Alexander I. Serykh

Unusual localization of zinc cations in MFI zeolites modified by different ways of preparation

Comparison of DRIFT spectra of OH groups in MFI with framework Si/Al ratios equal to 25 or 40 with the spectra of the same zeolites after reaction with zinc vapour at 773 K indicated quantitative substitution of acidic hydroxyls by Zn2+ ions. Complete substitution of protons indicates that at high Si/Al ratios some of the Zn2+ cations are localized at the isolated aluminium occupied oxygen tetrahedra with single negative charges. This results in the formation of sites with only partially compensated excessive positive charges of bivalent cations. In parallel, the same amount of negatively charged aluminium occupied oxygen tetrahedra remains without compensating cations or protons. Their negative charges are neutralized indirectly by the electrostatic interaction with the surrounding positively charged cationic sites. Zinc or other bivalent ions with only partially compensated positive charges exhibit an unusually high chemical activity. This explains the specific catalytic and adsorption properties of high-silica zeolites modified with the bivalent transition metal cations. In the present study, this was demonstrated by very large red shifts of H–H stretching vibrations of H2 adsorbed by zinc cations at 77 K and by heterolytic dissociative adsorption of molecular hydrogen at moderately elevated temperatures. Application of low temperature H2 adsorption as a molecular probe indicated that similar sites of unusual localization of bivalent zinc cations also exist in zeolites modified by ion exchange or incipient wetness impregnation, but the amount of such sites is less than for the samples modified by reaction with zinc vapour.

Unusually strong adsorption of molecular hydrogen on Cu+ sites in copper-modified ZSM-5

The copper-modified ZSM-5 zeolites, prepared by ion-exchange in an aqueous solution of Cu(CH3COO)2 and subjected to different treatments were studied by FTIR spectroscopy using molecular hydrogen adsorbed at 77 K and at room temperature as a probe. For reduced and calcined CuZSM-5 samples broad range IR spectra were measured and discussed in detail. At least two different Cu+ sites in CuZSM-5 reduced with CO were distinguished using molecular hydrogen as a probe. It was established that the copper adsorption sites which strongly perturb hydrogen molecules are Cu+ ions that also strongly adsorb dinitrogen molecules. For reduced CuZSM-5 adsorption isoterms of H2, N2 and CO were measured. A comparison of the amounts of strongly adsorbed H2, N2 and CO has also been made. The amounts of adsorbed molecules decreased in the following sequence: CO > N2 > H2. Based both on the results of FTIR studies and measurements of amounts of adsorbed H2, N2 and CO it was concluded that there are different Cu+ sites which differ in the strength of interaction with adsorbed molecules probably due to different localization in the zeolite framework.

Diffuse reflectance IR study of noble metals supported on basic carriers. Part I : Pt supported on Al-Mg hydrotalcite

The state of metallic Pt supported on alumina-magnesia hydrotalcite was investigated using diffuse reflectance FT-IR spectra of adsorbed CO and terminal Pt hydrides formed upon hydrogen dissociative chemisorption on metal at room temperature. On hydrotalcite surface metallic Pt exists in three different forms, such as neutral and negatively charged metallic particles as well as slightly negatively charged clusters encapsulated between layers of support. Negative charging on Pt particles results in decrease of singleton frequency of linearly adsorbed CO by 70 cm-1 and frequency of Pt-H bond vibration by 50-100 cm-1. It was also established that negatively charged Pt is less sulfur resistant than neutral Pt particles.

On the role of zinc oxide nanometric clusters in preparation of ZnNaY zeolite by ion exchange

Dehydration of Zn2+ exchanged NaY zeolite was studied by means of DRIFT spectra of adsorbed dihydrogen and carbon monoxide as molecular probes. Unlike alkaline earth forms of faujasites dehydration of ZnY zeolite at moderately high temperatures results in the formation of bridging protons and nanometric ZnO or mixed hydroxylated ZnO microclusters instead of bridging hydroxy groups and MeOH+ cations. After subsequent dehydration at above 573 K the zinc oxide clusters interact with acidic protons resulting in Zn2+ ions localized at SII sites. These sites adsorb hydrogen in its molecular form. Heterolytic dissociative adsorption does not take place even at elevated temperatures.

First observation of the broad-range DRIFT spectra of carbon dioxide adsorbed on NaX zeolite

A first observation of the broad-range DRIFT spectra of carbon dioxide adsorbed on NaX zeolite allowed detection of the ν2 CO2 bending frequency of 619–625 cm-1 and the ν1 symmetric stretching frequency of 1358 cm-1 calculated from the combination modes of adsorbed carbon dioxide stretching and bending vibrations. The vibrational frequencies of adsorbed molecules as a whole relative to adsorption sites were also obtained from positions of satellites of the ν3 asymmetric stretching band of adsorbed molecules. The observation of several sets of satellites definitely indicates the existence in the NaX zeolite of at least two types of CO2 adsorption sites.

Stable subnanometre Pt clusters in zeolite NaX ia stoichiometric carbonyl complexes: Probing of negative charge by DRIFT spectroscopy of adsorbed CO and H2

Negatively charged platinum carbonyl Chini complexes [Pt3(CO)6]n2− with n = 2–4 were synthesized by reductive carbonylation of Pt2+ in NaX zeolite and subsequently decomposed in vacuum at temperatures up to 773 K. The Pt clusters obtained preserved a low nuclearity, evidenced by rapid recarbonylation resulting in regeneration of the initial carbonyl Chini complexes. The electronic state of the metal clusters was probed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of chemisorbed CO and H2. A strong electron donor capacity of Pt was indicated by the bands of (i) the linearly bonded CO at 1958–1976 cm−1 and (ii) the stretching frequency of terminal platinum hydrides at 2040 cm−1. It was concluded that the strong donor properties have to be attributed to negative charging of the Pt clusters, originating from the precursor carbonyl complex.

Diffuse-Reflectance IR Spectra of ortho-Hydrogen and para-Hydrogen Adsorbed on Zeolite BaX at 77 K and Their Interpretation in Terms of the Stark Effect

The vibrational and vibrational–rotational spectra of hydrogen adsorbed on zeolite BaX at 77 K were examined by diffuse-reflectance IR spectroscopy over a wide range of wavenumbers. The use of hydrogen enriched in the para isomer and of molecular deuterium allowed us to reliably assign the absorption bands to orthohydrogen and para-hydrogen and compare the experimentally observed band splitting with the theoretically calculated shifts of the corresponding levels resulted from the Stark effect. In the calculations, an adsorption site was simulated as a positive point charge. Although this approximation is rough, the IR spectra of adsorbed hydrogen and deuterium are adequately described in terms of the Stark effect. A decrease in the frequency of stretching vibrations of H–H (D–D) bonds in the molecules coordinated to Ba2+ cations, a decrease in the intensity of vibrational–rotational absorption bands, and the band splitting in the spectra of adsorbed para-hydrogen and orthohydrogen were explained.

Characterization of silica-gel supported Pt–Cu alloy particles prepared ia the sol–gel technique

Silica-gel-supported Pt–Cu alloy particles (diameter 4–8 nm) were prepared ia sol–gel processing from mixtures of tetraethoxysilane, 3-(2-aminoethylamine)propyltriethoxysilane as anchors for metal ions, and acetates of Cu2+ and [Pt(NH3)4]2+ as metal sources. The calcined and reduced materials were characterized by transmission electron microscopy (carrier morphology and metal dispersion), photoelectron spectroscopy (global charge states) and diffuse reflectance infrared spectroscopy of chemisorbed CO to probe the electronic structure of the metal particles. The results obtained by the combined techniques provide evidence for the formation of supported Pt–Cu alloy particles, the detection of geometric and ligand effects, and an unusual redistribution of intensity between the bands corresponding to CO adsorbed on Cu and Pt atoms.

Study of the state of nickel in ion-exchanged NiNaY zeolite by IR spectroscopy using molecular hydrogen adsorbed at low temperatures

The state of nickel cations in NiNaY zeolite subjected to thermal vacuum and oxidative-reductive treatment was analyzed using diffuse reflectance IR spectra of molecular hydrogen, CO, and hydroxy groups. In a sample dehydrated at 570 K, nickel forms oxide particles, which are presumably located in big cavities of a zeolite. Heating of the sample at 670 K results in decomposition of the oxide with the participation of hydroxy groups followed by the formation of stabilized Ni2+ cations in large cavities of the zeolite. Calcination of NiNaY zeolite in a hydrogen atmosphere at 730 K leads to Ni2+ cation reduction to the metal state and to the formation of acidic hydroxy groups. Nevertheless, Ni2+ cations can be regenerated by reoxidation of the reduced sample with oxygen at 670 K and further evacuation at 870 K. Thermal destruction of NiO in the reoxidized sample also proceeds on acidic hydroxy groups, but the temperature of this process is higher than the temperature of oxide decomposition in the initial NiNaY zeolite. The IR spectra of molecular hydrogen complexes with Ni2+ cations were observed for the first time. A considerable decrease in the H-H stretching vibration frequency for the coordinated molecules (up to 210 cm’1) can be explained by electron density redistribution between a hydrogen molecule and the cation in the complex

12-O-04 - DRIFT study of dinitrogen and dihydrogen adsorption on Li- and Na- forms of LSX zeolite

Publisher Summary This chapter presents diffuse reflectance infrared Fourier transform (DRIFT) study of dinitrogen and dihydrogen adsorption on lithium (Li + ) and sodium (Na + ) forms of low silica X (LSX) zeolite. Adsorption of H 2 at 77 K on Li–LSX zeolite with DRIFT control of the perturbation of adsorbed molecules has discriminated three different types of adsorption sites. Two of them are attributed to H 2 adsorption on Li + ions localized in the supercages of the zeolite framework at S III and S III ’ sites while the latter one belongs to H 2 adsorption on Li + ions at S II sites. Adsorption of N 2 by Li + ions at S III and S III ’ sites are the strongest resulting in a decrease of intensity of the corresponding DRIFT bands from adsorbed H 2 .