Red. Trav. Chim. Pays-Bas 113, 465-474 (1994) SSDl 0 1 6 5 -0 5 1 3 ( 9 4 ) 0 0 0 3 8 -4
Abstract.A LEED and XPS study of the adsorption of naphthalene, quinoline, and 10,11-dihydrocinchonidine on Pt(ll1) at 300K has shown that only naphthalene forms an ordered ad-layer, and that quinoline and the alkaloid adsorb in a disordered state and without decomposition. These experiments do not support the hypothesis of ordered adsorption of alkaloid that forms the basis of the template model for the interpretation of enantioselectivity in Pt-catalysed pyruvate hydrogenation. The model is accordingly reviewed. Molecular modelling studies show that a highly specific 1 : 1 interaction between cinchonidine (or cinchonine) and pyruvate interprets the observed sense of the enantioselectivity, provided relative energy relationships derived for purely intermolecular interactions are valid for the same molecules in the adsorbed state. Moreover, the 'product' of this 1: 1 interaction is a satisfactory precursor to the H-bonded state considered responsible for the greatly enhanced rate that always accompanies enantioselective reaction over cinchona-modified Pt. The previously published dependencies of optical yield on ( a ) surface concentration of adsorbed cinchonidine modifier, and ( 6 ) modifier composition for mixtures of quinine and quinidine, are shown to be in quantitative agreement with the proposed 1 : 1 interaction model and at variance with the ordered adsorption model. Catalysts modified and used under strictly anaerobic conditions show negligible activity and enantioselectivity demonstrating that oxygen plays a crucial role in successful catalyst preparation. XPS experiments confirm that adsorption of cinchonidine from air-saturated ethanolic solution on Pt(l11) provides an adlayer containing both alkaloid and adsorbed oxygen. (S)-(-)-l-benzyl-pyrrolidine-2-rnethanol, various configurations of ephedrine, D-and L-histidine and the methyl esters of D-and L-tryptophan have been examined as modifiers for supported Pt. Although there is evidence that these compounds can provide chiral direction to pyruvate hydrogenation, rate enhancement is slight and enantioselectivity is correspondingly low.
Nickel oxides characterised by electron microscopy and with different defect concentrations have been studied by X-ray photoelectron spectroscopy. Oxides preannealed in air at 700, 1 100 and 1450 "C were examined after heating in uacuo in the spectrometer between 20 and 500 "C. High-binding-energy components of O( 1s) spectra at 531.4 eV and Ni(2p3,,) spectra at 856.1 eV can be correlated with the oxide defect structure. Loss of electron-acceptor surface hydroxyl groups as water during evacuation and heating results in the development of surface charge owing to increased band bending at the surface. Above 300 "C, carbon impurity present at < 0.15 of a monolayer is removed predominantly as CO. With the defective ' 700 "C annealed' nickel oxide this results in a major increase in the intensity of the O(ls) peak at 531.4 eV, but the other two samples show a continuous decrease in this peak's intensity. The O(ls) peak at 53 1.4 eV is attributed initially to OH, and after evacuation at 500 "C to an 0surface species.O/Ni atom ratios > 1.0 are found on all surfaces, even after evacuation at 500 "C, but higher values, up to 2.5, are found after evacuation at 25 "C. Spectra from a NiO(100) single-crystal surface evacuated at 600°C and heated in oxygen at 450 "C show enhancement of the high-binding-energy O( 1s) and Ni(2p3,?) peaks at low electron take-off angles, indicating stabilisation of defect 0and Ni3+ species at the solid/vacuum interface.
The interaction of 10,ll-dihydrocinchonidine (DHC) with a Pt(ll1) surface has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). From a saturated ethanolic solution DHC is chemisorbed in multilayers whereas from more dilute solutions adsorption is limited to a monolayer. When exposed to ethanol, the multilayers are removed leaving a strongly chemisorbed monolayer of DHC. A comparison of the LEED patterns of the structurally related compounds naphthalene, quinoline, and DHC shows that only naphthalene forms an ordered adlayer (3 x 3) whereas quinoline and DHC are adsorbed in a disordered state and without decomposition. The enhanced hydrogenation rate and the optical yield for the methyl pyruvate hydrogenation reaction at a cinchona alkaloidmodified EUROPT-1 catalyst, and the absence of ordered adsorption of the alkaloid on a Pt(ll1) surface, support the 1 : 1 interaction model for the alkaloid and pyruvate.Data are also reported for the interaction of Pt(ll1) with ethanol, both in the liquid and vapour state; dissolved oxygen is shown to influence the surface chemistry and lead to the formation of surface acetate species.
The formation of cesium−gold alloys by the deposition of cesium
on to a Au(100) surface has been investigated
by both core-level (X-ray induced) and valence-level (He(II) radiation)
spectroscopies. The stoichiometries
of the alloy overlayer indicate the preferential segregation of cesium
to the surface of the thinner films.
Oxygen chemisorption generates three distinct oxygen species
their O(1s) binding energies and is accompanied by dealloying.
With increasing oxygen exposure, the peroxo
species is the major one present at 298 K. Dealloying is
temperature dependent and is not complete at 80 K.
Two of the oxygen states, Oδ-(a) and
δ-(a), are reactive to carbon
monoxide, forming carbonate species.
Peroxo-type species are also formed readily by oxidation of
pre-carbonated surfaces. The products formed
from CO−O2 mixtures depend on the mixture composition:
with a 3:1 (CO:O2) mixture at 298 K,
and CO3 species are observed, while for very much richer
CO-mixtures (60:1) only O2- and
form; at 80 K both mixtures result in only peroxo
δ-) and carbonate
The interaction of a number of nitrogen containing molecules (NO, NH3 and N2H4) with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K ; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with NOs-(ads) and NOd+(ads) species, the former dissociating and the latter desorbing on thermal activation of the adlayer.Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(ls) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and I1 radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.Ammonia dissociates at 295 K on copper while with oxidized copper the surface oxide is replaced by an imide-type surface. With hydrazine, adsorption is molecular over the temperature range 85 to 295 K, in contrast to our observations with iron.
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