Effects of Morphology of Cerium Oxide Catalysts for Reverse Water Gas Shift Reaction

Reverse water gas shift reaction (RWGS) was investigated over cerium oxide catalysts of distinct morphologies: cubes, rods and particles. Catalysts were characterized by X-ray diffraction, Raman spectroscopy and temperature programmed reduction (TPR) in hydrogen. Nanoshapes with high concentration of oxygen vacancies contain less surface oxygen removable in TPR. Cerium oxide cubes exhibited two times higher activity per surface area as compared to rods and particles. Catalytic activity of these nanoshapes in RWGS reaction exhibited a relation with the lattice microstrain increase, however a causal relationship remained unclear. Results presented in this study suggest that superior catalytic activity of ceria cubes in RWGS originates from the greater inherent reactivity of (100) crystal planes enclosing cubes, contrary to less inherently reactive (111) facets exposed at rods and particles

The formation of a nanocarbon from lignocellulose with a sea anemone appearance

A filamentous carbon nanomaterial having morphology, elemental compositions and growth conditions similar to those of a sea anemone was formed during pyrolysis of lignocellulose in the presence of water and sodium. We call the material “carbon nano-anemones” (CNAs). Well known carbon nano filamentous materials such as carbon nanotubes find extensive applications in electronics, hydrophobic coatings, catalysis and sensor technology. However, they are inert, non-polar, hydrophobic and surface chemical modification is often necessary before use. CNAs, on the other hand, contain oxygen, are reactive, polar and may be suitable, without modification, for applications e.g., catalysis, sensors.\ud \ud \ud Increasing energy demands, concerns over environmental issues and depletion of fossil resources are amongst the driving forces to use renewable feedstock to meet the future energy needs [1]. Lignocellulosic biomass is a renewable source for fuels [2]. It can be converted to a bio-crude oil via pyrolysis at atmospheric pressure and temperatures around 500 °C [3]. Currently, there is tremendous interest in developing catalysts to make this conversion efficient and to improve the properties of bio-oil to suit application as feedstock in a conventional oil refinery. Formation of solid carbonaceous material is typical and acceptable during pyrolysis reaction as it helps in improving the energy density of the resulting bio-crude oil. However, these carbonaceous materials are usually in the form of char, a low value by-product material. We have recently shown that sodium modified amorphous silica alumina (Na/ASA) catalyst is promising for the pyrolysis conversion of lignocellulosic biomass (Canadian Pinewood) to bio-crude oil having a higher energy content (24 MJ kg−1) than a non-catalytic thermal process (18 MJ kg−1) [4]. During this pyrolysis experiment (450 °C, 1 bar, 20 min), a new type of carbonaceous material having filament-like morphology similar to sea anemones was observed (Fig. 1a and c) (for detailed reaction and catalyst preparation procedures please see Supplementary material). The features of sea anemones are strikingly similar to carbonaceous material observed in the pyrolysis experiment (Fig. 1b and d). For this reason we term them as carbon nano-anemones (CNAs). The anatomy of sea anemones (Fig. 1a and c) contains elongated flexible extensions called tentacles. Carbon nano-anemones have very similar filamentous features (Fig. 1b and d). They have tips at the free end of their structure similar to that observed for sea anemones at the end of their tentacles. Furthermore, similar to sea anemones, CNAs do not have any unique spatial orientation\u

The Effect of CO Adsorption at Room Temperature on the Structure of Supported Pt Particles

To improve the understanding of the applicability of CO-FTIR spectroscopy for probing the electronic properties of catalysts, the effect of CO adsorption on the geometry of small metal particles in Pt/LTL and Pt/SiO2 catalysts with varying support acidities was determined by comparison of X-ray absorption spectra before and after CO adsorption. At room temperature, the platinum particles (first shell coordination number N > 5) supported on SiO2 were stable under CO atmosphere. By contrast, the smaller platinum particles (N < 5) in zeolite LTL reconstructed with the formation of very small platinum-CO aggregates upon admission of CO at room temperature. For both Pt/SiO2 and Pt/LTL the linear-to-bridge ratio (L/B) of the CO infrared bands is a function of the support acidity/alkalinity. In the case of Pt/SiO2, the L/B ratio directly reflects the electronic properties of the catalytically active metal, since the metal particles do not reconstruct. The results show that the structure of the platinum particles in LTL zeolite, which participate in catalytic reactions is not the same as the Pt-CO aggregate analyzed by FTIR. Nevertheless, both the catalytic activity and L/B ratio are a function of support acidity

A New Model Describing the Metal-Support Interaction in Noble Metal Catalysts

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by the acidity/alkalinity of the support but are relatively independent of the metal (Pd or Pt) or the type of support (zeolite LTL or SiO{2}). As the alkalinity of the support increases, the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resonance in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt-H antibonding orbital and the Fermi level decreases as the alkalinity of the support increases. Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals. The EXAFS structural analysis indicates that the small metal particles are in contact only with the oxide ions of the support. Finally, a new spectroscopic characterisation, Atomic XAFS, is presented which provides new insights into the origin of the electronic changes in the metal. As the alkalinity of the support increases, there is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal-support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structural properties of the supported metal particles induced by support acidity ranging from acidic to neutral to alkaline

A New Model for the Metal-Support Interaction : Evidence for a Shift in the Energy of the Valence Orbitals

The catalytic and spectroscopic properties of Pt supported on LTL zeolite are greatly affected by the acidity/alkalinity of the support. The turnover frequency (TOF) for neopentane hydrogenolysis and isomerization decreases from acidic to neutral to alkaline. In addition, in the infrared spectra, there is a decrease in the linear to bridging ratio of adsorbed CO which parallels the catalytic activity, indicating that the changes in TOF are due to a modification of the electronic properties of the Pt particles resulting from the metal-support interaction. The local structure of the Pt particles has also been determined by EXAFS spectroscopy. The Pt atoms are in contact with the oxygen ions of the support. None of the Al, Si or K ions are within bonding distance of the Pt. In addition, analysis of the L{I}{I}{I} and L{I}{I} near-edge spectra suggest that, contrary to the generally accepted model, the number of electrons in the valence band is unchanged by the support interaction. Furthermore, at the L{I}{I}{I} edge in the presence of chemisorbed hydrogen, a Pt-H antibonding orbital is observed near the Fermi level. Isolation of this shape resonance indicates that the energy difference between the antibonding orbital and the Fermi level increases with increasing acidity of the support and correlates with the TOF. Based on the analysis of the Pt-H shape resonance, a new model for the metal-support interaction is proposed where the binding energy of the Pt valence orbitals increase as the charge of the support oxide ion becomes more positive, i.e., becomes more acidic. The catalytic and spectroscopic properties are discussed in the context of this new model