The role of carbon surface chemistry in N2O conversion to N2 over Ni catalyst supported on activated carbon

The effect of acidic treatments on N2O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N-2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N2O-carbon reaction catalyzed by Ni catalyst. HNO3 treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N2O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

Effects of acidic treatments on the pore and surface properties of Ni catalyst supported on activated carbon

Activated carbon as catalyst support was treated with HCl, HNO3, and HF and the effects of acid treatments on the properties of the activated carbon support were studied by N-2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). Ni catalysts supported on untreated and treated activated carbons were prepared, characterized and tested for the reforming reaction of methane with carbon dioxide. It is found that acid treatment significantly changed the surface chemical properties and pore structure of the activated carbon. The surface area and pore volume of the carbon supports are generally enhanced upon acid treatment due to the removal of impurities present in the carbon. The adsorption capacity of Ni2+ on the carbon supports is also increased, and the increase can be closely correlated with the surface acidity. The impregnation of nickel salts decreases the surface area and pore volume of carbon supports both in micropores and mesopores. Acid treatment results in a more homogeneous distribution of the nickel salt in carbon. When the impregnated carbons are heated in inert atmosphere, there exists a redox reaction between nickel oxide and the carbon. Catalytic activity tests for methane reforming with carbon dioxide show that the activity of nickel catalysts based on the acid-treated carbon supports is closely related with the surface characteristics of catalysts. (C) 1998 Elsevier Science Ltd. All rights reserved.

Effects of acid treatments of carbon on N2O and NO reduction by carbon-supported copper catalysts

The influences of HCl, HNO3 and HF treatments of carbon on N2O and NO reduction with 20 wt% Cu-loaded activated carbon were studied. The order of activity in both N2O and NO is as follows: Cu20/AC-HNO3>Cu20/AC>Cu20/AC-HF>Cu20/AC-HCl. The same sequence was also observed for the amount of CO2 evolved during TPD experiments of supports acid for the catalyst dispersion. On the other hand, N2O exhibited a higher reaction rate than NO and a higher sensitivity to acid treatments, and the presence of gas-phase O-2 had opposite effects in N2O and NO reduction. The key role of carbon surface chemistry is examined to rationalize these findings and the relevant mechanistic and practical implications are discussed. The effects of oxygen surface groups on the pore structure of supports and catalysts are also analyzed, (C) 2000 Elsevier Science Ltd. All rights reserved.

Role of oxygen in the nitrous oxide/carbon reaction

An investigation of the role of oxygen in the nitrous oxide/carbon reaction was carried out on various carbon samples (both graphitic and nongraphitic) over a range of temperatures and partial pressures. Previous work reported that oxygen strongly inhibited the nitrous oxide/carbon reaction. Large ratios of O-2/N2O were used in all previous work. In this work, the O-2/N2O ratio was kept below 1, and we found that oxygen did not inhibit the rate of the C + N2O reaction. Instead, the rate of the reaction in the presence of oxygen was essentially that predicted by the two independent reactions, nitrous oxide/carbon and oxygen/carbon, occurring simultaneously. A simple theoretical explanation is given for the observations, both past and present, on the basis of competitive chemisorption of nitrous oxide and oxygen on active sites.

The chemistry of copper patination

The chemistry of copper patination was investigated by two series of experiments. The chemistry of an aqueous copper-sulphate solution was studied at concentrations and pH values near those predicted in an electrolyte on copper exposed to the atmosphere. The electrochemical reactions in an electrolyte in contact with cuprite were investigated in a reaction vessel which used cuprite powder in artificial rainwater to study the electrochemistry of the atmospheric corrosion and patination of copper. Typical sulphate concentrations in rainwater are sufficient to precipitate posnjakite (Cu4SO4(OH)(6)2H(2)O)), a possible precursor to brochantite, within an hour of wetting a cuprite surface. Brochantite (Cu4SO4(OH)(6)), the most commonly found copper salt in natural patinas is responsible for their green appearance. Precipitation of brochantite from the electrolyte resulted from an increase in pH due to the cathodic reduction of oxygen and an increase in cupric ion concentrations by cuprite oxidation. (C) 1998 Elsevier Science Ltd. All rights reserved.

Sorption and desorption behaviour of acetochlor in surface, subsurface and size-fractionated soil

Pesticides leaching through a soil profile will be exposed to changing environmental sorption and desorption conditions as different horizons with distinct physical and chemical properties are encountered. Soil cores were taken from a clay soil profile and samples taken from 0.0 to 0.3 m (surface), 1.0-1.3 m (mid) and 2.7-3.0 m (deep) and treated with the chloroacetanilide herbicide, acetochlor. Freundlich isotherms revealed that sorption and desorption behaviour varied with each depth sampled. As soil depth increased, the extent and strength of sorption decreased, indicating that the potential for leaching was increased in the subsoils compared with the surface soil. Hysteresis was evident at each of the three depths sampled, although no significant correlations between soil properties and the hysteresis coefficients were evident. Desorption studies using soil fractions with diameters of > 2000, 250-2000, 53-250, 20-53, 2-20, 0-2 and 0-1 mum separated from each of the three soil depths showed that differential desorption kinetics occurred and that the retention of acetochlor significantly correlated (R-2 = 0.998) with organic matter content. A greater understanding of the influence of soil components on the overall sorption and desorption potential of surface and subsurface soils is required to allow accurate prediction of acetochlor retention in the soil. In addition, it is likely that the proportion of each size fraction in a soil horizon would influence acetochlor bioavailability and movement to groundwater.

Dissipation of acetochlor and its distribution in surface and sub-surface soil fractions during laboratory incubations

Pesticides in soil are subject to a number of processes that result in transformation and biodegradation, sorption to and desorption from soil components, and diffusion and leaching. Pesticides leaching through a soil profile will be exposed to changing environmental conditions as different horizons with distinct physical, chemical and biological properties are encountered. The many ways in which soil properties influence pesticide retention and degradation need to be addressed to allow accurate predictions of environmental fate and the potential for groundwater pollution. Degradation and sorption processes were investigated in a long-term (100 days) study of the chloroacetanilide herbicide, acetochlor. Soil cores were collected from a clay soil profile and samples taken from 0-30cm (surface), 1.0-1.3m (mid) and 2.7-3.0m (deep) and treated with acetochlor (2.5, 1.25, 0.67 mu g acetochlor g(-1) dry wt soil, respectively). In sterile and non-sterile conditions, acetochlor concentration in the aqueous phase declined rapidly from the surface and subsoil layers, predominantly through nonextractable residue (NER) formation on soil surfaces, but also through biodegradation and biotic transformation. Abiotic transformation was also evident in the sterile soils. Several metabolites were produced, including acetochlor-ethane sulphonic acid and acetochlor-oxanilic acid. Transformation was principally microbial in origin, as shown by the differences between non-sterile and sterile soils. NER formation increased rapidly over the first 21 days in all soils and was mainly associated with the macroaggregate (> 2000 mu m diameter) size fractions. It is likely that acetochlor is incorporated into the macroaggregates through oxidative coupling, as humification of particulate organic matter progresses. The dissipation (ie total loss of acetochlor) half-life values were 9.3 (surface), 12.3 (mid) and 12.6 days (deep) in the non-sterile soils, compared with 20.9 [surface], 23.5 [mid], and 24 days [deep] in the sterile soils, demonstrating the importance of microbially driven processes in the rapid dissipation of acetochlor in soil.

Synthesis and structure characterization of chromium oxide prepared by solid thermal decomposition reaction

Mesoporous chromium oxide (Cr2O3) nanocrystals were first synthesized by the thermal decomposition reaction of Cr(NO3)(3)(circle)9H(2)O using citric acid monohydrate (CA) as the mesoporous template agent. The texture and chemistry of chromium oxide nanocrystals were characterized by N-2 adsorption-desorption isotherms, FTIR, X-ray diffraction (XRD), UV-vis, and thermoanalytical methods. It was shown that the hydrate water and CA are the crucial factors in influencing the formation of mesoporous Cr2O3 nanocrystals in the mixture system. The decomposition of CA results in the formation of a mesoporous structure with wormlike pores. The hydrate water of the mixture provides surface hydroxyls that act as binders, making the nanocrystals aggregate. The pore structures and phases of chromium oxide are affected by the ratio of precursor-to-CA, thermal temperature, and time.