Nanoporous Gold-Supported Ceria for the Water–Gas Shift Reaction: UHV Inspired Design for Applied Catalysis

Abstract

Inspired by model studies under ultrahigh vacuum (UHV) conditions, inverse monolithic gold/ceria catalysts are prepared using thermal decomposition of a cerium nitrate precursor on a nanoporous gold (npAu) substrate. Cerium oxide deposits throughout the porous gold material (pores and ligaments 30−40 nm) are formed. npAu disks and coatings were prepared with loadings of about 3 to 10 atom % of ceria. The composite material was tested for the water−gas shift (WGS) reaction (H 2 O + CO → H 2 + CO 2 ) in a continuous flow reactor at ambient pressure conditions. Formation of CO 2 was observed at temperatures as low as 135°C with excellent stability and reproducibility up to temperatures of 535°C. The considerably increased thermal stability of the material can be linked to the presence of metal oxide deposits on the nanosized gold ligaments. The loss of activity after about 15 h of catalytic conversion with heating to 535°C was only about 10%. Photoemission spectroscopy indicates a defect (Ce 3+ ) concentration of about 70% on the surface of the cerium oxide deposits, prior to and after WGS reaction. Raman spectroscopic characterization of the material revealed that the bulk of the oxide is reoxidized during reaction. ■ INTRODUCTIONDuring the last decades increasing demand for a novel type of water−gas shift (WGS) catalyst in the context of mobile and green energy harvesting such as in fuel cells surfaced. 1,2 For either low-temperature fuel cells (polymer electrolyte membrane fuel cells, PEMFC) or high-temperature fuel cells in mobile applications such as in cars a novel type of WGS catalyst is required. These catalysts need to be highly active at low temperatures, shifting CO almost quantitatively to hydrogen (<10 ppm for PEMFCs). They need to be safe, nonpyrophoric and oxidation resistant upon exposure to air (excluding, thus, traditional WGS catalysts), highly durable and long-lasting and have to withstand fast deactivation and shut down procedures. Last but not least they have to be readily applicable into small scale reactor designs, ideally as coatings or monolithic catalyst beds. The focus of this latest surge in research regarding WGS catalysts are precious metal based catalysts. 1,3Among possible candidates, gold is of particular interest as it is cheaper than, e.g., platinum and also a very selective catalyst for the oxidation of CO in the presence of H 2 , making it an ideal catalyst material for procession of hydrogen gas for fuel cells. 4 Recently, Au-CeO 2 nanomaterials have been reported to be very efficient catalysts for the WGS reaction. 2 The interplay between the oxide and the Au provides reactivity toward the dissociation of water. Similarly, as reported for oxidation reactions using molecular oxygen, the synergy between the two partners is critical for the catalytic activity. 5,6 Oxygen vacancies existing in the oxide nanoparticles are suggested to play a key role for the dissociation of water. 7 A blend of catalysts consisting of metal nanoparticles dispersed on an oxide support has been report...

Correlative Multiscale 3D Imaging of a Hierarchical Nanoporous Gold Catalyst by Electron, Ion and X‐ray Nanotomography

“…Further examples of the 2D data obtained can be found in the Supporting Information. The 2D images clearly indicate the characteristic sponge‐like structure of CeO x /np‐Au and the presence of pores and ligaments of varying size . Notably, the samples appeared to contain finely structured ligaments together with sections of more dense or thicker material, indicative of mild coarsening which may have occurred during the process of ceria addition and low temperature heat treatment (280 °C) to remove organic residues.…”

“…% Ce within a structure dominant in gold, which is within the range reported previously for this material (2–10 At. % via SEM‐EDX analysis) . Additional possibilities for analysing and visualising the sample ceria content are discussed later.…”

“…This results in a large fraction of accessible gold surface atoms, particularly at kink and step sites, which makes np‐Au very interesting for catalysis and sensing applications . As with all functional materials, it is important to understand physical characteristics such as surface area, porosity, shape and mechanical stability, in order to: (i) define structure–activity relationships; (ii) identify and control synthesis parameters which lead to such structures; (iii) design future catalysts for target applications . When applied to catalysis, analytical methods based on structural imaging or microscopy are therefore required to incorporate large fields of view, high spatial resolution, and to extend across multiple length scales .…”

“…The characteristic sponge‐like structure of np‐Au has been characterised primarily on the nanoscale using scanning (SEM) or transmission electron microscopy (TEM). These have proven successful in analysing structural stability, ligament coarsening at high temperature, and morphological changes after incorporation of metal oxide dopants such as CeO 2 or TiO 2 , for example . In particular, modification of np‐Au with metal oxides has been proposed as a method of inhibiting sintering and pore collapse at the elevated temperatures often required for catalysis.…”

Nanoporous Gold Films as Catalyst

“…Not only hydrogen oxidation, but also hydrogen production using water-gas shift (WGS) reaction was reported on NPG decorated with ceria [36]. In WGS reaction, one water molecule reacts with one CO molecule and produces one hydrogen molecule and one CO 2 molecule.…”