Charles M. Truong

Adsorption and reaction of formic acid on NiO(100) films on Mo(100): Temperature programmed desorption and high resolution electron energy loss spectroscopy studies

Adsorption and reaction of formic acid on well‐defined NiO(100) films prepared on Mo(100) have been investigated using combined temperature programmed desorption (TPD)/high resolution electron energy loss spectroscopy (HREELS). A novel approach to HREELS of oxide materials is utilized to acquire spectral data. This approach enables the direct observation of weak loss features due to the excitation of adsorbates without serious interference from intense multiple surface optical phonon losses associated with oxide materials. The results indicate that formic acid initially adsorbs associatively on NiO(100) surfaces at 90 K and undergoes heterolytic dissociation upon heating to ≥200 K to form a formate intermediate. The adsorbed formate species is proposed to bond to a cation site via one of the oxygen atoms of the formate in a monodentate configuration. The presence of a dimeric formic acid species is also indicated and a bonding configuration for this species has been proposed.

Infrared vibrational studies of CO adsorption on Cu/Pt(111) and CuPt(111) surfaces

Adsorption of CO on ultrathin Cu films supported on Pt(111) has been studied using infrared reflection absorption spectroscopy (IRAS). Our results indicate that the infrared intensities of adsorbed CO are not representative of the relative composition of the Cu<1.0/Pt(111) surfaces. The Cu‐bonded CO molecules screen CO molecules bonded to Pt, making them invisible in the infrared spectrum. The ‘‘screening’’ effect depends on the morphology and polarizability of the Cu overlayer. Changes in the morphology of the Cu adlayer produce large variations in the position and line shape of the Cu–CO signal in the infrared spectrum. CO molecules bonded to small Cu clusters show a higher (∼40 cm−1) C–O stretch frequency than CO molecules adsorbed on large Cu islands. The present results were compared with those reported in the literature for the CO/Cu/Ru(0001) and CO/Cu/Rh(100) systems. For CO adsorbed on supported monolayers of Cu, a correlation was found between the strength of the Cu–CO bond, the amount of π backd...

CO adsorption isotherms on Cu(100) at elevated pressures and temperatures using infrared reflection absorption spectroscopy

Abstract Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Cu(100) surface. Adsorption isotherms were determined at CO pressures from 10−6 to 10 Torr, and at temperatures from 115 to 340 K, and the isosteric heats of adsorption (δEads) evaluated as a function of CO coverage. For increasing CO coverages between 0-0.15 monolayers (ML), δEads decreases sharply from 16.7 to 12.7 kcal/mol. From 0.15 to 0.35 ML, δEads remains approximately 12.7 kcal/mol and exhibits little coverage dependence. These results are in excellent agreement with previously reported data for the CO/Cu(100) system acquired at much lower pressures ( 10−4, significant bathochromic shifts of the CO frequency to lower wavenumbers are observed.

FT-IRAS studies of CO adsorbed on Ag/Pt(111): anomalous behavior of vibrational cross-sections

Abstract In CO/Ag/Pt(111) surfaces the interaction between Ag and Pt enhances the strength of the AgCO bond. The CO desorption temperature of one monolayer of silver supported on Pt(111) is ∼ 70 K higher than that of Ag(111). Our results indicate that the infrared intensities of adsorbed CO are not representative of the relative composition of Pt(111) surfaces containing submonolayer coverages of silver. The Ag-bonded CO molecules screen CO molecules bonded to Pt, making them invisible in the infrared spectrum. The “screening” effect depends on the morphology and polarizability of the silver overlayer.

A FT-IRAS study of ammonia adsorbed on Ru(0001)

Abstract The interaction between ammonia and Ru(0001) has been studied by means of Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS). Chemisorption of NH 3 on Ru(0001) enhances the IR cross section of the umbrella mode of the molecule. For the first adsorption layer, changes in the IR intensity of the umbrella mode correlate with variations in the orientation of the molecules observed in ESDIAD and work function measurements. Formation of hydrogen bonds between first- and second-layer NH 3 molecules reduces drastically the IR cross section of the umbrella mode of chemisorbed ammonia.

Surface spectroscopic studies of the deposition of TiN thin films from tetrakis‐(dimethylamido)‐titanium and ammonia

The adsorption and pyrolysis of tetrakis‐(dimethylamido)‐titanium (TDMAT), Ti[NMe2]4, on several metal substrates, were studied using x‐ray photoelectron spectroscopy, Auger electron spectroscopy, infrared reflection absorption spectroscopy and thermal desorption mass spectrometry. TDMAT was found to decompose readily above ∼480 K on metallic substrates, producing TiCxNy films exhibiting a carbon‐rich interface. However, in the presence of NH3, well below the threshold of gas‐phase reactions (<10−4 Torr), the growth of low‐carbon‐content (≤8 at. %) titanium nitride films proceeds readily, via surface mediated reaction(s) of TDMAT and NH3 between 550 and 750 K. The effects of surface temperature and reagent pressures are reported and discussed.

Nature of active sites in the oxidative coupling of methane to ethane over Li/MgO catalysts

The partial oxidation of methane to ethane over a model Li/MgO catalyst has been studied using combined surface science techniques/elevated pressure kinetic measurements. The results indicate that [Li+O−] centers are not likely directly involved in the methane activation step. Rather, addition of lithium promotes the production of F centers which are considered to be responsible for this key step in the methane coupling reaction.

Electronic and chemical interactions between boron and carbon monoxide on Ru(0001)

The interaction between B and CO on Ru(0001) has been studied by means of thermal desorption mass spectroscopy, Fourier transform infrared reflection absorption spectroscopy, and x‐ray photoelectron spectroscopy. Boron adatoms poison CO chemisorption approximately on a one‐to‐one basis. No reaction or direct bonding between B and CO was observed. The B⋅⋅⋅CO interaction is repulsive due to the electron‐acceptor nature of both adsorbates. Boron adatoms modify the electronic and chemical properties of first and second nearest‐neighbor metal atoms. In the presence of B, the CO desorption temperature decreases, whereas the O(1s) binding energy and CO stretching frequency increase. These trends are a consequence of (1) a reduction in 2π* back donation caused by competition for metal electrons between CO and B and (2) repulsive electrostatic interactions between the negative charges on CO and B. The infrared results indicate that metal atoms strongly affected by B are only occupied when no more unperturbed Ru si...

Patient-Reported Usability of Positive Airway Pressure Equipment Is Associated With Adherence in Older Adults

Study objectives To examine the usability of positive airway pressure (PAP) devices and its association with PAP adherence among older adults with sleep-disordered breathing. Methods We mailed questionnaires to patients aged ≥65 years prescribed PAP therapy during the prior 36 months from two large healthcare systems. Survey participants completed the Usability of Sleep Apnea Equipment-Positive Airway Pressure (USE-PAP) questionnaire, which assessed the usability of their PAP device. Other questionnaire items included demographics and self-rated health. We also abstracted adherence data (mean nightly hours of PAP use available from one site) and interface type from the electronic health record. Results Five hundred sixty-four patients completed the survey (response rate = 33%). The mean USE-PAP score (0 = best to 100 = worst) was 20 (SD ± 20). Mean duration of PAP use (available in 189 respondents) was 5.2 hours per night (SD ± 2.0). In a nested regression model predicting nightly hours of PAP use, a 10-point (0.5 SD) increase in USE-PAP score corresponded to a 0.37 hour/night reduction in PAP use. The model including the USE-PAP score explained a significant proportion (R2 = 15%) of the variation in nightly hours of PAP use above and beyond demographics, self-reported health, and interface type (∆R2 = 12%). Conclusions Our results demonstrate that PAP usability varies among older patients and is associated with PAP adherence, above and beyond other predictors of adherence. These results support measuring and improving PAP usability to further improve PAP adherence for older patients.

Synthesis of boron nitride ultrathin films: The bonding and chemistry of ammonia and hydrazine on Ru(0001) and B/Ru(0001) surfaces

The bonding and chemistry of NH3 and N2H4 on clean and B‐covered Ru(0001) surfaces have been studied using surface analytical techniques (thermal desorption spectroscopy, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and Fourier transform infrared reflection absorption spectroscopy) and molecular orbital calculations (INDO/1 method). Both molecules act as electron donors on Ru(0001). Their absorption bonds are mainly a consequence of interactions between the N‐lone pairs and the Ru(5p,4d) orbitals. On the basis of these MO calculations, possible UPS spectra for adsorbed ammonia and hydrazine are discussed. B adatoms enhance the binding energies of NH3 and N2H4 on Ru(0001). This is a consequence of attractive interactions (through‐metal and through‐space) between the negatively charged B adatoms and the positively charged admolecules. The dissociation of NH3 on B/Ru(0001) surface is negligible under ultrahigh vacuum conditions. On the other hand, hydrazine reacts extensively with B, produc...