Elsevier

Journal of Catalysis

Volume 389, September 2020, Pages 229-240
Journal of Catalysis

Atmosphere induced amorphous and permeable carbon layer encapsulating PtGa catalyst for selective cinnamaldehyde hydrogenation

https://doi.org/10.1016/j.jcat.2020.05.036Get rights and content

Highlights

  • C@PtGa interface structure is constructed by H2/CO2 atmosphere treatment using LDH as precursor.

  • PtGa alloys were covered by thin carbon layer with the characteristics of defective and permeable.

  • Preferred conversion compared to oxide layer catalyst is owing to high accessibility of metallic sites.

  • High selectivity is due to optimized adsorption mode of CAL on C@PtGa interface sites.

  • Amorphous carbon layer is stable after 5 times cycles under selective cinnamaldehyde hydrogenation reaction.

Abstract

In this work, an alloy catalyst with C@PtGa interface structure was synthesized under H2/CO2 atmosphere by using PtCl62−/MgAlGa-LDHs as precursor. Cs-corrected STEM, EELS and CO chemisorption clearly confirmed the PtGa alloy particles were well covered by amorphous carbon thin layer with the characteristics of porous and permeable. In selective cinnamaldehyde hydrogenation, the C@PtGa catalyst showed a higher selectivity than both bare PtGa catalyst and Ga2O3@PtGa catalyst. The enhanced selectivity was attributed to the geometric decoration of carbon layer as well as the formation of electron-rich Pt sites at C@PtGa interface, which optimized the adsorption mode of cinnamaldehyde. More importantly, the permeable characteristic of carbon layer which maintained the accessibility of reactants to the Pt sites, contributed to the higher conversion compared to Ga2O3@PtGa catalysts with traditional oxide interface layer. Additionally, C@PtGa catalyst exhibited a good reusability with conversion of 88.3% and selectivity of 91.9% after 5 cycles. This work provides an alternative way to realize the boost of activity and selectivity in selective hydrogenation of C=O bonds by intelligently fabricating non-oxides@metal interface.

Introduction

Selective hydrogenation of α, β-unsaturated aldehyde to unsaturated alcohols has attracted much more attention because it is widely used for production of pharmaceuticals, perfume and other important chemical products [1], [2]. Among various α, β-unsaturated aldehydes, cinnamaldehyde (CAL) is a presentative probe molecule used for revealing the competition mechanism of hydrogenating conjugated C=C and C=O bond. Theoretically, hydrogenating the conjugated C=C bond is both thermodynamically and kinetically favored than the C=O bond [3]. Therefore, the selective hydrogenation of C=O bond without attacking C=C bond is a knotty problem.

Supported metal-oxide catalysts are well-known as the most active materials for α, β-unsaturated aldehyde hydrogenation. However, even over the most selective metals Ir, Ru or Pt, there is a lack of selectivity to C=O bonds [4], [5]. Therefore, the importance of oxides support has been drawing more attention, as the oxides not just help the dispersion of metal nanoparticles, but rather interact with metal nanoparticles and affect the catalysis. Specially, the critical role of oxides support can become very prominent when oxides cover metal surfaces forming the oxide/metal interface structure [6]. Extensive reports demonstrated oxide/metal interface structure significantly alters the electronic structure through charge transfer and decorates geometric structure to enable the steric effect, which contributes to the improvement of selectivity in hydrogenation of α, β-unsaturated aldehyde [7], [8]. For example, Qin fabricated a catalyst with FeOx@Pt interface structure, which enhanced the C=O selectivity from 47 to 84% in cinnamaldehyde hydrogenation compared with Pt/Al2O3 catalyst, resulted from the electron transfer from Fe to Pt and obvious steric hindrance effect by FeOx [9]. Similarly, Zaera found SiO2-Pt interface significantly facilitated the selectivity of C=O bonds from 20% to 85%, respect to Pt/Al2O3 catalyst, while the TOF declined from 200 h−1 to 25 h−1 due to the decrease of Pt exposure degree by 33% [10]. In this regard, although the construction of traditional metal-oxide interface structure plays an important role in improving selectivity towards C=O bond, it is still a great challenge to maintain or enhance the catalytic activity due to the coverage of active metals by the oxide layers.

Following progressive reports in some other heterogeneous catalysis, the nature of interface structure also attracts more attention due to the remarkable influence on the catalytic performance. Li’s group demonstrated the formation of Ni-WC interface obviously strengthened the electronic transfer in the supported Ni catalyst, in which charge-riched Ni sites activated unsaturated bonds more efficiently and therefore resulted in lower reaction energy of O=N-O to H-N-H group in nitroarene reduction [11]. Moreover, during the CO2-reduction process, Christopher et al. found the reaction intermediate HCOx decorated the TiOx@Rh interface to form an encapsulated HCOx-TiOx@Rh structure [12]. The modification of Rh by the amorphous and porous HCOx-TiOx layer greatly improved the selectivity towards CO from 2% to 89%. Inspired by the advantages of carbon-containing layers, we intend to directly fabricate permeable C@metal interface structure which possess more beneficial electronic effect and superior steric hindrance to realize simultaneous improvement of selectivity and activity by introducing CO2 into the thermal treatment process of the catalyst.

Layered Doubled Hydroxides (LDHs) is a class of two-dimensional materials with the formula of [M1-x2+Mx3+OH2]x+(An-)x/nmH2O, which are widely applied in the field of catalysis [13], [14], [15]. The M2+ and M3+ cations are homogeneously dispersed within the layers and some of the divalent cations have been replaced by trivalent ions giving positively charged sheets [16]. By taking advantage of these characteristics, introducing catalytically active metals into the layers of LDHs can generate highly dispersed metal catalysts supported on mixed metal oxides (MMO) after calcination and reduction. Recently, we further revealed the superior of LDHs precursor method in the synthesis of uniform alloy catalysts [17], [18], [19]. Considering alloy structure also exhibits excellent catalytic performance compare to monometallic catalyst in selective hydrogenation of α, β-unsaturated aldehyde reaction, in this work, we propose to fabricate an alloy catalyst with C@alloy interface structure to simultaneously optimize activity and selectivity in cinnamaldehyde hydrogenation reaction by utilizing the dual synergic effect of alloy and non-oxide layer. To be specific, we prepared PtCl62−/MgAlGa-LDH as catalyst precursor which was then pretreated under H2/CO2 atmosphere to synthesize C@PtGa/MgAlGaOx catalyst. As expected, the Cs-corrected STEM images, electron energy-loss spectroscopy (EELS), Raman spectra and CO chemisorption clearly confirmed the PtGa alloy particles were well covered by amorphous carbon thin layer with the characteristics of porous and permeable. It should be noted that we also synthesized Ga2O3@PtGa interface catalyst as contrast under cycling H2-O2 atmosphere. In this case, the C@PtGa interface catalyst showed a conversion of 90.7% with C=O selectivity of 93.6%, which is higher than bare PtGa alloy catalyst (58.7%, 71.7%) and Ga2O3@PtGa interface catalyst (46.6%, 87.5%). A combined characterization revealed the superior selectivity of C=O bonds over C@PtGa/MgAlGaOx catalyst was ascribed to the C@PtGa interface structure optimizing the CAL adsorption mode (di-σco to on-top) by the geometric decoration of carbon layer and formation of electron-rich Pt sites. The higher conversion could be attributed to the permeable characteristic of carbon layer which maintained the accessibility of reactants to the Pt sites and the promotional C=O activation ability. Moreover, after 5 times cycles, C@PtGa catalyst exhibited a satisfying reusability. The main strategies of our work can be summarized to the following points: (1) intelligently fabricating C@alloy interface layer by combining H2/CO2 atmosphere pretreatment and LDHs precursor method; (2) in contrast to traditional oxide interface layer, the influence of interface nature on catalytic performance was clarified; (3) the construction of this permeable non-oxides@metal interface structure provides a new idea to realize the boost of activity and selectivity in hydrogenation of α, β-unsaturated aldehyde reaction which could also draw some reference to other selective hydrogenation reactions.

Section snippets

Synthesis of PtCl62−/MgAlGa-LDHs precursor

The MgAlGa-LDHs was obtained via a coprecipitation method. Mg(NO3)2·6H2O, Al(NO3)3·9H2O and Ga(NO3)3·9H2O with Mg2+/Al3+/Ga3+ molar ratio of 4:1:1 were dissolved in 60.0 mL deionized water as salt solution, and the total metal cation concentration is 0.70 M. Then, NaOH and Na2CO3 were dissolved in 60.0 mL deionized water as alkali solution (concentration of NaOH and Na2CO3 is 1.12 M and 0.70 M respectively). The salt solution and alkali solution were added simultaneously dropwise into flask at

Structure and morphological characterization

Fig. 1A depicts the XRD patterns of PtCl62−/MgAlGa-LDHs precursor and the catalysts synthesized under different pretreatment atmosphere at 600 °C (other XRD patterns are shown in Figure S1). Obviously, in case of PtCl62−/MgAlGa-LDHs, the pattern shows the characteristic reflections of LDH materials (JCPDS Card 89–5434) including (0 0 3), (0 0 6), (0 0 9), (1 1 0) and (1 1 3) facet, which suggests we obtained well-structured ternary MgAlGa-LDHs materials. After pretreated at 600 °C, the

Conclusion

In this work, amorphous carbon layer encapsulated PtGa alloy catalyst was synthesized under H2/CO2 pretreatment atmosphere by using PtCl62−/MgAlGa-LDHs as precursor. Cs-corrected STEM and EELS clearly confirmed the construction of C@PtGa interface structure, and CO-chemisorption indicated the thin amorphous carbon layer has the characteristics of porous and permeable. In selective cinnamaldehyde hydrogenation reaction, under similar conversion, the selectivity towards cinnamyl alcohol of

Declaration of Competing Interest

None.

Acknowledgment

This work was supported by the National Natural Science Foundation (21878016), National Key Research and Development Program of China (2016YFB0301601), the Fundamental Research Funds for the Central Universities (BHYC1701B, JD2004) and PetroChina Science and Technology Management Department (2016E-0703). The Pt L3-edge X-ray adsorption spectra (XAS) were performed at 1W1A beamline at Beijing Synchrotron Radiation Facility (BSRF).

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