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Reactions of n-hexane on three bimetallic (alloy) systems: Pt-Re, Pt-Ir, Pt-Co have been studied with sulphur-free and sulphided catalysts. A shift is observed in the selectivity pattern from “metallic: (on sulphur free catalysts) to “acid catalysed” reactions (on sulphur modified catalysts), which is explained by the shift in the formation of certain intermediates of the reaction.

Reactions of three /sup 13/C-labeled methylpentanes, namely, 2-(2-/sup 13/C)methylpentane, 2-(4-/sup 13/C)methylpentane, and 3-(3-/sup 13/C)methylpentane, were performed on a series of Pt-Cu alloy catalysts prepared by coimpregnation in order to determine the mechanistic change in isomerization caused by the dilution of Pt in Cu. The results of this research are explained by an ensemble size effect which causes a shift from multisite mechanisms for isomerization (on Pt-rich alloys) toward one-site mechanisms for isomerization (on Cu-rich alloys).

Isomerization, dehydrocyclization and hydrogenolysis reactions of n-pentane and n-hexane were studied on supported Pt and Pt-Au alloys. The alloys studied were all in the composition range where this system is completely miscible (18% Pt in Au). Characteristic changes in selectivity towards the different reactions were observed when Pt was compared with alloys in the composition range 1–12.5% Pt and at temperatures 250–400 °C: (a) extremely diluted Pt in Au (1–4%) mainly catalyses isomerization; (b) alloys with a composition around 10% Pt favor dehydrocyclization; (c) pure Pt shows mainly isomerization. From the comparison of the n-pentane and n-hexane reactions it is possible to suggest a self-consistent picture of the mechanisms involved. For the alloys in (a) above the reaction mainly proceeds via a one-site isomerization mechanism. Catalysts in (b) are comparable to small particle catalysts and show cyclization and isomerization mainly via a cyclic mechanism. Pure Pt catalyses isomerization mainly via the bond shift mechanism (αα-γ adsorption) while ring closure probably proceeds via an αα-Ω adsorbed intermediate.

The exchange reaction of cyclopentane and deuterium on Pt and Pt--Au alloys was studied, and information on reactions of C--H bonds and reaction mechanisms was provided. Product distribution data for the Pt and Pt--Au catalysts at various temperatures was tabulated. It was concluded by comparing the alloy and Pt films at the same conditions that the activity decreases on alloys by up to one tenth of that of pure Pt films. (DDA)

Palladium-silver alloys have been prepared by chemical reduction at 353 K and by equilibration in hydrogen at 673 K. They have then been investigated by Auger spectroscopy. The equation derived by Gallon has been used for the determination of the “monolayer intensity” and for the evaluation of data on the surface composition of alloys. The surface composition measured experimentally has been compared with the theoretical predictions. It appears that in compliance with the latter, the surface of equilibrated alloys is strongly enriched in silver. This silver enrichment is of the same order of magnitude as indicated earlier by other methods. The implications of this surface enrichment for chemisorption and catalysis are discussed.

A series of Pt-Cu alloys on a silica carrier has been prepared and investigated by infrared spectroscopy. It is observed that the position of the infrared bands is a function of the alloy composition. Comparison of the data obtained with other systems reported in the literature excludes the explanation of the shift in terms of electron transfer between alloy components (changes in n d , the number of d electrons) or in terms of bond strength to the nearest neighbors of the metal atoms in alloys. A tentative explanation is suggested according to which changes in the collective properties of metals (local density of states) are responsible for the shift of the infrared band maxima.

Methanation of CO and CO 2 , disproportionation of CO, and hydrogenation of carbon deposited on the surface of catalysts have been studied on Ni and Ni-Cu alloy films at low pressures, at temperatures of 250–350 °C. Under these reaction conditions, two steps of the overall mechanism have been identified; namely, dissociation of CO and hydrogenation of deposited carbon by adsorbed hydrogen. CO can be dissociated only on places where carbon atoms can be bound to several Ni atoms simultaneously; therefore addition of Cu to Ni strongly reduces the rate of methanation.

The extent of adsorption at 295 and 78 K and the thermal desorption of CO from Pd, Ag, and PdAg alloy films have been measured in an ultrahigh vacuum system. The adsorption data indicated that the surface of the alloys studied in this paper was enriched by Ag. Desorption spectra revealed several adsorption/desorption states, designed as γ 1 , γ 2 , α 1 , α 2 , and β . The nonspecific, most weakly bound γ 1 -state was observed with all adsorbents. The γ 2 -state, also weakly bound, was observed only on Pd. The α 1 - and α 2 -states were observed on Pd and PdAg alloys; they were completely missing on Ag. The population of α-states decreased with increasing Ag content. This is explained by assuming that upon alloying of Pd with Ag the number of large ensembles necessary for strong adsorption decreases.

Reactions of butanal and butanol and hydrogenation of cyclopentanone have been studied in a flow apparatus in mixtures with hydrogen and at a total pressure of 1 atm. The catalysts used were Ni and NiCu powders without carrier and Pt and PtAu on silica. Hydrogenation/ dehydrogenation of the compounds is only marginally influenced by alloying, while the inter- and intramolecular dehydration (formation of olefins and ethers from alcohols) is suppressed strongly. The selectivity can be controlled by the availability of ensembles of active sites. A mechanism is suggested which can rationalize the influence of alloying, including some data from the literature.

The effect of alloying on the absorption band frequency of CO adsorbed on Pt , v ( CO Pt ) , has been investigated with the alloys Pt-Pb, Pt-Sn, and Pt-Re. By use of the isotopic dilution method, it has been established that the downward shift of v ( CO Pt ) observed upon alloying Pt with Pb is caused partly by a dilution effect and partly by another, possibly electronic effect. Alloying Pt with Sn or Re, on the other hand, caused only a small shift of v ( CO Pt ) , which, moreover, could also be explained at least partly by a dilution effect. Oxidation of the alloy samples resulted in a phase separation of the components; the alloy situation could be restored again by a reduction procedure.

Cyclopentane-deuterium exchange has been followed on pure Pd and PdNi alloy films in the temperature range 200–430 K and at hydrogen/cyclopentane ratios of ~50. Conclusions on activity changes are limited because of the self-poisoning of alloys and Ni catalysts. However, the selectivity of the catalysts is much less influenced by self-poisoning; the initial product distributions are well reproducible and independent of these side processes. The main result of alloying of Pd with Ni is the gradual disappearance of one low temperature mechanism which leads on Pd to D10 products. This fact can be easily understood if we assume that it is the “roll-over” mechanism which is responsible for these products.

The reactions of neohexane were studied on a series of Pt-Cu alloy catalysts. It appears that from the three possible adsorption modes (the αγ, αγ′, and αβ complexes) which neohexane offers when two carbon atoms are bound to the surface, the αγ′ mode is preferred to the others and this tendency is independent of alloy composition. In this way, the already known preference of Pt to form αγ and αγ′ complexes is fully confirmed. The activity and selectivity pattern found with other hydrocarbons studied earlier is also confirmed: with increasing Cu content, the selectivity for hydrogenolysis increases, whilst that for isomerisation decreases. An explanation is suggested according to which the chemisorption complex yields isomers upon further reaction on a pure Pt ensemble, whereas on a mixed ensemble (Pt and Cu) hydrogenolytic products result.

The temperature programmed desorption of hydrogen from platinum films has been measured in a U.H.V. system by means of a mass spectrometer. The parameters varied were the extent of adsorption, the duration of the adsorption process and the heating rate during desorption. For maximum coverage three desorption peaks have been observed in temperature-programmed desorption: γ (120 K), β1 (200 K) and β2 (330 K). The population of the γ state varies with the duration of pumping at 78 K before adsorption is started, and the population of the β2 state is dependent on the duration of adsorption at room temperature. The isosteric heat of adsorption for high coverages has been measured and varies from 8 to < 2 kcal/Mol for coverages from 7 to 12.1014 mol/cm2 geometric surface.

Coadsorption of ethylene or oxygen with carbon monoxide has been investigated using ir transmission spectroscopy. Information on the extent of “electronic” and “dgeometric” (dilution) effects on the ir stretching frequency of CO has been obtained by the isotopic dilution method. The increase of this frequency (blue shift) caused by oxygen can be explained mainly by a compression of the CO layer. The decrease of the frequency (red shift) caused by ethylene can be explained for more than 60% by a dilution of the CO layer.

The work function Φ, the emission constant M , and the increase in work function after CO adsorption ΔΦ have been measured as a function of the overall composition of well annealed Ni-Al alloy films. The work function of the Ni-Al alloys is not constant within the several miscibility gaps of the Ni-Al phase diagram, and is not lower than the work function of the respective pure metals, as was found with Ni-Cu and Pt-Au alloy films within the miscibility gap. Of all compositions studied only the 25% Ni alloy (NiAl 3 ) distinguishes itself by an increased Φ-value. The higher Φ is explained by the higher atomic density of the NiAl 3 compound compared with pure Al and Ni 2 Al 3 . As with other metal alloy systems, an enrichment of the surface by the component with the lower surface energy (Al) is also found with the ordered Ni-Al alloy system. From ΔΦ measurements it appears that up to about 40% Ni the surface of the Ni-Al alloy is composed entirely of Al. The behavior of Φ and ΔΦ in the Ni-rich part is explained by the enrichment of the surface by Al. The tendency to form the surface from Al atoms is strengthened by the existence of suitable crystallographic structures. The emission constant does not distinguish any of the intermetallic compounds and resembles that of the Pt-Au and Pd-Ag alloy films.

Catalysts containing 15% loading of 4% Pt-Au alloy on silica or ..gamma..-alumina were prepared by co-reduction with hydrazine, reduced in hydrogen, and tested for activity for n-hexane isomerization and cyclization at 350/sup 0/C. The catalysts were then subjected to two oxidation-reduction cycles, and tested with the same reaction. Over alumina-supported alloy, the initial conversion was 2.9%/g catalyst and the selectivities for isomerization and cyclization were approx. 90 and approx. 5%, respectively. After the oxidation-reduction treatment, conversion was 6.0%/g catalyst, and isomerization and cyclization selectivities were 30 and 56%, respectively. The silica-supported alloy had similar initial activity and selectivity as the alumina-supported catalyst, but did not change significantly on oxidation-reduction. A comparison with pure platinum on the same carriers suggested that the platinum segregated from the alloy during oxygen treatment and formed platinum oxide, that the platinum oxide migrated to the alumina surface where it formed separate particles on reduction, but it was unable to ''wet'' the silica surface and migrated to the alloy particle surface in the silica-supported catalyst, where it forms bulk alloy again, on reduction.

The work function Φ and the change in work function ΔΦ caused by the adsorption of CO or C2H4 have been measured as a function of the overall composition of equilibrated Ni-Cu and Ni-Au alloy films. The measurements reveal that Φ as a function of bulk (average) composition of the alloy films has the same character for alloy films annealed below the critical temperature Tc, where two phases coexist (a Ni-rich phase and a Cu- or Au-rich phase), as well as for films annealed above Tc where only one phase is found. There always appears to be a broad range of concentrations where Φ is independent of the composition. Both the Φ and ΔΦ measurements indicate the surface in this concentration range to be Cu rich (about 20 atom% Ni) or Au rich (about 10 atom% Ni). The critical temperature Tc of Ni-Cu alloys was checked. The results agree with the value predicted by Meijering. The C2H4 adsorption experiments reveal that on Ni-Au surfaces less deep dissociation occurs and more partially hydrogenated C2H4 species are present than in the case of Ni-Cu alloys. The results indicate that the surface of Ni-Cu alloys within the concentration range of constant surface composition contains larger Ni clusters than in the case of Ni-Au alloys.

The adsorption of CO on alumina-supported Pt-Cu alloys was studied. The results were compared with those obtained on the unalloyed Al 2 O 3 -supported metals as well as with the results on alloys supported on silica. By use of the isotopic dilution method it has been established that the downward shift of the frequency of the CO Pt band observed upon alloying Pt with Cu is “geometrical,” caused by dilution of Pt by Cu; an electronic or ligand effect was not observed (within the limits of error). The upward shift of the frequency of the CO/Cu band upon alloying Cu by Pt is most probably caused by “individualization” of the Cu atoms in the Pt layer.

In this article an attempt is made to establish the role of Cu in the activity of Cu alloy catalysts for hydrogenolysis. For this purpose a comparison was made between supported Pt, Ir, Ni, and Pd catalysts and their Cu alloys. In all cases the amount of active metal (10 μmole) per gram of catalyst was the same. It was observed that all highly diluted Cu alloys showed an increased activity for hydrogenolysis. Two explanations for this behavior are suggested, but both involve the “inactive” Cu atoms in the formation of the cracking products.

The extent of CO adsorption has been measured at 295 K and the thermal desorption of CO has been followed on Ni and NiCu films prepared in an ultrahigh vacuum apparatus. The total extent of adsorption revealed a different behavior than the extent of hydrogen adsorption studied previously; an explanation involving gas-induced surface enrichment of alloys is suggested. An irreversible gas-induced enrichment has also been observed upon repeated runs of the thermal desorption of CO. The thermal desorption reveals the existence of two weakly bound states γ1 and γ2 and two more strongly bound states α1 and α2 on Ni and Ni alloys. Cu reveals only the weakly bound states γ1 and γ2. The population of various states varies with the Cu content as well as the chemisorption bond strength of the α2-species. Suggestions are made on the adsorption modes of CO.

Hydrogenation of acetylene and reactions of hexane were studied on Pd and Pd-Au alloys. Alloying increases the selectivity for nondestructive reactions of hexane, and the activity in hydrogenation of acetylene as a function of alloy composition shows a maximum. The present results and the comparison with results obtained on Ni-Cu alloys show that the changes in the population of the transition metal orbitals cannot alone be responsible for the variations in the kinetic parameters and product distributions.

Adsorption of nitrogen on evaporated iron films has been studied in the range of temperatures 78–273 °K. Films with clean surfaces and with surfaces precovered by hydrogen have been used. The adsorption of hydrogen has also been studied on clean surfaces and surfaces with preadsorbed nitrogen. On the basis of volumetric measurements of amount adsorbed, work function, and film resistance changes a conclusion has been made on the mechanism of nitrogen adsorption. Three forms of nitrogen adsorption have been detected: physical adsorption, weak, and strong chemisorption. Weak chemisorption is an unavoidable precursor for strong chemisorption.

The sorption of atomized hydrogen on films of transition metals and of copper and its reaction with oxygen have been studied by volumetric measurements of the extent of sorption and by following the changes in the electric resistance of the films. The extent of hydrogen atom sorption indicates the dependence on the metal-metal bond strength; the reactivity of adsorbed gases is in some way correlated with the heat of adsorption. An attempt has been made to locate the activation energy barrier in the hydrogen-oxygen interaction.

The exchange reaction of methylcyclopentane with deuterium has been followed on Ni and Ni-Cu alloy films. Reactions of methylcyclopentane with hydrogen have been followed on Ni and Ni-Cu alloy powders. The results confirm that 1. 1. the reactions at CH bonds are less influenced by alloying than the reactions at CC bonds; 2. 2. alloying with Cu favors the nondestructive reactions (e.g., isomerization) and suppresses the hydrogenolytic cracking reactions. The results can be rationalized assuming that the reactivity and flexibility of adsorbed molecules is inversely related to the heat of adsorption (bond strengths) and the degree of dehydrogenation.

An improved technique of calorimetric measurements on evaporated metal films is described and results of measurements of heats of hydrogen sorption on molybdenum films at 20°C are presented. The initial heat of hydrogen adsorption on molybdenum is 40 kcal mole−1, and integral heat of adsorption (for surface coverage up to ϑ = 0.9) is 31 kcal mole−1. Molybdenum ranks between tungsten and iron as regards the value of the heat of hydrogen adsorption and the dependence of the heat on surface coverage. A partial coverage of the molybdenum surface by oxygen leads to a very slight increase of the heat of the following hydrogen sorption (by several per cent). A dependence of this increase on the extent of oxygen preadsorption was not found. No signs of a reaction of preadsorbed oxygen with hydrogen were detected calorimetrically.

Cyclopentane-deuterium exchange has been followed on nickel and some nickelcopper alloys in the temperature range 200–430 °K. In the whole range, the reaction is accompanied by self-poisoning, and on Ni at 340–430 °K also by hydrogenolysis. The catalytic effect of alloying is most pronounced on hydrogenolysis and selfpoisoning, but is rather small with respect to multiple exchange. In having a low activity with respect to the CC bond fission, the NiCu alloys are more reminiscent of Pt than Ni.

Chemisorption and mutual interaction of hydrogen and oxygen has been measured on platinum films, the resistance changes have been followed simultaneously. By means of these experiments conditions have been found under which gases react on platinum. On the basis of the present results and of some data from the literature a model for the hydrogen chemisorption bond is suggested for transition metals.

Chemisorption of cyclopropane was investigated by measuring the extent of chemisorption on nickel film of known sufrace area, measuring the changes of electrical resistance of the film during chemisorption, and performing mass spectrometric analysis of the gas phase. It was found that during interaction of cyclopropane with nickel the CH and CC bonds are split. Some measurements of cyclopropane hydrogenation on nickel at 273 °K and under static conditions were made. Cyclopropane reacts with hydrogen in a rather complicated way as is shown by the presence of ethane in the reaction mixture. After pumping off the reaction mixture from the apparatus and admitting a starting mixture, hydrogenation on the same film proceeds with the same velocity. By poisoning nickel with oxygen before and after the first reaction, it was shown that only a small part of the surface and/or of the adsorbed layer participates in the reaction under given conditions.

The reaction of cyclopropane with hydrogen has been studied on nickel and nickel-copper alloy powders in a pulse reactor in the temperature range 40–350 °C. The reaction proceeds via two parallel routes, namely an addition and a hydrocracking reaction. Alloying with copper influences both reactions in a different way, most probably because the intermediates of cyclopropane addition reactions are different from those which play a role in hydrocracking reactions.

A comparative study was made of the cyclopropane chemisorption on pure nickel film surfaces and of the hydrogenation and hydrocracking of cyclopropane on both the films and a reduced nickel catalyst. In a range of higher coverages, the chemisorption of cyclopropane at 273 °K was accompanied by selfhydrogenation to propane, and, as a characteristic feature of the process on nickel, cracking to methane and ethane was also observed. Some further data concerning composition and properties of the chemisorbed layers are also given. Cyclopropane was reacted with hydrogen on nickel films in an UHV apparatus at 195 and 273 °K, and in the range of 300–600 °K using a microcatalytic-pulsed reactor coupled to a gas chromatograph. For the purpose of comparison, hydrocracking of propane was also investigated under the same conditions on the nickel powder catalyst. From the temperature variation of the product composition thus obtained, two types of cyclopropane cracking could be distinguished. With the low-temperature selective hydrocracking, the percentage of methane and ethane in overall product was constant over the range of 195–500 °K. Beyond this range the nonspecific hydrocracking began, characterized by the increasing proportion of cracking in products and by the prevalence of methane over ethane. No distinction could be made between the latter process and the propane or propylene hydrocracking. Some mechanistic consequences of the experimental findings are discussed. A concerted simultaneous splitting of two bonds in the chemisorbed cyclopropane ring is suggested as the reaction path for the low-temperature fragmentation.

Calorimetric curves corresponding to those observed in adsorption calorimeters, were simulated by means of an analogue computer assuming first order, Elovich, and Bangham kinetics for the adsorption process. The parameters of the calorimeter and the rate of the adsorption process were varied independently so as to show their influence on the shape of the calorimetric curve

To resolve discrepancies in reported methanation rates for several catalysts, three procedures were used to verify literature values for molybdenum (Mo), tungsten (W) and nickel (Ni). According to all three aspects checked, Mo and even more so W, are worse functioning methanation catalysts than is Ni. Attempts to determine if the dissociation of carbon monoxide was a necessary step in the methanation reaction were unsuccessful.

The reaction of 5:1 hydrogen/carbon monoxide on carbon-13-covered, evaporated films of cobalt and ruthenium (which represented typical Fischer-Tropsch catalysts) at 250/sup 0/-300/sup 0/C and Vertical Bar3; 1 mm Hg total pressure, produced about equal amounts of labeled and unlabeled methane. The higher initial yields of labeled methane obtained on nickel were probably due to a lower degree of deactivation of adsorbed carbon-13, or possibly to methane formation via a different intermediate.