Adsorption and reaction of methanol on Cu(977) and Pd/Cu(977) surfaces

抄録

Introduction

Methanol is a valuable feedstock for various chemical products, including formaldehyde, formic acid, and acetic acid ^[1]. Copper is an essential metal in catalytic reactions involving methanol synthesis, methanol dehydrogenation, water gas shift reactions etc. Among these reactions, the dehydrogenation of methanol where formaldehyde is formed, is particularly important.

Methoxy species is an important intermediate for conversion of methanol to formaldehyde. However, the dissociation does not take place on the clean Cu flat surfaces [2]. In this study, we conducted the following experiments using the modified Cu surfaces to investigate the effects of steps and additives.

The first model catalyst is the Cu(977) step surface. It is known that the step sites exhibit different catalytic activity from those of the terrace sites [3]. The second one is the Pd deposited Cu(997) surface. Since Pd is known to have a strong interaction with hydrogen, we expect that a small amount of deposited Pd atoms on a Cu surface would facilitate the dehydrogenation of methanol with the advantage of retaining the catalytic properties of Cu metal [4].

In this work, we have investigated the adsorption, dissociation and desorption of methanol and its reaction products on the Cu(977) and Pd/Cu(977) surfaces by using TPD and infrared reflection absorption spectroscopy (IRAS).

Experimental

All experiments were performed in an ultra-high vacuum (UHV) chamber. The Cu(977) clean surface was prepared by repeated cycles of Ar+ sputtering and annealing at 645 K. The Pd/Cu(977) surface was prepared by depositing Pd atoms on the Cu(977) surface at 380 K in UHV. The prepared surfaces were cooled to 85 K with liquid nitrogen, and then gaseous methanol was introduced on the surfaces. We have conducted TPD and IRAS measurements on these surfaces.

Results and Discussion

From the TPD spectra of methanol adsorbed Cu(977) surfaces, three desorption maxima by methanol were observed. Based on the comparison with the previous study [5], the peaks at 141 K, 153 K, and 195 K can be attributed to the methanol desorption from multilayer, terrace and step, respectively.

On the clean Cu(977) surface, a desorption peak for m/e=30 was little observed, indicating most of methanol were molecularly desorbed without any reaction. On the other hand, on the Pd/Cu(977) surface, a significant desorption peak was observed at 350 K. This peak can be assigned to the desorption of formaldehyde formed via methoxy decomposition [3].

A part of methanol molecules adsorbed on Pd/Cu(977) led to methoxy species formation (Equation (1)), followed by decomposition-limited desorption as formaldehyde and hydrogen above 300 K (Equation (3)).

CH₃OH(ad) → CH₃O(ad) + H (ad) (1)

H(ad) → 1/2 H₂(g) (2)

CH₃O(ad) → CH₂O(g) + 1/2 H₂(g) (3)

The present experimental results indicate that the presence of Pd on Cu(977) lowers the activation barrier of methanol decomposition.

Figure shows the TPD spectra of methanol adsorbed Cu(977) and Pd/Cu(977) for m/e=2. The 250 K peak is due to hydrogen desorption by the dissociation of methanol into methoxy and hydrogen (equation(2)), while the peak at 350 K is associated with methoxy decomposition (equation(3)); these peak intensities were nearly equal. From these results, we conclude that the methoxy species selectively decomposes into formaldehyde and hydrogen with almost 100% efficiency.

In the presentation, the experimental results by IRAS measurements will be discussed including the intermediate species of methanol dissociation as a function of temperature.

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