Full Length ArticleFirst-principles investigation of CO and CO2 adsorption on -Al2O3 supported monoatomic and diatomic Pt clusters
Graphical abstract
Introduction
Transition metals are often used as the active component of catalysts [1], [2], [3], [4], [5], [6], [7], [8]. A high degree of metal dispersion on the support must be maintained in order to maximize the efficiency. Due to recent advances in fabrication techniques, the size of the clusters deposited on support surfaces can now be controlled and characterized very precisely, down to the level of a single atom [9], [10]. In fact, there is a tremendous amount of current interest in supported single-atom catalysts (SACs) [11], [12], [13], [14], [15], [16], [17], which refer to a class of catalysts involving single metal atoms dispersed on suitable surfaces. A great challenge associated with the use of SACs is the loss of dispersion over time as the catalyst goes through regeneration cycles. If not anchored properly, small metal particles tend to migrate as a result of their facile diffusion behavior and agglomerate into larger clusters degrading the activity [2], [18], [19]. In order to prevent this, the substrate surface is often modified with dopants or defects that act as pinning centers for the single atoms [16], [17]. Once synthesized, SACs must be characterized to ensure that the desired high level of dispersion has been achieved. While high resolution direct imaging methods such as aberration-corrected STEM are available for this purpose [12], they must be supplemented with vibrational analysis techniques such as FTIR and DRIFTS for a more precise characterization [20]. Using these techniques, shifts in the fundamental frequencies of simple probe molecules are measured and information is inferred on the size of the clusters.
Metal oxide-supported transition metal clusters are known to have high catalytic activity towards many industrially relevant reactions including water-gas shift [6], [21], [22], methane oxidation [23], NO oxidation [24], and syngas production [25]. In addition to providing structural integrity, the support often significantly alters the electronic properties of the metal component [12], [26], [27], [28]. Furthermore, the oxide/metal interface may have even larger activity than either component [2]. -Al2O3 surfaces are commonly used for ensuring good noble metal dispersion, since they readily provide anchoring sites for small metal clusters due to their large surface area and high porosity [29]. As an added benefit, the acidity of the -Al2O3 surface can be tuned by introducing metallic or oxide additives [30].
CO adsorption on metals and alloys has been extensively studied both experimentally [31], [32] and computationally [33]. Several phenomenological models have been developed over the years with the aim of predicting the reactivity of transition metal surfaces in the pure and alloy form. With reference to CO adsorption, models have been developed that correlate the changes in the CO adsorbate orbitals and the catalyst d-band center with changes in the CO bond strengths via variations in their CO stretching frequencies. These include Blyholder-type models [34], [35], - models [36], [37], [38] and d-band center-based models [39].
In the current work, we present an ab initio investigation of the energetics of adsorption of CO and CO2 on atomic Pt and the Pt2 cluster supported on the (1 0 0) surface of -Al2O3. The particular choice of the molecular species is motivated by their ubiquitous presence as reactants, intermediates, and products in many important reactions such as CO oxidation [12], [40], [41], CO2 dissociation [42], and carbonate formation [43]. In addition to their active roles in key reactions, CO and CO2 are frequently used as probes to identify the charge state and coordination of active sites on surfaces [43]. We therefore aim to study in detail the interaction of CO and CO2 with a single Pt atom supported on -Al2O3 and compare it to that of the Pt2 cluster.
In this paper, the activity of Pt toward the aforementioned probe molecules is characterized through an analysis of the energetics of adsorption, the extent of substrate-to-metal charge transfer in addition to such tools as partial density of states (PDOS) profiles and charge density differences. We also compare the relative stability of adsorbed atomic Pt and Pt2, before and after the adsorption of the probe molecules.
The plan of the paper is as follows. Section 2 contains the computational details while the results are discussed in Section 3. We start Section 3 with the identification of the most stable adsorption sites for the Pt clusters on the fully dehydrated -Al2O3 (1 0 0) surface. This is followed by a discussion of the energetics of adsorption of the probe molecules on the bare, completely dehydrated -Al2O3 (1 0 0) surface. In the final part of Section 3, the energetics of adsorption of CO and CO2 adsorbates on the supported metallic clusters along with their electronic and vibrational properties are investigated. A summary is presented in Section 4.
Section snippets
Method
The calculations were performed using density functional theory (DFT) as implemented in the Quantum ESPRESSO code suite [44], using the generalized gradient-corrected exchange correlation functional [45], [46] by Perdew, Burke and Ernzerhof (PBE). A kinetic energy cutoff of 35 Ryd was used in the plane wave expansion, which ensures good convergence of energy differences. The interaction between nuclei and electrons were modeled by ultrasoft pseudopotentials [47]. Brillouin zone integrations
Atomic Pt and Pt2 cluster adsorption on the dehydrated -Al2O3 (1 0 0) surface
Fig. 1 displays the optimized adsorption geometries of atomic Pt and Pt2 on -Al2O3 identified in our calculations, whereas the energetics of adsorption for all stable geometries reported in Fig. 1 are presented in Table 1 along with computed Bader charges on the noble metal atoms.
For atomic Pt, the most stable adsorption configurations correspond to geometries embedded within the characteristic surface voids, shown in Fig. 1(a–c). The adsorption energies corresponding to these embedded
Summary and conclusion
The interaction of small molecular species with supported transition metal atoms is an active research field with numerous potential applications in heterogeneous catalysis. In this work, the structural, electronic and vibrational properties of CO and CO2 on preadsorbed atomic Pt and Pt2 clusters supported on the -Al2O3 (1 0 0) surface were investigated. Based on the energetics of the interaction of atomic Pt and the (1 0 0) surface of -Al2O3, we have shown that the formation of embedded
acknowledgement
This work is financially supported by TÜBITAK, Scientific and Technological Research Council of Turkey (Grant No: 112T542). D.T. also acknowledges partial funding from the project Small molecules: keys for sustainable development (Finanziamento per Ricerca di Ateneo 2016–2017) of the University of Trieste, Italy.
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