Philip J. Pomonis

Removal of chlorinated phenols from aqueous solutions by adsorption on alumina pillared clays and mesoporous alumina aluminum phosphates

Abstract Alumina pillared montmorillonite (AIPMt) and mesoporous alumina aluminum phosphates (AAPs) were contacted with aqueous, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol solutions, at a concentration range between 25 and 250 μg/litre, in batch equilibrium experiments, in order to determine their adsorption properties. The removal of chlorophenols by the adsorbents increase with increasing chlorine substitution in their molecules. In the case of pentachlorophenol, the increased affinity allows adsorption to occur much more efficiently than in the case of other compounds. AIPMt material adsorbs 26.3% of 2,4-dichlorophenol, 75.6% of 2,4,6-trichlorophenol and 95.2% of pentachlorophenol at equilibrium. The adsorption of chlorophenols on mesoporous AAPs is much less pronounced as compared to clays but increases with the ratio of P Al , as the surface acidity of those solids also increases. The AAP mixture with a ratio P Al = 0.6 adsorbs 14.8% of 2,4-dichlorophenol, 27.1% of 2,4,6-trichlorophenol and 58.3% of pentachlorophenol. The amounts of chlorophenols decomposed during the treatment increase in AAPs and especially in those with a higher P Al ratio (=0.6).

Effects of substitution in perovskites La2−xSrxNiO4−λ on their catalytic action for the nitric oxide+carbon monoxide reaction

Abstract The catalytic activity of the perovskite series La 2− x Sr x NiO 4−γ ( x = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) has been studied for the reduction of nitric oxide by carbon monoxide in equimolecular mixture. The conversion ( X %) of reactants approaches the value X NO / X CO = 2 at low temperatures and low degree of substitution of lanthanum by strontium but tends to unity at high temperatures and high degree of substitution. This behaviour is due to the change of the rate determining step of the reaction which at low temperature is the oxygen formation through decomposition of nitric oxide while at high temperatures it is the reaction between the adsorbed oxygen species and carbon monoxide. The apparent activation energies for the nitric oxide elimination drop with the process of substitution from x = 0.00 to x = 1.00 and increase thereafter. The controlling factor of the catalytic activity of the series seems to be the strength of adsorption of oxygen on the perovskite surface. Thus it is shown that the sum of the enthalpies of the metal-oxygen bonds in the material is closely related to the temperature where reaction switches mechanism. Besides, the differential mean electronegativity of the solids is related both to the reaction rate as well as to the apparent activation energy for nitric oxide reduction.

Structure and catalytic activity of perovskites La-Ni-O supported on alumina and zirconia

Lanthanum nickelate binary oxidic species supported on alumina and zirconia were tested for their catalytic activity for the NO + CO reaction. The support took place by impregnation of Al2O3 or ZrO2 without any pretreatment or after addition of lanthana on the support surface. The zirconia based solids show a surface area (BET) of 5–7 m2 g−1, one order of magnitude smaller than those based on alumina (40–60 m2 g−1). The crystal phase developed on ZrO2 was only La2NiO4 while on Al2O3 the phases found by X-rays were LaNiO3 and LaAlO3. The solids La2NiO4/ZrO2 containing nickel in 2 + state, were more active than LaNiO3/Al2O3 containing Ni3+. The smaller activity of the La-Ni-O solids supported on Al2O3 is attributed to the stronger adsorption of oxygen, or oxygenated species, on Ni3+ as compared to the weaker adsorption on Ni2+ existed in the La-Ni-O solids supported on ZrO2. Especially the stronger adsorption of nitric oxide in the first case results in a reaction route where an excess of nitric oxide as large as 25–30% is eliminated as compared to the reacting carbon monoxide. The excess of eliminated nitric oxide undergoes disproportionation towards nitrous oxide and nitrogen, the former undergoing further decomposition towards nitrogen and oxygen. In the second case of ZrO2/La-Ni-O the weaker adsorption of nitric oxide results in a nearly equimolecular elimination of the two reactants and the excess of nitric oxide reacting does not exceed a 12–14% of carbon monoxide. The Arrhenius plots for the nitric oxide reduction on the alumina-based solids show a unique behaviour for the whole temperature range examined while similar plots for the zirconia-based solids show a two stage behaviour corresponding to different surface mechanism. The activation energies of nitric oxide elimination with carbon monoxide on the above solids are appreciably smaller than activation energies of nitrous oxide decomposition on the same materials, in accordance with previous observations. Finally the activity of La2NiO4/ZrO2 solids calculated per mass of perovskite is nearly five times that of La2NiO4 pure perovskites prepared through the nitrate or citrate routes although their surface areas are equal for the citrate and only double for the nitrate solids.

Influence of Cr3+ and Fe3+ cations on the structure of alumina–aluminium phosphate solids and their catalytic activity for N2O decomposition

Phosphate salts of the metals Al, Fe and Cr supported on alumina were prepared by co-precipitation with NH3 from solutions containing calculated amounts of the metal nitrates and phosphoric acid. The atomic ratio of the elements participating in the solids was Al : P : M = 100 : x : 20 where M = Al, Fe, Cr and x= 0, 4.5, 9, 18, 36, 72 and 144. The parent compounds Al100PxAl20 are formed by an endothermic decomposition of NH4NO3, which exists in the dried precipitant, to NH3 and HNO3. The resulting solids for x= 0 contain just γ-Al2O3, for x= 144 AlPO4 plus an amorphous phase, while for intermediate values of x a totally amorphous phase is formed. Substitution of 20 % Al by Fe alters slightly the route of decomposition, the endo-effect noticed above is followed by a small exothermic one, meaning probably the decomposition of part of the resulting nitric acid through anion breakdown to N2, H2O and O2. The structure of the resulting solids corresponds to α-M2O3 for x= 0 and MPO4 for x= 144, while for intermediate values amorphous materials are formed. An addition of Cr instead of Fe alters completely the route of decomposition of NH4NO3 to N2, H2O and O2 for reasons similar to those noticed for Fe. For x= 0 the resulting solids contain α-M2O3 oxides alone, for x= 144 they are totally amorphous and for intermediate values of x an amorphous phase plus crystalline Cr2O3 is formed. The behaviour of these systems for a simple redox reaction, namely N2O decomposition, shows a continuous drop of activity with phosphorous content. An analysis of the results according to the kinetic theory of poisoning indicates an order of deactivation between one and two, depending on the catalyst and the temperature. A mechanism which may explain this result is proposed. Thiele modulus and effectiveness factors which have been calculated show that for AlPAl an AlPCr species internal diffusion is the rate-determining step and the reaction occurs mainly on the external surface. For the AlPFe catalysts the rate of diffusion is almost equal to that of reaction.

Catalytic combustion of methane on La2–xSrxNiO4–λ(x= 0.00–1.50) perovskites prepared via the nitrate and citrate routes

The catalytic combustion of CH4 with stoichiometric amounts of oxygen to CO2 and H2O has been studied over two series of perovskite solids La2–xSrxNiO4–λ(x= 0.00, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50) prepared via the nitrate and citrate methods. The reaction rate consists of two rate components, one suprafacial employing oxygen from the gas phase and active at low temperatures and another intrafacial, employing oxygen from the perovskite lattice and apparent mainly at high temperatures. The reaction is first order relative to CH4 for the two series and over the whole range of reaction temperature. The reaction order for oxygen decreases from 0.5 to 0.2 as the temperature increases. The Arrhenius-type temperature dependence of the reaction rate for the suprafacial and the intrafacial process have been determined for the La1.25Sr0.75NiO4 samples to be 47.4 and 110.8 kJ mol–1 for the nitrate solid and 61.5 and 52.5 kJ mol–1 for the citrate solid, showing that the oxygen is much more strongly bound on the catalyst of nitrate origin. The catalytic activity of both series is uniquely related to %Ni3+ in the perovskite.

Controlled architecture of solids with micro- and meso-porosity obtained by pillaring of montmorillonite with an LaNiOx binary oxide

High-surface-area solids containing both micropores and mesopores have been synthesized by intercalation of the heterobinuclear cation of the complex NiLa(fsaen)NO3 between the layers of montmorillonite clay and calcination at 500 °C. The adsorption mechanism of the complex follows an exchange reaction involving up to 2 mmol of complex cations per g of clay. Further amounts of complex are adsorbed in sites other than exchange ones as shown by zeta-potential experiments. The IR spectra of the intercalated complex showed no major alterations of the complex during adsorption and a stabilization effect and enhancement of the parallel orientation of the clay plates. X-Ray diffraction (XRD) measurements at low angles showed that the pillared clay possesses an inter-layer distance of 13.4 A at 500 °C. Its BET surface area reaches a maximum of 220 m2 g–1 at this temperature. The as plots showed a maximum microporosity at ⩽2 mmol g–1. Additional loading results in transformation to a mesoporous delaminated structure. X-Ray photoelectron spectra (XPS) of the composite materials showed that the ratios La/Si and Ni/Si are lower than those determined by chemical analysis, owing to the shielded environment of the LaNiOx pillars in the clay. The same method indicated that for a high degree of loading the aggregates seem to be of perovskite origin.

Low-temperature synthesis of perovskite solids LaMO3(M = Ni, Co, Mn)via binuclear complexes of compartmental ligand N,N′-bis(3-carboxysalicylidene)ethylenediamine

A new method has been developed for the low-temperature synthesis of perovskite solids LaMO3 where M = Ni, Co, Mn, using binuclear complexes of the ligand N,N′-bis(3-carboxysalicylidene)ethylenediamine (H4fsaen). The thermal decomposition of such complexes was studied using a thermobalance. The solids that formed after thermal treatment from 400 to 1000 °C were characterized by X-ray diffraction. Using this method perovskites LaNiO3, LaCoO3 and LaMnO3 have been synthesized at temperatures of ca. 500–600 °C. The above procedure yields solids with specific surface areas substantially higher than those obtained from ordinary ceramic methods, such as citrate and the nitrate routes.

Threshold limits and kinetics of the non-isothermal decomposition of ammonium nitrate catalysed by chromium ions.

Abstract Ammonium nitrate (AN) samples, containing varying amounts of Cr 3+ cations were prepared and their mode of thermal decomposition was analysed under non-isothermal conditions in a thermogravimetric balance. Pure AN decomposes endothermically, but addition of Cr 3+ in amounts larger than one mole of Cr 3+ per 3000 moles of AN acts catalytically, changing the decomposition to an exothermic mode. At concentrations lower than this, the diffusion limitations prevent the condensation of Cr 3+ species into Cr 2 O 7 2− which are the catalytically active species. The rates of decomposition, described by a typical Coats-Redfern kinetic equation, show two stages. Satisfactory results were obtained using a modified Coats-Redfern equation and by taking into account the continuous increment of chromium catalyst in the melt. The second stage of the decomposition shows an increased activation energy compared with the first. This is probably caused by a change in the rate-determining step of the reaction path, which is most likely the decomposition of dichromates.

Synthesis and properties of some mesoporous aluminophosphates with acidic surface sites

Mesoporous solids of the general formula Al100PxFe5-y[x= 0, 15, 60, and y(the final firing temperature in °C)= 400 or 600] have been prepared and characterised by surface area and porosity measurements, X-ray diffraction (XRD), Mossbauer spectroscopy and solid-state NMR (27Al and 31P). The surface acidities of the solids were also determined by ammonia adsorption, and the ζ-potential of the particles was measured. The gradual incorporation of phosphate groups into the alumina results in an increase of the surface area. The materials possess a pore-size distribution approximated by a mixed Gaussian and Lorentzian component which varies in a controllable manner according to the elemental composition. The material Al100P15Fe5 corresponds to that with the maximum surface area and pore volume. The XRD analysis showed that the materials without phosphorus possess the γ-Al2O3 structure, but they become amorphous upon addition of 15% P. A further increase in the amount of phosphorous to 60% initiates the transformation of the solids to AlPO4. The Mossbauer spectra suggest that, in the samples without phosphorus, the iron species are segregated, but the addition of phosphorus results in a dispersion of the iron into the aluminium phosphate matrix. The 27Al MAS NMR spectra indicate that the aluminium atoms are predominately in octahedral environments in the samples Al100P0Fe5-600 and Al100P15Fe5-600 but they become equally distributed between octahedral and tetrahedral sites in Al100P60Fe5-600. The ζ-potential of the particles in aquatic suspension indicates that this parameter is sensitive to the temperature of thermal treatment. The surfaces of the solids showed increased acidity which is linearly related to the phosphorus content.

Effect of the first row transition metal cations on the mode of decomposition of ammonium nitrate supported on alumina-aluminum phosphate and the final products obtained

Abstract The mode of decomposition of ammonium nitrate in mixtures with 80% Al 2 O 3 and 20% of first row transition metal oxides MOx (M = Mn, Cr, Fe, Co, Ni, Cu, Zn) and containing different amounts of phosphorus (Al:M:P = 100:20:18 and 100:20:144) has been examined under non-isothermal conditions in a thermogravimetric balance. Pure ammonium nitrate is decomposed endothermically while addition of Al 2 O 3 shows a stabilizing action of this endo effect. Substitution of 20% of aluminum by a first row transition metal (M = Mn, Cr, Fe, Co, Ni, Cu and Zn) shows that zinc does not alter the route of decomposition while the rest show a tendency to decompose ammonium nitrate exothermically. The catalytic action of the cations toward such an exothermic route is Fe = Ni 4 NO 3 are primarily related to electrochemical Gibbs free energy Δ G = − nFE of the process M oxidized /M reduced . On top of this the crystal field stabilization effect for each particular cation should be taken into account. The extent of exothermic decomposition of the ammonium nitrate seems to influence the specific surface area of the final product in a positive manner in the absence of phosphorus but in a negative manner in its presence. The reasons underlying this effect should be attributed to the stabilizing action of phosphorus on the decomposition route.