Relative Adsorption Strength Research Articles

Strong promotional effects of Li, K, Rb and Cs on the Pt-catalysed reduction of NO by propene

The catalytic activity and selectivity of Pt dispersed on γ-Al2O3 for the reduction of NO by propene is promoted extremely strongly by Li, K, Rb and Cs alkalis in a wide temperature range ca. 450–800K. Remarkable and unprecedented effects on both activity and selectivity are found. The best promotion effects are achieved by Rb-promotion, for which rate increases as high as 420-, 280- and 25-fold are obtained for the formation rates of N2, CO2 and N2O, respectively, in comparison with the performance of un-promoted (alkali-free) Pt/γ-Al2O3. From the other hand, the selectivity towards N2 is improved from ∼20% over the alkali-free unpromoted Pt catalyst, to >90% over the optimally alkali-promoted catalyst. These effects are understandable in terms of the effect of alkali promoter on the relative adsorption strengths of reactant species. Alkali promotes the adsorption, and consequently the dissociation, of electronegative adsorbates (NO) and inhibits the adsorption of electropositive adsorbates (propene), on a catalyst surface predominantly covered by propene and its fragments.

Removal of aquaculture therapeutants by carbon adsorption: 2: Multicomponent adsorption and the equilibrium behaviour of mixtures

This paper presents experimental data on the batch equilibrium adsorption behaviour of binary and ternary mixtures of aquaculture therapeutants (Malachite Green, Chloramine-T, and Oxytetracycline) and dissolved organic carbon (DOC) onto the coal-based granular activated carbon 207EA (Sutcliffe-Speakman). These data indicate directly the relative strengths of adsorption of some of the components under the conditions studied and confirm that DOC is generally less strongly adsorbed than most of the therapeutants at the concentrations of interest. This paper also reviews existing multicomponent equilibrium adsorption models and concludes that, for aquaculture applications, the Ideal Adsorbed Solution Theory (IAST) with the Freundlich isotherm is the model that is most suitable. This model was then tested using the experimental mixture data. The results show that IAST with the Freundlich isotherm is very successful at predicting the reduction in adsorption of DOC caused by the presence of competing therapeutants. It tends, however, to underestimate the amounts adsorbed of the therapeutants, particularly when these are present at higher concentrations. In general, the drop in DOC removal caused by the presence of competing therapeutants is quite small. IAST is clearly a good model to use in the design of batch carbon adsorption treatments for therapeutant removal.

Strong Promotion by Na of Pt/γ-Al2O3 Catalysts Operated under Simulated Exhaust Conditions

The catalytic performance of Pt dispersed on γ-Al2O3 is very strongly promoted by sodium in the temperature range 200–500°C under simulated exhaust conditions at the stoichiometric point. Remarkable and unprecedented effects on NO, CO, and C3H6 conversions, as well as enhanced selectivity towards N2, are found. Addition of Na causes a significant decrease in the temperature of 50% conversion (T50) for NO, CO, and C3H6. For optimally promoted Pt this temperature is ∼100° lower than for unpromoted Pt. Similar decreases are observed for the temperatures required for 100% conversion of NO, CO, and C3H6. The selectivity towards N2 is improved from ∼75% over unpromoted Pt to ∼100% over the optimally Na-promoted catalyst. These effects are understandable in terms of the effects of sodium on the relative adsorption strengths of reactant species.

00/01365 Calculation of phase equilibria and physicochemical properties of gas hydrates in electric field

The catalytic behavior of Pt and Rh catalysts during the gas-phase oxidation of H2–CO mixtures was studied with commercial Pt/C, Pt–Ru/C and Pt–Sn/C electrocatalysts, as well as laboratory prepared catalysts of Pt and Rh supported on various carriers (γ-Al2O3, TiO2, TiO2(WO3), SiO2, WO3, MoO3 and carbon). The goal of this study was to assess the CO-tolerance of the catalysts, i.e. the lowering of the H2 oxidation rate in the presence of CO, generally observed both in gas-phase and electrochemical environments. A significant drop of hydrogen oxidation rate, up to 5 orders of magnitude, was observed in the presence of CO. CO inhibition was strongly dependent on the nature of the support. The standard Pt/C electrocatalyst was the least CO-tolerant catalyst among those tested, while Pt–Sn/C, as well as Pt and Rh supported on MoO3 and TiO2-based carriers were the most active and CO-tolerant catalysts. The experimental findings indicate that CO inhibition is due to CO adsorption in linear and/or bridged form depending on the type of metal and support employed. The CO tolerance of the catalysts depends on the relative adsorption strength of CO and hydrogen on the metal surface. Simulation of CO inhibition showed that ΔHCO (or the relative values of ΔHCO and ΔHH2) may vary up to 20–40 kJ/mol between the most and least CO-tolerant catalysts.

The effect of carbon monoxide on the oxidation of four C6 to C8 hydrocarbons over platinum, palladium and rhodium

The oxidations of benzene, toluene and 1-hexene, alone and in mixtures with isooctane and carbon monoxide, have been investigated over alumina-supported platinum, palladium and rhodium under three different conditions. In the absence of CO, oxidation of all hydrocarbons is complete at the lowest temperature with platinum. However, inclusion of CO greatly inhibits hydrocarbon oxidation on platinum. There is also moderate inhibition of palladium, but with rhodium oxidation of the aromatics is somewhat enhanced by the presence of carbon monoxide. The activity order then becomes Rh>Pd>Pt. Conversely, unsaturates inhibit CO oxidation to a lesser extent, this effect being greatest for hexene and toluene with palladium and least with benzene on rhodium. In mixtures of the four hydrocarbons, with or without CO present, the order of removal is always hexene followed by toluene, benzene and isooctane with rhodium exhibiting the largest separation between toluene and benzene. This order appears to be largely governed by competitive adsorption between hydrocarbons of different adsorption strength since the pattern of reactivity is very different when the unsaturates are oxidised alone. Under very lean conditions platinum is the most active for the oxidation of benzene alone, palladium for toluene and rhodium for hexene alone. However, relative activities are strongly dependent on conditions since the reactions exhibit very different kinetic orders in oxygen with each metal. The oxidations of hexene and toluene are positive order in oxygen over Pt and Pd but negative order over Rh. On the other hand the oxidation of benzene seems to be positive order in oxygen for Pt, negative order with Pd and near zero order with Rh. Some of these trends can also be rationalised in terms of relative strengths of adsorption.

Coupled radiotracer and voltammetric study of the adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid on polycrystalline gold

Abstract Coupled application of a version of the in-situ radiotracer ‘foil’ method and voltammetry provided information on the time-, potential-, concentration- and pH-dependent adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid (HEDP) on a polycrystalline gold electrode, and on the effect of Zn 2+ ions on the adsorption phenomena. Adsorption processes on the oxide-free surface of gold were observed to be potential-dependent in the potential range 0.05–1.00 V (versus RHE), while formation and irreversible accumulation of oxidation products of HEDP could be detected at E >1.00 V. The relative adsorption strength of HEDP (its dissociation and/or oxidation products) was found to be higher on an oxide-free gold surface than on an oxide-covered one. The surface excess of HEDP increased with increasing pH. Addition of Zn 2+ ions to the solution exerted a substantial effect on the HEDP accumulation. Namely, significant differences in the surface coverage, as well as in the kinetics and mechanism of HEDP adsorption could be detected in the potential regions below and above E =0.2 V. Reduction of Zn(II) species at E ≤0.1 V is probably coupled with the induced adsorption of HEDP on an Au electrode, leading to the formation of a polymolecular HEDP–Zn surface complex layer.

A Test and Demonstration Unit for Concentrating Sulfur Dioxide from Flue Gas

A bench-scale system to continuously separate and concentrate SO{sub 2} from flue gas has been developed and tested. The separation is accomplished due to the relative adsorption strengths of SO{sub 2} and water on a synthetic mordenite, a phenomenon known as rollup. The continuous concentration system (CCS) is comprised of two pairs of packed beds that cycle between cleaning, roll-up, and regeneration modes. The bed pairs behave identically, with cycles staggered by half of one adsorption-desorption-regeneration period. The cycle for each pair involves regeneration of one bed with dry air, while the other is cleaning the flue gas and subsequently rolling up the SO{sub 2}. This bench-scale CCS is able to split a 6 standard liters per minute (SLPM) gas stream containing 2,000 ppm SO{sub 2} into two streams of equal flowrate, one containing 3,400 ppm SO{sub 2} and the other 70 ppm SO{sub 2}. The influence of water is strong, with the rollup being optimal at 13% moisture in the flue gas. Adsorption starting with the bed at 75 C is optimal, and air regeneration at 200 C and 8 SLPM for 76 min is adequate. An economic analysis of an industrial-scaled CCS capable of treating a 1,000,000 scfmmore » flue gas stream containing 2,000 ppm SO{sub 2} (typical of a 400-MW coal-fired power plant) was conducted. The CCS installation permits smaller flue gas scrubbers to be installed and can potentially save a 400-MW power plant $2.6 million in annual operating costs.« less

Influence of alloying platinum for the hydrogenation of p-chloronitrobenzene over PtM/Al 2O 3 catalysts with MSn, Pb, Ge, Al, Zn

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension, at 303 K and atmospheric pressure, over a series of Pt/Al 2O 3 and PtM/Al 2O 3 catalysts (MSn, Pb, Ge, Al, Zn). The bimetallic catalysts were prepared by the so-called controlled surface reaction method by reacting organometallic compounds on the well-dispersed Pt/Al 2O 3 parent sample. The kinetics obeys a modified Langmuir-Hinshelwood model with competitive adsorption between CNB and hydrogen. The competition of adsorption no longer holds at high CNB concentration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law is then proposed which allows both adsorption and rate constants to be determined. Upon addition of the second metal M, the specific activity per Pt surface atom goes through a maximum value around a chemical composition M/Pt≌0.1–0.2. It is suggested that an electron-deficient species of the second metal promotes the reaction rate by activating the nitrogenoxygen bond; ionic tin species are better promoters for this purpose. At high CNB conversion (> 98%) the yield of the desired product, p-chloroaniline (CAN), increases from 82% on pure Pt/Al 2O 3 to 97.5% on PtSn/Al 2O 3 (Sn/Pt = 0.36). The improvement of selectivity to CAN is not due to a decrease in CAN dehalogenation rate, which is not affected by alloying. Actually, the promoting effect of the second metal decreases the relative strength of adsorption between CAN and CNB up to a factor of 10: CAN is then easily removed from the surface by CNB, thus preventing hydrodechlorination.

Monte Carlo Simulation of Binary Gas Adsorption in Zeolite Cavities

Abstract Grand canonical Monte Carlo simulations have been performed for binary adsorption of Lennard-Jones molecules with point multipole moments in zeolite cavities of type X. Fluid-solid electrostatic interactions were taken into account. Phase diagrams and total coverage were calculated for three binaries and compared with experimental measurements. MC simulations gave good agreement with experiment for two mixtures (C2H4-CO2 and CO2-CH4) but there were discrepancies between simulation and experiment for the system i-C4H10-C2H4. The dependence of excess Gibbs free energy on the composition and pressure was studied. Negative deviations from ideality are due to energetic heterogeneity and size effects. Unlike liquid-vapor equilibrium, deviations from the Lorentz-Berthelot mixing rules for the adsorbates have little effect upon the phase behavior. Density distributions show that the components compete for the high energy sites inside the cavity; depending on its relative strength of adsorption, one compo...

Particle size effect on the kinetics of p-chloronitrobenzene hydrogenation over platinum/alumina catalysts

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension at 303 K and atmospheric pressure, over a series of Pt/Al 2O 3 catalysts of varying dispersion (0.07 < H/Pt < 1.26). The kinetics obeys a modified Langmuir—Hinshelwood model with competitive adsorption between CNB and hydrogen. The adsorption competition no longer holds at high CNB concentration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law is then proposed which allows determination of both adsorption and rate constants. The specific rate constant (expressed on a per surface Pt atom basis as a turnover frequency) increases by one order of magnitude when the Pt dispersion decreases. This behaviour is interpreted in terms of a transition state bearing a weak negative charge more stabilized on the large Pt particles. This hypothesis agrees with the effect of substituents on the aromatic ring. The conversion of CNB leads to p-chloroaniline (CAN) as main product, p-chloronitrosobenzene and p-chlorophenylhydroxylamine as intermediates, with nitrobenzene, aniline and chlorobenzene as byproducts. The product selectivity depends on the Pt particle size. At high conversion, the highest selectivity in CAN (>98%) is reached on large Pt particles. This can be interpreted by a lower relative adsorption strength between CAN and CNB on poorly dispersed catalysts; CAN is then easily removed from the surface by CNB, which prevents hydrodechlorination.

Alumina-supported bimetallics of palladium alloyed with germanium, tin, lead, or antimony from organometallic precursors II. gas-phase hydrogenation of 2-methyl-1-buten-3-yne (valylene) and 2-methyl-1,3-butadiene (isoprene)

A series of PdGe, PdSb, PdSn, and PdPb/α-Al 2O 3 catalysts, prepared by the controlled surface reaction (CSR) technique from organometallic precursors have been tested in the gas-phase hydrogenation of 2-methyl-1-buten-3-yne (valylene) and 2-methyl-1,3-butadiene (isoprene). On catalysts reduced at 573 K, the turnover frequencies for valylene (TON V) and isoprene (TON I) hydrogenation were not modified by addition to the 0.09 wt% Pd/α-Al 2O 3 base catalyst of Ge, Sb, Sn, or Pb, up to 0.1 wt% (at 293 K: TON V = 20 s −1 and TON I = 33 s −1). Pd/α-Al 2O 3 reduced at 773 K was severely sintered ( d TEM increased from 2.8 run to 12.4 nm) and TON V and TON I at 293 K increased to 190 and 283 s −1, respectively, as a result of an apparent crystal size effect: the reactants adsorb more strongly on the smaller Pd particles. Upon alloying with Ge, Sb, Sn, or Pb and a subsequent reduction at 773 K, a modest decrease of both TON V (by a factor of 2) and TON I (by a factor of 2–5) was observed. At high conversion, both the selectivity to isoprene (S I) in valylene hydrogenation, and to olefins ( S O ) in isoprene hydrogenation on Pd/α-Al 2O 3 were improved upon alloying with Sb, Sn, or Pb (Ge had no effect). In addition, the isomerization of 2-methyl-1-butene and 3-methyl-1-butene (double bond migration reaction) during isoprene hydrogenation was partially suppressed on PdSn and PdPb/α-Al 2O 3. The improvement in selectivities was interpreted in terms of a change in the relative adsorption strength of the reactants and intermediate products over the new bimetallic sites.

Theoretical study of system peaks and elution profiles for large concentration bands in the case of a binary eluent containing a strongly sorbed additive

The profile of system peaks when using high concentrations and a binary mobile phase was studied using the semi-ideal model of chromatography. The profile of the sample band depends on the concentration of the strong solvent and on the sample size. It is very strongly influenced by the relative adsorption strength of the strong solvent and the sample. This relative strength is measured by the ratio of (a) the origin slope of the equilibrium isotherm of the strong solvent between the stationary phase and the pure weak solvent to (b) the origin slope of the isotherm of the sample ( i.e., the ratio between their respective column capacity factors in the pure weak solvent). When this ratio is smaller than 0.2, the sample band profile depends only on the equilibrium isotherm of the sample in the binary mixture. If the ratio becomes larger, the competition between the strong solvent and the sample molecules for interaction with the stationary phase becomes more intense. For Langmuir-competitive isotherms of the strong solvent and the sample, if the concentration of the strong solvent and/or its strength are progressively increased at constant sample size, the retention time of the band decreases (as expected), whereas its typical Langmuirian asymmetry decreases and, past a narrow transition range, reverses. Bands traditionally associated with an anti-Langmuir isotherm are then obtained. Extremely unusual, broad, characteristic profiles, with a sharp front and a sharp rear, which sometimes exhibit two maxima, are obtained in the transition region. When the origin slope ratio is large, the profile of a high-concentration-sample band in the transition region has two shock layers. The occurrence of such band profiles, under experimental conditions similar to those predicted here, have been reported previously, but their origin has remained unexplained until now.

La 0.7A' 0.3MnO 3 catalysts by MS- Xα and CNDO/2 methods

This paper presents the orbital energies and the densities of states near the Fermi level of the pero Vskite type catalysts La 0.7A' 0.3MnO (A' = Sr, Ba, Ca), calculated by the MS- Xα method. It compares the frontier orbital electronegativities of the catalysts to those of the adsorbed molecules (O 2 and CO). This work also includes the total energy curves of the catalyst model with the adsorbed molecules calculated by the CNDO/2 method, compares the relative strength of adsorption of both O 2 and CO, and discusses the catalytic mechanism of the oxidation of CO. It is promising that the noble-metal catalysts used in the oxidation of CO for purification of automobile exhausts and for other purposes could be replaced by the peroVskite type catalysts La 0.7A' 0.3MnO 3 (A' = Sr, Ba, Ca). The catalysts, which are very active in such an oxidation reaction, have been studied for years by many authors, and considerable headway has been made experimentally. Theoretically, however, interpretation of the catalytic mechanism in terms of molecular interactions has not been available. This paper presents some theoretical results of the adsorption and activity of the catalysts studied by the MS- Xα and CNDO/2 methods.

The effect of coordinatively unsaturated surface Cr 3+ on the adsorption of gases on chromia

Clean chromias of smaller and larger degrees of dehydroxylation, i.e., smaller and larger surface concentrations in Cr 3+( cus), were prepared by activation in flowing helium to 315°C (microporous, amorphous) and to 520°C mesoporous, microcrystalline). These chromias were then exposed to argon, hydrogen, methane, or nitrogen at −160 to −78°C, flushed with helium, and the residual amount of adsorbate was determined after various times. From these data and from displacement experiments in which pulses of one gas were passed over chromia on which another gas was adsorbed, the relative strengths of adsorption at Cr 3+( cus) are Ne ⪡ Ar ⪡ H 2 ⪡ CH 4 < N 2. Converting Cr 3+( cus) to Cr 3+CO by adsorption of carbon monoxide or to Cr 3+OH − by rehydroxylation with water somewhat decreased the strength of adsorption of argon and greatly decreased those of nitrogen and methane. On microporous chromia, reduction in the strength of adsorption of nitrogen and methane was less for poisoning by carbon monoxide than it was for poisoning by water. It is suggested that this resulted from Cr 3+CO projecting more into the micropores than Cr 3+OH −.

Potassium and Rubidium Adsorption and Diffusion in Soil

AbstractUptake of cations by plant roots growing in soil is believed to be influenced by the relative adsorption strengths of cations by soil exchange sites. The adsorption of K and Rb by four soils was investigated to determine the differences in adsorption of K and Rb which may occur and how they are affected by soil properties with the view of using this information to evaluate the uptake mechanisms for K and Rb by plant roots growing in soil.Rubidium was adsorbed preferentially to K by all four soils. Selectivity coefficient kRb/K, indicating the size of the effect ranged from 1.9 to 4.4. Adding K decreased kRb/K, increased the amount of nonexchangeable Rb, and decreased exchangeable Rb. There was a negative correlation (r2 = 0.78) between exchangeable K in the soil and kRb/K. The level of K in the soil had a greater effect on kRb/K than varying types of exchange sites present due to differences in clay and amounts of organic matter in the four soils investigated. The size of the selectivity coefficient indicated that the difference between K/Rb ratios in the solution and exchange phases of these soils used and differences between soils made them suitable for use in plant uptake studies of the mechanisms of K and Rb uptake.

Soil Adsorption and Volatility of Dinitroaniline Herbicides

Studies were conducted to determine both the extent and relative strengths of adsorption of twelve dinitroaniline herbicides to a Piano silt loam soil. The effects of soil on the growth of foxtail millet [Setaria italica(L.) Beauv.] in vitro on agar media containing 1.0 and 0.1-μM concentrations of the various herbicides and on the relative phytotoxicities of herbicide vapors arising from the agar media to the same species were also measured. Although considerable differences in the extents and strengths of adsorption of the respective herbicides to soil were noted, correlations with herbicide performance under field conditions were low. Extrapolation of laboratory results to a Piano silt loam soil under field conditions suggests that field applications of 0.32 kg/ha of the respective herbicides might result in herbicide concentrations between 0.00062 and 0.026 ppmw in the soil solution. These concentrations appeared to be less than those necessary to produce substantial plant injury from most of the dinitroaniline herbicides. High correlations between the effects of herbicide vapors on growth of the foxtail millet under laboratory conditions and the relative effectiveness of seven of the herbicides under field conditions suggest that absorption of vapors of the dinitroaniline herbicides by plants may be more important than absorption of the herbicides from soil solutions.

Study on Adsorption of Sodium Dinonylnaphthalenesulfonate on Steel by Radiotracer Method

Adsorption of sodium dinonylnaphthalenesulfonate (NaDNNS) tagged with radioactive sulfur 35 was studied at the steel-benzene interface at room temperature. The adsorption increased with time, requiring 20hrs to reach a maximum value, and its equilibrium adsorption was about one tightly packed monolayer. Adsorbed NaDNNS molecules were not desorbed by benzene rinsing but 65% of adsorbed NaDNNS molecules were desorbed by exchanging with untagged NaDNNS molecules in benzene solution. The competitive adsorption of NaDNNS and Pb Naphthenate was not Langmuirian and the relative strength of adsorption was NaDNNS>Pb Naphthenate.

Nuclear Magnetic Resonance Measurements of Line Width, Chemical Shift, and Intensity for Molecules Adsorbed on Silica

Abstract High-resolution NMR spectra were taken of molecules adsorbed on silica to determine the effects of specific surface interactions on line widths, chemical shifts, equilibria, and freezing points. Line widths were measured for cyclohexane, benzene, acetone, and methanol adsorbed at monolayer coverage. Hydrogen bonding to the surface was evident for the latter two compounds from the decrease in line width when the silica was silanized. In addition, the hydroxyl hydrogen of methanol was found to undergo exchange with the surface. After silanization, chemical-shift changes were noted for cyclohexane, p-xylene, mesitylene, acetone, and methyl acetate. Attempts were made to explain the magnitude of the changes by comparison with data from infrared spectroscopy and gas-solid chromatography. Chemical shifts of mixtures of adsorbates were studied to gain additional information on relative adsorption strengths. The tautomeric equilibrium of adsorbed acetylacetone was measured from the relative integrated intensities of the keto and enol resonances. The enol form was favored more on silanized silica than on hydrated silica. Relative intensities were also used to study the freezing of adsorbates.

Adsorption of gases on 4A synthetic zeolite

Equilibrium data are presented for the adsorption of permanent gases and C1 and C2 hydrocarbons on Linde 4A synthetic zeolite from 50 °C down to the boiling point of the adsorbate or to a temperature at which the rate was very slow. The Freundlich equation often fits isotherms accurately; however, these linear logarithmic plots seem to be only portions of a more general non-linear relationship. Generalized Polanyi plots did not yield a unique characteristic curve for individual adsorbates at different temperatures nor for groups of adsorbates; nevertheless, these plots are useful in estimating the relative strength of adsorption of gases. Ar, O2, CH4, and C2H6 were the least strongly adsorbed, followed in order of increasing strength by N2, CO, CO2, and C2H2. Ethylene was strongly but anomalously adsorbed. For the gases studied, the quadrupole moment seems to be the principal factor determining strong adsorption.

An Explanation for the Variability in Sr-Ca Exchange Selectivity of Soils, Clays and Humic Acid

The reason for the variability among soils in their effect on the Sr-Ca exchange selectivity was investigated in systems where Sr and Ca were the only competing ions and Sr was present in small amounts. The exchange selectivity theory of Eisenman was used to explain the results. The dominant factor governing the relative strengths of adsorption by the clay minerals studied was the relative energy of hydration of the two cations, so that Sr, the more weakly hydrated, was adsorbed more strongly than Ca. However on humic acid, where the carboxyl group was mainly responsible for the exchange adsorption, the field strength of the exchange site was important so that a reversed sequence occurred and Ca was adsorbed more strongly than Sr. Investigations with soils confirmed the results using soil components. As the fraction of Sr on the exchanger phase was reduced, the relative Sr adsorption increased indicating that variable sites were present on all of the materials studied.