LETTER pubs.acs.org/JPCL
Identification of the Active Species in Photochemical Hole Scavenging Reactions of Methanol on TiO2 Mingmin Shen and Michael A. Henderson* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-87 Richland, Washington 99352, United States
bS Supporting Information ABSTRACT: Molecular and dissociative forms of adsorbed methanol were prepared on the rutile TiO2(110) surface to study their relative photocatalytic activity for hole-mediated oxidation. Molecular methanol is the dominant surface species on the vacuum-annealed TiO2(110) surface in ultrahigh vacuum (UHV). Coadsorption of methanol with oxygen results in ∼20% of the adsorbed methanol decomposing to methoxy and OH. Subsequent heating of the surface to ∼350 K desorbs unreacted methanol and OH (as water), leaving a surface with only adsorbed methoxy groups. Using temperature-programmed desorption (TPD), we show that adsorbed methoxy is at least an order of magnitude more reactive than molecularly adsorbed methanol for hole-mediated photooxidation. Methoxy photodecomposes through cleavage of a CH bond forming adsorbed formaldehyde and a surface OH group. These results suggest that methoxy, and not molecular methanol, is the effective hole scavenger in photochemical reactions of methanol on TiO2. SECTION: Surfaces, Interfaces, Catalysis
M
ethanol is widely employed as a hole scavenger in photocatalytic studies on TiO2.13 Photocatalysis of methanol on TiO2 is also prominent in environmental remediation411 and reforming reactions.1215 Despite the importance of methanol photocatalysis on TiO2, the mechanism of methanol photooxidation remains unclear.1,5,7,8,1618 One unresolved issue is the relative reactivities of the molecular versus dissociative forms of adsorbed methanol for hole-mediated photooxidation. In this study, we employ the rutile TiO2(110) surface and UHV conditions to identify the active form of adsorbed methanol. The TiO2(110) surface has received considerable attention in the literature as a model TiO2 catalyst and photocatalyst.1,19 Many experimental and theoretical studies have been performed to understand the thermal chemistry of methanol on TiO2(110).16,2035 Results show that both molecular and dissociative forms of methanol exist on TiO2(110), with dissociation occurring primarily at defect sites or through the influence of coadsorbed oxygen. For example, coadsorbed oxygen can be used to prepare a surface that is populated exclusively with methoxy groups and no molecular methanol species.20 Using this approach, we show that the methoxy group and not molecularly adsorbed methanol is the reactive species for hole-mediated oxidation of methanol on TiO2(110). The coverage of methanol in a saturated first layer on TiO2(110) has been estimated at between 0.67 and 0.77 ML with TPD, varying to a small extent depending on the oxygen vacancy coverage.20,36 Using these estimates as a gauge, we prepared surfaces with similar coverages of either molecular methanol or methoxy. The left plot of Figure 1 shows mass 29 TPD results from 0.18 ML (1 monolayer is defined as 5.2 1014 sites cm2) r 2011 American Chemical Society
methanol adsorbed at 100 K on the TiO2(110) surface followed by various UV irradiation periods in UHV. Mass 29 was selected because it conveys a signal from any CHxO-containing species (e.g., from QMS cracking of methanol and formaldehyde). Without UV irradiation, TPD showed three methanol desorption features from the vacuum-annealed surface (Figure 1a). The main TPD peak was centered at 308 K, with two weaker states at ∼450 and 640 K. As previously reported,20 the 308 K TPD peak is due to desorption of molecularly adsorbed CH3OH. The broad TPD state at ∼450 K was attributed to recombinative desorption of some methoxy groups, resulting from either dissociation at vacancy or nonvacancy sites.20,21 The peak at 640 K was observed in a previous work as a result of coadsorbed oxygen.20 The small amount of dissociated methanol (∼0.01 ML) seen in Figure 1 at 640 K was likely due to trace amounts of background oxygen adsorption. No significant change was seen in the TPD after 0.5 min of UV irradiation (Figure 1b); however, a weak shoulder at 270 K on the main CH3OH TPD peak grew in after more extensive periods of UV irradiation (Figure 1cg). Analysis of the QMS mass 2830 signals for this shoulder indicated that it was due to formaldehyde desorption. This was confirmed by TPD of H2CO dosed directly on TiO2(110) (Supporting Information) and found to be consistent with a previous TPD study of H2CO on TiO2(110) by Kim and coworkers.37 As shown in the right side of Figure 1, the impact of UV irradiation on CH3OH TPD was seen mostly in a decrease Received: September 12, 2011 Accepted: October 10, 2011 Published: October 10, 2011 2707
dx.doi.org/10.1021/jz201242k | J. Phys. Chem. Lett. 2011, 2, 2707–2710
The Journal of Physical Chemistry Letters
LETTER
Figure 1. Left: Mass 29 TPD spectra from 0.18 ML methanol on TiO2(110) after various UV irradiation times: (a) no UV irradiation and (bg) 0.5, 1, 2, 5, 10, and 20 min, respectively. Right: Comparison of mass 29 TPD traces with no UV irradiation (black line, same as panel a) and 10 min UV irradiation (red line, same as panel f).
Figure 2. Left: Mass 29 TPD spectra from 0.15 ML methoxy on TiO2(110) after various UV irradiation times: (a) no UV irradiation and (bg) 0.5, 1, 2, 5, 10, and 20 min, respectively. Right: Comparison of mass 29 TPD traces with no UV irradiation (black line, same as panel a) and 10 min of UV irradiation (red line, same as panel f).
in the 640 K peak and possibly some attenuation in the broad feature at ∼450 K. There was no discernible change in the main CH3OH peak at 308 K. (The black trace shows the TPD spectrum without UV irradiation, and the red trace shows the TPD spectrum after UV irradiation for 10 min.) These data provide evidence of the photochemical conversion of adsorbed methanol to adsorbed formaldehyde on TiO2(110) but likely only through the species associated with the 640 K TPD state (i.e., adsorbed surface methoxy). To confirm the results of Figure 1 that suggest molecular methanol is photoinactive and the species associated with the 640 K TPD state (methoxy) is photoactive, we prepared adlayers on TiO2(110) with only methoxy present. Previous work has shown that the 640 K mass 29 TPD peak in Figure 1 was from concomitant desorption of methanol and formaldehyde as a result of methoxy disproportionation (Reaction 1).20,25 Coadsorbed oxygen enhances methoxy formation by promoting CH3OH bond cleavage,20 with hydrogen and unreacted methanol removed from the surface by heating to ∼350 K, according to Reactions 24
shown in Figure 2a (left side). The two TPD features in this trace, a weak desorption state between 350 and 450 K and the main state at 640 K, correspond to small amounts of methanol dissociation at defects and methoxy disproportionation, respectively. The coverage of methoxy was clearly enhanced as a result of oxygen coadsorption. (The scale is the same as that in Figure 1.) The methoxy-covered surface prepared for Figure 2a was irradiated in UHV at 120 K with UV for various time periods (Figure 2bg). After 0.5 min of UV irradiation (Figure 2b), TPD showed two new features at 270 and 320 K. The peak intensities of these two desorption features increased with increasing UV irradiation time at the expense of the surface methoxy coverage (Figure 2cg). The right plot of Figure 2 shows a more detailed comparison from TPD results of the adsorbed methoxy group photoreaction. After UV irradiation for 10 min, TPD showed that the surface methoxy coverage decreased by >70% (from ∼0.15 ML to
