Gary W. Simmons

Catalytic synthesis of methanol from COH2: I. Phase composition, electronic properties, and activities of the Cu/ZnO/M2O3 catalysts

The low pressure methanol synthesis catalysts CuZnO, Cu/ZnO/Al2O3, and Cu/ZnO/Cr2O3 were found to contain a new compound identified as a CuI solution in ZnO, which is also an active component of the above catalysts. Combined X-ray diffraction, optical, and XPS—Auger studies are presented that describe the formation, electronic structure, and surface composition of these catalysts. In particular, the surface of the working catalyst is free of carbon, both in the presence and absence of CO2 in the feed gas. A synthesis mechanism is proposed whereby the CuI centers nondissociatively chemisorb and activate carbon monoxide and the ZnO surface activates hydrogen. Catalyst deactivation in COH2 mixture is explained as the reduction of CuI to inactive copper metal, while the rate enhancing effect of O2, H2O, and CO2 is due to the maintenance of an oxidation potential high enough to keep the copper in the active CuI state. No special pore distribution or presence of crystalline phases such as spinels is necessary for selectivity of the CuZnO catalyst to the formation of methanol.

Catalytic synthesis of methanol from COH2: IV. The effects of carbon dioxide

The effects of carbon dioxide on the catalytic synthesis of methanol over the copper-zinc oxide catalysts were investigated for CO2/CO/H2 ratios between 0/30/70 and 30/0/70. A maximum synthesis rate was observed-at CO2/CO/H2 = 2/28/70. At lower concentrations of CO2 the catalyst is deactivated by overreduction and at higher concentrations of CO2 the synthesis is retarded by a strong adsorption of this gas. A kinetic model is presented which quantitatively describes the observed patterns in the indicated range of synthesis gas compositions and at temperatures between 225 and 250 °C. This model is consistent with all physical characteristics of the CuZnO catalysts and corroborates earlier findings that an intermediate oxidation state of the catalyst is its active state. The adsorption enthalpies and entropies for the reactants indicate that carbon dioxide is strongly bound and immobile while carbon monoxide and hydrogen are chemisorbed with intermediate strength and experience a considerable mobility in the adsorbed layer. At concentrations of CO2 greater than 10%, methane is a side product. Mechanistic implications of this finding are that there is a nonselective pathway parallel to the CO hydrogenation; this pathway may involve formate and methoxy intermediates.

A LEED-AES study of the initial stages of oxidation of Fe (001)

Abstract LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and oxygen at room temperature. The AES oxygen signal was quantified by using a two-dimensional oxide layer as a calibration point. This reproducible oxide layer was prepared by the high temperature reaction of H2O at 10−6 torr with Fe(001). The initial oxygen sticking coefficient was observed to be close to unity, which suggests that the chemisorption is non-activated and involves a mobile adsorption step. The rate of chemisorption decreased as (1-Θ) and exhibited a minimum at Θ = 0.5. LEED data indicate that the minimum value of the sticking coefficient corresponded to the completion of a c (2 × 2) surface structure. Upon additional exposure to oxygen, an increase in the sticking coefficient was observed in conjunction with the disappearance of the c (2 × 2) and a gradual fade out of all diffraction features. After mild heating, epitaxial FeO (001) and FeO (111) structures were observed. The simultaneous appearance of a shifted M2,3M4,5M4,5 iron Auger transition with the increase in the sticking coefficient and the disappearance of the c (2 × 2) indicated that oxide nucleated on the surface after the complete formation of the c (2 × 2) structure. The relatively high sticking coefficient during the initial oxidation indicates that formation of a mobile adsorbed oxygen state precedes the formation of oxide.

Mössbauer spectroscopic study of the formation of Fe(III) oxyhydroxides and oxides by hydrolysis of aqueous Fe(III) salt solutions

Abstract Mossbauer spectroscopy has been used to investigate the precipitates formed by hydrolysis of 0.1 M solutions of Fe(NO 3 ) 3 , FeCl 3 , Fe 2 SO 4 ) 3 , and NH 4 Fe(SO) 2 at 90°C. The isomer shifts, electric quadrupole splittings, and nuclear magnetic splittings were used for the qualitative and quantitative identification of the hydrolysis products. Proposals were made concerning the mechanism of formation of the oxides and hydroxyoxides of iron. Hydrolysis in the nitrate and chloride solutions proceeds by the formation of monomers and dimers of iron III) ions, followed by the formation of polymeric species. The polymers formed in the nitrate solution are not presumed to include the nitrate ion in the polymer chain, whereas the polymers formed in the chloride solution contain some chloride ions in place of the hydroxyl ion. The next step in the precipitation process is the formation of oxybridges and the development of α-FeOOH and β-FeOOH structures. This step is followed by loss of water and internal crystallization of α-FeOOH to α-Fe 2 O 3 in nitrate solution or by dissolution of β-FeOOH and growth of α-FeOOH in chloride solution. In sulfate solutions the formation of an FeSO 4 + complex suppresses the polymerization process and the formation of oxyhydroxides and oxides. Basic Fe(III) sulfates are formed instead.

A study of the initial reaction of water vapor with Fe(001) surface

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and water vapor at temperatures from 298 to 473 K. A disordered c(2 × 2) structure was formed at all temperatures, and only 80% of the total number of sites were filled at saturation. The initial sticking coefficient was 0.56 ± 0.03, and the reaction rate increased with increasing temperature. A model was proposed that successfully accounted for these experimental observations. Irreversible chemisorption of water is proposed to take place via a precursor of physically adsorbed water molecules. The precursor, which is adsorbed on both bare surface and surface covered by chemisorbed species, is mobile and retains most of its degrees of rotational freedom. Water molecules in the precursor state can either desorb or dissociate, and the difference in activation energies for these reactions was found to be 5.7 ± 0.5 kcalmol. Only 80% of the available c(2 × 2) sites are filled and the surface layer is disordered because the chemisorbed species are immobile, and because each one blocks four nearest neighbor sites for further adsorption. The chemisorbed species occupy the fourfold symmetric sites either above the iron atoms or above the interstitial “holes” betweeh iron atoms.

Fracture Mechanics and Surface Chemistry Studies of Fatigue Crack Growth in an Aluminum Alloy.

Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851 aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs through a “hydrogen embrittle ment” mechanism.

Precursors of the copper-zinc oxide methanol synthesis catalysts

Abstract The coprecipitated hydroxycarbonate precursor of the methanol synthesis and shift reaction catalyst based on 30 at.% copper and 70 at.% zinc oxide, which was previously reported to be a mixture of hydrozincite Zn5(CO3)2(OH)6 and rosasite (Cu,Zn)2(CO3)(OH)2 (R. G. Herman, K. Klier, G. W. Simmons, B. P. Finn, J. B. Bulko, and T. P. Kobylinski, J. Catal. 56, 407, 1979) or a single-phase hydrozincite (G. Petrini, F. Montino, A. Bossi, and G. Gaybassi, in “Studies in Surface Science and Catalysis. Preparation of Catalysis III” (G. Poncelet, P. Grange, and P. A. Jacobs, Eds.), Vol. 16, p. 735. Elsevier, The Netherlands, 1983), is herein shown to be a single-phase aurichalcite (Cu0.3Zn0.7)5(CO3)2(OH)6. The orthorhombic B2212 aurichalcite is crystallograpically distinct from the monoclinic C 2 m hydrozincite, although these two compounds have the same ratio of metal ions to carbonate and hydroxyl anions. Both aurichalcite and hydrozincite are chemically and structurally distinct from the monoclinic P2 1 a rosasite. The earlier erroneous assignments are attributed to the structural similarity of the three hydroxycarbonates in question. An energy-dispersive characteristic X-ray emission analysis of individual particles in the scanning transmission electron microscope reveals a uniform distribution of copper and zinc at the analytical concentration Cu Zn = 30 70 . Precursors with less than 30% copper consist of mixtures of aurichalcite and hydrozincite.

High temperature oxidation of Nb, NbC and Ni3Nb and oxygen enhanced crack growth

Previous studies have shown that oxygen increased the rates of crack growth in Ni base superalloys at high temperatures (> 800 K) by up to four orders of magnitude over those observed in an inert environment for alloys such as inconel 718, Rene-95 and x 750. In this study, high temperature oxidation studies of pure Nb, NbC and Ni{sub 3}Nb using XPS were undertaken (a) to provide direct confirmation of the oxidation of NbC, (b) to explore the possible role of Ni{sub 3}Nb and (c) to assess the relative reactivity of the latter two phases, along with Nb, with oxygen. Nb, NbC and Ni{sub 3}Nb were oxidized as a function of temperature and constant oxygen exposure, and the results are presented in this communication.

Mechanism for oxygen enhanced crack growth in inconel 718

Department of Mechanical Engineering and Mechanics, Zettlemoyer Center forSurface Studies, Lehigh University, Bethlehem, PA 18015, USA(Received September 25, 2000)(Accepted in revised form January 11, 2001)Keywords: Oxygen enhanced crack growth; X-ray photoelectron spectroscopy (XPS); Niobium;Crack growth mechanism; Ni base alloyIntroductionNickel based superalloys are used extensively in high temperature applications, for example, turbinedisks in jet engines. Since turbine discs operate in air at 700 to 973 K, these alloys have receivedconsiderable attention with respect to their sensitivity to environmentally enhanced crack growth(EECG). Previous studies have shown that oxygen increased the rates of crack growth in niobiumcontaining alloys (such as Inconel 718, Rene-95 and3750) at high temperatures (.800K) by up to 4.5orders of magnitude over those observed in an inert environment (1–7). Several mechanisms have beensuggested for this crack growth enhancement which include: (a) the oxidation of metallic carbides orcarbon at the grain boundaries to form high pressures of CO and CO

Characterization of cu/ZnO methanol synthesis catalysts by analytical electron microscopy

Abstract The structure, morphology and composition have been determined for 30/70 and 67/33 Cu/ZnO catalysts to establish whether activity for methanol synthesis could be promoted by the presence of copper in the zinc oxide phase. Analysis of pure zinc oxide for copper in CuO/ZnO physical mixtures demonstrated that unwanted copper X-ray signals did not interfere with the analysis of ZnO in the catalyst specimens as long as adjacent copper particles were at a distance of more than 1.5-times the electron probe size used in the analysis. The morphology of the 67/33 catalyst permitted maintenance of this ‘safe distance’, and the results of the analysis indicated that the zinc oxide phase contained an average copper concentration of approximately 2 to 3 at.% for the calcined, reduced and tested states of the catalyst. Because of the high degree of interdispersion of phases, ‘safe distances’ were more difficult to achieve in the analysis of the 30/70 catalyst. Analysis of thin areas of the zinc oxide with low density of copper particles indicated an average copper concentration of 15 at.% in the reduced 30/70 catalyst which could not be accounted for on the basis of the observed number of copper particles in the analyzed area. This result suggests that up to about one-half of the copper in the 30/70 catalyst is unresolved by electron microscopy.