Surface Reactions on Pt during NO_<i>x</i> Storage−Reduction Studied by Polarization-Modulation Infrared Reflection−Absorption Spectroscopy

NO_<i>x</i> storage−reduction (NSR) cycles on a model catalyst, consisting of Pt particles on a BaO thin film deposited on a polished Al substrate, have been investigated by means of in situ polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS). The method affords simultaneous time-resolved detection of bulk Ba species and species in gas-phase and on the Pt surface. The study revealed the special role of atop NO adsorption sites on platinum for NO oxidation leading to efficient NO_<i>x</i> storage on the Ba component, and a gradual change in adsorbate composition resulting in a transition from N₂O- to NO₂-formation reaction on Pt during lean periods

Synergistic Effects of Au and FeO_<i>x</i> Nanocomposites in Catalytic NO Reduction with CO

Promotion of Au/TiO₂ by FeO_<i>x</i> addition is shown to result in a very strong enhancement of activity and selectivity in the NO reduction with CO. The nanocomposite Au-FeO_<i>x</i>/TiO₂ catalysts were prepared by deposition–precipitation (Au/TiO₂) and subsequent impregnation (FeO_<i>x</i>). Structural characterization of the nanocomposite catalysts using XRD, TEM, TPR, XPS, and DRIFTS revealed that they consist of titania-supported Au particles of about 4.4–4.8 nm size, which were partially covered with a thin layer of FeO_x,, mainly made up of Fe²⁺ (FeO), in addition to a small fraction of Fe³⁺ (Fe₂O₃). In situ DRIFTS studies showed that, in comparison to Au/TiO₂, on the Au-FeO_<i>x</i>/TiO₂ nanocomposites new adsorption sites were created, which led to enrichment of CO and NO molecules on the Au surface and at its interfaces with FeO_<i>x</i> and TiO₂. At the reaction temperature (250 °C), a new surface NO–CO complex, where both Fe and Au are proposed to be involved in the adsorptive interaction, was discovered. The titania-supported Au-FeO_<i>x</i> nanocomposites considerably facilitated the formation of this complex and contributed to the striking enhancement of the catalytic performance (activity and selectivity) of the Au-based catalyst

Hydrogenation of Acetophenone on Pd/Silica–Alumina Catalysts with Tunable Acidity: Mechanistic Insight by In Situ ATR-IR Spectroscopy

Understanding the cooperative action of metal and acid sites of bifunctional catalysts is essential for developing more efficient catalysts for greener chemical processes. We used in situ ATR-IR spectroscopy in tandem with modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) to examine the functioning of Pd/silica–alumina (Pd/SA) catalysts with different acidity of the support in the liquid-phase hydrogenation of acetophenone (AP). The spectroscopic studies revealed that AP was adsorbed on the Pd surface in η¹ (O) configuration and initially hydrogenated to 1-phenylethanol (PE) on the metallic Pd sites. On the Pd surface, PE was less strongly adsorbed than AP. PE was preferentially adsorbed on the acidic silica–alumina support via the C–OH group and subsequently dehydrated to styrene on the acidic sites. Hydrogen originating from dissociative adsorption on Pd sites is proposed to hydrogenate part of the formed styrene to ethylbenzene (EB). The intermediate styrene had a short lifetime under hydrogenation conditions. Increasing the support acidity by raising the atomic fraction of aluminum (Al × 100%/(Al + Si)) in SA from 0 to 70% promoted the styrene production, which in turn strongly enhanced the EB yield from 17.3% on Pd/silica to 54.3% on Pd/SA-70, respectively

Heterogeneous Asymmetric Hydrogenation of Prochiral Alkenoic Acid: Origin of Rate and Enantioselectivity Enhancement by Amine Addition

The origin of rate and enantioselectivity enhancement by achiral amine addition in the asymmetric hydrogenation of alkenoic acid on chirally modified Pd has been a long-debated subject. We show by means of in situ attenuated total reflection infrared spectroscopy combined with modulation excitation spectroscopy that the rate enhancement in the presence of an amine originates from restructuring of surface acid–base type adducts dynamically involving the substrate, product, chiral modifier, and amine. Phase-sensitive detection provides insight into the surface dynamics at the solid–liquid chiral interface: the addition of the achiral amine changes the adsorption configuration of the chiral modifier on the Pd surface, leading to a better stereochemically controlled surface

Molecular Insight into Pt-Catalyzed Chemoselective Hydrogenation of an Aromatic Ketone by In Situ Modulation–Excitation IR Spectroscopy

Chemoselective platinum-catalyzed liquid-phase hydrogenation of aromatic ketones is an important reaction in the production of fine chemicals and pharmaceuticals. A typical example of this class of reactions is the hydrogenation of acetophenone (AP) over a Pt/Al₂O₃ catalyst. We investigated the adsorption behavior of the different reaction components and their reaction pathways using in situ attenuated total reflection infrared spectroscopy in combination with modulation excitation spectroscopy and phase sensitive detection. AP adsorbed on both Pt and the alumina support. On Pt, AP adsorbed in the η¹ (O) configuration prevailed, whereas on alumina, AP bound to Lewis acid sites was predominant. In the presence of hydrogen, η¹ (O) AP adsorbed on Pt was hydrogenated to the main product, 1-phenylethanol (PE), with high selectivity (82.5%). The produced PE was more strongly adsorbed on the alumina support than on Pt, leading to replacement of AP and accumulation of PE on alumina. Co-adsorption experiments of AP with its products PE, 1-cyclohexylethanol, and ethylbenzene revealed that AP adsorbed in the η¹ (O) configuration was always the prevalent adsorption mode of AP on Pt, which may partly explain the high selectivity to PE observed. Co-adsorption of AP and CO did not significantly affect the adsorption of AP; however, CO adsorption strongly suppressed the adsorption and dissociation of H₂. The studies revealed a striking difference in the selectivity behavior between the gas-phase and liquid-phase hydrogenation. Although in the gas-phase hydrogenation of AP, a significant effect of decomposition/hydrogenolysis products on the chemoselectivity of AP hydrogenation was reported, these fragmentation reactions were barely observed in the liquid phase

Selectivity-Controlling Factors in Catalytic Methanol Amination Studied by Isotopically Modulated Excitation IR Spectroscopy

Present mechanistic models for catalytic amination of methanol by zeolites focus on shape-selectivity of monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) in the micropores. However, a rational explanation of the uniquely high selectivity to MMA and DMA achieved over Na⁺-exchanged mordenite (Na⁺-MOR) requires the consideration of additional selectivity-controlling factors. We have applied modulation–excitation diffuse reflectance IR Fourier transform spectroscopy with periodic perturbation by the isotope CD₃OD to realize a chemically steady state but isotopically transient condition during methylamines synthesis from methanol and ammonia. These studies proved that the H-bonded network of methanol agglomerates and open dimers in the micropores can readily be replaced by NH₃ at 623 K, probably leading to a decrease in the methanol concentration around catalytically active sites and, thus, suppressing the consecutive reaction of MMA to DMA and, finally, to TMA. Concentration modulation between NH₃ and MMA indicated weaker adsorption of MMA, which was largely replaced by NH₃ at the reaction temperature, thereby hindering further methylation of MMA

Heterogeneous Asymmetric Hydrogenation of Activated Ketones: Mechanistic Insight into the Role of Alcohol Products by in Situ Modulation-Excitation IR Spectroscopy

Present mechanistic models for the rationalization of enantiodifferentiation on cinchona-modified Pt focus on the activated ketone–modifier interaction, while the possible role of the product alcohol is largely ignored. Here we used in situ attenuated total reflection infrared (ATR-IR) spectroscopy combined with modulation-excitation spectroscopy (MES) and catalytic (kinetic) study to clarify the role of the two enantiomers of the alcohol products at the surface of chirally modified Pt/Al₂O₃. In situ monitoring of the solid–liquid interface proved that chiral modification of Pt with cinchonidine (CD) significantly reduced the amount of adsorbed (<i>R</i>)-methyl mandelate ((<i>R</i>)-MM), which is the major enantiomer in the asymmetric hydrogenation of methyl benzoylformate (MBF). Trace amounts of (<i>R</i>)-MM product on the surface were found to decrease significantly the hydrogenation rate of MBF. In situ ATR-IR spectroscopy with absolute configuration modulation indicated that an N–H–O type H bonding forms between CD and (<i>R</i>)-MM, whose structure is analogous to that of the diastereomeric CD–MBF complex. The rate deceleration is, therefore, considered to arise from competitive adsorption of the prochiral ketone and the product alcohol at the chirally modified surface. This conclusion is further supported by extending the spectroscopic study to (<i>R</i>)-ethyl lactate, (<i>R</i>)-pantolactone, and (<i>R</i>)-α-(trifluoromethyl)­benzyl alcohol