Adsorption selectivity of Lewis acids and bases on an oxidized Mo(100) surface studied by LEED, Auger, and XPS

Henry, R. M.; Walker, B. W.; Stair, Peter C.

doi:10.1016/0039-6028(85)90025-1

Cited by 21 publications

(9 citation statements)

References 26 publications

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“…There are several important points to note in Figure . Ethylene is known to adsorb molecularly on Mo(100) surfaces at 80 K, but heating leads to carbon−carbon bond scission, with no molecular C 2 H 4 remaining on the surface above room temperature. ,, Thus, the ethylene that is observed in the TPD spectra of Figure A must be due to decomposition reaction of either molecular TEOS or of ethoxysilyl intermediates, as previously observed. ,,,− , Similarly, alcohols adsorbed on molybdenum and molybdenum oxide surfaces below room temperature are known to decompose to alkoxide intermediates by ∼300 K, with no molecular species remaining on the surface at or above room temperature. − By extension, then, the ethanol observed in the TPD spectra of Figure B must also be formed via decomposition of TEOS and/or the ethoxysilyl intermediates. The second significant item observed in Figure is that total desorption product yield is greater for surfaces exposed to TEOS at lower temperatures vs those at higher temperatures.…”

Section: Resultsmentioning

confidence: 65%

“…2,16,49,[58][59][60]64 Similarly, alcohols adsorbed on molybdenum and molybdenum oxide surfaces below room temperature are known to decompose to alkoxide intermediates by ∼300 K, with no molecular species remaining on the surface at or above room temperature. [67][68][69] By extension, then, the ethanol observed in the TPD spectra of Figure 4B must also be formed via decomposition of TEOS and/or the ethoxysilyl intermediates. The second significant item observed in Figure 4 is that total desorption product yield is greater for surfaces exposed to TEOS at lower temperatures vs those at higher temperatures.…”

Section: B Tpd Parts a And B Ofmentioning

confidence: 94%

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Pyrolysis of Tetraethoxysilane on Mo(100) at Low Temperatures

Jurgens-Kowal

Rogers

1998

J. Phys. Chem. B

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Deposition of ultrathin silicon dioxide films on Mo(100) substrates by pyrolysis of tetraethoxysilane (TEOS) vapor has been investigated at temperatures between 300 and 860 K with X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and infrared reflection−absorption spectroscopy (IRRAS). Up to temperatures of ∼600 K, TEOS adsorbs on the Mo surface forming an ethoxysilyl intermediate, whereas at higher temperatures SiO2 is formed during the initial exposure, as evidenced by both XPS and IRRAS data. Deposition of silicon dioxide is reaction-limited in the temperature range studied with roughly a monolayer forming on the surface at 860 K. Heating the TEOS-exposed Mo(100) surfaces to ∼1000 K yields ethylene as the predominant gas-phase decomposition product and improves both the stoichiometry and order of the films as indicated by an increase in the stretching frequency of the Si−O IRRAS peaks. A decrease in the amount of desorbed ethylene is observed as the deposition temperature increases from 300 to ∼600 K, and no significant desorption of any other decomposition products was detected at higher deposition temperatures. Carbon contamination is minimal in these SiO2 films.

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Section: Resultsmentioning

confidence: 65%

Section: B Tpd Parts a And B Ofmentioning

confidence: 94%

Pyrolysis of Tetraethoxysilane on Mo(100) at Low Temperatures

Jurgens-Kowal

Rogers

1998

J. Phys. Chem. B

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“…A clean Mo (100) surface is extremely reactive and will dissociatively adsorb gases at room temperature until a passivation layer is formed. 29 The annealing temperatures used in the study temporarily removed the carbon, but were not high enough to completely desorb all of the oxygen. The base Mo surface in these experiments was passivated by both carbon and oxygen, as shown in the representative XPS data in Figure 1 for the base substrate used in the 573 K experiments.…”

Section: ■ Resultsmentioning

confidence: 99%

“…A clean Mo (100) surface is extremely reactive and will dissociatively adsorb gases at room temperature until a passivation layer is formed . The annealing temperatures used in the study temporarily removed the carbon, but were not high enough to completely desorb all of the oxygen.…”

Section: Resultsmentioning

confidence: 99%

Alternative Low-Pressure Surface Chemistry of Titanium Tetraisopropoxide on Oxidized Molybdenum

Johnson

Stair

2014

J. Phys. Chem. C

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Titanium tetraisopropoxide (TTIP) is a precursor utilized in atomic layer depositions (ALDs) for the growth of TiO 2 . The chemistry of TTIP deposition onto a slightly oxidized molybdenum substrate was explored under ultrahigh vacuum (UHV) conditions with X-ray photoelectron spectroscopy. Comparison of the Ti(2p) and C(1s) peak areas has been used to determine the surface chemistry for increasing substrate temperatures. TTIP at a gas-phase temperature of 373 K reacts with a MoO x substrate at 373 K but not when the substrate is at 295 K, consistent with a reaction that proceeds via a Langmuir−Hinshelwood mechanism. Chemical vapor deposition was observed for depositions at 473 K, below the thermal decomposition temperature of TTIP and within the ALD temperature window, suggesting an alternative reaction pathway competitive to ALD. We propose that under conditions of low pressure and moderate substrate temperatures dehydration of the reacted precursor by nascent TiO 2 becomes the dominant reaction pathway and leads to the CVD growth of TiO 2 rather than a self-limiting ALD reaction. These results highlight the complexity of the chemistry of ALD precursors and demonstrate that changing the pressure can drastically alter the surface chemistry. ■ INTRODUCTIONAtomic layer deposition (ALD) is a technique that relies on two sequential gas−surface reactions to grow materials in a layer-by-layer fashion. This method is employed in the semiconductor industry and is most commonly used to deposit metal oxides. 1 However, ALD has become an emerging technique for the fabrication of novel and complex catalytic systems and in the process has been extended to the growth of metal films and nanoparticles on catalytic supports. 2,3 ALD employs multiple cycles that are combined to build materials with precise thickness control. One ALD cycle is composed of two half cycles that result in the regeneration of the initial surface groups, providing new reactive sites for the following cycle. Given a finite number of surface reaction sites and steric constraints imposed by the ligands, only a finite number of precursor molecules can react, resulting in the selflimiting nature of the technique. 4 Reactive sites on oxides include hydroxyl groups, bridging oxygen atoms, and defect sites.There has been a significant amount of work directed toward elucidating ALD chemistry, utilizing techniques such as X-ray photoelectron spectroscopy (XPS), mass spectrometry, quartz crystal microbalance (QCM), temperature-programmed desorption (TPD), DFT, and ab initio calculations. 5,6 The most studied system is the deposition of Al 2 O 3 from trimethylaluminum (TMA) and H 2 O because it is widely used and considered an "ideal" deposition process. Significant work has been done to establish a mechanism for the reaction, identify the source of self-limiting behavior, and understand the reaction kinetics. 4,7,8 Many of these experiments are summarized in a dedicated review by Puurunen. 4 Studies of other ALD metal precursors and corresponding growth processes of...

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“…9 Subsequent modifications of this reactor cup design have been used by a number of research groups. 6,8,[10][11][12][13][14][15][16][17] The main advantage of the reactor cup design is that high pressures of 100 atm can be achieved; the reactor cup is sealed against the sample holder by using a hydraulic piston to apply 2000 lb/in. 2 to the metal gasket seal.…”

Section: Introductionmentioning

confidence: 99%

Novel recirculating loop reactor for studies on model catalysts: CO oxidation on Pt/TiO2(110)

Tenney

Xie

Monnier

et al. 2013

Review of Scientific Instruments

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A novel recirculating loop microreactor coupled to an ultrahigh vacuum (UHV) chamber has been constructed for the kinetic evaluation of model catalysts, which can be fully characterized by UHV surface science techniques. The challenge for this reactor design is to attain sufficient sensitivity to detect reactions on model single-crystal surfaces, which have a low number of active sites compared to conventional catalysts of equivalent mass. To this end, the total dead volume of the reactor system is minimized (32 cm(3)), and the system is operated in recirculation mode so that product concentrations build up to detectable levels over time. The injection of gas samples into the gas chromatography column and the refilling of the recirculation loop with fresh feed gas are achieved with computer-controlled, automated switching valves. In this manner, product concentrations can be followed over short time intervals (15 min) for extended periods of time (24 h). A proof of principle study in this reactor for CO oxidation at 145-165 °C on Pt clusters supported on a rutile TiO2(110) single crystal yields kinetic parameters that are comparable to those reported in the literature for CO oxidation on Pt clusters on powdered oxide supports, as well as on Pt(100). The calculated activation energy is 16.4 ± 0.7 kcal/mol, the turnover frequency is 0.03-0.06 molecules/(site·s) over the entire temperature range, and the reaction orders in O2 and CO at 160 °C are 0.9 ± 0.2 and -0.82 ± 0.03, respectively.

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Adsorption selectivity of Lewis acids and bases on an oxidized Mo(100) surface studied by LEED, Auger, and XPS

Cited by 21 publications

References 26 publications

Pyrolysis of Tetraethoxysilane on Mo(100) at Low Temperatures

Pyrolysis of Tetraethoxysilane on Mo(100) at Low Temperatures

Alternative Low-Pressure Surface Chemistry of Titanium Tetraisopropoxide on Oxidized Molybdenum

Novel recirculating loop reactor for studies on model catalysts: CO oxidation on Pt/TiO2(110)

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