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Deactivation of Ceria Supported Palladium Through C-C Scission During Transfer Hydrogenation of Phenol With Alcohols Nicholas C Nelson, Juan Sebastián Manzano, and Igor I. Slowing J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09828 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016
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Deactivation of Ceria Supported Palladium Through
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C-C Scission During Transfer Hydrogenation of
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Phenol with Alcohols
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Nicholas C. Nelson, J. Sebastián Manzano, Igor I. Slowing*
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US DOE Ames Laboratory, Ames, Iowa, 50011, United States
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Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
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ABSTRACT
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The stability of palladium supported on ceria (Pd/CeO2) was studied during liquid flow transfer
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hydrogenation using primary and secondary alcohols as hydrogen donors. For primary alcohols,
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the ceria support was reduced to cerium hydroxy carbonate within 14 h and was a contributing
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factor toward catalyst deactivation. For secondary alcohols, cerium hydroxy carbonate was not
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observed during the same time period and the catalyst was stable upon prolonged reaction.
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Regeneration through oxidation/reduction does not restore initial activity likely due to
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irreversible catalyst restructuring. A deactivation mechanism involving C-C scission of acyl and
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carboxylate intermediates is proposed.
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INTRODUCTION
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Ceria is ubiquitous in catalysis science due to its inherent redox properties. The rare earth
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oxide is most known for its oxidizing properties owed to the facile release and storage of oxygen.
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The oxygen storage properties of ceria rely on the cerium redox cycle which is closely related to
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the concentration and type of lattice defects.1 Oxygen vacancies are the most common defect and
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are often correlated to catalytic activity. The vacancies facilitate Ce3+/Ce4+ redox cycling2, 3 and
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can also activate molecules through adsorption at the defect site.4,
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during chemical transformations involving molecular oxygen and organic oxygenates.6,
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example, methanol has been observed to adsorb dissociatively at defect sites forming methoxy,8
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and the adsorption energy is calculated to be higher than on a defect-free site.9 Analogously,
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longer chain alcohols (i.e. ethanol, propanol) can also dissociatively adsorb on ceria. The
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resulting alkoxy species are activated toward further chemical reaction, which at low temperature
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(90 % selectivity). The
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phenol conversion rates were similar using methanol and ethanol as hydrogen donors. For 1-
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propanol, the conversion rate was much lower than the other primary alcohols. 2-propanol gave a
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higher rate than 1-propanol, but lower than methanol and ethanol. Ethanol was chosen for an 8
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day stability study and showed a monotonic decrease in phenol conversion rate for the duration
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of the experiment (Figure 1b).
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Figure 1. Phenol conversion rate as a function of time over Pd/CeO2 using (a) primary and
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secondary alcohols and (b) ethanol. Reaction conditions: 50 mM phenol, 30 v/v % aqueous
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alcohol, T = 130 °C, 0.1 mL min-1, Vbed = 0.8 mL, 1.0 g Pd/CeO2.
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Post-reaction PXRD analysis of Pd/CeO2 after the 8 day reaction showed formation of cerium
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hydroxy carbonate (Ce(CO3)(OH)) polymorphs (Figure 2a).21, 22 The hydroxy carbonate phase
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was observed after 14 h TOS for all primary alcohols, but was not observed for 2-propanol
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(Figure 2b). Furthermore, the catalytic activity was stable for at least 7 days using 2-propanol.18
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Figure 3a shows the Ce 3d spectral region of Pd/CeO2 before (fresh) and after (aged) the 8 day
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reaction with ethanol. The Ce 3d spectral region for the fresh catalyst showed characteristic
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peaks attributed to Ce(IV),23-25 while the aged catalyst exhibited bands that can be attributed to
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Ce(III) in a carbonate environment.26 The reduction of cerium cations was consistent with the
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cerium hydroxy carbonate phase observed from PXRD. The O 1s spectral region of the fresh and
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aged catalyst showed a band shift from 529.6 eV to 531.4 eV (Figure 3b), consistent with O 1s of
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cerium(IV) oxide27 and cerium(III) carbonate,26, 28 respectively. An additional band was observed
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in the C 1s spectrum for the aged catalyst at 289.7 eV and agrees well with ceria surface
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carbonate species.28-31 Furthermore, a CO2 evolution event around 400 °C was observed during
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temperature programmed desorption (TPD) analysis of Pd/CeO2 after 14 h TOS (Figure S3a).
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The evolved CO2 was likely a decomposition product of cerium hydroxy carbonate,32,
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suggested by XPS analysis (Figure S3b). The PXRD pattern of Pd/CeO2 after the thermal
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desorption experiment showed attenuation of the peaks attributed to the hydroxy carbonate phase
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(Figure S4). The results together indicate that the transfer hydrogenation of phenol with ethanol
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over Pd/CeO2 reduces ceria to cerium hydroxy carbonate polymorphs.
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as
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Figure 2. PXRD pattern of Pd/CeO2 after transfer hydrogenation with (a) ethanol for 8 days and
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(b) primary and secondary alcohols for 14 h. The reference patterns for (a) are color coded the
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same as in (b).
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Figure 3. XPS spectra of Pd/CeO2 before (fresh) and after (aged) 8 day phenol transfer
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hydrogenation with ethanol in the (a) Ce 3d, (b) O 1s, and (c) C 1s spectral region.
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The increasing concentration of the hydroxy carbonate phase (Figure 2) corresponded to the
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decreasing activity (Figure 1b) upon prolonged TOS. Thus, the catalyst deactivation can be
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attributed to the formation of cerium(III) hydroxy carbonate and/or changes in properties upon
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catalyst restructuring. In a recent publication, it was demonstrated that the redox properties of
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ceria influence the turnover rate during phenol transfer hydrogenation using 2-propanol.18 Hence,
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the reduction of redox-active ceria to redox-inactive cerium hydroxy carbonate should decrease
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phenol turnover. However, the transformation of ceria to cerium hydroxy carbonate is a
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reversible process and its deactivating effect could be mitigated through reoxidation. The pure
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ceria phase for the aged catalyst was regenerated by thermal treatment under air (450 °C) (Figure
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S5), followed by Pd activation34 with H2 at 350 °C. Figure S6 indicates that oxidation of cerium
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hydroxy carbonate to cerium oxide did not restore the initial activity of Pd/CeO2. The
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regenerated catalyst was further analyzed through H2 chemisorption and temperature
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programmed reduction (H2-TPR). The textural properties are summarized in Table S2. There was
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a decrease in the Pd dispersion from 20 % to 4 % between the fresh and regenerated catalyst
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which indicated irreversible catalyst restructuring (i.e. metal-support interactions35,
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restructuring of the catalyst with TOS was also evident through its ability to activate hydrogen.
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The H2-TPR profile for the fresh catalyst showed a large hydrogen uptake around 10 °C (Figure
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S7). This peak was essentially absent for the regenerated catalyst and indicated the catalyst has
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decreased hydrogen activation ability. Thus, it appears that the Pd/CeO2 catalyst is irreversibly
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deactivated during phenol transfer hydrogenation with ethanol. This is likely caused by or
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mediated through steps leading to cerium hydroxy carbonate formation.
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). The
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A likely intermediate during transfer hydrogenation using primary alcohols are aldehydes. A
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control experiment was run by replacing ethanol with acetaldehyde and did not result in phenol
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conversion. This indicated that acetaldehyde decomposition does not yield hydrogen atoms that
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are active for phenol reduction. It also indicated that the hydrogen necessary to reduce phenol
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was obtained from alcohol dehydrogenation. However, the PXRD pattern of the catalyst after
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treatment with acetaldehyde showed cerium hydroxy carbonate phases (Figure S8).
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Acetaldehyde trapping experiments with 2,4-dinitrophenylhydrazine (DNPH) during transfer
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hydrogenation with ethanol did not produce the expected hydrazone. Considering acetaldehyde
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resulted in hydroxy carbonate formation and the hydrazone product was not detected in the
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trapping experiment, it appears that most acetaldehyde evolved from ethanol dehydrogenation
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undergoes further reaction with the ceria surface. In contrast, when the reaction was performed
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using 2-propanol, the formation of the hydrazone product was observed.18 From these results,
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Pd/CeO2 becomes deactivated during transfer hydrogenation with primary alcohols due to
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aldehyde reaction with the ceria support, while Pd/CeO2 is more stable using secondary alcohols
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(i.e. ketone formation).
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The formation of carbonates during transfer hydrogenation indicated C-C cleavage precedes
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catalyst deactivation. Ethanol TPD reactions over Pd/CeO2 confirmed C-C scission reactions
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through the evolution of CO, CO2, and CH4 (Figure 4a). Evolution of H2 was not observed which
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could be due to reduction of the ceria support and result in water formation (Figure S9).
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Desorbed ethanol (m/z = 31) was also not observed, while a peak around 350 °C corresponding
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to m/z = 29 was evident (Figure S9). This could be attributed to acetaldehyde or alternatively to
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crotyl alcohol/crotonaldehyde. However, at the temperature used during transfer hydrogenation,
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acetaldehyde was not observed (Figure S9) and was in agreement with DNPH trapping
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experiments. Methane (m/z = 15) desorbed around 100 °C and 400 °C (Figure 4a). There was
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one CO (m/z = 28) desorption around 400 °C (Figure 4a). CO2 desorption began at 100 °C, with
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maxima centered around 200 °C and 325 °C (Figure 4a).
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Figure 4. (a) TPD of ethanol adsorbed onto Pd/CeO2 under He flow (40 mL min-1) at 10 °C min-
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DRIFTS of ethanol adsorbed onto Pd/CeO2 under He flow (30 mL min-1) after heating to the
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indicated temperature and cooling to room temperature.
. The H2, CH4, CO, and CO2 signals correspond to m/z = 2, 15, 28, and 44, respectively. (b) TP-
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Temperature programmed DRIFTS (TP-DRIFTS) analysis of adsorbed ethanol on Pd/CeO2
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was used to identify surface intermediates during TPD experiments (Figure 4b). The room
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temperature spectrum indicated adsorbed ethoxy species identified by the ν(CO) bands at 1052
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cm-1 and 1091 cm-1 corresponding to bidentate and monodentate coordinated ethoxy.11 The band
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at 1389 cm-1 was attributed to δsym(CH3) of ethoxy.37 The bands at 2891 cm-1, 2927 cm-1, and
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2975 cm-1 are recognized as the νsym(CH3), νasym(CH2), and νasym(CH3) of ethoxy species,
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respectively.38 Increasing the temperature to 100 °C resulted in significant decrease of ethoxy
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species and formation of acetate. The bands around 1017 cm-1, 1345 cm-1, 1435 cm-1, and 1547
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cm-1 can be assigned to the vsym(CH3), δsym(CH3), νsym(OCO), and νasym(OCO) modes of acetate,
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resepectivley.11, 39 Due to the similar vibrational signature between acetates and carbonates,40, 41
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it is possible that underlying carbonate bands are present.42 The C-H stretch region showed low
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intensity, which can be explained by the weak acetate C-H stretching.39 Heating to 150 °C
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resulted in growth of a carbonate band11, 41 at 1223 cm-1 and the acetate band at 1017 cm-1, while
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the ethoxy bands at 1052 cm-1 and 1091 cm-1 continued to decrease. The emergence of the band
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at 1704 cm-1 could indicate the v(C=O) mode of acetaldehyde.43-45 Upon increasing the
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temperature to 250 °C, acetate species were still present while formation of two bands at 1389
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cm-1 and 1620 cm-1 appeared. These bands can be attributed to hydrogen carbonates.40 At 450
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°C, acetate species were largely removed, clearly shown by the disappearance of the band at
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1017 cm-1 assigned to the vsym(CH3) mode of acetate. The remaining bands can be attributed to
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carbonates.40, 41
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DISCUSSION The reaction of primary alcohols over ceria-based materials has been studied extensively in the
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context of material property probes,46-52 partial oxidation,11,
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processes.56-61 It is generally accepted that the pathway for ethanol steam reforming or partial
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oxidation over ceria-supported platinum group metals (PGM/CeO2) begins with dissociative
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adsorption to form cerium-coordinated ethoxy.54,
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ethoxy over PGM/CeO2 is through dehydrogenation to acetaldehyde.55, 60, 63 The acetaldehyde
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intermediate has been observed to follow several different reaction pathways over PGM/CeO2
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that can be broadly classified as reduction, oxidation, C-C coupling, and C-C cleavage
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reactions.44,
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dehydrogenation to adsorbed acetyl (CH3CO-) species.44, 53, 54, 62, 65, 67, 68 The acetyl species can
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be decomposed to CH4, CO, CO2, and H2 through various pathways (e.g. water-gas-shift,
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methane reforming, etc.).53, 56, 62, 69 Alternatively, the acetyl species can be oxidized by lattice
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oxygen forming adsorbed acetate species which decompose to CH4, CO, CO2, and H2 through
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55, 62-65
53-55
and steam reforming
The dominant reaction pathway for
The C-C cleavage reaction is proposed to proceed through acetaldehyde
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various pathways.44, 53-56, 62, 65 Acetyl/acetate decomposition is promoted by PGM56 and has been
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observed to yield surface carbonates.56, 68
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Ethanol TPD over Pd/CeO2 showed decomposition to CH4 and CO2 in the low temperature
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regime (< 200 °C) while acetate formation was observed during DRIFTS at 100 °C. At room
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temperature, DRIFTS provided evidence for acetyl and bridging CO species shown by the band
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at 1680 cm-1 (ν(C=O)) and 1923 cm-1, respectively (Figure S10).44 The species were likely
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formed over Pd, which is known to decompose ethanol to H2, CO, and CH4 at room temperature
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through acetyl intermediates.70, 71 Furthermore, IR spectra of the Pd/CeO2 catalyst after reaction
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(Figure S11) clearly showed linear CO (2141 cm-1) adsorbed onto Pd.43 This suggests that
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decomposition through acetyl species was a relevant reaction pathway in this system. From these
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results, ethanol decomposition during transfer hydrogenation likely occurred through acetyl
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and/or acetate intermediates.
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Based on the data obtained for the phenol transfer hydrogenation system and literature results
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for ethanol decomposition over PGM/CeO2, the following deactivation pathway is proposed.
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Ethanol adsorbed dissociatively to form ethoxide and hydrogen (1). The ethoxide intermediate
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was dehydrogenated to yield adsorbed acetaldehyde and hydrogen (2). The hydrogen produced
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from the preceding two steps resulted in phenol turnover (3). The inability for acetaldehyde to
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provide phenol turnover suggests any subsequent reaction pathways involving evolution of
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molecular hydrogen were not relevant.
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(1)
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(2)
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(3)
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As observed by DRIFTS, two possible reaction routes exist for acetaldehyde. For both
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pathways, adsorbed acetyl and hydrogen are proposed to form through acetaldehyde
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dehydrogenation. This reaction is catalyzed by Pd at low temperatures.72 DFT calculations show
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that Pd clusters can lower the C-H dissociation energy by about 50 kcal mol-1 (Table S3). The
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acetyl intermediate can undergo C-C scission to methane and carbon monoxide (4). Hydroxy
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carbonate formation indicates CO was oxidized by lattice oxygen resulting in ceria reduction and
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oxygen vacancy (VO) formation (4). A competing pathway for the acetyl intermediate is through
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oxidation to acetate, again resulting in cerium reduction and oxygen vacancy formation, followed
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by C-C scission to yield methane and carbon dioxide (5).
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(4)
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(5)
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The reaction pathways for methane over Pd and PGM/CeO2 involve desorption, reforming
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(yielding COx, H2, or H2O), and/or dehydrogenation (yielding C and H2). The low temperatures
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used during transfer hydrogenation make reforming unlikely. Dehydrogenation is also
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improbable since phenol turnover was not observed using acetaldehyde. Thus, methane likely
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desorbed, which is consistent with the results of the ethanol TPD experiments (Figure 4a). The
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adsorbed CO2 from (4) and (5) can desorb or exist on the surface as a carbonate. The
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deactivation pathway resulting in cerium hydroxy carbonate formation necessitates CO2 to
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remain adsorbed on the surface in the form of carbonate (6). Hydroxyl formation likely resulted
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from dissociation of water at a nearby oxygen vacancy (7).73,
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carbonate phases using the other primary alcohols suggest the deactivation mechanism proposed
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is general for primary alcohols. The apparent difference between the two proposed pathways
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(Eq. 4 and 5) would be the order of the oxidation and C-C cleavage events, i.e. the likelihood of
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acetyl C-C cleavage preceding CO to CO2 oxidation versus acetyl oxidation to acetate followed
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by C-C cleavage to yield CO2.
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The formation of hydroxy
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(6)
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(7)
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For 2-propanol, the dehydrogenation product (i.e. acetone) was observed during TPD (Figure
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S12), which is consistent with lower adsorption energies for acetone than acetaldehyde on the
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metal surface (Table S4).75 Thus, acetone (or ketone) is less likely to react than acetaldehyde (or
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aldehyde) at low temperature. This could be related to the ability of Pd to activate aldehydes
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through dehydrogenation (i.e. acyl formation) via Pd insertion. For ketones, the absence of H-
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atom on the carbonyl group blocks this pathway.70, 76 The cerium hydroxy carbonate phase was
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not present after 14 h TOS using 2-propanol (Figure 2b); however, its formation was only
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observed after 7 d TOS using a higher alcohol concentration (90 v/v % for 2-propanol versus 30
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v/v % for ethanol, Figure S13). Yet, the transfer hydrogenation rate decreased by only 10 %
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using 2-propanol18 compared to 80 % for ethanol (Figure S14) which likely reflects the degree of
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hydroxy carbonate formation. Still, based on this observation it is expected that longer TOS will
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result in continued hydroxy carbonate formation leading to much lower activity analogous to the
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trend observed for ethanol (Figure 1b). TP-DRIFTS analysis suggests the precursor for hydroxy
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carbonate formation using 2-propanol is acetate as evidenced by the bands at 1019, 1344, 1431,
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and 1544 cm-1 (Figure S15). The relative amount of acetate and dehydrogenated product (i.e.
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acetone or acetaldehyde) adsorbed on the surface is less for 2-propanol than for ethanol. This
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likely contributes to higher catalyst stability using 2-propanol. Zaki et al. proposed that acetate
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formation proceeds through an acetone-like intermediate to yield gas-phase methane (Scheme
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S1).12, 77
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The formation of acetate during 2-propanol TPD over Pd/CeO2 and the lower concentration of
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cerium hydroxy carbonate phase after reaction (Figure 2a and S13) suggest that acetate C-C
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cleavage (Eq. 5) was neither kinetically nor thermodynamically favored under reaction
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conditions. Thus, for ethanol, C-C cleavage events leading to deactivation seem to occur
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preferentially through acetyl species (Eq. 4). Acetate species likely exist primarily as spectators
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during the reaction which is consistent with their decomposition above 250 °C (Figure 4b and
271
S15). The difference between acetyl and acetate decomposition barrier could relate to C-H
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activation events that precede C-C cleavage. The C-C cleavage barrier for acetaldehyde
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(decarbonylation) and acetate (decarboxylation) species is lowered upon dehydrogenation (i.e.
274
through metal insertion).67,
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(BDE = 11 kcal mol-1)81 through metal insertion and formation of acetate represents a
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thermodynamic sink toward C-C cleavage (BDE = 92 kcal mol-1)81 through elimination of metal-
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carbon bonds.
71, 78-80
Acetyl species are already activated toward C-C cleavage
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CONCLUSION
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Transfer hydrogenation using primary and secondary alcohols was studied using Pd/CeO2
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catalyst. For primary alcohols, the ceria support was reduced to cerium hydroxy carbonate within
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14 h TOS and led to irreversible restructuring of the catalyst. C-C scission of primary alcohols,
283
primarily mediated through acyl intermediates, resulted in CO formation. The adsorbed CO
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reacted with the ceria support forming carbonates. This led to cerium reduction and oxygen
285
vacancy formation. The latter provided dissociation sites for water and completed cerium
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hydroxy carbonate formation. Pd/CeO2 showed much higher stability during transfer
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hydrogenation with 2-propanol. However, small amounts of cerium hydroxy carbonate were
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observed after 7 d TOS at higher alcohol concentrations. Hydroxy carbonate formation using 2-
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propanol resulted from decarboxylation of acetate intermediates. The difference in cerium
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hydroxy carbonate formation rates using primary and secondary alcohols was attributed to the
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ability of Pd to activate the carbonyl C-H bond in aldehydes along with the higher C-C cleavage
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barrier for carboxylate (secondary alcohols) compared to acyl (primary alcohols) intermediates.
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Understanding of the deactivation pathway is expected to guide the design of more stable
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catalysts for transfer hydrogenation and low-temperature dehydrogenation of primary alcohols.
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ASSOCIATED CONTENT
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Supporting Information. The following files are available free of charge.
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Materials and methods, material characterization data and graphs (XRD, TEM, TPD, H2-TPR,
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DRIFTS), conversion rate plots, and computational data. (PDF)
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AUTHOR INFORMATION
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Corresponding Author
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*E-mail:
[email protected] 304
Author Contributions
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The manuscript was written through contributions of all authors. All authors have given approval
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to the final version of the manuscript.
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ACKNOWLEDGMENT
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This research is supported by the U.S. Department of Energy, Office of Science, Basic Energy
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Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, through the Ames
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Laboratory Catalysis Science program. The Ames Laboratory is operated for the U.S.
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Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358.
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(78) Lu, J.; Behtash, S.; Heyden, A. Theoretical Investigation of the Reaction Mechanism of the Decarboxylation and Decarbonylation of Propanoic Acid on Pd(111) Model Surfaces. J. Phys. Chem. C 2012, 116, 14328-14341. (79) Lamb, H. H.; Sremaniak, L.; Whitten, J. L. Reaction Pathways for Butanoic Acid Decarboxylation on the (111) Surface of a Pd Nanoparticle. Surf. Sci. 2013, 607, 130-137. (80) Hansen, E.; Neurock, M. First-Principles Based Kinetic Simulations of Acetic Acid Temperature Programmed Reaction on Pd(111). J. Phys. Chem. B 2001, 105, 9218-9229. (81) Luo, Y. R. Comprehensive Handbook of Chemical Bond Energies. CRC Press: 2007.
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Graphic for the TOC
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Phenol conversion rate as a function of time over Pd/CeO2 using (a) primary and secondary alcohols and (b) ethanol. Reaction conditions: 50 mM phenol, 30 v/v % aqueous alcohol, T = 130 °C, 0.1 mL min-1, Vbed = 0.8 mL, 1.0 g Pd/CeO2. Figure 1 685x254mm (72 x 72 DPI)
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PXRD pattern of Pd/CeO2 after transfer hydrogenation with (a) ethanol for 8 days and (b) primary and secondary alcohols for 14 h. The reference patterns for (a) are color coded the same as in (b). Figure 2 716x292mm (72 x 72 DPI)
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XPS spectra of Pd/CeO2 before (fresh) and after (aged) 8 day phenol transfer hydrogenation with ethanol in the (a) Ce 3d, (b) O 1s, and (c) C 1s spectral region. Figure 3 101x211mm (300 x 300 DPI)
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(a) TPD of ethanol adsorbed onto Pd/CeO2 under He flow (40 mL min-1) at 10 °C min-1. The H2, CH4, CO, and CO2 signals correspond to m/z = 2, 15, 28, and 44, respectively. (b) TP-DRIFTS of ethanol adsorbed onto Pd/CeO2 under He flow (30 mL min-1) after heating to the indicated temperature and cooling to room temperature. Figure 4 711x342mm (72 x 72 DPI)
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Graphic for TOC 64x44mm (300 x 300 DPI)
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