Carbon−Oxygen Coupling in the Reaction of Formaldehyde on Ge

J. Phys. Chem. C 2007, 111, 1739-1746

1739

Carbon-Oxygen Coupling in the Reaction of Formaldehyde on Ge(100)-2×1 Michael A. Filler, Charles B. Musgrave, and Stacey F. Bent* Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305 ReceiVed: July 28, 2006; In Final Form: NoVember 4, 2006

Although catalytic properties are usually ascribed to transition metals and their oxides, the present work identifies a catalytic carbon-oxygen coupling reaction on a group-IV (100)-2×1 semiconductor surface. The reactions of formaldehyde and formaldehyde-d2 on Ge(100)-2×1 were investigated with multiple internal reflection infrared (MIR-IR) spectroscopy, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Formic acid was also studied for comparison. Infrared data indicate that formaldehyde adsorbs in a dative-bonded state and also forms a minority C-H dissociation product. In addition, theory predicts that a CdO [2+2] cycloaddition product may form, although the infrared spectra are inconclusive as to its presence. Time-dependent and annealing infrared studies suggest that the dative-bonded species undergoes a subsequent coupling reaction with a nearby surface adduct, resulting in a bidentate formate group as evidenced by an intense ν(O-C-O) stretching mode near 1500 cm-1. The desorption of CO2 during TPD experiments confirms the carbon-oxygen coupling reaction.

I. Introduction Transition metals and their oxides have long been studied to gain a greater understanding of their catalytic properties as well as to improve the yields and selectivities of various industrially important reactions.1 Various bond breaking and forming processes have been observed depending on the metal and surface crystal plane, and these processes are influenced by the presence of partially filled d-orbitals located in a band near the Fermi level.1 While the electronic structure, and thus chemical properties, of group-IV atoms are significantly different from those of transition metals, their respective surfaces both have the ability to either donate or accept electron density from adsorbates. Germanium is a group-IV atom, which usually forms strong covalent bonds with neighboring atoms and possesses filled 3d orbitals. However, the structure of the Ge(100)-2×1 surface leads to donor/acceptor properties similar to those of transition metal surfaces. After proper preparation, the reconstructed Ge(100)-2×1 surface exhibits rows of surface dimers,2 each with a strong σ bond and a weak π bond. The surface dimers tilt out of the surface plane,2 which creates an uneven charge distribution, resulting in an electron-rich, nucleophilic “up” atom and an electron-deficient, electrophilic “down” atom. Thus, the surface dimer possesses both Lewis acidic and basic character, allowing it to both accept and donate charge, in a manner analogous to that of many transition metal surfaces. Examples of dative bonding, where an impinging molecule such as ammonia donates its lone pair to the electrophilic dimer atom on Ge(100)-2×1, have been reported.3-5 In addition to their ability to accept and donate charge, many industrially useful catalysts form adsorbate-surface bonds of moderate strength, an observation commonly referred to as Sabatier’s Principle.6 Sufficiently weak bonding is unlikely to dissociate chemical bonds, while prohibitively strong bonding prevents desorption of the reaction product and eventually leads to catalyst fouling. Previous work on Si(100)-2×1, which possesses chemical characteristics similar to those of Ge(100)* Corresponding author. E-mail: [email protected].

Figure 1. Surface-bound bidentate formate on a transition metal or transition metal oxide. The details of the reaction mechanism leading to this surface structure depend on the specific transition metal or oxide, crystal plane, and adsorbing species.

2×1, has revealed that many molecules, including alkene-,7 amine-,8 and carbonyl-containing9 compounds, form strongly bound surface products. Because of the strong bonds formed on Si(100)-2×1, heating of the surface usually leads to molecular decomposition and carbide formation, which limits its potential for catalytic coupling reactions. On the other hand, the binding energies of adsorbates on Ge(100)-2×1 are typically found to be 20-30 kcal/mol weaker than those on Si(100)-2×1 due to the reduced strength of Ge-X bonds (where X ) C, O, H, etc.).10-12 Several examples of molecular desorption processes on Ge(100)-2×1 have been reported in the literature,3,13-15 and we postulate that the weaker surfaceadsorbate bonds formed on Ge(100)-2×1 enable the reaction described in the current work. Interest in the water gas shift reaction, which converts CO and H2O into CO2 and H2, led to the identification of formate intermediates, as illustrated in Figure 1, which have been studied on transition metal surfaces and their oxides.16,17 Two methods are commonly employed to obtain and study surface-bound formate species. The most direct procedure involves the O-H cleavage of formic acid.17 In the other method, oxygen atoms pre-adsorbed on the surface either by direct exposure or by prior decomposition processes act as nucleophiles and attack the electron-deficient carbonyl carbon atom of adsorbed formaldehyde.18-21 Such a carbon-oxygen coupling reaction often leads to elimination of hydrogen and formation of the bidentate formate structure (Figure 1). In the present work, we explore aldehyde chemistry on Ge(100)-2×1 and provide evidence indicating that formaldehyde undergoes a carbon-oxygen coupling reaction analogous to

10.1021/jp064820v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007

1740 J. Phys. Chem. C, Vol. 111, No. 4, 2007 those observed on transition metals and metal oxides. Previous investigations of aldehyde adsorption on Si(100)-2×19,22 and Si(111)-7×723 indicate that this functional group forms [2+2] CdO cycloaddition products as well as a mixture of minor surface adducts, but does not form coupling products. Hence, the chemistry observed for formaldehyde on Ge(100)-2×1 is markedly different from that on silicon. Similarities in the infrared spectra of formaldehyde and formic acid adsorbed on Ge(100)-2×1, as well as the evolution of their infrared modes upon annealing, provide evidence of a bidentate formate species. Desorption of CO2 during temperature-programmed desorption studies confirms that a carbon-oxygen coupling reaction has occurred. II. Experimental and Computational Details All experiments were completed under ultrahigh vacuum conditions (UHV) in a reaction chamber described previously.24 The base pressure is less than 1 × 10-10 Torr. The surface was prepared by Ar+ sputtering at room temperature (20 mA emission current, 0.5 keV accelerating voltage, 7-8 µA sample current) for 20 min followed by annealing to 900 K for 5 min. Low-energy electron diffraction (LEED) confirmed that the proper surface reconstruction was achieved, and Auger electron spectroscopy (AES) verified that carbon, oxygen, and nitrogen surface concentrations were at undetectable levels. Infrared spectra were collected in a multiple internal reflection (MIR) geometry by employing a BioRad FTS-60A Fourier transform infrared (FTIR) spectrometer equipped with a liquid nitrogen cooled narrow-band mercury-cadmium-telluride (MCT) detector. The spectral range of the collected infrared data is limited by the absorption of the CaF2 chamber windows, resulting in a low-frequency cutoff of 1100 cm-1. We do not show regions above 2100 cm-1 because absorption in the ν(C-H) stretching regions was not intense enough to be observed with our setup for these molecules. All spectra were corrected for baseline sloping. Formaldehyde and formaldehyde-d2 were purchased as paraformaldehyde (95%, Aldrich) and paraformaldehyde-d2 (99 atom % D, Cambridge Isotopes), respectively. Both paraformaldehyde compounds are white powders, while formic acid (99%, Acros) is a clear liquid under ambient conditions. Transfer of all compounds to sample vials was completed under dry air purge. Paraformaldehyde and paraformaldehyde-d2 were outgassed using a water bath held at approximately 340 K for several hours prior to cracking into gaseous formaldehyde and formaldehyde-d2, respectively, at 380 K. Formic acid was purified by several freeze-pump-thaw cycles before exposure to the crystal surface. Exposure of gas to the crystal surface was accomplished through a variable leak valve and directed doser combination. Surface exposures are reported in langmuirs (1 L ) 10-6 Torr-s), and pressures were not corrected for ionization gauge sensitivity. Following each surface exposure, the manifold lines were pumped out to less than 20 mTorr before refilling for the following dose. An in situ quadrupole mass spectrometer confirmed the molecular identity and purity of each compound after introduction to the chamber. Water, from terminal group condensation during the paraformaldehyde cracking process,25 was largely undetectable during formaldehyde exposure. In addition, the low sticking probability of water on Ge(100)-2×1 at room temperature26,27 leads us to believe that a small water partial pressure will not impact the results presented here. Temperature-programmed desorption (TPD) experiments were conducted using a shielded quadrupole mass spectrometer

Filler et al. (Vacuum Generators). The ionizing filament is enclosed in a stainless steel shroud to minimize readings from stray desorption from the sample holder and heater. A 3 mm aperture at the tip of the shield was positioned approximately 1 cm from the crystal surface. Additional openings in the shield were located 2 cm behind the filament as well as near the chamber flange to facilitate pumping inside the shield. TPD experiments were performed by adsorbing 0.2 L of formaldehyde or formaldehyde-d2 at 310 K. Following this exposure, the chamber pressure returned to ∼5 × 10-10 Torr within 1-2 min, at which time a linear surface temperature ramp of 1 K/s was initiated. Up to five different masses were recorded simultaneously during a single TPD experiment. The masses selected for monitoring were based upon a review of formaldehyde adsorption, desorption, and coupling studies in the literature. For formaldehyde, TPD traces for masses 2, 15, 16, 17, 18, 28, 29, 30, 40, 42, 43, 44, 45, 46, 56, and 60 were recorded in a series of experiments. The temperature scale is calibrated with H2 desorption from a monohydride Ge(100)-2×1:H surface.28 Electronic structure calculations were completed with the Gaussian 03 software package29 using the Becke3 Lee-YangParr (B3LYP) density functional theory.30 The B3LYP functional is composed of the Lee-Yang-Parr and VWN correlation functionals31,32 in addition to the Becke hybrid gradientcorrected exchange functional.33 Previous studies of B3LYP indicate that it provides predictive results for similar systems7,34,35 and is in good agreement when experimental results are available.3,15,36,37 In the present work, single and double dimer cluster models were employed to capture local bonding trends as well as inter-dimer interactions, respectively. Because the larger cluster models are computationally intensive, their use was limited to an investigation of a few inter-dimer and inter-adsorbate interactions. To maximize accuracy and computational feasibility, different levels of theory were used for single and double dimer calculations. For single dimer calculations, unconstrained Ge9H12 and Ge9D12 cluster models were employed. Geometries of important local minima on the potential energy surface were calculated with the polarized double-ζ, 6-31G(d) basis set without geometrical constraints followed by a single point calculation at the 6-31G(d) geometry with the triple-ζ, 6-311++G(2df,pd) basis set. For intra- and inter-row calculations, Ge15H16 and Ge23H24 clusters were employed, respectively. Geometry optimizations for both twodimer clusters were completed at the B3LYP level of theory with a split basis set, where the 6-311++G(d,p) basis set, the LANL2DZ pseudo-potential, and the 6-31G(d) basis set describe the chemically active atoms, the subsurface Ge atoms, and the subsurface hydrogen atoms, respectively. To minimize aphysical cluster geometries, the positions of the third and fourth layer Ge atoms of both two-dimer clusters were fixed in their positions following the optimization of unconstrained, bare Ge15H16 and Ge23H24 dimer clusters. Zero and one imaginary frequency was found for all local minima and transition state structures, respectively. All reported energies have been zero-point corrected. III. Theoretical Results and Discussion Figure 2 illustrates the calculated stationary points for two formaldehyde adsorption pathways on a Ge9H12 dimer cluster model. The theoretically determined fundamental vibrational modes below 2000 cm-1 of each minimum are listed in Table 1. Both pathways lead to the products expected for carbonyl-containing compounds on Si(100)-2×1 or Ge(100)2×1 based on previously published literature.15,22,38,39 The

Reaction of Formaldehyde on Ge(100)-2×1

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1741 TABLE 2: Calculated [2+2] CdO Cycloaddition Binding Energies of Carbonyl-Containing Compounds on a Ge9H12 Dimer Cluster

Figure 2. Theoretically determined stationary points for the reaction of formaldehyde on a Ge9H12 dimer cluster: (a) carbonyl oxygen dativebonded state, (b) [2+2] CdO cycloaddition product, and (c) direct C-H dissociation product.

TABLE 1: Theoretically Determined Vibrational Modes of the Formaldehyde Oxygen Dative Bond, [2+2] CdO Cycloaddition, and Direct C-H Dissociation Productsa frequencies (cm-1) adsorption structure dative bond

[2+2] CdO cycloaddition

direct C-H dissociationb

formaldehyde/ Ge9D12

formaldehyde-d2/ Ge9H12

ν(CdO) 1656 δ(CH2) 1463 Fr(CH2) 1226 Fw(CH2) 1148 δ(CH2) 1442 Fw(CH2) 1202 Ft(CH2) 1164 ν(C-O) 935 ν(Ge-H) 1886 ν(CdO) 1699 δ(C-H) 1293

ν(CdO) 1605 δ(CD2) 1074 Fr(CD2) 966 Fw(CD2) 917 δ(CD2) 1068 Fw(CD2) 942 Ft(CD2) 859 ν(C-O) 911 ν(Ge-D) 1342 ν(CdO) 1684 δ(C-D) 993

a Frequency calculations for formaldehyde and formaldehyde-d were 2 completed on Ge9D12 and Ge9H12 clusters, respectively, to minimize unwanted coupling of dimer and subsurface Ge-H modes (scaling factor ) 0.96).41 b The calculated ν(Ge-H) and ν(Ge-D) vibrational frequencies are red-shifted from their experimentally determined locations near 1960 and 1420 cm-1, respectively, and we believe that this is likely an artifact of employing a single scaling factor for all vibrational frequencies.

thermodynamically favored surface adduct is the direct C-H dissociation product, with a binding energy of 30.6 kcal/mol. However, formation of this product requires passage over a significant activation barrier, located 18.2 kcal/mol above the entrance channel. Previous work in our group suggests that pathways with barriers of this magnitude do not proceed appreciably on Ge(100)-2×1 near room temperature,3,35,40 and we do not expect to observe a significant quantity of this surface product. Although the binding energy of the [2+2] CdO cycloadduct is calculated to be more than 8 kcal/mol weaker than the direct C-H dissociation product, the activation barrier is favorable for formation. Formaldehyde initially adsorbs in a barrierless dative-bonded precursor state with a binding energy of 8.2 kcal/mol. The formation of dative bonds is common during the reaction of lone pair-containing organic compounds on Si(100)2×1 and Ge(100)-2×1.42 In the present case, a bond is created by donation of an oxygen lone pair to the electrophilic dimer atom. Several attempts to isolate a stable minimum for a dative

molecule

Ebind (kcal/mol)

formaldehyde acetaldehyde acetone

22.0 14.5 11.3

bond between the nucleophilic dimer atom and the carbon atom of the formaldehyde molecule were unsuccessful. An activation barrier located 4.8 kcal/mol below the entrance channel leads from the oxygen dative-bonded state to the [2+2] CdO cycloaddition product, which exhibits a binding energy of 22.0 kcal/mol. In contrast, previous theoretical work has found that [2+2] CdO cycloadducts of related carbonyl-containing compounds, including ketones,38 carboxylic acids,40 and tertiary amides,15 exhibit considerably weaker binding energies (near 10 kcal/mol). The weak binding energies calculated for most carbonyl-containing compounds have led to the conclusion that the [2+2] CdO cycloaddition product is likely unobservable at room temperature due to rapid molecular desorption from the surface.15,38,40 Table 2 lists the binding energies calculated for formaldehyde, acetaldehyde, and acetone at the same level of theory employed in the present work. A clear increase in binding energy is seen upon decreasing methyl substitution. The effect of the methyl groups on the binding energy of the [2+2] CdO cycloaddition product is not likely to be steric, because the orientation of methyl substituents in the [2+2] CdO cycloaddition product suggests that there is little interaction of these groups with the surface. However, methyl substituents are known to reduce the reactivity of carbonyl bonds by stabilizing the partial positive charge on the carbonyl carbon atom,43 and we therefore attribute the observed trend to the electronic effects of the methyl substituents. Although the [2+2] CdO cycloaddition product is less thermodynamically favorable than the direct C-H dissociation pathway, the low activation barrier from the dative-bonded state suggests that this surface product will dominate on Ge(100)2×1 near room temperature. However, in the sections that follow, we will show that a single molecule, single dimer adsorption model is insufficient to capture the chemistry experimentally observed for formaldehyde on Ge(100)-2×1. IV. Experimental Results A. Infrared Spectroscopy. Infrared spectra were collected as a function of time following exposure of the Ge(100)-2×1 surface to formaldehyde and formaldehyde-d2. Figures 3a and 4a show the time-dependent spectra following the adsorption of 0.2 L of formaldehyde and formaldehyde-d2, respectively, at 310 K. For formaldehyde, vibrational modes are observed at 1960, 1705, 1641/1622, 1520, 1270, and 1213 cm-1 immediately following adsorption (t ) 0 min). For formaldehyde-d2, fewer vibrational features located at 1672, 1620/1598, and 1250 cm-1 are visible in the spectrum at t ) 0 min. Upon the acquisition of subsequent spectra with no additional formaldehyde or formaldehyde-d2 exposure, an interesting time dependence is observed as evident in Figures 3 and 4. Figures 3b and 4b show the overall time change for formaldehyde and formaldehyde-d2, respectively. For the case of formaldehyde, the doublet at 1641/1622 cm-1 as well as modes at 1270 and 1213 cm-1 attenuate, while peaks at 1520 and 1354 cm-1 increase in intensity. For formaldehyde-d2, the doublet at 1620/ 1598 cm-1 attenuates, while a doublet at 1520/1503 cm-1 increases in intensity.

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Figure 3. (a) Overall time-dependent infrared spectra following the exposure of 0.2 L of formaldehyde on clean Ge(100)-2×1 at 310 K and (b) the total incremental spectral change.

Figure 5. (a) 10 L saturation exposure of formaldehyde on Ge(100)2×1 at 310 K followed by annealing for 5 min to (b) 350 K, (c) 390 K, (d) 440 K, and (e) 490 K. Spectrum a is ratioed to the clean surface, while spectra b-e are incrementally ratioed to the previous annealing temperature. All spectra were recorded at 310 K.

Figure 4. (a) Overall time-dependent infrared spectra following the exposure of 0.2 L of formaldehyde-d2 on clean Ge(100)-2×1 at 310 K and (b) the total incremental spectral change.

The assignment of the spectral features is assisted by isotopic labeling. The presence of intense modes from 1500 to 1700 cm-1 for both the normal and the deuterated formaldehyde molecules strongly suggests that these modes result from either a CdO or C-O bond and do not arise from C-H bonds. Furthermore, because C-H vibrations are expected to red-shift upon isotopic substitution, the lack of a peak near 1350 cm-1 in the formaldehyde-d2 spectra indicates that the mode at 1354 cm-1 for formaldehyde (Figure 3a) is due to a δ(C-H) deformation mode. Similarly, the peak at 1213 cm-1 for formaldehyde (Figure 3a) is also assigned to a δ(C-H) deformation mode because it is not present in the

formaldehyde-d2 spectrum (Figure 4a). The presence of a ν(Ge-H) peak at 1960 cm-1 for formaldehyde provides evidence of at least some C-H dissociation. Interestingly, unlike the other modes, the ν(Ge-H) peak at 1960 cm-1 as well as the weak mode at 1705 cm-1 do not appear to undergo a time-dependent change, suggesting that these modes result from a separate surface species that is not changing with time. Figure 5 follows the thermal evolution of adsorbed formaldehyde on Ge(100)-2×1 with infrared spectroscopy. After a saturation exposure of formaldehyde on the Ge(100)-2×1 surface at 310 K (Figure 5a), peaks are present at 1641/1622 cm-1 in agreement with Figure 3a. The ratio of the two peaks at 1641/1622 cm-1 is near unity following the 10 L exposure shown in Figure 5a. However, the ratio after the 0.2 L exposure shown in Figure 3a reveals a peak ratio closer to 2. While we did not explicitly study the effect of exposure on the peak ratio of the doublet at 1641/1622 cm-1, we found that exposure influenced their intensity. Annealing to 350 K (Figure 5b) leads to the complete attenuation of the 1641/1622 cm-1 doublet in favor of strong stretching modes between 1518 and 1545 cm-1. As in Figure 3a, the mode at 1352 cm-1 is again assigned to a δ(C-H) bending mode. Similar to the time-dependent spectra shown above (Figure 3), the modes near 1500 and 1350 cm-1 grow in simultaneously upon annealing to 350 K, indicating that they are likely due to the same surface adduct. The ratio of the peaks comprising the doublet near 1500 cm-1 was consistently near unity for a range of initial formaldehyde exposures. No increase in the ν(Ge-H) stretching mode is visible upon annealing to 350 K, suggesting that hydrogen does not transfer to the surface upon annealing to 350 K. At 390 K (Figure 5c),

Reaction of Formaldehyde on Ge(100)-2×1

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1743 350 K. However, there are indirect indications that molecular desorption of formaldehyde does not occur to a significant extent below 350 K. The integrated intensity and peak temperature of the CO2 parent mass (m/z ) 44) in TPD traces recorded following formaldehyde adsorption at 310 K (Figure 6) and 350 K (not shown) were identical, providing evidence that the majority of formaldehyde does not desorb molecularly between 310 and 350 K. Special care was also taken to ensure that the observed desorption fragments resulted from surface-induced reactions and not from adsorbed paraformaldehyde oligomers. Mass spectra recorded during formaldehyde exposure do not reveal ion fragments heavier than mass 30. In addition, no desorption at mass 56, indicative of paraformaldehyde (-OC-O-C-), was observed during any TPD run. V. Discussion

Figure 6. TPD spectra of key mass fragments following the exposure of 0.2 L of formaldehyde on Ge(100)-2×1 at 310 K.

a sharp negative peak is visible at 1525 cm-1. Although difficult to see, this may be a derivative feature with a weak growth near 1545 cm-1. Upon annealing to 440 K (Figure 5d), clear loss peaks are observed at 1543, 1518, and 1354 cm-1, while growth is seen at 1962 cm-1. These changes suggest that the surface species associated with the modes near 1500 and 1350 cm-1 desorbs and/or decomposes leaving new Ge-H bonds at the surface at this temperature. Finally, a weak negative ν(GeH) mode is observed at 1959 cm-1 upon annealing to near 500 K (Figure 5e). B. Temperature-Programmed Desorption. The TPD spectra shown in Figure 6 are consistent with the infrared annealing data presented above and provide evidence for a C-O coupling reaction. Two major desorption channels were observed in the reaction of formaldehyde with Ge(100)-2×1 (Figure 6). Appearance of masses 12, 16, 28, and 44 peaked at 465 K with intensities consistent with the cracking pattern of CO2. Desorption of CO2 (m/z ) 44) following formaldehyde (m/z ) 30) adsorption provides strong evidence of a carbon-oxygen coupling reaction between two adsorbates on Ge(100)-2×1. This is an important finding that we will fully address in the discussion. The H2 desorption peak visible near 575 K indicates that direct recombinative desorption of hydrogen is likely occurring, and not desorption of another reaction product of which hydrogen is a cracking fragment. A high-temperature desorption channel is also visible at 700 K (Figure 6) for formaldehyde, and ion fragments were observed at 2, 15, 16, 28, 40, and 42. The origin of these masses is unclear. While the presence of masses 15 and 16 suggests the desorption of methane, this assignment is not supported by the deuterated experiments in which an expected CD4 peak at mass 20 was not observed during TPD studies of formaldehyded2. Weak peaks at masses 40 and 42 suggest that some quantity of ketene is created, but ketene would not account for the presence of mass 15. Several points must be made with regard to the TPD data discussed above. Stray desorption from hot surfaces near the crystal substantially obscured the spectra below 350 K. Hence, although it was not observed above 350 K, we were unable to probe directly whether molecular formaldehyde evolved below

The infrared and TPD results presented above for the reaction and subsequent evolution of formaldehyde on the Ge(100)-2×1 surface reveal that a bimolecular surface reaction between two surface adducts is likely occurring. We begin the discussion with an assignment of the surface species present following the initial adsorption of formaldehyde and formaldehyde-d2 at 310 K (Figure 3 and 4) followed by analysis of bidentate formate structure formation and its subsequent desorption as CO2. The presence of a ν(Ge-H) peak at 1960 cm-1 for formaldehyde provides evidence of at least some C-H dissociation. Furthermore, the modes at 1960 and 1705 cm-1 do not evolve with time, suggesting they result from the same surface adduct. The ν(Ge-H) stretching mode is more than 10 times weaker than that reported following the adsorption of ethylenediamine,24 indicating that the C-H dissociation pathway is only a minor pathway. While no ν(Ge-D) stretching features are observed near 1420 cm-1 44 following the adsorption of formaldehyde-d2, the dipole moment of this stretch is weaker than that of ν(Ge-H), and therefore may be present but not discernible above the noise. A 33 cm-1 red-shift is observed between the mode at 1705 cm-1 for formaldehyde and that at 1672 cm-1 for formaldehyde-d2. This difference is consistent with the 15 cm-1 red-shift predicted by our theoretical calculations (Table 1) as well as the 45 cm-1 red-shift reported for the carbonyl stretch upon the deuteration of gas-phase formaldehyde.45,46 This comparison, together with the location of the mode within the typical carbonyl stretching region, led us to assign this peak to a ν(CdO) stretch. Because of its correlation with the ν(Ge-H) mode, we propose that this species is a C-H dissociation product. The location of the ν(CdO) mode is in excellent agreement with the calculated frequency for this product (Table 1), although the barrier to direct C-H dissociation is calculated to be significant (Figure 2). The reaction may occur at steps and/or defect sites. Although our DFT calculations predict that formation of a [2+2] cycloaddition product at the interface is likely (section III), its presence following adsorption at 310 K (Figure 3) is difficult to experimentally ascertain. The Fw(CH2) wagging mode of the [2+2] CdO cycloadduct is calculated to be near 1200 cm-1 and is consistent with a peak observed at 1213 cm-1 immediately following formaldehyde adsorption. On the other hand, no mode is seen near the calculated δ(CH2) frequency of 1442 cm-1. The ν(C-O) stretch of its strained four-membered ring is calculated to fall below the cutoff of our chamber windows, near 935 cm-1 (Table 1), preventing its observation. Thus, we cannot reach a definitive conclusion as to the presence of the [2+2] CdO cycloaddition product following adsorption at 310 K.

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Figure 7. Illustration of (a) a dative-bonded formaldehyde species, which is stabilized by interaction with the nucleophilic atom of an adjacent surface dimer. Stabilization of the dative bond on a single dimer may also result from adsorbate-adsorbate interactions (not shown). (b) A bidentate formate structure due to a carbon-oxygen coupling reaction.

The identification of the surface adduct responsible for the intense modes observed for both formaldehyde and formaldehyde-d2 near 1620 cm-1 is possible by comparison with the literature and theory. From previous reports of organics adsorbed on Ge(100)-2×1, features between 1600 and 1700 cm-1 likely result from ν(CdC) and ν(CdO) stretching modes.11,15,38,40,47 Because formaldehyde contains only one carbon atom and the observed mode is extremely intense, we rule out the presence of a ν(CdC) stretching mode and focus on ν(CdO) stretching vibrations. For the case of formaldehyde, the doublet at 1641/1622 cm-1 is red-shifted by more than 60 cm-1 as compared to the peak at 1705 cm-1, previously assigned as the ν(CdO) mode of the direct C-H dissociation product. This shift suggests that the carbonyl bond leading to the absorbance at 1641/1622 cm-1 is electron deficient. Such an effect can be the result of a dative bond, where one of the carbonyl oxygen lone pairs donates to the electrophilic dimer atom (Figure 2a). For the case of acetone, this charge donation was found to result in a 70 cm-1 red-shift of the ν(CdO) stretching vibration from the condensed multilayer value.38 Although we were unable to obtain a condensed multilayer of formaldehyde on Ge(100)-2×1, our calculations indicate a 125 cm-1 red-shift of the ν(CdO) stretching frequency upon adsorption in the dative-bonded state. Consistent with these calculations, the experimentally observed peaks at 1641/1622 cm-1 for formaldehyde and 1620/1598 cm-1 for formaldehyded2 are red-shifted approximately 100 cm-1 from their location in the gas phase.45 Therefore, on the basis of the experimental and theoretical evidence outlined here, we conclude that the doublet near 1620 cm-1 for formaldehyde and formaldehyded2 results from an oxygen dative bond. Despite the assignment of a dative-bonded species, carbonyl dative bonds are calculated to only be weakly adsorbed. Our calculations indicate that the binding energy of the dativebonded species is only 8.2 kcal/mol below the entrance channel (Figure 2a). A first-order kinetic analysis assuming a preexponential factor of 1013 s-1 yields a surface lifetime of 50 ns at 310 K. With such kinetics, the dative-bonded product would not be expected to remain on the surface long enough to be observable. However, single dimer calculations cannot capture the nonlocal charge-transfer effects found for dative bonds with larger cluster models on Si(100)-2×148,49 and to a lesser extent on Ge(100)-2×1.50 In addition, single dimer clusters cannot incorporate bonding interactions between neighboring dimers and adsorbates, which may be important in this system. For example, Figure 7a shows a formaldehyde dative bond where a neighboring dimer in the adjacent row donates charge from its nucleophilic atom to the electron-deficient carbonyl carbon. This type of interaction is analogous to the classic attack of the electron-deficient carbonyl carbon by a nucleophile in the

Figure 8. Infrared spectra of (a) 5 L of formaldehyde on Ge(100)2×1 at 350 K and (b) 0.5 L of formic acid on Ge(100)-2×1 at 310 K. Dashes lines indicate spectral features common to both spectra and believed to be responsible for bidentate formate surface species. Both exposures are at saturation.

solution phase.51 Preliminary calculations on a Ge23H24 dimer cluster indicate that this type of interaction stabilizes a dative bond by 2.1 kcal/mol and suggest that nonlocal interactions may play a role in the experimental observation of the dative bond near room temperature. We also speculate that adsorbateadsorbate interactions may also lead to stabilization of the dative-bonded state on a single dimer. At the present time, we do not have the capability to complete an extensive study of inter-dimer or inter-adsorbate interactions, and we leave these calculations for further theoretical study. We now turn our discussion to the observed conversion of formaldehyde adsorbed in the dative-bonded state to the surface product responsible for the vibrational modes observed near 1500 cm-1, which increase as a function of time (Figures 3 and 4) at 310 K or upon annealing to 350 K (Figure 5), following formaldehyde and formaldehyde-d2 adsorption on Ge(100)-2×1. Similar to the modes near 1620 cm-1 previously assigned to a dative bond, only a small shift is observed upon deuteration, providing evidence that the modes near 1500 cm-1 are likely due to a type of C-O bond. Yet, few vibrational modes of C-O containing molecules fall in this region of the infrared spectrum.52 Several pieces of evidence suggest that the absorption feature near 1500 cm-1 is due to the presence of bidentate formate at the surface, a product which can only result from reaction between two surface adsorbates. First, the frequency is close to that for formate species on metals. As illustrated in Figure 7b, bidentate formate contains two oxygen atoms, which bond to the surface to create an O-C-O structure. The νs(O-C-O) and νas(O-C-O) infrared absorption modes of formate on various metal surfaces often fall between 1300 and 1550 cm-1.17,53 Second, similar structures have recently been identified by our group following the adsorption of carboxylic acids on Ge(100)-2×1,40 and the spectra of these structures match those for formaldehyde extremely well. Figure 8 compares the infrared spectra of formaldehyde and formic acid adsorbed on Ge(100)-2×1 at 350 and 310 K, respectively. As shown by dashed lines, modes at 1518 and 1354 cm-1 in Figure 8a are

Reaction of Formaldehyde on Ge(100)-2×1 strikingly similar to modes at 1508 and 1356 cm-1 in Figure 8b. The mode at 1508 cm-1 following the reaction of formic acid has previously been assigned to a ν(O-C-O) stretching mode of a bidentate formate structure,40 and the similarities suggest that formate exists on Ge(100)-2×1 following formaldehyde adsorption at 350 K. The relatively simple infrared spectrum observed in Figure 8a indicates that the bidentate formate is the dominant species upon adsorption at 350 K and is also the dominant species at long times after adsorption at 310 K (Figures 3 and 4). The weak mode centered at 1657 cm-1 in Figure 8a may result from a formate structure where one Ge-O bond has broken to form a surface-bound ester, consistent with the ν(CdO) stretch observed for the O-H dissociation product of formic acid at 1663/1653 cm-1 (Figure 8b).40 While the present work identifies a bidentate formate product at the Ge(100)-2×1 surface, several structural and electronic properties of this species cannot be determined with the experimental techniques available to us: (1) It is not possible for us to determine whether the bidentate surface adduct observed in the present work is an ionic formate species or simply an O-C-O bridging structure (the term “formate” has been employed in the present work for simplicity). (2) Although Figure 7b shows a formate adduct bridging two surface dimers, we cannot spectroscopically determine whether formate exists on a single dimer, between two dimers in the same row, between two dimers across the trench, or in another orientation. Because the location of formate ν(O-C-O) stretching modes is strongly influenced by its environment, the presence of more than one peak near 1500 cm-1 provides evidence that multiple related structures are present on Ge(100)-2×1. (3) The ν(O-C-O) bidentate modes near 1500 cm-1 cannot be specifically assigned as symmetric or asymmetric. Because of screening by the surface image charge, reflection-absorption IR spectroscopy studies of formate species on transition metal surfaces often report only the νs(O-C-O) near 1300 cm-1, and not the νas(O-C-O) stretch near 1500 cm-1.17,54 While the multiple internal reflection geometry employed here allows modes both parallel and perpendicular to the surface to be probed, only one group of ν(O-C-O) stretching peaks are visible near 1500 cm-1. Although their location suggests they result from a νas(O-C-O) mode, the lack of additional peaks corresponding to the νs(O-C-O) stretch prevents us from making a more specific assignment. The room-temperature infrared spectra presented above indicate that a carbon-oxygen coupling reaction leads to the formation of formate on Ge(100)-2×1, and the TPD data combined with the annealing IR data further support this conclusion. Although heating Si(100)-2×1 often leads to decomposition rather than desorption at moderate temperatures, various molecular desorption processes have been reported on Ge(100)-2×1.3,13-15 Therefore, assuming that adsorbed formate desorbs predominantly intact upon heating, the measurement of ion fragments with a mass greater than that of formaldehyde (m/z ) 30) would offer compelling evidence of a coupling reaction. As shown in Figure 6, CO2 desorbs from the surface near 465 K. The concomitant growth of a ν(Ge-H) mode near 1960 cm-1 and attenuation of ν(O-C-O) and δ(C-H) vibrational peaks near 1500 and 1350 cm-1, respectively, seen at 440 K in Figure 5d are consistent with the elimination of hydrogen from a bidentate formate surface species and subsequent CO2 desorption near this temperature. Furthermore, a comparison of the intensity of the H2 desorption peak at 575 K for formaldehyde with that from a saturated monohydride surface yields a hydrogen coverage of 0.4 ML (where 1.0 ML

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1745 ) one hydrogen atom per surface atom) immediately prior to desorption. This coverage is greater than that which would be expected on the basis of the intensity of the ν(Ge-H) peak in Figure 3 and is consistent with hydrogen elimination from formate prior to desorption as CO2. While the TPD data and IR data support the conclusion that a bidentate formate species forms on Ge(100)-2×1 and subsequently desorbs as CO2 upon annealing, not all features of the spectra are understood. Although the TPD peak for recombinative H2 desorption appears near 575 K, a minor attenuation of the ν(Ge-H) mode is observed at a lower temperature (near 500 K in Figure 5e). In addition, the loss or shift at 1525 cm-1 observed upon annealing to near 400 K suggests a minor side reaction, surface rearrangement, desorption, or decomposition of the formate species is occurring. The sources of the peaks that comprise the doublets located near 1500 and 1600 cm-1 are difficult to definitively ascertain. However, we postulate that interactions similar to that discussed above between the nucleophilic dimer atom and the carbon atom of formaldehyde in the dative-bonded state (Figure 7a) may explain the doublet near 1600 cm-1, while formate adducts bonded in different configurations on single or multiple surface dimers potentially lead to the doublet near 1500 cm-1. Additional study is required to determine the source of the fragments observed at high temperature during the TPD study. Despite an incomplete understanding of some features of the spectra, overall the infrared and TPD data presented above collectively point to a major reaction pathway where dativebonded formaldehyde reacts via a carbon-oxygen coupling reaction to form a bidentate formate species, which desorbs from the surface as CO2 at 465 K. This is a new reaction that has not been previously observed on a group-IV semiconductor surface. While we are currently working to develop a full understanding of the reaction pathway, we now speculate on two potential mechanisms that are consistent with the data available to us. One mechanism involves the stabilization of a dative-bonded product with a neighboring [2+2] CdO cycloaddition product. Formaldehyde adsorption on Ge(100)-2×1 differs from other carbonyl-containing compounds due to the strength of the [2+2] CdO cycloaddition product. Although this product is difficult to observe spectroscopically with our experimental setup, formation of a [2+2] CdO cycloadduct may initially help to stabilize a neighboring dative-bonded formaldehyde molecule by donation of the oxygen lone pair donation from the [2+2] CdO cycloadduct to the carbonyl carbon of an adjacent dativebonded formaldehyde molecule. Furthermore, this configuration may enable a facile cleavage of the [2+2] CdO cycloadduct C-O bond and formation of a new C-O bond with the dativebonded species. A hydrogen transfer would then lead to the observed formate species in addition to a surface-bound methyl group. Another reaction pathway involves surface radicals resulting from partially reacted surface dimers and/or surface defects. Preliminary experiments where atomic hydrogen was adsorbed under conditions that produced surface dimers with one hydrogen atom as well as a radical (H-Ge-Ge‚) prior to formaldehyde exposure lead to an increased rate of conversion from the dative-bonded state to formate. Surface radicals may lower the energy of the transition state for the cleavage of C-H or C-O bonds of dative-bonded or [2+2] CdO cycloaddition adducts during the surface reaction leading to formate. It is important to note the above mechanistic themes are speculative, and studies are ongoing to elucidate the complete mechanism of formate formation on Ge(100)-2×1.

1746 J. Phys. Chem. C, Vol. 111, No. 4, 2007 VI. Conclusions Infrared spectroscopy, temperature-programmed desorption, and density functional theory were used to investigate the adsorption of formaldehyde and formaldehyde-d2 on the Ge(100)-2×1 surface. Infrared spectroscopic evidence indicates that a species with an electron-deficient carbonyl group, likely a dative bond, initially exists near room temperature, together with a minority C-H dissociation product. In addition, theory predicts that a CdO [2+2] cycloaddition product may form, although the infrared spectra are inconclusive as to its presence. A time- and temperature-dependent carbon-oxygen coupling reaction then leads to the formation of surface-bound formate species. TPD experiments confirm that a carbon-oxygen coupling reaction occurs, as evidenced by the desorption of CO2 at elevated surface temperatures, and that it is a major pathway on Ge(100)-2×1. To our knowledge, the present work identifies the first catalytic reaction on a group-IV semiconductor surface. Although catalytic properties are usually ascribed to transition metal surfaces, the present work suggests that germanium, under certain circumstances, can also form coupling products. Further studies are required to determine the reaction mechanism that leads to carbon-oxygen coupling and whether the catalytic properties observed here extend to other group-IV semiconductor surfaces, particularly the Si(100)-2×1 surface. Acknowledgment. This work was supported by the National Science Foundation (CHE 0245260) (S.F.B.) and the Initiative for Nanoscale Materials and Processes (C.B.M). M.A.F. thanks the National Science Foundation for support in the form of a Graduate Research Fellowship. Scientific discussions with Dr. David Rampulla, Prof. Thomas J. Wandless, Albert J. Keung, Soon Jung Jung, and David W. Porter were insightful and enjoyable. Supporting Information Available: Complete ref 29. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley: New York, 1993. (2) Duke, C. B. Chem. ReV. 1996, 96, 1237. (3) Mui, C.; Han, J. H.; Wang, G. T.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 4027. (4) Hong, S.; Cho, Y. E.; Maeng, J. Y.; Kim, S. J. Phys. Chem. B 2004, 108, 15229. (5) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. J. Phys. Chem. B 2005, 109, 19817. (6) Chorkendorff, I.; Niemantsverdiet, J. W. Concepts of Modern Catalysis and Kinetics; Wiley-VCH Verlag GmbH: Weinheim, 2003. (7) Konecny, R.; Doren, D. J. Surf. Sci. 1998, 417, 169. (8) Mui, C.; Wang, G. T.; Bent, S. F.; Musgrave, C. B. J. Chem. Phys. 2001, 114, 10170. (9) Huang, H. G.; Zhang, Y. P.; Cai, Y. H.; Huang, J. Y.; Yong, K. S.; Xu, G. Q. J. Chem. Phys. 2005, 123, 104702. (10) Mui, C.; Bent, S. F.; Musgrave, C. B. J. Phys. Chem. A 2000, 104, 2457. (11) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 8990.

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