Dissociative Adsorption Of Formic Acid Research Articles

Formic acid generating in situ H2 and CO2 for nitrite reduction in the aqueous phase

Adsorbed NO suppresses dissociative adsorption of formic acid and thus reduction of nitrite.

Building a Differential Electrochemical Mass Spectrometry (DEMS): A Powerful Toll for Investigation of (photo)Electrochemical Processes

The development of electrochemical devices more efficient for energy conversion and storage are a challenge for modern society. Among this class of devices, fuel cells, batteries, and photovoltaic cells stand out. Despite their importance and practicality, the electrochemical processes that take place in these devices are not always easily understood. In addition, the detection and identification of the products and intermediaries, as well as the reaction paths usually are extremely complex, due to the different species produced and also several reaction steps that take place simultaneously in this type of system. In this context, analytical techniques have been combined to electrochemistry systems to investigate the electrochemical processes in real-time. FTIR (infrared spectroscopy with Fourier transform), MS (mass spectrometry) are some of examples that coupled techniques. Therefore, these setups provide great opportunities for the electrochemistry field, which also includes catalysts design improvement and reaction investigations under different experimental conditions.1-3 The electrochemical mass spectrometry (EMS) technique has stood out as an important tool for the identification and analyses of qualitatively/quantitatively gaseous, volatile intermediates, and products generated during electrochemical reactions. The interface between the mass spectrometer’s high-vacuum system and the electrochemical cell’s room pressure was composed of a hydrophobic porous membrane connected to the inlet system. The ionic current intensity associated with the products was monitored and correlated to the faradaic current3. In 1984 Wolter and Heitbaum4 refined the Electrochemical Mass Spectroscopy technique, employing a new vacuum system design consisting of two pumping stages, giving high sensitivity to the technique and allowing the on-line detection of gaseous with response-time ≤ 0.5 s, four times faster than the first setup developed. The Wolter and Heitbaum’s method was called Differential Electrochemical Mass Spectroscopy (DEMS). Here, differential pumping is related to a two-stage pressure reduction, where the gas flow is initially expanded to a low vacuum and later to a high vacuum, where the mass spectrometer is located4. This work presents the instrumental design used to construct a DEMS to investigate and elucidate (photo)electrochemical reactions. The DEMS design selected was composed of a two-stage differentially pumped vacuum system (Figure 1(I)). The instrument is composed of three main sections: electrochemical cell; hydrophobic microporous membrane, and mass spectrometer, including the vacuum system (turbomolecular and pre-vacuum pumps). The electrochemical cell used, was employed initially by Baltruschat5, this cell use a modified Polytetrafluoroethylene (PTFE) porous membrane interface as the working electrode, which is prepared by metal sputtering onto a microporous membrane.An example of the relation between cyclic voltammogram (CV) and mass-spectrum cyclic voltammograms (MSCV) using the DEMS constructed in this work is shown in Fig. 1((II)a-b), respectively (CVs only in H2SO4 is presented as an insert in Fig. 1(a)). The working electrode was sputtered of platinum (50 nm thickness) onto the PTFE microporous membrane interface. The counter electrode was a platinum mesh with a high surface area, the reference electrode was a reversible hydrogen electrode (RHE). The electrochemical cell was directly attached to the pre-vacuum chamber of the mass spectrometer High purity N2 was used to de-aerate the solution.The cyclic voltammograms were recorded at a sweep rate of 10 mV s-1. While the faradaic current and the ion mass current for the generated product, CO2 (m/z = 44 (CO2 +), were monitored at the same time. Fig. 1(c-e) shows the faradaic (d) and ionic currents (e) profiles vs. time obtained for 6 CVs, the potential is also presented. As can be observed, the faradaic current has an excellent correlation with the ionic current, conducting to a great synchronization between the voltammetric scans and the ionic signal profile. Besides, the results show a significant deactivation of the electrocatalyst, which is followed by the ionic signal (m/z = 44). As reported in the literature6, the formic acid electrooxidation produces a poison intermediate, which is formed from the dissociative adsorption of formic acid on the electrode surface, this poison specie was posteriorly identified as CO. Therefore, the DEMS technique can provide important information to determine the reaction mechanisms. Bashyam, R.; Zelenay, P., Nature 2006, 443 (7107), 63-66. Bruckenstein, S.; Gadde, R. R., Journal of the American Chemical Society 1971, 93 (3), 793-794. Chen, A.; Lipkowski, J., The Journal of Physical Chemistry B 1999, 103 (4), 682-691. Wolter, O.; Heitbaum, J., 1984, 88 (1), 2-6. Baltruschat, H., Journal of the American Society for Mass Spectrometry 2004, 15 (12), 1693-1706. Herrero, E.; et al.., Journal of Electroanalytical Chemistry 1993, 350 (1), 73-88. Figure 1

Heterogeneous Adsorption and Local Ordering of Formate on a Magnetite Surface.

We report a novel heterogeneous adsorption mechanism of formic acid on the magnetite (111) surface. Our experimental results and density functional theory (DFT) calculations give evidence for dissociative adsorption of formic acid in quasibidentate and chelating geometries. The latter is induced by the presence of iron vacancies at the surface, making oxygen atoms accessible for hydrogen atoms from dissociated formic acid. DFT calculations predict that both adsorption geometries are energetically favorable under our experimental conditions. The calculations prove that the locally observed (√3 × √3)R 30° superstructure consists of three formate molecules in a triangular arrangement, adsorbed predominantly in a chelating geometry. The results show how defects can stabilize alternative adsorption geometries, which is a crucial ingredient for a detailed atomistic understanding of reaction barriers on magnetite and other oxide surfaces, as well as for the stability of carboxylic acid based nanocomposite materials.

Formic Acid Dissociative Adsorption on NiO(111): Energetics and Structure of Adsorbed Formate

The dissociative adsorption of carboxylic acids on oxide surfaces is important for understanding adsorbed carboxylates, which are important as intermediates in catalytic reactions, for the organo-functionalization of oxide surfaces, and in many other aspects of oxide surface chemistry. We present here the first direct experimental measurement of the heat of dissociative adsorption of any carboxylic acid on any single-crystal oxide surface. The enthalpy of the dissociative adsorption of formic acid, the simplest carboxylic acid, to produce adsorbed formate and hydrogen (as a surface hydroxyl) on a (2 × 2)-NiO(111) surface is measured by single crystal adsorption calorimetry. The differential heat of adsorption decreases with formic acid coverage from 202 to 99 kJ/mol at saturation (0.25 ML). The structure of the adsorbed products is clarified by density functional theory (DFT) calculations, which provide energies in reasonable agreement with the calorimetry. These calculations show that formic acid readily...

Energetics of adsorbed formate and formic acid on Ni(111) by calorimetry

Surface carboxylates are common intermediates in many catalytic reactions and are also used for organo-functionalization of surface. Herein, adsorbed formate, the simplest carboxylate, was produced by dissociative adsorption of formic acid on the Ni(111) surface and the heats of adsorption were measured by single-crystal adsorption calorimetry (SCAC). At 240K, the integral heat of dissociative adsorption to make adsorbed bidentate formate and adsorbed hydrogen is 117.1kJ/mol at 1/7ML. The estimated accuracy of measured heats is within 3kJ/mol. Therefore, the standard enthalpy of formation of bidentate formate on Ni(111) is derived as −449.3kJ/mol and the bond enthalpy to the Ni(111) surface is 319.6kJ/mol. Comparison to our earlier results for Pt(111) shows that bidentate formate is 54kJ/mol more stable on Ni(111). This is the first experimental measurement of the energy of any molecular fragment bonded to any Ni surface through metal-oxygen bonds, so it is an important benchmark for validating the accuracy of quantum mechanical calculations of a wide range of adsorbed oxygenates on Ni catalysts. Compared to previous density functional theory (DFT) calculations, we found that DFT calculations systematically underestimate the bond energy of formate to Ni(111), but correctly predict that it is stronger to Ni(111) than to Pt(111). The integral heat of HCOOH adsorption to make molecularly adsorbed formic acid was also measured and found to be 63.2kJ/mol at 0.45ML and 120–155K.

Formic acid decomposition over V-Ti oxide catalyst: Mechanism and kinetics

A detailed kinetic model for formic acid (HCOOH) decomposition on oxide vanadium-titanium (7.3%V2O5/92.7%TiO2) catalyst has been developed based on transient kinetic studies and IR spectroscopy in situ. Formic acid decomposition to CO2 (dehydrogenation) and to CO (dehydration) proceeds via different mechanistic routes. Dehydrogenation proceeds via the conventional “formate” mechanism and include reversible dissociative adsorption of formic acid on vanadyl group V=O. The oxygen of the group acts as a proton acceptor whereas a vanadium cation is a formate anion stabilizer. Formate complexes can recombinate with protons and desorb in the form of formic acid or decompose to CO2. Dehydration reaction follows a different mechanism. An interaction of the coordination unsaturated sites V+-O− with HCOOH leads to the synchronous cleavage of the COH and CH bonds. Meanwhile the two OH hydroxyl groups are formed and CO molecule is evolved into gas phase. The path is completed by water desorption.The rate constants and activation energies of elementary steps are calculated for a dehydrogenation and dehydration reactions. The rate-limited steps of the reaction are formate decomposition stage for dehydrogenation path (Ea=60kJ/mol) and water desorption stage for dehydration rout (Ea=100kJ/mol). The difference in the values of activation energies results in an increase of selectivity towards CO with reaction temperature elevation. Water inhibits formic acid decomposition. In the case the dehydrogenation, it connected with adsorption displacement of formates with water, whereas, in the case of dehydration, it is associated with an active centers blocking by adsorbed water. The developed kinetic model takes into account the water effect of both routs and satisfactorily describes the steady state reaction behavior.

Kinetics of gas phase formic acid decomposition on platinum single crystal and polycrystalline surfaces

Formic acid dehydrogenation turnover rates (TORs) were measured on Pt(111), Pt(100), and polycrystalline Pt foil surfaces at a total pressure of 800Torr between 413 and 513K in a batch reactor connected to an ultra-high vacuum (UHV) system. The TORs, apparent activation energies, and reaction orders are not sensitive to the structure of the Pt surface, within the precision of the measurements. CO introduced into the batch reactor depressed the formic acid dehydrogenation TOR and increased the reaction's apparent activation energies on Pt(111) and Pt(100), consistent with behavior predicted by the Temkin equation. Two reaction mechanisms were explored which explain the formic acid decomposition mechanism on Pt, both of which include dissociative adsorption of formic acid, rate limiting formate decomposition, and quasi-equilibrated hydrogen recombination and CO adsorption. No evidence was found that catalytic supports used in previous studies altered the reaction kinetics or mechanism.

Energetics of Formic Acid Conversion to Adsorbed Formates on Pt(111) by Transient Calorimetry

Carboxylates adsorbed on solid surfaces are important in many technological applications, ranging from heterogeneous catalysis and surface organo-functionalization to medical implants. We report here the first experimentally determined enthalpy of formation of any surface bound carboxylate on any surface, formate on Pt(111). This was accomplished by studying the dissociative adsorption of formic acid on oxygen-presaturated (O-sat) Pt(111) to make adsorbed monodentate and bidentate formates using single-crystal adsorption calorimetry. The integral heat of molecular adsorption of formic acid on clean Pt(111) at 100 K is 62.5 kJ/mol at 0.25 monolayer (ML). On O-sat Pt(111), the integral heat of the dissociative adsorption of formic acid to make monodentate formate (HCOOmon,ad) plus the water-hydroxyl complex ((H2O-OH)ad) was found to be 76 kJ/mol at 3/8 ML and 100-150 K. Similarly, its integral heat of dissociative adsorption to make bidentate formate (HCOObi,ad) plus (H2O-OH)ad was 106 kJ/mol at 3/8 ML and 150 K. These heats give the standard enthalpies of formation of adsorbed monodentate and bidentate formate on Pt(111) to be -354 ± 5 and -384 ± 5 kJ/mol, respectively, and their net bond enthalpies to the Pt(111) surface to be 224 ± 13 and 254 ± 13 kJ/mol, respectively. Coverage-dependent enthalpies of formation were used to estimate the enthalpy of the elementary reaction HCOOHad → HCOObi,ad + Had to be -4 kJ/mol at zero coverage and +24 kJ/mol at 3/8 ML.

Kinetics of dissociative adsorption of formic acid on electrodes of tetrahexahedral platinum nanocrystals

In the present paper we study the kinetics of dissociative adsorption of formic acid on the electrode of tetrahexahedral platinum nanocrystals (THH Pt NCs). In situ FTIR spectroscopic results demonstrate that HCOOH can be oxidized to CO2 at a low potential (−0.2 V(SCE)) on the THH Pt NCs electrode, and the chemical bonds inside formic acid molecule are broken to form adsorbed COL species. The kinetics of the dissociative adsorption of HCOOH was quantitatively investigated by employing programmed potential step technique. It has been determined that, in 5 × 10−3 mol·L−1 HCOOH + 0.1 mol·L−1 H2SO4 solution, the maximal value of the average rate (υ a max ) of dissociative adsorption of HCOOH on a commercial Pt/C catalyst electrode is 8.58 × 10−10 mol·cm−2·s−1, while on the THH Pt NCs the υ a max is 1.5 times larger than the υ a max measured on Pt/C and reaches 13.19 × 10−10 mol·cm−2·s−1. The results have revealed that the reactivity of the THH Pt NCs is much higher than that of the Pt/C catalysts.

Electrochemical characterisation of platinum–palladium nanoparticles prepared in a water-in-oil microemulsion

A detailed electrochemical characterisation of platinum-palladium nanoparticles prepared by reduction with hydrazine of H2PtCl6 and K2PdCl4 in a water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ®30)/n-heptane is reported. XPS, AAS and UV–vis experiments were carried out to determine the atomic composition of the bimetallic nanoparticles obtained in the whole composition range. Cyclic voltammograms of clean nanoparticle alloys were obtained after a controlled decontamination procedure of their surfaces. CO adsorption–oxidation and dissociative adsorption of formic acid and methanol were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. FTIRS experiments of adsorbed CO were also carried out giving information on the relative amount of linearly and bridge-bonded CO, which is known to depend on the surface distribution of the two elements. Experimental results show how the electrochemical adsorption behaviour and electrocatalytic properties of the particles are under the control of their composition and, in a case studied in this work, of their size.

In situ FTIRS studies of kinetics of HCOOH oxidation on Pt(110) electrode modified with antimony adatoms

The electrochemical behavior of irreversibly adsorbed antimony on a Pt(110) electrode (Pt(110)/Sb) with various coverages was studied using cyclic voltammetry. The kinetics of HCOOH oxidation via reactive intermediates on Pt(110)/Sb were investigated quantitatively by employing the potential step technique and in situ FTIR spectroscopy. The results demonstrated that Sb adatoms were stable on Pt(110) when the electrode potential was below 0.45 V (SCE). It has been revealed that the dissociative adsorption of formic acid can be inhibited by the presence of Sbad on the Pt(110) surface. The electrocatalytic effects of Sbad towards HCOOH oxidation consist in a negative shift of the oxidation potential (about 350 mV) and the enhancement of the oxidation current. Based on the data processing method of integration transform developed in our previous papers, the kinetics of HCOOH oxidation on Pt(110)/Sb electrodes of different θSb have been investigated quantitatively, and both the rate constant kf and the transfer coefficient β were determined and reported.

First-principle analysis of the dissociative adsorption of formic acid on rutile TiO 2(110)

The adsorption of formic acid (HCOOH) on the rutile TiO 2(110) surface, where HCOOH dissociates into formate (HCOO) and hydrogen (H), has been analyzed with first-principles density functional calculations. A p(2×1) geometry can be observed for high exposure, while only local c(4×2) domains are detected for lower exposure. After desorption of some HCOOH at higher temperatures the p(2×1) geometry becomes unstable and the remaining molecules form a disordered structure. A direct and simultaneous observation of H and HCOO is difficult, but the calculations show that coadsorption of H is necessary to stabilize the p(2×1) geometry. Some of the vibrational modes also show a slight dependence on the supply of H and might give experimentally accessible hints concerning the role of H. An interesting aspect concerns the surface diffusion of HCOO: while HCOO molecules behave rather immobile on the surface in the c(4×2) domains and in the disordered structure, they show a high mobility along the [001] direction in a p(2×1)-like environment. As it turns out, the reason for these findings can be found in the complex movement of HCOO molecules, which includes the coadsorbed HCOOH compound as a whole.

Dissociative adsorption of formic acid and diffusion of formate on the TiO 2(110) surface: the role of hydrogen

Formic acid (HCOOH) dissociates on the rutile TiO 2(110) surface into formate (HCOO) and H. Certain ordered structures for HCOO have been observed for different coverages. In the present work, the role of hydrogen in the stability of these structures and also in the process of HCOO diffusion has been studied by means of ab initio density functional calculations. Our calculations show that coadsorption of H is necessary to stabilize the high-coverage p(2×1) geometry. They furthermore explain the observed high mobility of HCOO in a p(2×1)-like environment which is caused by a rather complex movement of HCOO together with H. This process cannot operate at lower coverages and for isolated molecules, which therefore makes HCOO immobile, as suggested by experiment.

Poison Formation upon the Dissociative Adsorption of Formic Acid on Bismuth-Modified Stepped Platinum Electrodes

The amount of poison formed and oxidatively desorbed upon the dissociative adsorption of formic acid on bismuth-modified Pt[n(111) × 100 and 110] electrodes has been investigated and found to decre...

Fourier Transform Reflection−Absorption IR Spectroscopy Study of Formate Adsorption on TiO2(110)

The adsorption of the formate species produced by dissociative adsorption of formic acid on TiO2(110) has been investigated using a combination of X-ray photoelectron spectroscopy (XPS) and Fourier transform reflection−absorption IR spectroscopy (FT-RAIRS). Adsorption at 300 K produces a formate coverage of 0.59 ML, in agreement with previous estimates and higher than that expected for a perfect (2 × 1) overlayer observed in low-energy electron diffraction (LEED). p-Polarized FT-RAIRS measurements carried out as a function of azimuthal angle (Φ) reveal the presence of two formate species through the coupling of both the tangential (It) and normal (In) components of the radiation to the symmetric (νsym(OCO)) and antisymmetric (νasym(OCO)) vibrational modes of the molecule. Species A has the OCO plane aligned in the 〈001〉 direction with C2v symmetry; νasym(OCO) = 1566 cm-1 is observed as an absorption band (coupled to It) at Φ = 90°, and νsym(OCO) = 1363 cm-1, observed as a transmission band (coupled to In). Species B has the OCO plane aligned in the 〈110〉 direction with Cs symmetry; νasym(OCO) = 1535 cm-1, observed as an absorption band (coupled to It) at Φ = 0°, and νsym(OCO) = 1393 cm-1, observed as a transmission band (coupled to In). Changes in the region of the νsym(OCO) at 1363 cm-1 for species A as a function of azimuthal angle (this band should be insensitive to Φ) are associated with an absorption band resulting from a coupling of νsym(OCO) of species B to It. This is a result of the reduction in symmetry from C2v to Cs and, as expected from the adsorption geometry, is observed at Φ = 0° and not at Φ = 90°. We estimate that the coverages of species A and species B at 300 K are 0.4 and 0.2 ML, respectively. Species A is the adsorbate responsible for the formation of the (2 × 1) ordered structure and adsorbed between the TiO2(110) oxygen rows. Species B we suggest is an adsorbed formate molecule with one of the oxygens incorporated in the TiO2(110) oxygen rows and is the species associated with the so-called disordered phase. It is clear from FT-RAIRS that the coverage of species B is increased by adsorbing formic acid at higher surface temperatures. We suggest that species B adsorbs at defects (oxygen vacancies) created through dissociative adsorption of formic acid to produce species A and the subsequent reaction of two hydroxyl groups to produce water. The existence and interdependence of the two adsorbed formate species provide an explanation for the complex activity observed for various catalytic reactions of formic acid over titania.

Numerical simulation of the voltammetric electrooxidation of CO adsorbed on Pt(111)

Numerical simulations of the voltammetric electrooxidation of CO adsorbed on Pt(111) surfaces are presented. For CO dosage, the numerical results confirm previous proposals, based upon current experimental information, concerning the CO distribution on the Pt surface. Specifically, whereas at low coverages (below 1/3) CO forms easily oxidizable open structures, at high coverages compact domains hinder the oxidation process. The two peaks observed in the voltammetric profiles are thus a consequence of topological differences between the CO distributions at low and high coverages. We also present new experiments on the electrooxidation of CO adlayers on Pt(111) formed upon the dissociative adsorption of formic acid. In this case our results indicate that a single oxidation peak remains up to coverages near 2/3. This striking new result is also reproduced in our simulations by taking the same kinetic parameters as in the previous case, and assuming that the CO are distributed on a honeycomb lattice, an assumption consistent with the characteristics of the dissociative adsorption of formic acid.

Monte Carlo simulation of CO adlayers electrooxidation on Pt(111)

Numerical simulations of the electrooxidation of CO adsorbed on Pt(111) surfaces are presented. For adlayers prepared by CO dosage, the numerical results confirm previous proposals, based upon current experimental information, concerning the CO distribution on the Pt surface. Specifically, whereas at low coverages (below 1/3) CO forms open and easily oxidizable structures, at high coverages compact domains hinder the oxidation process. The two peaks observed in the voltametric profiles are thus a consequence of topological differences between the CO distributions at low and high coverages. We also present new experiments on the electrooxidation of CO adsorbed on Pt(111) upon the dissociative adsorption of formic acid. In this case our results indicate that a single oxidation peak remains up to coverages below 2/3. This striking new result is also reproduced in our simulations by taking the same kinetic parameters as in the previous case, and assuming that the CO are distributed on a honeycomb lattice, an assumption consistent with the characteristics of the dissociative adsorption of formic acid.

Adsorption and Reaction of Formic Acid on a (2 × 2) NiO(111)/Ni(111) Surface. 3. IRAS Studies on the Characterization of Reaction Sites Using CO and the Behavior of Surface Hydroxyl Species

The decomposition of formic acid on (2 × 2) NiO(111)/Ni(111) was studied by infrared reflection absorption spectroscopy (IRAS). Formic acid molecules adsorbed dissociatively on the surface to form formate which decomposed to H2, CO2, H2O, and CO at 340−415 and 520 K. The vacant sites after the decomposition of formate were probed by the adsorption of CO at 100 K. On clean NiO(111) at 100 K, IRAS peaks of two types of adsorbed CO were observed at 2146 and 2079 cm-1, which were assigned to the CO on fully-oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The CO peaks were not observed on the formate precovered surface. The IRAS peak of adsorbed CO at 100 K on the fully-oxidized Ni cation sites appeared when the surface was heated to decompose formate around 340−415 K. After decomposition at 520 K, the intensities of both peaks of CO were recovered to the initial intensities. It was thus concluded that the sites of decompositions at 340−415 and 520 K are the fully-oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The behavior of hydroxyl species produced by dissociative adsorption of formic acid was next investigated using deuterated formic acid (DCOOD). The hydroxyl species (−OD) formed by the dissociative adsorption of formic acid (DCOOD) was observed at 2536 cm-1 below 343 K, while the isolated OD groups appeared at 2713 cm-1 and around 500 K. The mechanism of decomposition of formate at each temperature is discussed.

Adsorption and decomposition of formic acid on MgO(001) surface as investigated by temperature programmed desorption and sum-frequency generation spectroscopy: Recurrence induced defect sites

The change of the MgO(001) surface during the adsorption and thermal desorption of formic acid was investigated by temperature programmed desorption (TPD) and infrared-visible sum-frequency generation SFG spectroscopy. The TPD results revealed that the feature of desorption signals changed as the adsorption/thermal desorption process was repeated, despite no significant change of the low energy electron diffraction (LEED) pattern. The surface formate, produced by dissociative adsorption of formic acid, decomposed at 560 K and 700 K, giving H2O+CO and H2+CO2 as primary and minor products, respectively, but the broad desorption peak of H2O started to appear at 300 K. The 560 K peak appeared from an early stage, while the 700 K peak gained intensity as the experiment was repeated and the density of defects increased. The adsorption site for the 700 K peak was ascribed to the catalytic reaction of magnesia powder. The SFG spectra gave three resonance peaks of CH stretching bands at 2850 cm−1, 2870 cm−1, and 2920 cm−1 with a different dependence on the dose time, history of surface, and polarization of incident visible beam. The former two bands were assigned to the formates with bridged configuration, and the third band to the formate with unidentate configuration. The polarization dependence suggested that the CH bonds of bridged formates stood perpendicular to the surface, and that of the unidentate formate tilted approximately 30° from the surface normal. A vibration nonresonant background signal was present on the SFG signal, showing characteristic dependence on the polarization of incident light beams, as well as the coverage and history of the surface, and was interpreted as originating from the defect sites on the surface.

Two routes to formaldehyde from formic acid on TiO 2(001) surfaces

Two routes were observed for formaldehyde formation from formic acid on TiO 2(001) single crystal surfaces. The first, on reduced surfaces, involves the reduction of formic acid to formaldehyde accompanied by the oxidation of surface Ti cations. Surface oxidation was evident from the increase in the portion of the Ti ( 2 p 3 2 ) signal corresponding to Ti 4+ and the decrease in that corresponding to Ti x+ (0< x<4) in XPS. XPS after formic acid adsorption on both reduced and oxidized TiO 2 surfaces showed the presence of formate species having a C(1s) binding energy of 289.3 eV; these species are formed by dissociative adsorption of formic acid at room temperature. A second route for formaldehyde formation, operative on fully surfaces, involves bimolecular coupling of two formates. This reaction occurs only on the reconstructed, fully oxidized surface; the {114}-faceted TiO 2(001) structure (obtained by annealing at 900 K or above), as previously reported by Kim and Barteau [Langmuir 6 (1990) 1485].