Nobutaka Maeda

Asymmetric Hydrogenation on Chirally Modified Pt: Origin of Hydrogen in the N–H–O Interaction between Cinchonidine and Ketone

An understanding of the chiral site-substrate interaction is a necessary prerequisite for the rational design and development of efficient heterogeneous asymmetric catalysts. For the enantioselective hydrogenation of α-ketoesters on cinchona-modified platinum, it has earlier been proposed that the crucial interaction is an N-H-O type hydrogen bonding between the quinuclidine N atom of cinchonidine and the α-carbonyl O atom of the substrate. The involved hydrogen atom has been proposed to originate either from protonation (in protic solvent) or from dissociatively adsorbed hydrogen (in aprotic solvent), but experimental evidence for the latter was lacking so far. In this study, in situ attenuated total reflection infrared spectroscopy combined with modulation excitation spectroscopy and phase sensitive detection provides clear evidence that in aprotic media, hydrogen dissociated on Pt is involved in the N-H-O interaction between the chiral modifier, cinchonidine, and the ketone. In the absence of Pt (pure alumina support), no such interaction occurs, indicating the crucial role of dissociated hydrogen in the formation of the diastereomeric transition complex.

Influence of support acidity on the performance of size-confined Pt nanoparticles in the chemoselective hydrogenation of acetophenone

Size-confined Pt nanoparticles of about 1.5 nm have been introduced into [Al]MCM-41 supports with similar acid strength but various population densities of acid sites by means of wet impregnation. The Pt nanoparticles covered preferentially the surface Bronsted acid sites (BAS) of the supports or were located near acid sites rather than on the bigger free space between acid sites even at a very low acid density (6 BAS per 1000 nm2). The free BAS around the Pt particles did not interact with Pt atoms and the electronic properties of the Pt nanoparticles as probed by DRIFTS combined with CO adsorption were similar for Pt/[Al]MCM-41 with and without nearby free BAS. Ionic effects were generated by the Pt-covered acid sites, whereas the population of BAS did not contribute significantly to the ionic effects induced on the Pt nanoparticles. The coverage of BAS of similar strength by platinum nanoparticles led to similar chemoselectivity and product distribution in acetophenone (Aph) hydrogenation, though the density of BAS on the supports increased by more than 11 times. However, increasing the number of BAS on the supports significantly changed the hydrogenation rate. TOFs continuously increased from 125 h−1 up to 534 h−1, when the population of free BAS increased from 18.2 BAS per 1000 nm2 to 39.9 BAS per 1000 nm2. When the free BAS density was further increased to 70.4 BAS per 1000 nm2, the TOF then dropped to 176 h−1. The hydrogenation pathway is similar for both monofunctional (Pt covering all BAS) and bifunctional catalysts (Pt with free BAS), and the reaction was initiated on the Pt surface. This finding indicates that proper tuning of the population density of acid sites on the support can significantly improve the catalytic performance of the supported metal catalysts while keeping similar product selectivities.

Spectroscopic detection of active species on catalytic surfaces: steady-state versus transient method.

Discrimination between active and spectator species is an important and demanding task in catalysis research. A comparative study of the Pd-catalyzed CO hydrogenation using in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) in steady-state and dynamic (transient) experiments shows that the information on surface species differs significantly depending on the type of experiment. In order to discriminate between active species and spectator species not involved in the surface reactions, DRIFTS was combined with a transient technique, modulation excitation spectroscopy (MES). This approach allows the detection of surface species responding to a specific periodic external stimulus, i.e.achieved by concentration modulation, and thereby offers excellent potential to unveil features of the surface processes, which are not accessible by steady-state experiments. However, the example of CO hydrogenation shows that the perturbation imposed to the system has to be chosen properly to benefit from the transient technique. Modulation of the CO concentration did not provide deeper insight into the reaction mechanism, whereas periodic changes of the hydrogen concentration provided valuable information concerning the active surface species and the reaction pathway. The study revealed that only a small fraction (about 4%) of CO molecules adsorbed on specific Pd sites reacted with hydrogen, while the majority of adsorbed CO was inactive. The inactive CO molecules overwhelmingly contributed to the spectra measured under steady-state conditions.

Insight into the Mechanism of the Preferential Oxidation of Carbon Monoxide by Using Isotope-Modulated Excitation IR Spectroscopy

The preferential oxidation of carbon monoxide (PROX) in excess hydrogen is one of the key techniques for the industrial application of residential polymer electrolyte fuel cells (PEFCs). The PROX reaction removes detrimental CO from H2 that is produced from the steam reforming of natural gas or from the water gas-shift reaction. Various catalytic formulations, including noble metals on non-reducible and reducible metal-oxide supports, and metal alloys, have been developed to meet industrial needs, that is, less than d= 10 ppm CO in the reformate gas to avoid CO poisoning of the anode. Although great progress has been made in enhancing catalytic performance and decreasing the use of noble metals, the governing reaction mechanisms are still the subject of ongoing debate. Depending on the catalytic materials that are employed, the proposed mechanisms differ and can be roughly divided into two categories: 1) A classical CO-oxidation mechanism on noble-metal surfaces, which obeys a Langmuir–Hinshelwoodtype mechanism; and 2) a reaction pathway that involves adsorbed formate (HCOO(a)) or carboxy groups (COOH(a)) at the metal/support interfaces. 4] Recently, much attention has been devoted to gaining insight into the role of hydrogen in the PROX mechanism because, for most of the catalysts that have been studied, the rate of CO oxidation is considerably enhanced in the presence of excess hydrogen gas under lowtemperature reaction conditions (353–453 K). In situ IR spectroscopic studies confirmed the involvement of surface formate and bicarbonate species in the PROX reaction on Pt/CeO2 and FeOx/Pt/TiO2 catalysts. [4a,b,f,g, 5] This result has led to the assumption that surface hydroxy groups react with adsorbed CO molecules and undergo regeneration by water that is formed from H2 oxidation. The involvement of surface OH groups in the oxidation of CO is generally accepted for various catalysts that are used for the PROX reaction and also for the water gasshift reaction. 6] Some promoted catalysts, such as Pt Fe/ mordenite, are known to show high selectivity for CO oxidation, accompanied by rate enhancement in the presence of H2. [7] However, a simple Pt/Al2O3 catalyst, which is widely used in residential PEFC systems, can also offer selectivities of 50– 98 %, depending on the reaction conditions. However, the reason for this behavior is not yet clearly understood, thus prompting us to investigate the dynamic surface processes that occur during the PROX reaction over a commercial Pt/galumina catalyst by means of in situ IR spectroscopy. Beside mechanistic information, further insight into the dynamic behavior of this catalyst may provide relevant information regarding its “start-up” and “shut-down” operations. To monitor the dynamic surface processes that occur during the heterogeneous catalysis, vibrational spectroscopy with a high time resolution and a high signal-to-noise (S/N) ratio has to be applied under actual working conditions. Over the last decade, modulation excitation spectroscopy (MES) has been shown to offer such features and has been successfully applied for investigating heterogeneous catalysis at solid/liquid and solid/gas interfaces. 14] In particular, phase-sensitive detection (PSD) allows further enhancement of the S/N ratio and time resolution. On the other hand, steady-state isotopic transient kinetic analysis (SSITKA) is a powerful method for studying reaction mechanisms, the surface concentration of intermediates, and reaction rate constants by exchanging isotopes in the gas phase. In particular, its combination with in situ IR spectroscopy provides a better understanding of the elementary steps in a reaction by inspection of the isotope exchange of the surface species. Herein, we extend the scope of combining MES and PSD techniques by applying isotope modulation (H2/D2 and CO/CO) as an external stimulus. We demonstrate the benefits of isotope-modulated phase-domain IR spectroscopy and its application for identifying the role of water-assisted CO oxidation in catalytic cycles. The difficulty in isotope-modulation IR spectroscopy lies in differentiating between species that respond to the isotope exchange and those that remain unchanged under chemically steady-state reaction conditions. To overcome this problem, the gas mixtures that were fed into the spectroscopic reactor cell were periodically switched (CO+O2+D2

In Situ Modulation Excitation IR Study on the Dominant Product of Pt-Catalyzed Aromatic Ketone Hydrogenation

CO+O2+H2); the final spectrum in the CO+O2+D2 period (t = 113.5 s) was used as a background reference. In this way, the spectroscopic [a] Dr. N. Maeda, F. Meemken, Prof. Dr. A. Baiker Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences ETH Zurich, Hçnggerberg, HCI Zurich CH-8097 (Switzerland) E-mail : baiker@chem.ethz.ch [b] Dr. N. Maeda Current address : Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) School of Environmental Science and Technology Dalian University of Technology Linggong Road 2, Dalian 116024 (China) E-mail : nobutaka.maeda@dlut.edu.cn [c] Prof. Dr. A. Baiker Chemistry Department Faculty of Science, King Abdulaziz University P.O. Box 80203, Jeddah 21589 (Saudi Arabia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201300172.

Space‐ and Time‐Resolved Combined DRIFT and Raman Spectroscopy: Monitoring Dynamic Surface and Bulk Processes during NOx Storage Reduction

Chemoselective hydrogenation of aromatic ketones plays an important role in producing fine chemicals and pharmaceuticals. One of the simplest model reactions is acetophenone (AP) hydrogenation to corresponding alcohol 1-phenylethanol (PE). We studied the role of dominant product 1-phenylethanol (PE) on a Pt/Al2O3 catalyst. In situ attenuated total reflection infrared spectroscopy (ATR-IR) in combination with modulation excitation spectroscopy (MES) and phase sensitive detection (PSD) revealed that PE was more strongly adsorbed on Al2O3 than on Pt. PE was hardly hydrogenated to 1-cyclohexylethanol (CE) on the support. CO from AP decomposition didn’t inhibit PE adsorption on the support. The strong adsorption and accumulation of PE on the support allows active sites on Pt always accessible to AP, achieving efficient Pt-catalyzed catalysis.