Integrated Experimental And Dft Study Of Palladium And Nickel Metals Supported On Reducible Metallic Oxides For Selective Hydrogenation Of Alkynes And Dienes Bonds

الموضوعات

علوم تطبيقية وتكنولوجية

الملخص

This dissertation investigates the capacity of ceria (CeO 2 ) to serve as a stand-alone reactive catalyst, and to act as catalyst support for different catalyst configurations. Experimental and density functional theory (DFT) approach were used to critically predict and to design ceria-containing catalysts. Ceria owes the catalytic propensity to the ability to undergo switch in the oxidation state from the +4 to +3 states. Motivated by experimental studies involving CeO 2 catalysts, defects CeO 2 surface with an oxygen vacancy (CeO2(111)_Vo) was used to explore the decomposition chemistry of methanethiol based on DFT. The potential formation pathways revealed that the initial uptake of CH 3 SH takes place via either C-S or S-H bond fissions through modest activation barriers. In the initial decomposition pathways, Vo spots represent strong acidic sites, and thus facilitating rupture of C-S and S-H fissions. Adjacent HS* and HO* sites undergo a hydrogen diffusion reaction to ultimately produce H 2 S rather than water. The outcome of this study necessitated the actual formation of vacancies on ceria surface via the inclusion of dopants into the ceria lattice from experimental and DFT perspectives. Niobium materials are evolving and their use in catalysis is being widely investigated. Thus, this dissertation reported the structural and electronic properties of various configurations of mixed Ce-Nb oxides. Tetrahedral O atoms demonstrate an increase in electronegativity and this in turn facilitates catalytic propensity of the material because the O atoms will exhibit higher affinity for adsorbed reactants. CeO 2 after Nb doping displays a noticeable band gap narrowing, confirming the improvement in the catalytic behavior. The 4 d states of the Nb 2 O 5 is found to fill up the 4f states of CeO 2 around the Fermi energy level promoting electrons excitations of the CeO 2 . Pertinent industrial reactions that involves the selective hydrogenation of alkynes and dienes bonds were tested on either CeO 2 or Nb 2 O 5 -CeO 2 supports. The presence of phenylacetylene (PA) in the styrene (ST) stream poisons the catalyst during polymerization reaction, increasing the operational cost. Different loadings of nickel and niobium metals supported on cerium (IV) oxide were prepared and characterized. H2-TPR reveals that Nb offers high interaction with the CeO 2 support that promoted O vacancies necessary to induce facile switch in the Ce oxidation states from +4 to +3. Gas phase PA Hydrogenation shows steady increase in the conversion with rise in temperature (150-300°C). The Nb catalysts display highest conversion at the optimum value of 5% Nb content. The catalysts selectivity towards ST was highest on the Nb catalysts reaching 96% and 91% on the Ni-5%NbCe and Ni-10%NbCe catalysts, respectively. Desorption of ST from the catalyst surface requires a mere energy value of 15 kJ/mol. This easy desorption of ST from the catalysts surface enhances the selectivity due to the electron transfer effect of Nb atoms to CeO 2 support. Selective hydrogenation of 1,3-butadiene to more valuable butenes products was investigated on supported palladium and nickel-palladium catalysts on ceria. H 2 pretreatment of the samples confer slight increase in the lattice constants and the crystallite sizes, which is associated with the formation of oxygen vacancies. The Ce 3+ concentration determined by XPS shows that the palladium catalysts is more decorated with the reduced Ce state than the palladium-nickel catalysts owing to the PdO and NiO species that demands reduction. Butadiene conversion shows steady rise with temperature (40-120°C) until the full conversion was reached on all the catalysts. The products distribution reveals that 4% Pd content directs the products toward butane when 40°C is exceeded. Lowering the Pd content and the simultaneous inclusion of Ni into the catalysts preserve the activity. Constructed mechanisms by DFT calculations feature low reaction barriers for involved surface hydrogenation steps; and thus, accounts for the observed low temperature of the surface hydrogenation activity. Selective formation of 1-butene partially stems from its relatively weaker binding to Ni sites in reference to Pd sites. The mapped out mechanisms entail higher reaction barrier for the formation of 2-butene; in an agreement with experimental observation pertinent to its formation at higher temperatures when compared with that of 1-butene. Finally, the selective hydrogenation of 1,3-butadiene was tested between 50-300°C on Ni catalysts supported on Nb 2 O 5 -CeO 2 . XRD results reveal the formations of nickel oxide and nickel metals for the calcined and reduced sample, respectively. The catalysts exhibit total conversion even at lower Ni loading, suggesting the high activity of the catalysts as confirmed from the measurement of the turn over frequency (TOF). The selectivity potential of the catalysts assumes a rather different configuration not previously reported for pure butadiene stream. The catalysts show selectivity to the expected butenes but rather than forming butane, propene and propane were formed at higher reaction temperature.

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التعريف والنوع

رقم الوثيقة

005e67c8-4221-486c-bb99-3c4e41e624d6

رقم العقد

0

نوع الوسائط

Crawler

نوع المحتوى

الرسائل العلمية

صيغة المصدر

رسائل دكتوراة

نوع الملف

pdf text

أسماء الملفات

2325573_2.pdf

بيانات النشر

ألقاب المؤلفين

[{"name_ar":"Mohammednoor Altarawneh","title_ar":"اشراف","title_en":"Supervision"},{"name_ar":"Toyin Daniel Shittu","title_ar":"اعداد","title_en":"Preparation"}]

اللغة

English

المصدر والدورية

اسم المصدر

Integrated Experimental And Dft Study Of Palladium And Nickel Metals Supported On Reducible Metallic Oxides For Selective Hydrogenation Of Alkynes And Dienes Bonds

المحتوى والصفحات

عدد الصفحات

0

إشراف وإعداد

الإشراف

Mohammednoor Altarawneh

الإعداد

Toyin Daniel Shittu

الاقتباسات الببليوغرافية