ARTICLE pubs.acs.org/JPCC
Polymerization of Formaldehyde and Acetaldehyde on Ordered (WO3)3 Films on Pt(111) Zhenjun Li, Zhenrong Zhang,† Bruce D. Kay,* and Zdenek Dohnalek* Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352, United States
bS Supporting Information ABSTRACT: Polymerization of formaldehyde, H2CO, and acetaldehyde, CH3CHO, was studied under ultrahigh vacuum conditions on a model catalyst consisting of an ultrathin WO3 film supported on Pt(111). The onset of polymerization is observed at very low temperatures of 70 and 80 K for H2CO and CH3CHO, respectively, as documented by the evolution of the infrared reflectionabsorption spectra. The amount of polymer increases with increasing coverage and saturates at 5 and 8 monolayers (ML) for the H2CO and CH3CHO multilayer films, respectively. Upon heating, the polymers decompose around 250 and 190 K for H2CO and CH3CHO, respectively, as evidenced mass spectrometrically by the desorption of their monomers and oligomers into the gas phase. The heats of H2CO and CH3CHO sublimation and polymerization determined on the basis of our experiments are in good agreement with previously published values.
1. INTRODUCTION Transition-metal oxides are widely used in technology, environmental protection, and catalytic applications. In environmental applications, metal oxides can destructively adsorb various toxic and odorous compounds from air and water as well as remove heavy metals. In heterogeneous catalysis, they either serve as supports for dispersed metal clusters and/or are directly involved in reactions as catalysts. Among all transition-metal oxides, tungsten trioxide is of particular interest. It has been found to promote several reactions including isomerization of alkanes and alkenes,1,2 partial oxidation of alcohols,3,4 selective reduction of nitric oxide,57 and metathesis of alkenes.811 Formaldehyde (H2CO) and acetaldehyde (CH3CHO), studied here, are common building blocks for the synthesis of more complex compounds and materials. They can undergo several surface reactions, for example, decomposition to CO, H2, and hydrocarbon species on several transition-metal surfaces such as W(111),12 W(100),12,13 Pd(111),14 Pd(110),15 Ru(001),16,17 Ni(110),18 Pt(111),19,20 Pt(110),21 and Fe(100).22 They are also readily oxidized to formate, acetate, and/or CO2 on Pt(110),21 Pt(111),23 Pt(100),23 Cu(110),24 Cu(111),25 and TiO2(001).26 Surface-induced polymerization of formaldehyde and acetaldehyde, the focus of this article, has also been observed on several transition-metal surfaces (Pd(111),27 Ru(001),17 Ni(110),18 Pt(111),19 Cu(111),28,29 Ag(111)3032) and transition-metal oxide surfaces (NiO(100),33 CeOx(111),34 TiO2(110)35). On the basis of prior studies on stoichiometric oxide surfaces, H2CO polymerizes, likely as a result of Lewis acid/base interactions. Introduction of surface defects, such as oxygen vacancies, leads to enhanced H2CO binding that ultimately results in H2CO decomposition.3335 r 2011 American Chemical Society
The overall surface morphology, oxidation states, and surface defects are important factors that affect the reactivity of metal oxides. Recently, well-ordered tungsten trioxide thin films have been prepared in our laboratory via deposition of monodispersed (WO3)3 clusters on Pt(111).36 It was found that the first layer is partially reduced to W5þ, while the second layer is composed of fully oxidized (W6þ), molecularly bound (WO3)3 trimers ordered into a (3 3) overlayer. In this study, we use two-layer thick (WO3)3 films supported on Pt(111) having a (3 3) structure as a model catalytic system to study the polymerization of formaldehyde and acetaldehyde. The polymerization is probed by using a combination of temperature-programmed desorption (TPD) and infrared reflection adsorption spectroscopy (IRAS). It is found that both molecules start to polymerize between 70 and 80 K. The polymerization yield is found to be 5 and 8 monolayers (ML) for formaldehyde and acetaldehyde, respectively. Low thermal stability of the polymers leads to their decomposition and desorption of monomers and oligomers of formaldehyde and acetaldehyde at 250 and 190 K, respectively. The enthalpy of polymerization estimated from our results for both molecules is in a good agreement with previously published data.3739
2. EXPERIMENTAL DETAILS The experiments were conducted in an ultrahigh vacuum (UHV) molecular beam-surface scattering apparatus (base pressure 99%) was further purified by means of freezepumpthaw cycles. Both molecules were subsequently introduced using a neat, 300 K, quasi-effusive molecular beam directed normal to the (WO3)3/Pt(111) surface at 25 K. All TPD spectra were acquired using a line-of-sight quadrupole mass spectrometer and a linear temperature ramp rate of 1.0 K/s. To quantify the coverages of H2CO and CH3CHO, a chemically inert graphene monolayer was prepared on Pt(111),43,44 and the coverage-dependent TPD spectra of molecularly bound formaldehyde were acquired (spectra shown in Figure S1 of the Supporting Information). The area of the monolayer features at 107 and 127 K for H2CO and CH3CHO, respectively, were used to define their monolayer coverages on the (WO3)3 films.
3. RESULTS AND DISCUSSION 3.1. H2CO Adsorption on WO3/Pt(111): TPD and IRAS Results. In this section, we focus on the interaction of formalde-
hyde with (WO3)3 films on Pt(111). TPD is employed to monitor desorption products that result from reactions of formaldehyde with the (WO3)3/Pt(111) surface. Figure 2A displays a series of TPD spectra collected at 30 amu (the parent mass of H2CO) as a function of formaldehyde coverage. At the lowest coverage (0.4 ML), a single broad TPD peak is observed between 140 and 280 K. When the coverage is increased to 0.8 ML, a second peak develops at ∼123 K. Upon further coverage increase (1.2 8 ML), the peak at ∼123 K continuously shifts to lower temperatures and the peak at ∼212 K shifts to higher temperatures. An additional shoulder develops at ∼260 K on the high-temperature side of the high-temperature peak. At 10 ML, the high-temperature peak maximizes at ∼249 K and does not change with further coverage increase while the low-temperature peak at 98 K continues to grow. In agreement with the literature,34 we assign the low-temperature peak to the desorption of formaldehyde from multilayers. Figure 2B displays a series of different mass fragments for TPD spectra acquired after adsorption of 8.0 ML of H2CO. Note that only TPD peaks at 29, 30, and 31 amu are detected at ∼98 K. Their relative intensities (1, 0.72, and 0.01) are comparable with the tabulated cracking intensities (1, 0.58, and 0.005) of H2CO monomer45 as expected. Interestingly, the relative intensities of masses 29, 30, and 31 amu TPD peaks (1:0.65:0.43) at ∼245 K are very different from those at ∼98 K. Specifically, mass 31 amu is significantly more intense, indicating desorption of H2CO oligomers, probably in the form of 1-,3-,5-trioxane, (H2CO)3, which has the most intense cracking fragment at 31 amu (0.26, 0.15, and 1).45 To further explore the presence of higher cracking fragments of 1-,3-,5-trioxane and possibly larger H2CO oligomers, we also followed higher masses accessible to our mass spectrometer (
