Under a pressure tuned supercritical fluid mixture of pentane/hexane as the media and using Co/A12O3 as the catalyst, the Fischer–Tropsch synthesis (FTS) reaction was conducted in a fixed bed reactor. 14C-labeled decene, tetradecene, and nonadecene were chosen as probe compounds to study the chemical behavior of the added mid-carbon number olefin under FTS supercritical reaction conditions. In the case of the run with decene, 21.8% of the added decene was reduced to decane, 73.5% remained unchanged, and only 4.7% of the added decene was incorporated into higher carbon number compounds. For the tetradecene and nonadecene runs, the percentages of reduction of the added olefin are 23.4 and 2.5%, while the percentages of incorporation are 2.6 and 1.5%, respectively. The molar radioactivity of the incorporated compounds decreases with increasing molecular size, indicating the presence of some accumulated products under these reaction conditions. The α value calculated based on the relative radioactivity of the incorporated compounds is 0.62, which is smaller than the value obtained from normal FTS under the same reaction conditions. These results indicate that the added mid-carbon number olefin cannot significantly alter the product distribution in favoring the heavier products.

In situ, steady-state diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements for adsorption of CO and for water-gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, being formed by reaction with geminal OH groups that are present after reduction of the ceria surface shell. The process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little if at all the catalysis of bulk reduction. Gold was found to reduce the surface ceria at a lower temperature than that of platinum, but platinum gave a slightly higher degree of surface shell reduction. Under steady-state WGS at a high H2O/CO ratio, the concentrations of surface formates are strongly limited at high CO conversions, while metalCO was not. Since under these conditions, CO exhibits a first order rate dependency, the active site should move to sparser coverages of CO, indicating that a formate mechanism is more likely the correct one. At low temperatures and conversions, the formates were close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. In situ X-ray absorption near edge spectroscopy (XANES) directly links the metal to its ability to aid in catalyzing reduction of the surface shell of ceria. After surface shell reduction of ceria by hydrogen, addition of water to the hydrogen stream gave no indication of reoxidation whatsoever, as would be necessary under a ceria-mediated redox process. The reoxidation of ceria by water under helium alone was very slow, and only slight changes were recorded at 350°C. Therefore, the results strongly favor a formate mechanistic scheme for low temperature water-gas shift. To date, most researchers have claimed a ceria-mediated redox process operating to describe the mechanism. Both mechanisms require reduction of the ceria surface.

Steady-state IR measurements for adsorption of only CO and under WGS reaction indicate that formates are present on the surface of partially reduced ceria, in contrast to a recent study, and that they are strongly limited at high CO conversions. At low temperatures and conversions, the formates are close to the equilibrium adsorption/desorption coverages obtained from CO adsorption alone. The formates are close to saturation at low temperatures. These IR results favor the bidentate formate mechanism in explaining WGS. However, more kinetic studies are required and over a wider range of temperatures. While low-temperature kinetic studies have found a zero-order dependency for CO and related this to saturation of a noble metal surface, this study indicates that one cannot rule out the possibility of the formate mechanism on this basis, as CO is also close to saturation as an adsorbed formate at the low temperatures used in previous studies.

Modeling of the supercritical fluid mixture indicated that an important increase in density occurs above a threshold of approximately 4 MPa for the reaction temperature of 220degreesC studied. While transport parameters of the fluid are largely retained, the observed improvement in wax solubility was noteable.A cobalt catalyst (25%Co/-gamma-Al2O3) was used in a fixed bed reactor under a pressure/density tuned supercritical fluid mixture of n-pentane/n-hexane. By using inert gas as a balancing gas to maintain a constant pressure, the density of the supercritical fluid could be tuned near the supercritical point while maintaining constant space velocity within the reactor. The benefits of the mixture allowed for optimization of transport and solubility properties at an optimum reaction temperature for Fischer-Tropsch synthesis with a cobalt catalyst. Indeed, above 4 MPa, increases in wax yields from sampling and carefully controlled gas measurements using an internal standard demonstrated an important increase in conversion due to greater accessibility to active sites after extraction of heavy wax from the catalyst. Additional benefits included decreased methane and carbon dioxide selectivities. Decreased paraffin/(olefin + paraffin) selectivities with increasing carbon number were also observed, in line with extraction of the hydrocarbon from the pore. Faster diffusion rates of wax products resulted in lower residence times in the catalyst pores, and therefore, decreased probability for readsorption and reaction to the hydrogenated product. Even so, there was not an observable increase in the alpha value for higher carbon number products over that obtained with just the inert gas. (C) 2003 Elsevier Science Ltd. All rights reserved.

In situ diffuse reflectance mode (DRIFTS) measurements for adsorption of CO and under the water-gas shift (WGS) reaction revealed that formates emerge on the surface of reduced ceria after the reaction of CO with geminal OH groups. These groups are formed after reduction of the ceria surface shell. X-ray absorption near edge spectroscopy (XANES) results demonstrated that the process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little, if at all, the catalysis of bulk ceria reduction. For 1% Pt/ceria, under steady state WGS at a high H2O/CO ratio, surface formate concentrations were strongly limited at high CO conversions, while Pt-CO was H2O/CO ratios, CO exhibits a first order rate dependency, and therefore, the active site is expected to move affected only slightly. Under high H to sparser coverages of CO, which suggested that the WGS mechanism likely proceeded via formates. Later, XANES work gave no evidence for the reoxidation of ceria surface by water under a hydrogen environment, which would be necessary to substantiate an alternate mechanism, referred to as the ceria-mediated redox process. Later, isotope switching from H2O to D2O was carried out to validate the possibility that decomposition of surface formates could be the rate limiting step for the mechanism, as was proposed and demonstrated earlier by Shido and Iwasawa. In agreement with their findings, we also observed a normal isotope effect, consistent with a link between the activation energy barrier of the rate limiting step to the breaking of the C-H bond of the formate.In this study, a variety of metals were screened to try to gain further insight into the role played by the metal in the catalysis of metal/ceria systems for WGS. One group of metals was selected on the basis of reduction temperature, since spillover from reduced metal likely catalyzes the reduction of ceria surface. Therefore, we tested the following metals, moving from lowest to highest reduction temperature: Pt < Ni < Co = Fe. These were found to catalyze reduction of ceria surface by the same trend, as well as the WGS rate. In each case, DRIFTS showed that as reduction of ceria surface occurred, marked increases in geminal OH intensities occurred, which yielded formates upon adsorption of CO. The second group of metals was a comparative study between Pt and Group 11 metals that have been purported to catalyze surface reduction at the same or even lower temperatures than Pt. Indeed, the surface reduction occurred at a lower temperature than for Pt with An and at a similar temperature with Cu. However, on an equivalent atomic basis, the depth of reduction of ceria was found to be higher when Pt was used over Au, and the WGS rate was about 20 times higher with Pt than by Group 11 metal promotion at 250degreesC and higher. WGS feed conditions were carefully chosen to mimic conditions found in a fuel cell reformer. (C) 2003 Elsevier B.V. All rights reserved.