SFG study of unstable surface species by picosecond pump–probe method

doi:10.1016/S0039-6028(99)00301-5

Surface Science

Volumes 427–428, 1 June 1999, Pages 349-357

https://doi.org/10.1016/S0039-6028(99)00301-5 Get rights and content

Abstract

We succeeded in identifying the intermediates in thermal decomposition reactions by utilizing the combination of an instantaneous temperature jump induced by the irradiation of picosecond laser pulses and the subsequent observation by time-resolved sum-frequency generation (TR-SFG) spectroscopy. The short-lived reactive intermediates in the decomposition of formate on NiO(111) and Ni(111) surfaces were identified. The irradiation of 1064 nm laser pulses caused the vibrational peak of the CD stretching mode ν_CD of bidentate formate on NiO(111) to weaken and the ν_CD band of monodentate formate to appear. The result on Ni(111) showed the weakening of the ν_CD band of bridging formate and the appearance of the CO stretching mode ν_CO of monodentate formate. The spectral changes recovered on a 100 ps time scale but not fully above 400 K for the NiO(111) system and 320 K for the Ni(111) system, indicating the onset of thermal decomposition in the high-temperature period. The observations suggested that the formate in the stable bidentate/bridging configurations transformed to unstable monodentate formate prior to decomposition. Temperature- and time-dependent features indicated that the two types of formate were in equilibrium and the equilibrium shifted towards the monodentate form by the rapid laser-induced temperature jump of about 250–300 K.

Introduction

The verification of reactive intermediates is indispensable for the understanding of reaction mechanisms. However, the identification of short-lived and unstable reactive intermediates of surface reactions is difficult by means of conventional surface vibrational spectroscopic techniques such as high-resolution electron energy-loss spectroscopy (HREELS) and infrared reflection–absorption spectroscopy (IRAS) [1], [2], [3], [4], [5]. The pulsed IRAS method developed by the NIST group [6] and sum-frequency generation (SFG) spectroscopy [7] are capable of providing such a technique. We recently demonstrated that the combination of an instantaneous temperature jump induced by a picosecond laser pulse and subsequent observation by time-resolved sum-frequency generation (TR-SFG) spectroscopy makes possible to achieve the task [8], [9].

Photo-induced processes at surfaces have been investigated predominantly by way of state-resolved observation of desorbing molecules [10]. The investigation has shown that many of the processes induced by visible and ultraviolet (UV) photons proceed through hot-electron-mediated processes. Information on the processes promoted by the temperature jump at surfaces has also been obtained from the desorbing species [10], [11], [12], [13]. However, information about the species that are produced on the surfaces during the reaction cannot be obtained from desorbed molecules and observation of the surface itself is required. In photo-induced thermal processes in metal or semiconductor substrates, for example, absorbed optical energy is converted into thermal energy within picoseconds and surface temperatures of several hundred degrees can be attained within a short period of time. The outcome of the reactions under such conditions can be different from that of reactions under conventional experimental conditions used in experiments such as temperature-programmed desorption (TPD) [11], [12], [13]: the period of high surface-temperature jump lasts only for a hundred picoseconds or so due to the rapid diffusion of heat and some reaction steps which normally confine us to the detection of only the final products do not proceed appreciably.

The decomposition of formic acid on single-crystal metal and metal oxide surfaces has been investigated for the past three decades [14], [15], [16], [17], [18]. TPD experiments have revealed the decomposition pathways and surface vibrational spectroscopic techniques such as HREELS and IRAS have identified stable surface species [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. The occurrence of formate species either after the adsorption of formic acid at room temperature or on annealing a formic-acid-covered surface has been shown. In the thermal decomposition of formate on metal surfaces, dehydrogenation (HCOOH→CO₂+H₂) is the only reaction that takes place on copper [16], [28] and platinum [29], while both dehydrogenation and dehydration (HCOOH→CO+H₂O) reactions occur on nickel [14], [15], [22], [27], [30], [31], ruthenium [25], [26] and iron [32] surfaces. The selectivity is dependent on reaction conditions; e.g., only the dehydrogenation reaction takes place in the decomposition of formic acid on nickel surfaces [33] under the steady-state condition. In the decomposition of formate on the NiO(111) surface, where formate is anchored to a nickel cation site by two formate oxygen atoms in a bidentate configuration [34], [35], [36], dehydrogenation and dehydration reactions occur on heating the surface to above ∼350–400 K. No recombinative desorption of formate [HCOO(a)+H(a)→HCOOH(g)] was observed on Ni(111) and NiO(111) [35]. Our IRAS investigation under a heavy flow of formic acid at 10⁻³ Pa [34] has identified a new species, which was assigned to formate with monodentate configuration, to appear at the onset of the decomposition reaction. However, the IRAS investigation failed to verify the status of the new species – i.e., whether the new species was in equilibrium with the stable bidentate formate or independent from bidentate on the isolated sites. In any event, both the dehydrogenation and dehydration reactions of bidentate/bridging formate require the cleavage of CH bonds and thus we have questioned whether they proceed by a one-step or another mechanism.

In this paper, we describe the new insights obtained by applying the combined use of TR-SFG spectroscopy and the temperature-jump technique to the investigation of the thermal decomposition of formic acid (deuterated DCOOD) on NiO(111) and Ni(111) surfaces. The transient SFG signals generated by the CD and CO stretching modes of monodentate formate were observed at the instant of the laser-induced temperature jump to reveal that the stable bidentate/bridging formates are transformed to unstable monodentate formate prior to decomposition. The two species were in equilibrium. The similarities and differences between the reactions on the metal and oxide surfaces are discussed.

Section snippets

Experimental

The experiments were performed in an ultrahigh vacuum (UHV) chamber consisting of two sections. One section was for the cleaning and characterization of the sample, and was equipped with an argon-ion bombardment gun, low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) devices. The other section, with CaF₂ windows for the introduction and extraction of light beams, was equipped with a quadrupole mass spectrometer (QMS) for TPD measurements. A Ni(111) substrate was

Results and discussion

The laser-induced temperature jump was estimated to be about 300 K by the observation that the SFG band of the surface OD group at 2719 cm⁻¹ shifted by 7 cm⁻¹ to lower frequencies at the instant of irradiation, the IRAS experiment having shown the peak to shift by 10 cm⁻¹ on raising the substrate temperature by 400 K [8].

The value of the irradiation-induced jump of the surface temperature was also rationalized by calculation using the one-dimensional heat diffusion equation [37], [38] $C ∂T(z, t) ∂t =K ∂^{2}$

Summary

The combination of a laser-induced temperature jump and subsequent observation by time-resolved SFG spectroscopy enabled us to verify the decomposition route of formate on the NiO(111) and Ni(111) surfaces. The transformation of the bidentate/bridging formate to unstable monodentate formate occurred at the instant of irradiation and the feature was ascribed to the shift of the chemical equilibrium caused by the rapid laser-induced temperature jump at the surfaces. On the NiO(111) surface, the

Acknowledgments

One of the authors (A.B.) gratefully acknowledges the Japan Society for the Promotion of Science for awarding a Research Fellowship. This work was supported by Grant-in-Aids for Scientific Research from the Ministry of Education Science and Culture, Japan (Nos. 06239110 and 08404039).

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SFG study of unstable surface species by picosecond pump–probe method

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Summary

Acknowledgments

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