Robert J. Madix

The selective oxidation of CH3OH to H2CO on a copper(110) catalyst

The oxidation of methanol to formaldehyde was studied on a Cu(110) single crystal by flash decomposition spectroscopy. The Cu(110) surface was preoxidized with oxygen-18; deuterated methanol, CH3OD, was used to distinguish the hydroxyl hydrogen from the methyl hydrogens. Very little methanol chemisorbed on the oxygen-free Cu(110) surface, but the ability of the copper surface to chemisorb methanol was greatly enhanced by surface oxygen. This enhancement was absent subsequent to reduction of the surface by methanol itself. CH3OD was selectively oxidized upon adsorption at 180 °K to adsorbed CH3O and D218O. The methoxide was the most abundant surface intermediate and decomposed at about 365 °K to formaldehyde and hydrogen with a first-order rate constant equal to 5.2 ± 1.6 × 1012 exp(−22.1 ± 0.1 kcal/mole/RT) s−1. To a lesser extent some methanol was oxidized to HC16O18O which subsequently decomposed to C16O18O and hydrogen with a first-order rate constant equal to 8 ± 2 × 1013 exp(−30.9 ± 0.2 kcal/mole/RT) s−1. A pronounced maximum in oxidation activity with surface oxygen coverage was observed.

The effects of carbon, oxygen, sulfur and potassium adlayers on CO and H2 adsorption on Fe(100)

Abstract The adsorption and desorption of CO and hydrogen was studied on clean Fe(100) and Fe(100) with adiayers of carbon, oxygen, sulfur, and potassium using X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption. Hydrogen was found to be dissociatively adsorbed on Fe(100) with a binding energy of 86 kJ/mole. CO absorption was found to be complex, involving several molecular binding states as well as dissociated CO. The activation energy for dissociation of CO on Fe(100) was estimated to be 105 kJ/mole. Sulfur, oxygen, and carbon adlayers all reduced the binding energies for CO and hydrogen and inhibited CO dissociation. Potassium was observed to enhance the CO and hydrogen binding strengths and increase the amount of CO dissociation relative to the clean surface. These results have been compared to LCAO calculations for CO adsorption on an Fe(100) surface with adiayers. The model calculations showed that adiayers of carbon, oxygen, and sulfur reduced the CO binding energy on Fe(100) by bonding with the same Fe(3d) orbitals as the CO(2π ∗ ) orbitals would. Potassium was found to enhance the CO binding energy on Fe(100) due to interactions between the K(4s) orbital and the CO(2π ∗ ) orbitals.

The oxidation of methanol on a silver (110) catalyst

The oxidation of methanol was studied on a Ag(110) single-crystal by temperature programmed reaction spectroscopy. The Ag(110) surface was preoxidized with oxygen-18, and deuterated methanol, CH3OD, was used to distinguish the hydroxyl hydrogen from the methyl hydrogens. Very little methanol chemisorbed on the oxygen-free Ag(110) surface, and the ability of the silver surface to dissociatively chemisorb methanol was greatly enhanced by surface oxygen. CH3OD was selectively oxidized upon adsorption at 180 K to adsorbed CH3O and D218O, and at high coverages the D218O was displaced from the Ag(110) surface. The methoxide species was the most abundant surface intermediate and decomposed via reaction channels at 250, 300 and 340 K to H2CO and hydrogen. Adsorbed H2CO also reacted with adsorbed CH3O to form H2COOCH3which subsequently yielded HCOOCH3 and hydrogen. The first-order rate constant for the dehydrogenation of D2COOCH3 to DCOOCH3 and deuterium was found to be (2.4 ± 2.0) × 1011 exp(−14.0 ± 0.5 kcalmole · RT)sec−1. This reaction is analogous to alkoxide transfer from metal alkoxides to aldehydes in the liquid phase. Excess surface oxygen atoms on the silver substrate resulted in the further oxidation of adsorbed H2CO to carbon dioxide and water. The oxidation of methanol on Ag(110) is compared to the previous study on Cu(110).

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

Abstract 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 1 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.

Oxygen induced adsorption and reaction of H2, H2O, CO and CO2 on single crystal Ag(110)

Abstract The adsorption and reaction of H2, H2O, CO, and CO2 were examined on clean and oxygen-dosed Ag(110) surfaces. Limited adsorption with no reaction of these species was observed at surface temperatures down to 160 K on the clean surface. Although no adsorption of H2 was observed on the oxygen-dosed surface, the adsorption and reaction of the remaining species were strongly affected by the presence of oxygen. The sticking probability and desorption rate measurements for oxygen were in reasonable agreement with the results of other workers. For sequential exposure of the surface to 16O2 and 18O2, random isotopic mixing of the O2 desorbed from the surface was observed, indicating appreciable mobility within the adlayer. At 160 K ten H2O molecules could be adsorbed for each oxygen atom initially present on the surface; experiments with isotopically labelled oxygen revealed that complete isotopic mixing of oxygen between adsorbed H2O and the surface had occurred suggesting the formation of adsorbed OH groups. During the adsorption of CO on the oxygen-dosed Ag(110) surface, CO was rapidly oxidized via a chemisorbed state to CO2. The negative apparent activation energy for this process is shown to be the difference between the activation energies for the reaction step and for the desorption of CO from the surface. In the presence of surface oxygen CO2 was adsorbed to form a surface carbonate species which decomposed at 485 K to yield CO2 and O(a).

Vibrational spectra of molecular and atomic oxygen on Ag(11O)

Abstract Atomic and molecular states of bonding of oxygen have been resolved on Ag(11O) by electron energy loss vibrational spectroscopy. The O-O bond order is reduced below unity, indicating appreciable charge transfer into the antibonding orbitals of O 2 by analogy with O 2 on Pt(111). Dissociation of the molecular oxygen into the atomic form was observed above 200 K.

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

Abstract The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H 2 O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm −1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H 2 O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H 2 O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H 2 16 O (a) and 18 O (a) .

XPS, UPS and thermal desorption studies of the reactions of formaldehyde and formic acid with the Cu(110) surface

Formaldehyde adsorbs on the clean copper (110) surface in a weakly bound state which interacts with the metal primarily through its most weakly bound orbital, the in-plane orbital composed mainly of O(2p). When oxygen is preadsorbed on the surface, the X-ray photoelectron spectrum clearly shows the formation of two distinct species when formaldehyde is subsequently adsorbed. One species is formaldehyde as on the clean surface and the other is H2CO2. The latter species decomposes at around 230 K, leaving the formate, HCOO, which then decomposes at 500 K, to produce H2 and CO2. The formate species is also produced by the adsorption of DCOOH on Cu(110) at 400 K. At 140 K formic acid adsorbs molecularly but UPS indicates that its molecular structure is already strongly perturbed by adsorption; dissociation of the acid hydrogen takes place during heating at around 250 K. Oxygen preadsorption increases the amount of formate produced from formic acid, abstracting the acid hydrogen atoms to produce H2O.

Flash desorption activation energies: DCOOH decomposition and CO desorption from Ni (110)

Abstract Accurate values of activation energies were measured by flash desorption methods without assumptions about preexponential factors, reaction orders or specific reaction mechanisms. The activation energies were determined by two methods; one method employed a relationship for the shift in peak temperature with change in heating rate, and the other utilized the change in peak amplitude with shift in peak temperature for different heating rates. Agreement between the two methods was excellent . A series of flash curves at different heating rates were obtained for the CO 2 and CO products from DCOOH flash decomposition following adsorption on Ni (110) at 37°C. Adsorbed DCOOH decomposed autocatalytically with an activation energy of 26.6 kcal/mol to form CO 2 and D 2 . Carbon monoxide formation from DCOOH decomposition, which corresponded identically to CO desorption from this surface, showed a first order activation energy of 32.7 kcal/mol; this activation energy was used to fit a series of CO flash desorption curves obtained for CO adsorption at −55°C. The preexponential factor was found to be 8.5 × 10 15 s −1 . The desorption was first order with a slightly coverage dependent desorption energy. In addition the CO flash curves showed additional binding states at coverages at which changes in isosteric heats of adsorption have been observed. The results illustrate the sensitivity of flash desorption for the determination of binding energies over a wide range of coverages.

The adsorption of CO, O2, and H2 on Pt(100)-(5×20)

Abstract The adsorption/desorption characteristics of CO, O 2 , and H 2 on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol −1 . These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O 2 fluxes up to coverages of 2 3 of a monolayer with a net sticking probability to ssaturation of ⩾ 10 −3 . Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O 2 , and H 2 , the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.