Mark A. Barteau

Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals

The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.

Oxygenate reaction pathways on transition metal surfaces

Abstract The importance of various oxygenates as fuels and as chemical intermediates and products continues to grow. Alcohols and aldehydes have also been the subjects of numerous surface reactivity studies. We review here the decomposition mechanisms of oxygenates on transition metal surfaces focusing primarily on metals of Groups VIII and IB. Common pathways as well as deviations from these serve to illustrate the patterns of oxygenate reactions. Several major divisions in the preferred pathways can be rationalized in terms of the affinities of metals for making metal–oxygen and metal–hydrogen bonds. Other important factors determining oxygenate reactivities include surface crystallographic structure and the detailed molecular structure of the oxygenate. Differences in product distribution between metals are frequent, even in cases where many of the reaction steps are common, primarily because of the plethora of elementary reaction steps usually involved in oxygenate decomposition on transition metal surfaces. As a result, differences late in the reaction sequence can obscure important similarities in the overall reaction network. Spectroscopic identification of common surface reaction intermediates including alkoxides, acyls, and oxametallacycles, has become increasingly important in revealing the underlying similarities in seemingly diverse oxygenate reaction pathways on transition metal surfaces.

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).

Spectroscopic identification of alkoxide, aldehyde, and acyl intermediates in alcohol decomposition on Pd(111)

Abstract The reactions of primary and secondary alcohols were studied on clean Pd(111) surfaces with High Resolution Electron Energy Loss Spectroscopy (HREELS). Emphasis was placed on elucidating the reaction pathways involved in alcohol decomposition and upon determining the influence of different substituents on the stability of adsorbed intermediates. On the clean Pd(111) surface, both primary and secondary alcohols were found to react via decarbonylation pathways. Although the initial step in alcohol activation was alkoxide formation via O-H bond cleavage, the stabilities of the intermediates involved in subsequent reactions differed among the alcohols studied. Methoxides generated by the reaction of methanol underwent selective dehydrogenation to η2(C,O)-formaldehyde species which ultimately decomposed to adsorbed carbon monoxide and hydrogen atoms. Analogous behavior was observed for ethanol and 1-propanol, with the exception that partial dehydrogenation of the η2-aldehyde species derived from these alcohols produced stable acyl intermediates. Decomposition of these higher acyls required temperatures above 260 K and was the rate-limiting step in higher alcohol decomposition. The secondary alcohol 2-propanol reacted via similar pathways, although some desorption of the intermediate dehydrogenation product (acetone) occurred.

Decarbonylation and decomposition pathways of alcohols on Pd(111)

The reactions of methanol, ethanol, 1-propanol, and 2-propanol were examined on the clean Pd(111) surface using temperature programmed desorption (TPD). Methanol decomposed to CO and H 2 while ethanol and 1-propanol underwent cleavage of the α carbon-carbon bond to form CO, H 2 , and the hydrocarbons methane and ethylene, respectively. No selective dehydrogenation reactions of the primary alcohols to aldehyde products were detected on clean Pd(111). The elimination of CO to form a hydrocarbon one carbon atom shorter than the parent alcohol is essentially the reverse of steps proposed for higher alcohol synthesis on transition metals via CO insertion terminating hydrocarbon chain growth. In contrast to the primary alcohols, 2-propanol underwent selective dehydrogenation to acetone in addition to decomposition to methane, CO, and H 2 . A reaction scheme involving first dehydrogenation to alkoxides followed by formation of ν 2 (O, C) aldehyde and ketone intermediates is proposed to account for this behavior. Decomposition of the higher alcohols was also found to deposit residual carbon on the surface of the Pd(111) crystal. For both ethanol and 2-propanol, C 1 surface hydrocarbon species decomposed at 410 K to deposit residual surface carbon and evolve reaction-limited H 2 , whereas the C 2 surface hydrocarbons from 1-propanol decomposed at 450 K.

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.

Structure and composition requirements for deoxygenation, dehydration, and ketonization reactions of carboxylic acids on TiO2(001) single-crystal surfaces

Abstract The reactivities of TiO2(001) single-crystal surfaces for acetic acid decomposition were studied by TPD and XPS. Surface structure and composition were varied by a combination of argon ion bombardment and annealing in order to investigate reaction selectivity and to identify the active sites responsible for adsorption and decomposition. Acetic acid adsorbed both molecularly and dissociatively at 200 K, but only dissociatively at 300 K. The molecular species desorbed readily below 300 K. Approximately 15% of the dissociatively adsorbed species (acetates) desorbed as acetic acid at 390 K via recombination with surface hydroxyls. The remaining acetates decomposed at higher temperatures via three different reaction pathways with selectivities dependent on surface composition and structure. Reduced surfaces containing Ti cations of lower valences favored direct deoxygenation of the acetates to fill the oxygen vacancies of the surface. This reaction occurred even at room temperature and deposited atomic carbon on the surface. Oxidized surfaces containing only Ti4+ cations favored reactions to form volatile molecular products. On the {011}-faceted surface, surface acetates decomposed via net unimolecular dehydration to ketene. On the {114}-faceted surface, the bimolecular reaction of surface acetates to form acetone also took place. The reactions of propionates formed by dissociation of propionic acid were directly analogous to those of acetates: methyl ketene was produced by net unimolecular dehydration on the {011}-faceted surface and 3-pentanone was the bimolecular ketonization product on the {114}-faceted surface. Since the {011}-faceted surface exposes only Ti4+ cations of fivefold oxygen coordination, these sites were determined to be responsible for the net unimolecular dehydration. Bimolecular ketonization required the coordination of two acetates to a common Ti4+ cation on a stoichiometric surface. Of the single-crystal surfaces examined, only the {114}-faceted surface presented the necessary fourfold oxygen-coordinated Ti4+ sites for this reaction.

The adsorption of molecular oxygen species on Ag(110)

Abstract The adsorption of oxygen on the Ag(110) surface was examined at temperatures down to 123 K. In addition to the dissociatively adsorbed state which desorbed at 590 K, a second oxygen state desorbed at 190 K following adsorption at 150 K and below. This high temperature state appeared to form prior to the development of the low temperature state. The ratio of coverages of the two states was a strong function of both exposure and adsorption temperature. Isotopic exchange experiments indicated that the low temperature state was molecularly adsorbed. The desorption of the molecularly adsorbed oxygen exhibited complex kinetics due to interaction with adsorbed oxygen atoms.

Acid-base reactions on solid surfaces: The reactions of HCOOH, H2CO, and HCOOCH3 with oxygen on Ag(110)

The effect of oxygen adsorbed on a Ag(110) surface on the adsorption and reaction characteristics of HCOOH, H2CO, and HCOOCH3 was examined. No adsorption or reaction of these species was observed on the clean Ag(110) surface. All three reacted readily with adsorbed oxygen, however, to produce stable adsorbed intermediates. HCOOH reacted with adsorbed oxygen to produce two adsorbed formates for each oxygen atom initially adsorbed plus H2O, which desorbed from the surface during adsorption. H2CO reacted with surface oxygen to form an H2CO2 intermediate, which decomposed at 240 K to liberate hydrogen and produce the stable formate. Surface oxygen attacked HCOOCH3 at the acyl carbon to form adsorbed CH3O and HCOO species. The HCOO(a) intermediates formed from HCOOH, H2CO, and HCOOCH3 were indistinguishable. HCOO(a) decomposed with first-order kinetics at 410 K to yield CO2, H2 and traces of HCOOH. These patterns of reactivity clearly identify the chemical nature of adsorbed oxygen as a strong base.

Band gap tailoring of Nd3+-doped TiO2 nanoparticles

Undoped and Nd3+-doped TiO2 nanoparticles were synthesized by chemical vapor deposition in order to tailor the band gap of TiO2. The doping reduced the band gap. The band gap was measured by ultraviolet-visible light absorption experiments and by near-edge x-ray absorption fine structure. The maximum band gap reduction was 0.55 eV for 1.5 at. % Nd-doped TiO2 nanoparticles. Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method were used to interpret the band gap narrowing. The band gap narrowing was primarily attributed to the substitutional Nd3+ ions which introduced electron states into the band gap of TiO2 to form the new lowest unoccupied molecular orbital.