Carlos De La Cruz

Infrared spectroscopic comparison of the chemisorbed species from ethene, propene, but-1-ene and cis- and trans-but-2-ene on Pt(111) and on a platinum/silica catalyst

The chemisorption of ethene, propene, but-1-ene, cis- and trans-but-2-ene and but-2-yne has been studied on a Pt(111) surface using reflection absorption infrared spectroscopy (RAIRS). The results have been compared with transmission infrared spectra of the alkenes adsorbed on a finely divided impregnated Pt/SiO2 catalyst.On the Pt(111) surface ethene, propene and but-1-ene are adsorbed in the corresponding n-alkylidyne form CH3(CH2)nCPt3(n= 0, 1, 2) at room temperature. The cis- and trans-but-2-enes are adsorbed in the form of but-2-yne bonded to the surface in a di-σ/π fashion, as was confirmed by adsorption of but-2-yne itself.The room-temperature spectra of ethene, propene and but-1-ene on Pt/SiO2 all exhibited prominent absorptions from the n-alkylidyne, but other adsorbed species also contributed to the spectra, considered to be predominantly the appropriate di-σ species PtCH2CH(R)Pt (R = H, CH3, C2H5) together with smaller amounts of the relevant π-complexes. In the case of the cis- and trans-but-2-enes, virtually identical spectra were obtained on Pt/SiO2 but with many of the same prominent features as from but-1-ene. Clearly the finely divided catalyst had sites which led to double-bond isomerization. A smaller proportion of di-σ/π bonded but-2-yne was also present.The infrared spectra proved to be very effective at delineating the similarities and differences between the species chemisorbed on Pt(111) and on the Pt/SiO2 catalyst.

A numerical procedure for curve fitting of noisy infrared spectra

A method for the detection of overfitting of noisy spectra is presented. When fitting data that contains random noise, the autocorrelation function (RL) of residuals at lag 1 (R1) approaches zero and then shows a tendency toward more negative values, while the Wald–Wolfowitz test tends to give more positive values, as the data is overfitted. Models that give residuals with a non-random ordering may be rejected. The use of these functions for the determination of the “best” fitting of several approximations (Savitzky–Golay smoothing, segmented osculating polynomials (SOPA), Fourier series, and Lorentzian bands) to a noisy synthetic spectrum, and the application to infrared spectra of coal, is shown.

IN SITU IR SPECTROSCOPIC STUDY OF THE ADSORPTION AND DEHYDROGENATION OF ETHENE ON A PLATINUM-ON-SILICA CATALYST BETWEEN 100 AND 294 K

A new cell capable of obtaining infrared spectra of molecules adsorbed on catalysts within a wide temperature range has been described. The cell was used to obtain spectra between 100 and 294 K from physically and chemically adsorbed ethene on a 16% w/w platinum-on-silica catalyst with metal particle sizes predominantly in the range 5–15 nm.Between 129 and 189 K, the spectra of the chemisorbed species were attributed to the presence of two closely related π-adsorbed ethene species with absorptions at 3022(medium)/1500(medium) and 3016(medium to weak)/1496(medium to weak) cm-1, respectively; and to another pair of MCH2CH2M adsorbed species (M=metal atom), designated di-σ* and di-σ, respectively, with absorptions at 2922(medium to strong)/2830(weak)/ca. 1428(medium to strong, broad) and 2906(medium to strong)/ca. 2820/ca. 1419(medium to strong, broad) cm-1.Between 206 and 294 K, the spectrum of the di-σ species was gradually replaced by new absorptions at 2883(strong)/2795(weak)/1342(very strong) cm-1 attributed to the ethylidyne species CH3CM3 that widely occurs on triangular adsorption sites of [111] faces. It is deduced that the di-σ species also occurs on facets of this type on the metal particles. It is assumed that the more stable di-σ* species occurs on other types of facet such as [100] or [110]. An alternative, but less preferred attribution to the di-σ* designated spectrum is to an ethylylidyne species, M3CCH2M.A strong shoulder to the 2922/2906 cm-1 di-σ*/di-σ absorptions at ca. 2958 cm-1 is tentatively attributed to another ν(CH2)-s mode of the π-adsorbed species. A less-preferred alternative would be to a ν(CH3)-as of an ethyl group. Reasons are given for not attributing this absorption to an ethylidene species, CH3CHM2.

The prediction of coal properties using compressed infrared data from osculating polynomials

Abstract Reduction of the representation of infrared spectra from coal samples by osculating polynomials of degree nine is discussed. The reduced representation contains polynomial coefficients of order zero to four. Mathematical models of the original spectra are obtained by linear combination of the coefficients. These compressed models are statistically correlated to coal properties, namely, volatile matter, fixed carbon, ash content, heating value, hydrogen, carbon, sulphur, nitrogen, and maximum vitrinite reflectance, and the results are compared with those previously obtained from second derivatives of the same spectra. The use of compressed data, while giving slightly better correlations for some of the properties, has the advantage of requiring less computational time.

Determination of chemical properties of pyrolysis products from coals by diffuse-reflectance infrared spectroscopy and partial least squares

Abstract Typical coal samples from Venezuela were pyrolysed in nitrogen atmosphere at temperatures between 150 and 900°C. Structural changes were monitored by diffuse-reflectance infrared Fourier transform spectroscopy, i.e. DRIFTS. Segmented osculating polynomial approximation (SOPA) and numerical differentiation by the Savitzky–Golay procedure were used as spectroscopic data pre-treatment. The data was then correlated by partial least squares (PLS) and iterative predictor weighting (IPW) with volatile matter, hydrogen, carbon, nitrogen, and total sulphur contents. The method was found to be useful for the prediction of volatile matter and hydrogen contents, but not to the other physical and chemical properties of the pyrolysed products.

Adsorption and dehydrogenation of ethane on a silica-supported platinum catalyst monitored in the temperature range 90–294 K by FTIR spectroscopy

FTIR spectroscopy has been applied to study the adsorption and dehydrogenation of ethane on a silica-supported platinum (Pt/SiO2) catalyst with metal particles diameters mostly in the range 5–15 nm and with predominantly [111] micro-facets. At ca. 90 K, there is clear spectroscopic evidence for the formation of a first monolayer of di-σ C2H4 species on the platinum particles, overlaid with a multilayer of ethane. On warming towards room temperature the multilayer gradually evaporates into the gas phase, and from ca. 130 K the di-σ C2H4 species transforms to ethylidyne, reaching saturation by 294 K. As the di-σ C2H4 to ethylidyne transformation is well known to occur on [111] surfaces, and as dissociation of ethane is not observed to occur on a single crystal Pt[111] surface at low temperature, it seems that the initial dissociation on the finely divided Pt/SiO2 catalyst occurs on non-[111] sites, followed by migration to the [111] facets.

An infrared spectroscopic comparison of cis- and trans-CHCH and vinyl CHCH2 group frequencies of some hexenes, heptenes, and stereospecific and non-stereospecific polybutadienes

Abstract The infrared bands of cis - and trans -CHCH and vinyl CHCH 2 groups of 1-hexene, trans -2-hexene, trans -3-hexene, 1-heptene, trans - and cis -2-heptene and cis -3-heptene are compared spectroscopically with the bands of the analogous chemical groups in six stereospecific and non-stereospecific polybutadienes with different cis -, trans - and vinyl microstructural composition. Special attention is given to the bands associated with the vibrations of the in-plane and out-of-plane CH deformation modes. The bands of the cis - and trans -CHCH groups appear at 1411-1405, 1310-1300, 1313-1310 and ≈ 1298 cm −1 for the cis ρ(CH)a, cis ρ(CH)s, trans ρ(CH)s and trans ρ(CH)a modes respectively of the olefins and polybutadienes. The trans ω(CH) mode appears at 975 cm −1 (with a band-width at half height of 16 cm −1 ) or 967 cm −1 (with a band-width of 21 cm −1 ) in the spectra of the olefins and the polybutadienes respectively. These values contrast sharply with those of the cis ω(CH) mode. The spectra of the olefins show that this mode absorbs infrared radiation at 713 cm −1 and has a band width at half height of 32 cm −1 for cis -2-heptene, or at 698 cm −1 with 44 cm −1 band-width for cis -3-heptene. The spectra of the polybutadienes show that the maximum absorption of this mode appears between 728 and 739 cm −1 with band-widths that vary between 70 and 40 cm −1 . These observations indicate that the exact position and band-width of the cis ω(CH) is therefore very dependent on the configuration of the adjacent units, and possibly of units further removed. The bands of the vinyl group appear at 1422-1419, 1308-1295, 995 and 913 cm −1 for the δ(CH 2 ), ρ(CH), trans ω(CH)ip and ω(CH 2 ) modes of the olefins and polybutadienes. The cis ω(CH)ip mode splits into two bands at 632 cm −1 and 554 cm −1 in the spectra of the olefins; these bands are associated with the cis and skew rotational isomers of the olefins. Unfortunately these bands are not observed in the spectra of the polybutadienes because of the strong and broad absorption of the cis ω(CH) mode.

Fourier-transform surface enhanced Raman scattering: A quantitative technique?

Abstract Iron(III) meso-tetraphenylporphine chloride when sampled as a compacted disc in silver powder is shown to display quantitative Fourier transform surface enhanced Raman scattering.

A Fourier-transform infrared spectroscopic study of the adsorbed species from perdeuteroethene on a deuterium-exchanged Pt-on-silica catalyst

Fourier-transform infrared spectroscopy has been applied to study the adsorption and deuteration of perdeuteroethene (C2D4) at 298 K on a deuterium-exchanged Pt/SiO2 catalyst. The spectra show absorptions associated withπ- and di-σ-bonded (C2D4) and with (CCD3), and a trace of (CCD2H), ethylidyne surface species. These were identified by comparison with the closely-corresponding pattern of absorptions from the adsorption of C2H4. They are also correlated with previous spectroscopic results from C2D4 adsorption on Pt single-crystals and from analogous ligands in organometallic compounds. Deuteration of the initially-adsorbed species gives only gas-phase C2D6 and a trace of C2D5H.

An infrared spectroscopic investigatio of the species chemisorbed from ethylene over a platinum/silica catalyst between 100 and 294 K; the observation of di-σ adsorbed PtCH2CH2Pt surface complexes

Changes in the i.r. spectra of chemisorbed species from ethylene on a Pt/SiO2 catalyst between 100 and 294 K allow the identification of the di-σ adsorbed species (3) at low temperatures with partial transformation to the ethylidyne species (2) at room temperature.