The Role of Electronegative and Electropositive Modifiers in the Adsorption and Decomposition of Acetaldehyde on Rh(111) Surface ^†

Abstract

Rhodium is an effective catalyst in the CO+H₂ reaction into C₂-oxygenates. Among the products, acetaldehyde (AA) is an important hydrogen carrier, which is also produced in the decomposition of ethanol on metal surfaces. The chemisorbed acetaldehyde starts to decompose at 200 K. The main products of the chemisorbed AA are CO, H₂ and CH₄. In the chemisorbed layer η₁-(O)-CH₃CHO_a and η₂-(O,C)-CH₃CHO_a adsorption forms have been identified. Electronegative and electro positive modifiers (oxygen and potassium) suppress the oligomerization of aldehyde, they may influence the stability of these surface complexes such as: (i) surface C_a acts as a simple contaminant by site blocking mechanism. (ii) A direct surface reaction with O_a led to the formation of acetate. (iii) K_a increases the thermal stability of acetaldehyde and the decomposition products by an extended electronic interaction.

1. Introduction

Aldehydes (AA) are gas-phase products of the decomposition of primary alcohols on metal surfaces [1,2]. Acetaldehyde has an environmental risk because of its carcinogen effect [3]. On the other hand, AA as an intermediate or end material can be a good candidate for the production of more valuable compound from COx hydrogenation [4]. In the ethanol steam reforming reaction, the aldehyde an important intermediate releasing H₂ for fuel cell applications. [5]. Transformation of adsorbed AA into paraldehyde (CH₃CHO)₃ enables us to use this couple of compounds in heat pumps [6]. In the present study, the effect of different additive like electronegative oxygen and electropositive potassium is investigated on the adsorption and decomposition of acetaldehyde by AES, TDS, high resolution electron energy loss spectroscopy (HREELS) and work function (ΔΦ) methods.

2. Materials and Methods

For our experiments, the Rh single crystal from Materials Research Corporation (purity 99.99%) was purchased with (111) orientation (the accuracy of the orientation was within 1⁰). After inserting into the UHV chamber, it was sputtered with Ar+ ions (Typically 1 kV, 1 × 10⁻⁷ mbar and 2 mA for 10–30 min), and finally it was heated to 900–1100 K and this was repeated until no contamination was detected by Auger electron spectroscopy, AES. Two UHV chambers were used. The crystal was cooled to 100 K and 150 K for TDS and HREELS measurements, respectively. Acetaldehyde (CH₃CHO) of 99% purity was purchased from Sigma-Aldrich and it was purified by several freeze–pump–thaw cycles and for adsorption it was dosed through a capillary, which was placed 1.5 cm from the sample.

Oxygen (from Messer-Griesheim, 99.99% purity) was used for cleaning after each desorption through a second capillary. A potassium source (produced by SAES company) was used—after several minutes degassing—for K atom evaporation. Carbon deposition was obtained by high temperature (1000 K) decomposition of ethylene. The carbon, oxygen and potassium coverages were determined by combined AES and TDS measurements.

The first UHV system for mainly TDS experiment with a base pressure of 5 × 10⁻¹⁰ mbar, was described previously [7,8,9]. The UHV chamber equipped with AES, ELS with work function measurements and mass spectrometer facilities for TDS. A Vacuum Generators (VG) mass spectrometer was placed ca. 1 cm from the single crystal. For TDS, a 5 K/s heating rate was chosen. The electron energy loss spectra (ELS in the electronic range) were taken by means of a cylindrical mirror analyzer (Physical Electronics Instruments, PHI). The low energy cut off in the ELS was used for ΔΦ measurements. The second UHV system was equipped with AES and electron energy analyzer (LKH product) for measuring the vibrational state of adsorbed molecules.

3. Results

3.1. Reaction on Clean and Carbon Contaminated Rh(111)

The decomposition of AA on clean rhodium has been studied in detail previously by AES, EELS, TDS and work function methods [10]. Aldehyde adsorbs in two forms; the η₁-(O)-CH₃CHO_a form produces oligomers, while the η₂-(O,C)-CH₃CHO_a decomposes to different products. The η₂-(O,C)-CH₃CHO_a bonds with oxygen and carbon to the surface. It started to decompose at 200 K, producing mainly adsorbed CO, hydrogen and CH₄, which desorbed at Tp = 260 K. Minor product was surface C, too. A weakly adsorbed η₁-(O)-CH₃CHO_a (bonds with oxygen) desorbed in a sharp peak centered at 150 K, its oligomerized fraction was also found ([CH₃CHO]_n⁺, 1 < n < 5) in the gas phase with Tp = 225–235 K. The low temperature state was a monomer, while we observed oligomers, namely paraldehyde. On the carbon contaminated surface our HREELS measurements also confirmed the presence of η₁-(O)-CH₃CHO_a and η₂-(O,C)-CH₃CHO_a in the chemisorbed layer [10]. Carbon contamination has a site blocking effect on the adsorption of AA from the gas phase. The amount of the decomposition products and the oligomers decreased rapidly with a small shift in Tp values. This effect is less pronounced in the case of CO desorption and at the same time hydrogen desorption suggests the presence of some CxHy moieties.

3.2. Reaction with Oxygen Precovered Rh(111)

The preadsorbed oxygen changed the product distribution, and new products, like water, CO₂ and acetic acid (CH₃COOH) could be detected in TDS (Figure 1a). Water desorbed in a sharp peak, with Tp = 416 K. Two minor peaks centered at 256 K and 306 K also appeared. By recording the CO₂ desorption at amu 44 one can see the evaluation of acetaldehyde as well, since it has a contribution at that mass number too. The higher temperature peaks, with Tp = 412 K and 480 K can be considered as CO₂ (red line) and the peaks at Tp = 175 K and 242 K as CH₃CHO (green line). The desorption at mass 60 is connected to CH₃COOH with peaks at Tp = 330 K and 420 K. From the HREEL spectra we may conclude that the two adsorption forms of aldehyde and its oligomerized species do not exist above 170 K (no bands at 940 and 2980 cm⁻¹ due to p(CH₃) and ν_s(CH₃), respectively, for aldehyde). We observed the formation of acetate as an intermediate at 1420 (δ(CH₃), 1660 ν_a(OCO) and ~3000 cm⁻¹ (ν_s(CH₃)) [11] (Figure 1b). In addition, an increase in the thermal stability of acetate (up to T = 325 K) was also observed with increasing oxygen coverage (not shown). The appearance of the loss at 2050 cm⁻¹ is attributed to the adsorbed CO derived from the decomposition of acetate.

3.3. The Effect of Potassium on the Adsorption of AA on Rh(111)

The preadsorbed K atoms dramatically changed the stability of adsorbed molecules and its desorption products. Figure 2a shows the products from clean Rh(111) (blue line) compared to their desorption from the monolayer K covered surface. The hydrogen desorption from clean surface presented at Tp = 292 K was transferred to 395 K and 596 K. Methane desorption suffered only a minor shift, from 260 K to 300 K. CO desorption from clean surface occurs at 483 K, which was pulled to 596 K and 645 K. The K desorption curves represent its desorption from bare Rh(111) (blue line) and from the co-adsorbed layer (red line). The broad features at Tp = 368 K and 546 K were shifted to 510 K, 596 K and 645 K. Here, we have to emphasize the observed TPD data that at 596 K H₂, CO and K desorb together, suggesting one common surface complex (potassium compounds), which is responsible for these desorption. Due to the increased electron concentration from potassium to the adsorb species, the amount of adsorbed material increased and a certain surface complex formed between potassium and the decomposition (products) intermediates of acetaldehyde. The stabilization of CO (as a decomposition product) by potassium is also evident from TPD. TPD peaks are detected at 596 and 645 K (Figure 2a). The interaction between CO and K is also proved by HREELS. This is indicated by the significant shift and stability of the ν(C-O) valence vibration (1680–1600 cm⁻¹) on the HREEL spectra due to the interaction with potassium (Figure 2b). The peaks at 1810–2100 cm⁻¹, which are characteristic of the CO adsorption on the pure rhodium surface depending on the adsorption center [12], were significantly shifted in that case together with the desorption temperatures of the decomposition products. The electronic interaction between K and CO and the stabilization effect of K on the adsorbed CO was separately investigated previously [13,14].

4. Discussion

4.1. The Reaction with Preadsorbed O Compared to Bare and Carbon Contaminated Layer

The product distribution of acetaldehyde decomposition reflects to the fact that partially and fully oxidized processes took place on the surface. Some of the original AA molecules must have decomposed. Not only C-H bonds were broken, giving H_a atoms on the surface but C-C bond ruptured as well. The latter makes CO₂ desorption possible. The fact that its peak temperature is the highest among the products supports the oxidation of surface carbon. The acetic acid desorption nearly identical with the one after direct CH₃COOH adsorption (not shown).

4.2. The effect of K Atoms on the Reaction Routes

Surface potassium has a very strong stabilization effect on the surface layer. The different products appeared at significantly higher temperatures, these shifts were as large as 40–300 K. The decomposition mechanisms must have changed, too. The coincidence Tp values suggest the decomposition of a common state. To the interpretation of this finding, we may refer to the similarities with our previous studies [8,9]. The K atoms donate electrons to the underlying surface and to the adsorbed molecules. The electron distribution in the molecule might have changed, and therefore it will influence the stability of η₁-(O)-CH₃CHO_a and η₂-(O,C)-CH₃CHO_a forms.

5. Conclusions

The effect of preabsorbed carbon, oxygen and potassium was investigated on the adsorption and decomposition of acetaldehyde, AA, on Rh(111) surface by means of TDS and HREELS. The electronegative and electropositive additives suppressed the oligomerization of AA. The preabsorbed oxygen opened a new reaction path in the transformation of aldehyde; the oxidation products were acetate, CO₂ and water. Preabsorbed potassium markedly stabilized the reaction products due to electron donation to the adsorbed molecules. The observations presented in this study supplemented with DFT calculations may contribute to understanding the effect of additives and the mechanism of aldehyde transformation in real catalytic circumstances.