We have investigated the photocatalysis of partially deuterated methanol (CD3OH) and H2O on TiO2(110) at 400 nm using a newly developed photocatalysis apparatus in combination with theoretical ...calculations. Photocatalyzed products, CD2O on Ti5c sites, and H and D atoms on bridge-bonded oxygen (BBO) sites from CD3OH have been clearly detected, while no evidence of H2O photocatalysis was found. The experimental results show that dissociation of CD3OH on TiO2(110) occurs in a stepwise manner in which the O–H dissociation proceeds first and is then followed by C–D dissociation. Theoretical calculations indicate that the high reverse barrier to C–D recombination and the facile desorption of CD2O make photocatalytic methanol dissociation on TiO2(110) proceed efficiently. Theoretical results also reveal that the reverse reactions, i.e, O–H recombination after H2O photocatalytic dissociation on TiO2(110), may occur easily, thus inhibiting efficient photocatalytic water splitting.
Material response models for phenolic-based thermal protection systems (TPSs) for atmospheric entry are limited by the lack of knowledge of the nonequilibrium processes that may govern the ...decomposition pathways of phenolic resin at heating rates up to tens of degrees Celsius per second. We have investigated the pyrolysis of phenolic impregnated carbon ablator (PICA) by measuring the molar yields of the volatile decomposition products as a function of temperature at four nominal heating rates of 3.1, 6.1, 12.7, and 25 °C s–1, over the temperature range of 100–1200 °C. A mass spectrometer was used to probe the 14 significant gaseous products directly as PICA samples were heated resistively in vacuum. Four products, H2, CH4, H2O, and CO, overwhelmingly dominated the molar yields. However, in terms of mass yield, phenol and its methylated derivatives, cresol and dimethyl phenol, were significant. The temperature-dependent molar yields of the observed products exhibited a marked dependence on heating rate. The heating-rate-dependent behavior of the molar yields has been attributed to two main competing decomposition processes that occur as the temperature passes from roughly 300 to 500 °C: (1) cross-linking reactions that produce ether functional groups and carbon–carbon bonds and eliminate H2O and (2) breakdown of the polymer backbone through scission of methylene bridges and liberation of phenol and its methylated derivatives. The latter process competes more effectively with the former as the heating rate increases. The relative rates of these processes appear to have a significant effect on the molar yields of volatile products from subsequent decomposition processes as the temperature is increased further. Thus, the heating rate strongly affects the pathways taken during the pyrolysis of the phenolic resin in PICA. The new data may be used to test nonequilibrium models that are designed to simulate the response of TPS materials during atmospheric entry of spacecraft.
The environment encountered by space vehicles in very low Earth orbit (VLEO, 180–350 km altitude) contains predominantly atomic oxygen (AO) and molecular nitrogen (N2), which collide with ram ...surfaces at relative velocities of ∼7.5 km s–1. Structural, thermal-control, and coating materials containing organic polymers are particularly susceptible to AO attack at these high velocities, resulting in erosion, roughening, and degradation of function. Copolymerization or blending of a polymer with polyhedral oligomeric silsesquioxane (POSS) yields a material that can resist AO attack through the formation of a passivating silicon-oxide layer. Still, these hybrid organic/inorganic polymers become rough through AO reactions as the passivating layer is forming. Surface roughness may enhance satellite drag because it promotes energy transfer and scattering angle randomization during gas-surface collisions. As potential low-drag and AO-resistant materials, we have investigated POSS-containing films of clear and Kapton-like polyimides that have an atomically smooth AO-resistant coating of Al2O3 that is grown by atomic layer deposition (ALD). Coated and uncoated films were exposed to hyperthermal molecular beams containing atomic and molecular oxygen to investigate their AO resistance, and molecular beam-surface scattering studies were conducted to characterize the gas-surface scattering dynamics on pristine and AO-exposed surfaces to inform drag predictions. The AO erosion yield of Al2O3 ALD-coated films is essentially zero. Simulations of drag on a representative satellite structure that are based on the observed scattering dynamics suggest that the use of Al2O3 ALD-coated POSS-polyimides on external satellite surfaces have the potential to reduce drag to less than half of that predicted for diffuse scattering surfaces. These smooth and AO-resistant polymer films thus show promise for use in an extreme oxidizing and high-drag environment in the VLEO.
Molar yields of the pyrolysis products of thermal protection systems (TPSs) are needed in order to improve high fidelity material response models. The volatile chemical species evolved during the ...pyrolysis of a TPS composite, phenolic impregnated carbon ablator (PICA), have been probed in situ by mass spectrometry in the temperature range 100 to 935 °C. The relative molar yields of the desorbing species as a function of temperature were derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The temperature-dependent product evolution was consistent with earlier descriptions of three stages of pyrolysis, with each stage corresponding to a temperature range. The two main products observed were H2O and CO, with their maximum yields occurring at ∼350 °C and ∼450 °C, respectively. Other significant products were CH4, CO2, and phenol and its methylated derivatives; these products tended to desorb concurrently with H2O and CO, over the range from about 200 to 600 °C. H2 is presumed to be the main product, especially at the highest pyrolysis temperatures used, but the relative molar yield of H2 was not quantified. The observation of a much higher yield of CO than CH4 suggests the presence of significant hydroxyl group substitution on phenol prior to the synthesis of the phenolic resin used in PICA. The detection of CH4 in combination with the methylated derivatives of phenol suggests that the phenol also has some degree of methyl substitution. The methodology developed is suitable for real-time measurements of PICA pyrolysis and should lend itself well to the validation of nonequilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft.
We have conducted beam-surface scattering experiments with the intent of identifying the reaction mechanisms that are relevant to the ablation of carbon by N atoms at surface temperatures relevant to ...hypersonic flight. A pulsed molecular beam containing N and N2 with translational energies of 460 and 808 kJ mol-1, respectively, was directed at a vitreous carbon surface held at temperatures in the range 1023–1923 K. Time-of-flight distributions were collected for scattered products with fixed incidence and final angles of θi = θf = 45°. Inelastically scattered N and N2 and reactively scattered CN were the only products observed. The scattering dynamics of N and N2 were independent of surface temperature and were indicative of purely impulsive scattering. The scattering dynamics of CN suggested that this reaction product is formed by a mechanism that occurs in thermal equilibrium with the surface – i.e., a Langmuir-Hinshelwood mechanism. The reaction probability to produce CN has an Arrhenius temperature dependence with an activation energy of 207 kJ mol-1. The relevance of the current results to the hypersonic ablation of carbon in the presence of dissociated air is discussed.
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Molecular beam-surface scattering experiments were used to obtain fundamental data on gas-surface interactions that are central to the ablation of carbon during hypersonic flight through air. ...Continuous beams containing O or N atoms with incident velocities of ∼2000 m s−1 were directed at a vitreous carbon surface at temperatures in the range, 800–1873 K, and the products that desorbed from the surface were detected with a rotatable mass spectrometer detector as a function of their velocity and scattering angle. All products exhibited the dynamical characteristics of thermal desorption. The efficiencies of the gas-surface interactions, both reactive and non-reactive, were quantified as a function of surface temperature. In addition to reacting with carbon to produce CO2 (minor product) and CO (major product), oxygen atoms may recombine on the surface to produce O2 with an efficiency that is somewhat lower than that to produce CO. Nitrogen atoms may recombine on the surface to produce N2 or react to produce CN. The recombination efficiency of N atoms is generally more than an order of magnitude higher than the reaction efficiency to produce CN. The quantitative reaction efficiencies reported here are useful for the development of air-carbon models for hypersonic ablation.
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Previous observations of methyl formate (HCOOCH3) during the photo-oxidation of methanol (CH3OH) on TiO2 catalysts suggested that photocatalysis on TiO2 could be used to build up complex molecules ...from a single precursor. We have investigated the mechanism of HCOOCH3 formation by irradiating a CH3OH-adsorbed TiO2(110) surface with 400 nm light at low surface temperatures. Through the detection of volatile products after irradiation by temperature programmed desorption, we have found, as previously reported Phillips et al. J. Am. Chem. Soc. 2013, 135, 574–577 that HCOOCH3 is formed by the cross-coupling reaction of CH3O and CH2O, which are products of the first and second dissociation steps, respectively, in the stepwise photocatalysis of CH3OH on TiO2(110). Unlike the previous study, we have observed the photocatalytic production of HCOOCH3 without preoxidation of the surface, and we have concluded that the final reaction step to produce HCOOCH3 (i.e., the cross-coupling reaction of CH2O with CH3O) does not involve a transient HCO intermediate.
Copolymers of polyhedral oligomeric silsesquioxane (POSS) and polyimide (PI) have shown remarkable resistance to atomic oxygen (AO) attack and have been proposed as replacements for Kapton on the ...external surfaces of spacecraft in the harsh oxidizing environment of low Earth orbit (LEO). POSS PI blends would be an economical alternative to the copolymers if they also resisted AO attack. Thus, blends of trisilanolphenyl (TSP) POSS and PI with different weight percentages of the Si7O9 POSS cage were cast into films and exposed to a hyperthermal AO beam, and they were characterized in terms of their recession, mass loss, surface morphology, and surface chemistry. In order to compare the AO resistance of the blends with POSS PI copolymers, samples of previously studied copolymers were also investigated in parallel with the blends. For all POSS PI materials, the AO resistance increased with increasing AO fluence and with increasing POSS cage loading. At similar POSS cage loadings and exposure conditions, the TSP POSS PI blends showed comparable erosion yields to the POSS PI copolymers, with specific samples of blends and copolymers achieving erosion yields as low as 0.066 × 10–24 cm3 atom–1 with an AO fluence of 5.93 × 1020 O atoms cm–2. SEM and XPS analyses indicated that passivating SiO x layers were formed on the surfaces of all POSS-containing polymers during AO exposure. Thus, a TSP POSS PI blend is proposed as a low-cost variant of a POSS polyimide for use in extreme oxidizing environments, such as LEO.
The dynamics of O(
P) + NO collisions were investigated at a collision energy of ⟨
⟩ = 84.0 kcal mol
with the use of a crossed molecular beams apparatus employing a rotatable mass spectrometer ...detector. This experiment was performed with beams of
O atoms and isotopically labeled
N
O molecules to enable the products of reactive and inelastic scattering to be distinguished. Three scattering pathways were observed: inelastic scattering (
O +
N
O), O-atom exchange (
O +
N
O), and O-atom abstraction (
O
O +
N). All product channels exhibited a preponderance of forward scattering, but scattering over a broad angular range was also observed for all products. For inelastic scattering, an average of 90% of the collision energy is retained in the translation of
O and
N
O. On the other hand, for O-atom exchange (which also leads to O + NO products), the collision energy is partitioned roughly evenly between the translation of
O +
N
O and the internal excitation of
N
O. The available energy for O-atom abstraction is significantly lower than the collision energy because of the endoergicity of this reaction, but the available energy is again roughly evenly partitioned between the translation of
O
O +
N and the internal excitation of the molecular (O
) product. The relative yields of the three scattering pathways were determined to be 0.751 for inelastic scattering, 0.220 for O-atom exchange, and 0.029 for O-atom abstraction.
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