L. González Tejuca

Non-stoichiometric surface behaviour of LaMO3 oxides as evidenced by XPS

Abstract The non-stoichiometry of the surface of LaMO3 perovskites was found to be much more marked than that in the bulk. In general, the non-stoichiometric character of these oxides changes from oxidative to reductive when passing from LaCrO3 to LaNiO3. This may be related to the reducibility of these compounds.

Catalytic activity of perovskite-type oxides LaMeO3

Catalytic activities for CO oxidation of a series of LaMeO3(Me3+=V3+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+) perovskite-type oxides were measured. Maxima for Mn3+ and Co3+ were found. The relation between local symmetry of solid surfaces and chemisorption and catalysis is discussed.AbstractБыли измерены каталитические активности ряда окислов, типа перовскитов LaMeO3 (Me+3=V+3, Cr+3, Mn+3, Fe+3, Co+3, Ni+3) в окислении CO. Максимумы были найдены для Mn+3 и Co+3. Обсуждается зависимость между местной Симметрией твердых поверхностей, хемосорбцией и катализом.

Surface properties of LaNiO3: Kinetic studies of reduction and of oxygen adsorption

Abstract The kinetics of reduction and the kinetics of oxygen adsorption on LaNiO 3 were studied in a wide range of temperatures. Temperature-programmed reduction showed two reduction steps: (a) of 1 e − per molecule between 425 and 675 K and (b) of 3 e − per molecule between 675 and 900 K. In reduction step (a) a distorted LaNiO 3 (because of the presence of anion vacancies) was identified by X-ray diffraction; after sintering in He at 1073 K the perovskite structure was destroyed and La 2 NiO 4 and NiO were formed. In reduction step (b) La 2 O 3 and Ni were formed, these species being better crystallized after sintering at 1173 K. Steps (a) and (b), in the nonsintered samples, were reversible, i.e., after oxidation at 973 K the perovskite structure was newly formed. Reduction in step (a) takes place according to the contracting sphere model while reduction in step (b) is controlled by formation and growth of reduction nuclei. Experimental data in step (b) fit Avrami-Erofeev′s equation. Activation energies for reduction steps (a) and (b) were 108 and 221 kJ mol −1 , respectively. The kinetics of oxygen adsorption obeyed Elovich′s equation, t 0 being calculated by extrapolation to Z ( t ) = 0 of t vs Z ( t ) plots. The activation energy of adsorption was 38 kJ mol −1 . Within the LaMeO 3 series, the more easily reducible oxides (LaNiO 3 , LaCoO 3 , LaMnO 3 ) were found to be better adsorbents of oxygen and better catalysts for oxidation than the less reducible One (LaCrO 3 ).

Surface interactions of NO and CO with LaMO3 oxides

The surface interactions of NO and CO with LaMO3 perovskites (M = Cr, Mn, Fe, Co, Ni) have been studied by volumetric adsorption and infrared spectroscopy. NO adsorption at 200 mm Hg on LaNiO3 was found to be constant over the temperature range 200–750 K. NO adsorption on LaMO3 oxides at room temperature as a function of the atomic number of M showed maxima for LaMnO3 and LaCoO3 while CO adsorption exhibited one maximum for LaFeO3. The inhibiting effect of NO preadsorption on the subsequent adsorption of CO was found to be larger than the inhibiting effect of CO on NO adsorption. NO is apparently more strongly adsorbed than CO on the surface of these oxides. Infrared spectra after simultaneous adsorption of NO + CO on LaFeO3 contained bands between 1650 and 850 cm−1 due to species formed by the interaction of NO and CO with oxygen ions in the oxide surface, and a band at 2170 cm−1 assigned to isocyanate species. Spectra in the region 2300-1900 cm−1 after simultaneous adsorption of NO + CO on LaCoO3 contained bands at 2175 and 2000 cm−1, the latter being assigned to NO adsorbed via a donor-type or coordinative bond. The numbers of NO molecules adsorbed at monolayer coverage on the LaMO3 oxides did not equal estimated numbers of transition metal ions exposed in the oxide surfaces. The observed constancy of NO adsorption on LaFeO3 and LaNiO3 over a wide range of temperatures could provide a method for oxide characterization.

Physicochemical properties of LaMnO3: reducibility and kinetics of O2 adsorption

Abstract The kinetics of reduction in H 2 of the perovskite LaMnO 3 and also the kinetics of O 2 adsorption on this oxide were studied. The reduction process starts at 755 K; up to 950 K the reduction rate is low because of the slowness of nuclei formation and growth (nucleation). Once reduced nuclei are formed, the reduction process occurs at a higher rate (950–1050 K). At 1100 K a stable reduced state of ca. le − per molecule, accompanied by the disappearance of the perovskite structure and formation of La 2 O 3 and MnO, was reached. The reduction isotherms (873–1013 K), of sigmoidal form are typical of a reduction process controlled by formation and growth of reduction nuclei on the surface. An apparent activation energy of reduction of ca. 225 kJ mol −1 was found. The kinetics of O 2 adsorption showed (by means of plots b( dq d ln t) vs at) an Elovichian zone preceded (followed) by pre- and post-Elovichian regions. The results were fitted to a model of adsorption on a heterogeneous surface developed by M. Ungarish and C. Aharoni ( J. Chem. Soc. Faraday Trans. 1 79 , 119, 1983). The isobar of adsorption showed, in the range 350–700 K, two different activated processes which were associated to dissociative adsorption of oxygen (350–525 K) and incorporation of oxygen into the oxide lattice (above 600 K). A parallellism among oxygen adsorption, reducibility, and catalytic activity for total oxidation on LaMeO 3 oxides was found.

Adsorption of CO on the Perovskite-Type Oxide LaCoO3

— 273 K, of 15 to 5 kJ mol~1 point to physisorption. Between 573 and 648 K, the isosteric heat was 49 kJ mol-1, and the entropy values show that the adsorbed species has translational mobility in two dimensions. Adsorption of CO at 673 and above caused reduction of the bulk. CO adsorption at 298 gives rise to IR bands at 1495, 1450, 1175, 1110, 1070 and 850cm-1, attributed to bidentate carbonates. CO adsorption at 298 on a surface with preadsorbed 02 was found to be practically equal to the adsorption measured on a clean surface. On the contrary, preadsorption of C02 decreased the subsequent adsorption of CO to 1.2 %. It is concluded that CO and C02 adsorb on surface O2

An XPS and reduction study of PrCoO3

The perovskite-type oxide PrCoO3 has been studied by means of X-ray photoelectron spectroscopy (XPS), reduction in H2 and X-ray diffraction. Two types of oxygen were detected: lattice oxygen (binding energy = 528.4 eV) and adsorbed oxygen (binding energy = 530.9 eV). The increase in relative intensity of the peak corresponding to the latter species after reduction of PrCo03 to 3e− per molecule is assigned to the formation of hydroxyl groups. Temperature-programmed reduction (TPR) results showed two reduction steps: to 1 e− per molecule (Co3.1 → Co2+) at 475 to 635 K, and to 3e− per molecule (Co21 → Co0) at 725 to 815 K. Reduction in the first and second steps occurs according to the contracting sphere model and the nucleation mechanism, respectively. Reduction of Co3+ to Co2+ causes minimal structural changes in the perovskite. Reduction to 3e− per molecule yielded Pr2O3 and metallic cobalt. After this reduction and reoxidation at 973 K, the perovskite structure was regained. By XPS and TPR it was shown that PrCo03 is more easily reducible than LaCo03. It is concluded that the cation in the A position of the structure plays a significant role in the bulk and surface properties of LnCo03 (Ln, lanthanide elements) oxides.

A study of NO and CO interactions with LaMnO3

Abstract The surface of LaMnO 3 perovskite and its interactions with NO and CO were studied by means of X-ray photoelectron spectra (XPS), volumetric adsorption, and infrared spectroscopy. XP spectra show two photolines of lattice O 2− and adsorbed O − . Atomic ratios O/La and O/Mn show oxidative nonstoichiometry of the surface (LaMnO 3+ x ), the excess oxygen decreasing with increasing heating temperature. Volumetric measurements indicate that the number of adsorbed NO molecules at 373–673 K is remarkably higher than the number of exposed Mn 3+ ions. NO adsorption on this oxide is not as independent of temperature as that on LaFeO 3 or LaNiO 3 , showing a moderate decrease between 373 and 673 K. NO interacts with metal cations as well as with oxygen ions, giving place to dinitrosyls and nitrates which are stable up to 773 K. Adsorption of CO after preadsorbing NO yields labile carbonate species which decrease in concentration and eventually disappear with increasing temperature. NO appears to be bonded more strongly than CO to the surface of LaMnO 3 . These results support previous data suggesting that NO may be useful for oxide characterization.

CO, O2, and CO2 adsorption on scandium oxide

Abstract The adsorption of O 2 , CO, and CO 2 on Sc 2 O 3 was studied in the temperature range −195 to 550 °C. Isobars and isosteric heats were calculated, and ir spectra were measured for the identification of surface species. Oxygen is physically adsorbed at low temperatures. A slow chemisorption of oxygen begins about 100 °C, while above 350–400 °C there appears to be incorporation of oxygen into the lattice. Carbon monoxide is physically and reversibly adsorbed at temperatures up to 25 °C; however, chemisorption sets in above 100 °C and the ir spectrum shows the presence of surface carbonate at 210 °C. The adsorption of carbon dioxide is fast, giving high surface coverage; the spectra show the presence of both symmetric and bidentate surface carbonate. The slow oxygen chemisorption obeys a pseudo-Langmuirian isotherm. The large CO 2 chemisorption and the CO physical adsorption are better fit by a Freundlich model.

Particle size determination of palladium supported on sepiolite and aluminum phosphate

Abstract Chemical and physical methods for the determination of the particle size ( d ) of palladium metal supported on sepiolite and aluminum phosphate were studied. Differences among d values of Pd/sepiolite determined by H 2 and O 2 adsorption and O 2 -H 2 and H 2 -O 2 titrations were of 16% or lower, which proves that these three methods give a reliable estimate of metal particle sizes in these catalysts. CO adsorption and transmission electron microscopy (TEM) yielded, generally, remarkably higher values. In the case of Pd/AlPO 4 catalysts, d values obtained by H 2 chemisorption and wide-angle X-ray scattering were in good agreement. However, other methods of chemisorption, titration, and TEM gave noticeably higher d values. Formation of Pd n O in Pd/AlPO 4 catalysts points to a preferential growth on the support of the (111) and (100) faces of palladium. The observed sequence of sintering of Pd on both supports was H 2 O > high vacuum > H 2 .