Surface Science of Catalysis - American Chemical Society

Chapter 1

The Promise of Surface Science in Catalysis Success or Failure? 1

2

Friedrich M . Hoffmann and Daniel J. Dwyer 1

Downloaded by 193.9.158.98 on June 16, 2016 | http://pubs.acs.org Publication Date: January 23, 1992 | doi: 10.1021/bk-1992-0482.ch001

2

Exxon Research and Engineering Company, Annandale, NJ 08801 Laboratory for Surface Science and Technology, University of Maine, Orono, M E 04469-0107

Heterogeneous catalysis is an important part of the technology that supports industrially developed societies. Production of transportation fuels, pollution control, production of low-cost and high-quality raw materials are just some of the areas in which heterogeneous catalysis impacts. The Council on Competitiveness, a non-profit and non-partisan organization of chief executives from business, higher education and organized labor, has identified catalysis as one of the technologies critical to international competitiveness [1]. Over the past two decades, the importance of heterogeneous catalysis resulted in considerable research and development activity, especially in such areas as synthetic fuels and pollution control. These efforts have significantly advanced the field and have lead to a new generation of catalysts ranging from the three-way automotive exhaust catalyst to new selective Fischer-Tropsch catalysts. Heterogeneous catalysis, however, is an interesting mixture of engineering, science and "art". In fact, much of the catalyst development since the beginning of this century has used a catalyst screening process, where literally thousands of catalysts were evaluated for their activity and selectivity. This has had some remarkable success, for example with the ammonia synthesis catalysts, which were developed 75 years ago at BASF or with the Fischer Tropsch catalysts developed during World War II in Germany. Even to date new or improved catalysts are often discovered by apparent intuition or happenstance rather than by design and it is often difficult to evaluate the contribution of science to advancement of the field. This is especially true when one considers the contributions of surface science. In the early to mid seventies, the advent of UHV surface characterization and the emergence of modern surface science promised to revolutionize the field by unlocking the details of the surface processes that control heterogeneous catalysis. Optimistic proponents of this new thrust promised that studies of surface structure, adsorption, chemisorption and catalytic reactions over well characterized surfaces would reveal the all important surface structure-function relationships. That knowledge then would revolutionize the development of "tailored", highly product

0097-6156/92/0482-0001$06.50/0 © 1992 American Chemical Society

Dwyer and Hoffmann; Surface Science of Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


2

SURFACE SCIENCE OF CATALYSIS

specific catalysts. In spite of some remarkable successes in elucidating the structural factors of catalytic reactions, these predictions have proven too optimistic, at least in a short-term time frame. Indeed, to our knowledge, no new catalyst system has been developed solely on leads originating from surface science. Nevertheless, surface science has been very successful in a less obvious way by making major inroads in the understanding of the elementary steps of many catalytic reactions. Just a casual examination of the literature will reveal that surface science increasingly has an important impact on the way the catalysis community approaches the art of catalysis. Surface science has changed the way researchers in catalysis address their field, it has shaped the way they think and it has made immeasurable contributions to the knowledge base from which their intuitive leaps are made. Many of these changes were brought about by a new approach of synthesizing model catalysts, whose surface composition and structure can be controlled and characterized with a variety of surface probes from UHV to atmospheric conditions. Some of the advances gained from this new approach have led to: 1. A new understanding of the interaction of molecules with surfaces giving new insights into the adsorption properties and dynamics of molecules. In many cases, surface studies provide reference datafromsingle crystals surfaces such as vibrational or electronicfingerprintdata, which are useful to characterize more complex supported catalysts. Thermodynamic and kinetic data can be used to predict more complex catalytic processes, as discussed further below. Many new concepts have emerged or were unequivocally established from this new understanding. Examples are the characterization of molecular precursors, e.g. molecular oxygen, or reactions intermediates such as ethylidine, methoxy, formate species. Many of those species have been characterized in great detail with respect to their structure and adsorption geometry by a variety of powerful surface probes. 2. Information on the mechanism of reactions from characterization ofthe elementary steps and intermediates. Examples are the investigation of the role of kinetic energy in the dissociation of molecules with molecular beams, which have contributed among others to the understanding of CH bond activation of methane [2], [3], or the CO oxidation reaction (discussed further below). 3. The application of new surface spectroscopies in catalysis, many of which have originated in surface science and which are now commonly used both in catalysis and surface science. In this respect surface physics has played an important role by providing a fundamental understanding of these new spectroscopies. Examples are electron spectroscopies (Photoemission, AES, X-Ray Absorption), synchrotron probes (NEXAFS, FYNES, X-Ray scattering, Microtomography), Thermal Desorption Mass Spectroscopy, SIMS, vibrational spectroscopies, Scanning Tunneling Microscope (STM, AFM) etc. In this chapter we present examples which demonstrate the strong mutual interaction between catalysis and surface science and which one might tentatively term "success stories of surface science in catalysis". These examples are by no means a complete list nor are they presented in order of importance. Our choice of


1.

Surface Science in Catalysis: Success or Failure?

HOFFMANN & DWYER

examples was driven primarily by our own research interests and therefore represent only a small fraction of the total body of literature that has been published. For a more detailed discussion of catalytic reactions over single crystal surfaces we refer to reviews by Somorjai [4], Campbell [5] and Rodriguez and Goodman [6],

Hydrogenation of Carbon Monoxide over Single Crystals The energy crises of the early seventies led to a reconsideration of various alternative energy sources including the production of transportable fuels manufactured by synthesis gas chemistry. In this technology, synthesis gas (CO/H ) prepared by gasification of coal is converted to liquid hydrocarbons over iron or cobalt catalysts in the Fischer-Tropsch reaction (CO + H + ....) or to methane over nickel catalysts (CO + 3H CH4 + H 0). These forms of synthetic fuel production had been extensively studied in the forties and early fifties but interest waned upon the discovery of oil in the middle east after World War II. The extensive studies of this era, expertly summarized by Storch et al. [7], raised a number of interesting mechanistic issues involving the surface reaction pathways. Two general classes of reaction mechanisms were considered as possibilities at that time, the carbide model of Craxford and Rideal [8] and the hydroxymethylene model of Storch et al. [7]. One important distinction between the two concerned whether the carbon-oxygen bond of CO broke prior to or after hydrogen attack. In the carbide model CO was thought to be dissociatively chemisorbed on the catalysts surface forming a metal carbide, i.e. CO -* CO, -* C, 4- O . The metal carbide was subsequently hydrogenated to form the hydrocarbon intermediates which polymerize to form hydrocarbon products. The hydroxymethylene model, on the other hand, involved direct reaction between chemisorbed atomic hydrogen and chemisorbed molecular CO to form a hydroxymethylene intermediate. The postulated hydroxymethylene intermediate then underwent a stepwise polymerization process which lead to products. Support for the carbide model of Rideal was severely undermined by the radiotracer studies performed by Kummer, DeWitt and Emmett [9] which indicated that only a small fraction of carbidic carbon could be incorporated into the products. This study was widely cited as evidence that the hydrogenation of carbidic carbon was not a dominant reaction channel in the overall process. This tracer study coupled with the ability of the hydroxymethylene model to explain the presence of oxygenated hydrocarbons in the products resulted in the carbide mechanism being shelved for more than twenty years. However, when the oil embargoes of the seventies motivated renewed interest in the catalytic production of fuels, the carbide mechanism reemerged with the help of surface science and is today the most widely accepted. In the seventies a new and promising approach appeared, which combined traditional high pressure catalytic reaction studies with modern UHV surface analytical tools. In this approach the catalytic response of well characterized single crystal surfaces was used to investigate the role of surface structure and composition in determining the catalytic response of hydrocarbon synthesis catalysts. This approach was pioneered by Somorjai and coworkers [10-12] who evaluated methanation and Fischer-Tropsch kinetics over atomically clean metal surfaces. These studies showed that the low surface area metal catalysts displayed product selectivities


2

2

2

f

2

a



4


and catalytic rates that were directly comparable to those observed for high surface area supported metal catalysts. The studies also showed the importance of surface carbon deposits in determining the rates and selectivities of the reactions. Working hydrocarbon synthesis catalysts were found to be covered by a mixture of carbidic and hydrocarbon fragments, whereas deactivation of the catalyst was accompanied by deposition of graphitic type of carbon. Interest in the nature of this carbonaceous deposit and the role it played in the overall reaction mechanism became a topic of some interest. The work of Biloen et al. [13], Kellner, Bell [14] and Brady, Pettit [IS] indicated that surface carbon deposits on a number of transition metals could be hydrogenated to form a slate of products virtually identical to those encountered in conventional Fischer-Tropsch catalysis. The outcome of this work was the rebirth of the surface carbide mechanism and the definition of the terms reactive surface carbon (carbidic) and inactive carbon (graphitic). The surface science approach to CO/H catalysis was substantiated by Goodman et al. [16] who showed that the rate of methane formation over a Ni(100) single crystal surface was identical to that observed over high surface area supported Ni catalysts. This study was an important milestone in the establishment of low surface area single crystals as valid models of high surface area catalysts. The reason that the Ni(100) surface successfully reflected, on a per surface atom basis, the more complex surface of the supported metal particles was clearly demonstrated by Kelly and Goodman [17] in a subsequent paper. In this study the authors investigated the role of surface structure in the methanation reaction by comparing reaction rates obtained over two different single crystal faces of Ni with those obtained over supported metals. The data of Kelley and Goodman are reproduced in Figure 1 where the methanation kinetics for the Ni(100), Ni(l 11) and two high surface area materials are compared in an Arrhenius format. It can be seen in this figure that all of the rates are virtually identical indicating that the reaction is surface structure insensitive, at least within the experimental error. This structure insensitivity of the methanation reaction was unanticipated and resulted in much discussion within the catalysis community. The results, however, have stood up to the test oftimeand represent one of the important contributions of surface science to catalysis. These studies also substantiated the surface carbide mechanism by correlating the methanation rate to the amount and type of surface carbon as shown in Fig. 2. The surface carbon level was determined by Auger electron spectroscopy. From a line shape analysis of the Auger spectra these authors also could distinguish different forms of carbon, reactive surface carbide and unreactive graphite, which deactivated the catalyst. These findings clearly supported the hypothesis that surface carbide formation was an intermediate step to methane formation. Bonzel and coworkers [18-21], using a similar approach, investigated the reaction of CO and H over iron single crystals. These surfaces produced the traditional Fischer-Tropsch products, straight chained alkanes. The iron surfaces were found to be quite susceptible to the deposition of carbon which had a substantial impact on the reaction rates. The investigators were able to identify with X-ray Photoelectron Spectroscopy (XPS) three types of carbon on the surface after reaction. The working catalytic surface was found to be covered by a mixture of hydrocarbon fragments as well as carbidic carbon. Once again, deactivation of the surface under reaction conditions was associated with excessive carbon deposition in the form of graphitic carbon. 2

2



1.

HOFFMANN & DWYER


800K 700K

600K

1.2

1.6

1.4

1/TxlO

500K

2.0

1.8 3

450K

2.2

1

(K~ )

Figure 1: Comparison of methane synthesis rate over Ni(100) and Ni(lll) single crystal catalysts and supported Ni catalysts for reaction at 120 Torr and a H^CO ratio of 4:1. (Reproduced with permission from ref. 17. Copyright 1982 Elsevier.)




6

\

T=625K

0.40 0.30

13«\ •9 \ \ \ l 4 2

\3

0.20 10«

I

I

1

10

Ratio

Press. (Ton)

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 1.0 1.0 1.0 1.0 0.1 0.4 2.7

1 10 36 61 SO 120 500 1000 10 50 200 500 110 34 89

2

\ 11 •

0.101-

H /CO

PI. No. 1 2 3 4 5 5 7 3 9 10 11 12 13 14 15

0.50 - « 1

•15 5

•8 12 6 ^7 I 100 2

Methanation Rate (N C H x 10* ) 4

Figure 2: Methanation rate versus surface carbon level for CO hydrogenation at 625 K over a Ni(100) catalyst. (Reproduced with permission from ref. 17. Copyright 1982 Elsevier.)


1.


HOFFMANN & DWYER

Clearly, surface science has had an impact on the CO/H catalysis. The use of well characterized surfaces as models for high surface area materials has contributed extensively to our knowledge base in this area. The definition of reactive surface carbon, the role of alkali promoters, the structure insensitivity of methanation, surface reconstruction under reaction conditions are just a few of the concepts that surface science has introduced into the language of catalysis. These types of studies are continuing to uncover new and exciting leads in the area of CO/H catalysis such as the creation of bimetallic model systems where ultrathin metal overlayers exhibit highly unusual chemisorptive and catalytic behavior [22]. 2

2


Ammonia Synthesis The ammonia synthesis (N + 3H -+ 2NH ) is an example of a structure sensitive reaction. The development of a commercial catalyst for this process at the beginning of this century has been one of the early successes of catalysis. The catalyst, iron promoted with alumina and potassium, is still commercially used today. Its composition has been found by Mitasch, Haber and Bosch at BASF through extensive screening of about 2500 catalysts [23]. In spite of the early success in developing a working catalyst and the intensive research which followed, the mechanism of the reaction was not established unequivocally until the late seventies with pioneering work by the groups of Ertl [24], [25] and Somorjai [4]. The development of new surface science techniques permitted to establish the structural sensitivity of this reaction as a key factor controlling the activity. Detailed investigations of the adsorption of nitrogen on iron single crystal surfaces by Ertl and collaborators [26], [27] showed that the rate of dissociative nitrogen chemisorption depends strongly on the surface structure. Fig. 3 shows that the rates of dissociative adsorption of nitrogen on the three low-indexed iron surfaces are found to be in the sequence (111) > (100) > (110). From the initial rates it can be seen that the open (111) surface is about 60 times more active for nitrogen dissociation than the closed packed (110) surface (for bcc structure models see Fig. 4). Moreover, Boszo et al. found that the sticking coefficient of nitrogen was very low in the order of 10*. The latter indicates that the dissociation of nitrogen is die rate-limiting step in this reaction. The relevance of these findings to the synthesis of ammonia has been demonstrated by the Somorjai group [28],[29]. Studies of the ammonia synthesis reaction over single crystal surfaces at high pressure (20 bar) and temperature (500°C) revealed that the activity varied for iron by two orders of magnitude in the same order (111) > (100) > (110) as shown in Fig. 4. These results confirm that the Fe(lll) surface is the most active of the basal orientations and hence suggest that this surface might be a good model catalyst. In fact, high resolution Transmission Electron Microscope images obtained together with the corresponding electron diffraction pattern of an activated catalyst particle show that the catalyst primarily consists of small crystallite particles of (111) orientation [30]. The knowledge of the parameters describing the kinetics of the elementary steps on the Fe(l 11) model catalyst has enabled Stoltze and Norskov [31] to predict theoretically the ammonia yield of a commercial catalyst in high pressure industrial 2

2

3

6


7


8


Figure 3: Adsorption of nitrogen on the Fe(lll), (100), and (110) surfaces measured as a function of N exposure. (Reproduced with permission from ref. 27. Copyright 1977 Academic.) 2



1.

HOFFMANN & DWYER


(111)

Fed 11)

(211) (100) (210) Surfac* Oritntation

(110)

Fe(210)

Figure 4: Structure sensitivity of the rate of ammonia synthesis studied over different iron single crystal surfaces at 673 K and 20 atm (H :N =3:1). The bottom shows structure models of the low index surfaces of bcc iron. (Reproduced with permission from ref. 29. Copyright 1987 Academic.) 2

2


9

10


reactors. As shown in Fig. 5, the theoretical predictions show excellent agreement with experimental values covering a pressure range from 1 atm to 150 atm. These calculations demonstrate that from the knowledge of the elementary reaction steps a quantitative description of a complex technical process can be obtained. Thus the ammonia synthesis reaction is a particularly good example demonstrating the successful interaction of surface science and catalysis, where studies on model catalysts preformed under UHV and atmospheric conditions allow to elucidate the elementary steps of a catalytic reaction.


CO Oxidation Reaction The catalytic oxidation of CO is an economically important reaction due to its relevance for automotive emission control. Moreover, it is considered as a model for a "simple" surface reaction since the classic work of Langmuir [32]. The large number of studies following Langmuir*s work, however, show that even "simple" surface reactions can be quite complex. At low pressure, the reaction mechanism on platinum group metals has been explored in detail [33] and proceeds via the following steps CO 0 CO*, + 0* 2

-* -*

CO* 20* co 2

Pioneering molecular beam studies by Engel, Ertl and collaborators [34], [35] have unambiguously established the Langmuir-Hinshelwood mechanism as the only reaction path on platinum metals at low pressure, i.e. the reaction occurs at the surface with both reactants in the chemisorbed state. In detailed UHV studies, using a variety of surface probes, Ertl and collaborators also have characterized the adsorption energetics and kinetics of oxygen and carbon monoxide on various single crystal surfaces of Pd and Pt [33]. These and other studies have led to a detailed understanding of the CO oxidation on platinum metals at low pressure. This is illustrated in Fig. 6 with the energetics of the reaction on Pt(l 11) in the limit of low coverage [36,37]. The potential energy diagram shows that the transition state between the chemisorbed state of CO,,, + O^ and C 0 is about 30 kcal/mole above that of the adsorbed C 0 molecule. This leaves the C 0 molecule with considerable excess energy which can be either transferred to the substrate or leave a vibrationally and translationally "hot" molecule as demonstrated by Haller and collaborators [38] and discussed in detail in Chapter 4. Models based on parameters from the adsorption energetics and kinetics determined in UHV studies have been successful in predicting most of the reaction behavior at high pressure as demonstrated for the reaction on rhodium by Oh et al. [39] and by Schwartz et al. [40]. Measurements of the steady-state kinetics of the CO oxidation reaction at high pressures by Peden, Goodman and collaborators [41-45] demonstrate the validity of single crystal models. The comparison of reaction rates over single crystal surfaces for Ru and Rh in Fig. 7 shows excellent agreement with reaction rates measured over supported metals. These high pressure experiments with 2 M

2

2



1.

HOFFMANN & DWYER

Surface Science in Catalysis: Success or Failure? 1

EXPERIMENTAL EXIT NH

3

MOLE FRACTION

Figure 5: Comparison of calculated vs. experimentally determined ammonia yields from a commercial catalytic reactor. The data span a broad range of pressures from 1 atm to 300 atm. The calculation is based on values obtained from single crystal model studies. (Reproduced with permission from ref. 31. Copyright 1985 American Physical Society.)

Figure 6: Potential energy diagram for the CO oxidation reaction on Pt(lll) in the limit of low coverage. (Reproduced with permission from ref. 37. Copyright 1984 Elsevier.)


12


• i r

-I—I—I

I

I

I

I

I

100.00 -

U. O

Ru(0001) . P =16Torr c o

Pn =• 8 Torr

LU 10.00

Surface Science of Catalysis - American Chemical Society

Recommend Documents