Dissociative adsorption of methane on the Cu and Zn doped (111) surface of CeO2

The development of economical heterogeneous catalysts for the activation of methane is a major challenge for the chemical industry. Screening potential candidates becomes more feasible using rational catalyst design to understand the activity of potential catalysts for CH4 activation. The focus of the present paper is the use of density functional theory to examine and elucidate the properties of doped CeO2. We dope with Cu and Zn transition metals having variable oxidation state (Cu), and a single oxidation state (Zn), and study the activation of methane. Zn is a divalent dopant and Cu can have a +1 or +2 oxidation state. Both Cu and Zn dopants have an oxidation state of +2 after incorporation into the CeO2 (111) surface; however a Hubbard +U correction (+U = 7) on the Cu 3d states is required to maintain this oxidation state when the surface interacts with adsorbed species. Dissociation of methane is found to occur locally at the dopant cations, and is thermodynamically and kinetically more favorable on Zn-doped CeO2 than Cu-doped CeO2. The origins of this lie with the Zn(II) dopant moving towards a square pyramidal geometry in the sub surface layer which facilitates the formation of two-coordinated surface oxygen atoms, that are more beneficial for methane activation on a reducible oxide surface. These findings can aid in rational experimental catalyst design for further exploration in methane activation processes

Enhancing the oxygen vacancy formation and migration in bulk chromium(iii) oxide by alkali metal doping: a change from isotropic to anisotropic oxygen diffusion

Oxygen vacancy formation and migration are vital properties for reducible oxides such as TiO2, CeO2 and Cr2O3 as the oxygen storage capacity (OSC) of these materials are important for a wide range of applications in photovoltaics, oxidative catalysis and solid oxide fuel cells. Substitutional doping these transition metal oxides enhances their OSC potential, in particular for oxygenation and surface reaction chemistry. This study uses density functional theory with on-site Coulomb interactions (PBE+U) for Cr 3d states (+U = 5 eV) and O 2p states (+U = 5.5 eV) to calculate the oxygen vacancy formation energy and oxygen diffusion pathways for alkali metal (Li, K, Na, Rb) doping of bulk chromium(III) oxide (α-Cr2O3). Substitutional doping of the lattice Cr3+ cations with alkali metals that have a +1 oxidation state, creates two hole states on the neighbouring lattice O atoms, and removal of a lattice oxygen charge compensates the dopants by filling the holes. The removal of the next oxygen describes the reducibility of doped Cr2O3. The oxygen vacancy formation energy is greatly promoted by the alkali dopants with a correlation between the ionic radius of the dopant cation and vacancy formation energy; larger dopants (K, Rb) improve the reducibility more than the smaller dopants (Li, Na). The activation barriers for oxygen migration along different directions in the alkali metal doped Cr2O3 bulk were also calculated to examine the effect of doping on the oxygen migration. The calculated activation energies for the undoped chromia are symmetric in three dimensions (isotropic) and the presence of the dopants break this isotropy. Alkali dopants promote oxygen migration in the oxygen intra-layers while suppressing oxygen migration across the Cr cation layers. The smaller dopants (Li, Na) facilitate easier migration in the oxygen intra-layers to a greater extent than the larger dopants (K, Rb). The Na–Cr2O3 bulk promotes both oxygen vacancy formation and migration which makes it a novel candidate for anode materials in medium temperature SOFCs and battery applications

Low valence cation doping of bulk Cr2O3: Charge compensation and oxygen vacancy formation

The different oxidation states of chromium allow its bulk oxide form to be reducible, facilitating the oxygen vacancy formation process, which is a key property in applications such as catalysis. Similar to other useful oxides such as TiO2, and CeO2, the effect of substitutional metal dopants in bulk Cr2O3 and its effect on the electronic structure and oxygen vacancy formation are of interest, particularly in enhancing the latter. In this paper, density functional theory (DFT) calculations with a Hubbard + U correction (DFT+U) applied to the Cr 3d and O 2p states, are carried out on pure and metal-doped bulk Cr2O3 to examine the effect of doping on the electronic and geometric structure. The role of dopants in enhancing the reducibility of Cr2O3 is examined to promote oxygen vacancy formation. The dopants are Mg, Cu, Ni, and Zn, which have a formal +2 oxidation state in their bulk oxides. Given this difference in host and, dopant oxidation states, we show that to predict the correct ground state two metal dopants charge compensated with an oxygen vacancy are required. The second oxygen atom removed is termed "the active" oxygen vacancy and it is the energy required to remove this atom that is related to the reduction process. In all cases, we find that substitutional doping improves the oxygen vacancy formation of bulk Cr2O3 by lowering the energy cost

Historical dry and wet periods in Colorado: (Part A: Technical Report)

Includes bibliographical references (page 31).July 1999.Funded by: Office of Emergency Management under P.O. #PD97SEM000015

Climate data continuity with ASOS: 1993 annual report for the period September 1992-August 1993

Includes bibliographical references (page 28).February 1994.The research was supported by NOAA, NWS, Office of Meteorology under grant number NA90RAH00077.Annual

Colorado temperatures with degree day and growing season data

July 1989

Climatic data representativeness in western Colorado

June 1990.For U.S. Department of the Interior, Bureau of Land Management, Colorado State Office, Lakewood, Colorado

Modelling and Simulation of the Power Take-Off System for a Hinge-Barge Wave-Energy Converter

The McCabe Wave Pump (MWP) has possibilities for the conversion of wave energy into electrical energy and the production of potable water. However, in order to optimise the dynamics and operation of this device in the face of a wide variety of sea conditions, a number of important control issues must be addressed. The first step, which is addressed in this paper, is the production of a dynamical model of the MWP, which can provide a basis for both control design and simulation. Since the power take-off (PTO) system provides the main damping in the system, the paper places particular emphasis on the PTO model and how it couples to the main rig dynamics. The control problem formulation is also briefly addressed

Modelling and Simulation of the Power Take-Off System for a Hinge-Barge Wave-Energy Converter

The McCabe Wave Pump (MWP) has possibilities for the conversion of wave energy into electrical energy and the production of potable water. However, in order to optimise the dynamics and operation of this device in the face of a wide variety of sea conditions, a number of important control issues must be addressed. The first step, which is addressed in this paper, is the production of a dynamical model of the MWP, which can provide a basis for both control design and simulation. Since the power take-off (PTO) system provides the main damping in the system, the paper places particular emphasis on the PTO model and how it couples to the main rig dynamics. The control problem formulation is also briefly addressed

Cooperative solar radiation data collection program, Fort Collins, Colorado, June 1985-May 1986

September 1986.Funding shared by the Colorado Agricultural Experiment Station and Fort Collins Light and Power