Quantum Chemical Molecular Dynamics Study of the Water–Gas Shift Reaction on a Pd/MgO(100) Catalyst Surface

The water–gas shift (WGS) reaction on a Pd/MgO(100) catalyst surface was studied using the tight binding-quantum chemical molecular dynamics (TB-QCMD) method. Molecular adsorption of CO was observed. In contrast, we observed that H₂O adsorption occurs first molecularly but the molecule then dissociates on the surface. The resultant hydroxyl group reacts with preadsorbed CO to form an OCOH intermediate and a single H atom. This process is relevant as the initial hydroxylation step, and it is part of the catalyzed hydrolysis mechanism. During the molecular dynamics simulation the OCOH intermediate inverted into an H–CO₂ like molecule and finally HCO₂ decomposed to CO₂ and H. Later on, the resultant H interacts with the previously dissociated single H atom (H released from the H–OH dissociation) and forms the WGS product H–H molecule. It was observed that the CO₂ desorbed from the supported Pd cluster while the H₂ molecule remains attached to the Pd cluster during the simulation. The geometries and dissociation energies of water molecules were obtained and the type of adsorption assessed. Chemical changes, changes in electronic and adsorption states, and structural changes were also investigated through TB-QCMD calculations, which indicate that the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and the formation of the OCOH intermediate. The present study indicates that the MgO(100) support has a strong interaction with the Pd catalyst, which may cause an increase in Pd activity as well as enhancement of the metal catalyst dispersion, hence, increasing the rate of the WGS reaction. Furthermore, from the molecular dynamics and electronic structure calculations, we have identified a number of consequences for the interpretation and modeling of the WGS reaction

Characterization of Acid-Soluble Oxidized Asphaltenes by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Insights on Oxycracking Processes and Asphaltene Structural Features

The dissolution of organic matter into water via oxidative processes, named oxycracking, has been practiced for a long time for the removal of organic pollutants, in which oxygen induces breakage and functionalization of organic molecules. Recently, oxycracking has been explored as an alternative approach to handling the increased amount of solid residues produced in oil sands upgrading activities that involve carbon rejection in solvent deasphalting units. This study uses an asphaltene-rich feedstock, operationally known as petroleum pitch, isolated from an Athabasca bitumen vacuum residue, which was submitted to oxycracking reactions at 200 and 220 °C. The feed and water-soluble fractions isolated at pH 1, termed acid-soluble oxidized asphaltenes (ASOA), were analyzed by ultrahigh-resolution mass spectrometry [Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS)] using electrospray and atmospheric pressure photoionization ion sources. FTICR-MS analysis revealed extensive oxidation of all compound classes originally present in the asphaltene-rich feed. Double bond equivalent (DBE) distribution plots show that sequential carboxylation (formation of a carboxyl group) occurs progressively with an increasing reaction temperature, leading to the incorporation of up to 15 oxygen atoms per molecule, whereas simultaneous decarboxylation reactions produce a CO₂-rich gas phase. ASOA samples also show lower overall carbon number distributions than the asphaltene feed, which is direct evidence of C–C bond cleavage during the oxycracking process. In addition, molecular fragments detected in ASOA after carbon–carbon bond cleavages showed not only lower carbon numbers but also lower DBEs per molecule, consistent with a more dominant archipelago architecture for the parent asphaltene molecules