Asymmetric Hydrogenation of α‑Amino Ester Probed by FTIR Spectroscopy

Asymmetric hydrogenation reaction of dehydro-α-amino acid (i.e., α-amino ester) over cinchonidine (CD) modified Pd catalyst has been studied by an array of in situ infrared spectroscopic methods, including transmission, diffuse reflectance (DR), and attenuated total reflectance (ATR). Transmission FTIR spectra probed the hydrogenation reaction process, revealed OH–O and NH–N hydrogen bonding interactions between the adsorbed CD and during the reaction. DR and ATR spectra of the hydrogenation reaction under different conditions, which are consistent with but slightly different from the transmission spectra, evidenced the successful hydrogenation of the compound. The incorporation of DR and microfluidics flow-through design allowed us to investigate the adsorption of CD on the Pd surface efficiently. The results revealed that the N-bonded CD on Pd surface in a tilted configuration had increased abundance on the Pd surface with high coverage. These valuable insights provided an image of the reaction pathway to the prochiral structure (precursor state)

Asymmetric Hydrogenation of α‑Amino Ester Probed by FTIR Spectroscopy

Asymmetric hydrogenation reaction of dehydro-α-amino acid (i.e., α-amino ester) over cinchonidine (CD) modified Pd catalyst has been studied by an array of in situ infrared spectroscopic methods, including transmission, diffuse reflectance (DR), and attenuated total reflectance (ATR). Transmission FTIR spectra probed the hydrogenation reaction process, revealed OH–O and NH–N hydrogen bonding interactions between the adsorbed CD and during the reaction. DR and ATR spectra of the hydrogenation reaction under different conditions, which are consistent with but slightly different from the transmission spectra, evidenced the successful hydrogenation of the compound. The incorporation of DR and microfluidics flow-through design allowed us to investigate the adsorption of CD on the Pd surface efficiently. The results revealed that the N-bonded CD on Pd surface in a tilted configuration had increased abundance on the Pd surface with high coverage. These valuable insights provided an image of the reaction pathway to the prochiral structure (precursor state)

<i>In Situ</i> Infrared Study of the Effect of Amine Density on the Nature of Adsorbed CO₂ on Amine-Functionalized Solid Sorbents

<i>In situ</i> Fourier transform infrared spectroscopy was used to determine the nature of adsorbed CO₂ on class I (amine-impregnated) and class II (amine-grafted) sorbents with different amine densities. Adsorbed CO₂ on amine sorbents exists in the form of carbamate–ammonium ion pairs, carbamate–ammonium zwitterions, and carbamic acid. The adsorbed CO₂ on high-amine density sorbents showed that the formation of ammonium ions correlates with the suppression of CH stretching intensities. An HCl probing technique was used to resolve the characteristic infrared bands of ammonium ions, clarifying that the band observed around 1498 cm^–1 is a combination of the deformation vibration of ammonium ion (NH₃⁺) at 1508 and 1469 cm^–1 and the deformation vibration of NH in carbamate (NHCOO^–) at 1480 cm^–1. Carbamate and carbamic acid on sorbents with low amine density desorbed at a rate faster than those on sorbents with high amine density after switching the flow from CO₂ to Ar at 55 °C. Evaluation of the desorption temperature profiles showed that the temperature required to achieve the maximal desorption of CO₂ (<i>T</i>_max. des) increases with amine density. The adsorbed CO₂ on sorbents with high amine density is stabilized via hydrogen bonding interactions with adjacent amine sites. These sorbents require higher temperature to desorb CO₂ than those with low amine density

Nature of Active Sites and Surface Intermediates during SCR of NO with NH₃ by Supported V₂O₅–WO₃/TiO₂ Catalysts

Time-resolved in situ IR was performed during selective catalytic reduction of NO with NH₃ on supported V₂O₅–WO₃/TiO₂ catalysts to examine the distribution and reactivity of surface ammonia species on Lewis and Brønsted acid sites. While both species were found to participate in the SCR reaction, their relative population depends on the coverage of the surface vanadia and tungsta sites, temperature, and moisture. Although the more abundant surface NH₄⁺_,ads intermediates dominate the overall SCR reaction, especially for hydrothermally aged catalysts, the minority surface NH_3,ads intermediates exhibit a higher specific SCR activity (TOF). The current study serves to resolve the long-standing controversy about the active sites for SCR of NO with NH₃ by supported V₂O₅–WO₃/TiO₂ catalysts

Surface Structure Dependence of SO₂ Interaction with Ceria Nanocrystals with Well-Defined Surface Facets

The effects of the surface structure of ceria (CeO₂) on the nature, strength, and amount of species resulting from SO₂ adsorption were studied using in situ IR and Raman spectroscopies coupled with mass spectrometry, along with first-principles calculations based on density functional theory (DFT). CeO₂ nanocrystals with different morphologies, namely, rods (representing a defective structure), cubes (100 facet), and octahedra (111 facet), were used to represent different CeO₂ surface structures. IR and Raman spectroscopic studies showed that the structure and binding strength of adsorbed species from SO₂ depend on the shape of the CeO₂ nanocrystals. SO₂ adsorbs mainly as surface sulfites and sulfates at room temperature on CeO₂ rods, cubes, and octahedra that were either oxidatively or reductively pretreated. The formation of sulfites is more evident on CeO₂ octahedra, whereas surface sulfates are more prominent on CeO₂ rods and cubes. This is explained by the increasing reducibility of the surface oxygen in the order octahedra < cubes < rods. Bulk sulfites are also formed during SO₂ adsorption on reduced CeO₂ rods. The formation of surface sulfites and sulfates on CeO₂ cubes is in good agreement with our DFT results of SO₂ interactions with the CeO₂(100) surface. CeO₂ rods desorb SO₂ at higher temperatures than cubes and octahedra nanocrystals, but bulk sulfates are formed on CeO₂ rods and cubes after high-temperature desorption whereas only some surface sulfates/sulfites are left on octahedra. This difference is rationalized by the fact that CeO₂ rods have the highest surface basicity and largest amount of defects among the three nanocrystals, so they bind and react with SO₂ strongly and are the most degraded after SO₂ adsorption cycles. The fundamental understanding obtained in this work on the effects of the surface structure and defects on the interaction of SO₂ with CeO₂ provides insights for the design of more sulfur-resistant CeO₂-based catalysts

Reaction Pathways and Kinetics for Selective Catalytic Reduction (SCR) of Acidic NO_<i>x</i> Emissions from Power Plants with NH₃

Selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃ by supported vanadium oxide catalysts is an important technology for reducing acidic NO_<i>x</i> emissions from stationary sources and mobile diesel vehicles. However, rational design of improved catalysts is still hampered by a lack of consensus about reaction pathways and kinetics of this critical technology. The SCR fundamentals were resolved by applying multiple time-resolved in situ spectroscopies (ultraviolet–visible light (UV-vis), Raman and temperature-programmed surface reaction (TPSR)) and isotopically labeled molecules (¹⁸O₂, H₂¹⁸O, ¹⁵N¹⁸O, ND₃). This series of experiments directly revealed that the SCR reaction occurs at surface V⁵⁺O₄ sites that are maintained in the oxidized state by O₂ and the rate-determining step involves the reduction of V⁵⁺O₄ sites by NO and NH₃, specifically the breaking of N–H bonds during the course of formation or decomposition of the NO–NH₃ intermediate

Synergistic Effects of Water and SO₂ on Degradation of MIL-125 in the Presence of Acid Gases

The behavior of metal–organic frameworks (MOFs) in the presence of acid gases may be decisive in their suitability for industrial applications. In this study, MIL-125 and MIL-125-NH₂ were investigated with SO₂ exposure in dry, humid, and aqueous environments. MIL-125 was found to be unstable in both humid and aqueous acidic environments, while MIL-125-NH₂ was stable under these exposure conditions, showing no change in textural properties or visual degradation, as observed through SEM. Both materials were stable in the presence of water and dry SO₂, suggesting that the reaction of these molecules to form an acidic species is likely a key factor in the degradation of MIL-125. In situ IR experiments confirmed the presence of sulfite species, supporting the hypothesis that the presence of an acidic sulfur species likely leads to the degradation of the MIL-125 structure. Computational investigation of several potential reaction mechanisms in MIL-125 indicated reactions involving the bisulfite ion are favored over reactions with water or SO₂. DFT simulations support the observation that MIL-125-NH₂ is stable in humid conditions, as all reactions are less favorable with the functionalized framework compared to the unfunctionalized framework. This combined experimental and computational study advances the fundamental understanding of MOF degradation mechanisms during acid gas exposure