Luis G. Tejuca

Structure and Reactivity of Perovskite-Type Oxides

Publisher Summary This chapter discusses the structure and reactivity of perovskite-type oxides. Perovskite-type oxides have the general formula ABO 3 (A, cation of larger size) and are structurally similar to CaTiO 3 , the mineral that gave its name to that group of compounds. These materials are first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure. The chapter reviews only the study of oxides and their behavior in the gas solid interface and in heterogeneous catalysis. An important characteristic of perovskites, mentioned in the chapter, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions can be studied, where these oxides can participate as catalytic agents.

Preparation and characterization of LaMn1−xCUxO3+λ perovskite oxides

Abstract Compounds of nominal composition LaMn 1− x Cu x 0 3+λ , (0 ⪯ × ⪯ 1) have been prepared and characterized by means of X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). Substitution of manganese by copper up to x = 0.6 preserves the perovskite structure. For x = 0.8, in addition to the perovskite, La 2 CuO 4 and CuO were formed. For x = 1, the only phases present were La 2 CuO 4 and CuO. The perovskites showed oxidative nonstoichiometry for x ⪯ 0.4 and reductive nonstoichiometry for x = 0.6. Stoichiometry was reached for x = 0.5. XPS results for LaMn 0.6 Cu 0.4 O 3+λ reduced at 473–753 K are indicative of sintering of the copper particles when reduction is effected above 673 K. The TPR curve for LaMnO 3+λ showed an inflection point at ca. 0.5 e − per molecule which nearly corresponds to reduction of the estimated oxygen excess in the sample. Samples with substitutions 0 ⪯ × ⪯ 0.6 showed two reduction steps of Cu 2+ to Cu 0 and of Mn n + ( n ⪰ 3 ) to Mn 2+ . In the fully substituted sample ( x = 1) reduction of CuO and reduction of La 2 CuO 4 can be clearly distinguished. Reduction of perovskites of low copper content ( x

Chemisorption and catalysis on LaMO3 oxides

The adsorption of oxygen and isobutene and the catalytic activity for propene and isobutene oxidation have been studied on a series of LaMO3(M = Cr, Mn, Fe, Co and Ni) perovskite oxides. Coadsorption results point to the non-competitive adsorption of oxygen and isobutene; i.e. these molecules adsorb on different centres. Oxygen adsorption underwent a remarkable increase after isobutene had been preadsorbed on these oxides (enhanced adsorption). Activation energies for complete oxidation ranged between 16 kcal mol–1(LaMnO3, LaCoO3 and LaNiO3) and 31 kcal mol–1(LaFeO3). LaCrO3 showed some activity for methacrolein formation. Adsorption and catalytic-activity profiles showed maxima for LaMnO3 and LaCoO3. These results are discussed within the framework of the ideas of Dowden and Wells on the local symmetry of surface cations and its influence on chemisorption and catalysis and show the importance of localized interactions in the processes studied.

Adsorption of CO2 on the perovskite-type oxide LaCoO3

Adsorption isotherms and i.r. spectra of CO2 on the perovskite-type oxide LaCoCO3 are given. Coverages at 50 mmHg are above half a monolayer. At 195 K the B.E.T. multilayer adsorption isotherm fits the data satisfactorily while at 623–673 K the Freundlich isotherm is obeyed. At lower temperatures (195–373 K) mainly physical adsorption, with a constant isosteric heat of 18.8 kJ mol–1, takes place. Entropy calculations show this adsorbed species to be in a supermobile state with a vibration perpendicular to the surface at a frequency of 4.2 × 1011 s–1. At higher temperatures (373–673 K) chemisorption occurs; the heat of adsorption of the adsorbed species at 623–673 K was found to be 33.5 kJ mol–1 for a coverage of 0.5. At 298 K i.r. bands at 1110, 1070 and 840 cm–1 were observed. At 423, 573 and 773 K bands were found at 1770, 1635, 1465, 1355, 1105, 1060 and 850 cm–1, attributed to carbonate species. Coadsorption data show that CO2 and CO adsorb on the same sites while O2 adsorbs on a different type of site. It is concluded that CO2 adsorbs on O2– ions and O2 adsorbs on surface metal ions.

Properties of perovskite-type oxides II: Studies in catalysis

Abstract A review is presented of some representative reactions where perovskite oxides (mostly with a rare earth element in the A sites) have been used as catalytic agents. These are mainly reactions with oxygen or hydrogen participating as one of the reactants and/or products, and they include oxygen equilibration and exchange, oxidation of CO, hydrocarbons, oxygenated compounds, H 2 and NH 3 , and also the reduction of NO and SO 2 , hydrogenation, hydrogenolysis and the decomposition of 2-propanol, H 2 O 2 and N 2 O. A study of the role of the cations in positions A and B has been attempted.

XPS and TPD probe techniques for the study of LaNiO3 perovskite oxide

Abstract The surface of LaNiO 3 and the interactions of CO, CO 2 and hydrogen with this perovskite were studied as a function of the reduction temperature of the oxide. The evolution of the oxygen and lanthanum X-ray photoelectron spectroscopy (XPS) photolines is consistent with the formation and decomposition of an oxyhydroxide LaO(OH) species. CO adsorption yields CO temperature-programmed desorption (TPD) peaks at 365–385 K (assigned to CO linearly bonded to Ni 2+ ) and 745–930 K (assigned to bridged CO on Ni 0 centres) and CO 2 peaks at 500 K and above (assigned to surface carbonates). CO 2 adsorption yields CO 2 TPD peaks at 340–375 K and above 600 K which are assigned to monodentate and bidentate or bridged carbonates respectively. There are no significant differences between the hydrogen and CO TPD spectra obtained after CO-H 2 and H 2 -CO adsorption sequences and after the adsorption of CO or hydrogen on LaNiO 3 reduced at 773 K. These results indicate that no interaction (or a weak interaction) occurs between CO and hydrogen in the adsorbed state. This is in contrast with adsorption results on LaCrO 3 reduced at 923 K. This different behaviour appears to be related to the relative strengths of the interactions of CO and hydrogen with the surfaces of these reduced oxides.

Properties of perovskite-type oxides I: Bulk and surface studies

Abstract The methods of preparation and some bulk and surface properties of perovskite oxides, mainly lanthanide perovskites, are reviewed. These properties include oxidative and reductive non-stoichiometry, formation of cation and anion vacancies, behaviour in a reducing atmosphere and gas adsorption with reference to the characterization of perovskites and the role of adsorbed species in their catalytic activity. Emphasis has been laid on the role of the A and B cations in the reactivity of these compounds.

Surface reactivity of reduced LaFeO3 as studied by TPD and IR spectroscopies of CO, CO2 and H2

CO, CO2 and H2 reactive adsorption on LaFeO3 at 298 K has been studied as a function of the reduction temperature of the perovskite oxide by means of temperature programmed desorption (TPD) and infrared (IR) spectroscopies. TPD spectra of CO after CO adsorption contained peaks at 365 to 440 K assigned to linearly adsorbed CO and at 495 to 540 K assigned to bridged CO. TPD spectra of CO2 after CO adsorption presented broad peaks centred at 570 and 715 K assigned to monodentate and bidentate carbonates, respectively. TPD spectra of CO2 obtained after CO2 adsorption contained peaks at 375 to 425 K and at 570 to 675 K. These were associated to infrared bands of monodentate and bidentate carbonates, respectively. In the CO-H2 and H2-CO successive adsorption on the reduced surface of LaFeO3 the TPD peak of H2 at 345 to 360 K is strongly inhibited and a new desorption peak appeared at 585 to 590 K. This is assumed to be due to CO adsorption on metallic Fe0 sites (CO-H2 coadsorption) or to a displacement of adsorbed hydrogen from Fe0 to a new adsorption site (H2-CO coadsorption). CO was found to interact more strongly than hydrogen with the adsorbent surface.

Physicochemical properties of LaFeO3. Kinetics of reduction and of oxygen adsorption

The reduction with hydrogen of LaFeO3 and the kinetics of oxygen adsorption on this perovskite have been studied. The results of temperature-programmed reduction suggest that the starting sample was reduced first to nearly stoichiometric LaFeO3(600–850 K) and then to α-Fe and La2O3(850–1250 K). The reduction process at 1063–1163 K is controlled by formation and growth of nuclei of metallic iron. The kinetic data for reduction fit the Avrami–Erofeev equation. The kinetic data for oxygen adsorption fit the Elovich equation using t0 values as calculated by the method of Aharoni and Ungarish. Activation energies for reduction and for oxygen adsorption were 194 and 16 kJ mol–1, respectively. Oxygen adsorption was very low in comparison with other LaMO3 oxides, as would be expected from its low catalytic activity for total oxidation.

Infrared spectroscopic study of the adsorption of CO, CO2 and NO on fluorinated alumina and supported molybdenum-nickel catalysts

Adsorption of CO, CO2, NO and O2 probe molecules has been used to study the surface of fluorinated alumina and of supported Mo–Ni catalysts. Addition of F to alumina drastically inhibited formation of carbonates from CO (at 673 K) and CO2(at room temperature) adsorption and also inhibited the formation of nitrates from NO adsorption at room temperature. Reduction with H2(773 K) followed by O2 and NO adsorption at low temperatures suggest that the active-phase Mo–Ni is more highly dispersed in the catalyst with higher F content. These data are in agreement with spectroscopic results for NO adsorption. This effect of F on the degree of metallic dispersion is probably associated with the anchoring of the polymeric ion Mo7O246– to the support, which is highly unfavoured by the partial substitution of hydroxyl groups by F–.