Norman Sheppard

Possible importance of a “metal-surface selection rule” in the interpretation of the infrared spectra of molecules adsorbed on particulate metals; infrared spectra from ethylene chemisorbed on silica-supported metal catalysts

The infrared spectrum attributed to the presence of an MCH2CH2M species (M = surface metal atom), formed by chemisorption of ethylene on the particulate (silica-supported) metals Pt, Pd, Ni and Rh, is remarkably simple. This simplicity can be fully accounted for if it is assumed hat only vibrations which give dipole changes perpendicular to the metal surface are infrared ctive. There are well-understood physical reasons why the latter is the case for molecules dsorbed on bulk metals, but such a “metal-surface selection rule” has not previously been used in interpreting spectra from molecules adsorbed on finely-divided metals. It is shown that the majority of previous interpretations of infrared spectra of species adsorbed on supported etal catalysts are unaffected by the suggested additional “selection rule”. On the other hand a number of anomalies in the literature can be accounted for by this means. Reasons are given for considering that such a selection rule may be operative for molecules adsorbed on rather small metal particles. The hypothesis appears to merit further investigation.

An infrared spectroscopic study of the adsorption and mechanism of surface reactions of 2-propanol on ceria

An infrared spectroscopic study was made of the adsorbed species and the gas-phase products from the dehydrogenation and dehydration reactions of 2-propanol over CeO2 catalysts calcined at different temperatures. The adsorbed species observed included hydrogen-bonded alcohol molecules, alcoholate ions, and carboxylate ions. The dehydrogenation reaction to form acetone was followed by a transformation of this product to give isobutene and methane. The dehydration reaction gave propene as the sole product. Mechanisms for the various surface reactions are proposed which are consistent with the infrared results.

Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 2.—Methanol over TiO2, ZrO2 and HfO2

Infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 °C from the dehydrogenation/dehydration reactions of propan-2-ol over a series of differently calcined catalysts of TiO2, ZrO2 and HfO2. The ZrO2 and HfO2 results were independent of the calcination pretreatment, and the surfaces of these oxides, like that from a TiO2 sample calcined at 800 °C, were dehydroxylated. Different results were obtained from a TiO2 sample calcined at 300 °C which had a hydroxylated surface. The acidic sites and reactivities of the surfaces of TiO2(300 °C) and TiO2(800 °C) were explored by pyridine adsorption and infrared spectroscopy. Only Lewis-acid sites were detected by pyridine.On raising the reaction temperature, in all cases the dehydrogenation reaction to give acetone occurred either before or simultaneously to the onset of the dehydration reaction to give propene. Acetone production was most pronounced over ZrO2 and HfO2 but also occurred more with TiO2(800 °C) than with TiO2(300 °C). The dehydrogenation reaction was largely quenched by pre-adsorbed pyridine on both TiO2 samples. The TiO2(300 °C) catalyst showed the presence of adsorbed propan-2-ol and 2-propoxide groups at room temperature. The dehydroxylated ZrO2, HfO2 and TiO2(800 °C) samples only showed appreciable amounts of 2-propoxide groups. In each case the 2-propoxide ions occurred in two different forms, presumably formed by adsorption on different types of sites.Both the acetone and propene products appeared as absorptions from 2-propoxide surface species decreased in intensity, so the latter are clearly reactive species. Gas-phase acetone production was followed by the chemisorption of acetone at a higher temperature. This subsequently decomposed to give surface acetate species, and finally at 400 °C to give CO2 and methane in the gas phase. Propene did not give rise to adsorbed species, or to further products in the gas phase.At the higher temperatures, above 300 °C, the reaction was always selective in favour of the dehydration reaction. However, each of the dehydroxylated catalysts showed some selectivity in favour of the dehydrogenation reaction over the earliest temperature range for alcohol decomposition, between 200 and 250 °C.A discussion is given of possible mechanistic pathways for the production of surface 2-propoxide species and the two types of products, based on the infrared-supported assumption that the different adsorbed forms of 2-propoxide [and possibly adsorbed propan-2-ol on TiO2(300 °C)] are reactive intermediates.

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.

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C2D2 spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.

Infrared study of the surface reactivity of hematite

Abstract Infrared spectra of α-Fe 2 O 3 powders (obtained by thermal decomposition of crystalline goethite α-FeOOH) and of α-Fe 2 O 3 with adsorbed electron-donor organic molecules (pyridine, acetonitrile, acetamide, acetic acid, acetic anhydride, acetyl chloride, acetaldehyde and ethyl alcohol) were measured in a study of surface structure and reactivity. The experimental results give evidence that organic molecules put into contact with α-Fe 2 O 3 activated in air or oxygen can generally be both physisorbed and chemisorbed on the surface. However, several molecules can also undergo chemical transformations by reaction with surface OH groups or by oxidation. Two different kinds of oxidizing sites are shown to exist on the activated surface, the first corresponding to structural oxygen atoms and the second to molecular oxygen chemisorbed on Lewis acid sites.

Vibrational spectroscopic characterisation of hydrogen bridged between metal atoms: A model for the adsorption of hydrogen on low-index faces of tungsten

Abstract Vibrational spectra of hydrogen chemisorbed on metal surfaces are discussed in the context of data from model cluster compounds and adsorption sites for hydrogen on low index planes of tungsten are suggested on the basis of reported electron energy loss spectra. A hydrogen adsorption site bridging two metal atoms is proposed for both the low coverage, β 2 , and the high coverage, β 1 , states of hydrogen on W(100) and the change in the symmetric metal-hydrogen stretching frequency is correlated with the reconstruction of the tungsten surface in the β 2 state.

Reduction of fluorescence from high-area oxides of the silica, γ-alumina, silica-alumina, and Y-zeolite types and Raman spectra for a series of molecules adsorbed on these surfaces

It is shown that strong fluorescence that occurs with untreated high-area samples of porous silica-glass, silica gel, Cab-O-Sil, γ-aluminas, a silica-alumina and sodium Y zeolite is in all cases eliminated or greatly reduced by heating in oxygen for several hours at 500 °C. This fluorescence is attributed to traces of hydrocarbons decomposed on the acidic oxide surfaces that burn away in oxygen. Some degree of fluorescence returns when alumina-containing oxides are heated at elevated temperatures in a normal high vacuum; this is attributed to slow migration of hydrocarbon impurities. Certain organic molecules, such as furan and acetone, also give rise to new fluorescence, through decomposition, on contact with porous glass surfaces or while a Raman spectrum is being obtained. After heating in oxygen at 500 °C the aluminas or alumina-containing oxides exhibit a residual weaker fluorescence between 13,000 and 14,600 cm−, which may be attributed to traces of Fe3+ impurities. Raman scattering has been observed for all of the main bands active in pyridine, and variations in position, half-height width and relative intensity were observed. The bands in the 3000 and 1000 cm−1 regions from pyridine on the several forms of silica, when considered in relation to parallel infra-red studies, can be interpreted in terms of an initially adsorbed species involving a strong hydrogen bond between surface OH groups and the pyridine nitrogen lone-pair. Physically adsorbed pyridine, with a spectrum similar to that of the pure liquid, occurs at higher coverage. Three species are observed from pyridine on BDH γ-alumina, two of them similar to those found on silica, and the third probably held to the surface by a coordinate linkage involving the nitrogen atom. Pyridine adsorbed on NaY zeolite appears to be held to the cationic sites. Raman spectra have been obtained by adsorption of a number of other molecules on porous glass, including benzene, methyl iodide, and several substituted benzenes. In the early stages of adsorption benzene and aniline gave spectra notably different from those of the pure liquids, and possibly attributable to species held to the surface through hydrogen-bonding involving surface OH groups.

Infrared and Raman spectra and the vCC stretching frequencies of some silver-olefin and platinum-olefin complexes

Abstract Raman spectra of aqueous solutions of some silver-olefin complexes and the i.r spectrum of a new platinum-olefin complex, [PtCl 2 (Me 2 CCMe 2 )] 2 , have been studied in an attempt to enlarge upon a recent suggestion that v CC in platinum-olefin complexes could be preferably assigned to a Raman band in the 1240 cm −1 region rather than at about 1500 cm −1 . We conclude that while it is possible to assign the 1240 cm −1 band in the platinum-ethylene complex to a mainly CC stretching vibration in the complexes with substituted olefins the 1500 cm −1 band should be assigned to a mainly CC stretching mode. However both these statements are over simplications, and the true situation is best discussed in terms of the percentage charge of each frequency on complex formation. The summed percentage charges give the best spectroscopic measure of the modification of the CC bond.

Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 3.—Ethanol over TiO2, ZrO2 and HfO2, and general conclusions from parts 1 to 3

The dehydrogenation and dehydration reactions of ethanol over TiO2, ZrO2 or HfO2 catalysts has been monitored in the gas phase and on the surfaces by infrared spectroscopy. The reaction pathways closely parallel those of methanol reported in the prevous paper (Part 2) with the addition of the direct dehydration reaction C2H5OH → C2H4+ H2O and the production of benzene as a minor product. The infrared spectroscopic analysis of the decomposition of diethyl ether as initial reagent over the TiO2(500) catalyst confirms that the ethane product is derived from the ether precursor. As with methane from methanol, it is probably produced by reduction of the ether to the alkane plus water by hydrogen derived from the parallel dehydrogenation reaction.A summary is given of probable mechanisms for the catalysed reactions of the three alcohols, methanol, ethanol and propan-2-ol based on the gas-phase products and surface species identified by infrared spectroscopy. The general importance of alkoxide surface intermediates is emphasised. Alkoxides of a given formula occur on different spectroscopically distinguishable sites with different reactivities.