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In this review, the most relevant aspects of skeletal isomerization of n-butene to isobutene are discussed: the nature of the active sites, the prevailing mechanism of the skeletal isomerization, and the relation of this to that of n-butane. It is concluded that the prevailing mechanism of skeletal isomerization of n-butene is monomolecular (in contrast to butane isomerization) and requires Br⊘nsted acid (OH) active sites. The selectivity and catalytic stability can be influenced by the shape selectivity of zeolites and zeotypes. These effects are explained on the basis of the knowledge on the prevailing mechanism.

Adsorption of acetic acids has been studied by FT-IR on a series of adsorbents, known as good catalysts for the selective hydrogenation of carboxylic acids to corresponding aldehydes: SnO2, TiO2, Pt SnO 2 , Pt TiO 2 . Very similar adsorbed species are observed by FT-IR with all mentioned catalysts. The catalysts differ mainly by their behaviour upon increasing temperature program. The strongly bound unidentate or the bidentate, bound to the oxide, are the possible intermediates of the catalytic reaction to acetaldehyde.

The present study gives the results of our attempts to gain more knowledge on the interaction between an aromatic carboxylic acid and an oxide catalyst. There is quite a difference in the coordination of benzoic (and salicylic) acid with an acid-base catalyst (γ-Al2O3) or a redox one (α-Mn3O4). When a substituent is incorporated into the ring, the difference becomes even larger. These differences can perhaps explain the difficulties in the catalytic reduction from (substituted) benzoic acids to the corresponding aldehydes.

The structure of metallic copper oxide La2Sr6Cu8O18-δ (δ≈0.4) has been determined by electron diffraction and neutron powder diffraction. The correct space group was found to be P 4/mbm with lattice parameters a=10.8502 (3) A≈2√2ap and c=3.7658 (1) A≈ap, where ap is the lattice parameter of the cubic perovskite. The oxygen vacancies are ordered both in the basal plane, along [001], and in the z = 1 2 plane, along [110]. The idealised framework of [Cu8O18] consists of square planar CuO4 groups and CuO5 square pyramids by sharing their corner oxygen, leading to the formation of hexagonal tunnels along the c-axis. La3+ and Sr2+ ions are located in these tunnels in a disordered manner.

The geometric and promoter effects on the selectivity to furhryl alcohol were studied in the hydrogenation of furfural over various series of platinum catalysts. The results obtained with the PtCu catalysts indicate that the size of the active site does not affect the selectivity. Promotion of the platinum catalyst can lead to a considerable rise in the selectivity to furfuryl alcohol. The hydrogenation over the series of PtSn catalysts showed the influence of the reaction conditions. The experiments under isothermic conditions resulted in up to 80% selectivity for hrfuryl alcohol, while the selectivity dropped to approximately 40% if non-isothermic conditions were applied. This change in selectivity is attributed to the self-poisoning of the catalyst at high temperatures.

The changes in work function φ upon adsorption of C 2 H 4 on clean film surfaces of six fcc metals (Ni, Pd, Pt, Cu, Au and Al) have been followed by means of photoelectron emission at 293 K. A marked difference was observed in the behaviour between Ni, Pd and Al on the one side and on Cu, Au and Pt on the other side: while with Ni, Pd and Al, φ as a function of coverage goes through a maximum, with Cu, Au and Pt, φ only decreases. In the discussion, the data obtained by work function measurements are related to other literature data. Several films covered with C 2 H 4 species were also submitted to a heat treatment while in other experiments H 2 was admitted to the surface covered by C 2 H 4 species. In some experiments C 2 h 4 was admitted to surfaces covered by H 2 . In all cases φ was measured. The experiments reveal that C 2 h 4 is absorbed only reversibly on Cu and Au. On Ni, Pd and Pt, C 2 H 4 is adsorbed initially with dissociation and this leads to an increase in φ on Ni and Pd and a decrease on Pt. Hydrogenated reactive species contribute to the lowering of φ observed with Ni, Pd and Pt. As with Cu and Au also on Ni, Pd and Pt a weakly bound C 2 h 4 is observed which leads to a decrease in φ as well. The behaviour of φ indicates that upon Al, C 2 h 4 adsorbs first dissociatively to a small extent, while the weakly bound C 2 H 4 species act as intermediates for strongly adsorbed species which were observed after some time.

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).

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)

The IR Spectra of carbon monoxide adsorbed on different forms of platinum, iridium and copper (flat and rough surfaces) have been investigated. The results of IR transmission experiments on the silica supported metals are presented and compared with the data from IRAS (IR reflection adsorption spectroscopy) experiments on the unsupported metals. It is shown that the coverage dependency of the IR band frequency ΔνCO (θ) is independent of the state of the metal if the formation of islands of CO is taken into consideration. This was proven by isotopic dilution experiments with12 CO/13 CO mixtures. It is suggested that many apparent discrepancies and irreproducibilities in the literature can be explained by an uncontrolled formation of islands. At a given surface coverage. Group VIII metals reveal a lower νco value when CO is adsorbed on rough surfaces (or small particles) than on smooth surfaces (or large particles). The reverse is true for the lb metal Cu. The differences between supported and unsupported metals can be ascribed to differences in the electronic and geometric structure of the surface. The differences between adsorption of CO on Group VIII and lb metals are discussed in terms of a difference in bonding of CO on these two groups of metals.

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.

When a mixture of carbon monoxide and hydrogen is contacted with some metals (Co, Ru, Ni, Fe), a great number of reactions occur. Some of them are schematically described by the following stiochio-metric equations Reaction (1) with m=1 was discovered by Sabatier [1] and is usually called methanation. Reactions (1) to (3) with m < 1 are the Fischer-Tropsch [2] synthesis of hydrocarbons. There are several very good review papers [3–8] on this subject, and it is not intended to add another paper to this collection. Rather, results obtained during the last few years at Leiden will be reviewed. The mechanism suggested is still somewhat speculative; however, some experimental evidence exist for all aspects of the mechanism.

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 chemical characteristics and the reactivity of carbon on polycrystalline Rh and Pt surfaces towards hydrogen have been studied by Auger Electron Spectroscopy. Auger spectra of carbon on both metals have been subjected to spectrum subtraction techniques in order to reveal the chemical characteristics of it, as reflected by the fine structure of the carbon KVV Auger spectra. Two classes of carbon can be identified by AES: (A) amorphous and graphitic carbon; (B) molecular (or molecular fragments) and carbidic carbon, the first being almost unreactive in H 2 . Carbon B transforms into A and this transformation is faster on Pt than on Rh and is also faster in the absence than in the presence of H 2 .

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.

A short study on the title reaction confirms that at low hydrogen pressures no ether formation occurs, while when a higher hydrogen pressure suppresses the fast dehydrogenation, ethers are formed. A comparison of Pt and Pt alloys shows that ether formation is faster on large ensembles of Pt atoms, requirements of decarbonylation are less in this respect. Results of earlier studies on Ni/Cu alloys have thus been confirmed.

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.

Pt-Pd alloys were prepared as powders by a chemical reduction method and as films by evaporating the specpure metals onto a quartz substrate. Results were evaluated by means of the Gallon equation; the parameters of this equation — the signal intensity of a monolayer and of the bulk material — were determined experimentally for all relevant Auger peaks. The backscattering factor for the alloy system was evaluated from experiments in which a layer of one metal was evaporated on top of the other. The value determined in this way was compared with results obtained in the literature on similar systems. The theoretical surface compositions are calculated for several models and compared with the experimental results. Experimental results reveal surface enrichment in Pd, in good agreement with the theoretical calculations. Interactions of alloys with carbon monoxide and propane are discussed and a possible explanation of the effects observed is suggested.

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.

Pt-Ir alloys were prepared by evaporating the specpure metals onto a quartz substrate and equilibration at 700 K. The Auger signal intensity of a monolayer of the elements was determined for the relevant peaks. The results are evaluated by means of the Gallon equation. The theoretical surface composition is calculated for the ideal solution model and compared with the experimental resultd. A pronounced surface enrichment in Pt is observed, in agreement with the theoretical predictions. Experiments with propane adsorption have been performed and a possible explanation of the results is offered. Results obtained are found to correspond well with literature data on catalytic reactions.

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.

There are probably very few other systems in industrial chemistry which equal the CO/H2 reaction system in the variety of possible products. When using pure Ni or Co catalysts one can produce almost pure CH, or a mixture of saturated hydrocarbons; a promoted (K2O) iron catalyst produces, in addition to these products, also alcohols, aldehydes, and other oxygen-containing products (oxygenates); and almost pure CH3OH can be obtained when applying Cu/ZnO/Al2O3 or Pd-MgO promoted SiO2 catalysts. Some catalysts (Rh/V2O3) can lead the reaction of syngas toward C2-oxygenates, and others to a mixture of C2+-alcohols and aldehydes (Co, K, Cr, Fe multicomponent catalysts). All this is schematically shown in Fig. 1. It is a remarkable, but not unusual, situation that while the economic interest follows the order (Fig. 1) III > II > I, the fundamental knowledge on the reactions mentioned follows just the reverse order: I > II > III. Although the leading ideas for preparing selective catalysts are onIy slowly...

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.

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.

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 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.

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.

Both Pt/Al2O3 and (Pt+Re)/Al2O3 industrial catalysts contain, after reduction (at 450 °C) ions, extractable by acetylacetone. However, a substantial part of Re is reduced.

Hydrogenation of benzene and cyclohexene has been studied on Pt and four Pt-Au alloys. The overall behaviour −- activity, selfpoisoning and regeneration was similar to that of Ni−Cu alloys. The surface of the two-phase Pt−Au alloys had, according to the kinetic data a composition of the β-alloy (Au-rich phase). Hydrogenation of benzene was 103 to 104 times slower than the hydrogenation of cyclohexene. The most dilute alloys are active in cyclohexene hydrogenation but they are completely inactive in the low temperature hydrogenation of benzene. The main effect of alloying is the decrease in the pre-exponential term of the rate. The effect is explained in geometric terms: activation of a benzene molecule for the low temperature hydrogenation requires larger ensembles than those for cyclohexene activation and the larger ensembles are missing on the surface of the most dilute alloys.

A series of Pt catalysts with varying particle size was prepared and studied in the reforming reactions of hexane. Activity and selectivity of the catalysts were determined under well defined conditions on two kinds of surfaces - either “clean” or deliberately modified by carbon deposition at elevated temperatures. It appeared that the effects of carbon deposition were different for catalysts with different metal particle size and the effects of carbon deposition were, with our series of catalysts, more pronounced than a possible influence of the particle size itself.

Publisher Summary This chapter discusses Fischer–Tropsch synthesis (FTS), which consists of complex syngas reactions occurring with metallic, though not necessarily pure metal, catalysts. The Fischer–Tropsch-type reactions lead to very broad product distributions, including alkanes, alkenes, oxygen-containing derivatives, CO 2 , and H 2 O. The components involved in the FTS are hydrogen, hydrocarbons, and carbon monoxide. Adsorption characteristics of pure components and the adsorption features of mixtures are of importance. All metals active in hydrocarbon synthesis adsorb hydrogen at low temperatures up to full coverage. The hydrogen/surface atoms ratio can reach one, which can be also possible among other metals derived from the phase diagrams of H 2 adsorption on single crystals. Adsorption on clean surfaces is fast, nonactivated, and dissociative. There is always atomically dispersed carbon on the metal surfaces, or a two-dimensional carbide sets in which it is sufficiently reactive to play the role of an intermediate in the FTS. The faster it can react with hydrogen, the lower the probability that it will coagulate and deactivate on the surface or become dissolved in the bulk.

Resume Plusieurs catalyseurs au Palladium sur silice avec ou sans promoteurs (Mg2+ et La3+) ont ete prepares et etudies. La selectivite pour l'hydrogenation de CO en methanol ainsi que l'activite sont plus elevees pour les catalyseurs contenant des ajouts. Par l'application de plusieurs techniques experimentales (extraction des especes Pdn+ par l'acetylacetone; M.E.; spectroscopies: U.V.-visible, R.P.E., I.R.) il a ete demontre que la promotion du Palladium par Mg ou La, provoque la formation ou la stabilisation des centres positifs: Pdn+, qui comme nous le suggerons, sont essentiels pour la production de methanol a partir de CO et H2. Abstract Several Palladium catalysts on silica with and without promotors (Mg2+ and La3+) were prepared and studied. By catalytic activity measurements of the synthesis gas reaction over these catalysts it was found that the addition of the promotors increases both the selectivity for methanol and the activity of the Palladium catalysts. Using several experimental techniques (extraction of Pdn+ by acetylacetone; E.M.;U.V./VIS;E.P.R.;I.R.) it was shown that promotion of Palladium with Mg2+ or La3+ gives rise to the formation and stabilization of positive centres: Pdn+, which we suggest are essential for the production of methanol from synthesis gas.

In this paper the isomerisation of n-pentane and the hydrogenolysis of methylcyclopentane were studied on a series of Pt-Cu alloy catalysts and were compared with earlier obtained results concerning the isomerisation of n-hexane. A comparison between the reactions of these hydrocarbons is necessary to determine the relative contribution of the various mechanisms for isomerisation that are known to operate simultaneously on a Pt catalyst. It was found that alloying Pt with Cu caused considerable shifts between the various mechanisms and the conclusion is that alloying (decreasing the ensemble size of Pt) causes a change from mechanisms requiring large ensembles of Pt atoms towards mechanisms requiring small ensembles, possibly even one Pt atom.

During a study of the isomerisation of n-butane on Pt–Au films we found besides the usual products methane, ethane, propane, and isobutane, also an appreciable amount of n-pentane; pentane appears at 300 °C; below this temperature only cracking and isomerisation products are observed.