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Multi-component heterogeneous catalysts are among the top candidates for converting greenhouse gases into valuable compounds. Combinations of Cu, Zn, and ZrO2 (CZZ) have emerged as promisingly efficient catalysts for CO2 hydrogenation to methanol. To explore the catalytic mechanism, density functional theory (DFT) calculations and the energetic span model (ESM) were used to study CO2 conversion routes to methanol on CuZn–ZrO2 interfaces with a varying Zn content. Our results demonstrate that the presence of Zn sites at the interface improves CO2 binding. However, the adsorption and activation energies are insensitive to Zn concentration. The calculations also show that the hydrogenation of adsorbate oxygen atoms at the interface is kinetically more favourable and requires hydrogen spillover from the metal to the zirconia. This leads to barriers that are lower than those reported on interface or metal-only sites in previous literature. While DFT calculations alone are unable to identify which one of the competing pathways is more favourable, the ESM model predicts that the carboxyl pathway has a higher turnover frequency than the formate route. Our findings also show the importance of considering effects such as hydrogen spillover which take place at a metal-oxide interface when modelling complex catalytic environments. peerReviewed

Funding Information: The project was funded by the Academy of Finland projects 307853 (M.M.M.), 338228 (M.M.M.), 310574 (M.A.C.), 330488 (M.A.C.), 317739 (M.M.M., N.M., and K.H.), and 332290 (N.M.). M.M.M. and K.H. also acknowledge Jane and Aatos Erkko Foundation for funding to the LACOR project. The computational resources were provided by CSC-IT Center for Science Ltd through the pilot project initiatives (H2OINTE and FLUXMD). We also thank Professor Jakob Schiøtz for helpful discussions related to the thermostats implemented in ASE. Prof. Gerrit Groenhof and Dr. Dimitry Morozov are acknowledged for careful reading of the work and their fruitful comments. Publisher Copyright: © 2022 American Chemical Society. All rights reserved. Density functional theory-based molecular dynamics (DFT-MD) has been widely used for studying the chemistry of heterogeneous interfacial systems under operational conditions. We report frequently overlooked errors in thermostated or constant-temperature DFT-MD simulations applied to study (electro)catalytic chemistry. Our results demonstrate that commonly used thermostats such as Nosé-Hoover, Berendsen, and simple velocity-rescaling methods fail to provide a reliable temperature description for systems considered. Instead, nonconstant temperatures and large temperature gradients within the different parts of the system are observed. The errors are not a "feature"of any particular code but are present in several ab initio molecular dynamics implementations. This uneven temperature distribution, due to inadequate thermostatting, is well-known in the classical MD community, where it is ascribed to the failure in kinetic energy equipartition among different degrees of freedom in heterogeneous systems (Harvey et al. J. Comput. Chem. 1998, 726-740) and termed the flying ice cube effect. We provide tantamount evidence that interfacial systems are susceptible to substantial flying ice cube effects and demonstrate that the traditional Nosé-Hoover and Berendsen thermostats should be applied with care when simulating, for example, catalytic properties or structures of solvated interfaces and supported clusters. We conclude that the flying ice cube effect in these systems can be conveniently avoided using Langevin dynamics.

CO2 adsorption and activation on a catalyst are key elementary steps for CO2 conversion to various valuable products. In the present computational study, we screened different Cu–ZrO2 interface structures and analyzed the influence of the interface structure on CO2 binding strength using density functional theory calculations. Our results demonstrate that a Cu nanorod favors one position on both tetragonal and monoclinic ZrO2 surfaces, where the bottom Cu atoms are placed close to the lattice oxygens. In agreement with previous calculations, we find that CO2 prefers a bent bidentate configuration at the Cu–ZrO2 interface and the molecule is clearly activated being negatively charged. Straining of the Cu nanorod influences CO2 adsorption energy but does not change the preferred nanorod position on zirconia. Altogether, our results highlight that CO2 adsorption and activation depend sensitively on the chemical composition and atomic structure of the interface used in the calculations. This structure sensitivity may potentially impact further catalytic steps and the overall computed reactivity profile. peerReviewed

Raman signal is monitored after 248 nm photodissociation of formaldehyde in solid Ar at temperatures of 9–30 K. Rotational transitions J = 2 ← 0 for para-H2 fragments and J = 3 ← 1 for ortho-H2 are observed as sharp peaks at 347.2 cm−1 and 578.3 cm−1, respectively, which both are accompanied by a broader shoulder band that shows a split structure. The rovibrational spectrum of CO fragments has transitions at 2136.5 cm−1, 2138.3 cm−1, 2139.9 cm−1, and 2149 cm−1. To explain the observations, we performed adiabatic rotational potential calculations to simulate the Raman spectrum. The simulations indicate that the splitting of rotational transitions is a site effect, where H2 molecules can reside in a substitution site, in addition to an interstitial site. In the former site, rotational motion is unperturbed by the electrostatic field of the host atoms, while the latter site splits the excited rotational manifolds, J = 2 and 3, into doublet and triplet structures, respectively. For CO, the spectrum can be ascribed to monomeric species in single- and double-substitution sites, to a dimeric species (CO)2, and to a CO–H2O complex. The simulations show that a nearest-neighbor molecular complex CO–H2 is not responsible for any of the observed spectral fingerprints. The cause of the exit of the molecular hydrogen from the initial cage can be traced to high translational energy of the fragment after the photodissociation. After the matrix has reached a thermal equilibrium, a diffusion driven formation of the complex is possibly hindered by the high rotational zero-point energy developed upon complexation. peerReviewed