Creating alternate fuels, cleaning the environment, dealing with the causes of global warming, and keeping safe from toxic substances and infectious agents are the new challenges faced by humanity. To combat all these problems hydrogen is claimed to offer a potential solution for satisfying many of our energy needs while reducing and eventually eliminating carbon dioxide and other greenhouse gas emissions. Water-gas-shift is one of the important reactions by which hydrogen is produced from most of the synthetic gases.
Water-gas-shift reaction
In the water-gas-shift (WGS) CO and H2O are converted into CO2 and H2 by the reactions:
CO + H2O → CO2 + H2, and
2CO + O2 → 2CO2
It is primarily used to produce higher H2 content and to reduce the CO content of syngas. The WGS reaction has gained more interest due to its application for onboard purification and production of H2 for fuel cell vehicles. For industrial operations, Cu-based catalysts are used for of the WGS reaction which occurs at temperature range of 470 - 520K.
WGS catalysts
Although WGS catalyst have been used commercially for many decades, the conventional catalysts are not sufficient to meet the rigorous performance targets required for fuel processor systems.
But when used for automotive applications, Cu-based catalysts result in condensation of water and need subsequent deactivation treatment of the catalysts. Copper based WGS catalysts have been shown to be very active when precipitated with zinc and aluminum oxide or when supported on cerium. However copper catalysts remain to be very sensitive to sulfur poisoning whereas precious metal catalysts are somewhat tolerant. This makes a need for the development of advanced WGS catalysts that include high activity and stability.
Nano catalysts
Researchers have shown that Au or Cu nanoparticles supported on the oxides, such as CeO2 and TiO2, possess higher activity even better than commercial catalysts in the WGS reaction where as bulk gold, ceria, and titania are not known as WGS catalysts.
CeriaCerium oxide (CeO2) / ceria are an important inorganic material having the cubic fluorite type crystal structure. Ceria either in the pure form or doped with other metals (Cu, Ni, etc) / metal ions (Mg2+, La2+, Sc2+, Gd3+, Y3+, Zr4+ etc.), potentially has a wide range of applications including gas sensors, electrode materials for solid oxide fuel cells, oxygen pumps, amperometric oxygen monitors and three way catalytic supports for automobile exhaust gas treatment. The nanoceria has attracted much attention because of the improved physical and chemical properties compared to the bulk ceria material.
Synthesis of CeO2 based nanoparticles
Solution based techniques are used for the synthesis of pure ceria and transition metals, rare earth metals, or metal ions doped ceria materials. Various synthesis routes include co precipitation, hydrothermal, microemusion, solgel, combustion of aqueous solutions of the metal acetate without addition of any extra fuel in a methane oxygen flame and electrochemical methods. In these methods several steps have to be followed, more time will be taken for the completion of reaction or control of the product composition may be difficult.
Researcher Pati and his associates give the following procedure.The precursors used were; cerium acetate as the cerium source, copper acetate as the copper source, nickel acetate as the nickel source and iron acetate as the iron source. The precursors were dissolved in deionized water to make solutions of each. The solutions were filtered through a membrane filter before filling the nebulizer. Liquid precursor feed was then atomized with compressed air resulting in a fine spray. In the reactor the flame was made by methane, oxygen and nitrogen and the flow rate of gas and precursors were controlled. After burning the fine spray, the particles were collected on a water-cooled surface.
Uses of Cerium
Cerium oxide (CeO2) is widely used as a promoter and an oxidation catalyst because of its unique redox properties and high oxygen storage capability. Cerium oxide has potential applications for UV blocks, polishing materials, the three-way catalysts and in solid oxide fuel cell. In addition, supported CeO2 and CeO2-based mixed oxides are effective catalysts for the oxidation of different hydrocarbon and for the removal of organics from wastewater from different sources.
Gold ceria or titania nanoparticles
Researcher Rodriguez and associates have found a good performance of Au-CeO2 and Au-TiO2 catalysts in the water-gas shift reaction. Although gold is not catalytically active for the WGS, gold surfaces that are 20 to 30% covered by ceria or titania nanoparticles have activities comparable to those of good WGS catalysts. In TiO2-x/Au(111) and CeO2-x/Au(111), water dissociates on O vacancies of the oxide nanoparticles, CO adsorbs on Au sites located nearby, and subsequent reaction steps take place at the metal-oxide interface.Researcher Nan Yi and associates studied steam reforming of methanol over ceria and gold-ceria nanoshapes and found that a small amount of gold deposited on ceria nanorods exhibited excellent catalytic activity for the low-temperature steam reforming of methanol. Gold clusters dispersed on the faces of ceria nanorods catalyze the reaction in a cooperative mechanism with ceria.
Mechanism
Neither CeOx/TiO2(110) nor Au(111) was able to catalyze the WGS. However, Au/CeOx/TiO2(110) surfaces are outstanding catalysts for the WGS. The deposition of gold NPs on CeOx/TiO2(110) yield surfaces with an extremely high catalytic activity for the water–gas shift reaction and the oxidation of CO.In principle, the combination of two metals in an oxide matrix can produce materials with novel structural and/or electronic properties. At structural level, a dopant can introduce stress into the lattice of an oxide host, inducing in the formation of defects.
Ceria and titania adopt different crystal lattices in their most stable bulk phases, fluorite and rutile, respectively. Within the fluorite structure each Ce atom is bonded to 8 O atoms, whereas 6 O atoms surround the Ti atoms in the rutile structure. One of the most interesting properties of ceria is its ability to undergo a conversion between “+4” and “+3” formal oxidation states. The surface chemistry and catalytic properties of CeO2 depend on the formation of Ce3+ ions.