Deactivation of Ceria Supported Palladium through CâC Scission

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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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The Journal of Physical Chemistry

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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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1

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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.

74

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

268

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

272

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

278 279

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

284

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

286

hydroxy carbonate formation. Pd/CeO2 showed much higher stability during transfer

287

hydrogenation with 2-propanol. However, small amounts of cerium hydroxy carbonate were

288

observed after 7 d TOS at higher alcohol concentrations. Hydroxy carbonate formation using 2-

289

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

292

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

294

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)

300 301

AUTHOR INFORMATION


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Corresponding Author

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*E-mail: [email protected]

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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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REFERENCES

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(1) Trovarelli, A. Catalysis by Ceria and Related Materials. Imperial College Press: London, 2002. (2) Liyanage, A. D.; Perera, S. D.; Tan, K.; Chabal, Y.; Balkus, K. J. Synthesis, Characterization, and Photocatalytic Activity of Y-Doped CeO2 Nanorods. ACS Catal. 2014, 4, 577-584. (3) Khan, M. M.; Ansari, S. A.; Pradhan, D.; Han, D. H.; Lee, J.; Cho, M. H. Defect-Induced Band Gap Narrowed CeO2 Nanostructures for Visible Light Activities. Ind. Eng. Chem. Res. 2014, 53, 9754-9763. (4) Li, C.; Domen, K.; Maruya, K.; Onishi, T. Dioxygen Adsorption On Well-Outgassed And Partially Reduced Cerium Oxide Studied By FT-IR. J. Am. Chem. Soc. 1989, 111, 7683-7687. (5) Pushkarev, V. V.; Kovalchuk, V. I.; d'Itri, J. L. Probing Defect Sites on the CeO2 Surface with Dioxygen. J. Phys. Chem. B 2004, 108, 5341-5348. (6) Paier, J.; Penschke, C.; Sauer, J. Oxygen Defects and Surface Chemistry of Ceria: Quantum Chemical Studies Compared to Experiment. Chem. Rev. 2013, 113, 3949-3985. (7) Mullins, D. R. The Surface Chemistry Of Cerium Oxide. Surf. Sci. Rep. 2015, 70, 42-85.


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Graphic for the TOC

531 532 533 534


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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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