Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode Part I. EMIRS spectra in acid and alkaline solutions

Electrochemically modulated infrared spectroscopy (EMIRS) was applied to the study of adsorption of CO on gold in 0.5 M sulfuric acid and 1 M sodium hydroxide solutions. A CO stretch band was observed with a peak intensity of ca. 0.2 % between 1850 and 2000 cm⁻¹ in 1 M NaOH, while a very weak band was detected between 1950 and 2050 cm⁻¹ in 0.5 M H₂SO₄. The bands were assigned to linear CO species adsorbed on the gold surface. In 1 M NaOH, electrooxidation of the strongly adsorbed CO species, which was detected by EMIRS, starts from ca. 0.5 V (RHE) with a sharp voltammetric current peak at 1.0 V at 50 mV/s, while electrooxidation of the bulk CO starts from ca. 0 V in the absence of the strongly adsorbed CO species on Au. The strongly adsorbed CO species acts as a poison for the electrooxidation of CO in the lower potential region.     

A FTIR study of ethylene oxidation on an Ru/ZnO system

The interaction of ethylene with a highly dispersed and oxidized Ru/ZnO catalyst has been studied at room temperature (RT) by FTIR spectroscopy. A fast oxidative reaction at RT produces a glycolate-like species strongly adsorbed on the surface, and at the same time the depletion of a strong and narrow band assigned to an RuO surface group. The assignment of the spectrum of a C₂H₄O₂ complex, characterized by two intense bands at 1035 and 890 cm⁻¹ and by 10 other bands in the range 3000-700cm⁻¹, is performed on the basis of a C_2ν symmetry and confirmed by C₂D₄ oxidation data; this assignment is performed by comparison of our spectra with the literature data for ethylene glycol and related complexes. A long oxidative reaction at RT with formation of formate and carboxylate-like species at the expense of the glycolate was also observed.     

An exploration of the surfaces of some Pt/SiO2 catalysts using CO as an infrared spectroscopic probe

Infrared spectra in the bond-stretching, ν CO, region have been measured for CO adsorbed on an impregnated Pt/SiO₂ catalyst (16% Pt) with the aim of characterizing the adsorption sites present. This catalyst has previously been widely used for the study of the spectra from adsorbed hydrocarbons. It has relatively large metal particles, typically in the diameter range of 5 to 15 nm. Samples were studied which were hydrogen-covered, hydrogen-depleted, oxygen-covered and “mature”, the latter in the sense that the catalyst had undergone a number of repeated adsorption/desorption/re-reduction cycles. The spectra were interpreted in terms of the adsorption sites available by using recent correlations between ν CO wavenumbers and different patterns of CO metal interactions in metal clusters of structures known from X-ray crystallography.The strongest ν CO absorptions were from linearly adsorbed OCPt species, but these were relatively uninformative (maxima in the region 2087 to 2084 cm⁻¹ in all cases at saturation coverage) because strong dipolar coupling causes intensity-distortion and the merging-together of absorptions from different types of sites. Small proportions of sites, probably of an atomically rough nature from curved areas of the crystallites, gave weaker absorptions near 2050 and 2030 cm⁻¹. On the oxygen-covered surface a sharp absorption at 2099 cm⁻¹ denoted CO adsorption adjacent to sites of adsorbed oxygen.The weaker absorption bands in the ν CO bridged region were more informative. The strongest, near 1850 cm⁻¹ was correlated with 2-fold bridged species adsorbed on (111) surfaces. Weaker, overlapping features near 1880 and 1795 cm⁻¹ (separated from an overall contour with the aid of computer analysis) were correlated, respectively with 2-fold bridged species on (100) or (110) planes, and 3-fold bridged species on (111) planes. Weaker absorptions near 1700 cm⁻¹ were considered to correspond to different types of adsorption sites involving unequal interactions of CO with at least two metal atoms.The “mature” Pt/SiO₂ samples gave notably different spectra in both regions and this was tentatively attributed to the effect of residual carbon atoms on or near the Pt surface.A comparison was made of the present spectra with those previously published from similar experiments on a small-particle (ca 2 nm) Pt/SiO₂ catalyst, EUROPT-1. The spectral differences could be well accounted for in terms of the reported raft-like (111)-based morphology of the small metal particles of the EUROPT-1 catalyst.     

FT-IR study of the interaction of oxygen,argon, helium,nitrogen and xenon with hydroxyl groups in H-Y zeolite at low temperatures

Adsorptions of O₂, Ar, He, N₂ and Xe on H-Y zeolite at low temperatures were studied by transmission Fouriertransform infrared spectroscopy. For the O₂ adsorption, a weak v(OO) band was observed at 1550 cm⁻¹. The formation of a weak hydrogen-bonding of O₂ and Ar with the bridging OH-groups in the supercage was observed, while these molecules did not interact chemically nor physically with the bridging OH-groups in the small cavities. These results indicate that O₂ and Ar can approach the acid sites in the supercages, but cannot approach those in the small cavities. However, a slight physical interaction (not the H-bonding) of He with the OH groups in the small cavities was observed, suggesting that He would be able to approach near the sites. N₂ adsorption gave two v(NN) bands at 2353 and 2338 cm⁻¹, which have been attributed to the N2 species adsorbed on Lewis acid sites and on Br0nsted acid sites, respectively. It was observed that N₂ and Xe interact strongly with the bridging OH-groups in the supercages as well as weakly with the silanol groups. On the basis of the IR-spectroscopic data on the strength of the interaction with the small and nonpolar gases, the acidic properties of H-Y was compared with those of H-MOR and H-ZSM-5.     

FT-IR emission studies of chemisorbed species on supported metal catalysts.

Infrared emission spectra of CO adsorbed on Rh/Al₂O₃ and Pt/Al₂O₃ were studied at 420K. The bands at 2092 and 2031 cm⁻¹ were assigned to symmetric and asymmetric stretching modes of surface Rh(CO)₂ groups, respectively. The RhC stretching modes are expected just below the range of observations (420 cm⁻¹. On Pt/Al₂O₃ the bands at 2043 and 457 cm⁻¹ were assigned to CO and PtC stretching vibrations of Pt-CO surface groups, respectively. Force constant calculations give 17.3 and 16.9 Ncm⁻¹ for CO stretching and 2.55 and 3.53 Ncm⁻¹ for MC stretching on the Rh and Pt catalysts, respectively.     

Study of secondary porosity by SAXS and N2 adsorption in composite materials obtained from a Cuban natural clinoptilolite

In this work, a study of the secondary porosity of two zeolite-based composites is carried out by small angle X-ray scattering (SAXS) and N₂ adsorption at 77 K. The composites were obtained by the inclusion of ZnO nanoparticles in a Cuban natural clinoptilolite by mechanosynthesis and ‘in situ’ methods. It was observed a decrease in the specific surface area as a result of ZnO nanoparticles inclusion from 149 m² g⁻¹ in the started material to 60 m² g⁻¹ in the composite prepared by in situ method, whereas the mesopore diameter remained almost constant. The results confirmed the presence of mesopores with diameter between 3 and 36 nm, with good match by both methodologies. These materials were developed in view of their future application as catalysts and adsorbents, where the presence of secondary porosity is key to favor the diffusion processes.     

H2O and H2 interaction with ZnO surfaces: A MNDO,AM1, and PM3 theoretical study with large cluster models

We investigated the adsorption and heterolytic dissociation of H₂O and H₂ molecules on a (ZnO)₂₂ cluster corresponding to ZnO (0001), (000(OVERBAR)1), and (10(OVERBAR)10) surfaces using MNDO , AM 1 and PM 3 semiempirical procedures. The geometry of the adsorbed molecule has been optimized in order to analyze binding energies, charge transfer, and preferential sites of interaction. The adsorbed species interact most strongly when it is bonded to the twofold coordinated zinc atom of the cluster surface. The interaction of the H₂O molecule with the surface of ZnO has a charge transfer from H₂O to the surface ranging between 0.17 and 0.27 au. The neighboring atoms of the surface are the main receptors during the process of charge transfer. Our results indicate that there is a weak bonding of the hydrogen atom from OH with the oxygen surface atom that could produce the O(SINGLE BOND)H·O band. The interaction of the H₂ molecule with the surface is generally weak and only the PM 3 method yields a strong binding energy for this interaction. There is a charge transfer from the H₂ molecule to the surface. The chemisorption of H on oxygen atom of the surface transfer charge from the surface to the H. We also calculated the vibrational analyses for these interactions on ZnO surface and compared our results with available experimental data. © 1996 John Wiley & Sons, Inc.     

Non equivalency of oxygen nuclei in 17O−2 adsorbed on oxides

Oxygen enriched with ¹⁷O has been used to investigate the nature of oxygen species adsorbed on oxides. The variations in the hyperfine constants of O^?₂, small (<3%) for MgO and ZnO, are due to different adsorption sites. They become large (>15%) for supported molybdenum due to the different spin densities on the two nuclei showing that O^?₂ probably adsorbed at an angle to the surface.     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.     

Infrared Spectroscopic Studies on the Surface Chemistry of High‐Surface‐Area Gallia Polymorphs

Polymorphs α, β, and γ of Ga₂O₃ having hexagonal (corundum‐type), monoclinic and cubic (spinel‐type) structure, respectively, were prepared in a high‐surface‐area form, and characterized by powder X‐ray diffraction. Nitrogen adsorption at 77 K showed these gallia samples to have specific surface areas of 77 (α‐Ga₂O₃), 40 (β‐Ga₂O₃) and 120 m² g^?1 (γ‐Ga₂O₃). Fourier transform infrared spectroscopy of adsorbed carbon monoxide (at 77 K) and pyridine (at room temperature) showed that the three gallia polymorphs have a very similar surface Lewis acidity, regardless of their different crystal structures. This Lewis acidity was assigned, mainly, to coordinatively unsaturated tetrahedral Ga³⁺ ions situated on the surface of the small crystallites which constitute the different metal oxide varieties. Ga³⁺···CO adducts formed after CO adsorption gave (in all cases) a characteristic C–O stretching band at 2195–2200 cm^?1, while Lewis‐type adducts formed with adsorbed pyridine were characterized by IR absorption bands at 1610–1612 and 1446–1450 cm^?1. The three (partially hydroxylated) gallia polymorphs showed also a very weak Brønsted acidity, which they manifested by forming hydrogen‐bonded adducts with both CO and pyridine; however no protonation of adsorbed pyridine occurred.     

Surface cars spectroscopy of pyridine and phenol on ZnO optical waveguides

Using planar optical waveguide geometry, surface CARS spectra of phenol and pyridine adsorbed on ZnO surfaces have been observed under high-vacuum conditions. When most of the CARS contribution from the ZnO film was eliminated by an interference condition, submonolayer quantities were detectable. The v₁₂ mode of phenol is insensitive to the nature of the surface bond and leads to a single line at 1002 cm⁻¹. The v₁ mode of pyridine gives rise to three lines at 993, 1000 and 1008 cm⁻¹ corresponding to sites for physisorption, hydrogen-bonding and protonation. At high surface power densities, photoinduced desorption of pyridine was observed. Two-photon absorption in the ZnO leads to a power-dependent index of refraction.     

Probing the Reactivity of ZnO and Au/ZnO Nanoparticles by Methanol Adsorption: A TPD and DRIFTS Study

The adsorption of methanol on pure ZnO and Au‐decorated ZnO nanoparticles and its thermal decomposition monitored by temperature‐programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C? O stretching bands. During the subsequent TPD experiments two different H₂ peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJ mol^?1 and 127 kJmol^?1 for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non‐polar ZnO(${10\bar 10}$ ) surface and on the highly reactive polar ZnO(${000\bar 1}$ ) surface. The simultaneous formation of H₂, CO, and CO₂ at about 550–600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO₂ ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au‐decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies.     

IR-Spectroscopic Study of Complex Formation of Nitrogen Oxides (NO,N2O) with Cationic Forms of Zeolites and the Reactivity of Adsorbed Species in CO and CH4 Oxidation

The formation of complexes and disproportionation of nitrogen oxides (NO, N₂O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N₂O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν¹_0–2 and the combination bands of the stretching vibrations with other vibrational modes for N₂O complexes with cationic sites in zeolites (ν³_0–1 + ν¹_0–1, ν¹_0–1 + δ_0–2) are more significantly influenced by the nature of the zeolite. The presence of several IR bands in the region of 2400–2600 cm⁻¹ (the ν¹_0–1 + δ_0–2 transitions) for different zeolite types was explained by the availability of different localization sites for cations in these zeolites. The frequencies in this region also depend on the nature of the cation (its charge and radius). The data can be explained by the specific geometry of the N₂O complex formed, presumably two-point adsorption of N₂O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH₄ on the samples with preliminarily adsorbed N₂O at 20–180 °C does not result in any oxidation of these molecules. NO⁺ and N₂O₃ species formed by disproportionation of NO are capable of oxidizing CO and CH₄ molecules to CO₂, whereas NO_x is reduced simultaneously to N₂ or N₂O. The peculiarities in the behavior of cationic forms of different zeolites with respect to adsorbed nitrogen oxides determined by different density and localization of cations have been established.     

ZnO Quantum Dots Coupled with Graphene toward Electrocatalytic N2 Reduction: Experimental and DFT Investigations

Electrochemical reduction of N₂ to NH₃ is a promising method for artificial N₂ fixation, but it requires efficient and robust electrocatalysts to boost the N₂ reduction reaction (NRR). Herein, a combination of experimental measurements and theoretical calculations revealed that a hybrid material in which ZnO quantum dots (QDs) are supported on reduced graphene oxide (ZnO/RGO) is a highly active and stable catalyst for NRR under ambient conditions. Experimentally, ZnO/RGO was confirmed to favor N₂ adsorption due to the largely exposed active sites of ultrafine ZnO QDs. DFT calculations disclosed that the electronic coupling of ZnO with RGO resulted in a considerably reduced activation-energy barrier for stabilization of *N₂H, which is the rate-limiting step of the NRR. Consequently, ZnO/RGO delivered an NH₃ yield of 17.7 μg h⁻¹ mg⁻¹ and a Faradaic efficiency of 6.4 % in 0.1 m Na₂SO₄ at −0.65 V (vs. RHE), which compare favorably to those of most of the reported NRR catalysts and thus demonstrate the feasibility of ZnO/RGO for electrocatalytic N₂ fixation.     

Rare Earth Evoked Subsurface Oxygen Species in Platinum Alloy Catalysts Enable Durable Fuel Cells

Alleviating the degradation issue of Pt based alloy catalysts, thereby simultaneously achieving high mass activity and high durability in proton exchange membrane fuel cells (PEMFCs), is highly challenging. Herein, we provide a new paradigm to address this issue via delaying the place exchange between adsorbed oxygen species and surface Pt atoms, thereby inhibiting Pt dissolution, through introducing rare earth bonded subsurface oxygen atoms. We have succeeded in introducing Gd−O dipoles into Pt₃Ni via a high temperature entropy-driven process, with direct spectral evidence attained from both soft and hard X-ray absorption spectroscopies. The higher rated power of 0.93 W cm⁻² and superior current density of 562.2 mA cm⁻² at 0.8 V than DOE target for heavy-duty vehicles in H₂-air mode suggest the great potential of Gd−O−Pt₃Ni towards practical application in heavy-duty transportation. Moreover, the mass activity retention (1.04 A mg_Pt⁻¹) after 40 k cycles accelerated durability tests is even 2.4 times of the initial mass activity goal for DOE 2025 (0.44 A mg_Pt⁻¹), due to the weakened Pt−O_ads bond interaction and the delayed place exchange process, via repulsive forces between surface O atoms and those in the sublayer. This work addresses the critical roadblocks to the widespread adoption of PEMFCs.     

Surface Adsorbed Hydroxyl: A Double-Edged Sword in Electrochemical CO2 Reduction over Oxide-Derived Copper

Oxide-derived Cu (OD−Cu) featured with surface located sub-20 nm nanoparticles (NPs) created via surface structure reconstruction was developed for electrochemical CO₂ reduction (ECO₂RR). With surface adsorbed hydroxyls (OH_ad) identified during ECO₂RR, it is realized that OH_ad, sterically confined and adsorbed at OD−Cu by surface located sub-20 nm NPs, should be determinative to the multi-carbon (C₂) product selectivity. In situ spectral investigations and theoretical calculations reveal that OH_ad favors the adsorption of low-frequency *CO with weak C≡O bonds and strengthens the *CO binding at OD−Cu surface, promoting *CO dimerization and then selective C₂ production. However, excessive OH_ad would inhibit selective C₂ production by occupying active sites and facilitating competitive H₂ evolution. In a flow cell, stable C₂ production with high selectivity of ∼60 % at −200 mA cm⁻² could be achieved over OD−Cu, with adsorption of OH_ad well steered in the fast flowing electrolyte.     

Study of anion adsorption on polycrystalline Pt by electrochemical quartz crystal microbalance

The changes observed on Pt surfaces during a potential incursion into the underpotentially deposited (UPD) hydrogen and double layer ranges (0.05 to 0.8 V versus HESS) were analyzed in perchloric, sulfuric, phosphoric and hydrochloric acid media. Surface occupation after hydrogen desorption was verified in terms of both mass variations and voltammetric charges measured by EQCM and cyclic voltammetry. Firstly, mass incorporation due to the adsorption of water molecules (approximately 39 ng cm⁻², corresponding to a full monolayer of adsorbed water) replacing the UPD H atoms was observed in every case. The potential range associated with water adsorption varied from 0.05 V to a final value that depended on the strength of anion adsorption on Pt (0.4 V for ClO₄⁻ and 0.3 V for Cl⁻). Secondly, the mass incorporations in the potential region between 0.4 and 0.8 V were associated to adsorption of the corresponding hydrated anions, i.e., ClO₄⁻·2H₂O, HSO₄⁻·2H₂O, HPO₄²⁻ and Cl⁻·6H₂O. Calculated anion coverage values varied from 7 (perchlorate) to 19% (phosphate) on the Pt surface.     

Counting of Oxygen Defects versus Metal Surface Sites in Methanol Synthesis Catalysts by Different Probe Molecules

Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N₂O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N₂O, temperature programmed desorption of H₂, and kinetic experiments. The presence of ZnO dramatically influences the N₂O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu‐ZnO interface and can be used to easily quantify the intensity of Cu‐Zn interaction. N₂O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H₂.     

ZnO and TiO2 clusters as catalyst in the addition and abstraction reaction of acrylic acid with the OH radical

The present work displays the theoretical analysis on the role of metal oxide clusters as an effective catalyst in the reaction between acrylic acid and OH radical, which has an energy barrier of 12.4 kcal/mol. The formation of metal oxide cluster such as ZnO and TiO₂ with varying size from monomer to hexamer is analyzed using cohesive energy, which increases with cluster size. Adsorption of acrylic acid on clusters reveals that dimer ZnO and tetramer TiO₂ are good adsorbed entities. The dimer ZnO and tetramer TiO₂ clusters have reduced the barrier height. However, from the thermodynamical analysis of H-abstraction and OH addition reaction, the dimer ZnO cluster is found to be a good catalyst than a tetramer TiO₂ cluster. The favorable H abstraction and OH addition reactions are feasible at the active methylene group (–CH). OH addition reactions dominate over the H abstraction reaction. Further, the presence of metal oxide clusters enhances the rate of the reaction between acrylic acid and OH radical. The kinetics of the favorable reaction with a dimer ZnO cluster has a rate constant of 7.80 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which is higher than the literature report (1.75 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹). Overall, ZnO and TiO₂ metal oxide clusters can be effectively utilized as catalyst.     

In situ infrared and EXAFS studies of an Ag cluster in zeolite X

The in situ measurements of infrared spectra and the Ag K-edge EXAFS spectra of the fully Ag⁺ exchanged zeolite X (Ag₈₆–X) were carried out from room temperature to 300 °C under vacuum. By evacuation at room temperature the O–H stretch vibration (ν(O–H)) mode around 3 μm disappears and the coordination number of oxygen around Ag, N_Ag–O, decreases due to removal of water molecules. The T–O asymmetric stretch (ν_as(T–O)) mode associated with zeolite framework oxygen appears around 10 μm. These infrared spectra are fitted by summing up Gaussian peaks. The positions of the main two peaks are 1000 and 1100 cm⁻¹ at room temperature. At 100 °C, a third infrared peak appears at around 955 cm⁻¹, the total N_Ag–O becomes small and the coordination number of Ag around Ag, N_Ag–Ag, is 0.5. These results suggest that Ag atoms change sites in the zeolite and play an important role as a precursor of the Ag clusters. At 300 °C, the peaks around 1000 and 1100 cm⁻¹ shift to 1050 and 1140 cm⁻¹, respectively, and N_Ag–Ag becomes 2.9, which indicates that the Ag clusters attached to the zeolite framework are stabilized at high temperature. When the zeolite with Ag clusters is exposed to atmosphere, it is found that: (1) the ν(O–H) mode around 3 μm appears again, (2) there are two main peaks (1000 and 1100 cm⁻¹) and a small peak around 856 cm⁻¹ and (3) the local structure of the Ag clusters formed at 300 °C never reverses.