Application of thermal desorption methods in studies of catalysis: II. The oxidation of carbon monoxide on platinum

One of the major difficulties encountered in catalytic research has been the experimental characterization of the catalyst surface in its working state. Using thermal desorption techniques, we have developed a method capable of providing a quantitative measure of the concentrations of certain adsorbed reactants during catalysis. In a study of the oxidation of CO on polycrystalline platinum, three distinct reaction mechanisms have been distinguished: (1) the interaction of an oxygen molecule with a vacant Pt surface site and an adjacent adsorbed CO molecule forming a complex which dissociates to form a CO₂ molecule and an adsorbed oxygen atom; (2) the reaction between adsorbed CO and an adsorbed oxygen atom, a process in which surface transport is required to bring the reactants together; and (3) the reaction between a colliding CO molecule and an adsorbed oxygen atom. Rate constants for the reaction steps have been measured, and the active surface sites and reactive surface states on the Pt catalyst have been characterized in terms of the adsorption properties of CO.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

A photoemission and thermal desorption study of carbon monoxide and oxygen adsorbed on platinum

We have studied the changes in the photoelectron spectra of platinum (for photon energies of 21.2 and ? 10.2 eV), in conjunction with thermal desorption experiments, for coverages of carbon monoxide and oxygen of up to ~0.25 monolayer (saturation coverage at room temperature). Based on a comparison of the photoemission and thermal desorption results, we suggest that the less tightly bound of the two adsorption states observed in the thermal desorption data is due to adsorbate-adsorbate interactions. We further suggest that a relatively delocalized chemisorption bond plays an important role in this interaction.     

Reflection-absorption infrared and thermal desorption studies of carbon monoxide and hydrogen adsorbed on rhodium

Reflection-absorption infrared spectroscopic and thermal desorption techniques have been used to study the interaction of mixtures of carbon monoxide and hydrogen with evaporated rhodium films. For equimolar mixtures near 10^?9 Torr, hydrogen adsorbed much more rapidly, but long exposure times or increases in CO pressures to 10^?6 Torr led to its partial, but never complete, displacement by adsorbed carbon monoxide. Hydrogen desorption spectra taken during the displacement process showed two peaks which was consistent with a cooperative interaction between adsorbed CO and H species. In contrast to previous transmission studies of CO adsorption on small rhodium particles, the present reflection—absorption infrared study of the film system showed a single absorption band at 2075 ±10 cm^?1. While explanations for the discrepancy in terms of particle size effects are possible it is considered more likely that all CO molecules are linearly bound to individual Rh atoms in the present situation. In our work, increases in CO pressure (especially above 10^?6 Torr) were accompanied by an upward frequency shift (from 2065 cm^?1 to 2085 cm^?1) and a narrowing in half width (from 25 to 17 cm^?1). Several possible explanations for the latter unusual effect are discussed.     

Chemisorption studies on cobalt single crystal surfaces: I. Carbon monoxide on Co(0001)

The chemisorption of CO on Co(0001) and on a polycrystalline specimen has been studied by LEED, Auger spectroscopy, and thermal desorption measurements. Annealing of the polycrystal was found to result in a surface dominated by crystallites of (0001) orientation in the surface plane, along with a few (101̄2) oriented crystallites. CO adsorbs on the clean surface at 300 K with an initial sticking probability of 0.9 and the system follows precursor state kinetics. The saturation coverage under UHV conditions corresponds to a well-ordered (√3 × √3)R30° structure; with P_CO>5 × 10^-9 a uniform compression of the adlayer takes place and a (√7 × √7)R19.2° structure begins to form. Models are proposed for these two ordered phases which are in agreement with the observed relative coverage data and the appearance of the corresponding desorption spectra. The desorption enthalpy of CO at low coverages is 103 ± 8 kJmol^-1, and a fairly sharp fall in this enthalpy occurs for coverages $\text{>}\text{1}\text{3}$. In many respects, the system's behaviour closely resembles that of Ni(111)-CO. Oxygen contamination leads to the appearance of a strongly adsorbed CO state with a desorption enthalpy of ~170 kJmol^-1. This is reminiscent of a strongly adsorbed non-dissociated state of CO on Ru(101̄1) which occurs under similar conditions.     

Chemisorption of carbon monoxide on copper-nickel alloy surfaces

The adsorption of CO on Cu-Ni alloy surfaces has been studied at 300 and 120 K using LEED, AES, TDS, and work function measurement. The alloys have been prepared as thin (111) epitaxial films evaporated on mica, and as poly crystalline foils. At 300 K the alloy surfaces show an adsorption behavior similar to that of pure Ni: the work function increases to a saturation value which is higher for Ni-rich surfaces than for Cu-rich. The isosteric heat of adsorption (106 kJ/mole) is nearly as high as with pure Ni. At 120 K the alloys exhibit a more Cu-like adsorption behavior: the work function passes through a minimum which becomes deeper at higher Cu surface concentration. The isosteric heat of adsorption at low temperatures (50 kJ/mole) is nearly as low as for pure Cu. From TDS it can be shown, that the binding energy of the highest (Ni-like) adsorption states increases with increasing Ni surface concentration. At the (111) alloy surfaces no LEED superstructures due to CO adsorption could be observed.     

Transient studies of carbon monoxide oxidation over platinum catalyst

The applicability of transient techniques to the study of the catalytic oxidation of carbon monoxide is discussed. It is shown that at 100–150°C adsorption and desorption equilibria between the gas phase and catalyst cannot be assumed, and an elementary step formulation may be used to predict both multiple steady states and transient behaviour.     

Chemisorption and dissociation of carbon monoxide on rhodium surfaces

The adsorption, desorption and decomposition of CO on Rh surfaces have been investigated using field emission microscopy and thermal desorption spectroscopy. Thermal dissociation of CO cannot be detected on clean Rh surfaces at pressures up to 10^?1 Torr and temperatures below 1000 K. This holds also for atomically rough surfaces like (210). CO dissociation can be promoted under the influence of an electron beam directed to the surface, a high electric field in the presence of CO in the gas phase and by means of discharge techniques. The growth of crystallites formed by CO dissociation and the diffusion of carbon into the bulk has been followed as a function of temperature and surface structure. The tip regions around (110) are very active in these processes. Carbon crystallites on these surfaces disappear around 1000 K by diffusion into the lattice whereas crystallites present around (311) surfaces persist up to 1150 K. The results are discussed in relation to the activity of Rh in CO/H₂ reactions.     

Adsorption of hydrogen and carbon monoxide on platinum

TDS spectra and ESD ion energy distributions have been measured for coadsorption of H₂ and CO on recrystallized platinum. Sample exposure to a 90% H₂?10% CO gas mixture results in appearance of structurein both TDS spectra of H₂ and CO. Coadsorption also results in an O⁺ ion energy distribution which is much narrower compared to the distribution resulting from pure CO adsorption. These results are interpreted as evidence for the formation of an HCO surface complex.     

Laser induced thermal desorption of carbon monoxide from Fe(110) surfaces

Thermal desorption of CO is induced by bombarding an Fe(110) surface with pulses of a neodymium glass laser. The maximum amplitude of the desorption signal is recorded by a mass spectrometer as a function of the laser pulse intensity and of the CO coverage for both single pulses and sequences of pulses. Since the half width of the laser pulses is only 30 ns the shape of the desorption signal is mainly determined by the time-of-flight of the desorbed particles. There is strong evidence that the latter obey a Maxwell-Boltzmann distribution of temperature T_d, identical in the low temperature range with the maximum surface temperature T_s. Above T_s = 600 K, however, T_d is smaller than T_s. The experimental observations are analyzed successfully with the first order rate equation for desorption.     

Studies on self-sustained reaction-rate oscillations: I. Real-time surface infrared measurements during oscillatory oxidation of carbon monoxide on platinum

Time-resolved infrared absorption features in the 1800–2400 cm^?1 region during a typical cycle in the oscillatory oxidation of CO over a platinum foil were obtained by Fourier transform infrared reflection absorption spectroscopy. Pretreatment of the foil in an oxidizing environment at high temperatures was found to be necessary to induce large-amplitude, stable oscillations. The oscillations are approximately square-wave in shape, with a high and a low reaction-rate branch. The level of chemisorbed CO in the high reaction-rate branch is typically below the noise level, while in the low reaction-rate branch substantial substantial surface coverages of CO can be observed. No evidence for CO bridge-bonded to the platinum substrate or chemisorbed in the presence of a subsurface Pt oxide could be found at any time during the oscillation cycle. Evidence is presented for the existence of CO islands in the low reaction-rate branch. It is also shown that the low reaction rate realized in this branch is not due to blocking of the surface by chemisorbed CO.     

XPS,UPS and thermal desorption studies of alcohol adsorption on Cu(110): I. Methanol

The adsorption of methanol on clean and oxygen dosed Cu(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Methanol was adsorbed on the clean surface at 140 K in monolayer quantities and subsequently desorbed over a broad range of temperature from 140 to 400 K. The UPS He (II) spectra showed the 5 highest lying emissions seen in the gas phase spectrum of methanol with a chemisorption bonding shift of the two highest lying orbitais due to bonding to the surface via the oxygen atom with which these orbitals are primarily associated. A species of quite a different nature was produced by heating this layer to 270 K. Most noticeably the UPS spectrum showed only 3 emissions and the maximum coverage of this state was approximately $\text{1}\text{2}$ monolayer. The data are indicative of the formation of a methoxy species, thus showing that methanol is dissociated on the clean Cu(110) surface at 270 K. The same dissociated species was observed on the oxygen dosed surface, the main difference in this ease being the production of large amounts of H₂CO observed in TPRS at 370 K.     

Theory of oscillatory oxidation of carbon monoxide over platinum

It is shown that the finite rate of decay for temperature fluctuations of the catalytic surface in the Langmuir-Hinshelwood reaction: 2 CO + O₂ → 2 CO₂ over Pt can give rise to a self-sustained oscillatory regime. The waveforms and periods are in good qualitative agreement with experiments by Dauchot and Van Cakenberghe.     

Bistability of the reaction rate in the oxidation of carbon monoxide by nitrogen monoxide on polycrystalline platinum

Multiplicity of the stationary reaction rate of CO oxidation by NO is observed under reducing conditions at pressures below 4×10^?4 mbar in the temperature range 500<T<525 K. The two branches of the reaction rate coalesce at a point where sustained oscillations are observed.     

Studies on self-sustained reaction-rate oscillations: II. The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum

CO oxidation on a platinum foil was studied in a high pressure flow cell (10²−10² Pa) and an UHV chamber (10⁻⁸ −5 × 10⁴ Pa) both interfaced to a surface infrared spectrometer. Real-time surface infrared and calorimetry experiments performed in the cell during oscillatory oxidation indicated a slow periodic variation (∼ 40%) in the number of active sites, the period of which was commensurate with that of the reaction-rate oscillations. Auger spectroscopy performed in the UHV chamber showed that surface carbon quantitatively accounted for the surface deactivation, as evidenced by the inverse correlation of the number of surface sites active towards CO adsorption with the surface carbon concentration and by the demonstration that, at the oscillation temperatures, carbon can diffuse from the bulk to the surface, oxygen can remove surface carbon and adsorbed CO can block carbon diffusion. Although silicon oxide was always detected on the surface with infrared spectroscopy, no periodic variation in it could be observed during the reaction-rate oscillations. Auger studies confirmed that the maximum and the variations in surface concentration of silicon oxide could not account for the variations in the number of active sites. A mechanism is therefore proposed in which carbon is driving the long-period self-sustained oscillations in the rate of CO oxidation on Pt.     

Chemisorption of halogens on silver a critical comparison of results from photqemission and thermal desorption spectroscopy

Conflicting thermal desorption (TDS) results from submonolayer coverages of atomic Cl, Br or I on Ag(100), Ag(110) or Ag(111) stimulated us to check for the possible influence of different sample preparation procedures. We present experimental evidence, that a combined approach using TDS and angle-resolved ultraviolet photoemission may help to distinguish between better and worse surface preparation. We present detailed new results for the Cl/Ag(110) system. From our observations we conclude that all three halogens listed above give one and only one TDS peak in the submonolayer regime and show very similar adsorption/desorption-characteristics. This fact removes the apparent contradictions reported in the literature and demonstrates that the halogen-Ag bond is not very sensitive to the surface crystallography.     

The displacement of hydrogen by carbon monoxide on the (100) face of tungsten: A photoemission and thermal desorption study

Photoelectron spectra (hv = 21.22 eV) and thermal desorption data were obtained for CO and H coadsorbed on W(100) at 80 K. When the clean surface is exposed to a saturation dose of H₂, subsequent exposure to CO results in the formation of a state whose emission spectrum is similar to that of molecular α-CO. Upon heating to ~280 K, a structural rearrangement occurs in which most of the adsorbed CO is converted to the strongly bound β form as the hydrogen is simultaneously desorbed. These data plus the observation that H₂ cannot be adsorbed to any significant degree on a saturated layer of β-CO suggest that adsorbed β-CO and H occupy the same atomic sites on the W(100) surface. The distinction between long and short range repulsive COH interactions is discussed. For CO adsorbed on clean W(100), the range of activation energies for vigin to β conversion is calculated from the UPS data to be 45–62 kJ/mol.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.