The surface chemistry of hydrocarbon fragments on transition metals: towards understanding catalytic processes

Zaera, Francisco

doi:10.1080/00268970210130254

Cited by 29 publications

(28 citation statements)

References 121 publications

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“…Numerous species of carbon are observed on the surface of catalysts, ranging from carbides to hydrogen-rich aliphatic polymers. The chemical states of accumulated carbonaceous species on model palladium surfaces were often a matter of debate [38][39][40][41][42]. The majority of these studies were performed exsitu or in ultra-high vacuum (UHV), far away from the real catalytic conditions (this is often referred to as an example of the "pressure gap").…”

Section: Introductionmentioning

confidence: 99%

High-pressure X-ray photoelectron spectroscopy of palladium model hydrogenation catalysts.Part 1: Effect of gas ambient and temperature

Teschner

Pestryakov

Kleimenov

et al. 2005

Journal of Catalysis

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In light of accumulating evidences highlighting the major role of operational conditions (gas composition, pressure, temperature) on the surface/bulk structure of catalytic materials, their characterization should involve more and more in-situ methods. We constructed a synchrotronbased high-pressure XPS (X-ray photoelectron spectroscopic) instrument, allowing us to investigate the surface and near-surface state of a catalyst in the mbar pressure range. We discuss here the surface characteristics of palladium samples as a function of gas-phase (hydrogen, oxygen) and temperature. We demonstrate that the surface region of catalytic materials behaves dynamically in its composition, always reflecting on its environment. For example, surface oxide can be formed on Pd(111) in oxygen which decomposes rapidly as the gas supply is switched off. The chemical nature of carbonaceous deposits depends also strongly on the operational conditions (gas-phase hydrogen, temperature). It is the first time that XPS investigation on palladium β-hydride was performed at RT. The possible drawbacks of using non-UHV setup e.g. fast carbon accumulation are also discussed.

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Section: Introductionmentioning

confidence: 99%

High-pressure X-ray photoelectron spectroscopy of palladium model hydrogenation catalysts.Part 1: Effect of gas ambient and temperature

Teschner

Pestryakov

Kleimenov

et al. 2005

Journal of Catalysis

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“…The chemical state of accumulated carbonaceous species on model palladium surfaces was often a matter of debate [36][37][38][39][40]. The majority of these studies were performed ex-situ or in UHV conditions, far away from the real catalytic conditions (pressure gap).…”

Section: Introductionmentioning

confidence: 99%

High-pressure X-ray photoelectron spectroscopy of palladium model hydrogenation catalysts. Part 2: Hydrogenation of trans-2-pentene on palladium

Teschner

Pestryakov

Kleimenov

et al. 2005

Journal of Catalysis

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We have performed the first "high-pressure" XPS study on the palladium, hydrogen and olefin (trans-2-pentene) system in order to gain a better insight into the hydrogenation reaction. We report here data collected using a Pd(111) single crystal and a polycrystalline foil. Hydrogenation was observed on polycrystalline foil (RT and 373 K), but not on Pd(111) single crystal, revealed by on-line mass-spectrometry. We observed the reaction in the presence of huge amount of carbon (up to 73 %) in the information depth of XPS. On Pd(111) mainly graphite was present while other components C-H and C-Pd were also formed on the foil in much higher extent. C-Pd characterizes a carbon species in the interaction with palladium whereas C-H represents hydrogenated carbon including chemisorbed species. The d-band of the foil showed a remarkable up-shift towards E FERMI as compared to Pd(111). We concluded that both differences found in the valence and in C 1s region are indicators for different electronic structures that contribute to the variation in activity. The palladium foil lost its activity at elevated temperature (523 K), most probably due to desorption of hydrogen. Using additional UPS measurements we concluded that trans-2-pentene is hydrogenated in σ-bonded chemisorption modus, at least in UHV conditions.

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“…However, the coverage and nature of those ''spectator'' surface species is typically hard to control (other than by the use of additives or modifiers, or by tuning the reaction conditions). In this respect, the formation of complex surface carbonaceous deposits is of particular relevance to the passivation of the activity of metal catalysts for the promotion of facile hydrocarbon conversion catalysis [22,23,179,180]. Nevertheless, none of these differences preclude us from taking advantage of the knowledge available from organometallic chemistry and homogeneous catalysis to advance our understanding of surface chemistry and our ability to design better heterogeneous catalysts.…”

Section: Discussionmentioning

confidence: 98%

“…Under the reducing conditions used in most of those processes, however, the extent of the dehydrogenation is small. This step is also directly connected to the academic H-D exchange reaction in alkanes and alkenes [52][53][54], and is key in the mechanism for double bond migration and cis-trans isomerization [22,55,56].…”

Section: B-hydride Eliminationsmentioning

confidence: 99%

“…There have been some clear correlations identified between surface reactions and those seen with organometallic ligands [8,9,17,18], but much more remains unanswered in terms of reaction mechanisms and reaction kinetics on solid substrates. It has been one of the main aims of our research group to use the ideas of organometallic chemistry to better understand the thermal chemistry of adsorbates, of hydrocarbon moieties in particular, on transition metal surfaces [19][20][21][22][23]. Below we provide a brief summary of the results from that work so far.…”

Section: Introductionmentioning

confidence: 99%

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Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistry

Zaera

2005

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Catalysis is central to most industrial processes for chemical manufacturing. As catalytic processes have become more complex and more demanding, selectivity has become the central issue in their design. Selectivity is defined by the relative rates of competing reaction pathways available to crucial intermediates, and can be controlled by subtle changes in the nature of the catalyst, the reactants, and/or the reaction conditions. In order to be able to do this in a systematic manner, a good understanding of the catalytic reaction mechanisms is needed. Here a connection is drawn between the key elementary steps comprising hydrocarbon conversion reactions on surfaces and those known to occur on discrete organometallic complexes. This way, the hydrogenation, dehydrogenation, hydrogenolysis, chain growth, and isomerization reactions typical in heterogeneous catalysis are redefined in terms of hydride elimination, oxidative addition, reductive elimination, migratory insertion, and . 1, 2-shift elementary steps, among others. It is suggested that the knowledge already available from organometallic chemistry can be used to further advance the understanding of the surface science involved in heterogeneous catalysis. Thanks to the commonality of the chemistry involved, a better synergy could also be established between homogeneous and heterogeneous catalytic development. These ideas are discussed in this article in a critical and personal way.

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The surface chemistry of hydrocarbon fragments on transition metals: towards understanding catalytic processes

Cited by 29 publications

References 121 publications

High-pressure X-ray photoelectron spectroscopy of palladium model hydrogenation catalysts.Part 1: Effect of gas ambient and temperature

High-pressure X-ray photoelectron spectroscopy of palladium model hydrogenation catalysts.Part 1: Effect of gas ambient and temperature

High-pressure X-ray photoelectron spectroscopy of palladium model hydrogenation catalysts. Part 2: Hydrogenation of trans-2-pentene on palladium

Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistry

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