Ethylidyne Species Research Articles

Reflection-absorption infrared spectroscopy of ethylene on palladium (111) at high pressure

The adsorption of ethylene adsorbed on Pd(111) at ∼ 300 K is studied using reflection-absorption infrared spectroscopy which confirms the formation of an ethylidyne species because of the presence of vibrational mode at 1329 cm −1 with a less-intense peak at 1089 cm −1. The 1329 cm −1 methyl mode is well away from any vibrational modes of gas-phase ethylene, which allows the spectrum of the surface species to be collected in the presence of high pressures (up to ∼ 1 torr) of ethylene. These results reveal that ethylidyne is present on the surface in the presence of gas-phase ethylene and that there may be a slight increase in coverage. The width of the line, however, increases substantially by 5.3±0.4 cm −1 torr −1. This effect is ascribed to a loss of order in the ethylidyne layer probably caused by co-adsorption of ethylene.

Behaviour of a cyanide-derived Fe/Al 2O 3 catalyst during Fischer-Tropsch synthesis

The interaction of carbon monoxide and hydrogen with an alumina-supported iron catalyst has been studied at temperatures ranging from 300 to 523 K and at a pressure of 103 kPa. The species produced and adsorbed on the catalyst surface have been examined by infrared spectroscopy, while the chemical state of the iron within the catalyst was investigated by magnetization measurements and Mössbauer spectroscopy. At 300 K the main type of adsorbed species on the iron particle surface appeared to be molecular carbon monoxide and formate groups. The amount of the latter species increases considerably at elevated reaction temperatures up to 473 K. Changes in the infrared absorption bands of CO, and in the magnetization and Mössbauer data indicate that under the applied conditions the surface properties of the iron particles are affected by the synthesis gas, but that the bulk remains metallic. The increased Fischer-Tropsch activity at more elevated temperatures, viz. 473 and 523 K, is accompanied by significant changes in the in situ recorded infrared spectra. The formation of hydrocarbons is evident from the well developed absorption bands in the 3000-2800 cm −1 range, due to (a)symmetric stretching vibrations of CH 2 and CH 3 groups. Assignment of the various absorption bands in the 1700-1200 cm −1 wavenumber range is speculative due to (partial) coincidence of the bands and the gradual change in chemical composition of the catalyst particles. At higher temperatures the bulk of the iron particles reacts rapidly to a mixture of carbides. Initially the unstable ε-Fe 2C is formed which reacts to ε′-Fe 2.2C upon prolonged periods of time of reaction and/or increased reaction temperatures. These carbides appeared to be thermally unstable: upon heating in helium they are converted into a mixture of χ-Fe 5C 2 and metallic Fe. Interaction of the reduced iron catalysts with ethylene or methane at 523 K gives rise to absorption bands at 1555, 1345 and 1050 cm −1, which are assigned to CH x adsorbed species. Upon reaction with ethylene also a band at 2160 cm −1 was observed, which was assigned to an ethylidyne species.

The role of co-adsorbed metal atoms in the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury and dimethylzinc

The chemistry of methyl species resulting from the decomposition of dimethylmercury (DMM) and dimethylzinc (DMZ) on Pt(111) in the range 300–400 K has been investigated by temperature prograrnmed desorption (TPD) and Auger electron spectroscopy (AES). In each case at 300 K, dissociative adsorption of the precursor results in the formation of an adlayer of methylmetal (CH 3M) moieties. These species are thermally stable to around 350 K before decomposing to yield mainly gaseous products, methane and hydrogen, and surface bound metal atoms. For DMM, subsequent heating to 400 K or direct dissociative adsorption at 400 K results in the formation of ethylidyne species. Ethylidyne formation is not observed in the thermal chemistry of DMZ at temperatures below 400 K and only transiently in the chemistry at 400 K. Complementary TPD and AES data indicate that, for DMM, desorption of the mercury atoms produced by CH 3Hg decomposition is the limiting factor in allowing the prevailing C 1 species to couple to form ethylidyne. In contrast, AES evidence indicates that zinc atoms remain on the surface to temperatures in excess of 750 K and hence prevent CC coupling by blocking surface sites.

An infrared study of the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury

The chemistry of dimethyl mercury on a Pt(111) single crystal surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS). Dimethyl mercury appears to be highly reactive on Pt(111) and readily decomposes on the surface at temperatures of 100 K and above. Adsorption at 100 K initially occurs in a dissociative manner to produce CH 3 and CH 3Hg species on the surface, both of which are identified as having C 3v local symmetry. At higher exposures, molecular adsorption dominates with the HgCHg axis initially oriented parallel to the surface. This preferred orientation, however, does not persist into the multilayer. Thermal treatment of the surface layer results in multilayer desorption between 130 and 135 K, and no parent molecular species are observed beyond 160 K. Adsorption at 200 and 300 K produces an overlayer consisting primarily of CH 3Hg species, which are thermally stable to about 350 K. Subsequent heating to 400 K results in the formation of ethylidyne species which are characterised by RAIRS. Adsorption at 400 K results in the direct formation of an ethylidyne layer estimated to be about 85% of saturated coverage.

Reaction of Ethylene with Clean and Carbide-Modified Mo(110): Converting Surface Reactivities of Molybdenum to Pt-Group Metals

A comparative investigation of the surface reaction of ethylene with clean Mo(110) and carbide-modified Mo(110) has been carried out using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). As typically observed for early transition metals, the clean Mo(110) surface interacts very strongly with ethylene, as indicated by the decomposition of ethylene to produce C2H2 surface species at temperatures as low as 80 K. The surface acetylene species further decompose to atomic carbon and hydrogen at higher temperatures. The strong reactivity of the Mo(110) surface can be modified by the formation of carbide. The surface reactivity is modified in such a way that the reaction mechanism of ethylene on C/Mo(110) is very similar to those typically observed on Pt-group metal surfaces: At 80 K, ethylene molecules bond to the C/Mo(110) surface in the di-σ bonded configuration; a new surface reaction intermediate, which can be best described as ethylidyne species, is d...

The chemistry of methyl and ethyl radicals on Pt(111) from the decomposition of tri-alkyl bismuth compounds

The chemistry of trimethyl and triethyl bismuth compounds adsorbed on Pt(111) have been studied using thermal desorption spectroscopy, Auger electron spectroscopy, and high resolution electron energy loss spectroscopy. The tri-alkyl bismuth compounds adsorb molecularly on Pt(111) at 110 K. Upon heating, the R 3Bi compound decomposes and alkyl radicals are introduced to the surface. Alkyl radicals and bismuth atoms are delivered to the Pt(111) surface in a 3 : 1 (alkyl : bismuth) ratio. In the presence of Bi, it is expected that the alkyl radical chemistry is not perturbed substantially. Below multilayer coverage, the methyl and ethyl radicals, delivered to the surface by thermal decomposition of the parent, show similar trends in their chemistry. There is some fraction of parent (BiR 3) desorption at ~ 190 K The remaining methyl or ethyl radicals follow two reaction pathways. The first reaction channel is dehydrogenation of the methyl or ethyl radical leading to CH or ethylidyne species, respectively, remaining on the surface. The second channel is hydrogenation involving surface hydrogen produced by the dehydrogenation channel which leads to CH 4 or CH 3CH 3 formation, respectively. This pathway leads to reaction rate limited desorption of CH 4 with a peak temperature at 280 K and CH 3CH 3 with a peak temperature at 295 K.

Hydrogenation of isolated atoms and small clusters of carbon on Pt(111) surface: HREELS/TDS studies

The reaction of hydrogen with isolated atoms and small clusters of carbon adsorbed on Pt(111) surface was investigated by HREELS and TDS. The carbon adsorption layers were prepared by evaporation of carbon atoms onto the metal surface cooled down to 100 K. The surface carbon produced by this method reveals a high activity towards hydrogen: the reaction occurs at T ⩾ 170 K. The initial carbon concentration is found to determine the chemical content of products in the adlayer. At n c < 2 × 10 14 at/ cm 2, when isolated atoms C ads prevail in the initial adlayer, methine CH ads formation is chiefly observed revealing two bands in HREELS: δ(CH) at 800 and v( CH) at 2960 cm −1. The CH ads particles dissociate at 510 K leading to hydrogen evolution and formation of carbon islands with a graphite-like structure. At higher concentration, the carbon adlayer contains small clusters C x ads in addition to the isolated atoms leading to more complex reaction products. An ethylidyne species, ▪, is detected among the products with characteristic bands δ s(CH 3) at 1360 and v( CC) at 1130 cm −1. It is assumed that ethylidyne molecules are produced in the course of a consecutive hydrogenation of the C 2 ads cluster. Dehydrogenation of hydrocarbon surface species causes hydrogen evolution at T≈350, 410, 450, 510 and 600–700 K. Ethylidyne dissociation is associated with the desorption peak at T≈ 450 K; an ethynyl CCH ads being the product. It is shown that the highest temperature stage of the dehydrogenation in the adlayer occurring at T > 600 K is accompanied by an increase of the carbon content in the C x CH ads molecules, finally resulting in the formation of islands with a graphite-like structure.

Adsorption and thermal decomposition of ethene and propene on Ru(0001), studied by RAIRS

Ethene and propene adsorption on Ru(0001) has been studied using reflection-absorption infrared spectroscopy (RAIRS). Low temperature (∼ 100 K) adsorption of ethene is presumed to produce a di-σ adsorbed species but this was not detected. Low temperature adsorption of propene produced a physisorbed layer but at 130 K a weak spectrum attributed to a di-σ adsorbed species was observed. Ethene and propene both converted to alkylidyne species at higher temperature and propylidyne decomposed via the ethylidyne species.

Generation of alkyl fragments on Rh/SiO2 and VO x -promoted Rh/SiO2

The adsorption of ethylene on Rh/SiO2 and Rh-VOx/SiO2 has been studied by transmission FTIR spectroscopy. For C2H4 chemisorption at 298 K on Rh/SiO2 molecularly adsorbed C2H4 and ethylidyne species are identified. It has been found that ethylidyne can be hydrogenated toward surface ethyl species in a reversible reaction at room temperature. On Rh-VOx/SiO2 the fragmentation of C2H4 into methylene species has been observed during C2H4 adsorption at 298 K.

Reactions of halogenated hydrocarbons at platinum group metals. Part I: A DEMS study of the adsorption of CH 3CCl 3

The electrochemical adsorption of 1,1,1, tichloroethane on Pt in sulphuric acid was studied by on-line mass spectrometry. The adsorption is highest in the hydrogen region suggesting a reductive adsorption process. About 75% of the adsorbate can be desorbed cathodically; the products being ethane, butane and, to minor degree, hexane and possible higher alkanes. The ratio of these products depends on the adsorption potential. The only oxidation products is CO 2. H/D-exchange experiments suggest that the adsorbate is an ethylidyne species.

Vibrational coupling between ethylidyne species on platinum particles

Vibrational coupling between neighboring ethylidyne species adsorbed on a Pt/Al{sub 2}O{sub 3} catalyst has been detected using an equimolar mixture of {sup 13}CH{sub 3}C(a) and {sup 12}CH{sub 3}C(a). The coupling is detected by means of the development of increased intensity sharing in the symmetric C-H deformation mode for the chemisorbed ethylidyne species as the ethylidyne coverage increases. The results indicate that ethylidyne is produced from ethylene on (111) facets of small Pt particles, rather than on random trimer Pt sites which might also exist on the catalyst. Experiments on highly dispersed Pt catalyst surfaces have failed to detect ethylidyne production. 37 refs., 10 figs.

Diffuse reflectance infrared spectroscopy of adsorbates on supported metal catalysts

DRIFTS spectra of ethene adsorbed on a Ni/Al 2 O 3 catalyst show excellent sensitivity for adsorbed hydrocarbons across the mid infrared to 1100 cm − 1 , the black out of the support. For the Pt/SiO 2 catalyst EUROPT-1, the lower limit of the useful wavelength range in the DRIFTS spectra is 1400 cm − 1 , compared with 1300 cm − 1 in transmission. Spectra were recorded as a function of time after exposure of the Ni/Al 2 O 3 catalyst to ethene, revealing a very different chemistry on the supported catalyst to that expected from vibrational studies of ethene adsorbed on Ni single crystals. An ethylidyne species forms, rapidly decaying to three other surface fragments. One shows frequencies characteristic of the methyl group, while the others show methylene group vibrations.

Application of HREELS to model catalysts: carbon monoxide and ethene adsorption on platinum/alumina

Abstract : A model catalyst made by platinum vapor deposition onto a thin alumina film is investigated with Transmission Electron Microscopy and High Resolution Electron Energy Loss Spectroscopy. The preparation techniques are fully described, then the results of TEM analysis for platinum deposits from 0.3 monolayers to 1.7 monolayers are reported, for which clusters varying from 1.0 to 1.6 nanometers in diameter are observed. High Resolution Electron Energy Loss Spectroscopy spectra are reported for the cases of carbon monoxide and ethylene adsorption onto the model. For the case of CO adsorption, it is shown that the catalyst is stable when heated to 500 K. For the case of ethylene adsorption, pi and di-sigma species are identified for adsorption at 165 K, but on warming to 325 K, ethylidyne is only observed for the larger platinum deposits. This is first High Resolution Electron Energy Loss Spectroscopy observation of the ethylidyne species on alumina supported catalysts. (JG)

Hexakis(tert-butyldimethylsiloxy)ditungsten and its reaction with ethyne. Hydrogen atom transfer reactions involving bridging ethynyl, ethyne, vinyl and ethylidyne ligands. Crystal structures of [W2(OSiButMe2)6], [W2(OSiButMe2)6(µ-C2H2)(C5H5N)] and [W2(OSiButMe2)5(µ-CCH)

In hydrocarbon solvents [W2(OBut)6] and ButMe2Si(OH)(6 equivalents) react at room temperature to give [W2(OSiButMe2)6]1, with the elimination of ButOH. Compound 1 is a red, crystalline, hydrocarbon-soluble, air-sensitive compound and is a member of the family of ethane-like X3MMX3 compounds with W–W 2.324(3), W–O 1.89(1)–1.96(1)A and W–W–O 98–104°. Compound 1 and ethyne react in hexane–pyridine (py)(50:50 v/v) at –10 °C to give a dark brown compound, [W2(OSiButMe2)6(µ-C2H2)(py)]2, which has been fully characterized and shown to contain a central pseudo-tetrahedral W2(µ-C2) unit [W–W 2.653(2), W–C 2.05(2)–2.18(2) and C–C 1.45(3)A]. Compound 2 reacts in hydrocarbon solvents at room temperature to give [W2(OSiButMe2)5(µ-CCH)]3, with the elimination of one equivalent of ButMe2Si(OH) and py. The conversion of 2 to 3 in hydrocarbon solvents is suppressed by added py and the rate of reaction is negligible at and below –45 °C. Compound 3 contains a σ,π-ethynyl ligand and the central W2C2 moiety is planar and unsupported by bridging alkoxides [W–W 2.481(1)A, W–σ-C 1.97(2), W–η2-C 1.96(2) and 2.07(2) and C–C 1.41(3)A]. The conversion of 2 to 3 accounts for ca. 90% of the µ-C2-containing compounds but minor competition reactions yield both a σ,π-vinyl and an ethylidyne species formulated as [W2(OSiButMe2)7(µ-CHCH2)]4 and [W2(OSiButMe2)7(µ-CCH3)]5, respectively. Neither 4 nor 5 has been isolated in a pure form and evidence for the µ-vinyl and µ-ethylidyne complexes rests on 1H and 13C NMR data. From studies of reactions employing labelled ethyne it is shown that the formation of 3 with elimination of ButMe2Si(OH) is irreversible and that the µ-vinyl complex is formed by the reaction between the µ-ethyne complex 2 and ButMe2Si(OH). The µ-vinyl and µ-ethylidyne complexes are formed by independent paths and though they are isomers, being related by a 1,2-H atom shift, they are not interconverted in these reactions. It is proposed that a reactive µ-vinylidene intermediate is responsible for the formation of the ethylidyne ligand: W2(µ-HCCH)→ W2(µ-CCH2); W2(µ-CCH2)+ H → W2(µ-CCH3).

A HREELS investigation of ethylene on Pt model catalysts

The adsorption of C 2H 4 on supported Pt model catalysts is investigated with high resolution electron energy loss spectroscopy for the cases of Pt vapor deposited on an oxidized Al foil and a single crystal of TiO 2. At 160 K the HREELS spectra show evidence of the di-σ bonded ethylene species present on the supported Pt clusters, but upon warming to 325 K, only the TiO 2 supported model catalyst shows evidence of forming the ethylidyne species commonly seen on Pt(111) single crystals. The oxidized Al supported species begins to decompose by 250 K, and no spectra characteristic of ethylidyne has been seen. In the case of the TiO 2 supported model catalyst, we believe this is the first HREELS observation of ethylidyne on supported metal clusters.

Adsorption and thermal decomposition of ethylene on clean and carbon monoxide-covered Ir(110) surfaces

The adsorption and the thermal decomposition of ethylene have been investigated on clean and CO-covered Ir(110) surfaces at temperatures ranging from 170 to 600 K. HREELS, TDS and UPS methods are used. Adsorption on a clean surface at 170 K is found to lead to the formation of ethylidyne (≡C-CH3) whereas on a surface with a saturated CO coverage small amounts of ethylene are adsorbed molecularly at the same temperature forming a di-σ-bond. After heating, ethylidyne decomposes to CxH fragments (x = 1, 2), the process taking place at lower temperatures than on the (111) iridium surface. The presence of carbon monoxide on the surface enhances the temperature stability of the ethylidyne species by about 50° compared to that on a clean surface and hinders the formation of fragments. The stabilizing effect of carbon monoxide also concerns the decomposition products of ethylidyne.

Hydrogen isotope exchange in a coverage obtained after ethylene adsorption on an Ir(110) surface

The hydrogen isotope exchange in a coverage obtained after ethylene adsorption on Ir(110) has been investigated by HREELS, TDS and SIMS within a temperature range of 170–500 K at hydrogen (deuterium) pressures up to 1 × 10 −6 Pa. Hydrogenation products are found neither after ethylene adsorption on a hydrogen-precovered surface nor on a hydrocarbon coverage exposed to hydrogen (up to 2000 L). The exchange of hydrogen in the ethylidyne species formed on the surface after ethylene adsorption is a slow process, contrary to the exchange in the products (C x H fragments) of their decomposition. The exchange in CCH and CH species is rapid and takes place with the participation of atomically adsorbed hydrogen at temperatures up to about 400 K. The exchange processes are compared with those earlier observed on the Ir(111) surface.

ChemInform Abstract: Solid‐State NMR Study of Acetylene Adsorbed to Platinum on Alumina as Followed by Direct Observation Using CP MAS Methods. Differentiation Between Acetylene Adsorbed to Alumina and Platinum.

In a recent series of papers Slichter and co-workers have reported solid-state {sup 13}C and {sup 195}Pt NMR investigations of small molecules, e.g., acetylene and ethylene, adsorbed to platinum on {eta}-alumina. Of particular interest to us is the work involving the adsorption of acetylene. This work is significant because it verified that the principal species on the surface was not the ethylidyne, {triple bond}CCH{sub 3}, species proposed by Somorjai et al. The authors confirm that ethylidyne species are not present on Pt/{gamma}-Al{sub 2}O{sub 3} surfaces. However, we also prove that acetylene does chemisorb to alumina in the presence and absence of platinum metal. Finally, we conclude the principal species bound to the platinum is such that the bonding between the carbon atoms is intermediate between double and triple bonds. This latter conclusion is supported by IR experiments on the Pt/{gamma}-alumina surface.

The infrared spectra of some deuterium-substituted ethenes chemisorbed on Pt(111) and on a Pt/SiO 2 catalyst

Infrared spectra from the deuterium substituted ethenes, H 2CCD 2 and D 2CCD 2 adsorbed on Pt(111) at 360 K, and these plus HDCCDH on a finely-divided Pt/SiO 2 catalyst at ca. 300 K, allow a detailed analysis of the spectra from the isotopically-substituted ethylidynes. The RAIRS spectra from Pt(111) and the transmission spectra from Pt/SiO 2 (limited to > 1300 cm −1) are complementary in providing information about the vCC/δCH 3/δCD 3 and the vCH/ vCD regions respectively. The transmission spectra on Pt/SiO 2 also provide information about additional di-σ and π-adsorbed species on the finely-divided catalyst. The spectra from H 2CCD 2 show that there is isotopic scrambling between the adsorbed ethylidyne species, possibly mediated by the coexisting PtH/PtD species. There are indications that this does not apply to the nondissociative di-σ adsorbed species.

Solid-state NMR study of acetylene adsorbed to platinum on alumina as followed by direct observation using CP MAS methods. Differentiation between acetylene adsorbed to alumina and platinum

In a recent series of papers Slichter and co-workers have reported solid-state {sup 13}C and {sup 195}Pt NMR investigations of small molecules, e.g., acetylene and ethylene, adsorbed to platinum on {eta}-alumina. Of particular interest to us is the work involving the adsorption of acetylene. This work is significant because it verified that the principal species on the surface was not the ethylidyne, {triple bond}CCH{sub 3}, species proposed by Somorjai et al. The authors confirm that ethylidyne species are not present on Pt/{gamma}-Al{sub 2}O{sub 3} surfaces. However, we also prove that acetylene does chemisorb to alumina in the presence and absence of platinum metal. Finally, we conclude the principal species bound to the platinum is such that the bonding between the carbon atoms is intermediate between double and triple bonds. This latter conclusion is supported by IR experiments on the Pt/{gamma}-alumina surface.