enantioselectivity. While hydrogen-bonding interactions between modifier and probe molecule are believed to induce enantioselectivity on Pd surfaces (Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240-15249), such critical interactions may be missing on Cu surfaces where hydrogen-bonding interactions are believed to occur between adjacent modifier molecules, enabling them to form clusters or islands.     

Requirements for the formation of a chiral template

The chemisorptive enantioselectivity of propylene oxide is examined on Pd(111) surfaces templated by chiral 2-methylbutanoate and 2-aminobutanoate species. It has been found previously that chiral propylene oxide is chemisorbed enantiospecifically onto Pd(111) surfaces modified by either (R)- or (S)-2-butoxide. The enantiomeric excess (ee) varied with template coverage, reaching a maximum of approximately 31%. Templating the surface using 2-methylbutanoate, where the chiral center is identical to that in the 2-butoxide species, but is now anchored to the surface by a carboxylate rather than an alkoxide linkage, shows no enantiospecificity. The enantioselectivity is restored when the methyl group is replaced by an amine group, where a maximum ee value of approximately 27% is found. DFT calculations and infrared measurements suggest that the structures of the butyl group on the surface are similar for both 2-butoxide and 2-methylbutanoate species, implying that gross conformational changes are not responsible for differences in chemisorptive enantioselectivity. There is no clear correlation between the location of the chiral center and enantioselectivity, suggesting that differences in the template adsorption site are also not responsible for the lack of enantioselectivity. It is proposed that the 2-butyl group in 2-methylbutanoate species is less rigidly bonded to the surface than that in 2-butoxides, allowing the chiral center to rotate azimuthally. It is postulated that the role of the amino group in 2-aminobutanoate species is to anchor the chiral group to the surface to inhibit azimuthal rotation.     

Enantioselectivity of adsorption sites created by chiral 2-butanol adsorbed on Pt(111) single-crystal surfaces

The adsorption and thermal chemistry of 2-butanol and propylene oxide, each individually and when coadsorbed together, were characterized on Pt(111) single-crystal surfaces by using temperature programmed desorption and reflection-adsorption infrared spectroscopies. The formation of chiral superstructures on the surface upon the deposition of submonolayer coverages of enantiopure 2-butoxide species, produced by thermal dehydrogenation of 2-butanol, was highlighted by their difference in behavior toward the adsorption of the two enantiomers of propylene oxide. It was found that a significant enhancement in adsorption is possible on surfaces with the same chirality of the probe molecule, that is, for (R)-propylene oxide adsorption on (R)-2-butoxide layers and for (S)-propylene oxide adsorption on (S)-2-butoxide layers. The propylene oxide probe was found to also adsorb with the ring closer to the surface in those cases. Finally, less butoxide decomposition is seen at higher temperatures from the homochiral pairing, presumably because the coadsorbed propylene oxide forces the alkoxides into a more compact and better packed structure on the surface.     

Chemical speciation of adsorbed glycine on metal surfaces

Experimental studies have reported that glycine is adsorbed on the Cu(110) and Cu(100) surfaces in its deprotonated form at room temperature, but in its zwitterionic form on Pd(111) and Pt(111). In contrast, recent density functional theory (DFT) calculations indicated that the deprotonated molecules are thermodynamically favored on Cu(110), Cu(100), and Pd(111). To explore the source of this disagreement, we have tested three possible hypotheses. Using DFT calculations, we first show that the kinetic barrier for the deprotonation reaction of glycine on Pd(111) is larger than on Cu(110) or Cu(100). We then report that the presence of excess hydrogen would have little influence on the experimentally observed results, especially for Pd(111). Lastly, we perform Monte Carlo simulations to demonstrate that the aggregates of zwitterionic species on Pt(111) are energetically preferred to those of neutral species. Our results strongly suggest that the formation of aggregates with relatively large numbers of adsorbed molecules is favored under experimentally relevant conditions and that the adsorbate-adsorbate interactions in these aggregates stabilize the zwitterionic species.     

Reaction of tributyl phosphite with oxidized iron: surface and tribological chemistry

The surface chemistry of a model lubricant additive, tributyl phosphite (TBPi), is investigated on Fe3O4 in ultrahigh vacuum. A portion of the TBPi desorbs molecularly following adsorption at approximately 200 K, the remainder decomposing either by C-O bond scission to form 1-butyl species or by P-O bond cleavage to form butoxy species. Adsorbed butyl species either undergo beta-hydride elimination to desorb 1-butene or decompose to deposit carbon and hydrogen on the surface. The resulting adsorbed hydrogen reacts with the oxide to desorb water or with the butoxy species to form 1-butanol. Butoxy species are stable up to approximately 600 K at which temperature they also undergo beta-hydride elimination to form butanal and the released hydrogen reacts with other butoxy species to form 1-butanol. Only a small amount of carbon is deposited onto the surface following adsorption at approximately 200 K, which then desorbs as CO above approximately 750 K. Adsorbing TBPi at 300 K results in the deposition of more carbon and an Auger depth profile reveals that the carbon is located predominantly on the surface, while the phosphorus is rather uniformly distributed throughout the oxide film. This result is in accord with previous near-edge X-ray absorption fine structure measurements, which show the formation of phosphates and polyphosphate glasses. The resulting tribological film appears to be composed of a relatively hard polyphosphate glass formed by rapid diffusion of POx species into the oxide, covered by a low shear strength graphitic layer.     

Chemisorption of CO2 on nickel surfaces

CO2 chemisorption on the Ni(111), Ni(100), and Ni(110) surfaces was investigated at the level of density functional theory. It was found that the ability of CO2 chemisorption is in the order of Ni(110) > Ni(100) > Ni(111). CO2 has exothermic chemisorption on Ni(110) and endothermic chemisorption on Ni(111), while it is thermally neutral on Ni(100). It is also found that there is no significant lateral interaction between the adsorbed CO2 at 1/4 monolayer (ML) coverage, while there is stronger repulsive interaction at 1/2 ML. On all surfaces, the chemisorbed CO2 is partially negatively charged, indicating the enhanced electron transfer, and the stronger the electron transfer, the stronger the C=O bond elongation. The bonding nature of the adsorbed CO2 on nickel surfaces has been analyzed. The thermodynamics of CO2 dissociative chemisorption, compared with CO and O adsorption, has been discussed, and the thermodynamic preference is in the sequence Ni(100) > Ni(111) > Ni(110).     

乙腈、苯基氰在Cu(111)与Pd(100)表面上的吸附与反应

用高分辨电子能量损失谱(HREELS~*)对CH_3CN(乙腈)及C_6H_5CN(苯基氰)在清洁与氧覆盖的Cu(111)及Pd(100)表面上的吸附及其反应进行了研究。从198 K时CH_3CN吸附在Cu(111)及Pd(100)表面上的高分辨电子能量损失谱(HREELS)中观察到v(C≡N)几乎消失, 并在195 meV处出现一个较弱的v(C=N)谱带, 表明CH_3CN在吸附过程中C≡N再杂化为C=N,C,N原子分别与金属表面原子键合并C=N平行于表面。从198 K时C_6H_5CN在Pd(100)及Cu(111)上的HREELS表明C_6H_5CN的环平面与CN平行于金属表面。在185K时C_6H_5CN在氧覆盖的Pd(100)表面上的HREELS与其在清洁表面上的相似。并未观察到覆盖氧增强了C_6H_5CN在Pd(100)上的吸附及其它效应。C_6H_5CN吸附在氧覆盖的Cu(111)表面上产生了C_6H_5CNO的特征谱带。     

UHV–EC and EC–STM studies of molecular chemisorption at well-defined surfaces: hydroquinone and benzoquinone on Pd(hkl)

The chemisorption, from aqueous solutions, of hydroquinone and benzoquinone onto well-defined Pd(111) and Pd(100) electrode surfaces has been studied by tandem electrochemistry, high-resolution electron energy loss spectroscopy and in situ scanning tunneling microscopy. The results indicate that hydroquinone is oxidatively chemisorbed to form a quinonoid species that is oriented parallel to the surface but with a slight tilt in which one of the para-oxygens is topographically just higher than the other. The same surface species is obtained when adsorption is from aqueous benzoquinone.     

Direct IR observation of vibrational properties of carbonyl species formed on Pd nano-particles supported on amorphous carbon: comparison with Pd/SiO2-Al2O3

By diluting optically opaque carbon-supported Pd particles in silica Aerosil we succeeded in observing the IR bands of adsorbed carbonyls and extracting information on the particle dispersion. Comparison with literature single crystal data and with silica-alumina supported Pd allowed us to make an assignment in terms of linear and 2-fold bridged carbonyls formed on Pd(111) and Pd(100) faces. Two Pd/C samples have been investigated. The relative intensities of the two carbonyl families observed on the two samples are consistent with the Pd dispersion independently measured with CO chemisorption, TEM and EXAFS analysis.     

A molecular beam study of the NO + CO reaction on Pd(111) surfaces

Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state conditions. Finally, the results are discussed with respect to the rate-controlling character of the different elementary steps of the reaction system.     

Comparison of the reactivity of different Pd-O species in CO oxidation

The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.     

Isocyanate formation in the catalytic reaction of CO + NO on Pd(111): an in situ infrared spectroscopic study at elevated pressures

The catalytic CO + NO reaction to form CO2, N2, and N2O has been studied on a Pd(111) surface at pressures up to 240 mbar using in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS). At 240 mbar, for a pressure ratio of PCO:PNO = 3:2 and under reaction conditions, besides adsorbed CO, the formation of isocyanate (-NCO) was observed. Once produced at 500-625 K, the isocyanate species was stable within the entire temperature range studied (300-625 K). On the other hand, its formation required a total CO + NO pressure of at least 0.6 mbar, illustrating the importance of in situ infrared experiments under high-pressure conditions. The significance of the isocyanate formation for the CO + NO reaction on Pd(111) is discussed.     

The Use of Thin‐Layer Electroanalysis in the Study of the Chemisorption and Anodic Oxidation of Aromatic Molecules at Smooth Polycrystalline Palladium

Thin‐layer electroanalytical chemistry has been used in the study of the chemisorption and anodic oxidation of hydroquinone and benzoquinone at smooth polycrystalline palladium electrode surfaces in aqueous sulfuric acid solutions. The results were reminiscent of those obtained previously on smooth polycrystalline platinum: i) At low aqueous‐solution concentrations, the diphenol is oxidatively chemisorbed to form surface‐coordinated benzoquinone oriented parallel to the surface. ii) At higher concentrations, the oxidative chemisorption occurs via C? H activation to yield an edge‐oriented diphenolic species. iii) Chemisorption from benzoquinone solutions leads to species identical to those from hydroquinone solutions. iv) The extent of anodic oxidation of the chemisorbed organic depends upon the initial adsorbed‐molecule orientation: the flat‐adsorbed species are oxidized completely to carbon dioxide, whereas oxidation of the edge‐chemisorbed species yields other (unidentified) products that are chemisorbed upon regeneration of the oxide‐free surface.     

Adsorption of carbon on Pd clusters of nanometer size: a first-principles theoretical study

Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd(55), Pd(79), Pd(85), Pd(116), Pd(140), and Pd(146), were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600 kJ mol(-1)) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group O(h) to D(4h). On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.     

Time-lapse STM studies of diastereomeric cinchona alkaloids on platinum metals

The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In time-lapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.     

Selectivity control for the catalytic 1,3-butadiene hydrogenation on Pt(111) and Pd(111) surfaces: Radical versus closed-shell intermediates

The hydrogenation of 1,3-butadiene to different C4H8 species on both Pd(111) and Pt(111) surfaces has been studied by means of periodic slabs and DFT. We report the adsorption structures for the various mono- and dihydrogenated butadiene intermediates adsorbed on both metal surfaces. Radical species are more clearly stabilized on Pt than on Pd. The different pathways leading to these radicals have been investigated and compared to those producing 1-butene and 2-butene species. On palladium, the formation of butenes seems to be clearly favored, in agreement with the high selectivity to butenes observed experimentally. In contrast, the formation of dihydrogenated radical species seems to be competitive with that of butenes on platinum, which could explain its poorer selectivity to butenes and the formation of butane as a primary product.     

Characterization of trimetallic Pt-Pd-Au/CeO2 catalysts combinatorial designed for methane total oxidation

In the present work, the role and the effect of platinum and gold on the catalytic performance of ceria supported tri-metallic Pt-Pd-Au catalysts have been studied. The optimum composition of these tri-metallic supported catalysts has been discovered using methods and tools of combinatorial catalyst library design. Detailed catalytic, spectroscopic and physico-chemical characterization of catalysts in the vicinity of the optimum in the given compositional space has been performed. The temperature-programmed oxidation of methane revealed that the addition of Pt and Au to Pd/CeO2 catalyst resulted in higher conversion values in the whole investigated temperature range compared to the monometallic Pd catalyst. The time-on-stream experiments provided further evidence for the high-stability of tri-metallic catalysts compared to the monometallic one. Kinetic studies revealed the stronger adsorption of methane on Pt-Pd/CeO2 catalysts than over Pd/CeO2. XPS analysis showed that Pt and Au stabilize Pd in a more reduced form even under condition of methane oxidation. FTIR spectroscopy of adsorbed CO and hydrogen TPD measurements provided indirect evidences for alloying of Pt and Au with Pd. CO chemisorption data indicated that tri-metallic catalysts have increased accessible metallic surface area. It is suggested that advantageous catalytic properties of tri-metallic Pt-Au-Pd/CeO2 catalysts compared to the monometallic one can be attributed to (i) suppression of the formation of ionic forms of Pd(II), (ii) reaching an optimum ratio between Pd0 and PdO species, and (iii) stabilization of Pd in high dispersion. The results also indicate that Pd0 - PdO ensemble sites are required for methane activation.     

Monte Carlo simulation of catalytic CO oxidation

A simple Monte Carlo model of the CO oxidation on a single-crystal catalyst surface is presented. The simulation model considers the following elementary reaction steps:

1. (1) chemisorption of a CO molecule, its surface migration and possible desorption

2. (2) physisorption of an O₂ molecule to a precursor state and its subsequent dissociative chemisorption

3. (3) activated reaction of adsorbed O and CO (the Langmuir - Hinshelwood reaction mechanism), formation of CO₂ and its rapid desorption.

The changes in the activation energy of reaction and in the adsorption energy of CO resulting from the interactions between adsorbed species are also considered. The model makes possible to monitor temperature programmed reaction spectra or reaction spectra obtained during changes of the ratio of the partial pressures of CO and O₂. The results of simulations for a Pd(111) single-crystal plane are compared with experiment.     

Ethene adsorption, dehydrogenation and reaction with Pd(110): Pd as a carbon 'sponge'

The interaction of ethene with the Pd(110) surface has been investigated, mainly with a view to understanding the dehydrogenation reactions of the molecule and mainly using a molecular beam reactor. Ethene adsorbs with a high probability over the temperature range 130 to 800 K with the low-coverage sticking probability dropping from 0.8 at 130 K to 0.35 at 800 K. The adsorption is of the precursor type, with a weakly held form of ethene being the intermediate between the gas phase and strong chemisorption. Dehydrogenation begins at approximately 300 K and is fast above 350 K. If adsorption is carried out at temperatures up to approximately 380 K, adsorption saturates after about 0.25 monolayer have adsorbed, but above approximately 450 K, adsorption continues at a high rate with continuous hydrogen evolution and C deposition onto the surface. It appears that, in the intermediate temperature range, the carbonaceous species formed is located in the top layer and thus interferes with adsorption, whereas the C goes subsurface above 450 K, the adsorption is almost unaffected, and the C signal is significantly attenuated in XPS. However, the deposited carbon can easily be removed again by reaction with oxygen, thus implying that the carbon remains in the selvedge, that is, in the immediate subsurface region probably consisting of a few atomic layers. No well-ordered structures are identified in either LEED or STM, though some evidence of a c(2 x 2) structure can be seen. The Pd surface, at least above 450 K, appears to act as a "sponge" for carbon atoms, and this effect is also seen for the adsorption of other hydrocarbons such as acetaldehyde and acetic acid.