Ammonium Chromate Research Articles

Dissociation of the hyperchromate ion into the chromic ion in crystals: An electron spin resonance study

Abstract By heating LiCsSO4 crystals which have been doped with ammonium chromate and γ-irradiated, Cr3+ species with large zero-field splittings were detected by electron spin resonance (ESR). The Cr3+ species result from an ammonium-hydrogen-assisted dissociation of the CrO43− and CrO3− species produced by irradiation. The same oxygen-stripping effect was observed by ESR in γ-irradiated LiNH4SO4 and (NH4)2SO4 crystals doped with CrO42−. Dissociation of CrO42− itself into Cr3− was observed by ESR in CrO42−-doped ammonium sulfate crystals heated at temperatures well below the dissociation temperature of this compound.

Study of copper-chromium oxide catalyst: II . Crystal data and thermal decomposition of basic copper(II) ammonium chromate

Basic copper(II) ammonium chromate (BCAC) of the composition CuCrO 4 (Cu(OH) 2 ) x -(NH 4 OH) y , (NH 3 ) z has been prepared. Depending on the pH of the precipitation reaction, quotients x , y and z vary in the intervals 0 < x < 0.3, 0.6 < y < 0.9 and 0 < z < 0.2. The rational formula has been established from the elemental analysis, TG measurements, and from infrared and electronic spectra. The monoclinic unit cell parameters of BCAC were determined by X-ray powder diffraction ( a = 1.0235(15) nm, b = 0.654(2) nm, c = 0.7277(12) nm, β = 103.5(1) deg, V = 0.47364 nm 3 , the space group is C 2 m , Z is 4 and D m = (2.98±0.04) Mg m −3 ). The thermal decomposition of BCAC proceeds in four steps. The activation parameters, E a , A , Δ H ≠ and Δ S ≠ , of the first step of the thermal decomposition were derived.

Investigation of deactivation mechanisms of ASC whetlerite charcoal

Abstract ASC whetlerite, prepared by the impregnation of CWS grade activated charcoal with ammoniacal carbonate solutions of copper(II), chromium(VI), and silver salts, is an effective material for the removal of many low-molecular-weight gases (e.g., cyanogen chloride and hydrogen cyanide) from air streams. Whetlerite loses its effectiveness when exposed to extremes in temperature and/or humidity or by excessive periods of usage. Associated with this deactivation/deterioration process are changes in metal oxidation state(s) and impregnant morphology. This investigation examines the behavior of the metal impregnants on ASC whetlerite when subject to certain extreme conditions and, as a result, develops an understanding of the deactivation mechanism(s) that occur. The deterioration of whetlerite due to thermal deactivating conditions as well as that due to exposure to hydrogen involves the decomposition of the supported CuOHNH 4 CrO 4 (basic copper ammonium chromate) species. This complex chromate decomposes into a CuOCuCr 2 O 4 (copper-chromite) species upon thermal treatments up to 490 K. At temperatures above 490 K the CuO (cupric oxide) sinters away from the chromite and is subsequently reduced to Cu 1+ -containing and Cu 0 species. When exposed to high humidities for long periods of time (i.e., more than 2 days) it is hypothesized that the whetlerite surface experiences a decrease in pH. This results in the supported CuOHNH 4 CrO 4 species converting to Cu 4 (OH) 6 CrO 4 (brochantite chromate). The brochantite chromate gradually decomposes into less complex crystalline compounds such as CuO, Cu 2 Cr 2 O 4 , and Cr 2 O 3 which are not effective for destroying irritating gases.

Effect of pressure on the radiation annealing of recoil atoms in chromates

The effect of pressure on the annealing of recoil atoms by gamma radiation in neutron irradiated potassium chromate, ammonium chromate and ammonium dichromate has been studied. In potassium chromate pressure applied before the gamma-irradiation was found to retard the radiation annealing process. In ammonium chromate and ammonium dichromate the radiation annealing was found to be enhanced in the compressed samples in comparison to the noncopmressed ones.

Study of copper chromite catalysts, III. Structure and catalytic activity of copper chromite catalyst in reductive alkylation reaction

The catalytic activity of copper chromite catalysts prepared by thermal decomposition reaction of basic copper(II) ammonium chromate, CuCrO4[Cu(OH)2]0.17−(NH4OH)0.78, and copper(II) chromate CuCrO4, has been studied. The catalysts with highly disordered structure, prepared from basic copper(II) ammonium chromate at 270–320 °C, show the highest catalytic activity in reductive alkylation reaction. The activity of recrystallized catalysts, prepared from basic copper(II) ammonium chromate or CuCrO4 at higher decomposition temperature, is significantly lower.

The effect of pressure on neutron irradiated ammonium chromates

The effect of pressure on neutron irradiated ammonium chromate and ammonium dichromate has been studied. Compression was found to have no effect on the retention and on the M and DP yields in both salts. Pressure applied before heating was found to retard annealing in ammonium chromate, while it appeared to have no influence on the annealing of ammonium dichromate.

The effect of pressure on neutron irradiated ammonium chromates

The effect of pressure on neutron irradiated ammonium chromates

Decomposition mechanism of copper ammonium chromate: A basic material for copper chromite catalyst

The mechanism of thermal decomposition of basic copper ammonium chromate has been investigated using TG/DTG/DSC and X-ray studies. The decomposition is a three-step process. Calcination at lower temperatures can be achieved by taking advantage of the exothermicity of the decomposition process.

ChemInform Abstract: THERMAL DECOMPOSITION OF AMMONIUM CHROMATE

AbstractDie Untersuchungen zum Mechanismus der Zers. Alassen auf eine Reaktions?äche schließen, die von der ursprünglichen Kristalloberfläche aus einwärts wandert.

Thermal decomposition of ammonium chromate

Kinetic evidence has shown that the decomposition of ammonium chromate [→(NH4)2Cr2O7+ H2O + 2NH3] obeys the contracting cylinder equation. Evidence that a reaction interface advances progressively inwards from the original crystal surfaces was obtained from microscopic examination of cross-section surfaces revealed by fracture of the lath-shaped crystals of reactant after partial decomposition. Consistent and complementary results were obtained from quantitative studies using three different salt preparations. It is concluded that two distinct chemical steps contributed to reaction. These were reversible proton transfer, which was opposed by higher pressures of accumulated product gas, followed by water elimination, which was not reversible under reaction conditions. The decomposition is very slightly endothermic but is irreversible, an unusual combination.

Concentration profiles in impregnated chromium and copper on alumina

Chromium and copper catalysts were prepared by adding known amounts of aqueous solutions of chromium and copper compounds to dry γ-alumina spheres. After drying and calcining, the spheres were sectioned, and concentration profiles were determined along the diameter of the circular cross section by an electron probe microanalyzer. Isotherms for the adsorption on the alumina of aqueous solutions of ammonium chromate and ammonium dichromate were determined. Concentration profiles were functions of the solute and the concentration and amount of solution. It is inferred that the order of adsorption on the alumina of aqueous solutions is: chromium III nitrate > copper II nitrate > chromic acid > ammonium dichromate > ammonium chromate = copper II ammine nitrate. Moderately accurate quantitative analyses can be made on impregnated catalysts with the electron probe; the amount of metal found was usually within 10% of the amount introduced.

Electron transmission microscopy of charcoals impregnated with ammonium salts Of Cu(II) and Cr(VI)

Gas adsorbent charcoals were impregnated with simplified whetlerizing solutions and the results compared with commercial whetlerites. Electron micrographs of the products showed a segregation of a considerable fraction of the added salts into crystalline areas, the latter ranging from 50 to 200 Å in the short dimension. Impregnation with ammonium chromate alone gave single-crystal electron diffraction patterns having planar separations corresponding almost entirely to Cr 2 O 3 , hexagonal rhombic. The spacings for charcoals impregnated with ammoniacal basic copper carbonate agreed best with alimited defect structure of Cu 2 O. An ammoniacal solution of the crystals, formed in the slow evaporation of a whetlerizing solution, was impregnated on charcoal; the resultant planar separationscould be correlated with known X-ray diffraction spacings of (NH 4 ) 2 CrO 4 , Cu 2 O and Cr 2 O 3 . The impregnated copper salts thus appear to protect the chromate ion to a considerable extent. One ASC commercial whetlerite gave spacings principally of Cr 2 O 3 and Cu 2 O. There is goodevidence that impregnated charcoal dried below 150° is not an inert support,but it takes part in a redox system in which Cu(I) and Cr(III) are formed and the carbon networks oxidized. When a whetlerite was exposed to intensified beam current for short periods, small particles were observed to deposit on the support film. The particles ranged in size from about 100 Å to much smaller diameters; the diffraction patterns were polycrystalline Cu 2 O and Cu. The residue left in the charcoal gave 19 interplanar spacings that closely correlated with Cr 2 O 3 hexagonal rhombic.

Thermal and Radiation Annealing of Chromium-51 in Ammonium Chromates

Thermal and Radiation Annealing of Chromium-51 in Ammonium Chromates

Temperature Variation of Principal Moments of Some Polyatomic Crystals

Principal moments of some of the polyatomic single crystals such as ammonium dichromate, ammonium chromate, potassium chromate and potassium permanganate have been determined from room temperature down to liquid air temperature. These have been found to decrease with the decrease of temperature. This variation is smaller in magnitude. It has been observed that the temperature coefficient of the principal moments has almost the same value over the entire range of temperature. This small variation of paramagnetism has been attributed to the variation of absorption frequency in solid state.

Thermal Decomposition of Ammonium Salts of Transition Metal Oxyacids. III. Thermogravimetric Analysis of Ammonium Chromate

Abstract Thermal decomposition of ammonium chromate (AC) and ammonium dichromate (ADC) has been studied by means of TG, DTA, and DTG. It showed four steps in thermal decomposition of AC and three steps in that of ADC. Decomposition product in each step of the thermal decomposition was examined by X-ray diffraction method. From the results of both thermogravimetric analysis and X-ray study, it was concluded that the first decomposition product of AC was ADC, the second was CrO3 and the final product was Cr2O3. The third product gave the X-ray patterns of the mixture of Cr2O3 and CrO3, and the mole ratio of O/Cr in the temperature range for the existence of the product (300–390°C) was 2.1–2.0. The kinetics in the second decomposition step for AC and in the first decomposition step for ADC were studied, giving the values of the energy of activation of 24.2 and 22.4 kcal/mol, respectively.

Thermal Decompsition of Ammonium Salts of Transition Metal Oxyacids. IV. Determination of Constituents of Gases Evolved in Decomposition of Ammonium Chromate

Abstract Constituents of gases evolved in thermal decomposition of ammonium chromate and ammonium dichromate have been studied by an omegatron mass spectrometer. From the data of gas analysis, the following conclusions have been obtained. The decomposition of the first step started with proton transfer mechanism and the ratio of NH3 to H2O was not 2 : 1 at the beginning. The second step corresponded to the formation of N2, N2O, and H2O through oxidation of NH3 by the chromate anion. The decomposition in the third step was accompanied by the evolution of oxygen molecules. The production of CrO3 was confirmed by comparing the dissociation pressure of CrO3 produced in AC decomposition with that of pure CrO3. The decomposition in the last step corresponded to the process from CrO2 to Cr2O3, with the evolution of O2, CO2, and CO.

Solid-state vibrational spectroscopy. Part II. The vibrational spectra of some chromates and dichromates

The solid-state (powder) i.r. (1000–40 cm–1) and Raman (1000–130 cm–1) spectra of sodium, potassium, and ammonium chromate(VI) and dichromate(VI) are reported and compared with the previously reported solution Raman spectra. Considerable splittings of the degenerate modes are observed and an attempt is made to account for these splittings by site- and factor-group analysis.

Thermal Decomposition Reaction of Basic Salts: Nickel Carbonate and Copper Ammonium Chromate

Thermal Decomposition Reaction of Basic Salts: Nickel Carbonate and Copper Ammonium Chromate

α-annealing of recoil atoms in chromates

Annealing studies of recoil chromium-51 in mixtures of chromates and dichromates with boron compounds by α-particles produced from the 10B(n,α)7Li nuclear reaction, are reported. Only potassium chromate showed a considerable increase of retention, while α-particles had no direct effect on ammonium chromate or caused a decrease of retention in potassium dichromate. The post-irradiation isothermal annealing seems to be affected by the dose given to the lattice by α-particles.

Enrichment of Chromium‐51 by Szilard‐Chalmers Process

AbstractEnrichment of chromium‐51 by Szilard‐Chalmers process was studied, using potassium and ammonium chromate as target materials. Separation of enriched phase from the target was carried out by ion exchange method. Enrichment factor as high as 3,000 was attained for a potassium chromate target. When ammonium chromate was used as the target, the enrichment factor was much lower, amouting only to 150.