The Catalytic Hydrogenation of Maleic Anhydride on Ceo2−Δ-Supported Transition Metal Catalysts

catalysts

Article The Catalytic Hydrogenation of Maleic Anhydride on CeO2−δ-Supported Transition Metal Catalysts

Xin Liao 1,2, Yin Zhang 1, Junqiu Guo 2, Lili Zhao 1, Martyn Hill 2 ID , Zheng Jiang 1,2,* ID and Yongxiang Zhao 1,* 1 Engineering Research Center of Ministry of Education for Fine Chemicals, School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China; [email protected] (X.L.); [email protected] (Y.Z.); [email protected] (L.Z.) 2 Faculty of Engineering and the Environment, University of Southampton, Highﬁeld, Southampton SO17 1BJ, UK; [email protected] (J.G.); [email protected] (M.H.) * Correspondence: [email protected] (Z.J.); [email protected] (Y.Z.); Tel.: +44-0-238-059-4893 (Z.J.)

Academic Editor: Leonarda F. Liotta Received: 11 August 2017; Accepted: 8 September 2017; Published: 14 September 2017

Abstract: The proper selection of transition metals and support is pivotal to the design of active and selective catalysts for maleic anhydride hydrogenation (MAH). Herein, the M/CeO2−δ (M = Co, Ni, Cu, respectively) catalysts with pre-optimised metal loading of 10 wt % were prepared via a wet impregnation method and well characterized to corroborate their MAH performance with the properties of metal, support and the M/CeO2−δ catalysts. The results revealed that the metal dispersion on the catalyst declines in the order of Ni/CeO2−δ > Co/CeO2−δ > Cu/CeO2−δ, similar to the apparent activity for maleic anhydride (MA) transformation to succinic anhydride (SA). The hydrogenolysis of SA to γ-butyrolactone (GBL) occurs on Ni/CeO2−δ and Co/CeO2−δ only when the MA → SA transformation completing. The Ni/CeO2−δ displays superior activity and selectivity to Co/CeO2−δ in both MA → SA and SA → GBL reactions, while the Cu/CeO2−δ and CeO2−δ are both inert for SA → GBL hydrogenolysis. The MA hydrogenation to SA follows the ﬁrst order kinetic law on the Ni/CeO2−δ and Co/CeO2−δ catalysts yet a more complex kinetic characteristics observed on the Cu/CeO2−δ. The distinct catalytic hydrogenation behaviours of the M/CeO2−δ catalysts are assigned to the synergism of dispersion and electronic conﬁguration of the transition metals and oxygen vacancies.

Keywords: hydrogenation; maleic anhydride; transition metal catalysts; ceria; oxygen vacancy

1. Introduction Catalytic hydrogenation of maleic anhydride (MA) is an essential route for the current industrial production of the succinic anhydride (SA), γ-butyrolactone (GBL) and tetrahydrofuran (THF), which are value-added intermediates for pharmaceuticals and bio-degradable polymers and highly-demanded solvents for various industrial processes [1–3]. However, the MA hydrogenation (MAH) is a rather complex process that requires deliberately selecting metal catalyst to avoid excessive hydrogenation and concomitance of ring-open side reaction [1]. Currently, the main catalysts widely applied in the MAH processes are noble metal catalysts (Pd, Ru and Au) [4] and transition metal copper chromite (Cu-Cr) catalysts [5], though they are either costly, less selective or toxic. For developing environmentally-friendly and low-cost alternative MAH catalysts, extensive efforts have been made on: (1) exploring various transition metal Ni [1,2,6], Co [3], Cu [7–9] and bimetallic catalysts [10] with controlled particle sizes and surface dispersion; (2) modifying the traditional porous Al2O3 [11], SiO2 [3], zeolite [2] and SiO2-Al2O3 composite oxide supports [8] using functional promoters (e.g., CeO2 [12], TiO2 [13] and ZrO2 [14], etc.) to moderate the

Catalysts 2017, 7, 272; doi:10.3390/catal7090272 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 272 2 of 15 surface acidity and redox properties [8,15,16]; and (3) employing new catalyst supports, for example the structure-defective TiO2 [17] and CeO2 [1] have emerged as excellent supports for metallic Ni catalysts and demonstrated exceptional selectivity in MAH. The previous research advances in MAH catalysts revealed that their activity and selectivity depend greatly on the proper selection of metal and support and on the corresponding metal-support interaction. However, the systematic and comparative studies of the inﬂuence of transition metals and their supports on the MAH performance have not been well documented in literature, in particular the emerging ceria and titania supports. In our previous work, the superior activity and selectivity in MAH on the Ni/CeO2−δ catalyst to those on the Ni/Al2O3 catalyst was attributed to the synergism of the Ni-CeO2−δ interaction, the oxygen vacancies and developed porosity of the CeO2−δ support [1]. In theory, the electron-rich Ovac in CeO2−δ may modify the geometric and electronic structure of the active metal [18,19], as well as enforce the metal-support interaction [20]. It is thought that the Ovac defects could synergistically enhance the catalytic performance of M/CeO2−δ catalysts, through coordination with the oxygen-containing group in OH cleavage [21], NO2 reduction [22] and CO2 methanation [23]. Moreover, the ionic TM δ+ (M ) species can diffuse into the CeO2 lattice and modulate the population of Ovac in mixed oxides with dependence of the ionic radius and the valence state of the M [24,25]. Although M/CeO2−δ catalysts have been examined in various catalytic reactions, there is very limited knowledge of the metal inﬂuences on the CeO2−δ supports, particularly for the activity and selectivity in MAH. In this paper, various TMs (Co, Ni, Cu) supported on ceria were prepared by a simple wet impregnation method and employed in liquid phase hydrogenation of MA at 210 ◦C and 5 MPa. Through careful characterizations, the catalyst activity and product selectivity in MAH on the M/CeO2−δ were investigated and correlated to the properties of the materials.

2. Results and Discussion

2.1. Structure and Morphology

Figure1A shows the powder X-ray diffraction (XRD) data of CeO 2 and MOx/CeO2 (M = Co, Ni, Cu, respectively) precursors. For the CeO2 and MOx/CeO2 samples, the major Bragg diffraction peaks at 28.5◦, 33.1◦ and 47.5◦ are clearly resolved and attributed to the diffractions from the (111), (200) and (220) planes of the cubic fluorite-type ceria (JCPDS file 34-0394), respectively. The weak diffraction peaks are associated with isolated metal oxides and indexed to CuO (JCPDS file 48-1548), NiO (JCPDS file 78-0643) and Co3O4 (JCPDS file 43-1003) (Figure1A). The diffraction peaks of CeO 2 in the MOx/CeO2 catalysts slightly shift to greater 2θ angles than those of bare CeO2, revealing that +δ small portions of the M ions in the loaded MOx have diffused into the ceria matrix and partially substituted cerium cations (radius of Ce4+ = 0.084 nm) [1,25,26]. Indeed, the metal doping induces lattice distortion and shrinkage of the CeO2 lattice to different levels (Table1) due to the smaller radii of the doping cations (R(Co2+) = 0.074 nm and R(Co3+) = 0.063 nm, R(Ni2+) = 0.072 nm and 2+ R(Cu ) = 0.073 nm), but the changes of lattice parameters of CeO2 for the samples are insignificant because of the low doping level. The calculated mean crystalline sizes of Co3O4, NiO, CuO and CeO2 are listed in Table1. It is found that NiO possesses the smallest crystallite size, while CuO the largest. The CeO2 supports exhibit different crystallite sizes even though the MOx are loaded at similar amounts. The varying crystallite sizes of MOx and CeO2 are due to their different MOx-CeO2 interaction and the induced lattice distortion. Catalysts 2017, 7, 272 3 of 15 Catalysts 2017, 7, 272 3 of 15

 CuO  NiO Co O (A)  3 4  CeO2   (002) (111) CuO/CeO2

(111) (200) NiO/CeO  2  Intensity(a.u.) (311) Co3O4/CeO2 

CeO2 20 25 30 35 40 45 50 55 2theta

 (B)  Co  Ni Cu  CeO2-

  (111) Cu/CeO2-

(111) Ni/CeO2- 

Intensity (a.u.) Intensity (111)  Co/CeO2-

CeO2-

20 25 30 35 40 45 50 55 2theta

FigureFigure 1. 1.XRD XRD pattern pattern of of ( A(A) the) the MO MOx/CeOx/CeO22precursors precursors and and ( B(B)) the the M/CeO M/CeO2−2−δδ catalystscatalysts (M = Co, Ni,Ni, Cu,Cu, respectively). respectively).

TableTable 1. 1.The The properties properties of of the the MO MOx/CeOx/CeO2 2precursors precursors and and corresponding corresponding M/CeO M/CeO22−−δδ catalysts.catalysts.

a(CeO2) a(CeO2−δ) Loading Crystallite Size (nm) a(CeO ) a(CeO ) Loading Crystallite Size (nm) SampleSample 2 2−δ (nm) (nm) (wt %) D(CeO2) D(CeO2−δ) D(MOx) D(M) (nm) (nm) (wt %) D(CeO2) D(CeO2−δ) D(MOx) D(M) CeO2 0.5438 0.5438 ‐ 12.5 12.9 ‐ ‐ CeO2 0.5438 0.5438 - 12.5 12.9 - - Co/CeO2−δ 0.5430 0.5442 10.3 14.3 15.2 15.7 12.8 Co/CeO2−δ 0.5430 0.5442 10.3 14.3 15.2 15.7 12.8 Ni/CeONi/CeO2−δ2−δ 0.54240.5424 0.5441 0.5441 9.8 9.8 12.5 12.5 16.5 16.5 13.2 13.2 9.8 9.8 Cu/CeOCu/CeO2−δ2−δ 0.54130.5413 0.5442 0.5442 9.7 9.7 13.2 13.2 15.7 15.7 16.7 16.7 13.4 13.4 Note:Note: a(CeO a(CeO2)2 and) and a(CeO a(CeO2−δ)2 denote−δ) denote the latticethe lattice parameter parameter of CeO 2ofin theCeO MO2 inx/CeO the 2MOandx/CeO reduced2 and√ M/CeO reduced2−δ, h k l a = d × h2 + k2 + l2 respectively,M/CeO2−δ, calculatedrespectively, using calculated interplanar distanceusing interplanar (d) and Miller distance indices ( (d)) inand the Miller equation indices (h k l) in the. Metal loading amount was determined by ICP-OES. D(CeO2) and D(CeO2−δ) are the crystalline size of CeO2 in equation √ . Metal loading amount was determined by ICP‐OES. D(CeO2) and the MOx/CeO2 and M/CeO2−δ; D(MOx) and D(M) denote the crystallite size of metal oxide and metal calculated usingD(CeO the2−δ Scherrer) are the equation. crystalline size of CeO2 in the MOx/CeO2 and M/CeO2−δ; D(MOx) and D(M) denote the crystallite size of metal oxide and metal calculated using the Scherrer equation. After H reduction treatments, metal phases of Ni, Co and Cu emerge in the XRD patterns of the After H22 reduction treatments, metal phases of Ni, Co and Cu emerge in the XRD patterns of the reduced M/CeO catalysts (Figure1B), while the MO diffraction peaks vanish since the crystalline reduced M/CeO22−−δδ catalysts (Figure 1B), while the MOxx diffraction peaks vanish since the crystalline MO species are converted to corresponding metals. Compared to Co and Cu, the Ni species display MOxx species are converted to corresponding metals. Compared to Co and Cu, the Ni species display relatively weaker diffraction, suggesting that Ni might be highly dispersed on the surface of CeO relatively weaker diffraction, suggesting that Ni might be highly dispersed on the surface of CeO2−2δ−δ withwith a a smaller smaller crystallite crystallite size size (Table (Table1 ).1). We We suppose suppose that that the the smaller smaller crystallite crystallite sizes sizes of of metals metals relative relative to their metal oxides (Figure2) are induced by the H reduction treatment because the M-CeO to their metal oxides (Figure 2) are induced by the H2 2 reduction treatment because the M‐CeO2−2δ−δ would inherit the strong interaction between MO and the CeO . The proportional reliance of metal would inherit the strong interaction between MOxx and the CeO22. The proportional reliance of metal crystallitecrystallite sizes sizes on on their their corresponding corresponding metal metal oxides oxides is further is further conﬁrmed confirmed by the crystalliteby the crystallite size changes size changes of Ni/CeO2−δ and NiO/CeO2 samples (Figure S1 in Supplementary Materials), where the

Catalysts 2017, 7, 272 4 of 15

Catalysts 2017, 7, 272 4 of 15 Catalysts 2017, 7, 272 4 of 15 of Ni/CeO2−δ and NiO/CeO2 samples (Figure S1 in Supplementary Materials), where the strong strong dependence of the Ni0 crystallite sizes on their oxide precursors showed a linear correlation strongdependence dependence of the Ni of0 thecrystallite Ni0 crystallite sizes on sizes their on oxide their precursors oxide precursors showed showed a linear correlationa linear correlation with the with the Ni loading up to 20 wt %. withNi loading the Ni uploading to 20 wtup %.to 20 wt %.

13.5 Cu~CuO 13.5 Cu~CuO 13.0 13.0

12.5 Co~Co3O4 12.5 Co~Co3O4 12.0 12.0 11.5 11.5 11.0 11.0 10.5 10.5 10.0 10.0 Ni~NiO

Metal crystallite size (nm) size crystallite Metal Ni~NiO Metal crystallite size (nm) size crystallite Metal 9.5 9.5 13 14 15 16 17 13 14 15 16 17 MOMO crystallitecrystallite sizesize (nm)(nm)

Figure 2. The dependence of metal crystallite sizes on those of metal oxides. FigureFigure 2.2. TheThe dependencedependence ofof metalmetal crystallitecrystallite sizessizes onon thosethose ofof metalmetal oxides.oxides.

As shown in Table 1, the lattice parameters of ceria in the M/CeO2−δ catalysts appear quite As shown in TableTable1 ,1, the the lattice lattice parameters parameters of of ceria ceria in in the the M/CeO M/CeO2−2−δδ catalystscatalysts appear quite similar, but become larger than those of MOx/CeO2 samples. The increasing crystallite sizes of ceria similar,similar, but but become become larger larger than than those those of of MO MOx/CeOx/CeO2 samples.2 samples. The increasing The increasing crystallite crystallite sizes sizesof ceria of in M/CeO2−δ catalysts may be induced by comprehensive factors, including a fraction of Ce4+ cations4+ inceria M/CeO in M/CeO2−δ catalysts2−δ catalysts may be mayinduced be induced by comprehensive by comprehensive factors, factors,including including a fraction a fraction of Ce4+ cations of Ce being reduced to Ce3+ of a larger3+ ionic radius (RCe(III) = 0.102 nm [27]) that expands the ceria lattice beingcations reduced being reduced to Ce3+ toof Cea largerof a ionic larger radius ionic radius(RCe(III) = (R 0.102Ce(III) nm= 0.102 [27]) nm that [27 expands]) that expands the ceria the lattice ceria parameter,parameter,lattice parameter, thethe thermalthermal the thermal sinteringsintering sintering ofof ceriaceria of ceria andand andthethe themetalmetal metal exitingexiting exiting fromfrom from ceriaceria ceria matricesmatrices matrices duringduring during thethe the reductionreductionreduction process.process. Figure 3 shows the transition electron microscopy (TEM) images of reduced the M/CeO2−δ Figure 33 showsshows the the transition transition electron electron microscopy microscopy (TEM) (TEM) images images of of reduced reduced the the M/CeO M/CeO2−2−δδ catalysts. It is found that most CeO2 crystallites observed in the M/CeO2−δ samples are of spherical or catalysts. It It is is found found that that most most CeO CeO22 crystallitescrystallites observed observed in in the the M/CeO M/CeO2−δ2 −samplesδ samples are are of spherical of spherical or irregularirregularor irregular polyhedralpolyhedral polyhedral morphologies,morphologies, morphologies, independentindependent independent ofof the ofthe the depositeddeposited deposited metals.metals. metals. FromFrom From thethe the TEMTEM TEM images,images, images, itit isis it difficult to distinguish the metal particles because the sizes of metals are quite similar to the CeO2 difficultis difﬁcult to todistinguish distinguish the the metal metal particles particles because because the the sizes sizes of of metals metals are are quite quite similar similar to to the CeO2 support (shown in Table 1) [25]. In addition, a larger CeO2−δ particle is seen in the Co/CeO2−δ catalyst supportsupport (shown inin TableTable1 )[1) 25[25].]. In In addition, addition, a a larger larger CeO CeO2−2δ−δ particleparticle is is seen seen in in the the Co/CeO Co/CeO2−2−δδ catalystcatalyst compared with Ni/CeO2−δ and Cu/CeO2−δ, which is likely associated with the slight sintering of the compared withwith Ni/CeONi/CeO2−δ− δandand Cu/CeO Cu/CeO2−δ2,− whichδ, which is likely is likely associated associated with with the the slight slight sintering sintering of of the the ceriaceria supportsupport duringduring thethe reductionreduction atat higherhigher temperature. temperature.

(A)(A) (B)(B) (C)(C)

FigureFigure 3. TEM 3. TEM images images of M/CeO of M/CeO−δ2−δsamples. samples. ( A(A)) Co/CeO Co/CeO2−−δδ; ;((BB) )Ni/CeO Ni/CeO2−δ −andδ and (C) ( CCu/CeO) Cu/CeO2−δ. −δ. Figure 3. TEM images of M/CeO2 2−δ samples. (A) Co/CeO2 2−δ; (B) Ni/CeO2−δ2 and (C) Cu/CeO2−δ. 2

2.2. Reduction Behaviour of the MOx/CeO2 Catalyst Precursors 2.2. Reduction Reduction Behaviour Behaviour of the the MO xx/CeO/CeO22 CatalystCatalyst Precursors Precursors

Figure 4 shows the hydrogen temperature‐programmed reduction (H2‐TPR) profiles of Figure4 4shows shows the hydrogenthe hydrogen temperature-programmed temperature‐programmed reduction reduction (H 2-TPR) (H proﬁles2‐TPR) of profiles MO/CeO of2 MO/CeO2 samples. The TPR profiles at the temperature◦ below 400 °C are due to the reduction of MO/CeOsamples.2 The samples. TPR proﬁles The TPR at the profiles temperature at the temperature below 400 C below are due 400 to the°C reductionare due to of the supported reduction MO ofx supported MOx in the three MOx/CeO2 samples. For the CuO/CeO2 sample, the strong peak centred◦ supportedin the three MO MOx inx/CeO the three2 samples. MOx/CeO For2 thesamples. CuO/CeO For the2 sample, CuO/CeO the2 strongsample, peak the strong centred peak at 160 centredC is at 160 °C is attributed to the reduction of isolated CuO species with weak CuO‐CeO2 interaction, at 160 °C is attributed to the reduction of isolated CuO species with weak CuO‐CeO2 interaction, while the shoulder peak located at 130 °C is due to the reduction of adsorbed O species on Ovac sites while the shoulder peak located at 130 °C is due to the reduction of adsorbed O species on Ovac sites

Catalysts 2017, 7, 272 5 of 15

attributedCatalysts 2017 to, 7 the, 272 reduction of isolated CuO species with weak CuO-CeO2 interaction, while the shoulder5 of 15 ◦ peak located at 130 C is due to the reduction of adsorbed O species on Ovac sites and the highly dispersedand the highly CuO dispersed with strong CuO CuO-CeO with strong2 interaction CuO‐CeO [262 ].interaction For the NiO/CeO [26]. For 2thesample, NiO/CeO the2 reduction sample, the of surfacereduction adsorbed of surface O species adsorbed starts O from species an evenstarts lower from temperaturean even lower (~100 temperature◦C) [28] with (~100 a sharp °C) [28] reduction with a ◦ peaksharp at reduction 195 C, which peak canat 195 be related°C, which to the can reduction be related of highlyto the reduction dispersed NiOof highly on CeO dispersed2 [28]. The NiO main on ◦ TPRCeO2 peak [28]. at The 260 mainC is TPR assigned peak at to 260 the reduction°C is assigned of NiO to the clusters reduction aggregated of NiO on clusters the CeO aggregated2 surface [on1]. Onthe CeO the Co2 surface3O4/CeO [1].2 Onsample, the Co three3O4/CeO distinct2 sample, TPR three peaks distinct are observed, TPR peaks which are observed, could be which ascribed could to 3 4 →◦ →◦ thebe ascribed reductions to the of reductions Co3O4 → CoOof Co (220O C), CoO CoO (220→ °C),Co CoO (270 C) Co [ (27029,30 °C)] and [29,30] isolated and isolated CoO species CoO ◦ (380speciesC), (380 respectively °C), respectively [25]. It is worth[25]. It noting is worth that thenoting CeO 2thatreduction the CeO occurs2 reduction at a lower occurs temperature at a lower for Cotemperature3O4/CeO2 forrelative Co3O to4/CeO other2 relative samples, to becauseother samples, CoOx can because enhance CoO Hx2 canspillover enhance from H2 the spillover reduced from Co speciesthe reduced towards Co CeOspecies2, which towards decreases CeO2, the which CeO 2decreasesreduction the temperature CeO2 reduction [31]. The temperature different interaction [31]. The betweendifferent theinteraction MOx and between CeO2, asthe indicated MOx and by CeO their2, as distinct indicated reduction by their behaviours distinct reduction of the MO behavioursx/CeO2 samples,of the MO wouldx/CeO determine2 samples, the would dispersion determine of the the corresponding dispersion of metals the corresponding on CeO2−δ and metals thus affecton CeO their2−δ abilityand thus in hydrogenaffect their activation ability in (seehydrogen Section activation 2.4 for the (see H2 Section-TPD analysis). 2.4 for the H2‐TPD analysis).

160

130

710 CuO/CeO2 260

195

NiO/CeO2

220 270 380

consumption (a.u.) Co O /CeO

2 3 4 2 690 H 530 705 X 3 CeO2

100 200 300 400 500 600 700 800 Temperature (oC)

FigureFigure 4. 4.The The H H22-temperature-programmed‐temperature‐programmed reduction reduction (TPR) (TPR) proﬁles profiles of of MO MOxx/CeO/CeO2 samples.samples.

The two broad reduction peaks appearing at 530 and 705 ◦°C for CeO2 are attributed to the The two broad reduction peaks appearing at 530 and 705 C for CeO2 are attributed to the respectiverespective reductionsreductions of of surface surface oxygen oxygen species species and theand bulk the lattice bulk oxygenlattice withinoxygen ceria within [32 ].ceria However, [32]. However, for the three MOx/CeO2 samples, their reduction peaks due to the removal of lattice oxygen for the three MOx/CeO2 samples, their reduction peaks due to the removal of lattice oxygen from the from the CeO2 support are displayed in a similar temperature region◦ (600~800 °C), suggesting that CeO2 support are displayed in a similar temperature region (600~800 C), suggesting that the involved the involved MOx species slightly impact the reducibility of the lattice oxygen of CeO2. In contrast, MOx species slightly impact the reducibility of the lattice oxygen of CeO2. In contrast, the reduction the reduction of surface oxygen◦ (~530 °C) is absent for the MOx/CeO2 since both the strong M‐ceria of surface oxygen (~530 C) is absent for the MOx/CeO2 since both the strong M-ceria interaction interaction and the H2 spillover from metal onto the vicinity surface oxygen species can improve the and the H2 spillover from metal onto the vicinity surface oxygen species can improve the surface surface oxygen removal from the CeO2 support [33]. In other words, the metal on the support can oxygen removal from the CeO2 support [33]. In other words, the metal on the support can catalyse the ceriacatalyse reduction. the ceria reduction

2.3. Surface Structural Analysis

Raman spectroscopy is employed to investigate the surface structure of the MOx/CeO2 Raman spectroscopy is employed to investigate the surface structure of the MOx/CeO2 precursors precursors and the M/CeO2−δ catalysts. As shown in Figure 5A, the strong vibration mode−1 and the M/CeO2−δ catalysts. As shown in Figure5A, the strong vibration mode (F 2g, ~466 cm ), due(F2g, to~466 the cm symmetrical−1), due to the stretching symmetrical vibration stretching of Ce-O vibration bonds, of dominates Ce‐O bonds, the dominates Raman spectrum the Raman of spectrum of CeO2 [34]. Upon metal loading, the F2g modes red shift to −4541 cm−1 with peak broadening CeO2 [34]. Upon metal loading, the F2g modes red shift to 454 cm with peak broadening due due to the strong interaction between MOx and CeO2 that gives rise to distortion of the CeO2−δ lattice to the strong interaction between MOx and CeO2 that gives rise to distortion of the CeO2−δ lattice and creates electron‐rich Ovac for maintaining the system charge neutrality [34]. Besides the F2g band, and creates electron-rich Ovac for maintaining the system charge neutrality [34]. Besides the F2g band, twotwo broadeningbroadening bandsbands withinwithin thethe 600~700 600~700 cm cm−−11 and 1100~12501100~1250 cmcm−11 regionsregions are are observed on thethe MOx/CeO2 samples, which can be attributed to the Frenkel defect‐induced modes (D band) and the MOx/CeO2 samples, which can be attributed to the Frenkel defect-induced modes (D band) and the second order longitudinal optical mode (2LO), respectively [35]. The D and 2LO bands are related to the Ovac that originated from reducing Ce4+ to Ce3+. Compared to CeO2, MOx/CeO2 samples show broader and stronger vibrations in the D bands and 2LO modes, suggesting that the loading of MOx species facilitates the creation of Ovac thanks to metal substitution of lattice Ce, which is in good

Catalysts 2017, 7, 272 6 of 15 second order longitudinal optical mode (2LO), respectively [35]. The D and 2LO bands are related 4+ 3+ to the Ovac that originated from reducing Ce to Ce . Compared to CeO2, MOx/CeO2 samples Catalystsshow broader 2017, 7, 272 and stronger vibrations in the D bands and 2LO modes, suggesting that the loading6 of 15 of MOx species facilitates the creation of Ovac thanks to metal substitution of lattice Ce, which is in agreementgood agreement with the with discussion the discussion of XRD of XRD results. results. The The quasi quasi density density of ofOvac O vacin inceria ceria due due to to metal metal incorporationincorporation is is tentatively tentatively quantified quantiﬁed through through the the relative relative ratio ratio of (I ofD + (I ID2LO+)/I I2LOF2g, )/Ias insertedF2g, as inserted in Figure in 5A.Figure The5 A.(ID The+ I2LO (I)/IDF2g+ in I2LO the)/I MOF2gx/CeOin the2 MOsamplesx/CeO is 2moresamples than is three more‐fold than that three-fold in bare CeO that2, in namely, bare CeO the2 , Onamely,vac density the descends Ovac density in the descends order of in NiO/CeO the order2 > of CuO/CeO NiO/CeO2 >2 Co>3 CuO/CeOO4/CeO2 >>2 CeO> Co23. OThe4/CeO results2 >> clearly CeO2 . reflectThe results that the clearly Ovac reﬂectconcentration that the created Ovac concentration by the MOx created loading by highly the MO dependsx loading on highlythe metal depends species, on likelythe metal due species,to their different likely due ionic to their radius different and the ionic oxidation radius state and the of the oxidation dopants state [24]. of the dopants [24].

2LO 466 (B) 464 (A) D band CeO F 2LO F 2- 2g CeO 2g 2 Co/CeO 454 2-

Ni/CeO2- Co3O4/CeO2 Intensity (a.u.) Intensity Intensity (a.u.) NiO/CeO 2 Cu/CeO2- CuO/CeO 2 444 400 600 800 1000 1200 1400 D band 900 1000 1100 1200 1300 1400 -1 Raman shift (cm-1) Raman shift (cm ) CeO (ID+I2LO)/IF2g=0.051 2- (ID+I2LO)/IF2g=0.033 CeO2 (I +I )/I =0.46 Co/CeO Intensity (a.u.) Intensity(a.u.) Co O /CeO D 2LO F2g 2- (ID+I2LO)/IF2g=0.118 3 4 2 (ID+I2LO)/IF2g=0.58 Ni/CeO (ID+I2LO)/IF2g=0.137 NiO/CeO2 2- (I +I )/I =0.125 CuO/CeO D 2LO F2g 2 (ID+I2LO)/IF2g=0.54 Cu/CeO2-

400 600 800 1000 1200 1400 400 600 800 1000 1200 1400 -1 Raman shift (cm ) Raman shift (cm-1)

Figure 5. Raman spectra of the MOx/CeO2 precursors (A) and the reduced M/CeO2−δ catalysts (B). FigureThe inserted 5. Raman zoom-in spectra ﬁgures of the underline MOx/CeO the2 precursors existence ( ofA) 2LO and bands.the reduced M/CeO2−δ catalysts (B). The inserted zoom‐in figures underline the existence of 2LO bands.

The modes of F2g, D band and 2LO can be clearly identified in the reduced M/CeO2−δ catalysts The modes of F2g, D band and 2LO can be clearly identified in the reduced M/CeO2−δ catalysts and CeO2−δ (Figure5B) but much weaker than those of the MO x/CeO2 precursors, in particular the and CeO2−δ (Figure 5B) but much weaker than those of the MOx/CeO2 precursors, in particular the Ovac-related D and 2LO bands. Moreover, the F2g modes of M/CeO2−δ catalysts further red shift from Ovac‐related−1 D and 2LO−1 bands. Moreover, the F2g modes of M/CeO2−δ catalysts further red shift from 454 cm to 444 cm . The relative changes of Raman signals for the M/CeO2−δ catalysts to their 454 cm−1 to 444 cm−1. The relative changes of Raman signals for the M/CeO2−δ catalysts to their precursors are the consequence of their decreased structure symmetry. However, the (ID + I2LO)/IF2g precursors are the consequence of their decreased structure symmetry. However, the (ID + I2LO)/IF2g ratios of the M/CeO2−δ catalysts and CeO2−δ are much greater than those in their corresponding ratios of the M/CeO2−δ catalysts and CeO2−δ are much greater than those in their corresponding oxides oxides (inserted in Figure5A,B), suggesting that the H 2 reduction significantly promotes Ovac density. (inserted in Figure 5A,B), suggesting that the H2 reduction significantly promotes Ovac density. It is It is worth noting that Ovac concentration of the metal-loaded catalysts is enhanced approximately worth noting that Ovac concentration of the metal‐loaded catalysts is enhanced approximately nine‐ nine- to ten-fold that of CeO2−δ, further confirming that metal loading can dramatically improve to ten‐fold that of CeO2−δ, further confirming that metal loading can dramatically improve the Ovac the O formation. The results are in good accordance with the TPR characterization. We also formation.vac The results are in good accordance with the TPR characterization. We also note that the note that the Ovac density variation of the M/CeO2−δ catalysts follows the descending order of Ovac density variation of the M/CeO2−δ catalysts follows the descending order of Ni/CeO2−δ > Cu/CeO2−δ Ni/CeO2−δ > Cu/CeO2−δ > Co/CeO2−δ >> CeO2−δ, which inherits the Ovac concentration order of > Co/CeO2−δ >> CeO2−δ, which inherits the Ovac concentration order of their precursor oxides. their precursor oxides.

2.4. Hydrogen Activation on the M/CeO2−δ Catalysts 2.4. Hydrogen Activation on the M/CeO2−δ Catalysts Hydrogen temperature programmed desorption (H2‐TPD) is a powerful tool to gain insights Hydrogen temperature programmed desorption (H2-TPD) is a powerful tool to gain insights into into hydrogen activation and metal‐support interaction on the catalysts. As shown in Figure 6, the hydrogen activation and metal-support interaction on the catalysts. As shown in Figure6, the H 2-TPD H2‐TPD profiles of the M/CeO2−δ are deconvoluted into three peaks. The α peaks for the M/CeO2−δ proﬁles of the M/CeO2−δ are deconvoluted into three peaks. The α peaks for the M/CeO2−δ catalysts catalysts appear within a very similar temperature region, which is identical to the temperature range appear within a very similar temperature region, which is identical to the temperature range of the of the sole H2‐TPD peak on CeO2−δ. Such α peaks are therefore assigned to the desorption of H2 sole H2-TPD peak on CeO2−δ. Such α peaks are therefore assigned to the desorption of H2 up-taking up‐taking surface Ovac sites in CeO2−δ [27]. The difference in the integrated areas (Table 2) reveals that surface Ovac sites in CeO2−δ [27]. The difference in the integrated areas (Table2) reveals that the the amount of Ovac of the M/CeO2−δ is dependent on the metallic species, and the Ni/CeO2−δ and amount of Ovac of the M/CeO2−δ is dependent on the metallic species, and the Ni/CeO2−δ and Cu/CeO2−δ possess a similar amount of Ovac, but much larger than those of the Co/CeO2−δ and CeO2−δ supports. The results are consistent with the Raman and TPR analyses.

Catalysts 2017, 7, 272 7 of 15

Cu/CeO2−δ possess a similar amount of Ovac, but much larger than those of the Co/CeO2−δ and CatalystsCeO2− δ2017supports., 7, 272 The results are consistent with the Raman and TPR analyses. 7 of 15

α 0.02 β γ Cu/CeO2-δ

0.01 Ni/CeO2-δ uptake (a.u.) 2 Co/CeO2-δ H

x 3 CeO2-δ 0.00 100 200 300 400 500 600 Temperature (oC)

Figure 6. H temperature programmed desorption (TPD) proﬁles of CeO −δ and M/CeO −δ catalysts. Figure 6. H22 temperature programmed desorption (TPD) profiles of CeO2 2−δ and M/CeO2−δ catalysts.

Table 2. H uptake, metal dispersion and hydrogenation rate constants on the M/CeO catalyst. Table 2. H22 uptake, metal dispersion and hydrogenation rate constants on the M/CeO22−−δ δcatalyst.

Hα Hβ Hγ Σ(Hβ + Hγ) ΣH Rate Coefﬁcient,Rate Sample Hα Hβ Hγ Σ(Hβ + Hγ) ΣH DM (%) −1 Sample (µmol/g) (µmol/g) (µmol/g) (µmol/g) (µmol/g) DM (%) Coefficienk (min )

(μmol/g) (μmol/g) (μmol/g) (μmol/g) (μmol/g) −1 Co/CeO2−δ 17.28 24.04 11.84 35.9 53.16 3.9t, k 0.0588 (min ) Ni/CeOCo/CeO2−δ2−δ 28.3517.28 22.6 24.04 14.6 11.84 37.2 35.9 65.55 53.16 4.4 3.9 0.0216 0.0588 Cu/CeO −δ 28.74 14.2 8.2 22.4 52.14 2.9 - Ni/CeO2 2−δ 28.35 22.6 14.6 37.2 65.55 4.4 0.0216 CeO2−δ 7.87 / / / 7.87 / 0.0037 Cu/CeO2−δ 28.74 14.2 8.2 22.4 52.14 2.9 - Note: DM denotes the metal dispersion, calculated via DM = (Msurf/Mtotal), where Msurf and Mtotal are the amount CeO2−δ 7.87 / / / 7.87 / 0.0037 of surface-exposed metal and total metal loading, respectively. Assuming H/M = 1 and Msurf = 2 × (H2 desorption Note:from metal DM denotes surface), the Msurf metalcan be dispersion, calculated from calculated the amount via ofDM hydrogen = (Msurf/M (β totaland),γ wherepeaks) M uptakensurf and on M thetotal metal are the [1]. amount of surface-exposed metal and total metal loading, respectively. Assuming H/M = 1 and Msurf

= 2 × (H2 desorption from metal surface), Msurf◦ can be calculated from the amount of hydrogen (β and The β and γ peaks located in 100–250 C are due to H2 desorption from the corresponding γ peaks) uptaken on the metal [1]. metal species. The β peaks can be ascribed to the dissociative H2 adsorbed on the highly dispersed metal species with a large density of surface defects, which can enhance the hydrogen diffusion and The β and γ peaks located in 100–250 °C are due to H2 desorption from the corresponding metal reduce the energy barrier for hydrogen dissociation [36]. The γ peaks are associated with chemisorbed species. The β peaks can be ascribed to the dissociative H2 adsorbed on the highly dispersed metal hydrogen on isolated metal clusters [37]. The quantitative analyses of peak β and γ (Table2) clearly species with a large density of surface defects, which can enhance the hydrogen diffusion and reduce show that Co/CeO2−δ and Ni/CeO2−δ catalysts possess comparable amounts of β-H2 and γ-H2, the energy barrier for hydrogen dissociation [36]. The γ peaks are associated with chemisorbed which are more than those of Cu/CeO2−δ. The low uptake of β and γ hydrogen on Cu/CeO2−δ may hydrogen on isolated metal clusters [37]. The quantitative analyses of peak β and γ (Table 2) clearly be due to the weak capability of Cu for hydrogen activation caused by its closed d10 orbitals [38]. show that Co/CeO2−δ and Ni/CeO2−δ catalysts possess comparable amounts of β-H2 and γ-H2, which According to the assignment of the H2-TPD peaks, the metal dispersion (DM) can be quantiﬁed are more than those of Cu/CeO2−δ. The low uptake of β and γ hydrogen on Cu/CeO2−δ may be due to using only the metal-related hydrogen activation (sum of the β- and γ-H2), which is different from the the weak capability of Cu for hydrogen activation caused by its closed d10 orbitals [38]. overall H2 activation amount of the catalysts. As listed in Table2. The D M of the M/CeO2−δ catalysts According to the assignment of the H2-TPD peaks, the metal dispersion (DM) can be quantified is found to be decreasing in the order of Ni/CeO2−δ > Co/CeO2−δ > Cu/CeO2−δ. using only the metal-related hydrogen activation (sum of the β- and γ-H2), which is different from the2.5. overall Catalytic H2 Hydrogenation activation amount of Maleic of the Anhydride catalysts. As listed in Table 2. The DM of the M/CeO2−δ catalysts is found to be decreasing in the order of Ni/CeO2−δ > Co/CeO2−δ > Cu/CeO2−δ. Figure7A presents the MA conversion (X MA) along the hydrogenation courses on M/CeO2−δ ◦ 2.5.catalysts Catalytic in a batch Hydrogenation reactor at 210 of MaleicC and hydrogenAnhydride pressure of 5.0 MPa. Among the examined catalysts, Ni/CeO2−δ is found to possess the highest activity of MA conversion, reaching ~100% MA conversion Figure 7A presents the MA conversion (XMA) along the hydrogenation courses on M/CeO2−δ in 60 min. However, the MA conversion on Cu/CeO2−δ and Co/CeO2−δ in the same period only catalysts in a batch reactor at 210 °C and hydrogen pressure of 5.0 MPa. Among the examined achieves 32% and 70%, respectively. The MA transformation completes in 3 h on Co/CeO2−δ and catalysts, Ni/CeO2−δ is found to possess the highest activity of MA conversion, reaching ~100% MA conversion in 60 min. However, the MA conversion on Cu/CeO2−δ and Co/CeO2−δ in the same period only achieves 32% and 70%, respectively. The MA transformation completes in 3 h on Co/CeO2−δ and 4 h on Cu/CeO2−δ. Moreover, the turnover frequencies of MA transformed to SA (TOFMA→SA) over M/CeO2−δ catalysts are also estimated. The Ni/CeO2−δ catalyst records the highest TOFMA→SA of 61.7

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Catalysts4 h on Cu/CeO2017, 7, 2722 −δ. Moreover, the turnover frequencies of MA transformed to SA (TOFMA→SA)8 overof 15 M/CeO2−δ catalysts are also estimated. The Ni/CeO2−δ catalyst records the highest TOFMA→SA of −1 −1 −1 Cu/CeO61.7 min2−δ ,(TOF nearlyMA→ twiceSA = that13.2 of min the Co/CeO). The considerable2−δ catalyst (TOFdifferenceMA→SA of= 31.2the minMA conversion) and five-fold on thatthe −1 M/CeOof Cu/CeO2−δ catalysts2−δ (TOF cannotMA→SA only= 13.2 be attributed min ). The to considerablethe particle size difference or metal of dispersion the MA conversion effects because on the theM/CeO activity2−δ differencecatalysts cannot is much only more be attributed significant. to We the speculate particle size that or there metal exist dispersion other factors, effects becausesuch as thethe activityintrinsic difference properties is (e.g., much electronic more significant. configuration) We speculate of individual that there metals, exist M other‐support factors, interaction such as andthe intrinsic Ovac amount, properties to influence (e.g., electronic the catalytic configuration) performance of individualsynergistically. metals, M-support interaction and Ovac Figureamount, 7B to presents influence the the curves catalytic of −ln(1 performance − XMA) versus synergistically. time within the first 60 min, which are fitted accordingFigure to7 Bthe presents first order the kinetic curves law of in− ln(1terms− ofX MA) conversion versus time on within the metal the‐based first 60 catalysts min, which [17]. Theare fittedlinear accordingkinetic plots to over the first Ni/CeO order2−δ kineticand Co/CeO law in2−δ termscatalysts of MAsuggest conversion that the C=C on the hydrogenation metal-based followscatalysts the [17 quasi]. The‐first linear order kinetic reaction plots in over terms Ni/CeO of MA 2conversion,−δ and Co/CeO while2 −theδ catalysts nonlinear suggest kinetic thatplot theon Cu/CeOC=C hydrogenation2−δ reflects that follows its initial the hydrogenation quasi-first order follows reaction a complicated in terms of kinetic MA conversion, mechanism. while The MA the hydrogenationnonlinear kinetic rate plot coefficients on Cu/CeO (k21−) δonreflects the Ni/CeO that its2−δ initial and hydrogenationCo/CeO2−δ catalysts follows are a complicateddetermined accordingkinetic mechanism. to the gradients The MA of hydrogenation their linear plots rate and coefficients summarised (k1) on in the Table Ni/CeO 2. The2− δk andfor Ni/CeO Co/CeO2−δ2 −isδ muchcatalysts greater are determined than that accordingfor Co/CeO to2−δ the, indicating gradients that of their metallic linear Ni/CeO plots and2−δ summarisedis more reactive in Table than2. Co/CeOThe k for2−δ Ni/CeO. 2−δ is much greater than that for Co/CeO2−δ, indicating that metallic Ni/CeO2−δ is moreIn reactive principle, than the Co/CeO electronic2−δ .configurations of the active metals are profound in the activation of MA forIn principle,the hydrogenation the electronic reaction. configurations The previous of theDFT active simulation metals demonstrated are profound that in the the activation adsorption of andMA foractivation the hydrogenation of MA on the reaction. metal Thesurface previous are primarily DFT simulation controlled demonstrated by the back that‐donation the adsorption of the electronand activation from ofthe MA metal on theonto metal the surfaceanti‐bonding are primarily π* orbital controlled of maleic by the anhydride back-donation [39]. Therefore, of the electron the metallicfrom the Cu metal with onto a full the‐filled anti-bonding 3d10 electronicπ* orbital configuration of maleic without anhydride the [unpaired39]. Therefore, electron the possesses metallic weakerCu with capability a full-filled of electron 3d10 electronic‐donation configuration to the MA molecule. without theIn contrast, unpaired the electron unpaired possesses electrons weaker of Co (3dcapability7) and Ni of (Ni electron-donation 3d8) would favour to the the MA coordination molecule. In and contrast, activation the unpaired of MA, thus electrons exhibit of much Co (3d higher7) and activity.Ni (Ni 3d 8) would favour the coordination and activation of MA, thus exhibit much higher activity. Besides the intrinsic properties, the Ovac isis regarded regarded as as another factor factor to enhance the C=C hydrogenation because the O vac cancan enrich enrich the the electron electron density density of of active active metals, metals, which which promotes promotes the electronelectron-donating‐donating abilityability andand promotes promotes H H22dissociation dissociation [ 1[1,40].,40]. In In our our cases, cases, the the Ni/CeO Ni/CeO2−2−δδ catalystcatalyst has has richerricher O vac and,and, thus, thus, may may donate donate more electrons toto metallicmetallic Ni Ni in in comparison comparison to to Co/CeO Co/CeO22−−δδ. .Further, Further, thethe smaller particle size of Ni (XRD section) also strengthens the short-rangeshort‐range electron transferring fromfrom O Ovacvac ontoonto the the adjacent adjacent metal metal clusters. clusters. Hence, Hence, the abundant the abundant Ovac and Ovac theand direct the or direct indirect or indirect Ovac → MOvac electronic→ M electronic donation donation are responsible are responsible for the for fact the that fact thatNi/CeO Ni/CeO2−δ shows2−δ shows higher higher activity activity in MA in hydrogenationMA hydrogenation than than Co/CeO Co/CeO2−δ. However,2−δ. However, it is noted it is noted that Ni/CeO that Ni/CeO2−δ and2− δCu/CeOand Cu/CeO2−δ catalysts2−δ catalysts have a similarhave a similarOvac amount, Ovac amount, but different but different activity in activity MA hydrogenation, in MA hydrogenation, suggesting suggesting that the intrinsic that the electron intrinsic structureelectron structure of active ofmetal active dominates metal dominates the MA hydrogenation. the MA hydrogenation.

110 (A) 3.5 (B) 100 y=0.0588x 90 3.0 R2=0.996 Co/CeO 80 2- 2.5 y=0.0216x Ni/CeO2- 2 70 )  R =0.960

60 MA 2.0 Cu/CeO2-

CeO y=0.0102x 50 1.5 2- R2=0.83 40 Co/CeO2- -ln(1- X 1.0 y=0.00369x 30 Ni/CeO 2 2- R =0.999 Cu/CeO 20 2- 0.5 MA conversion (%) conversion MA

10 CeO2- 0.0 0 0 102030405060 0 60 120 180 240 300 360 420 480 Time (min) Time (min)

Figure 7. (A) The conversion of maleic anhydride (MA) and (B) the kinetic curves of the

FigureM/CeO 7.2− δ(Acatalysts.) The conversion of maleic anhydride (MA) and (B) the kinetic curves of the M/CeO2−δ catalysts.

Figure 8A shows the SA selectivity of the M/CeO2−δ catalysts in the prolonged MAH for 8 h. The Cu/CeO2−δ and CeO2−δ catalysts exhibit 100% selectivity to SA in the 8‐h continuous MAH, without GBL observed in the products, indicating they are inert for SA hydrogenolysis to GBL. The TOFSA→GBL

Catalysts 2017, 7, 272 9 of 15

Figure8A shows the SA selectivity of the M/CeO 2−δ catalysts in the prolonged MAH for 8 h. The Cu/CeO2−δ and CeO2−δ catalysts exhibit 100% selectivity to SA in the 8-h continuous MAH, Catalystswithout 2017 GBL, 7, 272 observed in the products, indicating they are inert for SA hydrogenolysis to9 GBL. of 15 −1 −1 The TOFSA→GBL values over Ni/CeO2−δ, Co/CeO2−δ and Cu/CeO2−δ are 3.08 min , 0.87 min and values0, respectively, over Ni/CeO indicating2−δ, Co/CeO that Ni/CeO2−δ and2− δCu/CeOshows the2−δ are highest 3.08 activitymin−1, in0.87 the min SA −→1 andGBL 0, transformation. respectively, indicatingIt is worth that noting Ni/CeO that2−δ GBL shows was the generated highest activity only if MAin the fully SA converts→ GBL transformation. to SA on the Ni/CeO It is worth2−δ, notingas evidenced that GBL by was the SAgenerated selectivity only over if MA Ni/CeO fully converts2−δ, which to SA remains on the 100% Ni/CeO in the2−δ, as initial evidenced 40 min by MAH the SAtime-on-stream selectivity over before Ni/CeO declining2−δ, which due remains to the following100% in the GBL initial generation. 40 min MAH The time generation‐on‐stream of GBL before on decliningCo/CeO2 −dueδ also to the commences following while GBL generation. the conversion The of generation MA to SA of is GBL completing. on Co/CeO GBL2−δ yields also commences (Figure8B) whilereach ~25%the conversion on Ni/CeO of 2MA−δ and to SA ~5% is completing. on Co/CeO2 GBL−δ in yields 6 h, which (Figure are 8B) much reach slower ~25% than on Ni/CeO the MA2−δ→ andSA ~5%transformation. on Co/CeO2 The−δ in results 6 h, which suggest are that much MAH slower on the than M/CeO the MA2−δ is→ a SA multiple-step transformation. process: The MAH results to suggestSA occurs that ﬁrst MAH and on rapidly, the M/CeO while2 GBL−δ is a generates multiple‐ fromstep process: the hydrogenolysis MAH to SA of occurs SA only first if noand MA rapidly, exists whilein the GBL system. generates A similar from phenomenon the hydrogenolysis has been observedof SA only on if the no TiO MA2-supported exists in the Ni-based system. catalysts A similar by phenomenonTorres et al. [17 has]. been observed on the TiO2‐supported Ni‐based catalysts by Torres et al. [17]. OwingOwing to to the complexity of the MAH reaction, its hydrogenation mechanism has has not been explicatedexplicated in the literature,literature, inin particularparticular forfor the the M/CeO M/CeO22−−δδ systems,systems, although although the the theoretical theoretical and experimentalexperimental understanding understanding of of H H2 adsorption,2 adsorption, activation activation and and cycling cycling on CeO on CeO2 and2 andnoble noble metals metals was attemptedwas attempted for a forvariety a variety of redox of redox reactions reactions [38,41]. [38 ,Even41]. Even fewer fewer investigations investigations of MA of adsorption MA adsorption and hydrogenationand hydrogenation over overnoble noble metals metals are areopenly openly accessible accessible [39,42]. [39,42 ].However, However, on on the the basis basis of of our experimentalexperimental characterization characterization data data and and the the reported reported fundamental data data regarding regarding MAH reaction thermodynamicsthermodynamics andand kinetics, kinetics, we we may may explicitly explicitly rationalize rationalize the stepwise the stepwise MAH MAH with a distinctwith a reactiondistinct reactionrate and rate selectivity and selectivity on the M/CeO on the 2M/CeO−δ catalyst2−δ catalyst systems. systems.

100 30 (B) 95 (A) 25

Co/CeO2- 20 Ni/CeO 90 2- Cu/CeO2- 15 85

10 80 Co/CeO2- SA selectivity (%) Ni/CeO GBL yield (%) 2- 5 75 Cu/CeO2- 0 0 60 120 180 240 300 360 420 480 0 60 120 180 240 300 360 420 480 Time (min) Time (min)

Figure 8. (A) The selectivity of succinic anhydride (SA) and (B) the yield of γ‐butyrolactone (GBL) Figure 8. (A) The selectivity of succinic anhydride (SA) and (B) the yield of γ-butyrolactone (GBL) over over M/CeO2−δ catalysts in the 8 h time. M/CeO2−δ catalysts in the 8 h time. The preferential hydrogenation of C=C bonds rather than C=O groups in the initial MAH stage can beThe implicitly preferential interpreted hydrogenation from kinetics of C=C and bonds thermodynamic rather than C=O perspectives. groups in The the initial possible MAH reaction stage routescan be for implicitly MAH to interpreted SA and GBL from can kinetics be written and as: thermodynamic perspectives. The possible reaction routes for MAH to SA and GBL can be written as: MA + H2 = SA ΔHMA→SA = −133.8 kJ/mol (1)

MA + H2 = SA ∆HMA→SA = −133.8 kJ/mol (1) SA + 2H2 = GBL + H2O ΔHSA→GBL = −108.7 kJ/mol (2)

SAMA + 2H + 3H2 =2 GBL= GBL + + H H2O2O∆ ΔHHSAMA→→GBLGBL = −−241.7108.7 kJ/mol kJ/mol (3) (2)

Here, the free enthalpyMA + 3H change2 = GBL data + H 2Oof Δ∆HHMAMA→→GBLSA and= − Δ241.7HSA kJ/mol→GBL are adopted from the (3) literature [43], while the ΔHMA→GBL of Global Reaction (3) is calculated applying Hess’s law to Here, the free enthalpy change data of ∆H and ∆H are adopted from the Reactions (1) and (2). Since the molecule kinetic characteristicsMA→SA determineSA→GBL the rate and selectivity, the literature [43], while the ∆H of Global Reaction (3) is calculated applying Hess’s law to MA → SA transformation wouldMA→ beGBL more favourable and faster than the SA → GBL and MA → GBL Reactions (1) and (2). Since the molecule kinetic characteristics determine the rate and selectivity, the transformations that involve more H2 molecules than the former reaction. This interpretation is well MA → SA transformation would be more favourable and faster than the SA → GBL and MA → GBL evidenced by the fact that the GBL generation initiates only if complete MA → SA transformation on the M/CeO2−δ catalyst systems occurs, which also reveals that Reaction (3) is least viable on all of the catalysts investigated. Indeed, it was claimed that the C=C group is kinetically more active than the C=O group [44]. As for the distinct selectivity of the chosen M/CeO2−δ systems, we suppose that their different surface electron configurations of the catalysts, particularly the metal d orbitals, play essential roles, because the overall hydrogenation reaction is usually determined by H2 activation on the

Catalysts 2017, 7, 272 10 of 15

transformations that involve more H2 molecules than the former reaction. This interpretation is well evidenced by the fact that the GBL generation initiates only if complete MA → SA transformation on the M/CeO2−δ catalyst systems occurs, which also reveals that Reaction (3) is least viable on all of the catalysts investigated. Indeed, it was claimed that the C=C group is kinetically more active than the C=O group [44]. As for the distinct selectivity of the chosen M/CeO2−δ systems, we suppose that their different surface electron configurations of the catalysts, particularly the metal d orbitals, play essential roles, because the overall hydrogenation reaction is usually determined by H2 activation on the catalysts [42,45]. This argument is also evidenced by our TPD data (Figure6 and Table2) where the Cu/CeO2−δ displays a smaller amount of β and γ hydrogen, namely the weaker ability to activate hydrogen. For the transformation of SA → GBL, the adsorption of the C=O group was also considered to be of profound importance for the hydrogenolysis process [46]. The closed d10 shell of metal Cu is not beneficial to C=O hydrogenolysis due to its low interstitial electron density and weak energy for adsorption and activation of the C=O group [47]. In contrast, metallic Ni and Co with partially filled d-orbitals are prone to coordinate with the C=O groups of organic adsorbates and thus facilitate C=O hydrogenolysis [47]. The beneficial contributions of Ovac in C=O hydrogenolysis have been widely accepted [48,49]. The Ovac in the non-stoichiometric oxides, such as ZnO1−x [50] and CeO2−x [1], can polarise and activate the C=O group and thus reduce the energy barrier of C=O conversion to display reasonable hydrogenolysis selectivity. In this work, the amount of Ovac of the Ni/CeO2−δ catalyst is about 1.5-times that in the Co/CeO2−δ catalyst, suggesting that Ni/CeO2−δ catalyst can offer more active sites for C=O activation. However, the calculated TOFSA→GBL of Ni/CeO2−δ is nearly triple that of Co/CeO2−δ, which is larger than the relative Ovac ratio between the two catalysts. Therefore, the electronic configuration of the active metal is the key factor to determine the catalytic hydrogenolysis of SA to GBL, while Ovac can facilitate C=O conversion on Ni/CeO2−δ due to the electron-donating and selective adsorption effects.

3. Materials and Methods

All chemical reagents, Ce(NO3)3·6H2O, Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Co(NO3)2·6H2O and citric acid (CA). purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), were analytic grade and used as-purchased without pre-puriﬁcation.

3.1. Ceria Support Preparation

Ceria supports were prepared by the sol-gel process. First, 5.00 g Ce(NO3)3·6H2O were dissolved in 20 mL deionized water containing 6.56 g CA under rigorous stirring. The solution was heated in a water bath at 80 ◦C until a porous dry gel was obtained. During this process, the colourless solution became light yellow and ﬁnally brown. The obtained gel was then dried at 120 ◦C for 8 h to form a spongy material, which is subject to calcination for 3 h at 500 ◦C with a heating ramp of 3 ◦C/min. The resulting cerium oxide was used as the support to prepare the corresponding catalysts.

3.2. M/CeO2−δ (M = Co, Ni and Cu) Catalysts Preparation

The M/CeO2−δ (M = Co, Ni, Cu) catalysts (metal loading = 10 wt %) were prepared by an incipient wetness impregnation method as reported in our previous research paper [1]. For the preparation of the Ni/CeO2−δ catalyst, 1.01 g Ni(NO3)2·6H2O were dissolved in 10 mL of the ethanol/DI water (1:1 volumetric ratio) solution, and then, 2.00 g of CeO2 support were added into the mixed solution. The mixed solution was heated on a hotplate at 80 ◦C under rigorous stirring to evaporate the solvent before drying at 120 ◦C for 8 h in a convection oven. The obtained sample was calcined at 450 ◦C for 3 h in a mufﬂe furnace with the heating ramp of 3 ◦C/min. The received oxide sample was reduced ◦ ◦ at 350 C (heating ramp of 2 C/min) in H2 ﬂow for 3 h to obtain the Ni/CeO2−δ catalyst for MA hydrogenation. The Cu/CeO2−δ and Co/CeO2−δ catalysts were prepared using a similar procedure, Catalysts 2017, 7, 272 11 of 15

◦ ◦ but Cu/CeO2−δ and Co/CeO2−δ were reduced at 250 C and 450 C, respectively, which ensured that all of the transition metal species fully reduced at the optimum temperature as determined by H2-TPR.

3.3. Catalyst Characterizations X-ray diffraction data were acquired by powder X-ray diffraction using a D8 Advance (Bruker, Billerica, MA, USA) (Cu Kα1 radiation, λ = 0.15418 nm) equipped with a Vantec detector and Ni filter. Data were collected between 10◦ and 80◦ 2θ degrees via a continuous scan mode with a scanning rate at 4◦/min. The mean crystallite size oof the pertinent species were calculated employing the Scherrer equation: D = kλ/(βcosθ), where k is 0.9, β is the full width of the diffraction line at half of the miximum intensity and θ is the diffraction angle. The Ni loading of the catalysts was carried out by means of inductively-coupled plasma (ICP) spectroscopy with the iCAP 7400 ICP-OES (Thermo Fisher Scientific, Waltham, MA, USA) equipment. The Raman spectrum was recorded on a LabRAM HR Evolution Raman microscope with a laser wavelength of 532 nm (HORIBA, Tokyo, Japan) for surface characterization. The morphology of the materials was visualized using a JEOL JEM-2010 transmission electron microscope (TEM) (JEOL, Tokyo, Japan) operated at 120.0 kV. The samples were dispersed in ethanol assisted by the ultrasonic technique. Hydrogen-temperature programed reduction (H2-TPR) experiments were performed using the Micromeritics AutoChem II 2950 system (Norcross, GA, USA) equipped with a thermal conductivity detector (TCD). Typically, 30 mg of catalyst were first degassed at 300 ◦C for 1 h with argon flushing. After cooling down to ambient temperature in argon, H2/Ar (10 vol % H2) at a flow rate of 50 mL/min was passed through the catalysts, and the TPR profiles were recorded from 50 to 800 ◦C at a ramp of 10 ◦C/min. The output signal of the gas mixture was measured continuously using the TCD as a function of temperature. Hydrogen temperature programmed desorption (H2-TPD) measurements were carried out on the ◦ same apparatus as TPR. In the H2-TPD test, 100 mg catalyst were first reduced in situ at 350 C for 1 h ◦ in pure H2, followed by purging with Ar for 1 h at 360 C to remove the excess hydrogen adsorbed on ◦ the surface and cooling down to 50 C in Ar flow. Then, 10 vol % H2 in Ar was injected until saturation at 50 ◦C and then purged with Ar until the baseline was stable, prior to carrying out TPD up to 600 ◦C ◦ at a heating rate of 10 C/min. Ag2O was taken as the standard material to correlate the peak areas to volume data.

3.4. Catalyst Test

Liquid-phase hydrogenation of MA was carried out over M/CeO2−δ catalysts in a batch reactor at 210 ◦C and 5 MPa. The reduced catalysts (0.1 g) and MA (4.9 g) were put in an autoclave (100 mL) containing THF (40 mL) solvent and purged five times with nitrogen to remove air before pressurizing up to 4.0 MPa using hydrogen. The diffusional limitation including internal and external diffusion were ruled out by performing experiments varying the particles size of the catalyst (0.425–0.250 mm) and adjusting the string rate of 500 rpm. Simultaneously, the hydrogen pressure was raised up to and maintained at 5.0 MPa. The reaction effluents were analysed using a 7890A gas chromatograph (Agilent, Santa Clara, CA, USA) equipped with an SE-52 (30 m × 0.32 mm × 1.0 µm) capillary column and a flame ionization detector (FID). The programmed temperature from 100 ◦C–120 ◦C with a ramp of 5 ◦C/min was used to ensure complete separation of individual components in the effluents. The temperatures of the injector and detector were set at 260 ◦C and 190 ◦C, respectively. MA conversion and yield of the product were calculated according to the following equations:

XMA (%) = (CSA + CGBL)/(CMA + CSA + CGBL) × 100% (4)

SMA (%) = (CSA + CGBL)/(CSA + CGBL) × 100% (5)

YSA (%) = CSA/(CMA + CSA + CGBL) × 100% (6) Catalysts 2017, 7, 272 12 of 15

YGBL (%) = CGBL/(CMA + CSA + CGBL) × 100% (7) where CMA, CSA and CGBL represent the percent content of the reactant and products in the reaction efﬂuent, respectively. XMA, SSA, YSA and YGBL represent the MA conversion, SA selectivity and the SA and GBL yields. The turnover frequency (TOF) of the MA hydrogenation to SA (in the ﬁrst 10 min) and the subsequent hydrogenation of SA to GBL (in the following 60 min) were calculated using the following formulae: TOFMA→SA = n(MAint.) × XMA/(t × n(Msurf.)) (8)

TOFSA−GBL = n(MAint.) × YGBL/(t × n(Msurf.)) (9) where n(MAint.) denotes the initial amount of MA in mol, XMA the conversion of MA, YGBL the yield of GBL (%), t the reaction time (min) and n(Msurf.) the surface metallic M atom number on the corresponding catalyst.

4. Conclusions

Ni/CeO2−δ, Co/CeO2−δ and Cu/CeO2−δ catalysts for maleic anhydride hydrogenation were prepared via a simple wet impregnation synthesis and well characterised applying XRD, TEM, Raman, TPR and TPD techniques, in order to unravel the dependence of the MAH performance over the redox properties and their M-CeO2−δ interaction and, thus, guide the future design of robust and highly selective transition metal catalysts for MAH. It was found that metal dispersion of M/CeO2−δ catalysts declines as Ni/CeO2−δ > Co/CeO2−δ > Cu/CeO2−δ, in opposition to the order of metal crystallite sizes. The small metal crystallite sizes of the M/CeO2−δ catalysts are found proportional to their oxide precursors, revealing that the strong MOx-CeO2 interaction plays a crucial role in governing the metal dispersion and particle sizes of the M/CeO2−δ catalysts. The Ovac concentrations in the M/CeO2−δ catalysts are observed to be enhanced one magnitude relative to CeO2−δ, in the increasing sequence of Cu/CeO2−δ > Ni/CeO2−δ > Co/CeO2−δ >> CeO2−δ. The increased Ovac concentration in M/CeO2−δ is mainly due to the H2 spillover from the metals to ceria, which favours the creation of Ovac in the ceria matrices. The H2 reduction-induced metal exit from ceria matrices, sintering and Ovac is responsible for the increased crystallite size of the CeO2−δ support. The CeO2−δ itself is evidenced an active support for SA generation since it shows notable activity in MAH to SA, yet is inert in SA hydrogenolysis to GBL. The metal loading greatly enhances the MAH activity and the SA selectivity on the M/CeO2−δ catalysts. The apparent activity and turn over frequencies of MAH to SA (TOFMA→SA) increase following the trend of Ni/CeO2−δ > Co/CeO2−δ > Cu/CeO2−δ >> CeO2−δ. Among the examined catalysts, the Ni/CeO2−δ displays the highest activity and selectivity to SA in the initial 40 min and the following SA hydrogenolysis to GBL. TOFMA→SA −1 −1 (61.7 min ) on Ni/CeO2−δ is twice- and five-times those on Co/CeO2−δ (TOFMA→SA = 31.2 min ) −1 and Cu/CeO2−δ (TOFMA→SA = 13.2 min ), respectively. Ni/CeO2−δ also possesses higher activity and selectivity than Co/CeO2−δ in SA → GBL. The SA hydrogenolysis to GBL occurs on Ni/CeO2−δ and Co/CeO2−δ catalysts only when the conversion to SA completes, though no SA hydrogenolysis is observed on the Cu/CeO2−δ and CeO2−δ catalysts. The MAH to SA on the Ni/CeO2−δ, Co/CeO2−δ and CeO2−δ catalysts follows the first order kinetic law, but Cu/CeO2−δ displays a more complex kinetic mechanism. CeO2−δ may catalyse MAH to SA and favours the MAH on Ni/CeO2−δ, though Ovac is not a dominant factor for the enhanced MAH performance, as shown by the facts that both CeO2−δ and Cu/CeO2−δ are inert in SA hydrogenolysis and the Ovac-richest Cu/CeO2−δ is not robust in SA generation. The electronic configurations of the metals are supposed crucial for the activation of H2, MA and SA in the MAH. Therefore, the distinct catalytic hydrogenation behaviour on the catalysts arises from the synergistic Catalysts 2017, 7, 272 13 of 15

contributions of the metal dispersion, Ovac concentration and the metal electronic conﬁgurations, which determine the activation of hydrogen and double bonds in MA and SA.

Supplementary Materials: The following are available online at www.mdpi.com/2073-4344/7/9/272/s1, Figure S1: The dependence of metal crystallite sizes on those of metal oxides in the xNi/CeO2−δ catalysts (x represents the loading amount (wt %) of Ni in the xNi/CeO2−δ catalysts). Acknowledgments: This work was partially supported by the Natural Science Foundation of China (No. 21503124), the Key Coal Technologies R&D Program of Shanxi Province (No. MH2014-06), the Science and Technology Program of Shanxi Province (No. 20140321013-02) and the Royal Society International collaboration award. X.L. appreciates the Eustice studentship from Southampton University. We sincerely appreciate Dr Kevin Chung-che Huang (Optical Research Centre, University of Southampton) for his kind help in Raman characterization. Author Contributions: The idea was conceived of by Y.Z. and Z.J. X.L. performed the experiments and drafted the paper under the supervision of Y.Z., Z.J. and M.H. L.Z. and Y.Z. helped to collect and analyse some characterization data. The manuscript was revised through the comments of all authors. All authors have given approval for the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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