Morphology, oxygen vacancies, and other factors affecting the performance of supported bimetallic ceria catalysts

Summary Here in we report the development of a Pt-V/CeO2 catalyst performing under mild conditions in amide hydrogenation. Ceria with different morphologies was employed as support in this study. We further developed a glycol-thermal technique that yields thermally stable quantum dot ceria, which can be applied as a support. A systematic investigation revealed the importance of proximity between the small crystalline hydrogenating sites (Pt) and oxophilic sites (V). The study showed that oxygen vacancies on the ceria surface oxidize both Pt and V, poisoning the hydrogenation reaction. In contrast, the absence of oxygen vacancies promoted the hydrogenating ability of Pt sites and also improved their ability to participate in the H2 spillover mechanism and in situ formation of oxophilic V3+. This study demonstrates how the engineering of the oxygen vacancies on the surface of the redox support can manipulate the nature of active sites toward specific reactions.


INTRODUCTION
The reduction reaction is a commonly used transformation in organic chemistry and is extensively applied in laboratory and industrial processes. 1 Considering the reduction of carbonyls within different functional groups, the carbonyl bonds within amides (carboxamides) are the hardest to reduce compared to other functional groups containing carbonyl groups. Therefore, high temperatures (230-350 C) and H 2 pressures of 200-300 bar are required for their catalytic reduction using hydrogen. 1,2 However, this reduction is important in the pharmaceutical, dye, and polymer industries. 1,3,4 The difficulties associated with the reductive deoxygenation of amides are due to the tautomerization of this functional group between amide and imine structures. Therefore, both C-O and C-N bonds are partially unsaturated. Thus, it is of great importance to design a catalyst that directs the tautomerization reaction toward C=N formation (imine) and dehydration of the O-H group if one aims for selective reductive deoxygenation. 1 Otherwise, a mixture of alcohols, products of C-N cleavage, and amines, products of C-O scission, will be obtained. As shown in Figure 1, most of the catalysts applied for amide hydrogenation are bimetallic catalysts composed of hydrogenating and oxophilic metals. Recent advancements in catalyst development for this application have helped overcome the thermodynamics for this challenging reaction. [5][6][7][8] Work done by Mitsudome et al. using state of the art Pt-V/HAP (hydroxyapatite) drastically lowered both the temperature and pressure required for amide hydrogenation compared to what has been reported in the past. 8 In their study, Pt sites were decorated with V. They showed that both Pt and V have small crystallite sizes and have proximity. Pt was found to assist in situ reduction of V 5+ to V 3+ with high oxophilicity. 8 the oxidation states of both platinum and vanadium on the surfaces of these catalysts, but also control the performance of the hydrogen spillover mechanism during the hydrogenation of 4-acetylmorpholine. Our study provides direct evidence of the extraordinary ability of ceria (redox support) in controlling the properties of the loaded metals, which is very important for heterogeneous catalysts applied in hydrogenation or oxidative reactions.

Active metal distribution and supports' morphologies
To first establish the effect of preparation techniques on the properties of the final catalysts, the reference catalyst Pt-V/HAP was prepared via the reported method of Mitsodume et al. 8 We obtained a catalyst where Pt and V showed close proximity as per the reference study (Figures S1E and S1F). In contrast, the application of H 2 PtCl 6 as a Pt precursor and water as the reaction medium resulted in a catalyst with totally different properties (Figures S1C and S1D). The crystallite sizes of Pt were much larger, and the closeness of Pt and V was not apparent. Thus, the preparation technique using the organometallic precursors (as done in the study by Mitsodume et al.) 8 and organic solvents as the media, appears to be a powerful technique in synthesizing bimetallic catalysts where the synergistic relationship between these two metals is essential. According to fundamental studies on the acac precursors used in this study, Pt(acac) 2 and VO(acac) 2 have similar structural geometries of square planar and square pyramidal, respectively. 20,21 This allows them to adsorb on the same sites on the surface of the support, where they remain even after calcining and decomposing the ligands coordinated to these metals in their complex forms. 20,21 Therefore, Pt and V remain in close proximity on the final catalyst surface. Furthermore, the use of a low boiling point organic solvent like acetone (in the impregnation technique) with minimal interaction with applied supports (due to the hydrophobic nature of acetone), allowed for synthesizing supported materials containing active sites with small crystallite sizes, because the solvent can be removed at room temperature and there is minimal effect of the solvent altering the surface properties of the support during catalyst preparation. The obtained crystallite sizes for Pt correspond well to the Pt size measured by Mitsudome et al. where Pt-V/HAP was studied. Therefore, we decided to use the acac method to load Pt and V on the support (including the reference catalyst) for this study. Ceria (as support) with different morphologies, was prepared using the hydro-thermal and glycol-thermal methods. [22][23][24][25] In the naming applied for these catalysts, LS, NS, MM, NR, and NC refer to catalysts containing ceria with large sphere, nanosphere, mixed morphology, nanorods, and nanocube morphologies, respectively. The ceria with different morphologies is in the form of the fluorite phase with a face-centered cubic Bravais lattice (fcc). 26 Different morphologies allow the exposure of different facets on their surfaces.
The final catalysts Pt-V/CeO 2 (with different morphologies) showed different porous structures based on the obtained physisorption data (BET surface areas, pore sizes, and pore volumes are provided in  Table S3). N 2 adsorption-desorption isotherms of Pt-V/CeO 2 (LS), Pt-V/CeO 2 (NR), Pt-V/CeO 2 (NC) consisted of hysteresis loops at high relative pressures indicating that the voids between the particles resulted in their porosity ( Figure 2). In contrast, Pt-V/CeO 2 (NS) and Pt-V/CeO 2 (MM) showed type IV isotherms with H1 hysteresis loops with capillary condensation at lower P/P 0 . This indicates the presence of mesopores within the particles present in Pt-V/CeO 2 (NS) and Pt-V/CeO 2 (MM) as viewed using SEM ( Figure 2).
The HR-TEM and SEM images of these catalysts ( Figure 3) showed that the targeted surface morphologies were obtained and remained intact during the material preparation. As evidenced by the EDX line scan of Pt-V/CeO 2 (MM) shown in Figure 3, close proximity of Pt and V sites exists for these catalysts. Interestingly, the nanospheres within the Pt-V/CeO 2 (NS) catalyst, prepared via a new modified glycol-thermal method, 22 is composed of a grouping of several thermally stable unsupported quantum dot (QD) ceria units which are known to have extraordinary properties (particle size distribution is provided in Figure S2). 27, 28 Having nanospheres made of QD ceria resulted in a high surface area of 107 m 2 /g for this catalyst. To the best of our knowledge, obtaining unsupported QD ceria that is thermally stable (due to calcination at high temperature) has not been reported before. Applying this material as a support (carrier) will likely offer a new field for ceria chemistry in material sciences and heterogeneous catalysis.

Catalytic deoxygenative hydrogenation of amide to amine
The catalysts were tested for the hydrogenation of 4-acetylmorpholine to n-ethyl morpholine as a model for the amide hydrogenation reaction. The results obtained for amide hydrogenation using heterogeneous iScience Article catalysts in a high-pressure reactor depend highly on the reactor design. 1 Therefore, we prepared the Pt-V/ HAP catalyst via the procedure reported by Mitsudome et al. 8 and tested it as a reference catalyst to establish a baseline for our study. In our reactor setup, we obtained a 45% yield to n-ethyl morpholine at 90 C and 30 bar of H 2 pressure (reference values: yield = 70% at T = 70 C and p = 30 bar). The difference in reactor setup required us to operate at 90 C to obtain a comparable yield to that reported at 70 C over the same catalyst. Consequently, we chose 90 C for further testing. Furthermore, we tested the catalysts prepared using H 2 PtCl 6 instead of Pt(acac) 2 and observed no activity, indicating the critical role of the synthetic technique in obtaining a synergistic bimetallic catalyst.
Interestingly, Pt-V/CeO 2 (NS) and Pt-V/CeO 2 (MM) provided the highest yield to N-ethylmorpholine, 73%, and 79%, respectively (Table 1). These values are higher than the reference catalyst 8 tested in our study. The monometallic catalysts did not show any activity signaling the importance of having bimetallic (bifunctional catalysts). Further optimization was done, and the results are provided in Table S1.

Correlation between oxygen vacancies and ease of hydrogenation
Raman analysis was done ( Figure 4A) to study the concentration of the oxygen vacancies (O v ) in the different catalysts. The peak at 590-595 cm À1 is due to the presence of O v in the ceria lattice. The F 2g peak that appears at 500 cm À1 is assigned to the fluorite structure of ceria (the symmetric bending of the oxygen ions surrounding the cerium ions). 29 O v /F 2g can be used to establish the relative concentration of O v in the structure of ceria-based catalysts. Based on Raman analysis the concentration of O v in the ceria lattice of the studied catalysts followed the order of Pt-V/CeO 2 (NR) > Pt-V/CeO 2 (NC) > Pt-V/CeO 2 (NS) > Pt-V/CeO 2 (MM) > Pt-V/CeO 2 (LS) ( Table S2).
H 2 -TPR analysis was done to gain insight into different types of PtO x in these catalysts ( Figure 4 and Table S4). Based on XPS data, it was found that the majority of VO x species in these catalysts were V 2 O 3 which was difficult to reduce, in agreement with Mitsudome et al.'s findings who used the same technique iScience Article and precursor to load V over HAP. 8 This phase of vanadia reduces at temperatures higher than 500 C. Also, ceria reduces at temperatures higher than 450 C. Since the focus of this H 2 -TPR study was to investigate the ease of reduction of loaded PtO x over ceria with different morphologies, the H 2 -TPR experiments were carried out in the temperature range of 50-300 C. The large differences between the acquired H 2 -TPR curves indicate the significant contribution of the support morphology to the performance of the catalysts by providing them with different concentrations of active sites, oxygen vacancies, and so forth. The exposed facets on the surface of the support appear to influence the metal-support interactions between PtO x species and ceria. It is known that bulk PtO and PtO 2 with no metal support interactions (MSI) reduce at temperatures below room temperature. Therefore, these peaks are assigned to Pt 2+ that interact with ceria in the form of Pt-O-Ce. 30,31 Considering this fact, the differences noted in the reduction temperature of PtO x species in these catalysts are merely the result of MSI, thus it was found more applicable to present them as hard and easy to reduce PtO x species. Therefore, the catalysts composed of ceria with large spheres, nanocubes, and nanorods mainly contained hard to reduced PtO x species. In contrast, catalysts with mixed morphologies and nanospheres containing ceria of QD size showed a high concentration of easily reducing PtO x on their surfaces. In addition, the catalysts comprising the highest concentration of oxygen vacancies within their structures (Pt-V/CeO 2 (NR) and Pt-V/CeO 2 (NC)) were found to delay the reduction of PtO x species. However, the catalysts with a relatively low concentration of oxygen vacancies in their structures (Pt-V/ CeO 2 (MM) and Pt-V/CeO 2 (NS)) facilitated the low-temperature reduction of PtO x species on the surfaces of these catalysts. Therefore, an inverse relationship between the ease of reduction of PtO x and both crystallite size of ceria (support) and the concentration of O v vacancies in these Pt-V/CeO 2 catalysts can be concluded.
Effect of oxygen vacancies on oxidation states of loaded metals on the surface  iScience Article data clearly shows that PtO x and VO x on the surface of CeO 2 cause the reduction of ceria on the point of contact between the active metal (oxides) and the support with redox properties via MSI (metal support interaction). However, this phenomenon was not seen when close proximity of Pt and V was established, suggesting the electronic exchanges between two metals (oxides) minimizes their electronic requirements from the support. Thus, similar concentrations of oxygen vacancies on both CeO 2 (MM) and Pt-V/CeO 2 (MM) surfaces were noted. However, this discussion does not apply when the concentration of oxygen vacancies on CeO 2 (NR) and Pt-V/CeO 2 (NR) surfaces are compared. These results indicate that the measured concentration of oxygen vacancies in the final catalysts (Raman) does not necessarily come from the original support. The ceria with different exposed facets interacts differently with the redox-active sites (Pt and V in this case), which alters the concentration of oxygen vacancies detected in the original support.

Contributing factors in determining the ability of catalysts in amide hydrogenation
According to published articles, ceria with nanorod morphology has exposed (110) facets on its surface, whereas this facet is much less exposed in the catalyst with mixed morphology. [32][33][34] The interaction between the exposed (110) facet on the surface of CeO 2 (NR) and PtO x and VO x loaded over it (in Pt-V/ CeO 2 (NR)) resulted in different charge transfer effects to what was discussed for Pt-V/CeO 2 (MM). In the case of Pt-V/CeO 2 (NR), the proximity of Pt and V (oxides) does not switch the electron exchange between PtO x -CeO 2 or VO x -CeO 2 to PtO x -VO x modes. As a result, the bimetallic Pt-V/CeO 2 (NR) showed to have a high surface oxygen vacancy content since both PtO x and VO x contributed toward ceria reduction to Ce 3+ (as they did in the case of the monometallic CeO 2 (MM) catalysts) ( Figures 5A and 5B). These results support stronger MSI in Pt-V/CeO 2 (NR) than Pt-V/CeO 2 (MM). Studying the 4f orbital of PtO x in the Pt/CeO 2 (MM) ( Figure 5C) catalyst using XPS showed that more than 50% of Pt is in the metallic state, while approximately 20% each is present as PtO and PtO 2 . However, these ratios were different for Pt-V/CeO 2 (MM). The co-addition of V resulted in lowering of the Pt content (40%), XPS analysis of V 2p orbitals ( Figure 5D) in V/CeO 2 (MM) and Pt-V/CeO 2 (MM) showed V 4+ as dominating species (89 and 94%, respectively). The remaining V species were V 3+ which has high oxophilicity. The coexistence of Pt with V favored less formation of V 3+ on the catalyst surface. Conversely, no V 3+ was detected in Pt-V/CeO 2 (NR), and high V 4+ (58%) and V 5+ (42%) contents on the surface of this catalyst were measured. This surprising result highlights the extraordinary capability of ceria with nanorods morphology and exposed (110) facets to oxidize the loaded metals (Pt and V) over it at the price of O v formation on its surface. This discovery explains well why ceria with nanorod (or nanocube) morphology is popular for oxidative catalysis but not for hydrogenation. 35 The formation of high in abundance V 5+ with poor oxophilic properties and hard to reduce PtO x species on the surface of this catalyst explains the poor catalytic data obtained for this catalyst.
Both XPS results and H 2 -TPR data showed that the presence of oxygen vacancies on the surface of ceria results in the oxidation of the hydrogenating sites and makes their reduction more difficult. This correlation is better shown in Figure 6. An inverse relationship between the concentration of oxygen vacancies (calculated using Raman) and the specific reaction yield (yield/surface area) was noticed. Furthermore, ceria with a high oxygen vacancy content can oxidize the Pt and V loaded on it (VO x with oxophilic properties in this study). These two metals (Pt and V) thus consume the oxygen from the surface of the ceria to remain in the oxidized phase. Therefore, using ceria with a high oxygen vacancy concentration has an adverse effect on reduction reactions where the active sites are metals in the reduced state. In contrast, literature has shown that it provides a significant advantage to oxidative reactions (metal oxides as active sites). [36][37][38] Ceria is well known to be a redox support and an oxygen reservoir. 15 Therefore, the oxygen vacancies on its surface act differently from other materials because these defect sites become centers to promote the migration of lattice oxygen from the bulk of ceria iScience Article to the surface. 39 As a result, the surface of ceria is highly oxidized, and these oxidizing properties increase as the concentration of oxygen vacancies on the surface increases. The oxygen vacancies act as oxygen shuttles to the surface of the ceria, as discussed in part, in other literature. Now considering that the oxygen vacancies on the surface of ceria induce the movement of oxygen from the bulk to the surface of this support, it expresses why the lower concentration of oxygen vacancies eases the reduction of supported metal oxides. This is indeed a very ceria-specific case. The effect of oxidizing support (ceria) can only be discussed for catalysis that follows a heterogeneous mechanism with an interaction between the support and the active metals. Recyclability tests were performed to confirm whether this catalyst activates amide via heterogeneous or homogeneous pathways. Interestingly, the best catalyst (Pt-V/CeO 2 (MM)) in this study could maintain its performance for at least three cycles. This further supports that this reaction proceeds via a heterogeneous mechanism since the discussed support's effects in the formation of desired active sites cannot be argued if any metal ions dissolved in the solution were responsible for the achieved recyclability results. Besides this fact, there is no study showing that Pt in ionic forms (dissolved in the reaction media) can catalyze this challenging hydrogenation reaction. 40 Based on other reports 15 and our work, we find the oxygen vacancies to be key to the unique chemistry that ceria offers to materials when used as a support. They strongly contribute to the oxidation states of the supported metals on them. Therefore, carefully engineering them is significant to their enhanced catalytic iScience Article activity. Hence, ceria, with no oxygen vacancies can sometimes be a better support for hydrogenation reactions than carbon-based materials. On the contrary, it can be an excellent support for oxidation catalysis if it consists of a high concentration of oxygen vacancies on its surface.

Conclusion
Successful recyclable catalysts were developed for the hydrogenation of 4-acetylmorpholine to n-ethyl morpholine. The crystallite sizes of Pt and V sites and their proximity were found to be the key to achieving any activity over these Pt-V/CeO 2 catalysts. In experiments toward preparing ceria with different morphologies, we discovered a new procedure to prepare unsupported thermally stable quantum dot ceria that can be employed as a support in heterogeneous catalysis. Furthermore, we found that oxygen vacancies are responsible for the oxidation of the active sites, which is undesired for the hydrogenation reaction but beneficial for the oxidation reaction. Ceria with nanorod morphology interacts strongly with Pt and V, thus forming a high concentration of oxygen vacancies on its surface to strengthen the bonding and oxidizing ability. In contrast, ceria with mixed morphology does not catalyze the oxygen vacancy formation and, thus, maximizes the synergistic interactions between Pt and V, with minimal interactions with the support. Therefore Pt-V/CeO 2 with mixed morphology outperformed the other catalysts during the hydrogenation of the amide to the amine.

Limitations of the study
This catalytic reaction generates water (in situ), which poisons the catalytic sites. Therefore, molecular sieves should be included in the catalytic testing. Furthermore, the generality of the catalytic hydrogenation with other amides has not been investigated.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following: Data and code availability d All data reported in this paper will be shared by the lead contact upon request.
d This paper does not report original code.
d Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request

General information
The elemental compositions of the fresh and used catalysts were determined using a Perkin Elmer Optima 5300 DV inductively coupled plasma-optical emission spectrometer (ICP-OES). The nitrogen physisorption analyses were done in a Micromeritics Tri-Star II. The samples were degassed (Micromeritics FlowPep 060 instrument) under nitrogen flow at 200 C overnight before the physisorption analysis. Raman spectra of the fresh catalysts were obtained using a Renishaw inVia Raman microscope at l = 514 nm, 50% laser power, and 10 s of exposure time. The catalyst morphology was viewed using a Zeiss Ultra plus Scanning Electron Microscope (SEM