Article pubs.acs.org/JPCC
Low-Temperature Reductive Coupling of Formaldehyde on Rutile TiO2(110) Ke Zhu,† Yaobiao Xia,† Miru Tang,‡ Zhi-Tao Wang,§ Igor Lyubinetsky,§ Qingfeng Ge,‡ Zdenek Dohnálek,∥ Kenneth T. Park,*,† and Zhenrong Zhang*,† †
Department of Physics, Baylor University, Waco, Texas 76798, United States Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States § Environmental Molecular Sciences Laboratory and ∥Fundamental and Computational Sciences Directorate, Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡
S Supporting Information *
ABSTRACT: The formation and coupling of methylene upon dissociation of formaldehyde on reduced TiO2(110) are studied using variable temperature scanning tunneling microscopy (STM). In agreement with prior studies, formaldehyde preferably adsorbs on the bridging-bonded oxygen vacancy (VO) defect site. VO-bound formaldehyde couples with Ti-bound formaldehyde forming a diolate species, which appears as the majority species on the surface at 300 K. Here, STM images directly visualize a low-temperature coupling reaction channel. Two VO-bound formaldehyde molecules can couple and form Tibound species, which desorbs above ∼215 K. This coupling reaction heals both VO sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty VO sites upon dissociation of the C−O bond in VO-bound formaldehyde, which suggests that the ethylene formation occurs via coupling of the methylene groups. Statistical analysis shows that the sum of visible reaction products on the surface can only account for a half of the consumption of the initial VO coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposure between 215 and 300 K.
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aldehyde.11 On reduced TiO2(110) surfaces, several ensemble-averaged experimental studies reported the C−C coupling of CH2O forming a diolate (−OCH2CH2O−) species, followed by ejection of the olefin at high temperature (∼600 K).18,23 There is currently a controversy regarding the role of the defects including the bridging-bonded oxygen (Ob) vacancy (VO) and Ti interstitial in this coupling reaction.6,18,19,23 The low-temperature coupling after formaldehyde decomposition has not been reported on oxide surfaces. Our recent studies on the reactions of formaldehyde provide the molecular-level information on adsorption, diffusion, and coupling reaction of formaldehyde on reduced TiO2(110).25,26 Scanning tunneling microscopy (STM) images have directly visualized that formaldehyde preferentially adsorbs on VO sites, which agrees with the ensemble-averaged experimental studies,18,23 but are at odds with the theoretical results.19,25
INTRODUCTION Chemical reactions of aldehyde on reducible metal oxides are important in numerous catalytic reactions in the applications related to the degradation of toxic compounds, air purification, and chemical synthesis.1−3 For instance, reductive coupling of carbonyl species has proven to be a versatile strategy for the functionalization of vicinal carbon atoms in organic molecules.3−5 Adsorption and reaction of various oxygenated hydrocarbons on well-defined surfaces have been extensively studied to provide unparalleled understanding of the reaction mechanisms.6−10 Production and conversion of simple C1 molecules such as methane, methanol, formaldehyde and formic acid are involved in many of these reactions. Hence the reactivity of formaldehyde has been studied on many metal11−17 and metal oxide surfaces18−24. On metal surfaces, formaldehyde generally dehydrogenates to form CO via various intermediates such as formyl, methoxy, and formate. The coupling of aldehydes to form alkenes has been identified on TiO2,6,9,18,23 UO2,20 V(100),11 and Mo(110).12 On these surfaces, the coupling reaction proceeds through the formation of diolates via C−C coupling of two aldehydes.6,9,12,18,20 A distinct low-temperature pathway was determined on V(100), which occurs via the coupling of methylene groups formed upon dissociation of the C−O bond in adsorbed form© XXXX American Chemical Society
VO + CH 2OTi → CH 2Ob
(1)
Received: June 12, 2015 Revised: July 16, 2015
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DOI: 10.1021/acs.jpcc.5b05639 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C The Ti-bound CH2OTi and the VO-bound CH2Ob further couple and form a diolate intermediate, which is the dominant surface species at room temperature. CH 2Ob + CH 2OTi → −ObCH 2CH 2OTi −
(2)
However, the reaction mechanism is different from the proposed coupling between two CH2Ob molecules forming a diolate bound to two Ob ions (−ObCH2CH2Ob−).18,23 The diolate species has been determined as the key reaction intermediates in the formation of ethylene at high temperature (∼600 K).18,23 In this study, we present the direct visualization of the lowtemperature coupling reaction of formaldehyde on reduced TiO2(110) using STM. Two VO-bound molecules couple and form Ti-bound ethylene at ∼175 K, which desorbs above ∼215 K. The position and the appearance of Ti-bound ethylene indicate that the coupling happens after the dissociation of formaldehyde.
Figure 1. Three isothermal STM images taken at (a) t = 0, (b) t = 4.3 min, and (c) t = 8.6 min on a same area of reduced TiO2(110) at 175 K after 0.02 ML formaldehyde was dosed at 75 K. The models illustrate the corresponding diffusion processes and the coupling reaction of two VO-bound CH2Ob molecules. Additional examples are provided in the Supporting Information as Figure S1.
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EXPERIMENTAL SECTION Experiments were performed in two different ultrahigh vacuum chambers at Baylor University and Pacific Northwest National Laboratory (PNNL). The UHV system (base pressure
