Surface structure effects in the adsorption and desorption of nitric oxide on rhodium

The adsorption, desorption and decomposition of NO on Rh surfaces have been investigated using field electron microscopy (FEM) and thermal desorption spectroscopy (TDS). At 77 K NO is molecularly adsorbed on all surfaces of Rh. At room temperature, however, about 30% of NO adsorbed on the rough surfaces is dissociated. The work function change Δφ due to NO adsorption increases as the surface becomes rougher. The results suggest the following order in Δφ: 0.93 eV = (100) < (111) < (511) < (410) < (331), (533) < (321) < (110) < (650) < (531), (210) = 1.4 eV. Upon heating the tip covered with molecular NO the FEM results suggest that the (321) surface is most active in the NO bond scission. The smooth surfaces are least effective in NO dissociation. The most likely interpretation of the FEM results is that the activity in NO bond scission increases in the following order: (111), (110) < (100), (511) < (650) < (410) < (210) < (331), (533) < (321). These results are discussed in relation to literature data concerning the dissociation of NO on the noble metals of Group VIII.     

Coadsorption of carbon monoxide and hydrogen on polycrystalline rhodium

The coadsorption of carbon monoxide and hydrogen on polycrystalline rhodium filament has been studied by thermal desorption mass spectrometry. From a series of thermal desorption spectra of CO and H₂ from rhodium as a function of the exposure time to the gas mixture of CO and H₂, it is indicated that there are a single broad peak for CO and three peaks designated as β₁, β₂, β₃ for hydrogen. Thermal desorption of hydrogen is complex. CO and β₁-hydrogen coadsorb on the rhodium surface with their partial pressures in the initial stage of the exposure to the gas mixture and then the β₁ -hydrogen adsorbed on the surface is replaced by CO with the further exposure time. The kinetics for the replacement of β₁-hydrogen by CO may be discussed from the standpoint of the L-H reaction process. The β₂-and β₃-hydrogen are observed with a longer exposure time after the elimination of β₁ -hydrogen. It may be suggested that the β₃-hydrogen peak is attributed to the desorption of hydrogen absorbed in the bulk. The nature of β₂ -hydrogen is also briefly mentioned in possible implications.     

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.     

A reflection-absorption infrared study of carbon monoxide and nitric oxide adsorption on platinum (100)

The adsorption of NO, CO, and NO/CO mixtures, onto Pt(100), is studied by RAIRS. CO and NO are found to adsorb into islands at 300 K, but the islands breakup upon heating to 400 K. Dosing with a mixture of NO and CO at temperatures below 325 K is found to produce a mixed NO/CO island. There is a shift in the CO peak and NO peak during mixed island formation which is attributed to a strong chemical interaction between the adsorbed NO and CO. This interaction is found to produce an increase in the desorption temperature of CO. Autocatalytic behavior is found to arise because of an enhanced reactivity when CO enters a mobile state. The autocatalytic behavior could be responsible for the “surface explosion” reported by Lesly and Schmidt.     

Isotope effects in the interaction of hydrogen and deuterium with a rhodium (110) surface

The adsorption of H₂ and D₂ on a Rh (110) surface at 100 K leads to a sequence of ordered phases, among others 1×2 phases at _H =0.5 and at _H =1.5 which likely involve a partial surface reconstruction consisting of a small perpendicular displacement of Rh surface atoms. The structure of the adsorbate phases is strongly correlated with the binding energy of the adsorbed phases. Three H (D) binding states (₁,₂ and) are populated at saturation as determined by thermal desorption spectroscopy (TDS). Whereas the peak temperature of the state is invariant with the hydrogen isotope, the D ₁ state appears at a 8 Klower and theD ₂ state at a 5 Khigher temperature than the respective H states. Generally the D phases exhibit a better long-range order than the H phases. The rate of adsorption is identical for the first three adsorbed phases but D₂ adsorbs appreciably faster in the 1×2–3H and the final l×1–2H phases.Zero point energy effects as well as a H coverage dependent local interaction model could account for the observed effects.     

The oxidation of carbon monoxide on polycrystalline rhodium under knudsen conditions: II. Reaction with nitrogen monoxide

The reaction between carbon monoxide and nitrogen monoxide on a polycrystalline rhodium ribbon under stationary conditions is followed by mass spectrometry. In the temperature range 300 to 1100 K the ratio of the partial pressures of the reactants varies between 0.1 < p_NO/p_CO < 100 at values of the total pressure in the reactor from 10^?4 to 10^?5 Torr. The results can be interpreted qualitatively by a simple elementary reaction sequence. Simulation using literature values of the kinetic constants leads to semi-quantitative agreement with experimental results. No isothermal oscillations of the reaction rate could be observed under the stated conditions.     

The oxidation of carbon monoxide on polycrystalline rhodium under knudsen conditions: III. Simultaneous reaction with oxygen and nitrogen monoxide

The CO oxidation using mixtures of oxygen and nitrogen monoxide has been studied under stationary conditions. The measured selectivities could not always be foreseen from the results obtained in the binary systems CO/O₂ and CO/NO. At high temperatures the amount of NO reduced is directly proportional to its fraction in the oxidizing gas. At low temperatures, NO may be reduced with high selectivity even in the presence of high oxygen concentration.     

Chemisorption,surface structural chemistry,and electron impact properties of carbon monoxide on rhodium (110)

The adsorption, desorption, surface structural chemistry, and electron impact properties of CO on Rh(110) have been studied by LEED, Auger spectroscopy, thermal desorption, and surface potential measurements. At 300 K, CO adsorbs into a single chemisorbed state whose desorption energy (E_d) is ~130kJmol^-1. The initial sticking probability is unity, and at saturation coverage a (2 × 1)plgl ordered phase reaches its maximum degree of perfection, thus demonstrating that this CO structure is common to the (110) faces of all the cubic platinum group metals. The saturated adlayer corresponds to θ = 1 and shows a surface potential of Δ? = +0.97 V. Under electron impact, desorption and dissociation of CO occur with about equal probability, the relevant cross sections being ~10^-22 m² in each case. Slow thermal dissociation of CO occurs at high temperature and pressure, leaving a deposit of C and O atoms on the surface. The thermal, electron impact, and Δ? properties of Rh(110)CO resemble those of Ni(110)CO rather closely, and are very different from those of Pt(110)CO. Surface carbon is shown to inhibit CO chemisorption, whereas surface oxygen appears to lead to the formation of a new more tightly bound form of CO with a considerably enhanced desorption energy (E_d ~ 183 kJmol^-1). Similar oxygen-induced high temperature CO states have been reported recently on Co(0001) and Ru(101&#x0304;1).     

An XPS study of carbon monoxide and nitric oxide adsorption on platinum (410): Unusual dissociation activity

The adsorption of NO and CO on platinum (410) is studied by X-ray photoemission spectroscopy (XPS). It is found that the (410) surface shows unusual activity for NO and CO bond breaking. CO is found to adsorb molecularly on Pt (410) at 300 K, but it partially dissociates upon heating to 500 K. NO is found to adsorb dissociatively under all conditions studied. By comparison, the low index faces of platinum and stepped surfaces considered previously give negligible dissociation of CO and negligible dissociation of NO up to 450 K. The (410) surface is also unusual from an orbital symmetry standpoint; in fact, it was predicted to be unusually active for NO and CO bond breaking based on Woodward-Hoffman ideas. The present results show, then, that Woodward-Hoffman (i.e., orbital symmetry conservation) methods could yield valuable insights into the nature of active sites on single crystal catalysts.     

The oxidation of carbon monoxide on polycrystalline rhodium under knudsen conditions: I. Reaction with oxygen

The reaction between oxygen and carbon monoxide on a polycrystalline rhodium ribbon under stationary conditions is followed by mass spectrometry. At temperatures from 300–1100 K the ratio of the partial pressures of reactants varies between 0.1 < p_O2/p_CO < 100. The value of the total pressure in the reactor varies between 10^?5 and 10^?4 Torr. The reaction on rhodium shows similar features as in the case of platinum. The results are consistent with a simple elementary reaction sequence but quantitative agreement by model calculations was not obtained.     

Alkali promotion on cobalt: Surface analysis of the effects of potassium on carbon monoxide adsorption and Fischer-Tropsch reaction

A surface study is made of CO adsorption and the Fischer-Tropsch (CO hydrogenation) reaction on Co foils with K precoverages of from 0 to 0.8 monolayers (ML, 1 ML = saturation K coverage ≈ 5 × 10¹⁴ K/cm²). X-ray photoemission (XPS) and Auger spectroscopy (AES) are used to characterize the adsorbed surface species. The adsorption and reaction occur in an atmospheric-pressure microreactor coupled to the surface analysis vacuum chamber by a rapid, valveless sample-transfer mechanism. After CO exposure (548 K, 100 kPa, 10 s) core levels are seen corresponding to molecular and dissociated CO. Molecular CO 1s levels exhibit a K-dependent binding energy decrease attributed to a surface-to-molecule charge transfer enhanced by the alkali. Potassium enhances the amount of CO adsorbed and thermally stabilizes the adsorbed molecule. Surface analysis after Fischer-Tropsch reaction (523–548 K, 100 kPa total reactant pressure) characterizes the amount and nature of the C deposited. The results are compared with earlier measurements on Fe surfaces. For identical reaction conditions, clean Co shows much less deposited C than clean Fe. It also forms surface carbidic C deposits much less readily than Fe, the C deposited being predominantly graphitic. Potassium predosing of the surfaces enhances the amount of C deposited, attributable to its enhancement of the CO adsorption step. It also changes the nature of the C, inducing carbidic C. When the carbidic C is induced (by a K precoverage of ∼ 0.7 ML), the amount of C deposited by reaction actually decreases, compared to that produced by reactions on surfaces with less K, an effect not previously observed during K-promoted reactions on Fe or Ni. The reactivity of carbidic C to hydrogen on both Co and Fe is reduced by K, helping to explain the lower methanation rate that has been observed after K promotion of Fischer-Tropsch catalysts.     

P-odd effects observed in neutron-induced reactions and isospin structure of weak nucleon-nucleon interaction

It is shown that, by introducing a resonance phase for two opposite-parity quasistationary states of the same spin, the observed sign dependence of P-odd effects in neutron-induced reactions can be matched with theoretical predictions. The proposed approach makes it possible to deduce information about the isospin structure of weak nucleon-nucleon interaction.     

An FTIR study of the bonding of methoxy on Ni(100): effects of coadsorbed sulfur, carbon monoxide and hydrogen

The vibrational spectra of CH₃O_(a), CD₃O_(a), CDH₂O_(a) and CD₂HO_(a) on Ni(100) are analyzed and interpreted in terms of resonances between fundamental modes and either combinations or overtones. Analysis of the symmetry of the modes observed suggests that methoxy binds normal to the surface with C_s symmetry, at least at low coverages. Two distinct vibrational bands emerge in the vibrational spectrum of methoxy in the v(CO) region as the coverage increases which are attributed to bonding in four-fold hollow sites and bridging sites. These bands exhibit blue shifts of about 25 cm⁻¹ with increasing coverage up to the saturation coverage. The vibrational bands in the v(CH) region appear concomitantly at all coverages and shift down 12 cm⁻¹ as the coverage is increased. These shifts are attributed to changes in the metal-oxygen bond which are reflected in changes in the strength of the C---O and C---H bonds. Affects on the bonding also appear to occur with the coadsorption of hydrogen or CO with methoxy. Coadsorption of 0.36 ML hydrogen with 0.04 ML methoxy induces blue shifts of 15 and 7 cm⁻¹ for the v(CO) bands at 949 and 984 cm⁻¹, respectively. Adsorbing 0.43 ML of CO with 0.04 ML methoxy (and 0.04 ML hydrogen) causes a red shift of 20 and 12 cm⁻¹ for these bands. A drastic drop in mode intensities for methoxy when CO is coadsorbed suggests that the methoxy tilts away from the surface normal. Pre-adsorbing sulfur on the Ni(100) surface reduces the amount of methoxy formed from methanol, but the v(CO) methoxy bands are unshifted in frequencies relative to their position for the same methoxy coverage on the clean surface.     

Work function changes and surface chemistry of oxygen, hydrogen, and carbon on indium tin oxide

The surface chemistry of indium tin oxide (ITO) has been investigated with Auger electron spectroscopy (AES) and high resolution electron energy loss spectroscopy (HREELS). A vibrating Kelvin probe (KP) with a graphite reference was used to monitor the absolute work function (Φ) of ITO as a function of chemical modification. The ITO was exposed in situ to molecular hydrogen (H₂), hot-filament-activated oxygen (O₂^*), and hot-filament-activated deuterium (D₂^*). The initial Φ of ITO was determined to be 5.2 eV, and surface chemical changes had strong effects on this value, as seen by KP. Exposure of clean ITO to O₂^* increased Φ to 5.6 eV, but the increase was short-lived. The changes in Φ over time were correlated with the uptake of carbon impurities in ultra high vacuum (UHV), as monitored by AES.

The HREELS of ITO revealed significant hydrocarbon impurities. Chemical reduction of ITO produced a metallic surface and dehydrogenated the adsorbed hydrocarbons. Both re-oxidation of metallic ITO and oxidation of clean ITO temporarily removed adventitious carbon from the surface, but oxidized ITO adsorbed an even larger quantity of carbon over time.     

Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films

H₂ interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes ΔΦ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10⁻³ Pa, increasing the work function. Under higher H₂ pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.     

Adsorption of hydrogen and carbon monoxide on Rh(1 1 1)/V surface alloys

A detailed study of the interaction of hydrogen and carbon monoxide with two different Rh(1 1 1)/V surface alloys (1/3 monolayer of V in the second atomic layer or 1/3 monolayer of V in form of islands on the surface) is presented in comparison to the clean Rh(1 1 1) surface. For hydrogen a decrease in the sticking coefficient is found for both alloy surfaces. The sticking coefficient of H₂ as a function of the translational energy is similar to the Rh(1 1 1) surface, showing a direct activated adsorption mechanism. For low translational energies hydrogen adsorption is dominated by dynamical steering on Rh(1 1 1) and by a precursor mechanism on the Rh(1 1 1)/V subsurface alloy. The H₂ TPD desorption peaks are shifted to lower temperatures on the alloy surfaces, caused by the downshift of the metal d-band due to V alloying. On all three surfaces the saturation coverage of hydrogen was measured, giving 1.2, 1.0 and 0.8 monolayer for Rh(1 1 1), the Rh(1 1 1)/V subsurface alloy and for the Rh(1 1 1)/V islands, respectively. For CO the sticking coefficients and the saturation coverages are basically the same on the Rh(1 1 1) and the alloy surfaces. There is an extrinsic precursor on the ordered CO (√3×√3) phase on the Rh(1 1 1) surface, but there is no evidence for such a precursor on the Rh(1 1 1)/V subsurface alloy. On the Rh(1 1 1)/V islands surface, the extrinsic precursor exists on the Rh(1 1 1) surface between the V islands. Apparently this precursor is only stable on the ordered CO layer on Rh(1 1 1).     

The interaction of hydrogen and carbon monoxide with oxygen on Cu(110)-Fe surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(110)-Fe surfaces has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and Fe_0.95O partially. With a model which is only an extension of the scheme for the reduction of pure Cu(110) by CO, the reduction of Cu(110)-Fe can be simulated. The lateral orientation of Fe_0.95O with respect to the copper matrix changes during repetitive oxidation-reduction cycles. At 725 K oxygen deficient iron oxide segregates to the surface. With hydrogen all oxygen can be removed.     

The interaction of hydrogen and carbon monoxide with oxygen on Cu(111)-Fe Surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(111)-Fe surfaces containing different amounts of iron has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and if, at larger iron deposits, γ-Fe₂O₃ is present, γ-Fe₂O₃ can be reduced to Fe₃O₄. The maximum reaction rate is proportional to the square of the total copper surface. With hydrogen all oxygen can be removed. The reduction proceeds via a number of different stages. This is explained by the subsequent occurrence of γ-Fe₂O₃, Fe₃O₄ Fe_0.95O and Fe.     

A study of the infrared spectra of carbon monoxide adsorbed on metals of the copper subgroup and on cobalt,ruthenium, rhodium,and palladium

A study has been made of the infrared spectra of carbon monoxide CO, adsorbed on Cu, Ag, Au, Co, Ru, Rh and Pd at temperatures from –160° C to 200° C and pressures from 10^–5 to 1–10 mm Hg. The spectra of CO adsorbed on Cu, Ag, and Au show one absorption band, characteristic of a surface compound of carbon monoxide, with linear structure. It has been found that in the stable chemosorption of CO on Co, Ru, Rh, and Pd in the range of temperatures and pressures studied, two types of surface compound-linear and bridged-are produced.