Low-Temperature Hydrogenation of Acetaldehyde to Ethanol on H

LETTER pubs.acs.org/JPCL

Low-Temperature Hydrogenation of Acetaldehyde to Ethanol on H-Precovered Au(111) Ming Pan, David W. Flaherty, and C. Buddie Mullins* Departments of Chemical Engineering and Chemistry and Biochemistry, Center for Nano and Molecular Science and Technology, Texas Materials Institute, and Center for Electrochemistry, University of Texas at Austin, Austin, Texas 78712-0231, United States

bS Supporting Information ABSTRACT: Gold-based classical high surface area catalysts have been widely investigated for hydrogenation reactions, but fundamental studies on model catalysts are lacking. We present experimental measurements of the reaction of hydrogen adatoms and adsorbed acetaldehyde on the Au(111) surface employing temperature-programmed desorption. Here, we show that chemisorbed hydrogen adatoms bind weakly with desorption peaks at ∼110 K, indicating an activation energy for recombinative desorption of ∼28 kJ/mol. We further demonstrate that acetaldehyde (CH3CHO) can be hydrogenated to ethanol (CH3CH2OH) on the H-atom-precovered Au(111) surface at cryogenic temperatures. Isotopic experiments employing D atoms indicate a lower hydrogenation reactivity. SECTION: Surfaces, Interfaces, Catalysis

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ince the high catalytic activity of nanoscale gold was discovered by Haruta,1
3 Hutchings,4 and Bond,5 gold catalysts have been widely studied for many oxidation reactions including CO oxidation,6,7 the water
gas shift reaction,8,9 the oxidation of alcohols,10,11 and epoxidation of propylene.12,13 In addition, high surface area nanoscale gold catalysts also promote hydrogenation of unsaturated chemical bonds (e.g., CO,14 CO2,15 acyclic alkenes,16,17 cyclic alkenes,18 alkynes,19 and derivatives of benzene20
22). Fundamental studies of gold catalysts regarding oxidation reactions are relatively abundant, and this research has led to enhanced understanding of the related chemistry.23
37 On the other hand, insights into the mechanisms of hydrogenation reactions on gold model catalysts are lacking. As the most stable and readily formed facet on classical catalysts, the Au(111) single crystal is a planar representative of gold-based catalysts. However, it is well-known that gold has a high barrier for the dissociative adsorption of molecular hydrogen,38 which necessitates employing H atoms to populate the surface in vacuum. While Sault, Madix, and Campbell were able to adsorb hydrogen onto the Au(110) surface using a hot filament, there have been no investigations of the hydrogenation reactions on Au surfaces at low pressures.39 Furthermore, there have been no experimental reports concerning adsorption and desorption of hydrogen or investigations of hydrogenation reactions on Au(111). In this Letter, we report on our investigation of hydrogenation chemistry via H adatoms on Au(111) by employing acetaldehyde as a probe molecule. We show that H2 and D2 recombinatively desorb from the clean Au(111) surface with peaks for both between ∼108 and ∼116 K, significantly lower temperatures r 2011 American Chemical Society

than that for Au(110) (216 K).39 We also demonstrate that the hydrogen-covered Au(111) surface can mediate hydrogenation of acetaldehyde to ethanol at 180 K (or lower). Hydrogenation using adsorbed deuterium atoms results in a lower production of CH3CHDOD, suggesting a primary kinetic isotope effect. All experiments were conducted in a supersonic molecular beam apparatus under ultrahigh vacuum (UHV) conditions with a base pressure of ∼2  10
10 Torr.40
43 H atoms were generated by a homemade device in which molecular hydrogen is converted to H atoms and excited molecules [which likely dissociate to H atoms on the Au(111) surface at 77 K] via an electron-beam-heated tungsten capillary held at high temperature.44,45 The Au(111) sample was cleaned by exposure to NO2 at 800 K prior to each experiment.46 A detailed description of the apparatus and experimental procedures is contained in the Supporting Information. Figure 1 displays H2 and D2 temperature-programmed desorption (TPD) spectra from Au(111). Note that exposures of 50 L saturate the sample with H or D; thus, we define a coverage relative to saturation θH,rel (or θD,rel) for lower exposures, as displayed in Figure 1 and referred to later. Hydrogen shows a desorption feature with peak temperatures in the range of 108
111 K in Figure 1a. This small range suggests that the peak temperatures are nearly constant, indicative of first-order desorption kinetics, although we expect second-order kinetics from the recombination of hydrogen atoms and subsequent Received: April 30, 2011 Accepted: May 19, 2011 Published: May 19, 2011 1363

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The Journal of Physical Chemistry Letters

Figure 1. H2 (a) and D2 (b) TPD spectra from Au(111) following adsorption of various relative coverages at 77 K. Coverages of hydrogen (θH,rel) and deuterium (θD,rel) are relative to the saturated surface. Note that (a) and (b) have the same scale on the Y-axis. The heating rate was 1 K/s.

desorption of H2. It has been reported previously that the recombinative desorption rate of oxygen from Au(111) is firstorder or zero-order, as Koel et al.47 and Gong et al.48 observed for O adatom recombination on Au(111). On the basis of the Redhead approximation49 using a frequency factor of 1013 s
1 and desorption temperature of 110 K, the desorption activation energy of H2 is estimated to be ∼28 kJ/mol50 [lower than that on Au(110), 51 ( 4 kJ/mol39], yielding a H
Au surface binding energy of ∼231 kJ/mol, in good agreement with density functional theory (DFT) calculations (214 kJ/mol).51 Additionally, DFT calculations indicate that H atoms favor adsorption on three-fold fcc hollow sites on the Au(111) surface.51 Figure 1b displays D2 TPD spectra from clean Au(111) with desorption peaks centered between 111 and 116 K. This series of TPD spectra show desorption peaks very slightly increasing in temperature with higher deuterium coverages, again suggesting first-order desorption kinetics as with hydrogen (Figure 1a). The desorption activation energy of deuterium is ∼30 kJ/mol52 via the Redhead equation with a frequency factor of 1013 s
1 and a desorption peak temperature of 114 K. Note that the desorption activation energy for deuterium is slightly higher than that for hydrogen, in agreement with previous studies on other coinage metals (Cu53,54 and Ag55). For the same exposures, D2 yields slightly smaller relative coverages than those from H2 desorption

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Figure 2. TPD of ethanol [(a) mass 31 and (b) mass 33] from 0.35 ML acetaldehyde adsorbed on Au(111) with coadsorption of a variety of relative coverages (exposures) of (a) hydrogen and (b) deuterium. Note that (a) and (b) have the same scale on the Y-axis. All species were adsorbed on the surface at 77 K. The heating rate was 1 K/s.

from the Au(111) surface. We believe that this difference is likely due to the efficiency of our H/D thermal cracker for the different isotopes. In order to study hydrogenation chemistry on Au(111), we coadsorbed acetaldehyde on the H-atom-precovered surface at 77 K and subsequently annealed the sample at a ramp rate of 1 K/s. We expect near-unity adsorption of acetaldehyde molecules using a neat molecular beam with a kinetic energy of ∼9.6 kJ/mol56,57 for exposing both the clean and H/Dprecovered Au(111) sample. Figure 2a shows the temperatureprogrammed reaction of coadsorbed acetaldehyde [0.35 ML (monolayer)] and hydrogen (θH,rel = 0.98) on Au(111). Ethanol, produced by hydrogenation of acetaldehyde, begins desorbing at ∼180 K and reaches a maximum at ∼210 K. We also monitored the other characteristic mass fragments for ethanol by QMS including masses 45 and 46, which show a desorption peak at the same temperature as mass 31, confirming the production of ethanol. Multiple control experiments have been performed, as shown in Figure S1 in the Supporting Information, ensuring that ethanol can accurately be identified from mass 31 as due to the hydrogenation of CH3CHO on Au(111) rather than from impurities from our H-atom doser and/or background contaminants. Note that molecularly adsorbed ethanol has two desorption features on the clean Au(111) surface, one at ∼145 K attributed to multilayer desorption and the second at ∼175 K due to monolayer desorption, as shown in Figure S2 (Supporting Information).58 1364

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The Journal of Physical Chemistry Letters We examined the effect of varying hydrogen coverage with a fixed acetaldehyde coverage (0.35 ML of CH3CHO). As shown in Figure 2a, increasing coverages of hydrogen promote higher production of ethanol [i.e., with increasing θH,rel, the relative values of the integrated mass 31 (the
CH2OH fragment of CH3CH2OH) signals in Figure 2a are 0.50, 0.80, 0.89, and 1.00]. We estimate that the acetaldehyde conversion is ∼10%, at most. A better estimate of the conversion is complicated by (i) a negligible (i.e., immeasurable) amount of acetaldehyde being consumed by reaction with atomic hydrogen and (ii) a calculation of the conversion based on the quantity of ethanol produced involving significant uncertainty stemming from the different QMS ionization sensitivities and mass fragmentation ratios of ethanol and acetaldehyde and their pumping speeds. Deuterium was also employed in the reaction in order to confirm our observations on the H-precovered surface and achieve a better understanding of CH3CHO hydrogenation kinetics on Au(111).59
61 As Figure 2b shows, a mass 33 signal (the
CHDOD fragment) is detected with a peak at ∼210 K (similar to the mass 31 signal in the case of adsorbed hydrogen atoms), which is the largest mass fragment for CH3CHDOD. This result and control experiments (not shown here) further confirm the production of ethanol via a reaction between H/D and acetaldehyde. Similar to the hydrogen-precovered surface, increasing deuterium coverage results in more reacted acetaldehyde producing larger amounts of ethanol [i.e., with increasing deuterium coverage, the relative values of the integrals of the mass 33 (CH3CHDOD) signals are 0.41, 0.58, 0.66, and 1.00 in Figure 2b]. Clearly, as illustrated in Figure 2, more acetaldehyde can be hydrogenated on the H-precovered surface than on the deuterated Au(111) surface (i.e., the integrated areas under mass 31 TPD spectra are greater than those under mass 33 signals). We estimate that the deuterium-covered surface yields CH3CHDOD with a production equivalent to roughly one-quarter that for CH3CH2OH formed with the hydrogen-precovered surface under the same reaction conditions. In addition, during the hydrogenation reaction, no H
D scrambling products (such as deuterated acetaldehydes) have been detected. The effect of acetaldehyde coverage as displayed in Figure 3a shows higher production of ethanol with increasing CH3CHO coverage over the range from 0.1 to 1.27 ML (coadsorbed with hydrogen at θH,rel = 0.98). Figure 3b displays the relationship between ethanol production and acetaldehyde coverage, which is linear at low CH3CHO coverages. The production of ethanol plateaus when the CH3CHO coverage reaches ∼1 ML and remains approximately constant with further increases in exposure, suggesting that 1 ML of acetaldehyde can saturate the adsorption/active sites while further adsorption of CH3CHO is incorporated into the multilayer and is not involved in the surface reaction with adsorbed H. The amount of H that desorbs during our temperature-programmed reaction measurements is approximately equivalent to what would be measured without coadsorbed acetaldehyde, further suggesting a relatively low reaction probability for acetaldehyde hydrogenation on Au(111). Similar results concerning D are shown in Figure S3 (Supporting Information), where larger amounts of acetaldehyde cause more ethanol to be formed on the deuterium-coadsorbed surface. To access mechanistic information about acetaldehyde hydrogenation on Au(111), we first investigated adsorption and desorption of acetaldehyde from the clean surface. Indeed, desorption of acetaldehyde from our clean Au(111) surface displays a complicated spectrum as shown in Figure S4 of the Supporting

LETTER

Figure 3. (a) Selected TPD spectra of ethanol from 25 L of hydrogen (θH,rel = 0.98) precovered Au(111) with coadsorption of a variety of amounts of acetaldehyde. (b) Ethanol production with varying CH3CHO coverages on 25 L of hydrogen-precovered Au(111). All species were adsorbed on the surface at 77 K. The heating rate was 1 K/s.

Information, likely indicating four adsorption states including multilayer CH3CHO, monolayer CH3CHO, 3-D polyacetaldehyde, and 2-D polyacetaldehdye.62,63 The interaction of acetaldehyde with other transition-metal surfaces has also been studied,64
66 and polymerization has also been observed at low surface temperatures.62,63 Interestingly, the desorption spectra undergo a complicated transformation when acetaldehyde is coadsorbed with atomic hydrogen on Au(111), indicating a strong interaction between CH3CHO and hydrogen on the surface. Notably, a new desorption feature for acetaldehyde occurs concomitantly with product ethanol desorption, as shown in Figure S5 (Supporting Information), and leads us to speculate that an intermediate complex is created, which decomposes to both CH3CHO and CH3CH2OH beginning at ∼180 K and reaches the highest reaction rate at ∼210 K. We speculate that polyacetaldehyde species form on our Au(111) surface62,63 and are attacked by H/D adatoms to generate intermediate complexes (i.e., H/D modified 2-D polyacetaldehyde) prior to complete H2/D2 desorption. The decomposition of the partially hydrogenated intermediate at 180 K likely leads to the prompt and simultaneous desorption of ethanol and acetaldehyde because this temperature is higher than either molecularly adsorbed CH3CHO or CH3CH2OH (CH3CHDOD) desorption from the clean Au(111) surface. The observation that CH3CHDOD evolves at the same temperature 1365

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The Journal of Physical Chemistry Letters as CH3CH2OH suggests that the barrier for decomposing the modified 2-D polymer is independent of the identity of the hydrogenating species (H or D). Thus, the lower reactivity of hydrogenation using D is a result of a lower probability of forming D-modified polymer in the first step of this reaction. These complicated mechanistic details are not currently fully understood and likely will require further study employing computational modeling and vibrational spectroscopy (these studies are underway). However, our primary message in this Letter is to report H/D adatom recombinative desorption and low-temperature hydrogenation of acetaldehyde to ethanol. In summary, atomic hydrogen and deuterium were populated on the clean Au(111) surface at 77 K under UHV conditions with subsequent recombinative desorption upon annealing to yield desorption peaks in the range of ∼108
116 K with activation energies of ∼28 (H) and ∼30 kJ/mol (D), suggesting that H/D is very weakly bound to the Au(111) surface. Additionally, acetaldehyde is hydrogenated to ethanol on H/D-precovered Au(111) at low temperature with a small conversion (10% at most). Compared to hydrogen, deuterium shows a lower reactivity for the reaction with a smaller production of ethanol. We are hopeful that this study will yield insights into gold as a hydrogenation catalyst.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed experimental procedures, description of control experiments, TPD spectra of ethanol and acetaldehyde on the clean Au(111) surface, and effects of acetaldehyde coverage for the reaction on 25 L of deuterium-precovered Au(111). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: þ1 512 471 7060. Tel: þ1 512 471 5817.

’ ACKNOWLEDGMENT We thank the Department of Energy (DE-FG02-04ER15587) and the Welch Foundation (F-1436) for their support. M.P. acknowledges the William S. Livingston Fellowship for financial support. ’ REFERENCES (1) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153–166. (2) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel Gold Catalysts for the Oxidation of Carbon-Monoxide at a Temperature Far below 0 Degree C. Chem. Lett. 1987, 405–408. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. Gold Catalysts Prepared by Coprecipitation for Low-Temperature Oxidation of Hydrogen and Carbon-Monoxide. J. Catal. 1989, 115, 301–309. (4) Nkosi, B.; Adams, M. D.; Coville, N. J.; Hutchings, G. J. Hydrochlorination of Acetylene Using Carbon-Supported Gold Catalysts — A Study of Catalyst Reactivation. J. Catal. 1991, 128, 378–386. (5) Bond, G. C.; Sermon, P. A.; Webb, G.; Buchanan, D. A.; Wells, P. B. Hydrogenation over Supported Gold Catalysts. J. Chem. Soc.: Chem. Commun. 1973, 444–445. (6) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.; Ichikawa, S.; Haruta, M. Au/TiO2 Nanosized Samples: A Catalytic,

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