Approximate Model for Estimating the Potential Reduction of Malnutrition and Mortality by Various Interventions K. SARRIS, K. SARRIS Massachusetts Institute of TechnologyCambridge, Mass. 02139, U.S.A. Search for other works by this author on: Oxford Academic PubMed Google Scholar R. E. STICKNEY R. E. STICKNEY Massachusetts Institute of TechnologyCambridge, Mass. 02139, U.S.A. Search for other works by this author on: Oxford Academic PubMed Google Scholar Journal of Tropical Pediatrics, Volume 21, Issue 6, December 1975, Pages 334–344, https://doi.org/10.1093/tropej/21.6.334 Published: 01 December 1975

The apparent equilibrium relationship between adsorption and desorption distributions measured in recent molecular beam experiments is discussed. Through an equilibrium synthesis and the application of detailed balancing, the energy and angle dependence of the dissociative adsorption probability of hydrogen on copper is shown to predict the non-cosine angular distributions of desorption. In a similar construction, the velocity distribution of H2 desorbing from nickel predicts the peaked desorption angle distributions observed. The implications of the apparent local equilibrium and detailed balancing at low pressure on the mechanism of surface catalyzed isotopic exchange and on the dynamics of chemical processes on surfaces are discussed.

The angular distributions of hydrogen molecules desorbed from copper have been measured for single-crystal surfaces of three different orientations: (100), (110), and (111). The surfaces were cleaned to the degree that the impurities were below the level of detection by Auger electron spectroscopy. The angular distributions were all significantly more peaked at the surface normal than the distribution corresponding to diffuse emission, i.e., where the molecular flux is proportional to cos θ, where θ is the angle measured from the surface normal. We have characterized the observed angular distributions by fitting the emperical expression cosdθ to the data, where d is an adjustable parameter. The results are: d ≈ 5 for (100); d ≈ 2.5 for (110); d ≈ 6 for (111). The distributions are nearly symmetric about the azimuthal angle between the plane of detection and the principal axes of the surface lattice. Possible models are considered.

Molecular beam techniques are employed to study the adsorption and desorption of H2 on the (100), (110), and stepped (310) crystal faces of copper. Each crystal is exposed simultaneously to a supersonic molecular beam of H2 (energy variable from 1.6 to 10.7 kcal/mole) and a highly dissociated beam of deuterium. The majority of the H2 molecules are scattered from the surface (i.e., are not adsorbed), while a portion of the remaining molecules adsorb dissociatively and react catalytically with adsorbed deuterium atoms to form HD molecules. These HD molecules desorb, and their angular distribution is measured by a rotatable mass spectrometer. For all three crystal faces, the distributions of desorbed HD deviate significantly from diffuse emission and are in excellent agreement with the results of our previous permeation study. From the dependence of the HD signal on the energy and incident angle of the H2 beam, it appears that there are substantial energy barriers to adsorption, with these barriers depending on crystallographic orientation and acting essentially perpendicular to the surfaces. Both the energy dependence of the dissociative adsorption probability and the shapes of the HD angular distributions are nearly identical for the stepped (310) and (100) surfaces, thereby suggesting that ledge sites are not the principal regions responsible for adsorption of hydrogen on copper. The estimated adsorption probabilities versus energy are “S” shaped curves which appear to level off at values considerably less than unity. A comparison of our results with a very simple model with a single energy barrier to adsorption is qualitatively but not quantitatively satisfactory. An interpretation which includes a distribution of energy barriers is suggested.

Auger electron spectroscopy (AES) has been employed to determine the relative coverage of oxygen on polycrystalline tungsten at high temperatures (1200 ⩽T ⩽ 2500 K) and low O2 pressures (5 × 10−9 ⩽po2 ⩽5 × 10−6 Torr). We believe that this is the first demonstration that chemical analysis of solid surfaces by AES is possible even at temperatures as high as 2500 K. It is assumed that the relative oxygen coverage is directly proportional to the peak-to-peak amplitude of the first derivative of the 509 eV oxygen Auger peak. The experimental results illustrate the dependence of coverage on temperature and pressure, and it is shown that the results for low coverages may be described reasonably well by a simple first-order desorption model plus a semi-empirical expression for the equilibration probability (or sticking coefficient). On the basis of this approximate model, the binding energy of oxygen on tungsten is estimated as a function of coverage, giving a value of ∼ 140 kcalmole in the limit of zero coverage.

The spatial distributions of H2 molecules desorbed from polycrystalline Fe, Pt, Cu, Nb and stainless steel (type 304) surfaces were measured with a rotatable ionization gauge. Prior to cleaning the surfaces by ion bombardment or by oxidation, the AES (Auger electron spectroscopy) spectra indicated that all of the sample surfaces were highly contaminated. (S, C, and P were the predominant impurities.) The spatial distributions measured under these conditions were considerably narrower than the commonly-assumed diffuse distribution; i.e., the concentration of desorbed molecules was higher in the vicinity of the surface normal than for the case of diffuse emission represented by cos θ, where θ is the angle of inspection measured from the surface normal. These measured distributions may be described approximately by the form cosdθ, where d ranges from ∼ 2.5 for Nb to ∼ 7 for Fe. However, as the surfaces were cleaned, the spatial distributions for Fe, Pt, and Nb tended to become diffuse (i.e.,d → 1), which is consistent with the trend we observed previously for polycrystalline and single-crystal Ni. Copper proved to be an exception to this trend, since its distribution was far from being diffuse (i.e., d≅ 4) when the surface appeared to be clean according to our AES measurements. We suggest that this unusual characteristic of Cu is a consequence of an activation energy barrier associated with the dissociative adsorption of hydrogen on Cu.

The quasiequilibrium treatment of gas-solid reactions (J. C. Batty and R. E. Stickney, J. Chem. Phys. 51, 4475 (1969)] is employed to develop a semiquantitative description of existing experimental data on the “pumping” of O2 by a hot tungsten filament [J. H. Singleton, J. Chem. Phys. 45, 2819 (1966)]. The results indicate that the present analysis provides a useful means for predicting the dependence of the O2 “pumping” speed on the operating conditions (e.g., O2 partial pressure, filament temperature) and on system design (e.g., conductance, pumping speed, wall material). The analysis is applied to estimate the error of an O2 partial pressure measured by a partial pressure analyzer or mass spectrometer having a tungsten cathode. It is also shown that the analysis provides a semiquantitative explanation of existing experimental data on the effects of O2 pressure and filament temperature on the rate of emission of ions (impurities) from a hot tungsten filament [H. F. Winters et al., J. Appl. Phys. 34, 1810 (1963)].

The effects of surface composition and crystal orientation on the spatial distribution of H 2 molecules desorbed from Ni surfaces have been examined. In this continuation of the work of Dabiri et al., we have obtained data for both polycrystalline and singlecrystal [(111) and (110)] Ni surfaces that are contaminated to varying degrees with impurities. The surface composition was detected by Auger electron spectroscopy and varied by ion bombardment and by controlled deposition of S, C, Si and Au. The spatial distribution of the desorbed molecules was measured with a rotatable ionization gauge. The measured spatial distributions may be described approximately by the form cos d θ, where θ is the angle of inspection measured from the surface normal. Prior to cleaning the Ni surfaces by ion bombardment, the distributions are essentially the same (i.e., d ≈ 4) for the polycrystalline and single-crystal samples and for sample temperatures ranging from 925 to 1225°K. In this case, however, the Auger spectra indicate that the surfaces are unintentionally contaminated with S and, to a lesser degree, with C. After the surfaces are cleaned by ion bombardment, the distributions are essentially diffuse (i.e., d ≈ 1). Diffuse distributions are also observed when pure layers of S, C, or Si are deposited on the Ni surfaces. These results indicate that the spatial distribution of H 2 desorbed from Ni is independent of crystal orientation and temperature, but strongly dependent upon surface composition. Non-diffuse distributions (i.e., d ≠ 1) were observed only for impure surfaces (i.e., surfaces composed of more than one element). Based on these results, we suggest that the non-diffuse distributions reported by Van Willigen, Palmer et al., and Dabiri et al. correspond to impure surfaces.

The rates of desorption of Cs+ from W(100) and W(110) crystals have been measured by the modulated molecular beam method. The Cs coverage was extremely low (≲ 5 × 1011 cm−2), thereby insuring that: (a) the desorption rate of Cs atoms was negligible relative to that of Cs ions; (b) the desorption rate depended primarily on the interaction of Cs with W rather than the interaction of Cs with Cs. Assuming that the dependence of the Cs+ desorption rate on coverage n and temperature T may be represented by the form R+ = nν+exp( − χ0+kT), we determined ν+ and χ0+ from the measured rates. For W(100) in the range 900–1250 K, ν+ = (1.0 ± 0.2) × 1013 sec−1 and χ0+ = 2.05 ± 0.02 eV; for W(110) in the range 880–1050 K, ν+ = (1.7 ± 0.4) × 1012 sec−1 and χ0+ = 1.77 ± 0.02 eV. The results are compared with existing experimental data and theoretical predictions. The predictions of the image potential model appear to be consistent both with the binding energies inferred from the present results and with the dipole moments inferred from previous contact-potential measurements for Cs on W(100) and W(110) crystals.

Berichte der Bunsengesellschaft für physikalische ChemieVolume 76, Issue 8 p. 760-760 Kurzvorträge Permeation of hydrogen through Ni, Fe, Pt, Nb, Cu, and stainless steel at elevated temperatures: Surface effects† R. E. Stickney, R. E. Stickney Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this authorT. L. Bradley, T. L. Bradley Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this authorT. E. Kenney, T. E. Kenney Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this authorR. L. Levin, R. L. Levin Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this author R. E. Stickney, R. E. Stickney Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this authorT. L. Bradley, T. L. Bradley Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this authorT. E. Kenney, T. E. Kenney Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this authorR. L. Levin, R. L. Levin Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139, U.S.A.Search for more papers by this author First published: August 1972 https://doi.org/10.1002/bbpc.19720760822 † Work supported in part by the Advanced Research Projects Agency and the Joint Services Electronics Program. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References 1 A. E. Dabiri, T. J. Lee, and R. E. Stickney, Surface Sci. 26, 522 (1971). 2 T. L. Bradley, A. E. Dabiri, and R. E. Stickney, Surface Sci. (submitted). Volume76, Issue8August 1972Pages 760-760 ReferencesRelatedInformation

A quasi-equilibrium model which provides semiquantitative predictions of the oxygen reaction with refractory metals was developed at high temperature and low pressure. Extensive experimental data was obtained on adsorption and work function properties for a wide variety of adsorbates (Cs, K, Na, I, Br, Cl, and O) on several refractory metals (W, Ta, Mo, and Re). Conclusions and recommendations for research on alkali metal adsorption, oxygen adsorption, and adsorption of cesium - oxygen mixtures are included.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation R. E. Stickney, D. V. Tendulkar, S. Yamamoto; Scattering of Atoms and Molecules from Tungsten: Effect of Surface Composition and Crystal Orientation. Journal of Vacuum Science and Technology 1 March 1972; 9 (2): 819–824. https://doi.org/10.1116/1.1317793 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAVS: Science & Technology of Materials Interfaces and ProcessingJournal of Vacuum Science and Technology Search Advanced Search |Citation Search

A review of the major contributions from modern surface science to the understanding of the chemistry of heterogeneous catalysis is provided. A number of basic concepts, including the nature of surface bonds, the dynamics of surface reactions, and the identification of the intermediates and elemental steps that comprise the catalytic processes, are surveyed. The localized nature of the surface chemical bond, the drastic changes that varying surface coverages introduce on surface energetics and kinetics, and the key role that subtle energy barrier differences among competing reactions play in defining the overall selectivity of catalytic processes are particularly noted. A discussion on the implications of these microscopic concepts to catalysis is then provided. A critical analysis of the limitations encountered when extrapolating results from well-defined model systems to more realistic reaction conditions using supported catalysts and on the ways those problems have been addressed is also presented. A more comprehensive summary of the knowledge acquired to date on the surface chemistry of hydrocarbon conversion reactions on transition metals is provided. Finally, some ideas on the future directions of surface science in connection with catalysis are advanced.

The desorption of hydrogen (H2 and D2) from a polycrystalline nickel surface has been investigated by measuring the spatial and speed distributions of the desorbed molecules. The Ni specimen was constructed as a membrane with one side exposed to hydrogen at ~ 1 atm pressure and the other side exposed to vacuum, thereby enabling us to supply hydrogen to the test surface via permeation of atoms through the membrane. These atoms recombine on the surface to form molecules that desorb into the evacuated chamber. The spatial distribution of the desorbed molecules was measured with a rotatable ionization gauge, whereas the speed distribution of molecules desorbed along the surface normal was determined by means of a time-of-flight detector in a second apparatus.

Molecular beam techniques have been employed in an investigation of the scattering of helium atoms from the (112) face of a tungsten crystal. The resulting data are believed to be the first observation of the diffraction of atoms from a clean metal surface. When the azimuthal angle of the incident beam relative to the surface lattice corresponds to the [11̄0] direction of the lattice, the zeroth and first order diffraction peaks are sharp and well resolved and their positions agree satisfactorily with the predictions of simple diffraction theory. However, no first ordei peaks are observed when the azimuthal angle is changed to correspond to the [111̄] direction of the surface lattice. It is suggested that this result arises from the fact that the surface structure is far more pronounced in the [11̄0] direction than in the [111̄] direction, thereby causing the strength of the periodic interaction potential for He-W(112) to be significantly greater when the azimuthal angle corresponds to [11̄0] rather than to [111̄].

Theoretical predictions computed on the basis of the quasiequilibrium treatment of gas-solid reactions are compared with existing experimental data on the rate of volatilization (erosion) of solid tungsten by reaction with gaseous O2 at high temperature (∼ 1300° ≲ T ≲ 3600° K) and low pressure (4.5 × 10−7 ≤\(p_{O_2 } \) ≤11.5 Torr). The only unknown parameter in the analysis is the equilibrium probability,\(\zeta '_{O_2 } \), defined as the fraction of the impinging O2 molecules that attain thermochemical equilibrium at the tungsten surface rather than undergoing nonreactive scattering (e.g., reflection). An approximate expression for\(\zeta '_{O_2 } \) is estimated by a straightforward empirical procedure that is consistent with the quasiequilibrium treatment. The theoretical results based on this expression for\(\zeta '_{O_2 } \) T because appears to be an exponential function ofT; (b) In the intermediate region, the formation of volatile oxides decreases sharply with increasingT because atomic oxygen becomes the thermodynamically favored reaction product, thereby causing ΣW to decrease with increasingT; (c) In the highest region, ΣW again increases withT as a result of the formation of WO and the sublimation of W.