Transient Structures of PdO during CO Oxidation over Pd(100)

Abstract

In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semirealistic conditions, i.e., 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas flow and in a gradually changing gas composition with a time resolution of 0.5 s. The experimental technique allows for rapid reciprocal space mapping providing the complete information on structural changes of a surface with unprecedented time resolution in harsh conditions. Our results show that the (√5 × √5)R27°-PdO(101) surface oxide forms in a close to stoichiometric O 2 and CO gas mixture as the mass spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O 2 in the gas stoichiometry, islands of bulk oxide grow epitaxially in the same (101) crystallographic orientation of the bulk PdO unit cell according to a Stranski−Krastanov type of growth. The morphology of the islands is analyzed quantitatively. Upon further increase of the O 2 partial pressure a polycrystalline Pd oxide forms on the surface. ■ INTRODUCTIONFor more than a century heterogeneous catalysis has been extensively exploited by the industry, and as a consequence it has been intensively studied. 1 One of the most prominent examples is the CO oxidation reaction, CO + 1 / 2 O 2 → CO 2 . This process transforms highly toxic carbon monoxide, formed e.g. as a byproduct during incomplete combustion of the fuel in internal combustion engines, to less harmful carbon dioxide gas. However, the reaction is very slow under the operational conditions in the gas phase and requires thus the presence of a solid catalysts to proceed at a sufficiently high rate. Because of its importance and relatively simple mechanism, this reaction has become the subject of numerous studies aiming to resolve the atomic-scale processes that occur on the surface of catalysts. 2 Supported nanoparticles of late transition metals represent a well-known and efficient type of oxidation catalyst and are currently widely used in catalytic converters. 3,4 Hence, a deep understanding of the fundamental processes proceeding in such systems is important for improvement of existing catalyst-based solutions and development of new potential approaches. For this purpose, studies of atomic-scale surface structure and determination of the active phase of catalysts under working conditions are essential. However, the complexity of such systems and the inability of many experimental techniques to work under realistic pressuresthe challenges known as material and pressure gapssignificantly narrow the selection of available methods for structural determination and necessitate the use of model systems. One of the commonly used approaches is to study single crystals with different surface crystallographic orientation...

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

“…These changes were explained as the possible formation of incommensurate structures or as CO bonded to Pt steps or kinks. Although CO seems to increase the mobility of Pt atoms at the step and induce the formation of kink sites, 55,58,59 large-scale roughening of the Pt (111) surface in CO was not observed at a pressure of around 100 kPa.…”

“…It is believed to use both physisorbed and precursor states. 19 After dissociation, the O atoms are separated by two lattice constants on the Pt (111) surface, thus requiring a large surface area to dissociatively adsorb, 20 whereas CO needs only a single free site to adsorb.…”

“…The epitaxial oxide had a thickness of 5 AE 2 nm and an island width of approximately 47 AE 1 nm. 112 It coexisted with the surface oxide thus showing a Stranski-Krastanov, i.e., layerplus-island, growth mode. Additional structural information on the transition from a reduced to an oxidized surface can be obtained from a reported work combining HP STM and QMS.…”

Time Resolved Operando X-ray Techniques in Catalysis, a Case Study: CO Oxidation by O2 over Pt Surfaces and Alumina Supported Pt Catalysts

“…Approaches Using High Energy (>50 keV): SXRD and Total X-ray Scattering/PDF High energy scattering methods are also coming of age, as hopefully the study shown in Section 2.7 has made clear in the case of supported Pt catalysts for fast PDF studies. However, SXRD [177,178] made upon metal surfaces and a sophisticated variant of it due to Stierle and co-workers [179][180][181][182] using high energy X-rays to investigate supported nanoparticles has much to offer; and there are good reasons to think that in the near future both high energy PDF, XRD and even high energy SAXS measurements are going to be able to play ever more important roles in the wider scheme of catalytic science.…”

Near Ambient Pressure XPS Investigation of CO Oxidation Over Pd3Au(100)

“…Surface oxides rather than surfaces covered by chemisorbed oxygen have been 35 observed as the most active towards CO oxidation under near ambient as well as more realistic 36 conditions (above ambient pressure) [5][6][7][8][9][10][11][12][13][14][15][16]. However, some studies also indicate Pd surfaces covered 37 by atomic oxygen as highly active [8,17], and generally both will exhibit activity.…”