Haihua Qi

Structure sensitivity of methane oxidation over platinum and palladium

A series of supported platinum and palladium catalysts were tested for methane oxidation at 260 to 370°C, 50 Torr methane, 110 Torr oxygen, 900 Torr helium, and conversions below 2%. The intrinsic rate varies by more than 5000 from the least active to the most active catalyst, indicating that the reaction is structure sensitive. The catalytic activity of platinum depends on the distribution of the metal between a dispersed and a crystalline phase. These two phases are distinguished by absorbances at 2068 and 2080 cm−1, respectively, in the infrared spectrum of adsorbed carbon monoxide. The catalytic activity of palladium depends on the metal particle size. For the different classes of catalysts, the mean steady state turnover frequency (TOF) at 335°C and the mean apparent activation energy (Ea) are as follows: dispersed phase of platinum, TOF = 0.005 s−1 and Ea = 36 kcal/mole; crystalline phase of platinum, TOF = 0.08 s−1 and Ea = 28 kcal/mole; small particles of palladium, TOF = 0.02 s−1 and Ea = 27 kcal/mole; and large particles of palladium, TOF = 1.3 s−1 and Ea = 29 kcal/mole. The structure sensitivity may be explained by differences in the reactivity of the adsorbed oxygen on these metal surfaces.

Effect of catalyst structure on methane oxidation over palladium on alumina

Abstract A series of palladium on alumina catalysts were prepared and tested for methane oxidation at 300°C, 50 Torr methane, 110 Torr oxygen, 900 Torr helium, and conversions less than 2%. During the reaction the palladium particles are oxidized. The extent of oxidation increases with decreasing particle size and increasing number of crystal imperfections. Palladium oxidation breaks apart the crystals, so that all the oxide is exposed and participates in catalysis. Turnover rates for methane oxidation, based on the oxygen uptake at the temperature and oxygen pressure of the reaction, vary from 0.004 to 0.15 s −1 . Our results suggest that the palladium oxide dispersed over the alumina is much less active than the oxide dispersed over the surface of the palladium crystallites. Also, chlorine deposited during catalyst preparation may inhibit the rate of methane oxidation.

Scanning tunneling microscopy of chemically cleaned germanium (100) surfaces

Abstract The effects of HF/H 2 O 2 etching, UV/ozone oxidation, and ultrahigh vacuum annealing on the composition and structure of Ge(100) surfaces has been studied by X-ray photoemission and scanning tunneling microscopy. Our results indicate any carbon impurities left on the surface after the etch and oxidation steps cannot be completely removed by heating in vacuum. On flat Ge(100), carbon pins the steps in place during annealing. Germanium terraces pile up at these locations, producing a mountain-and-valley structure between 20 and 30 atomic layers in height. On Ge(100) 9° off-axis, step pinning by carbon generates a faceted surface covered with V-shaped ridges.

Carbon monoxide restructuring of palladium crystallite surfaces

Six palladium on alumina catalysts were prepared with dispersions between 3 and 86%. Over several cycles of carbon monoxide exposure at 25°C and reduction at 300°C, the carbon monoxide, oxygen, and hydrogen adsorption capacities decreased by 20 to 50%. The loss of surface sites was higher on the small crystallites. The adsorption capacity recovered with oxidation at 550°C and reduction at 300°C. The carbon monoxide and oxygen treatments did not affect the hydrogen solubility in the palladium particles. However, infrared spectroscopy of adsorbed carbon monoxide revealed that the surface structure changed with the gas treatments. When the catalyst was converted from the carbon monoxide-exposed state to the oxidized state, the band at 1970 cm−1 for bridge-bonded carbon monoxide on (100) facets decreased, while the band at 2095 cm−1 for linearly bonded carbon monoxide on low-coordination sites increased. In some instances, the bands at 1995 and 1885 cm−1 increased after oxidation. These latter features are tentatively assigned to bridged and threefold-bridged bonded carbon monoxide adjacent to linearly bonded carbon monoxide on low coordination sites. A model is presented to explain these results: cycles of carbon monoxide exposure and reduction smooth the crystallite surfaces into well-ordered facets, whereas cycles of oxidation and reduction roughen the surfaces.

Sites for hydrogen adsorption on GaAs(001)

Abstract The adsorption of hydrogen atoms on the c(2 × 8) and (1 × 6) reconstructions of GaAs(001) has been studied by multiple internal-reflection infrared spectroscopy. A series of vibrational bands are observed which are due to arsenic hydrides, bridging gallium hydrides and terminal gallium hydrides. These species result from hydrogen adsorption on arsenic dimers, second-layer As atoms, gallium dimers, and second-layer Ga atoms, respectively. The hydrides on these sites form at about the same rate during dosing and disappear at about the same rate during heating. The increase in coverage with dosage follows a Langmuir adsorption isotherm with an initial sticking probability of 0.1–1.0 at 303 to 433 K. Assuming a second-order reaction, the average frequency factor and activation energy for H 2 desorption are 1.0 ± 0.5 × 10 8 s −1 and 14 ± 2 kcal/mol (60 ± 8 kJ/mol). However, these rate constants may be shifted below their true values as a result of hydrogen desorbing off multiple surface sites.

Observation of the atomic surface structure of GaAs(001) films grown by metalorganic vapor-phase epitaxy

We present atomically resolved scanning tunneling micrographs of the surfaces of GaAs(001) films grown by metalorganic vapor-phase epitaxy (MOVPE). Thin films deposited in an MOVPE reactor were transferred to an (ultra high) vacuum system without air exposure. After heating the samples from 480 to 580°C, high-quality images of the (2 × 4)/c(2 × 8), (1 × 6)/(2 × 6) and (4 × 2)/c(8 × 2) reconstructions were obtained. In addition, a new Ga-rich (3 × 2)/(3 × n) phase was observed that forms during annealing at 540°C. This structure consists of single dimer rows running along the [110] direction with a spacing of 12 A. The rows vary in length, and are separated by line defects which occur on average every 20 A (n = 5). A model is proposed for the (3 × 2) which consists of rows of Ga dimers alternating between the first and third layers. Since this structure exhibits a deficit of one electron, line defects are required to expose As dimers in the second layer and neutralize the surface charge.

Sites for trimethylgallium adsorption on GaAs(001)

Abstract The sites for trimethylgallium (TMGa) adsorption on c(2 × 8) GaAs(001) have been identified by hydrogen titration of the surface before and after adsorbing the organometallic molecules. There are four main adsorption sites on GaAs(001): As dimers, Ga dimers, second-layer As atoms, and second-layer Ga atoms. Hydrogen bonds to each of these sites, producing AsH and GaH species with characteristic stretching vibrations in the infrared spectrum. By monitoring the changes in the vibrational spectra of the hydrides upon coadsorption of TMGa, it has been discovered that this molecule dissociatively adsorbs onto an arsenic dimer. One methyl group is transferred to the As atom of the dimer, while at least one other methyl group is transferred to a second-layer Ga atom. At TMGa coverages ≥ 0.9, the adsorbed gallium atoms combine to form dimers. The distribution of the three methyl groups between one As site and two Ga sites is also consistent with the CH stretching vibrations recorded of these species. This model of trimethylgallium adsorption is further supported by the fact that the total number of valence electrons are conserved throughout the reaction.

The reaction of carbon tetrachloride with gallium arsenide (001)

Carbon tetrachloride dissociatively adsorbs on the Ga-rich (4×2) reconstruction of GaAs (001) at 200 °C. Upon heating to 440 °C, the chlorine desorbs as GaCl, which etches the surface. Scanning tunneling micrographs reveal that this reaction transforms the (4×2) into a Ga-rich (3×2) structure that is interlaced with As-rich (2×4) phases. The (3×2) is well ordered, while the (2×4) phases exhibit a high degree of disorder. This work establishes the surface reaction pathway for carbon doping of GaAs with CCl4.

Sites for arsine adsorption on GaAs(001)

Abstract Arsine adsorption on c(2 × 8) and (1 × 6) GaAs(001) at 303–573 K has been studied by internal-reflection infrared spectroscopy. We have discovered that arsine adsorbs onto two sites: second-layer Ga atoms and Ga dimers. On the c(2 × 8), arsine dissociatively adsorbs on second-layer Ga atoms, forming arsenic monohydrides and transferring two H atoms each to nearby As dimers. On the (1 × 6), arsine dissociatively adsorbs on Ga dimers, and also transfers H atoms to As sites. The saturation coverage of arsine at 303 K on the (1 × 6) is twice that on the c(2 × 8). Also, more As-H infrared bands are observed, indicating that several AsH x s are formed. Dosing the surfaces with arsine at successively higher temperatures from 303 to 473 K leads to the loss of adsorbed AsH x species. At 573 K, no change in the c(2 × 8) occurs upon extended exposure to arsine. However, on the (1 × 6), the Ga dime are replaced by As dimers during arsine dosing, and at 573 K, 1800 L of AsH 3 is sufficient to convert the (1 × 6) into the c(2 × 8) reconstruction. We conclude that during vapor-phase epitaxy of GaAs with trimethylgallium and arsine, AsH 3 decomposes on second-layer Ga atoms and Ga dimers. However, on the latter sites the arsenic is more likely to incorporate into the crystal, instead of desorbing as As 2 or As 4 .

Effect of catalyst structure on the rate of alkane oxidation over platinum

Abstract The effect of catalyst structure on the intrinsic activity of platinum for methane and heptane oxidation has been determined. The reactions were carried out in 5% excess oxygen and at low conversion. Depending on the support and method of catalyst preparation, the platinum may be distributed between a dispersed and a crystalline phase. These phases show characteristic absorbance at 2067 and 2085 cm −1 , respectively, in the infrared spectrum of adsorbed carbon monoxide. These phases catalyze methane oxidation at different rates. The turnover frequency for methane oxidation at 608 K is between 0.001 and 0.01 s −1 for the dispersed platinum and between 0.05 and 0.10 s −1 for the crystalline platinum. The turnover frequency for heptane oxidation is insensitive to the platinum phase present, and varies from 0.003 to 0.011 s −1 at 413 K. For platinum dispersions ranging from 6 to 100%, the specific activities for methane and heptane oxidation are not significantly affected by crystallite size.