Formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys

The formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys were investigated by X-ray photoelectron spectroscopy (XPS) and by adsorption of CO probe molecules, which was characterized by temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The PdAg/Pd(1 1 1) surface alloys were prepared by annealing (partly) Ag film covered Pd(1 1 1) surfaces, where the Ag films were deposited at room temperature. Surface alloy formation leads to a modification of the electronic properties, evidenced by core-level shifts (CLSs) of both the Pd(3d) and Ag(3d) signal, with the extent of the CLSs depending on both initial Ag coverage and annealing temperature. The role of Ag pre-coverage and annealing temperature on surface alloy formation is elucidated. For a monolayer Ag covered Pd(1 1 1) surface, surface alloy formation starts at ∼450 K, and the resulting surface alloy is stable upon annealing at temperatures between 600 and 800 K. CO TPD and HREELS measurements demonstrate that at 120 K CO is exclusively adsorbed on Pd surface atoms/Pd sites of the bimetallic surfaces, and that the CO adsorption behavior is dominated by geometric ensemble effects, with adsorption on threefold hollow Pd₃ sites being more stable than on Pd₂ bridge sites and finally Pd₁ a-top sites.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

Adsorption of carbon monoxide Au/Pd(1 0 0) alloys in ultrahigh vacuum: Identification of adsorption sites

The adsorption of carbon monoxide is studied on Au/Pd(1 0 0) alloys by means of reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The alloy was formed by adsorbing a four-monolayer thick gold film on a Pd(1 0 0) substrate and by heating to various temperatures to form alloys with a range of palladium coverages. The alloy was characterized using X-ray photoelectron spectroscopy and the composition of the outermost layer measured using low-energy ion scattering spectroscopy. CO adsorbs on palladium bridge sites only for palladium coverages greater than 0.5 monolayers (ML) suggesting that next-nearest neighbor sites are preferentially populated by palladium atoms. CO adsorbs on atop palladium sites and desorbs at ∼350 K corresponding to a desorption activation energy of ∼117 kJ/mol. However, at lower palladium coverages, these sites are not occupied and CO desorption states are detected 170 and 112 K corresponding to desorption activation energies of ∼53 kJ/mol and ∼35 kJ/mol, respectively, for these states. It is suggested that these states are due to a restructuring of the surface to form low-coordination gold sites that obscure the atop palladium site.     

The adsorption of ethylene on Au/Pd(1 1 1) alloy surfaces

The adsorption of ethylene on gold-palladium alloys formed on a Pd(1 1 1) surface is investigated using a combination of temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Various alloy compositions are obtained by depositing four monolayers of gold on a clean Pd(1 1 1) surface and annealing to various temperatures. For gold coverages greater than ∼0.7, ethylene adsorbs primarily on gold sites, desorbing with an activation energy of less than 55 kJ/mol. At gold coverages between ∼0.5 and ∼0.7, ethylene is detected on palladium sites in a π-bonded configuration (with a σ-π parameter of ∼0.1) desorbing with an activation energy of between ∼57 and 62 kJ/mol. Further reducing the gold coverage leads to an almost linear increase in the desorption activation energy of ethylene with increasing palladium content until it eventually reaches a value of ∼76 kJ/mol found for ethylene on clean Pd(1 1 1). A corresponding increase in the σ-π parameter is also found as the gold coverage decreases reaching a value of ∼0.8, assigned to di-σ-bonded ethylene as found on clean Pd(1 1 1).     

Growth of epitaxial thin Pd(1 1 1) films on Pt(1 1 1) and oxygen-terminated FeO(1 1 1) surfaces

Thin Pd films (1-10 monolayers, ML) were deposited at 35 K on a Pt(1 1 1) single crystal and on an oxygen-terminated FeO(1 1 1) monolayer supported on Pt(1 1 1). Low energy electron diffraction, Auger electron spectroscopy, and Kr and CO temperature programmed desorption techniques were used to investigate the annealing induced changes in the film surface morphology. For growth on Pt(1 1 1), the films order upon annealing to 500 K and form epitaxial Pd(1 1 1). Further annealing above 900 K results in Pd diffusion into the Pt(1 1 1) bulk and Pt-Pd alloy formation. Chemisorption of CO shows that even the first ordered monolayer of Pd on Pt(1 1 1) has adsorption properties identical to bulk Pd(1 1 1). Similar experiments conducted on FeO(1 1 1) indicate that 500 K annealing of a 10 ML thick Pd deposit also yields ordered Pd(1 1 1). In contrast, annealing of 1 and 3 ML thick Pd films did not result in formation of continuous Pd(1 1 1). We speculate that for these thinner films Pd diffuses underneath the FeO(1 1 1).     

Chemistry of Alanine on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The adsorption of alanine is studied on a Pd(1 1 1) surface using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is found that alanine adsorbs into the second and subsequent layers prior to completion of the first monolayer for adsorption at ∼250 K, while at ∼300 K, alanine adsorbs almost exclusively into the first monolayer with almost no second-layer adsorption. Alanine adsorbs onto the Pd(1 1 1) surface in its zwitterionic form, while the multilayer contains about 30-35% neutral alanine, depending on coverage. Alanine is thermally stable on the Pd(1 1 1) surface to slightly above room temperature, and decomposes almost exclusively by scission of the CCOO bond to desorb CO₂ and CO from the COO moiety, and the remaining fragment yields ethylamine and HCN.     

Electronic properties of thin Zn layers on Pd(1 1 1) during growth and alloying

The growth and alloying of thin Zn layers on Pd(1 1 1) was investigated using X-ray and ultraviolet photoelectron spectroscopy as well as low energy electron diffraction and correlated with density functional calculations. At 105 K, the formation of a pseudomorphic Zn monolayer is observed. Upon heating this layer to 550 K or upon deposition of 1 ML at 550 K, an ordered p(2 × 1) PdZn surface alloy with a Pd:Zn ratio of ∼1:1 is formed, with a characteristic Pd 3d_5/2 peak at a binding energy of ∼335.6 eV. For deposition of 3 ML Zn at 550 K or by heating 3 ML, deposited at low temperature, to 500 or 600 K, a PdZn alloy with a Pd:Zn ratio of again ∼1:1 is found in the surface region, with a Pd 3d_5/2 peak at ∼335.9 eV; the direct preparation at 550 K leads to a more homogeneous and better ordered alloy. The valence band spectrum of this alloy with a low density of states at the Fermi level and pronounced maxima due to the “Pd 4d” band at ∼2.4 and 3.9 eV closely resembles the spectrum of Cu(1 1 1), in good agreement with the calculated density of states for a PdZn alloy of 1:1 stoichiometry. The shift of the “Pd 4d” band to higher binding energies as compared to Pd(1 1 1) indicates a charge transfer from Zn to the Pd 4d levels. Overall, the similarity between the ultraviolet photoelectron spectra for the PdZn alloy and for Cu(1 1 1) is taken as explanation for the similar chemical activity of both systems in methanol steam reforming.     

The adsorption of acetic acid on Au/Pd(1 1 1) alloy surfaces

The adsorption of acetic acid is studied as a function of gold content by temperature-programmed desorption and reflection-absorption infrared spectroscopy on Au/Pd(1 1 1) alloys formed by depositing 5 ML of gold onto a Pd(1 1 1) surface and heating to various temperatures. For mole fractions of gold greater than ∼0.5, acetic acid adsorbs molecularly and desorbs intact with an activation energy of ∼52 kJ/mol. This acetic acid is present as catemers, where the nature of the catemer is found to depend on gold concentration. When the relative gold concentration is less than ∼0.33, adsorption of acetic acid at 80 K and heating to ∼207 K forms η¹-acetate species on the surface. On further heating, these can either thermally decompose to eventually evolve hydrogen, water and oxides of carbon, or form η²-acetate species, where the coverage of reactively formed η²-acetate species increases with decreasing gold concentration in the near surface region.     

The reaction of vinyl acetate with Pd(1 1 0) studied with TPD and molecular beams

The adsorption and reaction of vinyl acetate with the clean Pd(1 1 0) surface has been investigated using temperature programmed desorption and molecular beam reaction measurements. These show that, under low pressure conditions, the main reaction pathway above 400 K is total dehydrogenation to yield hydrogen and carbon dioxide in the gas phase, and surface carbon. This occurs at a steady state, notwithstanding the fact that carbon is being deposited continuously onto the surface. The reaction continues because the vast majority of this carbon is lost from the surface to the bulk of the sample. Between about 320-380 K the reaction profile is somewhat different; the molecule dissociates at the CH₃COOCHCH₂ bond, producing the most stable intermediate, the acetate, and the reaction stops after the build-up of adsorbed acetate and surface carbonaceous species. At ∼300 K, the products are very similar to those for acetaldehyde adsorption (namely, methane, CO and some surface carbon), and they evolve in a non-steady state manner due to the build up of adsorbed CO on the surface. Thus the mechanism is dominated here by dissociation at the CH₃COOCHCH₂ bond, and formation of the acetyl intermediate. Consideration is given to the connection between these data and vinyl acetate synthesis.     

Comparison of the interaction of Pd with positively and negatively poled LiNbO3(0 0 0 1)

To investigate the possibility of manipulating the surface chemical properties of finely dispersed metal films through ferroelectric polarization, the interaction of palladium with oppositely poled LiNbO₃(0 0 0 1) substrates was characterized. Low energy ion scattering indicated that the Pd tended to form three-dimensional clusters on both positively and negatively poled substrates even at the lowest coverages. X-ray photoelectron spectroscopy (XPS) showed an upward shift in the binding energy of the Pd 3d core levels of 0.9 eV at the lowest Pd coverages, which slowly decayed toward the bulk value with increasing Pd coverage. These shifts were independent of the poling direction of the substrate and similar to those attributed to cluster size effects on inert supports. Thus, the spectroscopic data suggested that Pd does not interact strongly with LiNbO₃ surfaces. The surface chemical properties of the Pd clusters were investigated using CO temperature programmed desorption. On both positively and negatively poled substrates, CO desorption from freshly deposited Pd showed a splitting of the broad 460 K desorption peak characteristic of bulk Pd into distinct peaks at 270 and 490 K as the Pd coverage was decreased below 1.0 ML; behavior that also resembles that seen on inert supports. It was found that a small fraction of the adsorbed CO may dissociate (<2%) for Pd on both positively and negatively poled substrates. The thermal response of the smaller Pd clusters on the LiNbO₃ surfaces, however, was different from that of inert substrates. In a manner similar to Nb₂O₅, when CO desorption experiments were carried out a second time, the adsorption capacity decreased and the higher temperature desorption peak shifted from 490 K to below 450 K. This behavior was independent of the substrate poling direction. Thus, while there was evidence that LiNbO₃ does not behave as a completely inert support, no significant differences between positively and negatively poled surfaces were observed. This lack of sensitivity of the surface properties of the Pd to the poling direction of the substrate is attributed to the three-dimensional Pd clusters being too thick for their surfaces to be influenced by the polarization of the underlying substrate.     

Acetaldehyde adsorption and catalytic decomposition on Pd(1 1 0) and the dissolution of carbon

The reaction of acetaldehyde with the Pd(1 1 0) surface has been studied using a molecular beam reactor, TPD and LEED. Below 270 K acetaldehyde sticks to the surface with a high initial probability (∼0.8), but no gas phase products evolve. When the reaction is run at >270 K, hydrogen evolves into the gas phase early in the reaction together with methane in a non-steady-state fashion, but above 300 K there is a very efficient steady-state catalytic reaction at the surface; this reaction is the decarbonylation of acetaldehyde to produce methane and carbon monoxide in the gas phase. This behaviour continues up to about 400 K. However, when acetaldehyde is dosed at 423 K, the reaction rate slowly evolves through a maximum to a very low catalytic rate. Upon carrying out reactor experiments at 473 K and above, the reaction mechanism changes to total dehydrogenation, and CO and H₂ are produced at high steady-state rate, not withstanding the fact that carbon is continually being deposited onto the surface. This carbon does not appear to affect the reaction, which takes place on a surface with a c(2 × 2)-C layer present, since the extra carbon is lost from the reaction zone by diffusion into the bulk of the crystal.     

Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Infrared reflection absorption spectral study for CO adsorption on Pd/Pt(1 1 1) bimetallic surfaces

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on 0.15 nm-thick-0.6 nm-thick Pd-deposited Pt(1 1 1) bimetallic surfaces: Pd_x/Pt(1 1 1) (where x is the Pd thickness in nanometers) fabricated using molecular beam epitaxial method at substrate temperatures of 343 K, 473 K, and 673 K. Reflection high-energy electron diffraction (RHEED) measurements for Pd_0.15-0.6 nm/Pt(1 1 1) surfaces fabricated at 343 K showed that Pd grows epitaxially on a clean Pt(1 1 1), having an almost identical lattice constant of Pt(1 1 1). The 1.0 L CO exposure to the clean Pt(1 1 1) at room temperature yielded linearly bonded and bridge-bonded CO-Pt bands at 2093 and 1855 cm⁻¹. The CO-Pt band intensities for the CO-exposed Pd_x/Pt(1 1 1) surfaces decreased with increasing Pd thickness. For Pd_0.3 nm/Pt(1 1 1) deposited at 343 K, the 1933 cm⁻¹ band caused by bridge-bonded CO-Pd enhanced the spectral intensity. The linear-bonded CO-Pt band (2090 cm⁻¹) almost disappeared and the bridge-bonded CO-Pd band dominated the spectra for Pd_0.6 nm/Pt(1 1 1). With increasing substrate temperature during the Pd depositions, the relative band intensities of the CO-Pt/CO-Pd increased. For the Pd_0.3 nm/Pt(1 1 1) deposited at 673 K, the linear-bonded CO-Pt and bridge-bonded CO-Pd bands are located respectively at 2071 and 1928 cm⁻¹. The temperature-programmed desorption (TPD) spectrum for the 673 K-deposited Pd_0.3 nm/Pt(1 1 1) showed that a desorption signal for the adsorbed CO on the Pt sites decreased in intensity and shifted ca. 20 K to a lower temperature than those for the clean Pt(1 1 1). We discuss the CO adsorption behavior on well-defined Pd-deposited Pt(1 1 1) bimetallic surfaces.     

Adsorption of water on thin V2O3(0 0 0 1) films

V₂O₃(0 0 0 1) films have been grown epitaxially on Au(1 1 1) and W(1 1 0). Under typical UHV conditions these films are terminated by a layer of vanadyl groups as has been shown previously [A.-C. Dupuis, M. Abu Haija, B. Richter, H. Kuhlenbeck, H.-J. Freund, V₂O₃(0 0 0 1) on Au(1 1 1) and W(1 1 0): growth, termination and electronic structure, Surf. Sci. 539 (2003) 99]. Electron irradiation may remove the oxygen atoms of this layer. H₂O adsorption on the vanadyl terminated surface and on the reduced surface has been studied with thermal desorption spectroscopy (TDS), vibrational spectroscopy (IRAS) and electron spectroscopy (XPS) using light from the BESSY II electron storage ring in Berlin. It is shown that water molecules interact only weakly with the vanadyl terminated surface: water is adsorbed molecularly and desorbs below room temperature. On the reduced surface water partially dissociates and forms a layer of hydroxyl groups which may be detected on the surface up to T ∼ 600 K. Below ∼330 K also co-adsorbed molecular water is detected. The water dissociation products desorb as molecular water which means that they recombine before desorption. No sign of surface re-oxidation could be detected after desorption, indicating that the dissociation products desorb completely.     

Sulphur overlayers on the Au(1 1 0) surface: LEED and TPD study

The adsorption and desorption of sulphur on the clean reconstructed Au(1 1 0)-(1 × 2) surface has been studied by low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. The results obtained show a complex behaviour of the S/Au(1 1 0) system during sulphur desorption at different temperatures. Two structures of the stable ordered sulphur overlayer on the Au(1 1 0) surface, p(4 × 2) and c(4 × 4), were found after annealing the S/Au(1 1 0) system at 630 K and 463 K, respectively. The corresponding sulphur coverage for these overlayers was estimated by AES signal intensity analysis of the Au NOO and S LMM Auger lines to be equal to 0.13 ML and 0.2 ML, respectively. Both sulphur structures appear after removing an excess of sulphur, which mainly desorbs at 358 K as determined from TPD spectra. Furthermore, it was not possible to produce the lower coverage p(4 × 2) sulphur structure by annealing the c(4 × 4) surface. In the case of the p(4 × 2) S overlayer on the Au(1 1 0)-(1 × 2) surface it is proposed that the sulphur is attached to “missing row” sites only. The c(4 × 4) S overlayer arises via desorption of S₂ molecules that are formed on the surface due to mobility of sulphur atoms after a prolonged anneal.     

CO2 adsorption on the bimetallic Zn-on-Cu(1 1 0) system

Adsorption probability measurements (molecular beam scattering) have been conducted to examine the adsorption dynamics (i.e. the gas-surface energy transfer processes) of CO₂ adsorption on the Zn-on-Cu(1 1 0) bimetallic system. The results indicate surface alloy formation, which is in agreement with prior studies. Depositing Zn at 300 K on Cu(1 1 0), above the condensation temperature of CO₂, leads to a “blocking” of CO₂ adsorption sites by Zn which is incorporated in the Cu(1 1 0) surface. This apparent site blocking effect indicates a lowering of the CO₂ binding energy on the alloyed surface as compared with the clean Cu(1 1 0) support. The Zn coverage has been calibrated by Auger electron spectroscopy and thermal desorption spectroscopy.     

An IRAS study of CO bonding on Sn/Pt(1 1 1) surface alloys at maximal pressures of 10 Torr

The adsorption of CO on Pt(1 1 1), (2 × 2) and (√3 × √3)R30° Sn/Pt(1 1 1) surface alloys has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED) and infrared reflection adsorption spectroscopy (IRAS). The presence of Sn in the surface layer of Pt(1 1 1) reduces the binding energy of CO by a few kcal/mol. IRAS data show two C-O stretching frequencies, ∼2100 and ∼1860 cm⁻¹, corresponding to atop and bridge bonded species, respectively. Bridge bonded stretching frequencies are only observed for Pt(1 1 1) and (2 × 2) Sn/Pt(1 1 1) alloy surfaces. A slight coverage dependence of the vibrational frequencies is observed for the three surfaces. High pressure IRAS experiments over a broad temperature range show no indication of bridge bonded CO on any of the three surfaces. Direct CO adsorption on Sn sites is not observed over the measured temperature and pressure ranges.     

Methanol adsorption on Pd(1 1 0) and Ag/Pd(1 1 0) studied by high-resolution photoelectron spectroscopy

Adsorption of methanol on clean Pd(1 1 0) and on an alloyed Ag/Pd(1 1 0) surface has been studied by high-resolution photoelectron spectroscopy. On Pd(1 1 0) two different chemisorbed methanol species were observed for temperatures up to 200 K, with the one at lower binding energy remaining at low coverage. These species were attributed to methanol adsorbed in two different adsorption sites on the Pd(1 1 0) surface. As is well established for this system, heating to 250 K resulted in decomposition of methanol into CO. The adsorption and decomposition behaviour of methanol on the Ag/Pd(1 1 0) surface alloy formed by depositing Ag on Pd(1 1 0) at elevated temperature was similar to that of the pure Pd(1 1 0) surface. This suggests that the amount of Ag present in the Pd(1 1 0) surface in this study does not affect the decomposition behaviour of methanol as compared to pure Pd(1 1 0). Complementary density functional theory calculations also show little influence of Ag on the binding of methanol to Pd. These calculations predict an on-top adsorption site for low methanol coverages.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

Chemistry of l-proline on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The surface chemistry of proline is explored on Pd(1 1 1) using a combination of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy. Proline adsorbs on Pd(1 1 1) at temperatures of 250 K and below into second and subsequent layers prior to the saturation of the first layer, where approximately 70% of the adsorbed proline is present in its zwitterionic form. Molecular proline desorbs between ∼315 K and ∼333 K depending on coverage. When adsorbed at ∼300 K, only the first monolayer is formed, and the proline is present as zwitterions, oriented such that all of the carbons are detected equally by XPS. Proline decomposes by scission of the C-COO bond, where the carboxylate moiety desorbs as carbon monoxide and carbon dioxide, while the nitrogen-containing moiety desorbs as to HCN, and evolves pyrrole at ∼390 K, pyrrolidine at ∼410 K, and final species that desorbs at ∼450 K that cannot be unequivocally assigned but may be 2-butenenitrile (CH₃-CHCH-CN), 3-butenenitrile (CH₂CH-CH₂-CN), 2-methyl-2-propenenitrile (CH₂C(CH₃)-CN) or cyclopropanecarbonitrile.