Spectroscopic and kinetic evidence for origins of selective bond activation over transition metal catalysts

Sampath, Abinaya

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https://hdl.handle.net/2142/115722 Copy

Description

Title

Spectroscopic and kinetic evidence for origins of selective bond activation over transition metal catalysts

Author(s)

Sampath, Abinaya

Issue Date

2022-04-20

Director of Research (if dissertation) or Advisor (if thesis)

Flaherty, David

Doctoral Committee Chair(s)

Flaherty, David

Committee Member(s)

• Yang, Hong
• Jain, Prashant
• Peters, Baron

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Hydrogenation
• dehydrogenation
• oxidation
• phosphorus
• promoters
• support
• interface
• Palladium
• Ruthenium
• Gold
• Silica
• Cerium Oxide
• Aluminium oxide
• Raman spectroscopy
• Interface
• CO oxidation
• Water

Abstract

Promoters and supports for transition metal catalysts affect heterogeneous catalysis by altering the reactivity towards for the activations of specific bonds in small molecules relevant for hydrogenation and oxidation reactions. Promoters induce electronic and ensemble changes on the catalytic metal surfaces and understanding of these changes is important for reactive and selective catalyst design. Often, the metal-support interface sites play a crucial role in bond activation in supported transition metal catalysts for selective oxidation reactions. However, it is very challenging to characterize the properties of these sites. Here, we use surface science techniques to probe the role of phosphorus as a promoter for C-C and C-H bond activation steps for hydrogenation and dehydrogenation reactions; use in situ Raman spectroscopy to examine the activation of O-O bonds of H2O2 and O2 over Au nanoparticles stabilized by distinct forms of supports. Transition-metal and metal phosphide nanoparticles catalyze hydrogen-transfer steps that reduce biomass-derived aldehydes using either gaseous H2 or organic hydrogen donors (e.g., alcohols) and produce valuable chemicals and fuels to replace petroleum derivatives. Moreover, transition metal phosphides are more selective for non-oxidative dehydrogenation than the corresponding unmodified transition metals, however there are few detailed experiments probing thermodynamics and kinetics of adsorption and reactions of hydrocarbons on these surfaces. Chapter 1 discusses the current importance of studying hydrogenation/dehydrogenation reactions over Ru and Pd model catalysts and the plausible effects of P as a promoter on the selectivity of these reactions using probe reactions. Supported Au nanoparticles are more reactive for selective oxidation reaction when compared to bulk Au. Similar to the presence of promoters, the metal-support interface sites in supported transition metal catalysts provide catalytic functions different from that of the metal that improve catalyst reactivity. Chapter 1 also illustrates the importance of studying the role of these interfacial sites in O-O bond activation in H2O2 using in situ spectroscopic techniques. Later, the discussion investigates the various effects of presence of supports for Au catalysts in the context of H2O-assisted O2 activation over Au surfaces. Chapter 2 presents our examination of the reaction pathways of aromatic aldehydes (furfural, benzaldehyde) on Ru(0001) and on P0.4–Ru, a surface representative of the (0001) facet of Ru2P, which show how the addition of phosphorus influences activation barriers and flux along competing reaction pathways. Although Ru(0001) entirely decomposes furfural to CO, H2, and surface carbon, P0.4–Ru primarily decarbonylates furfural to form CO and furan. Similarly, benzaldehyde decarbonylates to benzene with a selectivity that is 7-fold greater over P0.4–Ru than on Ru(0001). These observations are consistent with weaker interactions between adsorbates and P0.4–Ru than on Ru that reflect charge transfer from Ru to P that in turn reduces electron back donation from Ru to the adsorbates and leads to selective decarbonylation of aromatic aldehydes over P0.4–Ru. The distribution of product isotopologues from temperature-programmed reactions (TPR) of selectively deuterated forms of furfural on P0.4–Ru shows that furan forms by catalytic transfer hydrogenation (CTH) on P0.4–Ru. The rate-determining step that forms furan from a furanyl surface species involves hydrogen transfer, but H atoms transfer directly from either the aldehydic group or the decomposed aromatic ring of the parent molecule and not from H* atoms chemisorbed to Ru. These findings show that modifying Ru with phosphorus results in the selective rupture of C–C bond required for decarbonylation while facilitating CTH of reactive intermediates from aromatic aldehydes in the absence of an external hydrogen source. Chapter 3 uses a combination of TPR and reactive molecular beam scattering (RMBS) of cyclohexene (C6H10) and benzene (C6H6) and temperature programmed desorption of CO and H2 to examine how the addition of P-atoms alters the catalytic properties of Pd(111) for non-oxidative dehydrogenation of C6H10 to C6H6. Quantitative analyses of TPR results demonstrate that a phosphorus modified Pd(111) surface (P0.34–Pd) dehydrogenates C6H10 to C6H6 with a selectivity approaching 100%, which is ∼3-fold greater than that of the unmodified Pd(111) surface (∼30%). These differences in selectivity correlate with a significantly reduced desorption energy for small molecules (e.g., CO, C6H6, C6H10, H2) on P0.34–Pd in comparison to Pd(111). Together these comparisons suggest that adsorbates exchange less charge with P0.34–Pd than with Pd(111), which agrees with prior computational predictions. Diminished electron exchange between Pd atoms and bound reactive intermediates leads also to greater near steady-state selectivities for dehydrogenation of C6H10 to C6H6, decreased rates of C–C bond rupture, and increased catalyst stability. Apparent activation energies for dehydrogenation of C6H10 are lower for Pd (−13 kJ mol−1) than P0.34–Pd (−7 kJ mol−1), while those for C6H6 decomposition seem to differ more significantly (∼90 kJ mol−1 on Pd; immeasurable rates and barrier for P0.34–Pd) as shown by the RMBS of C6H10 and C6H6 respectively. As a consequence, P also increases apparent barriers for catalyst deactivation observed during RMBS of C6H10 (1 kJ mol−1 on Pd to 38 kJ mol−1 on P0.25–Pd) due to the decreased tendency to decompose C6H6 to more hydrogen-deficient forms of carbon. These results demonstrate that the presence of P-atoms on and within the near surface region of Pd significantly impacts the energetics of C–H and C–C bond rupture pathways in ways that lead to greater selectivities and stabilities for hydrocarbon dehydrogenation reactions. Chapter 4 discusses the incorporation of phosphorus into the Ru(0001) surface that increases the selectivity of C6H10 dehydrogenation to C6H6 by 100-fold when compared to Ru(0001) under steady-state conditions. We propose a series of elementary steps for the reactions of C6H10 over Ru(0001) and P0.4-Ru(0001) based on TPR of C6H10, 1,3-cyclohexadiene and RMBS of C6H10 on Ru(0001) and P0.4-Ru(0001). TPR of 1,3-cyclohexadiene shows that P atoms alter the kinetically relevant step for C6H10 dehydrogenation from C–H activation in 1,3-cyclohexadiene on Ru(0001) to C–H activation in 2-cyclohexenyl on P0.4-Ru(0001). During TPR of C6H10, C6H6 forms over P0.4-Ru(0001) with an intrinsic activation energy that is 40 kJ mol–1 lower than that for Ru(0001). In addition, the presence of P atoms increases the apparent activation energy for deactivation by 21 kJ mol–1 during RMBS of C6H10. The increase in the barrier for deactivation, presumably by C–C bond rupture steps, significantly reduces the quantity of coke formed by consecutive TPR of C6H10 and contributes to greater selectivities for C6H6 formation. These observations suggest that the addition of P atoms to Ru(0001) introduces both electronic and geometric effects that alter the metal–adsorbate interactions. These findings indicate that transition-metal phosphides may be useful for selective dehydrogenation reactions important for reforming light hydrocarbons (e.g., ethane, propane, and cycloalkanes) to increase the yield of valuable alkenes and arenes. Distinct catalytic functions at interfaces between oxides and Au nanoparticles assist in the activation of O2 and H2O2 during oxidation reactions. Chapter 5 shows how we use in situ surface-enhanced Raman spectroscopy (SERS) to identify how oxide supports (SiO2, γ-Al2O3) change the stability of monatomic and diatomic oxygen intermediates from H2O2 activation on Au in aqueous phase. Au nanoparticles on SiO2 weakly bind diatomic oxygen intermediates and do not show significant quantities of atomic oxygen (O*). Au on γ-Al2O3 more strongly binds diatomic oxygen species (O2*, OOH*), and stabilizes significant hydroxyl (OH*) coverages near Au nanoparticles that facilitate O-O bond cleavage leading to formation small quantities of O*. While Au(111) does not activate O-O bond, electrochemical roughening introduces significant OH* coverages to Au, which in the absence of a support, activate H2O2 to produce high O* coverages, in addition to stabilizing diatomic oxygen intermediates. These results provide direct spectroscopic evidence that OH* present near Au active sites change the distribution of reactive oxygen species present during catalytic oxidation reactions. Furthermore, we illustrate the use of transient spectroscopic measurements on the Au extended surfaces to successfully spectroscopically distinguish different monoatomic oxygen intermediates present on Au. H2O adsorption over supported Au nanoparticles facilitates O2 adsorption and activation during partial oxidation reactions. Chapter 6 presents the effects of reducible and irreducible Au-support interface sites (nanoporous Au, Au-CeO2, and Au-SiO2) influence the manner by which H2O molecules assist in the adsorption and dissociation of O2 on Au nanoparticles in contact with gas-phase reagents using in situ SERS developed in Chapter 5. The pressures of H2O and O2 affect binding energies and coverages of O2-derived intermediates on all Au surfaces examined. Specifically, H2O facilitates O2* adsorption and increases the binding energies with the formation of H2O-activated O2 complexes on nanoporous Au at lower O2 pressures and temperatures. Higher ratios of O2 to H2O pressures facilitate O-O bond activation in these H2O-activated O2 complexes and the formation of monoatomic species over nanoporous Au. Diatomic oxygen intermediates bind the most strongly to Au-CeO2 when compared to Au-SiO2 and nanoporous Au and the coverage increases with O2 and H2O pressures. Prior oxidative/reductive treatments influence the distribution of oxidized Au on Au-CeO2 that increase the diatomic oxygen coverage. H2O stabilizes these oxidized Au and further increase the diatomic oxygen intermediates on Au-CeO2. Au-SiO2 binds negligible quantities of monoatomic and diatomic oxygen intermediates than nanoporous Au or Au-CeO2 at all conditions, apparently because Au-SiO2 interface interacts weakly with O2 and H2O molecules. These results provide direct spectroscopic evidence for the differences in the form and coverage of O2-derived surface intermediates on supported Au nanoparticles and the effects of H2O pressure on these quantities. adsorption and activation over Au surfaces. Analysis of these findings clearly shows that the sensitivity of O2 adsorption and activation to the presence of H2O depends strongly on composition of the supporting oxide, which indicates the significance of Au-support interfacial sites. Chapter 7 summarizes the findings from Chapter 2-6 on the effects of promoters and supports on the selective activation of bonds in small molecules over transition metal and transition metal phosphide catalysts. Chapter 7 also includes the proposed potential future research directions that seek to develop knowledge of O2 activation and the effects of H2O on Au and Au-based alloy catalysts during selective oxidation reactions using in situ SERS methods. H2O acts as a co-catalyst for reactions such as CO oxidation but there is limited in situ spectroscopic evidence for H2O-assisted aerobic CO oxidation and how it differs with metal-support interface sites. In situ Raman spectroscopic and in operando measurements of CO oxidation over supported Au nanoparticles would elucidate the purportedly co-catalytic role of H2O in this reaction. We look to compare changes in the forms and coverages of reactive oxygen species in response to CO, O2, and H2O pressures and correlate these measurements to rates for CO oxidation over Au catalysts to expand existing knowledge of these processes. Au-based alloy catalysts are increasingly used for many partial oxidation reactions such as H2, alkene, and alcohol oxidation. Prior work shows that alloying transition metals such as Pd and Pt with Au suppresses O-O bond activation steps that form atomic oxygen and facilitate undesirable reaction pathways that completely oxidize (i.e., combust) hydrocarbon and oxygenated reactants. Nevertheless, the presence of the active metal is crucial for higher rates of oxidation reactions. The current challenges are to identify the changes in the binding energies of oxygen intermediates and the active sites for O-O bond activation with alloy composition in situ that affect reaction selectivities. Electrochemical deposition of Pd on roughed Au extended surfaces will produce varying composition of Au-Pd surfaces. SERS can be used to study O2 binding and activation over these Au-Pd alloy catalysts at different O2 pressures. In operando measurements of ethylene oxidation over Au-Pd alloy surfaces would inform us the reactive oxygen species induced changes in rates and selectivities. The changes in the reactive oxygen species and the active sites with alloy composition correlated to ethylene oxidation rate measurements will help us in designing future alloy catalysts for epoxidations using transition metals. Collectively, these results demonstrate how promoters such as phosphorus and the nature of metal-support interface sites affect the activation of bonds in small molecules. Fundamental understanding of these roles can facilitate rational choice of catalytic materials for various industrial reactions.

Graduation Semester

2022-05

Type of Resource

Thesis

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Copyright 2022 Abinaya Sampath

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