Christophe Escriba

Fulcrand Rémy

Artillan Philippe

Jugieu David

Bancaud Aurélien

Boukabache Ali

Gue Anne-Marie

Fourniols Jean-Yves

Trapping Biological Species in a Lab-On-Chip Microsystem : micro inductor optimization design and SU8 process

Keywords: Analytical model, Finite element method, Magnetic field / force, Magnetic actuators, magnetic bead separation

Micro-spiral inductor dedicated to microbeads manipulation in a fluidic channel had been optimized by analytical modelling correlated to multi physics fem numerical Maxwell®3D L software. Main advantage of the analytical model described below is time analysis calculus decrease of and the capability offered to optimize geometrical and electrical parameters of the inductor. First experimental results show a good correlation between simulation and realized integrating micro-devices in a fluidic channel.

I. INTRODUCTION

Lab-On-Chip micro-systems with embedded analysis are the two major concepts of MEMS dedicated to fluidic applications. In order to realize biochemical analysis or pharmacological screening, separation of biological species, we propose magnetic bead as a very useful technique. In most cases, the magnetic activation is macroscopic and positioned outside of the system. As these approaches limits strongly the MEMS integrating process, we develop in LAAS-CNRS lab, a multi-functional magnetic source in order to realize a complete fluidic micro-system for handling paramagnetic beads.

By reporting here first works dedicated to model the behaviour of magnetic sources and the forces generated on microbeads, we present two methods allowing to determine magnetic field generated by a planar inductor: one entirely analytical and other by using the multi-physics software Maxwell®3D. Comparisons are discussed in terms of performances, accuracy, and CPU time computation. As a complementary part, we present first technological results of research (in progress) in order to integrate a family of inductors in channels. These preliminary devices confirm the feasibility of our approach.

II. DETERMINATION OF MAGNETIC FIELD DISTRIBUTION IN A 3D SPACE. ANALYTICAL MODELING

The magnetostatic field induced by a DC current flowing in an electric conductor is given by the following two Maxwell's equations:

∇ × � �⃗ = ⃗ (1) ∇ • �⃗ = 0 (2)
With the following constitutive (material) relationship being also applicable:

�⃗ = µ 0 µ � �⃗ + µ �����⃗ (3)
• H(x, y, z) is the magnetic field strength • B(x, y, z) is the magnetic flux density.

• J(x, y, z) is the conduction current density.

• Mp(x, y, z) is the permanent magnetization.

• µ 0 = 4 * * 10 -7 . -1 is the permeability of vacuum.

• µ is the relative permeability.

First, we consider all the segments constituting the inductor which coordinates are [α i ω i Β ] in frame (Figure 1). Geometrical and electrical characteristics are:

• L 1 In order to evaluate quickly magnetic field generated by the induction coil, we consider the inductor in frame Β like a set of elementary segments where the magnetic field B is first obtained for each segment

[α i ω i Ά Β
], and the total magnetic field will be determined by summation of each elementary field calculated in the frame .

A. STEP1: Magnetic field expression B( x',y',z') r created by a single segment in Ά Β We consider M point, included in the (x0y) area where we define [α, ω] segment. We define the orthonormal frame Ά Β with α origin, and first axis [α, ω], Figure 2. Β frame is obtained by a translation of T vector and a linear transformation through [P] matrix (Figure 3). = ΄ u r u r u u r [START_REF] Ramadan | On-chip micro-electromagnets for magnetic-based bio-molecules separation[END_REF] Where (΄ is vector product). We can observe that the z component present an extrema above the center of the inductor and a value close to zero along Ox and Oy.

III. VALIDATION OF THE ANALYTICAL APPROACH

In order to check the validity of the previous analytical modelling, we compare results obtained width the Maxwell ®

The electric field is restricted to the objects modelled as real (non ideal) conductors and is totally decoupled from the magnetic field. We consider that there no time variation effects and objects are considered to be stationary. So previously, the magnetostatic field solution verifies the Maxwell's equations. The MAXWELL3D magnetostatic solver considers the magnetic field H with the following components: 3D software and hence a FEM approach with those obtained with our model.

H ��⃗ = H p �����⃗ + ∇φ + H c ����⃗ (11)
Where is the magnetic scalar potential, H p �����⃗ is a particular solution constructed by assigning values to all the edges in the mesh in such a way that Ampere's law holds on all contours of all tetrahedral faces in the mesh, and H c ����⃗ accounts for permanent magnet if any. Thus, the Degrees Of Freedom (DOFs) are the nodal values of the magnetic scalar potential with ten values per tetrahedron at each of the four vertices and all six mid edge nodes, ensuring a quadratic approximation inside each finite element.

There are major advantages of this formulation over other existing ones, including using considerably fewer computational resources (due to the scalar nature of the DOFs), not requiring a gauge due to excellent numerical stability, significantly reducing cancellation errors, and capably of automatically multiplying connected iron regions. The magnetostatic solver handles both 3D linear and nonlinear problems. The magnetostatic solver calculates the magnetic field distribution produced by a combination of known DC current density vector distribution (Figure 6).

Various coil designs have been simulated with the use of MAXWELL3D. For example, Figure 6 shows the magnetic field calculated for a micro-coil with 5 turns, a conductor square section of 5µm × 5µm and a 50mA current. The comparison between analytical and FEM modelling of magnetic field B r is detailed in Figure 7, where we have reported the values of B calculated at different heights z from the coil.

IV. APPLICATION: MAGNETIC BEADS TRAPPING

Using our analytical model, we have optimized the design of an integrated magnetic actuator dedicated to the manipulation of microbeads in lab on chip devices. The criterion of optimization was to obtain magnetic force allowing the trapping super paramagnetic microbeads in micro-channels. The magnetic forces have been calculated starting from the simulation results. For this purpose, we have integrated some microinductors in a micro-fluidic device made entirely with a biocompatible SU-8 polymer. This prototype will undergo a series of characterization and validation.

V. MAGNETIC FORCE EXERTED ON A MAGNETIC MICROBEAD

The force ⃗

exerted by a magnetic field on a particle is represented by the gradient of the magnetic interaction energy of the particle immersed in the magnetic field.

F �⃗ p m = -∇U � �⃗ p m ( 12
)
The magnetic energy can be expressed by:

U ��⃗ p m = - 1 2 µ 0 ∫ M ���⃗ p .H ��⃗ . dv (13)
Where µ 0 = 4π. 10 -7 H. m -1 is the vacuum permeability and ��⃗ the magnetizing of this particle by the magnetic field of excitation ���⃗ .

In the case of a particle with very small dimension, the integration of ( 13) is then replaced by the value of the field in the center of the particle multiplied by the volume of this particle:

U ��⃗ p m = - 1 2 V p µ 0 M ���⃗ p .H ��⃗ (14)
In air or the vacuum, a diamagnetic or paramagnetic micro particle acquires a magnetizing ��⃗ which a function of the magnetic susceptibility , the field of excitation � �⃗ and the demagnetization coefficient D (D = 1/3 for a sphere, D=1 for a parallelepiped, D = 0 long bar).

M ���⃗ p = χ p 1+χ p .D H ��⃗ (15)
It is considered that 1 + . ≈ 1 and equation ( 15) can be simplified:

M ���⃗ p = χ p H ��⃗ (16)
Equation ( 14) becomes then:

U � �⃗ p m = - 1 2 V p µ 0 χ p |H|² (17)
The expression of the magnetic force deduced from ( 12) and ( 17) is given by:

F �⃗ p m = µ 0 V p χ p (H ��⃗ . ∇)H ��⃗ (18)
This expression shows that direction of the force does not depend on the field sign but of the product of field with its gradient. Its orientation is also related to the susceptibility of the particle. The total magnetic force exerted on the particle is given by:

F �⃗ MagTtl = F �⃗ p m + F �⃗ f m = µ 0 V p (χ p -χ f )(H ��⃗ . ∇)H ��⃗ (19)
Because of very low value of susceptibility of the immersing medium (χ f = -9.048 * 10 -6 for water) we assume that the magnetic field induced by the medium can be neglected and that :

B � �⃗ = B � �⃗ coil = μ 0 H ��⃗ (20)
The equation of the total magnetic force becomes:

F �⃗ MagTtl = V p χ p µ 0 (B � �⃗ coil . ∇)B � �⃗ coil (21)
The components of the magnetic force in a Cartesian reference mark are expressed by:

F �⃗ MagTtl = V p µ 0 �χ p �( B x B y B z ) ⎝ ⎜ ⎛ d dx d dy d dz ⎠ ⎟ ⎞ ( B x B y B z ) (22)
Thus in the case of a super paramagnetic particle in a conveying fluid like water, the particle is attracted towards magnetic field maxima. By convention, the magnetic actuator (coils) being in (X, Z) plan, the force is taken into account only in the (X, Z) plan (Figure 8) and:

F MagTtlx (x, z) = V p µ 0 �χ p � �B x dB x dx + B z dB x dz � (23)

VI. TECHNOLOGICAL REALIZATIONS

VI.1 Process Flow For The Fabrication Of Integrated Microsystem

One of the main tasks of this work was to develop a generic polymer based technology in order to manufacture a microfluidic device able to trap functionalized micro beads. As it will be shown below, this technology combines standard electroplating technique and lamination technologies in order to achieve flexible and transparent full polymer systems with the precision and reliability of "stat-of-the-art" microsystems. The requirements for the integrated microsystem are:

→ accurate alignment between the fluidic levels → high reliability process → flexible microsystem → optically transparent material → biocompatibility.

According to these criteria, we have chose to utilize the polyethylene terephthalate layer (PET sheet, for the flexibility) as a "sacrificial" material, the epoxy-based negative photoresist SU-8 on the structural material for the microfluidic network (transparency, biocompatibility, very high aspect ratio…) and copper for the electroplating process of the micro-coil. The technological process flow can be described in Figure 9. As a first step, a PET sheet (50µm Polyethylene Terephthalate film) is laminated on the top of a silicon wafer. The PET was chosen for its poor adhesion properties and transparency. Then, conductive tracks are patterned (Ti/Au) by lift-off processing (Figure 10 (a)). A layer of SU-8 (Microchem 3005) is deposited on top of the structure in order to perform electrical insulation between conductive tracks and coils, and to create electrical VIA (Figure 10 (b)). A seed layer of Ti/Cu (500/500 Å) is deposited followed by the deposition and patterning of a positive photoresist (AZ4562) in order to create the electroplating mold. Then, copper coils are electroplated (5µm) into the resist mould (Figure 10 (c)) and the photoresist and seed layers (Ti/Cu) are removed.

VI.2 Magnetic Microbeads Trapping

Magnetic micro and nano-beads have proven to be very interesting and reliable tool in biological and chemical analysis in recent years. Separation or purification are often practiced using magnetic labeled beads in biological laboratory and sometimes for biomedical diagnosis [START_REF] Norgall | Elevated expression of VEGFR-3 in lymphatic endothelial cells from lymphangiomas[END_REF][START_REF] Safarik | Magnetic techniques for the isolation and purification of proteins[END_REF]. The experimental procedure for the realization of an experimental bead separation is illustrated in Figure 12. In general, bead based bioanalysis protocols use permanent magnet [START_REF] Pamme | On-chip free-flow magnetophoresis: Separation and detection of mixtures of magnetic particles in continuous flow[END_REF][START_REF] Kim | Droplet-based Magnetically Activated Cell Separation[END_REF] and repeat a specific sequence manipulation. In a first step, magnetic microbeads are functionalized with a probe (antibody) specific of the targeted molecule. Then, the target (antigen) contain in the sample is recognized by the probe and adsorbs specifically on the microbead surface. Finally, microbeads are trapped by an permanent magnet. The goal of our study was to implement such kind of protocol in a lab on chip device integrating a microelectromagnet. Compared to permanent magnets, electromagnets offer a higher flexibility and a higher control of magnetic field. Since small dimensions are able to be manufactured in microfluidic system, the combination of microelectromagnets and microfluidic network offers an interesting approach to this integrated microsystem [5, 6, 7 and 8].

VI.3 Real Time Experimentation And Validation

The choice of the type of magnetic microbeads depends on the application specifications. In this study, we choose to use Dynabeads® M500 Subcellular microbeads and Dynabeads® M270 Carboxylic Acid with diameter of respectively 5 µm and 2.8µm. Figure 13 shows a SEM photograph of Dynabeads® M270 and an optical picture of microbeads in microfluidic channel. In order to validate our technological concept, chips integrating microelectromagnets in a microfluidic channel have been fabricated. Magnetic bead separation has been performed using these systems. Figure 14 shows a schematic overview (a) and cross-section (b) of the system. The experimental setup is shown in Figure 15. The instrumental bench includes a CCD camera for recording of bead trapping, a syringe for dispensing the liquid sample and a DC power supply. A driving current of 50mA was applied to the inductor. Experimentation were performed using a square micro-coils with 5 turns and a square section (5µm*5µm).

The video frame is constituting by 12 frames per second. The main steps of trapping are reported on Figure 16:  Firsr we present the magnetic microbead contained in the microfluidic channel before supplying current to the microelectromagnet (0s).  Then, current was applied to the microelectromagnet and microbeads are trapped at the micro-channel bottom just above the micro-coils (transient analysis from 10s to 100s). This example put the stress on the good correlation between our Finite Element approach and our experimental realization. Future work will be done to further improve the trapping and sorting efficiency of the flexible integrated microsystem by a design optimization of our microelectromagnets and microfluidic network.

VII. CONCLUSION

In this study, we describe magnetic field modelling created by a planar micro-inductor. A 3D model of the micro-spiral inductor was been realized by using our own analytical model and by using a second method based on COMSOL Multiphysics software. These two approaches allow us to check the magnetic field distribution generated by the inductor and the force exerted on magnetic bead. All these studies show that many parameters can influence the interaction between the bead and the micro-spiral inductor. The model offer the behaviour response to any electrical current and can optimized geometrical coil parameters respect to microbeads and fluid characteristics. The first experimental results with a fluidic structure fabricated entirely in a biocompatible polymer give to our approach a novel dynamic and validate the concept and its analytical model used for actuating the biological species.

Figure 1 .

 1 Figure 1. Equivalent topology of planar inductor and display results for five spires inductor.

Figure 2 .

 2 Figure 2. Magnetic field created by the [α ω] segment at point M

  uuur , y' z' x'

Figure 3 .Figure 4 .Figure 5 ,

 345 Figure 3. Β« Ά Β frame definitions.Combining M coordinates with α and ω points, we obtain scalar elements of [P] matrix by concatenation of x', y', z' uur uur ur :

Figure 5 .

 5 Figure 5. Magnetic field computed: (a) modulus; (b) along Oz axis, (c) along Ox axis, (d) along Oy axis.

Figure 6 .

 6 Figure 6. 3D representation of the magnetic field. a) in (X, Z) plan and b) in (X, Y) plan

Figure 7 .

 7 Figure 7. Magnetic field calculated by analytical and finite element methodsThese results confirm the good correlation between the two methods and fully validate the efficiency of the analytical model. Thus we can assume that the analytical model developed is powerful for parametric optimization.

FFigure 8 .

 8 Figure 8. (a) Sight 3D of the device and the plan (X, Z) where is represented the distribution of the magnetic field B. In red, the fluidic channel where calculations of the forces for different height are localized. (b) Graphic of the magnetic forces according to X and Z obtained by the post-processing

Figure 9 .

 9 Figure 9. Process flowchart for the fabrication of the flexible integrated microsystem

Figure 10 .Figure 11 .

 1011 Figure 10. SEM pictures of the different steps processing

Figure 12 .

 12 Figure 12. Procedural sequences for magnetic bead separation. a) Magnetic bead labeled incubation with antigen target. b) Separation of magnetic bead to the solution

Figure 13 .

 13 Figure 13 . SEM photographs of Dynabeads M270 (a, b and c), and optical picture of flow microbeads in micro-channels (d)

Figure 14 .

 14 Figure 14. The complete microsystem. a) An overview of the chips with three microelectromagnets, the micro-channel with flow inlet and outlet. b) A cross-section of the chip

Figure 15 .

 15 Figure 15. Experimental setup to test and demonstrate our microbead handling

Figure 16 .

 16 Figure 16. Magnetic beads trapping using our microelectromagnets. (0s) without excitation applied. (10s to 100s) after excitation (I=50mA) applied to the microelectromagnet: trapping beads.

Figure 17

 17 Figure17shows a comparison between real time magnetic beads trapping and modeling to magnetic field distribution around the microelectromagnets.

Figure 17 .

 17 Figure 17. Comparison between our real time experimentation and modeling: magnetic field repartition around the microelectromagnet.


John R Venable

Jan Pries-Heje

Deborah Bunker

Nancy L Russo

Research for Human Benefit

Keywords: Creation, Transfer, and Diffusion

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Herbert [START_REF] Simon | The Sciences of the Artificial[END_REF] defined the science of design as the study of the artificial and properly "the way in which that adaptation of means to environments is brought about" (p. 113). Melding science with design provides a means for designing at higher levels of abstraction, designs that are more universal and address a general class of problems rather than a single, unique design problem. In their seminal papers on DSR, Nunamaker et al. (1991), [START_REF] Walls | Building an Information System Design Theory for Vigilant EIS[END_REF], and [START_REF] March | Design and Natural Science Research on Information Technology[END_REF] laid the foundation for DSR. The consolidation of DSR into mainstream Information Systems research came with [START_REF] Hevner | Design Science in Information Systems Research[END_REF] article in MIS Quarterly. In information systems, [START_REF] Vaishnavi | Design Research in Information Systems[END_REF] define design science research as "the analysis of the use and performance of designed artifacts to understand, explain and very frequently to improve on the behavior of aspects of Information Systems." Central to these notions is the design of an artifact that is meant to have a presence in the real world. This artifact could be conceived as a construct, a model, a method, or a material instantiation [START_REF] March | Design and Natural Science Research on Information Technology[END_REF].

The present conference expands on DSR research approaches in IS in three key areas. First, the conference assumes that designing and developing a new technology without also considering the transfer, diffusion, and adoption of that new technology risks producing research that is irrelevant. Both design practice and DSR projects are conducted within an organizational and societal environment, but DSR outcomes (artifacts) must be diffused and adopted into a broad variety of organizational and societal settings. While the importance of such issues has long been the concern of both IFIP Working Groups 8.2 and 8.6, the relationship to DSR has heretofore not explicitly been considered. [START_REF] Bunker | A Perspectival Punctuated Action Approach to Policy Development in Information Technology and Systems[END_REF] have suggested that DSR has been predominantly concerned with building and evaluating artifacts aimed at achieving humandefined goals [START_REF] Simon | The Sciences of the Artificial[END_REF], the operationalization of which is affected by contextual complexity. Adoption and diffusion are particularly important to the success of DSR in the context of information systems, due to the complex interactions of design artifacts with social and organizational contexts. Understanding of implementation context is critical to the appropriate design and use of artifacts [START_REF] March | Design and Natural Science Research on Information Technology[END_REF].

Second, the conference considers that there is a need to emphasize research in which new technologies (e.g., information systems) are designed expressly for human benefit. This theme is one that is well established in IFIP WG 8.2, for example the theme of the 1987 IFIP WG 8.1 and 8.2 Joint Working Conference addressed "Information Systems Development for Human Progress in Organizations" (Klein and Kumar 1987). As they are considered in the current conference, technologies for human benefit are concerned with improving the human condition, for example, by improving the quality of working life, improving health, improving the environment, reducing poverty, improving social conditions, or improving participation in and service provision by government or not-orprofit organizations. Until now, the emphasis on DSR in the IS field has been on meeting business needs (as in [START_REF] Hevner | Design Science in Information Systems Research[END_REF])-that is, on the development of new technologies that are primarily aimed at enabling increased business profit. The choices of goals and problems addressed by DSR, and who is entitled to make those choices, are seminal and should be examined critically. For example, [START_REF] Venable | Identifying and Addressing Stakeholder Interests in Design Science Research: An Analysis Using Critical Systems Heuristics[END_REF] suggests that the critical systems heuristics framework (Ullrich 1981(Ullrich , 1987(Ullrich , 2002) ) could be used to guide proper identification of stakeholders and the selection of problem domain and system boundaries when conducting design science research. In our view, DSR cannot and should not ignore such concerns and a greater proportion of design science research, including that conducted in the IS field, should focus on human benefit (not-for-profit) rather than purely on business (for-profit). Much of the public policy debate and creation, which is so critical to resourcing of the design of IS artifacts for human benefit (i.e., IS and communications infrastructure and policy encouraging adoption practices ), is directly related to human benefit and social outcomes. Bunker and Campbell, for example, in their examination of a public consultation process in the development of a B2G online authentication framework using the DSR-based approach of perspectival punctuated action (PPA), ask how we get good policy, which leads to the more pressing question: how do we get good policy design? PPA is based on distinct decision-making configurations of intelligence, choice, and design by extending [START_REF] Boland | Design in the Punctuation of Management Action[END_REF] articulation of [START_REF] Simon | The New Science of Management Decision[END_REF] decision-making theory.

Third, design (and DSR) is informed by organizational and societal needs, is conducted in an organizational and societal environment, is diffused and adopted into organizational and societal settings, has organizational and societal consequences, and is (ideally) evaluated for how well it works and solves problems in the real (organizational and societal) world. Therefore, IS research needs to take a (more) holistic perspective, which integrates DSR and behavioral research in the creation of the IS artifact. [START_REF] Venable | The Role of Theory and Theorising in Design Science Research[END_REF] suggests that the core activity of DSR, technology invention and design, should be integrated with two other main activities in the field of applied research: problem diagnosis (i.e., understanding the causes and consequences of problems) and technology evaluation (i.e., understanding whether technologies are efficient, effective or efficacious-and why). The current emphasis in DSR is on design and evaluation (only) as the key activities [START_REF] Hevner | Design Science in Information Systems Research[END_REF][START_REF] March | Design and Natural Science Research on Information Technology[END_REF]. [START_REF] Baskerville | Soft Design Science Research: Extending the Boundaries of Evaluation in Design Science Research[END_REF][START_REF] Baskerville | Soft Design Science Methodology[END_REF] recommend the development and use of a soft design science approach to realistically address organizational and societal issues in DSR.

While software and hardware are regarded as the core "working" artifacts in information systems and technology [START_REF] Orlikowski | Research Commentary: Desperately Seeking the 'IT' in IT Research-A Call to Theorizing the IT Artifact[END_REF][START_REF] Weber | Editor's Comments: Still Desperately Seeking the IT Artifact[END_REF], there are other artifacts that are also important components in the creation of innovative ITS. These artifacts are constructs, models, methods [START_REF] Hevner | Design Science in Information Systems Research[END_REF][START_REF] March | Design and Natural Science Research on Information Technology[END_REF] and better theories [START_REF] Rossi | Design Research Workshop: A Proactive Research Approach[END_REF]. Constructs define the conceptual vocabulary of a domain, models contain an expression of how constructs are related, methods provide a description on how to perform a specific task, and better theories are derived from experiment-like proof of concept. To address these needs, the theme of this working conference incorporates issues related to the integration and cross-fertilization of DSR with the organizational and societal research areas traditionally covered by IFIP Working Groups 8.2 (Information Systems and Organizations) and 8.6 (Diffusion, Transfer, and Implementation of Information Technology).

THIS BOOK

This book is the result of the IFIP WG 8.2/8.6 Joint International Working Conference on Human Benefit Through the Diffusion of Information Systems Design Science Research, held in Perth, Western Australia, March 30-April 1, 2010. This chapter introduces the theme of the conference and provides an overview of the research contributions that are included in the book. While not all of the issues mentioned above are explicitly addressed by the papers selected for these Proceedings, the breadth and variety of the research and ideas presented represent a major step forward in our understanding of the design, development, adoption, and diffusion of IT/IS artifacts that can provide human benefits.

In this book, we have grouped the papers into six areas. The first part looks at design, organization, and adoption. The second part we have called design exemplars. The third part discusses the notion of human benefit. The fourth part is about designing adoption and diffusion. The fifth part looks at the core of DSR and discusses design as science. And the sixth part focuses on participation in design. Finally, at the end of the book we have included three panels descriptions.

Part 1: Design, Organizations, and Adoption

The first part of the book aims exactly at the area of overlapping interest between the two Working Groups that have responsibility for this conference. IFIP WG 8.2 looks at organizations and information systems and IFIP WG 8.6 looks at diffusion and adoption of information technology.

In Chapter 2, Dirk Hovorka aims at expanding DSR into the broader organizational and societal research domains. "Care must be taken," says Hovorka, to comprehend and articulate the philosophical underpinnings of theory building and evaluation in DSR, or we will end up creating incoherent design theory. The approach suggested by Hovorka is a so-called multi-paradigm grounding to ensure that the DSR approach remains a legitimate approach to knowledge creation.

Mutaz Al-Debei and Guy Fitzgerald look more specifically at design problems in mobile data services. In Chapter 3, they develop an ontology based on business model thinking. The research approach to building the ontology essentially follows the designscience paradigm but also incorporates other research methods. The developed ontology identifies four primary dimensions in designing business models for mobile data services: value proposition, value network, value architecture, and value finance. Within these dimensions, 15 key design concepts are identified along with their interrelationships and rules. The resulting ontology is of value to academics and practitioners alike, particularly those interested in strategy and telecommunication in relation to IS.

In Chapter 4, Heidi Tscherning and Lars Mathiassen present detailed insights into why and how five closely related individuals decided to adopt the iPhone before it was available through traditional supply chains. Discussing the role played by social networks, Tscherning and Mathiassen analyze how adoption threshold, opinion leaders, social contagion, and social learning shape adoption. The chapter confirms that network structures in fact have an impact, and it shows that the adoption decisions emerged as a combined result of individual adoption reflections and major influences from the social network.

Part 2: Design Exemplars

An exemplar is a model or pattern to be copied or imitated. The second part of this book contains three very interesting exemplars of design.

In Chapter 5, Ivan Aaen asks, "How can we facilitate innovative software development in teams?" He comes up with an answer in the form of a work-in-progress design called Essence made with inspiration from role-play and improvisational theater. Based on agile principles, Essence is designed for teams of developers and an onsite customer. Essence has been applied in teaching at Aalborg University, where different roles were assigned to team members. This provided valuable insights into the design of roles in Essence. These insights were then used for redesigning the roles in Essence, thereby emphasizing the iterative nature of DSR.

In Chapter 6, Michael Hicks, Graham Pervan, and Brian Perrin explore the criteria of effective IT governance processes employed in universities and their impact on the diffusion of appropriate technology to users. A case study was conducted at a large Australian university that is currently undergoing a major restructure of its IT governance process. The case study found significant improvement in key areas of IT governance and the University realized that IT governance is an ongoing design process.

In Chapter 7, Carl Lawrence, Tuure Tuunanen, and Michael Myers propose an extension of DSR by integrating critical ethnography into the evaluation phase. Critical ethnography provides a way for IS researchers using DSR to better understand culture, and may help to ensure that IT artifacts are designed for a variety of cultural contexts. This is a very important theme to discuss, as the creation and design of artifacts lies at the heart of DSR. However, with an increasingly connected and globalized world, designing IT artifacts for a multicultural world is a challenge. This chapter offers a way to do so.

Part 3: Human Benefit?

Is there human benefit from design? This was the key question we raised when we gave the name to this conference. In this part of the book, three chapters discuss that question from three quite different angles.

Imran Khan and Elaine Ferneley, in Chapter 8, look at the UK National Health Service, which is currently is undergoing tremendous IS led change. In the concrete, the case presented is about the design of an electronic single patient care record system. The chapter examines the extent to which persuasive discourse, or rhetoric, influences and affects the adoption of IS. Further, the chapter explores the ways in which various actors use rhetoric to advance their own agendas and the impact this has. The chapter concludes that rhetoric is an important and effective persuasive tool, employed by system trainers to coax users into not only adopting the system but also into using the system in a predefined manner.

Chapter 9 by Ramanjit Singh and Trevor Wood-Harper is a study of the challenges faced by knowledge workers in a Swedish company TeliaSonera when using wireless technology on the move. The chapter identifies five problem areas: (1) work and life balance, (2) addiction, (3) organizational involvement, (4) nomadic work and control, and (5) individual productivity. Each problem area is subsequently analyzed using sociotechnical design principles. The chapter concludes that better role boundary management, self-discipline, work negotiation, and e-mail communication skills may be required for the knowledge workers to manage the demands of nomadic working.

In Chapter 10, Mikael Lind, Daniel Rudmark, and Ulf Seigerroth look at the design of business processes in another Swedish company, Intersport. The study was carried out as an action research undertaking where the purpose was to design a new process aligned with the strategic goals of Intersport. The chapter address the question of how design science research can contribute to business process design. Three heuristic guidelines for creating organizational commitment and strategic alignment in process design are presented-derived from successful actions taken. Finally these guidelines are used as a basis to reflect on the contribution of DSR in relation to business process design.

Part 4: Designing Adoption

This part of the book contains four papers of a more classic WG 8.6 nature in that they have adoption as their primary topic. In all four chapters we can, however, find elements of design, hence the title for this part: designing adoption.

In Chapter 11, Mohammad Hossain and Mohammed Quaddus look at the adoption process and the subsequent diffusion and extended usage of RFID (radio frequency identification) for Australian livestock. A research model is designed based on Rogers' innovation-diffusion theory and Oliver's expectation-confirmation theory. The model posits that while adoption of RFID may result from legislative pressure, its further diffusion is an evaluative process, which is judged against "satisfaction" and "performance" derived from RFID systems. The implications of the designed model are then discussed and some hypotheses developed.

In Chapter 12, Yogesh Dwivedi, Navonil Mustafee, Michael Williams, and Banita Lal examine the factors affecting the consumer adoption of broadband in the UK. A conceptual model of broadband adoption is designed by selecting and justifying a number of relevant constructs from the technology adoption literature. Findings from the testing of the model suggest that relative advantage, utilitarian and hedonic outcomes, primary influence, facilitating conditions, resources, and self-efficacy all have an influence. The potential implication of this chapter is that stakeholders in broadband adoption can use the findings to encourage and promote the adoption and usage of broadband among the general population in the UK as well as in other parts of the world.

In Chapter 13, Gasparas Jarulaitis takes a closer look at the adoption of Microsoft SharePoint in a global oil company. Longitudinal data from the period 2007-2009 are analyzed focusing on two parts of the organization, R&D and oil and gas production. As a result, Jarulaitis found that the different ways in which the technology is managed and used in these contexts results in uneven diffusion.

Chapter 14 concludes this part of the book with an analysis by Yogesh Dwivedi, Linda Levine, Michael Williams, Mohini Singh, David Wastell, and Deborah Bunker of the research published in the previous 11 IFIP WG 8.6 conferences held between 1993 and2008. Their analysis of the published material includes examining variables such as the most active authors, citation analysis, universities associated with the most publications, geographic diversity, and authors' backgrounds. The keyword analysis suggests that the work in WG 8.6 has evolved from examining basic issues such as organizational impact of technology adoption and technology transfer to contemporary issues such as open innovation.

Part 5: Design Science

This fifth part of the book returns to design as a science. Three interesting approaches are given, spanning from situational design to designing management.

In Chapter 15, Daniel Stock, Robert Winter, and Jörg Mayer lay the foundation for the design of a situational method for functional service domain architecture management. Based on a review of current literature, a framework is proposed. In this framework, the authors find that situational method engineering for functional domains can be applied by identifying context types and goal vectors, designing fragments, and associating successfully adopted method fragments with specific situations. Finally, the validity of the proposed framework is tested by five case studies.

In Chapter 16, Jan Pries-Heje and Richard Baskerville elaborate a DSR approach for management planning anchored to the concept of a management design theory. Unlike the notions of design theories arising from information systems, management design theories can appear as a system of technological rules, much as a system of hypotheses or propositions can embody scientific theories. This chapter illustrates this form of management design theories with three grounded cases: process improvement, user involvement, and organizational change.

In Chapter 17, Colin Armstrong and Helen Armstrong present an overview of the application of DSR to the management of forensic evidence processing. The chapter begins with a discussion of DSR and socio-technical IS research in relation to the processing of forensic evidence. The discussion then presents the current problems faced by those dealing with evidence and a conceptual meta-model for a unified approach to forensic evidence is developed. Finally, Armstrong and Armstrong state that practical application of the suggested model would have to be predominantly driven by law enforcement.

acceptance tests structured the customer and user involvement. Kautz finds genuine customer and user involvement in the design activities in the form of both direct and indirect participation in the agile development project. Further, the involved customer representatives played informative, consultative, and participative roles in the project. This led to their functional empowerment: the users were enabled to carry out their work to their own satisfaction and in an effective, efficient and economical manner.

In Chapter 19, Birgitta Bergwall-Kåreborn, Debra Howcroft, Anna Ståhlbröst, and Anna Wikman employ a case study using a "living lab." The starting point is taken in the observation that while participation is established and has been reported successful in many cases, some now see it as an "old, tired concept" that is in need of revitalization in order to cater for changing IS practices. Thus the authors look at the process of participation during the design stages of a health care project for the elderly in Sweden. In this chapter, Bergwall-Kåreborn et al. reflect on how participation materializes in a context that is quite dissimilar from more traditional development settings and report on the kinds of practices that may be used to assist design with users

In Chapter 20, Thomas Østerlie, Ole Martin Asak, Ole George Pettersen, and Håvard Tronhus describe a study of ICT use at an airport security checkpoint to explore the paradox that travelers find existing airport security measures inadequate while at the same time believing air travel to be sufficiently secure. Østerlie et al. pursue this paradox by showing that for the security checkpoint to function properly in relation to the overall function of the airport, travelers have to be enrolled in a particular program of action, one in which travelers are both ethically and morally challenged (in their own view). Nevertheless, their active participation makes it difficult for them to object to the moral and ethical issues. Thus the explanation for the paradox presented is that travelers have been made accomplices.

Technology (Information Systems) from Binghamton University (New York) in 1994. He has taught and researched in IS at Binghamton University and Central Connecticut State University in the United States, at Aalborg University in Denmark, at the University of Waikato in New Zealand, and at Murdoch University and Curtin University of Technology in Australia. He has consulted on ICT and organizational change with large and small business organizations, government agencies, and not-for-profit organizations. His main research interests are in IS development and planning methods and practice, modeling of organizations, IS and data, problem solving methods, organizational change, group support systems, knowledge management and organizational learning, IS in the not-for-profit sector, and IS research methods, including critical research and design science research. John can be reached by e-mail at John.Venable@ cbs.curtin.edu.au.

Jan Pries Heje is a professor in Information Systems, Department of Communication, Business and Information Technologies, Roskilde University, and head of the User Driven IT-Innovation Research Group. His research focuses on designing and building innovative solutions to managerial and organizational IT problems. Previous and current projects explore process improvement as design, the ability for an organization to improve, and how one can design a process for making better sourcing decisions. From January 2010, Jan is the Chair of the IFIP Technical Committee 8 on Information Systems. He can be reached by e-mail at janph@ruc.dk.

Deborah Bunker is a senior lecturer in the discipline of Business Information Systems at the University of Sydney in Australia. She holds a Ph.D. in Information Systems Management. Her research interests are in IS management frameworks and approaches, IS innovation, adoption and diffusion, interorganizational IS and philosophical foundations of IS (phenomenology, systems thinking, system of systems) and she has published widely in these areas. Deborah is president of the Australasian Association of IS (AAIS) and is a founding member and the current Vice Chair of IFIP WG 8.6 on the adoption and diffusion of IT. She can be reached by e-mail at D.Bunker@econ.usyd.edu.au.

Nancy L. Russo is the Pavlović Professor of Information Systems in the Department of Operations Management & Information Systems at Northern Illinois University. She also serves as the Vice-Rector for International Cooperation at Slobomir P University in Bosnia & Herzegovina. She received her Ph.D. in Management Information Systems from Georgia State University in 1993. Her research has addressed the use and customization of system development methods, factors related to information systems success, IT innovation, and issues related to research and education in the information systems field. Her work has appeared in Information Systems Journal, European Journal of Information Systems, Information Technology & People, Journal of Information Technology, and other international journals and conference proceedings. Nancy is currently the Chair of IFIP WG 8.2. She can be reached by e-mail at nrusso@niu.edu.

Part 6: Participation in Design

The this sixth part of the book, another theme at the core of design, namely, participation, is examined. Again we have three chapters. These span from a look at agile methods, through a living laboratory with users, to design of airport security.

In Chapter 18, Karlheinz Kautz provides a case study of a large agile development project and focuses on how customers and users participated in agile development and design activities in practice. The investigated project utilized the agile method eXtreme Programming. Planning games, user stories and story cards, working software, and


Damien Cornu

Hazar Guesmi
email: hazar.guesmi@upmc.fr

Jean-Marc Krafft

Hélène Lauron-Pernot

Lewis acido-basic interactions between CO2 and MgO surface: DFT and DRIFT approaches

Keywords: MgO(111), MgO(110), MgO(100), defects, charge density difference, thermodynamic calculation, Lewis basicity, Infra-red

come   L'archive ouverte pluridisciplinaire

Introduction

Due to environmental questions, the interest for developing the use of basic catalysts is increasing [1][2]. One of the main drawbacks of these basic catalysts is that they are naturally polluted by water and carbon dioxide forming, by the way of an acid-base reaction, hydroxyl and carbonate groups on the catalyst surface. These adsorbates are usually considered as poisons of the reactivity even if they can be desorbed at specific temperatures depending on their nature and local environment [3]. In order to understand the catalytic properties of the basic catalysts linked to the pretreatment conditions, it is important to be able to characterize the adsorbates remaining on the surface before the catalytic reaction and, as a consequence, to know which basic sites are available after the thermal treatment.

Such an attempt has already been made with hydroxyl groups of magnesium oxide [4][5] which is a well-known basic catalyst involved in numerous studies of industrial interest [3,[6][7][8][9]. The aim of this paper is to investigate, by a mixed experimental-theoretical approach, the nature of the carbonates formed by interaction with CO 2 on MgO surface defects and their stability upon thermal treatment. As CO 2 is a Lewis acid while H 2 O or methanol used in previous studies are Brønsted acidic molecules, the discussion of the results will involve a comparison between Lewis and Brønsted basic properties of MgO.

Infra-Red spectroscopy is one of the more informative techniques for the characterisation of adsorbed molecules [10], as carbonate species, formed on MgO surfaces [11][12][13][14][15][16][17][18][19]. The literature describes the carbonates present on MgO surface as monodentates or bidentates, but their detailed pictures have been unclear so far and the interpretation of IR band spectra and their attributions in connection to specific adsorbed species are questions which are far from being resolved [2]. In this work, using DRIFT spectroscopy we have followed, for different temperature ranges, the interaction of CO 2 with MgO powder sample, prepared by precipitation. The originality of the pre-treatment operated here resides in covering the surface previously cleaned up from hydroxyl and carbonates groups by different amounts of CO 2. In addition, we have attempted to specify adsorbed CO 2 species by ab-initio DFT method. The theoretical investigation of CO 2 interactions with MgO surface has often concerned (100) flat surface [20][21][22], step and corner defects [20,[22][23]. Due to their lower stability compared to the (100) surface, the (110) [24] and (111) [23] lattice planes have attracted much less attention. Nevertheless, as the adsorption of molecules such as water or CO 2 can modify the relative stabilities of the lattice planes [25], this work presents results obtained on all these surfaces and also includes kinks and divacancies which can be formed on the (100) surface.

In the following the configuration and energetic stability as well as electronic structures of the different carbonate species which could be formed over MgO support are investigated using DFT calculations. In addition by mean of thermodynamic model, the stabilities of carbonates are drawn as a function of temperature. Thus the comparison of DFT calculated frequencies and measured IR data leads to a detailed description of the surface state depending on the temperature treatment.

Experimental and Computational details

Materials and experiments

2.1.1. Preparation of the catalyst: Mg(OH) 2 was obtained from Mg(NO 3 ) 2 (Aldrich, 99.99%) solution using ammonium hydroxide (Aldrich, 99%) (8 mol.L -1 ) as precipitating agent. Thus, the latter was added to a magnesium nitrate solution (1 mol.L -1 ) and the pH value adjusted to 10 at 303 K. After stirring for 30 min, the precipitate was washed several times with deionized water, and then filtered. The hydroxide sample was finally treated in vacuum (10 -6 Torr) up to 1273 K (ramp 1 K.min -1 ) and maintained at this temperature for 2 h.

Thermal treatments:

Figure 1 reports the different steps of the thermal treatments of the magnesium oxide sample. During the pre-treatment phase, the sample is "cleaned up" 2h at 1023 K under nitrogen flow (Air Liquide, U nitrogen >99.995% pure) (20 cm 3 .min -1 ), then cooled down to 323 K. During the treatment phase, the sample is first contacted with carbon dioxide (Air Liquide, Alphagaz, >99.9% pure) diluted in nitrogen (5% in a 20 cm 3 .min -1 .flow) for 15 min. Then, it is once again heated at intermediate temperature (473 K < temp < 1073 K) in a nitrogen flow (20 cm 3 .min -1 ) for 2 h. In the following, only the final temperature "temp" that determines the degree of carbonatation of the sample will be indicated.

Insert Figure 1 2.1.3. Spectroscopic measurement: DRIFT spectra were recorded on a Brüker IFS 66 V spectrometer in the 4000-600 cm -1 range (4 cm -1 resolution, 256 scans/spectrum) using a Thermo Spectra-Tech high temperature cell. All spectra were converted into Kubelka-Munk units after subtraction of the spectrum recorded on dehydrated KBr sample. All spectra are recorded in situ after the pretreatment procedure described in section 2.1.2 and the cooling down of the sample to 333K. Finally, Raman spectra were recorded on a Model HL5R of Kaiser Optical System Inc. in the the 3450-100 cm -1 range (4 cm -1 resolution). It is equipped with a high-powered near-IR laser diode working at 785 nm.

Computational details

2.2.1. Methods. All calculations were performed using ab-initio plane-wave pseudopotential approach as implemented in VASP [26][27]. The generalized gradient approximation exchange-correlation functional of Perdew and Wang PW91 [28] was chosen to perform the periodic DFT calculations with reliable accuracy on the convergence criterions. The one electron wave function was developed on a basis set of plane waves and the interaction between the core and the valence electrons is described by the projector augmented waves (PAW) approach [29]. The selected description of the oxygen atomic core allows a good convergence on the energy for a cutoff of 400 eV. The convergence criterion for the electronic self-consistent cycle was fixed to 10 -6 eV per supercell. Geometry optimizations were performed within a conjugate-gradient algorithm until the convergence criterion on forces (10 -2 eV.Å -1 ) was reached. A dipolar correction along the perpendicular to the slab was applied in order to remove the effect of electrostatic interaction between the slab and its periodic images along the z axis. The adsorption energy of carbonates on the MgO surfaces was calculated as:

E ads (0K) = E CO2/surf -( E surf + E CO2free )
eq. 1

Vibrational spectra were calculated for selected surface species within the harmonic approximation. In addition to CO 2 molecules, magnesium and oxygen surface atoms in direct interaction with adsorbed species were allowed to relax. The Hessian matrix was computed by the finite difference method followed by a diagonalization procedure. The eigenvalues of the resulting matrix lead to the frequency values. The assignment of the vibrational modes is done by inspection of the corresponding eigenvectors. Free carbon dioxide molecule is used as a reference in order to estimate the systematic deviation between experimental and calculated νCO values. The DFT calculated vibration frequencies for free CO 2 are 2366 (2349), 1318(1340) and 637(667) cm -1 (experimental frequencies [START_REF] Herzberg | Molecular Spectra and Molecular Structure[END_REF] are indicated in parenthesis).

As the average of calculated frequencies fit quite well the average experimental values, no empirical scaling factors were applied to the DFT reported frequencies.

Surfaces.

The (1.29 J.m -2 [34])). This difference reflect well the instability of the polar terminated surface compared to the (100) one.

Thermodynamic

In order to study the adsorption phase diagram of CO 2 molecule on the different considered surfaces, a model of atomistic thermodynamics was developed. This model was inspired from the one developed by D. Loffreda et al. [37] for a realistic description of adsorption thermodynamics of multifunctional molecules on metal surfaces.

The Gibbs free adsorption energy depends on the adsorption internal energy and on the vibrational entropy change during the adsorption process. Thus, for an accurate description of the free Gibbs energy, the vibrational contributions of the initial (bare surface and gas phase)

and final system (adsorbed system) should be included in the formalism. In this work the Gibbs free adsorption energy was calculated as follows:

) P / P ln( kT ) config S rot , trans S vib S ( T E ads E ads G 0 0 - + + + + = eq. 2
Where E ads is the adsorption energy calculated from DFT (see eq. 1), E 0 is the zero point energy (ZPE) change during adsorption process and S vib , S trans,rot are the vibrational and transitional, rotational entropy changes, respectively. The calculated vibrational properties in carbonated systems included the interacting molecule and all top-most surface atoms.

Similarly for decarbonated systems the first surface layer was considered.

When several adsorption sites (Nsite) are available on the surface, an additional term called configurational or positional entropy, S config is included in the formalism. It defines the logarithm of all possible arrangements of the molecule on the surface. Its value is given by the equation 3.

S config = -k ln[ ! )! (N ! N site site ads ads N N - ] eq. 3
Finally, S vib is derived from the reduced vibrational partition functions Q vib of adsorbate system, gas phase and surfaces (eq. 4) and S trans,rot is calculated (eq. 5) on the basis of the transitional partition function of the adsorbate system trans ads Q and the gas phase trans gas Q as well as the rotational partition function of gas phase rot gas Q .

) ln(

vib gas vib surf vib ads vib Q Q Q k S - = eq. 4 ) ln( , rot gas trans gas trans ads rot trans Q Q Q k S - =
eq. 5

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Results and discussion

Experimental IR data

The effect of the pretreatment (1023 K, 2h under N 2 ) on the adsorbed species over MgO surface is shown by the comparison of the DRIFT spectra of the initial sample and of the treated one (Figure 2). The considered high temperature process cleans up the surface from almost all hydroxides and carbonates. The IR spectra (black line) shows a weak remaining absorption band at 3750 cm -1 which is attributed to strongly bounded hydroxides [5], and some carbonates with IR bands at 1657 and 1329 cm -1 .

Insert Figure 2 After the addition of CO 2 , DRIFT data show strong modifications in the IR range 1800-1000 cm -1 . Figure 3 gathers the set of IR spectra obtained after the surface treatments at temperatures temp = 523 K, 673 K, 873 K. As expected, the spectra show the formation of carbonate species the amount of which decreases upon the successive thermal treatments.

After the treatment at 873 K, the obtained spectra show the same absorption in the 1000 -2000 cm -1 range as the one recorded after the pre-treatment at 1023 K. In addition, the successive thermal treatments (temp) do not affect the recorded high frequencies (> 2000 cm - 1 ) on magnesium dioxide surface. In the lower frequency ranges, the absorptions are always very high due to the bulk infrared absorption of MgO with a lateral phonon frequency at 800 cm -1 [38].

Insert Figure 3 According to the literature, adsorption of carbonate species leads to the splitting of the ν 3 stretching vibration band of the free ion, located at 1415 cm -1 , and gives different contributions depending on the geometry of the adsorbed ion [2,13]. The carbonate ν 3 stretching mode is split to ν 3high and ν 3low contributions. Generally a consensus is gained that the more coordinated the carbonate ion the more split the two contributions. In addition the average ν for each species has also been determined [13]. As reported in Table 1, which summarizes the IR results from numerous experimental studies, the vibrational frequency bands recorded on magnesium oxide surfaces have been assigned either to monodentate, bidentate or hydrogen carbonate species [11-12, 14-16, 18]. This table also evidences many discrepancies relative to the IR values assigned to different carbonate species, which may be due to the different states of the MgO surfaces obtained after the thermal treatment such as hydration state, morphology etc… Insert Table 1 In this work (see figure 3), MgO surface is almost free of residual adsorbates. Therefore no hydrogen carbonate can be found on the surface even at temp = 523 K (no contribution at 1220 cm -1 ). Comparing the position of our recorded spectra with IR attributions reported in Table 1, one can assign, "at first sight", all the identified bands in the range of 1370 -1590 cm -1

to monodentate species and those in the ranges of 1270 -1390 cm -1 and 1620 -1710 cm -1 to bidentate species. Moreover, two minor contributions located at 1252 and 1275 cm -1 do not change with the thermal treatment and can be linked to the small amount of silicon which is a residual impurity in the sample. This attribution of the carbonate bands goes on line with the evolution of the spectra with temp: the most stable species are reported as bidendate [11] and are the only ones remaining on MgO surface after a treatment at 873 K, giving contributions at 1661 cm -1 and 1321 cm -1 . Nevertheless, for each carbonate species, several contributions are obtained. For instance, in the infrared range attributed to bidentate species, two bands at 1680 and 1651 cm -1 (split bands at 1321 and 1304 cm -1 ) can be attributed to two different bidentate species. The same analysis can be made with the monodentate species. These different contributions are probably linked to the carbonate formation on different MgO defects. Because in catalytic applications these defects may behave differently [START_REF] Alghamdi | Metal Oxide Catalysis[END_REF][START_REF] Trionfetti | [END_REF][41] it is important to know which of them are poisoned by carbonates as a function of the thermal treatment process. Modelling is thus necessary to calculate the theoretical position of the IR band linked to carbonates species depending on their topological environment.

Computational results

On the nomenclature of carbonates

The earlier work of Fukuda and Tanabe [11] described the carbonates as "bidentate" or "unidentate" for the carbonate linked with only one magnesium ion, and "bridging" for the one bonded with two magnesium ions (see scheme 1). Busca and Lorenzelli [13] added some complexity to this model by considering "polydentate" species. The improvement in the description of the carbonates by the way of computational methods allows us to give a more precise description of the carbonates. Those carbonate should be given an unambiguous name.

We propose that the carbonates should be ranked by the number of oxygen atoms making connection with the surface (ionic or covalent bonds), such connections being the bonds that lengths are less or equal to the Mg-O bond length in the bulk magnesium oxide (≈ 2.1 Å). Let us add that a thorough description of the nature of carbonate bonds will be evaluated through DFT electronic analysis. Thus, mono-, di-and tri-dentate carbonates refer, from now on, to carbonate species in which one, two and three oxygen atoms are connected to the surface, respectively. In addition among a general type of carbonate species, for example bidentate, distinctions will be made considering the type of plan or surface defect on which the interaction occurs. The carbonates will be named by the combination of a letter for each type (M : monodentate, B : bidentate, T : tridentate) and of the name of defect or plane. For instance, T-S1 will refers to a tridentate carbonate formed on a S1 step, and M-(111) will refers to a monodentate carbonate on a (111) plane.

Inset scheme 1

CO 2 species adsorbed on MgO(100) surface

In table 2 are reported geometric and energetic parameters as well as ν CO frequencies of all optimized configurations of CO 2 species adsorbed on the MgO (100) plane and its various defects (mono (S1) and diatomic steps (S2), corner (C3), kink (K) and divacancy (D)).

Comparisons with available values from the literature are also considered [14,20,[22][23][24].

Insert Table 2 Adsorption on the flat surface: The low calculated adsorption energy (-0.09 eV) for linear This can be reached by calculating the difference charge electron density obtained by subtracting from the charge electron density of the adsorbed carbonate system both the charge electron density of the clean MgO surface and that of an isolated CO 2 . The atomic positions of the clean surface and of the isolated gas are taken to be the same as those of the relaxed adsorbate system [START_REF] Scheffler | Theory of adsorption on Metal substarates[END_REF]. Insert Figure 5 In order to investigate the effect of coverage on the surface interactions of carbonates we report in table 3 the calculated adsorption energies of carbonates with increasing coverage.

With respect to the adsorption energy of single T-S1 (S1 surface includes 3 adsorption sites), the adsorption of a second and a third carbonate is lowered by 1.01 and 1.94 eV, respectively (the adsorption energies are calculated by considering the surface already adsorbing CO 2 molecules). Because of the strong repulsive interaction between oxygen carbonates, the second adsorbed T-S1, which has a similar geometry, adopts an orientation shifted with regard to the first carbonate. In that conformation (see figure 6), one magnesium ion of the edge is connected to two carbonates and so tries to reach the MgO bulk octahedral geometry.

The calculated Hessian matrix (table 2) of the adsorbed tridentate species indicates wavenumbers of 1603 cm -1 (T-S1), 1639 cm -1 (T-S2) and 1300 cm -1 (T-S1), 1287 cm -1 (T-S2)

for ν 3h and ν 3l , respectively. As shown in table 3, these later frequencies are affected by the presence of the second and the third neighbouring carbonate surface (θ = 2/3 and θ = 1 ML).

Thus, compared to the case of isolated carbonate, the calculated vibrations vary by less than 100 cm -1 for θ = 2/3ML and highly change for the monolayer coverage. The split between low and high frequencies grows when the coverage increases, due to the coupling between the adsorbed carbonates sharing a Mg 2+ cation. Similar results were also observed for CO 2 adsorbed on ZnO surface [START_REF] Saussey | [END_REF].

Insert table 3 and Figure 6 Considering these results, the experimental bands (Figures 2 and3) observed around 1660 and 1320 cm -1 , that remain clearly visible whether the considered temperature, can be attributed to oxygen does not make any connection with the surface. On the corner, the interaction of B-C3 induces a strong modification on the substrate. The surface oxygen atom (Oβ) is strongly displaced in order to insure the double connection of carbonate species with the surface. IR νCO (ν 3hν 3l ) frequencies of carbonates adsorbed on corners, kinks and divacancies, are calculated to be of 1727-1198 cm -1 (B-C3), 1679-1125 cm -1 (B-K) and 1649-1149 cm -1 (B-D), respectively. These resulted values can predict that the experimental band observed at 1680 cm -1 on the sample treated at temp = 523 and 673 K is the signature of bidentate carbonate species adsorbed on corner, kink and/or divacancie defects. The ν 3l band frequencies are out of the range of this study. As there is only one adsorption site, the effect of coverage is not considered here.

Finally, it remains the question of the observed IR bands between 1350 and 1600 cm -1

(namely located at 1516 cm -1 , 1423 cm -1 and 1367 cm -1 ) for the sample treated at temp = 523 and 673 K which cannot be explained by the presence of carbonate species on the MgO (100) defects. Moreover, Yanagisawa et al. [14] have already considered the adsorption of CO 2 on cubic crystallites of the MgO smoke, which exhibit well-defined (100) facets, and they have not observe any IR adsorption between 1350 and 1600 cm -1 . This means that the recorded IR frequencies on our samples would results from different adsorption configurations on other crystallographic planes even if they are known to be less stable in vacuum conditions compared to the (100) plane. In fact, several theoretical [34,[46][47] and experimental [48] studies have shown the change in the stability of surface orientations after the addition of adsorbates such as water. As IR spectroscopy showed that, in our conditions, few OH groups remain on the surface, we investigate, in the following section, the adsorption configurations and properties of CO 2 on facetted (110) and (111) MgO.

CO 2 species adsorbed on MgO(111) and (110) surfaces

In figures 4f and 4g are reported the configurations of carbonate species formed over the modelled MgO(111) 5) show similar isosurface shape of the electron rich zones of tridentate species, the adsorption is energetically favourable, by 0.49 eV, on the (110) than on the stepped (100).

The vibrational frequency calculations of T-110 indicate values at 1559 and 1331 cm -1 for the νCO 3h and νCO 3l , respectively. This carbonate type may correspond to the species vibrating around 1510 and 1360 cm -1 on our sample treated at temp = 523 and 673 K.

As for the stepped (100) surface, in presence of neighbouring carbonates the νCO strongly vary and the split between low and high frequencies of T-110 grows as the coverage increases (Table 3).

Over the (111) surface terminated by oxygen atoms, the carbonate lies to the surface through a single connection. Thus, the carbonates adsorbs as monodentate species on the MgO(111)

surface. In spite of the short d C-O(β) bound distance of 1.31 Å, which is the same than for the tridentate configuration, the small O-C-O angle of 115° illustrate the repulsive interactions that undergoes between the oxygen atoms (Oγ) and the surface (Figure 4g). These repulsive interactions can clearly be attested in figure 5 by the charge density difference on M-111 configuration that evidences the low electron density between (Oγ) and the surface. This may explains the low energetic stability of these species. In addition, as the carbonate presents a symmetry close to D 3h , the split between νCO 3h and νCO 3l is very low: 1440 and 1378 cm -1 (1415 cm -1 for the free carbonate [49]). These values are smaller than the frequency values calculated by Allen et al [23] who predicted a vibrational frequencies for adsorbed carbonates on MgO(111) surface in the ranges of 1440 -1459 and 1572 -1631 cm -1 . This is probably due to the empirical method used by these authors. In this work, the resulted monodentate carbonate, with weekly degenerated IR-band, could be attributed to the species vibrating at 1423 cm -1 on the DRIFT spectra for the sample treated at temp = 523 and 673 K.

Nevertheless, the low bonding energy of -0.8 eV is not consistent with the persistence of the IR bands for high temperature treatments. This point will be discussed in the next section. In table 3 are also reported the adsorption energies and νCO frequency values of carbonate M-111 for increasing coverage. The high coverage was not reached because of the strong repulsive interactions occurring between surface/carbonates and carbonate/carbonate species.

Thus, as depicted in table 3, for coverage of 3/4 monolayer the adsorption is highly endothermic and the calculated frequency values are highly different from those of single carbonate calculations.

Adsorption thermodynamics

In order to get a more precise picture of the respective stabilities of adsorbed carbon dioxide on different plans and defects and more particularly, to determine the temperature of desorption of the different adsorbed carbonates, the Gibbs free adsorption energy diagram was plotted for each considered system (see figure 7). The partial pressure is set to 0.001 Pa and desorption is presumed to occur when the Gibbs free adsorption energy becomes positive.

Insert Figure 7 As reported in figure 7, Gibbs free adsorption energies of carbonates follow the same linear trend. On the (100) plan, bidentate carbonate species which populate the corners, kinks and divacancies are predicted to leave the MgO support for temperature range of 722-822 K.

Tridentate carbonates formed on the Mg(100) steps would desorb at higher temperature and remain on the surface for temperatures as high as 908 K. This is in agreement with the assignment we made for the IR bands recorded at temp = 873 K to highly adsorbed tridentate species

The same kind of tridentate species is also strongly adsorbed on (110) surface thus should not disappear at temp < 1200 K. But experimentally, it is shown that the corresponding IR bands (1516 and 1367 cm-1) disappear from the spectrum at 673 < temp < 873 K. We suppose that this contradiction may be explained considering the well known stability of the (100) surface compared to the (110): surface recombination may thus occur at temp < 873 K, leading to the disappearance of the (110) plane and to the desorption of these species at lower temperature than predicted by our thermodynamic calculations. Moreover, the surface reconstruction of the (110) planes into reconstructed stepped (100) surface [50] may also explain the enhancement of the IR-adsorption at 1321 cm -1 assigned to T-S1. Let us add that this reconstruction phenomenon should be confirmed by an appropriate kinetic study.

Finally, on the (111) surface, the desorption of monodentate carbonates is predicted to occur at 314 K. Regarding this result, the IR band observed at 1440 cm -1 for temperature as high as temp = 673 K can not be attributed to the monodentate species. Du and co-workers have attributed similar infrared recorded bands of νCO 3h =1445 cm -1 and νCO 3l = 1375 cm -1 to bulk-like carbonates which could desorb at high temperature [19]. Mekhemer et al. [17] have reported similar suggestion. In order to characterize such carbonates, we have performed Raman spectra on a sample treated at 673 K. Nevertheless, no absorption band at 1100 cm -1 , characteristic of the ν 1 Raman active bulk carbonates [51][52] was observed on this sample. As bulk carbonates can be present but their amount may be under the detection limit of the Raman spectrometer, a conclusion on that point is difficult to be made.

New insights on basicity characterization

As underlined in the introduction, in order to study the catalysis, we need to know the nature of the basic sites available after the pretreatment step. For that purpose, the data obtained in this study on carbonates species, giving their nature, localisation, desorption temperature and IR vibration bands (calculated and experimentally detected) are summed up in Table 4. The new nomenclature of these species is also reported in order to take into account not only their mono-, bi-or tri-dentate character but also the topology of their adsorption site.

Insert Table 4 Moreover, the literature dealing with basic catalysts reactivity shows that the characterization of the basic sites is a key point for the interpretation of the catalytic results. One of the most popular method to determine these basic properties is CO 2 TPD that consists in following the progressive desorption of pre-adsorbed CO 2 with increasing temperature (see as an example,

Hattori's review [53]). An attempt to discuss the relevance of this method has already been made [54] and 740K, respectively [4]. Thus, it can be seen that the strongest interaction between water and the surface is achieved on kinks and divacancies whereas the most stabilizing sites for CO 2 adsorption are predicted here to be the steps. By consequence, direct correlation between the information obtained by CO 2 TPD and a reaction which first step is a deprotonation has to be made very cautiously.

Conclusion

Attribution of the IR bands of the carbonates adsorbed on MgO is a very important tool to identify the nature of the active basic sites available after a pretreatment. DFT calculations combined with the DRIFT measurement on MgO sample after peculiar pretreatments allow us to give a new assignment to these IR bands considering the adsorption on (100) defects and

(110) and (111) planes. These attributions are confirmed by the thermodynamic calculations of the stability of all carbonate species. Moreover, these results give a new tool to identify the crystallographic plane and defects by analysing the formation of IR-band when absorbing the CO 2 on it. Results show that the strongest basic site versus CO 2 , which is a Lewis acid, is not the same that the strongest basic site for the deprotonating adsorption of Brønsted acids. Thus, even of the same nature, Lewis and Brønsted basic sites differ by their basicity strength scale.

This conclusion should have consequences on the use of CO 2 TPD, when it has to be correlated with kinetic studies of basic Brønsted reactivity.

Tables: Carbonates: Gibbs free adsorption energy

species ν 3high ν 3low ν 1

CO 2

 2 molecule on flat (100) terraces T confirms that five coordinated O 2- 5C ions of MgO surface are totally unreactive for the formation of carbonates in agreement with literature data[21,42].Adsorption on surface defects: DFT calculations of CO 2 interactions with four and three coordinated oxygen ions (O 2- 4C and O 2- 3C ), involved in the defects of the MgO(100) plan, show largely more stable adsorbed carbonate species compared with flat MgO(100), (Table2). Pacchioni has investigated the interaction of CO 2 on regular and defect sites of the MgO(100) surface by mean of ab-initio cluster model SCF calculations[42]. He explained that because of the considerably large Madelung potential of a surface five coordinated oxygen ion (O 2- 5C site) compared to the lower coordinated site as the four coordinated O 2- 4C sites, the electron cloud of the O 2- 4C ion is more spatially diffuse and can easily overlap with the CO 2 empty orbitals interacting with the surface. Insert Figure 4 DFT optimizations predict that CO 2 molecule adsorbs with energy values of -2.26 and -2.16 eV on the S1 and S2 step systems, respectively (Figures 4.a and 4.b). The molecule interacts with O 2- 4C ions with an average d C-O distance of 1.37 Å and assumes an orientation "parallel" to the edge which allows a direct electrostatic interaction of its O atoms with the Mg 2+ 4C cations of the surface. The O-C-O angle is predicted to be of about 130°. These results are in line with those of Olsbye and co-workers [20] who have called a similar configuration of adsorbed carbonate species as "monodentate". Nevertheless, as in a such configuration the carbonate species form three connections (short bonds < 2.10 Å) with the MgO surface (one O 2-is already inserted in the magnesium oxide network (Oβ) and two Mg 2+ are linked to the oxygen ions (Oα) of CO 2 ). Thus we propose (see section 3.2.1) to call them as tridentate carbonates (T-S1 and T-S2).One way in which one can gain further insight into the molecule/surface interactions as well as the character of the bonding from the results of the DFT calculations is to investigate the change in the electron density distribution in the surface as a result of the adsorbate bonding.

Figure 5

 5 depicts the charge density difference of computed carbonate species graphically represented [44] by isosurface contours. These 3D visualizations reveal that all relevant chemical changes are suffered by the O valence of carbonates. Concerning the T-S1 configuration, there is clearly evidence of three connections occurring between carbonate and the edge of the MgO step. The electron charge appearing into O2p orbitals pointing towards Mg 2+ 4C cations indicates the formation of two polar covalent bonding. The third connection is the ionic one between (Oβ) and the magnesium cations of the oxide network.

Figure captions:Figure 1 :

 1 Figure captions:

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2: DRIFT spectra of the adsorbed species over magnesium oxide before (red line) and after (black line) pre-treatment at 1023 K (arbitral unit for absorbance)

Figure 5 :

 5 Figure 5: 3D visualization of the charge density difference of tridentate (T-S1 and T-110), bidentate (B-C3 and B-K) and mondentate (M-111) carbonate species. Pertinent isosurface values = -3.7 10 -2 e/Å 3 for T-S1 and T-110; -20.10 -2 e/A° for B-C3 and B-K; 12 10 -2 for M-111.

Figure 6 :Figure 7 :

 67 Figure 6 : Minimum energy configuration of the adsorption of two carbonates over the stepped MgO(100) defects; As the S1 has 3 adsorption sites on the edge, the coverage is supposed to be 2/3 ML.

Figure 3 Page 34 of 39 ACS

 339 Figure 3

Figure 4 PageFigure 5 Figure 6 Page 36 of 39 ACS

 45639 Figure 4

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	single tridentate species strongly attached on the steps defects of the magnesium oxide
	surface.
	The modelled corner, kink and divacancy systems (Figure 4.c, 4.d, 4.e) involve a three
	coordinated ion O 2-3C which interacts directly with the adsorbed CO 2 molecule. On this sites,
	the adsorption energy of carbonate species are found to be lower than on the four coordinated
	sites. The analysis of the distances between Oα and the surface (table 2) show that carbonates
	are doubly bounded to the surface through the network oxygen (Oβ) and one oxygen atom
	(Oα) from CO 2 . The bond character of these bidentate carbonates (B-C3, B-K and B-D) can

be analysed from the charge density difference graphic represented in figure

5

. On the kink (the same trend is observed for divacancy (not represented)), the adsorption does not strongly modify the slab. The electron rich zones appearing into O2p orbitals indicate only one polar covalent bond between (Oα) from CO 2 and Mg 2+ 4C . The electron rich zone around the third oxygen atom, the (Oγ) of the carbonate molecule, is orientated out of the plane and thus, that

  underlining that CO 2 adsorption probes the Lewis basic character of the surface whereas Brønsted basicity is mainly involved in catalytic reaction. Let us remind that Lewis basicity is the ability of the surface to give an electronic pair to a Lewis acid molecule while Brønsted basicity is, according to Brønsted definition [55], the ability to deprotonate a molecule. It implies that all Lewis bases are also Brønsted bases and reciprocally. But we demonstrate here, in the particular case of oxides ions, that the strength scale is different depending on the nature of the considered basicity. This demonstration is made from the comparison of the deprotonating adsorption energy of water (a Brønsted acid) and the formation of the Lewis adducts of CO 2 (a Lewis acid) on MgO (100) plane defects. Using similar theoretical approach, C. Chizallet and co-workers have established the desorption temperature of H 2 O from the MgO (100) defects. According to these authors water leaves S1, K, D and C3 defects at temperatures of 620K, 805K, 1110K

Table 1 :

 1 Review of the literature experimental attributions of IR-bands assigned to carbonates and hydrogen carbonates species adsorbed on MgO

	Hydrogen carbonates:
	ν 2	ν 3	ν 4	Work references
	1655 -1658 1405 -1419 1220 -1223	[16]
		1480	1250	[15]
	1650	1510,1408	1220	[18]

Table 2 :

 2 Geometric (distances (Å) and angles (degrees °)), energetic (E ads (eV)) and frequency values (νCO (cm -1 )) of optimized CO 2 species adsorbed on the (100) plan and its defects as well as the (110) and (111) surfaces. Values in brakets are reported from the litterature[20], b[22], c [14] , d[24], f[23]

		E ads (eV)	O(α)-Mg (Å)	O(α)-C (Å)	O(β) -C (Å)	O(γ)-C (Å)	OCO (degree)	ν 3h (cm -1 )	ν 3l (cm -1 )
	Terrace (T)	-0.09		2.70		1.18						177.8		2346		1308
	Monoatomic step (S1)	-2.26		2.10		1.28		1.36		-		129.9		1603		1300
	Diatomic step (S2)	-2.16 (-1.58) (-2.16)	a b	2.09 (2.14) (2.08)	b c	1.27 (1.25) (1.26) (1.26)	a b c	1.38 (1.38) (1.37) (1.38)	a b c	-		130.6 (130.6) (131) b	a	1639 (1784) (1606)	a c	1287 (1288) (1237)	a c
	Corner (C3)	-1.53 (-1.18) (-1.87)	a b	1.95 (1.97)	b	1.31 (1.29) (1.29)	b c	1.40 (1.42) (1.40)	b c	1.24 (1.21) (1.22)	a b	126.9 (129) a (128) b		1727 (1778)	b	1198 (1243)	b
	Kink (K)	-2.02		2.04		1.34		1.37		1.23		125.8		1679		1125
	Divacancy (D)	-1.97		2.05		1.33		1.37		1.24		125.9		1649		1149
	(110)	-2.75 -2.16 d	2.08		1.28		1.34		-		128.9		1559		1331
	(111)	-0.79 (0.28) d (-3.97) f			1.31		1.31		-		115.8		1440 (1572-1631) f	1378 (1440 -1459)

f Ref. a

Table 3 :

 3 Calculated DFT adsorption energies (eV) and ν 3h and ν 3l IR vibrations (cm -1 ) for different coverage of carbonate species adsorbed on MgO(100) steps and MgO (110) and (111) surfaces. Values in brakets are reported from the litterature Ref. G[START_REF] Scheffler | Theory of adsorption on Metal substarates[END_REF]

		Coverage ML	E ads (eV)	ν 3h (cm -1 )	ν 3l (cm -1 )
		1/3	-2.26	1603	1300
	S1 (3 sites)	2/3	-1.25	1691, 1626 (1588-1638) G	1284, 1259 (1520-1564) G
		1	-0.32	1754, 1710, 1657	1260, 1255, 1248
		1/3	-2.16	1639	1287
	S2 (3 sites)	2/3	-0.90	1728, 1691	1280, 1262
		1	+0.93	1824, 1775, 1695	1284, 1235, 1224
		1/9	-2.75	1559	1331
		1/3	-2.49	1554, 1552, 1525	1316, 1302, 1301
	(110) (9 sites)	2/3 1	-0.93 +2,70	1688, 1687, 1660, 1609, 1608, 1594 1796, 1795, 1759, 1759, 1743, 1742, 1742, 1721, 1720	1287, 1272, 1271, 1254, 1253,1250 1270, 1258,1258, 1252, 1252, 1251, 1251, 1246, 1246
	(111) (9 sites)	1/4 2/4 3/4	-0.85 -0,34 +0,10	1440 1523, 1463 1850, 1824, 1789	1378 1399, 1236 1161, 1077, 1034

Table 4 :

 4 Summary of experimental and theoretical data: Nomenclature, Desorption temperatures (K), and frequencies of all computed carbonate species adsorbed over MgO support.

	experimental data	DFT Calculations

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  Scheme 1: Description of the carbonate species adsorbed on magnesium oxide according to the literature and as revisited in this study

		O		O		O
							O	O
		C		C		C	C
		O	O	O	O	O	O
							Mg	O	Mg
		Mg		Mg	Mg	Mg
	Described as	Monodentate [2], unidentate [6]	Bridging [2,6]	Bidentate [2,6]	Polydentate [2] monodentate [15]
	Our description	Monodentate	Bidentate	Bidentate	Tridentate

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Acknowledgments: The authors kindly wish to acknowledge David Loffreda for the thermodynamic phase diagram calculations of adsorption of carbonates and Konstantin Hadjiivanov, Françoise Delbecq, Frederik Tielens as well as Guylène Costentin for the fruitful discussions. This work was granted access to the HPC resources of [CCRT/CINES/IDRIS] under the allocation 2010 [x2010086395] made by GENCI (Grand Equipement National de Calcul Intensif].

The graphic for the TOC