Structural Origin: Water Deactivates Metal Oxides to CO Oxidation and Promotes Low‐Temperature CO Oxidation with Metals

Abstract

Water is perhaps the most common molecule in heterogeneous catalysis, as it is unavoidable in almost any system. Interestingly, it can play completely different roles in the presence of either metals or metal oxides, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] which are the two most common types of the catalysts. On one hand, a moderate amount of water on the surface of late-transition metals such as Au, Pt, and Pd, can promote low-temperature CO oxidation, which is one of the hottest topics in catalysis because of the environmental concerns. [1][2][3][4][5][6] On the other hand, it can be a devastatingly poisonous species on the surface of metal oxides, the best example being the waterinduced deactivation on tricobalt tetraoxide (Co 3 O 4 ). [7][8][9][10][11][12][13][14][15] Specifically, morphology-controlled Co 3 O 4 [7, 16, 17] displays extraordinarily high catalytic activity for CO oxidation at very low temperatures (ca. À77 8C). However, in the presence of trace amounts of water its activity is dramatically reduced. [7][8][9][10][11][12] It is worth emphasizing that transition-metal oxides have received an increasing amount of attention for CO oxidation because of their unexpectedly high catalytic activities, low price, and especially the rich surface chemistry which affords the potential to tune the catalytic properties to a considerable degree. [18][19][20] Moreover, the poisoning effect of H 2 O has also been reported for other oxide-based catalysts such as CuO and MnO x , and it may well be a common issue in many oxide systems. [13][14][15] To comprehend the fundamental role of water in heterogeneous catalysis in general, the following questions need to be answered: What is the mechanism of H 2 O deactivation on Co 3 O 4 oxide? How can one rationalize such a difference between metal and metal oxide systems regarding H 2 O effects? Herein we report a thorough investigation uncovering the origin of the deactivation of Co 3 O 4 by H 2 O and addressing the general effect of H 2 O on metal and metal oxides by using first principles calculations.The deactivation resulting from the presence of water is the main obstacle currently limiting the application of Co 3 O 4 to CO oxidation, and the deactivation mechanism is much debated. The following suggestions regarding the water poisoning effect have been proposed: [7][8][9][10][11][12] 1) water molecules strongly adsorb at the active site, thus blocking the CO adsorption; or 2) water dissociation occurs on the catalyst surface to form a surface OH group that inhibits the adsorption of CO or O 2 ; or 3) the formation of graphitetype carbon deposits or surface carbonate (CO 3 2À ) species. However, no consensus has been reached. To the best of our knowledge, there is only one theoretical study reported concerning the deactivation mechanism at the molecular level, and it focused on the competing effect of the molecular adsorption of H 2 O at active Co 3+ sites. [11b] In this work, almost all the possible deactivation pathways of Co 3 O 4 with regard to wa...

A comparative study of the catalytic oxidation of HCHO and CO over Mn0.75Co2.25O4 catalyst: The effect of moisture

“…7.However, in dry conditions, the catalystsurface are gradually covered by the carbonate species accumulated during the reaction (step 5b), so that the activity steadily decreases. having been shown that surface OH groupsare inert with respect to CO oxidation at low temperature [47]. However, sufficient drying of the catalyst ensures the availability of active sites for the adsorption of carbon monoxide (Fig 9), the increased CO conversion obtained with increase of temperature from 90 to 100 o C under humid conditions being in agreement with thishypothesis [48].Notably, the removal of carbonates, corresponding to the reverse of step 1c,which results in regeneration of the catalyst, requires the involvement of water.…”

Promotion of CO oxidation on PdO(101) by adsorbed H2O

“…Water is present in many catalytic 44 applications, and can influence CO oxidation in multiple ways. In gener-45 al, adsorbed water has been found to enhance the oxidation of CO on 46 oxygen-covered metal surfaces, including Pt, Pd and Au surfaces 47 [7][8][9][10][11][12]. For example, Ojifinni et al [8] report that exposing O-covered 48 Au(111) to water enhances CO oxidation rates and present evidence 49 that direct reactions between co-adsorbed CO and OH groups are re-50 sponsible for the improved reactivity.…”

A strategy for improving deactivation of catalytic combustion at low temperature via synergistic photocatalysis

“…[4,19,43]. A third explanation is certain kind of surface poisoning from unavoidable water molecules [46][47][48]. In this section, the origin of catalyst deactivation and regeneration are discussed in detail.…”

“…Chemisorbed H 2 O over the surface of transition metal oxides has been widely reported to be poisonous during various processes [47,48,53]. The dissociation of water (H 2 O) molecule over the catalyst surface creates chemically bonded hydroxyl ( OH), blocking the adsorption of reactants and O 2 [8].…”

“…The competitive adsorption of H 2 O inhibits the Mars-van Krevelen cycles. Furthermore, the reactivity of surface OH is proved to be much lower than that of lattice O, thus accelerating the catalyst deactivation [48]. The O 1s XPS was used to identify the coexistence of various O species [53][54][55].…”