The reactions of carboxylic acids on UO2(111) single crystal surfaces. Effect of gas-phase acidity and surface defects

S V Chong, Hicham Idriss

Research output: Contribution to journalArticle

28 Citations (Scopus)

Abstract

The interaction of formic acid with the stoichiometric and oxygen-defected (1 1 1) surfaces of UO2 single crystal (the [I 1 1] plane is the natural cleavage of uranium dioxide) has been investigated. HCOOH adsorbs with a high sticking coefficient (s(10.331) = 0.6 at 305 K). The sticking coefficients for acetic and propionic acids were also measured for comparison. Both showed lower values, as expected from their weaker gas-phase acidity. The surface coverage and the degree of surface stoichiometry affected the reaction products of formic acid. On the stoichiometric surface, formate species decomposed via both dehydrogenation (CO2 and H-2) and dehydration (CO and H2O) pathways. with the yield of the latter far higher than that of the former. The desorption profile of these products was found to be very complex. Each route desorbed in two distinct temperature domains (ca. 630 K-the beta-state-and ca. 700 K-the beta-state) and with two distinct CO to CO2 ratios. The CO2 yield of the alpha-state (small) "as relatively insensitive to surface coverage while that of the beta-state (large) dramatically increased with increasing adsorbate surface coverage. Conversely, the CO yield of the a-state (large) was sensitive to surface coverage while that of the P-state (small) was relatively independent. On the oxygen-deficient surface the main route was dehydrogenation to CO2, although reduction to formaldehyde (together with considerable amounts of ethylene) occurred. Formaldehyde desorbed in two different temperature domains, at ca. 550 and 630 K. The first is attributed to U cations with multiple coordinative Lin sat Li rations. while the second is most likely due to reduction of formates by randomly distributed surface defects. (C) 2002 Published by Elsevier Science B.V.

Original languageEnglish
Pages (from-to)145-158
Number of pages14
JournalSurface Science Reports
Volume504
Issue number1-3
Publication statusPublished - 20 Apr 2002

Keywords

  • uranium oxide
  • sticking
  • carboxylic acid
  • single crystal surfaces
  • surface defects
  • RAY PHOTOELECTRON-SPECTROSCOPY
  • FORMIC-ACID
  • URANIUM-DIOXIDE
  • ACETIC-ACID
  • ZINC-OXIDE
  • ELECTRON SPECTROSCOPY
  • REACTION PATHWAYS
  • POLAR SURFACES
  • DECOMPOSITION
  • ADSORPTION

Cite this

The reactions of carboxylic acids on UO2(111) single crystal surfaces. Effect of gas-phase acidity and surface defects. / Chong, S V ; Idriss, Hicham.

In: Surface Science Reports, Vol. 504, No. 1-3, 20.04.2002, p. 145-158.

Research output: Contribution to journalArticle

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AU - Idriss, Hicham

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N2 - The interaction of formic acid with the stoichiometric and oxygen-defected (1 1 1) surfaces of UO2 single crystal (the [I 1 1] plane is the natural cleavage of uranium dioxide) has been investigated. HCOOH adsorbs with a high sticking coefficient (s(10.331) = 0.6 at 305 K). The sticking coefficients for acetic and propionic acids were also measured for comparison. Both showed lower values, as expected from their weaker gas-phase acidity. The surface coverage and the degree of surface stoichiometry affected the reaction products of formic acid. On the stoichiometric surface, formate species decomposed via both dehydrogenation (CO2 and H-2) and dehydration (CO and H2O) pathways. with the yield of the latter far higher than that of the former. The desorption profile of these products was found to be very complex. Each route desorbed in two distinct temperature domains (ca. 630 K-the beta-state-and ca. 700 K-the beta-state) and with two distinct CO to CO2 ratios. The CO2 yield of the alpha-state (small) "as relatively insensitive to surface coverage while that of the beta-state (large) dramatically increased with increasing adsorbate surface coverage. Conversely, the CO yield of the a-state (large) was sensitive to surface coverage while that of the P-state (small) was relatively independent. On the oxygen-deficient surface the main route was dehydrogenation to CO2, although reduction to formaldehyde (together with considerable amounts of ethylene) occurred. Formaldehyde desorbed in two different temperature domains, at ca. 550 and 630 K. The first is attributed to U cations with multiple coordinative Lin sat Li rations. while the second is most likely due to reduction of formates by randomly distributed surface defects. (C) 2002 Published by Elsevier Science B.V.

AB - The interaction of formic acid with the stoichiometric and oxygen-defected (1 1 1) surfaces of UO2 single crystal (the [I 1 1] plane is the natural cleavage of uranium dioxide) has been investigated. HCOOH adsorbs with a high sticking coefficient (s(10.331) = 0.6 at 305 K). The sticking coefficients for acetic and propionic acids were also measured for comparison. Both showed lower values, as expected from their weaker gas-phase acidity. The surface coverage and the degree of surface stoichiometry affected the reaction products of formic acid. On the stoichiometric surface, formate species decomposed via both dehydrogenation (CO2 and H-2) and dehydration (CO and H2O) pathways. with the yield of the latter far higher than that of the former. The desorption profile of these products was found to be very complex. Each route desorbed in two distinct temperature domains (ca. 630 K-the beta-state-and ca. 700 K-the beta-state) and with two distinct CO to CO2 ratios. The CO2 yield of the alpha-state (small) "as relatively insensitive to surface coverage while that of the beta-state (large) dramatically increased with increasing adsorbate surface coverage. Conversely, the CO yield of the a-state (large) was sensitive to surface coverage while that of the P-state (small) was relatively independent. On the oxygen-deficient surface the main route was dehydrogenation to CO2, although reduction to formaldehyde (together with considerable amounts of ethylene) occurred. Formaldehyde desorbed in two different temperature domains, at ca. 550 and 630 K. The first is attributed to U cations with multiple coordinative Lin sat Li rations. while the second is most likely due to reduction of formates by randomly distributed surface defects. (C) 2002 Published by Elsevier Science B.V.

KW - uranium oxide

KW - sticking

KW - carboxylic acid

KW - single crystal surfaces

KW - surface defects

KW - RAY PHOTOELECTRON-SPECTROSCOPY

KW - FORMIC-ACID

KW - URANIUM-DIOXIDE

KW - ACETIC-ACID

KW - ZINC-OXIDE

KW - ELECTRON SPECTROSCOPY

KW - REACTION PATHWAYS

KW - POLAR SURFACES

KW - DECOMPOSITION

KW - ADSORPTION

M3 - Article

VL - 504

SP - 145

EP - 158

JO - Surface Science Reports

JF - Surface Science Reports

SN - 0167-5729

IS - 1-3

ER -