Unusual Forms of Molecular Hydrogen Adsorption by Cu+1 Ions in the Copper-Modified ZSM-5 Zeolite

DRIFT and IR transmittance spectra of H₂ adsorbed at 77 K or at room temperature by the copper-modified ZSM-5 zeolite pre-evacuated or pre-reduced in CO at 873 K indicated several unusual forms of adsorbed hydrogen. H–H stretching frequencies of adsorbed species at 3075–3300 cm^–1 are by about 1000 cm<>^–1<> lower than in the free hydrogen molecules. This indicates unusually strong perturbation of adsorbed hydrogen by reduced Cu<>⁺¹<> ions that has been never before reported neither for hydrogen nor for adsorption of other molecules by any cationic form of zeolites or oxides.     

The adsorption of Sn on Pt(1 1 1) and its influence on CO adsorption as studied by XPS and FTIR

The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5×10⁻⁵ M Sn²⁺ in 0.5 M H₂SO₄ has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of ?_max=0.35 (Sn to surface Pt atoms).Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm⁻¹ band associated with bridge-bonded CO and the 2070 cm⁻¹ band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm⁻¹ band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2×2)-3CO saturation structure on Pt(1 1 1). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV.     

Infrared Study of Benzene Hydrogenation on Pt/SiO2 Catalyst by Co-adsorption of CO and Benzene

FT-IR spectra of the co-adsorption of benzene and CO have been performed to identify the preferred adsorption sites of hydrogen and benzene on a Pt/SiO₂ catalyst for hydrogenation of benzene. Results of CO adsorbed on atop sites on Pt/SiO₂ includes: an α peak at 2091 cm⁻¹, a β peak at 2080 cm⁻¹ and a γ peak at 2067 cm⁻¹ indicating three kinds of adsorption sites for dissociative hydrogen on Pt/SiO₂. The site of lowest CO stretching frequency offers stronger adsorbates–metal interaction for benzene and hydrogen. Hydrogen binding on the site of lowest CO stretching frequency before benzene adsorption significantly enhances the reaction rate of benzene hydrogenation.     

Ion-dipole interactions between adsorbed CO and support cations in Pt/K-LTL

The adsorption of carbon monoxide on a non-acidic Pt/K-LTL catalyst has been studied by diffuse reflectance and transmission IR spectroscopy. The CO spectrum is strongly dependent on the experimental conditions. Adsorption on the small Pt clusters in the presence of water gives linear-CO bands between 2060 and 1990 cm^–1 and a bridging-CO band around 1800 cm^–1. In the absence of water, the linear bands are red shifted to about 1940 and 1720 cm^–1, respectively. The frequency shift is attributed to an ion-dipole interaction between adsorbed CO and support cations. The ion-dipole interaction is screened by the adsorbed water leading to a smaller red shift in the CO stretching frequency.     

Effects of Water and Ion-Exchanged Counterion on the FTIR Spectra of ZSM-5. II. (Cu+–CO)-ZSM-5: Coordination of Cu+–CO Complex by H2O and Changes in Skeletal T–O–T Vibrations

The 2157 cm^–1 (strong) and 2108 cm^–1 (very weak) (CO) IR bands due to Cu⁺–CO in ZSM-5 zeolite with ¹²C and ¹³C isotopes, respectively, are reversibly red-shifted by subsequent adsorption of H₂O at 293 K. On the contrary, the locally perturbed internal (T–O–T) asymmetric stretching framework vibration [ _as ^int (TOT)(Cu⁺–CO)=965 cm^–1] is reversibly blue-shifted. The courses of the band shifts revealed notable features. Charge transfers from water to Cu⁺ ions, changes in coordination spheres of Cu⁺(CO)(H₂O) _n aqua complexes and secondary (solvent-like) effects were considered to explain the results.     

CO adsorption on supported and promoted Ag epoxidation catalysts

CO was used to probe the nature of adsorption sites on Ag/α-Al₂O₃ epoxidation catalysts and to investigate the effect of Cs and Cl promoters by employing diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and chemisorption measurements. In contrast to previous studies, IR absorption bands for CO chemisorbed on reduced, supported Ag crystallites were observed; however, CO adsorption occurred on only 3–7% of the total Ag surface at 300 K and coverage depended on both the pretreatment and CO pressure utilized. No irreversible CO adsorption occurred on the alumina, whereas linearly bonded CO was the dominant species on the metallic Ag sites. After a 30-min purge, the bands due to these chemisorbed forms of CO decreased in intensity while a band due to bridge-bonded CO increased in intensity, which implies that CO reoriented as the surface concentration of CO decreased. In the presence of Cs, similar behavior was observed and the band intensity of the bridge-bonded CO increased. After reduction at 673 K, cesium suboxides appeared to be formed based on the formation of carbonyl complexes at 2028, 1950, and 1869 cm⁻¹. On reduced Ag catalysts, electronic effects of Cs and Cl were observed and adsorbed CO gave a lower frequency, i.e., 2018 and 2009 cm⁻¹ for Cs-promoted samples reduced at 473 and 673 K, respectively, due to an increase in the electron density on surface Ag atoms, while this band occurred at a higher frequency of 2129 cm⁻¹ with a CsCl-promoted Ag catalyst due to a net decrease in the electron density on surface Ag atoms. After CO adsorption on O-covered Cs-promoted and CsCl-promoted catalysts, a band between 1520 and 1491 cm⁻¹ existed which was assigned to a COO⁻ stretching mode in a carbonate species formed on composite AgCs_xO_y sites. These studies with CO provide evidence that reduction at 673 K following a calcination step can lead to redistribution of Cs atoms.     

In-situ FT-IRAS study of the CO oxidation reaction over Ru(001): III. Observation of a 2140 cm−1 C-O stretching vibration

Utilizing Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), we have investigated the CO oxidation reaction in-situ on a Ru(001) surface at high ( 10 Torr) pressures. Under certain temperature and reactant (CO and O₂) partial pressure conditions, we observe for the first time on unsupported Ru a weakly adsorbed CO species which is characterized by an unusually high C-O stretching frequency of 2140 cm^–1. A similar feature has been identified previously on small Ru particles in supported catalysts and attributed by some to a multicarbonyl species (–Ru(CO) _n ,n > 1). By following the intensity of this feature on Ru(001) relative to other peaks in the spectra, we believe that the 2140 cm^–1 peak observed here is most likely due to a highly perturbed linearly adsorbed monocarbonyl on partially oxidized Ru sites generated by locally high concentrations of coadsorbed oxygen.Sandia National Laboratories is supported by the United States Department of Energy under contract number DE-AC04-76DP00789.     

FTIR Characterization of Fe3+–OH Groups in Fe–H–BEA Zeolite: Interaction with CO and NO

Fe³⁺–OH groups of a Fe–H–BEA sample prepared by conventional ion-exchange method are characterized by an IR band at 3686–3684 cm^?1. They exhibit a weak acidity: upon low-temperature CO adsorption the O–H stretching modes are blue shifted by 100 cm^?1 and the respective carbonyl adducts are observed at 2158 cm^?1. The Fe³⁺–OH groups are reduced at room temperature by NO to form Fe²⁺–NO species and NO⁺ groups in cationic positions. Desorption of pre-adsorbed NO at temperatures above 373 K regenerates the Fe³⁺–OH groups. The relation of the Fe³⁺–OH species to the so-called α-oxygen is discussed.     

NO Adsorption on Ex-Framework [Fe,X]MFI Catalysts: Novel IR Bands and Evaluation of Assignments

IR spectra of NO adsorbed on isomorphously substituted [Fe,Al]MFI, [Fe,Ga]MFI and [Fe]MFI after steaming at 873 K in 30 vol% H₂O are presented. On ex-[Fe,Al]MFI, NO adsorption leads to bands at 2133 cm^-1 and a doublet at 1886 and 1874 cm^-1. The 2133 cm^-1 band is assigned to NO⁺ occupying cationic positions in the zeolite structure. Of the doublet, the 1874 cm^-1 band is much more susceptible to reaction with O₂ than the 1886 cm^-1 band, yielding adsorbed NO₂ with an absorption frequency of 1635 cm^-1. After evaluation of the constitution of the catalyst and (sometimes contradictory) literature assignments, the 1886 cm^-1 band is assigned to NO adsorbed on Fe ions located in isolated positions, and/or (FeO)_n clusters inside the zeolite channels, whereas the 1874 cm^-1 band is proposed to be induced by 2 nm FeAlO_x nano-particles. The ex-[Fe,Ga]MFI catalyst showed a similar absorption pattern (doublet), which is shifted to lower wavenumbers (1881 and 1867 cm^-1), suggesting that both frequencies are affected by the vicinity of Ga (or Al) to the Fe site involved. The absence of bands at 1765 and 1835 cm^-1 suggests that the isolated sites causing these absorptions are in the Fe^III state in ex-[Fe,Al]MFI and ex-[Fe,Ga]MFI. For the ex-[Fe]MFI sample, which did not contain any 2 nm FeO_x nano-particles, an NO absorption band at 1854 cm^-1 is assigned to mono-nitrosyl on extra-framework oligonuclear (Fe^IIO)_n species in the zeolite channels.     

Characterizing electrocatalytic surfaces: Electrochemical and NMR studies of methanol and carbon monoxide on Pt/C

We use cyclic voltammetry (CV) on fuel cell electrodes to elucidate the important differences between adsorbates resulting from carbon monoxide adsorption and methanol adsorption onto commercial Pt/C electrocatalysts in a sulfuric acid electrolyte. Under open circuit conditions, methanol was found to adsorb preferentially onto the Pt sites associated with “strongly bound” hydrogen. The sites associated with “weakly bound” hydrogen adsorbed methanol more slowly. In the case of CO adsorption, which requires no adsorbate dehydrogenation, all adsorption sites showed similar affinity towards the adsorbate. Electrochemical oxidation of the adsorbates derived from both methanol and CO exposure exhibit slower oxidation when the adsorbate is associated with cubic-packed-like sites than from close-packed-steps and other sites. NMR of a ¹³CO-adlayer prepared by electrochemical adsorption from low concentration ¹³CH₃OH shows a lower NMR shift and smaller linewidth than the previously reported values for electrochemically adsorbed ¹³CO gas. These results are interpreted in terms of adsorbate motion on the electrocatalyst surface.     

Formation of Ni(CO)4 during the Interaction between CO and Silica-Supported Nickel Catalyst: an FTIR Spectroscopic Study

The formation of Ni(CO)₄ during interaction of CO with silica-supported highly dispersed nickel metal (d _av4 nm) was investigated by FTIR spectroscopy. At temperatures below 145 K, in addition to linear and bridged nickel carbonyls, CO adsorption on Ni⁰/SiO₂ leads to the formation of Ni(CO) _x (x=2, 3) subcarbonyls (band at ca. 2090 cm^–1) and negligible amounts of Ni(CO)₄ adsorbed on SiO₂ (band at 2048 cm^–1). Up to this temperature CO causes no detectable erosion of the metal surface. Above 145 K the rate of interaction between CO and the nickel particles significantly increases. Until 235 K Ni(CO)₄ mainly remains in the adsorbed state, while at still higher temperatures the equilibrium between adsorbed and gaseous Ni(CO)₄ (band at 2058 cm^–1) is shifted towards the latter. It is assumed that subcarbonyls formed on defect sites of the metal surface are precursors of the nickel tetracarbonyl. Successive adsorption–evacuation cycles of CO at room temperature result in a decrease in the amount of the Ni(CO)₄ formed, probably due to a reduction of the number of defect metal sites. On the basis of ¹²CO and ¹³CO coadsorption, an alternative interpretation of the band at 2048 cm^–1 to species containing isolated Ni(CO)₃ groups is proposed.     

Adsorption/oxidation of CO on highly dispersed Pt catalyst studied by combined electrochemical and ATR-FTIRAS methods: Part 1. ATR-FTIRAS spectra of CO adsorbed on highly dispersed Pt catalyst on carbon black and carbon un-supported Pt black

Elecrochemical ATR-FTIRAS measurements were conducted for the first time to investigate nature of CO adsorbed under potential control on a highly dispersed Pt catalyst with average particle size of 2.6 nm supported on carbon black (Pt/C) and carbon un-supported Pt black catalyst (Pt-B). Each catalyst was uniformly dispersed by 10 μg Pt/cm² and fixed by Nafion^® film of 0.05 μm thick on a gold film chemically deposited on a Si ATR prism window. Adsorption of CO was conducted at 0.05 V on the catalysts in 1 and 100% CO atmospheres, for which CO coverage, θ_CO, was 0.69 and 1, respectively. Two well-defined ν(CO) bands free from band anomalies assigned to atop CO (CO(L)) and symmetrically bridge bonded CO (CO(B)_sym.) were observed. It was newly found that the CO(L) band was spitted into two well-defined peaks, particularly in 1% CO, from very early stage of adsorption, which was interpreted in terms of simultaneous occupation of terrace and step-edge sites, denoted as CO(L)_terrace and CO(L)_edge, respectively. This simultaneous occupation was commonly observed in our work both on Pt/C and Pt-B. A new band was also observed around 1950 cm⁻¹ in addition to the bands of CO(L) and CO(B)_sym., which was assigned to asymmetric bridge CO, CO(B)_asym., adsorbed on (1 0 0) terraces, based on our previous ECSTM observation of CO adsorption structures on (1 0 0) facet. The CO(B)_asym. on the Pt/C, particularly in 100% CO atmosphere, results in growth of a sharp band at 3650 cm⁻¹ accompanied by a concomitant development of a band around 3500 cm⁻¹. The former and the latter are assigned to ν(OH) vibrations of non-hydrogen bonded and hydrogen bonded water molecules adsorbed on Pt, respectively, interpreted in term of results from a bond scission of the existing hydrogen bonded networks by CO(L)s and from a promotion of new hydrogen bonding among water molecules presumably by CO(B)_asym..It was found that the frequency ν(CO) of CO(L) both on Pt/C and Pt-B is lower than that on bulky polycrystalline electrode Pt(poly) or different crystal planes of Pt single-crystal electrodes by 30-40 cm⁻¹ at corresponding potentials, which implies a stronger electronic interaction between CO and Pt nano-particles and/or an increased contribution of step-edge sites on the particles. Determination of the band intensities of CO(L), CO(B)_asym. and CO(B)_sym. has led us to conclude a much higher bridged occupation of sites at Pt nano-particles than Pt(poly) electrodes.     

Infrared spectroscopic and voltammetric study of adsorbed CO on stepped surfaces of copper monocrystalline electrodes

Voltammetric and infrared (IR) spectroscopic measurements were carried out to study adsorbed CO on two series of copper single crystal electrodes n(1 1 1)-(1 1 1) and n(1 1 1)-(1 0 0) in 0.1 M KH₂PO₄ + 0.1 M K₂HPO₄ at 0 °C. Reversible voltammetric waves were observed below −0.55 V versus SHE for adsorption of CO which displaces preadsorbed phosphate anions. The electric charge of the redox waves is proportional to the step atom density for both single crystal series. This fact indicates that phosphate anions are specifically adsorbed on the step sites below −0.55 V versus SHE. Voltammetric measurements indicated that (1 1 1) terrace of Cu is covered with adsorbed CO below −0.5 V versus SHE. Nevertheless, no IR absorption band of adsorbed CO is detected from (1 1 1) terrace. Presence of adsorbed CO on (1 1 1) terrace is presumed which is not visible by the potential difference spectroscopy used in the present work. IR spectroscopic measurements showed that CO is reversibly adsorbed with an on-top manner on copper single crystal electrodes of n(1 1 1)-(1 1 1) and n(1 1 1)-(1 0 0) with approximately same wavenumber of CO stretching vibration of 2070 cm⁻¹. The IR band intensity is proportional to the step atom density. Thus CO is adsorbed on (1 1 1) or (1 0 0) steps on the single crystal surfaces. An analysis of the IR band intensity suggested that one CO molecule is adsorbed on every two or more Cu step atom of the monocrystalline surface. The spectroscopic data were compared with those reported for uhv system. The CO stretching wavenumber of adsorbed CO in the electrode-electrolyte system is 30-40 cm⁻¹ lower than those in uhv system.     

Spectroscopic features and reactivity of CO adsorbed on different Au/CeO2 catalysts

An FTIR study of CO adsorption from 120 K up to room temperature on a series of Au–ceria samples is presented. Samples with low gold content (0.7 and 0.6 at%) were prepared by urea gelation/co-precipitation and by cyanide leaching of the high-gold content (5.8 at%) material prepared by deposition–precipitation on La-doped CeO₂. The samples were subjected to different pretreatments to collect information on the surface composition under working conditions. An absorption band at 2130–2140 cm⁻¹, not reversible on outgassing and more resistant to oxidation than the usual carbonyl band on Au⁰ sites, was present due to CO adsorbed on cationic gold clusters. This highly stable species is relevant for hydrogen gas upgrade by removing CO from reformate-type gases at low temperatures. In addition, a broad absorption band in the 2000–2100 cm⁻¹ range was observed after reduction in hydrogen, due to structural and electronic changes of gold. Interestingly, the reduced gold species in ceria can be reoxidized at mild conditions. Light-off of the CO oxidation reaction took place below room temperature on the metallic gold-containing ceria but was delayed until 310 K on the ionic gold-containing sample. TPR and XPS analysis of the fresh and used catalysts corroborated the stability of ionic gold in ceria up to 393 K in the reaction gas mixture.     

The Sites of Molecular and Dissociative Hydrogen Adsorption in High-Silica Zeolites Modified with Zinc Ions. III DRIFT Study of H2 Adsorption by the Zeolites with Different Zinc Content and Si/Al Ratios in the Framework

study of dihydrogen adsorbed at 77K by the zinc-modified hydrogen forms of mordenite and ZSM-5 zeolites with the different Si/Al ratios in the framework indicated the existence of several adsorption sites connected with the modifying zinc ions. The fraction of the sites of the strongest perturbation of adsorbed molecular hydrogen with the H–H stretching frequency of ca. 3930–3950 cm^-1 is the highest at the highest Si/Al ratios of 41 or 80. These sites dissociatively adsorb hydrogen at moderately elevated temperatures. Adsorption of hydrogen with the higher H–H stretching frequency of about 3970–4000 cm^-1 predominates at the lower Si/Al ratios. It does not result in the dissociation of adsorbed molecules. It was concluded that the most active sites are most probably connected with the zinc ions, which compensate two distantly separated negatively charged [AlO₄]^- tetrahedra localized in the adjacent five- or six-membered rings on the walls of the large channels of the pentasil's framework. The sites of the weaker perturbation of adsorbed hydrogen are most probably connected with Zn⁺² ions at the conventional exchangeable cationic positions with two aluminum atoms in the same five- or six-membered ring.     

CO adsorption and correlation between CO surface coverage and activity/selectivity of preferential CO oxidation over supported Ag catalyst: an in situ FTIR study

In situ IR measurements for CO adsorption and preferential CO oxidation in H₂-rich gases over Ag/SiO₂ catalysts are presented in this paper. CO adsorbed on the Ag/SiO₂ pretreated with oxygen shows a band centered around 2169 cm^–1, which is assigned to CO linearly bonded to Ag⁺ sites. The amount of adsorbed CO on the silver particles (manifested by an IR band at 2169 cm^–1) depends strongly on the CO partial pressure and the temperature. The steady-state coverage on the Ag surface is shown to be significantly below saturation, and the oxidation of CO with surface oxygen species is probably via a non-competitive Langmuir–Hinshelwood mechanism on the silver catalyst which occurs in the high-rate branch on a surface covered with CO below saturation. A low reactant concentration on the Ag surface indicates that the reaction order with respect to Pco is positive, and the selectivity towards CO₂ decreases with the decrease of Pco. On the other hand, the decrease of the selectivity with the reaction temperature also reflects the higher apparent activation energy for H₂ oxidation than that for CO oxidation.     

Mass spectrometric investigation of the electrochemical behavior of adsorbed carbon monoxide at platinum in 0.2 M sulphuric acid

The technique of Electrochemical Mass Spectrometry is applied to the cathodic adsorption of CO at platinum in 0.2 M H₂SO₄ and the anodic desorption of the reaction product CO₂. From the data obtained by CO shielding during the adsorption reaction and by CO₂ collection during the desorption process, one site adsorption of the principal CO adsorbate at the electrode surface is determined to occur over the whole range of CO coverage. A two electron transfer is involved in the oxidation of this adsorbed CO species to CO₂. CO adsorbed in the hydrogen adsorption region produces a small amount of adsorbate requiring a three electron transfer to produce CO₂.     

Electronic state and location of Pt metal clusters in KL zeolite: FTIR study of CO chemisorption

Electronic state and location of Pt metal clusters supported on KL zeolite are studied by FTIR spectroscopy of adsorbed CO. Investigation of the CO adsorption was performed within the wide CO pressure range (from 4×10^?3 to 10² Pa) and supplemented by the study of the CO desorption at elevated temperature. Comparison of the data on CO adsorption and desorption at increased temperature reveals the existence of two groups of Pt particles in the sample. The first group of the particles is localized on the outer surface of the zeolite microcrystals and in the near surface region; they exhibit CO bands at 2060-2050 cm^?1 close to those of Pt supported on conventional supports. The particles of the second group are encaged inside zeolite channels and their electronic structure is presumably strongly perturbed by the zeolite framework. CO adsorbed on the Pt particles of this group exhibits coverage dependent bands at frequencies in the range 1960-1920 cm^?1. The marked downward shift of thevCO band is attributed to the increase of electron density on these particles.     

A DRIFTS study of the morphology and surface adsorbate composition of an operating methanol synthesis catalyst

The nature of the species adsorbed on a Cu/ZnO/Al₂O₃ catalyst while it was producing methanol has been elucidated in this study using DRIFTS. The species are carbonates, formate, CO, oxygen atoms ( 2% of a monolayer) and methoxy on the Cu and methoxy on the ZnO. The frequencies observed for the C-O stretch on Cu, 2076, 2092, 2105 and 2132 cm^–1, have revealed the morphology of the copper component of the operating catalyst. The surface of the copper is predominantly the (111) face ( 65%) (the 2076 cm^–1 peak) with the (755) (the 2092 cm^–1 peak) and the (311) (the 2105 cm^–1 peak) faces occupying roughly 20% and 15%, respectively, of the copper area. The 2132 cm^–1 peak derives from CO adsorbed on Cu⁺ site on the copper which is 2% of a monolayer.