J. Phys. Chem. B 1998, 102, 5951-5954
5951
Short-Lived Reactive Intermediate in the Decomposition of Formate on NiO(111) Surface Observed by Picosecond Temperature Jump Athula Bandara,† Jun Kubota,† Ken Onda,† Akihide Wada,† S. S. Kano,‡ Kazunari Domen,*,† and Chiaki Hirose*,†,§ Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, and College of Engineering, Hosei UniVersity, Koganei, Tokyo 184, Japan ReceiVed: March 25, 1998; In Final Form: May 14, 1998
Unstable intermediates are indispensable ingredients of chemical reactions, but their identification at solid surfaces has been hampered by the lack of techniques to detect short-lived and fractionally generated species. We succeeded in identifying the intermediate in a thermal decomposition reaction by utilizing the picosecond temperature jump induced by the irradiation of laser pulses and the observation by sum-frequency generation spectroscopy. We verified that the decomposition of formate on the NiO(111) surface is preceded by the transformation of stable bidentate formate to unstable monodentate formate, which is the reactive intermediate, and two species are found to be in equilibrium before decomposition.
Introduction Reaction intermediates are key elements in chemical reactions, and their verification is indispensable for the understanding of reaction mechanism. However, clear evidence of the species participating in chemical reaction in the catalytic reactions on solid surfaces has been lacking despite intensive investigation for the past 30 years.1-5 Such intermediates are very low in concentration and short-lived, defying the observations by conventional vibrational spectroscopies such as high-resolution electron energy loss spectroscopy (HREELS) 1,2,6 and infrared reflection absorption spectroscopy (IRAS).1,2 We expect that an appreciable amount of the species will exist only for a short period of time, and they are produced at the temperature that is appropriate for production but unfit for decomposition. The failure to detect them by recently developed rapid-scan IRAS, for example, by gradually increasing the substrate temperature is thus rationalized. Pulsed lasers provide the way to probe such a species. The rate of heating by ultrafast laser pulses is sufficiently fast, attaining the temperature jump of about 300 K in a few picoseconds in our experiment, to make it possible to accumulate short-lived intermediate species and to follow the chemical reactions occurring at the surfaces. By the studies that have been carried out so far of laser-induced thermal desorption (LITD), the desorption of methanol from Ni surfaces instead of decomposition has been observed as the favorable reaction channel under the irradiation of nanosecond laser pulses 7 but the identification of short-lived intermediates has not been reported. Formate has been found in various catalytic reactions such as water-gas shift reaction and methanol synthesis on solid surfaces.1-5 On the NiO(111) surface, formate is anchored to a Ni cation site by two formate oxygen atoms as illustrated in Scheme 1, and the species with this configuration is named bidentate formate.8,9 The adsorbed formate decomposes giving †
Tokyo Institute of Technology. Hosei University. § E-mail:
[email protected]. ‡
SCHEME 1: Adsorption Configurations of Bidentate and Monodentate Formates
out either H2 and CO2 or H2O and CO on heating the surface above 350 K, but the reactions cannot proceed by a single elementary step. Our IRAS investigation under heavy flow of formic acid at 10-3 Pa identified a new species,8 which was assigned to the formate with monodentate configuration (see Scheme 1), to appear at the onset of decomposition reaction. The nature of the experiment prohibited us, however, from verifying whether the new species was the reactive intermediate or just a spectator. In this study, we have succeeded in accumulating the shortlived reactive intermediate for the first time by using picosecond laser pulses; instantaneous temperature jump by laser irradiation and subsequent observation by infrared-visible sum-frequency generation (SFG) spectroscopy10-16 enabled us to identify a reaction channel in the decomposition of formate on NiO(111) surface. In situ time-resolved SFG spectroscopy made of the formate (DCOO)-covered surface under the irradiation of picosecond 1064 nm laser pulses revealed that the irradiationinduced transformation of the stable formate in bidentate configuration (left-hand side figure of Scheme 1) to unstable monodentate formate (right-hand side figure of Scheme 1) occurred prior to the decomposition, and the two species were found to be in equilibrium. Experimental Section Our experiment was performed on the deuterated formate (DCOO(a)) adsorbed on the NiO(111)/Ni(111) surface using an ultrahigh vacuum (UHV) chamber8,9 and the optical setup depicted in Figure 1. The NiO(111) layer was prepared on
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5952 J. Phys. Chem. B, Vol. 102, No. 31, 1998
Figure 1. Optical setup of the SFG spectroscopy. The visible 532 nm and frequency-tunable IR pulses for SFG were generated from a modelocked Nd:YAG laser having the pulse width of 35 ps and pulse repetition rate of 10 Hz; the visible pulses were the second harmonic generation (SHG) output from a KH2PO4 (KDP) crystal, and the IR pulses were generated by the difference frequency generation (DFG) from near-IR and the 1064 nm pulses in a AgGaS2 (AGS) crystal. (The near-IR pulses were frequency-tunable and were generated by optical parametric generator/amplifier using β-BaB2O4 (BBO) crystals from a separately generated SHG output.) Mildly focused IR pulses (fwhm; 3.0 cm-1 at 2200 cm-1; pulse width, 18 ps, pulse energy, ∼60 µJ) and visible pulses (pulse energy, ∼200 µJ) were crossed at the sample surface at the incident angle of about 75°. A portion of the 1064 nm pulses with 10 mJ/pulse energy was passed through a variable optical delay (OD) line to irradiate the sample with prescribed delay time. The zero delay time between pump and probe pulses was set by monitoring the SFG signal from the reference sample of GaAs mounted to the same sample holder. The diameters of the IR, visible, and 1064 nm beams were 2, 3, and 5 mm, respectively. The SFG pulses were passed through optical filters and a monochromator (MC), detected by a photomultiplier tube (PMT), and normalized by the intensity of the IR pulses.
Ni(111) surface according to the literature prescription17 and had a well-defined (111) structure as judged by low-energy electron diffraction (LEED) image. The thickness of the oxide layer was 1-2 nm as estimated by Auger electron spectroscopy (AES).8,9 The SFG measurement was carried out under the continuous flow of 3 × 10-5 Pa of deuterated formic acid (DCOOD) to maintain constant coverage of the surface. Results and Discussion Compared in Figure 2 are the SFG spectra observed for the DCOO(a)-covered NiO(111) surface under the irradiation of 1064 nm picosecond laser pulses of 10 mJ/pulse energy and 5 mm diameter at the substrate temperatures of 325 and 400 K. The spectra observed by p-polarized visible pulses and ppolarized infrared (IR) pulses (denoted as (pp) polarization combination, the SF signal was p-polarized) and those observed by s-polarized visible and p-polarized IR pulses ((sp) polarization combination, the SF output was s-polarized) are shown in parts a and b of Figure 2, respectively, where the results obtained at 100 ps before the irradiation of the 1064 nm pump pulses (-100 ps delay time) and 0 and 100 ps after the moment of the irradiation are shown by open circles. Solid lines denote the deconvoluted components obtained by the simulation of observed profiles.16 The spectra observed without the irradiation of the pump pulses were identical to those at -100 ps delay time indicating that the induced change was restored during the 0.1 s interval of the pulse repetition. It was confirmed that the effect of the irradiation of the pump pulses was the transient temperature jump of about 300 K by the observation that the SFG band of the surface OD group at 2719 cm-1 shifted to low frequency by 7 cm-1 at the instant of the irradiation while
Letters
Figure 2. SFG spectra of DCOO(a)/NiO(111) under the irradiation of 1064 nm pump pulses of 10 mJ/pulse energy and 30 ps width. The oxide surface was kept under the continuous flow of DCOOD vapor of 3 × 10-5 Pa pressure, and the measurement was made at the substrate temperature of 325 and 400 K and at 100 ps before, right at, and 100 ps after the pump irradiation. The results obtained by using p-polarized visible and p-polarized IR pulses and those obtained by s-polarized visible and p-polarized IR pulses are shown in (a) and (b), respectively. As no SFG peak was observed at the CdO stretching band for the both polarization combinations, the CdO bond of the monodentate is postulated to be parallel to the surface while the signal-giving C-D bond to be oriented significantly away from the surface normal as depicted in Scheme 1.
the IRAS experiment has shown the peak to shift by 10 cm-1 on raising the substrate temperature by 400 K.18 The irradiationinduced spectral change was not observed when the substrate temperature was lower than 250 K and the amplitude of the intensity change started to decrease at 475 K. The vibrational peak at 2160 cm-1, which is persistent on all spectra and assigned to the C-D stretching band of bidentate formate,8,9 weakened, and a transient peak appeared at 2190 cm-1 instead by the irradiation of the 1064 nm pulses. Furthermore, the relative intensity of the peak observed for the (sp) with respect to that observed for the (pp) combinations is considerably larger than that for the 2190 cm-1 peak, testifying that the CD bonds associated with the 2160 and 2190 cm-1 peaks are oriented by different angles from the surface normal and thus the two peaks arose from different chemical species. The frequency of the transient 2190 cm-1 peak coincides with that of the C-D stretching band of monodentate formate,8 and we postulate that a part of the bidentate formate was converted to monodentate formate by the irradiation-induced temperature jump and that the amount of thus produced species was appreciable only when the temperature of the substrate was between 250 and 500 K. To examine the dynamical aspect of the phenomenon, we measured the temporal profiles of the irradiation-induced changes; the recovery of the irradiation-induced loss of the signal intensities at 2160 cm-1 and the dissipation of the transient signal at 2190 cm-1 were measured at the temperatures from 275 to 475 K. The results at 325 and 400 K for the (pp)
Letters
Figure 3. Temporal recovery profiles of the transient SFG signals obtained for the (pp) polarization combination. The irradiation-induced intensity decrease at 2160 cm-1 and the signal intensity at 2190 cm-1 were monitored at various delay times after the moment of irradiation of 1064 nm with 35 ps fwhm. The zero delay time between pump and probe pulses was set by monitoring the SFG signal from the reference sample of GaAs mounted to the same sample holder. The results obtained at the substrate temperature of 325 and 400 K are shown in (a) and (b), respectively. The longer rise time and recovery time at higher temperature are due to the temperature dependencies of heat capacity and thermal conductivity of the substrate.
polarization combination are shown in parts a and b of Figure 3, respectively. We note a reversibility in the sense that the intensity loss of the 2160 cm-1 peak and the intensity gain of the 2190 cm-1 peak took place in the same time scale indicating the presence of rapid equilibrium between the bidentate and monodentate formates. The presence of the equilibrium was also supported by the observation of the reversibility on the results measured at different temperatures and excellent fit of the temperature-dependent ratio of the monodentate formate with respect to the bidentate formate to the van’t Hoff equation. When the substrate temperature was higher than 400 K, the intensity loss did not fully recover at the delay time of 250 ps, the longest delay time of the experiment, and we ascribe this to the decomposition reaction during the high-temperature period. Present observations suggested that the one of the two NiO(formate) bonds of bidentate formate breaks prior to the decomposition to produce monodentate formate and the cleavage of the C-D bond of the monodentate formate must follow to complete the decomposition. The observation and the identification as reaction intermediate of the monodentate formate have thus been achieved by way of the picosecond temperature jump and SFG spectroscopy. Combining together the results of the frequency- and time-axis experiments, we conceive that the accumulation with significant amount of monodentate formate was realized for a brief duration of up to 100 ps when the temperature jump caused the equilibrium point to shift instantaneously toward the monodentate configuration while leaving the decomposition to go rather slowly because of higher activation energy. In other words, short duration of high substrate temperature kept most of the molecules from decomposing and left them in the configurations at instantaneous equilibrium. Depicted in Figure 4 is the energy curve drawn to explain this postulate; the temperature jump shifts the equilibrium from the condition that is dominated by the stable bidentate formate to the one that allows for the presence of significant amount of monodentate formate, and the decomposition of the monodentate formate goes over a
J. Phys. Chem. B, Vol. 102, No. 31, 1998 5953
Figure 4. Energy scheme postulated to explain the observation. Stable surface species (bidentate formate) is located about 19 kJ/mol below the monodentate formate, which is the intermediate for the decomposition that proceeds over the barrier of about 30 kJ/mol above it. A potential hump with modest height is pictured between the two types of formates to ensure the equilibrium. The reaction scheme consisting of two elementary steps is shown at the bottom.
potential hump getting significant only at a higher-temperature period. The equilibrium constants at each temperature were estimated from the relative amounts (derived from the deconvolution of SFG spectra) of the two species, and the use of van’t Hoff equation for the temperature dependence of equilibrium constants led to the estimated value of 19 ( 5 kJ/mol as the energy difference ∆H between the formates with bidentate and monodentate configurations. Further, an approximate value of about 30 ( 5 kJ/mol was obtained as the activation energy of the decomposition of monodentate formate from the integration of the temporal profiles measured in the 300∼500 K region over the time. To summarize, we have succeeded in identifying the reactive intermediate and new reaction step involved in the thermal decomposition of formate at solid surface by using a rapid laserinduced temperature jump. The identified monodentate formates are in equilibrium with the stable bidentate formate, and the activation energy for the decomposition reaction of the monodentate formate is considerably higher than the height of the potential barrier separating the two types of formate. The bidentate formate on the surface was found to transform, on heating the surface, to monodentate formate prior to decomposition. Acknowledgment. The authors gratefully acknowledge Dr. J. N. Kondo of our Research Institute for her helpful comments and discussions. A.B. is grateful to the support and fellowship by the Japan Society for the Promotion of Sciences. This work was supported by the Grants-in-Aid for Scientific Research from the Ministry of Education Science and Culture, Japan (Nos. 06239110 and 08404039). References and Notes (1) Duke, C. B., Ed. Surf. Sci. 1994, 299/300. (2) Yates, J. T., Jr., Ed. Chem. ReV. 1996, 96. (3) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; (John Wiley & Sons Inc.: New York, 1994. (4) King, D. A., Woodruff, D. P., Eds., The Chemical Physics of Solid Surfaces and Heterogeneous catalysis; Elsevier: Amsterdam, 1981-88; Vol. 1-5. (5) Tamaru, K., Ed. Dynamic Processes on Solid Surfaces; (Plenum Publishing Corp.: New York, 1993.
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Letters (14) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. (15) Klu¨nker, C.; Balden, M.; Lehwald, S.; Daum, W. Surf. Sci. 1996, 360, 104. (16) Hirose, C. AdV. in multiphoton processes and spectroscopy; Lin, S. H., Villayes, A. A., Fujimura, Y., Eds.; World Scientific: Singapore, 1995; Vol. 9, pp 143-197. (17) Rohr, F.; Wirth, K.; Libuda, J.; Cappus, D.; Ba¨umer, M.; Freund, H.-J. Surf. Sci. 1994, 315, L977. (18) Matsumoto, T.; Bandara, A.; Kubota, J.; Domen, K.; Hirose, C., in preparation.
