Reflection absorption infrared spectroscopy of the surface chemistry of furfural on Pd(111)

In order to produce value-added products from lignocellulosic biomass, the raw materials (cellulose, hemicellulose) are typically hydrolyzed to the corresponding carbohydrate monomers, glucose, fructose, and xylose, using acid catalysis.^1–4 Subsequent acidic dehydration leads to the formation of smaller oxygenates such as furfural, 5-hydroxymethylfurfural, and levulinic acid. These compounds can be used to produce fuels and polymers or for synthesizing other monomers relevant to the polymer industry.^2,4–9 However, many of the current methods for transforming such feedstocks are often dirty and energy consuming;^1–3,7–9 for example, FF is upgraded using a toxic copper chromite catalyst^10,11 at high pressures.

It would thus be advantageous to develop catalysts for converting furfural to more valuable feedstocks,^8,12 and palladium has been identified as a selective catalyst for the hydrogenation of furfural to methyl furan and dimethylfuran.^7,13–16 This has prompted investigations of the chemistry of furfural on Pd(111) single-crystal model catalysts.¹⁷ In particular, it has been proposed experimentally^18,19 and theoretically¹⁸ that the adsorbate coverage controls the selectivity, with flat-lying species leading to decarbonylation and the formation of furan and tetrahydrofuran, while tilted conformations result in furfural hydrogenation to furfural alcohol or 2-methyl furan.

However, there have been no detailed infrared spectroscopic investigations of the adsorption of furfural into the monolayer and its structure and surface chemistry on Pd(111) that would provide direct experimental verification of the reaction pathways and afford measurements of the orientation of the adsorbates from the surface infrared selection rules.²⁰ This is addressed in the following by investigating the surface chemistry of furfural on a Pd(111) single crystal in ultrahigh vacuum (UHV) as a function of coverage and temperature by using reflection-absorption infrared spectroscopy (RAIRS).

Previous temperature-programmed desorption (TPD) experiments of furfural on Pd(111)¹⁷ show molecular desorption in a broad state centered at ∼365 K that extends between ∼300 and 450 K. The formation of a small amount of furan is found at 295 K with the majority desorbing at 350–375 K, where preadsorbed deuterium results in the incorporation of two deuterium atoms in the product, and some propylene is formed in a broad peak at ∼370 K, ascribed to a furan ring-opening reaction.

Detailed density functional theory (DFT) calculations propose a number of orientations of furfural on Pd(111).²¹ At lower coverages, the most stable conformation has the furyl ring lying flat on the surface, centered over a hollow site, while more weakly bound tilted species form at higher coverages, in accord with previous work.^17,22,23 Another weakly adsorbed species, namely, a bent configuration, with a furyl ring tilted away from the surface and an upright structure, has been proposed, which may be stabilized at higher coverages.²⁴ 

In order to more clearly identify the surface reaction pathways of furfural, the adsorption and reaction of furan, 2-methyl furan, and furfuryl alcohol were also investigated. Previous work has shown that furan adsorbs on Pd(111) in a tilted geometry at a saturation coverage and decomposes to form carbon monoxide and hydrogen for furan doses below 0.25 L, while for exposures >0.25 L, some furan desorbs molecularly.²⁴ The decomposition pathway has been studied using laser-induced thermal desorption (LITD) to show that furan decomposes between 280 and 320 K.²⁵ A DFT investigation of the reaction pathway²⁶ suggests that furan decomposition is initiated by C–O bond scission, followed by the removal of two hydrogens to produce a stable C₄H₂O intermediate.

Experiments were carried out in UHV chambers operating at base pressures of ∼2.0 × 10⁻¹⁰ Torr after bakeout. Infrared spectra were collected using a Bruker Vertex 70 infrared spectrometer and a liquid-nitrogen-cooled, mercury cadmium telluride detector.²⁷ The complete light path was enclosed and purged with dry, CO₂-free air. Spectra were collected for 1000 scans at 4 cm⁻¹ resolution, and the surface was exposed to the indicated compound using a precision variable leak valve connected to the infrared cell.

The Pd(111) sample was cleaned using a standard procedure of Ar⁺ bombardment with subsequent heating in oxygen to remove any strongly bound surface carbon. Ar⁺ bombardment was performed at a background pressure of ∼3.0 × 10⁻⁵ Torr at a 1 kV potential while maintaining a ∼2 μA sample current. Cycles of heating were performed in a background pressure of ∼3.0 × 10⁻⁸ Torr of oxygen at a sample temperature of 780 K for 10 min. The sample was cooled to 690 K and flash annealed to 850 K to desorb any remaining adsorbates. After completing three cycles of heating in oxygen, cooling, then annealing, the sample was allowed to cool to ∼400 K, after which the oxygen exposure was stopped and the sample was flash annealed to 1000 K while subsequently scanning using a Hiden mass spectrometer while monitoring the CO (28 amu) versus O₂ (32 amu) ratio. When the oxygen signal intensity significantly exceeded the CO signal, the Pd(111) sample was considered to be free of carbon, particularly in the subsurface region of the sample. This process was repeated three times before conducting each experiment. The CO infrared spectrum was also used to confirm that the Pd(111) surface was clean.²⁸ The sample temperature was monitored by means of a chromel/alumel thermocouple spot-welded to the edge of the single crystal.

Furfural (Aldrich, >99.9% purity), furan (Aldrich, >99.9% purity), 2-methyl furan (Aldrich, >99% purity), and furfuryl alcohol (Aldrich, >98% purity) were purified using several freeze-pump-thaw cycles, and the cleanliness judged by mass spectroscopy, and then dosed using a variable leak valve via a 3.2-mm internal diameter tube with the end located approximately 2 cm from the front of the sample. This arrangement limits the contamination of other parts of the UHV chamber and enhances the local pressure at the sample, while still obtaining a relatively uniform flux across the sample. An enhancement factor of 100 was applied to calculate all exposures. Exposures in Langmuirs (1 L = 1 × 10⁻⁶ Torr s) were not corrected for ionization gauge sensitivities.

The infrared spectra of furfural adsorbed on Pd(111) at 90 K are shown in Fig. 1(a) and the assignments are summarized in Table I. At the lowest exposures, up to 1.6 L, only out-of-plane modes with A″ symmetry in the C_S point group²⁹ are detected and indicate that furfural lies with its molecular plane parallel to the surface in a flat-lying geometry at lower coverages. At higher coverages, both the A′ modes and the intense carbonyl modes at 1676–1699 cm⁻¹ appear indicating that, as the surface becomes more crowded, other configurations occur.

Furfural can adopt two distinct conformers depending on whether the aldehyde carbonyl group is cis or trans to the furyl ring oxygen [Fig. 1(b)] to cause a shift in the frequencies of some vibrational modes.^29–32 This effect is most prominent in the carbonyl stretching modes, where the 1703 cm⁻¹ mode is due to the cis conformer, while the 1676 cm⁻¹ peak is assigned to the trans structure. Other features that show similar cis-trans splitting are the 1475/1465 cm⁻¹ doublets due to a furyl ring breathing mode, as well as the 1279/1251, 1230/1207, and 929/947 cm⁻¹ doublets due to C–H in-plane bending vibrations. Here, the first vibrational frequency is due to the cis conformer and the second to the trans geometry. As previously reported, approximately ∼25% of the furfural adsorbs as the cis conformer and ∼75% as trans on the palladium surface at 90 K.²⁹ 

The average furfural tilt angle with respect to the surface can be estimated from the relative intensities of the A′ (1470 cm⁻¹) and the A″ modes (789 cm⁻¹)²⁹ by using calculated absorbance values in the literature,³² and Fig. 1(c) shows the tilt angle of furfural as a function of exposure. As noted above, furfural initially adsorbs with a flat-laying geometry at low coverages. As the exposure increases to 2.9 L, the furfural A′ modes as well as the carbonyl stretching vibrations become visible, where the tilt angle tends toward ∼35°, close to the value predicted for the so-called tilted-2 species from DFT calculations.^18,21 The tilt angle increases further as the furfural film grows and reaches a plateau above an exposure of 8 L saturating at a tilt angle of ∼54°, indicating a random orientation.²⁹ This indicates that furfural adsorbs directly onto the surface at exposure below ∼ 4 L, with multilayer furfural forming at higher coverages.

The temperature dependence of a saturated furfural overlayer on Pd(111), formed using a 5 L exposure, is shown in Fig. 2. The spectra were collected by heating to the selected temperature (indicated adjacent to each spectrum), then cooling to 175 K and the spectrum subsequently collected. The vibrational frequencies for spectra obtained with annealing temperatures below ∼150 K are essentially identical to those found for the multilayer (Fig. 1), indicating that the furfural adsorbs molecularly at this temperature.³⁰ The presence of both cis and trans carbonyl stretching modes is observed, indicating that the monolayer contains both conformers.

However, the furfural adsorbed at ∼175 K induces the formation of an additional vibrational mode at 1614 cm⁻¹ and is assigned to a carbonyl mode shifted by interacting with the palladium surface to form an η¹(O) species,^33–35 identified by Vlachos as a Tilted-1 species.^18,21 Interestingly, this mode is not observed when furfural is adsorbed at 90 K, consistent with previous high-resolution electron energy loss results.³⁶ 

Upon annealing to 200 K, the peak at 786 cm⁻¹ disappears, indicating that the aldehyde group is deprotonated and this observation is consistent with the first step predicted in the calculated pathway for furfural conversion to furan.²¹ Deprotonation induces an orientation change so that the furfuryl carbonyl group is closer to parallel to the surface as demonstrated by the decrease in intensity between ∼1600 and ∼1700 cm⁻¹, while the furyl ring starts to tilt away from the surface by ∼200 K as evidenced by the increase in intensity of modes at 1483, 1410, and 1371 cm⁻¹ characteristic of a furyl ring.³⁷ In particular, the 1614 cm⁻¹ feature has also almost completely disappeared as the sample is heated to 200 K, suggesting that the η¹(O) species is a precursor to the dehydrogenated furfural.

These changes become more pronounced on heating to 225 K, indicating the completion of this process. Heating to 275 K or above causes the deprotonated furfural to react to form a furyl species and CO so that the furyl group lays completely flat and thus becomes infrared invisible³⁸ in agreement with the reaction pathway predicted by theory.²¹ This reaction is confirmed by the detection of CO formation from peaks between 1745 and 1873 cm⁻¹. Heating to above 450 K causes all features to disappear, consistent with the temperature at which CO desorbs from Pd(111).^39,40

While the assignment made above of peaks to adsorbed carbon monoxide is consistent with the DFT calculations²¹ and TPD experiment,¹⁷ the lower frequencies are below the range generally found for CO on Pd(111) surfaces.^39,40 This is further illustrated by absorbing CO onto a clean Pd(111) surface at 175 K and collecting exposure- and temperature-dependent infrared spectra [Figs. S1(a) and S1(b)].⁴⁷ At the lowest coverages, the CO vibrational mode is centered at 1832 cm⁻¹ and increases in frequency with increasing coverage due to lateral interactions and is assigned to CO adsorbed in the threefold site.^39,40 At higher coverages, an additional CO feature peak appears at ∼2069 cm⁻¹, which increases in frequency with increasing coverage and is attributed to atop binding of CO. Thus, the 1745 cm⁻¹ peak seems to be too low to be assigned to carbon monoxide on the surface and may imply the existence of an intermediate species as a precursor to total decomposition at higher temperature. Alternatively, the lower frequency may be caused by CO interacting with furfural-derived intermediates. This possibility was investigated by monitoring the infrared spectra in the CO stretching region as a function of the coverage of either furfural decomposition products or furfural itself. Figure S2(a)⁴⁷ shows the infrared spectra around 1800 cm⁻¹ after saturating the Pd(111) surface with furfural and then taking a background spectrum. It was then heated to 350 K, cooled to 180 K, and the spectrum scanned and displayed relative to the initial, furfural-saturated surface. The spectrum, labeled as cycle number “1,” reflects the changes caused by this procedure. The sample was then heated to 500 K to desorb all the CO, but to leave any strongly bound decomposition products. The sample was again cooled to 180 K, and the process was repeated without additional cleaning between each iteration and the resulting spectra are indicated as “2,” 3,” etc. This procedure caused the buildup of adsorbed furfural decomposition products and enabled the evolution in CO coverages and vibrational frequency to be monitored as a function of the coverage of these decomposition products. Figure S2(b)⁴⁷ displays the frequency of the resulting vibrational modes in the CO stretching region as a function of the number of cycles. A clear trend is observed where the infrared peak starts at ∼1850 cm⁻¹ but decreases to 1720 cm⁻¹ as the amount of CO produced by the reaction decreases [Fig. S2(c)],⁴⁷ confirming that the low-frequency features at ∼1745 cm⁻¹ in Fig. 2 are likely to be due to co-adsorbed CO. To further confirm that the low-frequency vibrations are due to shifts in CO vibrational frequency induced by interaction with co-adsorbed molecular adsorbates on the surface, 0.18 ML of CO was adsorbed on Pd(111), and the surface was exposed to furfural and the resulting spectra are displayed in Fig. S2(d)⁴⁷ as a function of furfural coverage, where the furfural coverages are displayed adjacent to the corresponding spectra. The CO peak intensity decreases and is broadened with increasing furfural coverage. The CO vibrational frequency decreases from 1832 cm⁻¹ in the absence of co-adsorbed furfural to ∼1768 cm⁻¹ as the furfural coverage increases. This frequency is relatively close to the reactively formed peak in Fig. 2, thereby confirming that it is due to carbon monoxide formed from the decomposition of furfural on the surface.

Having unequivocally confirmed that peaks in this region of the infrared spectra are due to reactively formed carbon monoxide, the effect of furfural coverage on CO production was investigated by adsorbing various coverages of furfural onto cleaned Pd(111) at 180 K, collecting a background spectrum, heating to 350 K, then collecting an infrared spectrum at 180 K relative to a background spectrum of a furfural-covered surface. The results are shown in Fig. 3(a), where the furfural coverages are displayed adjacent to the corresponding spectrum, and show a CO peak centered at ∼1820 cm⁻¹ for furfural coverages below 0.42 ML, shifting abruptly to ∼1850 cm⁻¹ at higher coverages. The integrated CO absorbance is plotted as a function of furfural coverage in Fig. 3(b), where the CO yield varies linearly with furfural coverage up to ∼0.5 ML, after which the CO yield remains constant. This transition coincides with an abrupt change in CO vibrational frequency and suggests that this change is associated with the transition from flat-lying to tilted furfural.

To further investigate the decomposition pathways of the surface-bound furyl species formed by furfural decarbonylation, the temperature dependences of the infrared spectra of furan, 2-methyl furan, and furfuryl alcohol were also investigated. The infrared spectrum of furan adsorbed on Pd(111) at 90 K is shown in Fig. 4 where the frequencies are summarized in Table II and are in good agreement with those for gas-phase furan.⁴¹ Heating to 125 K causes a decrease in intensity of the in-plane modes and a corresponding increase in out-of-plane intensity, indicating that the molecule lies almost parallel to the surface. Heating to ∼150 K and above significantly broadens the infrared features and a much less intense spectrum indicates that that the furyl vibrations are sufficiently weak that they are not easily detected for the monolayer. A small amount of CO forms at 250 K at 1728 cm⁻¹, which, as discussed above, is shifted to lower frequencies due to interaction with co-adsorbed hydrocarbons. This is in agreement with the results of LITD experiments which show the onset of CO desorption at slightly above 250 K and the formation of a species with C₃H₃ stoichiometry²⁵ subsequently suggested to be due to a propargyl species⁴² and is consistent with the detection of propylene during TPD of furfural.¹⁷ 

The temperature-dependent spectra of 2-methyl furan are shown in Fig. 5, and the assignments are given in Table III. The multilayer species adsorbed at 90 K desorbs to form a flat-lying species on heating to 150 K with the molecular plane close to parallel to the surface,²¹ again producing weak infrared modes. The ring again opens to produce CO at a vibrational frequency of ∼1718 cm⁻¹ occurring at a slightly lower temperature (∼225 K) than for furan (235 K, Fig. 4), indicating that the presence of a methyl group in the 2 position facilitates C–O bond scission.

The analogous infrared spectra for furfuryl alcohol are shown in Fig. 6 and the corresponding assignments are given in Table IV. The multilayer is stable up to 225 K, then desorbs to leave a monolayer with a flat-lying geometry in which the C–O bonds are parallel to the surface¹⁸ and so is infrared invisible.³⁸ Furfuryl alcohol decomposes to form CO upon heating to 300 K, which continues upon heating to 350 K. Upon heating to 400 K and higher, the CO infrared feature decreases in intensity, connoting CO desorption. This process is complete by 500 K to produce a featureless spectrum. The conversion of furfuryl alcohol to furfural on Pd(111) has been previously investigated and the suggested pathway has a DFT predicted energy barrier of 49 kJ/mol, indicating the furfuryl alcohol could follow the decomposition pathway of furfural on Pd(111) after the removal of two hydrogen.²¹ This process is evidenced by the formation of CO at 300 K, higher than the temperatures at which CO is formed from furan (Fig. 4) and 2-methyl furan (Fig. 5). This picture is consistent with temperature-programmed desorption experiments, where hydrogen desorbs at ∼320 K,¹⁷ close to the temperature at which hydrogen desorbs from clean Pd(111),⁴³ suggesting that deprotonation has occurred at lower temperatures. This is confirmed by the detection of some furan at ∼355 K due to the hydrogenation of the furyl ring. This indicates that furfuryl alcohol deprotonates at between 275 and 300 K and then undergoes a rapid decarbonylation reaction to form a furyl species that can then undergo ring opening to desorb further CO.

The resulting furfural reaction pathway is summarized in Fig. 7. The picture that emerges is that furfural adsorbs at low temperature with an orientation that depends on the coverage, adopting a planar geometry at low coverages, but the tilting to accommodate more furfural as the coverage increases, in a behavior that is common to planar molecules adsorbed on transition-metal surfaces.^24,44 The monolayer flat-lying furfural carbonyl modes of 1674 and 1700 cm⁻¹ are unshifted compared to the multilayer and indicate a carbonyl mode that is remote from the Pd(111) surface. The tilted furfural has a tilt angle of 35° and is in accord with the tilted-2 structure predicted by DFT calculations.^18,21 The presence of a mode at 1614 cm⁻¹ indicates the presence of an η¹(O) binding geometry and agrees with the tilted-1 structure predicted by DFT calculations.^33–35 

Heating to 200–250 K causes some furfural to desorb and the majority of the adsorbed species to react by dehydrogenation of the aldehyde group with a calculated activation barrier of ∼91 kJ/mol, followed by a decarbonylation reaction with a calculated energy barrier of 65 kJ/mol.²¹ This process is evidenced by the appearance of a small amount of CO at 257 K. The resulting reaction at temperatures above 275 K follow that of furan on Pd(111), which proceeds by a ring-opening reaction to form CO as evidenced in the infrared spectra and forms a C₃H₃ intermediate, postulated to be a propargyl (CH₂–CCH) species. The TPD results¹⁷ reflect the subsequent reactions of these fragments, with the furyl species hydrogenating between 295 and 375 K, and incorporate deuterium when adsorbed on a deuterium-covered surface. Propylene is formed between 340 and 400 K from the C₃H₃ intermediate, which incorporates a significant amount of deuterium when the reaction is carried out on deuterium-covered Pd. Finally, CO desorbs molecularly at 450 K causing it to disappear from the infrared spectra by 500 K, and hydrogen desorbs in a broad feature between 300 and 500 K as the carbonaceous fragments dehydrogenate.

The surface chemistry of furfural is studied on Pd(111) as a function of coverage and temperature using infrared spectroscopy. This aldehyde is one of the compounds formed by biomass dehydration and is a potential feedstock for value-added products. Information on the surface reaction pathways of furfural are corroborated by comparison with the chemistry of furan, 2-methyl furan, and furfuryl alcohol. The molecular adsorption geometry depends on the coverage and forms a flat-lying species for furfural exposures below ∼2.9 L, but furfural starts to tilt to a maximum of ∼35° at saturation. Heating this surface to ∼175 K results in the formation of an η¹(O) species. Heating to a slightly higher temperature (∼200 K) induces deprotonation of the aldehyde group, which causes additional intensity changes that indicate that the carbonyl group lines are close to flat to the surface, while the furyl group is more tilted. This species is the precursor decarbonylation and the formation of a flat-lying furyl group, which is infrared invisible. The carbon monoxide formed on the surface appears at a lower frequency than that found on clean Pd(111), but experiments on a carbonaceous layer-covered surface confirm that the shifts to lower frequencies are due to the interaction with the carbonaceous layer and confirm that carbon monoxide is indeed formed.

The reaction sequence, in particular, the fate of the furyl ring, is corroborated by studying the surface chemistry of furan and 2-methyl furan and shows that the presence of the methyl group stabilizes it toward decarbonylation. Furfuryl alcohol reaction is initiated by dehydrogenation to furfural, after which the reaction follows that of furfural itself. Finally, the reaction pathway is consistent with the products observed in temperature-programmed desorption. These studies will provide a basis for understanding furfural hydrogenation reactions, in particular, and biomass conversion processes, in general.

We gratefully acknowledge the support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Grant No. DE-FG02-00ER15091, and the National Science Foundation (NSF) under Grant No. CHE-1855199. We also thank Pat Thiel for her stimulating discussions, advice, and friendship over many years, which will be sorely missed.

The data that support the findings of this study are available from the corresponding author upon reasonable request.