Crystal-Facet Effect of Î³-Al2O3 on Supporting CrOx for Catalytic

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Crystal-Facet Effect of #-Al2O3 on Supporting CrOx for Catalytic Semi-hydrogenation of Acetylene Yibo Wang, Jie Yang, Rongtian Gu, Luming Peng, Xuefeng Guo, Nianhua Xue, Yan Zhu, and Weiping Ding ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01619 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Crystal-Facet Effect of γ-Al2O3 on Supporting CrOx for Catalytic Semi-hydrogenation of Acetylene Yibo Wang, Jie Yang, Rongtian Gu, Luming Peng, Xuefeng Guo, Nianhua Xue, Yan Zhu*, and Weiping Ding* Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

ABSTRACT: With the successful preparation of gamma alumina with high-energy external surfaces such as {111} facets, the crystal-facet effect of γ-Al2O3 on surface loaded CrOx has been explored for semi-hydrogenation of acetylene. Our results indeed demonstrate that the harmonious interaction of CrOx with traditional γ-Al2O3, of which external surfaces are typically low-energy{110} facets, has caused a highly efficient performance for semi-hydrogenation of acetylene over CrOx/(110)γ-Al2O3 catalyst, whereas the activity of the CrOx/(111)γ-Al2O3 catalyst for acetylene hydrogenation is suppressed dramatically due to the limited formation of active Cr species, restrained by the high-energy {111} facets of γ-Al2O3. Furthermore, the use of inexpensive CrOx as the active component for semi-hydrogenation of acetylene is an economyfriendly alternative relative to commercial precious Pd catalysts. This work sheds light on a strategy for exploiting the crystal-facet effect of the supports to tailor the catalytic properties purposefully.

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KEYWORDS: γ-Al2O3, facet-effect, CrOx, selective hydrogenation, acetylene TOC Graphic


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INTRODUCTION Crystal-facet effect is critical to the improvement and design of efficient catalysts, therefore great efforts have been made for the fundamental understanding of surface structure-catalytic performance relationships and recent advances in controllable synthesis of catalysts with designed surface structures enable further tuning the catalytic performances by the crystal-facet effect.1-3 Gamma alumina has been widely used as a catalytic support for metals and oxides,4-6 whose catalytic performances have been strongly affected due to the distinctive physicochemical properties of the exposed crystal facets of supports. Commonly, the mainly exposed surfaces of γ-alumina prepared from the dehydration of boehmite powders are considered as {110} planes on the basis of non-spinel model proposed by Digne et al.7,8 We have recently succeeded in preparing the γ-alumina nanotubes with high energy {111} facets as main external surfaces in the light of a “bottom-up” route, employing particular surfactants to protect related crystal surfaces.9 On the basis of the theoretical calculation, the surface energy of {101} planes of boehmite turned to be the lowest one due to the adsorption and protection of carboxylic acid on {101} planes, remaining the dominating external surfaces after hydrothermal treatment. The obtained γ-Al2O3 thus exhibited external surfaces mainly with {111} planes according to the topotactic transformation during the calcination process.7-9 The systematic investigation on γ-alumina with high-energy {111} facets has inspired us to devise various catalysts supported on γ-alumina with different external surfaces to tailor their catalytic properties via the crystal-facet effect. In our previous work, tiny Pd particles supported on the γ-alumina nanotubes exposed with {111} facets, relative to {110} surfaces, gave rise to the higher catalytic performance for selective hydrogenation of alkynes because of the stronger interaction between Pd and {111} facets of γ-Al2O3 nanotubes.9 Now that the strong interfacial


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interaction between metal and γ-alumina with high-energy {111} facets can improve the catalytic activity of the related catalysts,9,10 it may also promote or hamper the catalytic activity of the supported metal oxides by the strong interactions. It is well-known that the closeness of the ionic radii of Cr6+ (0.052 nm) and Cr3+ (0.062 nm) with that of Al3+ (0.053 nm) and the structural similarity between γ-Al2O3 and Cr2O3 may lead to an epitaxial growth of Cr2O3 clusters on the γAl2O3 surface.11 We anticipate that the intriguing role of exposed crystal facets of γ-Al2O3 such as high-energy {111} facets and relatively more stable {110} facets supporting chromium oxide may dissimilarly facilitate the reversible change of valence states of Cr6+ to Cr3+ during catalytic reactions. It has been a controversy for many years regarding to the nature of active chromium species,12-18 however according to some authors, the Cr3+ reduced from the Cr6+ species in oxidized catalyst are considered to be catalytically active for dehydrogenation12,13 and alkene hydrogenation18 reactions, and interestingly, Cr3+ species existed in original oxidized catalyst, identified as “non-redox Cr3+”, are relatively inactive after the same pretreatment. Our results indeed illustrate that the exposed surfaces of γ-Al2O3 have a substantially different influence on the supporting and interchanging of Cr6+ and Cr3+ species, where the higher relative amounts of active Cr species appear on the {110} surfaces rather than on the high-energy {111} surfaces of γ-Al2O3 after pretreatment for semi-hydrogenation of acetylene to ethylene, which is an important reaction in industry.19-22 The findings give us new insights into the crystal-facet effect of catalytic supports. EXPERIMENTAL SECTION Preparation of catalysts: Traditional γ-Al2O3 dominated with {110} external surfaces were synthesized by the direct dehydration of boehmite purchased from Aluminum Corporation of Shandong and denoted as (110)γ-Al2O3. And the nanotubular γ-Al2O3 exposed with higher


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energy {111} facets was obtained in a procedure similar to that reported previously with some modification.9 The CrOx/γ-Al2O3 catalysts were prepared by an incipient wetness impregnation method with Cr(NO3)3∙9H2O as the precursor and the theoretical weight percent of Cr2O3 was 10 % deposited on 1 g support for both catalysts. For example, 0.5265 g Cr(NO3)3∙9H2O was dissolved with about 1 mL ethanol, then the solution was added to 1 g corresponding γ-Al2O3 ((110)γ-Al2O3 or (111)γ-Al2O3), after that the resulting sample was kept at room temperature for 12 h and then dried at 363 K for 2 h, later was calcined at 450 °C, 550 °C or 700 °C for 3 h respectively to obtain the corresponding catalysts CrOx/(110)γ-Al2O3-xxx (hereafter referred to as chromium oxide supported on {110} surface of γ-Al2O3) or CrOx/(111)γ-Al2O3-xxx (hereafter referred to as chromium oxide supported on {111} surface of γ-Al2O3). The xxx denoted the temperatures used to treat the catalysts in air after deposition. The BET surface areas of the obtained γ-Al2O3 were 236 m2∙g-1 for (110)γ-Al2O3 and 170 m2∙g-1 for (111)γ-Al2O3, respectively, and the surface areas of the corresponding CrOx/γ-Al2O3 catalysts fluctuated around the related γ-Al2O3 support due to the different calcination temperatures, that were 237, 185, 171 m2∙g-1 for CrOx/(110)γ-Al2O3 catalysts and 142, 141, 120 m2∙g-1 for CrOx/(111)γ-Al2O3 catalysts calcined at 450, 550, 700 °C separately. The XRD patterns of CrOx/γ-Al2O3 catalysts (Figure 1) implied that CrOx was highly dispersed on the surface of γ-Al2O3 since we did not detect any crystalline phase of chromia except gamma alumina.


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Figure 1. XRD patterns for the obtained CrOx/γ-Al2O3 catalysts. Characterization: Nitrogen adsorption isotherms were measured at 77 K on ASAP 2020 volumetric adsorption analyzers manufactured by Micromeritics. Before adsorption measurements, each sample was degassed under a vacuum for 12 h at 293 K. The specific surface areas of the samples were calculated using the Brunauer-Emmett-Teller (BET) method within the relative pressure range of 0.05−0.20. Powder XRD measurements were performed with a Philips X’Pert MPD Pro X-ray diffractometer using Cu Kα radiation (1.54 Å) at 40 KV and 40 mA. X-ray photoelectron spectroscopy (XPS) tests were done with Thermo ESCALAB 250 equipped with monochromatic Al Kα (hv =1486.6 eV) X-ray exciting source, and the Cr 2p region was acquired at the beginning of each experiment and in the shortest time in order to avoid the X-ray induced reduction of Cr6+.14,23,24 Hydrogen temperature-programmed reduction (H2-TPR) profiles were recorded with a thermal conductivity detector by putting the sample in a gaseous flow of 5 vol % H2/N2. 0.1 g catalyst was pretreated in 5% O2/He or pure O2 flow at 300 °C for 1 h to remove moisture and then cooled to room temperature. After that, the temperature was raised from 30 °C to 700 °C at a rate of 10 °C∙min-1 under a 5 vol % H2/N2 mixed gas flow (20 mL∙min-1). H2-TPR of CuO was


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also measured and the hydrogen consumption of CuO was used as a calibration to calculate the hydrogen consumption per Cr atom thus obtaining more information of the surface Cr species. Hydrogen temperature-programmed desorption (H2-TPD) experiments were recorded using a gas chromatograph equipped with a thermal conductivity detector. 0.1 g catalyst was pretreated in Ar flow at 550 °C for 30min to remove moisture and reduced in pure H2 at 350 °C for 1 h, then cooled to room temperature during which the adsorption of H2 occurred. And then, the gas flow was switched to Ar to remove any physisorbed H2. TPD analysis was carried out from ambient temperature to 550 °C at a rate of 10 °C ∙min-1 under a pure Ar gas flow (30 mL∙min-1). D/H and H/D exchanges: The as-prepared samples (180 mg) were pretreated under O2 (10 mL·min−1) at a heating rate of 10 °C·min−1 to 550 °C and kept at that temperature for 0.5 h and then the sample was cooled under the same atmosphere to room temperature. Deuterium exchanged with the surface proton took place by increasing the temperature from ~r.t. to 550 °C at a heating rate of 10 °C·min−1 under the 10 vol % D2/Ar flow and the signal of HD was monitored by a mass spectrometry. It should be pointed out that the sample was also reduced during the first run of D/H exchange, after which, the sample was cooled to room temperature in the same atmosphere of 10 vol % D2/Ar flow and the chemical adsorption of D2 also took place on the reduced catalysts during the cooling process. After cooled to room temperature, Ar (25 mL∙min-1) was switched in to remove the physically adsorbed D2. Subsequently, the second run of H/D exchange was measured following the similar procedure under a flow of 5 vol % H2/Ar and the signal of HD was monitored by the same mass spectrometry. Catalytic Tests: The selective hydrogenation of acetylene in an ethylene-rich stream was investigated in a heated fixed-bed reactor under continuous flow conditions. Prior to catalytic tests, 0.15 g catalysts with 20-40 mesh size were pretreated in pure H2 with a flow rate of 20 mL


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∙min-1 at 350 °C for 1 h.25 After cooling to room temperature, a gas mixture composed of 0.475% acetylene and 9.25% ethylene, with the balance being Ar was introduced into the reactor with the ratio of H2 and C2H2 being about 77 and a space velocity of 21660 mL∙h-1∙g-1. The reaction temperature was held constant for 20 min prior to ramping to the next temperature point. The gas stream at the reactor outlet was analyzed by online gas chromatography using a GC 9560 chromatograph equipped with a TCD detector and a column of Porapak Q. Acetylene conversion and relative ethane selectivity were defined as follows:

Acetylene Conversion(%)=

Ethane Selectivity(%)=

C2H2(inlet)-C2H2(outlet) ×100% C2H2(inlet)

C2H6(outlet) ×100% (C2H2+C2H4+C2H6)(outlet)

RESULTS AND DISCUSSION The catalytic performances of CrOx/γ-Al2O3 catalysts pretreated with air at different temperatures for acetylene semi-hydrogenation are shown in Figure 2. Whatever the airtreatment temperatures were, CrOx/(110)γ-Al2O3 catalysts were more effective than CrOx/(111)γAl2O3 catalysts for the catalytic semi-hydrogenation of acetylene. The complete conversion of acetylene could be achieved over CrOx/(110)γ-Al2O3 catalyst when the reaction temperature was up to 140 °C, while only 20% or even less than 40% conversion of acetylene was obtained on CrOx/(111)γ-Al2O3 catalyst at a temperature range of 40-200 °C. Interestingly, it appeared that the different crystal-facet of γ-Al2O3 did not significantly affect the selectivity for ethylene from the semi-hydrogenation of acetylene. With respect to the selectivity, CrOx/(111)γ-Al2O3 catalysts gave almost 100% selectivity for ethylene at three cases, and CrOx/(110)γ-Al2O3 catalysts


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calcined in air at 450 and 550 °C exhibited a slightly lower selectivity to ethylene compared to the corresponding catalysts on (111)γ-Al2O3, whereas the ethylene selectivity for CrOx/(110)γAl2O3-700 catalyst was the lowest due to the highest air-pretreatment temperature accompanied with the higher conversion which is the common phenomenon in semi-hydrogenation of acetylene.19-22

Figure 2. Catalytic performances for acetylene semi-hydrogenation (0.15 g cat.; 0.475% C2H2, 9.25% C2H4 with balance Ar; space velocity of 21660 mL∙h-1∙g-1). (a,b,c) Acetylene conversion and (d,e,f) ethane selectivity as a function of reaction temperature over CrOx/(110)γ-Al2O3 and CrOx/(111)γ-Al2O3 catalysts (450, 550 and 700 are identified as air-treatment temperatures of the catalysts before reaction).


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Figure 3. H2-TPR curves for (a) CuO, (b) CrOx/(111)γ-Al2O3 pretreated in pure O2 atmosphere at 300 oC for 1 hour, (c) CrOx/(110)γ-Al2O3 pretreated in pure O2 atmosphere at 300 oC for 1 hour, (d) CrOx/(111)γ-Al2O3 pretreated in 5% O2/He atmosphere at 300 oC for 1 hour, and (e) CrOx/(110)γ-Al2O3 pretreated in 5% O2/He atmosphere at 300 oC for 1 hour. (The numbers in the bracket are the hydrogen consumption per Cr atom of the catalysts). The distinct performance of CrOx/(110)γ-Al2O3 versus CrOx/(111)γ-Al2O3 prompted us to explore the facet-dependent catalytic properties. The H2 temperature-programmed reduction (TPR) was conducted to determine the redox properties and the hydrogen consumption of the two catalysts. In Figure 3b, c, d, e, both CrOx/(110)γ-Al2O3 and CrOx/(111)γ-Al2O3 showed a reduction peak at the temperature span of 300~400 °C, which was associated with the reaction of CrOx with H2. The CuO reduction was used as calibration to quantify the hydrogen consumption from the TPR results as exhibited in Figure 3a. The hydrogen molecule consumption per Cr atom


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for the catalysts was quite interesting and 1.0 hydrogen molecule consumption per Cr atom for CrOx/(110)γ-Al2O3 while 0.8 H2 consumption per Cr atom for CrOx/(111)γ-Al2O3 were revealed. It implied the diverse valences of surface Cr species on the two γ-Al2O3 supports. The valence of Cr surface species on the high energy {111} surfaces of γ-Al2O3 was lower than that on the {110} surfaces of γ-Al2O3 right after calcination in air so that when exposed in a reductive atmosphere the catalysts could consume less hydrogen as we have observed. Further the electronic properties of catalysts were studied by X-ray photoelectron spectroscopy, as presented in Figure 4a. Noted that the peak at 576.9 eV is assigned to Cr3+ and the peak at 579.4 eV is related to Cr6+, indicating the presence of Cr3+ and Cr6+ in the two samples before H2-treated.14,23,24,26,27 The mole ratio of Cr3+/Cr6+ of the two catalysts was 0.41 for CrOx/(110)γ-Al2O3 and 1.06 for CrOx/(111)γ-Al2O3, respectively, as listed in Table 1, which revealed that Cr6+ species dominated the surface of CrOx/(110)γ-Al2O3, whereas both Cr6+ and Cr3+ species occupied the surface of CrOx/(111)γ-Al2O3 nearly equally, which was consistent with the observation from H2-TPR. While the surface composition of samples pre-reduced in H2 were displayed in Figure 4b and Table 1 as well, revealing that the mean valence states of CrOx/(110)γ-Al2O3-reduced and CrOx/(111)γ-Al2O3-reduced were around 3, with negligible Cr6+ species on the surface after H2-pretreatment, verifying the importance of reduction procedure and the subsequent production of active chromium species.


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Table 1. XPS results of CrOx/(110)γ-Al2O3 and CrOx/(110)γ-Al2O3 catalysts. Sample

Cr3+(eVa)

Cr6+(eVa)

Cr3+/ Cr6+ b

Mean valencec

Cr/Ald

CrOx/(110)γ-Al2O3-550

577.1

579.4

0.41

5.1

0.55

CrOx/(111)γ-Al2O3-550

577.1

579.5

1.06

4.4

0.58

CrOx/(110)γ-Al2O3-550reduced

577.3

579.3

6.75

3.3

0.88

CrOx/(111)γ-Al2O3-550reduced

577.3

579.5

6.15

3.3

0.81

a

Binding energy of Cr 2p3/2 peak; b Calculated according to the integrated area of the corresponding deconvoluted peak; c Calculated according to the ratio of Cr3+/Cr6+; d Calculated according to the area ratio of Cr 2p and Al 2p.

Figure 4. (a) XPS spectra of CrOx/(110)γ-Al2O3 and CrOx/(111)γ-Al2O3 calcined in air at 550 °C for 3 hours. (b) XPS spectra of CrOx/(110)γ-Al2O3-550 and CrOx/(111)γ-Al2O3-550 pre-reduced in H2 at 350 °C for 1 hour. (The gray lines in the spectra give the original data).


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(a) D2

α

HO

HD

(b) D2 HO

α

HD

β

(c) D2 HO

HD Evolution

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γ

α (d)

HD

HO Cr

δ D2 HO

γ (e)

HD

HO Cr

δ β

H2

Cr D

HD

ε

Cr D

α̍

(f) Cr D Cr D

H2 HD

ε

α̍

β

Figure 5. (a-d) D/H exchange profiles of γ-Al2O3 and CrOx/γ-Al2O3. (e, f) H/D exchange profiles of CrOx/γ-Al2O3 (The weight percent of chromia is 10 and the calcination temperature is 550 °C).


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D/H and H/D exchange studies provided very important information on the catalytic mechanism of the catalysts,28 giving us deep insight into how the exposed different facets of γAl2O3 control the surface properties of the supported chromia. Figure 5 displayed the very interesting and important results of D/H and H/D exchanges. During the first run of D/H exchange, four types of surface H species associated with two samples could be differentiated, i.e., the peak α was corresponded to the hydroxyls of γ-Al2O3, peak β was related to the specific H species connected with {111} facets and the new peaks γ and δ were ascribed to the surface hydroxyls attached to Cr species with higher or lower oxidation states, since CrOx was loaded on the catalyst surfaces and nearly covered the alumina surfaces. For the CrOx/(110)γ-Al2O3 catalyst, small amounts of residual hydroxyls belonging to alumina (peak α), due to whose relatively larger surface area, were still retained. Right after the exchange of D2 with OH and the cooling down of catalysts in D2 stream, H2 was introduced to perform the second run of H/D exchange. The peak α’ of HD evolution can be considered originating from the OD groups attached mainly to the surface of chromia. Meanwhile, a new peak ε was observed at much lower temperatures with regard to the reduced catalysts, which was tentatively assigned to the chemically adsorbed deuterium on reduced Cr sites, as H2 + Cr-D*→HD + Cr-H*, generated from the reduction of higher valence Cr species.28-30 The chemically adsorbed hydrogen was most likely related to the catalytic activity of the catalysts for the semi-hydrogenation of acetylene, thus the amount of D species adsorbed on surface Cr sites, determined by the areas of the peak ε, were calculated according to the Ar signal as inner standard and exhibited in Table 2.


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Table 2. H/D exchange data of CrOx/(110)γ-Al2O3 and CrOx/(111)γ-Al2O3 catalysts. Catalyst samples Chemical adsorption of D per Cra CrOx/(110)γ-Al2O3-550-H/D

0.15

CrOx/(111)γ-Al2O3-550-H/D

0.06

a

Calculated from the integrated area of the corresponding peak after deconvolution using the flow rate of Ar as the inner standard.

The CrOx/(110)γ-Al2O3 catalyst appeared to have ~2.5 times more chemically adsorbed deuterium species, compared with CrOx/(111)γ-Al2O3 catalyst, revealing that the reduced chromia species on {110} surfaces of γ-Al2O3 were much easier to adsorb and activate H2 than the corresponding catalysts supported on γ-Al2O3 preferentially exposed {111} facets, which provided crucial implications for much better performances of semi-hydrogenation of acetylene over CrOx/(110)γ-Al2O3 catalyst. The measurement of H2 temperature-programmed desorption gave similar results, as shown in Figure 6. It was evident that the peak located at 270 °C for CrOx/(110)γ-Al2O3 was related to chemical adsorption of hydrogen whereas CrOx/(111)γ-Al2O3 showed only one peak of hydrogen desorption at 430 °C, and the relatively larger peak area of the former catalyst in comparison to the latter demonstrated that the chemical adsorption capacity of hydrogen over CrOx/(110)γ-Al2O3 was much larger than that of the parallel catalyst on {111} surfaces of γ-Al2O3, thus the higher hydrogenation activity of the former catalyst. The assignment and discussion of the catalytically active chromia catalysts seemed in accordance with the previous investigations for alkane dehydrogenation and alkene hydrogenation reactions.12,13,18 However it is very strange that the selective hydrogenation activity of CrOx/Al2O3 catalyst was scarcely mentioned in literatures, though the investigations on its dehydrogenation activity were well documented.12,14-16,31,32


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Figure 6. H2-TPD curves for CrOx/(110)γ-Al2O3-550 and CrOx/(111)γ-Al2O3-550 catalysts after H2 pretreatment at 350 °C for 1 hour. The discrepancy of the above results leads us to believe that the crystal-facet effect of γAl2O3 indeed decides the semi-hydrogenation efficiency of acetylene over two catalysts. Figure 7b showed the proposed mechanism of acetylene hydrogenation over CrOx/γ-Al2O3 catalyst. CrOx species were essentially reduced before catalytic tests, regarded as activated to generate coordinatively unsaturated Cr3+ and O2- species. After exposed to the reaction atmosphere, the heterolytic dissociative adsorption of H2 took place at the pair sites of coordinatively unsaturated surface Cr3+ and O2-,33,34 leaving a hydride ion that hydrogenated the triple bond of acetylene adsorbed on the coordinatively unsaturated Cr3+ sites, and then the formed carbanion would attract the proton atom attached to the O2- sites to generate the ethylene, followed by the desorption of the ethylene, regenerating the active sites for readsorption. This mechanism reasonably explains the active role of the reduced chromia supported on alumina, in the meantime, the catalytic results together with the XPS, TPD, TPR, and HD exchange findings help us further understand the divergent activities of the related Cr3+, on account of the specific interactions with the predominated surfaces of γ-Al2O3 supports. As depicted in Figure 7a, the


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higher energy {111} facets of γ-Al2O3 tend to stabilize the lower valence of Cr species after deposition, which are referred to as “non-redox Cr3+” inclined to be catalytically inactive,13,31,32 whereas the Cr3+ ions formed through reduction of chromium with higher oxidation states stabilized by the lower energy {110} surfaces of alumina are referred to be “redox Cr3+” which could be catalytically active when exposed on the surface. Thus, the more the amount of reduced Cr species, the more active the supported CrOx catalysts are. Nevertheless, the contribution of coordination states of surface chromium species to the diverse performances in acetylene semihydrogenation, in virtue of distinctive surfaces of γ-Al2O3 supports should not be ruled out either, which is in need of other characterization tools31 to gain more insights into the different nature of active Cr species of CrOx/(110)γ-Al2O3 and CrOx/(111)γ-Al2O3 catalysts involved in selective hydrogenation.

Figure 7. (a) Schematic depiction of crystal-facet effect of γ-Al2O3 on supporting CrOx catalysts for semi-hydrogenation of acetylene. (b) The proposed mechanism of acetylene semihydrogenation over CrOx supported on γ-Al2O3.


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CONCLUSIONS In summary, we have investigated the crystal-facet effect of γ-Al2O3 on the catalytic performances of CrOx/γ-Al2O3 catalysts for selective hydrogenation of acetylene to ethylene. The excellent catalytic reactivity of CrOx supported on stable {110} surfaces of γ-Al2O3 is attributed to the presence of relative higher amounts of catalytically active Cr species in lower valence states, reduced from high valence chromium, in favor of the activation of H2, while the suppression of the formation of catalytically active Cr sites on high-energy {111} facets of γAl2O3 results in the adverse activity of the relevant supported chromia catalysts. It is imperative to investigate the effect of exposed crystal facets on the catalysis, leading to a better understanding of the interaction between support and main catalyst, and thus will be favorable to the synthesis and control of the suitable catalysts, predominated with particular facets, and exploited in many chemical reactions for improving the conversion of reactants. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ORCID Weiping Ding: 0000-0002-8034-5740 Yan Zhu: 0000-0002-5993-4110 Author Contributions W.D. conceived the work. Y.B. did the experiment and characterization. J. Y. and R. G. helped to prepare the alumina. X.F., N.H. and L.M. gave helpful suggestions. Y.Z. joint the analysis of data and the manuscript writing. W.D. and Y.Z. guided and finalized the work.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (21273107, 21773109, 91434101, 91745108). And the financial support from the Ministry of Science and Technology of China (2017YFB0702900) is also thanked.


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Crystal-Facet Effect of Î³-Al2O3 on Supporting CrOx for Catalytic

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