Visualizing Gas Molecules Interacting with Supported Nanoparticulate Catalysts at Reaction Conditions (original) (raw)
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The surface structure of catalytic gold nano-particles was dynamically observed for the first time during CO oxidation reaction by using a dedicated environmental-cell transmission electron microscope (E-cell TEM). Under the reaction environment, the shape of the particle was drastically changed, for example, facets on the gold particles dominantly disappeared and sometimes recovered. Since such surface shape changes became remarkable when the reaction gas pressure increased, it is predicted that this phenomena was caused by heat of reaction and may be related to the catalytic behavior.
The recent availability of aberration corrected analytical electron microscopes (ACAEM) is revolutionizing our ability to characterize nanostructured catalyst materials. Some recent case studies are presented whereby the application of the high angle annular dark field (HAADF) imaging technique, coupled with STEM-XEDS analysis, has given us a more detailed and realistic view of the catalyst morphology. The examples chosen include supported Au catalysts for low temperature CO oxidation and supported AuPd bimetallic alloy catalysts for the direct production of H 2 O 2 .
The Journal of Physical Chemistry C, 2011
The origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO 2 (110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O 2 , Ar, or a mixture of O 2 þ CO and of Ar þ CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle's diameter of 2.1 ( 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both "model" and "real" catalysts.
ChemCatChem, 2018
Tomographic imaging of catalysts allows non-invasive investigation of structural features and chemical properties by combining large fields of view, high spatial resolution, and the ability to probe multiple length scales. Three complementary nanotomography techniques, (i) electron tomography, (ii) focused ion beam-scanning electron microscopy, and (iii) synchrotron ptychographic X-ray computed tomography, were applied to render the 3D structure of monolithic nanoporous gold doped with ceria, a catalytically active material with hierarchical porosity on the nm and μm scale. The resulting tomograms were used to directly measure volume fraction, surface area and pore size distribution, together with 3D pore network mapping. Each technique is critically assessed in terms of approximate spatial resolution, field of view, sample preparation and data processing requirements. Ptychographic X-ray computed tomography produced 3D electron density maps with isotropic spatial resolution of 23 n...
CO oxidation on gold surfaces studied on the atomic scale
Catalysis Letters, 2001
The interaction of small gold crystal tips with oxygen gas and CO/O 2 gas mixtures was studied by means of field ion microscopy (FIM). High-resolution FIM-images of clean tips were obtained with hydrogen and neon as imaging gas. At temperatures between 300 and 450 K the exposure of a clean Au sample to O 2 gas at 100-1000 mbar, in the absence of an electric field, led to oxygen chemisorption and formation of a "surface oxide". The presence of an electric field of 12-15 V/nm was found to enhance the oxidation process. Exposure to CO gas at 300 K led to the removal of the surface oxide. This was associated with the occurrence of a wave front which started in the apex centre and extended to the outskirts of the tip sample. The build-up of the surface oxide and its titration by carbon monoxide was completely reversible. Our results strongly suggest that pure gold crystals are active catalysts for the CO oxidation at 300 K.
Journal of Catalysis, 2019
A parallel thermographic screening methodology has been developed which allows the measurements of the particle size and support influences on model planar heterogeneous catalysts. A screening chip was designed and fabricated in order to produce multiple fields of low stress silicon nitride membranes that exhibit low thermal conductivity and heat capacity. The heat generated on supported model catalysts in an exothermic reaction on the membranes was measured using a thermal (infra-red) imaging camera, which in turn provided a measure of the turn over frequency (TOF) for the reaction. The catalytic activity for CO oxidation on titania supported gold model catalysts with varying particle size has been measured on 100 catalysts simultaneously. The reaction has been investigated at 80 o C and 170 o C, and pressures ranging between 0.06 mbar and 1.5 mbar for various O2:CO ratios. Under all conditions investigated, a monotonic increase in the TOF is observed with decreasing particle diameter (d) which is proportional to ca. d-1.8 in the range 6 > d/nm > 1.5. This is in the opposite direction to the number of potentially active perimeter sites which increases linearly with increasing particle size on these catalysts. We show that the surface area specific activity of the gold is increasing even more steeply with reduced particle size, and is proportional to ca. d-4. This rate of increase in activity is significantly higher than one would expect by any increase one may expect as a result of more active low coordinate sites on the gold. The steep increase in activity is ascribed to an electronic interaction between the substrate and the particle.
Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation
Journal of Physical Chemistry C, 2007
The catalytic activity of gold-based catalysts for CO oxidation is influenced by gold particle size, dispersion, and redox properties of the support. The nature of the active site (gold oxidation state) and its modification along the course of the reaction are under discussion. In this work, we studied the modifications of a Au/ CeO 2 catalyst in the presence of gas-phase CO by simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFT-MS) in isothermal conditions. Redox processes involving surface hydroxyl groups, gold atoms, and gas-phase CO molecules play a determinant role in surface gold dynamics. The interaction of CO with the catalyst surface results in the evolution of CO 2 and H 2 through both the decomposition of the formate species and the formation of the [Au(CO) 2 ] + species, which accounts for the redispersion of gold atoms. The identification of the involved surface species by DRIFT lets us state that a deep reduction of the surface (oxygen vacancy creation) changes the gold dispersion and migration of oxygen atoms to the surface, generating an oxidized gold species. This result is also confirmed by XRD, where a decrease in the intensity of all metallic gold diffraction peaks and the relative intensity among them is evidenced in the used catalyst. Therefore, this work provides evidence for the surface dynamics of gold in this Au/CeO 2 catalyst and hence provides clues for understanding the modification of the catalytic activity of gold catalysts under cyclic operations.
Surface oxygen vacancies in gold based catalysts for CO oxidation
Experimental catalytic activity measurements, diffuse reflectance infrared Fourier spectroscopy, and density functional theory calculations are used to investigate the role and dynamics of surface oxygen vacancies in CO oxidation with O 2 catalyzed by Au nanoparticles supported on a Y-doped TiO 2 catalyst. Catalytic activity measurements show that the CO conversion is improved in a second cycle of reaction if the reactive flow is composed by CO and O 2 (and inert) while if water is present in the flow, the catalyst shows a similar behaviour in two successive cycles. DRIFTS-MS studies indicate the occurrence of two simultaneous phenomena during the first cycle in dry conditions: the surface is dehydroxylated and a band at 2194 cm À1 increases (proportionally to the number of surface oxygen vacancies). Theoretical calculations were conducted in order to explain these observations. On one hand, the calculations show that there is a competition between gold nanoparticles and OH to occupy the surface oxygen vacancies and that the adsorption energy of gold on these sites increases as the surface is being dehydroxylated. On the other hand, these results evidence that a strong electronic transfer from the surface to the O 2 molecule is produced after its adsorption on the Au/TiO 2 perimeter interface (activation step), leaving the gold particle in a high oxidation state. This explains the appearance of a band at a wavenumber unusually high for the CO adsorbed on oxidized gold particles (2194 cm À1) when O 2 is present in the reactive flow. These simultaneous phenomena indicate that a gold redispersion on the surface occurs under reactive flow in dry conditions generating small gold particles which are very active at low temperature. This fact is notably favoured by the presence of surface oxygen vacancies that improve the surface dynamics. The obtained results suggest that the reaction mechanism proceeds through the formation of a peroxo-like complex formed after the electronic transfer from the surface to the gas molecule.
Gold catalysts: A new insight into the molecular adsorption and CO oxidation
Chemical Engineering Journal, 2009
The molecular adsorption and CO oxidation on a gold-deposited TiO 2 catalyst were investigated by means of molecular dynamics simulation. The results indicate that the molecules (i.e., O 2 , CO, and H 2 O) are selectively adsorbed on the specific locations such as gold particle, gold-support perimeter interface, and support surface. The adsorption and dissociation of H 2 O molecules at the perimeter interface enhance the supply of oxygen, thus promoting the oxidation of CO on the Au/TiO 2 catalyst. However, the presence of Cl − ions could significantly impede CO oxidation due to their competition with O 2 , CO, and H 2 O for the adsorption sites. A reaction mechanism of CO oxidation is postulated on this basis. The findings are useful in developing a comprehensive picture about CO oxidation on gold-deposited TiO 2 and in the design of new gold catalysts with high catalytic activity.