Ni (111

Matthias Schwarz , Chantal Hohner , Susanne Mohr , and Jörg Libuda. The Journal of Physical Chemistry C 2017 121 (51), 28317-28327. Abstract | Full T...
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J. Phys. Chem. B 1997, 101, 361-368

361

Adsorption and Reactions of Formic Acid on (2×2)-NiO(111)/Ni(111) Surface. 2. IRAS Study under Catalytic Steady-State Conditions Athula Bandara, Jun Kubota, Akihide Wada, Kazunari Domen, and Chiaki Hirose* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan ReceiVed: April 5, 1996; In Final Form: October 7, 1996X

Product gas analysis using quadrupole mass spectrometer (QMS) and infrared reflection absorption spectroscopy (IRAS) have been performed to investigate the catalytic decomposition of formic acid on NiO(111) film grown on Ni(111) surface under steady-state conditions and the results have been compared with the previous results obtained in vacuum condition. The rates of the catalytic reactions as measured by product gas analysis at various pressures of formic acid and also at different substrate temperatures revealed two reaction paths for the decomposition of surface formate under equilibrium with gas-phase formic acid. The dehydrogenation producing H2 and CO2 with the activation energy of 22 ( 2 kJ/mol was preferred even at the low temperature of 373 K over the dehydration producing CO and H2O which occurred at a higher temperature of above 423 K with the activation energy of 16 ( 2 kJ/mol, where the activation energy refers to the reaction of formic acid in gas phase to the product substances. The apparent reaction order was 0.5 with respect to the pressure of formic acid for both reactions. IRA spectroscopy using both normal and deuterated formic acid revealed the presence of two kinds of formate species: bidentate formate aligned normal to the surface and monodentate formate. The result is in contrast with our previous finding of only the tilted-bidentate formate species on the surface under vacuum. Pressure-dependent features of the IRA spectra suggested that the monodentate formate is the intermediate for the catalytic decomposition reactions. The combined use of gas analysis and IRAS results enabled us to derive the values of 58 ( 3 and 49 ( 3 kJ/mol as the activation energy for the dehydrogenation and dehydration reactions, respectively, of the monodentate formate on the surface.

Introduction The catalytic reactions on the structure-specified surfaces of single crystalline metal oxides are the subject of profound attention in the community of surface science. The past studies of polycrystalline powder catalysts have shown that the catalytic property depends strongly on the preparation procedure and history of the catalyst used, and attempts have been made to pursue the subject on the well-defined surfaces of single-crystal samples with molecule sensitivity.1-6 However, the application of such molecule-sensitive techniques like high-resolution electron energy loss spectroscopy (HREELS) and infrared reflection absorption spectroscopy (IRAS) has been defied by the low conductivity and nonmetallic reflection of the surfaces and high-vacuum condition required for the application of electron spectroscopy posed additional problem for the promotion of the subject which require the in situ investigation of surface reactions under ambient gas. An acute way to deter these difficulties has been provided by the recently devised technique of growing single crystalline oxide layers on top of metal surfaces.1-3,5 A significant number of studies has been reported on the synthesis of single crystalline NiO(100) thin films on Ni(100), Ni(111) and Ni(110).7,8 The metastable NiO(111) thin films were grown on the NiO(100) and Ni(111) substrates at elevated temperatures.9 Rohr et al. reported that (2×2) reconstruction occurred when the polar NiO(111) films on Ni(111) were heated under ambient conditions through the elimination of the surface hydroxyl groups.10 As for the chemical reactions of organic substances on solid surfaces, carboxylate species adsorbed on the surface are regarded as common intermediates in many catalytic reactions X

Abstract published in AdVance ACS Abstracts, December 15, 1996.

S1089-5647(96)01032-2 CCC: $14.00

such as water-gas shift reaction, methanol synthesis, and selective oxidation on the surfaces of transition metal oxides. Typically, surface carboxylates have been postulated as reaction intermediates in the reactions on the powder of ZnO,11 TiO2,12 MgO,13 NiO,14 and SnO215 and the decomposition on metal oxides, for example, generally follows dehydration and dehydrogenation pathways depending on the reaction conditions. Although ketonization of carboxylic acids has also been observed on certain metal oxides,16 the selectivity for the dehydration versus dehydrogenation reactions of carboxylic acids and alcohols has been used as a probe of the acid-base properties of metal oxide surfaces. In the reactions proceeding under continuous flow of reactant gas, dehydrogenation is said to be favored on the surfaces of basic oxides such as MgO while dehydration is said to take place on the surfaces of acidic oxides such as Al2O3.17 Similar tendency has been observed in the decomposition of alcohols as well, and it has been reported that the dehydration is the major reaction in the decomposition of ethanol on the surfaces of MgO powder.17 Vohs and Barteau investigated the decomposition of alcohols adsorbed on ZnO(1000) surface18 to observe substantial amount of dehydration products even though flow reactor studies of steady-state alcohol decomposition on ZnO powder generally have shown high selectivity toward dehydrogenation.19 Formate is the postulated intermediate of these reactions and it has been revealed that the surface formate is also produced when methanol is adsorbed on the powder surfaces of various metal oxides such as alumina, magnesia, and zinc oxide at elevated temperatures although its stability depends largely on the oxide employed.19 Tamaru and co-workers employed infrared spectroscopy to investigate the behavior of surface formate species formed during the decomposition of methanol © 1997 American Chemical Society

362 J. Phys. Chem. B, Vol. 101, No. 3, 1997 over Cr2O3 and found that the decomposition of formate was accelerated remarkably in the presence of gaseous methanol.20 Recently, Shido et al. used infrared spectroscopy to identify the reaction intermediates of the water-gas shift reaction on powders of MgO,21 ZnO,22 and CeO223 and disclosed that surface formate was the intermediate for the reaction on all the examined catalysts. On the other hand, rather few works have been reported on the catalytic reactions on the surfaces of single crystal oxide. Vest et al. investigated the catalytic decomposition of 2-propanol on different surfaces of single crystalline ZnO by exposing them to a molecular flux of 2-propanol to reveal that the reaction rate was sensitive to the structure of the surface under certain conditions.24 Onishi et al. reported that both dehydration and dehydrogenation reactions occurred during catalytic decomposition of formic acid on the TiO2(110) surface.25 Turning to the application of the techniques of surface vibrational spectroscopy, Petrie and Vohs reported pioneering works on the application of HREELS to the investigation of the adsorption and decomposition of formic acid on ZnO(1000) surface by eliminating the scattering by surface optical phonon modes using Fourier deconvolution.26 HREELS was further used without serious interference from intense multiple surface optical phonon losses to investigate the adsorption and reaction of formic acid on NiO(100)27 and NiO(111)28 films grown on Mo(100) substrate. Recently, Xu and Koel employed IR spectroscopy in transmission mode to examine the adsorption of acetic acid on MgO(100) surface and proved that the multiphonon cross section of infrared absorption is lower than the scattering cross section of HREELS and the surface phonon cannot be directly excited by IR.29 Hoffmann and co-workers employed in situ Fourier transform IRA spectroscopy to reveal the formation of formate intermediate during the CO hydrogenation reaction at high pressures over a potasium-promoted Ru(001) surface.30,31 However, there have been no reports on the application of surface vibrational spectroscopy to the in situ investigation of the reaction intermediates present under catalytic steady-state conditions on the surfaces of single crystalline metal oxides. We have reported previously on the results of our investigation, using temperature-programmed desorption (TPD) and IRAS, of the adsorption and decomposition of formic acid on the NiO(111) layers grown on the Ni(111) surface in UHV condition.32 The present paper describes about the investigation extended to the reaction of the same oxide layer under continuous flow of reactant formic acid by using the same apparatus. The quadrupole mass spectrometer (QMS) was used this time to measure the partial pressure of reaction products and IRAS to observe the vibrational spectrum of surface species. Stated briefly, the surface formate was found to decompose through two reaction pathways: dehydrogenation and dehydration and the formation of two kinds of formate species. Monodentate and bidentate formate, instead of only the tiltedbidentate formate found in vacuum condition, were identified on the surface. The activation energies for the gas-todecomposition products and the adsorbed monodentate-toproducts reactions have been derived and the mechanisms of the catalytic reactions are proposed. Experimental Section The experiments were performed in an ultra high vacuum (UHV) system detailed description of which will be found elsewhere.32,33 In brief, the UHV system consisted of two sections: the preparation chamber is for sample cleaning and characterization and equipped with Ar ion bombardment gun,

Bandara et al.

Figure 1. Intensities of QMS signals of decomposition products of formic acid as a function of reaction time at 298 and 423 K and at 1 × 10-4 Pa of formic acid. Rate of production of each product corresponds to the increment of the partial pressure.

Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED); and the infrared chamber is equipped with NaCl windows for the introduction of infrared beam and quadrupole mass spectrometer (QMS) to monitor partial pressures of reaction products. The whole system was evacuated by oil-diffusion pump to the base pressure of 1 × 10-10 Torr. The speed of evacuation was estimated to be ∼200 L s-1. The single crystalline Ni(111) sample piece was mounted to the sample holder by Ta wires and could be heated to above 1000 K by resistive heating and cooled to ∼120 K by flowing liquid nitrogen. The temperature was measured using chromel-alumel thermocouple spot welded to the back face of the sample. Gas introduction was carried out by backfilling the chamber to an uniform distribution through a variable leak valve. As for the preparation of NiO(111) surfaces, the Ni(111) surface was first cleaned by Ar ion bombardment and annealed at 1000 K, next exposed to the repeated cycles of the 1000 L (1 L ) 1 × 10-6 Torr s; 1 Torr ) 133 Pa) exposure of oxygen at 570 K, and finally annealed at 650 K in vacuum. Three such oxidation cycles gave sharp LEED spots, indicating the growth of NiO(111)-(2×2) phase and the Auger signals was obtained with the peak-to-peak intensity ratio of 1.75 for the OKLL peak at 510 eV and the NiLMM at 848 eV under the conditions of primary electron energy of 3 keV, modulation width of 5 eV, and scan rate of 0.1 keV/min. The resulted oxide surface dominated by fully oxidized sites was further characterized by the IRA spectra of the surface adsorbed by CO as reported previously.33 Commercial formic acid of 99% purity was further purified by repeated freeze-pump-thaw cycles prior to use. The measurements of the reaction rates of the catalytic decomposition proceeded as follows: first the NiO(111) surface at a prescribed reaction temperature was exposed to a continuous flow of formic acid and QMS signals of H2 (m/e ) 2), CO2 (44), H2O (18), CO (28), and HCOOH (46) were recorded after the steady state was reached. The same measurements were carried out next at 298 K where the catalytic reaction was not taking place. The reaction-induced increase of the intensity of the QMS signal was linearly related to the partial pressure of the relevant species. A typical example of the measurement is shown in Figure 1 where the intensities of QMS signals are plotted against the reaction time at different substrate temperature. At 298 K, the QMS signals were regarded as the fragmentations of formic acid and the signal intensities of H2, CO2, CO, and H2O increased significantly when the sample was heated to 423 K as shown in Figure 1. One notes that the reaction-induced increase of QMS signals of each species is enough to ascribe it as being produced continuously by the reaction under the steady flow of formic acid. The increase of

Formic Acid on (2×2)-NiO(111)/Ni(111) Surface

J. Phys. Chem. B, Vol. 101, No. 3, 1997 363

thus derived partial pressure at the reaction temperature should correspond to the rate of production. The conversions of the dehydrogenation and dehydration rections at 423 K were estimated as ∼3.8% and ∼2.0%, respectively, from the analysis of QMS signals. In the analysis, the amount of the parent formic acid was derived from its QMS signal (not shown) and low-temperature QMS signals of fragmentation products, and the corrections for the detection sensitivities were made. IRA spectra were obtained by a JEOL JIR-100 Fourier transform infrared (FTIR) spectrometer at incident angle of 82° and spectral resolution of 4 cm-1. A liquid nitrogen-cooled HgCdTe (MCT) and InSb composite detector was used for the spectral regions of 400-1800 and 1800-4000 cm-1, respectively. A wire grid polarizer, which was revolved by a stepping motor, was used to obtain the ratio spectra, the spectra obtained for the p-polarized infrared beam divided by the ones obtained for the s-polarized beam. Below 650 K, the composition of the NiO(111) surface remained unchanged as examined by AES after each experiment proving that formate decomposed catalytically in the investigated temperature range. Results and Discussion Temperature Dependence of Reaction Rates. The measurement of the QMS signals at various temperatures revealed two reaction pathways: dehydrogenation reaction producing H2 and CO2 started at low temperature of 373 K and dehydration reaction producing H2O and CO operated above 423 K. Under the condition of dehydration reaction, the amount of the continuously produced water molecule was much larger than the amount of background water, giving significant intensity of the QMS signal. The rate of the dehydrogenation reaction was approximately twice that of the dehydration reaction for the carbon-containing produts in the temperature range between 423 and 473 K and the temperature dependence of the rate for the latter reaction deviated from the straight line below 423 K. It is noted that the slope for the production of H2 coincides with that for the CO2 production in the dehydrogenation and that for the CO production with that for the H2O production in the dehydration reaction. Shown in Figure 2 are the Arrhenius plots for the rate of the dehydrogenation reaction (Figure 2a) and for the dehydration reaction (Figure 2b). The activation energies were derived from the plots as 22 ( 2 kJ/mol for the dehydrogenation reaction in the 373-473 K region and 16 ( 2 kJ/mol for the dehydration in the region between 423 and 473 K. The values of the presently derived activation energy are different from those of the corresponding reactions on the NiO(111) surface in vacuum.32 The values should not be taken as the real activation energy for the decomposition of formate on the NiO(111) surface. The interpretation of the derived values, which are lower than anticipated, will be postponed until we describe the reaction order and the IRAS data but we state here. We have reported that under ultrahigh vacuum conditions32 the dehydrogenation reaction occurred at 340, 390, and 520 K and dehydration reaction at 415 and 520 K, both reactions taking place on the formate-covered NiO(111)-(2×2) surface. Activation energy of 90-100 and ∼110 kJ/mol were derived from the desorption temperatures of 340-390 and 415 K for the dehydrogenation and dehydration reactions, respectively, by assuming the reported value of 1013 s-1 as pre-exponential factor for the thermal decomposition of formate in vacuum.34 The breakpoint at around 420 K of the dehydration reaction shown in Figure 1b was found to coincide with the temperature

Figure 2. Arrhenius plots for the reaction products of (a) dehydrogenation and (b) dehydration reactions of the adsorbed formate on NiO(111)/Ni(111) surface under the stream of gaseous formic acid at 5 × 10-4 Pa pressure. The logarithms of the intensity V (in arbitrary units) of reaction-induced QMS signals are plotted against the reciprocals of substrate temperature T. See Experimental Section for the basis and correction procedure of deriving reaction rate from partial pressure which in turn was estimated from QMS signals.

at which the TPD signal of the CO appeared from the formatecovered surface in UHV condition, suggesting that the surface was covered by the reactant formate until that temperature and that the formate on the surface participated to the reaction after the temperature exceeded the point. Higher activation energy of about 80 kJ/mol was estimated for the dehydration reaction in the temperature region between 353 and 423 K. Apparent Reaction Order. The rate of both reactions increased with the pressure as shown in Figure 3 where the relations between the pressure of formic acid and the rates of catalytic decomposition at 423 K through dehydrogenation (Figure 3a) and dehydration (Figure 3b) reactions are plotted. The overall reaction rate, V, is expressed as V ) kPfan, where k, Pfa, and n represent the rate constant, pressure of formic acid, and apparent value of reaction order, respectively. The apparent value of 0.5 was derived as the reaction order n for both the dehydrogenation and the dehydration reactions with respect to the pressure of formic acid. The value of 0.5 suggests that the surface was covered with reactive intermediates and their decomposition determine the rate of both reactions. The conformity of the reaction order for the both reactions indicates that the rate-determining steps were catalyzed by common intermediate. IRAS Measurements. Catalytic decomposition of formic acid on NiO(111) surface has been further examined by IRAS to identify the reactive intermediates responsible for the reactions. The IRA spectra from the NiO(111) surface kept at 423 K and under a continuous flow of formic acid at various pressure are shown in Figure 4. Each spectrum was measured after the steady state was reached. The absorption bands at 777, 1355, and 2850 cm-1 as observed when the surface was exposed to the formic acid at 1 × 10-6 Pa (bottom curve), were assigned to the OCO deformation [δ(OCO)], the symmetric OCO stretching [νs(OCO)], and the CH stretching [ν(CH)] modes, respectively, of adsorbed formate having bidentate configuration

364 J. Phys. Chem. B, Vol. 101, No. 3, 1997

Bandara et al. SCHEME 1

Figure 3. Change of partial pressures at 423 K of the reaction products by the pressure of flowing formic acid. Logarithms of the intensity (in arbitrary units) of QMS signals are plotted against logarithms of pressure (in Pa) for the (a) products of dehydrogenation reaction and (b) products of dehydration reaction.

Figure 4. IRA spectra of NiO(111)/Ni(111) surface exposed at 423 K to the continuous flow of formic acid (HCOOH) of various pressures. Absorption bands are all assigned to formate (HCOO).

as observed under ultrahigh vacuum conditions.32 The intensity of the 2850 cm-1 band reached its maximum when the pressure was 1 × 10-5 Pa while the intensities of the 777 and 1355 cm-1 bands reached their maximum when the pressure was 5 × 10-4 Pa, further increase of the pressure led to the decrease of the intensity. Upon increasing the pressure to 5 × 10-5 Pa, new bands appeared at 2940 and 1253 cm-1 and the 2850 cm-1 band started to weaken. On further increase of the pressure, the newly appeared bands continued to gain intensity. The 2940 and 1253 cm-1 bands were assigned to the ν(CH) and the ν(CO) modes, respectively. It has been known that the higher frequency around 2940 cm-1 of the ν(CH) is characteristic of the bidentate and/or bridged formate on metal surfaces, the monodentate

formate on oxide surfaces, and the formic acid molecule. The appearance of the ν(CO) band alongside with the ν(CH) band and the absence of the band corresponding to OH bending mode indicated that the newly observed two bands originated from the formate species having monodentate configuration. The absence of the absorption band corresponding to the CdO stretching mode ν(CdO) of monodentate formate, which should be located around 1700 cm-1 suggests that the CdO bond was directed parallel to the surface (see Scheme 1) to make the ν(CdO) inactive to IRAS. Table 1 summarizes the presently derived and previously reported values of the vibrational frequencies and their assignments for the formate on various single crystalline surfaces of metal oxide. Figure 5 shows the IRA spectra observed on using deuterated formic acid DCOOD and the absorption bands of deuterated formate (DCOO), produced when NiO(111) surface at 423 K was exposed to DCOOD at various pressures. The bands at 2160, 1330, and 767 cm-1 were assigned to the ν(CD), νs(OCO), and δ(OCO) modes, respectively, of bidentate formate while the bands at 2190 and 1267 cm-1 assigned to the C-D stretching ν(CD) and C-O stretching ν(C-O) modes of monodentate formate. The broad band centered at 2640 cm-1 was attributed to the O-D stretching mode of surface hydroxyl groups since the position and broadness of the band indicated that the band originated from hydroxyl species interacting with neighboring species by hydrogen bonding. One notes that the absorption band by the OCO asymmetric stretching mode νa(OCO) of formate, which was observed under ultrahigh vacuum conditions,32 was not observed at the expected frequency of 1570 cm-1 under the continuous-flow condition, and this fact led us to postulate that the formate was oriented normal to the surface with bidentate configuration. Actually, Figures 4 and 5 indicate the presence of another kind of formate species, the monodentate formate under steady-state conditions. Only the tilted-bidentate formate species existed on the NiO(111) surface under ultra-high vacuum condition.32 The orientational change of the bidentate formate, by the introduction of ambient formic acid suggested that some structural change took place on the examined NiO(111) surface. As a matter of fact, the oxygen-terminated NiO(111)-(2×2) surface has been regarded as being preferred in vacuum on the basis of STM observation,36 and we have postulated in our previous paper that the adsorbed bidentate formates under UHV condition are located along the slope of the oxygen-terminated NiO(111)-(2×2) surface.32 It is impossible for a bidentate formate to adsorb with its orientation normal to the surface when the Ni cations are not located on the top layer of oxygenterminated NiO(111)-(2×2) surface. In other words, the presence of Ni cation at the outermost layer is the prerequisite for the vertical adsorption of bidentate formate. Thus, the occurrence of bidentate formate with surface normal orientation should indicate that the Ni-terminated surface appeared in the steady-state condition. The surface reconstruction from the (2×2) phase to the (1×1) phase, which has the needed Ni cations on the top layer, by the presence of surface hydroxyl has been postulated.8,37 In the present study, surface hydroxyl groups may have been produced from the protons liberated by dis-

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TABLE 1: Vibrational Frequencies (cm-1) and Assignments of Adsorbed Formate on Various Surfaces of Single Crystalline Metal Oxides NiO(111)a vibrational mode

{ a

ν(CH) νa(OCO) νs(OCO) ν(CO) assignment method

}

steady-state

vacuum

NiO(100)b

ZnO(0001)c

ZnO(0001)c

MgO(100)d

2940 1253 monodentate IRAS

2850 1355

2860 1570 1360

2901 1594 1377

2939 1612 1387

2951 1634 1349

bidentate

bidentate IRAS

monodentate HREELS

monodentate HREELS

monodentate HREELS

Present work. b Reference 27. c Reference 26. d Reference 35.

Figure 5. IRA spectra of NiO(111)/Ni(111) surface 423 K under continuous flow of deuterated formic acid (DCOOD) of various pressure. The observed bands are due to the formate DCOO.

sociative adsorption of formic acid and surface oxygen atoms causing the surface structure to change from waved (2×2) to flat (1×1) structure. As described above the adsorption of deuterated formic acid gave rise to the ν(OD) band on the IRAS spectrum although the band was not detected on using normal formic acid presumably because the signal was weaker than the noise level of our spectrometer around 3600 cm-1. Figure 6, parts a and b, display in logarithmic scale the pressure dependence of the integrated peak intensities of the ν(CH) and ν(CD) bands of monodentate formate denoted by ν(CH)m and ν(CD)m, respectively, and ν(CH), ν(CD), and νs(OCO) bands of bidentate formate, denoted by ν(CH)b, ν(CD)b, and νs(OCO)b, respectively. The traces of the ν(CH)m and ν(CD)m bands are similar to each other and the straightlined parts of the plots for the ν(CH)m and ν(CD)m bands give the slopes of 0.41 and 0.47, respectively. The values are very close to 0.5 which was derived from the QMS measurement (see Figure 3) as the reaction order of the decomposition of formic acid. These values suggested strongly that the monodentate formate behaved as an intermediate in the presently investigated decomposition reaction of formic acid on the NiO(111) surface under the steady-state condition. The order of 0.5 suggests that the surface was covered by a considerable amount of monodentate formate decomposition of which determined the rate of reactions. The intensity of the band due to monodentate formate was almost saturated when the pressure was above 1 × 10-2 Pa making it the possibile to estimate the coverage of monodentate formate on the surface.

Figure 6. Variation by the pressure of flowing formic acid of integrated peak intensities (in arbitrary units) of IR absorption bands of (a) ν(CH)m, ν(CH)b, and νs(OCO)b formate species and (b) ν(CD)m, ν(CD)b, and νs(OCO)b modes of formate species on NiO(111)/Ni(111) surface at 423 K. Plots are given in logarithmic scale and the subscripts m and b denote monodentate and bidentate formate, respectively.

The intensity of the νs(OCO)b band increased slightly by the pressure of formic acid, and the slope of 0.15 was estimated from the linear region for both the HCOO and the DCOO. The intensity of ν(CH)b and ν(CD)b bands apparently saturated and decreased slightly at higher pressure although the dependence on the pressure differed between the two bands. Incidentally, Onishi et al.25 investigated the catalytic decomposition of formic acid, on the TiO2(110) surface to reveal that both dehydrogenation and dehydration reactions occurred. Unlike in the presently investigated reaction on NiO(111) surface, the dehydrogenation reaction proceeded through low activation energy of 15 ( 10 kJ/mol and the dehydration reaction had the higher activation barrier of 120 ( 10 kJ/mol. Unimolecular decomposition through the dehydration of formate has been reported on the single crystalline surfaces of TiO2,38 ZnO,39 SnO2,40 and MgO.41 On the other hand, bimolecular reaction of a formate with an impinging formic acid molecule was proposed for the dehydrogenation reaction on ZnO21 and MgO22 powder catalyst. Benziger and Schoofs derived as the apparent reaction order of 1 in the low-pressure region and zero in the high-pressure region for the decomposition of formic acid on the metallic Ni wire under steady-state condition and their infrared spectroscopic study identified the decomposition of surface formate as the rate-limiting step.42 Noto et al.43 reported the zeroth order reaction for the dehydrogenation of formic acid on zinc oxide and revealed that the rate was independent of the

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Bandara et al.

pressure of formic acid. They also noted that the evolution of CO2 arose from the decomposition of formate and that the H atoms remained on the surface in the absence of formic acid and evolved as gaseous hydrogen when the formic acid molecules were present on the surface. The presently proposed mechanisms for both the dehydrogenation and the dehydration are distinctly different from the above-summarized cases. Kinetics of the Catalytic Decomposition Reactions. Figure 7 is the IRA spectra of surface formate (HCOO) at various temperatures as taken after the NiO(111) surface was exposed at 423 K to the continuous flow of formic acid at 5 × 10-4 Pa and subsequently heated to the stated temperature. The observed bands are due to the surface monodentate and bidentate formates as already assigned. The intensities of all bands are seen to weaken at increased temperature and the following part of this article deals with the quantitative analysis of the above observations. We have postulated monodentate formate as the reaction intermediate for both the dehydrogenation and the dehydration reactions occurring on the catalytic decomposition of formic acid on the NiO(111) surface under steady-state condition. The derived reaction order of 0.5 with respect to the pressure of formic acid clearly suggested that the rate-determining step is the decomposition of monodentate formate. The reaction steps conceivable for the decomposition reaction of formic acid on the surface are Kads

HCOOH(g) {\} HCOOH(R) k1

HCOOH(a) 98 m-HCOO(a) + H(a) k2

m-HCOO(a) + H(a) 98 CO2(g) + H2(g) k3

m-HCOO(a) + H(a) 98 CO(g) + H2O(g)

(A) (B) (C)

Figure 7. Temperature-dependent features of the IRA spectra of formate adsorbed on NiO(111)/Ni(111) surface. The surface at 323 K was initially exposed to the continuous flow of formic acid at 5 × 10-4 Pa and subsequently heated to the stated temperature with the reactant gas still flowing.

dehydration D reactions as

V1 ) k2θmf

(1)

V2 ) k3θmf

(2)

(D)

Kads is the equilibrium constant for adsorption equilibrium, and ki (i ) 1-3) are the rate constants of the associated reactions where m-HCOO(a) denotes the monodentate formate. It should be mentioned, however, that the reactions C and D may actually consist of many elementary steps like

m-HCOO(a) f CO2(a) + H(a) CO2(a) f CO2(g) 2H(a) f H2(g) H2(a) f H2(g) and

m-HCOO(a) f CO(a) + OH(a) CO(a) f CO(g) OH(a) + H(a) f H2O(g) H2O(a) f H2O(g) for the dehydrogenation and dehydration reactions, respectively. We postulate that both the formation of formate in step B and the decomposition of monodentate formate in steps C and D are the slow steps of the reaction scheme under the investigated experimental conditions and we express by V1 and V2 the rates of the two surface reaction steps of dehydrogenation C and

where θmf is the coverage of monodentate formate and can be estimated from the relative intensity of the ν(CH) band of monodentate formate at 2940 cm-1 with respect to the saturated intensity. The rate constants k2 and k3 are expressed in terms of the effective activation energy E2‡ and E3‡, of the steps C and D, respectively, as follows

( ) ( )

k2 ) A2 exp

-E2‡ RT

(3)

k3 ) A3 exp

-E3‡ RT

(4)

where A2 and A3 are the preexponential factors. Thus, the values of E2‡ and E3‡ can be derived from the slopes of the ln(θmf/V1) and ln(θmf/V2) , respectively, against the reciprocal temperature. The plots shown in Figure 8, where the experimental data of V and θmf were obtained from the QMS signals of reaction products and the intensities of the CH stretching band of monodentate formate, respectively, gave the values of 58 ( 3 kJ/mol and 49 ( 3 kJ/mol as E2‡ and E3‡, respectively. It should be noted that these values are derived from the data for the temperature region where both reactions are taking place. The deviations of the plots from straight lines are seen at lower temperature, suggesting that the activation energy is somewhat dependent on the coverage of the surface. From QMS data the values of 22 ( 2 and 16 ( 2 kJ/mol for the activation energy of the HCOOH(g) f H2(g) + CO2(g) and HCOOH(g) f CO(g) + H2O(g) and reactions, respectively, and

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J. Phys. Chem. B, Vol. 101, No. 3, 1997 367

Figure 8. The plots of ln(v/θmf) against 1/T for the decomposition of monodentate formate under steady state conditions at 5 × 10-4 Pa of formic acid flow, where θmf is the coverage of monodentate formate as estimated from the intensity of the ν(CH) band at 2940 cm-1 and V is the rate of reaction estimated from the intensity of QMS signals as the partial pressure of the products. Open circles and squares refer to the dehydrogenation and dehydration reactions, respectively.

Figure 10. The plot of (1 - θmf)/θmf against 1/T, where θmf is the coverage of monodentate formate species calculated from the intensity of the ν(CH) band at 2940 cm-1 under the continuous flow of formic acid at 5 × 10-4 Pa.

Figure 11. A conceptual drawing of the potential energy diagram for the dehydrogenation and dehydration reactions of formic acid on the NiO(111)/Ni(111) surface.

The rate constant k1 and the equilibrium constant are expressed in terms of the activation energy E1‡ and energy of adsorption Eads, respectively, as follows, Figure 9. The plot in logarithmic scale of θmf/(1 - θmf) against the pressure where θmf is the coverage of monodentate formate species calculated from the intensity of the ν(CH) (open circles) band at 2940 cm-1 of HCOO and ν(CD) (closed circles) band at 2190 cm-1 of DCOO under the continuous flow of formic acid at 423 K.

we have derived the values of 90-100 and ∼110 kJ/mol as the activation energy of the decomposition under vacuum of bidentate formate through dehydrogenation and dehydration reactions, respectively. The presently derived smaller values for the decomposition under steady flow of reactant gas indicate that the monodentate formate decomposed more readily. When we consider the formation and decomposition of monodentate formate together and assume that the coverage of adsorbed formic acid θfa on the surface is negligible and that adsorbed bidentate formate does not affect the adsorption sites of the monodentate formate, the following relation arises for the system with continuous and steady flow of reactant

KadsPfaki(1 - θmf) ) (k2 + k3)θmf

(5)

where Pfa denotes the pressure of formic acid and θfa is defined as

θfa ) KadsPfa(1 - θmf)

(6)

Figure 9 is the plot against the pressure in logarithmic scale of θmf/(1 - θmf) for both normal and deuterated formic acid, and the linear dependence of the θmf/(1 - θmf) on the pressure with the slope of ∼1 demonstrates the validity of the model which we used for the extraction of kinetc parameters.

( )

k1 ) A1 exp

-E1‡ RT

(7)

( )

(8)

Eads RT

Kads ) K0 exp

where A1 and K0 are the pre-exponential factors. Equations 5 and 8 lead to the following relation:

[ (

)

-E2‡ + E1‡ - Eads 1 - θmf 1 ) A2 exp + θmf A1K0Pfa RT

(

A3 exp

)]

-E3‡ + E1‡ - Eads RT

(9)

Figure 10 is the plot of the (1 - θmf)/θmf against the reciprocal temperature of the surface, and computer fitting of the data was made by substituting the above derived values of E2‡ and E3‡ to give the result shown by the solid line in Figure 10. The value of 14 ( 5 kJ/mol was derived as the (E1‡ - Eads). We have derived, as the effective activation energies of the dehydrogenation and dehydration reactions, the values of (1) 22 ( 2 and 16 ( 2 kJ/mol, respectively, for the formic acid (gas) to products (gas) reactions, and (2) 58 ( 3 and 49( 3 kJ/mol, respectively, for the monodentate formate(a) to product reactions. Combining the above-derived value of (E1‡ - Eads ), the energy diagram shown in Figure 11 is proposed for the catalytic decomposition of formic acid on the NiO(111)/Ni(111) surface under the steady-state conditions.

368 J. Phys. Chem. B, Vol. 101, No. 3, 1997 We now turn to the inspection of rate-determining steps in the above-depicted reaction schemes. Actually, Fukuda et al.44 previously suggested that the desorption of surface hydroxyl groups as water molecule was the rate-determining step of the catalytic dehydration reaction of formic acid on alumina and silica and similar postulate has been suggested for the catalytic dehydration of formic acid on TiO2 and Cr2O3. In the cases of the reaction of formic acid over Fe3O4 and MnO, however, the breaking of the C-O bond of the formate has been suggested to be rate-determining.45,46 In the present study, however, no significant change in the amount of surface oxygen was detected on the AES signals as examined after each experiment and thus the reduction of the surface during the studied catalytic reaction must have been negligible. This observation may suggest that the oxygen defect sites, if produced, were recovered by the decomposition of formic acid. In any event, both of hithertodepicted rate-determining steps can be eliminated from the present reactions. Well-designed experiments which enable distinctive examination of the two reaction pathways are required for further details.

Bandara et al. (3) Akther, S.; Cheng, W. H.; Lui, K.; Kung, H. H. J. Catal. 1984, 85, 437. (4) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1988, 201, 481. (5) Mokwa, W.; Kohl, D.; Heiland, G. Surf. Sci. 1982, 117, 659. (6) D’Amico, K. L.; Trenary, M.; Shinn, N. D.; Soloman, E. I.; McFeely, F. R. J. Am. Chem. Soc. 1982, 104, 5102. (7) Kuhlenbeck, H.; Odo¨rfer, G.; Jaeger, R.; Illing, G.; Menges, M.; Mull, Th.; Freund, H.-J.; Po¨hlohen, M.; Staemmler, V.; Witzel, S.; Scharfschwerdt, C.; Wennemann, K.; Liedtke, T.; Neumann, M. Phys. ReV. 1991, B43, 1969. (8) Langell, M. A.; Nassir, M. H. J. Phys. Chem. 1995, 99, 4162. (9) Cappus, D.; Xu, C.; Ehrlich, D.; Dillmann, B.; Ventrice C. A.; AlShamery, K., Jr.; Kuhlenbeck, H.; Freund, H.-J. Chem. Phys. 1993, 177 , 533. (10) Rohr, F.; Wirth, K.; Libuda, J.; Cappus, D.; Ba¨umer, M.; Freund, H. -J. Surf. Sci. Lett. 1994, 315 , L977. (11) Akther, S.; Lui, K.; Kung, H. H. J. Phys. Chem. 1985, 89, 1958. (12) Groft, R. P.; Manogue, W. H. J. Catal. 1984, 87, 461. (13) Spitz, R. N.; Barton, J. E.; Barteau, M. A.; Staley, R. H.; Sleight, A. W. J. Phys. Chem. 1986, 90, 4.67. (14) Kishi, K.; Ikeda, S. Appl. Surf. Sci. 1980, 5, 7. (15) Thoren, W.; Kohl, D.; Heiland, G. Surf. Sci. 1985, 162, 402.

Conclusions The catalytic decomposition of formic acid on the NiO(111) layers grown on Ni(111) surface was studied using IRAS and gas analysis by QMS and under steady-state conditions. The IRA spectra of the adsorbed formate suggested the presence of two types of formate species, bidentate with vertical to the surface and monodentate formates in contrast to the surface under UHV where only the tilted-bidentate formate was observed. The formic acid underwent decomposition through two parallel reaction pathways: dehydrogenation to produce H2 and CO2 started at 373 K and dehydration to produce CO and H2O occurred above 423 K with the activation energies of 22 ( 2 and 16 ( 2 kJ/mol, respectively. The order of the both reactions was derived as 0.5 with respect to the pressure of formic acid. Pressure dependent features of the IRA spectra suggested strongly that the monodentate formate is the intermediate of the investigated reactions on the NiO(111) surface. The activation energies of 58 ( 3 and 49 ( 3 kJ/mol were derived for the decomposition of monodentate formate through dehydrogenation and dehydration, respectively. The change of the orientation of the bidentate formate from the one found in UHV conditions was attributed to the change of the surface structure from oxygen-terminated (2×2) structure in UHV to the nickel-terminated (1×1) structure due to the production of the hydroxyl groups during the decomposition of formic acid. The catalytic reaction rate as measured by QMS agreed with the rate of decomposition of monodentate formate measured by IRAS to indicate considerably lower reactivity of the bidentate formate the presence of which was also identified by IRAS.

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Acknowledgment. The authors are indebted to Dr. J. N. Kondo for her helpful comments and suggestions. One of the authors (A.B.) gratefully acknowledges the Ministry of Education, Science and Culture (MONBUSHO) of Japan for awarding a Japanese Government (Monbusho) Scholarship to study at Tokyo Institute of Technology. This work was supported by the Grant-in-aid on Priority-Area-Research “Photoreaction Dynamics” from the Ministry of Education, Science and Culture, Japan (no. 06239110).

(43) Noto, Y.; Fukuda, K.; Onishi, T.; Tamaru, K. Trans Faraday Soc. 1967, 63, 3081.

References and Notes

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