TPD Spectra Research Articles

STM studies of oxygen-induced reconstruction on a PtRh(100) alloy surface

Oxygen-induced reconstruction of a PtRh(100) alloy surface was studied by scanning tunneling microscopy (STM). When a clean Pt-enriched PtRh(100) surface was exposed to O 2 at more than 600 K, oxygen-induced reconstruction took place with segregation of Rh atoms to the surface. Depending on the coverage of oxygen, two types of reconstruction on the PtRh(100) alloy surface are induced. A p(3 × 1)-O structure appeared at relatively low oxygen coverage, and the TPD spectrum of the p(3 × 1)-O surface gave a desorption peak of O 2 at ca. 930 K. The p(3 × 1)-O structure showed several different STM images depending on the tip condition, from which we could distinguish the two component metals, RhO rows and Pt rows. Based on this, a model structure was deduced. At a high oxygen coverage, more Rh atoms were segregated to the surface and the STM image suggests a c(2 × 20)-O structure. The c(2 × 20)-O surface changed into the p(3 × 1)-O by heating with the concomitant desorption of O 2 at ca. 830 K. The STM images for this c(2 × 20)-O surface is analogous to the arrangement of Rh atoms on the top layer of a Rh(111) hexagonal lattice. Due to the lattice mismatch between the quasi-Rh(111) overlayer and PtRh(100) substrate, a moire pattern with 20 times periodicity along the 〈111〉 directions was observed. Both the p(3 × 1)-O and c(2 × 20)-O surfaces are easily reduced by H 2 even at room temperature.

Long-range ordered, commensurate superstructures of large organic adsorbates: a LEED and STM study of [formula omitted

The adsorption behaviour and especially the geometric structure of a large organic molecule, end-capped quaterthiophene (EC4T), on Ag(111) has been studied using TPD, LEED and STM. Three different peaks in the TPD spectra of the undissociated molecule represent the desorption of the multilayer, the bilayer and a small fraction of the monolayer during a structural phase transition. The majority of the monolayer cannot be desorbed thermally due to a covalent bonding of EC4T to the Ag substrate. This site-specific bonding is also responsible for the occurrence of a 2D-commensurate superstructure for coverages at and below 90% of a saturated monolayer and for the reversible phase transition which leads to a unidirectionally compressed, 1D-commensurate superstructure for a saturated monolayer.

Spatial distribution of N2 and NO desorbing from a Rh(533) surface

The spatial distribution of N2 and NO desorbing from Rh(533), Rh(S)-[4(111)×(100)], was measured along the [65̄ 5̄] direction (along the steps) after adsorption of NO at 320 K. The N2 desorption at 450 K and 600 K showed a specific spatial distribution, that is, there was no peak either normal to the (111) terraces (θ=+14.4°) or normal to the (100) steps (θ=−40.3°), but a desorption maximum around θ=−15°. Integrated TPD spectra of N2 from the NO preadsorbed Rh(533) surface were very similar to those on Rh(111), but the spatial distribution of N2 from Rh(533) indicates desorption of N2 from the boundary between the (111) terraces and the vacant (100) steps. The spatial distribution of NO desorbing from the Rh(533) surface at 420 K obeyed cos θ, indicative of desorption normal to the crystal surface. Contrary to this, the spatial distribution of the desorption of NO molecules produced by the recombination reaction of adsorbed N and O desorbed at 550 K exhibits a peak intensity at θ=+15° (normal to the (111) terraces).

Characterization of Pd and [formula omitted] catalysts. A TPR-TPD study

Low loaded (< 0.5 wt.-%) palladium on α-Al 2O 3 catalysts were prepared using palladium acetylacetonate as a chemical precursor. Temperature programmed reduction studies (TPR) indicate that in the initial oxidized state a fraction of palladium exhibits a strong interaction with the alumina support. Hydrogen treatments remove the interaction allowing the formation of palladium particles that upon reoxidation stabilize the palladium as PdO 1.4. Bimetallic PdPb catalysts were obtained by reacting the palladium catalysts with tetra-n-butyl lead. Hydrogen and carbon monoxide desorption studies (TPD) have shown that progressive lead addition suppresses the dissociative adsorption of hydrogen as well as the multiple coordinated forms of carbon monoxide. For hydrogen, the TPD spectra supports the existence of an ensemble effect upon alloying. On the other hand the increased adsorption strength of multiple coordinated carbon monoxide suggested the presence of a ligand effect. Another important effect of lead on the TPD of carbon monoxide is the inhibition of carbon dioxide formation. The role of lead in improving the selectivity of alkynes hydrogenation reactions is discussed.

THERMAL DESORPTION ACCOMPANIED BY DECOMPOSITION OF ADSORBED MOLECULES

The effect of adsorbate–adsorbate lateral interactions on TPD spectra is analyzed in the case where the desorption process is accompanied by decomposition of molecules remaining on the surface. General results are used to interpret the kinetics of NO desorption from Rh(111).

Mathematic modelling of the peculiarities of NO decomposition on Rh(111)

The complicated reaction kinetics of NO desorption and decomposition were adequately simulated within the framework of a deterministic model considering interactions in the adsorbed layer. The repulsive interactions between N and NO adparticles together with the strong inhibition of NO dissociation by NO, N and O adsorbates were shown to be responsible for the peculiarities of N2 and NO TPD spectra from the Rh(111) surface.

Modification of Ni(111) reactivity toward CH4, CO, and D2 by two-dimensional alloying

Alloying Au into the outermost atomic layer of Ni(111) significantly alters the physical and chemical properties of the surface. The reactivity is investigated by the use of seeded supersonic molecular beams of CH4 and it is found that the nobleness of Au reduces the overall reactivity toward CH4. This is accounted for in an ensemble model resolving the sticking probability on Ni atoms having different nearest neighbor surroundings. Although a mean field description of site distributions is found to be a very good approximation it is improved by using experimentally determined ensemble statistics from STM images. The strong influence of the vibrational temperature on the sticking coefficients of CH4 vs translational energy on the pure Ni(111) is also demonstrated. Desorption energies of CO and D2 is observed to decrease approximately 25–30 kJ/mole as the coverage of Au is increased from 0.0 to 0.7 ML. In TPD spectra of deuterium saturated surface alloys a new clearly resolved desorption state is observed at 180–220 K with maximum intensity around θAu=0.3–0.4 ML. This state is clearly related to chemisorption sites involving both Au and Ni. A site model based on mean field statistics adequately accounts for the appearance of this state. The effect of Au is also evident in the TPD spectra of CO saturated Au/Ni(111) surface alloys where the saturation coverage decreases and new structure develops.

Oxygen-Free Conversion of Methane to Higher Alkanes through an Isothermal Two-Step Reaction on Platinum (EUROPT-1): I. Chemisorption of Methane

When submitted to CH4at atmospheric pressure, EUROPT-1 (a 6.3 wt% Pt/SiO2catalyst) is able to chemisorb CH4with parallel evolution of H2atT≥ 150°C. C2H6is also produced but at a rate approximately 10 times lower, so that H-deficient hydrocarbonaceous fragments build up on the metal surface. Temperature-programed desorption spectra obtained just after chemisorption show that CH4can be removed from the surface mainly in two peaks, one at ≈60°C and the second at a temperature varying between 180 and 250°C. Formation of CH4, accompanied by small amounts of C2H6and traces of C3H8, is observed during subsequent temperature-programed surface reaction with H2of the adspecies remaining after each TPD interrupted at 300°C. The total amount of CH4initially chemisorbed can be easily derived from those experiments and the H/C ratio of the surface species formed during chemisorption can also be estimated. It has been shown that the amount of chemisorbed CH4increased and the H/C ratio strongly decreased when the temperature or the duration of exposure was increased. When EUROPT-1 was submitted to (Ar, CH4) mixtures instead of pure CH4, the increase of the CH4content had an effect similar to that of the increase of either the temperature or the duration of exposure. Moreover the TPD and TPSR spectra are very sensitive to the conditions of the exposure, so that they constitute some fingerprint of the adspecies at the end of the adsorption step.

Ethylene Oxidation over Platinum:In SituElectrochemically Controlled Promotion Using Na–β′′ Alumina and Studies with a Pt(111)/Na Model Catalyst

Electrochemically modified ethylene oxidation over a Pt film supported on the Na+ion conductor β′′ alumina has been studied over a range of conditions encompassing both promotion and poisoning. The system exhibits reversible behavior, and the data are interpreted in terms of (i) Na-enhanced oxygen chemisorption and (ii) poisoning of the surface by accumulation of Na compounds. At low Na coverages the first effect results in increased competitive adsorption of oxygen at the expense of ethylene, resulting in an increased rate. At very negative catalyst potentials (high Na coverage) both effects operate to poison the system: the increased strength of the Pt–O bond and coverage of the catalytic surface by compounds of Na strongly suppress the rate. Kinetic and spectroscopic results for ethylene oxidation over a Pt(111)-Na model catalyst shed light on important aspects of the electrochemically controlled system. Low levels of Na promote the reaction and high levels poison it, mirroring the behavior observed under electrochemical control and strongly suggesting that sodium pumped from the solid electrolyte is the key species. XP and Auger spectra show that under reaction conditions, the sodium exists as a surface carbonate. Post-reaction TPD spectra and the use of13CO demonstrate that CO is formed as a stable reaction intermediate. The observed activation energy (56±3 kJ/mol) is similar to that measured for CO oxidation under comparable conditions, suggesting that the rate limiting step is CO oxidation.

Selective Photobleaching of (−Ag−O−) Strings on the Ag(110) Surface

A p(2×3)-N Ag(110) surface obtained by N+ and/or N2+ ion bombardment gave a desorption peak of N2 at 530 K. When this p(2×3)-N Ag(110) surface was exposed to O2 at room temperature, the LEED pattern showed a combined structure of [p(2×3)-N + p(2×1)-O]. The STM image proved the growth of the (−Ag−O−) and (Ag2N) strings in the [001] and [11̄0] directions, respectively. The TPD spectrum of the combined surface gave the desorption of N2 at 530 K, NO at 490−520 K, and O2 at 600 K. When this [p(2×3)-N + p(2×1)-O] Ag(110) surface was illuminated with a UV light at room temperature, the (−Ag−O−) strings in the STM image were selectively erased although the (Ag2N) underwent little influence, and the LEED pattern changed to the p(2×3)-N structure. The TPD spectrum of the surface after the illumination gave only N2. These results indicate that surface migration of N atoms triggers the reaction between N and N yielding N2 as well as N and O yielding NO on the surface. Selective photobleaching for the combined structure may suggest a feasibility of atomic scale lithography of the surface by using photochemical reactions.

Activation of Hydrogen over Sulfate-Promoted Iron Oxide

The mechanism for activation of hydrogen over sulfate-promoted iron oxide has been investigated by means of hydrogen TPD and FTIR spectrometry of adsorbed pyridine. The catalyst activated isomerization of 1-butene to 2-butene and hydrogenation of 1,3-butadiene to form 1-butene, trans- and cis-2-butene, and small amounts of n-butane, but the isomerization activity decreased when the catalyst was prereduced and disappeared when the catalyst was reduced at 723 K although the hydrogenation activity still remained. In the section of the isomerization of 1-butene when the reaction was carried out in the absence of hydrogen, it is clear that the sulfate itself in the catalyst catalyzed the isomerization of 1-butene in the absence of hydrogen. However, in the presence of hydrogen, the bifunctional mechanism was observed. It should be, therefore, considered that the isomerization was promoted by the protonic acids formed on the Fe-sulfate catalyst by the dissociation of hydrogen molecules. In TPD experiments, the hydrogen TPD spectra appeared in a wide range of temperatures between 373 and 623 K or above. In FTIR spectrometry of the adsorbed pyridine on the catalyst, an increase in intensity of protonic acid sites as well as a decrease of Lewis acid sites was observed by heating the pyridine-covered catalyst in the presence of hydrogen at 423 K or over. This indicates the conversion of hydrogen molecules into protonic acids. Also, we tried to find hydride-like species on the surface of the catalyst but could not find them. In conclusion, the presence of sulfate species in the iron oxide catalyst is essential for the activation of hydrogen, promoting the bifunctional activity for the hydrogenation and the isomerization of olefins. The overall expression for the mechanism of the activation of hydrogen over the Fe-sulfate catalyst is explained by the following equation (Fe2+ + H2 = Fe0 + 2H+) followed by another equation ([Fe2+−O2- ] + H+ + H- = Fe+−H + H+ −O-).

Reactive oxygen atoms on Cu(110) formed at 100 K: vibrational spectra and CO oxidation

The dissociative adsorption of oxygen on Cu(110) at 100 K and its reactivity for CO oxidation were studied by HREELS, TPD and LEED. Oxygen dissociatively adsorbs on Cu(110) at 100 K. These oxygen adatoms, designated as the as-exposed oxygen, form no ordered structure. Annealing this surface above 200 K causes the appearance of the (2 × 1)-O ordered structure which corresponds to the added row type reconstruction. A characteristic change in the loss feature of ν(CuO) was found upon annealing Cu(110) covered with the as-exposed oxygen by HREELS, which corresponded to the change from the as-exposed oxygen to oxygen atoms in the (2 × 1)-O phase. The as-exposed oxygen on Cu(110) at 100 K oxidized CO to form CO 2, while oxygen atoms in the (2 × 1)-O phase were inactive in TPD conditions. It was found that the reactivity of the as-exposed oxygen was more than 25 times higher than that of oxygen in the (2 × 1)-O phase. Four peaks at 125 K (α), 150 K (β), 180 K (γ) and 210 K (δ) were found in the CO 2 TPD spectra. The features of these peaks were discussed based on TPD spectra with various CO coverages at constant coverages of oxygen. In the coadlayer of CO and oxygen, CO species with a ν(CuCO) peak at 285 cm −1 was observed in addition to the ν(CuCO) peak at 340 cm −1 observed for a pure CO adlayer on Cu(110). Both the as-exposed oxygen and the oxygen in the (2 × 1)-O phase caused this downward shift.

Structural effects on water formation from coadsorbed H + O on Rh(100)

The effect of adsorbate coverage, adsorption sequence and temperature on the structure, composition and reactivity of coadsorbed layers, produced by dissociative adsorption of O 2 and H 2 at 200 K on a Rh(100) surface, has been studied by combined TPD, XPS and LEED measurements. The emphasis is on the impact of the structure and composition of the mixed O + H layers on the synthesis of hydroxyl and water as a result of the O + H surface reaction. The difference in the O 1s binding energies of adsorbed O (529.9 eV) and OH species (530.8 eV) was used as a fingerprint to monitor the formation of the OH species. The H 2O TPD spectra show substantial variations of the desorption temperature range and the amount of water evolved with coadsorbate coverage and structure: from 270 to 350 K and from 0 to 0.08 ML, respectively. It has been found that dense O + H adlayers, where the O coverage is in the range 0.25-0.4 ML, favor the formation of stable OH species. The maximum amount of stable hydroxyl OH species (∼ 0.16 ML) can be produced by heating of these dense adlayers to ∼ 260 K. This results in reordering of the adspecies to form a new O + OH − (2 × 6) structure, where hydroxyls react readily to evolve 0.08 ML of water in a sharp desorption peak at ∼ 280 K. The effect of the adlayer density and restructuring on the production of OH and H 2O is discussed.

The influence of oxide surface structure on adsorbate chemistry: desorption of water from the smooth, the microfaceted and the ion sputtered surfaces of TiO 2(100)

The relationship between oxide surface structure and oxide surface chemistry was examined by studying the desorption of water from three structurally unique surfaces of the (100) face of TiO 2 (rutile): the smooth (1 × 1) surface, the microfaceted (1 × 3) surface and the ion sputtered surface. The (1 × 1) surface structure reflects bulk termination along the (100) plane. This surface possesses one type of surface Ti site. The (1 × 3) surface has {110} microfacets and possesses three distinctly different surface Ti sites. On each surface, water adsorbed at 130 K with unity sticking probability and was the only detectable desorption product between 130 and 800 K. Water exposures below 2.2 × 10 14 molecules cm 2 (0.30 ML) desorbed from the (1 × 1) and (1 × 3) surfaces in similar TPD peaks at 350 K. The zero coverage desorption energies from both surfaces were estimated to be 18 kcal mol . TPD spectra from higher water exposures were very different for the (1 × 1) and (1 × 3) surfaces. A single desorption feature was observed from the (1 × 3) surface which shifted from 350 K at low exposure to 238 K as the monolayer saturated. Two distinct TPD features were observed for submonolayer exposures on the (1 × 1) surface. The first feature saturated with an exposure of 2.2 × 10 14 molecules cm 2 at 350 K and the second grew in at 250 K with higher exposures. Assignment of these two peaks to dissociative and molecular states of water, respectively, was chosen over assignment to two different adsorption sites or phases of molecular water. Direct comparison of TPD data from the two surfaces indicates that the (1 × 1) surface binds more water above room temperature than does the (1 × 3) surface. There were subtle differences between water interactions with two-coordinate bridging O 2− sites (second layer water) on the two surfaces which suggested the bridging O 2− atoms on the (1 × 3) surface were more basic. In general, water was more weakly bound to the sputtered surface than to either the (1 × 1) or (1 × 3) surfaces, although some irreversible water decomposition ( 1.3 × 10 14 molecules cm 2 ) occurred.

The interaction of H 2O and Cl 2 on Fe polycrystalline surfaces

Temperature-programmed desorption and Auger spectroscopy were used to study the interaction of H 2O and Cl 2 on an Fe polycrystalline surface under UHV conditions over the temperature range of 90–900 K. The TPD spectra of H 2O on a clean Fe(poly) surface show the desorption of H 2O from two states at 175 and 205 K. There is no recombinant H 2O desorption from the Fe(poly), in contrast to the Fe(100) surface. The adsorption and dissociation of H 2O is significantly affected by the presence of Cl at the Fe(poly) surface. Pre-exposure of Cl 2 results in dissociatively adsorbed Cl (Cl a) at the surface. Cl a acts as a site blocker, preventing H 2O dissociation. Cl 2 adsorption on an Fe(poly) surface pre-exposed to H 2O at 90 K leads to a Cl 2-H 2O reaction which results in the formation of an OH-Cl complex. This complex is stabilized to temperatures above 400 K in UHV and results in recombinant H 2O desorption over a broad temperature range. Desorption of an FeOCl complex is observed at 240 K regardless of dosing sequence, but only at low adsorbate pre-coverages. These data demonstrate that Cl a inhibits hydroxylation of the Fe(poly) surface. The low temperature volatilization of the Fe surface via Fe oxychloride formation does not directly involve H 2O (a), but can only occur after H 2O dissociation.

The surface science based ammonia kinetics revisited

A previously developed microkinetic model of NH3 synthesis is extended to include recent experimental results on the kinetics of N2-dissociation obtained from molecular beam studies and on the NHx-K-repulsion obtained from TPD studies. Further, we show that agreement between the experimental and calculated TPD spectrum for N2 can be made perfect by including a small coverage dependency in the activation energy for N2 -dissociation. Finally we show that there is a close relation between the TPD spectrum for N2 used as input for microkinetic models and the calculated rate of NH3 synthesis calculated from the model.

CO adsorption on Rh(100) covered with ordered (3 × 1) and (2 × 1) nitrogen layers

Room temperature CO adsorption on Rh(100) modified with ordered (3 × 1) N and (2 × 1) N overlayers has been studied by means of XPS, TPD and LEED. The CO and N 2 TPD spectra and the O 1s and N 1s XPS spectra were used for measuring the coverage and determining the coadsorption effect on the CO and N adsorption states. It has been found that CO coadsorbed with N is characterised by a O 1s binding energy at ∼ 532.1 eV which is slightly higher than the value of 531.9 eV measured for on-top CO on a N-free surface. Both the adsorption rate and the adsorptive capacity of the surface towards CO are reduced by the presence of ordered N layers. The CO and N 2 TPD spectra from mixed CO + N layers show an enhanced desorption rate and exhibit new low-temperature features not present in the TPD spectra when each species is adsorbed alone. LEED data have revealed that CO coadsorption at 305 K leads to substantial changes of the initial N-related (3 × 1) and (2 × 1) patterns. When CO is coadsorbed on a surface precovered with a (3 × 1) N layer the CO-induced changes involve streaking of the 1 3 N-related fractional order spots and an appearance of weak extra spots indicating formation of poorly ordered c(2 × 2) and (2 × 1) p2 mg CO domains. CO coadsorption on a surface with a (2 × 1) N layer causes streaking in the [1 1 0] direction with an enhanced intensity at 1 4 order positions. Stepwise annealing and partial desorption of CO from dense CO + N layers lead to structural rearrangements and formation of new composite LEED patterns due to formation of separate ordered CO and N islands. The results are interpreted considering the possible interactions between CO and N in the coadsorbed layer.

Monte Carlo simulations of temperature programmed desorption spectra

We introduce a Monte Carlo algorithm which simulates first-order temperature programmed desorption spectra using the correct connection between Monte Carlo simulation time and real time. The TPD spectra obtained from the Monte Carlo simulations are exactly the same as those calculated from the governing continuum differential equation for a noninteracting system. We discuss in some detail the effects of lateral interactions between adsorbates on the temperature programmed desorption spectra. We find that peak splitting of the temperature programmed desorption spectra characterizes an adsorbate configuration transition on the surface. Furthermore, we analyze in detail the behavior of the coverage-dependent activation energy and preexponential factor for the case of a repulsive nearest-neighbor interaction and find that kinetic compensation occurs in this case.

The effect of boron modification of Pt/Al 2O 3 on hydrogen chemisorption

The effect of boron modification of Pt/Al 2O 3 on hydrogen chemisorption has been studied by the temperature-programmed desorption method. The X-ray diffraction technique was also used to investigate the crystal phase of alumina support and Pt metal particle size upon boron modification. The TPD spectra revealed a major peak accompanied by an unresolved high-temperature shoulder. The major peak is assigned to adsorption of hydrogen on a Pt metal site. The upward shift of this peak upon boron modification indicates a stronger Pt-H interaction. The high-temperature shoulder can be attributed to hydrogen spillover from the metal onto the support. The peak area of this high-temperature shoulder is increased upon boron modification, indicating that boron can enhance the spillover of hydrogen.

Interaction of CO with silicon single-crystal surfaces studied by HREELS, UPS and TPD

When 3 L CO was dosed on Si substrates at 100 K, the CO molecule readily adsorbed on Si(100)-2 × 1, but caused no obvious change in the HREELS of Si(111)-7 × 7. The interaction between CO and Si(100)-2 × 1, however, is rather weak as indicated by the low Si-C stretching frequency (51 meV) and the small red-shift (8 meV) of the C-O stretching mode from its free molecular value. Nearly all of the CO molecules desorbed from the Si(100) when the surface was warmed to T s = 220 K or when the surface was irradiated with 308 nm excimer laser for two 2 min. In the TPD spectrum, a desorption peak at 180 K was observed for m e = 29 ( 13CO) . We report, for the first time, the vibrational spectrum of the CO molecule adsorbed on a Si single-crystal surface.