E.A. Lombardo

The nature of cobalt species in Co and PtCoZSM5 used for the SCR of NOx with CH4

Abstract A thorough characterization of CoZSM5 and PtCoZSM5 before and after catalytic use was carried out using a battery of techniques. The bimetallic solid was more selective for N2 production. The TPR profiles showed significant differences. No solid, either fresh or used, exhibited any of the characteristic cobalt oxide X-ray reflections. The XPS data provided information concerning cobalt dispersion. The Raman spectroscopy clearly indicated that Co3O4 species were present only in the monometallic zeolites while a form of highly dispersed CoxOy moieties became dominant in the PtCoZSM5. The diffuse reflectance spectroscopy showed that Co2+ species in the monometallic solids were preferentially located at the main channels while in PtCoZSM5 these cations moved to higher coordination lattice sites. Through the combination of these tools, a much better understanding of the synergetic effect of Pt incorporated to CoZSM5 has been achieved. In view of these findings, related work previously published is revisited.

XPS studies of the surface oxidation states on vanadium-phosphorus-oxygen (VPO) equilibrated catalysts

Abstract The oxidation states of vanadium on the surface of VPO catalysts which show the characteristic diffraction pattern of vanadyl pyrophosphate have been investigated by X-ray photoelectron spectroscopy (XPS). The (VO)2P2O7 phase is obtained from crystalline precursors which show the X-ray pattern of VOHPO4·1/2H2O. The XPS spectra of 40 solids (precursors, catalysts, and model compounds) hinted the presence of low concentrations of VV centers on the surface of the bulk VIV catalysts. To further investigate this matter, the solids were oxidized or reduced in a pretreatment chamber attached to the spectrometer. These studies confirmed the presence of VV centers on the surface of the catalysts which may be initiated by strong oxygen chemisorption on exposed VIV sites of (VO)2P2O7. It is also shown that the presence of excess phosphorus prevents the oxidation of the surface (and the bulk) to β-VOPO4, a phase which has a deleterious effect on catalyst performance.

The nature of active sites for the oxidation of methane on La-based perovskites

Highly crystalline, monophasic LaFeO3 and LaCoO3 perovskites, prepared by the explosion method, are shown to be heterogeneous at surface level. The outmost atomic layers of these perovskites contain high concentrations of carbonate-type species. Their specific activities for methane combustion are in fact identical to La2O2CO3 and air-exposed La2O3. These results compared with pertinent data from the literature hint that surface heterogeneity may be often present in mixed oxides catalysts.

Acidity and catalytic behavior of vanadium-phosphorus-oxygen catalysts

Abstract The surface acidity of equilibrated and non-equilibrated vanadium-phosphorus oxide (VPO) catalysts was studied using basic probe molecules (ammonia, pyridine and acetonitrile) via Fourier transform-IR spectroscopy. The stronger bases detected the presence, in varying proportions, of both Lewis and Bronsted acid centers on all the solids assayed. All the catalysts used exhibited the X-ray diffraction pattern of vanadyl pyrophosphate and much higher Lewis/Bronsted ratios than the β-VOPO 4 compound. This indicates that the Lewis acidity observed on the used catalysts is not due to the presence of small amounts of V v species. The Lewis/Bronsted ratio almost invariably decreased with temperature, suggesting that the Bronsted sites are stronger than the Lewis centers. Acetonitrile served to differentiate Lewis centers of varying strength. The equilibrated catalysts which exhibited a better crystallized pyrophosphate phase showed an increase in very strong Lewis acidity whereas the stacking fold disordered non-equilibrated catalyst showed the presence of both medium strong and very strong Lewis centers. The oxidation of n-butane to maleic anhydride was studied using a plug flow reactor. Both the yield of maleic anhydride and the proportion of very strong Lewis sites increased with time-on-stream. These findings together with previous IR studies reported in the literature allow a better understanding of the relationship among structural features, acidity and catalytic behavior of VPO formulations.

Cobalt-containing catalysts for the high-temperature combustion of methane

Cobalt was supported on ZrO2, La-doped ZrO2 and La2O3 through atomic layer epitaxy (ALE) and wet impregnation. The rate data obtained at 770 K is compared with literature information about cobalt inserted in other matrixes. The ALE technique using ZrO2 and La-doped ZrO2 yielded the best cobalt-containing catalysts. Bulk and surface characterization techniques provided key clues to understand the origin of the large difference in catalytic activity reported for cobalt-containing formulations.

Chemistry of vanadium-phosphorus oxide catalyst preparation

Abstract The chemistry of the reduction process of V 2 O 5 with mixtures of aliphatic and benzyl alcohols was studied using several analytical techniques. The aromatic alcohol, which acted as the reducing agent, was invariably oxidized to benzaldehyde while further oxidation to benzoic acid often occurred. The presence of soluble vanadium (V) alkoxides was detected through IR spectroscopy. At the higher reduction temperatures, which varied between 353 and 393 K, V 2 O 4 , appeared in the solid residue. In no case, however, was the vanadium pentoxide completely reduced with the alcohol mixture. The nature and proportion of the aliphatic alcohol defines the reflux temperature and therefore the rate of the kinetically controlled redox process. The reduction of V 2 O 5 is completed upon addition of H 3 PO 4 (100%). The precursors obtained invariably showed the X-ray diffraction pattern of VOHPO 4 ·/2H 2 O. The extent of disorder of the lattice in the direction perpendicular to the (010) plane increased with a higher proportion of benzyl alcohol in the reduction mixture. The oxidation of n-butane to maleic anhydride was studied using a plug flow reactor. The equilibrated catalysts display very similar X-ray diffraction patterns characteristic of (VO) 2 P 2 O 7 . No correlation was found between the extent of disorder of the precursors prepared in organic medium and the catalytic performance of the equilibrated solids.

Surface chemistry and catalytic activity of La1 − yMyCoO3 perovskite (M = Sr or Th): 2. Hydrogenation of CO2

Abstract The partial substitution of La III by either Sr II or Th IV in lanthanum cobaltate perovskite affects both the rate of hydrogenation of carbon dioxide and the distribution of products. The reaction of cyclopropane with hydrogen was used as a parallel test reaction in order to ascertain the nature of the active sites developed on these solids upon reduction. The active site density was calculated from the measured amount of chemisorbed hydrogen at 25 °C on the oxides reduced at temperatures between 250 and 500 °C. The catalytic activity was measured in a standard recirculation system using a H 2 : CO 2 = 4:1 ratio, total pressure of 160 Torr, and 280 °C reaction temperature. The total conversion of CO 2 into products (activity) on LaCoO 3 is little affected by the extent of reduction of the sample. More sensitive to this parameter are both the rate of methanation and the production of C + 2 compounds. Reduced La 0.8 Th 0.2 CoO 3 is very stable giving a constant activity, the highest of all the solids assayed and almost exclusively producing methane. La 0.6 Sr 0.4 CoO 3 shows a sharp maximum in activity and high selectivity to methane when prereduced at 300 °C. However, when reduced at increasing temperatures the overall activity sharply drops while the selectivity to higher hydrocarbons increases very rapidly. In all cases the unreduced solids present induction periods, which indicate that the oxide is being reduced in situ by the reacting mixture. In runs designed to measure the extent of deactivation due to coke deposition it was found that the degree of activity decay was inversely correlated with the methanation selectivity. The test reaction was conducted in the same system at H 2 : cyclopropane = 1:1 ratio, P = 170 Torr, and 250 °C. The overall activity and the product distribution toward isomerization, hydrogenation, and hydrogenolysis is very sensitive to both the nature of the solid involved and the extent of reduction. The initial rate of formation of hydrogenation plus hydrogenolysis products when plotted vs extent of reduction produces curves which are similar to those observed in methanation activity. To gain further insight into both the matrix and promoter effect a series of catalysts were prepared containing different combinations of Co, Sr, La, and Th supported on either celite or La 2 O 3 . The matrix effect is most important in the Sr-substituted oxide, less so in LaCoO 3 , and unimportant in the Th-containing perovskite. The promoter effect for C + 2 production follows the order Sr > La ⪢> Th. This and previous studies made on crystalline mixed oxides, together with data available in the literature, allowed us to propose a model to interpret the effect of lanthanum replacement upon the catalytic activity, selectivity, and stability of these solids.

Effects of NO and solids on the oxidation of methane to formaldehyde

The selective oxidation of methane has been studied both in the presence and absence of solids (inert or catalysts) with and without NO added, at 1 bar of total pressure. NO enhances the yield to formaldehyde, while the solids favor its decomposition. These results, together with abundant literature data, show a maximum for formaldehyde yield of about 4.0%.

Characterization of hydrogenation active sites on LaCoO3 perovskite

Abstract LaCoO 3 becomes active for hydrogenation of ethene upon reduction in hydrogen at temperatures between 300 and 490 °C. Several aspects of the reacting system were studied in order to ascertain the nature of the active sites generated in this manner. Catalyst deactivation was evaluated by comparing rates between two successive experiments. An upper limit was estimated for the amount of polymeric residues formed after a single run: 1.1 ± 0.5 × 10 14 molecules of C 2 per square centimeter. Reduced LaCoO 3 also catalyzed the self-hydrogenation of ethene. When a mixture of C 2 H 4 :D 2 = 1:1 was reacted over LaCoO 3 reduced to varying extents multiple-exchanged ethenes and ethanes were formed. The exchange patterns were almost unaffected by the extent of reduction. The effect of pretreatment temperatures was also evaluated. The solid in its reduced form was particularly sensitive to high-temperature treatments. The amounts of CO chemisorbed when plotted vs extent of reduction gave curves that were almost identical to the activity plots. The results reported here, discussed in terms of the current literature, are consistent with a model in which finely dispersed Co 0 , formed in the oxide matrix upon reduction, is the locus of hydrogenation activity.

Kinetics and mechanism of CO oxidation over Cu mordenite

Cu mordenite (CuM) has proved to be highly active for the oxidation of CO with oxygen. The effect of pretreatment, the kinetics, and the mechanism of the CO + O2 reaction have been studied using a continuous-stirred tank reactor (CSTR), Bennett-type unit, and standard BET system. Redox cycles performed using COO2 showed that the sample was stable and could be reversibly reduced and oxidized many times at temperatures up to 500 °C. The extent of reduction was 0.8 e/Cu. Pretreatment in CO at 750 °C did not affect the reversibility of the redox cycles but produced a larger valence change, 1.8 e/Cu, even at reduction temperatures as low as 300 °C. XRD patterns show the appearance of finely dispersed CuO on the partially destroyed mordenite lattice. This solid, CuM∗, shows different catalytic behavior compared to CuM. The kinetic studies on the latter were performed in the range 200–340 °C. Between 200 and 250 °C the rate function was zero order in CO and close to first order in O2. In the upper temperature range this pressure dependency became first order in CO and zero order in O2. The Arrhenius plot shows a break at 250 °C. At temperatures higher than 250 °C the oxidation reaction on CuM is severely limited by mass transport. On CuM∗ the reaction rate was first order in CO and zero order in O2 over the entire temperature range, 200–325 °C. The reduction and catalytic behavior of CuOSiO2 and CuOγ-Al2O3 were also studied to confirm the important role played by copper oxide produced by the CO pretreatment on CuM∗. The results obtained are analyzed in terms of the reaction mechanisms, and the predominance of individual steps, due to either different pretreatments and/or operating conditions, is assessed.