INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 4 pdf

Chemistry of Sulfur Oxides on Transition Metal Surfaces Technology, Cambridge, Massachusetts, U.S.A.. Thus, a detailed understanding of the general sulfur poisoning effect on transition

Chemistry of Sulfur Oxides on

Transition Metal Surfaces

Technology, Cambridge, Massachusetts, U.S.A

I INTRODUCTION

Transition metals are currently the most widely used heterogeneous catalysts in industry In this article, we shall focus our attention primarily on the automobile engine-out exhaust emission catalysts for environmental concerns In order to meet automobile emission control requirements in the United States, so-called three-way catalysts, consisting of Rh, Pt, and Pd, were selected to simultaneously convert CO, hydrocarbons, and NOx to CO2, H2O, and N2[1] In this conversion process, both oxidation and reduction reactions take place on the same three-way catalyst surfaces Therefore, only a narrow range of air-to-fuel (A/F) ratios around the stoichiometric point should be taken as the operating “window” of the catalytic conversion However, it is certainly desirable to have a wider A/F range with a better rate of conversion of CO, hydrocarbons, and NOx

Lean-burning NOx-trapping catalysts were designed for this purpose The idea

is to separate the competing oxidation and reduction reactions by time, via period-ically alternating the A/F ratio between lean (high A/F ratio) and stoichiometric (A/F ratio⬃ 14.6) conditions in the combustion chamber Under lean conditions, both CO and hydrocarbons can be efficiently oxidized to CO2and H2O, while NOx will be oxidized to the unfavorable chemical NO2 At the same time, by introducing NOx-trapping materials such as BaO, NO2is trapped on the catalyst within this lean cycle When the stoichiometric cycle is alternatively switched

on, the trapped NO2is reversibly released from the BaO surface and further re-duced to N2 However, this novel lean-burn NOx-trap catalysis process has con-siderable practical problems due to serious sulfur poisoning issues (described below)

Before getting to the sulfur poisoning problem for the lean-burn NOx-trap catalyst, let us examine how the traditional three-way catalyst could get over this

sulfur problem Ever since the early 1980s, when three-way catalysts were first introduced, a substantial amount of research has been carried out to understand the interaction of SO2 with the catalysts, due to the concern that SO2might be catalytically oxidized to sulfuric acid and released to the environment together with the automobile exhaust Note that a typical concentration of 0.03 wt% (or

⬃200 mg L⫺1) sulfur is present in unleaded regular gasoline, which produces about 20 ppm of SO2 in the engine-out exhaust gas [1] However, it is quite surprising that little sulfuric acid was actually generated by these three-way cata-lysts It was thought that the use of Rh might help to lower the activity of the three-way catalysts for SO2oxidation, compared to a pure oxidation catalyst [2] More importantly, the stoichiometric air-to-fuel ratio helped to suppress the SO2 oxidation

Under lean-burning conditions, however, full oxidation of SO2to SO3or sulfu-ric acid is feasible when excess oxygen exists on the three-way catalysts Both

SO3and sulfuric acid can severely damage NOx-trapping materials, such as BaO This poisoning process is very difficult to reverse and therefore inhibits utilization

of the lean-burning NOx-trapping catalysts Thus, a detailed understanding of the general sulfur poisoning effect on transition metals and metal oxides is neces-sary for the development of next-generation automobile exhaust emission cata-lysts It was proposed that sulfur poisoning will not be serious if one manages

to block the oxidation channel of SO2 to SO3on these catalyst surfaces under lean conditions

Although the oxidation of SO2to SO3is not significant under stoichiometric conditions, early research did show that the presence of SO2in engine-out exhaust affected the reactivity of the three-way catalysts It was demonstrated that by increasing the sulfur concentration in gasoline from zero to 0.03 wt%, one ob-served lowering of the conversion efficiency of CO, hydrocarbons, and NOx [3] The effect of sulfur on the activity of three-way catalysts was found to be more pronounced under rich conditions This was attributed to a larger coverage of catalytic sites by atomic sulfur under rich conditions than under dynamic condi-tions around the stoichiometric point Laboratory durability studies also indicated

a faster drop in activity with time with sulfur-containing (0.03 wt%) fuel, com-pared to the sulfur-free fuel [3,4]

Moreover, SO2has been found to influence the selectivity of three-way cata-lysts The importance of sulfur chemistry on transition metal surfaces was re-cently highlighted in a series of extensive experimental studies geared to under-standing how SO2 poisons the oxidation of CO and propene but promotes the oxidation of alkanes, such as propane [5–7]

It is believed that the sulfur poisoning effect on transition metals is due mainly

to the high reactivity of sulfur with transition metals However, at this time, few details of the elementary reactions involving SO2 are known This is because

Sulfur Oxides on Transition Metal Surfaces 57

these processes are complex and frequently made even more complicated by the interaction of reactants and products with coadsorbed species

First-principles computational research has become an efficient and accurate technique, complementary to experimental work in terms of determining both the static and the dynamic properties of molecules on extended transition metal surfaces This approach was benefited mainly by the rapid progress of density-functional theory (DFT) [8,9] and pseudo-potential [10] techniques associated with the fast increase of computational power in the past couple of decades The static properties computed from first principles consist of a variety of elec-tronic and geometrical structures, including adsorbate configurations, surface reconstruction, electronic spin configurations, and adiabatic potential energy surfaces The dynamic processes comprise sticking, diffusion, desorption, and, most important and interesting, surface chemical reactions First-principles molecular dynamics on metal surfaces are still under development, but rough estimates of entropic effects may be obtained based on static adiabatic potential energy surfaces using the harmonic approximation and transition state theory [11]

Rather generally, theoretical studies for the sulfur poisoning effect on transi-tion metal surfaces indicated that the perturbatransi-tions caused by sulfur-containing molecules on the metal electronic structure reduce the ability of these metals to adsorb CO and dissociate hydrocarbons [12–14] Moreover, it was shown that these induced electronic perturbations could have a long-range character

II STATIC INTERACTIONS: EQUILIBRIUM

POSITIONS AND ADIABATIC POTENTIAL

ENERGY SURFACE

The major goal of research on the static properties of chemical systems is to obtain the adiabatic potential energy surface of the groundstate, which includes the following:

1 Equilibrium atomic positions, such as bond lengths, bond angles, and tor-sion angles, among both the adsorbates and possible substrate surface reconstruc-tion In surface science experiments, the atomic vibrational modes with certain underlying symmetries are the main observables and have been extensively inves-tigated through high-resolution electron energy loss spectroscopy (HREELS), surface-extended ray absorption fine structure (SEXAFS), and near-edge X-ray absorption fine structure (NEXAFS) techniques Referring to the gap-phased isolated adsorbate molecules, UV photoelectron spectroscopy (UPS), angle-resolved UPS (ARUPS), and X-ray photoelectron spectroscopy (XPS) can pro-vide hints for identifying adsorbate species However, the classic experimental

surface structure determination approach, low-energy electron diffraction (LEED), does not seem to be useful, because adsorbed sulfur-containing mole-cules, such as SO2, do not exhibit long-range order Even if they do, they would rapidly be destroyed upon an electron beam

2 The static properties also contain the electronic spin configurations, since many of the transition metals and their derivatives are ferromagnetic crystals It

is known that spin plays an essential role in the groundstate properties of small transition metal clusters [15,16] This spin effect might not be particularly impor-tant in the case of SO2, compared, for example, with NO [17], since the adsorbate molecule is spin-pared To our knowledge, no experimental work, such as elec-tron paramagnetic resonance (EPR), has been performed especially for the pur-pose of studying sulfur oxides on transition metal clusters

3 Thermodynamic properties are important by themselves, as well as serv-ing as the basis for many dynamic approaches, such as transition state theory Temperature-programmed desorption (TPD) is widely used, but the accuracy in many cases is only qualitative [16] A rather wide binding energy range from

100 to 150 kJ/mol is estimated from TPD data for SO2 on the Pt(111) surface [18] A single crystal adsorption calorimetry study [19] on SO2 has not been reported up to this time

A Gas-Phase Sulfur Oxides

Isolated gas-phase SO is linear, possessing the C ∞vpoint group symmetry The intramolecular SEO bond length is 1.48 A˚ The spin-polarized electronic con-figurations around the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown in Figure 1a From the frontier orbital point of view [20], these single-particle orbitals should be considered the most active ones in most kinds of chemical reactions, including surface reac-tions [21]

Gas-phase SO2 has an intramolecular SEO bond length of 1.43 A˚ and an OESEO angle of 120° As shown in Figure 1b, both the HOMO and LUMO are localized mainly on the sulfur atom, which suggests that bonding of SO2via the sulfur atom to the transition metal surfaces should be expected Note that a

䉴

FIG 1 Groundstate electronic configurations Spheres represent s-type and lobes repre-sents p-type atomic orbitals For the p-type atomic orbitals, white and dark regions stand

for different phases of the orbitals, while bigger lobes indicate larger participation in the

corresponding molecular orbitals (a) SO (triplet, C ∞v) Diagram on the left/right shows the majority/minority spin configurations The HOMO is theΠ bonding in the majority spin and the LUMO is theΠ antibonding in the minority spin (b) SO2(singlet, C 2v) (c)

SO3(singlet, C 3v)

(b)

(c)

FIG 2 Free energy/temperature plot of gas-phase SO2and SO3 (From Ref 22.)

larger overlap of the p-orbitals between the sulfur and oxygen atoms is expected

for the HOMO, compared to the LUMO, due to the in-plane OESEO bond angle

Gas-phase SO3 has the largest number of oxygen atoms among all neutral

sulfur oxides It has a planar structure and possesses a C 3vsymmetry The intra-molecular SEO bond length is 1.43 A˚, almost the same as that in the gas-phase

SO2molecule The OESEO bond angles are 120° The free energy versus tem-perature [22] is plotted in Figure 2 to show the thermodynamic stability of gas-phase SO2versus SO3 Note that two curves cross at⬃1100 K, which indicates that SO3is more stable than SO2under a typical engine-out exhaust temperature

⬃600 K Therefore the experimentally observed SO2in engine-out exhaust gas

is due to kinetic limitations under lean conditions We notice that the LUMO level of SO3possesses the same phases of the p-orbitals from three oxygen atoms and the opposite phase of the p-orbital from the central sulfur atom Therefore,

one should expect a bent molecular structure (sulfur atom protruding out of the plane containing the three oxygen atoms) when the LUMO of SO3accepts elec-trons from donors of the same phase

B SO2 on Pt(111)

SO2plays one of the most essential roles among all the sulfur oxides (SOx , x⫽

0, 1, 2, 3, 4), since it is readily formed by burning natural sulfur-containing

mate-Sulfur Oxides on Transition Metal Surfaces 61

rials or by roasting metal sulfides in air The most important intermediate process

in the manufacture of sulfuric acid is the oxidation of SO2to SO3in the presence

of transition metal catalysts, such as platinum, because platinum is a very effec-tive catalyst for SOxoxidation Thus the interaction of SO2on Pt(111) has re-ceived the widest attention of all of the transitions metals

It is generally agreed that the SO2 molecule adsorbs intact on the Pt(111) surface at low temperatures, typically 100–160 K Through XPS, UPS, TPD, and HREELS, Sun et al [23] found that the binding of the SO2molecule was through

anη2-S,O structure, while the SO2molecular plane was essentially perpendicular

to the Pt(111) surface Their further simple frontier molecular orbital analysis suggested a preferred configuration with the sulfur atom on a bridge site and one oxygen atom on a top site More recently, Polcik et al [24] claimed to have found a new flat-lying configuration of SO2on the Pt(111) surface at 150 K in their combined XPS and NEXAFS study and pointed out that this new flat-lying configuration was invisible in the HREELS experiments by Sun et al [23] But Polcik et al did not give any detailed structural information for this new flat-lying configuration Sellers and Shustorovich employed the empirical bond order conservation–Morse potential method [25,26] and concluded that the most stable configurations involved dicoordination binding through bothη2-S,O andη2-O,O structures on the Pt(111) surface but no flat-lying configurations

Our first-principles DFT calculations confirmed both of (and only) the two most stable structures found by Sun et al [23] and Polcik et al [24] (one perpen-dicular and the other parallel to the Pt(111) surface) at low temperatures We did not find any stableη2-O,O structure Detailed results will be published separately

C SO2 on Other Transition Metal Surfaces

Similar to the interaction of SO2on the Pt(111) surface, SO2follows either spon-taneous or thermally activated decomposition on all of the transition metals ex-cept Ag, on which SO2adsorbs and desorbs only molecularly [27] Temperature-programmed desorption, UPS, and XPS studies by Outka and Madix on Ag(110) showed that three cleanly distinct phases exist, depending on the temperature: (1) multilayer SO2under 120 K, (2) dual-layer SO2between 140 and 175 K, and (3) monolayer SO2between 175 and 275 K The clean Ag(110) surface can be restored at temperatures greater than 275 K, which indicates a complete molecular desorption of SO2[27]

Molecular SO2was detected intact on Pd(100) at temperatures below 120 K

in a TPD and EELS study by Burke and Madix [28] When heated up to 135 K, multilayer SO2desorbed and left a single layer of SO2on the Pd(100) surface The monolayer of SO2left consequentially decomposed at 240 K, forming chemi-sorbed SO, which led to atomic sulfur and oxygen on the surface at even higher temperatures Similarly, in a combined TPD, XPS, and ARUPS study [29],

Zeb-isch et al showed that the SO2 multilayer desorbed at about 130 K, and the remaining SO2monolayers desorbed at 360 K on a Ni(110) surface The heating left a large number of sulfur atoms on the surface An NEXAFS study indicated that SO2 adsorption at 170 K is partly dissociative on Ni(110), Ni(111), and Ni(100) surfaces [30,31] Partial dissociation of adsorbed SO2 also occurs on Cu(100) and Cu(111) surfaces at 180 K, although more detailed measurements indicated much less dissociation on Cu(111) than on Cu(100) [32]

As for the geometric structure of the chemisorbed SO2, it was suggested that the molecular plane of chemisorbed SO2 aligned perpendicular to the closed-packed rows, as is the case on the Pt(111) surface Detailed measurements also suggested that the SO2bonds to the surface through the S atom The C2axis was shown to be perpendicular to the surface via measurements such as on Ag(110)

in an NEXAFS study by Solomon et al [33], or the C2axis could also be tilted within the SO2molecular plane on Pd(100) [28] and Cu(111) [34], similar to that

on Pt(111) as described earlier This tilted axis was attributed to the additional O–substrate bonding interaction, which might lead to the dissociation of molecu-lar SO2

However, an XAFS study of low-coverage SO2 on Ni(110), Ni(111), and Ni(100) surfaces [30,31] suggested that SO2species orient themselves with mo-lecular planes approximately parallel to the surface, which is also similar to the second most stable structure on Pt(111), as discussed earlier Therefore, one may conclude that there are in general two stable SO2species present on various transi-tion metal surfaces, one perpendicular and one parallel to the surface Depending

on the symmetry restrictions in experimental techniques, one may not always be able to observe both of the species

Unfortunately, little knowledge has been obtained directly from experiments

on the surface adsorption site Although in general it might be quite misleading, making use of the surface-cluster analogy suggested an atop site of SO2on the Ag(100) surface [33], and a fourfold hollow site of SO2 with an oxygen atom close to a bridge site was suggested on the Pd(100) surface [28] It is noted that

a quite surprising location of SO2 has been suggested in which the sulfur atom

is equally distributed between the long- and short-bridge sites on Ni(100), Ni(111), and Ni(110) surfaces [30,31]

One recent NEXAFS and SEXAFS study by Polcik et al demonstrated the presence of a SO2-induced surface reconstruction of Cu(111) at 170 K, on which the sulfur atom of the molecular SO2 is located at a hollow site on a locally pseudo-(100) reconstructed surface [34] However, a later study by Jackson et

al using chemical-shift normal-incidence X-ray standing waves (CS-NIXSW)

on the identical system seemed to disagree with the proposed local pseudo-(100) reconstruction [35] A very recent scanning tunneling microscopy (STM) study

by Driver and Woodruff further demonstrated that the kind of pseudo-(100)

re-Sulfur Oxides on Transition Metal Surfaces 63

constructions on Cu(111) can be induced by atomic sulfur, formed by dissociated methanethiolate under an electron beam [36]

Recently Rodriguez et al performed a DFT calculation to examine the adsorp-tion of SO2 on Cu(100) and showed an increasing bonding energy in the order

ofη1-S⬍ η2-S,O⬍ η2-O,O⬍ η3-S,O,O To make comparison with experiments, Rodriguez et al further proposedη2

-O,O orη2

-S,O to be the most stable configu-ration under large coverage limit, by assuming the large surface SO2 coverage made theη3-S,O,O binding mode impossible [37]

D SO on Transition Metals

Dissociation of SO2, resulting in SO species, has been experimentally observed

on Pt(111) [23,24], Pd(100) [28], Cu(100) [32], Cu(111) [32], Ni(100) [30,31], Ni(111) [30,31], and Ni(110) [29] surfaces, as discussed earlier Our DFT studies suggested that the thermodissociation of SO2on Pt(111) to SO was energetically unfavorable at low temperatures Further dissociation of molecular SO to S and

O atoms would cost even more energy, therefore being even less favorable The dissociation of SO2has been observed at higher temperatures, for exam-ple, at 240–270 K on Pd(100) [28] A similar dissociation temperature of⬃300

K of SO2on Pt(111) and all the other transition metal surfaces is also reported The chemisorbed SO thus formed, sequentially recombined with other surface adsorbates to form higher oxidized species, such as SO4, at the same tempera-ture [23]

SO2adsorption on Cu(100) is partly dissociative, even at about 180 K An SEXAFS study suggested that the sulfur atom was located at a fourfold hollow site and that the oxygen atom was located at a near-bridge site [32] The recent DFT calculations by Rodriguez et al showed a cost or⬃67–111 kJ/mol in energy for this dissociation process [37] This is rather misleading, however, since a meaningful comparison must be done by allowing the separation of the dissoci-ated SO and O species instead of by constraining them in one small supercell

E SO3 on Transition Metals

Our DFT calculations showed that this oxidation reaction is energetically favor-able at low temperatures on the Pt(111) surface In experiments, following the dissociation of chemisorbed SO2on transition metal surfaces, such as Pd(100), Cu(100), and Ni(110) at⬃170 K, SO3is formed upon adsorption as well as after heating the SO2layers to room temperature On Ag(110), however, SO2can be oxidized to SO3only when preadsorbed oxygen is available

An NEXAFS and CS-NIXSW study of SO3 on Cu(111) shows that the C 3v

axis of the adsorbed SO3is perpendicular to the surface, located at atop sites,

with the sulfur atom pointing out of the plane formed by the three oxygen atoms, away from the surface [35]

The DFT calculation by Rodriguez et al showed that the bonding of SO3 to Cu(100) was through anη3-O,O,O configuration, with the C 3vaxis perpendicular

to the surface They again proposedη2-S,O as the most stable binding configura-tion in the high SO3surface coverage limit [37]

F SO4on Transition Metals

The oxidation of chemisorbed SO2to SO4species has been observed on essen-tially all the transition metal surfaces studied In addition to the oxidation of SO2

to SO3, our DFT calculations showed that this oxidation reaction, i.e., from SO3

to SO4, is also energetically favorable at low temperatures on the Pt(111) sur-face

SO4species have been observed via spectroscopic methods to be present on transitional metal surfaces, such as Pt(111) and Pd(100), at 300 K [23] It is believed that the dissociation of SO2must occur first in order to provide chemi-sorbed atomic oxygen on the surface, if no additional gas-phase oxygen was supplied These SO4 species on Pt(111) decompose when the temperature is above 418 K without increasing the amount of atomic sulfur on the surface [24] Under lean conditions, when oxygen is preadsorbed on Pt(111), chemisorbed

SO2readily reacts with preadsorbed oxygen to form SO4, which has been indi-cated as the key surface species responsible for SO2-promoted catalytic oxidation

of alkanes [5,6] When CO or propene are coadsorbed, the SO2overlayers would

be efficiently reduced to form atomic sulfur The latter contributed to the poison-ing of the oxidation of CO and propene in the presence of SO2under rich condi-tions At⬃550 K, adsorbed SO4is identified as the precursor to SO3 desorp-tion [6]

Similar to the SO2-induced Cu(111) reconstruction described earlier, it was observed in an STM study by Broekmann et al that the topmost layer of Cu(111) was reconstructed by sulfate when the Cu(111) surface was exposed to a dilute sulfuric acid solution [38]

G Modified Transition Metals Toward Designed

Reactivity

Beyond the simple single-crystal transition metal surfaces, possible modifications

on the activity of the transition metals toward reactions involving sulfur oxides consist of:

mixed layer by layer or mixed within layers and repeated through whole crystals

It is shown that tin, acting as a site blocker, forms a well-defined and stable alloy