John L. Falconer

Temperature-Programmed Desorption and Reaction: Applications to Supported Catalysts

Abstract In a typical temperature-programmed desorption (TPD) experiment on a supported metal catalyst, a small amount of catalyst (10–200 mg) is contained in a reactor that can be heated by a furnace. An inert gas, usually helium at atmospheric pressure, flows over the catalyst. Following pretreatment to obtain a reduced catalyst, a gas is adsorbed on the surface, usually by pulse injections of the adsorbate into the carrier gas upstream from the reactor. After the excess gas is flushed out, the catalyst is heated to create a linear rise in temperature with time. A small thermocouple inserted in the catalyst measures the temperature and a detector downstream measures the change in the inert gas stream. The ideal detector is a mass spectrometer which measures the composition of the effluent stream as a function of catalyst temperature. Because of the high carrier-gas flow rate, the detector response is proportional to the rate of desorption if diffusion and readsorption are not limiting.

Alumina-Supported SAPO-34 Membranes for CO2/CH4 Separation

SAPO-34 membranes were prepared by in situ crystallization on alpha-Al2O3 porous supports. The crystal size of the seeds was effectively controlled in the 0.7 to 8.5 micron range by employing different structure-directing agents. Seeds smaller than 1 micron produced membranes with CO2/CH4 separation selectivities higher than 170 and unprecedented CO2 permeances as high as 2.0 x 10(-6) mol/m2.s.Pa at 295 K and a feed pressure of 224 kPa. The membranes effectively separated CO2/CH4 mixtures up to 1.7 MPa.

Flash desorption activation energies: DCOOH decomposition and CO desorption from Ni (110)

Abstract Accurate values of activation energies were measured by flash desorption methods without assumptions about preexponential factors, reaction orders or specific reaction mechanisms. The activation energies were determined by two methods; one method employed a relationship for the shift in peak temperature with change in heating rate, and the other utilized the change in peak amplitude with shift in peak temperature for different heating rates. Agreement between the two methods was excellent . A series of flash curves at different heating rates were obtained for the CO 2 and CO products from DCOOH flash decomposition following adsorption on Ni (110) at 37°C. Adsorbed DCOOH decomposed autocatalytically with an activation energy of 26.6 kcal/mol to form CO 2 and D 2 . Carbon monoxide formation from DCOOH decomposition, which corresponded identically to CO desorption from this surface, showed a first order activation energy of 32.7 kcal/mol; this activation energy was used to fit a series of CO flash desorption curves obtained for CO adsorption at −55°C. The preexponential factor was found to be 8.5 × 10 15 s −1 . The desorption was first order with a slightly coverage dependent desorption energy. In addition the CO flash curves showed additional binding states at coverages at which changes in isosteric heats of adsorption have been observed. The results illustrate the sensitivity of flash desorption for the determination of binding energies over a wide range of coverages.

Organics/water separation by pervaporation with a zeolite membrane

Abstract Organic/water mixtures are separated at ambient temperature and pressure by pervaporation through a silicalite zeolite membrane supported on the inner surface of a porous stainless-steel cylindrical tube. Methanol, ethanol and acetone were preferentially separated from aqueous solutions. For methanol/water separations, a relatively constant separation factor between 11 and 14 was obtained over a wide range of methanol feed concentrations. Total mass fluxes of 1 to 2.7 kg/(m 2 h) were obtained. Water and methanol permeances were independent of methanol feed concentration, except at low concentrations. Pervaporation has a higher separation factor than expected for vapor-liquid equilibrium separation. The highest separation factor obtained for acetone/water was 255 at an acetone feed concentration of 0.8 wt% with an acetone flux of 0.20 kg/(m 2 h). The highest acetone flux of 0.95 kg/(m 2 h) was obtained at an acetone concentration of 43 wt%, when the separation factor was 37. Separation factors decreased with increasing acetone concentration. The production indices for the silicalite membrane were much higher then other membranes at similar feed concentrations. The silicalite membrane was unable to selectively remove acetic acid from aqueous solutions at low acid concentrations.

Ceramic-zeolite composite membranes and their application for separation of vapor/gas mixtures

Ceramic-zeolite composite membranes were prepared by in-situ synthesis of a thin (∼ 10μm) polycrystalline silicalite-1 layer on the inner surface of an alumina membrane tube. The inner surface is a λ-alumina coating that has 5-nm diameter pores. X-ray diffraction verified the presence of a pure silicalite phase in the layer, and SEM showed that individual silicalite crystals had grown together to form a continuous silicalite-1 layer. The addition of silicalite to the alumina membrane decreased the N2 permeance by a factor of 5, but it decreased the n-C4H10 permeance by a factor of 190, and n-C4H10 appeared to adsorb on the membrane. At room temperature, the permeance ratio of n-C4H10/i-C4H10 was one for the alumina membrane, but it was 3 for the zeolite membrane. Methanol was separated from H2 and from CH4 at 373 K and pressures from 110 to 1100 kPa by preferentially permeating CH3OH through the zeolite membrane. For some conditions the CH3OH/H2 separation factor was greater than 1000, and the CH3OH/CH4 separation factor was 190. Apparently, CH3OH adsorbs and blocks the pores for H2 or CH4 permeation.

Adsorption and methanation of carbon dioxide on a nickel/silica catalyst

The adsorption and methanation of carbon dioxide on a nickel/silica catalyst were studied using temperature-programmed desorption and temperature-programmed reaction. Carbon dioxide adsorption on nickel was found to be activated; almost no adsorption occurred at room temperature, but large coverages were obtained between 383 and 473 K. The data indicate CO2 dissociates upon adsorption at elevated temperatures to yield carbon monoxide and oxygen atoms. These oxygen atoms react with hydrogen at room temperature, so the methane and water peaks observed during programmed heating in flowing hydrogen are identical for adsorbed CO and adsorbed CO2. Single CH4 and H2O peaks, each with a peak temperature of 473 K, were observed. This peak temperature did not change with initial coverage, indicating methanation is first order in CO surface coverage. The activated adsorption of CO2 allowed these coverage variation experiments to be carried out. Thus, following adsorption, CO and CO2 methanation proceed by the same mechanism. However, the activated adsorption of CO2 may create a higher H2: CO surface ratio during steady-state hydrogenation, causing CO2 hydrogenation to favor methane over higher hydrocarbons.

High Density, Vertically-Aligned Carbon Nanotube Membranes

A method is presented to prepare high-density, vertically aligned carbon nanotube (VA-CNT) membranes. The CNT arrays were prepared by chemical vapor deposition (CVD), and the arrays were collapsed into dense membranes by capillary-forces due to solvent evaporation. The average space between the CNTs after shrinkage was approximately 3 nm, which is comparable to the pore size of the CNTs. Thus, the interstitial pores between CNTs were not sealed, and gas permeated through both CNTs and interstitial pores. Nanofiltration of gold nanoparticles and N(2) adsorption indicated the pore diameters were approximately 3 nm. Gas permeances, based on total membrane area, were 1-4 orders of magnitude higher than VA-CNT membranes in the literature, and gas permeabilities were 4-7 orders of magnitude higher than literature values. Gas permeances were approximately 450 times those predicted for Knudsen diffusion, and ideal selectivities were similar to or higher than Knudsen selectivities. These membranes separated a larger molecule (triisopropyl orthoformate (TIPO)) from a smaller molecule (n-hexane) during pervaporation, possibly due to the preferential adsorption, which indicates separation potential for liquid mixtures.

Desorption rate isotherms in flash desorption analysis

Abstract In most flash desorption and temperature programmed desorption experiments only a small fraction of the data obtained is used in the kinetic analysis. A technique which utilizes more of the desorption data from a series of desorption curves obtained at different initial surface coverages is discussed. A number of desorption rate isotherms can be obtained by measuring desorption rates and coverages at selected temperatures in the desorption range. Each isotherm is a plot of ln (rate) versus ln (coverage) measured at a given temperature and its slope corresponds to the order of the desorption. From a series of desorption rate isotherms, plots of ln (rate) against 1 T can be obtained; the activation energy as a function of coverage can be obtained from the slopes of these plots. The success of the method for assisting in distinguishing between simple first-order desorption, first-order desorption with a coverage dependent activation energy and second-order desorption is illustrated. Theoretical calculations show the technique is very sensitive for distinguishing desorptions with coverage dependent activation energy. Analysis by desorption rate isotherms requires few assumptions, is easy to apply and can be used to determine kinetic parameters for both unsupported and supported catalysis.

The kinetics and mechanism of the autocatalytic decomposition of HCOOH on clean Ni(110)

The flash decomposition of DCOOH was studied on a clean nickel (110) surface following adsorption at 37°C. The reaction proceeded by a two-dimensional autocatalytic mechanism to form D2, CO2 and CO products. The results indicated DCOOH adsorbed dissociatively at 37°C by splitting off H2O and forming an adsorbed molecule composed of DCO and DCOO. Above ten percent of saturation coverage these molecules formed a condensed phase or island structure. The decomposition of the molecules was rate determining for the formation of CO2 and D2 products. Theoretical calculations for branched chain mechanisms and coadsorption experiments with CO and H2 separately with DCOOH indicated the intermediate involved in the explosion was not associated with the observed product molecules. The intermediate in the explosive decomposition was shown by interrupted flashes to be stable at 37°C. The autocatalytic flash decomposition curves were explained by reaction occurring at bare metal sites within the islands, and as product molecules desorbed the number of sites increased, causing the rate to accelerate. The rate of decomposition was well described by the equation Rate = −k(ccI)(cI −c + fcI), where c is the surface concentration, cI is the initial surface concentration, and f is the density of initiation sites. The activation energy of 26.6kcal/mol was determined from heating rate variation. The narrow flash curves were fit with a first order pre-exponential factor of 1.6 × 1015 sec−1 with a density of initiation sites of 0.004.

Preparation and separation properties of silicalite composite membranes

Silicalite-alumina composite membranes were prepared by an in situ zeolite synthesis method using an alumina membrane tube with a 5-nm pore diameter, γ-alumina layer as a substrate. Single gas permeances of H2, Ar, n-C4H10, i-C4H10, and SF6 were measured and mixtures of H2i-C4H10 and H2SF6 were separated to characterize the silicalite membrane. These measurements were made from 300 to 737 K, and are compared to an alumina membrane without a silicalite layer. Permeances were lower in the silicalite membrane (a factor of 8 for Ar at 298 K). Permeances for the alumina membrane decreased as the temperature increased, and separation selectivities were lower than values expected for Knudsen diffusion. Transport through the alumina membrane was by Knudsen flow and surface diffusion. The silicalite membrane showed dramatically different behavior, and transport appeared to be controlled by molecular size and adsorption properties. Permeances of all components studied were activated in the silicalite membrane, and activation energies ranged from 8.5 to 16.2 kJ/mol. The ratio of single gas permeances was as high as 136 for H2 to SF6 and 1100 for H2 to i-C4H10 at 298 K. Separation selectivities at elevated temperatures were significantly above Knudsen diffusion selectivity for the silicalite membrane and were larger than ratios of pure gas permeances at the same temperature. The largest permeance ratio for the separation of mixtures was 12.8 for H2SF6 at 583 K. Separation selectivities for both membranes were higher when a pressure drop was maintained across the membrane than when an inert sweep gas was used because of counter diffusion of the sweep gas.