ARTICLE pubs.acs.org/JPCC
Determination of Absolute Coverages for Small Aliphatic Alcohols on TiO2(110) Zhenjun Li, R. Scott Smith, Bruce D. Kay,* and Zdenek Dohnalek* Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352, United States ABSTRACT: The absolute coverages of water and small aliphatic alcohols (C1 C4) were determined on TiO2(110) using a combination of temperature-programmed desorption and liquid nitrogen cooled quartz crystal microbalance measurements. The absolute saturation coverages of water on Ti4+ and bridging oxygen, Ob, sites are found to be equal to 1 monolayer with respect to the number of Ti4+ and/or Ob sites (1 ML 5.2 1014 cm 2) in good agreement with prior studies. The saturation coverages of primary alcohols on Ti4+ sites are found to be approximately constant and equal to 0.77 ML. This indicates that the increasing length of the alkyl chains does not contribute to added steric hindrance. Additional steric hindrance is observed with increased branching as shown for secondary alcohols and tertiary t-butanol where the saturation coverage decreases to 0.62 and 0.44 ML, respectively. On Ob rows, a monotonic decrease of the alcohol coverages is observed for both increasing length of the alkyl chains and the chain number.
1. INTRODUCTION Titanium dioxide (TiO2) has attracted a great deal of attention over the past decades due to its widespread applications in catalysis, photocatalysis, photovoltaic cells, sensors, and coatings.1 6 The rutile TiO2(110) surface, which is considered to be one of the most stable and easily prepared oxide surfaces, has served as a model in studies of the chemical and electronic properties of oxides.7,8 The surface is composed of alternating 5-coordinated Ti4+ and bridging oxygen (Ob) rows, which form frustrated pairs of Lewis acid base sites.4 A small fraction of bridging oxygen vacancies (VO’s) can be created on the surface by a slight reduction. These sites serve as model sites in studies where lattice oxygen participates in the reaction (Mars van Krevelen reactions). The adsorption and reactions of simple molecules such as water, O2, and organic compounds on TiO2 have been extensively studied.4,9 21 In particular, interactions with alcohols were thoroughly explored due to their importance as a source of renewable energy and/or as models in the studies of photooxidation of organic contaminants. On TiO2(110), alcohols exhibit a very rich surface chemistry.4,13,15 19,22 While most of the molecules desorb during temperature programmed desorption (TPD) in a molecular form, a fraction dissociates via O H bond cleavage forming alkoxy species that can further dehydrate at elevated temperatures to form alkenes, dehydrogenate to aldehydes or ketones, condense to form ethers, and/or rehydrogenate to alcohols.15,17 19,23,24 The initial dissociation of alcohols via O H bond cleavage on VO sites has been shown to result in the alkoxy species filling the VO, and hydroxy species binding on a neighboring Ob site.25 27 Detailed experimental studies have shown that the dehydration reaction is favored over dehydrogenation and condensation with increasing chain length and chain number.17,18 An additional low temperature dehydrogenation r 2011 American Chemical Society
channel has also been observed at 300 450 K and attributed to the dissociation of alkoxy species on Ti4+ rows.18,19 Although the surface chemistry of alcohols on TiO2(110) has been extensively studied, their absolute coverages are generally not known. For example, methanol saturation coverage was measured previously using a calibrated pinhole tube doser and temperature-programmed desorption (TPD) and was determined to be 3.2 1014 molecules/cm2,10 while values of 3.0 1014 and 2.6 1014 molecules/cm2 were obtained by X-ray photoelectron spectroscopy (XPS) measurements.21,28 The saturation coverage of ethanol at room temperature was found to be 2.6 1014 molecules/ cm2 based on an XPS measurement.29 Unfortunately, as reported previously, the saturation coverage values that are based on the attenuation of Ti 2p and O 1s peaks have a fairly high uncertainty of (1.0 1014 molecules/cm2.21 In this study, we used a liquid nitrogen (LN2) cooled quartzcrystal microbalance (QCM) in combination with TPD to determine the absolute monolayer (ML) coverages of small aliphatic alcohols (methanol, ethanol, 1-,2-propanol, and 1-,2-, t-butanol) on TiO2(110). We have found that the saturation coverages of primary alcohols do not change appreciably with increasing chain length and are equal to ∼4.0 1014 molecules/ cm2, which corresponds approximately to three alcohol molecules adsorbed per four Ti4+ sites. The saturation coverage further decreases to two alcohol molecules per three Ti4+ sites in the case of secondary alcohols and to one alcohol molecule per two Ti4+ sites for tertiary t-butanol. This trend is likely a result of the steric hindrance caused by the increasing number of alkyl chains. Received: August 25, 2011 Revised: October 6, 2011 Published: October 13, 2011 22534
dx.doi.org/10.1021/jp208228f | J. Phys. Chem. C 2011, 115, 22534–22539
The Journal of Physical Chemistry C
Figure 1. TPD spectrum from 2.0 ML H2O adsorbed on TiO2(110) at a sample temperature of ∼90 K and collected at m/e = 18 amu using a heating rate of 1.5 K/s. A coverage of 1 ML is defined relative to a Ti4+ concentration of 5.2 1014 cm 2.
In contrast, on Ob sites, the coverage decreases monotonically not only with the increasing number of chains but also with increasing chain length.
2. EXPERIMENTAL DETAILS The experiments were conducted in an ultrahigh vacuum (UHV) molecular beam surface scattering apparatus (base pressure
