PtOx Cly (OH)z (H2 O)n Complexes under Oxidative and Reductive Conditions: Impact of the Level of Theory on Thermodynamic Stabilities

Abstract

Platinum-based catalysts with Cl^- , OH^- , O^2- and H₂ O ligands, are involved in many industrial processes. Their final chemical properties are impacted by calcination and reduction applied during the preparation and activation steps. We investigate their stability under these reactive conditions with density functional theory (DFT). We benchmark various functionals (PBE-dDsC, optPBE, B3LYP, HSE06, PBE0, TPSS, RTPSS and SCAN) against ACFDT-RPA. PBE-dDsC is well adapted, although hybrid functionals are more accurate for redox reactions. Thermodynamic phase diagrams are determined by computing the chemical potential of the species as a function of temperature and partial pressures of H₂ O, HCl, O₂ and H₂ . The stability and nature of the Pt species are highly sensitive to the activation conditions. Under O₂ , high temperatures favour PtO₂ while under H₂ , platinum is easily reduced to Pt(0). Chlorine modifies the coordination sphere of platinum during calcination by stabilizing PtCl₄ and shifts the reduction of platinum to higher temperatures under H₂ .

Keywords: calcination and reduction conditions; density functional calculations; platinum complexes; random phase approximation; thermodynamic phase diagrams.

Figure 1

Optimized structures of a selection of the most stable considered platinum complexes ranked by their formal oxidation degree (Pt(IV): a–k and Pt(II): l–r) and hydration level.

Figure 2

Hirshfeld charges of the Pt(0), Pt(II) and Pt(IV) complexes according to their chlorination level and number of undissociated water molecules in their coordinative shell. Blue box is for Pt(0), purple one for Pt(II) and the red box for Pt(IV).

Figure 3

Averaged reaction energies classified per class of reaction and ranked according to the oxidation level of the platinum. ACFDT‐RPA (red) is taken as reference and compared to GGAs (green colors), hybrids (blue colors) and metaGGAs (yellow‐brown colors)

Figure 4

Platinum complexes considered in the thermodynamic models and the interconnected reactions: hydration, ligand exchange and reduction. For the sake of clarity, Pt(III) and Pt(I) are put aside but are also linked to Pt(IV), Pt(II) and Pt(0) through a linear combination of these reactions.

Figure 5

ACFDT‐RPA thermodynamic diagrams of the most stable species under oxidative conditions at P(O₂)=0.21 bar (a,b,c) and reductive conditions at P(H₂=1 bar) (d,e,f) for three chlorine loadings: a–d: without chlorine (P(HCl)=0 bar); b–e: P(HCl)=10⁻¹² bar and c–f: P(HCl)=10⁻⁵ bar. The gas phase domain of water is located above the black curved line.

Figure 6

Transition temperature according to the HCl pressure and temperature for the main stable species expected after calcination (a) and reduction (b).