Chemical Engineering
Raman and UV-Vis Spectroscopy Study of Vanadium-Containing Heteropoly Acids in Aqueous Solutions
To advance liquid phase spectroscopic techniques, we have selected two types of heteropoly acids (HPAs) in aqueous solutions to serve as our experimental catalysts: H3PW12O40, (TPA-tungstophosphoric acid) and H3PMo12O40, (MPA-molybdophosphoric acid). The cage-like structure that these HPAs assume is called the Keggin structure [1,2]. Distorted forms of the Keggin structure are also known to exist. For example, the Dawson structure is composed of two Keggin anions which have each expelled three WO3 or MoO3 units and joined together as a dimeric unit [1,3].
The initial objectives of this investigation were (1) to compare the ambient and aqueous solution spectra of the HPAs and (2) to determine differences or similarities in their structures between their ambient and aqueous states. Varying levels of vanadium were introduced into the primary and secondary structure of each HPA in order to investigate the influence of vanadium when the HPAs are in solution. TPA and MPA samples containing vanadium in the primary structure are denoted as TPAVx and MPAVx. The chemical formulas for solid TPAVx and MPAVx are H3+xPW12-xVxO40 and H3+xPMo12-xVxO40 (where x=1, 2, and 3). TPA and MPA which contain vanadia on the secondary structure are denoted as VOTPA and VOMPA. › Continue reading
Influence of ZrO2 Nanoligands on the Catalytic Performance of Supported Pt/ZrO2/SiO2 Catalysts
A series of double-supported Pt/ZrO2/SiO2 catalysts were prepared to determine the influence of ZrO2 nanoparticle (NP) domain size on the reactivity of the catalytic active Pt component. The catalysts were synthesized by first impregnating zirconium tert-butoxide, Zr[OC(CH3)3]4, in toluene, drying and then calcining at 500 C to form ZrO2. In a second step, aqueous platinum tetra-ammine nitrate, Pt(NH3)4(NO3)2, was impregnated, dried and calcined in air at 500 C to form the final double-supported Pt/ZrO2/SiO2 catalyst. The ZrO2 loading was varied between 1% and 50% and the Pt loading was maintained constant at 0.1%. In situ Raman and UV-vis spectroscopy and TEM microscopic characterization revealed that the supported ZrO2 phase varied its domain size from isolated surface species to polymeric surface species to NPs (1-3 nm). TEM microscopy revealed that the supported Pt phase was present as 10-70 nm NPs for 1-20% ZrO2/SiO2, where the surface ZrOx species was present (1-12%) and a combination of surface ZrOx species and ZrO2 NPs (15-20%). The Pt NPs, however, were completely absent for higher zirconia loading where ZrO2 NPs are present and reflect the presence of a highly dispersed Pt phase on the ZrO2 NPs. The reactivity of the supported Pt phase was chemically probed with CH3OH oxidation (both steady-state and CH3OH-temperature programmed surface reaction (TPSR) spectroscopy). The reactivity of the methanol oxidation reaction was found to increase with the dimension of the Pt NPs, which also corresponds to lower ZrO2 domain size. Thus, the reactivity of the Pt catalytic active sites could be tuned by the domain size of the ZrO2 nanoligands.
A Simple Gasifier Model That Runs in Aspen Dynamics
Gasification has been used in industry on a relatively limited scale for many years, but it is immerging as the premier unit operation in the energy and chemical industries. The switch from expensive and insecure petroleum to solid hydrocarbon sources (coal and biomass) is occurring due to the vast amount of domestic solid resources, national security and global warming issues. Gasification (or partial oxidation) is a vital component of “clean coal” technology. Sulfur and nitrogen emissions can be reduced, overall energy efficiency is increased and carbon dioxide recovery and sequestration are facilitated. Gasification units in an electric power generation plant produce a fuel gas for driving combustion turbines. Gasification units in a chemical plant generate synthesis gas, which can be used to produce a wide spectrum of chemical products. Future plants are predicted to be hybrid power/chemical plants with gasification as the key unit operation. › Continue reading
Materials for Reversible High Temperature Chemisorption of CO2
Results of experimental tests into the equilibrium and column dynamic data for the chemisorption of CO2 on two materials has identified the materials as potential candidates for the capture of CO2. The first of the materials is a K2CO3 – promoted hydrotalcite that displays good sorption capacity in 400-500 C range. The second is a Na2O -promoted alumina that has shown good sorption capacity in 250-400 C temperature range. The two materials both exhibited Langmuirian behavior in the low pressure region, but deviated substantially in higher pressure regions. A new analytical model that simultaneously accounts for Langmuirian chemisorption and an additional surface complexation reaction between gaseous and sorbed CO2 has been proposed to describe the measured equilibrium data for both materials. Experimental breakthrough tests showed fast kinetics and narrow mass transfer zones for CO2 adsorption. The isosteric heats of chemisorption and heats of additional complexation reaction on both materials were estimated to be low, indicating that desorption of CO2 from both materials could be achieved with relative ease. Tests have confirmed that both materials show stable sorption capacity after several sorption-desorption cycles. These characteristics make them attractive candidates for use in cyclic processes for the capture of CO2. The Na2O promoted alumina shows promise as a candidate for capture of CO2 from flue gas of a coal fired power plant, while the K2CO3 promoted hydrotalcite will be a better candidate for the Sorption Enhanced Reaction process to simultaneously produce fuel cell grade H2 and high purity CO2 at feed gas pressure.