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The discussion is devoted to some controversial points related to the aromatization of paraffins at high temperature (around 500°C), and low hydrogen pressure (1–8 bar) over non-acidic catalysts, in particular platinum plus a second component on a neutral(ized) support. Special attention is paid to those points which relate to the question why the second component enhances the stability of the dehydrocyclization (DHC) activity of supported platinum. We first consider the valence state of the second component. On the basis of, among other factors, the total hydrogen uptake we argue that at least part of the second component is present in the reduced state, while TPR points to alloy formation. A comparison of the catalytic and chemisorptive behaviour of bulk alloys with that of supported bimetallics indicates that modification (alloying) of the gas phase-exposed platinum surface is an essential action of the second component. This, in turn, suggests that the superior stability of bimetallic DHC catalysts is related to the catalytic properties of the alloys. Considering the changes in selectivity observed upon alloying we will argue that the most important one is the enhanced selectivity towards dehydrogenation of paraffins (olefins) to the corresponding olefins (poly-olefins), which leads to the question: “Are bimetallics under DHC conditions merely selective dehydrogenation catalysts?” From the invariance in TON for dehydrogenation per surface-exposed platinum atom up to very high dilution of platinum in gold (< 1% at Pt in Au), we infer that only one platinum atom is involved in the rate-determining step. We will extrapolate the aforementioned results to actual DHC catalysts and consider the proposition that the active site in PtSn, PtRe (S), Pt (S) and deactivated Pt (i.e. Pt (C)) DHC catalysts consists of one platinum atom embedded in an “inactive” matrix. Finally we will list some points deserving further study, amongst them the interpretation of the results of techniques such as TPR, the relative importance of different DHC pathways, the chemisorptive properties of alloys and the causes underlying the low hydrogen uptake of “overreduced” supported platinum catalysts.

We have determined the surface composition of three different solid solutions (5 at.% Ni-Au, 4 at.% Pd-Au and 14 at.% Pt-Au) and two stoichiometric compounds (Ni 3 Sn and Pt 3 Sn), all of them selected because of their significance in current catalytic work on group VIII/“inert metal” alloys. Low-energy ion scattering (LEIS), Auger spectroscopy (AES) and selective CO chemisorption, applied to Pt 3 Sn and Ni 3 Sn, yield consistent results. For both alloys the three techniques indicate surface enrichment in tin, which is more pronounced for Ni 3 Sn. Averaged over the three different techniques, the surface compositions work out at Pt 0.48 Sn 0.52 and Ni 0.21 Sn 0.79 respectively. When comparing these results with theoretical predictions we notice that the restricted bond-breaking theory, which takes into account the heat of sublimation of the pure elements only, predicts enrichment of the surfaces in tin. Satisfactory agreement between theory and experiments is found for Pt 3 Sn. LEIS applied to 4 at.% Pd-Au and 5 at.% Ni-Au yields surface compositions of 0.8 at.% Pd and 1.0 at.% Ni respectively, in fair agreement with predictions from the bond-breaking theory. AES is not suited to detect this segregation. LEIS is found to be inapplicable for 14 at.% Pt-Au. For this alloy AES and selective CO-chemisorption yield a surface composition between 5 and 10 at.% Pt, whereas the bond-breaking theory predicts that the surface should contain as little as about 0.01 at.% Pt. A comparison of the results with contemporary theories on surface segregation is included.

Abrupt changes in the reaction ambient from 12 CO H 2 to 13 CO H 2 are utilized to obtain transientkinetic information regarding the causes of activity decline in the methanation over Raney nickel. From the responses we infer that the methanation proceeds via at least two parallel pathways. The relative contribution of the two pathways changes with catalyst aging. The gradual development of a less reactive carbonaceous overlayer alone is insufficient to explain the observations. We conclude that aging affects the nickel catalyst itself.

The interaction of O 2 , CO 2 , CO, C 2 H 4 and C 2 H 4 O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage ( ϑ ). Up to ϑ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10 −4 Pa in an ultrahigh vacuum chamber and at pressures up to 10 3 Pa in a preparation chamber. After desorption at 10 3 Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO 2 is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O ad and CO 2 with an activation energy of 27 kcal/mol (1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO 2 molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO 2 adsorbed becomes smaller. CO readily reacts with O ad at room temperature to form CO 2 . This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO 2 that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C 2 H 4 O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C 2 H 4 O from clean silver is ∼9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.

An abrupt change in the reaction ambient from 12CO/H2 to 13CO/H2 leads to a gradual ingrowth of 13C in hydrocarbon chains. From the rate of 13C ingrowth we calculate C—C bonds to form on a time scale varying from ≤1 s (ruthenium) to ≥10s (cobalt).

An X-ray photoelectron spectroscopic study of the reducibility of Ptγ-Al2O3, Geγ-Al2O3, and Pt, Geγ-Al2O3 systems has shown that: (i) treatment with H2 at temperatures up to 650 °C leads to interaction between Pt and Ge, giving rise to the formation of PtGe alloys; (ii) in the absence of platinum, germanium is difficult to reduce and at 650 °C seems to dissolve in the alumina matrix whereas in the presence of platinum, it remains in a exposed state. The results strongly suggest that in both Ptγ-Al2O3 and Pt, Geγ-Al2O3 “platinum atoms” are present in an electron-deficient state.

To assess the role of carbidic intermediates in hydrocarbon synthesis we have studied the incorporation of surface carbon into hydrocarbon products. To this end, supported transitionmetal catalysts (Ni, Co, and Ru) have been precovered with carbon, deposited via the Boudouard disproportionation of 13CO. Subsequent exposure of these catalysts to 12CO and H2 led to the abundant production of 13CH4 and of hydrocarbons containing several 13C atoms within one molecule. From this we conclude that oxygen-free species CHx (x = 0 to 3) are possible intermediates in methanation and that they are capable of being incorporated into growing hydrocarbon chains. Our results suggest that in the Fischer-Tropsch synthesis CO dissociates in a fast step to give carbidic intermediates, from which both methane and the higher hydrocarbons are produced.

X-ray photoelectron spectra of the valence bands of NaCN, Li 4 Fe(CN) 6 , Li 3 Fe(CN) 6 , Ni(CN) 2 , Li 2 Pt(CN) 4 and LiSCN have been measured and interpreted. Normalization of the valence-band intensities by equalization of the nitrogen is intensities made it possible to distinguish between cyanide and transition-metal bands. The spectrum of CN − is similar to that of the iso-electronic molecules N 2 and CO. The transition-metal bands occur at a lower binding energy than the CN − bands and do not show structure. From the intensities we conclude that much less mixing of the metal and ligand orbitals takes place than indicated by recent MO calculations.

Exchange of silicate anions into hydrotalcite-type materials leads to the ready formation of well-ordered two-dimensional silicate sheets. The combined evidence from diffraction, NMR, and IR allows for a global identification of the reaction product. Its structure probably consists of positively charged brucitelike layers intercalated with puckered, silicic-acid-type macroanions.

Chemisorption, ir, and XPS data indicate that rhenium atoms covered with chemisorbed sulfur atoms divide the metal surface of platinum-containing catalysts into ensembles consisting of a small number of contiguous platinum atoms. Data on the catalytic conversion of n-hexane show that it is the combined action of rhenium and sulfur that converts platinum into a catalyst of very high selectivity for mild dehydrogenation. It thus appears that the beneficial effects on catalyst selectivity and stability which are brought about by adding widely different catalyst modifiers (gold, tin, sulfur, carbon) to platinum catalysts are largely due to one common cause, namely, the division of the platinum surface into very small ensembles of platinum atoms.

The rates of dehydrogenation of propane to propene over platinum and very dilute platinumin-gold alloys have been measured. In the composition range of 0.5–14.0 atom % platinum, the rates per unit surface area of the alloy powders vary linearly with the bulk platinum concentration in the alloys. From this it is concluded that only one platinum atom is involved in the rate-determining step. For both platinum and the alloys, activation energies of some 29 ± 2 kcal/mole were measured. The reaction rate order in hydrogen, however, is different. It is proposed that propane dehydrogenation over platinum and over the alloys occurs via the same reaction mechanism, namely, dissociative chemisorption of propane on a single platinum atom to which two adsorption sites are associated, one of which carries a hydrogen atom. The subsequent conversion of the propyl radical into π-bonded propene via β-hydrogen elimination appears to be rate determining. The last step, desorption of π-bonded propene, has a comparatively low activation energy. The difference in negative reaction order, with respect to hydrogen, between platinum and its diluted alloys reflects a lower steady-state θH on Pt atoms surrounded by Au atoms vis-a-vis Pt atoms on a Pt surface.

Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (X-PS) have been combined to study surface enrichment of two platinum/tin alloys, viz. PtSn and Pt3Sn, effected by reduction in hydrogen at 550°C. By making use of the different information depths of AES and X-PS for this alloy system, as determined by the corresponding mean free paths of inelastic scattering of the emitted characteristic electrons, a rather detailed picture has been obtained of the composition of the topmost atomic layers in these alloys. It is shown that both alloys exhibit phenomena of surface enrichment and depletion of the underlying layers over distances of the order of 10 A.

Adsorption of oxygen on Ag(110) has been studied by high resolution electron energy loss spectroscopy (ELS) and temperature programmed desorption (TPD) in the temperature range from − 160°C to 310°C. At − 160°C oxygen is absorbed as a diatomic species. The low vibrational frequency of the O-O stretch vibration is explained in terms of charge transfer from the metal into the π ∗ antibonding orbital and donation from the π bonding orbital to the metal. A tentative model is presented, according to which the molecule is adsorbed in the grooves of the (110) surface with its axis parallel to the surface. It is explicitly shown that this diatomic species is the precursor for dissociative adsorption of oxygen at temperatures above − 100°C. Upon dissociation part of the diatomic species is desorbed. Between − 100°C and + 310°C a single type of adsorbed atomic oxygen is observed which is desorbed at 310°C. Above 150°C adsorbed atomic oxygen also diffuses to subsurface sites. Below 450°C subsurface oxygen neither desorbs nor diffuses into the bulk, although it does exchange with adsorbed atomic oxygen at a temperature below 310°C. Therefore, both forms of atomic oxygen coexist at temperatures at which ethylene epoxidation occurs.

High resolution electron energy loss spectroscopy (EELS) has been used to study the interaction of oxygen and ethylene with the (110) plane of silver. The excitation mechanisms and corresponding selection rules important in the interpretation of the spectra are briefly outlined. Three adsorption states of oxygen on Ag (110) can be distinguished. Below -100°C oxygen is adsorbed as a diatomic species for which an adsorption model is proposed. This diatomic species has been shown to be the precursor for dissociative adsorption of oxygen at higher temperatures. Adsorption of ethylene is strongly promoted by the presence of dissociatively adsorbed oxygen, and we conclude that the adsorption takes place selectively on those silver atoms on which a positive charge has been induced by the oxygen atoms. By measuring the absolute intensities of the loss peaks as a function of the scattering angle the contributions from dipole and impact scattering could be separated. Reaction of dissociatively adsorbed oxygen with CO 2 results in the formation of an adsorbed carbonate ion CO δ- 3 . The relevance of these studies for the epoxidation of ethylene is briefly discussed.

Precipitation of Al3+ at pH = 10 in excess Li2CO3 leads to an anion exchanging compound, [Al2Li(OH)6]+2CO2−3. This compound exhibits, compared to [Mg3Al(OH)8]+2CO2−3, a higher degree of size selectivity in anion exchange. The structure of the [Al2Li(OH)6]+ layers is gibbsite-like, with a (110) diffraction feature at d = 4.35 A indicating a pronounced Al3+ ordering. As claimed originally by Serna et al., the structure is [ Al 2 Li(OH) 6 ] + A z− 1 z rather than [Al 2 (OH) 6 ]Li + A z− 1 z , with the Li+ coordinated in the octahedral positions left vacant by Al3+. This emerges from the details of a lithium-leaching process, which proposedly leads to a novel compound, [ Al 2 H(OH) 6 ] + A z− 1 z .

Stepwise switches from a 12 CO H 2 to a 13 CO H 2 feed and vice versa have been utilized to obtain transient kinetic information without disturbing the steady-state catalysis. In the context of methanation and Fischer-Tropsch catalysis it emerges that of the large carbidic overlayer which develops only a small part belongs to reaction intermediates proper. Under the prevailing conditions the fractional coverage in growing chains is low and, consequently, the chain growth relatively fast i.e., not rate-determining. Details in the relaxation behavior are suggestive of intermediates that are carbidic in nature.

An abrupt change of the reaction environment from 12COH2 to 13COH2 leads to a gradual ingrowth of 13C in hydrocarbons. This rate of ingrowth can be modeled in a manner which is essentially independent of the details of the reaction mechanism. It is calculated that the rate of CC bond formation varies from ≥1 s-1 (ruthenium) to ≤0. 1 s-1 (cobalt). Further findings include the experimental observation of surface heterogeneity and the relatively short residence times for the C1 monomers involved in chain growth.

Transient kinetic methods allow for assessment of the reactivity and the abundancy of catalyst-bound intermediates. These methods are therefore instrumental in placing the comparison between heterogeneous and homogeneous catalytic pathways on a quantitative basis. The following is a brief analysis of transient kinetic data and of the factors affecting transient kinetic experiments in ‘homogeneous’ and heterogeneous catalysis.

Distinct selectivity changes have been observed in the conversion of n-hexane over Pt upon its being alloyed with Sn. In systems consisting of PtSn on nonacidic supports, the presence of true alloys can be confirmed. Addition of Sn causes a drastic lowering of the rates of those reactions which produce catalyst poisons and hence gives an increase in stability. The change in selectivity can be interpreted in terms of a change in adsorption properties due to differences in the number of adjacent platinum atoms required for the various reaction pathways (ensemble effect). It is assumed that with these catalysts and at temperatures above 500 °C, secondary reactions in the gas phase become important. Hexatrienes formed on the catalyst surface might then contribute to the benzene production and radicals might initiate hydrogenolysis. The present results allow some speculations on the role of the metal component in bimetallic dual-functional Pt catalyst systems.

A comparison of the spectra of metals and their oxides reveals large differences between the chemical shifts in Auger and photoelectron lines. The data for magnesium are discussed in detail. An explanation is proposed which is based upon the difference in polarization energy between the double-hole state produced by the Auger electron emission and the single-hole state which results from the photoelectron emission.

THERMODYNAMIC ARGUMENTS TOGETHER WITH THE SOLUTIONS OF SOME DIFFUSION EQUATIONS LEAD TO THE THEORY THAT ONE OF THE CAUSES OF BLISTERING OF BITUMINOUS COATINGS IS THE VARIATION IN GAS PRESSURE. THIS THEORY HAS BEEN CONFIRMED BY SOME EXPERIMENTS. A MODEL IS DISCUSSED THAT GIVES A QUANTITATIVE DESCRIPTION OF THE BLISTERING. LABORATORY RESULTS ARE DISCUSSED IN VIEW OF THE MODEL PREDICTIONS. A PROCEDURE IS PROPOSED TO EXTRAPOLATE THE EXPERIMENTAL RESULTS UNDER LABORATORY CONDITIONS TO PRACTICAL CONDITIONS. /TRRL/

Calculations on the cation-induced shifts in the UV spectrum of the terphenyl mononegative ion suggest that the spectra of the ions of alternant hydrocarbons, as compared with those of the molecules, are rather insensitive to electric field perturbations. This feature only shows up if one includes enough (doubly) excited configurations to allow for the polarization of the excited states involved in the optical transitions.

An X-ray photoelectron spectroscopy study of supported tungsten oxide has indicated that calcination of WO 3 γ- Al 2 O 3 leads to formation of tungstate. Proof has been obtained that WO 3 γ- Al 2 O 3 cannot be appreciably reduced in hydrogen at 550 °C, whereas WO 3 SiO 2 is reduced to lower valence states and unsupported WO 3 is completely reduced to tungsten metal under these conditions.

(1963). Hydrocarbon ions with triplet ground states. Molecular Physics: Vol. 6, No. 6, pp. 633-635.

An X-ray photoelectron spectrscopy study has been made of the valence regions of some selected close-shell d 8 and d 6 transition-metal halide complexes, MX 2− and MX 2− 6 . It leads to a spectral assignment in which the lowest ionization energy corresponds to removal of metal electrons.

The accessibility of the interlayer space in layered double-metal hydroxides is being reflected by their sorption of nitrogen and hydrocarbons. The results indicate that the interlayer space of dehydrated materials intercalated with [Fe 11 (CN) 6 ] 4− and [Fe III (CN) 6 ] 3− anions is accessible to molecules as large as methyl eyelohexane whereas even nitrogen is excluded from the interlayer space in the material intercalated with NO 3 − anions. These findings confirm that their microporosity can be widely changed by varying the size and charge of the intercalating anions.

Experimental work is continuing on four separate projects related to coal conversion processes. The direct digital control of exothermic multiphase reactions is being studied in an experimental adiabatic flow reactor. The existence of two stable steady states for the Fischer-Tropsch reaction network at the same temperature and feed condition has been verified and quantified. Various absorbents for SO/sub 2/ and NO/sub X/ are being studied. The absorption of NO/sub 2/ by methanol and N-cyclohexyl-2-pyrrolidone has been extensively examined. Preliminary data have been obtained with triethylene-tetraamine. Hindered amines will be studied next. Procedures for the preparation of liquid membranes have been tested and the incorporation of hindered amines in them will now be examined. Isotopic switching is being used to study the way in which promoters affect supported metal catalysts. With improved resolution from the mass spectrometer, early quantitative results give indications of three different surface species and of non-statistical ingrowth of /sup 13/C into the product molecules. A program for the study of the extraction of coal and oil shale using supercritical fluids is being carried out. The effect of the presence of piperidine on the amount of toluene solubles produced by supercritical extraction of coal with toluene/piperidine mixture hasmore » been determined. A new kinetic model for the extraction/liquefaction of coal by supercritical toluene and THF has been developed and proven satisfactory. Bruceton coal and Hi Na lignite have been extracted with supercritical water. 3 references, 7 figures, 6 tables.« less

Publisher Summary This chapter discusses the mechanism of hydrocarbon synthesis over Fischer-Tropsch catalysts. The superiority of liquid over gaseous energy carriers, in particular for automotive purposes, resides in the high energy density of liquids and the relatively low weight of the required container. When compared to solid fuels such as coal, liquids have the advantage that pollutants can be removed more easily, and transport through pipelines or in tankers is cheap and efficient. The chemistry of the catalytic processes is, of course, independent of the way in which the synthesis gas was manufactured; besides coal gasification the steam re-forming of natural gas is a well-known route to produce synthesis gas, although with a higher H 2 /CO ratio. It is convenient to distinguish methanation, where methane is the predominant product, and the Fischer- Tropsck process directed toward the manufacture of predominantly liquid hydrocarbons. However, as the product composition for a given catalyst largely depends on the conditions, the term Fischer-Tropsck catalysis is used also for those cases in which, owing to the variation of one or more of these parameters, the product is mainly gaseous or solid at room temperature and atmospheric pressure.

Multilayer condensation of CBr4 on Pt causes the UPS bands to broaden by some 0.8 eV and to shift by 1 eV towards lower ionization energy. These observations are related to dielectric polarization in the adlayer. Larger shifts, observed in chemisorption, likewise can be explained from image charge screening of the support. For α-CO/Pt a two-band spectrum is observed, whereas for β-CO/Pt only one band is observable. This is ascribed to either the effect of heavy-metal 5d orbital admixture in the CO 2pσ-derived band, or the formation of bridge-bonded CO.Demuth and Eastman's results for C6H6/Ni(111) have been reproduced for polycrystalline nickel. Similar measurements on Pt, Cu and Cu/Ni, however, are severely hampered by the overlap in the spectrum of metal-d and carbon-2p derived bands. Annealing induces a change in the UPS spectrum of 0.33 Cu/0.67 Ni, probably due to enrichment of the surface in copper.