Dynamics of bound states of dihydrogen at Cu(I) and Cu(II) species coordinated near one and two zeolite framework aluminium atoms: A combined sorption, INS, IR and DFT study

Abstract Ambient conditions sorption isotherms of dihydrogen in a series of various levels of Cu-exchanged ZSM-5 zeolites, with two different Si/Al ratios, namely 11.5 and 25, show the presence of different amount of Cu centres able to strongly bind H2. Although the isosteric heats of adsorption derived from these isotherms are rather similar, of the order of 30 kJ/mol H2, Inelastic Neutron Scattering (INS) of adsorbed dihydrogen and Fourier-Transformed Infra-Red (FTIR) spectroscopy measurements of adsorbed CO and NO reveal that copper is encountered in two oxidation states. At least two types of Cu(I) ions are clearly detected as well as some heterogeneity of the Cu(II) species. The number of these Cu species is different in the two investigated ZSM-5 materials and depends on the Cu exchange level. With the aid of DFT model cluster calculations we find that under different coordination environments, determined by the Al distribution, both mono- and divalent Cu ions could bind H2 with a different strength. Unprecedentedly, we found that Cu-ions compensating two Al atoms, i.e. formally Cu(II) species, relatively far apart from each other, may behave very similarly to the monovalent Cu-species or alternatively viewed – as Cu(I) species that compensate for two framework Al-atoms. Such Cu-species also form stable η2 dihydrogen complexes

FTIR Evidence of Different Bonding of Methane to OH Groups on H–ZSM-5, HY, and SiO₂

Low-temperature adsorption of CH₄ and ¹⁵N₂ on H–ZSM-5, SiO₂, and HY is comparatively studied. Partly and fully deuteroxylated samples were also investigated. It was established that methane forms H bonds simultaneously with oxygen and hydrogen from the Si–OH groups, which reflects in enhanced shift of the OH modes to lower frequency as compared to the case if methane was bound to the proton only. In contrast, methane is attached to highly acidic hydroxyls forming a bond mainly with the proton. At high methane equilibrium pressure, a second methane molecule is bound to the same acidic OH group

Characterization of vanadium sites on vanadium-containing mesoporous silica catalysts and their catalytic behaviour in propane ODH

Parameters that affect the catalytic behaviour of vanadium-containing mesoporous silicas in oxidative dehydrogenation reaction of propane are mainly aggregation state of surface VOx species and reducibility of vanadium species. Type and conditions of synthesis have big impact on the nature of vanadium species and, therefore, influence significantly catalytic properties. Two vanadiumcontaining mesoporous silica catalysts were synthesized by different techniques (including direct synthesis from hydrogel containing vanadium precursor and wet impregnation of SBA-15 mesoporous silica by vanadyl acetylacetonate) and their physico-chemical and catalytic properties were studied and compared. Direct synthesis of vanadosilicate led to catalysts with non-uniform mesoporosity, but exhibiting better spreading of vanadium on the surface resulting in species with lower degree of polymerization compared to impregnated catalyst. It is reflected in higher iso-conversional selectivity of directly synthesized catalyst towards propene

Role of methods and conditions of preparation of vanadium species supported on lamellar zeolite MCM-36 in their catalytic behaviour in oxidative dehydrogenation of propane

In this work, we report successful preparation of hierarchical VOx-MCM-36 catalysts. Lamellar pillared zeolitic support (MCM-36) was prepared from layered precursor of MCM-22P zeolite by swelling and pillaring. Vanadium was introduced into MCM-36 support by different methods (including conventional impregnation and ion exchange) by means of different precursors and synthesis conditions. Textural properties of prepared catalysts were characterized by nitrogen adsorption-desorption isotherms, the concentration of vanadium was determined by X-ray fluorescence and vanadium complex speciation was investigated by diffuse reflectance UV-vis spectroscopy, hydrogen temperature programmed reduction (H2 -TPR), electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. Effect of synthesis method and conditions on catalytic behaviour was tested in the oxidative dehydrogenation of propane at 540 °C. The vanadium is present in oxidation state IV, instead of usual oxidation state V, in the cases of the catalyst with low content of vanadium. The best selectivity to propene with 13 % of conversion (43.9 %) was obtained when the acid center was neutralized with potassium cations and the content of vanadium was 2 wt %

OH/OD Isotopic Shift Factors of Isolated and H‑Bonded Surface Silanol Groups

The OH/OD isotopic shift factors (<i>i = </i>ν_OH/ν_OD) of isolated silanols on SiO₂ and [Si]­BEA are between 1.3563 and 1.3568, values lower than the theoretical shift of 1.3744. However, <i>i</i> of the harmonic OH modes almost coincides with the theoretical value which indicates that the experimental deviations in this case are mainly due to anharmonicity. The anharmonicity slightly decreases when the OH groups participate in weak H-bonding with adsorbed CH₄ or CO which should lead to an increase of <i>i</i>. However, contrary to these expectations, <i>i</i> additionally decreases. This is attributed to the lower acidity of the OD groups as compared to the respective OHs. The value of <i>i</i> is also lower for H-bonded silanols as compared to isolated SiOH groups. It is concluded that <i>i</i> depends on the extent of H-bonding which allows easy distinguishing between isolated and H-bonded surface hydroxyls

Pt/CeO_<i>x</i>/ZrO_<i>x</i>/γ-Al₂O₃ Ternary Mixed Oxide DeNO_<i>x</i> Catalyst: Surface Chemistry and NO_<i>x</i> Interactions

Surface chemistry and the nature of the adsorbed NO_<i>x</i> species on a Pt/CeO₂–ZrO₂/Al₂O₃ catalyst were investigated by IR spectroscopy, X-ray diffraction, H₂-temperature programmed reduction, and NO_<i>x</i>-temperature programmed desorption. Parallel studies were also carried out with benchmark samples such as CeO₂/Al₂O₃, ZrO₂/Al₂O₃, CeO₂–ZrO₂/Al₂O₃ and Pt-supported versions of these materials. All samples were studied in their reduced and nonreduced forms. The use of CO as a probe molecule revealed that during the synthesis of the mixed-metal oxide systems, deposited zirconia preferentially interacted with the alumina hydroxyls, while deposited ceria was preferentially located at the Lewis acid sites. Despite the limited extent of Zr⁴⁺ ions incorporated into the CeO₂ lattice, the reduction of ceria was promoted and occurred at lower temperatures in the presence of zirconia. When deposited on ZrO₂/Al₂O₃, platinum formed relatively big particles and existed in metallic state even in the nonreduced samples. The presence of ceria hindered platinum reduction during calcination and yielded a high platinum dispersion. Subsequent reduction with H₂ led to the production of metallic Pt particles. Consequently, NO adsorption on nonreduced Pt-containing materials was negligible but was enhanced on the reduced samples because of Pt⁰-promoted NO disproportionation. The nature of the nitrogen-oxo species produced after NO and O₂ coadsorption on different samples was similar. Despite the high thermal stability of the NO_<i>x</i> adsorbed species on the ceria and zirconia adsorption sites, the NO_<i>x</i> reduction in the presence of H₂ was much more facile over Pt/CeO₂–ZrO₂/Al₂O₃. Thus, the main differences in the NO_<i>x</i> reduction functionalities of the investigated materials could be related to the ability of the catalysts to activate hydrogen at relatively lower temperatures

Surprising Coordination Chemistry of Cu⁺ Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N₂, and H₂O on Cu–ZSM‑5

Cations exchanged in zeolites are generally characterized by a low coordination number and can thus attach simultaneously more than one small guest molecule. For instance, Cu⁺ ions in ZSM-5 can accept, at low temperature, up to three CO and up to two NO molecules. However, only one N₂ molecule can be coordinated to such sites. Although mixed aqua-carbonyl and aqua-dinitrogen complexes are formed, no mixed carbonyl-nitrosyl, carbonyl-dinitrogen, or nitrosyl-dinitrogen species can be produced. Thus, adsorption of NO on CO precovered sample results in segregation of the CO adsorption layer according to the reaction: 2Cu⁺–CO + 2NO → Cu⁺(CO)₂ + Cu⁺(NO)₂. Adsorption of N₂ on NO precovered sample leads to a similar process: 2Cu⁺–NO + N₂ → Cu⁺(NO)₂ + Cu⁺–N₂. No carbonyl-dinitrogen complexes are produced during CO–N₂ coadsorption. The role of the ligand and the nature of the bond in the formation of geminal and mixed-ligand complexes are discussed

Purification of Hydrogen from CO with Cu/ZSM-5 Adsorbents

The transition to a hydrogen economy requires the development of cost-effective methods for purifying hydrogen from CO. In this study, we explore the possibilities of Cu/ZSM-5 as an adsorbent for this purpose. Samples obtained by cation exchange from aqueous solution (AE) and solid-state exchange with CuCl (SE) were characterized by in situ EPR and FTIR, H2-TPR, CO-TPD, etc. The AE samples possess mainly isolated Cu2+ cations not adsorbing CO. Reduction generates Cu+ sites demonstrating different affinity to CO, with the strongest centres desorbing CO at about 350 &deg;C. The SE samples have about twice higher Cu/Al ratios, as one H+ is exchanged with one Cu+ cation. Although some of the introduced Cu+ sites are oxidized to Cu2+ upon contact with air, they easily recover their original oxidation state after thermal treatment in vacuum or under inert gas stream. In addition, these Cu+ centres regenerate at relatively low temperatures. It is important that water does not block the CO adsorption sites because of the formation of Cu+(CO)(H2O)x complexes. Dynamic adsorption studies show that Cu/ZSM-5 selectively adsorbs CO in the presence of hydrogen. The results indicate that the SE samples are very perspective materials for purification of H2 from CO

Low-Temperature Adsorption of H₂ and D₂ on Dehydrated and Water Precovered CPO-27-Ni

Metal–organic frameworks (MOFs) possessing open metal sites (e.g., from the CPO-27 series) are a promising class of materials for hydrogen storage. However, there is still no consensus on the vibrational signatures of H₂ adsorbed on different sites. In this work we report results of a combined Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) study on H₂ adsorption on dehydrated and D₂O-precovered CPO-27-Ni. For unambiguous interpretation of the results adsorption of CO was also studied. Low-temperature CO adsorption on dehydrated CPO-27-Ni results in formation of Ni²⁺–CO adducts stabilized by π-back-donation and characterized by a CO stretching frequency at 2182 cm^–1 (low coverage). With D₂O-precovered a sample CO replaces part of the preadsorbed D₂O molecules, and in this case the ν­(CO) is significantly lowered (2172 cm^–1). When H₂ is adsorbed at 100 K on dehydrated sample, complexes involving Ni²⁺ sites are formed and characterized by a ν­(H–H) band at 4031 cm^–1. A satellite band (Q_trans mode) is detected at 4249 cm^–1. Similar results were obtained after D₂ adsorption. However, D₂ was more strongly adsorbed than H₂ and the Q_trans mode was of lower relative intensity. Only at high H₂/D₂ equilibrium pressures (≥50 mbar) occupation of secondary sites was clearly detected by a ν­(H–H) band at 4118 cm^–1 or ν­(D–D) band at 2964 cm^–1. No significant differences in the strength of adsorption of H₂ and D₂ were detected in this case, suggesting the mode of adsorption is different from that realized for the complexes involving Ni²⁺ sites. Progressive filling of the Ni²⁺ sites by D₂O leads to a strong decrease in intensity of the H₂/D₂ bands associated with Ni²⁺ sites and red shift of their frequencies indicating a decrease of the interaction strength. On the contrary, the bands due to H₂/D₂ interaction with secondary sites appear with enhanced intensities and are blue-shifted, which suggests an increase of the interaction strength. These results were confirmed by DFT calculations showing an increase of the H₂ adsorption enthalpy on secondary sites (identified as framework oxygen atoms) in the presence of water due to attractive interaction of H₂ with the nearby water molecule. Spectral evidence of direct interaction of adsorbed H₂/D₂ with adsorbed water was also found. Our results allow confirming that H₂ adsorbed on open Ni²⁺ sites is characterized by a stretching frequency lower than 4100 cm^–1 and some proposed values of M^<i>n</i>+–H₂ adducts above 4100 cm^–1 are in fact due to H₂ interacting with secondary sites affected by H₂O located in the vicinity