Albert Sacco

The initiation and growth of filamentous carbon from α-iron in H2, CH4, H2O, CO2, and CO gas mixtures

The initiation and growth mechanisms of filamentous carbon over iron foils were studied at 900 K and 1 bar pressure. Various gas mixtures of CO, CO2, CH4, H2, and H2O were used to fix the solid phase compositions based on nonequilibrium phase diagrams. Solid phase compositions were verified using X-ray and electron diffraction. Gravimetric analysis indicated that in dry gas mixtures the initial rate of fractional weight gain was a direct function of the PCO PH2 product; for water containing experiments it was related to the PH2OP H2PCO ratio. X-Ray diffraction analysis of the solid suggested that the maximum rate of fractional weight gain coincided with complete carbiding of the “surface” layers. Examination of the foils in an electron microscope indicated the surface breaks up into a nodular morphology, and these nodules are comprised of filamentous carbon. An initiation mechanism is proposed which assumes that Fe3C acts to increase over all surface area through surface breakup and also acts as a catalyst for carbon deposition and subsequent filament growth.

The effects of the silica source on the crystallization of zeolite NaX

Abstract The effects of varying the starting silica source on the synthesis of molecular sieve zeolite NaX were investigated, while all other reaction parameters were kept constant. The silica sources were all powders of varying types and purity. Characterizations of the silica sources, the silicate solutions, and the synthesis products were completed by powder X-ray diffraction, electron microscopy, particle-size analysis, n.m.r, spectroscopy, laser light scattering, and elemental analysis. The use of different silica sources significantly influenced the outcome of the synthesis experiments. There were large differ- ences in the results from the various batch synthesis mixtures, even though the batch compositions were all the same (neglecting impurities). The experiments, all conducted at 115°C in sealed 6 ml autoclaves, yielded products of different particle sizes, different amounts of impurity zeolite phases, and different conversion rates to zeolite NaX. The n.m.r. spectra of the dissolved silica sources were all effectively the same; however, the levels of trace impurities were very different. The extent to which nuclei formed in these experiments was shown to correlate to the impurity level of any one of several elements in the silica sources, but not to the turbidity of the filtered solutions as noted by a simple light-scattering measurement.

The role of triethanolamine in zeolite crystallization

Abstract The role of triethanolamine (TEA) in zeolite NaA synthesis systems was investigated. Particle-size analyses on products formed in the presence of the amine revealed that TEA affected the nucleation and crystallization rates to some extent. Crystallization studies carried out in reduced aluminum and aluminum-free systems suggested an aluminum complexing role for TEA. The existence of a complex was confirmed by solution-phase 13C n.m.r. spectroscopy. Synthesis studies using close analogs of TEA revealed that all three hydroxyl groups as well as the amine group in TEA were necessary for successful complexation and nucleation suppression. Differential scanning calorimetry carried out on products crystallized in the presence of the amine showed that TEA did not act as a void filler during zeolite NaA crystallization.

Large zeolite crystals: their potential growth in space

Abstract The possibility of synthesizing molecular sieve zeolites in space permits one to take advantage of a micro-gravity environment. Particle settling and thermal convective currents can be avoided in space. Data from ground-based experiments are presented which suggest that large zeolite crystals can be produced in high yield in the microgravity environment of space. These experiments used triethanolamine (TEA) to suspend zeolite NaA nuclei in solution. This resulted in hindered settling and reduced the shear normally associated with settling. The average and maximum crystal sizes from these experiments are shown to be 50–100% larger than identical experiments with no TEA. Analysis of these data illustrate that there is an optimum TEA Al 2 O 3 ratio.

Carbon deposition and filament growth on Fe, Co, and Ni foils using CH4H2H2OCOCO2 gas mixtures

Non-equilibrium phase diagrams have been used to systematically investigate carbon deposition and filament growth on Fe, Ni, and Co foils. All three metals were observed to have similar slow rates of carbon deposition in the metal phase field. The carbon formed was primarily amorphous and platelet in type and morphology. In regions where carbides where thermodynamically favored to form, carbon deposition rates were orders of magnitude higher for Fe and Co foils. In the carbide phase field, the rate of fractional weight gain was highest at the highest carbide decomposition temperature and became progressively lower as the carbide decomposition temperature dropped. The order was Fe {much gt} Co {much gt} Ni. Carbon deposition in the carbide region for Fe and Co produced filaments and amorphous carbon and showed little or no site preference. Carbon deposition on Ni was plane specific and in the carbide region was primarily filamentous in morphology. The primary source of carbon on Ni is CH{sub 4}, while on Fe and Co it appears to be CO. The role of carbides on these foils is hypothesized to be to increase surface area through surface break-up, and to help set up the mass flux gradient for filament growth.

Carbon deposition and filament growth on Fe, Co, and Ni foils using CH sub 4 -H sub 2 -H sub 2 O-CO-CO sub 2 gas mixtures

Non-equilibrium phase diagrams have been used to systematically investigate carbon deposition and filament growth on Fe, Ni, and Co foils. All three metals were observed to have similar slow rates of carbon deposition in the metal phase field. The carbon formed was primarily amorphous and platelet in type and morphology. In regions where carbides where thermodynamically favored to form, carbon deposition rates were orders of magnitude higher for Fe and Co foils. In the carbide phase field, the rate of fractional weight gain was highest at the highest carbide decomposition temperature and became progressively lower as the carbide decomposition temperature dropped. The order was Fe {much gt} Co {much gt} Ni. Carbon deposition in the carbide region for Fe and Co produced filaments and amorphous carbon and showed little or no site preference. Carbon deposition on Ni was plane specific and in the carbide region was primarily filamentous in morphology. The primary source of carbon on Ni is CH{sub 4}, while on Fe and Co it appears to be CO. The role of carbides on these foils is hypothesized to be to increase surface area through surface break-up, and to help set up the mass flux gradient for filament growth.

Direct synthesis of zeolite Y with large particle size

Abstract Large zeolite Y particles have been synthesized directly from gels (4.76Na2O: 1.0Al2O3: xSiO2: 454H2O: 5TEA, x=5.25–8.75) aged at ∼293 K before static heating at 368 K. Product purity, size, and Si/Al ratio of zeolite Y depended on the gel composition and aging time. Two-day aging of gels (x=5.25 and 7.0) resulted in nearly pure zeolite Y (

The role of the dissolution of silicic acid powders in aluminosilicate synthesis mixtures in the crystallization of large mordenite crystals

The role of the dissolution of silicic acid powders in alkaline aluminosilicate synthesis mixtures in the crystallization of mordenite was investigated. Three different as-received lots of silicic acid (Aldrich lots 01807PW (lot A), 04720KX (lot B), and 18913LY (lot C)) were used to synthesize mordenite from the same initial batch composition (4.32 Na 2 O: Al 2 O 3 : 19 SiO 2 : 293.6 H 2 O). Different overall crystallization times (lot A ≈ 10 h, lots B and C ≈ 28 h), conversion rates at the 50% point (lot A ≈ 44% h 1 lots B and C ≈ 12% h 1 ) and maximum crystal sizes (lot A= 10–15 μn, lot B = 35–40 μm, lot C = 30–35 μm) were observed. The prismatic crystal morphologies were the same in all cases. Heat treatment of as-received lots A and B in air at 550°C before syntheses resulted in about 6- and 2-fold decreases of mordenite conversion rates, respectively; and in about 6- and 2-fold increases of mordenite crystal sizes, respectively. Heat treatment of asreceived lot C in air at 850°C resulted in about a 6-fold decrease of mordenite conversion rate and in about a 6-fold increase of mordenite crystal size. Induction periods and overall crystallization times also increased in these cases. Large crystals of mordenite with sizes of up to 250 μm were obtained using lot C heat treated at 850°C. These results reflect different silica dissolution rates that were correlated with the variation of specific surface area and average pore diameter of silicic acid powders. The dissolution of silicic acid in the investigated crystallization of mordenite intervened in the rate-determining step for nucleation and growth of mordenite crystals.

Hydrogen reduction mechanisms of ilmenite between 823 and 1353 K

In situ gravimetric measurements and microscopic examinations were used to determine the mechanisms of oxygen removal from synthetic ilmenite disks between 823 and 1353 K. Under a hydrogen atmosphere, iron was observed to form a layer of low porosity on the surface of samples early in the reduction. This created diffusion limitations for hydrogen to the reaction front and for the escape of water vapor. A shrinking core reduction model, modified to include the growth of this iron film, was capable of predicting the conversion-time relationships of ilmenite samples. An activation energy of 43.2 +/- 2.6 kcal/gmole was determined to be representative of reaction control over the temperature range 823-1023 K.

The role of an aluminum-tertiary

The role of 2, 2′, 2″-nitrilotriethanol (triethanolamine, TEA) as a solution additive in the synthesis of large crystal zeolite NaA has been determined by a combination of 27Al n.m.r. and zeolite syntheses using related compounds as solution additives. TEA was found to chelate aluminum, with its hydroxyl groups being directly involved in bonding to the metal ion. In addition, it was found that four additional tertiary alkanolamines to TEA were effective as solution additives in allowing the synthesis of large crystal zeolite NaA.