CuCeOx/CuO Catalyst Derived from the Layered Double Hydroxide Precursor: Catalytic Performance in NO Reduction with CO in the Presence of Water and Oxygen.

Valencies of metal species and lattice defects, such as oxygen vacancies, play a pivotal role in metal oxide-catalyzed reactions. Herein, we report a promising synthetic strategy for preparing CuO-supported CuCeOx catalysts (CuCeOx/CuO) by calcination of a hydrotalcite precursor [Cu6Ce2(OH)16]CO3·nH2O. The structural and chemical properties of catalysts were characterized by XRD, ICP-AES, TEM, TPR, NH3-TPD, XPS, Raman spectroscopy, and N2 adsorption, which revealed that the thermal pretreatment in an oxidative atmosphere caused segregation and reconstitution processes of the precursor, resulting in a mesoporous catalyst consisting of well-dispersed CuO-supported CuCeOx clusters of 1.8-3.2 nm in size with a high population of oxygen vacancies. The as-prepared catalyst shows excellent catalytic performance in the reduction of NO by CO in the absence as well as in the presence of water and oxygen. This behavior is attributed to its high oxygen defect concentration facilitating the interplay of the redox equilibria between Cu2+ and reduced copper species (Cu+/Cu0) and (Ce4+/Ce3+). The high surface population of oxygen vacancies and in situ-generated metallic copper species have been evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. The layered double hydroxide-derived CuCeOx/CuO also showed good water tolerance and long-term stability. In situ infrared spectroscopy investigations indicated that adsorbed hyponitrite species are the main reaction intermediates of the NO conversion as also corroborated by theoretical simulations.

Promoting Aromatic C-H Activation through Reactive Brønsted Acid-Base Pairs on Penta-Coordinated Al-Enriched Amorphous Silica-Alumina.

The surface acidity and local coordination environment of zeolites and amorphous silica-aluminas (ASAs) can promote acid-catalyzed C-H activation in many important hydrocarbon conversion reactions. Acid sites generated by penta-coordinated Al species (AlV) can lead to enhanced acidity and changes in the surface coordination. We evaluated the potential of flame-derived ASAs with enriched AlV for C-H activation using hydrogen/deuterium (H/D) exchange with benzene-d6. With increasing Al content of ASAs, the exchange rate increased, whereas the activation energy (Ea) slightly decreased due to the enhanced Brønsted acidity. The ASAs exhibited significantly higher exchange rates and lower Ea values than the sol-gel-derived ASAs and zeolite H-ZSM-5. The superior activity is attributed to the fact that more oxygen coordinated with AlV species on flame-made ASAs, which can act as acceptors for D+, enhancing the deuterium displacement. These findings could offer a valuable alternative strategy for tailoring high-performance solid acids to promote hydrocarbon conversion reactions.

Effect of the Configuration of Copper Oxide-Ceria Catalysts in NO Reduction with CO: Superior Performance of a Copper-Ceria Solid Solution.

Various copper-ceria-based composites have attracted attention as efficient catalysts for the reduction of NO with CO. In this comparative study, we have examined the catalytic potential of different configurations of copper oxide-ceria catalysts, including catalysts based on a copper-ceria solid solution, copper oxide particles supported on ceria, and ball-milled copper oxide-ceria. The structurally different interfaces between the constituents of these catalysts afforded very different catalytic performances. The solid solution catalyst outperformed the corresponding ceria-supported and ball-milled CuO-CeO2 catalysts. The copper cations incorporated into the ceria lattice strongly improved the activity, N2 selectivity, and water vapor tolerance compared to the other catalyst configurations. The experimental observations are supported by first-principles density functional theory (DFT) studies of the reaction pathway, which indicate that the incorporation of Cu cations into the ceria matrix lowers the energy required for activating the lattice oxygen, thereby enhancing the formation and healing of oxygen vacancies, and thus promoting NO reduction with CO.

Revealing Brønsted Acidic Bridging SiOHAl Groups on Amorphous Silica-Alumina by Ultrahigh Field Solid-State NMR.

Amorphous silica-aluminas (ASAs) are important acidic catalysts and supports for many industrially essential and sustainable processes. The identification of surface acid sites with their local structures on ASAs is of critical importance for tuning their catalytic properties but still remains a great challenge and is under debate. Here, ultrahigh magnetic field (35.2 T) 27Al-{1H} D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation) two-dimensional NMR experiments demonstrate two types of Brønsted acid sites in ASA catalysts. In addition to the known pseudobridging silanol acid sites, the use of ultrahigh field NMR provides the first direct experimental evidence for the existence of bridging silanol (BS: SiOHAl) acid sites in ASAs, which has been hotly debated in the past few decades. This discovery provides new opportunities for scientists and engineers to develop and apply ASAs in various reaction processes due to the significance of BS in chemical and fuel productions based on its strong Brønsted acidity.

Engineering the Distinct Structure Interface of Subnano-alumina Domains on Silica for Acidic Amorphous Silica-Alumina toward Biorefining.

Amorphous silica-aluminas (ASAs) are important solid catalysts and supports for many industrially essential and sustainable processes, such as hydrocarbon transformation and biorefining. However, the wide distribution of acid strength on ASAs often results in undesired side reactions, lowering the product selectivity. Here we developed a strategy for the synthesis of a unique class of ASAs with unvarying strength of Brønsted acid sites (BAS) and Lewis acid sites (LAS) using double-flame-spray pyrolysis. Structural characterization using high-resolution transmission electron microscopy (TEM) and solid-state nuclear magnetic resonance (NMR) spectroscopy showed that the uniform acidity is due to a distinct nanostructure, characterized by a uniform interface of silica-alumina and homogeneously dispersed alumina domains. The BAS population density of as-prepared ASAs is up to 6 times higher than that obtained by classical methods. The BAS/LAS ratio, as well as the population densities of BAS and LAS of these ASAs, could be tuned in a broad range. In cyclohexanol dehydration, the uniform Brønsted acid strength provides a high selectivity to cyclohexene and a nearly linear correlation between acid site densities and cyclohexanol conversion. Moreover, the concerted action of these BAS and LAS leads to an excellent bifunctional Brønsted-Lewis acid catalyst for glucose dehydration, affording a superior 5-hydroxymethylfurfural yield.

Formation and Location of Pt Single Sites Induced by Pentacoordinated Al Species on Amorphous Silica-Alumina.

Alumina and its mixed oxides are popular industrial supports for emerging supported metal catalysts. Pentacoordinated Al (AlV) species are identified as key surface sites for anchoring and stabilizing metal single-site catalysts; however, AlV is rare in conventional amorphous silica-alumina (ASA). Recently, we have developed AlV-enriched ASA, which was applied as a support for the synthesis of Pt single-site catalysts in this work. Each preparation stage and the interaction between Pt and surface Al species were explored by 1H and 27Al solid-state nuclear magnetic resonance spectroscopy, and the formation of the dominant Pt single sites on the surface of AlV-enriched ASA was confirmed by high-angle annular dark-field imaging scanning transmission electron microscopy and energy dispersive spectroscopy line scanning. On the surface of supports without a significant AlV population (Pt/Al2O3 and Pt/SiO2), mainly Pt nanoparticles were formed. This indicates that AlV contributes to the strong metal-support interaction to stabilize the Pt single sites on Pt/ASA, which was characterized by diffuse reflectance infrared Fourier transform spectroscopy combined with CO adsorption, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. Pt single sites supported on AlV-enriched ASA exhibit excellent chemoselectivity in the hydrogenation of C═O groups, affording 2-3-fold higher yields compared to those of Pt nanoparticles supported on Al2O3 and SiO2.

Synergistic Effects of Ternary PdO-CeO2-OMS-2 Catalyst Afford High Catalytic Performance and Stability in the Reduction of NO with CO.

We developed a robust ternary PdO-CeO2-OMS-2 catalyst with excellent catalytic performance in the selective reduction of NO with CO using a strategy based on combining components that synergistically interact leading to an effective abatement of these toxic gases. The catalyst affords 100% selectivity to N2 at the nearly full conversion of NO and CO at 250 °C, high stability in the presence of H2O, and a remarkable SO2 tolerance. To unravel the origin of the excellent catalytic performance, the structural and chemical properties of the PdO-CeO2-OMS-2 nanocomposite were analyzed in the as-prepared and used state of the catalyst, employing a series of pertinent characterization methods and specific catalytic tests. The experimental as well as theoretical results, based on density-functional theory calculations suggest that CO and NO follow different reaction pathways, CO is preferentially adsorbed and oxidized at Pd sites (PdII and Pd0), while NO decomposes on the ceria surface. Lattice oxygen vacancies at the interfacial perimeter of PdO-CeO2 and PdO-OMS-2, and the diffusion of oxygen and oxygen vacancies are proposed to play a critical role in this multicenter reaction system.

Pentacoordinated Aluminum Species: New Frontier for Tailoring Acidity-Enhanced Silica-Alumina Catalysts.

Silica-alumina catalysts, including zeolites and amorphous silica-aluminas (ASAs), are among the most widely used solid acid catalysts and supports to produce petrochemicals, fine chemicals, and renewable energy. The coordination, distribution, and interactions of aluminum in ASAs have an enormous impact on their acidic properties and catalytic performance. Unsaturated tetracoordinated aluminum (AlIV) species are commonly accepted as the key sites in generating catalytically active Brønsted acid sites (BASs) in silica-alumina catalysts. Extensive efforts focus on increasing the concentration of AlIV as the main route to enhance their Brønsted acidity for efficient catalysis. However, increasing the AlIV concentration either weakens the acid strength in zeolites or lowers Brønsted acidity in ASAs at high Al/Si ratios, impeding acidity enhancement of these popular catalysts."Pentacoordinated aluminum (AlV) species" are potential unsaturated Al species like AlIV but rarely observed in silica-aluminas, and thus, are widely considered unavailable for BAS formation or surface reactions. In this Account, we will describe novel strategies for the controlled synthesis of AlV-enriched ASAs using flame-spray pyrolysis (FSP) techniques and highlight the contribution of AlV species in acidity enhancement, together with their structure-activity relationship in the conversion of biomass-derived compounds into valuable chemicals. Using various in situ and advanced 2D solid-state NMR (SSNMR) experiments, the studies of the acidic properties and local structure of AlV-enriched ASAs reveal that AlV species can highly populate on ASA surfaces, promote BASs formation, and facilitate adaptable tuning of BASs from moderate to zeolitic strength by synergy with neighboring Al sites. Moreover, the BASs with enhanced acidity can work jointly with surface Lewis acid sites or metal active species for bifunctional catalysis on AlV-enriched ASAs. Compared to zeolites, these AlV-enriched ASAs are highly active in acid-catalyzed biomass conversion, including alcohol dehydration and sugar conversion reactions, as well as in promoting the performance of supported metal catalysts in chemoselective hydrogenation of aromatic ketones. These new insights provide a state-of-the-art strategy for strongly enhancing the acidity of these popular silica-alumina catalysts, which offers an interesting potential for a wide range of acid and multifunctional catalysis.

Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks.

Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C-H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis.

Shedding light on the atomic-scale structure of amorphous silica-alumina and its Brønsted acid sites.

In spite of the widespread applications of amorphous silica-aluminas (ASAs) in many important industrial chemical processes, their high-resolution structures have remained largely elusive. Specifically, the lack of long-range ordering in ASA precludes the use of diffraction methods while NMR spectroscopy has been limited by low sensitivity. Here, we use conventional as well as DNP-enhanced 29Si-29Si, 27Al-27Al, and 29Si-27Al solid-state NMR experiments to shed light on the ordering of atoms in ASAs prepared by flame-spray-pyrolysis. These experiments, in conjunction with a novel Monte Carlo-based approach to simulating RESPDOR dephasing curves, revealed that ASA materials obey Loewenstein's rule of aluminum avoidance. 3D 17O{1H} and 2D 17O{1H,27Al} experiments were developed to measure site-specific O-H and HO-Al distances, and show that the Brønsted acid sites originate predominantly from the pseudo-bridging silanol groups.

High population and dispersion of pentacoordinated AlV species on the surface of flame-made amorphous silica-alumina.

Pentacoordinated Al (AlV) species in silica-alumina are promising to promote the formation of acid sites or act as surface defects for tailoring single-atom catalysts. However, pentahedral coordination (AlV) is rarely observed in conventionally prepared silica-alumina. Here, we show that high population and dispersion of AlV species on the surface of amorphous silica-alumina (ASA) can be achieved by means of flame spray pyrolysis. High resolution TEM/EDX, high magnetic-field NMR and DFT calculations are employed to characterize the structure of as-prepared ASAs. Solid-state 27Al multi-quantum MAS NMR experiments show that most of the AlV species are formed independently from the alumina phase and are accessible for guest molecules on the surface. Upon water adsorption, these AlV species are transformed to AlVI species, structurally similar to surface AlIV species, as confirmed by DFT calculations. The outstanding catalytic activity of as-synthesized ASA is demonstrated using the in situ H/D exchange reaction with deuterated benzene as an example. The AlV-rich ASA provides a much lower activation energy (∼30 kJ/mol) than that reported for zeolite H-ZSM-5 (∼60 kJ/mol). The superior catalytic performance is attributed to the high AlV content promoting the surface active sites in ASA. The knowledge gained on the synthesis of AlV-rich ASAs and the nature of aluminum coordination in these materials could pave the way to more efficient silica-alumina based catalysts.

Recent Progress in Heterogeneous Asymmetric Hydrogenation of C═O and C═C Bonds on Supported Noble Metal Catalysts.

The ease of separation, simple regeneration, and the usually high stability of solid catalysts facilitating continuous production processes have stimulated the development of heterogeneous asymmetric hydrogenation catalysis. The simplest and so far most promising strategy to induce enantioselectivity to solid metal catalysts is their modification by chiral organic compounds, as most prominently represented by the cinchona-modified Pt and Pd catalysts for the asymmetric hydrogenation of activated C═O and C═C bonds. In this Review, we provide a systematic account of the research accomplished in the past decade on noble metal-based heterogeneous asymmetric hydrogenation of prochiral C═O and C═C bonds, including all important facets of these catalytic systems. The advances made are critically analyzed, and future research challenges are identified.

Brønsted acid sites based on penta-coordinated aluminum species.

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (AlIV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-AlIV and -AlV show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of 1H and 27Al sites demonstrates that BAS-AlV co-exists with BAS-AlIV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.

Synthesis of catalytic materials in flames: opportunities and challenges.

The proven capacity of flame aerosol technology for rapid and scalable synthesis of functional nanoparticles makes it ideal for the manufacture of an array of heterogeneous catalysts. Capitalizing on the high temperature environment, rapid cooling and intimate component mixing at either atomic or nano scale, novel catalysts with unique physicochemical properties have been made using flame processes. This tutorial review covers the main features of flame synthesis and illustrates how the physical and chemical properties of as-synthesized solid catalytic materials can be controlled by proper choice of the process parameters. Gas phase particle formation mechanisms and the effect of synthesis conditions (reactor configuration, precursor and dispersion gas flow rates, temperature and concentration fields) on the structural, chemical and catalytic properties of as-prepared materials are discussed. Finally, opportunities and challenges offered by flame synthesis of catalytic materials are addressed.

Chiral modification of platinum: ab initio study of the effect of hydrogen coadsorption on stability and geometry of adsorbed cinchona alkaloids.

The cinchona alkaloids cinchonidine and cinchonine belong to the most efficient chiral modifiers for the noble metal-catalyzed enantioselective hydrogenation of C=O and C=C bonds. Under reaction conditions these modifiers are coadsorbed on the noble metal surface with hydrogen. Using density functional theory, we studied the effect of coadsorbed hydrogen on the adsorption mode of cinchonidine and cinchonine on a Pt(111) surface at different hydrogen coverages. The theoretical study indicates that the presence of coadsorbed hydrogen affects both the adsorption geometry as well as the stability of the adsorbed cinchona alkaloids. At all hydrogen coverages the cinchona alkaloids are found to be adsorbed via anchoring of the quinoline moiety. In the absence of hydrogen as well as at low hydrogen coverage the quinoline moiety adsorbs nearly parallel to the surface, whereas at higher hydrogen coverage it becomes tilted. Higher hydrogen coverage as well as partial hydrogenation of the quinoline part of the cinchona alkaloid and hydrogen transfer to the C[double bond, length as m-dash]C double bond at 10, 11 position of the quinuclidine moiety destabilize the adsorbed cinchona alkaloid, whereas hydrogen transfer to the nitrogen atom of the quinoline and the quinuclidine moiety stabilizes the adsorbed molecule. The stability as well as the adsorption geometry of the cinchona alkaloids are affected by the coadsorbed hydrogen and are proposed to influence the efficiency of the enantiodifferentiating ability of the chirally modified platinum surface.

Enantioselection on Heterogeneous Noble Metal Catalyst: Proline-Induced Asymmetry in the Hydrogenation of Isophorone on Pd Catalyst.

In the (S)-proline-mediated asymmetric hydrogenation of isophorone (IP) on supported Pd catalyst, excellent enantioselectivity is achieved, with an enantiomeric excess of up to 99%. The role of the heterogeneous catalyst has been the subject of a controversial debate, and the current mechanistic understanding cannot explain the observed enantioselectivity of this catalytic system. The lack of in situ information about the role of the heterogeneous catalyst has prompted us to investigate the surface processes occurring at the methanol-Pd catalyst interface using attenuated total reflection infrared spectroscopy. Time-resolved monitoring of the homogeneous solution and of the catalytic solid-liquid interface coupled with catalytic data provides crucial information on the catalytically relevant enantiodifferentiating processes. While the condensation of IP and the corresponding chiral product 3,3,5-trimethylcyclohexanone with the chiral amine is connected to the enantiodifferentiation, it was found that the crucial enantioselectivity-controlling steps take place on the metal surface, and the reaction has to be classified as heterogeneous asymmetric hydrogenation. The presented spectroscopic and catalytic results provide strong evidence for the existence of two competing enantioselective processes leading to opposing enantioselection. Depending on surface coverage of the Pd catalyst, the reaction is controlled either by kinetic resolution ((S)-pathway) or by chiral catalysis ((R)-pathway). Steering the hydrogenation on the (R)-reaction pathway requires sufficient concentration of IP-(S)-proline condensate, as this chiral reactive intermediate becomes the most abundant surface species, inhibiting the competing kinetic resolution. The unraveled (R)-reaction pathway emphasizes an intriguing strategy for inducing chirality in heterogeneous asymmetric catalysis.

Crucial aspects in the design of chirally modified noble metal catalysts for asymmetric hydrogenation of activated ketones.

In view of the importance of optically pure chiral products there is ample reason to develop methods that facilitate their efficient production. Compared to the mostly applied homogeneous catalysts based on transition metals coordinated to suitable chiral ligands, heterogeneous chiral catalysts could offer several features that are beneficial for practical application such as stability, ease of catalyst separation and regeneration as well as straightforward access to continuous process operation. Various strategies have been developed for imparting chirality to catalytic active surfaces, among which the chiral modification of active metal surfaces by adsorption of suitable chiral organic compounds has so far been among the most successful. In this tutorial review lessons learned from research on asymmetric hydrogenation on chirally modified noble metals will be presented. Key aspects for the design of such catalysts will be elucidated using chirally modified platinum catalysts for the asymmetric hydrogenation of α-activated ketones as an example.

Simultaneous probing of bulk liquid phase and catalytic gas-liquid-solid interface under working conditions using attenuated total reflection infrared spectroscopy.

Design and performance of a reactor set-up for attenuated total reflection infrared (ATR-IR) spectroscopy suitable for simultaneous reaction monitoring of bulk liquid and catalytic solid-liquid-gas interfaces under working conditions are presented. As advancement of in situ spectroscopy an operando methodology for gas-liquid-solid reaction monitoring was developed that simultaneously combines catalytic activity and molecular level detection at the catalytically active site of the same sample. Semi-batch reactor conditions are achieved with the analytical set-up by implementing the ATR-IR flow-through cell in a recycle reactor system and integrating a specifically designed gas feeding system coupled with a bubble trap. By the use of only one spectrometer the design of the new ATR-IR reactor cell allows for simultaneous detection of the bulk liquid and the catalytic interface during the working reaction. Holding two internal reflection elements (IRE) the sample compartments of the horizontally movable cell are consecutively flushed with reaction solution and pneumatically actuated, rapid switching of the cell (<1 s) enables to quasi simultaneously follow the heterogeneously catalysed reaction at the catalytic interface on a catalyst-coated IRE and in the bulk liquid on a blank IRE. For a complex heterogeneous reaction, the asymmetric hydrogenation of 2,2,2-trifluoroacetophenone on chirally modified Pt catalyst the elucidation of catalytic activity/enantioselectivity coupled with simultaneous monitoring of the catalytic solid-liquid-gas interface is shown. Both catalytic activity and enantioselectivity are strongly dependent on the experimental conditions. The opportunity to gain improved understanding by coupling measurements of catalytic performance and spectroscopic detection is presented. In addition, the applicability of modulation excitation spectroscopy and phase-sensitive detection are demonstrated.

Monitoring surface processes during heterogeneous asymmetric hydrogenation of ketones on a chirally modified platinum catalyst by operando spectroscopy.

Surface processes occurring at the catalytic chiral surface of a cinchona-modified Pt catalyst during the asymmetric hydrogenation of activated ketones have been monitored for the first time using operando ATR-IR spectroscopy. Fundamental information about this catalytic system could be gained, including the chiral modification process of the catalyst, the surface interaction of reactant ketone with preadsorbed chiral modifier, the role of hydrogen as well as the influence of the product enantiomers in the catalytic cycle. The formation of a diastereomeric transient surface complex between ketone and chiral modifier was found to be related to the ketone consumption. Among the studied activated ketones, a correlation between stereoselection and the strength of the intermolecular hydrogen bond was identified. Dissociated hydrogen from the catalytic surface is found to play a crucial role in the formation of the diastereomeric surface complex.

Chiral modification of platinum by co-adsorbed cinchonidine and trifluoroacetic acid: origin of enhanced stereocontrol in the hydrogenation of trifluoroacetophenone.

Cinchonidine (CD) adsorbed onto a platinum metal catalyst leads to rate acceleration and induces strong stereocontrol in the asymmetric hydrogenation of trifluoroacetophenone. Addition of catalytic amounts of trifluoroacetic acid (TFA) significantly enhances the enantiomeric excess from 50 to 92%. The origin of the enantioselectivity bestowed by co-adsorbed CD and TFA is investigated by using in situ attenuated total reflection infrared spectroscopy and modulation excitation spectroscopy. Molecular interactions between the chiral modifier (CD), acid additive (TFA) and the trifluoro-activated substrate at the solid-liquid interface are elucidated under conditions relevant to catalytic hydrogenations, that is, on a technical Pt/Al2O3 catalyst in the presence of H2 and solvent. Monitoring of the unmodified and modified surface during the hydrogenation provides an insight into the phenomenon of rate enhancement and the crucial interactions of CD with the ketone, corresponding product alcohol, and TFA. Comparison of the diastereomeric interactions occurring on the modified surface and in the liquid solution shows a striking difference for the chiral preferences of CD. The spectroscopic data, in combination with calculations of molecular structures and energies, sheds light on the reaction mechanism of the heterogeneous asymmetric hydrogenation of trifluoromethyl ketones and the involvement of TFA in the diastereomeric intermediate surface complex: the quinuclidine N atom of the adsorbed CD forms an N-H-O-type hydrogen-bonding interaction not only with the trifluoro-activated ketone but also with the corresponding alcohol and the acid additive. Strong evidence is provided that it is a monodentate acid/base adduct in which the carboxylate of TFA resides at the quinuclidine N-atom of CD, which imparts a better stereochemical control.