Geri Dön

Effect of adsorbate interactions on catalytic reactivity: Elementary surface reactions on rhodium and cobalt

Başlık çevirisi mevcut değil.

PDF İndir

  1. Tez No: 402323
  2. Yazar: ALİ CAN KIZILKAYA
  3. Danışmanlar: PROF. DR. J. W. NIEMANTSVERDRIET
  4. Tez Türü: Doktora
  5. Konular: Metalurji Mühendisliği, Metallurgical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2014
  8. Dil: İngilizce
  9. Üniversite: Technische Universiteit Eindhoven
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 199

Özet

Özet yok.

Özet (Çeviri)

Kinetics on heterogeneous catalyst surfaces are usually represented by oversimplified models, which leads to an incorrect description at the atomic scale. The main assumptions that are used in traditional models are that the adsorption sites on the surface are equivalent, the adsorbates do not interact and the catalyst surface remains unchanged. Although such models work reasonably well to describe macroscopic kinetics of a supported catalyst in a reactor, detailed information at the atomic level as obtained by surface science studies can give insights on adsorbate interactions and their effect on reaction rate. The aim of this thesis is to investigate the role of adsorbate interactions on phenomena such as adsorbate configurations, surface restructuring, and kinetics. In the research project that was chosen as the subject of this thesis, experiments were performed on Rh(100) and flat, sputtered and defective Co(0001) surfaces. This combination of surfaces allows us to understand the role of undercoordinated sites in catalysis and to make better projections about nanoparticles. Theoretical calculations were used to study a stepped Co(211) surface, as a model for surfaces that contain undercoordinated sites. A number of experimental and theoretical tools have been employed. On the experimental side, Temperature Programmed Desorption (TPD) is the tool that has been used most frequently, as it can provide indirect information on adsorbate coverages, adsorbate interactions and activation barriers. Work Function (WF) measurements supply direct information on the coverage of adsorbates and they were often performed simultaneously with the TPD measurements to confirm or enrich the information supplied indirectly by TPD. Low Energy Electron Diffraction (LEED) was utilized to monitor ordered structures. Scanning Tunneling Microscope (STM) measurements provided invaluable local information about the exact structure of the surface on the atomic scale, such as steps, defects, etc. and on the ordered structures formed by adsorbates on the surface. Such information is otherwise inaccessible by techniques that generally provide information averaged over the surface. Computationally, Density Functional Theory (DFT) and mean field kinetic modelling were utilized.DFT simulations were used to further confirm the experimental results, as the surface models that are used are very closely related to experimentally used surfaces. Furthermore, DFT simulations provided information that is otherwise very difficult to obtain by experimental tools. Kinetic modelling was used to extract activation barriers from an experimental dataset and to extrapolate the results that are obtained under Ultra High Vacuum (UHV) conditions to practical conditions for Fischer-Tropsch Sytnhesis (FTS), that is, high pressure and isothermal operation. Chapter 3 presents an example on how changes in adsorbate structure influence reactivity. CO oxidation on Rh(100), which is relevant for automotive exhaust catalysis, serves as an example. A combination of LEED, TPRS, RAIRS and DFT modelling was used. Repulsive lateral interactions are found to decrease the stability of CO+O adlayer, resulting in different adsorbate structures at increasing coverages, and a significant modification of the reactivity for CO oxidation. Chapter 4 provides a direct relation between adsorbate interaction, configurations and reactivity by investigating CO oxidation on Rh(100) and Rh(111) surfaces covered by various amounts of CO and O by DFT simulations. The effect of the adsorbate interactions varies between two surfaces, demonstrating the structure sensitivity of the CO oxidation reaction. The results demonstrate the importance of adsorbate configurations for the rate of a surface reaction, and illustrate the sensitivity of a relatively simple reaction such as CO oxidation to the exact structure of the metal surface. Chapter 5 deals with the surface chemistry of NH3 on Co(0001), investigated experimentally under UHV conditions and theoretically with Density Functional Theory calculations. NH3 is found to adsorb molecularly on Co(0001), as the activation barrier for decomposition is higher than its adsorption energy. In relation to its relevance for Fischer-Tropsch Synthesis (FTS), where it is known to act as a poison, co-adsorption of NH3 with CO and atomic H is investigated. The results showed that there is a mutual stabilization of NH3 and CO on Co(0001) and preadsorbed CO does not inhibit NH3 adsorption. Pre-adsorbed NH3, NH and N are found to decrease the quantity of CO and H that can be adsorbed on Co(0001) due to lowering of their sticking coefficient (in case of NH3) and the blocking of sites (NH, N) necessary for CO adsorption or H2 dissociation. Chapter 6 extends the understanding about NH3 chemistry on cobalt surfaces by investigating NH3 decomposition experimentally on defective Co(0001) under UHV conditions and computationally by DFT modelling on the Co(211) surface. This approach aims to model nanoparticles which consists mainly of flat and stepped (kinks, defects,etc.) surfaces. Afterwards, the DFT results obtained on flat Co(0001) and stepped Co(211) surface are exploited to build a kinetic model to make projections about the prevailing NHx species at temperature and pressure conditions that are relevant for FTS. It is shown that defects on cobalt surfaces are very active sites for NH3 decomposition. Kinetic modelling indicated that the strongly bound NH2 and NH species will be present on stepped cobalt surfaces at typical FTS conditions, inhibiting CO adsorption and dissociation on cobalt surfaces. Chapter 7 covers the adsorption and removal of oxygen on flat and sputtered cobalt surfaces due to its relevance for FTS as a major elementary reaction that is generally overlooked in modeling. LEED, STM, XPS, temperature programmed and isothermal WF measurements were performed under UHV conditions and hydrogen pressures up to 1x10-5 mbar. It was found that chemisorbed oxygen forms islands on Co(0001) at low temperature. Surface oxygen can be fully reduced on both flat and sputtered Co(0001) surfaces under hydrogen pressure of 1x10-5 mbar at 450 K and 550 K respectively. The experimental activation barrier for the water formation removal reaction is found as 128 kJ/mol for the flat Co(0001) and 134 kJ/mol for the sputtered Co(0001) by kinetic modelling of the isothermal hydroxyl removal reaction. The removal of oxygen under CO pressure was not possible under the same temperature regimes used for oxygen removal under hydrogen pressure. 8.2. General conclusions Throughout the thesis, a variety of different approaches have been combined during the investigation of each subject, such as UHV experiments, DFT modelling and mean field kinetic modelling. Each of these approaches have their own limitations, but by combining the results of the different techniques, a very detailed insight about the catalytic problem could be obtained. For example, experimental data obtained in the UHV chamber, like TPD spectra, are often influenced by background desorption and may result from different surface process, e.g. free hydrogen desorption, NHx decomposition, etc. DFT modelling can provide adsorption energies and activation barriers on a chosen surface and allow a more detailed interpretation of the data. However due to the inherent errors in DFT, for example, inability to correctly predict CO adsorption sites and energies on transition metal surfaces, van der Waals forces, and arbitrary choice of surface structure, the value of the results are questionable unless experimental confirmation is obtained. Using a combination of experiment and theory, it becomes possible to obtain a detailed molecular level understanding about the catalytic surface reaction. Kinetic modelling offers the opportunity to estimate the desired catalytic parameters such as surface concentrations and reaction rates at catalytically relevant conditions, thereby providing a first approximation to overcome the pressure gap between surface science and the catalytically relevant conditions. An important effect that is caused by the strong adsorption energy of atomic adsorbates and strong lateral interactions between them is surface restructuring. This is observed for 0.50 ML, saturation coverage, oxygen chemisorbed on Rh(100). The Rh surface atoms undergo a clockwise rearrangement for this coverage. A restructured surface can either be more or less reactive, depending on the surface structure, which is an important consequence of adsorbate interactions. The reconstruction brought about by a high chemisorbed oxygen coverage on Rh(100) is not highly reactive for CO oxidation. Upon CO co-adsorption, the surface is restored to its original structure and CO oxidation can proceed at a higher rate afterwards. An important role that lateral interactions can play for heterogeneous catalytic reaction kinetics is altering the adsorbate configurations and in turn influence catalytic activity. If the adsorbates interact repulsively, the effect of such interactions on configurations and reactivity is only observed at high coverages, when adsorbates are forced to adsorb in close proximity. This is the phenomenon that is observed on Rh(100) and Rh(111) surface during CO oxidation at high coverages. Repulsive interactions destabilize O and CO atoms at different configurations and thus decrease the activation barrier. Attractive interactions, however, are observed already at low coverages and result in the formation of adsorbate islands. The island formation limits the contact of species in a surface reaction, since homogeneous mixing is hindered. This then translates to a lower reaction rate. This effect has been observed for oxygen chemisorbed on Co(0001). Oxygen islands tend to react with hydrogen with a slow initial rate due to limited contact between O and H atoms. Attractive interactions also play an important role on the co-adsorption characteristics of species. Species with attractive interactions stabilize each other and can form stable coadsorption structures on the surface. The case of NH3 and CO coadsorption, together filling almost a complete monolayer on the Co(0001) surface, as described in Chapter 5, forms a remarkable example of this effect.

Benzer Tezler

  1. Tez No

    255899

    A theoretical study on catalytic steps of hydrogen production and purification

    Hidrojen üretimi ve saflaştırılması aşamalarında kullanılan katalitik sistemlerin kuramsal olarak incelenmesi

    ASLIHAN SÜMER

    Doktora

    İngilizce

    İngilizce

    2010

    KimyaBoğaziçi Üniversitesi

    Kimya Mühendisliği Ana Bilim Dalı

    PROF. DR. AHMET ERHAN AKSOYLU

  2. Tez No

    129226

    Identification of NOx species adsorbed on cobalt(II)-supported zirconia and sulfated zirconia and investigation of their reactivity toward methane

    Kobalt(II) depolanmış zirkonyum ve zirkonyum sülfat üzerine adsorbe olmuş NOx moleküllerinin tanımlanması ve metan ile tepkimesinin araştırılması

    AHMET SELİM VAKKASOĞLU

    Yüksek Lisans

    İngilizce

    İngilizce

    2002

    Kimyaİhsan Doğramacı Bilkent Üniversitesi

    Kimya Ana Bilim Dalı

    DOÇ. DR. MARGARİTA KANTCHEVA

  3. Tez No

    868594

    Comprehensıve analysıs of vıtamın D3 adsorptıon and monıtorıng usıng QCM wıth hydrophobıc algınate-halloysıte nanoclay composıte bılayers

    Hidrofobik aljinat-halloysit nanokil kompozitleri QCM kullanılarak D3 vitamini adsorpsiyonu ve izlenmesinin kapsamlı analizi

    MERVENUR KİRAZOĞLU

    Yüksek Lisans

    İngilizce

    İngilizce

    2024

    Biyomühendislikİstanbul Teknik Üniversitesi

    Nanobilim ve Nanomühendislik Ana Bilim Dalı

    DOÇ. DR. BİRGÜL BENLİ

  4. Tez No

    305295

    Fe(II) ve Mn(II)'nin birlikte giderimine DOM etkisi ve batık membran filtrelerle ileri arıtımı

    Effect of NOM on the removal of Fe(II) and Mn(II) and advanced treatment with immersed membrane system

    SUNA ÖZDEN ÇELİK

    Doktora

    Türkçe

    Türkçe

    2011

    Çevre Mühendisliğiİstanbul Üniversitesi

    Çevre Mühendisliği Ana Bilim Dalı

    DOÇ. DR. NEŞE TÜFEKÇİ

  5. Tez No

    854294

    Gün ışığı altında fotokatalitik yöntemle atık sudan siprofloksasin (CIP) giderilmesi

    Removal of ciprofloxacin (CIP) from wastewater by photocatalytic method under daylight

    ZOZAN GÜL

    Yüksek Lisans

    Türkçe

    Türkçe

    2024

    Kimya Mühendisliğiİstanbul Teknik Üniversitesi

    Kimya Mühendisliği Ana Bilim Dalı

    PROF. DR. GÜLHAYAT NASÜN SAYGILI

Geri Dön