J. Phys. Chem. B 1997, 101, 11119-11128
11119
Methyl Formate on Ag(111). 2. Electron-Induced Surface Reactions A. L. Schwaner and J. M. White* Center for Materials Chemistry, Department of Chemistry and Biochemistry, UniVersity of Texas, Austin, Texas 78712 ReceiVed: May 16, 1997; In Final Form: October 13, 1997X
Methyl formate, HCOOCH3, molecularly adsorbed at 120 K on Ag(111), readily undergoes electron-induced chemistry. Working mainly with monolayer coverages and 50 eV electrons, we have characterized the products ejected during electron irradiation and desorbed thermally after irradiation. Controlled electron irradiation at 120 K dissociates HCOOCH3, but more than 95% of the C and O is retained, even when the original monolayer is completely dissociated. H2, CH4, CH2O, CO2, and HOCH2CHO (glycolaldehyde) desorb as the temperature is raised. Heating to 450 K leaves a clean Ag(111) surface as measured by X-ray photoelectron spectroscopy and work function change measurements. Primary products are themselves altered by electron irradiation; no new products appear in desorption, but the distribution changes. The data support a two-channel electron dissociation model, one forming methyl (H3C(a)) and formates (HCOO(a) and HCO(a)O(a)), the second forming methoxy (CH3O(a)) and formyl (HC(a)O).
1. Introduction In the companion paper (part 1),1 we reported the thermal characteristics of methyl formate, HCOOCH3, adsorbed on a relatively inert metal substrate, Ag(111). Here, we report on the chemistry induced by electron irradiation of adsorbed HCOOCH3. This work continues our research employing nonthermal methodssphoton or electron excitationsto activate adsorbatesubstrate interfaces and drive surface reaction chemistry using electronically active and excited states. One goal, and the purpose of this paper, is to describe the nonthermal reaction processes that occur. In those cases where nonthermal excitation at low substrate temperatures leads to selective bond breaking, a second goal is to prepare spectroscopically significant concentrations of important catalytic intermediates, e.g., vinyl and phenyl.2 In this paper, we extend previous work with controlled electron activation of hydrocarbons on Ag(111) and Pt(111) to heteroatom hydrocarbons containing oxygen. We have also studied electron irradiation of methanol (CH3OH),3 acetone (CH3COCH3),4 and biacetyl (CH3COCOCH3) on Ag(111).5 Relevant results from part 1 can be summarized as follows. On the basis of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and work function change (∆Φ) data, no thermal dissociation occurs during either adsorption below 120 K or subsequent heating. The monolayer adsorption energy is smallsthe desorption activation energy is 37.4 kJ mol-1sand only slightly larger than the multilayer value of 34.2 kJ mol-1. Thus, strong chemisorption forces are absent. For monolayer HCOOCH3, the absolute coverage calculated from the C(1s) XPS peak area is 7 × 1014 molecules cm-2, close to what is expected for crowded, aligned, but weakly bound adsorbates; an estimate based on the liquid-phase density is 5 × 1014 molecules cm-2. Reflection-absorption infrared vibrational spectra (RAIRS) are consistent with monolayer cis-HCOOCH3 aligned with its molecular symmetry plane perpendicular to the surface, the carbonyl (CdO) bond and the methyl group toward the substrate, and the C-O ester linkage nearly parallel to the surface. X
Abstract published in AdVance ACS Abstracts, December 1, 1997.
S1089-5647(97)01650-7 CCC: $14.00
TABLE 1: Electron Impact Fragmentation of Gas-Phase HCOOCH3 fragment
mass (amu)
relative intensity
lit. (ref 15)
CH3O HCO+ CH3OH+ CH3+ HCOOCH3+
31 29 32 15 60
1.00 0.64 0.39 0.37 0.35
1.00 0.65 0.35 0.30 0.40
+
In this paper, low-energy (ca. 50 eV) electrons are used to excite and dissociate monolayer methyl formate on Ag(111). The products are examined using TPD, XPS, UPS, work function measurements (∆Φ), and isothermal mass spectrometry (ISOMS). Extensive fragmentation is evidenced by C(1s) and O(1s) XPS after electron exposure and by the appearance in TPD of methane (209 K), hydrogen (217 K), formaldehyde (∼210 and 243 K), and carbon dioxide (∼385 K), as well as some undissociated methyl formate (∼147 and 167 K). A new species, identified as glycolaldehyde (226 K), was detected in the postirradiation TPD spectra for long electron exposures. On the basis of these measurements, the products are ascribed to two dominant dissociation channels that follow electron excitation, each breaking one C-O bond to form either (1) H3C(a) + HCOO(a) and HCO(a)O(a) or (2) CH3O(a) + HC(a)O.6 2. Experimental Section The experiments were performed in a stainless steel ultrahigh vacuum (UHV) chamber,7-10 equipped for residual gas analysis (RGA), ISOMS, TPD, Auger electron spectroscopy (AES), XPS, and UPS. Work function changes were calculated from shifts of the measured secondary emission onsets in UPS. The Ag(111) crystal was prepared and cleaned by standard methods and its cleanliness confirmed by AES. Methyl formate, HCOOCH3 (Aldrich), was subjected to several freeze-pumpthaw cycles at 77 K before use. The RGA fragmentation pattern is given in Table 1. Gas was dosed through a leak valve using a directed capillary array; the delivery tube terminated 0.5 cm away from the Ag(111) surface. With the sample turned away from the doser, the ion gauge reading was increased, ∆P ) 5 × 10-10 Torr, by opening the leak valve. The sample was then rotated in front of the doser for a period, predetermined to give the desired coverage (e.g., ∼60 s to adsorb a monolayer). © 1997 American Chemical Society
11120 J. Phys. Chem. B, Vol. 101, No. 51, 1997 For electron irradiation, the sample was rotated into line-ofsight with the mass spectrometer for a predetermined time. The mass spectrometer has an open ion source, and current flows from the filament to the sample in this position. The incident energy of these electrons was 50 ( 2 eV as measured using a energy analyzer located across the chamber from the filament. For subsequent TPD, the sample was rotated 90° away from line-of-sight of the QMS to minimize electron irradiation;11 the heating rate was 1.5 K/s between ∼100 and 690 K. To identify species ejected during irradiation, we collected ISOMS while dosing with electrons. To execute these experiments, a negatively biased sample with monolayer coverage was positioned in line-of-sight with the mass spectrometer, typically for 60 s, while we monitored several mass signals. Then, while continuing to monitor the mass spectra, we removed the bias, allowing the electron flux (∼1014 e cm-2 s-1) to irradiate the adsorbate-substrate interface. For XPS, various pass energies were used, typically 50 or 100 eV, depending on the signal intensity. For a given pass energy, our fitting procedure employed fixed full widths at halfmaximum (fwhm) established in calibration data from known sampless2.2 ( 0.1 and 2.0 ( 0.1 eV for C(1s) and O(1s) at 50 eV pass energy and 3.1 ( 0.1 and 3.0 ( 0.1 eV for 100 eV pass energy. Using these widths and a Shirley background subtraction, Mg(KR) excited XPS core level spectra for C(1s) and O(1s) were fit to Gaussian-Lorentzian line shapes (G-L) functions (85% G-15% L) using standard least-squares numerical analysis. To fit each of the measured spectra, we used the minimum number of G-L functions required to match the overall spectral width and structure. This procedure does not exclude the likely possibility of contributions from more than one species to each of the fitted peaks. UPS spectra were taken using a conventional helium discharge lamp. With the sample biased negatively, so the secondary onset was easy to identify, we measured work function changes from shifts in this onset. 3. Results The electron-induced chemistry of HCOOCH3 on Ag(111) provides evidence for (1) electron-induced decomposition of HCOOCH3, (2) negligible electron stimulated desorption of the parent during electron exposure, (3) barely detectable ejection of decomposition products during electron irradiation, (4) desorption of hydrogen, methane, formaldehyde, carbon monoxide, and glycolaldehyde in the postirradiation TPD, and (5) return to a clean surface by annealing to 450 K. Unless otherwise stated, the initial coverage of HCOOCH3 was 1 ML, as defined in the companion paper,1 and the incident electron energy was 50 ( 2 eV. 3.1. TPD. Fluence Dependence at 50 eV. To initiate this study, we examined the TPD spectra of parent HCOOCH3 after several 50 eV electron fluences from 0.0 to 6.0 × 1016 e cm-2 (Figure 1). Beginning with monolayer coverage, curve a, the parent intensity (60 amu) drops steadily and redistributes thermally as the electron dose increases. Without irradiation, there is a single peak at 145 ( 2 K and a shoulder toward higher temperature (∼160 K), the latter attributed to desorption from defect sites.1 After an electron fluence of 1.3 × 1016 e cm-2 (curve b), the 145 K peak decays and a new peak appears at 170 K. The total HCOOCH3 peak area (sum of both peaks) also decays with electron dose; about half the initial coverage remains for 3 × 1016 e cm-2 (curve c) and less than 10% after 6 × 1016 e cm-2 (curve e). At the latter electron fluence, there is no detectable 145 K parent desorption peak. The inset of Figure 1 shows the relative loss of total parent peak area (145 plus 170 K peaks) as a function of fluence. From
Schwaner and White
Figure 1. TPD spectra, 60 amu, of methyl formate as a function of 50 eV electron fluence: (a) 0, (b) 1.3, (c) 3, (d) 4.6, and (e) 6 × 1016 e cm-2. The heating rate was 1.5 K s-1. The dotted baselines are offset for clarity. Inset: Semilogarithmic plot of the fractional decrease of the HCOOCH3 TPD peak area with increasing electron fluence. The indicated cross section is calculated from the slope and the measured current from the sample to ground.
Figure 2. Plot of the electron energy dependence of the cross section determined for irradiation of 1 ML HCOOCH3. The dashed line is a guide through the data points.
the linearity of this semilogarithmic plot over a factor of 10 decay, the calculated total cross section for loss of parent TPD intensity is 3.9 (( 0.2) × 10-17 cm2. This indicates that molecularly adsorbed methyl formate is quite vulnerable to 50 eV electrons; about 1 in 10 incident electrons initiates events leading to parent loss. The cross section for loss of monolayer HCOOCH3 varies with incident electron energy (Figure 2). At 9.5 eV, the lowest energy feasible in our apparatus, the cross section is not detectable. At 13.5 eV the cross section is measurable, and it rises steadily up to 53 eV, the highest energy used here. The steady rise suggests contributions from secondary electrons. Extrapolating the low-energy portion of this curve (solid line in Figure 2) gives an effective threshold between 11 and 12 eV, slightly above the 11.0 eV ionization potential of molecular HCOOCH3.12 Below the effective threshold, electron attach-
Methyl Formate on Ag(111). 2
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11121
Figure 3. Isothermal spectra of masses monitored during ∼50 eV electron irradiation of 1 ML methyl formate. The substrate temperature was held at 125 ( 5 K and the electron flux was 1.1 × 1014 e cm-2 s-1. Prior to to, the electron beam was repelled from the sample, and the dashed lines describe the baseline trends during the 30 s period before the irradiation commenced.
Figure 4. TPD spectra, background subtracted, obtained after 1 ML HCOOCH3 was exposed to an electron fluence of 3 × 1016 e cm-2. Masses 31, 29, 32, 15, and 60 correspond to desorption of methyl formate, HCOOCH3. The heating rate was 1.5 K s-1. Inset: TPD spectra of ion signals with no peaks.
ment processes may contribute, but cross sections for loss of HCOOCH3 associated with them lie below 5 × 10-9 cm2. Having established the electron energy threshold of the cross section, the remaining experiments used 50 ( 2 eV electrons. Insight is gained by gathering ISOMS data (Figure 3). In this experiment, a negatively biased sample with monolayer coverage was positioned in line-of-sight with the mass spectrometer while we monitored several signals. At 60 s, the bias was removed, allowing the electron flux (1.1 × 1014 e cm-2 s-1) to irradiate the adsorbate-substrate interface for 600 s (fluence ) 6.6 × 1016 e cm-2). Since no changes were observed beyond 150 s, we show only the first 225 s in Figure 3. Reflecting fragment desorption, the 15, 16, and 29 amu signals rise slightly above their baselines. This is not due solely to HCOOCH3 ejection: were the 15 amu signal attributable to HCOOCH3 ejection we would expect (contrary to observation) a 31 amu increase three times the 15 amu increase. Similar arguments apply to other signals. While other products may be ejected, note that none of the measured increases is large; on the basis of XPS, very little material (
