Adsorption Of Probe Molecules Research Articles

Periodic Hybrid DFT Approach (Including Dispersion) to MgCl2-Supported Ziegler–Natta Catalysts. 2. Model Electron Donor Adsorption on MgCl2 Crystal Surfaces

The adsorption of small probe molecules (H2O, NH3 and EtOH) and the small model silane Me2Si(OMe)2 on (104) and (110) surfaces of α-MgCl2 have been studied using periodic DFT calculations including a classical correction (of the type f(R)/R6) for dispersion. The results reveal that donors strongly stabilize both crystal surfaces relative to the bulk solid. Moreover, coordination of two donor molecules to the four-coordinate exposed Mg atom of MgCl2 (110) causes this surface to become preferred over MgCl2 (104) surface with only a single donor per exposed Mg. However, coverage also plays an important role. The model silane preferentially adsorbs in bidentate mode on MgCl2 (110), provided that coverage is 0.5 or lower; at full coverage, there is not enough space for such an arrangement, and only a monodentate binding mode is obtained. Such coverage effects should be even more pronounced for the bulkier silanes used as external donors in real MgCl2-supported Ziegler–Natta systems, as tailored experiments seem to confirm.

Parts-per-Million of Polyethylene Glycol as a Non-Interfering Blocking Agent for Homogeneous Biosensor Development

Many homogeneous assays are complicated by the adsorption of probe molecules by the surface of reaction vessels, which are often made of polypropylene or polystyrene-based plastics. To solve this problem, many protein and surfactant-based blocking agents are used. However, these blockers may interfere with intended assays by sequestering transition-metal ions, inducing protein denaturing, generating air bubbles or making pores in membranes. Coating surfaces with polyethylene glycol (PEG) through covalent linkages has been proven to be an effective method to minimize protein adsorption. However, this method is more difficult to apply on plastic surfaces and is quite expensive. While unmodified PEG is often considered as a nonadsorbing polymer, in this Technical Note, we report that PEG at very low concentration (ppm level) can still effectively block plastic surfaces. This method works for DNA, protein, and liposome-based assays as long as the molecular weight of PEG is greater than 2000. PEG works because of multivalent hydrophobic interaction from its repeating methylene units. This Technical Note will not only facilitate biosensor development, but also enhance our understanding of the interaction between various molecules and plastic surfaces.

The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammett's indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions. Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.

Interdependence between porosity, acidity, and catalytic performance in hierarchical ZSM-5 zeolites prepared by post-synthetic modification

Adopting a systematic demetallation strategy to prepare hierarchical ZSM-5 zeolites with a wide range of concomitant micro- and mesoporosity, we precisely examine the relation between variations in the porous and acidic properties. A comparative assessment of the type, concentration, and strength of acid sites is attained through the infrared study of adsorbed probe molecules (pyridine, 2,4,6-trimethylpyridine, 2,6-di-tert-butylpyridine, and d3-acetonitrile), the temperature-programmed surface reaction (TPSR) of n-propylamine and the cracking of n-hexane. The impact of these core intrinsic properties on the hierarchical zeolite performance is quantified in liquid-phase reactions with distinct acidity demands and diffusion constraints, including the alkylation of toluene with isopropanol or benzyl alcohol and the esterification of hexanoic acid with benzyl alcohol. Optimal post-synthetic modification greatly improves the initial turnover rates per Brønsted acid site. The relative performance is strongly influenced by both the mesopore surface area and by the associated concentration and strength of acid sites. The need for improved characterization of the mesopore surface acidity is highlighted.

Porosity and accessibility of acid sites in desilicated ZSM-5 zeolites studied using adsorption of probe molecules

Porosity of zeolites ZSM-5 (Si/Al=32 or 164) desilicated with NaOH or NaOH/TBAOH (tetrabutylammonium hydroxide) solutions was studied by means of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) of selected hydrocarbons. QE-TPDA of n-nonane confirmed substantial formation of mesopores upon desilication, observed also in N2 adsorption/desorption isotherms. Very narrow mesopore size distributions with maxima at about 4nm, typical for partially constricted mesopore networks, were obtained for all desilicated zeolites. It was also observed that desilication with NaOH/TBAOH results in formation of additional slightly larger mesopores (ca. 4–6nm) while even larger mesopores (6–10nm) were found after leaching with NaOH only. Thermodesorption of n-hexane revealed that the micropores in the desilicated zeolites remained unchanged. Acidity of the zeolites was characterized by quantitative IR measurements of pyridine and collidine (2,4,6-trimethylpyridine) adsorbed on acid sites. Increased numbers of Brønsted acid sites accessible for large collidine molecules found for the modified zeolites indicate that during desilication redistribution of the framework Al atoms to the surface of mesopores occurred. The influence of TBAOH presence in desilicating mixture on porosity and accessibility of the acid sites was also discussed.

In-Situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

Elucidation of complex heterogeneous catalytic mechanisms at the molecular level is a challenging task due to the complex electronic structure and the topology of catalyst surfaces. Heterogeneous catalyst surfaces are often quite dynamic and readily undergo significant alterations under working conditions. Thus, monitoring the surface chemistry of heterogeneous catalysts under industrially relevant conditions such as elevated temperatures and pressures requires dedicated in situ spectroscopy methods. Due to their photons-in, photons-out nature, vibrational spectroscopic techniques offer a very powerful and a versatile experimental tool box, allowing real-time investigation of working catalyst surfaces at elevated pressures. Infrared reflection absorption spectroscopy (IRAS or IRRAS), polarization modulation-IRAS and sum frequency generation techniques reveal valuable surface chemical information at the molecular level, particularly when they are applied to atomically well-defined planar model catalyst surfaces such as single crystals or ultrathin films. In this review article, recent state of the art applications of in situ surface vibrational spectroscopy will be presented with a particular focus on elevated pressure adsorption of probe molecules (e.g. CO, NO, O2, H2, CH3OH) on monometallic and bimetallic transition metal surfaces (e.g. Pt, Pd, Rh, Ru, Au, Co, PdZn, AuPd, CuPt, etc.). Furthermore, case studies involving elevated pressure carbon monoxide oxidation, CO hydrogenation, Fischer–Tropsch, methanol decomposition/partial oxidation and methanol steam reforming reactions on single crystal platinum group metal surfaces will be provided. These examples will be exploited in order to demonstrate the capabilities, opportunities and the existing challenges associated with the in situ vibrational spectroscopic analysis of heterogeneous catalytic reactions on model catalyst surfaces at elevated pressures.

Spectroscopic Evidences of Charge Transfer Phenomena and Stabilization of Unusual Phases at Iron Oxide Monolayers Grown on Pt(111)

Strong electrostatic coupling between the local structure of metal-supported oxide ultra-thin films and the charge state of adsorbed species (such as Au atoms and O2 molecules) is at the origin of a co-existence of alternative adsorption configurations, with drastically different structural and electronic characteristics. This effect may contribute also to the stabilization of oxide phases of unusual stoichiometries, where the ion valences are compensated by an electron exchange with the metal substrate. We report an analysis of the spectroscopic signatures of such differently charged adsorbates and unusual oxide phases, including valence and core electrons spectroscopies and vibrational characteristics. We show that the adsorbate charge states can be identified by their signature in the electronic states around the Fermi level and by the modification of the vibrational frequencies of adsorbed probe molecules. Moreover, core level analysis may contribute to the identification of the oxygen-rich phase, formed at high oxygen exposure. The detailed theoretical investigation of the spectroscopic signatures of alternative adsorption configurations and adsorbate charge states supplies a set of data, which may help the experimental identification of such species.

Momentary Equilibrium in Transient Kinetics and Its Application for Estimating the Concentration of Catalytic Sites

We describe the novel concept of momentary equilibrium (ME), a special event during a pulse-response transient experiment in which the non-steady-state rates of adsorption and desorption of a probe molecule are instantaneously balanced. In the absence of other reactions, any system with reversible adsorption will always pass through ME during a pulse-response experiment with effusion. We also suggest a new method for measuring the concentration of adsorption sites on heterogeneous catalysts and the corresponding equilibrium constant by observing momentary equilibrium in thin-zone temporal analysis of products (TZTAP) pulse-response experiments with modulated pulse intensity. The suggested method employs reversible adsorption of probe molecules, contrary to traditional methods of counting adsorption sites which utilize irreversible reactions.

Effect of Surface Acid Properties of Modified VOx/Al2O3 Catalysts on Methanol Selective Oxidation

Effect of surface acid properties of HNO3 and NaOH modified VOx/Al2O3 catalysts on methanol selective oxidation was studied by solid-state nuclear magnetic resonance and X-ray powder diffraction (XRD) spectroscopy. The surface acidity including type and strength of the acid sites in the catalysts was characterized by the adsorption of probe molecules. Compared with the VOx/Al2O3 catalyst, stronger Bronsted acid sites on the acid-treated VOx/Al2O3 catalyst lead to higher selectivity for DMM, while the base-treated VOx/Al2O3 catalyst with no Bronsted acid site gives rise to formate instead of DMM.

Graphene-Thickness-Dependent Graphene-Enhanced Raman Scattering

Graphene-enhanced Raman scattering (GERS), enhancing Raman signals on graphene surface, is an excellent approach to investigate the properties of graphene via the Raman enhancement effect. In the present study, we studied the graphene-thickness dependent GERS in detail. First, by keeping molecule density on few-layer graphene using vacuum thermal deposition method, GERS enhancement was found to be the same for all graphene layers (one to six layers). While adsorbing probe molecules by solution soaking, the GERS enhancing factor was different on monolayer and bilayer graphene. By soaking in low concentration solutions, the GERS intensity on bilayer graphene was stronger than that on monolayer graphene, whereas by soaking under high concentration solutions, the GERS intensity difference was much less for that on monolayer and on bilayer. Molecule density, molecular configuration, and GERS enhancing factor are further discussed for molecules on monolayer and bilayer graphene. It was finally concluded that th...

Study of thermodynamic characteristics of CO adsorption on Li exchanged zeolites

The experimental thermodynamic characteristics of CO probe molecule adsorption on lithium exchanged zeolites of MFI, FAU, MCM-58, MCM-22, MCM-36 and MCM-49 structure was investigated by joint volumetry-calorimetry method. Consideration was given to the interpretation of the heat evolved when a probe molecule was adsorbed on the surface. In particular, the number and strength of adsorption sites were discussed as functions of zeolite structure and concentration of extra-framework cation. In addition, the obtained thermodynamic data and energy distribution functions were discussed together with results obtained from FTIR spectroscopy and theoretical DFT calculations.

The effect of MFI zeolite lamellar and related mesostructures on toluene disproportionation and alkylation

The effect of MFI zeolite nanosheet morphology and related mesostructures on the accessibility of acid sites evaluated from FTIR spectra of different adsorbed probe molecules and conversions and selectivities in toluene disproportionation and alkylation with isopropyl alcohol was investigated. Two-dimensional MFI lamellar zeolite (1.5 nm thickness of layers) and related mesoporous sponge morphologies, most probably with 10-ring channel system not yet structurally fully determined, were synthesized and characterized by XRD, SEM, TEM, and adsorption measurements. FTIR spectroscopy using pyridine, deuterated acetonitrile and 2,6-di-tert-butyl pyridine evidenced high concentrations of accessible Lewis acid sites at the expense of Bronsted acid sites. Ordered mesoporous zeolites exhibited slightly higher toluene conversion in its alkylation with isopropyl alcohol than lamellar MFI or conventional MFI due to easy access to active sites. The lamellar zeolite consisting of less than a unit cell layer was slightly more active than MFI but still exhibited high selectivity to p-cymene. It evidences that alkylation of toluene with isopropyl alcohol proceeds mainly in the 10-ring channel system of MFI zeolite, although the effect of large external surface cannot be neglected. Increased para-selectivity compared with thermodynamic value is achieved even in the case of extremely thin zeolite nanosheets.

Understanding CeO2 as a Deacon catalyst by probe molecule adsorption and in situ infrared characterisations

CeO(2) has been identified as an efficient catalyst for HCl oxidation in the temperature range of 623-723 K provided that the oxygen content in the feed mixture was sufficiently high to avoid bulk chlorination and thus deactivation. Here we characterise ceria in its fresh and post-reaction states by adsorption of CO(2), NH(3) and CO. Micro-calorimetry, FTIR and TPD experiments are complemented by DFT calculations, which assess adsorption energies and vibrational frequencies. The calculations were performed on the lowest energy surface, CeO(2)(111), with perfect termination and with various degrees of hydroxylation and/or chlorination. Both experiments and calculations suggest that the basic character of the ceria surface has been eliminated upon reaction in HCl oxidation, indicating that most of the basic lattice O sites are exchanged by chlorine and that the OH groups formed are rather acidic. The density and the strength of surface acidic functions increased significantly upon reaction. An in situ FTIR reaction cell has been designed and constructed to study the evolution of OH group density of the ceria surface during HCl oxidation. The effect of experimental variables, such as pO(2), pHCl and temperature, has been investigated. We found that the OH group density positively correlated with the reactivity in the pO(2) and temperature series, whereas negative correlation was observed when pHCl was varied. Implications of the above observations to the reaction mechanism are discussed.

Influence of Surface Acidity of Y Zeolites on the Adsorption of Organic Molecules by <em>In situ</em> Fourier Transform Infrared Spectroscopy

The surface acidity of Y-type zeolites (HY and NaY) modified by solid state ion exchange (SSIE) and liquid phase ion exchange were characterized by in situ Fourier transform infrared (FTIR) spectroscopy using pyridine as the probe molecule (Py-FTIR). The adsorption of single probe molecules (thiophene, cyclohexene and benzene) and double probe molecules (thiophene and cyclohexene, thiophene and benzene) compared with sorbents were studied using in situ FTIR spectroscopy. The results indicated that the Bronsted acid (B acid) of HY (L-CeY) is the catalytic center of cyclohexene dimerized alkenyl carbenium ions and thiophene oligomerization, while the Lewis acid (L acid) is the major center of adsorption of thiophene, cyclohexene, and benzene. In addition, there is strong chemisorption and competitive adsorption of cyclohexene and thiophene, which provides evidence for the poor performance [Article] doi: 10.3866/PKU.WHXB201303183 www.whxb.pku.edu.cn 物理化学学报(Wuli Huaxue Xuebao) Acta Phys. -Chim. Sin. 2013, 29 (6), 1273-1280 June Received: January 2, 2013; Revised: March 18, 2013; Published on Web: March 18, 2013. ∗Corresponding author. Email: lsong56@263.net; Tel/Fax: +86-24-56860658. The project was supported by the National Natural Science Foundation of China (20976077, 21076100), Major Program of Petroleum Refining of Catalyst of Petro China Company Limited (10-01A-01-01-01), Innovation Team of Liaoning Province Colleges, China (200T110), and Construction Project of Liaoning Province High-end Talent Team. 国家自然科学基金(20976077, 21076100),中国石油天然气股份有限公司炼油催化剂重大专项(10-01A-01-01-01),辽宁省高校创新团队项目 (200T110)及辽宁省高端人才队伍建设项目资助 C Editorial office of Acta Physico-Chimica Sinica 1273 Vol.29 Acta Phys. -Chim. Sin. 2013 of removing sulfur. The S-CeY zeolite has abundant of weak Lewis acid sites. The sorbent is good at absorbing thiophene, while the influence of the competition adsorption of cyclohexene was not predominately. As to NaY zeolite, there is no preference for adsorption of thiophene, cyclohexene and benzene.

Low-Pressure Hysteresis in Adsorption: An Artifact?

Hysteresis phenomena are usually observed in the characterization of porous solids using gas adsorption of polar and nonpolar probe molecules. Commonly, hysteresis phenomena take place at high relative pressures due to the presence of metastable states associated with the capillary condensation of the probe molecule on mesopores. However, low-pressure hysteresis phenomena have also been reported for porous solids such as ordered mesoporous silicas, zeolites, and activated carbons. Unfortunately, the reason behind these processes taking place at low pressure is still unclear. Here we prove that the low-pressure hysteresis is rather an artifact associated with the lack of equilibrium in the adsorption isotherm and/or the lack of proper outgassing, mainly found in porous solids where narrow pore constrictions are expected.

The effect of synthesis gel dilution on the physico-chemical properties of acid saponite clays

Acid saponite samples prepared by varying H2O/Si molar ratio of the synthesis gel are reported in this work aiming to tune their particles size and surface properties. In particular, H2O/Si molar ratio of 20, 50, 100 and 150 were used and the obtained samples were submitted to an ion exchange procedure in 0.01M HCl solution in order to replace the interlayer Na+ ions by protons. A combined experimental approach (XRD, HRTEM, N2 physisorption and solid-state MAS NMR) was used to finely characterize the synthesized materials.FTIR spectroscopy of adsorbed probe molecules with different basicity (e.g., CO and NH3) was used to monitor the surface acid properties (i.e. number and strength of acid sites) and to assess the acid site distribution from weak to strong acidity. Structural data evidenced that samples prepared by using H2O/Si ratio of 20, 50 and 110 are characterized by a layered structure typical of saponite. Moreover, the increase of synthesis gel dilution leads to a progressive reduction of the particles sizes along with cation exchange capacity (CEC), which depends on the gradual decrease of aluminum concentration in the tetrahedral sheets as found by 27Al MAS NMR. In addition, the presence of an amorphous phase was observed for the saponite sample prepared with H2O/Si=150.IR spectroscopy of adsorbed NH3 and CO probes pointed out that the H-exchanged samples prepared with higher synthesis gel dilution presented a reduced concentration of Brønsted acid sites, according to CEC data.

Total oxidation of propane on Pt/WOx/Al2O3 catalysts by formation of metastable Ptδ+ species interacted with WOx clusters

A series of Pt/Al2O3 catalysts with various tungsten oxide loadings were prepared by a stepwise wet impregnation method. The catalysts were characterized by X-ray diffraction, nitrogen physisorption, Raman, UV–vis diffuse reflectance, transmission electron microscopy and infrared spectroscopy of adsorbed probe molecules (CO, NH3 or C3H8). The propane oxidation activity of Pt/Al2O3 catalyst is significantly improved by the addition of tungsten oxide. The tungsten oxide overlayer is presented as monomeric/polymeric WOx clusters and WO3 crystals depending on the loading amount. The most active catalyst occurs at an intermediate surface tungsten density corresponding to the maximum of polytungstate species. The electronic interactions between Pt and WOx clusters lead to the generation of more reducible Ptδ+ species which are suggested to be active sites for propane oxidation. Basically, a simple model is proposed involving the initial CH bond activation at the platinum–tungsten oxide interface.

Surface-Enhanced Raman Scattering on Hierarchical Porous Cuprous Oxide Nanostructures in Nanoshell and Thin-Film Geometries.

Understanding the mechanism of surface-enhanced Raman scattering (SERS) of molecules on semiconductor nanostructures is directly related to our capabilities of designing and optimizing new SERS-active substrates for broad applications in the field of molecular detection and characterization. Here, we present an exploration of using cuprous oxide nanostructures with hierarchical porosity for enhancing Raman signals of adsorbed probe molecules. Distinct SERS signals were detected on both individual polycrystalline nanoshells and porous thin films composed of cuprous oxide nanocrystals. The observed enhancement of SERS signals can be interpreted as synergistic effects of strong chemical interactions between the probe molecules and cuprous oxide surfaces, localized electromagnetic field enhancement, and the unique hierarchical porosity of the nanostructures. Our work introduced a novel type of semiconductor substrates for high-performance SERS and extended the applications of cuprous oxide nanostructures to spectroscopy-based molecular sensing and characterizations.

Transesterification of rapeseed oil under flow conditions catalyzed by basic solids: M[sbnd]Al(La)[sbnd]O (M = Sr, Ba), M[sbnd]Mg[sbnd]O (M = Y, La)

Binary composites on basis of MAlO (M=Sr, Ba), MMgO (M=Y, La), MLaO (M=Sr, Ba) prepared by precipitation and calcined at 700–1200°C were characterized by thermal analysis (TG–DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy of adsorbed probe molecule CDCl3 (FTIR), low temperature nitrogen adsorption and heat desorption of argon and their catalytic activity towards the transesterification reaction of rapeseed oil was studied in the flow fixed bed reactor at 200°C and 20atm. The phase composition, texture and surface composition of the catalysts are determined by their chemical nature and calcination temperature. The performance of the catalysts calcined at 700–750°C diminishes in the order of SrLaO>BaLaO>LaMgO≥YMgO>BaAlO700>SrAlO700 and is in a straight correlation with the integrated intensity of absorption of surface carbonate species representing the amount of the strong basic sites on the surface of the catalysts. Thermal treatment of MAlO (M=Sr, Ba) catalysts at 1200°C exerts minor influence on their performance.

Assessing the adsorption selectivity of linker functionalized, moisture-stable metal–organic framework thin films by means of an environment-controlled quartz crystal microbalance

The stepwise thin film deposition of the robust, hydrophobic [Zn(4)O(dmcapz)(3)](n) (dmcapz = 3,5-dimethyl-4-carboxy-pyrazolato) is reported. The adsorption of small organic probe molecules, including alkanols, toluene, aniline and xylenes, was monitored by an environment-controlled quartz crystal microbalance setup. The adsorption selectivity was tuned by introducing alkyl side chains in the dmcapz linker.