Highly Dispersed PdNPs / α-Al 2 O 3 Catalyst for the Selective Hydrogenation of Acetylene Prepared with Monodispersed Pd Nanoparticles

Pd nanoparticles (PdNPs) stabilized by methyl cellulose (MC) were synthesized in an aqueous solution, which are monodispersed nanoparticles. PdNPs/α-Al2O3 catalyst was prepared with monodispersed PdNPs and showed better catalytic performance than Pd/α-Al2O3 catalyst prepared by the incipient wetness impregnation method using Pd(NO3)2 as a precursor. The catalysts were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS). It was found that monodispersed PdNPs were spherical or elliptical nanoparticles with exposed (111) and (100) facets, and the PdNPs/α-Al2O3 catalyst showed a more concentrated distribution of Pd particles on the surface of α-Al2O3 support than the Pd/α-Al2O3 catalyst. The preparation method achieved the highly dispersed PdNPs/α-Al2O3 catalyst with smaller Pd particle size and decreased the aggregation of Pd active sites, which was responsible for higher acetylene conversion and ethylene selectivity.


Introduction
Industrially, ethylene is an important chemical for polymerization to polyethylene.However, ethylene-rich feed stream contains trace amount of acetylene (0.5-2% by volume) for the tail-end hydrogenation reactor.The small trace of acetylene as impurity has to be reduced to less than 5 ppm [1][2][3].For the selective hydrogenation of acetylene, supported Pd catalysts are used for the catalytic removal process.Therefore, supported Pd catalysts with low Pd loading still have attracted much attention in recent years [4][5][6][7][8][9][10].The recent advance on supported Pd catalysts is to increase their ethylene selectivity and enhance their long-term stability.For improving the performance of supported Pd catalysts, bimetallic Pd catalysts have been investigated extensively with the addition of a second metal as a promoter to dilute Pd ensemble size [11][12][13][14][15][16][17][18][19][20][21][22].
For monometallic Pd catalysts, the shape and size of Pd nanoparticles (PdNPs) are very important for Pd catalysts such as core-shell Pd nanocatalysts, magnetic Pd catalysts, porous carbon nanocomposite Pd catalysts and so on, which also have attracted much attention in the last few years [23,24].For the selective hydrogenation of acetylene, Yarulin et al. [25] investigated the structure sensitivity of acetylene hydrogenation on catalysts with controlled shape of PdNPs by a colloidal method using (polyvinyl pyrrolidone) PVP as the stabilizer.The result shows that the shape of PdNPs does not affect the catalyst selectivity, but the activity decreases in the order Pdoct > Pdco > Pdcub.Crespo-Quesada et al. [26] also prepared Pd nanocubes using PVP as the stabilizer and investigated the effect of stabilizer removal and catalytic performance.In addition, He et al. [27] reported that supported Pd nanowire and cuboctahedron catalysts were synthesized in an ethylene glycol-poly (vinylpyrrolidone)-KBr system using a precipitation-reduction method.The activities of the Pd nanowire catalysts were significantly higher than those of the cuboctahedron catalysts owing to the exposure of larger numbers of Pd atoms, which resulted in excessive hydrogenation and a decrease in ethylene selectivity.Kim et al. [28] reported that cubic PdNPs were synthesized and supported on Al 2 O 3 , which was used as a catalyst for acetylene hydrogenation.It was found that the easier decomposition of subsurface H in Pd(100) facets contributed to its higher activity and the strong adsorption of reactants on Pd(111) facets surface decreased ethylene selectivity.
In this paper, PdNPs stabilized by methyl cellulose (MC) were synthesized in an aqueous solution, which are monodispersed nanoparticles.PdNPs/α-Al 2 O 3 catalyst was prepared with monodispersed PdNPs stabilized by MC.Monodispersed PdNPs were spherical or elliptical nanoparticles with exposed (111) and (100) facets.Moreover, the PdNPs/α-Al 2 O 3 catalyst showed a more concentrated distribution of Pd particles on the surface of α-Al 2 O 3 support than the Pd/α-Al 2 O 3 catalyst prepared by the incipient wetness impregnation method using Pd(NO 3 ) 2 as a precursor.The preparation method in this work achieved the highly dispersed PdNPs/α-Al 2 O 3 catalyst with smaller Pd particle size and decreased the aggregation of Pd active sites.Therefore, the PdNPs/α-Al 2 O 3 catalyst had higher acetylene conversion and ethylene selectivity.

Catalyst Characterization
The transmission electron microscopy (TEM) image shown in Figure 1a indicates that the PdNPs stabilized by MC are monodispersed spherical or elliptical nanoparticles.The average particle size is 3.2 nm and the deviation from the mean diameter is about ±0.4 nm.As shown in Figure 1b, the high-resolution TEM (HRTEM) image provides the crystal structure of PdNPs.It shows that PdNPs exposed two kinds of crystal facets, which are attributed to Pd(111) and Pd(100) facets.Moreover, the (111) and (100) lattice fringes in PdNPs exhibited a clearly ordered continuous fringe pattern corresponding to the face-centered cubic (fcc) structure [28].According to the literature, for the selective hydrogenation of acetylene in ethylene-rich stream, the shape of PdNPs had no impact on ethylene selectivity.Moreover, Pd(111) facets had strong adsorption to the target reactant, and are thus responsible for a higher acetylene conversion than Pd(100) facets [25].
For supported Pd/Al 2 O 3 catalysts, isolated Pd active sites dispersed on the surface of the Al 2 O 3 support as egg-shell catalysts exhibited high activity for acetylene removal, which also inhibited the over-hydrogenation of ethylene to ethane.Therefore, it is significant to decrease Pd active site ensembles [5,7,11,16].Figure 2a shows that the PdNPs on the PdNPs/α-Al 2 O 3 catalyst were smaller particles than those on the Pd/α-Al 2 O 3 catalyst (Figure 2b).This means that the PdNPs/α-Al 2 O 3 catalyst prepared with monodispersed PdNPs can achieve highly dispersed active sites.As can be seen in Figure 2c, the PdNPs on the PdNPs/α-Al 2 O 3 catalyst are isolated Pd particles, which decrease the Pd ensemble size.In contrast, the Pd particles on the Pd/α-Al 2 O 3 catalyst can form large Pd ensembles, which are the aggregation of Pd active sites.Also, the Pd dispersion of the PdNPs/α-Al 2 O 3 catalyst was 25.12%.The Pd dispersion of the Pd/α-Al 2 O 3 catalyst was 20.36%, which was lower than that of the PdNPs/α-Al 2 O 3 catalyst.[12,15,16,20,22].Therefore, it is significant to achieve a more concentrated distribution of Pd particles on the surface of the α-Al 2 O 3 support.

Catalytic Performance
Figure 6a shows that the catalytic activity of the PdNPs/α-Al 2 O 3 catalyst was a little higher than that of the Pd/α-Al 2 O 3 catalyst for the selective hydrogenation of acetylene in the temperature range of 50-120 • C. As shown in Figure 6b, the PdNPs/α-Al 2 O 3 catalyst also possesses higher ethylene selectivity than that of the Pd/α-Al 2 O 3 catalyst at the same reaction condition.Both of these two catalysts present a similar tendency for ethylene selectivity to decrease as acetylene conversion increased, when the reaction temperature increased from 50 to 120 • C. The selective hydrogenation of acetylene in the ethylene-rich stream was a typical consecutive reaction.Ethylene was produced as an intermediate during the reaction.Therefore, ethylene selectivity decreased as acetylene conversion increased [28,32].The stabilities of the PdNPs/α-Al 2 O 3 catalyst and Pd/α-Al 2 O 3 catalyst were tested online by using a microreactor-GC system.As shown in Figure 7, for 60 h on stream, the acetylene conversion of the PdNPs/α-Al 2 O 3 catalyst was always kept at 99%, which showed a stable catalytic activity.Its selectivity to ethylene was around 40%.In contrast, the catalytic performance of the Pd/α-Al 2 O 3 catalyst was also stable at the beginning of the 24 h.Its acetylene conversion was almost kept at 97% and its selectivity to ethylene was around 34%, which was 6% lower than that of the PdNPs/α-Al 2 O 3 catalyst.However, it can be seen that the acetylene conversion of the Pd/α-Al 2 O 3 catalyst decreased slightly after 24 h.Meanwhile, ethylene selectivity also increased gradually.It is demonstrated that the catalytic performance of the PdNPs/α-Al 2 O 3 catalyst is more stable than the Pd/α-Al 2 O 3 catalyst.The excellent catalytic performance may be ascribed to the preparation method in this work, which achieved the highly dispersed PdNPs/α-Al 2 O 3 catalyst with a more concentrated distribution of isolated Pd active sites on the surface of the α-Al 2 O 3 support.

Synthesis
The aqueous solution of Pd(NO 3 ) 2 (1 mg/mL) was obtained from Sinopec Beijing Research Institute of Chemical Industry.Methyl cellulose (average Mw: 40,000) was supplied by Sigma-Aldrich (Saint Louis, MO, USA).In a typical process, a 60-mL aliquot of a 1 mg/mL solution of Pd(NO 3 ) 2 was added to 140 mL of a 0.043 wt % aqueous solution of soluble MC.Meanwhile, the pH was adjusted to about 8.5 by dropwise addition of an aqueous solution sodium hydroxide (NaOH, 0.5 M).The stirring speed was 300 rpm.The mixture was treated with ultrasonic for 30 min.Then the mixture in a stainless steel autoclave was treated with a gas flow (10% H 2 in Ar) passing through the solution at 40 • C for at least 30 min at atmospheric pressure.The stirring speed was 300 rpm.Finally, monodispersed PdNPs stabilized by MC were synthesized in the aqueous solution.
The support of α-Al 2 O 3 consisted of spherical pellets (average diameter 3.5 mm, BET surface area 30.9 m 2 /g), which were produced by Sinopec Beijing Research Institute of Chemical Industry.The 200.0 g α-Al 2 O 3 support was put in a laboratory coating machine (rotating speed: 100 rpm).Then the above solution of monodispersed PdNPs stabilized by MC was mist-sprayed onto the support using a home-made glass sprayer actuated by compressed air (pressure: 0.1 MPa).Then, the catalyst was dried at 120 • C for 6 h.The catalyst was subsequently calcined in air at 450 • C for 4 h to remove the MC and re-expose the palladium active sites.The obtained catalyst was marked as the PdNPs/α-Al 2 O 3 catalyst.For comparison, the Pd/α-Al 2 O 3 catalyst (0.03 wt % Pd loading) was prepared by the same method using an aqueous solution of the desired amount of Pd(NO 3 ) 2 .

Characterizations
The catalysts were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS).TEM images were recorded by a JEOL JEM-3010 (200 kV) (JEOL, Tokyo, Japan).The SEM observation was performed on FEI XL30 ESEM-FEG (FEI, Eindhoven, The Netherlands) and energy dispersive X-ray analysis was coupled to SEM.X-ray photoelectron spectroscopy (XPS) measurements were measured on a Perkin-Elmer PHI 5600 spectrophotometer (Perkin Elmer Limited, Waltham Mass, Waltham, MA, USA) with the MgK α radiation.BET surface area was determined by an ASAP 2020C instrument (Micromeritics Instruments, Norcross, GA, USA).Pd dispersion was measured using CO pulse chemisorption by an AutoChem II 2920 instrument (Micromeritics Instruments, Norcross, GA, USA).XRD analysis was performed on a Bruker AXS D8 Advance instrument (λ = 1.5406Å; tube voltage, 40 kV; tube current, 300 mA) (Bruker, Karlsruhe, Germany).The ICP analysis was performed by taking a 1.0-g sample into a Teflon crucible, adding 20 mL HNO 3 (BV-III) and leaving one night, followed by ultrasonic treatment for 3 h, and subsequent 50-fold dilution with 2% HNO 3 .Finally, this was analyzed with an Agilent 7500CX ICP-MS (Agilent Technologies, Santa Clara, CA, USA).

Catalytic Tests
For catalytic performance tests, the selective hydrogenation of acetylene in the ethylene-rich stream was performed by using a microreactor-GC system with 1 mL of the catalyst.The microreactor was an 8 mm (i.d.) stainless steel tube.The catalyst was completely immersed in a continuous ethylene-rich gas flow.Before the reaction, the catalyst was reduced by H 2 (gas hourly space velocity: 300 h −1 ) at 180 • C for 2 h.A feed gas consisting of 0.61% H 2 , 0.42% C 2 H 2 , 7.69% C 2 H 6 , and balanced with C 2 H 4 was used for the reaction.The total gas hourly space velocity (GHSV) was 10,000 h −1 .The products were analyzed online by gas chromatography (Agilent 7890, Agilent Technologies, Palo Alto, CA, USA) with FID and TCD detectors.According to the literature, acetylene conversion and ethylene selectivity were calculated by the following equations [32,33]:

Conclusions
In summary, monodispersed PdNPs stabilized by MC were successfully synthesized in an aqueous solution.The monodispersed PdNPs were supported on an α-Al 2 O 3 catalyst to prepare the PdNPs/α-Al 2 O 3 catalyst, which had a more concentrated distribution of Pd particles on the surface of the α-Al 2 O 3 support.Compared with the Pd/α-Al 2 O 3 catalyst, which was prepared by the incipient wetness impregnation method using Pd(NO 3 ) 2 as a precursor, the PdNPs/α-Al 2 O 3 catalyst exhibited better catalytic performance for the selective hydrogenation of acetylene.The enhanced catalytic property can be attributed to the preparation method, which can achieve the highly dispersed PdNPs/α-Al 2 O 3 catalyst with smaller Pd particle size and decreased the aggregation of Pd active sites.

Figure 1 .
Figure 1.(a) TEM image of Pd nanoparticles (PdNPs); the inset is the corresponding Pd particle size distribution; (b) HRTEM image of PdNPs; the inset is the corresponding fast Fourier transform image.

Figure 7 .
Figure 7.The stabilities of PdNPs/α-Al 2 O 3 catalyst and Pd/α-Al 2 O 3 catalyst during the reaction with time on stream: (a) Acetylene conversion and (b) ethylene selectivity.(Reaction conditions: the ethylene-rich stream containing 0.61% H 2 , 0.42% C 2 H 2 , 7.69% C 2 H 6 , and balanced C 2 H 4 was used for the reaction at atmospheric pressure.The reaction temperature was kept at 110 • C. Total gas hourly space velocity was 10,000 h −1 ).

Table 1 .
The Pd elemental contents of the catalysts (measured by inductively coupled plasma-mass spectrometry (ICP-MS).