Sergio De Rossi

CATALYTIC ACTIVITY OF CO-ZSM-5 FOR THE ABATEMENT OF NOX WITH METHANE IN THE PRESENCE OF OXYGEN

Abstract The abatement of NO with CH4 in the presence of oxygen ([NO] = [CH4] = 1000 or 4000 ppm, [O2] = 0 to 2%, by volume) was studied on Co-ZSM-5 catalysts (Co content 0.29 to 4.1 wt.-%), prepared from H-ZSM-5 or Na-ZSM-5 by the ion-exchange method. On all samples, the amount of CO and NO adsorbed at room temperature was proportional to the cobalt content (CO/Co⋍ 0.5 and NO/Co⋍ 1.6), with the exception of the Co-ZSM-5 sample with Co 4.1 wt.-%, on which the adsorption was only slightly higher than that on Co-ZSM-5 with Co 2.0 wt.-%. Infrared spectroscopy (FTIR) showed the formation of carbonyls (one type only, on cobalt equivalent sites), cobalt mononitrosyls (two types) and dinitrosyls (two types). The intensity of bands from carbonyls and nitrosyls was about proportional to the cobalt content, with the exception of the Co-ZSM-5 sample with Co 4.1 wt.-%, on which the bands were roughly as intense as in the sample with Co 2.0 wt.-%. In the Co-ZSM-5 sample with Co 4.1 wt.-%, after heating with O2 at 773 K, or after its use in catalysis, diffuse reflectance spectroscopy (DRS) showed the presence of Co3O4, not detected by X-ray diffraction. In the presence of O2, the NO reduction rate was proportional to the Co content, except for the sample containing Co 4.1 wt.-%. The CH4 oxidation rate was proportional to the Co content, in the entire range of Co concentrations. The selectivity of catalysts for NO abatement (selective catalytic reduction, SCR), was nearly independent of Co content but was markedly lower on the sample with Co 4.1 wt.-%. The results suggest that only CoII ions exchanged in the framework of the ZSM-5 matrix are active in CO and NO adsorption and in the SCR reaction, whereas also the cobalt of the dispersed Co3O4 phase contributes to CH4 oxidation with O2.

The adsorption of O2 and NO on CrOx/ZrO2 catalysts, as investigated by IR and ESR spectroscopies

Abstract The adsorption of O 2 at 195 K and that of NO at 298 K on CrO x /ZrO 2 samples have been investigated by the double isotherm method. The nature of the surface species formed before and after adsorption has been studied by means of IR and ESR spectroscopies. Prior to O 2 or NO adsorption, CrO x /ZrO 2 samples were heated in O 2 at 773 K (average oxidation number of Cr, n =5.5) and reduced with CO at 423 to 623 K to controlled extents ( n =5.5 to 2.5). The results show that the adsorption of O 2 at 195 K oxidizes to Cr(VI) the Cr(II) present in the sample after the reduction. The oxidation of Cr(II) to Cr(VI) is quantitative and nearly selective. Only a small fraction of Cr(III), also present in the reduced sample, is oxidized to Cr(V) (ESR, γ-signal). The adsorption of NO on the sample heated in O 2 yields mono- and dinitrosyls of Cr(III), arising from the reduction of Cr(V). The reduction of Cr(V) to Cr(III) is evidenced by both ESR and IR data. The adsorption of NO on the reduced sample ( n =2.5) leads, on average, to 1.7 to 2.0 NO molecules adsorbed per Cr atom. In the same conditions, dinitrosyls and mononitrosyls of Cr(II) and Cr(III) are formed, in addition to N 2 O and nitrites (and/or nitrates). Upon evacuation of the latter sample at 423 K, the adsorbed NO oxidizes most of the Cr(II) to Cr(VI) ( n =4.2) and, in fact, the nitrosyls of Cr(III) are the only species detected by IR on NO re-admission after the evacuation treatment. Blank experiments on pure ZrO 2 show that the dismutation of NO leading to N 2 O and nitrites (and/or nitrates) takes place on sites of the ZrO 2 support. When the reduced sample is further reacted with H 2 O at 853 K, only the dinitrosyls and mononitrosyls of Cr(III) are detected upon exposure to NO, since the reaction with H 2 O at 853 K selectively oxidizes the Cr(II) to chromia-like species. It is suggested that coordinatively unsaturated Cr(III) ions, two vacancies at least on IR evidence, are the active sites for the hydrogenation of propene and H 2 D 2 equilibration reactions, previously investigated on the same CrO x /ZrO 2 catalysts.

Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: I. X-ray characterization of iron-containing bismuth molybdovanadate catalysts☆

Abstract The selective oxidation of propene to acrolein has been studied on iron-containing bismuth molybdovanadates with the scheelite structure and general formula Bi 1 − x 3 x 3 − y Me y ( V 1 − x Mo x − y Fe y ) O 4 , where Me = Fe or Bi , = cation vacancy, with x = 0.45 and 0.60 and 0 ⩽ y ⩽ x 3 , using the pulse technique in the temperature range 573–673 K. Total conversion, rate constant, and selectivity were found to depend on catalyst composition, in particular cation vacancy concentration, content, and coordination symmetry of the iron ions. The results, discussed in terms of compositional parameter y and surface geometry of BiO and MoO species, confirm the importance of the cation vacancies in the selective oxidation process and suggest a role of the eight-coordinated cations in the allyl formation.

XAS characterization and CO oxidation on δ-alumina supported La, Mn, Co and Fe oxides

δ-Al2O3 supported La, Mn, Co and Fe containing catalysts were prepared by impregnation of δ-Al2O3 with citrate-type precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. XRD revealed the presence of δ-Al2O3 in all cases and, at 30 wt.% of metal loading, of other single (α-Mn2O3 and α-Fe2O3) or mixed (LaAlO3, LaMnO3, CoAl2O4 and LaFeO3) oxide phases. XAS suggested the formation of some oxide phases also at lower loading. In particular, all Mn and 10 wt.% La–Mn containing samples revealed the formation of α-Mn2O3, while the 30 wt.% La–Mn bimetallic sample showed the formation of LaMnO3 perovskite. All Co containing samples revealed the presence of CoAl2O4 spinel. Fe containing samples showed the formation of α-Fe2O3, while La–Fe containing ones, that of LaFeO3 perovskite. Catalytic tests of CO oxidation were performed in the temperature range 300–800 K. The sample containing 30 wt.% of La and Mn in the form of LaMnO3 perovskite dispersed on δ-Al2O3 was found the most active among all the examined catalysts. Most of the Co containing catalysts were found active at RT too, but they deactivated rapidly. None of the Fe-based samples was active at RT and these catalysts were found, on average, to be substantially less active than the Mn- and Co-based ones.

Characterization of copper-zinc mixed oxide system in relation to different precursor structure and morphology

Abstract Hydrocarbonate and hydroxynitrate precursors of CuO-ZnO catalyst (Cu/Zn atomic ratio=67/33) have been prepared by two different methods; the precursor obtained by precipation at 333 K and constant pH =8 from mixed nitrate solution with excess of sodium bicarbonate consisted of zincian malachite and aurichalcite, while that obtained by addition of sodium carbonate solution to Cu-Zn nitrate solution is essentially copper hydroxynitrate plus some amount of aurichalcite. By thermal decomposition at 623 K both types of precursor gave a mixture of CuO and ZnO. The mixed oxides were then treated at 873, 1073 and 1273 K in air. X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy and surface area determination were used to characterized the mixed oxide systems. The precursor containing zincian malachite plus aurichalcite, after calcination at 623 K gave rise to well dispersed and much smaller particles of CuO and ZnO than the precursor containing copper hydroxy nitrate plus aurichalcite. No Cu 2+ in solid solution in the ZnO zincite structure or Zn 2+ in the CuO tenorite lattice were detected by reflectance spectroscopy up to 873 K; the presence of tetrahedral copper (Cu x Zn 1- x O solid formation at least at the surface) was evident only in samples calcined at temperatures higher than 1073 K. X-ray diffraction analysis for lattice parameter determination showed that only for samples treated at 1273 K both Cu x Zn 1- x O and Zn y Cu 1- y O solid solution formations are detectable. An unexpected volume decrease of Zn y Cu 1- y O with respect to pure CuO was revealed; the introduction of zinc in the tenorite structure probably changes the local metal symmetry from nearly square planar towards octahedral, producing an overall less distorted and more compact structure.

LaAl1−xCrxO3 perovskite-type solid solutions: Structural, electronic, magnetic properties and catalytic activity towards CO oxidation

Perovskite-type LaAl1−xCrxO3 samples were prepared from citrate precursors and calcined at several temperatures up to 1073 K. The materials were characterised by XRD, diffuse reflectance spectroscopy (DRS), EPR and magnetic susceptibility measurements. At 773 K all samples are amorphous, at 923 K the perovskite structure starts to form in addition to other La–Cr oxides such as LaCrO4 and La2CrO6, whereas at 1073 K all samples are single phase LaAl1−xCrxO3 perovskite-type solid solutions. EPR analysis confirmed the presence of both Cr3+ and Cr5+ in the materials fired at 923 K. DRS patterns, magnetic susceptibility measurements and EPR analysis confirmed the presence of Cr3+ alone in all the perovskite-type solid solutions obtained at 1073 K. For these samples: (i) the particle sizes and surface areas were found to be in the 610 to >1000 A and 3–7 m2 g−1 ranges, respectively; (ii) a linear increase of the unit cell volume, V, with x was observed, in agreement with expectations based on the ionic size of interchanging Al3+ and Cr3+ ions. The perovskite samples calcined at 1073 K were tested for CO oxidation. All Cr-containing materials are much more active than LaAlO3 pointing to the essential role of the transition metal ion in developing highly active catalysts. The activity trend and the variation of the activation energy with the increase in chromium content suggest a change in the active site from mononuclear to polynuclear.

Nature of the active site for propene hydrogenation on CrOx/ZrO2 catalysts

Abstract The catalytic activity of CrOx/ZrO2 for propene hydrogenation has been investigated at 195 K in a flow apparatus. Samples containing 0.05 to 0.8% chromium by weight (0.1 to 1.9 Cr atoms nm−2) were prepared by the equilibrium adsorption method. After heating in O2 at 773 K (average oxidation number of Cr, n=5.5), samples were reduced to a controlled extent with CO at 393 to 623 K. The activity at 195 K of partially reduced samples (n=4.4) is low, but it increases by nearly two orders of magnitude when n is further decreased by 0.2 units. Thereafter the activity changes only slightly with increasing reduction (down to n=2.5). The concentration of the Crv species observed by ESR after heating the samples with O2 was monitored after each reduction treatment, and it was found to decrease by 17% in the same reduction region in which the catalytic activity increases sharply. The average turnover frequencies per total chromium (NCr/molecules s−1) atom−1 of CrOx/ZrO2 catalysts reduced to the same extent (n=2.5) at 623 K differ by a factor 2.6, in spite of the large variation (factor of 20) of Cr content. The CrIII(NO)2 complex formed on the surface of reduced CrOx/ZrO2 by adsorption of NO at room temperature was reacted with CO at room temperature. By IR spectroscopy, it was found that a fraction of the CrIII(NO)2 species leads to mixed ligand complexes, CrIII(NO)2CO. Relying on IR, ESR and catalytic results, it is inferred that active sites for the low temperature hydrogenation of propene on reduced CrOx/ZrO2 catalysts are surface mononuclear CrIII, species with three coordinative vacancies.

CrOx/SiO2 catalysts prepared using chromium recovered from tanning sewage

Abstract Catalysts at different Cr loading ( x , in the range 0.18–3.8 wt% Cr) were prepared by (i) impregnation of SiO 2 Aerosil directly with the waste water (SC x (I) samples); or (ii) mixing Aerosil with chromium oxo-hydroxides, precipitated from the waste water to eliminate the sulphate and sodium ions (SC x (II) and SC x (III) samples). Preparations (II) and (III) only differ for the accuracy in sulphate and sodium ions elimination. Catalyst surface characterization was performed by FT-IR spectroscopy using CO, adsorbed at room temperature, as test molecule, and by UV-Vis-NIR DR spectroscopy. Their activity towards ethylene polymerization were tested by FT-IR spectroscopy, monitoring the time dependence of the CH 2 stretching modes intensity. The obtained results were compared with those obtained for simplified Phillips catalysts with comparable chromium contents, SC x (R). SC x (I) catalysts were not active in the ethylene polymerization, while the SC x (II) and SC x (III) ones showed lower activity (1/10 at maximum) compared to that of simplified Phillips catalysts with comparable chromium contents. SC x (II) and SC x (III) catalysts were active in the isobutane dehydrogenation reaction and showed activity in the range of those of standard catalysts, while SC x (I) ones showed activity at 873 K, but very low at the usual temperatures (773–823 K).

Butane isomerization on several H-zeolite catalysts*

H-ZSM-5, H-Beta, H-Mordenite and H-Y zeolites are active catalysts for n-butane isomerization. The level of activity and resistance to poisoning are a function of the concentration of the Bronsted acid sites and framework structure. H-L and H-Ferrierite zeolites have a very low activity. Silicalite-1 (S-1) and mesoporous silica-alumina (MSA) are not active. The isobutane yield on ultrastable H-Y (Si/Al=3.15) catalyst is lower in comparison to tungsta supported on zirconia (WOx/ZrO2), H-Mordenite, H-Beta and H-ZSM-5 catalysts, however, H-Y catalysts are more resistent to poisoning and highly selective towards isobutane. In fact, no deactivation of H-Y catalysts was observed after 5 h of time on stream. On the contrary, WOx/ZrO2, H-Mordenite and H-Beta catalysts, under the same experimental conditions, are deactivated in less than 1 h of time on stream. It is suggested that the stability of the H-Y catalysts in comparison to the other acidic zeolites catalysts may be due to the 3-dimensional structure of H-Y zeolites, made of large supercages interconnected by apertures of 12 oxygen atoms. H-ZSM-5 catalysts are stable with time on stream but their selectivity to isobutane is very low. Active acidic molecular sieves with monodimensional structure (H-Mordenite, H-Beta) may favour the formation of polyenyl unsaturated chains, the precursors of the coke responsible for the catalysts deactivation.

Isomerization of n-butane over ultrastable H-Y zeolites with different Si/Al atomic ratio

Ultrastable H-Y zeolites with different Si/Al atomic ratios (3<Si/Al<11) are active and selective catalysts for the n-butane isomerization. The initial activity of these catalysts is lower than that measured on tungsta supported on zirconia catalysts (WOx/ZrO2) and acidic mordenite catalysts; however, the Brønsted acid sites of the ultrastable H-Y zeolites are stable and selective towards isobutane. No deactivation of the catalysts was observed after 5 h of time on stream. In contrast, WOx/ZrO2 and acidic mordenite catalysts under the same experimental conditions are largely deactivated in less than 1 h of time on stream. The stability of the ultrastable H-Y zeolite in comparison to H-mordenite catalysts may be due to the three-dimensional structure of H-Y made of large supercages interconnected by apertures of 12 oxygen atoms. This structure may favour the diffusion of reactant and product decreasing the residence time and the ensuing degradation to coke. Acidic molecular sieves with monodimensional structure may favour the formation of the precursors of the coke responsible of the catalyst deactivation.