Reverse hydrogen spillover during ethanol dehydrogenation on TiO2-supported gold catalysts

Molecular Catalysis 433 (2017) 391–402

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Reverse hydrogen spillover during ethanol dehydrogenation on TiO2 -supported gold catalysts Jorge Cornejo-Romero a , Alfredo Solis-Garcia a , Sofia M. Vega-Diaz b , Juan C. Fierro-Gonzalez a,∗ a b

Departamento de Ingeniería Química, Instituto Tecnológico de Celaya, Av. Tecnológico y Antonio García Cubas s/n, Celaya, Guanajuato, 38010, Mexico Cátedras CONACYT, Instituto Tecnológico de Celaya, Av. Tecnológico y Antonio García Cubas s/n, Celaya, Guanajuato, 38010, Mexico

a r t i c l e

i n f o

Article history: Received 26 October 2016 Received in revised form 16 February 2017 Accepted 26 February 2017 Keywords: Supported gold Ethanol dehydrogenation Reverse hydrogen spillover

a b s t r a c t Cleavage of the ␤ C H bond of ethanol during its dehydrogenation on TiO2 -supported gold samples was investigated by the combined use of infrared (IR) spectroscopy and mass spectrometry. IR spectra characterizing the samples during ethanol adsorption shows that the alcohol was adsorbed both molecularly and dissociatively in the form of ethoxy species bonded to Ti4+ sites. When a supported gold sample was exposed to consecutive pulses of ethanol 1,1-D2 (CH3 CD2 OH) at 220 ◦ C, mass spectra of the products indicated the formation of acetaldehyde 1-D1 (CH3 CDO) and three forms of hydrogen, namely H2 , HD and D2 . Because the formation of H2 was much higher than those of HD and D2 , it was inferred that cleavage of the ␤ C H bond did not necessarily occur on the gold particles, but on sites of the support. Bolstering this idea, IR spectra measured under reaction conditions indicate the appearance of a band at 2712 cm−1 , assigned to the OD vibration mode of deuteroxyl groups on TiO2 . The data are consistent with the abstraction of deuterium atoms from the ethoxy species by basic sites of the support. It is proposed that the presence of gold particles enhances the diffusion of hydrogen atoms on the sample, as evidenced by marked changes in the absorbance of IR and UV–vis radiation during the reaction, which indicate the presence of electrons trapped at defect sites on the metal oxide near the bottom of the conduction band. Those electrons might have been formed during the reverse spillover of hydrogen from the support to the gold particles, on which they were readily recombined and desorbed as H2 . © 2017 Elsevier B.V. All rights reserved.

1. Introduction Ethanol is an important organic compound that can be obtained from renewable resources and be used for the production of H2 through reforming processes on supported metal catalysts [1–3]. Attempts have been made to synthesize better catalysts for ethanol steam reforming (ESR) and to gain insight into the reactions that ethanol undergoes on the surfaces of supported metals [1–5]. At the typical conditions of ESR the alcohol is also dehydrogenated, which motivates research to understand the nature of the surface sites and the identity of ethanol-derived surface species that lead to the formation of acetaldehyde and H2 on the catalysts. Because the dehydrogenation of ethanol involves simple molecules (i.e., ethanol, acetaldehyde and H2 ) its reaction intermediates are also expected to be simple. Thus, the reaction could serve as good model

∗ Corresponding author. E-mail address: jcfi[email protected] (J.C. Fierro-Gonzalez). http://dx.doi.org/10.1016/j.mcat.2017.02.041 2468-8231/© 2017 Elsevier B.V. All rights reserved.

to explain the surface transformations of ethanol on catalysts that also function for ESR. Metal oxide-supported gold catalysts are active and selective for the dehydrogenation and oxidation of ethanol [6–10], but details on the way in which the reaction proceeds are still obscure. In some reports [9,11,12], ethanol is proposed to be dehydrogenated directly on the surface of the supported gold particles. This possibility has been tested by investigating the activity of unsupported gold (e.g., Au(111) single crystals) for the oxidative dehydrogenation of ethanol [13,14]. It was observed that when the surface of Au(111) was precovered with atomic oxygen it became catalytically active. However, the activity of such samples is typically lower than that of supported gold catalysts, and it has been reported that the nature of the support affects both the activity and selectivity of supported gold catalysts [15]. Recently, Panayotov and Morris [16] reviewed the role of the support on the oxidative dehydrogenation of alcohols catalyzed by supported gold. Some proposals suggest that alcohols are activated at the metal-support interface [17], whereas others have proposed that alcohols are activated on the support in the form of alkoxide species and the gold particles

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act as sites for hydrogen abstraction and recombination to give the corresponding aldehyde or ketone and hydrogen [8,10,18,19]. In either case, it is generally agreed that the rate-determining step for the reaction is the cleavage of the ␤ C H bond (␣ to the oxygen) of the alcohols. However, specific details about the identity of the sites where the bond is cleaved are still scarce. Here we show the combined use of infrared (IR) spectroscopy and mass spectrometry to monitor the dehydrogenation of ethanol and ethanol 1,1-D2 (CH3 CD2 OH) on TiO2 -supported gold samples. Our results indicate that the alcohol is adsorbed on the support in the form of ethoxy species, which are dehydrogenated to give acetaldehyde and hydrogen atoms. The latter migrate from the support to the gold particles, where they are recombined to give H2 . Our data are consistent with a process that involves reverse hydrogen spillover. The use of isotopically labeled ethanol allowed us to monitor the cleavage of the ␤ C D bond during the reaction. 2. Experimental 2.1. Synthesis of TiO2 -supported gold samples A deposition-precipitation method [20] was used to synthesize samples of TiO2 -supported gold. During the procedure, a 1 M solution of NaOH (Sigma-Aldrich) was added dropwise to a solution of AuHCl4 (Sigma-Aldrich) at 60 ◦ C under stirring until the pH of the resulting mixture reached a value of approximately 5.0. Then, TiO2 powder (Evonik, P25; 30% rutile and 70% anatase) was added, while the pH value of the slurry was adjusted to 6.0 by means of addition of NaOH at 60 ◦ C. The resulting solid was aged for 60 min and then it was filtered and washed with distilled water. Finally, it was dried at 110 ◦ C for 24 h. The concentration of the AuHCl4 solution and the amount of TiO2 were calculated to give TiO2 -supported gold samples with 5.0% wt Au. 2.2. Characterization of TiO2 -supported gold samples by transmission electron microscopy The TiO2 -supported gold samples were characterized by transmission electron microscopy (TEM) with a Jeol JEM-2100F HRTEM instrument. Prior to the measurements, each sample was ultrasonically dispersed in ethanol and then it was placed on a holey carbon film grid, which was transferred to the microscope. Images were acquired at 200 kV. 2.3. Characterization of TiO2 -supported samples by UV–vis spectroscopy UV–vis spectroscopy was used to characterize the TiO2 supported gold samples. The measurements were conducted in the diffuse reflectance mode with an EvolutionTM 300 (Thermo Scientific) instrument coupled with a Harrick cell. Spectra were recorded in a spectral region from 200 to 1100 nm at 500 nm min−1 . Experiments were also performed as samples of TiO2 -supported gold were treated with a sequence of pulses of ethanol (25 ␮L) at 220 ◦ C with He as a carrier gas. 2.4. Characterization of TiO2 -supported gold samples by X-ray absorption spectroscopy X-ray absorption spectra (XAS) at the Au LIII edge were measured at the X-ray beamline D04B-XAFS1 at the Brazilian Synchrotron Light National Laboratory. For the experiments, self-supporting wafers of the TiO2 -supported gold samples were placed in a stainless steel holder inside a quartz cell that was previously aligned in the beamline. XAS measurements were done in transmission mode for the initially prepared samples, for samples that had been

exposed to a flowing mixture of He at 220 ◦ C for 1 h and for the catalyst after it had been exposed to a flowing mixture of ethanol in He at 220 ◦ C. The beamline was equipped with a Si (111) monochromator and the spectra were calibrated by measuring the spectra of a gold foil in transmission mode. The software XDAP [21] was used for the data reduction and analysis. The data were normalized by dividing the absorption intensity by the height of the absorption edge, which is represented as the inflection point of the first absorption peak at nearly 11,919 eV. 2.5. Analysis of extended X-ray absorption fine structure (EXAFS) data Analysis of the EXAFS data was done by using a difference file technique [22] embedded in the software XDAP [21]. The small atomic X-ray absorption fine structure (AXAFS) [23] (the low-r portion; r is the distance from the absorber atom, Au) was not included in the fit other than by application of background subtraction methods [24]. Fitting was done iteratively until good agreement was attained between the calculated data in k-space (k is the photoelectron wave vector) and the experimental results. Reference files were prepared from EXAFS data of a gold foil and Na2 Pt(OH)6 to fit the phase shifts and backscattering amplitudes of the Au–Au and Au–Osupport interactions. The transferability of the phase shifts and backscattering amplitudes for neighboring atoms in the periodic table has been justified experimentally [25]. The number of parameters to fit the the data was justified statistically by the use of the Nyquist theorem [26]. 2.6. Infrared spectroscopic characterization of TiO2 -supported gold samples IR spectra were recorded with a Nicolet FTIR 6700 spectrometer equipped with a SpectraTech CollectorTM in the diffuse reflectance Fourier transform (DRIFT) mode with a wavenumber resolution of ±4 cm−1 . Each reported spectrum corresponds to 128 scans and KBr was used as a reference material. Prior to the experiments, samples of the bare support and TiO2 -supported gold were loaded into an IR cell that was closed and isolated with two standard three-way valves. The samples were treated in flowing N2 (50 mL NTP min−1 ) at room temperature for approximately 1 h to remove impurities in the environment of the cell. In some experiments, spectra were measured as samples of TiO2 and TiO2 -supported gold were exposed to 25 ␮L of either ethanol (99% purity, Sigma-Aldrich) or ethanol 1,1-D2 (98% D atom, SigmaAldrich) at room temperature with flowing N2 as a carrier gas. In other experiments, spectra were recorded as the samples were treated with a sequence of pulses of 25 ␮L of ethanol or ethanol 1,1 D2 at 220 ◦ C in flowing N2 . 2.7. Mass spectra characterizing the effluent gases from the flow reactor/DRIFT cell Mass spectra of the effluent gases from the flow reactor/DRIFT cell were recorded simultaneously with the IR measurements by means of an on-line Pfeiffer OmniStarTM mass spectrometer. The experiments were done using a secondary electron multiplier detector to monitor the intensities of the main mass fragments of ethanol (m/e = 31, 45), ethanol 1,1-D2 (m/e = 33, 47), acetaldehyde (m/e = 44, 29), acetaldehyde 1-D1 (m/e = 45, 30), H2 (m/e = 2), HD (m/e = 3), D2 (m/e = 4), water (m/e = 18, 17), DHO (m/e = 19), D2 O (m/e = 20), butene (m/e = 56, 41), butene-2,3-D2 (m/e = 43, 58), crotonaldehyde (m/e = 70, 39), and crotonaldehyde 2,4-D2 (m/e = 72, 40). All signals are reported relative to that of the N2 carrier gas (m/e = 28) to remove any effects of pressure fluctuations.

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3. Results

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3.2. Adsorption of Ethanol on TiO2 and TiO2 -supported gold at room temperature

3.1. Evidence of gold nanoparticles supported on TiO2 XANES spectra characterizing an initially prepared TiO2 supported gold sample include a whiteline, centered at approximately 11,923 eV and characteristic of cationic gold (Fig. 1A) [27]. Additionally, spectra include features at 11,934, 11,946 and 11,969 eV, characteristic of zerovalent gold [28]. Thus, it is concluded that the fresh sample contained mixtures of cationic and zerovalent gold, which is consistent with previous reports for supported gold samples prepared by deposition-precipitation [27,29]. XANES spectra of the sample after it had been treated in flowing N2 at 220 ◦ C for 1 h show the absence of a whiteline and the presence of features that resemble those of XANES spectra of a gold foil (Fig. 1A). Therefore, it is concluded that thermal treatment of the sample led to the reduction of the initially present cationic gold to give a sample containing predominantly zerovalent gold. UV–vis spectra of the treated sample show the presence of a surface plasmon resonance centered at 550 nm, which is characteristic of gold nanoparticles (Fig. 1B) [10,30]. Bolstering the UV–vis data, TEM images characterizing the treated sample (Fig. 1C) indicate the presence of gold nanoparticles with diameters ranging from 5 to 12 nm (mean diameter of 7.6 nm). Finally, EXAFS spectra characterizing the treated sample show a first shell Au–Au coordination number of 8.5 (Supplementary data), which is consistent with the TEM results.

IR spectra characterizing the bare TiO2 and the TiO2 -supported gold samples as they were treated in flowing N2 at room temperature are shown in Figs. 2a and 3a , respectively. In both cases, spectra include bands in the range between 3692 and 3630 cm−1 , which are assigned to O H stretching (OH ) vibration modes of isolated hydroxyl groups on the surface of TiO2 [31]. A broad band centered at approximately 3340 cm−1 is also observed. This band can be assigned to the symmetric and antisymmetric OH mode of water coordinated to Ti4+ sites or to OH vibration modes of H-bonded hydroxyl groups on TiO2 [31–33]. When the samples were exposed to 25 ␮L of ethanol, the bands assigned to isolated hydroxyl groups disappeared from their spectra and the band characteristic of hydrogen-bonded hydroxyl groups increased in intensity and became broader (Figs. 2b and 3b), suggesting that at least part of the alcohol was adsorbed by forming hydrogen-bonds with the initially present isolated hydroxyl groups, as others have proposed for the adsorption of various alcohols on supported metals [10,32,34]. Simultaneously, C H stretching (CH ) bands appeared at 2974, 2930, 2902 and 2870 cm−1 in the spectra of both samples, together with other bands at 1452, 1382 and 1272 cm−1 (Figs. 2b and 3b). Those bands are indicative of the presence of molecularly adsorbed ethanol on the samples, as their frequencies are very similar to those of gasphase ethanol [35]. Adsorption of ethanol also led to the appearance

Fig. 1. (A) XANES spectra characterizing a TiO2 -supported gold sample (a) before and (b) after it had been treated in flowing He at 220 ◦ C for 1 h; (c) XANES spectra of a gold foil. (B) UV–vis spectra characterizing a TiO2 -supported gold after it had been treated in flowing He at 220 ◦ C for 1 h. (C) TEM image characterizing the TiO2 -supported gold sample after it had been treated in flowing He at 220 ◦ C for 1 h.

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Table 1 Observed frequencies/cm−1 and band assignments of bands observed in IR spectra during the injection of 25 ␮L of ethanol or ethanol 1,1 D2 to the flow reactor/DRIFT cell containing samples of TiO2 and Au/TiO2 in the presence of flowing N2 as a carrier gas at room temperature and 760 Torr. Samples treated with 25 ␮L of ethanol

Samples treated with 25 ␮L of ethanol 1,1-D2

TiO2

Au/TiO2

TiO2

Au/TiO2

2974 2930 2902 2870

2974 2930 2902 2870

1452 1382 1270

1452 1382 1270

2974 2930 2902 2870 2180 2125 2096 1452 1373

2974 2930 2902 2870 2180 2125 2096 1452 1373

1158

1158

1123 1097 1074 1056

1123 1097 1074 1056

a b c

Assignment

Ref

a (CH3 )a  a (CH2 )a s (CH2 )a s (CH3 )a a (CD2 ) s (CD2 )a s (CD2 )a

[10,30,33–35] [10,30,33–35] [10] [30,33–35] [41] [41] [41] [24–26] [10,30,34,35] [10,33,35] [41] [30,34] [30,34,35] [10,30,33–35] [10,34,35]

␦a (CH3 )a ␦s (CH3 )a ␦ (OH)a ␦ (CD2 )a a (CCO)b (C − C)b,c (C − C)/a (CCO)c a (CCO)c

Molecularly adsorbed alcohol. Alkoxy species bonded linearly to Ti4+ sites. Alkoxy species doubly-bonded to Ti4+ sites.

of four bands between 1123 and 1056 cm−1 (Figs. 2 b and 3b), which are assigned to various vibration modes of ethoxy species bonded to Ti4+ sites [10,33–36]. A summary of the observed bands and their assignments is included in Table 1. The data are consistent with the adsorption of ethanol both molecularly (by formation of hydrogen-bonds with hydroxyl groups) and dissociatively (in the form of ethoxy species on sites of TiO2 ) on the samples. Because spectra of both samples are essentially undistinguishable from each other, it is concluded that the alcohol was preferentially adsorbed on sites of the support. Similar conclusions have been reached by others [10,37–39] who have studied the adsorption of alcohols on various oxide-supported metals. However, we must warn that, in spite of the resemblance between the spectra of the bare support and the supported gold sample, the presence of alcohol-derived

surface species on the gold nanoparticles cannot be completely ruled out. Liu et al. [40] used high resolution electron energy loss (HREEL) spectroscopy to investigate the adsorption of ethanol on the oxygen-precovered surface of Au(111) and observed bands at 1055, 1450 and 2930 cm−1 , which they assigned respectively to CO , ␦CH3 and CH vibration modes of adsorbed ethanol. The bands are similar to those present in our IR spectra (Fig. 2 and Table 1). To investigate the cleavage of the ␤ C H bond of ethanol during its dehydrogenation, experiments were done with ethanol 1,1-D2 . IR spectra recorded during adsorption of the labeled alcohol on the samples show the appearance of bands at 2180, 2125, 2096 and 1158 cm−1 (Figs. 2c and 3c). Street et al. [41] investigated the adsorption of ethanol 1,1-D2 on Cu(111) and observed the presence of bands at 2123 and 2098 cm−1 , which they attributed to

Fig. 2. IR spectra in the regions between (A) 4000 and 2000 cm−1 , and (B) 1500 and 1000 cm−1 characterizing a sample of TiO2 as it was treated at room temperature with (a) flowing N2 , and after it was exposed to pulses of 25 ␮L of either (b) ethanol or (c) ethanol 1,1 D2 .

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Fig. 3. IR spectra in the regions between (A) 4000 and 2000 cm−1 , and (B) 1500 and 1000 cm−1 characterizing a TiO2 –supported gold sample as it was treated at room temperature with (a) flowing N2 , and after it was exposed to pulses of 25 ␮L of either (b) ethanol or (c) ethanol 1,1 D2 .

the existence of two conformers of molecularly absorbed ethanol 1,1 D2 . By analogy with the results of Street et al. [41], we assign the bands at 2125 and 2096 cm−1 to the sCD vibration modes of 2 two conformers of the adsorbed alcohol. In turn, the bands at 2180 −1 and 1156 cm are assigned to the aCD and aCCO /␻CD2 vibration 2 mode of the adsorbed alcohol [41]. Therefore, IR spectra recorded during adsorption of ethanol 1,1 D2 allows for the identification of vibrations associated with the ␤ C D bonds of the alcohol, thus providing an opportunity to monitor their cleavage during ethanol dehydrogenation. 3.3. Reactions of ethanol on TiO2 and TiO2 -supported gold samples at 220 ◦ C Mass spectra of the effluent gases recorded as a TiO2 -supported gold sample was treated with a sequence of pulses of 25 ␮L of ethanol at 220 ◦ C in flowing N2 are shown in Fig. 4A. The results indicate the formation of H2 and acetaldehyde during the admission of each pulse, as evidenced by the changes in the intensities of the mass fragments at m/e = 2 and 44, respectively. The formation of both products was higher during the first pulse of ethanol, and the intensities of their mass fragments decreased with increasing number of pulses until a quasi-steady state was achieved after 15 pulses. H2 and acetaldehyde were also formed (but in a lower extent) when the experiment was done under the same conditions in the presence of the bare TiO2 support (Fig. 4B). These results indicate that the presence of gold favored the ethanol dehydrogenation, consistent with previous observations [8,36,42,43]. The data also show the formation butene and crotonaldehyde in the presence of the supported gold sample, as evidenced by the changes in the signals of the mass fragments at m/e = 56 and 71, respectively (Fig. 4A). The intensities of the mass fragments of those compounds were much lower than those of acetaldehyde and H2 (Fig. 4A) and their formation was essentially negligible in the presence of the bare support (Fig. 4B). It is observed that the formation of butene on the supported gold sample was significantly higher dur-

ing the first admission of ethanol, but that it decreased drastically in the subsequent pulses. The formation of butene could be explained by the reductive coupling of acetaldehyde [10,43–46], whereas that of crotonaldehyde might have occurred by the by ␤-aldolization of two molecules of acetaldehyde [45–47]. IR spectra characterizing the samples during the sequence of ethanol pulses at 220 ◦ C are shown in Fig. 5 (for clarity, data are shown for only the first three pulses and the complete set of data is included in Supplementary data). The data show that as soon as the alcohol was brought in contact with the TiO2 -supported gold sample bands characteristic of molecularly adsorbed ethanol appeared at 2969, 2932, 2876, 1258 and 1386 cm−1 (Fig. 5A). Simultaneously, bands of ethoxy species bonded to Ti4+ sites were observed at 1128, 1070 and 1050 cm−1 (Fig. 5A) [48]. It is observed that the bands at 1128, 1070 and 1050 cm−1 decreased in intensity rapidly between each pulse, consistent with the consumption of the ethoxy species bonded to Ti4+ sites. Accordingly, bands at 1735, 1704 and 1420 cm−1 appeared in the spectra (Fig. 5A). The former two bands are assigned to the CO vibration mode acetaldehyde in the gas-phase and linearly bonded to Ti4+ sites, respectively [10,46,47], whereas the latter might be assigned to the ␦CH3 vibration mode of adsorbed acetaldehyde [46,47,49]. The appearance of those bands is consistent with the dehydrogenation of ethoxy species on the sample, in agreement with mass spectra of the effluent gases from the flow reactor/DRIFT cell showing formation of acetaldehyde (Fig. 4A). The IR data also show the appearance of bands at 1520 and 1448 cm−1 (Fig. 5A), which might be assigned to the aCO and sCO vibration modes of acetate species, respectively [50–53]. The intensities of those two bands increased with increasing time on stream and with every pulse of ethanol, indicating the accumulation of acetate species on the surface of the sample. This observation could explain the decrease in the formation of gaseous products with increasing number of ethanol pulses (Fig. 4A). However, another explanation is the aggregation of the gold particles during the reaction, as evidenced by EXAFS spectra characterizing a used catalyst indicating an increase in the first shell Au–Au coor-

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Fig. 4. Mass spectral signals of the effluent gases from the flow reactor/DRIFT cell recorded as samples of (A) TiO2 -supported gold and (B) TiO2 were treated in a sequence of pulses of ethanol (25 ␮L) with N2 as a carrier gas at 220 ◦ C and 760 Torr. () H2 , (䊉) acetaldehyde, (䊐) butene, and () crotonaldehyde.

dination number from 8.5 to 10.1 (Supplementary data). Finally, the bands at 1647 and 1596 cm−1 could be attributed to C C and CO vibration modes of crotonaldehyde [46,47,54,55]. IR spectra of the bare support during the treatment in the sequential pulses of ethanol at 220 ◦ C are shown in Fig. 5B. Again, admission of each pulse of ethanol led to the appearance of bands characteristic of the adsorbed alcohol. However, in this case the decrease in the intensities of the bands characteristic of surface ethoxy species (at 1128, 1070 and 1048 cm−1 ) was much lower than that observed when the experiment was done in the presence of the supported gold sample. Also, no bands characteristic of acetaldehyde were observed in the spectra (Fig. 5B). These results suggest that less ethanol reacted in the presence of the bare support than on the supported gold sample, in agreement with the lower production of acetaldehyde and H2 (Fig. 4B). 3.4. Reactions of ethanol 1,1-D2 on TiO2 -supported gold samples To gain insight into the surface reactions of ethanol, experiments were performed by exposing the supported gold sample to a sequence of pulses of ethanol 1,1-D2 at 220 ◦ C. Mass spectra of the effluent gases recorded during the experiment are shown in Fig. 6. The results show the formation of acetaldehyde 1-D1 (CH3 CDO), as evidenced by the appearance of a signal at m/e = 45. The data also show the formation of H2 , HD and D2 , as evidenced by the changes in the signal intensities of mass fragments at m/e = 2, 3 and 4, respectively (Fig. 6). It is observed that among the various species of molecular hydrogen, the signal intensity of H2 was higher than those of HD and D2 in all the ethanol 1,1-D2 pulses. The results also show the formation of butene-2,3-D2 (CH3 CDCDCH3 ) and crotonaldehyde-2,4-D2 , as evidenced by the changes in the intensity of signal of mass fragments at m/e = 58 and 73, respectively (Supplementary data).

When the same experiment was done in the presence of the bare support, the production of acetaldehyde 1-D1 and H2 was lower than those observed in the presence of the supported gold sample (Supplementary data), and the formation of HD, D2 , butene-2,3-D2 and crotonaldehyde-2,4-D2 was almost negligible. IR spectra measured as the supported gold sample was treated with the sequence of pulses of ethanol 1,1-D2 at 220 ◦ C are shown in Fig. 7 (again, data are shown for the first three pulses and the complete set is included in the Supplementary data). The data show the rapid appearance bands characteristic of the adsorbed alcohol. It is observed that the bands at 2180, 2090 and 1152 cm−1 , attributed to the aCD ,sCD and aCCO /␻CD2 vibration modes of the alcohol 2 2 decreased in intensity between each pulse, which is consistent with cleavage of the ␤ C D bond. The data also show the appearance of CO bands of acetaldehyde (adsorbed and in gas-phase) at 1705 and 1736 cm−1 (Fig. 7), consistent with mass spectra of the effluent gases from the flow reactor/DRIFT cell showing formation of acetaldehyde 1-D1 during the experiment (Fig. 6). Similar to the results obtained when the experiment was done in the presence of the unlabeled ethanol, IR spectra also include bands of acetate species at 1510 and 1420 cm−1 (Fig. 7), which increased in intensity with increasing time on stream. Notably, admission of each pulse led to the appearance of a small band at 2712 cm−1 . Others [56–58] observed IR bands in the region between 2700 and 2740 cm−1 when samples of partially hydroxylated TiO2 were treated with D2 O and assigned them to ␯OD vibration modes of surface deuteroxyl groups. By analogy, we assign the band observed at 2712 cm−1 to an ␯OD vibration mode of deuteroxyl species. Therefore, it is concluded that surface OD groups were formed on the sample during the dehydrogenation of the labeled alcohol.

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Fig. 5. IR spectra characterizing samples of (A) TiO2 -supported gold and (B) bare TiO2 as they were treated in a sequence of pulses of ethanol (25 ␮L) with N2 as a carrier gas at 220 ◦ C and 760 Torr.

4. Discussion There is a general agreement in the fact that the ratedetermining step for the dehydrogenation and the selective oxidation of alcohols catalyzed by supported gold is the cleavage of the ␤ C H bond to give adsorbed aldehydes or ketones [9,20,59]. However, a debate exists regarding the specific sites on which that bond is cleaved. In part, the debate might arise from the fact that studies with supported gold catalysts have been conducted with samples that have significant structural differences with each other. For example, there are reports for alcohol oxidation and dehydrogenation reactions using catalysts that contain gold particles with average diameters that range from 2 to 20 nm [6–9]. Such structural differences complicate the comparisons between reported results, and might hinder the formulation of generalizations to explain the way in which supported gold particles catalyze specific reactions. In this study, the supported gold particles had an average diameter of approximately 8 nm. Therefore, our interpretation of the data might not apply directly to explain the way in which other supported gold catalyst (with smaller or larger gold particles) function for ethanol dehydrogenation. In general, the proposals that have been advanced to explain the dehydrogenation of alcohols on supported gold can be separated

into two categories: (a) those in which the reaction is proposed to occur directly on the surface of the supported gold particles [11,12], and (b) those suggesting that the alcohols are adsorbed on sites of the support, while the role of gold consists in providing sites for hydrogen abstraction and recombination [8,10,18,19]. The idea that the reaction might take place directly on the surface of the supported gold particles comes from reports [13,14] showing that alcohols can be selectively oxidized even on unsupported gold (i.e., gold single crystals). Gong and Mullins [14] investigated the selective oxidation of ethanol on Au(111) and found that acetaldehyde was formed only when the single crystals had been precovered with oxygen. In contrast, no reaction was observed when experiments were performed with clean surfaces of Au(111). Those results are consistent with other reports [13,14,60–62] showing that alkyl alcohols are only adsorbed molecularly at cryogenic temperatures on single-crystal coinage metals and that they desorb without reacting at relatively low temperatures in the absence of oxygen. Although studies of the reactions of ethanol on unsupported gold might provide insight into the way in which alcohols react on surfaces, the observations on those idealized samples might not necessarily explain the surface reactions that predominate on the more complex structures of powdered and hydroxylated metal oxide-supported gold catalysts. Mass spectra of the effluent gases

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Fig. 6. Changes in the mass spectral signals of the effluent gases from the flow reactor/DRIFT cell recorded as a sample of TiO2 -supported gold was treated in a sequence of pulses of ethanol 1,1-D2 (25 ␮L) with N2 as carrier gas at 220 ◦ C and 760 Torr. (䊉) H2 , () HD, () D2 , and () CH3 CDO.

recorded as our TiO2 -supported gold sample was exposed to pulses of labeled ethanol at 220 ◦ C allowed us to test whether cleavage of the ␤ C D bond occurred directly on the gold particles. In such a process, one would expect that deuterium atoms formed during the rate-determining step of ␤ C D bond cleavage would be recombined rapidly to give D2 . However, our results (Fig. 6) show that H2 was the main product formed during the experiment. Consequently, one might conclude that cleavage of the ␤ C D bond did not occur directly on the surface of the supported gold particles. However, two other possibilities need to be explored, namely: (a) a large isotope effect and (b) substantial scrambling of the molecules in the mass spectrometer. The first-order plots for ethanol and ethanol 1,1-D2 consumption at 220 ◦ C result in essentially straight lines (Supplementary

data). The values of calculated first-order constants for the experiments with unlabeled and labeled ethanol (i.e., kH and kD ) are 3.27 × 10−4 and 2.57 × 10−4 s−1 , respectively. Therefore, the kinetic isotope effect, represented as the kH /kD ratio, is 1.27. This value is relatively small, showing that the reaction of ethanol 1,1-D2 dehydrogenation is only slightly slower than that of the unlabeled alcohol. Thus, the predominant formation of H2 observed during the dehydrogenation of ethanol 1,1-D2 cannot be attributed to a large kinetic isotope effect. This conclusion is in agreement with other reports in which relatively small isotope effects have been observed for the dehydrogenation of alcohols on supported metal catalysts [62–65]. To further explore the relative importance of kinetic isotope effects, experiments were done as a TiO2 -supported Au sample was treated with a sequence of pulses of CD3 CH2 OH at 220 ◦ C. Mass

Fig. 7. IR spectra characterizing a TiO2 -supported gold sample as it was treated with a sequence of pulses of ethanol 1,1-D2 (25 ␮L) and N2 as a carrier gas at 220 ◦ C and 760 Torr.

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Scheme 1. Schematic representation of the dehydrogenation of ethanol 1,1-D2 on a TiO2 -supported gold sample.

spectra of the effluent gases (Supplementary data) show the preferential formation of H2 , with amounts that are comparable to those observed when the experiment were done in the presence of the unlabeled ethanol (Fig. 4). Thus, a secondary kinetic isotope effect is also small and our results thus confirm that kinetic isotope effects do not explain the preferential formation of H2 during the experiments with labeled ethanol. The signals of the mass fragments of H2 , HD and D2 measured during the experiment with CD3 CH2 OH are also useful to gain

information about the importance of the possible scrambling of the molecules during ionization in the mass spectrometer. Indeed, part of the signal of H2 at m/e = 2 in the mass spectra of the effluent gases when the catalyst was exposed to ethanol 1,1-D2 (Fig. 6) might be the result of isotope scrambling. However, if such an effect had dominated, one would expect that the ratios of the signals of HD, H2 and D2 would be similar when the catalyst was exposed to CD3 CH2 OH, and that was not the case (Supporting Information). Therefore, our analysis of the data indicates that kinetic isotope

Fig. 8. Normalized integrated absorbance in the region between 3500 and 1000 cm−1 of IR spectra characterizing samples of (A) TiO2 -supported gold and (B) bare TiO2 are they were exposed to a sequence of pulses of ethanol 1,1 D2 at 220 ◦ C with flowing N2 as a carrier gas.

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effects and scrambling at the mass spectrometer do not contribute significantly to the signals of H2 , HD and D2 in Fig. 6. Thus, we conclude that cleavage of the ␤ C D bond did not occur directly on the surface of the supported gold particles. There is extensive physical evidence of the dissociative adsorption of alcohols on the surfaces of metal oxides [20,33–36]. Our results (Fig. 2b), showing the presence of ethoxy species bonded to Ti4+ sites upon adsorption of ethanol on the TiO2 –supported gold samples are in agreement that evidence. It has been proposed that particles of noble metals (e.g., Pt [39,52], Rh [3], Ir [3], Au [8,10]) facilitate cleavage of the ␤ C H bond of neighboring alkoxide species bonded to metal oxide supports to form adsorbed aldehydes or ketones. The role of the metal particles is thought to consist in providing sites for hydrogen abstraction and recombination to form H2 . Nevertheless, details on the specific way by which the C H bonds are cleaved remain elusive. Specifically, one open question is whether the adsorbed alkoxy species need to be at the metal-support interface for the metal particles to abstract hydrogen atoms directly from them, or whether hydrogen atoms could be subtracted by the support and then migrate to the metal particles, on which they would be recombined and desorbed. Again, if the gold particles participated in the abstraction of deuterium atoms from labeled ethoxy species bonded at Ti4+ sites, one would expect to have observed the predominant formation of D2 and DH from the dehydrogenation of ethanol 1,1-D2 . However, that was not the case. Therefore, we propose that the supported gold particles did not necessarily participate directly in the ␤ C D bond cleavage of species adsorbed in their vicinity. Instead, the appearance of a band characteristic of surface deuteroxyl groups on TiO2 (at 2712 cm−1 ) in the spectra of the supported gold sample during the admission of ethanol 1,1-D2 at 220 ◦ C (Fig. 7) suggests that the support was involved in the reaction. Although the formation of acetaldehyde and H2 was favored when gold particles were present (Fig. 4A), both products were also formed with the bare support (Fig. 4B). Indeed, others [66–68] have observed the dehydrogenation of ethanol and other alcohols on the surfaces of metal oxides. Our results are consistent with those observations and indicate that TiO2 is capable to dehydrogenate the alcohol even in the absence of gold, suggesting that sites on TiO2 could be involved in the cleavage of the ␤ C H (or ␤ C D) bond of ethanol (or ethanol 1,1-D2 ). In such a process, the alcohol is first dissociatively adsorbed on the support in the form of ethoxy species, as represented in Scheme 1 for the adsorption of labeled ethanol. Neighboring basic sites can then abstract a ␤-deuterium atom, leading to the formation of acetaldehyde 1-D1 linearly bonded to Ti4+ sites and a surface deuteroxyl group (Scheme 1). Cosimo et al. [67] proposed an analogous route for the ethanol dehydrogenation on Mgy AlOx catalysts. Appearance of the OD band at 2712 cm−1 during the experiment (Fig. 7) is consistent with the participation of the support in the abstraction of deuterium atoms from the adsorbed species. Because hydrogen atoms can be recombined on the surface of supported metals [10,62,69,70], a reverse hydrogen spillover process to the supported gold is expected to enhance the mobility of hydrogen atoms from surface hydroxyl groups of TiO2 . Iglesia et al. [71] referred to the importance of diffusion of hydrogen atoms on metal oxides during the dehydrogenation of alcohols on supported metals. Therefore, we propose that when supported gold particles are present, hydrogen atoms from surface hydroxyl groups could spill over to them and then be recombined and desorbed as H2 (Scheme 1). In contrast, the lower formation of dehydrogenation products in the absence of gold can be rationalized in terms of the lower diffusion of hydrogen atoms on the surface of bare TiO2 . The reverse hydrogen spillover has been investigated for various dehydrogenation reactions, including those of alkanes [72,73], cycloalkanes [69,74] and alcohols [75,76]. In all those examples,

Fig. 9. UV–vis spectra characterizing a TiO2 -supported gold sample as it was treated in flowing N2 and during the admission of a sequence of pulses of ethanol 1,1-D2 (25 ␮L) at 220 ◦ C and 760 Torr.

it is proposed that organic molecules are adsorbed on sites of the support and cleavage of the C H bonds leads to the formation of hydrogen atoms that migrate to the metal particles, where they are recombined and desorbed in the form of H2 . During the abstraction of hydrogen atoms from the C H bonds, there is formation of: (a) protons that bind to oxygen anions on the metal oxide (in the form of hydroxyl groups) and (b) electrons that could be trapped at defect sites on the metal oxide near the bottom of the conduction band [77–79]. Thus, hydroxyl groups and electrons are expected to be both present on the surfaces of metal oxides during spillover processes. The formed hydroxyl groups are identified by their OH vibration mode in IR spectra, whereas the resulting electrons might be excited to the conduction band when samples are exposed to IR radiation, thus causing an increase in the absorbance in the region between 3500 and 1000 cm−1 . Consequently, IR spectroscopy can also be used as an indirect method to detect trapped electrons that are formed by hydrogen spillover, as others have proposed [80]. Fig. 8 shows the values of the normalized integrated absorbance at the level of the baseline as a function of time on stream in spectra characterizing a supported gold sample and the bare support as they were exposed to the sequence of pulses of ethanol 1,1-D2 . It is observed that the increase in the absorbance was much higher in spectra of the supported gold sample, indicating that more trapped electrons were excited during exposure of that sample to IR radiation. Because the presence of trapped electrons on TiO2 might lead to the partial reduction of the support to form sites containing Ti3+ , we measured UV–vis spectra of a TiO2 -supported gold sample as it was exposed to a sequence of pulses of ethanol 1,1-D2 at 220 ◦ C. Our results (Fig. 9) show that with the admission of each pulse of ethanol spectra of the sample shifted to greater wavelengths (indicating a decrease in the band gap) and the absorbance in the visible region increased. These observations are consistent with the changes in the IR spectra (Fig. 8) and indicate the partial reduction of TiO2 to give Ti3+ , as others have reported [81]. The data bolster our conclusion of the existence of a reverse spillover process during the ethanol dehydrogenation on supported gold. 5. Conclusions Isotopically labeled ethanol was used to investigate the cleavage of the C H bond during the ethanol dehydrogenation catalyzed by TiO2 -supported gold. The results show that the alcohol was

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adsorbed preferentially on sites of the support in the form of ethoxy species. The greater production of H2 with respect to HD and D2 at conditions of alcohol dehydrogenation suggests that the gold particles did not necessarily participate directly in the cleavage of the ␤ C D (or ␤ C H) bond from ethoxy species. Instead, the identification of deutroxyl species on TiO2 suggests that ethanol 1,1-D2 was dehydrogenated by neighboring basic sites of the support. It is proposed that hydrogen atoms removed from ethanol can migrate form the support to gold particles via reverse spillover. In the process, electrons are trapped at defect sites on the metal oxide near the bottom of the conduction band. Because those electrons can be excited by IR radiation, the increase in the absorbance in IR spectra of TiO2 -supported gold samples as they function for ethanol dehydrogenation might be interpreted as indirect evidence of hydrogen reverse spillover. In-situ UV–vis spectra bolster this conclusion by showing changes that are consistent with the formation of Ti3+ sites during the reaction. Those sites might have arisen during the reverse spillover of hydrogen on the functioning catalyst. Acknowledgments We acknowledge the Brazilian Synchrotron Light National Laboratory (LNLS) for financial support and the staff of beamline D04-XAFS1 for assisting with the XANES measurements. This work was supported by the Consejo Nacional de Ciencia y Tecnología (project number 219892 and scholarship awarded to J.C.-R.). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.02. 041. References [1] D.K. Liguras, D.I. Kondarides, X.E. Verykios, Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts, Appl. Catal. B Environ. 43 (2003) 345–354. [2] Agus Haryanto, Sandun Fernando, Naveen Murali, S. Adhikari, Current status of hydrogen production techniques by steam reforming of ethanol: a review, Energy Fuels 19 (2005) 2098–2106. [3] T. Hou, S. Zhang, Y. Chen, D. Wang, W. Cai, Hydrogen production from ethanol reforming: catalysts and reaction mechanism, Renew. Sustain. Energy Rev. 44 (2015) 132–148. [4] D. Zanchet, J.B.O. Santos, S. Damyanova, J.M.R. Gallo, J.M.C. Bueno, Toward understanding metal-catalyzed ethanol reforming, ACS Catal. 5 (2015) 3841–3863. [5] M. Ni, D.Y.C. Leung, M.K.H. Leung, A review on reforming bio-ethanol for hydrogen production, Int. J. Hydrog. Energy 32 (2007) 3238–3247. [6] L. Prati, M. Rossi, Gold on carbon as a new catalyst for selective liquid phase oxidation of diols, J. Catal. 176 (1998) 552–560. [7] S. Biella, M. Rossi, Gas phase oxidation of alcohols to aldehydes or ketones catalysed by supported gold, Chem. Commun. (2003) 378–379. [8] A. Gazsi, A. Koós, T. Bánsági, F. Solymosi, Adsorption and decomposition of ethanol on supported Au catalysts, Catal. Today 160 (2011) 70–78. [9] A. Abad, P. Concepción, A. Corma, H. García, A collaborative effect between gold and a support induces the selective oxidation of alcohols, Angew. Chem. Int. Ed. 44 (2005) 4066–4069. ˜ [10] E.O. Gonzalez-Yanez, G.A. Fuentes, M.E. Hernández-Terán, J.C. Fierro-Gonzalez, Influence of supported gold particles on the surface reactions of ethanol on TiO2 , Appl. Catal. A Gen. 464–465 (2013) 374–383. [11] T. Ishida, M. Nagaoka, T. Akita, M. Haruta, Deposition of gold clusters on porous coordination polymers by solid grinding and their catalytic activity in aerobic oxidation of alcohols, Chem. A Eur. J. 14 (2008) 8456–8460. [12] A. Abad, A. Corma, H. García, Catalyst parameters determining activity and selectivity of supported gold nanoparticles for the aerobic oxidation of alcohols: the molecular reaction mechanism, Chem. A Eur. J. 14 (2008) 212–222. [13] J.L. Gong, C.B. Mullins, Surface science investigation of oxidative chemistry on gold, Acc. Chem. Res. 42 (2009) 1063–1073. [14] J. Gong, C. Mullins, Selective oxidation of ethanol to acetaldehyde on gold, J. Am. Chem. Soc. 130 (2008) 16458–16459. [15] C. Della Pina, E. Faletta, M. Rossi, Update on selective oxidation using gold, Chem. Soc. Rev. 41 (2012) 350–369.

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