Compensation effect. A DFT study of the activation of N2O over M-CHA (M=Fe2+, Co2+, RuO2+, RuO+)

Towards operando computational modeling in heterogeneous catalysis

An increased synergy between experimental and theoretical investigations in heterogeneous catalysis has become apparent during the last decade. Experimental work has extended from ultra-high vacuum and low temperature towards operando conditions. These developments have motivated the computational community to move from standard descriptive computational models, based on inspection of the potential energy surface at 0 K and low reactant concentrations (0 K/UHV model), to more realistic conditions. The transition from 0 K/UHV to operando models has been backed by significant developments in computer hardware and software over the past few decades. New methodological developments, designed to overcome part of the gap between 0 K/UHV and operando conditions, include (i) global optimization techniques, (ii) ab initio constrained thermodynamics, (iii) biased molecular dynamics, (iv) microkinetic models of reaction networks and (v) machine learning approaches. The importance of the transition is highlighted by discussing how the molecular level picture of catalytic sites and the associated reaction mechanisms changes when the chemical environment, pressure and temperature effects are correctly accounted for in molecular simulations. It is the purpose of this review to discuss each method on an equal footing, and to draw connections between methods, particularly where they may be applied in combination.

The dependence on ammonia pretreatment of N-O activation by Co(II) sites in zeolites: a DFT and ab initio molecular dynamics study

This work is focused on the donor properties of cobalt-exchanged cationic sites in zeolites. It is based on cluster and periodic density functional theory modeling for relevant {[Co(II)(NH3)n]-NO} adducts, where Co(II) means a cobalt cation embedded either in a periodic model of chabasite (CHA) zeolite or in model clusters. NO stretching frequencies were derived from MD trajectories and compared to harmonic values from cluster calculations. By relating calculated NO frequencies to experimental FTIR spectra, it was shown that the forms of {Co(II)-NO} adducts comprising three or four ammonia co-ligands dominate the spectrum taken in ammonia-saturation conditions while forms with two NH3 ligands prevail under intermediate ammonia saturation. Finally, this work confirms the critical dependence of Co(II) activation ability towards NO upon the center donor properties, reinforced by ligation of strong donor ammonia ligands. However, strongly bound ligands appear also to compete with interaction of the center with the electron-rich framework, and a balance must be observed to maintain optimal activation ability. Graphical abstract A snapshot from MD trajectory showing a fragment of periodic framework with twoCo(II)-NO centers, bound to one framework oxygen and strongly coordinating three ammonia ligands with four others forming the second coordination sphere.

Ammonia-modified Co(II) sites in zeolites: spin and electron density redistribution through the Co(II)-NO bond

Electronic factors essential for the bonding of a non-innocent NO ligand to ammonia-modified Co(2+) sites in cobalt-exchanged zeolites are examined for small cluster models using DFT and advanced correlated wave function calculations. The analysis of charge transfer processes between the NO ligand and the cobalt center involves two protocols: valence-bond expansion of the multiconfiguration CASSCF wave function (in terms of fragment-localized active orbitals) and spin-resolved natural orbitals for chemical valence (SR-NOCV). Applicability of SR-NOCV analysis to transition metal complexes involving non-innocent fragments is critically assessed and the approach based on the CASSCF wave function turns out to be much more robust and systematic for all studied models. It is shown that the character and direction of electron density redistribution through the Co-N-O bond, quantified by relative share of the Co(II)-NO(0), Co(III)-NO(-), and Co(I)-NO(+) resonance structures in the total wave function, fully rationalize the activation of the N-O bond upon NH3 co-ligation (evidenced by calculated and measured red-shift of the NO stretching frequency and commonly ascribed to enhanced backdonation). The huge red-shift of νN-O is attributed to an effective electron transfer between the ammonia-modified Co(ii) centers and the NO antibonding π*-orbitals (related to the increased share of the Co(III)-NO(-) form). Unexpectedly, the effect is stronger for the singlet complex with three NH3 ligands than for that with five NH3 ligands bound to the cobalt center. Our results also indicate that high-efficiency electron transfers between the Co(ii) center and the NO ligand may be enabled for the selected spin state and disabled for the other spin state of the adduct. This illustrates how the cobalt center may serve to fine-tune the electronic communication between the NO ligand and its binding site.

Activity and deactivation of Ru supported on La1.6Sr0.4NiO4 perovskite-like catalysts prepared by different methods for decomposition of N2O

Ru/La_1.6Sr_0.4NiO₄ prepared by the addition of ethylene glycol shows excellent activity for N₂O decomposition, even in the presence of O₂ and water.