Reaction of Formic Acid over Amorphous Manganese Oxide Catalytic

Nov 5, 2010 - The interaction of formic acid with amorphous manganese oxide (AMO) is investigated using in situ photoelectron and infrared spectroscop...
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J. Phys. Chem. C 2010, 114, 20000–20006

Reaction of Formic Acid over Amorphous Manganese Oxide Catalytic Systems: An In Situ Study Jason P. Durand,† Sanjaya D. Senanayake,§ Steven L. Suib,*,†,‡ and David R. Mullins*,§ Department of Chemistry, Institute of Materials Science, UniVersity of Connecticut, Storrs, Connecticut 06269, United States and Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37830-6201, United States ReceiVed: May 20, 2010; ReVised Manuscript ReceiVed: October 12, 2010

The interaction of formic acid with amorphous manganese oxide (AMO) is investigated using in situ photoelectron and infrared spectroscopy techniques. Soft X-ray photoelectron spectroscopy (sXPS) and in situ FTIR illustrate two possible modes of formate bound species at the AMO surface. Two peaks in the IR region from 1340-1390 cm-1 are indicative of formate species bound to the surface in a bidentate configuration. However, a 224 cm-1 band gap between νsOCO and νasOCO suggests formate is bound in a bridging configuration. Temperature-programmed desorption studies confirm the formate bound species desorbs as carbon dioxide from the surface at multiple binding sites. At temperatures above 700 K, the presence of K+ · · · OC complex suggests the bound species interacts at vacant sites related to framework oxygen and cation mobility. 1. Introduction Organic acids act as contaminants in fat-based oils and crude petroleum. Metal oxides reduce the total acid by converting the carboxylic acid impurities to noncorrosive products via decarboxylation or by cracking. Patent literature discusses the need for manganese oxides to increase activity and selectivity toward decarboxylation of fatty acids.1,2 Manganese oxides also play a role in the preparation of phenols via the decarboxylation of benzenemonocarboxylic acid derivatives,3 as well as in the purification of crude oils.4 Manganese octahedral molecular sieves (OMS) have been utilized as very efficient heterogeneous catalysts in the selective oxidation of alcohols,5-7 oxidation of indene,8 oxidation of fluorene,9 condensation reactions,10 and tandem catalysis for direct catalytic synthesis of imines from alcohols.11 Isotopically labeled experiments have shown the involvement of surface oxygen in the lattice framework of the OMS materials in these types of reactions. Temperature-programmed desorption studies of manganese oxides illustrate the mobility of the oxygen from the gas phase into the framework as a mechanism for regeneration, via a Mars-van Krevelen mechanism.12 Manganese oxides have also been utilized for high-efficiency photocatalysts used in oxidation reactions,13,14 as active decomposition catalysts for nontoxic stimulants of extremely toxic organophosphorus nerve and blistering agents used in chemical warfare,15 as well as a partial oxidation of trifluoroacetic acid,16 oxidation of benzene with ozone,17 methane coupling with CO218 and dye molecules that could act as stain removing agents in detergents.19 Studying the surface reactivity of amorphous manganese oxide (AMO) is the goal of this work. The overall composition of AMO is K0.6Mn0.93O2. The framework structure has local * To whom correspondence should be addressed. (S.L.S.) E-mail: [email protected]. Tel.: 860.486.2797. Fax: 860.486.2981. (D.R.M.) E-mail: [email protected]. Tel.: 865.574.2796. Fax: 865.576.5235. † Department of Chemistry, University of Connecticut. ‡ Institute of Materials Science, University of Connecticut. § Oak Ridge National Laboratory (ORNL).

ordering such as in a glass structure. Desorption of the oxygen atoms from tetrahedrally coordinated silicon would disrupt glass in much the same way as decomposition of AMO.14 The interaction of formic acid with AMO is reported here. 2. Experimental Methods The experiments presented in this study were performed in two separate locations. The surface characterization and reactivity studies using soft X-ray photoelectron spectroscopy were performed using synchrotron radiation at beamline U12a at the National Synchrotron Light Source, Brookhaven National Lab. The synthesis, temperature programmed desorption-mass spectrometry (TPD-MS), thermogravimetric analysis (TGA), catalytic studies, and Fourier transform infrared (FTIR) studies were undertaken at the Department of Chemistry at the University of Connecticut. 2.1. Synthesis of Amorphous Manganese Oxide (AMO). The preparation of AMO samples used in this study is described in the literature.13 A typical synthesis of amorphous manganese oxide involves reduction of 1.58 g of potassium permanganate dissolved in 60 mL of distilled deionized water (DDW) with 2.28 g of oxalic acid dissolved in 100 mL of DDW. The mixture was agitated for 30 min until a brown precipitate appeared. The precipitate was filtered, washed with 1000 mL of DDW, and dried at 423 K overnight. The material was stored in a desiccator over calcium carbonate under vacuum for 2 days prior to pressing the pellet. About 600 mg of sample was pressed into a pellet using a die and hydraulic press (10 000 psi) for 6 min. The pellets were used to mount the samples vertically into the endstation at the U12a beamline. 2.2. Powder X-ray Diffraction (XRD). XRD patterns of the samples were taken to study the structural properties of the AMO material. The XRD patterns were collected using a Scintag XDS 2000 diffractometer equipped with Cu KR X-ray radiation (λ ) 1.5418 Å). A beam voltage of 40 kV and a 40 mA beam current were used. 2.3. Beamline Endstation. The endstation at beamline U12a is described in detail elsewhere20 and is maintained at ultrahigh

10.1021/jp104629j  2010 American Chemical Society Published on Web 11/05/2010

Reaction of Formic Acid over AMO Catalytic Systems vacuum (UHV) (1 × 10-10 Torr) with samples mounted inside the chamber with tungsten wires (0.01”) and a tantalum back plate (0.05 in.). The samples were heated resistively (up to 900 K) and cooled (to 125 K) with liquid nitrogen with temperature readings obtained using a K-type thermocouple with insulated chromel-alumel wires spot-welded to the back of the tantalum back plate. All reaction studies were performed in situ with formic acid (HCOOH) vapor introduced into the UHV system by way of “backfilling” with a high precision leak valve. Coverage of HCOOH on the surface was estimated with an approximation that 1 × 10-6 Torr · s is equivalent to one ML (monolayer). The monolayer formation time is the length of time required to cover a surface with an adsorbate. It is assumed that every molecule that comes in contact with the surface sticks without re-evaporation. The time required to cover a monolayer is 3 × 10-4/P, where time is in seconds and P is pressure in pascals. It takes about 1 s for a surface to be covered at a pressure of 300 µPa. One pascal ) 0.0075 Torr, therefore 300 µPa ) 2.25 × 10-6 Torr. Formic acid (Aldrich 98%) used in these experiments was purified using sequential freeze pump thaw cycles with liquid nitrogen to drive out any volatile contaminants such as CO2. 2.4. Soft X-ray Photoelectron Spectroscopy (sXPS). The sXPS spectra were recorded using different photon energies for O 1s and C 1s to optimize the sensitivity and resolution. The O 1s data were collected at hν ) 600 eV with an analyzer pass energy of 10 eV, and alternatively the C 1s data were recorded at hν ) 400 eV with a pass energy of 20 eV. The instrumental resolution was ∼0.5 eV. All scans were performed with a step size of 0.1 eV · step-1 and scan averaged for 15 scans. Adventitious carbon was used for calibration of the spectra. The corrected spectral data were referenced against the literature and found to be in good agreement.21 2.5. Temperature-Programmed Desorption with Mass Spectrometric Analysis. The exhaust gas was monitored to trace the consumption of formic acid and desorption of the species relative to temperature. Peaks at m/e of 2, 28, 44, and 18 were monitored to study H2, CO, CO2, and H2O. About 600 mg of AMO (pelletized) was placed into a 1 in. quartz tube and loaded into a Thermolyne tube furnace equipped with a Eurotherm temperature controller. The sample was purged with UHP He (Airgas) for 2 h at 450 K and then cooled to room temperature. Adsorption of the formic acid was carried out at 300 K by passing the carrier gas (He) through a bubbler containing the formic acid for 15 min at ambient temperature. After adsorption, the bubbler was bypassed and the reactor was allowed to purge with dry helium until all the physisorbed formic acid was removed. The exhaust gas was fed to an MKS Cirrus LM99 equipped with an IP Residual Gas Analyzer used to monitor evolved species during the temperature profile. The profile was run while the sample was heated from 300 to 1073 K at a rate of 20 K · min-1. 2.6. Thermal Analysis. Thermal analysis of the AMO was studied by thermogravimetric analysis using a TGA Model Q500. The samples were heated to 1073 K in argon flow of 60 mL · min-1 and at a rate of 20 K · min-1. 2.7. In situ Fourier Transform Infrared Spectroscopy (In situ FTIR). The AMO sample was mixed in a 1:10 ratio of sample to KBr and pressed into a pellet. The pellet was mounted vertically in a glass T-shaped cell equipped with an inlet and outlet for gases as well as an adjustable sample holder to move the sample into a heating zone for temperature treatment. Helium was used as a carrier gas to mimic conditions similar to the

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Figure 1. All of the possible modes of formate species bound to a surface in (a) monodentate, (b) bridging, and (c) bidentate configurations.

Figure 2. The spectra collected with a incident photon energy of 600 hν and a pass energy of 10 eV illustrates the oxygen 1s region of the AMO surface (a) untreated, (b) cleaned, (c) dosed with HCO2H at 300 K, (d) annealed to 400 K, (e) annealed to 500 K, (f) annealed to 600 K, (g) annealed to 700 K, and (h) annealed to 800 K. All the spectra were collected after the sample was cooled down to 300 K.

TPD-MS experiments. The sample was pretreated at 423 K and then cooled to 300 K before dosing with formic acid. HCOOH treatment similar to the TPD-MS was achieved after 5 min. Once all the gaseous HCOOH was purged from the system, the sample was moved into the beam path and measurements were taken. A Nicolet Magna-IR Spectrometer 750 was used to collect an average of 100 scans at a resolution of 4.0 cm-1. The sample was heated at 20 K · min-1 to the desired temperature followed directly by scanning in the mid infrared region. 3. Results 3.1. Photoelectron Spectroscopy. Soft X-ray photoelectron spectroscopy was utilized to probe the surface reactivity of amorphous manganese oxide with formic acid over a series of temperatures ranging from 125 to 800 K. No transitions for formate species were observed below 300 K. Possible configurations of bound formate (HCOO-) with a surface metal site (M) are depicted in Figure 1. The oxygen 1s and carbon 1s regions were probed in situ to monitor the surface species during thermal treatment. The 1s regions of oxygen and carbon are shown in Figures 2 and 3, respectively. The as-synthesized surface of AMO was surveyed, showing two peaks at 531.6 and 533.3 eV, the O 1s region in Figure 2a. Removal of the higher binding energy species was achieved by flashing the sample to 900 K (Figure 2b). Consecutive flashing

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Figure 3. The spectra collected with a incident photon energy of 400 hν and a pass energy of 20 eV illustrates the carbon 1s region of the AMO surface (a) cleaned, (b) dosed with HCO2H at 300 K, (c) annealed to 400 K, (d) annealed to 500 K, (e) annealed to 600 K, (f) annealed to 700 K, and (g) annealed to 800 K. All the spectra were collected after the sample was cooled down to 300 K.

had no further effect on cleaning the surface. Figure 3a shows the carbon 1s region (285.4 eV) of the cleaned surface. Formic acid reacted with the AMO surface at 300 K under ultrahigh vacuum conditions. After adsorption, peaks were observed at 287.6 eV in the carbon 1s region (Figure 3b) as well as at 533.6 eV in the oxygen 1s region (Figure 2c). Flashing the sample to 900 K completely cleans the surface of all adsorbed species deposited. sXPS was utilized to monitor the thermal behavior of the adsorbed species on the surface of amorphous manganese oxide in this manner. Heating the sample in 100 K increments from 300 to 800 K was chosen for these experiments. Each thermal treatment followed a cooling step to 300 K prior to scanning the oxygen and carbon regions. Changes in the adsorbed species started occurring at 500 K. The higher binding energy peak in the oxygen region (Figure 2) at 533.3 eV shifts up in binding energy, while the peak at 287.6 eV in the carbon region of Figure 3 decreases. 3.2. Temperature-Programmed Desorption. Carbon dioxide was the major species observed at m/e of 44 in the collection of the exhaust gas (Figure 4) by mass spectrometry. Figure 5 depicts the temperature stability of the adsorbed species, desorption of carbon dioxide, and peak area of the adsorbed species as detected by sXPS. 3.3. In Situ Fourier Transform Infrared. The reactivity of formic acid with amorphous manganese oxide was monitored using FTIR (Figure 6). After dosing formic acid onto the AMO surface at 300 K, absorbance bands were observed at 2840, 2340, 1705, 1584, 1387, and 1362 cm-1, seen in Figure 6a. Figure 6b depicts the sample after heating to 500 K. The peak at 2839 cm-1 has become more enhanced with an added feature at 2734 cm-1. The disappearance of the band at 1705 cm-1 and the formation of a shoulder peak at 1636 cm-1 occur. As the

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Figure 4. Thermal decomposition mass spectrometry traces for all desorbing products with respect to thermal activation.

Figure 5. The graph correlates the thermal stability of amorphous manganese oxide and the thermal activation adsorbed/desorbed formate species; a) (O) the partial pressure of carbon dioxide (normalized) followed by temperature programmed desorption. (b) (-0-) Normalized formate peak area taken from the carbon 1s region in sXPS. (c) (-) Thermogravimetric analysis of AMO in argon atmosphere.

sample is heated above 500 K, the peaks at 1387 and 1362 cm-1 have disappeared and two new peaks appear at 1400 and 1352 cm-1 (Figure 6c, 590 K). A small feature at 2449.5 cm-1, a doublet at 2170 and 2158 cm-1, and a feature at 1747 cm-1 are also present on the surface of amorphous manganese oxide at 709 K. These features are still present at 738 K with a loss of intensity at 1632 cm-1. 4. Discussion The overall composition of amorphous manganese oxide (AMO) is K0.6Mn0.93O2 and has a framework structure with local

Reaction of Formic Acid over AMO Catalytic Systems

Figure 6. In situ FTIR of the AMO (a) dosed with HCO2H at 300 K; heated to (b) 500 K, (c) 590 K, (d) 709 K, and (e) 738 K.

ordering such as in glass. Desorption of the oxygen atoms from tetrahedrally coordinated silicon would disrupt glass in much the same way as decomposition of AMO. Energy dispersive X-ray analysis and ICP microanalysis show that some K+ is incorporated in the AMO material due to reduction of some Mn4+ to Mn3+. The potassium is well dispersed throughout the bulk material.13 For every Mn3+ there must be a K+ for charge compensation.14 This charge compensation mechanism is very similar to that of crystalline octahedral molecular sieves.22 4.1. In Situ Fourier Transform Infrared. The reactivity of formic acid with amorphous manganese oxide was monitored using FTIR (Figure 6). The absorbance in the range of 1340-1390 cm-1 in the νs(COO-) region is characteristic of formates (HCOO-), whereas with carboxylate (COO-) the corresponding adsorption band occurs in the range of 1400-1480 cm-1.23 Larger splits related to monodentate configuration and smaller splits correlated to bidentate conformation, where splits correlated to bridging configuration falls in between. The splitting of the symmetric and asymmetric C-O modes is indicative of the type of coordination; lower than 80, ∼200, and higher than 300 cm-1 are associated with bidentate (chelating), bridging, and monodentate species, respectively.24 Also, for the formate species to coexist on the surface in two different orientations the νs(C-H) should occur as a doublet at 2840 and 2910 cm-1.25 After dosing HCO2H onto the AMO surface at 300 K, bands were observed at 2840 (C-H stretch) (Figure 6a), 2340, 1705 (CdO stretch), 1584 (C-O stretch), 1387, and 1364 cm-1 (both COO- rocking with C-H bending, in-plane), seen in Figure 6a. The latter two peaks are indicative of formate species on the AMO surface.23 The 224 cm-1 band gap between the νsOCO and νasOCO is indicative of a bridging orientation.24 However, the lack of a doublet feature at 2840 cm-1 suggests the formate is orientated in one configuration.25 The band at 2840 cm-1 is a broad, weak and the doublet feature could be buried in the noise. Figure 6b depicts the sample after heating to 500 K. The C-H stretch at 2839 cm-1 has become more enhanced, with an added feature at 2734 cm-1. The disappearance of the CdO vibration at 1705 cm-1 and a shoulder peak at 1636 cm-1 is indicative of

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20003 the νa(COO-) in a bidentate configuration. As the sample is heated above 500 K, the peaks at 1362 and 1387 cm-1 have disappeared and two new peaks appear at 1352 and 1400 cm-1 (Figure 6c, 590 K) indicative of the formate species undergoing deprotonation to a carboxylate species (COO-) bound to the surface.23 In Figures 4 and 5a, desorption of carbon dioxide starts to occur above 500 K. By 590 K, the νa(COO-) stretch at 1632 cm-1 has increased, with almost complete loss of the band at 1591 cm-1. The extremely small intensity of the asymmetric band is interpreted as screening of the vibrational dipole parallel to the surface in a bidentate bonding configuration.25 The doublet feature νs(COO-) at 1352 and 1400 cm-1, as well as the πCOH feature at 1049 cm-1 are features associated with deprotonation of the formate species.23 As the sample is heated to 700 K, the ν(COO-) rocking vibration and the νs(C-O) have disappeared. The bridging configuration is present at the surface due to the C-O-H inplane bending at 1476 cm-1 while the νs(COO-) and νa(COO-) are present at 1410 and 1636 cm-1 respectively. An O-H peak indicative of a dimer formation through hydrogen bonding occurs at 2660 cm-1, due to possible hydrogen consumption by the material during deprotonation. A small feature at 2450 cm-1, a doublet at 2170 and 2158 cm-1, and a feature at 1747 cm-1 are also present on the surface of amorphous manganese oxide at 709 K. These features are still present at 738 K, with a loss of intensity at 1632 cm-1, indicative of νa(COO-). The doublet feature in the carbonyl region of 2100-2200 cm-1 can be associated with a K+ · · · CO complex, based on work by others.26 The weak feature around 2450 cm-1 is most likely associated with C-H stretching of the surface compounds and band at 1747 cm-1 may be due to M-OC- species.27 As the sample is heated from 590 to 738 K, the peaks at 1632 cm-1 and at 2660 cm-1 indicative of the νa(COO-) in the bidentate configuration and the O-H peak, respectively, gradually decrease. Because of the limitations of the setup, the temperature was not raised above 738 K, however based on the trends observed in infrared experiments; as well as the sXPS (Figures 2 and 3) and TPD-MS (Figure 5) discussed previously, after 1000 K, the νa(COO-) band and the O-H dimer peaks would be completely gone. Identification of all peak vibrations agrees well with the literature.25,28,29 4.2. Photoelectron Spectroscopy. In the preparation of AMO, an acid precursor (oxalic acid) is used, resulting in the catalyst having a protonated surface, which is observed in the O 1s spectrum in Figure 2a. The hydroxyl species on the surface still remain, to some extent, even after rigorous cleaning, as suggested by the O 1s sXPS (Figure 2b). These surface hydroxyl species could assist in an acid-catalyzed surface hydrolysis of the formate species into a carboxylate species bound to the surface at different thermal energies. At 300 K, the sXPS spectra suggest that there is one binding configuration of the formate species that reacts with the AMO surface, confirmed by in situ FTIR. Thermal activation assists in deprotonation of the formate species (HCOO) forming a carboxylate (COO) and the hydrogen is consumed by the material, most likely at the surface. The change in the binding energy of the formate peak from 533.6 to 533.8 eV between 400 and 500 K suggests that this formate species is deprotonated to a carboxylate species at the surface. This is consistent with one carbon peak being observed at 287.6 eV in Figure 3. Loss of a proton leads to a more tightly bound oxygen at the surface. By 700 K, in both Figures 2 and 3, the formate/carboxylate species is no longer present on the AMO surface. However, a

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Figure 7. Thermal programmed oxidation mass spectrometry correlated to the thermal stability of amorphous manganese oxide. The trace for m/z ) 32 shows the oxygen uptake by the bulk AMO material while the trace for m/z ) 28 shows the release of carbon monoxide.

shoulder on the K 2p3/2 peak appears at 290.2 eV. This may be due to carbonate uptake by the surface of the amorphous manganese oxide at oxygen defect sites. This assignment comes from the observations made in Figure 7, when the surface is treated with oxygen after desorption of the formate species. Oxygen is adsorbed while carbon monoxide is desorbed from the surface. 4.3. Reactivity and Decomposition. The 224 cm-1 gap observed in the IR spectrum at 300 K of Figure 6 would suggest the presence of bridging or bidentate species.24 However, the existence of a νs(CdO) stretch at 1705 cm-1 suggests a small amount of carbonyl character even though the νs(OCO) mode does not show single bond character below ∼1300 cm-1, typical of monodentate species.30 According to Hirose et al., monodentate configuration was postulated for formate having a ∆ν value larger than 220 cm-1, which is the value for a free formate ion, while a formate with the ∆ν similar to or less than 220 cm-1 was assigned to bridging or bidentate configurations.30 The asymmetric mode has a broad peak centered at 1585 cm-1, with a shoulder at 1635 cm-1 that suggests higher carbonyl character, which is not typical for monodentate species.24 There are two ν(CdO) stretching modes at 1705 cm-1 and another around 1600 cm-1, buried under the broad feature at 1585 cm-1. These features are usually observed due to a multilayer resulting from individual formic acid molecules forming chains through hydrogen bonding interactions between the hydroxyl hydrogen of one molecule and the carbonyl group of an adjacent molecule. The formation of these chains results in two ν(CdO) stretching modes that are separated by approximately 100 cm-1.31 Thermal activation at 500 K caused loss of the 1705 cm-1 peak, sharpening of the peak at 1585 cm-1, and maintaining the shoulder at 1635 cm-1, suggesting that bond breaking of the chains formed between adjacent formate ions on the surface of AMO occurs. Sharpening of the νas(OCO) mode also occurs with thermal activation to 500 K. The simultaneous strengthening and narrowing of modes is consistent with a thermal ordering process toward a more homogeneous layer.24 Thermal activation to 590 K caused a broadening and shift in the νsym(OCO) doublet feature to 1352/1400, having a split of ∼48 cm-1, with a shoulder at 1472 cm-1. The shift in peak position changes from a sharp peak at 1585 with a shoulder at

Durand et al. 1630 cm-1, to a sharp peak at 1632 with a shoulder at 1591 cm-1. The sharpening and peak shifting with the antisymmetric and symmetric OCO features, respectively, are associated with a thermal ordering process of the formate to a carboxylate species at the surface of AMO as well as the desorption of carbon dioxide. The weak ν(O-H) mode appearing at ∼2662 cm-1 is accompanied by a weak π(COH) band at 1049 cm-1, confirming the presence of formic acid24 at the surface of AMO. The formation of formic acid as well as the shift in the symmetric feature coincides with the conversion of formate to carboxylate species.23 A νs(OCO) mode indicative of C-O single bond character at ∼1251 cm-1 may exist under the broad symmetric feature centered at 1400 cm-1. Further thermal activation to 709 K sharpens the antisymmetric peak at 1632 cm-1 with the loss of the shoulder at 1591 cm-1. A broadening of the symmetric peak at 1409 cm-1 with a shoulder on either side at 1354 and 1476 cm-1 and the loss of the splitting feature also occurs. The loss of the splitting feature could be a result of surface ordering of the formate/ carboxylate species to a more homogeneous conformation, since splitting of formate and carbonyl-containing compounds is indicative of subtle conformational differences.31,32 The peak broadening of the symmetric feature occurs above 500 K. This peak broadening, coupled with increased splitting of the symmetric bands, from 25 to 48 cm-1 is indicative of the bidentate formate in a tilted or bent configuration on the surface of AMO.30 Further thermal activation to 738 K reveals a decrease in the antisymmetric band at 1632 cm-1 and the shoulder at 1354 cm-1, relating to desorption of the carboxylate species as carbon dioxide. Scheme 1 depicts the proposed binding of formic acid with the surface of amorphous manganese oxide. On the basis of FTIR data, the bound species exhibits two types of binding to the AMO surface. Distinct bands in the νsOCO region suggest the formate binds to the AMO surface in a bidentate configuration, while the splitting between the νsOCO and νasOCO bands (224 cm-1) suggests the species is bound in a bridging configuration. The authors are postulating the binding of the formate species is dependent on the electronic state of the surface. Assuming the surface is electron neutral, the formate species should bind in a bidentate configuration (Scheme 1a). Oxygen vacancies, due to thermal treatment, allows for binding sites into the framework of the AMO structure. Oxygen vacancies (Scheme 1b-i) explain how the formate species could be bound in a bridging configuration. Cation mobility in the framework structure is known to occur. The doublet feature in the carbonyl region 2100-2200 cm-1 indicates binding interaction with the cation after thermal treatment.26 Depicted in Scheme 1b-ii is a proposed binding interaction of the carbonyl species with the potassium cation that may occupy the area near an oxygen vacancy, created through thermal desorption of the lattice oxygen. Desorption of carbon dioxide occurs in two stages that may be related to two distinct sites that the formate species is associated with at the surface of AMO. Kung33 has suggested that there are several types of oxygen that can desorb from metal oxides such as molecularly adsorbed oxygen, adsorbed atomic oxygen, and lattice oxygen which is evolved at high temperatures (∼800 K). The nature and origin of the peak for desorbed oxygen at 450 to 550 K may be adsorbed atomic oxygen. Kung suggests that adsorbed atomic oxygen is evolved in an intermediate temperature range for several metal oxides.33 Thermal analysis of AMO (Figure 5c) shows a weight loss starting at 800 K associated with the lattice oxygen. The

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SCHEME 1: Proposed Reactivity of Formic Acid on the Surface of Amorphous Manganese Oxide

temperature-programmed desorption profile of carbon dioxide illustrates this point. The rate of desorption increased 3-fold at 592, which is the maximum of the first desorption peak of CO2, related to surface adsorbed CO2. CO2 is then desorbed from lattice sites associated with oxygen defect sites. Oxygen defect site formation could be generated in one of two ways. One way involves oxygen release during the cleaning of the surface prior to formic acid adsorption. Another way occurs by formate species reacting with the surface, binding to framework oxygen, and release in the form of CO or CO2 at a lower thermal activation. Previous studies on these systems12,34 suggest a Mars-van Krevelen Mechanism where the involvement of the lattice oxygen was implicated as a factor for its selectivity. The surface reactivity studies between formic acid and amorphous manganese oxide is done in an oxygen free environment. The exchange between gas phase oxygen and lattice oxygen is not possible; however carbon can replace oxygen at the binding sites, due to the more strongly reactive carbon dioxide. This hypothesis is corroborated by the temperature-programmed oxidation (TPO) data of Figure 7. Following the temperature-programmed desorption experiment, the material was cooled in a stream of dry argon. Once the exhaust gases stabilized and the material was at ambient temperature, a gas mixture of 15% oxygen in argon was introduced and a temperature programmed oxidation profile was

recorded. The TPO data reveal consumption of oxygen with the release of carbon monoxide, due to loss of lattice oxygen. The unreleased carbon from the reactivity of formic acid is easily removed by molecular oxygen via a Mars-van Krevelen Mechanism. The selective oxidation reactions of alcohols to aldehydes using octahedral molecular sieves all have water as a byproduct. Experiments reported here suggest that hydrogen consumption from manganese oxides occurs and in the presence of O2, the hydrogen can be released as water, leaving O- at the surface. 5. Conclusions The work herein has focused on surface reactivity of formate species on amorphous manganese oxide using a variety of in situ methods. This in situ study shows that formate species move and interact with the surface of amorphous manganese oxide under different thermal conditions. The possible interactions of adsorbed formate species with the surface of amorphous manganese oxide are shown in Scheme 1. The adsorbate exhibits two types of binding configurations at the surface of amorphous manganese oxide, dependent on the electronic state of the surface. The adsorbate appears to bind in a bidentate configuration on an electron neutral surface, while surface defects like oxygen vacancies direct a bridging configuration.

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The observed bands at 1387 cm-1 and 1364 cm-1 are indicative of formate (HCOO-) species on the AMO surface at 300 K. As the sample is heated, the formate species is further deprotonated at 500 K under UHV conditions, and at slightly higher temperatures at ambient pressure. Deprotonation may occur through the C-H bond of the formate species beginning to bend out-of-plane with the observed increased splitting in the νsym(OCO) doublet feature from 25 cm-1 to ∼48 cm-1. Cation mobility in the framework structure influenced by thermal treatment is observed around 700 K. Infrared spectroscopy reveals a doublet feature in the carbonyl region of 2100-2200 cm-1 associated with a K+CO complex while a decrease in photon count is observed in sXPS. The cationcarbonyl complex coupled with a band at 1747 cm-1, indicative of a metal-carbonyl complex suggests binding in a bridging conformation due to electronic defects. This study corroborates earlier work that suggests that manganese oxide catalysts used in selective oxidation reactions follow the Mars-van Krevelen mechanism. In addition, the observed deprotonation of formate species without the presence of hydrogen or water in the effluent gas by mass spectrometry suggests the amorphous manganese oxides could be useful as hydrogen storage materials. Acknowledgment. The U12a beamline is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. J.D. and S.L.S. acknowledge the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences of the US DOE under contract DE-FG02-86ER13622.A000. References and Notes (1) Linchevskij, F. V.; Dyshlovoj, V. I. Catalyst for decarboxylation of fatty acids. Russian Patent, 1992, SU 365912 A1, October 23, 1992. (2) Dyshlovoj, V. I.; Makarov, S. V.; Bavika, V. I.; Gorbacheva, R. A.; Logvinova, N. I.; Smovzh, V. F.; Karyukin, E. S.; Yusupov, A. Sh.; Sadykov, A. S. Preparation of methyl esters of fatty acids. Russian Patent, 1992, SU 671223 A1, October 23, 1992. (3) Matsumura, Y.; Tanaka, T.; Wanyoike, G. N.; Maki, T.; Onomura, O. J. Electroanal. Chem. 2001, 507, 71–74. (4) (a) Deng, L.; Fu, Y.; Guo, Q. X. Energy Fuels 2009, 23, 564–568. (b) White, J. H.; Schutte, E. J.; Rolfe, S. L. Cyclic catalytic upgrading of chemical species using metal oxide materials. PCT Int. Appl., 2008, WO 2008036902 A2, March 27, 2008. (c) Grandvallet, P.; Hagan, A. P.; Huve, L. G. Upgrading of pre-processed used oils, PCT Int. Appl., 2003, WO 2003033630 A1, April 24, 2003.

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