Kinetic Coupling among Metal and Oxide Phases during CO Oxidation on Partially Reduced PdO(101): Influence of Gas-Phase Composition

Abstract

We used direct-rate measurements and reflection absorption infrared spectroscopy (RAIRS) to investigate CO oxidation on a PdO(101) film at 450 K and focused on characterizing how the gas-phase composition (O 2 + CO) influences the reaction kinetics and the coupling between the reaction and the surface phases that develop. We find that chemisorbed O atoms on Pd(111) are intrinsically more reactive than PdO(101) and that CO oxidation rates can be as much as 2−3 times higher on Pd(111) compared with PdO(101) for the conditions studied. Using RAIRS, we show that reduction of PdO( 101) by CO produces metallic Pd(111) domains at 450 K and identified several rate processes that couple the PdO(101) and Pd(111) phases during reaction, including the formation of Pd( 111) domains from O vacancies on PdO(101), oxygen transfer from the oxide to the metal to generate chemisorbed O atoms, and the healing of surface oxygen vacancies on PdO(101) via O atom migration from the bulk oxide as well as O 2 dissociative adsorption. We show that the CO oxidation rates increase autocatalytically as the surface fraction of metallic Pd initially increases during reaction because Pd( 111) is more reactive than PdO(101) over the range of conditions studied. Our results further demonstrate that the oxide and gaseous O 2 can act cooperatively to supply metallic Pd(111) domains with chemisorbed O atoms, resulting in higher CO oxidation rates achieved on mixtures of PdO(101) and Pd(111) compared with pure Pd(111). The present study clarifies key rate processes and the resulting kinetic couplings among the gas-phase and solid phases during CO oxidation on partially reduced PdO(101). We expect that our findings will provide useful guidance for advancing first-principles kinetic modeling of CO oxidation promoted by transition-metal catalysts.

Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal: Spatial Variation of Active and Poisoning Phases in Stationary Conditions

“…The CO oxidation (2 CO + O 2 → CO 2 ) on platinum group metal surfaces, such as palladium, has been intensively studied during decades, due to its enormous technological impact, and also as a model heterogeneous gas/surface catalytic reaction. 1 In the early times, 2,3 Surface Science experiments carried out in Ultrahigh Vacuum (UHV) played an essential role in the basic understanding of the CO oxidation process, but a much deeper atomic-scale insight is being lately gained through surface sensitive techniques that operate at millibar and bar pressures, such as high-pressure Scanning Tunneling Microscopy (STM), 4,5 Near-Ambient Pressure X-ray photoemission spectroscopy (NAP-XPS), [6][7][8] infrared reflection absorption spectroscopy, [9][10][11][12][13] and high-energy surface X-ray diffraction. [14][15][16][17][18][19][20][21][22][23] All these studies agree in the fundamental picture, namely the abrupt transition at the "ignition" temperature, from the low-temperature oxidation stage, when CO covers (or "poisons") the catalytic surface, to the high-temperature activity stage, when the CO-poisoning layer is displaced by chemisorbed oxygen, preceding the build-up, first, of two-dimensional (2D) surface oxides, and last, of bulk oxides.…”

“…In the case of Pd, the CO oxidation reaction has been investigated on a variety of high and low symmetry crystal surfaces under reaction conditions. [4][5][6][7][8]10,13,[21][22][23][24][25][26][27][28] This will help to separately understand the catalytic performance at crystal planes that shape technologically relevant nanoparticles. However, given the variety of reaction parameters and experimental constraints, comparative in-situ investigations among different surface orientations would be better suited.…”

Self‐sustained oscillations within the temperature hysteresis in CO oxidation over Pd: Mathematical model of a cascade of continuous stirred‐tank reactors

“…To study Models I and II, we used both the qualitative methods of theory of dynamical systems and numerical methods, which include the methods of continuation of the steady state and limit cycle on the parameter. The values of the rate constants of steps were previously determined in our works . The rate constants of steps were varied according to the experimental and density functional theory (DFT) data, which demonstrated that the rate of CO oxidation over PdO depends on the case where surface oxide, epitaxial oxide, or polycrystalline bulk oxide are considered.…”

“…The values of the rate constants of steps (1)-(3) were previously determined in our works. 9,10 The rate constants of steps (4)-(6) were varied according to the experimental 33 and density functional theory (DFT) data, 34,35 which demonstrated that the rate of CO oxidation over PdO depends on the case where surface oxide, epitaxial oxide, or polycrystalline bulk oxide are considered. In Ref.…”

“…In the case of CO oxidation over Pd, we can take into account all oxygen forms observed on the catalyst in kinetic experiments as well as their transitions into each other. [10][11][12] This gives us a large kinetic system of ordinary differential equations from which, using small parameters and the quasi-steady-state concentration method, we can obtain a differential-algebraic system of equations. As a rule, such sort of nonlinear algebraic equations can have multiple steady states.…”