Effect of Aldehyde and Carboxyl Functionalities on the Surface

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The Effect of Aldehyde and Carboxyl Functionalities on the Surface Chemistry of Biomass-derived Molecules Basar Caglar, J. W. (Hans) Niemantsverdriet, and C.J. (Kees-Jan) Weststrate Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02215 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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The Effect of Aldehyde and Carboxyl Functionalities on the Surface Chemistry of Biomass-derived Molecules Basar Caglar†,‡,*, J.W. (Hans) Niemantsverdriet‡,ӝ, C.J. (Kees-Jan) Weststrate‡, ӝ †

‡

Department of Energy Systems Engineering, Yasar University, 35100, Izmir, Turkey.

Laboratory for Physical Chemistry of Surfaces, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands. ӝ

Syncat@DIFFER, Syngaschem BV, De Zaale 20, 5612 AJ, Eindhoven, The Netherlands.

KEYWORDS: biomass conversion, C=O and COOH functionalities, coverage effect, C-O bond scission, RAIRS, TPRS.

The adsorption and decomposition of acetaldehyde and acetic acid were studied on Rh(100) to gain insight into the interaction of aldehyde and carboxyl groups of biomass-derived molecules with the surface. Temperature programmed reaction spectroscopy (TPRS) was used to monitor gaseous reaction products, whereas Reflection absorption infrared spectroscopy (RAIRS) was used to determine the nature of surface intermediates and reaction paths. The role of adsorbate interactions in oxygenate decomposition chemistry by varying the surface coverage. Acetaldehyde adsorbs in an η2 (C,O)-configuration for all coverages, where the carbonyl group

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binds to the surface via the C and O atoms. Decomposition occurs below room temperature (180280 K) via C-H and C-C bond breaking which releases CO, H and CHx species on the surface. At low coverage CHx dehydrogenation dominates and surface carbon is produced alongside H2 and CO. Instead, at high coverage about 60% of the CHx hydrogenates to form methane, whereas only 40% of the CHx decomposes further to surface carbon. Acetic acid adsorbs dissociatively on the Rh(100) surface via O-H bond scission, forming a mixture of mono- and bidentate acetate. Decomposition of acetate proceeds via two different pathways: (i) deoxygenation, via C-O and C-C bond scissions, and (ii) decarboxylation via C-C bond scission. At low coverage the decarboxylation pathway dominates, a process that occurs slightly above room temperature (280360 K) and produces CO2 and CHx, where the latter decomposes further to surface carbon and H2. At high coverage both decarboxylation and decarbonylation occur, slightly, above room temperature (280-360 K). The resulting O ad-atoms produced in the decarbonylation path react with surface hydrogen or CO to form water and CO2, respectively. The CHx species dehyrogenate to surface carbon for all coverages. Our findings suggest that oxygenates with C=O functionality and alkyl end reacts on the Rh(100) surface to produce synthesis gas and small hydrocarbons whereas CO2 and synthesis gas are produced when oxygenates with COOH functionality and alkyl end reacts with the Rh(100) surface. For both cases carbon accumulation occurs on the surface.

Introduction Biomass is an alternative feedstock, which can be used to replace petroleum for fuel and fine chemicals production.1–4 In order to understand the way how to convert biomass-derived


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molecules into desired chemical products, we study the surface chemistry of molecules with functional groups (e.g. -OH, C=O, -COOH) that are characteristic for biomass. Our previous studies report on the decomposition of ethylene glycol and glycol aldehyde on Rh(100), where they serve as model compounds for carbohydrates as they contain alcohol and aldehyde functionalities and each carbon atom is bound to an oxygen atom.5,6 The decomposition chemistry of ethylene glycol and glycol aldehyde on transition metals is compared with that of reference compounds with similar functionalities for better understanding of bond activation of these molecules. The present study is focused on the interaction of acetaldehyde and acetic acid with the Rh(100) surface, to investigate the effect of aldehyde (C=O) and carboxyl (COOH) functionalities on the bond breaking sequence and product selectivity. Aldehydes are key intermediates for oxygenate synthesis from CO and H2,7 which can be easily obtained from biomass-derived molecules. The coordination of acetaldehyde on transition metal surfaces and its decomposition pathways have been studied previously surfaces of Ru,8 Pd,9–11 Ni,12 and Rh13. In general, aldehydes can adsorb on metal surfaces via two different bonding configurations: (i) the η1(O)-configuration (Figure 1a), where acetaldehyde binds to the surface through the lone pair of the oxygen, and (ii) the η2(C,O)-configuration (Figure 1b), where acetaldehyde binds to the surface through the carbonyl π-orbital with overlap between the delectrons of metal and the carbonyl π*-orbital.7,8,10,13 On Ru (0001),8 acetaldehyde polymerizes in two dimensions as a consequence of the reaction of neighboring adsorbed η1(O)-acetaldehyde molecules and decomposes to η2 (C,O)-acetaldehyde at 250 K. On Rh(111),13 Pd(111)10 and Pd(110),9 acetaldehyde binds to the surface in the η1(O)- configuration, which desorbs at 150200 K and the rest is converted to η2(C,O)-acetaldehyde. Decarbonylation of the η2(C,O)acetaldehyde produces methyl groups on Pd(110) and Rh(111), whereas methylene forms


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following decarbonylation of the η2(C,O)-acetaldehyde on Pd(111). Decarbonylation yields CO, CH4, H2 and surface carbon. No methane formation is observed on Ru(0001). The surface chemistry of carboxylic acids is of interest as they are important intermediates for catalytic oxidation reactions, especially for vinyl acetate production used in the manufacture of adhesives, paints and coatings.14,15 The decomposition of acetic acid chemistry has been studied on Pd,14–17 Rh,18–20 Cu,21 Pt,22 Ru,23 Ni24 and Al25 surfaces. Acetic acid dissociates into acetate and hydrogen below 200 K for all surfaces except for Cu(110), on which acetate is observed at around 240 K. Acetate typically adsorbs in the bidentate form (Figure 1d), while monodentate (Figure 1c) formation is also identified on Rh(111)18–20 and Pd(111).14,16,26 Decomposition of acetate proceeds via decarboxylation on Rh(110)19 and Pd(110)15 to produce CO2, H2 and carbon at 350-450 K. C-O bond scission occurs in addition to decarboxylation on Rh(111) and Pd(111) to yield CO, CO2, H2, oxygen and carbon at 200-300 K. The same chemistry is found on Ni(111)24 with a higher decomposition temperature of acetate (450 K). On Pt(111)22 and Ru(0001),23 the decomposition of acetate results in CO, H2, oxygen and carbon ad-atoms at 200300 K. Coverage has a pronounced effect on the stability of acetate on Ru(0001). At high coverage, even at 500 K, there are still some acetate molecules present on the surface, like the Cu(110) and Al(111) surfaces on which acetate is rather stable. On Al(111),25 the complete decomposition of acetate yields H2, carbon and oxygen at 500 K, whereas on Cu(110),21 adsorbed acetate and acetic anhydride (surface intermediate) decompose at 590 K to yield CO2, CH4, ketene (H2C=C=O) and other carbon containing products.


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(a) H H

H

C

(c)

(b) H

H

H

C O η1(O)-acetaldehyde

C

H H H C O

η2(C,O)-acetaldehyde

H

H

C

(d) H

C O

H

C C

O η1(O)-acetate (monodentate)

H

O

O

η2(O,O)-acetate (bidentate)

Figure 1. Bonding configurations of a) η1(O)-acetaldehyde b) η2(C,O)-acetaldehyde c) monodentate acetate d) bidentate acetate on metal surface. The chemistry of acetaldehyde and acetic acid decomposition has not been studied to date on Rh(100). The purpose of this study is to explore the surface chemistry of acetaldehyde and acetic acid on Rh(100), which provides insight about the interaction of aldehyde and carboxyl functionalities with this surface. The decomposition was investigated for (i) low coverage - to exclude lateral interactions between adsorbates, and for (ii) high coverage - to understand the effect of coverage on the decomposition. For this purpose, we used two surface science techniques: Temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS). TPRS is used to monitor reaction products and RAIRS is used to determine reaction intermediates and decomposition pathways.

Experimental Methods All experiments were performed in a home-built, two-stage stainless steel ultrahigh vacuum (UHV) system with a base pressure of 3x10-10 mbar. The first stage is equipped with a quadrupole mass spectrometer (Prisma QME 200, Balzers) for TPRS and a Kelvin probe (UHV KP 4.5) for work function measurements. The first stage also contains a dual anode (Al/Mg) X-


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ray source and a hemispherical electron energy analyzer for XPS measurements. In addition to this, LEED optics is available. The second stage contains a Fourier transform infrared spectrometer (Nicolet iS10 FT-IR spectrometer, ThermoScientific) with a mercury cadmium telluride (MCT) detector flushed with dry nitrogen for RAIRS. The Rh(100) crystal was used as a catalyst surface for the study of surface reaction of acetaldehyde and acetic acid. The Rh(100) crystal used in the present study is the same crystal that was used in several other studies reported previously. In these previous studies,27,28 LEED data was reported which shows a Rh(100) surface of good quality. The Rh(100) crystal was mounted on a movable sample rod with two 0.3 mm tantalum wires allowing for resistive heating to 1400 K. The crystal was continuously cooled with liquid nitrogen, which enabled cooling down to 80 K. The temperature was monitored using a chromel–alumel thermocouple, spotwelded to the backside of the Rh crystal. The crystal was initially cleaned by cycles of argon ion sputtering (5 kV, 4 µA/cm-2) at 900 K and annealing in ~10-7 mbar of high purity synthetic air (20% O2/Ar) at temperatures of 900 to 1100 K and a final flash to 1400 K in a vacuum. Carbon diffusing to the surface at temperatures above 900 K during flashing was removed by adsorption of oxygen at 150 K followed by flash heating to 800 K. Finally, CO was dosed at 550 K and flashed to 800 K in a vacuum to remove the remaining oxygen. Extensive cleaning cycles on this surface ensure a low contaminant level, below the detection limit of the experimental techniques used, i.e. CO and H2 TPD, which have been shown to be sensitive to small quantities of carbon and oxygen.29 Acetaldehyde (Sigma-Aldrich ≥99.5%) and acetic acid (Sigma-Aldrich ≥99.7%) were purified by repeated freezing, pumping, and thawing cycles to remove high vapor pressure contaminants. Sample purity was confirmed by the mass spectrometer prior to experiments.


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TPRS experiments were performed to follow the acetaldehyde and acetic acid decomposition chemistry and determine surface coverages of those molecules. Coverages are reported in ML, that is, relative to the number of Rh surface atoms. The TPRS study also allowed quantifying the gaseous reaction products. In order to determine CO and H2 coverages, the TPD area for the saturation coverage of CO and H2 was determined and linked to the absolute coverage, which was obtained from LEED studies published earlier.30–34 Where the coverage of CO was known, the quantities of CH4, H2O, CH3CHO and CH3COOH were determined from their TPD peak areas after correcting for the mass spectrometer sensitivities for each molecule. The mass spectrometer sensitivities for each gas were determined by leaking a gas sample into the UHV chamber at a measured pressure rise of 1x10-8 mbar and collecting the MS signal intensity for the appropriate masses. Also, the relative ionization efficiencies of the pressure gauge were used to correct the pressure reading for each gas. Different from the other products, the amount of CO2 formed was determined from CO oxidation experiments, explained in detail elsewhere.35 For a typical TPRS experiment acetaldehyde or acetic acid was adsorbed on the clean surface at 100 K. The acetaldehyde or acetic acid covered surface was heated up to 700 K with a rate of 5 K/s. The desorption products were then monitored using a mass spectrometer, with a typical sampling rate of 1 s per data point. Each product was identified by its specific m/z ratio (28 for CO, 2 for H2, 16 for CH4, 18 for H2O, 29 for acetaldehyde, 43 for acetic acid and 44 for CO2). After the TPRS experiment, the amount of surface carbon was determined as follows: after cooling to 150 K, 10 L (saturation) of O2 was dosed to the surface, after which the crystal was heated to 800 K (10 K/s). The amount of CO and CO2 produced in this second heating step was then used to quantify the carbon deposits that remained after the first heating step.35


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RAIRS spectra were obtained using a Fourier-transform infrared spectrometer (Nicolet is10 FT-IR spectrometer, Thermoscientific), equipped with a KRS-5 wire grid polarizer allowing only the p-polarized component of the light to be detected. The infrared beam undergoes a single reflection from the crystal surface near the grazing angle (85°). A mercury cadmium telluride (MCT) detector was used with a spectral range of 4000–650 cm-1. The RAIRS spectra reported here consist of 512 scans taken at 4 cm-1 spectral resolution and subtracted by a stored background spectrum of a clean surface. After reaching the desired temperature the temperature program was halted and the sample temperature was kept constant for 5 minutes, the time required to record 512 scans.

Results The Adsorption of Acetaldehyde at 100 K The speciation and molecular configuration of acetaldehyde upon adsorption at low temperature were studied by RAIRS. Figure 2 shows the infrared spectra obtained after dosing various amounts of acetaldehyde at 100 K. TPRS was used to determine the acetaldehyde coverage. A dose of 0.08 and 0.32 L of acetaldehyde correspond to 0.07 ML (ca. 1/3 saturation coverage) and 0.25 ML (saturation of the chemisorbed layer), respectively, while 2.1 L of acetaldehyde produces multilayers of acetaldehyde. The infrared spectra show C-H stretching bands (3000-2800 cm-1), C-O stretching bands (1750-1400 cm-1), C-H deformation bands (14501300 cm-1), C-C stretching bands (1150-1050 cm-1) and C-H rocking bands (950-850 cm-1). Detailed vibrational assignments of acetaldehyde on Rh(100) are given in Table 1 in comparison to those of gas phase acetaldehyde and acetaldehyde adsorbed on Rh(111).


8

2998

1127 1113 1095

2821

2889

1423

1719

1681 1662

0.32 (sat) 2891

ρ(CH3)

ν(CC)

δs(CH3)

η2−ν(C-O)

2.1

νs(CH3) ν(CH)

Exposure (L)

η1−ν(C=O)

CH3CHO on Rh(100), Tads= 100 K νas(CH3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Absorbance ∆R/R (%)

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1346

936

924

1073 917

1416

908

1341

0.08

0.1%

1069

1341

3500 3250 3000 1750

1500

1064

1250 -1

909

1000

wavenumber (cm ) Figure 2. RAIRS spectra after various doses of acetaldehyde on Rh(100) at 100 K.

At low coverage (0.07 ML, Figure 2-bottom spectrum), the spectrum indicates the presence of acetaldehyde, binding to the surface via both oxygen and carbon atoms of the carbonyl bond (η2(C,O)-configuration). Three bands are resolved, at 1341, 1064 and 909 cm-1, which are assigned to the symmetric CH3 deformation (δs(CH3)), C-C stretching (v(C-C)) and CH3 rocking (ρr(CH3)) modes of acetaldehyde, respectively. No C-O stretching bands of acetaldehyde are observed. A C-O stretching band at around 1700 cm-1 is characteristic for η1(O)-acetaldehyde and the lack of this band suggests that acetaldehyde binds to the surface in a η2(C,O)configuration. In studies of acetaldehyde on Ru(0001)8 and Rh(111),13 the η2(C,O)-configuration is identified by bands at 1380 and 1450 cm-1, respectively. These bands are not observed in our spectrum, suggesting that C-O bond axis is aligned parallel to the surface.


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Table 1. Vibrational assignments of acetaldehyde adsorbed on Rh(100). Assignments for gas phase of acetaldehyde and acetaldehyde on Rh(111) are included for comparison. All frequencies are in cm-1. Mode

Vibrational Assignments (cm-1) Gas Phasea

Rh(100)

Rh(100)

low coverageb

saturation coverageb

Multilayerb

13

Rh(111)

Rh(100)

vas(CH3)

3005

3010

nr

nr

2998

νs(CH3)

2917

nr

nr

2891

2889

v(C-H)

2822

nr

nr

nr

2821

v(C-O)-η1

1743

1745

nr

nr

1719

1460

nr

1416

1423

v(C-O)-η2 δas(CH3)

1441, 1420

1380

nr

1416

1423

γ(C-H)

1400

nr

nr

nr

1388

δs(CH3)

1352

nr

1341

1341

1346

v(C-C)

1113

1135

1064

1069

1113,1095,1073

950

909

916

936,924,878

610

nr

nr

nr

919, ρr(CH3) 867 δ(CCO) a

657

NIST (National Institute of Standards and Technology). b present study. nr: not resolved

At saturation coverage (Figure 2-middle spectrum), acetaldehyde adsorbs in the η2(C,O) configuration as well. The only difference is the presence of the CH3 stretching and C-O stretching bands (or CH3 deformation) of acetaldehyde that are observed at 2891 and 1410 cm-1, respectively. This could be related to a change in molecular orientation with coverage increase.


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The reason for the absence of these bands at low coverage can also be attributed to their relatively low intensities compared to the observed vibrational bands (at 1341, 1064 and 909 cm1

). At coverages higher than saturation (Figure 2-upper spectrum), the spectrum is similar to that

of acetaldehyde in the gas phase, (Table 1), suggesting that a physisorbed acetaldehyde layer forms on top of the monolayer of η2(C,O)-acetaldehyde.

Acetaldehyde Decomposition at Low Coverage Low-coverage experiments were performed to exclude the effect of adsorbate interactions on the surface chemistry of acetaldehyde. TPRS was used to determine desorption products and their yields. The TPRS spectra following 0.05 ML acetaldehyde adsorption show production of H2 and CO (Figure 3a). H2 desorbs in two peaks at 300 and 335 K and CO desorption occurs at 505 K. TPD data for H2- and CO-dosed Rh(100)30,31,27,36 show similar desorption peaks in shape with a ≈10 K downward shift in desorption temperature. The similar peak shapes indicate that H2 and CO desorption peaks from acetaldehyde decomposition are desorption limited. The downward shift in the H2 desorption temperature can be attributed to the repulsive interaction between hydrogen and CO27 or other possible ad-species (CHx, C) that form as a result of acetaldehyde decomposition. Similarly, the downward shift in CO desorption temperature is assigned to the repulsive interaction between CO and surface carbon.29,37 Surface carbon is identified by subsequent temperature programmed oxidation (TPO) experiments. Peak areas in TPRS and TPO spectra are used to determine product yields, indicating that 0.05 ML of CO, 0.1 ML of H2 and 0.05 ML of surface carbon are produced as a result of acetaldehyde decomposition (Table 2).


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Temperature (K) 500

β = 5 K/s

335 300

H2 505

200

400


Rh(100), Tads= 100 K

450 400

1863 1994 1865 1996

280

1868 1997

240

1873 2006 1996

200

2005

180 140

CO

100

600

2400

Temperature (K)

1991

360 320

0.3%

2003

Integrated Intensity (a.u)

b) 0.05 ML CH3CHO on Rh(100)

a) 0.05 ML CH3CHO

desorption rate (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CO total CO top CO bridge

100

200

300

400

Temperature (K)

500

1872 1995

1868

1993 1868 1991

1340

1060

1983

1339

1060

2000

1200 1000 -1

wavenumber (cm )

Figure 3. a) TPRS and b) Temperature programmed infrared spectroscopy (TP-IR) spectra of 0.05 ML acetaldehyde adsorbed at 100 K. For the infrared experiments the surface was heated stepwise and spectra were recorded at the temperatures indicated. The inset shows the course of the integrated intensities of the CO top (2010-1980 cm-1), CO bridge (1880-1850 cm-1) and the summation of CO top and CO bridge called CO total.

The surface intermediates formed after acetaldehyde decomposition were analyzed by temperature programmed infrared spectroscopy (TP-IR). Figure 3b shows the infrared spectra obtained after 0.05 ML acetaldehyde adsorption and subsequent heating. Only the regions of the spectrum in which bands are resolved are included in the figure. At 100 K the spectrum indicates the presence of η2(C,O)-acetaldehyde, which is marked by the δs(CH3) and v(C-C) bands at 1339 and 1060 cm-1.10–13 One additional band is also resolved at 1983 cm-1 and is assigned to


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molecular CO adsorbed on top site,5,30,38 due to the background CO in the UHV system. After annealing to 180 K, the band related to CO on bridge site appears at 1868 cm-1 and increases in intensity. The slight increase in intensities for CO on top and bridge sites between 100-180 K is assigned to the additional adsorption from the background (Figure 3b-inset). Between 180-240 K the intensities of CO related bands increase rapidly, which is accompanied by the disappearance of acetaldehyde related bands at 1339 and 1060 cm-1. This indicates that acetaldehyde undergoes decarbonylation to yield CO, CHx and hydrogen via C-C and C-H bond scissions. The CO related bands persist till 450 K, after which CO desorbs.

Coverage Dependent Acetaldehyde Decomposition

CH3CHO on Rh(100), Tads= 100 K, β = 5 K/s 304

θCH3CHO (ML)

282 250

450

0.25 0.18 0.15 0.07

300 200

335 400

0.05

CH4 desorption rate (a.u)

b)

θCH3CHO

295

(ML)

0.25 0.18 0.15 0.07 0.05

600 100

Temperature (K)

c)

260

CO desorption rate (a.u)

a) H2 desorption rate (a.u)

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200

480

θCH3CHO (ML)

0.25 0.18 0.15 0.07 505

0.05 300

400

Temperature (K)

200

400

600

Temperature (K)

Figure 4. a) H2 b) CH4 and c) CO desorption spectra of various amounts of acetaldehyde adsorbed on Rh(100) at 100 K. The heating rate is 5 K/s.


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The gaseous decomposition products formed by acetaldehyde decomposition change with increasing coverage. TPRS and TPO were used to observe differences in decomposition products. Figure 4 shows desorption spectra of H2, CH4 and CO following various amounts of acetaldehyde decomposition. Different from the low coverage case, CH4 formation is observed at high coverage along with CO, H2 and surface carbon.

Table 2. TPRS yields of various amounts of acetaldehyde on Rh(100) and those of saturation coverage of acetaldehyde on Rh(111)

Surface

Rh(100)

Rh(111)13

TPRS and TPO Yields Coverage(ML) (ML) CO

H2

CH4

C

0.05

0.05

0.10

0

0.05

0.07

0.07

0.14

0

0.07

0.15

0.13

0.19

0.03

0.10

0.18

0.18

0.22

0.07

0.11

0.25 (sat)

0.25

0.20

0.15

0.10

0.17 (sat)

0.17

0.32

0.08

0.07

The methane formation observed at acetaldehyde coverage above 0.15 ML suggests a different chemistry from the low coverage case (Figure 4b). Methane forms due to the reaction between CHx species and hydrogen and desorbs in two peaks at 260 and 295 K. CHx species only decompose to hydrogen and surface carbon at low coverage, whereas at high coverage methane formation is favored due to relatively high concentrations of CHx species and hydrogen atoms. TPRS yields of products indicate that at saturation coverage (0.25 ML), 60% of acetaldehyde


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(0.15 ML) decomposes to yield methane (Table 2) and CO, while the remaining 40% (0.10 ML) decomposes to hydrogen, CO and surface carbon. Surface carbon remains on the surface, and repulsive interactions between the surface carbon and CO cause a slight downward shift in CO desorption temperature for all coverages (Figure 4c).

500 450 400

1876 2007 1997

360

2019

320

2020

280 240 200

Integrated Intensity (a.u)

0.25 ML CH3CHO on Rh(100) CO total Temperature (K)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CO top CO bridge

100

200

300

400

Temperature (K)

500

1892 2022 1907 2026 1343 1892 1874

1071

180 1073

140 100 0.3%

2400

2000

1342

1073

1342

1072

1200 1000 -1

wavenumber (cm ) Figure 5. TP-IR spectra of 0.25 ML acetaldehyde adsorbed at 100 K. The surface was heated stepwise and spectra were recorded at the temperatures indicated.

The decomposition of acetaldehyde at high coverage was studied by TP-IR. Figure 5 shows the infrared spectra following 0.25 ML (saturation coverage) of acetaldehyde adsorption and subsequent heating. Different from the low coverage case, the acetaldehyde related bands at


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1343 and 1072 cm-1 persist up to 240 K. Decomposition starts above 180 K, as evidenced by a dramatic increase in intensities of the CO related bands (Figure 5, inset), and is complete at 280 K, where acetaldehyde related bands disappear completely. CH4 and H2 desorption at 250-300 K (Figure 5a-b) as a result of acetaldehyde decomposition is consistent with the infrared results. Hydrogen desorption causes a site change of CO from bridge to top,30,27,36 which is seen in the inset of Figure 5 for T>280 K. The disappearance of the CO related band is accompanied by CO desorption at 480 K.

Adsorption of Acetic Acid at 100 K

2576 3027 2992 2925 2639

1793

1052

1436

γ(OH)

ρr(CH3)

δas(CH3)

2.40

ν(C=O)

Exposure (L)

δs(CH3)

CH3COOH on Rh(100), Tads=100 K ν(OH) ν(CH3)


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Page 16 of 41

950

1312 1022 1363

0.20 (saturation)

1730 1757 1645

2931

νs(OCO)

νas(OCO)

0.08

0.5% 3200

1410

2800

2400

1640 1406

2000

1600 -1

1200

800

wavenumber (cm ) Figure 6. RAIRS spectra of various amounts of acetic acid on Rh(100) at 100 K.

The surface intermediates, which form upon acetic acid adsorption on clean Rh(100) were investigated by RAIRS. Figure 6 shows the infrared spectra obtained after dosing different


16

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Langmuir

quantities of acetic acid at 100 K. TPRS was used for quantification of the resulting coverage. A dose of 0.08 L and 0.30 L of acetic acid produce 0.05 ML (1/5 saturation coverage) and 0.24 ML (saturation of first, chemisorbed layer), respectively, whereas 2.4 L acetic acid exposure results in multilayer of acetic acid on the surface. The spectra show coupled OH and CH3 stretching bands (3100-2500 cm-1), C=O stretching bands (1800-1700 cm-1), CH3 deformation bands (14501300 cm-1), CH3 rocking bands (1060-1000 cm-1) and OH bending bands (950-900 cm-1). Detailed vibrational assignments are given in Table 3 in comparison to IR spectrum of gas phase acetic acid and acetic acid adsorbed on Rh(111) and Pd(111). At low coverage (0.05 ML, Figure 6-bottom spectrum), the spectrum is very different from the gas phase spectrum. Only two vibrational bands are observed at 1640 and 1406 cm-1. The lack of CH3 and C=O stretching bands of acetic acid suggests that no molecular acetic acid is present on the surface. Moreover, the absence of O-H bending band (950-900 cm-1) is an indication for O-H bond breaking, which results in acetate formation. In line with this, the bands at 1640 and 1406 cm-1 can be assigned to the asymmetric and symmetric OCO stretching modes of acetate, as reported previously for acetate adsorbed onto Rh(111)18 and on Rh/SiO2.39 The symmetric OCO stretching band at 1406 cm-1 is indicating the presence of bidentate acetate species (η2-acetate), which binds to the surface via both oxygen atoms.14,16,18,26 The asymmetric OCO stretching band at 1640 cm-1 is only allowed when the two O-Rh bonds are not equivalent,23 and its appearance in the spectrum therefore indicatesthe presence of monodentate acetate adsorbed with C=O bond intact, as observed on Pd(111).14 The presence of both asymmetric and symmetric OCO stretching modes of acetate suggests a mixture of the monodentate and bidentate acetate.


17

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Page 18 of 41

Table 3. Vibrational assignments of acetic acid and acetate adsorbed on Rh(100) compared with those adsorbed on Pd(111) and Rh(111) and IR spectrum of gas phase acetic acid and sodium acetate. All frequencies are in cm-1. Vibrational Assignments (cm-1)

Mode Acetic acid

Gas Phasea Pd(111)14,16,26

Rh(111)18

Rh(100)b

v(OH)

3583

2525

2700,2500

2800-2500

νs(CH3)

3051

2965

2995

3027

νas(CH3)

2944

nr

nr

2925

v(C=O)

1788

1685

1685

1730

δas(CH3)

1430

1440

1425

1436

δs(CH3)

1382

1310

1310

1363,1312

v(C-O)

1182

1304,1284

nr

nr

ρr(CH3)

1067,1015

1055,1026

-

1052,1022

γ(O-H)

947

955

970

950

v(C-C)

847

901

nr

900

δ(OCO)

657

685

630

nr

Acetate

Sodium Acetate40

vas(OCO)

1585

nr

1600

1645

vs(OCO)

1408

1415

1425

1410

a

NIST. b present study.nr: not resolved

At saturation coverage (Figure 6–middle spectrum) both monodentate and bidentate acetate species are observed, marked by the bands at 1645 and 1410 cm-1. Moreover, two new bands


18

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become apparent at 2931 and 1757 cm-1. The former can be associated with the ν(CH3) mode of both acetate species, whereas the latter is assigned the ν(C=O) mode of the monodentate acetate. When the surface is exposed to 2.4 L acetic acid (Figue 6–upper spectrum), the emergence of molecular acetic acid related bands indicates the formation of acetic acid layers on the surface. Acetic acid is known to form dimers or catemers via hydrogen bonding on metal surfaces after high doses. The OH stretching (ν(O-H)) mode is a good indicator to identify these species. The ν(O-H) modes of acetic acid dimers and catemers show bands at 3193 and 2875 cm-1 on Pd(111), respectively,26 indicating that more hydrogen-bonding interactions cause a downward shift in band frequency. In this connection, the presence of bands at 2639 and 2576 cm-1 indicates that adsorbed acetic acid molecules form catemers (Figure 7) on Rh(100).

O

H

O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

H H

H

Figure 7. Schematic representation of acetic acid catemers

Acetic Acid Decomposition at Low Coverage Acetic acid decomposition at low coverage was studied by TPRS. The desorption spectra of 0.05 ML (ca. 1/5 saturation coverage) acetic acid show H2 (362 and 450 K) and CO2 (356 K)


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desorption, along with a small amount of CO (505 K) (Figure 8a). The presence of carbon was confirmed in a subsequent TPO experiment. TPRS and TPO yields of products reveal that 0.10 ML H2, 0.045 ML CO2, 0.045 ML surface carbon and 0.01 ML CO are produced (Table 4). H2 desorption after acetic acid decomposition occurs at higher temperatures (362 and 450 K) compared to H2 desorption from a surface only covered by H (305 and 345 K), suggests that H2 desorption is reaction limited.30,27,36 The simultaneous evolution of H2 and CO2, at 356 K, are indicative for acetate decarboxylation,18,19 which is consistent with TP-IR results (discussed in the following TP-IR study). As acetate decomposes, it releases CO2 and CHx ad-species on the surface. The resulting CHx species can either hydrogenate to methane or dehydrogenate to surface carbon. The lack of methane evolution and the observed surface carbon indicate that dehydrogenation is the dominant CHx decomposition pathway. Additional evidence for CHx dehydrogenation is the H2 desorption at 450 K. The H2 desorption at high temperature (400-500 K) is also observed after ethanol37 and acetaldehyde decompositions (see: coverage dependent acetaldehyde decomposition) due to CHx dehydrogenation, which produces H2 and surface carbon. A TP-IR study was conducted to determine surface intermediates and decomposition pathways. TP-IR spectra obtained after 0.05 ML acetic acid adsorption and subsequent heating are shown in Figure 8b. At 100 K, the spectrum shows the νas(O-C-O) and νs(O-C-O) related bands at 1640 and 1406 cm-1, indicative of a mixture of monodentate and bidentate acetate. Two more bands are also resolved at 1395 and 1345 cm-1, assigned to the CH3 deformation mode of acetate. Annealing to 140 K results in attenuation of the νas(O-C-O) related band and the intensified band of the νs(O-C-O) mode, suggesting (incomplete) conversion of monodentate acetate into bidentate. The disappearance of the νas(O-C-O) related band at 1640 cm-1 after


20

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heating to 180 K indicates complete conversion of monodentate into bidentate at this point. Concurrently, CO related bands at 1982 (CO top) and 1855 cm-1 (CO bridge) appear with low intensities, suggesting that a small portion of acetate decomposes to CO via C-O and C-C bond scissions.

a) TPRS

b) TP-IR

0.05 ML CH3COOH β = 5 K/s

H2

450

362

505

CO CO2 200

356


on Rh(100),Tads= 100 K


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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360

1862

320

1865

280

1868

240

1871

200

1865

180

1862

140

0.3% 400

600

on Rh(100)

400

100

Temperature (K)

0.05 ML CH3COOH

Temperature (K) 500 450

1982 1855

1416

1338 1380

1340 1382 1632 1411 1385 1345 1416

1345 1640 1406 1395

2400 2200 2000 1800 1600 1400 1200 -1

wavenumber (cm )

Figure 8. a) TPRS spectra obtained after 0.05 ML acetic acid adsorbed b) TP-IR of 0.05 ML acetic acid adsorbed at 100 K. The surface was heated stepwise and spectra were recorded at the temperatures indicated.

The decomposition of acetate mainly occurs after annealing to 280 K, as evidenced by a significant attenuation of the acetate related band at 1416 cm-1 and it is complete at 360 K where the band disappears. The disappearance of the acetate related band is accompanied by the simultaneous CO2 (356 K) and H2 (362 K) evolution, suggesting that acetate decomposes to CO2


21

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Page 22 of 41

and CHx species via C-C bond scission. Annealing to 280 K also results in a small intensity increase of the band of CO on bridge site at 1871 cm-1. Since this intensity increase coincides with the intensity decrease of the acetate related band, this is taken as evidence of a minor reaction route where acetate decomposes to CO via C-C and C-O bond scission. After further annealing to 360 K, CO molecules on bridge sites move to top sites due to H2 desorption (Figure 8b). At 360 K, CO molecules mainly occupy top sites. In literature, it was reported that the integrated band intensity of the CO on top site increases linearly with CO coverage (up to 0.3 ML).28 By using the correlation between the integrated band intensity of CO on top site and CO coverage, the amount of CO (on top site) formed due to acetate decomposition is determined as approximately 0.01 ML, consistent with the TPRS yield of CO. The CO related bands disappear with CO desorption after 450 K.

Table 4. TPRS and TPO yields of acetic acid on Rh(100) and Rh(111)

Surface

Coverage TPRS and TPO Yields (ML) (ML) CO CO2 H2 H2O C 0.05

0.010 0.045 0.100 -

0.045

Rh(100) Rh(111)18

0.24 (sat) 0.14

0.16

0.46

0.02

0.18

0.12 (sat) 0.05

0.09

0.24

-

0.09

At low coverage, two different pathways are observed for the acetate decomposition: (i) decomposition to CO via C-C and C-O bond scissions, and (ii) decomposition to CO2 via C-C bond scissions. We refer to the former as the deoxygenation pathway (CH3COO(ad) = CO(ad) + O(ad) + CHx(ad)) and the latter as the decarboxylation pathway (CH3COO(ad) = CO2 + CHx(ad)). The small amount of CO, formed by acetate decomposition suggests that


22

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deoxygenation is a minor pathway and the decomposition proceeds mainly via decarboxylation, which occurs around 360 K.

Coverage Dependent Acetic Acid Decomposition The change in surface chemistry with coverage increase is also observed for acetic acid. TPRS and TPO were used to identify decomposition products and to determine product yields at high coverages. Figure 9a shows TPRS spectra following saturation coverage (0.24 ML) of acetic acid decomposition. Different from the low coverage case, water and acetic acid desorption are observed in addition to H2, CO, CO2 and surface carbon when the initial acetate coverage is high. TPRS and TPO yields of products show that at saturation coverage (0.24 ML), acetic acid decomposition produces 0.46 ML H2, 0.16 ML CO2, 0.14 ML CO, 0.02 ML H2O and 0.18 ML surface carbon (Table 4). Acetic acid and water desorption are observed when the initial acetic acid coverage is higher than 0.15 ML (Figure 9a). At saturation coverage, acetic acid desorbs in two peaks at 200 and 240 K, while water desorption occurs at 325 K with a high temperature shoulder at 360 K. The acetic acid desorption at 200 and 240 K are attributed to the acetate+H recombination. H2O formation requires C-O bond scission and it can be linked to the deoxygenation pathway, which produces CO at low coverage. Deoxygenation releases O or in addition to CO and CHx on the surface. The resulting O or OH species are hydrogenated by surface hydrogen that is present in abundance due to the preceding decomposition steps. At high coverage, CO desorbs in two states at 480 and 600 K. The CO desorption at 480 K is mainly attributed to the decomposition of acetate via deoxygenation. This peak shows a 25 K downward shift by comparison to TPD data for CO-dosed Rh(100). As noted before, this


23

Langmuir

difference is due to the interaction between CO and surface carbon. The high temperature peak at 600 K is assigned to the reaction of atomic carbon and oxygen, another indication for the deoxygenation pathway.

CH3COOH on Rh(100), Tads= 100 K, β= 5 K/s a) 0.24 ML CH3COOH

460 325

480

CO

325 360

600 400 450

200 240

CH3COOHx4 200

400

600

Temperature (K)

375

θAA 280 (ML)

460

0.24 0.20 0.15 0.12 0.05 200

CO2 desorption rate (a.u)

H2

CO2

c)

H2

325

375

280

H2 O

b)

H2 desorption rate (a.u)

325


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Page 24 of 41

400

325

600

360

θAA (ML)

400 450

0.24 0.20 0.15 0.12 0.05

362

Temperature (K)

CO2

200

356

400

600

Temperature (K)

Figure 9. a) TPRS spectra of 0.24 ML acetic acid adsorbed on Rh(100) at 100 K. b) H2 and c) CO2 desorption spectra of various amounts of acetic acid adsorbed on Rh(100) at 100 K. The heating rate is 5 K/s.

Figure 9b-c shows how the H2 and CO2 desorption spectra develop as a function of acetic acid coverage. At low coverage, the reaction-limited H2 desorption occurs at 362 and 450 K. The former is due to the acetate decomposition via decarboxylation, whereas the latter is assigned to the decomposition of CHx species. As the coverage increases, the H2 desorption peaks at 362 and 450 K shift to higher temperatures (375 and 460 K) and new desorption states appear at 280 and 325 K. The H2 desorption at 325 and 375 K are accompanied by the CO2 desorption at 325 and


24

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360 K, suggesting that they are due the acetate decomposition via decarboxylation. The other H2 desorption at 280 K can be assigned to the recombination of hydrogen produced either from acetic acid decomposition via O-H bond scission or from acetate decomposition. At high coverage, apart from the acetate related CO2 desorption at 325 and 360 K, there are two additional CO2 desorption states at 400 and 450 K. They are assigned to CO oxidation since the same CO2 desorption is also observed when the surface is covered by the comparable coverages of CO and O.35 Acetate decomposition can also contribute to the CO2 desorption peak at 400 and 450 K but the extent is unclear due to complexity of surface reactions. TPRS indicates a complex reaction network to product formation, and TP-IR was used to identify surface intermediates and decomposition pathways. Figure 10b shows the infrared spectra obtained after saturation coverage of acetic acid adsorption and subsequent annealing. The spectra show that at 200 K, monodentate acetate converts into bidentate acetate, which is proved by the absence of the band at 1640 cm-1 and the presence of an intensified band at 1425 cm-1. A part of the disappearance of the monodentate related band at 1640 cm-1 could be also attributed to the acetic acid desorption observed at 200 and 240 K (Figure 10a) due to the reaction of some monodentate acetate molecules with H atoms. However, the TPRS yield of acetic acid desorbed shows that only a small amount of acetic acid (0.005 ML) desorbs due to this reaction. Acetic acid desorption can therefore only account for a part of the disappearance of the monodentate-related band. Different from the low coverage case, no CO formation is observed at low temperature. The CO related bands at 1993 (CO top) and 1876 cm-1 (CO bridge) appear at 280 K with the attenuation of the bidentate acetate related band at 1425 cm-1, suggesting that acetate decomposition via deoxygenation is underway. The H2O evolution at 325 and 360 K (Figure 9c)


25

Langmuir

due to the reaction of atomic hydrogen and oxygen supports this idea. Acetate decomposition via deoxygenation is complete at 360 K, since no more intensity increase is observed for the CO related band after this temperature. On the other hand, the simultaneous CO2 and H2 evolution between 325 and 400 K suggests that some acetate adsorbates decompose via the decarboxylation pathway to yield CO2 and CHx. This means that acetate decomposition proceeds via both pathways at 280-400 K. At 450 K, the CO related bands disappear. Since CO desorbs around 480 K, we expect to have these bands in 450 K spectrum as well. The lack of bands is evidence for CO oxidation, occurring between 400-500 K as seen by CO2 desorption at this temperature interval (Figure 9).

Temperature (K) 500


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Page 26 of 41

0.24 ML CH3COOH on Rh(100), Tads= 100 K

450 400 360 320

2011 2012

1894 1892

280

2007 1886

240

1993 1876

1417 1419

200 180

1425

140

1637

1422

100

1644

1415

1645

1410

0.3%

2200 2000 1800 1600 1400 1200 -1

wavenumber (cm )


26

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Langmuir

Figure 10. TP-IR of 0.24 ML acetic acid adsorbed at 100 K. The surface was heated stepwise and spectra were recorded at the temperatures indicated.

TPRS and TP-IR studies show that acetate decomposes via both deoxygenation and decarboxylation at high coverage. CO2 and CO formation are indicative of these pathways as long as no additional CO and CO2 are produced by different reactions. However, as some CO molecules are oxidized to CO2 between 400-500 K the amount of CO2 produced via CO oxidation cannot be determined accurately and it is therefore difficult to determine the relative contribution of these pathways to acetate decomposition in detail. Still, the high amounts of CO formation at high coverage suggests that a significant amount of acetate decomposes via deoxygenation, as opposed to the low coverage case.

Discussion In this study, we describe the surface chemistry of acetaldehyde and acetic acid on Rh(100) as a reference for understanding the behavior of aldehyde (C=O) and carboxyl (COOH) functionalities on bond-breaking sequences of biomass-derived molecules. This provides insights into the reactions involved in the catalytic conversion of these molecules on metal surfaces. Our main findings are: i.

Acetaldehyde binds to the surface via the aldehyde end and decomposes to synthesis gas, methane and surface carbon via C-C and C-H bond scissions below room temperature (180-280 K).

ii.

Acetic acid loses the hydroxyl hydrogen upon adsorption and binds to the surface via both oxygen atoms at the carboxylate end. It mainly decomposes to synthesis gas, CO2


27

Langmuir

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Page 28 of 41

and surface carbon via C-C, C-O and C-H bond scissions above room temperature (280360 K). iii.

The surface chemistry of both molecules depends on the surface coverage. For acetaldehyde, at high coverage, methane formation is favored due to a high concentration of CHx and hydrogen. For acetic acid, C-O bond scission is significantly promoted for higher coverages.

Decomposition pathways of Acetaldehyde on Rh(100) Acetaldehyde binds to the Rh(100) surface in the η2(C,O)-configuration, which leads to a stronger interaction with the surface compared to acetaldehyde in the η1(O)-configuration. Therefore the decomposition of acetaldehyde is favored over desorption on Rh(100). The decomposition of η2(C,O) acetaldehyde to CO occurs between 180-240 K via C-H and C-C bond breaking. No spectroscopic evidence is found for any surface intermediates between acetaldehyde and CO. Thus, it is not clear whether first C-H or C-C bond breaking occurs. On Pd(111)10 and Pd(110),9 η2(C,O)-acetaldehyde reacts to form acyl (CH3CO) species via α C-H bond scission and acyl species are identified by the characteristic C-O stretching band at 1595 and 1610 cm-1, respectively. We have not observed any band around these frequencies. It may be related to low stability of acyl intermediates because of the unsaturated carbon of their carbonyl group. The acyl related band is also not observed on Rh(111) surface. Houtman et al.13 proposed acyls as likely transient intermediates on Rh(111) due to similarities in the decomposition of acetaldehyde on Rh(111) and on the supported Rh/SiO2 (on which acyl intermediates are identified) and due to the high dehydrogenation activity of Rh surfaces. In the same manner we


28

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Langmuir

also regard acetyls as likely, but short-lived surface intermediates for acetaldehyde decomposition on the Rh(100) surface. The decarbonylation of acetaldehyde (or acetyl) can produce different CHx species. Isotope labeling experiments show that on Rh(111), methyl species (CH3) are produced after the decarbonylation of acetaldehyde.13 The similarities in the ethanol decomposition chemistry on both Rh(100) and Rh(111) surfaces37,41 and similar product distribution on both surfaces following the decarbonylation of acetaldehyde suggest that acetaldehyde decarbonylation on Rh(100) also proceeds via methyl elimination. High coverage hinders the decarbonylation of η2(C,O)-acetaldehyde, which is marked by the presence of acetaldehyde related band in the IR spectra up to 240 K. Even though the decomposition of acetaldehyde begins around the same temperature (180 K) for all coverages, there is a prolonged period for the acetaldehyde decomposition at high coverage. This can be related to the lack of available sites for bond activation. Since at high coverage the surface is fully covered by adsorbates which form as a result of acetaldehyde decomposition, the active sites for the decarbonylation are not available. Once H2 and CH4 start to desorb, at 250-260 K, free surface sites are created and the remaining acetaldehyde also decomposes at 280-290 K.

Decomposition Pathways of Acetic Acid on Rh(100) Acetic acid initially forms a mixed monodentate and bidentate acetate layer upon adsorption via O-H bond breaking. The monodentate species mostly transform into bidentate species by annealing to 200 K. Decomposition of bidentate acetate proceeds via two different reaction pathways: deoxygenation and decarboxylation. Deoxygenation is a minor pathway at low coverage, whereas at high coverage it is as active as decarboxylation. As noted before, the CO


29

Langmuir

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Page 30 of 41

related bands in the IR spectra are indicative of deoxygenation, whereas decarboxylation is identified by simultaneous CO2 and H2 desorption. At high coverage, first the CO related bands appear at 280 K concurrent with the intensity decrease in the acetate related band. Then the simultaneous CO2 and H2 desorption is observed at 325 K. This suggests that decomposition begins via deoxygenation at 280 K followed by decarboxylation at 325 K. One can argue that decarboxylation can occur below 325 K, but the sharp increase in the CO2 desorption peak at 325 K (Figure 9c) indicates that the decomposition via decarboxylation begins at this temperature. Acetate decomposition via decarboxylation occurs in multiple steps, as evidenced by multiple desorption peaks of CO2 (325 and 360 K) and H2 (325 and 375 K) around similar temperatures. This can be linked to the lack of available surface sites at high coverage, as observed for acetaldehyde decomposition. Once acetate decomposes via decarboxylation, available sites on the surface are blocked by decomposition products (CHx species and hyrogen), which stops the reaction until surface sites become free again. Free surface sites are created by the H2 and CO2 desorption at 325 K and the remaining acetate species decompose again via decarboxylation. Acetic acid decomposition on Rh(100) does not produce methane, as opposed to acetaldehyde decomposition. In principle the decomposition of both molecules produces CHx species on the surface, but since acetic acid decomposition occurs approximately 100 K higher than acetaldehyde decomposition, methane formation upon acetate decomposition is disfavored due to the high reaction rate of the competing CHx dehydrogenation followed by irreversible H2 desorption. The absence of methane production from acetic acid decomposition can be also linked to the bond breaking sequence of acetic acid on Rh(100). Acetic acid decomposes to acetate via O-H bond breaking. If acetate decomposition occurs via initial C-H bond scission, which releases less hydrogen containing CHx species (e.g. CH2, CH), then more hydrogen atoms


30

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Langmuir

are required to form methane. Since hydrogen atoms are also consumed to form H2, methane formation is disfavored. For the reasons discussed above, methane production is also not observed on Rh(111).18 The proposed reaction mechanisms for both acetaldehyde and acetic decomposition on Rh(100), at both high and low coverage, are summarized in Figure 11.

a)

b)

H

H

C

H

H H

H

C

H

180-280 K

180-280 K

H

C

H H

O C

H H O C

C H

H H

CH4, H2

280-450 K H

H

C

C C O O H O O H

C O

H

H

H

CH4, H2

H

C C O O

θAA = low H

H

H

CH3COOH, H2 H

280-450 K CO2, H2

C

450-550 K O C

C

H H H O C O C

CO2, H2O, H2

450-550 K CO

O

CO C

>550 K C

θAA = high

C C O

550-700 K C

CO C

Figure 11. Reaction mechanisms for a) acetaldehyde b) acetic acid decomposition.

Effects of C=O and COOH Functionalities on the Reaction of Oxygenates with the Rh(100) Surface The surface chemistries of acetaldehyde and acetic acid on the Rh(100) surface mimic the interaction of biomass-derived molecules containing aldehyde (C=O) and carboxyl (COOH) functionalities with the metal surfaces. The decomposition of both molecules on the Rh(100)


31

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surface involves O-H, C-H and C-C bond scissions. At high surface coverage, C-O bond scission is also observed for the decomposition of acetic acid. Both molecules bind to the surface via their functional groups, i.e., acetaldehyde binds via C and O atoms at the aldehyde end and acetic acid binds via both oxygen atoms at the carboxyl end. For acetic acid, the O-H bond is activated by the surface upon adsorption. The resulting acetate is more stable than the adsorbed acetaldehyde, so the decomposition of acetate occurs at higher temperatures than that of acetaldehyde. The latter produces synthesis gas (CO+H2), methane and surface carbon whereas the former produces CO2 and a minor amount of H2O as well. The decomposition of both molecules proceeds via CH and C-C bond scissions and leave the surface covered by surface carbon. The acetic acid decomposition produces more hydrogen (e.g. almost 3 times higher at saturation coverage) and surface carbon (e.g. almost 2 times higher at saturation coverage) per parent molecule at the expense of CH4 formation and less CO compared to the acetaldehyde decomposition. The difference in product composition is strongly related to binding geometries of molecules on the Rh(100) surface.

Reactivity of Metal Surfaces towards Oxygenates with C=O and COOH Functionalities The surface chemistry of acetaldehyde and acetic acid on metal surfaces are dependent on the type of metal and facet. On all surfaces for which data is reported in literature, acetaldehyde was found to decompose via C-H and C-C bond scissions. However, the nature of the gaseous products found afterwards shows significant differences depending on the surface. On Rh surfaces acetaldehyde decomposition produces CH4 and CO with a ratio of 0.5-0.6 whereas on Pd and Ni surfaces the CH4/CO ratio is between 0.8-0.95. This ratio is very close to 0 for the Ru(0001) surface, i.e., no methane formation is observed. As a result of this, a significant


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amount of carbon formation is observed on the Ru surface. In contrast to the Ru surface, only a negligible amount of carbon formation is observed on Pd surfaces. For acetic acid decomposition (100) and (111) facets of Rh and Pd shows similar characteristic in terms of bond scission. On these surfaces acetic acid decomposition occurs via decarbonylation and deoxygenation, resulting in CO, CO2, H2, surface carbon formation along with a minor amount of water production. Different from (100) and (111) surfaces, deoxygenation is not active on the more reactive (110) facets of Rh and Pd surfaces, accordingly, no CO and water formation are observed. This can be linked to the binding state of acetate, which forms upon acetic acid adsorption below the room temperature. For both acetaldehyde and acetic decomposition, the Rh(111) surface produces a gaseous mixture with a higher H2/CO ratio compared to the Rh(100) surface. Our findings in this study and literature studies on the surface chemistry of acetaldehyde and acetic acid suggest that Rhodium is more reactive towards synthesis gas production from oxygenates containing aldehyde and carboxyl functionalities rather than hydrocarbon production. Nevertheless, Rhodium also leads to high carbon deposition on the surface. This creates a problem in terms catalyst stability. This problem can be minimized by using Pd catalysts where the carbon preferentially forms methane rather than carbon deposits. This however lowers the production of H2.

Conclusions The adsorption and decomposition of acetaldehyde and acetic acid on Rh(100) have been studied by TPD and RAIRS. Upon adsorption at 100 K, acetaldehyde binds to the surface via C and O atoms at the aldehyde end and it decomposes between 180-280 K via C-H and C-C bond


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activation. Decomposition yields CO, H2 and surface carbon at low coverage. Methane formation is also observed along with CO, H2 and surface carbon at high coverage. Acetic acid dehydrogenates to monodentate and bidentate acetate upon adsorption at 100 K. At low coverage decomposition of acetate results in CO, CO2, H2 and surface carbon, while H2O formation is observed at high coverage. At low coverage, acetate mainly decomposes via decarboxylation to yield CO2 and CHx species at 280-360 K, whereas at high coverage C-O and C-C bond scissions occur to yield CO, CO2, CHx and oxygen atom at the same temperature interval (280-360 K). The O ad-atoms, which form via C-O bond scission react with hydrogen and CO to form water and CO2, respectively. The CHx species decompose further to surface carbon for all coverages.

AUTHOR INFORMATION Corresponding Author * Department of Energy Systems Engineering, Yasar University, 35100, Izmir, Turkey. E-mail: [email protected]; Fax: +902325707000; Tel: +902325708265

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †‡ ӝ These authors contributed equally.

Funding Sources Dutch National Research School Combination Catalysis Controlled by Chemical Design (NRSCCatalysis).


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ACKNOWLEDGMENT We gratefully acknowledge the Dutch National Research School Combination Catalysis Controlled by Chemical Design (NRSC-Catalysis) for funding of this research. Syngaschem BV acknowledges funding from Synfuels China Technology Co., Ltd, Beijing-Huairou, China.

ABBREVIATIONS TPRS, temperature-programmed reaction spectroscopy; RAIRS, reflection absorption infrared spectroscopy; TP-IR, temperature-programmed infrared spectroscopy; TPO, temperatureprogrammed oxidation; UHV, ultra-high vacuum.


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TABLE OF CONTENT / ABSTRACT GRAPHIC


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Effect of Aldehyde and Carboxyl Functionalities on the Surface

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