Operando ATR-FTIR analysis of liquid-phase catalytic reactions: can heterogeneous catalysts be observed?

The applicability of a React-IR™ system, containing a diamond attenuated total reflection (ATR) crystal, in heterogeneous liquid-phase catalytic reactions was evaluated by analysis of the catalytic esterification of 1-octanol and hexanoic acid over a Nafion/silica catalyst in an open reflux configuration at atmospheric pressure. The reaction was performed in either cumene (at 427 K) or n-decane (at 447 K). The concentration profiles of the esterification reaction, as determined by this real-time in situ IR spectroscopic technique, are in qualitative and quantitative agreement with those determined by conventional off-line GC analysis.Interestingly, besides the bands assigned to the ester, alcohol, and acid, an additional strong and broad absorption band was observed at around 1100 cm⁻¹ in the spectra during the esterification reaction in cumene (at 427 K). It was assessed by variation of the reaction mixture that this band is a result of the reaction of silica with octanol, yielding Si–O–R functionalities. The relevance of this reaction for the kinetics of the studied catalytic esterification, is discussed. More importantly, the contribution of solid particles and leached species to the 1100 cm⁻¹ band is evaluated. Strong indications exist that solid catalyst particles are contributing to the spectra, implying that on-line analysis of intermediate species adsorbed on heterogeneous catalysts is in specific cases possible using the React-IR™ technique.     

Construction of the reaction networks for heterogeneous catalytic reactions: Fischer—Tropsch synthesis and related reactions

The key approaches to the generation of reaction networks for the synthesis of products from CO and H₂ are considered. The selection rules for the elementary steps on the surface of heterogeneous catalysts are formulated. Data on the surface compounds and steps related to reactions of CO and H₂ are analyzed and a set of transforms (models of elementary steps) for generation of the reaction network are selected. Eight variants of generation of reaction networks for the formation of C1 products with different sets of transforms (12 to 31) were tested in computer experiments, and eight reaction networks comprising 34 substances and 132 to 1647 elementary steps were obtained. The pathways to CO₂, CH₄, and CH₃OH and pairs of compounds CH₄, CO₂ and CH₄, HCOOH obtained from the reaction network (220 elementary steps) are compared with the published schemes.     

Why the model of non-interacting chains yields adequate description of spin crossover in space structures?

Considering many-body interactions in tetrahedral structures as perturbations of binary potentials by third bodies yields a free energy functional of the binary mixture equivalent to one earlier derived for spin crossover equilibrium in one-dimensional chains. Formal non-ideality parameters of this functional, the excess energy and asymmetries of splittings can be expressed in terms of molecular parameters based on binary potentials.     

Does the Schulman's titration of microemulsions really provide meaningful parameters?

In 1955, Bowcott and Schulman coined the term microemulsions and, in passing, proposed a cosurfactant titration of interfaces tailored for water-in-oil systems (Bowcott, J. E.; Schulman, J. H. Z. Elektrochem. 1955, 59, 283). This procedure, elegant and inexpensive, is accomplished by dilution with both oil and cosurfactant up to the onset of microemulsion formation. The rationale they proposed for this method should furnish, with high accuracy, the composition of reverse micelles and continuous bulk in the presence of cosurfactant partition. The present paper demonstrates, by means of pulsed gradient spin-echo NMR, that the Schulman's titration quantitatively describes the cosurfactant partition and that the titration path really corresponds to a dilution path for reverse micelles (at constant composition) dispersed in a continuous bulk (at constant composition).     

Note on “Electrochemical promotion of catalytic reactions”

A recent review published in this Journal of the electrochemical promotion of catalysis (EPOC or NEMCA effect) is discussed. Some key aspects of the effect’s phenomenology and physicochemical origin are reviewed and clarified and the interplay of catalytic kinetics and mass transfer limitations in EPOC studies under high vacuum is discussed.     

Synthesis of some new germasesquioxides by intramolecular catalytic Mannich-type reactions

A convenient procedure for the preparation of some new germasesquioxides containing phosphonyl groups carried by an intramolecular catalytic Mannich-type reaction is reported.     

Does catalytic activity of bence-jones proteins contribute to the pathogenesis of multiple myeloma?

Some Bence-Jones proteins have been found to be capable of hydrolyzing DNA, chromogenic amide substrates, such as benzoylarginine p-nitroanilide, and natural oligopeptides, such as arginine vasopressin. Patients who excrete Bence-Jones protein with the DNA-nicking activity have shown moderately severe symptoms. When incubated with LLC-PK1 (porcine kidney proximal tubule) cells, some Bence Jones proteins penetrated the cytoplasm, and entered the nucleus with little or no degradation of epitopes. Intranuclear Bence Jones proteins ultimately induced DNA fragmentation in situ and cell death. This cytocidal activity was not directly associated with the DNA-nicking activity, since Bence Jones proteins with no detectable DNase activity also produced cell death. These results, however, suggest that the biological activities of Bence Jones proteins described here makes a significant contribution to the development and/or deterioration of multiple myeloma.     

Does the gradient-regulated connection improve the description of correlated metal bond properties?

Gradient-regulated connection (GRAC) is a generalized gradient approximation exchange density functional designed by combining the revPBE and PW91 exchange functionals to impose their behaviors in the slowly- and fast-varying density regions, respectively. Such a construction allows one single density functional to accurately estimate both covalent and weak interactions occurring in main-group-based molecular systems. For the first time, the assessment of the performance of the GRAC exchange functional is extended to the modeling of various metal bond energy and structure properties. This assessment shows that when GRAC is coupled with the Perdew, Burke, Ernzerhof (PBE) correlation, the resulting exchange-correlation density functional is an excellent alternative to global hybrids to model bond dissociation energy, atomic electronic excitation energy, and bond length structure properties of single-reference metal bonds. It also shows that coupling with the Tognetti, Cortona, Adamo (TCA) correlation constitutes a robust approach to tackle energy bond properties of organometallic complexes with multi-reference character.     

Exact transient solutions of kinetics of first‐order reactions with end effects

The finite set of rate equations C _m,n ^' =α _n,n-1 C _m,n-1 (t)+α _n,n C _m,n (t)+α _n,n+1 C _m,n+1 (t), $$0 \leqslant m \leqslant N,0 \leqslant n \leqslant N,$$ where $$\alpha _{i,j}$$ are $\alpha _{j,j - 1} = A,\alpha _{j,j} = - \left( {A + B} \right),\alpha _{j,j + 1} = B$ , with $\alpha _{0,0} = - \alpha _{1,0} = - \alpha$ and $\alpha _{N,N} = - \alpha _{N - 1,N} = - b,\alpha _{0, - 1} = \alpha _{N,N + 1} = 0$ , subject to the initial condition $C_{m,n} \left( 0 \right) = \delta _{n,m}$ (Kronecker delta) for some $m$ , arises in a number of applications of mathematics and mathematical physics. We show that there are five sets of values of $a$ and $b$ for which the above system admits exact transient solutions.     

Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?

This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X.     

Does a high UV environment ensure adequate vitamin D status?

This study assesses the Vitamin D status of 126 healthy free-living adults aged 18-87 years, in southeast Queensland, Australia (27 degrees S) at the end of the 2006 winter. Participants provided blood samples for analysis of 25(OH)D (the measure of an individual's Vitamin D status), PTH, Calcium, Phosphate, and Albumin, completed a questionnaire on sun-protective/sun-exposure behaviours, and were assessed for phenotypic characteristics such as skin/hair/eye colour and BMI. We found that 10.2% of the participants had serum 25(OH)D levels below 25 nmol/l (considered deficient) and a further 32.3% had levels between 25 nmol/l and 50 nmol/l (considered insufficient). Our results show that low levels of 25(OH)D can occur in a substantial proportion of the population at the end of winter, even in a sunny climate. 25(OH)D levels were higher amongst those who spent more time in the sun and lower among obese participants (BMI>30) than those who were not obese (BMI<30). 25(OH)D levels were also lower in participants who had black hair, dark/olive skin, or brown eyes, when compared with participants who had brown or fair hair, fair skin, or blue/green eyes. No associations were found between 25(OH)D status and age, gender, smoking status, or the use of sunscreen.     

Brdička-type catalytic currents of hemoglobins: Part III. pH dependence of the catalytic current - an indication of quaternary structure changes?

Changes of the catalytic current of bovine, ovine, rabbit, porcine, human, dog and pigeon hemoglobins (Hb), differing in the number of SH groups from 2 to 10 per tetramer, were studied in ammonia buffer solutions covering the pH range between 8.6 and 11.0. Two Hb concentrations were used: 5.0 × 10⁻⁷ and 1.54 × 10⁻⁶M. In all cases, the pH rise was accompanied by an increase in the catalytic current in the form of two steps, until a maximum value of I_catal at about pH 10.6–10.8 was attained, after which a decrease followed. A similar course of this relationship was observed for both concentrations of all the hemoglobins. The shape of the I_catal = f(pH) curves resembled titration curves, from which common characteristic points were evaluated for all the hemoglobins, corresponding to pH values of about 9.8 ± 0.1 and 10.5 ± 0.1, done in a similar way as for pK. In the discussion, ionization of hemoglobins, complex formation with Co(II), denaturation, adsorption phenomena, in connection with the charge of Hb, quaternary structure changes, the accessibility and reactivity of SH groups and possible changes of the preparations prior to catalytic wave registration were taken into account. The most probable reason for formation of the second step in the I_catal = f(pH) curves was considered to be the dissociation of deoxy-Hb tetramers to dimers. In the catalytic process, depending on the conditions at least two types of hemoglobin-Co(II) complex of different character may participate. The drop in I_catal at high pH values was assumed to be the result of the competing action of ammonia on complexation processes involving hemoglobin and Co(II).     

Oscillatory kinetics of the permanganate oxidation of oxalic acid?

The oxidation of oxalic acid by permanganate satisfies the mechanistic criteria of the oscillatory kinetics. However, the oscillatory changes in the absorbancy eventually found are due to formation of colloidal and coagulated MnO₂ and not to the chemical events.

---

. , , MnO₂, .

     

Supercritical carbon dioxide: an inert solvent for catalytic hydrogenation?

Various surface species originating from the reaction between CO2 and H2 over Al2O3-supported Pt, Pd, Rh, and Ru model catalysts were investigated by attenuated total reflection infrared (ATR-IR) spectroscopy under high-pressure conditions. Two different spectroscopic cells were used: a variable-volume view cell equipped with ATR-crystal and transmission IR windows (batch reactor) and a continuous-flow cell also equipped with a reflection element for ATR-IR spectroscopy. The study corroborated that CO formation from dense CO2 in the presence of hydrogen occurs over all Pt-group metals commonly used in heterogeneous catalytic hydrogenations in supercritical CO2 (scCO2). In the batch reactor cell, formation of CO was detected on all metals at 50 and 90 degrees C, with the highest rate on Pt. Additional surface species were observed on Pt/Al2O3 at 150 bar under static conditions. It seems that further reaction of CO with hydrogen is facilitated by the higher surface concentration at higher pressure. In the continuous-flow cell, CO coverage on Pt/Al2O3 was less prominent than that in the batch reactor cell. A transient experiment in the continuous-flow cell additionally revealed CO formation on Pt/Al2O3 at 120 bar after switching the feed from a H2-ethane to a H2-CO2 mixture. The in situ ATR-IR measurements indicate that CO formation in CO2-H2 mixtures is normally a minor side reaction during hydrogenation reactions on Pt-group metal catalysts, and dense ("supercritical") CO2 may be considered as a relatively "inert" solvent in many practical applications. However, blocking of specific sites on the metal surface by CO and consecutive products can affect structure sensitive hydrogenation reactions and may be at the origin of unexpected shifts in the product distribution.     

Does an exact local exchange potential exist?

In Kohn–Sham density functional theory, equations for occupied orbital functions of a model state are derived from the exact ground‐state energy functional of Hohenberg and Kohn. The exchange‐correlation potential in these exact Kohn–Sham equations is commonly assumed to be a local potential function rather than a more general linear operator. This assumption is tested and shown to fail for the exchange potential in a Hartree–Fock model for atoms, for which accurate solutions are known. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of the precursor structure on the catalytic properties of the nickel—chromium catalysts of hydrogenation reactions

The influence of the Ni²⁺/Cr³⁺ ratio in the precursor compound on the formation of the catalyst structure and its transformation upon the thermal treatment and reductive activation in hydrogen was studied. The precursors with the cation ratio Ni²⁺/Cr³⁺ = (2.3–3)/1 represent a homogeneous system of the stichtite-type structure. The treatment of the precursors at T ～400 °C in an inert atmosphere forms a nanosized phase of the NiO-type structure with the lattice parameter a = 4.186±0.005 Å. At 600 °C the lattice parameter of this phase decreases to the tabulated value (a = 4.177±0.005 Å). The phase of nickel chromite of the cubic spinel structure with the lattice parameter a = 8.320±0.005 Å is also observed. Hydrogen activation of the catalyst preheated at 300 °C in an inert gas leads to the formation of Ni⁰ crystallites with a size of ～5.5 nm and a specific surface area of ～7.0 m² g^?1. This catalyst exhibits high activity and selectivity in benzene hydrogenation and preferential CO hydrogenation in the presence of CO₂. The catalysts with the ratio Ni²⁺/Cr³⁺ = 1/(2?3) containing nickel and chromium hydroxocarbonates as precursors are less active in the hydrogenation of benzene to cyclohexane.