Article pubs.acs.org/JPCC
Shape Effect of Pd-Promoted Ga2O3 Nanocatalysts for Methanol Synthesis by CO2 Hydrogenation Jin Qu,†,‡,§ Xiwen Zhou,‡,§ Feng Xu,‡ Xue-Qing Gong,† and Shik Chi Edman Tsang*,‡ †
Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science & Technology, Meilong Road 130, Shanghai 200237, China ‡ Wolfson Catalysis Centre, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. S Supporting Information *
ABSTRACT: In this paper, we present a new approach to investigate metal−support interaction in catalysis. First, we have carried out a controlled growth of two semiconductive Ga2O3 nanocrystals in distinctive shapes, namely, plate and rod with the majority of their surfaces covered with polar and nonpolar facets, respectively. We have then placed the same contents of Pd on these nanocrystals and carried out a systematic testing and characterization for methanol synthesis from CO2 hydrogenation under industrial applicable conditions. It is found that a low indexed (002) polar Ga2O3 surface is highly unstable, which gives oxygen defects and mobile electrons in the conduction band more readily than those nonpolar (111) and (110) surfaces. A significantly strong metal−support interaction between the (002) polar Ga2O3 surface and Pd was determined, and it gave rise to higher metal dispersion and facilitated electron transfer between them, leading to the formation of PdGax. This renders such composite nanocatalysts active for methanol production.
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INTRODUCTION In recent years, carbon dioxide (CO2) has become the focus of attention because of the position of CO2 as the primary greenhouse gas and the implication of its emissions on the problem of climate change.1 Various sequestration technologies for CO2 abatement are at present being considered.2 Meanwhile, transformation to diversified chemicals, materials, and fuels is an ideal prospect of CO2 utilization.3 On the other hand, chemical synthesis of methanol from CO2 and H2 has been regarded as an important way in the conversion and transportation of hydrogen gas related energies derived from nonfossil sources such as solar, hydropower, and biomass means.4 Thus, the use of energetic but carbon neutral methanol molecules derived from renewable hydrogen sources for energy provision is strategically more desirable than the approach of post capture and storage of CO 2 . As a result, CO 2 hydrogenation to methanol is the key route to recycling of CO2 which has prompted recent research interests to develop effective catalysts for this reaction.5 Methanol as the main platform chemical for present fuel and chemical infrastructures6 can be produced from CO2 hydrogenation over Cu-based catalysts. Numerous investigations concerning various aspects of Cu on ZnO catalysts have been carried out during the past few decades, making this field relatively well established.7,8 With various modifiers or promoters, Cu is still identified as the main active catalyst component.9−11 Besides improving the dispersion and stabilization of Cu, ZnO could also provide lattice defects and modify the basicity/acidity of the catalyst.12−14 The strong metal support interaction (SMSI) plays an important role in this area, and many other oxides, especially ZrO2, were © 2014 American Chemical Society
extensively studied as support modifiers to promote this interaction.15−19 It is proposed that the SMSI led to Cu0, Cun+, and oxide basic sites at the interface which benefit the adsorption/activation of reactants.20 On the other hand, palladium (Pd), which was used as a modifier and played a synergetic effect on the acitve Cu sites,21,22 is also known to be active for methanol synthesis by CO2 hydrogenation. The activity and selectivity could be improved remarkably with a promotive SMSI.23,24 The report of a new Pd/Ga2O3 catalyst giving a higher activity than the Cu/ZnO formulation for methanol production from CO2 hydrogenation was first launched by Fujitani et al.25,26 This catalyst was also reported to be superior to Pd/SiO2 (inert support) and Cu/ZnO/Al2O3 as well as other supported Pd such as Pd/CeO2 or Pd/Al2O3 per gram or per metal surface area basis.25,26 It was proposed that the origin of the high activity might be related to the presence of Pdn+ species.27−29 This study has triggered further investigations of this new and related system. For example, adding gallium oxide to Pd/SiO2 was proved to enhance the reaction selectivity dramatically to 70%, and a 500-fold increase in turnover frequency (TOF) compared to unmodified Pd/SiO2 in CO2 hydrogenation to methanol was recorded.30 The reduction behavior on Ga2O3 was studied in H2, which showed that catalytic reduction of this stable oxide can only take place at a very high temperature. However, in the presence of Pd, catalytic reduction of the oxide can be facilitated at lower Received: June 25, 2014 Revised: September 14, 2014 Published: September 19, 2014 24452
dx.doi.org/10.1021/jp5063379 | J. Phys. Chem. C 2014, 118, 24452−24466
The Journal of Physical Chemistry C
Article
temperature.5,31,32 Meanwhile, in the cofeed of CO2 and H2, different adsorption species including (bi)carbonate, formate, and formyl species were clearly identified on the surface of gallium oxide rather than the weakly adsorbed CO2 on SiO2, and thus the surface defect(s) of Ga2O3 was proposed to play an important role in the CO2 hydrogenation.33,34 The above characterization could also be related to the photocatalytic reduction of CO2 in water by metal-promoted Ga2O3.35 The detailed mechanism of the CO2 hydrogenation on Pd/ Ga2O3 was studied by Fourier transform infrared spectroscopy (FTIR) and some other characterizations.36 The bifunctional mechanism was proposed. This suggested that active hydrogen atoms were generated on a metal surface in the hydrogen atmosphere, and they then would spill to the support surface to reduce the trapped CO2 in the form of (bi)carbonate species.37 In contrast, the high activity of Pd/Ga2O3 was attributed to the formation of active Pd−Ga bimetallic for the CO2 hydrogenation or for the reversed reaction of methanol steam reforming (MSR).38,39 In the latter reaction Pd−Ga was expected to provide active hydrogen species to reduce the strongly adsorbed methanol on the same metallic surface to CO2.40,41 High activity and selectivity were also reported over the same type of catalysts for some other reactions such as acetylene semihydrogenation.42 Density functional theory (DFT) is a powerful tool to look into the micromechanism at the atom scale. The reaction network of CO2 hydrogenation was extensively calculated on Cu(111) for traditional Cu/ZnO catalyst.43−45 Combining experimental work, there have also been some conputational studies on the Pd/Ga2O3 system. Adsorbate behaviors of reactants and intermediates such as CO2,46 H2,47−49 formate,50 and methanol51 were investigated by DFT. These studies also proposed that surface defect(s) played a crucial role to enhance CO2 adsorption and H2 dissociation.46−48 Most of these simulations were based on the most energetically favorable Ga2O3(100)-B facet.52 However, the adsorptions on Ga2O3 were found to be very sensitive to the surface terminations, and the less stable suface was more active.53 Moreover, DFT is a direct tool to look into SMSI, and many relative researches have been done.54−57 Apart from the pure metal or oxide surface, metal support interaction was also considered, and some calculations focued on the reaction at the interface were also performed.58−60 A new Cu−ceria interface was investigated with a model of a ceria cluster on a Cu surface.61 Metal carbides could also show significant SMSI and be excellent supports for nobal metals enhancing bond and activating CO2.62 Although the real metal−support interfaces are far more complex than theoretical models and no consensus of the mechanism of this reaction was made, these calculations still significantly boost our understanding of interface reactions and SMSI. Apart from the search for different catalyst formulations, the study of morphology (shape effect)−performance relationships is an emerging important field of research. Particularly, metal− support interaction plays an important role in heterogeneous catalysis, which stems from geometric and electronic perturbations between catalyst components. It has been clearly shown from the literature that by presenting different shapes (exposed crystallographic planes) of metal oxide support particles to host metal there is a great variation in their catalytic performances.63 This new approach adds an exciting variable in tailoring properties of nanocatalysts for various reactions,2 which may lead to rational design of novel highly active and selective nanocatalysts for many catalytic applica-
tions. However, little work has been done in this important area for methanol synthesis from CO2 hydrogenation. During recent years, various shapes of Ga2O3 nanostructures were synthesized such as nanowires, nanoribbons, nanosheets, and nanotubulars.64−68 We have recently reported in communication notes a remarkable activity dependence on the shape of using a ZnO plate nanocrystal in Cu/ZnO as well as β-Ga2O3 plate nanocrystals in Pd/β-Ga2O3, which give a higher yield for methanol production from CO2 hydrogenation than other corresponding oxide surfaces. This clearly indicates that the exposed crystallographic plane on their plate-form surface must show a stronger material synergy with metal than other facets, giving enhanced catalysis.2,69 Here, we compare two Ga2O3 nanoparticles of the same phase but with different shapes, namely, the β-Ga2O3 rod and β-Ga2O3 plate systematically. We have also doped these two Ga2O3 materials with Pd nanoparticles from low to high loadings and used them as model catalysts to study the morphology−performance relationships for the reaction of methanol synthesis from CO2 hydrogenation. We have then carried out careful testing of these model catalysts in combination with in-depth characterizations using cyclic voltammetry and X-ray photoelectron spectroscopy and computing modeling. From the above works and systematic comparisons, we aim to produce a more comprehensive understanding of the morphological effect of Ga2O3 for this important catalytic reaction.
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EXPERIMENTAL SECTION Catalyst Preparations. β-Ga2O3 rodlike materials were synthesized according to a reported method by Zhao et al.70 In a typical synthesis, 0.006 mol of Ga(NO3)3 was dissolved in ultrapure water to form a solution A with a metal ion to water molar ratio of 1:100 and heated to 80 °C. With stirring a 1.5 mol/L NaOH solution was added drop by drop into solution A. The added 0.018 mol portion of NaOH, which is three times of the amount of Ga(NO3)3, was used to form precipitate. The pH value after mixing was adjusted at around 6.3−6.5, which is nearly at the neutral point, in order to obtain a maximum precipitate. The mixture was then sealed and put in a shaking bath at a shaking rate of 100 rpm at 80 °C for 2 h. The resulting precipitate was recovered by centrifuge and washed with pure water at least 10 times to remove sodium ions (EDX showed the absence of Na signal). Then the precipitate was suspended in pure water again and vigorously stirred at room temperature for 2 h. The sample was hydrothermally treated at 100 °C for 2 days, washed by ultrapure water several times, and dried in air at 80 °C. The dried solid was then calcined in air to form Ga2O3 crystals at 900 °C for 2 h, with a heating rate at 2.5 °C/ min. β-Ga2O3 platelike nanomaterials synthesis was through a solid state synthesis, according to the method from Yan et al.,71 followed by subsequent treatments. KGaO2 (precursor) solid powders were prepared by heating a stoichiometric mixture of K2CO3 and Ga2O3 at 950 °C in air for 12 h. For the preparation of GaOOH, in a typical procedure, 20 mL of KGaO2 aquatic solution (0.2 mol/L) was added into 20 mL of CH3COOH (0.2 mol/L) aquatic solution and stirred for 3 h at room temperature to form GaOOH nanoplates, and then the sedimentation was separated by centrifugation, washed repeatedly by deionized water, and dried at 60 °C overnight. The β-Ga2O3 nanoplates were obtained by heating the asprepared GaOOH at 800 °C for 3 h with a heating rate of 5 °C/min in air. 24453
dx.doi.org/10.1021/jp5063379 | J. Phys. Chem. C 2014, 118, 24452−24466
The Journal of Physical Chemistry C
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The GC electrode was prepolished to a mirror-like finish using an aluminum oxide (
