J.O. Petunchi

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.

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.

The role of cobalt as promoter of equilibrated vanadium–phosphorus–oxygen catalysts

Catalysts were prepared containing between 1% and 6% of Co by weight. The cobalt was either added during preparation or impregnated on the dry VPO precursor. The catalytic evaluation showed that the cobalt-impregnated solids gave the highest yields to maleic anhydride. To understand the promoting effect of cobalt, the catalysts were characterized using XRD, FTIR, SEM, TPR, Raman spectroscopy and XPS. The solids obtained through the addition of Co during the synthesis of the VPO precursor exhibit less disordered crystalline phases in the (001) direction. No separate Co-containing phase could be detected through the techniques used. In the non-equilibrated catalysts, V(V)-containing phases were detected through Raman spectroscopy. After 500 h on stream under reaction conditions (equilibrated catalysts), vanadyl pyrophosphate was the only crystalline phase detected while V(IV) was the only surface species observed using XPS. XRD, FTIR and SEM coincidentally show that depending on the way cobalt is added the solids obtained develop different structural features.

Continuous-stirred tank reactor (CSTR) transient studies in heterogeneous catalysis: CO oxidation over CuY zeolite

Abstract CuY zeolites develop a high activity to catalyze CO oxidation upon reduction in CO at 750 °C. A continuous-stirred tank reactor (CSTR), Bennett-type unit, operated both in the transient and steady-state regimes was used to determine the rate constant, the adsorption equilibrium constant, and the effective diffusivity under reaction conditions. The reactor, loaded with powdered catalyst, was operated at reaction temperatures between 150 and 300 °C, atmospheric pressure, and usually in an oxydizing atmosphere ( CO O 2 ≤ 2 ). Under these conditions the rate function was found to be first order in CO pressure and zero order in oxygen pressure. The weighted, rather than ordinary moments method, was used for it was shown to give a better estimation of the reaction parameters. The use of step functions was useful to detect an activation process which occurs when the solid is contacted with the reacting mixture. The reaction was shown to be diffusion limited in the temperature range studied. The diffusivity obtained was D = 2.5 × 10 −7 exp(−4.0/ RT ), the adsorption constant K = 8.2 exp(3.9/ RT ) and the surface rate constant k = 1.6 × 10 5 exp(−13.4/ RT ). Energy values are given in kilocalories per mol. These data are analyzed in terms of relevant literature information on related systems.

Kinetics, acid sites and deactivation of H-mordenite during the SCR of NOx with CH4

H-mordenites are active for the SCR reaction but they suffer irreversible partial deactivation after being on stream for one hour at 650°C. The reaction orders and activation energies are not significantly affected by deactivation. This indicates that deactivation originates in a decrease in the number of active sites due to dealumination and possible pore blockage. The NO disappearance rate correlates with TPD NH3 between 300 and 700°C; FTIR confirms these results. 129Xe NMR of adsorbed xenon shows that pore blockage occurs and is due to the presence of aluminum species in the main zeolite channels. The overall deactivation process and the role of acid sites is discussed in terms of the current literature.

Ethylene hydrogenation over LaCoO3 perovskite

LaCoO3 is inactive for ethylene hydrogenation, but its activity sharply increases and goes through a maximum when the oxide, pretreated with hydrogen at temperatures between 300 and 490 °C, is reduced to different extents (up to 3 electrons/molecule).

The nature of the cobalt salt affects the catalytic properties of promoted VPO

Cobalt-impregnated VPO catalysts containing 2 and 4% of the metal by weight were prepared using two different cobalt salts. The catalytic tests showed that cobalt impregnation significantly increased the overall activity. The use of cobalt acetylacetonate led to a more selective high loading catalyst. To investigate the origin of the cobalt effect, the solids were characterized using XRD, Raman Spectroscopy, FTIR and XPS. No structural effects were detected through XRD. After several hundred hours on stream, the only phase detected in all cases was V(Iv) vanadyl pyrophosphate. The surface oxidation state of vanadium was V(Iv). The Co 2p XPspectrum has an intense shoulder at 788 eV, indicating that Co(II) species are present.

Characterization of the role of Pt on CoPt and InPt ferrierite activity and stability upon the SCR of NO with CH4

The selective catalytic reduction (SCR) of NO x with CH 4 in excess of oxygen was studied over a series of ferrierite-supported monometallic (Co, In or Pt) and bimetallic (Pt-Co or Pt-In) catalysts. Pt promoted the NO x to N 2 conversion of both Co and In ferrierite after the samples were reduced with H 2 at 350°C either under dry or wet reaction stream. The addition of 2% of water to the feed stream also increased the activity of the calcined PtlnFerrierite for the said reaction. Temperature Programmed Reduction and XPS results show the presence of Co ) , Pt ) , Co +2 , Pt +2 and In at exchange position in the samples with the highest activity and selectivity for the SCR of NO x .

CoPt clusters in NaMordenites as catalysts for SCR of NOx

A new material based on Pt and Co exchanged in NaMordenite for the selective catalytic reduction (SCR) of nitric oxide with methane in the presence of excess oxygen is studied. The incorporation of 0.5% weight of Pt and 2% weight of Co to the zeolitic matrix after calcination and reduction on H2 flow for 1 h yields a solid converting 100% of NO to N2 and, simultaneously, 100% of CH4 to CO2 with a CH4/NO ratio = 3 and 2% of oxygen in the feed at 450°C. When the oxygen concentration in the feed varies, the NO conversion goes through a maximum for 2% at 450°C. The incorporation of Pt also promotes Co reducibility; 1% is reduced to Coo in the monometallic sample and 13% in the bimetallic sample. XPS results reveal that in the calcined samples Co2+ is at exchange position and, after being reduced, there appear thinly dispersed Coo particles and exchanged Co2+ ions. A greater reducibility and a shift of the maxima in the temperature-programmed reduction profiles suggest a Pt-Co interaction. In order to get an efficient catalysts for nitric oxide abatement it is necessary that the highly dispersed Coo and Pto particles and the Co2+ and H+ ions at exchange positions be in intimate contact inside the mordenite channels.