CO Vibrational Frequency Research Articles

Comment on “surface characterization of cerium oxide catalysts using deep learning with infrared spectroscopy of CO”

We demonstrate that the predicted distribution of ceria facets in CeO2 nanorods, derived from the analysis of the measured IR spectrum of CO by fitting it using experimentally or DFT(HSE) calculated CO vibrational frequencies collected on all three low-index ceria surfaces, does not align with that derived using deep learning methods [1]. The latter utilizes a dataset based on extensive DFT(PBE) + U calculations, which are erroneous and have thus been scaled to the experimental values.

Adsorption and vibrational spectroscopy of CO on the surface of MgO from periodic local coupled-cluster theory.

The adsorption of CO on the surface of MgO has long been a model problem in surface chemistry. Here, we report periodic Gaussian-based calculations for this problem using second-order perturbation theory (MP2) and coupled-cluster theory with single and double excitations (CCSD) and perturbative triple excitations [CCSD(T)], with the latter two performed using a recently developed extension of the local natural orbital approximation to problems with periodic boundary conditions. The low cost of periodic local correlation calculations allows us to calculate the full CCSD(T) binding curve of CO approaching the surface of MgO (and thus the adsorption energy) and the two-dimensional potential energy surface (PES) as a function of the distance from the surface and the CO stretching coordinate. From the PES, we obtain the fundamental vibrational frequency of CO on MgO, whose shift from the gas phase value is a common experimental probe of surface adsorption. We find that CCSD(T) correctly predicts a positive frequency shift upon adsorption of +14.7 cm-1, in excellent agreement with the experimental shift of +14.3 cm-1. We use our CCSD(T) results to assess the accuracy of MP2, CCSD, and several density functional theory (DFT) approximations, including exchange correlation functionals and dispersion corrections. We find that MP2 and CCSD yield reasonable binding energies and frequency shifts, whereas many DFT calculations overestimate the magnitude of the adsorption energy by 5-15 kJ mol-1 and predict a negative frequency shift of about -20 cm-1, which we attribute to self-interaction-induced delocalization errors that are mildly ameliorated with hybrid functionals. Our findings highlight the accuracy and computational efficiency of the periodic local correlation for the simulation of surface chemistry with accurate wavefunction methods.

Pt-and Pd-doped carbon nanotubes with different diameters as CO sensors: A comparative DFT study

The density functional theory (DFT) is employed to assess the electronic characteristics and vibrational frequencies of CO adsorbed by several Zigzag carbon nanotubes with different diameters that are substitutionally doped using Pt and Pd. Computations were based on the Quantum Espresso (PBE) and Gaussian 16/6-31G(d,p) or LanL2DZ basis sets. The formation energy, adsorption energy, and interaction distances between CO and carbon nanotube as a function of the tube diameter were evaluated for Pt- and Pd- doped carbon nanotubes. Such doped nanotubes have relatively higher adsorption energy at smaller diameters, amounting to −2.784 eV and −2.635 eV for Pd-doped and Pt-doped carbon nanotubes, respectively. The adsorption energies of CO gas on Larger tubes Pt and Pd doped carbon nanotube are −2.207 eV, and −2.583 eV eV, respectively. Following CO adsorption, the smaller diameter Pt-doped carbon nanotube has a relatively greater change in electrical conductivity and chemical reactivity than other Pd- or Pt-doped nanotubes with relatively higher diameters. These changes are attributed to energy gaps concerning the alpha and beta HOMO and LUMO molecular orbitals. The computed Raman active mode frequencies extensively depend on diameter pre- and post-adsorption on the Pd- and Pt -nanotubes. This work offers several scientific suggestions concerning the use of Pt and Pd doped carbon nanotubes with different diameters for CO molecule sensor use cases. Such sensor materials having charge movement-based sensing might be employed as acceptors or donors.

Vibrational frequencies of CO bound to all three low-index cerium oxide surfaces: A consistent theoretical description of vacancy-induced changes using density functional theory.

The facet-dependent adsorption of CO on oxidized and reduced CeO2 single crystal surfaces is reviewed, with emphasis on the effect of CO coverage and the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the IR vibrational frequency of CO. Comparison with detailed, high-resolution experimental infrared reflection absorption spectroscopy data obtained for single crystal samples allows the assignment of the different CO vibrational bands observed on all three low-index ceria surfaces. Good agreement is achieved with the hybrid density functional theory approach with the HSE06 functional and with saturation coverage. It is shown that CO is very sensitive to the structure of cerium oxide surfaces and to the presence of oxygen vacancies. The combined theoretical-experimental approach offers new opportunities for a better characterization of ceria nanoparticles and for unraveling changes occurring during reactions involving CO at higher pressures.

Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT

Dynamic structural changes of single-atom catalysts (SACs) are key to many reactions that were reported to be catalyzed by supported single atoms. To understand these changes, systematic in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS) experiments were performed under reducing (1% CO) and oxidizing (1% CO + 1% O2) reaction atmospheres at room temperature over CeO2-supported late transition metals (Ru, Rh, Pd, Ir, and Pt) synthesized via two different methods (wet impregnation and precipitation) to account for the influence of surface properties. As a general trend, the CO vibrational frequencies downshifted under the CO atmosphere, which we assigned to the formation of clusters. Upon changing the gas mixture to more oxidizing (1% CO + 1% O2), single sites are retained as evidenced by the CO vibrational frequencies at higher wavenumbers. Among the investigated metals, Pt2+ and Pd2+ are more prone to cluster formation, and Rh3+ and Ru4+ are found to be stable as single sites following the order Rh > Ru > Ir > Pt > Pd. In combination with the density functional theory (DFT) calculations of CO vibrational frequencies, we were able to assign shifts to changes in the oxidation state of the metals. These findings thus serve as a benchmark for ceria-supported Pd, Pt, Ru, Ir, and Rh SACs.

The impact of the zeolite local environment on the stability and spectroscopy features of the (CO, NO, NO2)–Pd-(H-MOR, MOR) systems

The stability of the Pd, PdO and Pd3 species at different crystallographic sites on the channels of the protonated and deprotonated MOR zeolite and their capacity to adsorb CO, NO and NO2 were investigated as a function of the Al – Al distance using DFT. For the characterization of these species, we considered Hirshfeld charges, the UV–vis spectra, the vibration frequencies of the probe molecules adsorbed on the Pd-MOR systems, as well as a thermodynamic analysis of the adsorption processes. The DFT results show that the most stable Pd-MOR, OC-Pd-MOR and ON-Pd-MOR structures are those in which the Al atoms are two T-sites apart. From the TD-DFT calculations it was possible to distinguish the Pd species in the Pd-MOR systems. In general, the adsorption of CO and NO is a thermodynamic favored process. Conversely, the NO2 adsorption exhibits positive values for ΔΗads and ΔGads for almost 40% of the Pd/MOR structures considered. NO adsorbs preferably on Pd3/MOR systems than on those with one Pd atom. It was found that the stability of the NO-(OC)Pd/MOR and NO2–Pd/MOR systems is related to the oxidation state of the Pd atom. After analyzing one set of experimental CO and NO vibrational frequency results of Pd-MOR systems, it became clear that the determination of the Pd speciation must consider the particular experimental conditions under which the measurements have been performed.

Experimental and Theoretical Characterization of Rh Single Atoms Supported on γ-Al2O3 with Varying Hydroxyl Contents during NO Reduction by CO

Oxide-supported Rh catalysts are important components of commercial three-way catalysts for pollution abatement. Despite their universal application, many mysteries remain about the active structure of Rh on oxide supports as these materials often contain a mixture of nanoparticles and single-atom Rh species on the same support, even after aging. Probe molecule Fourier transform infrared (FTIR) spectroscopy in this work shows that atomically dispersed Rh on γ-Al2O3 prefer to strongly bind CO when exposed to NO and CO mixtures and that light-off of NO reduction occurs at temperatures similar to CO desorption, suggesting that the first and rate-determining step in NO–CO reactions may be the desorption of CO from single-atom Rh dicarbonyl complexes, Rh(CO)2. Two sets of symmetric and asymmetric stretching frequencies associated with distinct Rh(CO)2 species are observed in FTIR spectra at 2084/2010 and 2094/2020 cm–1. During temperature ramps, the latter pair of bands at 2094/2020 cm–1 converts to the 2084/2010 cm–1 bands at 463 K before all symmetric and asymmetric bands disappear at 573 K. Bands then appear in the range of 1975–1985 cm–1 associated with Rh monocarbonyl, Rh(CO), species upon the disappearance of the 2084/2010 cm–1 bands, suggesting that CO desorbs sequentially from Rh(CO)2 by forming Rh(CO) intermediates. Combined DFT and FTIR experiments suggest that local OH coverage on the γ-Al2O3 surface distinguishes the two Rh(CO)2 species: the higher frequency species resides on a less hydroxylated region and migrates to a more hydroxylated region at higher temperatures, causing the CO vibrational frequency to decrease by ∼10 cm–1. CO desorption occurs from this Rh(CO)2 structure with high local OH coverage, consistent with the DFT predicted trend of CO binding energies. Because of the coincidence of CO desorption with the light-off of NO reduction, local support hydroxylation of atomically dispersed Rh1/γ-Al2O3 catalysts likely affects both the Rh structure after CO desorption and the kinetics of NO reduction, studies of which are enabled by the Rh(CO)2 model developed here.

Metal-Support Interaction and Charge Distribution in Ceria-Supported Au Particles Exposed to CO.

Understanding how reaction conditions affect metal–support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure–activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal–support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO2 samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au+ atoms in contact with the ceria substrate and neutral Au0 atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Auδ− atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy.

Characterization of plasma catalytic decomposition of methane: role of atomic O and reaction mechanism

In this work, we investigated atmospheric pressure plasma jet (APPJ)-assisted methane oxidation over a Ni-SiO2/Al2O3 catalyst. We evaluated possible reaction mechanisms by analyzing the correlation of gas phase, surface and plasma-produced species. Plasma feed gas compositions, plasma powers, and catalyst temperatures were varied to expand the experimental parameters. Real-time Fourier-transform infrared spectroscopy was applied to quantify gas phase species from the reactions. The reactive incident fluxes generated by plasma were measured by molecular beam mass spectroscopy using an identical APPJ operating at the same conditions. A strong correlation of the quantified fluxes of plasma-produced atomic oxygen with that of CH4 consumption, and CO and CO2 formation implies that O atoms play an essential role in CH4 oxidation for the investigated conditions. With the integration of APPJ, the apparent activation energy was lowered and a synergistic effect of 30% was observed. We also performed in-situ diffuse reflectance infrared Fourier-transform spectroscopy to analyze the catalyst surface. The surface analysis showed that surface CO abundance mirrored the surface coverage of CH n at 25 °C. This suggests that CH n adsorbed on the catalyst surface as an intermediate species that was subsequently transformed into surface CO. We observed very little surface CH n absorbance at 500 °C, while a ten-fold increase of surface CO and stronger CO2 absorption were seen. This indicates that for a nickel catalyst at 500 °C, the dissociation of CH4 to CH n may be the rate-determining step in the plasma-assisted CH4 oxidation for our conditions. We also found the CO vibrational frequency changes from 2143 cm−1 for gas phase CO to 2196 cm−1 for CO on a 25 °C catalyst surface, whereas the frequency of CO on a 500 °C catalyst was 2188 cm−1. The change in CO vibrational frequency may be related to the oxidation of the catalyst.

Syngas molecules as probes for defects in 2D hexagonal boron nitride: their adsorption and vibrations.

Single-layer, defect-laden hexagonal boron nitride (dh-BN) is attracting a great deal of attention for its diverse applications: catalysis on the one hand, and single photon emission on the other. As possible probes for identifying some common defects in single-layer h-BN, we present results of ab initio calculations for the adsorption and vibrational characteristics of syngas molecules (H2, CO, CO2) on dh-BN containing one of four types of defects: nitrogen vacancy (VN), boron vacancy (VB), Stone-Wales defect (SW), and nitrogen substituted by boron (BN). Through a comparative examination of adsorption features, charge transfer, electronic structure, and vibrational spectrum, we obtain a deep understanding of the interaction of these molecules with dh-BN and the role of the defect states. We find that while CO, CO2 and atomic hydrogen chemisorb, molecular H2 physisorbs on dh-BN with the four considered defect types. VN and VB show strong affinity for CO and CO2 since the defect states induced by them lie close to the Fermi level. SW does not favor adsorption of these small molecules, as the process for each is endothermic. In the case of BN, CO adsorbs strongly but CO2 only weakly. Vibrational frequencies of notable modes localized at the adsorbed molecules are analyzed and suggested as measures for identification of the defect type. Through a simple comparison of adsorption characteristics of the molecules on these defects, we propose dh-BN with VN to be a good catalyst candidate for CO2 hydrogenation.

Reaction mechanism and kinetics for CO2 reduction on nickel single atom catalysts from quantum mechanics

Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni sites (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SACs has not been determined experimentally. Here, we apply the recently developed grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential (U) to determine faradic efficiency, turn over frequency, and Tafel slope for CO and H2 production for all three sites. We predict an onset potential (at 10 mA cm−2) Uonset = −0.84 V (vs. RHE) for Ni-N2C2 site and Uonset = −0.92 V for Ni-N3C1 site in agreement with experiments, and Uonset = −1.03 V for Ni-N4. We predict that the highest current is for Ni-N4, leading to 700 mA cm−2 at U = −1.12 V. To help determine the actual sites in the experiments, we predict the XPS binding energy shift and CO vibrational frequency for each site.

Photoinduced Terminal Hydride of [FeFe]-Hydrogenase Biomimetic Complexes.

The active site of the [FeFe]-hydrogenase ([FeFe]-H2ase) has a bridging carbonyl ligand and a terminal hydride in the key H-cluster intermediate Hhyd. However, nearly all of the synthetic mimics reported, so far, prefer a hydride bridging the two irons, and only few mimics with a terminal hydride were achieved by tuning the steric effects of bulky diphosphine ligands. Moreover, although intermediates with either a terminal hydride or a protonated bridging thiolate ligand were proposed to exist during protonation processes or hydrogen exchange in the [FeFe]-H2ase mimic, [Fe2(μ-pdt)(μ-H)(CO)4(PMe3)2]+ (1H+), only bridging hydrides were observed by time-resolved IR spectroscopy. In this report, FTIR spectroscopy of 1H+, under CO with longer irradiation time, revealed several new photoinduced species. In addition to the CO loss species, many of the photoinduced products can be assigned to 1H+ with a terminal hydride by comparison of their CO vibrational frequencies with density functional theory calculations.

First-principles investigation of CO and CO2 adsorption on [formula omitted]-Al2O3 supported monoatomic and diatomic Pt clusters

Supported clusters of transition metal atoms are key components of heterogeneous catalysts. Understanding their interaction with small molecular species is therefore an important step in catalyst design. In this work, we provide a detailed first-principles investigation of the adsorption properties of CO and CO2 on Ptn(n=1,2) clusters supported by a γ-Al2O3 (1 0 0) substrate. In particular, important parameters such as the stability of the Pt clusters, molecular adsorption energies and vibration frequencies were investigated. All Pt2 clusters exhibit lower adsorption energies than their monoatomic counterparts, especially single Pt atoms embedded in the surface of the support. Atomically preadsorbed Pt acts as an anchor for the CO molecule, increasing its adsorption energy compared to the bare surface. The support actively participates in stabilizing the adsorbates and a markedly different behaviour can be expected depending on the adsorption site being on the surface of small Pt clusters or on the Pt/support interface. Vibrational frequencies of CO and CO2 adsorbed on the supported clusters can be used to gain insight into the degree of dispersion of the metallic component of the catalyst, and can be profitably used in the design of novel single-atom catalysts (SACs).

Temperature-Controlled CO Adsorption Configurations on (2 × 1)Ni–O/Ni(110) Surfaces

CO adsorptions and configuration evolutions with temperatures on (2 × 1)Ni–O row structures on Ni(110) surfaces were studied by using ultrahigh vacuum–Fourier transform infrared spectroscopy, thermal desorption spectroscopy, and density functional theory calculations. Two kinds of adsorption configurations were identified. In the top configuration at low temperature (90 K), the carbon atom of one CO binds to one Ni atom in the Ni–O rows with a tilt angle of about 53°, and the vibration frequency of CO is 2100–2119 cm–1 depending on CO coverage. In the bridge configuration near room temperature (280 K), the carbon atom of one CO binds to two Ni atoms from the neighboring Ni–O rows, and the vibration frequency is 2030–2039 cm–1. By annealing the sample prepared at 90 to 280 K, the top adsorption configuration gradually evolves into the ordered bridge adsorption configuration via a disordered state. In the disordered state, the top and bridge configurations are distributed in disorder, induce strong transver...

Nature of stable single atom Pt catalysts dispersed on anatase TiO2

Pt single atom catalysts anchored on the surface of anatase TiO2 have recently been shown to be stable through oxidation and reduction conditions and exhibit higher catalytic activity in CO oxidation than Pt metallic clusters. In this work we unravel the atomistic nature of the isolated Ptiso species on anatase TiO2 thanks to the combination of CO probe molecule Fourier Transform Infrared (FTIR) and temperature programmed desorption (TPD) experiments with an extensive set of Density Functional Theory (DFT) calculations. We used the stretching frequencies and adsorption energies of CO bound to Pt as a fingerprint of the specific structure of the stable Ptiso species. These consist of Pt atoms bound to two excess O atoms on the surface (PtO2). The excess oxygen atoms on the surface arise from the formation of surface hydroxyl groups, and provide a solid anchor to the oxide support, which explains the high thermal stability of the single-atom Pt catalyst through oxidative and reductive treatments. Beside the single atom Pt species, also models of metallic, Ptmetal, and oxidized, Ptox, sub-nanometer clusters have been considered. Comparisons between characteristics of Ptiso species and sub-nanometer Ptmetal and Ptox clusters demonstrate that the combination of CO vibrational frequency and adsorption energy can be an effective approach to differentiate these species.

Quantum Chemical Study of the Redox Potential of the Co(OH2)62+/3+ Couple and the Singlet-Quintet Gibbs Energy Difference of the Co(OH2)63+ Ion.

The geometry and vibrational frequencies of Co(OH2)62+ in the quartet state and Co(OH2)63+ in the singlet and quintet states were computed with quantum mechanics/molecular mechanics (QM/MM), whereby the LC-BOP-LRD functional was used for the QM part involving the Co(OH2)6n+ ( n = 2, 3) ions. The surrounding 124 MM water molecules were treated with the MMFF94 force field. The hydration energy differences between low-spin Co(OH2)63+ and Co(OH2)62+ or Co(OH2)63+ in the quintet state were also calculated using this method. The electronic energy of the Co(OH2)6n+ ( n = 2, 3) ions was calculated with wave function theory, multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory and spin-orbit configuration interaction. The redox potential of the Co(OH2)62+/3+ couple, and the singlet-quintet (adiabatic) Gibbs energy difference of Co(OH2)63+, computed based on these data, agree with the experiment.

Supported Pt Nanoparticle Catalysts - Correlation of Electrochemical CO-Stripping Data and Gas Phase in-Situ Drifts Measurements

Although significant progress has been made over the last couple of years, acidic polymer electrolyte membrane fuel cells (PEMFC) still require a substantial amount of the precious metal platinum, which renders them too costly for their global commercialization. State-of-the-art electrocatalysts are Pt nanoparticles supported on low-cost and electron conducting carbon substrates, such as Vulcan XC-72. However, these Pt-C catalysts are not sufficiently stable during long-term fuel cell operation due to carbon corrosion at the cathode side and poisoning with carbon monoxide contained as a trace gas in the industrial fuel at the anode side. That is why a lot of effort has been dedicated to develop alternative support materials such as oxides, e.g. titania or Sb- and In-doped tin oxide. In contrast to carbon supports, oxides provide an inert substrate, which can also tune the catalysts’ CO tolerance, as indicated by a strong negative shift of the oxidation peak in classical electrochemical CO stripping experiments. The lowered CO poisoning tendency can be due to various reasons, for instance a) support material induced changes in the electronic structure of the catalytically active Pt centers reducing the CO binding strength, or b) a so-called bifunctional effect, in which the support material provides oxygen-containing functional groups accelerating the oxidative removal of adsorbed CO. It is, however, not an easy task to unravel whether an electronic and/or a bifunctional effect is responsible for the enhanced CO tolerance. In this paper, we use CO as a probe molecule to study the Pt nanoparticle-support interaction of various oxide-supported catalysts. We attempt to separate the electronic and the bifunctional contribution by using electrochemical CO stripping experiments and in-situ gas phase diffuse reflectance infrared absorption Fourier transform spectroscopy (DRIFTS). Hence, we compare the CO vibrational frequency of gas phase adsorbed carbon monoxide, sensitive to electronic changes, with the position of the CO stripping peak. Furthermore, we suggest an alternative solution to the peak multiplicity puzzle occurring in electrochemical CO stripping [1] by considering the presence of bridge- bonded CO, known from in-situ gas phase spectroscopy [2-4], also in an aqueous environment. [1] P. Urchaga, S. Baranton, C. Coutanceau, and G. Jerkiewicz, “Electro-oxidation of CO chem on Pt nanosurfaces: Solution of the peak multiplicity puzzle,” Langmuir, vol. 28, no. 7, pp. 3658–3663, 2012. [2] A. Bourane, O. Dulaurent, and D. Bianchi, “Heats of Adsorption of Linear and Multibound Adsorbed CO Species on a Pt/ Al2O3 Catalyst Using in Situ Infrared Spectroscopy under Adsorption Equilibrium,” vol. 125, pp. 115–125, 2000. [3] J. Xu, J. T. Y. Jr, J. Xu, and J. T. Yates, “Catalytic oxidation of CO on Pt ( 335 ): A study of the active site,” vol. 725, no. 335, 1998. [4] A. M. B. Heyden, B. E., “The Adsorption Of CO On Pt(111) Studied By Infrared Reflection-Absorption Spectroscopy,” Surf. Sci., vol. 125, pp. 787–802, 1983. Figure 1

Bias-Dependent Chemical Enhancement and Nonclassical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips.

Tip-enhanced Raman spectromicroscopy (TERS) with CO-terminated plasmonic tips can probe angstrom-scale features of molecules on surfaces. The development of this technique requires understanding of how chemical environments affect the CO vibrational frequency and TERS intensity. At the scanning tunneling microscope junction of a CO-terminated Ag tip, we show that rather than the classical vibrational Stark effect, the large bias dependence of the CO frequency shift is due to ground-state charge transfer from the Ag tip into the CO π* orbital softening the C-O bond at more positive biases. The associated increase in Raman intensity is attributed to a bias-dependent chemical enhancement effect, where a positive bias tunes a charge-transfer excited state close to resonance with the Ag plasmon. This change in Raman intensity is contrary to what would be expected based on changes in the tilt angle of the CO molecule with bias, demonstrating that the Raman intensity is dominated by electronic rather than geometric effects.

Nature of Sintering‐Resistant, Single‐Atom Ru Species Dispersed on Zirconia‐Based Catalysts: A DFT and FTIR Study of CO Adsorption

AbstractRecent studies on the upgrade of cellulosic biomass to renewable chemicals and fuels based on Ru/ZrO2 catalysts have shown that the catalyst contains atomically dispersed, highly stable Ru atoms. FTIR spectra after CO dosage reveal a complex manifold of bands resulting from Ru‐CO carbonyl species. The nature of the atomically dispersed Ru species on monoclinic zirconia, and the origin of their thermal stability are the object of this investigation. Combining density functional theory (DFT) calculations on model systems of both tetragonal and monoclinic ZrO2, with novel experimental data, we provide a basis for the identification of the monoatomic Ru species. Various candidates are explored based on DFT calculations of their intrinsic stability and of the vibrational properties of adsorbed CO probe molecules. The results allow us to discard a number of possible structures, and restrict the analysis to a few potential candidates. Most likely, the atomically dispersed sites result from the interaction of Ru atoms with one OH group of the surface with elimination of H2 by condensation, leaving RuO species strongly bound to the zirconia surface. CO binds strongly to the RuO units forming (RuO)(CO)2 geminal complexes with characteristic signatures in terms of CO vibrational frequency. Despite the formal positive oxidation state of the Ru species, large negative shifts are found in the CO stretching frequency compared with the free CO molecule.

Two dimensional infrared spectroscopic investigations of protein and enzyme dynamics

Ultra fast two dimensional infrared (2D IR) spectroscopy is used to measure dynamics and interactions in complex materials in the ground electronic state under thermal equilibrium conditions. Two observables, ultra fast chemical exchange and spectral diffusion, will be discussed. In a 2D IR experiment, three variably time delayed IR excitation pulses, tuned to the vibrational transition of interest, give rise to a fourth vibrational echo pulse, the signal in the experiment. The first two pulses in the sequence label the vibrations with their initial frequencies. After a waiting period, Tw, the third pulse induces the vibrational echo, which reads out the final vibrational frequencies. Structural changes of the system cause the vibrational frequencies to change. Therefore, measuring the time evolution of the vibrational frequencies reports on the time evolution of the structure. Folded proteins can exist in multiple conformational substates. Each substate reflects a local minimum on the free‐energy landscape with a distinct structure. Using 2D‐IR chemical exchange spectroscopy, conformational switching between two well defined substates of two myoglobin mutants were observed on the ~50 ps time scale. The conformational dynamics were measured through the growth of off‐diagonal peaks in the 2D IR spectra of CO bound to the heme. The conformational switching likely involves motion of the distal histidine/E helix that changes the position of the histidine's imidazole side group. These results demonstrate that interconversion between protein conformational substates can occur on very fast time scales. Detailed MD simulations are able to capture some of the observed behavior, but not quantitatively. Cytochrome (cyt) P450s hydroxylate a variety of substrates that can differ widely in their chemical structure. The importance of these enzymes in drug metabolism and other biological processes has motivated the study of the factors that enable their activity on diverse classes of molecules. 2D IR spectroscopy was used to measure the dynamics of cyt P450cam from Pseudomonas putida using CO bound at the active site as the vibrational probe. The experiments measure spectral diffusion, the time evolution of the CO vibrational frequencies within the in homogeneously broadened vibrational absorption line. The time evolution of the frequencies is caused by the time evolution of the enzyme structure. The enzyme bound to both its natural substrate, camphor, and a series of related substrates were investigated to explicate the role of dynamics in molecular recognition in cyt P450cam and to delineate how the motions may contribute to hydroxylation specificity. Correlations in the observed dynamics with the specificity of hydroxylation of the substrates, the binding affinity, and the substrates' molecular volume suggest that motions on the hundreds of picosecond time scale contribute to the variation in activity of cyt P450cam toward different substrates.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.