CORDIS - EU research results

Oxidation of Nanomaterials (NanO2)

Final Report Summary - NANO2 (Oxidation of nanomaterials)

The main objective of the NANO2 project was to understand and control the oxidation of nanomaterials in industrially and environmentally relevant conditions. NANO2 aimed overcome the pressure and materials gap from single crystalline surface oxidation studies at near-Ultra high vacuum (UHV) conditions to the ambient pressure oxidation of nanoparticles. The key parameters exploited in NANO2 to tailor nano-oxidations were the material, the size and the shape of nanoparticles as well as the substrate material. NANO2 focused on 4/5d transition metals (Pd, Rh, Ru and Pt) and 3d metals (Cu) which were already applied or will potentially be applied in industry, as in the catalytic oxidation of methane, in methanol and ammonia synthesis, or in hydrocarbon conversion processes for fuel cells.

As an important ingredient for the shape reconstruction by the Wulff diagram, individual facet orientations and vicinal orientations between the different facets were studied as a function of the gas pressure and temperature. NANO2 has bridged the pressure gap for a large variety of different materials, low index surfaces and vicinal surfaces. In particular, the following achievements have been made:
- to Increase knowledge on the oxidation of single crystal surfaces under industrially relevant conditions for the systems Rh(100), Rh(111), Ru(0001), Pd(111), Pd(100), Pt(110), Pt(111): the formation of a surface oxide trilayer O/ME ion/O is observed as a common feature before the bulk oxide formation sets in;
- oxidation of vicinal surfaces for the example of Pt(553), Rh(553), Pd(553); Rh(111) with holes: the role of steps is identified as nucleation centre for the growth of surface oxide trilayers: the trilayer stability can induce a reorganisation of the step structure;
- characterisation of the oxidation of vicinal and single crystal surfaces of Pd(553), Pd(223), Rh(223) and Pt(553), Pd(111) as a function of oxygen pressure and temperature by a multi-technique approach. As a general feature it is established, that the formation of ultrathin surface oxide layers on vicinal surfaces is strongly influenced by the atomic structure and periodicity of the steps. Oxide growth is observed to take place along the steps accompanied by a rearrangement of the step structure. The surface oxide structures differ from those observed on the corresponding '0' miscut single crystal surfaces;
- characterisation of the oxidation of vicinal and single crystal surfaces of Pd(553), Pd(110), Rh(110) and Rh(111), Pd(111), Ag(111) as a function of oxygen pressure and temperature by a multi-technique approach and their CO reduction. Development of novel surface oxide structural models for these systems;
- proposal of a novel structural model for the oxygen induced, p(4x4) reconstruction on Ag(111), which was in the last decades believed to be a prototype for a surface oxide playing a role in catalytic reactions.

To deliver a fundamental understanding of the so-called 'subsurface oxygen', NANO2 has undertaken several efforts to prepare artificial subsurface oxygen under controlled conditions. In all cases investigated, subsurface oxygen was not stable and the concept of subsurface oxygen in the sense of lattice dissolved oxygen that is made responsible for chemical reactions, has to be questioned. Instead, the formation of meta-stable oxide films was observed, which take over the role of an oxygen buffer. A strong influence of the system size on the reactivity has been observed, which exemplifies the importance of quantum size effects for the reactivity. The following points have been addressed in detail:
- oxidation of ultrathin epitaxial Rh films on Ru(0001) and on Au(110): an enhanced stability of chemisorbed oxygen phases is observed;
- observation of a strong thickness dependence of the oxidation rate of Mg films on Au(110): the role of film thickness dependent quantum well states for oxidation reactions is identified
- observation of a strong size dependence of the oxidation of Pd nanoparticles on MgO(100): particles smaller than 4 nm are much more reactive towards oxidation;
- characterisation of a artificial subsurface oxygen on Ru(0001) covered by epitaxial Rh films: subsurface oxygen is not stable and segregates to the surface;
- oxidation and reduction of Ru(0001) and comparison with industrial Ru catalysts: it was demonstrated that knowledge created on the oxidation of Ru(0001) and Rh(10-10) surfaces can be extrapolated to the oxidation of Ru nanoparticles;
- observation of novel surface oxides for ultrathin Pd films on Rh(111);
- investigation of the atmospheric pressure oxidation of Ir(111). As oxidation mechanism the formation of trilayer stacks was observed, transforming into rutile bulk oxide for thicker IrO2 films;
- the reduction of ultrathin Rh cap layers for Extreme ultraviolet lithography (EUVL) optics was investigated; H2 can be employed at room temperature to clean the Ru surface.

Due to their reduced size, the electronic properties of nanomaterials change and even undergo metal / insulator transitions, which strongly influence the adsorption properties of oxygen at a cluster. NANO2 has extended the theoretical work from single crystal surfaces at elevated pressures and temperatures using ab-initio and kinetic Monte Carlo techniques, towards the description of vicinal surfaces. NANO2 has embarked into ab-initio modelling of the oxidation of nanosized materials at high oxygen pressures and high temperatures, thereby allowing a rigorous comparison with experiment and enabling reliable prediction of oxidation properties. Based on the energetics known fro the single crystal and vicinal surfaces, the environmentally induced shape change of nanoparticles can be evaluated using the Wulff construction. The theory groups have addressed the following points in detail:
- The CO oxidation reaction over Pd(100) was investigated by ab initio Monte Carlo simulations, demonstrating that the sqrt 5 surface oxide is stable under industrial CO oxidation conditions.
- The adsorption of oxygen on various facets of Rh and Pd has been investigated by Density functional theory (DFT) and molecular dynamics simulations.
- First-principles kinetic Monte Carlo simulations on CO oxidation reactions allowing for the first time to predict reaction rates on single crystal surfaces.
- Theoretical treatment of the vicinal O/Pd(11N) surfaces, O/Rh(331), O/Rh(553) subsurface O/Pd(111), oxidation of Pd(100), stability of PdO nanoparticles
- Theoretical description of the particle shape as a function of the oxygen chemical potential by a combination of DFT results from the oxygen interaction with different single crystal facets and the Wulff construction.
- First-principles kinetic Monte Carlo simulations on CO oxidation reactions allowing for the first time to predict reaction rates on single crystal surfaces.

The following systems have been studied in detail under oxidation / reduction conditions by a multi-technique approach:
- In situ Transmission electron microscopy (TEM) study of the shape change of Cu nanoparticles on different substrates in H2 and H2O atmospheres.
- Demonstration that potassium acts as a promoter for the oxidation of Rh(110) surfaces.
- First observation of the formation of a novel surface oxide present during the CO oxidation on a Pt(110) single crystal, only present at high pressures close to industrial reaction conditions.
- The partial methanol oxidation over Ru/RuO2 was investigated.
- The oxidation of Rh microparticles was investigated by XPS. The results demonstrate that each particle acts as independent microreactor.
- Experiments on epitaxial Cu and Pd particles have been performed under industrially relevant reaction conditions.
- First structural in-situ investigation of a Pt25Rh75 alloy single crystal during a chemical reaction.

NANO2 has reached all milestones defined within the project and all deliverables were fulfilled. From the results obtained during the duration of NANO2, it becomes clear that the NANO2 consortium has performed work beyond the project's objectives and that NANO2 has created a new state of the art in the strongly expanding scientific field of the oxidation and chemical reactions on nanomaterials. As a result 50 publications are already accepted, and overall nearly 100 publications in highly ranked journals are expected. A huge number of 300 additional dissemination activities in the form of talks and posters at international conferences have been performed, which are expressing the very high impact that the work of NANO2 is expected to have.