James Highfield

Ultrahigh sensitivity of Au/1D α-Fe2O3 to acetone and the sensing mechanism.

Hematite (α-Fe(2)O(3)) is a nontoxic, stable, versatile material that is widely used in catalysis and sensors. Its functionality in sensing organic molecules such as acetone is of great interest because it can result in potential medical applications. In this report, microwave irradiation is applied in the preparation of one-dimensional (1D) α-FeOOH, thereby simplifying our previous hydrothermal method and reducing the reaction time to just a few minutes. Upon calcination, the sample was converted to porous α-Fe(2)O(3) nanorods, which were then decorated homogeneously by fine Au particles, yielding Au/1D α-Fe(2)O(3) at nominally 3 wt % Au. After calcination, the sample was tested as a potential sensor for acetone in the parts per million range and compared to a similarly loaded Pt sample and the pure 1D α-Fe(2)O(3) support. Gold addition results in a much enhanced response whereas Pt confers little or no improvement. From tests on acetone in the 1-100 ppm range in humid air, Au/1D α-Fe(2)O(3) has a fast response, short recovery time, and an almost linear response to the acetone concentration. The optimum working temperature was found to be 270 °C, which was judged to be a compromise between the thermal activation of lattice oxygen in hematite and the propensity for acetone adsorption. The surface reaction was investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and a possible sensing mechanism is proposed. The presence of Au nanoparticles is believed to promote the dissociation of molecular oxygen better in replenishing O vacancies, thereby increasing the instantaneous supply of lattice oxygen to the oxidation of acetone (to H(2)O and CO(2)), which proceeds through an adsorbed acetate intermediate. This work contributes to the development of next-generation sensors, which offer ultrahigh detection capabilities for organic molecules.

Kinetics studies on wet and dry gas–solid carbonation of MgO and Mg(OH)2 for CO2 sequestration

Mineral carbonation is a carbon dioxide capture and storage (CCS) route that warrants further investigation. Although most of the CCS research to date has been concerned with underground storage in liquefied form, mineral carbonation is the only method that disposes CO2 in a permanent and inherently safe manner. Here, we consider the gas–solid conversion of both MgO and Mg(OH)2 with CO2 in the presence and absence of steam in an attempt to model and predict the optimum conditions for rapid and complete carbonation. Results from pressurised thermogravimetric analysers (PTGA) and a laboratory scale pressurised fluidised bed (PFB) are presented. The results show that the carbonation of Mg(OH)2 is much faster (∼50% in 4 min) in a PFB than the carbonation of comparatively fine MgO ( 10%) accelerates the carbonation considerably. However, in the case of Mg(OH)2, the addition of steam to the CO2 is less important as it is provided intrinsically, as a result of the dehydroxylation of Mg(OH)2 at elevated temperatures. Still, humidifying the gas stream can help control dehydroxylation, thereby sustaining carbonation, which typically levels out short of completion. A careful control of the carbonation conditions (temperature, pressure, fluidising velocity, gas composition) and particle properties should allow for close to complete carbonation (>90%) without compromising the carbonation kinetics. Because the PFB carbonation step considered here is part of a larger CCS process (Mg extraction from a natural and abundant mineral followed by production of MgCO3), the precipitation stage [Mg(OH)2 formation] may be tailored to obtain the necessary particle properties (surface area, porosity).

Insights into the Oxidation and Decomposition of CO on Au/α-Fe2O3 and on α-Fe2O3 by Coupled TG-FTIR

CO oxidation and decomposition behaviors over nanosized 3% Au/alpha-Fe2O3 catalyst and over the alpha-Fe2O3 support were studied in situ via thermogravimetry coupled to on-line FTIR spectroscopy (TG-FTIR), which was used to obtain temperature-programmed reduction (TPR) curves and evolved gas analysis. The catalyst was prepared by a sonication-assisted Au colloid based method and had a Au particle size in the range of 2-5 nm. Carburization studies of H 2-prereduced samples were also made in CO gas. According to gravimetry, for the 3% Au/alpha-Fe2O3 catalyst, there were three distinct stages of CO interaction with the Au catalyst but only two stages for the catalyst support. At low temperatures (<or=100 degrees C), only the Au catalyst had a rapid weight loss, which confirmed that CO reacted with highly active absorbed oxygen species and/or OH species which were associated with and promoted by the Au nanoparticles. Around 300 degrees C, both the catalyst and support samples experienced the reduction of Fe2O3 to Fe3O4, while above 400 degrees C further reduction to FeO and Fe metal took place. Au played no part in the kinetics of Fe3O4 formation because lattice O mobility was rate-limiting. At higher temperature where Fe3O4 was further reduced to FeO and Fe 0, the initially formed metallic Fe 0 nuclei could decompose CO molecules and release O species. Both this coproduced O species and the lattice oxygen could react with CO molecules. Thus, the CO oxidation was not limited by the mobility of lattice oxygen, and the catalytic function of Au was revealed again. Carburization of metallic Fe, created by prereduction in H 2, revealed a distinct weight gain at 350 degrees C corresponding to Fe 3C formation, as subsequently confirmed by X-ray diffraction (XRD). Sustained carbon deposition ensued at 450 degrees C. In the cases of the 3% Au/gamma-Al 2O 3 and Au/ZrO 2 catalysts prepared by the same method, however, after exposure to CO in the same temperature range, no carbon deposit was observed, indicating that although Au nanoparticles could activate the absorbed oxygen molecules at low temperatures, they were not able to activate the lattice oxygen in the three catalyst supports or to dissociate the CO molecules directly.

Activation of serpentine for CO2 mineralization by flux extraction of soluble magnesium salts using ammonium sulfate

This paper concerns the growing role of cheap and potentially recyclable ammonium salts in CO2 mineralization. The powerful hyphenated technique TG-FTIR, along with XRD and ICP-AES, were used to shed light on the underlying chemistry and measure the efficiency of magnesium ion extraction from a Finnish serpentinite in contact with molten ammonium sulfate above 300 °C. From micro- and gram-scale tests, flux extraction as epsomite [MgSO4·7H2O] proceeds via the intermediacy of Tutton salts, NH4/Mg double sulfates increasingly rich in Mg. Extraction is effected through the agency of acidic derivatives, principally ammonium bisulfate and sulfamic acid, which are created sequentially from ammonium sulfate in situ. However, sulfamic acid volatilizes and/or decomposes at a significant rate by 400 °C. This loss mechanism is primarily responsible for the modest recovery of Mg (50–60%). An improved process would operate below 400 °C where Mg extraction as efremovite [(NH4)2Mg2(SO4)3] is effective. Future experiments evaluating the use of humid air to stabilize the bisulfate and impede the loss of flux are recommended.

Assembly of Au colloids into linear and spherical aggregates and effect of ultrasound irradiation on structure

In this work, we aimed to tune the aggregation state of Au colloids by judicious regulation of their surface charges and by introduction of ultrasound irradiation. Au colloids were prepared by reduction of HAuCl4 with NaBH4 in the presence of 3-mercaptopropionic acid (MPA) as the stabilizing ligand. The Au colloids capped with MPA molecules possessing a single –SH terminal group, can form either linear or spherical Au colloid aggregates depending on the MPA ∶ Au ratio, solution pH and Au colloid concentration. Adjustment of these parameters provides a convenient means of regulating surface charge on the Au colloid surface and, thus, the state and degree of aggregation. The spherical aggregates appeared to be thermodynamically stable, and ultrasound irradiation could promote their formation. Intense ultrasound irradiation resulted in the formation of well-crystallized nanospheres whilst mild irradiation tended to produce a more amorphous product. This is the first observation of a sonication effect on the crystallization state in freely suspended Au colloids in aqueous solution.

Mechanisms of serpentine–ammonium sulfate reactions: towards higher efficiencies in flux recovery and Mg extraction for CO2 mineral sequestration

There is a growing research interest in CO2 mineral sequestration methods that follow an intermediate Mg extraction step (from Mg-silicates, especially serpentinite rock) by fluxing with ammonium sulfate (AS) or ammonium bisulfate (ABS). This study reports the use of thermogravimetry (TG) combined with differential scanning calorimetry (DSC), mass spectrometry (MS) and/or Fourier-transform infrared spectrometry (FTIR), to explore the serpentinite/flux [(S)/AS and S/ABS] reaction chemistry in more detail and identify conditions under which flux losses are restricted. TG-DSC-MS results show that AS decomposition proceeds through a series of reactions leading to the formation of ammonium pyrosulfate [(NH4)2S2O7, APS] via an ABS intermediate. That APS is the key intermediate is attested by the fact that the analogous potassium salt is a well-known flux for metal oxides. As expected the mechanisms for S/AS reaction are more complex than those of thermal decomposition of pure AS or ABS compounds. Two likely possibilities were identified with S/AS thermolysis: formation of APS or sulfamic acid (SA) precursors that extract Mg/Fe cations from serpentinite above 400 °C. A sulfur dioxide peak was detected on the ensemble spectra at 280 °C. This indicates a loss of ABS through sublimation rather than a complete degradation of AS or ABS reagents. At a fast heating rate of 40 K min−1, tests on S/AS resulted in a significantly lower weight loss (ΔW) than at 10 K min−1 (46% vs. 54%), implying better retention of flux and higher extraction efficiency. From TG-FTIR tests, the presence of humidity has a suppressive effect on SA volatilization, stabilizing the hydrated intermediate APS and/or ABS. It also inhibits mineral transformation to the less reactive forsterite (Mg2SiO4). Extraction of magnesium is primarily dependent on serpentine particle size, but it can be increased significantly in the presence of humidity.

Mechanochemical processing of serpentine with ammonium salts under ambient conditions for CO2 mineralization

This paper assesses the suitability of mechanochemistry as a convenient low-energy processing option in CO2 mineralization. Whereas some success has been reported in milling alkaline earth-containing minerals under gaseous CO2, this work focuses instead on a purely solid-state approach towards two key objectives: (a) Mg extraction from serpentine using ammonium bisulfate; and (b) direct or indirect CO2 sequestration using ammonium bicarbonate in a natural extension of its role as “CO2 carrier” in the chilled ammonia scrubbing process. In Mg extraction work, dry milling of serpentine with ammonium bisulfate gave respectable yields (>60% Mg) as boussingaultite [(NH4)2Mg(SO4)2·6H2O] in 2 to 4 h. In CO2 sequestration, dry milling anhydrous magnesium sulfate with ammonium bicarbonate yielded only mixed sulfate products. Carbonation of the heptahydrate, epsomite, was found to proceed via ammonium magnesium carbonate hydrate [(NH4)2Mg(CO3)2·4H2O], which dissolves incongruently to yield nesquehonite [MgCO3·3H2O]. The modest conversion (∼30%) is probably due to equipartition of Mg into the double sulfate co-product. A similar route is followed in magnesia and brucite, in which the existence of an amorphous native carbonate precursor to nesquehonite in the same molar ratio (Mg : CO2 = 1) was inferred from inconsistency in the XRD intensities. This was largely responsible for the high carbonation yields in the unwashed products, ∼70% and ∼85% in MgO and Mg(OH)2, respectively, as confirmed by TG-FTIR. The same intermediate is probably formed in serpentine, but it is apparently soluble in the aqueous mineral environment. When the unwashed product is subjected to mild thermal consolidation, stable hydromagnesite [Mg5(CO3)4(OH)2·4H2O] is formed in ∼20% yield after milling for 16 h. Possible identities for the amorphous precursor are briefly considered.

Low-temperature gas–solid carbonation of magnesia and magnesium hydroxide promoted by non-immersive contact with water

From gas–solid carbonation studies and product characterization by XRD, carbon elemental analysis, and TG-FTIR profiling of evolved CO2, the presence of water vapour at high relative humidity (>25% RH) was shown to cause a drastic acceleration in the rate of CO2 absorption into MgO and Mg(OH)2 producing magnesite and hydrocarbonated precursors. From thermogravimetric experiments in the vicinity of the dew point, carbonation was shown to proceeded in steps triggered by spontaneous condensation (and re-evaporation) of water. Almost complete conversion to magnesite (MgCO3) and/or hydromagnesite [HM = 4MgCO3·Mg(OH)2·4H2O] was observed in a few hours under rather mild conditions of pressure (PCO2 ≤ 10 bar) and temperature (T ≤ 150 °C). Unwashed (sulphate-contaminated) Mg(OH)2 extracted from serpentinite mineral via the Abo Akademi sulphation/precipitation route yielded MgCO3 selectively. Washed extracts and commercial samples formed mainly HM. Carbonation was even measurable below 50 °C using sub-atmospheric pressures of CO2 typical of flue gas. The rate was quite insensitive to PCO2. Complete conversion to nesquehonite [MgCO3·3H2O] and/or dypingite [4MgCO3·Mg(OH)2·5H2O] was achieved by overnight treatment. High-pressure scale-up tests (1–35 g) on commercial Mg(OH)2 (Aldrich, >95%) from 70–150 °C under equilibrated batch conditions at 100% RH yielded HM in the 1st hour. At longer times, HM seemed to transform spontaneously into MgCO3. Judiciously pre-dampened samples were more reactive, re-affirming the importance of ambient water. Water as a promoter in the more speculative direct gas–solid mineral carbonation is briefly evaluated.