Oxidation Stability of Multiwalled Carbon Nanotubes for Catalytic Applications

The oxidation of multiwalled carbon nanotubes (CNTs) was investigated with regard to a detailed prediction of the lifetime of this material as a catalyst for oxidative dehydrogenations. A power-law kinetics is found to be adequate for the description of CO2 formation in the temperature range of 623−823 K and under O2 partial pressures of 0.025−0.6 bar. The stability against oxidation can be enhanced by passivation with B2O3 or P2O5 and by high temperature treatment. The progress of oxidative degradation was monitored by TEM and Raman spectroscopy. A mechanistic study supported by high pressure XPS and SSITKA reveals full agreement with the established model of the oxidation of conventional carbon materials; however, the theory of sequential layer degradation as observed for single crystal graphite is not transferable to a technical grade CNT material, and instead, various modes of propagation of combustion sites are identified

Enhancement of Stability and Activity of MnO_<i>x</i>/Au Electrocatalysts for Oxygen Evolution through Adequate Electrolyte Composition

Oxygen evolution and catalyst corrosion were studied side by side for electrodeposited MnO_<i>x</i> catalysts. Measurements using a combination of electrochemical flow cell, atomic absorption spectroscopy, and rotating ring disk electrode reveal a high sensitivity of oxygen evolution and of manganese oxide corrosion toward the presence of ions (alkali-metal cations and anions) in the electrolyte. The charge to radius ratio of alkali-metal ions affected the reactivity of the oxides and was seen to influence the reaction under both potentiostatic and potentiodynamic conditions, with Li⁺- and (K⁺,Cs⁺)-containing electrolytes showing the lowest and highest activities, respectively. Thermogravimetry in combination with mass spectrometry showed significant differences between samples treated in different electrolytes. Raman spectroscopy showed that the material transformed during the oxygen evolution reaction, with multiple phases α-MnO₂ and <i>birnessite</i>-MnO₂ being present in the catalyst during oxygen evolution reaction. Electronic structure (XANES) studies revealed the significant influence of alkali-metal ions on the oxidation state of Mn, with the OER-inactive Mn²⁺ oxidation state being stabilized with the Li⁺ ion. It was found that selected combinations of anions and cations in the electrolyte and suitable potential can significantly stabilize the electrode during OER application

In Situ Characterization of Alloy Catalysts for Low-Temperature Graphene Growth

Low-temperature (∼450 °C), scalable chemical vapor deposition of predominantly monolayer (74%) graphene films with an average D/G peak ratio of 0.24 and domain sizes in excess of 220 μm2 is demonstrated via the design of alloy catalysts. The admixture of Au to polycrystalline Ni allows a controlled decrease in graphene nucleation density, highlighting the role of step edges. In situ, time-, and depth-resolved X-ray photoelectron spectroscopy and X-ray diffraction reveal the role of subsurface C species and allow a coherent model for graphene formation to be devised

Introducing Carbon Diffusion Barriers for Uniform, High-Quality Graphene Growth from Solid Sources

Carbon diffusion barriers are introduced as a general and simple method to prevent premature carbon dissolution and thereby to significantly improve graphene formation from the catalytic transformation of solid carbon sources. A thin Al₂O₃ barrier inserted into an amorphous-C/Ni bilayer stack is demonstrated to enable growth of uniform monolayer graphene at 600 °C with domain sizes exceeding 50 μm, and an average Raman D/G ratio of <0.07. A detailed growth rationale is established via in situ measurements, relevant to solid-state growth of a wide range of layered materials, as well as layer-by-layer control in these systems

Oxygen-Doped Carbon Supports Modulate the Hydrogenation Activity of Palladium Nanoparticles through Electronic Metal–Support Interactions

In heterogeneous catalysis, synergies between the metal active phase and oxide support can enhance the catalytic activity through electronic metal–support interactions (EMSI). Such effects are unexpected for conventional carbon supports, and carbon is often viewed as an inert scaffold in catalysis. Here, we demonstrate that carbons do present EMSI that alter the intrinsic rate of palladium atoms near the interface by 200-fold compared to atoms at the apex of 5 nm particles. We also show that oxygen-containing functional groups, which are ubiquitous on carbon surfaces, are responsible for these EMSI. Controlling the scaffold’s surface chemistry allowed us to tune its work function from 5.1 to 4.7 eV, the intensity of the charge redistribution at the metal–carbon interface, and the catalytic activity of the corresponding metal atoms. The proposed platform can be applied to fundamentally understand EMSI effects for reactions and carbonaceous supports beyond those studied in the present work

Observing Graphene Grow: Catalyst–Graphene Interactions during Scalable Graphene Growth on Polycrystalline Copper

Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions. Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. The importance of these dynamic interactions is discussed for graphene growth, processing, and device integration

Co-Catalytic Solid-State Reduction Applied to Carbon Nanotube Growth

We report on a new class of cocatalysts for the chemical vapor deposition of carbon nanotubes, where the cocomponent (Ta) acts as a solid-state reducing agent for the active catalyst (Fe). The cocatalytic FeTa system enables carbon nanotube growth without the need for a reducing gas atmosphere such as H2 or NH3. In situ X-ray photoelectron spectroscopy reveals that the tantalum (oxide) getters the oxygen from the iron (oxide) by a diffusive solid-state process, driven by the much larger affinity to oxygen of Ta compared to Fe. We suggest that this redox-based mechanism is applicable to a wide range of metal (oxide)/catalyst systems and relevant to rational catalyst design in general heterogeneous catalysis

<i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth

The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni₂C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes

<i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth

The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed via complementary in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni2C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) via replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes

Nanoscale Zirconia as a Nonmetallic Catalyst for Graphitization of Carbon and Growth of Single- and Multiwall Carbon Nanotubes

We report that nanoparticulate zirconia (ZrO2) catalyzes both growth of single-wall and multiwall carbon nanotubes (CNTs) by thermal chemical vapor deposition (CVD) and graphitization of solid amorphous carbon. We observe that silica-, silicon nitride-, and alumina-supported zirconia on silicon nucleates single- and multiwall carbon nanotubes upon exposure to hydrocarbons at moderate temperatures (750 °C). High-pressure, time-resolved X-ray photoelectron spectroscopy (XPS) of these substrates during carbon nanotube nucleation and growth shows that the zirconia catalyst neither reduces to a metal nor forms a carbide. Point-localized energy-dispersive X-ray spectroscopy (EDAX) using scanning transmission electron microscopy (STEM) confirms catalyst nanoparticles attached to CNTs are zirconia. We also observe that carbon aerogels prepared through pyrolysis of a Zr(IV)-containing resorcinol−formaldehyde polymer aerogel precursor at 800 °C contain fullerenic cage structures absent in undoped carbon aerogels. Zirconia nanoparticles embedded in these carbon aerogels are further observed to act as nucleation sites for multiwall carbon nanotube growth upon exposure to hydrocarbons at CVD growth temperatures. Our study unambiguously demonstrates that a nonmetallic catalyst can catalyze CNT growth by thermal CVD while remaining in an oxidized state and provides new insight into the interactions between nanoparticulate metal oxides and carbon at elevated temperatures