State-of-the-art Nanofabrication in Catalysis

The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum-ceria catalyst. At low temperatures (25-100 °C and 1 mbar H2) surface O-H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O-H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring.

Microfabrication Enables Quantification of Interfacial Activity in Thermal Catalysis

A myriad of heterogeneous catalysts comprises multiple phases that need to be precisely structured to exert their maximal contribution to performance through electronic and structural interactions at their peripheries. In view of the nanometric, tridimensional, and anisotropic nature of these materials, a quantification of the interface and the impact of catalytic sites located there on the global performance is a highly challenging task. Consequently, the true origin of catalysis often remains subject of debate even for widely studied materials. Herein, an integrated strategy based on microfabricated catalysts and a custom-designed reactor is introduced for determining interfacial contributions upon catalytic activity assessment under process-relevant conditions, which can be easily implemented in the common catalysis research infrastructure and will accelerate the rational design of multicomponent heterogeneous catalysts for diverse applications. The method is validated by studying the high-pressure continuous-flow hydrogenation of CO and CO₂ over Cu-ZnO catalysts, revealing linear correlations between the methanol formation rate and the interface between the metal and the oxide. Characterization of fresh and used materials points to the model catalyst preparation as the current challenge of the methodology that can be addressed through further development of nanotechnological tools.

Plasma Generating—Chemical Looping Catalyst Synthesis by Microwave Plasma Shock for Nitrogen Fixation from Air and Hydrogen Production from Water for Agriculture and Energy Technologies in Global Warming Prevention

Simultaneous generation of plasma by microwave irradiation of perovskite or the spinel type of silica supported porous catalyst oxides and their reduction by nitrogen in the presence of oxygen is demonstrated. As a result of plasma generation in air, NOx generation is accompanied by the development of highly heterogeneous regions in terms of chemical and morphological variations within the catalyst. Regions of almost completely reduced catalyst are dispersed within the catalyst oxide, across micron-scale domains. The quantification of the catalyst heterogeneity and evaluation of catalyst structure are studied using Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and XRD. Plasma generating supported spinel catalysts are synthesized using the technique developed by the author (Catalysts; 2016; 6; 80) and BaTiO3 is used to exemplify perovskites. Silica supported catalyst systems are represented as M/Si = X (single catalysts) or as M(1)/M(2)/Si = X/Y/Z (binary catalysts) where M; M(1) M(2) = Cr; Mn; Fe; Co; Cu and X, Y, Z are the molar ratio of the catalysts and SiO2 support. Composite porous catalysts are synthesized using a mixture of Co and BaTiO3. In all the catalysts, structural heterogeneity manifests itself through defects, phase separation and increased porosity resulting in the creation of the high activity sites. The chemical heterogeneity results in reduced and oxidized domains and in very large changes in catalyst/support ratio. High electrical potential activity within BaTiO3 particles is observed through the formation of electrical treeing. Plasma generation starts as soon as the supported catalyst is synthesized. Two conditions for plasma generation are observed: Metal/Silica molar ratio should be > 1/2 and the resulting oxide should be spinel type; represented as MaOb (a = 3; b = 4 for single catalyst). Composite catalysts are represented as {M/Si = X}/BaTiO3 and obtained from the catalyst/silica precursor fluid with BaTiO3 particles which undergo fragmentation during microwave irradiation. Further irradiation causes plasma generation, NOx formation and lattice oxygen depletion. Partially reduced spinels are represented as MaOb–c. These reactions occur through a chemical looping process in micron-scale domains on the porous catalyst surface. Therefore; it is possible to scale-up this process to obtain NOx from MaOb for nitric acid production and H2 generation from MaOb–c by catalyst re-oxidized by water. Re-oxidation by CO2 delivers CO as fuel. These findings explain the mechanism of conversion of combustion gases (CO2 + N2) to CO and NOx via a chemical looping process. Mechanism of catalyst generation is proposed and the resulting structural inhomogeneity is characterized. Plasma generating catalysts also represent a new form of Radar Absorbing Material (RAM) for stealth and protection from radiation in which electromagnetic energy is dissipated by plasma generation and catalytic reactions. These catalytic RAMs can be expected to be more efficient in frequency independent microwave absorption.