Ab-initio study of Pd-based alloy catalysts for CO 2 hydrogenation to fuel
Igor Kowalec 1 , L. Kabalan 1 , Z. Lu 1 , C.R.A. Catlow 1,2,3 , A. Logsdail 1 1 Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, UK 2 UK Catalysis Hub, Research Complex at Harwell, RAL, Oxford, OX11 0FA, UK 3 Department of Chemistry, University College London, London, WC1H 0AJ, UK
Introduction Methanol synthesis by direct hydrogenation of CO 2 has been recognised as a potential route towards sustainable fuels for transport and a circular fuel economy. 1 The facilitation of direct CO 2 hydrogenation to methanol via a surface HCOO formate intermediate is crucial for desirable selectivity, as shown in the theoretical study of Cu catalysts by Medford et al. 2 Our previous work has shown that flat surfaces of a Pd catalyst have a high activation energy of initial CO 2 activation and hydrogenation to HCOO. 3 However, Pd-based catalysts supported on ZnO are potent catalysts for this reaction, with their reactivity attributed to the PdZn binary metallic phases. 4 Methodology The reactivity of Tetragonal Body-Centred (TBC) PdZn (110) and (101) and Body-Centred Cubic (BCC) CuPd (110) alloy surface facets towards CO 2 activation and hydrogenation has been investigated computationally with comparison against low-index monometallic Face-Centred Cubic (FCC) Cu, Pd and Hexagonal Close-Packed (HCP) Zn surfaces. We used periodic DFT to ensure accurate representation of the metallic and alloy structures and catalytic processes, using the semi-local Bayesian error estimation density functional (mBEEF). 5,6 All surface structures were verified against experimental observables. Transition states (TS) in the reaction pathway were investigated using NEB approaches to study the energy profile. Results and discussions CO 2 adsorption energy and activation energy of hydrogenation to the HCOO intermediate were calculated on Cu, Pd and Zn, CuPd and PdZn surfaces. The preferred starting CO 2 geometry was chemisorbed - a negatively charged and bent geometry, rather than physisorbed – a neutral and linear CO 2 geometry. 3 Chemisorption of CO 2 was exothermic on Pd (100) and Pd (110), and endothermic on Pd (111), Cu (100), (110), PdZn (101), (110) and CuPd (110) surfaces. The two largest TS energies were on surfaces that could only accommodate physisorbed CO 2 , i.e., HCP Zn (0001) and FCC Cu (111). The rate determining step varies across the metals where the surface with smallest TS energy is considered – for Cu it is CO 2 chemisorption, for Pd it is hydrogenation of chemisorbed CO 2 , and for Zn it is hydrogen adsorption. On Pd-M (M = Cu, Zn) alloys CO 2 preferably binds in a bidentate configuration via carbon to negatively charged surface Pd and via oxygen to the positively charged neighbouring surface M atom. CuPd shows a hydrogen adsorption energy between that of Cu and Pd metals, more stable CO 2 chemisorption, and a TS energy near that of Cu and Pd. Upon alloying Pd with Zn, the TS on PdZn (110) is reduced in energy below the net zero reaction energy, thereby highlighting the catalytic capability of this alloy. The study brings insight into rational design of modern Cu x Pd y Zn z alloy catalysts in CO 2 hydrogenation. References 1. Sustainable Synthetic Fuels for Transport | Royal Society. https://royalsociety.org/-/media/policy/projects/synthetic-fuels/
synthetic-fuels-briefing.pdf, Accessed 27 Jan. 2023 2. A. J. Medford et al. , J. Cat. , 2014 , 309, 397–407. 3. I. Kowalec et al., Phys. Chem. Chem. Phys., 2022 , 24, 9360-9373 4. N. Iwasa et al. , Catalysis Letters , 1998 , 54, 119–123. 5. J. Wellendorff et al. , J. Chem. Phys. , 2014 , 140, 144107. 6. L. Kabalan et al. , Phys. Chem. Chem. Phys. , 2021 , 23, 14649–14661.
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