Adsorption and reaction of CO on (Pd–)Al2O3 and (Pd–)ZrO2: vibrational spectroscopy of carbonate formation

γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm−1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface

Synthesis, Characterization and Application of intermetallic Pd-X (Ga, Zn) Nanoparticles derived from ternary Hydrotalcite-like precursors

A novel, feasible synthesis approach for supported intermetallic Pd2Ga and PdZn nanoparticles derived from Hydrotalcite-like compounds (HTlc) is introduced. Ternary HTlc with the nominal composition (Pd2+,M2+)0.70(M3+)0.30(OH)2(CO3)0.15 ∙ m H2O are synthesized by pH-controlled co-precipitation. Mg2+/Ga3+ and Zn2+/Al3+ are chosen as M2+/M3+ combinations to permit formation of the nanocrystalline Pd2Ga and PdZn intermetallic compounds on a porous MgO/MgGa2O4 and ZnO/ZnAl2O4 support, respectively. In addition, a PdMgAl HTlc is prepared as monometallic Pd reference compound on a MgO/MgAl2O4 support. Incorporation of Pd2+ into the HT structure requires octahedral coordination, while Pd2+ ions prefer square planar coordination in aqueous solution. At the same substitution degree of M2+ by Pd2+, complete insertion is achieved for PdZnAl HT. In case of PdMgGa and PdMgAl HT a minor fraction is present as segregated Pd2+ on the external surface of the platelet-like particles with a local environment similar to PdO, i.e. in a square planar coordination. A limit of incorporation into the HT lattice exists at < 1 mol% for the Pd2+ containing precursors. Upon thermal decomposition in reductive atmosphere, intermetallic and metallic nanoparticles ranging from below 2 nm to 6 nm in size and exhibiting monomodal particle size distributions are formed. Alloying of Pd with Ga and Zn changes the crystal structure as well as the electronic structure and leads to the increased formation of isolated adsorption sites at the surface. Furthermore, dynamic surface changes of intermetallic Pd2Ga nanoparticles were noticed at longer exposure time to CO and higher CO coverage. This is attributed to the decomposition into metallic Pd and Ga2O3. The nanostructured Pd2Ga catalyst shows excellent performance in the selective semi-hydrogenation of acetylene similar to a bulk Pd2Ga model catalyst. In comparison to the elemental Pd catalyst the selectivity to ethylene is drastically improved by formation of Pd2Ga. Interestingly, the nanostructered catalyst slowly activates in the feed gas. The activation is triggered faster by a treatment in oxidative atmosphere. These dynamics of the Pd2Ga nanoparticles can be explained by the interplay of surface decomposition into Pd0 and Ga2O3 in oxygen and reversal of the strong-metal support interaction state leading to an increased activity. Furthermore, increased activities and selectivities in methanol steam reforming and methanol synthesis from CO2 are observed for the Pd2Ga and PdZn nanoparticles in contrast to the unmodified Pd particles. These structurally modified Pd catalysts exhibit a considerably lower selectivity to CO and enhanced formation of methanol compared to the monometallic Pd catalyst.Abstract i Zusammenfassung iii Danksagung v List of Figures x List of Tables xiv List of Abbreviations xv Chapter 1: Introduction and Overview 1 1.1. Intermetallic compounds 1 1.2. Motivation 2 1.3. Synthesis strategy for binary Pd-X intermetallic nanoparticles 3 1.4. Pd-Ga system 6 1.5. The Pd-Ga intermetallic compound Pd2Ga 7 1.6. Pd-Zn phase system 9 1.7. Pd-Ga IMCs as selective acetylene hydrogenation catalysts 10 1.8. Pd based IMCs in methanol synthesis and methanol steam reforming 13 1.9. Aims of this work and thesis structure 15 1.10. References 17 Chapter 2: Intermetallic Compound Pd2Ga as a Selective Catalyst for the Semi-Hydrogenation of Acetylene: From Model to High performance Catalyst 21 2.1 Introduction 22 2.2 Experimental Section 24 2.2.1 Synthesis Procedures 24 2.2.2 Characterization Techniques 24 2.2.3 Catalytic Measurements 25 2.3 Results and Discussion 27 2.4 Conclusion 34 2.5 References 36 Chapter 3: Dynamic Surface Processes of nanostructured Pd2Ga catalysts derived from ternary Hydrotalcite-like Precursors 38 3.1 Introduction 39 3.2 Experimental 40 3.2.1 Synthesis conditions 40 3.2.2 Characterization 41 3.2.3 Catalytic performance in the selective hydrogenation of acetylene 43 3.3 Results and Discussion 44 3.3.1 Structural and textural properties of the precursor material 44 3.3.2 Reduction and intermetallic phase formation 49 3.3.3 Structural properties of Pd2Ga nanoparticles 54 3.3.4 IR characterization of supported Pd2Ga nanoparticles during exposure to CO 58 3.3.5 Catalytic performance 62 3.4 Conclusions 69 3.5 References 71 Supplementary Information 73 Chapter 4: Methanol Synthesis and Methanol Steam Reforming of Supported Pd2Ga and PdZn Intermetallic Nanoparticles 79 4.1. Introduction 80 4.2 Experimental 81 4.2.1 Synthesis conditions 81 4.2.2 Characterization 82 4.2.3 Catalytic performance 84 4.2.3.1 Methanol synthesis from CO2 84 4.2.3.2 Methanol steam reforming 85 4.3 Results and discussion 86 4.3.1 Properties of the HTlc precursors 86 4.3.2 Reducibility of the HTlc precursors and IMC formation 88 4.3.2.1 TPR and MS measurements 88 4.3.2.2 XANES measurements 91 4.3.3 Properties of the ex-HTlc samples after reduction 93 4.4 Catalytic properties of the IMCs 97 4.4.1 Methanol synthesis from CO2 97 4.4.2 Steam reforming of methanol 100 4.5 Conclusion 102 4.6 References 103 Supplementary Information 105 Chapter 5: Final summary and conclusion 108 Appendix xvii Curriculum vitae xvii Publications xvii Oral presentations xviii Patent application xviii Poster presentations xi

Single-Particle Catalysis: Revealing Intraparticle Pacemakers in Catalytic H2Oxidation on Rh

Self-sustained oscillations in H2 oxidation on a Rh nanotip mimicking a single catalytic nanoparticle were studied by in situ field emission microscopy (FEM). The observed spatio-Temporal oscillations result from the coupling of subsurface oxide formation/depletion with reaction front propagation. An original sophisticated method for tracking kinetic transition points allowed the identification of local pacemakers, initiating kinetic transitions and the nucleation of reaction fronts, with much higher temporal resolution than conventional processing of FEM video files provides. The pacemakers turned out to be specific surface atomic configurations at the border between strongly corrugated Rh{973} regions and adjacent relatively flat terraces. These structural ensembles are crucial for reactivity: while the corrugated region allows sufficient oxygen incorporation under the Rh surface, the flat terrace provides sufficient hydrogen supply required for the kinetic transition, highlighting the importance of interfacet communication. The experimental observations are complemented by mean-field microkinetic modeling. The insights into the initiation and propagation of kinetic transitions on a single catalytic nanoparticle demonstrate how in situ monitoring of an ongoing reaction on individual nanofacets can single out active configurations, especially when combined with atomically resolving the nanoparticle surface by field ion microscopy (FIM)

Local Catalytic Ignition during CO Oxidation on Low-Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study

Shedding light on light-off: Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis