In situ IR spectroscopy of intermediates in heterogeneous oxidative catalysis

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd

The use of visible photon fluxes to influence catalytic reactions on metal nanoparticle surfaces has attracted attention based on observations of reaction mechanisms and selectivity not observed under equilibrium heating. These observations suggest that photon fluxes can selectively impact the rates of certain elementary steps, creating nonequilibrium energy distributions among various reaction pathways. However, quantitative studies validating these hypotheses on metal nanoparticle surfaces are lacking. We examine the influence of continuous wave visible photon fluxes on the CO desorption rates from 1 to 2 nm diameter Pt and Pd nanoparticle surfaces supported on γ-Al2O3. Temperature-programmed desorption measurements quantified via diffuse reflectance infrared Fourier transform spectroscopy demonstrate that visible photon fluxes significantly enhanced the rate of CO desorption from Pt nanoparticles in a wavelength-dependent manner. 440 nm photons most efficiently promoted CO desorption from Pt nanoparticle surfaces, aligning with the excitation energy for the interfacial electronic transition within the Pt-CO bond. Conversely, visible photon fluxes had no measurable influence on CO desorption rates from Pd nanoparticle surfaces after accounting for photon-induced heating. Density functional theory calculations demonstrate that the Pt-CO bond exhibits a narrower LUMO resonance, stronger coupling between the photoexcitation and forces induced on the metal-C bond, and vibrational energy dissipation that more effectively couples to desorption as compared to Pd-CO. These results demonstrate the specificity photons provide in facilitating chemical reactions on metal nanoparticle surfaces and substantiate the idea that photon fluxes can steer processes and outcomes of catalytic reactions in ways not achievable by equilibrium heating.

Tailored activity of Cu–Fe bimetallic Beta zeolite with promising C3H6 resistance for NH3-SCR

The introduction of Cu has inhibited the polymerization of C₃H₆ and promoted the oxidation of C₃H₆, which alleviated competitive adsorption between C₃H₆ and NO_x, therefore results in the enhanced NH₃-SCR performance in the presence of C₃H₆.

Preparation of SnO2 nanotubes via a template-free electrospinning process

A facile and environmentally friendly template-free method is developed for the fabrication of SnO₂ nanotubes via electrospinning and precisely controlled heat treatment method. It is revealed that the as-spun solid SnO₂ precursor fibers gradually transformed into hollow-structured nanotubes when the temperature was controlled precisely from 200 °C to 600 °C. It was confirmed, that this remarkable structural evolution corporate the respective thermal decomposition of polyvinyl butyral (PVB) at the surface and inside of the fibers. The formation mechanism of the nanotubes has been clarified by systematically investigating the morphology, phase structure, chemical state, and decomposition of the organic compounds during the heat treatment. The as-prepared SnO₂ nanotubes exhibit a high specific surface area of 32.91 m² g⁻¹ and a porous structure with pore sizes of 2 nm and 10–25 nm. The SnO₂ nanotubes were assembled as a photosensor, which demonstrates a fast response upon UV light illumination at 254 nm. From this discovery, it is expected that a new method for fabricating nanotubes will be established and the development of materials with a higher functionality will be promoted.

The morphology of the prepared samples. (a) FESEM images of each temperature which shows the structural evolution of as-spun fibers to nanotube during the heat treatment process. (b) TEM images of 600 °C heat-treated sample.

Mechanistic study of 1,1-dimethylhydrazine transformation over Pt/SiO2 catalyst

Catalytic oxidation of 1,1-dimethylhydrazine (UDMH) with molecular oxygen over Pt/SiO₂ was studied by in situ FTIR spectroscopy coupled with online MS monitoring of the gas phase. An unusual two-step oxidation process was detected in experiments with the pulse UDMH feeding: initial UDMH oxidation over a fresh platinum surface quickly terminates due to the blockage of active sites; a time-separated second oxidation step corresponds to combustion of the surface residue. This residue consists of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N nitrile groups formed via decomposition of the products of non-oxidative UDMH conversion, such as dimethylamine. The two-step oxidation picture is observed over a broad range of reaction temperatures and oxygen to UDMH ratios.

Unusual two-step oxidation process of 1,1-dimethylhydrazine on Pt/SiO₂ catalyst.

Pitfalls and benefits of in situ and operando diffuse reflectance FT-IR spectroscopy (DRIFTS) applied to catalytic reactions

The procedures and conditions that need to be fulfilled to be able to carry out appropriate in situ and operando diffuse reflectance FT-IR (DRIFTS) analyses are discussed.

New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions

Infrared absorption spectroscopy has proven to be one of the most powerful spectroscopic techniques available for the characterization of catalytic systems. Although the history of IR absorption spectroscopy in catalysis is long, the technique continues to provide key fundamental information about a variety of catalysts and catalytic reactions, and to also offer novel options for the acquisition of new information on both reaction mechanisms and the nature of the solids used as catalysts. In this review, an overview is provided of the main contributions that have been derived from IR absorption spectroscopy studies of catalytic systems, and a discussion is included on new trends and new potential directions of research involving IR in catalysis. We start by briefly describing the power of Fourier-transform IR (FTIR) instruments and the main experimental IR setups available, namely, transmission (TIR), diffuse reflectance (DRIFTS), attenuated total reflection (ATR-IR), and reflection-absorption (RAIRS), for advancing research in catalysis. We then discuss the different environments under which IR characterization of catalysts is carried out, including in situ and operando studies of typical catalytic processes in gas-phase, research with model catalysts in ultrahigh vacuum (UHV) and so-called high-pressure cell instruments, and work involving liquid/solid interfaces. A presentation of the type of information extracted from IR data follows in terms of the identification of adsorbed intermediates, the characterization of the surfaces of the catalysts themselves, the quantitation of IR intensities to extract surface coverages, and the use of probe molecules to identify and titrate specific catalytic sites. Finally, the different options for carrying out kinetic studies with temporal resolution such as rapid-scan FTIR, step-scan FTIR, and the use of tunable lasers or synchrotron sources, and to obtain spatially resolved spectra, by sample rastering or by 2D imaging, are introduced.

Mechanistic insights into formation of SnO₂ nanotubes: asynchronous decomposition of poly(vinylpyrrolidone) in electrospun fibers during calcining process

The formation mechanism of SnO2 nanotubes (NTs) fabricated by generic electrospinning and calcining was revealed by systematically investigating the structural evolution of calcined fibers, product composition, and released volatile byproducts. The structural evolution of the fibers proceeded sequentially from dense fiber to wire-in-tube to nanotube. This remarkable structural evolution indicated a disparate thermal decomposition of poly(vinylpyrrolidone) (PVP) in the interior and the surface of the fibers. PVP on the surface of the outer fibers decomposed completely at a lower temperature (<340 °C), due to exposure to oxygen, and SnO2 crystallized and formed a shell on the fiber. Interior PVP of the fiber was prone to loss of side substituents due to the oxygen-deficient decomposition, leaving only the carbon main chain. The rest of the Sn crystallized when the pores formed resulting from the aggregation of SnO2 nanocrystals in the shell. The residual carbon chain did not decompose completely at temperatures less than 550 °C. We proposed a PVP-assisted Ostwald ripening mechanism for the formation of SnO2 NTs. This work directs the fabrication of diverse nanostructure metal oxide by generic electrospinning method.

Characterization of C5 hydrocarbons relevant to catalysis

A recent in situ infrared study on the selective hydrogenation of C5 dienes and monoenes over a Pd/Al(2)O(3) catalyst only reported incomplete vibrational assignments for some of the reagents, intermediates, and products encountered in that study. This work uses a combination of infrared absorption spectroscopy, Raman, and inelastic neutron scattering to characterize the vibrational spectra of pentane, 1-pentene, cis- and trans-2-pentene, cis- and trans-1,3-pentadiene, 1,4-pentadiene, cyclopentane, and cyclopentene. Ab initio calculations of the potential energy surface, geometry, and vibrational transition energies were performed and simulations of the vibrational spectra compared to the experimental data. Complete vibrational assignments for the majority of the molecules are presented. The potential for using gas-phase infrared measurements for studying heterogeneously catalyzed gas-phase reactions is also briefly considered.

Quantitative Analysis of Adsorbate Concentrations by Diffuse Reflectance FT-IR

Fully quantitative analyses of DRIFTS data are required when the surface concentrations and the specific rate constants of reaction (or desorption) of adsorbates are needed to validate microkinetic models. The relationship between the surface coverage of adsorbates and various functions derived from the signal collected by DRIFTS is discussed here. The Kubelka-Munk and pseudoabsorbance (noted here as absorbance, for the sake of brevity) transformations were considered, since those are the most commonly used functions when data collected by DRIFTS are reported. Theoretical calculations and experimental evidence based on the study of CO adsorption on Pt/SiO2 and formate species adsorbed on Pt/CeO2 showed that the absorbance (i.e., = log 1/R', with R' = relative reflectance) is the most appropriate, yet imperfect, function to give a linear representation of the adsorbate surface concentration in the examples treated here, for which the relative reflectance R' is typically > 60%. When the adsorbates lead to a strong signal absorption (e.g., R' < 60%), the Kubelka-Munk function is actually more appropriate. The absorbance allows a simple correction of baseline drifts, which often occur during time-resolved data collection over catalytic materials. Baseline corrections are markedly more complex in the case of the other mathematical transforms, including the function proposed by Matyshak and Krylov (Catal. Today 1995, 25, 1-87), which has been proposed as an appropriate representation of surface concentrations in DRIFTS spectroscopy.

Dissociative and associative attachment of NO to iron clusters

Electronic and geometrical structures of iron clusters with associative (FeNO, Fe2NO, Fe3NO, Fe4NO, Fe5NO, and Fe6NO) and dissociative (OFeN, OFe2N, OFe3N, OFe4N, OFe5N, and OFe6N) attachments of NO, as well as the corresponding singly negatively and positively charged ions, are computed using density functional theory with generalized gradient corrections. Both types of isomers are found to be stable and no spontaneous dissociation was observed during the geometry optimizations. The ground states correspond to dissociative attachment of NO for all iron clusters Fe(n), except for Fe and Fe+. All of the OFe(n)N clusters have ferrimagnetic ground states, except for OFe2N, OFe2N-, OFe4N, and OFe4N-, which prefer the ferromagnetic coupling. In the ferrimagnetic states, the excess spin density at one iron atom couples antiferromagnetically to the excess spin densities of all other iron atoms. Relative to the high-spin Fe(n) ground state, the lowest energy ferrimagnetic state quenches the total magnetic moments of iron clusters by 7, which is to be compared with a reduction in the magnetic moment of one in the lowest energy ferromagnetic states. Dissociation of NO on the iron clusters has a pronounced impact on the energetics of reactions; the Fe(n)NO+CO-->Fe(n)N+CO2 channels are exothermic while the OFe6N+CO--> Fe6N+CO2 channels are nearly thermoneutral.