Temperature Programmed Desorption Peak Research Articles

Effect of Bystander Hydrogen Atoms on Hydrogen Desorption on Single-Atom Alloy Surfaces: Insights from Simulated Temperature-Programmed Desorption Spectra.

Single-atom alloy (SAA) catalysts exhibit unique and excellent catalytic properties in heterogeneous hydrogenation/dehydrogenation reactions. A thorough understanding of the microscopic surface processes is essential to improve the catalytic performance. Here, from a new perspective of the temperature-programmed desorption (TPD) spectra of hydrogen (H) on two common SAA surfaces, Pt@Cu(111) and Pd@Cu(111), we reveal and confirm the key influence of H atoms attached to Pt/Pd dopants, i.e., the H atom bystander, on the desorption process of surface H atoms. It is found that only after considering the effect of the H atom bystander can the simulated TPD spectra well reproduce the experimentally observed higher desorption temperature on Pt@Cu(111) than on Pd@Cu(111) and the leftward shift of the TPD peak with increasing H atom coverage; otherwise, the features are inconsistent with experiments. Our work provides direct evidence for the effect of bystander H atoms from a simulation perspective.

Theoretical study of hydrogen desorption on the Ni(100) surface through simulated temperature programmed desorption spectra

Nickel has been widely used as heterogeneous catalysts in important reactions such as dehydrogenation and steam reforming, where the desorption of H2 molecules is one of the key steps. Herein, based on density functional theory calculations, we have simulated the temperature programmed desorption (TPD) spectra of hydrogen (H) on the Ni(100) surface using the mean-field approximation and a new HH dimer desorption method. Zero-point energy corrections are also considered. It is found that the centered-peaks of TPD obtained from simulations are in good agreement with the previous experimental results. However, the centered-peak position moves in an opposite direction to the experiments as atomic hydrogen coverage increases. We have systematically investigated the effects of the interaction between adsorbed H atoms and the diffusion of H atoms on the peak shifts, and found that they are not responsible for the experimentally observed leftward shift of the TPD peak. We then propose and confirm that the presence of subsurface H atoms plays a key role in the leftward peak shift. This work refines the interpretation of H2 TPD spectra and contributes to the understanding of microscopic behaviors of H atoms on the Ni(100) surface.

Adsorption and Oxidation of NO2 on Anatase TiO2: Concerted Nitrate Interaction and Photon-Stimulated Reaction

The catalytic and photon-induced oxidation of NO2 on anatase TiO2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO3. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO3– species, where NO2 bonds to the lattice oxygen, and freely adsorbed NO3– ions. These products can be obtained either by dissociative adsorption of HNO3 or by catalytic/photocatalytic oxidation of NO2, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO3– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N2O5) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO2 crystal. In particular, a stable surface–NO3– and NO3– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N2O during continued UV light illumination.

CO and H2 adsorption on Au-Ni bimetallic surfaces: a combined experimental and DFT theoretical study

Au-Ni bimetallic thin films were grown on refractory metal substrates. CO and H2 adsorption on Au-Ni bimetallic surfaces have been studied by a combination of in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. It is found CO desorption peak shifts from 413 K on pure Ni surfaces to 293 K on the isolated Ni atoms formed by alloying with Au atoms. The sharp decrease of CO desorption temperature on Au-Ni surfaces with increasing Au coverage is caused by the change of the favored CO adsorption sites from bridge/hollow sites on pure Ni surfaces to Ni top sites on Au-Ni bimetallic surfaces. In situ PM-IRRAS shows two CO adsorption bands on Au sites at 2119 cm−1 and 2103 cm−1 on Au-Ni surfaces at 80 K, which are due to CO bound on under-coordinated Au atoms and electron negatively charged Au sites modified with nearby Ni atoms, respectively. Even with the Au-Ni surface temperature at as low as 100 K, CO adsorption induced Ni surface segregation has been observed by in situ PM-IRRAS. Furthermore, DFT calculation results discover the adsorption energy of CO on Ni top sites continues to decrease with increasing Au coverage due to the geometric ensemble effect and the lowered d-band center after Ni alloying with Au. H2 desorption temperature decreases from 363 K on pure Ni thin films to 302 K with increasing Au coverage to 0.6 ML. A new H2 peak appears at around 170 K on the Au-Ni surfaces with Au coverages between 0.6 ML and 0.9 ML. This new H2 TPD peak is assigned to H2 desorption from the totally isolated Ni sites. With Au coverage above 1.5 ML, there is no any H2 desorption detected. The combined surface science studies and DFT calculations provide new insights into the surface structure-activity correlation of Ni-base bimetallic surface alloys.

Interaction of CO with Pt nanoclusters on a graphene-covered Ru(0001) surface.

The adsorption of CO on Pt nanoclusters on a single layer of graphene epitaxially grown on the Ru(0001) surface [Gr/Ru(0001)] was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The graphene layer was grown through exposure to ethylene using a method that has previously been shown to completely cover the surface. As CO adsorbs on Ru(0001) but not on graphene, the complete coverage of the Ru(0001) surface by graphene was verified with TPD as no CO adsorption was detectable. Previous work has demonstrated that Pt nanoclusters nucleate in the moiré unit cells of the Gr/Ru(0001) surface. Exposure of the Pt/Gr/Ru(0001) surface to CO gives rise to strong RAIRS peaks at 2065-2085 cm-1 assigned to CO at Pt atop sites and at 1848 cm-1 due to CO at Pt bridge sites. The CO TPD peak areas were used to quantify the CO coverage, which allowed for the determination of the RAIRS peak areas per CO molecule. It was found that the RAIRS intensity for CO on Pt/Gr/Ru(0001) is as much as nine times the intensity of CO on Ru(0001) on a per molecule basis. A more modest intensity enhancement was observed compared to CO on Pt islands on the Ru(0001) surface.

Molecular chemisorption of N2 on IrO2(110).

We investigated adsorption of N2 on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed desorption (TPD) experiments and density functional theory (DFT) calculations. TPD shows that N2 desorbs predominantly from the stoichiometric-IrO2(110) surface in a well-defined peak at 270 K for N2 coverages below about 0.5 ML and that a shoulder centered near 235 K develops in the N2 TPD traces as the coverage approaches saturation, indicating that adsorbed N2 molecules destabilize at high N2 coverages. Experiments of N2 adsorption onto O-rich IrO2(110) surfaces provide evidence that N2 adsorbs exclusively on the coordinatively unsaturated Ir atoms (Ircus) of the surface and that pre-adsorbed O-atoms ("on-top" oxygen) stabilize adsorbed N2 molecules, causing the main N2 TPD peak to shift toward higher temperature with increasing oxygen coverages. Consistent with prior results, our DFT calculations predict that an N2 molecule preferentially adsorbs into an upright configuration on an Ircus atom of the IrO2(110) surface and achieves a binding energy of about 100 kJ/mol. The computed binding energy agrees well with our experimental estimate of ∼90 kJ/mol for low N2 coverages on stoichiometric IrO2(110). The DFT calculations also quantitatively reproduce the observed stabilization of N2 by co-adsorption on-top O-atoms and predict the destabilization of N2 on IrO2(110) as the N2 adlayer becomes crowded at high coverages.

Surface modification of fly ash by non-thermal air plasma for elemental mercury removal from coal-fired flue gas

ABSTRACT The fly ash from a coal-fired power plants was modified with non-thermal plasma in air to improve the elemental mercury (Hg0) removal performance. The Hg0 adsorption experiments were implemented via a bench-scale fixed-bed reactor system. Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), ultimate, X-ray diffraction (XRD) and XRF analysis were employed to characterize the fly ash. The effect of non-thermal plasma voltage and time on Hg0 removal efficiency was investigated. The results showed that fly ash had better Hg0 removal performance when treatment voltage was 3.0 kV and treatment time was 7 min. Furthermore, the results showed that non-thermal plasma treatment had little influence on the specific surface area. However, non-thermal plasma treatment increased the relative content of oxygen. XPS and temperature programmed desorption results indicated that Hg0 removal process included adsorption and oxidation of Hg0. Moreover, ester and carbonyl groups played extremely vital roles in the improvement of Hg0 removal performance, and their temperature programmed desorption peak occurred at around 250°C and 320°C, respectively.

Water chemistry on two-dimensional silicates studied by density functional theory and temperature-programmed desorption

The interactions of water with two-dimensional (2D) silica and aluminosilicate bilayers were studied using density functional theory (DFT) and temperature-programmed desorption (TPD). The DFT calculations encompassed a systematic investigation of the impacts of silanol groups, Si:Al ratio, and the proximity of a Pd(111) growth substrate on water adsorption, permeation, and dissociation, as well as the thermal stability of charge balancing protons at the 2D aluminosilicate surface. Water adsorption on pure, defect-free 2D SiO2 was weak; however, the barrier for water penetration to the Pd(111) substrate was small. Water adsorption at the 2D SiO2/Pd(111) surface was slightly weaker than bare Pd(111) but once at the metal surface water dissociation energetics were scarcely affected by the SiO2 overlayer. Silanol groups at 2D SiO2 edges substantially strengthened water adsorption but by only roughly half as much as hydroxyls bridging Al and Si atoms in 2D aluminosilicates. The metal substrate did not fundamentally alter the surface chemistry of the outer 2D aluminosilicate surface; water adsorption energies on the bridging hydroxyls differed by only a few meV for freestanding versus Pd-supported bilayers. Removal of the charge balancing proton via a dehydration process that produces an oxygen vacancy was favored over a dehydrogenation pathway irrespective of the presence of the metal. The dehydration pathway is uphill by at least 3.37 eV suggesting that temperatures in excess of 1100 K would be required to remove charge balancing protons as neutral species. In contrast, chemical bonds between the 2D aluminosilicate and the metal substrate make it difficult to protonate the metal side of the 2D aluminosilicate bilayer via water dissociation. The TPD experiments for pure 2D SiO2/Pd(111) revealed desorption curves consistent with adsorption at the metal surface, supporting weak adsorption on a defect-free bilayer and permeation through and around the silica overlayer to the metal. Incorporating Al into the bilayer shifted the water TPD peak to higher temperatures but not as much as would be expected based on the computed adsorption strength at bridging hydroxyls on 2D aluminosilicate surfaces. No water or hydrogen desorption consistent with removal of charging balancing protons from 2D aluminosilicate surfaces via dehydration or dehydrogenation was observed. The results suggest thermally robust Brønsted acid sites on the surface of 2D aluminosilicates supported on Pd(111).

Temperature-programmed desorption of CO2, formed by CO photooxidation on TiO2 surface

Temperature-programmed desorption (TPD) spectra of CO2, remaining on the dehydroxylated powder TiO2 (anatase and rutile) surface after photooxidation of CO at T = 293 K, were analysed. It was shown that the CO2 TPD peak appears at T > 400 K, has a symmetric shape, and its maximum position (Tm) depends on the CO2 coverage: the lower is the CO2 amount, the higher is the Tm value. The CO2 TPD peak is well fitted with the second-order desorption kinetics with Edes = 1.15 ± 0.05 eV and ν = 0.32 ± 0.02 cm2 s−1 for anatase and Edes = 1.24 ± 0.05 eV and ν = 0.16 ± 0.01 cm2 s−1 for rutile. The desorption process can be interpreted as monomolecular desorption of carbonate species with two types of precursor states: 1) “over an empty site” and 2) “over a filled site”. It was supposed that CO2 thermal desorption resulted from migration of strongly bonded carbonate species (CO3) along the surface (via the precursor state “over an empty site”) and interaction of them (desorption from precursor state “over a filled site”).

DFT and TPD study of the role of steps in the adsorption of CO on copper: Cu(4 1 0) versus Cu(1 0 0)

Adsorption of carbon monoxide (CO) was studied on stepped Cu(4 1 0) by temperature programmed desorption (TPD) and density-functional-theory (DFT) calculations. For comparison, the adsorption of CO was characterized also on Cu(1 0 0) by DFT calculations. On Cu(4 1 0) TPD reveals two desorption peaks: a high temperature peak (∼210 K) is attributed to the desorption of CO from step-edge sites and low temperature peak (∼170 K) to desorption from terrace sites. According to DFT calculations, CO prefers to adsorb at step-edges of Cu(4 1 0), although the step-edge versus terrace site preference is rather small at low coverage of 1/16 ML, about 0.05 eV; the respective DFT predicted CO binding energies are −0.89 and −0.84 eV at the step-edge and terrace top sites, whereas the value calculated at top sites of Cu(1 0 0) is −0.86 eV. Although this small step-edge over terrace site preference of 0.05 eV cannot explain the temperature difference of 40 K between the two TPD peaks, when the lateral intermolecular interactions are neglected, it is sufficient that the CO adsorbs almost exclusively at step-edges at low coverage (at 200 K the 0.05 eV corresponds to 3 kT). The emergence of the two TPD peaks on Cu(4 1 0) is therefore attributed to a combination of step-edge preference and lateral repulsion between CO molecules, which increases with increasing coverages and diminishes the net desorption energy of CO. DFT calculations further reveal that the reason for the significant increase of saturation coverage on Cu(4 1 0) compared to Cu(1 0 0) is related to the geometry of the step-edge that allows the CO molecules adsorbed thereon to tilt away from the nearest neighboring CO molecules adsorbed at the terrace and therefore to effectively reduce the lateral repulsion.

D2O Interaction with Planar ZnO(0001) Bilayer Supported on Au(111): Structures, Energetics and Influence of Hydroxyls

We investigate the interaction between D2O and the planar ZnO(0001) bilayer grown on Au(111) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We show that D2O molecules adsorbed on this planar surface form two ordered overlayers, a (3 × 3) and a (√3 × √3)R30°, not seen before on any of the bulk ZnO single crystal surfaces. The apparent activation energies of desorption (Ed) estimated from TPD peaks are 15.2 and 16.7–17.3 kcal/mol for (3 × 3) and (√3 × √3)R30°, respectively, which agree well with the adsorption energy values calculated from DFT (14.9–15.6 kcal/mol and 16.8–16.9 kcal/mol, respectively). The DFT calculations reveal that the formation of the overlayers takes place at different packing densities and is mediated by extensive hydrogen bonding among the molecules. The hydroxyl groups, which accumulate very slowly on the ZnO(0001) bilayer surface under the standard ultrahi...

The Temperature-Programmed Desorption of H2 from Cu/ZrO2

The desorption kinetics of hydrogen from a polycrystalline Cu/ZrO2 catalyst was investigated under atmospheric pressure using temperature-programmed desorption (TPD) experiments in a microreactor set-up. Different heating rates were applied under equal conditions with a carefully reduced catalyst. The hydrogen TPD peaks were symmetric and centered slightly above 300 K indicating associative desorption of H2 from metallic Cu. Using heating rate variation, the kinetic parameters $$A_{des}$$ and $$E_{des}$$ were determined to be 1.24 × 109 s−1 and 68 kJ mol−1, respectively. As the modeling with constant values of $$A_{des}$$ and $$E_{des}$$ yielded signals which were too narrow, dependence of $$E_{des}$$ on coverage was introduced applying $$E_{des} - K (\Theta _H)^n$$ . By application of the “full-analysis” method an optimal fit to the experimental data was found. Setting n = 1 resulted in the best fit and a value of 61 kJ mol−1 − (6.25 kJ mol−1 × $$\Theta _H$$ ) for $$E_{des}$$ was determined.

A Zn-Offretite for the adsorption of thiophenic species under fluidized catalytic cracking conditions. Synthesis, characterization and reactivity

A zinc-containing offretite (Zn-OFF) was synthesized using a hydrothermal method. The goal was to prepare a Zn-OFF zeolite with adsorption properties for selective removal of thiophenic chemical species in gasoline. It was expected that this new sorbent would perform favorably under the conditions close to the ones of FCC units. Following the Zn-OFF synthesis, several characterization techniques were used such as: a) temperature programmed desorption (TPD), b) temperature programmed reduction (TPR), c) temperature programmed oxidization (TPO), d) X-ray diffraction, e) N2 isotherm, f) UV–vis spectra, g) Scanning Electron Microscopy (SEM) and h) Fourier Transform Infrared Spectroscopy (FTIR). In addition, chemical characterization was developed using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and X-Ray Fluorescence (XRF). It was proven on this basis, that zinc species are most likely included in the zeolite framework. Regarding the influence of Zn on the OFF zeolites, it was shown that Zn considerably increases (doubles) the weak acidity measured by TPD as well as shifts the TPD peak towards a higher temperature range. Additionally, FTIR pyridine adsorption also showed in the case of the Zn-OFF in comparison with the OFF, a 2.5 times higher abundance of Lewis acid sites versus Brönsted sites. It is speculated on this basis, that the Zn in the OFF structure increases species adsorption, changing the adsorption bonding strengths. Regarding the Zn-OFF performance, it was proven in a CREC Riser Simulator unit that one can achieve 37wt% of 2-methylthiophene removal with less than 1.3wt% being converted into coke.

Growth and termination of a rutile IrO2(100) layer on Ir(111)

We investigated the oxidation of Ir(111) by gas-phase oxygen atoms at temperatures between 500 and 625K using temperature programmed desorption (TPD), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEISS) and density functional theory (DFT) calculations. We find that a well-ordered surface oxide with (√3×√3)R30° periodicity relative to Ir(111) develops prior to the formation of a rutile IrO2(100) layer. The IrO2(100) layer reaches a saturation thickness of about four oxide layers under the oxidation conditions employed, and decomposes during TPD to produce a single, sharp O2 desorption peak at ~770K. Favorable lattice matching at the oxide-metal interface is likely responsible for the preferential growth of the IrO2(100) facet during the initial oxidation of Ir(111), with the resulting coincidence lattice generating a clear (6×1) moiré pattern in LEED. Temperature programmed reaction spectroscopy (TPRS) experiments reveal that CO and H2O molecules bind only weakly on the IrO2(100) surface and LEISS measurements show that the oxide surface is highly enriched in O-atoms. These characteristics provide strong evidence that the rutile IrO2(100) layer is oxygen-terminated, and thus lacks reactive Ir atoms that can strongly bind molecular adsorbates. Oxygen binding energies predicted by DFT suggest that on-top O-atoms will remain adsorbed on IrO2(100) at temperatures up to ~625K, thus supporting the conclusion that the rutile IrO2 layer grown in our experiments is oxygen-terminated. As such, the appearance of only a single O2 TPD peak indicates that the singly coordinate, on-top O-atoms remain stable on the IrO2(100) surface up to temperatures at which the oxide layer begins to thermally decompose.

Adsorption of alkanes on stoichiometric and oxygen-rich RuO2(110).

We investigated the molecular adsorption of methane, ethane, propane and n-butane on stoichiometric and oxygen-rich RuO2(110) surfaces using temperature-programmed desorption (TPD) and dispersion-corrected density functional theory (DFT-D3) calculations. We find that each alkane adsorbs strongly on the coordinatively-unsaturated Ru (Rucus) atoms of s-RuO2(110), with desorption from this state producing a well-defined TPD peak at low alkane coverage. As the coverage increases, we find that alkanes first form a compressed layer on the Rucus atoms and subsequently adsorb on the bridging O atoms of the surface until the monolayer saturates. DFT-D3 calculations predict that methane preferentially adsorbs on top of a Rucus atom and that the C2 to C4 alkanes preferentially adopt bidentate configurations in which each molecule aligns parallel to the Rucus atom row and datively bonds to neighboring Rucus atoms. DFT-D3 predicts binding energies that agree quantitatively with our experimental estimates for alkane σ-complexes on RuO2(110). We find that oxygen atoms adsorbed on top of Rucus atoms (Oot atoms) stabilize the adsorbed alkane complexes that bind in a given configuration, while also blocking the sites needed for σ-complex formation. This site blocking causes the coverage of the most stable, bidentate alkane complexes to decrease sharply with increasing Oot coverage. Concurrently, we find that a new peak develops in the C2 to C4 alkane TPD spectra with increasing Oot coverage, and that the desorption yield in this TPD feature passes through a maximum at Oot coverages between ∼50% and 60%. We present evidence that the new TPD peak arises from C2 to C4 alkanes that adsorb in upright, monodentate configurations on stranded Rucus sites located within the Oot layer.

Spreading of lithium on a stainless steel surface at room temperature

Lithium conditioned plasma facing surfaces have lowered recycling and enhanced plasma performance on many fusion devices and liquid lithium plasma facing components are under consideration for future machines. A key factor in the performance of liquid lithium components is the wetting by lithium of its container. We have observed the surface spreading of lithium from a mm-scale particle to adjacent stainless steel surfaces using a scanning Auger microprobe that has elemental discrimination. The spreading of lithium occurred at room temperature (when lithium is a solid) from one location at a speed of 0.62 μm/day under ultrahigh vacuum conditions. Separate experiments using temperature programmed desorption (TPD) investigated bonding energetics between monolayer-scale films of lithium and stainless steel. While multilayer lithium desorption from stainless steel begins to occur just above 500 K (Edes = 1.54 eV), sub-monolayer Li desorption occurred in a TPD peak at 942 K (Edes = 2.52 eV) indicating more energetically favorable lithium-stainless steel bonding (in the absence of an oxidation layer) than lithium–lithium bonding.

Adsorption and thermal chemistry of formic acid on clean and oxygen-predosed Cu(110) single-crystal surfaces revisited

The thermal chemistry of formic acid on clean and oxygen-predosed Cu(110) single-crystal surfaces was studied under ultrahigh-vacuum (UHV) conditions by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Key results reported in the past were confirmed, including the partial switchover from H2 to H2O desorption upon oxygen addition on the surface and the development of a second decomposition regime at 420K, in addition to the one observed at 460K on the clean substrate. In addition, new observations were added, including the previously missed desorption of H2 at 420K and the existence of a normal kinetic isotope effect in both TPD peaks. Peak fitting of the XPS data afforded the identification of an asymmetric geometry for the formate intermediate, which was established to form by 200K, and the presence of coadsorbed molecular formic acid up to the temperatures of decomposition, probably in a second layer and held by hydrogen bonding. Quantitative analysis of the TPD data indicated a one-to-one correspondence between the increase in oxygen coverage beyond θO=0.5 ML and a decrease in formic acid uptake that mainly manifests itself in a decrease in the decomposition seen in the 460K TPD peak. All these observations were interpreted in terms of a simple decomposition mechanism involving hydrogen abstraction from adsorbed formate species, possibly aided by coadsorbed oxygen, and a change in reaction activation energy as a function of the structure of the oxygen overlayer, which reverts from a O-c(6×2) structure at high oxygen coverages to the O-(2×1) order seen at θO=0.5 ML.

An XPS and TPD study of gold oxide films obtained by exposure to RF-activated oxygen

Low-temperature oxidation of the polycrystalline surface of bulk metallic gold with radio-frequency activated oxygen is used to obtain oxide films, which are investigated by a combination of X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The XPS estimate for the composition and thickness of the oxide films is close to Au2O3 and is 6–10 nm. The analysis of the Au4f and O1s lines reveals only one state of the oxidized gold, which is described by E b(Au4f) = 85.5 eV, and two nonequivalent states of oxygen with E b(O1s) = 529.3 eV and 530.3 eV, which are uniformly distributed over the oxide film and may be attributed to nonequivalent oxygen states in the gold oxide structure. The thermal stability study shows that the gold oxide decomposes in one stage into metal at a temperature of T des = 530 K. According to the TPD data, the desorption activation energy is E des = 140 kJ/mol. The high-temperature oxidation of the gold surface reveals the formation of dissolved oxygen in bulk gold. This oxygen is characterized by a broad TPD peak with a maximum at T des = 800 K. This oxygen state is not detected by XPS. The concentration of dissolved oxygen is supposed to be below the XPS sensitivity threshold and is estimated at about 1% or less.

Thermal chemistry of the Cu-KI5 atomic layer deposition precursor on a copper surface

The thermal chemistry of a Cu(I) ketoiminate complex, Cu-KI5, resulting from the modification of the known Air Products CupraSelect® copper CVD precursor Cu(hfac)(tmvs) designed to tether the two ligands via an isopropoxide linker, was studied under ultrahigh vacuum on a Cu(110) single-crystal surface by using a combination of temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy. Adsorption at low temperatures was determined to take place via the displacement of the vinyl ligand by the surface. Molecular desorption was seen at 210 K, and the evolution of Cu(II)-KI52 was established to take place at 280 K, presumably from a disproportionation reaction that also leads to the deposition of Cu(0). Other sets of desorption products were seen at 150, 250, and 430 K, all containing copper atoms and small organic moieties with molecular masses below 100 amu. The latter TPD peak in particular indicates significant fragmentation of the ligands, likely at the C–N bond that holds the vinylsilane-isopropoxide moiety tethered to the ketoimine fragment, and possibly also at the union between the vinylsilane and the alkoxide linker. The 430 K temperature measured for this chemistry may set an upper limit for clean Cu film deposition, but since reactivity on the surface was also found to be inhibited at higher surface coverages, it may be delayed to higher temperatures under atomic layer deposition conditions.

CO adsorption on regularly stepped Cu(410) surface

Carbon mono-oxide (CO) adsorption on a regularly stepped Cu(410) surface was investigated with infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). CO was adsorbed on Cu(410) molecularly at 96K and all species were desorbed below 230K. There were three desorption peaks of CO; high temperature TPD peak (~210K) corresponds to the adsorption on the step edge, while the lower temperature TPD peaks (~138 and ~170K) correspond to the adsorption on the terrace. IRAS measurements suggest that CO molecules are adsorbed on the on-top sites.