Oberflächenverbindungen von Übergangsmetallen. XVIII. Der Einfluß der Cr-Konzentration auf Oxydationszahl und „Qualität”︁ von Cr-Oberflächenverbindungen auf Silicagel

Surface Compounds of Transition Metals. XVIII. The Influence of Cr Concentration on Oxidation Number and “Quality” of Cr Surface Compounds on Silicagel Mean oxidation number (OZ ) and mean unsaturated character (q ) of reduced Cr surface compounds on silicagel depend on the concentration of Cr in the product. The first effect is caused by the decrease of thermally stable surface Cr^VI with increasing Cr concentration in the activated product, since surface Cr^VI is the only precursor of surface Cr^II (constant yield of reduction～90%); consequently OZ _red shifts to higher values with increasing concentration. — The measured decrease of q with increasing Cr content cannot be correlated with a possible coordination of neighbouring SiOH groups, as it was suggested for the influence of T_act and T_red; other explanations are proposed. For the analyzed product, the results support the postulated model of an ensemble of topologically different individual surface species of typical reactivity.     

Über Oberflächenverbindungen von Übergangsmetallen. X [1]. Zum Stöchiometrie-Problem der Bindung von Oberflächen-Chrom(II) an Silicagel

On Surface Compounds of Transition Metals. X. The stoichiometry Problem of Fixation of Surface Chromium (II) on Silicagel. Thermogravimetric analysis of a reduced Philips catalyst (support: silicagel “Merck 7733”) shows that the metal is bound prediminantly (if not only) in chromate-like units up to a concentration of 2,8% Cr^II. At higher Cr^II contents. up to the maximum of 3.5%, the additionally fixed chromium does not consume further OH^s, within the scope of the thermogravimetric method formation of polychromatelike structures and/or use of not-neighbouring OH groups. The analysis further demonstrates that not all the Si? Oh groups comprised by the method are able to accommodate surface chromium. The fixation of the metal seems to proceed most likely by reaction with the vicinal pairs of Si? OH, as a correlation of our data with silica models shows.     

Über Oberflächenverbindungen von Übergangsmetallen. V. Oligomere Reaktionsprodukte bei der Polyäthylen- Synthese mit Chrom-Oberflächenverbindungen

On surface compounds of transition metals. V. Oligomeric reaction products by the polymerization of ethylene with chromium surface compounds Short time polymerization of ethylene with Cr(II) and Cr(VI) surface species on oxide carriers produces oligomeric reaction products, when followed by protonolysis, that originate, at least in part, from the decomposition of a soluble organochromium complex. Different homologous series result according to the type of heterogeneous catalyst employed: with Cr(II) contact α-olefines are obtained (in presence of O₂ the corresponding n-aldehydes are formed), Cr(VI) contacts give straightchain methylketones. Whereas the aldehyde formation occurs in secondary reactions the CH₃CO endgroup is produced evidently through the primary oxidation of C₂H₄ with surface Cr(VI). A polymerization sequence is then discussed in terms of protonolysis products wherein the alkene and acetyl endgroups are attached at that chain end which is not bound to chromium.     

Über Oberflächenverbindungen von Übergangsmetallen. VIII. Die Komplexbildung von koordinativ ungesättigtem Oberflächen-Chrom (II) mit Oxiden des Stickstoffs

On Surface Compounds of Transition Metals. VIII. Complex Formation of a Coordinatively Unsaturated Cr^II Surface Compound with Nitrogen Oxides N₂O forms with surface-Cr(II) a relatively unstable light blue compound of the stoichiometry 1:1, while addition of NO results in formation of a very stable dark brown, diamagnetic surface complex . By reaction with O₂ this complex undergoes — depending on reaction temperature — either replacement of NO unter reoxidation of the metal (→Cr(VI)) or/and reaction of the ligand (→NO₂). Direct reaction of NO₂ with results in the same products as stepwise addition of NO and 1/2 O₂. reacts with HCl/ROH under formation of the soluble, paramagnetic kation [Cr(NO)(ROH)_n]²⁺, which is formulated as [Cr(II)(NO)]²⁺ ? [Cr(I)(NO⁺)]²⁺ accordingly to the e.s.r. spectra.     

Oberflächenverbindungen von Übergangsmetallen. XXX. Fischer-Tropsch-analoge Umsetzung von CO/H2 bzw. CO2/H2 an Oberflächen-Chrom(II)

Surface Compounds of Transition Metals. XXX. Fischer-Tropsch Analogous Reactions of CO/H₂ and CO₂/H₂ on Surface Chromium(II) Surface chromium(II)/silicagel catalyzes the reduction of CO and CO₂ by hydrogen to CH₄, C₂H₆ and C₃H₈. The catalyst is partially desactivated by this reaction, but a permanent activity of ca. 30% remains. – The reaction can be formulated via the sequence formaldehyde complex → <CH₂>-complex → alkane. If cycloalkenes or chlorobenzene are added simultaneously, these scavenger molecules are methylated by <CH₂>.     

New Chromium on Silica Gel Catalysts with very high activity for the polymerization of ethylene

Together with the known chromium (II)/silica gel catalyst (Phillips catalyst) for the polymerization of ethylene, two new ones have been investigated. It was found that a chromium(II)-“repoly” catalyst (prepared by short reaction of the chromium(II)/silica gel with ethylene at temperatures between 100 and 225°C) and a chromium(III)/silica gel catalyst have up to hundred times higher activity than the chromium(II) one. Activation energies were calculated as 54.6, 49.6 and 43.8 kJ per mol, respectively. The number of active sites was determined by measuring the integrated absorbance of the C? H and C?O stretching vibrations of the polymer. At low chromium concentration (0.056%) roughly 50% of all chromium was catalytically active in the case of chromium(II) and chromium(III) on silica gel. For the chromium(II)-“repoly” catalyst all chromium atoms can be active. The turnover numbers for the polymerization at 20°C were calculated as 0.1 (chromium(II)), 7.5 (chromium(II)-“repoly”) and 20 (sec^?1 atm^?1) (chromium(III)).     

Untersuchungen an katalytisch aktiven Oberflächenverbindungen. VII. Katalytische Eigenschaften von Vanadiumoxid-Chromoxid-Aufschicht-Katalysatoren

Studies on Catalytically Active Surface Compounds. VII. Catalytic Properties of Oxidic Vanadium-Chromium Catalysts The catalytic properties of silica supported vanadium-chromium catalysts for the oxidation of ethanol, butane, propene, and butene are described and completed by ESR measurements. Differences in catalytic activity of chromium-free vanadium catalysts which are caused by different values of the silanol group concentration of the support are shown to become levelled by the addition of chromium. V? O? Cr species of medium catalytic activity are considered to be the catalytic active center. ESR spectra taken after the reaction show the existence of Cr³⁺ and VO⁺² species. A correlation between the intensity of the VO²⁺ signal and the catalytic activity of the samples is observed.     

Analytical speciation of chromium in in-vitro cultures of chromate-resistant filamentous fungi

In this work, different analytical speciation schemes have been used to study the reduction of Cr(VI) by a chromate-resistant strain of filamentous fungi Ed8 (Aspergillus sp), indigenous to contaminated industrial wastes. As demonstrated previously, this strain has the capability to reduce chromate present in the growth medium without its accumulation in the biomass, yet the reduced chromium end-products have not been characterized. Liquid growth medium, initially containing 50 mg L(-1) Cr(VI), was analyzed for Cr(III)/Cr(VI) and for total Cr at different time intervals (0-24 h) after inoculation with fungi. Three hyphenated procedures, based on the Cr(III)-EDTA formation and species separation by anion-exchange or ion-pairing reversed-phase chromatography with ICP-MS or DAD detection were used. The results obtained for Cr(VI) in each case were consistent, demonstrating efficient reduction of chromate during 24 h of Ed8 growth. However, pre-column complexation with EDTA did not ensure complete recovery of the reduced forms of chromium in the above procedures. An alternative speciation scheme, based on extraction of Cr(VI)-benzyltributylammonium bromide (BTAB) ion pairs into chloroform and subsequent determination of residual chromium by ICP-MS has provided evidence on the effective conversion of chromate into reduced chromium species in the growth medium. The results indicate the feasibility of using Ed8 strain for chromate bioremediation purposes. Analytically it can be concluded that speciation of chromium in biological systems should not be limited to its two most common oxidation states, because the actual reduced chromium species are not converted quantitatively to Cr(III)-EDTA.     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Rapid leaching of Cr(VI) in soil with Na3PO4 in the determination of hexavalent chromium by electrothermal atomic absorption spectrometry

A method has been developed that leaches Cr(VI) selectively from soil samples. Hexavalent chromium was leached completely from soil with 0.01molL(-1) Na(3)PO(4). This was achieved by boiling the soil-reagent solution mixture for a period of 5min. The leached Cr(VI) was then quantified by electrothermal atomic absorption spectrometry (ET-AAS) after filtration of the sample solutions through Hydrophilic Millipore PVDF 0.45microm filter. Statistical evaluations indicated that the new developed method is reliable since neither its comparison with the established method nor the comparison of the sum of the concentrations of chromium species to that of the total concentration of chromium show any difference at 95% level of confidence. The spiking of soil samples with Cr(III) standards before pretreatment show that Cr(III) was not oxidized to Cr(VI) during leaching as the Cr(VI) content never increased. The detection limit established was 0.07microg g(-1), which is an improvement to that of the US EPA method 3060A by a factor of more than 500. The maximum concentrations of Cr(VI) found in soil samples collected around the new chromium mine was 8.0microg g(-1) and falls within acceptable level of 15microg g(-1) in accordance with the Italian Guidelines.     

Facile Oxidation of Aldehydes to Carboxylic Acids with Chromium(V) Reagents

Recently we reported a convenient method of oxidation of alcohols to carbonyl compounds using chromium(V) reagents.¹ Although a variety of reagents are available for effecting this transformation, there are only a few reagents which have been successfully used for the oxidation of aldehydes to carboxyllc adds. Chromic acid, silver oxide and potassium permanganate are commonly employed for this purpose and reactions are performed in protic media under conditions which are not that mild.² The “non-aqueous” chromium(VI) reagent, pyridinium dichromate, recently reported by Corey³ oxidises alcohols and aldehydes to carboxylic acids in DMF at room temperature. Although Cr(V) species is postulated as an intermediate in all oxidations with Cr(VI), no systematic oxidation studies have been reported with these reagents. This note reports the results of some fruitful investigations on aldehyde → carboxylic acid conversion involving some “non-aqueous” chromium (V) complexes 1, 2, 3 and 5 under anhydrous conditions.     

Determination of chromium(III) and total chromium in natural waters using a surface ion-imprinted silica gel as selective adsorbent

A sensitive and selective method has been developed to determine Cr(III) and total Cr in natural water samples by ICP-AES with a Cr(III)-imprinted aminopropyl-functionalised silica gel adsorbent. The Cr(III)-imprinted and non-imprinted adsorbent were prepared by an easy one-step reaction with a surface imprinting technique. Their maximum static adsorption capacities for Cr(III) were 11.12 mg g^?1 and 3.81 mg g^?1, respectively. The relative selectivity factors (α _r) for Cr(III)/Co(II), Cr(III)/Au(III), Cr(III)/Ni(II), Cr(III)/Cu(II), Cr(III)/Zn(II), and Cr(III)/Cr(VI), were 377, 21.4, 15.4, 27.7, 26.4, and 31.9, respectively. Under the optimal conditions, Cr(III) can be absorbed quantitatively, but Cr(VI) was not retained. Total chromium was obtained after reducing Cr(VI) to Cr(III) with hydroxyammonium chloride. The detection limit (3σ) for Cr(III) was 0.11 ng mL^?1. The relative standard deviation was 1.2%. The proposed method has been validated by analysing two certified reference materials and successfully applied to the determination and speciation of chromium in natural water samples with satisfactory results.     

Untersuchungen an oxidischen Katalysatoren. XV. Der Einfluß von Oberflächenhydratation und -acidität der Übergangsformen des Aluminiumoxids auf die katalytische Aktivität bei der Dehydratisierung von Isopropanol

Studies on Oxide Catalysts. XV. The Influence of Surface Hydration and Acidity of Transient Forms of Alumina on the Catalytic Activity in the Dehydration of Isopropanol Surface OH groups were determined quantitatively on a aeries fo different samples of transient aluminas by means of D₂O exchange as well as by interaction with thionyl chloride and potassium vapour, and the acid centers were determined by titration with potassium methoxide and high temperature chemisorption of ammonia. Only the deuterium exchange and chemisorption of ammonia proved to be suitable for a quantitative characterization of the surface chemical properties mentioned in the paper. The catalytic activity is dependent directly on the number of acid centers. Between the number of surface OH groups and the results of the catalytic dehydration of isopropanol, however, no correlation was found.     

Simultaneous determination of hexavalent and total chromium in water and plating baths by spectrophotometry

A new spectrophotometric determination method of hexavalent chromium in waste water and plating baths is described based on the oxidation of beryllon III by chromium(VI) in 0.02M sulphuric acid medium. The decrease in the absorbance of beryllon III was measured at 482 nm with an apparent molar absorptivity of 5.15 x 10(4)1.mole(-1).cm(-1). Beer's law was obeyed for chromium(VI) over the range 0-25 mug/25 ml. After the oxidation of Cr(III) to Cr(VI) by ammonium persulphate, total chromium can be determined. Therefore, chromium(III) can be calculated by subtracting chromium(VI) from total chromium. The detection limit is 0.015 and 0.020 mug/25 ml for chromium(VI) and total chromium, respectively. A sensitive spectrophotometric method for trace Cr(III) and Cr(VI) in waste water and plating baths was developed with good precision and accuracy. The reaction is also discussed.     

A potentiometric rhodamine-B based membrane sensor for the selective determination of chromium ions in wastewater.

The construction and performance characteristics of a novel chromate ion-selective membrane sensor are described and used for determining chromium(III) and chromium(VI) ions. The sensor is based on the use of a rhodamine-B chromate ion-associate complex as an electroactive material in a poly(vinyl chloride) membrane plasticized with o-nitrophenyloctyl ether as a solvent mediator. In a phosphate buffer solution of pH 6 - 7, the sensor displays a stable, reproducible and linear potential response over the concentration range of 1 x 10(-1) - 5 x 10(-6) mol l(-1) with an anionic Nernstian slope of 30.8 +/- 0.5 mV decade(-1) and a detection limit of 1 x 10(-6) mol l(-1) Cr(VI). High selectivity for Cr(VI) is offered over many common anions (e.g., I-, Br-, Cl-, IO4-, CN-, acetate, oxalate, citrate, sulfate, phosphate, thiosulfate, selenite, nitrate) and cations (e.g., Ag+, Ca2+, Sr2+, Co2+, Ni2+, Cu2+, Mn2+, Fe2+, Zn2+, Cd2+, Al3+, Cr3+). The sensor is used for determining Cr(VI) and/or Cr(III) ions in separate or mixed solutions after the oxidation of Cr(III) into Cr(VI) with H2O2. As low as 0.2 microg ml(-1) of chromium is determined with a precision of +/-1.2%. The chromium contents of some wastewater samples were accurately assessed, and the results agreed fairly well with data obtained by atomic absorption spectrometry.     

The use of 5-(4-pyridyl)nonane for the separation of chromium(vi) from fission products in hydrochloric acid media

The distribution of chromium(VI) between 5-(4-pyridyl)nonane in benzene and hydrochloric acid media has been studied as a function of the concentration of the acid, extractant, chromium(VI), chloride and a few other ions. The extraction mechanism and the composition of the extracted complexes of Cr(VI) have been proposed. The separation of Cr(VI) from uranium, thorium and fission products in 3M hydrochloric acid has been achieved.     

Sequential injection analysis for Cr(VI) and Cr(III) with renewable surface reflection spectrophotometry.

On the basis of the chromogenic reaction of chromium(VI) with 1,5-diphenylcarbohydrazide (DPC) on the surface of Polysorb C-18 beads and the sequential injection renewable surface technique (SI-RST), a highly sensitive reflect spectrophotometric method for the determination of chromium(III) and chromium(VI) was proposed. Considerations of system and flow cell design, and factors that influence the determination performance were discussed. With 300 microl of sample loaded and 0.6 mg of beads trapped, the linear response range was 0.02 - 0.5 mg l(-1) Cr(VI) with a detection limit (3 sigma) of 2.4 microg l(-1) Cr(VI). The method achieves a precision of 1.3% RSD (n = 11) and a throughput of 53 samples per hour. The determination of Cr(III) was based on the same reaction for the determination of Cr(VI) after being oxidized by (NH4)2S2O8. The precision of the oxidation method was evaluated using a 0.2 mg l(-1) Cr(III) standard, yielding an RSD of 2.5% (n = 11). The average recovery of Cr(III) oxidized was tested to be 99.1%. The proposed method was used in the simultaneous determination of Cr(VI) and Cr(III) in water samples, and the error was less than 3%.     

Speciation of chromium in seawater by ICP-AES with dual mini-columns containing chelating resin.

A method for the preconcentration and speciation of chromium in seawater was developed. On-line preconcentration and determination were carried out by using inductively coupled plasma atomic emission spectrometry (ICP-AES) with dual mini-columns containing a chelating resin. In this system, Cr(III) was collected on the first column. The effluent containing residual chromium from the first column was collected on the second column after passing through a reduction-switching unit, in which the reducing agent was introduced, or not, for the reduction of Cr(VI) to Cr(lII). Cr(VI) was determined as the difference between the concentration of pre-reduced Cr(VI) and Cr(III) in the effluent from the first column. The detection limits for Cr(III) and Cr(VI) were 0.04 and 0.09 microg l(-1), respectively.     

Metal speciation by flow-injection analysis

A two-channel switching valve is incorporated in the flow-injection manifold for on-line control of the metal speciation of Fe(II)/Fe(III) and Cr(III)/Cr(VI). 1,10-Phenanthroline is used for the iron determinations and diphenylcarbazide for chromium. The absorbances are measured at 512 and 540 nm, respectively. When a 30-mul injection loop is used, the response is linear for 0.5-30.0 ppm Fe and 0.5-40.0 ppm Cr. The relative standard deviation in each case is approximately 1%. The method allows at least 180 injections per hour.