Peter H. McBreen

Direct observation of molecular preorganization for chirality transfer on a catalyst surface.

Scanning tunneling microscopy and theoretical calculations shed light on an asymmetric heterogeneous catalyst. The chemisorption of specific optically active compounds on metal surfaces can create catalytically active chirality transfer sites. However, the mechanism through which these sites bias the stereoselectivity of reactions (typically hydrogenations) is generally assumed to be so complex that continued progress in the area is uncertain. We show that the investigation of heterogeneous asymmetric induction with single-site resolution sufficient to distinguish stereochemical conformations at the submolecular level is finally accessible. A combination of scanning tunneling microscopy and density functional theory calculations reveals the stereodirecting forces governing preorganization into precise chiral modifier-substrate bimolecular surface complexes. The study shows that the chiral modifier induces prochiral switching on the surface and that different prochiral ratios prevail at different submolecular binding sites on the modifier at the reaction temperature.

RAIRS and TPD study of CO and NO on β-Mo2C

Abstract The chemisorption of CO and NO on bulk β-Mo 2 C was studied using reflectance absorbance infrared spectroscopy (RAIRS) and thermal desorption measurements. The carbide foil substrate was cleaned in situ prior to each experiment by repeated annealing to 1400 K. Low coverage dissociation of CO yields a weak recombinative thermal desorption feature at approximately 1200 K and molecular desorption of CO occurs at 325 K for the lowest exposures studied. RAIRS results show that carbon monoxide on β-Mo 2 C at 100 K is characterized by a single CO vibrational stretching frequency, typical of on-top adsorption, which increases from 2057 to 2072 cm −1 as a function of increasing coverage. Preadsorption of oxygen leads to a broadening to higher frequencies but not to the appearance of any new absorbance bands. The vibrational spectrum for adsorbed NO at 100 K displays a band at 1780 cm −1 and a shoulder at 1820 cm −1 . RAIRS spectra acquired as a function of temperature were used to monitor the decomposition of NO. Decomposition occurs over the 250–450 K range and results in high temperature CO and N 2 desorption peaks. The results for the chemisorption of NO and CO on molybdenum carbide are compared with literature data for the adsorption of these probe molecules on molybdenum and ruthenium surfaces. This comparison is used to comment on the physical basis for the correspondence in catalytic properties between early transition metal carbides and noble metal based catalysts.

Activation in prochiral reaction assemblies on Pt(111).

Trifluoroacetophenone (TFAP) forms C O...H-C bonded dimers and trimers at room temperature on Pt(111). It is proposed that these systems mimic the prochiral carbonyl-chiral modifier interaction in the enantioselective hydrogenation of TFAP on cinchona-modified Pt catalysts. That is, the activation of TFAP in homomolecular assemblies at racemic sites is expected to be roughly the same as in the diastereomeric complex formed at chiral sites. This interpretation suggests a reason why alpha-phenyl ketones do not display a strong measured rate enhancement effect in the Orito reaction.

Formation of thermally stable alkylidene layers on a catalytically active surface

Materials containing organic–inorganic interfaces usually display a combination of molecular and solid-state properties, which are of interest for applications ranging from chemical sensing to microelectronics and catalysis. Thiols—organic compounds carrying a SH group—are widely used to anchor organic layers to gold surfaces, because gold is catalytically sufficiently active to replace relatively weak S–H bonds with Au–S bonds, yet too inert to attack C–C and C–H bonds in the organic layer. But although several methods of functionalizing the surfaces of semiconductors, oxides and metals are known, it remains difficult to attach a wide range of more complex organic species. Organic layers could, in principle, be formed on the surfaces of metals that are capable of inserting into strong bonds, but such surfaces catalyse the decomposition of organic layers at temperatures above 400 to 600 K, through progressive C–H and C–C bond breaking. Here we report that cycloketones adsorbed on molybdenum carbide, a material known to catalyse a variety of hydrocarbon conversion reactions, transform into surface-bound alkylidenes stable to above 900 K. We expect that this chemistry can be used to create a wide range of exceptionally stable organic layers on molybdenum carbide.

Stereodirection of an α-Ketoester at Sub-molecular Sites on Chirally Modified Pt(111): Heterogeneous Asymmetric Catalysis

Chirally modified Pt catalysts are used in the heterogeneous asymmetric hydrogenation of α-ketoesters. Stereoinduction is believed to occur through the formation of chemisorbed modifier-substrate complexes. In this study, the formation of diastereomeric complexes by coadsorbed methyl 3,3,3-trifluoropyruvate, MTFP, and (R)-(+)-1-(1-naphthyl)ethylamine, (R)-NEA, on Pt(111) was studied using scanning tunneling microscopy and density functional theory methods. Individual complexes were imaged with sub-molecular resolution at 260 K and at room temperature. The calculations find that the most stable complex isolated in room-temperature experiments is formed by the minority rotamer of (R)-NEA and pro-S MTFP. The stereodirecting forces in this complex are identified as a combination of site-specific chemisorption of MTFP and multiple non-covalent attractive interactions between the carbonyl groups of MTFP and the amine and aromatic groups of (R)-NEA.

Ru-Mo/HZSM-5 Catalyzed Methane Aromatization in Membrane Reactors

Oxygen-free methane conversion into benzene was carried out in a catalytic membrane reactor over 0.5%Ru-3%Mo/HZSM-5 in the temperature range 873-973 K following three reaction protocols: (i) straight-run catalytic reactor without hydrogen permeation (OFF), (ii) cycled OFF/ON hydrogen permeation sequences, and (iii) cycled OFF/ON hydrogen permeation sequences intertwined with CH4/H2 regenerative steps. X-ray photoelectron spectroscopy analysis of fresh and spent catalysts identified, in all cases, three types of carbon species that formed during aromatization, including carbide formation. The presence of a permeating membrane did not give rise to different chemical states of carbon and molybdenum on the catalyst from those known to form in straight runs under no hydrogen permeation. The ON mode, i.e., during permeation, led to the accumulation of graphite-like and aromatic-aliphatic (coke) species on the catalyst. However, both types of carbon were reduced during the OFF step either by autogenous hydrogen or via an external source of hydrogen under CH4/H2 regenerative steps.

Decomposition of diazirine on Pd(110) to yield adsorbed CH2 : XPS and TPD results

Abstract XPS and TPD measurements were used to study the interaction of cyclic CH 2 N 2 , diazirine, with a Pd(110) surface. One goal of the study was to prepare adsorbed methylene, a product of the gas phase thermolysis of diazirine, through thermal decomposition on the surface. Thermal decomposition of diazirine on Pd(110) results in both CN and NN bond rupture. CN bond cleavage, which occurs at 130 K, leads to the deposition of CH 2 on the surface as characterized by a C(1s) binding energy of 283.6 eV. TPD measurements show that the CH 2ads species reacts to form methane and ethylene which desorb at approximately 180 K and some partially hydrogenated carbonaceous residue which remains on the surface. The latter residue is characterized by a broad hydrogen desorption tail to above 600 K and a C(1s) value of 284.2 eV. CH 4 desorption is observed for all initial coverages of diazirine whereas C 2 H 4 desorption only becomes significant at high coverages. The temperature at which CH 4 and C 2 H 4 desorption takes place is found to coincide with a shift in the C(1s) value from 283.6 to 284.0 eV. This shift is interpreted as arising from the interaction of CH 2 units to form the C x H y species which comprise the carbonaceous residue. Effects due to coadsorbed species are discussed. The results are compared with previous C(1s) data for hydrocarbon surface species and other procedures for generating adsorbed C 1 fragments.

Single-chiral-catalytic-surface-sites: STM and DFT study of stereodirecting complexes formed between (R)-1-(1-naphthyl)ethylamine and ketopantolactone on Pt(111)

The formation of bimolecular complexes on metal surfaces through interaction between a single chemisorbed chiral molecule and a single chemisorbed prochiral substrate molecule can be considered as a preorganization step toward chirality transfer. In the case of asymmetric hydrogenation on chirally modified platinum catalysts, the metal surface dissociates H2 and provides atomic hydrogen for the desymmetrization step. Along the reaction path, the combined chemisorption and intermolecular interactions in the assembly formed between the modifier and the substrate determine which enantiomer is formed in excess. In this study, we use DFT calculations and STM measurements to describe chemisorption and intermolecular interactions in isolable structures formed between single ketopantolactone and single (R)-1-(1-naphthyl)ethylamine molecules on Pt(111). The study reveals several distinct complexation geometries at the sub-molecular level as well as the stereodirecting forces operating in the most abundant bimolecular assemblies. The comparison of theoretical and experimental data strongly suggests that partial hydrogenation of KPL occurs under the experimental conditions and that some of the most abundant complexes are formed by the hydroxy intermediate.

In‐Situ Spectroscopic Detection of Active Surface Species in Asymmetric Heterogeneous Catalysis

Heterogeneous asymmetric catalysis is an attractive, but as yet under-developed strategy for the production of enantiopure compounds. Its potential combination of enhanced catalytic performance, ease of separation and reuse of the catalyst make it a persistently active field of applied research. It is also a fascinating and challenging area in relation to chirality in 2D and chirality transfer at metal surfaces. Among the different approaches used to develop heterogeneous asymmetric catalysts, significant attention has been given to systems prepared by chemisorbing chiral auxiliaries on transition metal surfaces. The most widely studied example is the enantioselective hydrogenation of activated ketones, such as a-ketoesters and a,a,a-trifluoroketones on cinchona-modified platinum particles (Scheme 1). Orito and co-workers first reported this family of reactions and its significance within an industrial context was described in a recent review. The database on the reaction is complex, displaying several examples of solvent and substituent dependent stereoinversion, different optimisation conditions for different groups of

Scanning Tunneling Microscopy Measurements of the Full Cycle of a Heterogeneous Asymmetric Hydrogenation Reaction on Chirally Modified Pt(111)

The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (R)-(+)-1-(1-naphthyl)ethylamine, ((R)-NEA), as the modifier. On the nonmodified surface, introduction of H2 at a background pressure of ∼1 × 10(-6) mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAP-related images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H2 to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst.