Article pubs.acs.org/JPCC
Conversion of 1,3-Propylene Glycol on Rutile TiO2(110) Long Chen, Zhenjun Li,† R. Scott Smith, Bruce D. Kay, and Zdenek Dohnálek* 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 S Supporting Information *
ABSTRACT: The adsorption of 1,3-propylene glycol (1,3-PG) on partially reduced TiO2(110) and its conversion to products have been studied by a combination of molecular beam dosing and temperature-programmed desorption (TPD). When the Ti surface sites are saturated by 1,3-PG, ∼80% of the molecules undergo further reactions to yield products that are liberated during the TPD ramp. In contrast to ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG) that yield only alkenes and water at very low coverages (0.1 ML), propanal (CH3CH2CHO) and two additional products, 1-propanol (CH3CH2CH2OH) and acrolein (CH2CHCHO), are observed. The desorption of 1propanol is found to be coupled with the desorption of acrolein, suggesting that these products are formed by the disproportionation of two 1,3-PG molecules. The coverage-dependent TPD results further show that propylene formation dominates at low coverages (900 K. The temperature was measured by a chromel−alumel (type K) thermocouple glued to the edge of the crystal. The TPD spectra (80−870 K, 1 K/s) were acquired with the crystal in a line-of-sight geometry. The initial surface cleaning procedure comprised cycles of Ne+ sputtering (1.5 kV, 10 uA) at 300 K and annealing at 850−900 K for 10 min until a clean and ordered TiO2(110)-1×1 surface was obtained. To maintain the surface cleanliness and order, a brief, 3 min sputtering at 300 K, followed by 10 min of annealing at 870 K, was used on a daily basis. Reproducibility of the surface structure was further confirmed using H2O TPD.34 The population of VO’s on the surface was determined to be ∼5% based on the ratio of water recombination desorption peak at 500 K to the monolayer desorption peak at 270 K.34,35 The 1,3-PG, HO(CH2)3OH (99.8%), was obtained from Sigma-Aldrich and transferred into a round-bottom flask containing a baked molecular sieve to remove water. Before use, the 1,3-PG was further purified by pumping at room temperature and then stabilized in a water bath at 333 K for ∼5 h. The gas lines connected to the vacuum system were heated to avoid condensation of the molecules in the delivery system. The molecules were dosed onto the substrate using an effusive 23182
dx.doi.org/10.1021/jp507787m | J. Phys. Chem. C 2014, 118, 23181−23188
The Journal of Physical Chemistry C
Article
molecular beam. The molecular beam flux was stable as measured by QCM and repeated TPD experiments. Absolute coverages in ML for 1,3-PG and reaction products were defined relative to the number of Ti4+ sites (1 ML ≡ 5.2 × 1014 cm−2) on TiO2(110). The absolute 1,3-PG coverages were determined by correlating the C 1s XPS spectrum for 1,3-PG saturation coverage with that of methanol, which has a known saturation coverage of 0.77 ML.36 More details are provided in our prior study of hydrogen formation for a range of simple glycols.32 The desorption yield of unreacted 1,3-PG was quantified by comparing the TPD areas from two different multilayer doses where the amount of reacted 1,3-PG did not change (e.g., the difference in TPD area between 3 and 2 ML doses corresponds to a 1 ML desorption signal). Mass spectra and absolute desorption rates of products have been determined by dosing known amounts of these molecules with a flux calibrated molecular beam and measuring their TPD spectra.36 The resulting mass spectra were also used to determine the ionization fragmentation patterns of the reaction products in our experimental setup.
can be grouped into carbon-free and carbon-containing products. The carbon-free products include only H2O (18 amu) and H2 (2 amu), while the carbon-containing products consist of propylene (41 amu), propanal (58 amu), formaldehyde (30 amu), ethylene (25 amu), 1-propanol (31 amu), and acrolein (56 amu). In Figure 1, TPD spectra for several mass fragments that arise for each of the carbon-containing products are displayed (fragments from the same product are grouped together and have the same color coding). All the products were subsequently monitored by their most intense mass fragments (listed in the parentheses) except ethylene, which was monitored at 25 amu because the most intense mass fragment at 28 amu is common to many products. In contrast, only acrolein and ethylene contribute to the 25 amu signal, which allows us to distinguish ethylene desorption features in the TPD spectra without difficulty. In a similar way, mass 45 amu was selected to follow parent 1,3-PG desorption, since no other products contribute to this mass fragment. For clarity, the deconvolved product TPD spectra are shown in subsequent figures. There, the contributions of parent 1,3PG to 18, 25, 30, 31, 41, 56, and 58 amu were first removed by subtracting a cracking pattern (determined from the 1,3-PG multilayer desorption region of the TPD) scaled mass 45 amu signal from the raw spectra. In some cases, further deconvolution/subtraction was necessary to isolate the signal of individual products. For example, both acrolein and ethylene contribute to 25 amu, and both 1-propanol and propylene have fragments at mass 41 amu. The acrolein contribution to 25 amu and the 1-propanol contribution to 41 amu were further subtracted using their determined cracking pattern scaled masses 56 and 31 amu, respectively. An example of the deconvolution procedure for the propylene TPD spectra is provided in the Supporting Information (Figure S1). Representative TPD spectra of the resulting net products are shown in Figure 2a,b, which correspond to a low 1,3-PG coverage (0.045 ML) and a high coverage (0.7 ML), respectively. The desorption products observed at low coverages (comparable to the VO concentration) are water, propylene, and two smaller carbon-containing molecules, formaldehyde and ethylene (Figure 2a). This is in contrast to the EG and 1,2PG results on TiO2(110),13,31 where only water and alkenes were observed at similar coverages. The formation of formaldehyde and ethylene indicates reaction channels involving C−C bond scission, which were not found for EG and 1,2-PG on TiO2(110). At high coverage, the product distribution becomes more complex (Figure 2b). In addition to water, hydrogen (not shown; see further discussion below), propylene, formaldehyde, and ethylene that were detected at low coverages, propanal, acrolein, and 1-propanol along with unreacted 1,2-PG are observed at high coverages. Water desorption is observed at all 1,3-PG coverages and precedes the desorption of any carbon-containing products. At low coverages, a single peak is observed at ∼500 K, whereas, at high coverages, water desorption extends from 200 to 550 K. Generally, water desorption from 1,3-PG resembles that observed for EG and 1,2-PG on TiO2(110)13,31 and can be explained by the recombination of surface hydroxyls formed on the surface (HOb and terminal hydroxyl groups, HOt).16,37−39 In addition to water, D2 desorption from OD-labeled 1,3-PG has been also observed previously at ∼400 K at high 1,3-PG coverages (see also Figure S2, Supporting Information).32 Nondeuterated 1,3-PG has been employed in this detailed
3. RESULTS AND DISCUSSION To determine the gas-phase products from 1,3-PG on TiO2(110), we have acquired extensive sets of mass spectra in initial survey experiments while monitoring numerous mass fragments. A representative set of such raw spectra for HO(CH2)3OH is presented at a high coverage of 0.8 ML in Figure 1. Analogous to our previous studies with EG and 1,2PG,11−13,31 the desorption products from 1,3-PG on TiO2(110)
Figure 1. Survey TPD spectra from 1,3-propylene glycol (1,3-PG), HO(CH2)3OH, dosed on TiO2(110) at 80 K and heated at 1 K/s. The coverage of 1,3-PG is ∼0.8 ML, which exceeds the saturation coverage of Ti5c sites of 0.5 ML. The spectra are color-coded and grouped for the mass fragments that arise from the same products: 1,3-propylene glycol, HO(CH2)3OH, gray; water, H2O, red; propylene, CH3CH CH2, green; propanal, CH3CH2CHO, magenta; formaldehyde, HCHO, violet; ethylene, C2H4, olive; 1-propanol, CH3CH2CH2OH, cyan; acrolein, CH2CHCHO, orange. The fragments used in the detailed coverage-dependent studies described here were 18 amu for H2O, 41 amu for propylene, 30 amu for formaldehyde, 25 amu for ethylene, 31 amu for 1-propanol, 56 amu for acrolein, and 58 amu for propanal. The amount of desorbing 1,3-PG was quantified using an m/ z = 45 amu. 23183
dx.doi.org/10.1021/jp507787m | J. Phys. Chem. C 2014, 118, 23181−23188
The Journal of Physical Chemistry C
Article
Figure 2. TPD spectra of all observed desorption products (except hydrogen) from (a) 0.05 ML and (b) 0.7 ML of 1,3-PG, HO(CH2)3OH, dosed on TiO2(110) at 80 K: 1,3-propylene glycol, HO(CH2)3OH, gray; water, H2O, red; propylene, CH3CHCH2, green; propanal, CH3CH2CHO, magenta; formaldehyde, HCHO, violet; ethylene, C2H4, olive; 1-propanol, CH3CH2CH2OH, cyan; acrolein, CH2CHCHO, orange. The spectra were deconvolved (see text and captions of Figures 3−5 and 7 for details) to eliminate the contributions of overlapping masses and to isolate the net product specific spectra.
Figure 3. Coverage-dependent TPD spectra of 1,3-PG, HO(CH2)3OH, obtained using the CH2CH2OH+ mass fragment (m/z = 45 amu) following dosing on TiO2(110) at 80 K. The TPD traces for the coverages that correspond to VO concentration (∼0.05 ML) and saturation coverage on Ti5c sites (0.5 ML) are highlighted with blue and red, respectively. The 1,3-PG coverages and desorbing yields are listed in the figure.
coverage-dependent study, which precludes us from following the H2 formation because of a high H2 background that is liberated from the closed-cycle He cooled manipulator during the TPD ramp. Mechanistically, molecular hydrogen formation from glycols on TiO2(110) has been shown to result from the bimolecular reaction between two hydroxyl groups of neighboring Ti5c-bound glycols.32 Likewise, the hydrogen atoms in water also originate exclusively from the cleavage of hydroxyl groups in 1,3-PG, as confirmed by experiments with OD-labeled molecules (Figure S2, Supporting Information). Essentially identical TPD spectra are observed for the carboncontaining products from HO(CH2)3OH and DO(CH2)3OD, suggesting that hydroxyl hydrogens are not incorporated into the carbon-containing products. This is consistent with the conclusions for EG and 1,2-PG, as demonstrated in our previous studies.13,31 We further focus on the coverage-dependent TPD spectra of 1,3-PG and its carbon-containing products (summarized in Figure 2). Figure 3 displays TPD spectra of 1,3-PG after dosing at 80 K. In general, the coverage-dependent evolution of 1,3PG TPD spectra is similar to that obtained previously for EG and 1,2-PG.13,31 The absolute 1,3-PG coverages were determined by correlating the C 1s XPS spectrum for 1,3-PG saturation coverage with that of methanol, as discussed in the Experimental Section.36 At low coverages (
