Dispersed single-atom Co and Pd nanoparticles forming a PdCo bimetallic catalyst for CO oxidation
Introduction
Bifunctional catalysts comprising two metals and their oxides supported on inert matrixes are widely investigated in heterogeneous catalysis [1], [2], [3], [4]. This interest is derived from the synergic properties arising from metal-promoter interactions, which typically originates from the addition of a more oxyphilic metal onto a noble metal, such as AuCu [5], PdCo [6], and PtFe [7]. In these systems, the nature of the alloying and segregation of the nanoalloys (or active phase) plays a significant role in promoting or, in some cases, hindering catalytic activity. For example, Destro et al.[5] showed that catalysts comprising AuCu nanoparticles (NPs) supported on SiO2 are highly susceptible to changes in the pretreatment conditions which affect the alloy composition and either promotes or suppresses CO oxidation. The effect is partially linked to the formation of CuOx species adjacent to the (de)alloyed AuCu-NPs. Hence, careful consideration of these effects is important to understand the interaction between the metals and their behaviour under catalytic conditions. Furthermore, a variety of chemical and physical parameters relating to bimetallic catalysts have been investigated such as, alloy composition [1,5,[8], [9], [10]], core-shell formation [10,11], single atmos formation [12], [13], [14], [15], [16], [17], [18], [19], [20], [21],phase crystallinity [22], particle size and particle distribution [5] as well as the influence of the support on the catalytic performance [23,24].
In general, studies commence from samples with well-controlled composition and particle size, their response to thermal treatment, catalytic environment and interaction with the supports are then explored. Recently, Wu et al. [10] used surface characterization techniques and CO oxidation as a probe catalytic reaction to provide important insights into the promotional effect of adding Co to Pd based catalysts. Their valuable discoveries showed that Pd and Co play different roles in the catalytic reaction whereby CO is absorbed onto Pd, O2 is then activated at the Co sites, then, depending on the active particle boundaries and interfaces and the proximity of both metals, CO and O interaction is facilitated to form CO2. According to the studies, there is a fine balance between Pd and Co ratios which leads to improved CO conversions. Less than a monolayer of Co on Pd is required, otherwise, CoOx species will hinder CO adsorption onto Pd. The proximity between these metals and an optimal ratio between Pd and Co creates synergic effects which are not merely a linear combination of their monometallic counterparts and generate unique active sites to improve the efficacy of the catalytic reactions [10].
Probe reactions such as the oxidation of carbon monoxide can offer insights into the behaviour of active sites and aid in elucidating reasons for observed catalytic activity for more complex reaction systems, such as hydrodeoxygenation (HDO) of biomass-derived chemicals and various electrochemical reactions [1,22]. Although CO oxidation in the presence of O2 is considered a relatively simple reaction, the mechanistic steps occurring during this reaction and their correlation with the physical and chemical properties of the catalyst provide valuable information. However, the interpretation of the catalytic data relating to bimetallic catalysts is not trivial. For example, the hysteresis features of CO oxidation reactions over noble metal nanoparticles dispersed on a variety of supports have been studied [23,25] with no general agreement being reached. This lack of consensus arises from the complexity of the catalytic process that involves adsorption, diffusion, multiple reaction rate kinetics and surface interactions followed by desorption [23,25]. The mechanism underpinning hysteresis features has been explained via the existence of multiple steady states during CO interaction with the catalyst surface [23,[25], [26], [27]]. In the work published by Casapu et al. [26], a direct correlation between the hysteresis feature and the Pt nanoparticle size was determined. CO oxidation over noble metal nanoparticles of 2 nm resulted in an inverse hysteresis. In contrast, larger Pt nanoparticles resulted in typical hysteresis for the CO oxidation reaction. This was attributed to differences in the adsorption strength of CO onto the active sites of the catalyst with variations noted between surface/bulk sites, and the degree of oxidation of the Pt nanoparticles [26].
In this work, the CO oxidation reaction is used to probe PdCo bimetallic catalysts supported on activated carbon. Moreover, pourous, high surface area [28] activated carbon was chosen as a support due to its favourable chemical interaction with Co to produce highly dispersed cobalt anchored on carbon [29]. Furthermore, activated carbon exhibits acid-base chemical properties, can be easily functionalized, and presents electrical conductivity [28,30]. These characteristics make the catalysts studied herein a promising material for the HDO of biobased molecules [30,31] and show potential as catalysts for cathode materials in Li-air batteries [11,32]. The synergic and promotional effect played by Co when added to Pd based catalysts is studied via CO oxidation probe reactions. Characterization of the catalysts using in situ X-ray diffraction (XRD) and in situ X-ray absorption near-edge spectroscopy (XANES) was performed to investigate the correlation of the alloy formation and metal segregation during the temperature-programmed reduction (TPR) experiments and under CO oxidation reactions. In addition, transmission electron microscopy analysis was conducted to investigate morphological and chemical properties of the active catalysts as well as after catalytic reaction.
Section snippets
Catalyst synthesis
All reagents were used as received without further purification. Catalysts were synthesized via incipient wetness impregnation [7]. Briefly, the acetylacetonate complexes of each metal Pd(acac)2 and Co(acac)2) were dissolved at 70°C in a mixture of 75:2:2 (isopropanol:oleylamine:oleic acid) and were added dropwise to activated carbon (Merck, incipient point of 1.58 mL.g−1) at the same temperature. The solid was dried overnight at 110°C and was subsequently calcined at 300°C for 30 min under a
Catalytic testing – CO oxidation
Light-off curves, i. e. catalytic activity measured non-isothermally using a heating and cooling ramp, were collected for Pd/C, Co/Pd and bimetallic PdCo samples comprising various Pd/Co molar ratios as shown in Fig. 2. A blank reaction, with only activated carbon, was also measured for comparison. The temperature at which 50% CO conversion occurs is known as the catalyst light-off temperature (T1/2) and is a parameter used to characterize and compare CO conversion among the catalysts [25].
Conclusions
The synthesis methodology, using oleyamine and oleaic acid as capping agents for the optimized delivery of Pd and Co to the activated carbon surface, created single-atom, highly dispersed cobalt and nanosized particles of Pd over the support. Thermal activation of the catalysts, under reductive atmosphere, allowed Pd and Co mobility leading to the formation of a PdCo alloy, according to STEM-EDS, in situ XRD and XANES analysis. The intimate contact between Co and Pd facilitates rapid adsorption
CRediT authorship contribution statement
The project conceptualization, methodology, sample preparation, catalytic tests, data curation as well as data analysis, interpretation and discussion were conceived by I. B. Aragão under the supervision of C. B. Rodella. TEM and HAADF-STEM data curation, analysis and interpretation were performed by F. R. Estrada and discussion with C. B. Rodella. Data analysis, discussions and manuscript revision were conducted by D. H. Barrett. All authors contributed to the writing, review and editing of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research used facilities of the Brazilian Synchrotron Light Laboratory (LNLS) and Brazilian Nanotechnology National Laboratory (LNNano), part of the Brazilian Centre for Research in Energy and Materials (CNPEM), a private non-profit organization under the supervision of the Brazilian Ministry for Science, Technology, and Innovations (MCTI). We would like to thank the financial support of the CAPES/CNPEM, grant number 88887.177693/2018-00; XPD, DXAS and XAFS2 beamlines and staff and LNnano
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