Article pubs.acs.org/JPCC
Comparison of the Photodesorption Activities of cis-Butene, transButene, and Isobutene on the Rutile TiO2(110) Surface Michael A. Henderson* Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-87, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: The photodesorption properties of three butene molecules (cis-butene, trans-butene, and isobutene) were explored on the clean rutile TiO2(110) surface using temperature programmed desorption (TPD) and photon simulated desorption (PSD). At the low coverage limit, trans-butene is the most strongly bound butene on the TiO2(110) surface, desorbing at 210 K. Steric repulsions between neighboring molecules diminishes the binding of trans-butene at higher coverage. Both cis-butene and isobutene saturate the first layer on TiO2(110) at a coverage of ∼0.50 ML in a single TPD feature at 184 and 192 K, respectively. In contrast, the maximum coverage that trans-butene achieves in its 210 K peak is ∼1/3 ML, with higher coverages desorbing at ∼137 K. Coverages of these molecules above 0.50 ML populate second layer and multilayer states. The instability of trans-butene at a coverage of 0.5 ML on the surface is linked to the inversion center in its symmetry. The primary photochemical pathway of each butene molecule on the clean TiO2(110) surface is photodesorption. The photodesorption activities of these molecules on TiO2(110) at an initial coverage of 0.50 ML follows the trend: isobutene ≥ cis-butene > trans-butene. In contrast, the photodesorption activities at low initial coverage exceeds that measured at 0.50 ML, suggesting coverage-dependent effects on the photodesorption rate. The low photodesorption activity of trans-butene at a coverage of 0.50 ML may be linked to a weakened interaction between the molecule’s CC π bond and the surface Ti4+ adsorption site due to steric repulsions between neighboring molecules. These data suggest that steric repulsions may play a significant role in diminishing the photoactivities of weakly bound molecules on TiO2 photocatalysts. are similar, but slightly above that of isobutene.2 (The lowest energy singlet-to-triple transitions of all three molecules are about the same (∼4.2 eV).1) On the basis of these simple gas phase properties, one might expect that the general photoactivities of cis-butene and trans-butene to be similar and that of isobutene to be different. However, the arrangement of methyl groups about the alkene’s CC bond may also affect the degree of electronic coupling with the photocatalyst surface, and in turn photoactivity. In this study, the photoactivities of cis-butene, trans-butene, and isobutene are directly compared on the well-characterized rutile TiO2(110) surface using photodesorption as the comparative metric. The thermal and photochemical properties of isobutene have been explored previously,6,7 and will provide the basis for interpreting structural dependence in the photodesorption activities of the 2-butenes. These experiments were done in the absence of coadsorbed oxygen in order to avoid the effect of multiply partial oxidation pathways seen for isobutene6 that would complicate direct comparisons between the photoactivities of the three butenes. The main photochemical pathway of the butenes on TiO2(110) in the absence of coadsorbed oxygen is photodesorption, with trace amounts of photodecomposition resulting (presumably) in polymerized surface species. Direct comparison of the photodesorption
1. INTRODUCTION Molecules with wide HOMO−LUMO gaps can be difficult to photoactivate on a semiconducting oxide photocatalyst because their donor/acceptor molecular orbitals lie too far below/above the oxide’s band edges. For example, rates for hole-mediated oxidation of an adsorbed molecule will generally depend on the electron transfer rate between the molecule’s highest occupied molecular orbital (HOMO) and a hole at the semiconductor’s valence band edge. Similarly, rates of reduction will depend on the electron transfer rate from the semiconductor’s conduction band edge to the adsorbed molecule’s lowest lying electron acceptor state, which can be conceptualized as the molecule’s lowest unoccupied molecular orbital (LUMO). If the molecule’s orbital energies are not favorable for such transfers, then achieving reasonable photoactives may require “indirect” pathways (such as OH radical attack). Developing a better understanding of how a wide HOMO−LUMO gap molecule is activated on a photocatalyst requires detailed knowledge of the interactions between the molecule and the surface. Simple alkenes are an interesting test case for such an effort. For example, butenes show small variations in their electronic structures due to the arrangement of alkyl groups about the CC bond.1−4 The HOMO−LUMO gaps of the two 2butene molecules (cis-butene and trans-butene) are similar (∼7 eV), but slightly greater than that for isobutene (∼6.7 eV).1 In particular, the HOMO of isobutene is slightly lower in energy (∼0.1−0.2 eV) than those of cis-butene and trans-butene.3−5 Conversely, the LUMO energies of cis-butene or trans-butene © 2013 American Chemical Society
Received: August 22, 2013 Revised: October 16, 2013 Published: October 17, 2013 23840
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molecular beam doser (∼5 mm), which ensured uniformity of irradiation in the butene-covered region of the surface. The UV photon flux was ∼4 × 1016 photons cm−2 s−1, as determined with an Si photodiode. The sample experienced only a slight (a few degrees) rise in temperature at 35 K during irradiation.
activities of the three butenes reveals that the structural arrangement of the alkyl groups (methyls) about the CC bond is as important factor in influencing photodesorption activities as is the (gas phase) electronic structures of the molecules. Coverage is also seen to be important to the extent that packed surface situations accentuate the structural compatibilities (or incompatibilities) of each molecule on the TiO2(110) surface, revealing the influence of intermolecular repulsions on photocatalytic process.
3. RESULTS AND DISCUSSION 3.1. TPD Comparison of cis-Butene, trans-Butene, and Isobutene. Before examining the photochemical properties of the three selected butenes on TiO2(110), a baseline understanding of their surface chemical properties was obtained using TPD. The chemistry of isobutene on TiO2(110) has been examined previously,6,7 so this section will focus on cis-butene and trans-butene. Figures 1 and 2 present TPD data (mass 56)
2. EXPERIMENTAL SECTION The rutile TiO2(110) crystal, obtained from CrysTec, was mounted on a gold-coated Ta metal plate, which permitted cooling to 20 K (using a liquid He cryostat) and resistive heating to ∼1000 K. The surface was cleaning by cycles of ion sputtering (1 kV Ne+) and annealing, with cleanliness assessed using secondary ion mass spectrometry. The temperature during anneals was limited to 850 K, with the sample cooling to the base temperature in 10−15 min depending on the duration of the anneal. The crystal became pale blue, but remained transparent, as a result of the cleaning treatment. These conditions indicate a low level of bulk reduction, which was manifested by a low bridging oxygen vacancy concentration of ∼4%.8 (The surface oxygen vacancy population was determined using water TPD. Similarly, the crystal surface step density was estimated with water TPD to be below a few percent.9,10) The UHV system had a base pressure below 1 × 10−10 Torr, and was equipped with an apertured quadrupole mass spectrometer (QMS) and a three-stage molecular beam doser.6 TPD measurements were obtained with a sample heating rate of 2 K/s and in a line-of-sight orientation of the sample face and the QMS entrance aperture. Isobutene, cis-butene, and trans-butene were obtained from Aldrich. The as-received butenes all possessed significant amounts of impurity gases that were not condensed with liquid nitrogen (LN2), and therefore were easily removed by careful freeze−pump−thaw cycles. The desired butene was extracted from the supplied gas through vacuum distillation. Concerns that the cis-butene and trans-butene sources may have been compromised with the complementary isomer lead to additional effort being applied to the distillation process exploiting the small differences between the phase change properties of these molecules.11 While the mass spectra of these two molecules are nearly identical (see below) differences in their mass 29 to 56 ratios were successfully used to validate isomeric purities. Since all three butenes possessed the same molecular weight (∼56 g/mol), their fluxes exiting the molecular beam doser per unit backing pressure entering the first stage of the doser were the same. Exposures for all three molecules were obtained from a previous calibration of the doser with isobutene.6 The error in coverage based on this calibration was on the order of ±5%. Exposures were equated with coverages since the dosing temperature (35 K) enabled unity sticking. Coverages are expressed as monolayers (ML), where 1 ML = 5.2 × 1014 sites per cm2 based on the areal density of fivecoordinated Ti4+ (Ti5c) sites on the ideal surface. UV irradiation was accomplished using a 100 W Hg arc lamp that permitted irradiation of the sample via a fused silica fiber optic light delivery system. The sample was positioned with the incident light on the surface, resulting in PSD signals being registered with the sample rotated 45° from the TPD position. The diameter of the irradiation spot on the surface (∼7 mm) was slightly bigger than that of the dosing spot from the
Figure 1. Mass 56 TPD spectra from various coverages of cis-butene dosed on the clean TiO2(110) surface at 35 K. Blue traces: 0.03, 0.09, 0.17, 0.29, 0.35, and 0.51 ML; black traces: 0.55, 0.68, 0.75, 0.88, and 1.4 ML; inset traces: 0.88, 1.4, 2.0, and 3.1 ML.
from various coverages of cis-butene and trans-butene, respectively, dosed on the clean TiO2(110) surface at 35 K. Similar TPD plots for isobutene on TiO2(110) are available elsewhere for comparison.6 TPD traces from cis-butene coverages above and below 0.50 ML are shown in Figure 1 as black and blue traces, respectively. A broad feature was observed in TPD at ∼210 K for the lowest coverage of cisbutene explored (0.03 ML). Increased coverages of cis-butene resulted in this peak intensifying and shifting to lower temperature, eventually saturating at 188 K from a coverage of 0.50 ML. Coverages of cis-butene above 0.50 ML resulted in a low temperature shoulder on the 188 K peak that shifted down to ∼117 K with increasing coverage (up to ∼0.9 ML). Additional cis-butene resulted in two new TPD features at 107 and 100 K (see Figure 1 inset). The 107 K peak saturated at a cis-butene coverage of ∼2 ML, whereas the 100 K peak did not saturate with increasing coverage (data not shown) and was assigned to multilayer cis-butene desorption. No cis-butene thermal decomposition was detected in TPD, and the TPD uptake plot from the data in Figure 1 (not shown) exhibited 23841
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Figure 2. Mass 56 TPD spectra from various coverages of trans-butene dosed on the clean TiO2(110) surface at 35 K. Blue traces: 0.01, 0.03, 0.05, 0.14, 0.20, 0.28, 0.36, 0.43, and 0.50 ML; black traces: 0.59, 0.82, and 1.1 ML; inset traces: 0.82, 1.1, 1.7, and 3.1 ML.
Figure 3. Comparison of mass 56 TPD spectra from 0.50 ML of cisbutene, trans-butene, and isobutene dosed on the clean TiO2(110) surface at 35 K. Spectra were normalized by peak area for ease of viewing.
properties consistent with the absence of decomposition (i.e., all cis-butene dosed was recovered in TPD as cis-butene). The TPD behavior of trans-butene on the clean TiO2(110) surface (Figure 2) was noticeably different from that of cisbutene (Figure 1), although at low coverage the progression of trans-butene TPD features resembled that of cis-butene. For the lowest trans-butene coverage explored, there was a single desorption feature detected at ∼230 K similar to that observed for cis-butene. This feature shifted down in temperature to ∼210 K for trans-butene coverages up to ∼0.3 ML. At the 0.3 ML coverage limit, it would appear from TPD that trans-butene binds slightly more strongly to TiO2(110) than does cis-butene. However, the 210 K trans-butene TPD feature saturated as the coverage was increased above 0.3 ML, and additional transbutene appeared in TPD as a low temperature shoulder. This is apparent for trans-butene in Figure 2 by using the same color scheme as was used for cis-butene in Figure 1 (i.e., blue for coverages below 0.5 ML and black for coverages above 0.5 ML). These data suggest that the adsorption capacity of the TiO2(110) surface is greater for cis-butene than trans-butene, despite the fact that in the zero coverage limit trans-butene appears to bind more strongly to the surface than does cisbutene. As was the case with cis-butene, there was no evidence for thermal decomposition of trans-butene on the clean TiO2(110) surface. Additional trans-butene coverage between 0.5 and 1 ML desorbed in a new TPD feature peaked at ∼117 K. As shown in the inset of Figure 2, the 117 K peak was not due to multilayer trans-butene, which appeared in TPD at ∼109 K for coverages above ∼1.5 ML. To better illustrate the differences between the TPD behaviors of the butenes, a direct comparison of the TPD spectra of cis-butene, trans-butene, and isobutene on the clean TiO2(110) surface is presented in Figure 3 for identical coverages of 0.50 ML. In these data, the mass 56 spectra were normalized to equivalent peak areas to facilitate this
comparison because the per molecule QMS signals for these molecules differed somewhat. There are two notable differences in the TPD spectra of these molecules at 0.50 ML. The most obvious difference is that both cis-butene and isobutene evolved in single TPD feature with leading desorption edges at ∼150 K, whereas a substantial amount of trans-butene (∼1/6 ML) desorbed at ∼137 K with a leading edge at ∼110 K. This difference suggests that packing of cis-butene and isobutene on the TiO2(110) surface at a coverage of 0.50 ML is more efficient than for trans-butene. The second difference is that the main TPD peak temperatures for these three molecules followed the trend: cis-butene (184 K) < isobutene (192 K) < trans-butene (210 K). Similarly, the trailing edges for these TPD features, which provides insight into the zero coverage limit, follow the same trend. This implies that as the coverage diminishes via desorption the stability of the remaining molecules increased in this trend. Taken together, these data suggest that trans-butene molecules at low coverage bind more strongly to the clean TiO2(110) surface than do similar coverages of cis-butene or isobutene, but that the adsorption capacity of trans-butene in the first layer on TiO2(110) is significantly less (by ∼1/3rd) relative to that of either cisbutene or isobutene. The TPD data in Figures 1−3 reveal that the coverage dependence of the 2-butenes on the TiO2(110) surface (as well as in the layers above the first layer) depend on the isomeric configuration of the butene molecule. Figure 4 presents a schematic model that illustrates a likely explanation based on how the molecular structures of the three butene molecules might influence their packing on the TiO2(110) surface. This model is based on the proposition that the most stable adsorption structure involves centering of the molecule’s CC π bond over a Ti5c site.12,13 A simple conceptualization of the space filling molecular structures of trans-butene, cis-butene, 23842
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inversion symmetry of the molecule. In situations where neighboring trans-butene molecules are sterically constrained, the binding energy likely diminishes because the molecules are forced to adopt adsorption geometries that compromise the strength of the CC π bondTi5c interaction. For this reason, the leading edge for desorption of 0.50 ML trans-butene is shifted lower by ∼40 K than that of either cis-butene or isobutene. The model for trans-butene on TiO2(110) depicted in Figure 4 was tested by comparing TPD of trans-butene dosed at 35 K with that of trans-butene dosed at 140 K (see Figure 5). In
Figure 4. Space filling model depicting the intermolecular interactions between trans-butene, cis-butene, and isobutene molecules on the clean TiO2(110) surface at 0.50 ML. (Molecular dimensions are to scale relative to the surface dimensions.) Model assumes each molecule binds to surface Ti4+ sites through their CC π orbitals. The ability of each to respond to intermolecular repulsions is suggested (left) based on these bonding structures. Color scheme: O2− anions, Ti4+ cations, C atoms, and H atoms are gray, blue, black, and white, respectively. Horizontal lines are oriented along ⟨001⟩ directions and mark the surface bridging O2− sites.
and isobutene situated on the TiO2(110) surface indicates that only trans-butene has the means of adopting an “atop” orientation with minimal repulsive interactions between its methyl groups and bridging O2− (Obr) sites. As shown with the trans-butene molecule on the right in Figure 4, this is accomplished by rotating the molecule slightly about an axis through the CC bond normal to the surface such that CC bond direction is no longer along the ⟨001⟩ direction (i.e., the direction of the Ti5c rows). In contrast, there appear to be no such orientations of either cis-butene or isobutene over a Ti5c site that do not also involve some degree of repulsive interaction between a methyl group and a Obr site. These considerations may account for the apparent greater stability of trans-butene on TiO2(110) at low coverage in comparison to cis-butene and isobutene (Figure 3). However, as the coverage of each butene is increased to ∼0.5 ML, cis-butene and isobutene appear to be better adapt at accommodating steric repulsions than is trans-butene. The molecular orientations of each molecule depicted on the left side of Figure 4 offer a plausible explanation for this. At a 0.5 ML coverage, each butene molecule must occupy every other Ti5c site along each row of these sites. In cases where intermolecular interactions might exist between methyl groups on adjacent isobutene or cisbutene molecules, rotations of these molecules about the surface normal could relieve these repulsions. For example, if the methyl groups of neighboring isobutene molecules conflicted, then one molecule could rotate 180° to relieve these repulsions. In concept, such rotations for each molecule should have low barriers because interactions between the molecule’s system and a Ti5c site are retained as the molecules rotate, with rotational restrictions mainly resulting from passing the methyl groups by/over the Obr sites. In concept, a row of isobutene molecules or cis-butene molecules with minimal repulsions could be achieved through successive rotations. In contrast, if neighboring trans-butene molecules find themselves with opposing methyl groups (top left), rotations about the surface normal will not alleviate repulsions because of the
Figure 5. Mass 56 TPD spectra from trans-butene dosed on the clean TiO2(110) at 35 K (blue trace) and at 140 K (black trace).
concept, dosing at the leading edge of the main 184 K transbutene feature (at 140 K) should promote population of transbutene configurations with optimal intermolecular interactions and depopulate (through desorption) the more repulsive configurations depicted in Figure 4. Dosing an excess of trans-butene should also facilitate sufficient adsorption− desorption cycles to maximize the more stable trans-butene configurations. As shown in Figure 5, dosing a trans-butene exposure at 140 K equivalent to a ∼3 ML coverage at 35 K result in a ∼20% increased (from ∼0.36 to ∼0.44 ML) in the intensity of the 184 K peak relative to the “saturation” coverage in this peak obtained by dosing at 35 K. These data support the model in Figure 4 that the symmetry of the trans-butene molecule is the main factor responsible for weakening bonding as the first layer is populated at a dosing temperature of 35 K. 3.2. Photochemical Comparison of cis-, trans-, and isobutene. In this section, postirradiation TPD and PSD were used to compare the photodesorption activities of the three butene molecules on TiO2(110). Two initial coverages were used for these comparisons. An initial coverage of 0.50 ML was selected because it maximized the “chemisorbed” amounts of each butene and also provided a coverage in which the role of steric repulsions (in the trans-butene case) could be tested. The second coverage, 0.09 ML, provided a setting in which butene photochemistry could be examined in the absence of steric 23843
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repulsions. The photochemical properties of isobutene on TiO2(110) have been reported previously,6 but will be reapproached in this study to provide a better comparison with results for cis-butene and trans-butene. 3.2.1. Postirradiation TPD. Figure 6 presents postirradiation TPD results from UV irradiation (for 5, 15, and 50 min) of 0.50
Figure 7. Mass 56 TPD spectra after UV irradiation of 0.50 ML transbutene on the clean TiO2(110) surface at 35 K.
in Figure 2, these data show that photodesorption of only a small amount of trans-butene is needed to stabilize the remaining molecules, whereas the shoulder is still prominent in the thermal desorption spectrum of 0.43 ML trans-butene in Figure 2. The remaining trans-butene on the surface after photolysis (Figure 7) desorbed at 190 K instead of at 210 K (Figure 2) suggesting a slight destabilization of the remaining molecules due to photolysis. New trans-butene TPD features were also observed in TPD at higher temperatures (∼265 and ∼420 K) after UV irradiation. The rate of trans-butene depletion was slower compared to that for cis-butene (Figure 6) based on a comparison of the two sets of TPD data (see below). The effect of coverage on the photochemical properties of cis-butene and trans-butene is explored in Figure 8 using an initial coverage of 0.09 ML. Mass 41 was used in this figure instead of the weaker mass 56 because of the smaller coverages employed. At this coverage, trans-butene desorbed from TiO2(110) without the intermolecular repulsions that destabilized this molecule at a coverage of 0.50 ML. UV irradiation of 0.09 ML cis-butene (top panel) for only 5 min resulted in significant diminishment of the TPD signal, along with a positive shift in the peak temperature of the remaining cisbutene. This is consistent with the coverage-dependent TPD data seen at low coverages in the absence of UV irradiation (Figure 1). There was also evidence for a small amount of new desorption at ∼310 K. The depletion of 0.09 ML trans-butene (bottom panel) from UV irradiation was also significant after 5 min of UV, but the remaining trans-butene desorbed at the same temperature (219 K) with a more prominent new desorption feature seen at ∼260 K. These data suggest that trans-butene is more likely to be stabilized on the surface as a result of UV irradiation than is cis-butene. The nature of this stabilization (in the 260 K tail) is not clear. Figure 9 compares the rates of total photodepletion for the three butene molecules from the TiO2(110) surface at the two
Figure 6. Mass 56 TPD spectra after UV irradiation of 0.50 ML cisbutene on the clean TiO2(110) surface at 35 K.
ML cis-butene on the clean TiO2(110) surface at 35 K. As was observed for isobutene,6 UV irradiation of 0.50 ML cis-butene at 35 K diminished the main TPD feature at 184 K, and new minority features appeared at higher temperature (at ∼295 and ∼415 K). These two features were assigned in the isobutene case to higher binding at surface charge trapping sites and thermal decomposition of a photochemically generated polymerized form of the alkene, respectively. The latter phenomenon has been observed in the UV photolysis of butenes on various oxide surfaces.14−17 No other forms of cisbutene photodecomposition were detected. As was the case for isobutene,6 the majority of cis-butene depleted from the surface as a results of UV irradiation was photodesorbed rather than photodecomposed. No butene photooxidation products were detected in the absence of coadsorbed oxygen based on extensive search of potential decomposition products.6,7 The higher temperature states resulting from photolysis return cisbutene in TPD. These are interesting, but the main focus will be on the extent of cis-butene photodesorbed from the surface. The behavior of trans-butene photochemistry on TiO2(110) was different from that of cis-butene. Figure 7 presents postirradiation TPD results from UV irradiation (for 5, 15, and 50 min) of 0.50 ML trans-butene on the clean TiO2(110) surface at 35 K. After 5 min UV irradiation, the low temperature trans-butene TPD shoulder at 137 K was attenuated, accompanied by depletion of the main TPD feature at 210 K. Disappearance of the 137 K shoulder after only 5 min UV irradiation is remarkable in that only ∼0.04 ML of isobutene had desorbed. Compared with the TPD data shown 23844
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follows the trend: isobutene (diamonds) > cis-butene (triangles) > trans-butene (circles). Total photodepletion cross sections (i.e., photodesorption plus the trace photodecomposition) obtained from fitting these data to single exponential decay curves are displayed in Table 1. The single Table 1. Photodepletion Cross Sections (x10−20 cm2) from UV Irradiation of Butenes on the Clean TiO2(110) Surface at 35 K initial coverage (ML)
isobutene
cis-butene
trans-butene
0.50 0.09
1.2
0.9 2.0
0.3 1.2
exponentials adequately fit during the initial irradiation periods (below 20 min) for cis-butene and isobutene, but did not at longer time periods, which suggests that changes to the surface and/or overlayer as a result of irradiation alter the photodesorption activity of the remaining molecules. Examples of irradiation-induced surface changes that alter rates include charge trapping, coverage-dependent effects or accumulation of products/intermediates.18 The initial photodepletion cross section for 0.50 ML isobutene (1.2 × 10−20 cm2) was similar to that measured previously.6 The depletion cross sections for 0.50 ML cis-butene and trans-butene were ∼75% and ∼25%, respectively, of that for isobutene. These rates are over 1 order of magnitude less than some photochemical processes on TiO2(110) (e.g., acetaldehyde,19 oxygen,20 or acetone20,21) but higher than others (e.g., methylene bromide22). This variance indicates that the nature of the adsorbate is the deciding factor in influencing photoactivity on TiO2(110). The data from Figure 9 suggest that the structure of the butene molecule influences its photochemical activity on TiO2(110), particularly when the first layer is saturated. It is obvious that the physical structure of these molecules are different based on the arrangement of methyl groups about the CC bond. As discussed above, this physical structural variance affects the binding energy of each molecule on the surface, particularly at high coverage. The role of coverage in the photodepletion of butene is evident in a comparison of the high and low coverage data for cis-butene and trans-butene. The initial cross section increased by more than a factor of 2 for cisbutene and a factor of 4 for trans-butene by lowering the initial coverage. These measurements suggest that coverage plays a major role in the rates of butene photodesorption, particularly for trans-butene, where steric repulsions decrease rate. The small differences in the electronic structures of the butenes (see the Introduction section) does not appear to be a major factor in explaining the differences between the butenes. The two 2butenes have similar electronic structures, but different photodepletion rates at saturation coverage and similar rates at low coverage. Conversely, isobutene has the more distinctive electronic structure, but its photodepletion rate is similar to that of cis-butene. A more important factor influencing the rates of butene photodepletion appears to be how well the structure of the molecule permits it to electronically coupling to the surface and how coverage affects this interaction. On the basis of TPD, the weakest binding situation coincides with the lowest photodesorption cross section situation, which is contrary to the obvious relationship between binding and thermal desorption. Similarly, lower coverages which show higher binding (based on TPD) resulted in higher initial photodepletion rates. Incidentally, the fact that the photodepletion
Figure 8. Comparison of mass 41 TPD traces from UV irradiation of 0.09 ML cis-butene (top) and trans-butene (bottom) on the clean TiO2(110) surface at 35 K.
Figure 9. TPD yields after UV irradiation of cis-butene, trans-butene (both at 0.09 and 0.50 ML initial coverages), and isobutene (0.50 ML initial coverage) on the clean TiO2(110) surface at 35 K. (Dashed lines are from single exponential fits of the data.)
coverages explored (0.09 and 0.50 ML) using the TPD data in Figures 6−8 (along with additional data not shown in these figures). Data for 0.50 ML isobutene obtained under identical conditions as those for Figures 6−8 are also included (TPD data not shown6). The relative photodesorption activities from a starting coverage of 0.50 ML, as measured by the photodepletion of the three butenes in postirradiation TPD, 23845
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greater than that for cis-butene as the irradiation time period was extended beyond 2 min, explaining why the rate for isobutene was greater than cis-butene in as measured by postirradiation TPD (see Figure 9). 3.2.3. Photoisomerization of cis-/trans-Butene. The potential for photoisomerization of cis-butene or trans-butene was explored on the TiO2(110) surface using TPD, with the data presented in the Supporting Information. The challenge in this endeavor is differentiating between these two molecules based on their mass spectra, which are nearly identical.11 Differences in the intensities of the prominent QMS cracking fragments (masses 27, 28, 39, 41, 55, and 56 (parent)) were too small to reliably be used in distinguishing between the two molecules. (For example, the TPD peak area ratios from equivalent coverages of cis-butene and trans-butene for the respective mass 56 signals was ∼0.95 to 1, and essentially 1:1 for the mass 41 signals.) The most significant differences between the QMS signatures of these molecules was found with the mass 29 cracking fragment (C2H5+). The mass 29 to 56 were estimated (from many TPD experiments) at 0.77 for cisbutene and 0.93 for trans-butene. Unfortunately, the data presented in the Supporting Information suggest that photoisomerization of cis-butene or trans-butene is at most a minor process on TiO2(110). There is a possibility that photoisomerization occurs in the high temperature TPD states generated by irradiation (see Figures 6 and 7), or in the photodesorbing molecules. However, the signals are too weak in both these cases to discern whether photoisomerization occurs.
cross sections increased with lower initial coverage suggests that the deviations from a single exponential decay curve after long irradiation periods for the initial 0.50 ML coverage was not due coverage-dependent effects. 3.2.2. Photon Simulated Desorption. The relative photodesorption activities of the three butenes at 0.50 ML were also demonstrated in PSD, as shown in Figure 10. In these data,
4. CONCLUSIONS A comparison of the relative photochemical properties of isobutene, cis-butene, and trans-butene on the clean TiO2(110) surface suggests that steric repulsions play a significant role in influencing the photodepletion rates of these molecules on TiO2. trans-Butene, because of the inversion symmetry in its structure, is the least capable of adapting to intermolecular steric repulsions. As a result, this molecule couples weakly with the surface at saturation and shows the lowest level of photodesorption activity. In contrast, at low coverage (in the absence of significant intermolecular steric repulsions), the photodepletion rate of trans-butene is comparable to those of the other butenes. While optimal tuning of the electronic structure and optical properties of a photocatalyst is a major effect in achieving favorable photocatalytic efficiencies, results in this study suggest that “tuning” the steric interactions between weakly bound adsorbates can also have a significant influence on rates.
Figure 10. Normalized PSD signals from UV irradiation of 0.50 ML butene (isobutene, cis-butene, and trans-butene) on the clean TiO2(110) surface at 35 K.
mass 41 was used for each butene, with the QMS intensity for each normalized to account for the different cracking efficiencies of the three molecules. The mass 41 signals for each butene increased abruptly at time “0” as the surface was initially irradiated with UV. The initial “jump” was greatest for cis-butene and lowest for trans-butene, with the latter roughly 25% of the former. The PSD rates (averaged over the initial 2 min period) for these three molecules are summarized in Table 2. These rates were normalized using TPD. Also, the amounts Table 2. Initial Butene PSD Rates from UV Irradiation of 0.50 ML Butene on the Clean TiO2(110) Surface at 35 K butene
PSD rate (ML/sec)
iso cis trans
2.0 × 10−4 2.1 × 10−4 7.5 × 10−5
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ASSOCIATED CONTENT
S Supporting Information *
Experiments using postirradiation TPD conducted to test the possibility of photoisomerization of cis-butene and trans-butene on the TiO2(110) surface. This material is available free of charge via the Internet at http://pubs.acs.org.
of photodesorption in the initial 2 min period was typically small (
