Sándor Kristyán

Low temperature oxidation of CO over tin-modified Pt/SiO2 catalysts

Abstract Low temperature oxidation of CO over alloy type Sn–Pt/SiO 2 catalysts with different Sn/Pt ratios has been investigated at different CO partial pressure using thermal programmed oxidation (TPO) technique and time on stream (TOS) experiments. The introduction of tin into platinum strongly increased the activity of the catalyst. The activity had a maximum, which depended on both the Sn/Pt (at./at.) ratio and the CO partial pressure. TOS experiments revealed the aging of the Sn–Pt/SiO 2 catalysts. FTIR and Mossbauer spectroscopy has been used to follow compositional and structural changes of Sn–Pt/SiO 2 catalysts during the catalytic run. The results show that the in situ formed, highly mobile “Sn n + –Pt” ensemble sites are responsible for high activity, while formation of relatively stable SnO x type surface species are involved in the catalyst deactivation.

Theoretical study of the reaction of hydrogen with nitric acid: Ab initio MO and TST/RRKM calculations

Abstract The kinetics and mechanism of the H + HNO3 reaction have been elucidated with ab initio molecular orbital and statistical theory calculations. Our room temperature reaction rate results accord well with available experimental data. The reaction is dominated by an indirect metathetical process taking place via vibrationally excited dihydroxyl nitroxide, ON(OH)2, producing OH + cis-HONO. The excited ON(OH)2 also undergoes molecular elimination, yielding H2O + NO2 as a minor competing reaction. The direct H abstraction reaction forming H2 + NO3 was found to be the least important one. At atmospheric pressure, we recommend the following expressions for the three rate constants, in units of cm3/molecule s, from the 300–3000 K temperature range for H + HNO3 collision yielding the products H2 + NO3 by direct mechanism ka = (9.24 × 10−16)T1.53e−8253/T based on CTST calculations, OH + cis-HONO by indirect mechanism kb = (6.35 × 10−19)T2.30e−1.53/T), and H2O + NO2 by indirect mechanism kc = (1.01 × 10−22)T3.29e−3126/T, the latter two are based on Arrhenius fits to the solution of the master equation which includes RRKM microscopic rate constants and tunneling corrections.

IMMEDIATE ESTIMATION OF CORRELATION ENERGY FOR MOLECULAR SYSTEMS FROM THE PARTIAL CHARGES ON ATOMS IN THE MOLECULE

Abstract In the authors previous work (Chem. Phys. Lett. 247 (1995) 101 and Chem. Phys. Lett. 256 (1996) 229) a simple quasi-linear relationship was introduced between the number of electrons, N, participating in any molecular system and the correlation energy: −0.035 (N − 1) > Ecorr[hartree] > − 0.045(N −1). This relationship was developed to estimate more accurately correlation energy immediately in ab initio calculations by using the partial charges of atoms in the molecule, easily obtained after Hartree-Fock self-consistent field (HF-SCF) calculations. The method is compared to the well-known B3LYP, MP2, CCSD and G2M methods. Correlation energy estimations for negatively (−1) charged atomic ions are also reported.

Thermotropic and structural effects of poly(malic acid) on fully hydrated multilamellar DPPC–water systems

The thermotropic and structural effects of low molecular weight poly(malic acid) (PMLA) on fully hydrated multilamellar dipalmitoylphosphatidylcholine (DPPC)-water systems were investigated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and freeze-fracture transmission electron microscopy (FFTEM). Systems of 20wt% DPPC concentration and 1 and 5wt% PMLA to lipid ratios were studied. The PMLA derivatives changed the thermal behavior of DPPC significantly and caused a drastic loss in correlation between lamellae in the three characteristic thermotropic states (i.e., in the gel, rippled gel and liquid crystalline phases). In the presence of PBS or NaCl, the perturbation was more moderate. The structural behavior on the atomic level was revealed by FTIR spectroscopy. The molecular interactions between DPPC and PMLA were simulated via modeling its measured infrared spectra, and their peculiar spectral features were interpreted. Through this interpretation, the poly(malic acid) is inferred to attach to the headgroups of the phospholipids through hydrogen bonds between the free hydroxil groups of PMLA and the phosphodiester groups of DPPC.

Mechanistic study of the catalytic hydrogenolysis of ethane

A mechanistic investigation of the hydrogenolysis of ethane is presented. The chemisorptions of ethane and hydrogen produce a common surface species, adsorbed hydrogen, and the coverages of the two-carbon-atom surface compound and the adsorbed hydrogen are interdependent through the partial pressures of ethane and hydrogen. The kinetically slow rupture of the C—C bond takes place in an interaction with a free site, adsorbed hydrogen or molecular hydrogen. Based on a theoretical analysis and previous experimental results, our conclusion is that molecular hydrogen is the most probable agent in the bond splitting.

Fitting atomic correlation parameters for RECEP (rapid estimation of correlation energy from partial charges) method to estimate molecular correlation energies within chemical accuracy

The accuracy of the RECEP method [Chem Phys 1997, 224, 33 and Chem Phys Lett 1999, 307, 469] has been increased considerably by the use of fitted atomic correlation parameters. This method allows an extremely rapid, practically prompt calculation of the correlation energy of molecules after an HF‐SCF calculation. The G2 level correlation energy and HF‐SCF charge distribution of 41 closed‐shell neutral molecules (composed of H, C, N, O, and F atoms) of the G2 thermochemistry database were used to obtain the fitted RECEP atomic correlation parameters. Four different mathematical definitions of partial charges, as a multiple choice, were used to calculate the molecular correlation energies. The best results were obtained using the natural population analysis, although the other three are also recommended for use. For the 41 molecules, the G2 results were approached within a 1.8 kcal/mol standard deviation (the mean absolute difference was 1.5 kcal/mol). The RECEP atomic correlation parameters were also tested on a different, nonoverlapping set of other 24 molecules from the G2 thermochemistry database. The G2 results of these 24 molecules were approached within a 2.3 kcal/mol standard deviation (the mean absolute difference was 1.9 kcal/mol). This method is recommended to estimate total correlation energies of closed shell ground‐state neutral molecules at stationary (minimums and transition states) points on the potential surface. Extension of the work for charged molecules, radicals, and molecules containing other atoms is straightforward. Numerical example as a recipe is also provided.

New development in RECEP (rapid estimation of correlation energy from partial charges) method

Abstract We analyze the feasibility of extremely rapid estimation of correlation energy from the HF-SCF charge distribution in closed-shell molecules. In Kristyans previous work [Chem. Phys. 224 (1997) 33-51] a simple linear relationship using atomic correlations was developed in order to calculate correlation energy of molecules. This method has been further refined in this Letter. The proposed method is analyzed for 18 molecules and ions and the new results are compared to the B3LYP, CCSD and G2 results.

How can (semi)local density functional theory account for the ground‐state total energy of highly ionized atoms of the first three periods in the periodic table?

We investigate three generally used exchange‐correlation functions (good for most other properties) in order to discern their ability to reproduce ground‐state total energies of highly ionized atoms as well as the sum of their first two ionization energies. Total ground‐state energies of closed shell atoms with N electrons and Z atomic numbers are considered for 2≤N≤Z≤18, and N=2,4,6,8,10. The sum of the first two ionization energies, I1+I2, is calculated for closed shell atoms with Z=2,4,6,8,10. The density functional theory(DFT) methods investigated are remarkably successful in accounting for the ground‐state total energy of the ionized states of atoms, although their accuracy significantly varies with the positive charge of the ionized atom. Interestingly, the conventional Hartree–Fock self‐consistent field (HF‐SCF) method is more ‘‘rigid’’ with respect to this type of variance in accuracy. The Becke gradient corrected exchange function gives good results, but the Becke exchange with the Lee–Yang–Parr correlation function is better. However, there are some ionized states of atoms for which even the best density functional methods do not exceed the accuracy of the conventional Hartree–Fock SCF method. The simple Dirac–Slater functional gives poor results. The comparison of these methods to accurate ab initio calculations and experimental data are reported in detail. Interestingly, the accuracy of these methods (as a function of the degree of ionization) may reflect the shell structure of the atom.

Equipotential surfaces of the model reaction H+H 2 : struggling with three dimensions

The solution of the complete Schrodinger equation for scattering and electronic structure calculations is the objective of this work, focusing on the triatomic systems. The mathematical difficulties of solving the Schrodinger equation are analyzed, what can be done easily and what needs large scale computational work. An overview of basic atomic and molecular quantum mechanics is given, including how adiabatic and diabatic approximations are used in calculating scattering cross sections. Effective interaction potentials are discussed in detail, how to compute them and what they look like. Besides describing how computers have opened the door to real computation of the full spatial behavior of these multicomponent systems, some simple graphical images significantly aid the development of intuition of how the system actually behaves. The graphical representation focuses on the potential energy surfaces of the H3 electronic structure.

Specific behavior of the p-aminothiophenol--silver sol system in their Ultra-Violet-Visible (UV-Visible) and Surface Enhanced Raman (SERS) spectra.

The UV-Visible and Surface Enhanced Raman Spectroscopy (SERS) behavior of silver sol (a typical SERS agent) were studied in the presence of different bifunctional thiols such as p-aminothiophenol, p-mercaptobenzoic acid, p-nitrothiophenol, p-aminothiophenol hydrochloride, and 2-mercaptoethylamine hydrochloride in diluted aqueous solution. Our results confirm that the p-aminothiophenol induced aggregation of citrate stabilized silver colloid originates from its electrostatic nature, as well as the azo-bridge formation cannot be the reason of the observed time dependent UV-Visible spectra. Based on our parallel SERS and electrospray ionization mass spectrometry measurements, we have concluded that certain amount of oxidized form of the probe molecule has to be present for the so-called b2-mode enhancement in the SERS spectrum of p-aminothiophenol. Our findings seem to support the idea that the azo-bridge formation is responsible for the b2-mode enhancement in the SERS spectrum of p-aminothiophenol.