Influence of the surface area of the support on the activity of gold catalysts for CO oxidation

In the preparation of 1% Au/TiO₂ catalysts supported on either Degussa P-25 or anatase (90 m² g⁻¹) by deposition–precipitation, the gold content passes through a maximum at about the isoelectric point (pH 6), but maximum specific rates occur at pH 8–9 because the Au particle size becomes smaller as the pH is further increased. The gold uptake increases with the surface area of the support (anatase, rutile, P-25) and is complete above 200 m² g⁻¹; adsorption of the gold precursor at pH 9 is shown to be equilibrium-limited. Highest activities are found with supports of 50 m² g⁻¹. Catalysts made with high-area anatase (240 or 305 m² g⁻¹) are least active but show least deactivation.With Au/SnO₂ catalysts, gold uptake does not depend on the area of the support, and is highest at pH 7–8; very active catalysts (T₅₀ = 230–238 K) are obtained using SnO₂ of 47 m² g⁻¹. Storing a catalyst at 258 K for 1 week dramatically improves its stability. Results for Au/CeO₂ and Au/ZrO₂ catalysts confirm that moderate support areas give the most active catalysts, and suggest that surface area is often more important than chemical composition.     

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO₃ (A = La, Nd, Sm) and Sm_1−xSr_xMnO₃ (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm³ g⁻¹ h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v.

Specific surface areas of perovskites were in the range 13–20 m² g⁻¹. XRD analysis showed that LaMnO₃, NdMnO₃, SmMnO₃ and Sm_1−xSr_xMnO₃ (x = 0.1) are single phase perovskite type oxides. Traces of Sm₂O₃ besides the perovskite phase were detected in the Sm_1−xSr_xMnO₃ catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO₃. The fraction of Mn(IV) in the Sm_1−xSr_xMnO₃ samples increased with x. TPR measurements on AMnO₃ showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO₃ > NdMnO₃ > SmMnO₃. In Sm_1−xSr_xMnO₃ the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO₂ below 1023 K. The activation energies of the AMnO₃ perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO₃ perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.     

TPD and XRD studies of molybdenum nitride and its activity for hydrodenitrogenation of carbazole

The nitrided MoO₃ catalysts formed using two kinds of treatment with either NH₃ or He after nitriding were studied by temperature-programmed desorption and X-ray diffraction analyses. The catalysts were cooled to room temperature in either flowing NH₃ or He (NH or HE catalyst) after nitriding at 773, 973 and 1173 K with NH₃. The activities of the catalysts were determined during the hydrodenitrogenation of carbazole at 573 K and 10.1 MPa total pressure. MoO₂, γ-Mo₂N, and Mo metal were mainly formed in the NH catalysts nitrided at 773, 973 and 1173 K, respectively. Mo oxides and metals in the NH catalysts were nitrided to γ-Mo₂N and β-Mo₂N_0.78 with low crystallinity during TPD. The surface area of the NH and HE catalysts nitrided at 773 K increased to 66 and 59 m² g⁻¹ maximum from 1.1 m² g⁻¹ of fresh MoO₃, respectively, but decreased as the nitriding temperature increased to 973 K and 1173 K. The HE catalysts per surface area were more active than the NH catalysts for both the overall HDN reaction and hydrogenation, and the 1173 K-nitrided catalysts were highest. On the other hand, the NH catalysts were more active than the HE catalysts for C–N hydrogenolysis and the 973 K-nitrided catalyst showed a maximum activity for C–N hydrogenolysis.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Surfactant-mediated synthesis of metal substituted hexaaluminate from alumina sol

Manganese substituted hexaaluminate has been prepared using environmentally benign surfactants such as Triton X-100, under ambient condition with a commercial alumina sol and metal acetate precursors. The surface area of the pure alumina can be controlled to 10–70 m² g⁻¹ using cetyltrimethylammonium chloride after heating in oxygen flow at 1200°C for 6 h. The crystal structure of the obtained alumina was high purity θ-Al₂O₃. Incorporation of La and Mn leads to the formation of the high purity manganese substituted hexaaluminate with a surface area of 30–40 m² g⁻¹ which is also controllable using organic additives such as urea. The catalytic activity of the manganese substituted hexaaluminate was comparable to the sol–gel derived hexaaluminate catalyst from metal alkoxides.     

CVD preparation of alumina-supported niobium nitride and its activity for thiophene hydrodesulfurization

Niobium nitride was synthesized on a Si(400) substrate and a γ-alumina pellet using a CVD method with a stream of NbCl₅/Ar, NH₃, and H₂ gases at 723–973 K under reduced pressure. The composition and surface properties of the deposited niobium nitride were analyzed using XRD and XPS measurements. The activity of alumina-supported niobium nitrides for the hydrodesulfurization (HDS) of thiophene at 673 K and atmospheric pressure was determined. The alumina had a surface area of 177 m² g⁻¹ and the alumina-supported niobium nitride catalyst had surface areas of 179–190 m² g⁻¹. Although the catalysts had low activity in the initial stages, the activity increased after 200–300 min started to about three times the initial activities. XPS analysis indicated that the activity of the niobium nitride catalysts was decreased by sulfur accumulation on the surface and nitrogen released from niobium nitride. The relationship between the surface properties of the niobium nitride catalysts and the activities for thiophene HDS is discussed.     

High-quality, oriented and mesostructured organosilica monolith as a potential UV sensor

We synthesized high-quality and oriented periodic mesoporous organosilica (PMO) monoliths through a solvent evaporation process using a wide range of mole ratios of the components: 0.17–0.56 1,2-bis(triethoxysilyl)ethane (BTSE): 0.2 cetyltrimethylammonium chloride (CTACl): 0–1.8 × 10⁻³ HCl: 0–80 EtOH: 5–400 H₂O. X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images indicated that the mesoporous channels within the monolith samples were oriented parallel to the flat external surface of the PMO monolith and possessed a hexagonal symmetry lattice (p6mm). The PMO monolith synthesized from a reactant composition of 0.35 BTSE: 0.2 CTACl: 1.8 × 10⁻⁶ HCl: 10 EtOH: 10 H₂O had a pore diameter, pore volume, and surface area – obtained from an N₂ sorption isotherm – of 25.0 Å, 0.96 cm³ g⁻¹ and 1231 m² g⁻¹, respectively. After calcination at 280 °C for 2 h in N₂ flow, the PMO monolith retained monolith-shape and mesostructure. Pore diameter and surface area of the calcined PMO monolith sample were 19.8 Å, 0.53 cm³ g⁻¹ and 1368 m² g⁻¹, respectively. We performed ²⁹Si and ¹³C CP MAS NMR spectroscopy experiments to confirm the presence of Si–C bonding within the framework of the PMO monoliths. We investigated the thermal stability of the PMO monoliths through thermogravimetric analysis (TGA). In addition, rare-earth ions (Eu³⁺, Tb³⁺ and Tm³⁺) were doped into the monoliths. Optical properties of those Eu³⁺, Tb³⁺ and Tm³⁺-doped PMO monoliths were investigated by photoluminescence (PL) spectra to evaluate their potential applicability as UV sensors.     

Microwave-enhanced catalytic degradation of phenol over nickel oxide

A mix-valenced nickel oxide, NiO_x, was prepared from nickel nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by sodium hypochlorite. Further, pure nickel oxide was obtained from the NiO_x by calcination at 300, 400 and 500 °C (labeled as C300, C400 and C500, respectively). They were characterized by thermogravimetry (TG), X-ray diffraction (XRD), nitrogen adsorption at −196 °C and temperature-programmed reduction (TPR). Their catalytic activities towards the degradation of phenol were further studied under continuous bubbling of air through the liquid phase. Also, the effects of pH, temperature and kinds of nickel oxide on the efficiency of the microwave-enhance catalytic degradation (MECD) of phenol have been investigated. The results indicated that the relative activity affected significantly with the oxidation state of nickel, surface area and surface acidity of nickel oxide, i.e., NiO_x (>+2 and S_BET = 201 m² g⁻¹)  C300 (+2 and S_BET = 104 m² g⁻¹) > C400 (+2 and S_BET = 52 m² g⁻¹) > C500 (+2 and S_BET = 27 m² g⁻¹). The introduction of microwave irradiation could greatly shorten the time of phenol degradation.     

Supported Co-based perovskites as catalysts for total oxidation of methane

Supported LaCoO₃ perovskites with 2, 5, 10, 15, 20 and 30 wt.% loading were prepared by impregnation of a Ce_0.8Zr_0.2O₂ support (40 m² g⁻¹) with: (i) a solution of La and Co nitrates and (ii) a “citrate” solution, namely containing La and Co nitrates, and citric acid. All precursors were decomposed and calcined at 700 °C for 5 h. XRD investigations indicated the formation of a pure perovskite phase only if citrates were used. These materials were tested as catalysts for methane combustion in the temperature range 300–700 °C. All catalysts showed a lower T₅₀ (the temperature at which the conversion level of methane is 50%) than the Ce_0.8Zr_0.2O₂ support or non-supported LaCoO₃. The activity increased continuously with the perovskite loading. The samples prepared from citrates were slightly more active than from nitrates. This is due to a more homogeneous surface, as indicated by XPS measurements. The presence of a well-characterized perovskite phase (as opposed to highly dispersed elements) seems necessary for good activity. A higher reaction rate per perovskite weight is observed for low loadings when compared to bulk LaCoO₃, but the variation with perovskite loading presents a breakpoint, suggesting complex interactions in the catalysts or in the oxidation mechanism.

In spite of the experimental impossibility to evaluate the area developed by the supported perovskite, an approximative approach strongly suggests a synergy between the support and supported species.     

Characterization and activity in dry reforming of methane on NiMg/Al and Ni/MgO catalysts

Dry reforming of methane has been investigated on two series of catalysts either prepared by co-precipitation: n(Ni_xMg_y)/Al, Ni_xMg_y and Ni_xAl_y or prepared by impregnation: Ni/MgO (mol% Ni = 5, 10). The catalysts, calcined at 600–900 °C, were characterized by different techniques: BET, H₂-TPR, TPO, XRD, IR, and TEM-EDX analysis. The surface BET (30–182 m² g⁻¹) decreased with increasing the temperature of calcination, after reduction and in the presence of Mg element. The XRD analysis showed, for n(Ni_xMg_y)/Al catalysts, the presence of NiAl₂O₄ and NiO–MgO solid solutions. The catalyst reducibility decreased with increasing the temperature of pretreatment. The n(Ni_xMg_y)/Al catalysts were active for dry reforming of methane with a good resistance to coke formation. The bimetallic catalyst Ni_0.05Mg_0.95 (calcined at 750 °C and tested at 800 °C) presents a poor activity. In contrast, the 5% Ni/MgO catalyst, having the same composition but prepared by impregnation, presents a high activity for the same calcination and reaction conditions. For all the catalysts the activity decreased with increasing the temperature of calcination and a previous H₂-reduction of the catalyst improves the performances. The TPO profiles and TEM-EDX analysis showed mainly four types of coke: CH_x species, surface carbon, nickel carbide and carbon nanotubes.     

Evaluation of carbon monoxide oxidation over CeO2/Co3O4 catalysts: Effect of ceria loading

Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, S_BET = 100 m² g⁻¹) with different loading of ceria was proceeded using the impregnation method (assigned as CeX/R230, X = 4, 12, 20, 35 and 50 wt%). The CeX/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at −196 °C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area (S_BET = 109 m² g⁻¹) of cobaltic oxide, further weaken the bond strength of CoO and lower the activation of CO oxidation among CeX/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T₅₀ (temperature for 50% CO conversion) at 88 °C.     

Highly active sulfided CoMo catalyst on nano-structured TiO2

High surface area (>300 m² g⁻¹) nano-structured TiO₂ oxides (ns-T) were used as CoMo hydrodesulfurization catalyst support. Cylindrical extrudates were impregnated by incipient wetness with Mo (2.8 Mo at. nm⁻²) and Co (atomic ratio Co/(Co + Mo) = 0.3). Characterization of impregnated precursors was carried out by N₂ physisorption, XRD and atomic absorption and laser-Raman spectroscopies. Sulfided catalysts (400 °C, H₂S/H₂) were studied by X-ray photoelectronic spectroscopy. As indicated by XRD and after various preparation steps (extrusion, Mo and Co impregnation and sulfiding) the nano-structured material was well preserved. XPS analyses showed that Co and Mo dispersion over the ns-T support was much higher than that on alumina. Very high surface S concentration suggested that even ns-T was partially sulfided during catalyst activation. Dibenzothiophene hydrodesulfurization activity (5.73 MPa, 320 °C, n-hexadecane as solvent) of CoMo/ns-T was two-fold to that of an alumina-supported commercial CoMo catalyst. The improvement was even more remarkable in intrinsic pseudo kinetic constant basis. No important differences in selectivity over the catalysts supported on either Al₂O₃ or ns-T were observed, where direct desulfurization to biphenyl was favored. Both Mo dispersion and sulfidability were enhanced on the ns-T support where Mo⁴⁺ fraction was notably increased (100%) as to that found on CoMo/Al₂O₃.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.     

Catalytic combustion of methane over perovskites

Perovskite-type oxides of the series La_{1− x}A_xMnO₃ (A Sr, Eu and Ce) were prepared by the amorphous citrate process, leading to high surface area catalysts (up to 45 m² g⁻¹). They were tested in a flow reactor for the total combustion of methane. Complete conversion was obtained over all of the catalysts between 500 and 600°C and catalyst performance did not change significantly after 100 h on-stream. Specific activity was found to decrease monotonically with increasing the temperature of the O₂ TPD desorption peak maximum. The rate of methane combustion was low below 500°C, then grew very fast, showing that two kinds of oxygen are active in these catalysts: an adsorbed oxygen species, that reacts at low temperature, and a lattice oxygen species, that becomes available at high temperature, boosting the catalytic activity.     

CO2 reforming of CH4 over nanocrystalline zirconia-supported nickel catalysts

Mesoporous nanocrystalline zirconia with high-surface area and pure tetragonal crystalline phase has been prepared by the surfactant-assisted route, using Pluronic P123 block copolymer surfactant. The synthesized zirconia showed a surface area of 174 m² g⁻¹ after calcination at 700 °C for 4 h. The prepared zirconia was employed as a support for nickel catalysts in dry reforming reaction. It was found that these catalysts possessed a mesoporous structure and even high-surface area. The activity results indicated that the nickel catalyst showed stable activity for syngas production with a decrease of about 4% in methane conversion after 50 h of reaction. Addition of promoters (CeO₂, La₂O₃ and K₂O) to the catalyst improved both the activity and stability of the nickel catalyst, without any decrease in methane conversion after 50 h of reaction.     

Water-tolerant catalysis of a silica composite of a sulfonic acid resin, Aciplex

A silica composite of a perfluorocarbonsulfonic acid resin, Aciplex, has been used as a solid acid catalyst for a variety of reactions concerning water. The Aciplex–SiO₂ composite containing 20 wt% Aciplex has a surface area of 1.3 m² g⁻¹ and possesses an ion-exchanged capacity of 0.46 meq. g⁻¹ after pretreatment at 423 K, which is higher than that of 13 wt% Nafion–SiO₂ (0.12 meq. g⁻¹). The acid strengths estimated from an initial heat of adsorption of NH₃ were similar for these polymer resin composites. It was found that the Aciplex–SiO₂ was more active than typical solid acids such as Cs_2.5H_0.5PW₁₂O₄₀, H-ZSM-5, and SO₄²⁻/ZrO₂ for hydrolysis of ethyl acetate in excess water and esterification of acrylic acid with 1-butanol, while it was less active than Cs_2.5H_0.5PW₁₂O₄₀ for N-alkylation of acrylonitrile with 1-adamantanol and solid–solid hydrolysis of 2-naphthyl acetate. The Aciplex–SiO₂ was superior in activity to Nafion–SiO₂ for all the above reactions and in thermal stability. These results indicate that Aciplex–SiO₂ is a promising solid acid catalyst for reactions involving liquid phase water.     

1,2-Diolates of titanium as suitable precursors for the preparation of photoactive high surface titania

1,2-Diolates of titanium of stoichiometric formula [Ti(OCHRCH₂O)₂] (R = −H, −CH₃, −CH₂CH₃) were prepared by reacting a titanium source (titanium(IV) isopropoxide (TIP) or titanium(IV) chloride) with an excess of the respective 1,2-diol. The diol was employed in a great excess; no other solvent was present in the synthesis procedure. The titanium diolates obtained have a white powder appearance and show a high degree of crystallinity as deduced from powder X-ray diffraction. It has been observed that their stability towards moisture is determined by the R group (R = −H, −CH₃, −CH₂CH₃).

Abstract

Hydrolysis of these titanium 1,2-diolates in water was a simple way to produce high surface porous titania with BET surface area up to 300 m² g⁻¹. These as-synthesised titania contain anatase crystallites and show photocatalytic activity versus formaldehyde degradation prior to calcination. Nevertheless, the decomposition rate of formaldehyde increased notably when titania samples were annealed at 773 K. This thermal treatment increases their crystallinity at the expense of a BET surface area decrease down to 75 m² g⁻¹ or less. The photocatalytic performance of the annealed samples is superior than that of commercial TiO₂ (Degussa P-25, Hombikat UV-100).     

Activity, selectivity and durability of VO–ZSM-5 catalysts for the selective catalytic reduction of NO by ammonia

ZSM-5 zeolite was loaded with vanadyl ions (VO²⁺) by treatment of Na–ZSM-5 with aqueous VOSO₄ solution at pH 1.5–2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH₃ and 2% O₂ in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 620 l g⁻¹ h⁻¹ (≈400 000 h⁻¹) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N₂O formation at NH₃ excess. Admixture of SO₂ (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures ≥623 K. No SO₂ conversion was noted at T ≤ 723 K and 450 l g⁻¹ h⁻¹. The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water.     

Sorption kinetics of ammonia and ammonium ions on gel and macroporous sulphonic acid cation exchangers

Diffusion of ammonia and ammonium ions in sulphonic acid cation exchangers (gel Purolite SGC 100 × 10 MBH and macroporous Purolite C 160 MBH) from the solutions, representing the composition of “caustic condensate” (waste of nitrogen fertilizers production) is affected by pH of initial solution and structure of the matrix of cation exchanger. In gel matrix the effective intraparticle diffusivity (D_ef) depends greatly on the solution pH because of shrinkage in alkaline and swelling in acidic medium: on decreasing the initial concentration of ammonia from 0.214 to 0.003 and increasing that of ammonium nitrate from 0 to 0.214 mol l⁻¹ instead, the effect of ion exchange leads to a decrease in pH, resulting in swelling and increase in D_ef from 0.1 to 0.34 × 10⁻¹⁰ for gel Purolite SGC 100 × 10 MBH and variation of 0.18–0.11 × 10⁻¹⁰ m² s⁻¹ for macroporous Purolite C 160 MBH (resistant to shrinkage and swelling).

In Purolite C 160 MBH both macropore diffusivity (0.07–0.29 × 10⁻¹⁰ m² s⁻¹) and gel (solid phase) diffusivity (0.06–0.19 × 10⁻¹⁰ m² s⁻¹) are higher than micropore diffusivity (0.28–0.56 × 10⁻¹⁸ m² s⁻¹).

With respect to the effective intraparticle diffusivity, resistance to nitric acid, used for the regeneration, and high concentration of ammonium nitrate in eluate (up to 110 g l⁻¹), Purolite C 160 MBH has been installed for the conversion of ammonia and ammonium ions to ammonium nitrate reusable in the fertilizers production. This allows minimizing the economic loss and preventing the environmental contamination.     

D-tracer study of butadiene hydrogenation and tetrahydrothiophen hydrodesulphurisation catalysed by Co9S8

H₂/D₂ exchange (473–583 K), 1,3-butadiene hydrogenation (418–513 K) and tetrahydrothiophen hydrodesulphurisation (428–557 K) have been studied over powdered Co₉S₈ (surface area, 7 m² g⁻¹) using D₂ as an isotopic tracer. Hydrogen exchange proceeded as a first order process at a modest rate (k₅₄₀ = 1.0 h⁻¹ m⁻²) with an apparent activation energy of 67 kJ mol⁻¹. Butadiene hydrogenation was diagnostic as to the surface state of Co₉S₈; samples showed either predominant 1:2-addition or 1:4-addition of hydrogen, interpreted as indicating the presence in the surface of single sites or pair/ensemble sites, respectively. Reactions at 473 K in the presence of D₂ gave butenes containing 0–6 D-atoms: exchange patterns obtained from these D-distributions showed that a proportion of butadiene molecules underwent extensive dehydrogenation during the normal progress of hydrogenation. At 633 K this dehydrogenation activity was evident as self-hydrogenation which occurred in the absence of D₂. Tetrahydrothiophen was desulphurised in the presence of D₂ to thiophen (void of D), butadiene (containing 0–5 D-atoms) and 1-butene (containing mostly 0 and 4 D-atoms). Increase in temperature or in deuterium pressure favoured butene formation so that it became the dominant product (88%). Tetrahydrothiophen also underwent self-hydrodesulphurisation in the absence of D₂. A mechanism is proposed, consistent with this D-tracer information, that accommodates dehydrogenation, desulphurisation and hydrogenation steps in the overall process. The activity of powdered Co₉S₈ exceeded that of powdered MoS₂.