Surface Of Chromia Research Articles

The surface stability of Cr2O3(0 0 0 1)

The surface of chromia (Cr2O3) has a surface electronic structure distinct from the bulk and a packing density distinct from the bulk. More than a demarcation between the solid and the vacuum, the surface differs from the bulk of chromia, not just because of a partial occupancy of chromium sites, but also because of an increased number of unoccupied surface oxygen sites (vacancy sites), evident in angle-resolved core level photoemission. In spite of the structural differences that exist at the surface, there is, as yet, no evidence that these complications affect the surface Debye temperature beyond the most simple of assumptions regarding the lower coordination of the surface. Using low-energy electron diffraction (LEED), the effective surface Debye temperature (∼490 K) is found to be lower than the bulk (∼645 K) Debye temperature of Cr2O3(0 0 0 1). This surface effective Debye temperature, indicative of vibrations along the surface normal, uncorrected for anharmonic effects, has a value reduced from the effective bulk Debye temperature yet close to the value √2 expected from a simple mean field argument.

Spin polarization asymmetry at the surface of chromia

We demonstrate boundary spin polarization at the surface of a Cr2O3 single crystal using spin-polarized low-energy electron microscopy (SPLEEM), complementing prior spin polarized photoemission, spin polarized inverse photoemission, and x-ray magnetic circular dichroism photoemission electron microscopy measurements. This work shows that placing a Cr2O3 single crystal into a single domain state will result in net Cr2O3 spin polarization at the boundary, even in the presence of a gold overlayer. There are indications that the SPLEEM contrast for the two polarization states may be different, consistent with scanning tunneling microscopy spectroscopy results obtained from ultrathin films of Cr2O3.

Robust isothermal electric control of exchange bias at room temperature

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

Static TOF-SIMS Surface Analysis of a CCl2F2Activated Chromia Catalyst Used for a Cl/F Exchange Reaction

Static TOF-SIMS results strongly underpin earlier conclusions from ESCA, XANES, and tracer studies that the accumulation of fluorine and chlorine at the surface of chromia by a heterogeneous reacti...

Kinetic and Mechanistic Study of the Reaction of CCl4with Prefluorided Chromia to Form CCl3F and CCl2F2

The reaction of CCl4with prefluorided chromia (prefluorided to multilayer depth) has been studied using the transient techniques of temperature programmed reaction (TPR), temperature programmed desorption (TPD) and gas adsorption chromatography (GAC). The reaction has been shown to proceed by a Langmuir–Hinshelwood mechanism in which the chemisorbed CCl4molecule exchanges its chlorine atoms with the fluoride ions on the surface of fluorided chromia. Mono and di-exchange, forming CCl3F and CCl2F2, has been shown to occur simultaneously with an activation energies of ∼63 kJ mol−1for both. The facile reaction of the chemisorbed CCl4with the surface F−ion to form CCl3F is rationalised in a four centre exchange mechanism in which the CCl4is bonded to the fluorided chromia surface through a chlorine atom of the CCl4molecule bonding to a surface Cr3+ion. The C atom of CCl4molecule bonds to a surface F−species. Di-exchange which occurs simultaneously with mono exchange, the two reactions having the same activation energy, is accounted for by the CCl4being bonded through two Cl atoms to two surface Cr3+ions, with the carbon being loosely associated with two surface F−ions. This configuration which has been shown to be geometrically possible on the (001) surface of CrF3allows for simultaneous double exchange of the halogens.

Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide III. Diffuse reflectance ftir study of ammonia desorption from Brønsted and Lewis acid sites

Ammonia has been adsorbed onto oxidized and reduced surfaces of crystalline α-Cr 2O 3 and of amorphous chromia, respectively. Temperature-dependent changes in the diffuse reflectance FTIR spectrum are monitored and are correlated with temperature-programmed reaction and desorption (TPRD) spectra reported in part I of this study (1). Two types of NH3 molecules bound to Brønsted acidic sites on the surface of the chromia catalysts, as well as two species bound to Lewis sites, are identified from the IR spectra. The oxidized surface of α-Cr 2O 3 is characterized by a high surface coverage of strongly Lewis-bound ammonia molecules, which desorb at temperatures above 400 K. In contrast, weakly bound NH 3 prevails on the oxidized surface of amorphous chromia. The desorption of the latter species starts at 380 K and is completed around 480 K. A reductive pretreatment of the chromia surfaces by hydrogen (10% in Ar) decreases the number of surface hydroxyl groups and devoids the surface of adsorbed oxygen. As a consequence, the number of Brønsted acidic sites is reduced both on the crystalline and the amorphous catalyst. On the reduced surface of α-Cr 2O 3, strongly bound ammonia coordinated to Lewis acidic sites is observed to persist to temperatures beyond 560 K. In contrast, weakly Lewis-bound NH 3 prevails on reduced amorphous chromia, which is quantitatively desorbed at temperatures below 410 K. The formation of NH 3 oxidation products on α-Cr 2O 3 is manifested by the observation of two characteristic surface species, an N 2O 2 dimer and an NO 2 chelate surface complex. Corresponding signals are not observed on the surface of amorphous chromia. Apparently the removal of labile oxygen from the surface of α-Cr 2O 3 at moderate temperatures generates strongly Lewis acidic chromium sites, and the firm bonding of NH 3 to these sites is related to the undesired direct oxidation of ammonia.

Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide II. Diffuse reflectance FTIR study of thermal treatment and oxygen adsorption

The activation of crystalline and amorphous chromia surfaces by thermal pretreatment in argon and oxygen adsorption at 473 K has been studied by diffuse reflectance FTIR and Raman spectroscopy. The formation of coordinatively unsaturated chromium sites during thermal activation is monitored by observing the evolution of CrO stretching absorptions both in the fundamental and overtone regions of the FTIR spectrum. On α-Cr 2O 3, labile surface oxygen species are largely removed at 498 K, whereas on the amorphous chromia surface, labile oxygen is more tightly bound. As a consequence, coordinatively unsaturated chromium sites are generated on amorphous chromia to a lesser extent than on α-Cr 2O 3. Upon high-temperature oxygen treatment, O 2 is dissociatively adsorbed. Coordinatively unsaturated sites are occupied by the added oxygen, as manifested by an increase in the number of CIO oscillators. Fine structure in the CrO absorptions of the amorphous chromia is observed for the first time, and is tentatively assigned to various types of surface sites. Raman spectroscopic characterization of the amorphous chromia surface reveals laser-induced dehydration and creation of coordinatively unsaturated surface CrO sites, accompanied by progressive crystallization of the amorphous substrate. Differences between crystalline and amorphous chromia with respect to their SCR activity are correlated with the higher density of labile oxygen species available on the surface of amorphous chromia under SCR reaction conditions (423–473 K).

Surface and catalytic properties of chromia in dehydrogenation of secondary alcohols

The rates of dehydrogenation of a series of 2-alkanols, the selectivity of the decomposition of 3,3-dimethyl-2-butanol, adsorption heats of diethyl ether by gas chromatographic method, crystallite size, total water content and surface concentration of hydroxyl groups were measured with six samples of chromium(III) oxide differing in the method of preparation. Correlation of the results showed that the catalytic properties of chromium oxide depend mainly on its temperature of calcination which governs the surface concentration of hydroxyl groups and the state of coordination of surface Cr 3+ ions.

Studies on the ketonization of acetic acid on chromia: I. The adsorbate-catalyst interaction

The ketonization of acetic acid has been studied on a chromia catalyst at 460 °C and at a contact time of 0.7 sec at various partial pressures in presence of tertiary butyl alcohol and benzyl alcohol. There is no inhibition of the ketonization of acetic acid in presence of tertiary butyl alcohol which undergoes only dehydration giving isobutylene and water. Benzyl alcohol, which undergoes only dehydrogenation to give benzaldehyde and hydrogen, suppresses the ketonization of acetic acid. Comparative studies of the reaction of isopropyl alcohol in mixture with acetic acid and also with propionic acid, under identical conditions, confirm the conclusion that the ketonization is caused by the same activity of the catalyst that is responsible for dehydration and dehydrogenation. It is suggested that the ketonization occurs through the interaction of two acetic acid molecules adsorbed on the surface of chromia, one of them utilizing the dehydrogenation activity for adsorption and the other utilizing the dehydration activity.

Behavior of metal oxide additives in thermal decomposition of ammonia perchlorate

The reaction of CCl4with prefluorided chromia (prefluorided to multilayer depth) has been studied using the transient techniques of temperature programmed reaction (TPR), temperature programmed desorption (TPD) and gas adsorption chromatography (GAC). The reaction has been shown to proceed by a Langmuir–Hinshelwood mechanism in which the chemisorbed CCl4molecule exchanges its chlorine atoms with the fluoride ions on the surface of fluorided chromia. Mono and di-exchange, forming CCl3F and CCl2F2, has been shown to occur simultaneously with an activation energies of ∼63 kJ mol−1for both. The facile reaction of the chemisorbed CCl4with the surface F−ion to form CCl3F is rationalised in a four centre exchange mechanism in which the CCl4is bonded to the fluorided chromia surface through a chlorine atom of the CCl4molecule bonding to a surface Cr3+ion. The C atom of CCl4molecule bonds to a surface F−species. Di-exchange which occurs simultaneously with mono exchange, the two reactions having the same activation energy, is accounted for by the CCl4being bonded through two Cl atoms to two surface Cr3+ions, with the carbon being loosely associated with two surface F−ions. This configuration which has been shown to be geometrically possible on the (001) surface of CrF3allows for simultaneous double exchange of the halogens.